Optical fibers and their use

Abstract

An optical fiber is provided having a first portion and a second portion: the second portion is a void portion having one or more voids, the one or more voids extending longitudinally along the length of the optical fiber within the void portion; the first portion and the second portion are substantially mode-matched; and the one or more voids are at least partially filled with a filler having material properties different from the material properties of the optical fiber material.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber having machined voids therein, a method for manufacturing an optical fiber having voids therein, and applications of an optical fiber having voids therein.

Background Art

[0002] Optical fibers are used for a wide range of applications, including communication and various types of environmental sensing. The propagation of light in an optical fiber is greatly affected by the properties of the material of the optical fiber. Many studies have been conducted to optimize the materials and combinations of materials used in optical fibers intended for different applications. However, it can still be difficult to find a suitable material or combination of materials having the desired properties for a particular application. In particular, the properties of optical fiber materials can change with temperature and pressure, which can cause undesirable effects on the propagation of light in the fiber.

[0003]

Summary of the Invention

Problems to be Solved by the Invention

[0004] ​In light of these issues, it would be desirable to provide optical fibers that have more easily adjustable characteristics and can be manufactured more flexibly. [Means for solving the problem]

[0005] According to a first aspect of the present invention, an optical fiber having one or more voids is provided: the one or more voids extend longitudinally along the length of the optical fiber within the void portion, and the void portion is a continuous and integral part of the optical fiber; the longitudinal extent of each of the one or more voids is smaller than the longitudinal extent of the void portion; and the one or more voids are sealed from the outside of the optical fiber and are at least partially filled with a filler having material properties different from those of the material of the optical fiber.

[0006] Providing voids containing filler material allows for the tuning of optical fiber properties by varying the properties of the filler material and the arrangement of the voids. Providing voids within an integral portion of the fiber offers improved mechanical strength for spliced ​​fibers and greater flexibility in the design and arrangement of the voids.

[0007] Optionally, one or more voids may have two or more voids filled with different fillers. Optionally, the material properties of the fillers may differ between the different fillers. This provides greater flexibility in tuning the properties of the fibers.

[0008] Optionally, the material properties are optical properties, and optionally, the refractive index. Optionally, one or more voids are configured such that the optical properties of the filler affect the light transmitted through the optical fiber. This can enable a stronger, more direct effect on light in the fiber, rather than relying on indirect effects such as changing the expansion properties of the fiber material.

[0009] Optionally, the temperature-dependent changes in the material properties of the filler are different from, and optionally inversely, those of the optical fiber material. Optionally, one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the temperature-dependent changes in the material properties of the optical fiber material. This allows the voids to reduce the effect of temperature on the fiber, thereby further stabilizing its properties over a wider range of conditions.

[0010] Optionally, the cross-sectional area of ​​at least one of the one or more voids varies along the length of the optical fiber. Optionally, the cross-sectional area decreases along the length of the optical fiber, moving away from the center of the void for at least a portion of the void's length. Optionally, the central axis of one or more voids extends along a direction inclined with respect to the longitudinal axis of the optical fiber for at least a portion of the voids. This allows for variations in its properties along the length of the fiber and across the cross-section of the fiber, potentially achieving more localized effects.

[0011] Optionally, at least one of the voids has a non-circular cross-section. This can offer greater flexibility in how the filler affects light in the fiber, and how the void and filler respond to external conditions such as pressure, compared to existing fibers stretched from preforms with substantially circular drilled holes.

[0012] Optionally, the material properties are optical properties, and optionally, refractive index; one or more voids are configured such that the optical properties of the filler affect the light transmitted through the optical fiber; and variations in the cross-sectional area of ​​one or more voids and / or the orientation of the central axis of one or more voids are such that the light transmitted through the optical fiber experiences substantially continuous variations in the effect of the optical properties on the transmission of light along the length of the optical fiber. Continuous changes in optical properties reduce reflections caused by discontinuous interfaces within the fiber, thereby improving transmission performance.

[0013] Optionally, the optical fiber has a core and cladding surrounding the core, and one or more voids have at least one cladding void within the cladding. Optionally, the cladding voids are configured such that the transmission of light within the optical fiber is influenced by the filler within the cladding voids, and optionally, at least one cladding void is adjacent to the core. The cladding voids may allow for weaker interactions between light and filler in the fiber, which is primarily guided by the core, and may enable a more controlled effect on light in the fiber.

[0014] Optionally, the cladding void extends so that the filler material contacts the core. Optionally, the cladding void extends so that the filler material does not contact the core. Contact between the filler material and the core may be desirable or undesirable in different applications, depending on the desired strength of the interaction between the filler material and the light guided by the fiber core.

[0015] Optionally, one or more voids may have multiple cladding voids within the cladding, optionally, at least two cladding voids, optionally, at least four cladding voids, and optionally, at least six cladding voids. More cladding voids may allow for greater control over the effect of the filler on light propagating in the fiber.

[0016] Optionally, multiple cladding voids are arranged symmetrically around the core. Optionally, the distance between the cladding voids and the core varies along the length of the cladding voids. This may allow for greater control over the effect of the filler on light in the fiber and the variation of this effect along the length of the fiber or around the outer circumference of the fiber.

[0017] Optionally, the optical fiber may further have one or more access gaps extending from one or more voids toward the outer surface of the optical fiber. The access gaps allow voids within the fiber to be filled from the outer surface of the fiber during manufacturing.

[0018] Optionally, one or more access voids are sealed on the outer surface of the optical fiber. Optionally, the access voids are sealed by a block member within the access void, and optionally, the block member is entirely within the access void; or, they are sealed by melting the optical fiber material on the outer surface of the optical fiber. Sealing the access voids ensures that the voids remain filled after manufacturing and that the effect of the voids on light in the fiber is consistent throughout the fiber's use.

[0019] Optionally, one or more voids are formed by etching the optical fiber material through one or more access voids before sealing one or more access voids. Optionally, the optical fiber is formed by stretching a preform, and one or more voids are formed after the optical fiber is stretched. In particular, forming voids by etching after the fiber is stretched provides greater control over the shape, location, and extent of the voids compared to existing techniques in which the channel is determined before stretching.

[0020] Optionally, one or more voids extend entirely within the void portion, and / or the boundaries of one or more voids are defined entirely within the void portion. Optionally, the void portion has no interfaces with adjacent solid materials in the longitudinal direction. Forming voids within a continuous and integrated void portion as described above improves the mechanical strength of the fiber.

[0021] Optionally, one or more voids are completely filled with a filler material. This provides uniform properties throughout the void.

[0022] Optionally, the filler may be a gas, and optionally nitrogen or air. Optionally, the filler may be a non-gaseous material, and optionally a liquid. Optionally, the filler may be glycerol or a glycerol-water mixture. Different fillers may be suitable for different applications, for example, based on the required refractive index or other properties.

[0023] Optionally, the filler material may contain liquid crystals. This could be useful in applications where changing the properties of the filler material is advantageous.

[0024] Optionally, the refractive index of the filler is within 0.1 of the refractive index of the optical fiber material, optionally within 0.05, optionally within 0.01, optionally within 0.005, and optionally within 0.001. Optionally, the optical fiber has a core and cladding surrounding the core, and the refractive index of the filler is approximately equal to the refractive index of the cladding material at a reference temperature. Strictly matching the refractive index of the filler with that of the optical fiber material reduces disturbances to the waveguide properties and mode characteristics of the optical fiber.

[0025] Optionally, optical fibers have outer diameters of 25 μm to 300 μm, optionally 100 μm to 300 μm, and optionally approximately 125 μm or 250 μm. These diameters are useful for typical optical fiber applications.

[0026] Optionally, the optical fiber is a single-mode optical fiber. Optionally, the optical fiber is an anti-resonant fiber or a negative curvature fiber. These types of fibers may be suitable for specific applications.

[0027] Optionally, optical fibers contain silica. This is a common material for optical fibers because it is inexpensive to manufacture.

[0028] Optionally, optical fibers may have crystalline material, or optionally, single-crystal material. Optionally, optical fibers may be crystalline fibers, or optionally, sapphire-derived fibers. Including at least a portion of crystalline material in optical fibers allows some of the advantageous properties of crystalline material to be used in optical fibers.

[0029] Optionally, optical fibers are crystalline optical fibers, and optionally, single-crystal optical fibers. Optionally, optical fibers have sapphire, diamond, or yttrium aluminum garnet (YAG) crystals. Crystalline optical fibers may be more resistant to extreme temperature and pressure conditions than other common optical fiber materials.

[0030] The crystals can be optionally doped, and optionally doped with rare earth elements. This may provide further control over the optical properties of the optical fibers.

[0031] Optionally, optical fibers have a Bragg grating. The Bragg grating causes reflection of light at specific wavelengths, which can be affected by the fiber's characteristics. This allows the fiber to be used in a wide range of environmental sensing applications.

[0032] Optionally, the optical fiber has a core and cladding surrounding the core, and the Bragg grating is located at least partially on the core. Optionally, the Bragg grating is provided by periodic modification of the core material. This allows the Bragg grating to strongly interact with light in the fiber, typically most localized around the core.

[0033] Optionally, the Bragg grating is provided by periodic modification of the cladding material, and optionally, the periodic modification is adjacent to the core. This may be preferable when modifying the fiber core is difficult or undesirable.

[0034] Optionally, the material properties are optical properties, and one or more voids are configured such that the optical properties of the filler affect the Bragg wavelength of the Bragg grating. Optionally, one or more voids at least partially surround the Bragg grating. This may allow the filler to modify the optical behavior of the Bragg grating.

[0035] Optionally, the longitudinal extent of one or more voids is at least the same length as the longitudinal extent of the Bragg grating. Optionally, one or more voids extend up to 1 mm beyond the ends of the Bragg grating, and optionally up to 0.5 mm beyond those ends. This ensures a uniform effect of the filler along the entire length of the grating.

[0036] Optionally, one or more voids are configured such that the temperature-dependent change in the material properties of the filler at least partially compensates for the effect of the temperature-dependent change in the material properties of the optical fiber material on the Bragg wavelength of the Bragg grating, and optionally, the material properties are optical properties. This can stabilize the behavior of the Bragg grating with respect to temperature. Optionally, one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at a maximum of 5 pm / °C, optionally at a maximum of 2 pm / °C, and optionally at a maximum of 1 pm / °C over a temperature range of at least 10°C, optionally at least 20°C, and optionally at least 40°C. This can enable more accurate sensing of quantities such as pressure or strain, or more stable filtering, using the grating without the confounding effects of temperature shifts.

[0037] Optionally, one or more voids are configured such that the temperature-dependent change in the material properties of the filler increases the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating compared to optical fibers without one or more voids, and optionally, the material properties are optical properties. Optionally, one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 20 pm / °C, optionally at least 30 pm / °C, optionally at least 40 pm / °C, and optionally at least 50 pm / °C over a temperature range of at least 20°C, and optionally at least 40°C. This can make the effect of temperature on the grating more pronounced and enable more sensitive detection of temperature changes using the grating.

[0038] Optionally, the Bragg wavelength of a Bragg grating decreases with increasing temperature.

[0039] Optionally, the optical fiber has two separate Bragg gratings. This allows for greater design flexibility in the properties of the optical fiber.

[0040] Optionally, the material properties are optical properties, and one or more voids are configured such that the optical properties of the filler have different effects on the Bragg wavelengths of the two Bragg gratings. Optionally, one or more voids are configured such that the optical properties of the filler affect the Bragg wavelength of one of the two Bragg gratings, but not the Bragg wavelength of the other Bragg grating. This can enable differential sensing or other applications by observing how changes in conditions affect the two Bragg gratings differently, for example, one grating being stabilized against temperature effects while the other is not. Another example is when one or more voids are configured such that a temperature-dependent change in the material properties of the filler increases the magnitude of the temperature-dependent change in the Bragg wavelength for only one of the two separate Bragg gratings, and optionally, the temperature-dependent change in the material properties of the filler at least partially compensates for the effect of a temperature-dependent change in the material properties of the optical fiber material on the Bragg wavelength of the other of the two separate Bragg gratings. This latter example can be achieved by using different fillers in different voids.

[0041] Optionally, the optical fiber has a core and cladding surrounding the core, and two Bragg gratings are spaced apart longitudinally along the core. This may be a simple method for manufacturing multiple Bragg gratings in an optical fiber.

[0042] Optionally, the optical fiber has a core and cladding surrounding the core; one of the two Bragg gratings is placed in the cladding; and the other of the two Bragg gratings is placed in the core. This reduces the longitudinal spread of the two Bragg gratings as a whole, making the assembly more compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fiber and that both experience environmental conditions at the same point along the fiber. This provides improved consistency in sensing applications.

[0043] Optionally, one of the two Bragg gratings is placed in a waveguide optically coupled to the core, so that some of the light guided by the core is transferred into the waveguide. This allows both Bragg gratings to interact with the light in the fiber at equivalent intensity levels.

[0044] Optionally, the material properties are optical properties, and one or more voids have at least one void configured such that the transmission of light in the waveguide is affected by the optical properties of the filler within the void, and optionally, at least one void is adjacent to the waveguide. This allows both gratings to be affected by the filler.

[0045] Optionally, one or more voids may have cladding voids configured to provide waveguides within the cladding; the cladding voids are optically coupled to the core so that some of the light guided by the core is transferred into the cladding voids; and one of the two Bragg gratings is provided by periodic modification of the optical fiber material adjacent to the cladding void. This may allow the light to interact more directly with the filler in the waveguide.

[0046] Optionally, one or more air gaps are configured to isolate one of the Bragg gratings from strain within the optical fiber. Optionally, the isolation air gap surrounds one end of the Bragg grating. Optionally, the isolation air gap surrounds at least 50%, optionally at least 75%, optionally at least 90%, and optionally 100% of the length of one of the Bragg gratings. This allows for differential strain sensing between two Bragg gratings by isolating one grating from the effects of strain. Differential sensing can then be performed to account for the effects of other factors, such as temperature.

[0047] Optionally, the optical fiber has a core and cladding surrounding the core; one of two Bragg gratings is placed in the cladding; the other of two Bragg gratings is placed in the core; and one of the two Bragg gratings is placed in a waveguide optically coupled to the core, so that some of the light guided by the core is transferred into the waveguide. This allows both gratings to be placed in more similar longitudinal positions, making the assembly more longitudinally compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fiber and that both experience the same environmental conditions at the same point along the fiber. This provides improved consistency in sensing applications.

[0048] Optionally, optical fibers are configured to exhibit birefringence. Birefringence means that light with different polarizations is affected differently as it propagates through the fiber. This enables differential sensing because external conditions can affect the two propagation modes differently, and these conditions can be determined from the difference in polarization behavior.

[0049] Optionally, the optical fiber has a core and cladding surrounding the core, and the optical fiber has one or more stress-inducing regions positioned around the core to contribute to birefringence. Optionally, the stress-inducing regions include laser-exposed regions. This allows for tuning of the birefringence characteristics.

[0050] Optionally, the optical fiber has a core and cladding surrounding the core, and one or more voids are arranged symmetrically around the core to contribute to birefringence, etc. This allows for further methods of tuning the birefringence characteristics.

[0051] Optionally, the multiple voids have two voids positioned on either side of the core, along the first diameter of the optical fiber. Optionally, no voids are provided along the second diameter of the optical fiber perpendicular to the first diameter. This is a convenient arrangement for generating a birefringence effect.

[0052] Optionally, the optical fiber has a Bragg grating, and the birefringence is different for light with different polarizations and Bragg wavelengths of the Bragg grating; optionally, the polarizations are orthogonal. Using a Bragg grating generates easily distinguishable peaks that can be used in sensing applications.

[0053] Optionally, birefringence is such that when the pressure in one or more gaps is substantially equal to the pressure outside the optical fiber, the reflection peaks for light with different polarizations around the corresponding Bragg wavelength of the Bragg grating are resolvable, and optionally, the reflection peaks are separated by the total width of at least half of the reflection peak. This ensures that the peaks are resolvable even without an external pressure difference, making it easier to calibrate and measure the pressure using sensors.

[0054] Optionally, the material properties are optical properties, and one or more voids are configured such that the effect of the filler's optical properties on light guided by the optical fiber differs for light with different polarizations, and optionally the polarizations are orthogonal. This may allow for differential tuning of the properties of two birefringence modes by the filler.

[0055] Optionally, an optical fiber may have a waveguide separate from its core. This allows for the placement of additional features in the fiber at the same longitudinal positions as features in the core.

[0056] Optionally, a waveguide is formed by one or more modified regions of an optical fiber, in which the optical properties of the optical fiber differ from those of the optical fiber material surrounding the modified region, and optionally, the optical property is the refractive index. This creates a feature in the fiber that allows light to be guided.

[0057] Optionally, one or more modified regions may have one or more laser-exposed regions. Optionally, one or more modified regions may have one or more voids extending longitudinally along the length of the optical fiber. Both of these are convenient methods for creating regions that can be integrated into the void manufacturing process.

[0058] Optionally, the optical fiber is configured to exhibit birefringence, and one or more modified regions are configured to contribute to birefringence. This enables the birefringence effect described above.

[0059] A strain sensor having an optical fiber according to a first aspect of the present invention is also provided, wherein the optical fiber has a Bragg grating. By measuring the wavelength of the Bragg grating, it is possible to determine the strain applied to the optical fiber. Filling material in the void allows for tuning of the fiber's properties to enhance or enable strain sensing for specific environments and applications.

[0060] Optionally, the strain sensor may further have a controller configured to determine the strain applied to the optical fiber based on the Bragg wavelength of the Bragg grating. This allows for sensor readout.

[0061] Optionally, the optical fiber is configured to exhibit birefringence; and the controller is configured to determine strain based on the difference between Bragg wavelengths of the Bragg grating for light with different polarizations. This type of differential sensing may be more robust than sensing based on the absolute value of the Bragg wavelength.

[0062] A system for sensing strain and / or temperature is also provided, having an optical fiber according to a first aspect of the present invention, wherein the optical fiber has two separate Bragg gratings, and the temperature-dependent changes in the Bragg wavelengths of the two Bragg gratings are different. This may allow the effects of strain and temperature to be separated due to the different effects on the two gratings, so that both temperature and strain can be accurately determined.

[0063] Optionally, the system may further include a controller configured to determine the strain applied to the optical fiber and the temperature of the optical fiber based on the Bragg wavelengths of the two Bragg gratings. This allows for readout of the strain and temperature.

[0064] A pressure sensor having an optical fiber according to a first aspect of the present invention is also provided, the optical fiber having a Bragg grating. External pressure shifts the Bragg wavelength. The optical fiber may be particularly useful as a pressure sensor in constrained or extreme environments.

[0065] Optionally, the pressure sensor is configured such that the pressure difference between the pressure of the filler and the external pressure applied to the optical fiber affects the Bragg wavelength of the Bragg grating. This allows for control of pressure sensing based on the properties of the filler, for example, to increase sensitivity.

[0066] Optionally, the pressure sensor may further have a controller configured to determine the external pressure applied to the optical fiber based on the Bragg wavelength of the Bragg grating. This allows for pressure readout.

[0067] Optionally, the optical fiber is configured to exhibit birefringence; pressure differences affect birefringence; and the controller is configured to determine the external pressure applied to the optical fiber based on the difference between Bragg wavelengths of the Bragg grating for different polarizations of light. Differential sensing allows for more accurate and sensitive sensing than determining the absolute value of the wavelength. Birefringence can make this possible using different polarizations of light.

[0068] Optionally, the optical fiber may have a coating; temperature-dependent changes in the material properties of the coating affect the light transmitted through the optical fiber; and one or more voids are configured such that temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of temperature-dependent changes in the material properties of the optical fiber and the coating on the light transmitted through the optical fiber. Coatings are commonly used to protect optical fibers, but because the coating has a different coefficient of thermal expansion than the optical fiber material, for example, it can affect how the optical fiber responds to external conditions such as temperature. Compensating for this allows the coating to be used while stabilizing the fiber properties.

[0069] According to a second aspect of the present invention, an optical fiber is provided, which has: one or more voids at least partially filled with a filler, the filler having material properties different from those of the optical fiber material; and a coating, the temperature-dependent changes in the material properties of the coating affect the light transmitted through the optical fiber; and the one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of the temperature-dependent changes in the material properties of the optical fiber material and the temperature-dependent changes in the material properties of the coating on the light transmitted through the optical fiber. Coatings are commonly used to protect optical fibers, but they can affect their optical properties. Compensating for this allows the coating to be used while stabilizing the fiber properties.

[0070] Optionally, one or more voids extend longitudinally along the length of the optical fiber within the void portion, and the void portion is a continuous and integral part of the optical fiber. Optionally, the longitudinal extent of each of the one or more voids is smaller than the longitudinal extent of the void portion. Forming voids within a continuous and integral material avoids weaknesses that may be created by joining and splicing fibers.

[0071] Optionally, one or more voids are sealed from the outside of the optical fiber. This allows the effect of the filler to be consistently determined and controlled.

[0072] Optionally, the coating can be polyacrylate or polyimide. Optionally, the coating can be metallic. These are common coating types used for many optical fiber applications.

[0073] Optionally, material properties are optical properties, and optionally, refractive index. This makes it possible to control the propagation of light in a fiber by the filler material.

[0074] Optionally, optical fibers may have Bragg gratings. This generates clearly detectable peaks in the fiber spectrum, which can be used for various sensing applications.

[0075] Optionally, the temperature-dependent changes in the material properties of the coating affect the Bragg wavelength of the Bragg grating; and one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of the temperature-dependent changes in the material properties of the optical fiber and the coating on the Bragg wavelength of the Bragg grating. Optionally, one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 5 pm / °C, optionally at least 20°C, and optionally at least 40°C, over a temperature range of at least 10°C, optionally at least 20°C, and optionally at least 40°C. Compensating for the effect on the Bragg wavelength stabilizes the peak for sensing applications.

[0076] Optionally, an optical fiber has a core and cladding surrounding the core, and one or more voids within the core are core voids. Core voids allow for very strong interactions between the filler material and light in the fiber, which are usually most strongly localized in the core.

[0077] According to a third aspect of the present invention, an optical fiber is provided, which has: one or more voids; and a Bragg grating; one or more voids extending longitudinally along the length of the optical fiber; one or more voids being at least partially filled with a filler having different optical properties than that of the material of the optical fiber; the optical fiber having a core and cladding surrounding the core; and one or more voids having core voids within the core. The core voids allow for very strong interaction between the filler material and light in the fiber, which is usually most strongly localized in the core.

[0078] Optionally, the core consists substantially of core voids. This means that the propagation of light is primarily determined by the filler material.

[0079] Optionally, Bragg gratings are provided by periodic modification of the cladding. This allows for Bragg gratings with several types of packing materials, such as liquid or gaseous packing materials, which may not exhibit periodic modification.

[0080] Optionally, the core is aligned with the central axis of the optical fiber. This creates symmetrical cladding around the core.

[0081] Optionally, Bragg grating is provided by periodic modification of the filler in the core void, and optionally the filler is liquid crystal. Optionally, the filler is polymerizable, and the periodic modification involves periodic polymerization, and optionally, the filler contains monomers and photoinitiators, and the periodic polymerization is performed using a laser. This allows for Bragg grating in the core where the interaction with light is strongest, even when using liquid fillers.

[0082] Optionally, the optical fiber may further have one or more electrodes configured to apply an electric field to the filler material, thereby influencing the material properties of the filler. This may allow for adjustment of the filler material's properties even after manufacturing.

[0083] According to a fourth aspect of the present invention, an optical fiber having one or more voids is provided: one or more voids extending longitudinally along the length of the optical fiber; one or more voids being at least partially filled with a filler having material properties different from those of the optical fiber material; and the optical fiber further having one or more electrodes configured to apply an electric field to the filler to affect the material properties of the filler. The electrodes allow for adjustment of the properties of the filler even after manufacturing.

[0084] Optionally, the filler material may be liquid crystal. This is a class of material that is well understood to have properties that can be tuned by an electric field.

[0085] Optionally, the electric field is configured to affect one or more of the refractive index, absorption, and scattering losses of the filler. Optionally, the filler exhibits birefringence, and the electric field is configured to affect one or more of the magnitude of the birefringence and the angle of the optical axis of the birefringence. These properties would allow for flexibility in influencing the propagation of light in the fiber.

[0086] Optionally, the optical fiber may have a Bragg grating, and the electric field may be configured to affect the Bragg wavelength of the Bragg grating. The Bragg grating generates an easily detectable peak, which may be advantageous for sensing applications.

[0087] According to a fifth aspect of the present invention, a crystalline optical fiber having one or more voids is provided, the one or more voids extending longitudinally along the length of the optical fiber. The crystalline fiber is more durable than other types of optical fibers, such as silica, allowing them to be used under extreme conditions. Providing voids may allow the properties of the optical fiber to be tuned by varying the arrangement of the voids.

[0088] Optionally, one or more voids are at least partially filled with a filler having material properties different from those of the optical fiber material, and optionally, the material properties are optical properties, and optionally, the refractive index. Providing voids containing a filler makes it possible to adjust the properties of the optical fiber by varying the properties of the filler.

[0089] Optionally, the longitudinal extent of one or more voids is less than the length of the optical fiber. Closed voids within the fiber allow for easier bonding to other components that interface with the optical fiber. Closed voids are also advantageous for pressure sensing and for preventing fluid from entering the void and affecting the properties of the filler.

[0090] Optionally, one or more voids extend within the void portion, and the void portion is a continuous and integral part of the optical fiber. Optionally, the longitudinal extent of one or more voids is smaller than the longitudinal extent of the void portion. Providing voids within an integral portion improves the structural performance of spliced ​​or joined fibers.

[0091] Optionally, the central axis of one or more voids extends along a direction inclined with respect to the longitudinal axis of the optical fiber for at least a portion of one or more voids. This can be used to create a smoother transition in properties along the fiber and reduce loss and reflection around the voids.

[0092] Optionally, optical fibers have a Bragg grating. The Bragg grating causes reflection of light at specific wavelengths, which can be affected by the fiber's characteristics. This allows the fiber to be used for a wide range of environmental sensing applications.

[0093] Optionally, optical fibers are configured to exhibit birefringence. Birefringence means that light with different polarizations is affected differently as it propagates through the fiber. This enables differential sensing because external conditions can affect the two propagation modes differently, and these conditions can be determined from the difference in polarization behavior.

[0094] Optionally, optical fibers can be anti-resonant fibers or negative curvature fibers. These types of fibers may be suitable for specific applications.

[0095] Optionally, optical fibers may be single-crystal optical fibers, or optionally, sapphire, diamond, or yttrium aluminum garnet (YAG) crystals. These types of crystals are known for their use as optical fibers and therefore possess well-understood properties.

[0096] The crystals can be optionally doped, and optionally doped with rare earth elements. This may provide further control over the optical properties of the optical fibers.

[0097] A sixth aspect of the present invention provides a method for forming one or more voids in an optical fiber, the method comprising: selectively exposing an optical fiber to laser irradiation to define one or more exposed regions within the optical fiber; bringing the optical fiber into contact with an etching solution, the etching solution etching the exposed regions at a faster rate than the regions of the optical fiber not exposed to laser irradiation, so that one or more voids are formed by etching the exposed regions; at least partially filling one or more voids with a filler having material properties different from those of the optical fiber material; and sealing one or more voids from the outside of the optical fiber.

[0098] Existing methods for creating optical fibers with holes along their length typically rely on fabricating a fiber preform and then stretching the preform. This creates significant limitations on the structure of the holes that can be created in the final fiber. The method of the present invention makes it possible to form voids of almost any shape and size within an optical fiber with much higher precision than existing methods.

[0099] Optionally, one or more exposure areas may have high-exposure and low-exposure regions, with the high-exposure regions being exposed to higher doses of laser irradiation than the low-exposure regions. Optionally, even higher doses of laser irradiation can be achieved by varying one or more of the following: laser power, laser scanning speed, and laser beam profile. This may allow for more precise control of the manufacturing process.

[0100] Optionally, the etching solution etches higher exposure areas faster than lower exposure areas. This results in a differential etching rate during the etching step, allowing for more precise control over the void formation process.

[0101] Optionally, selectively exposing an optical fiber to laser irradiation further involves defining one or more access regions extending to the outer surface of the optical fiber, such that each of the one or more exposed regions is connected to the outer surface by at least one of the one or more access regions. This allows the void to be formed from any outer surface, not just from the ends of the fiber, as in common existing methods.

[0102] Optionally, during the step of bringing the optical fiber into contact with the etching solution, the access region is etched first to form one or more access voids, which allow the etching solution to come into contact with the exposed region within the optical fiber. This provides access for etching the exposed region that will form the voids.

[0103] Optionally, sealing one or more voids involves sealing one or more access voids on the outer surface of the optical fiber. This prevents subsequent changes to the contents of the voids.

[0104] Optionally, the access gap is sealed by inserting a block member into the access gap, with the block member entirely located within the access gap; or by melting the optical fiber material on the outer surface of the optical fiber, with the melting optionally performed using a laser or electric arc. These steps are easily integrated into the manufacturing process.

[0105] Optionally, this method further comprises the step of forming one or more stress-inducing regions in the optical fiber. Optionally, forming one or more stress-inducing regions involves selectively exposing the optical fiber to laser irradiation. The stress-inducing regions can be used to influence the optical properties of the fiber, providing greater flexibility in fiber design.

[0106] Optionally, one or more stress induction regions are configured to contribute to the birefringence of the optical fiber. Birefringence is advantageous for various sensing applications, as described above.

[0107] Optionally, the step of forming one or more stress-inducing regions is performed before the step of contacting the optical fiber with the etching solution, and the stress-inducing regions are separated from the exposed regions. Optionally, the step of forming one or more stress-inducing regions is performed after the step of contacting the optical fiber with the etching solution. If the stress-inducing regions are formed before etching, it should be determined to prevent the etching solution from affecting the stress-inducing regions. Otherwise, the stress-inducing regions may be formed after etching the voids.

[0108] Optionally, this method further comprises rinsing one or more voids with etching solution before filling one or more voids. This prevents contamination of the filler with etching solution or changes in the void shape over time.

[0109] Optionally, the etching solution contains potassium hydroxide, and optionally has a concentration of at least 5 moles, or optionally at least 8 moles. This etching solution is effective for etching typical optical fiber materials.

[0110] Optionally, the laser irradiation can be infrared or visible light. Optionally, the laser irradiation has wavelengths of 700nm–900nm, optionally 750nm–850nm, and optionally approximately 790nm. Optionally, the laser irradiation has wavelengths of 450nm–650nm, optionally 500nm–600nm, and optionally approximately 530nm. These wavelengths are effective in generating the typical modifications to the fiber material required to define the areas to be etched.

[0111] Optionally, laser irradiation is provided by a laser beam generated using a laser system. Optionally, the laser beam is a pulsed laser beam. Optionally, the pulses of the pulsed laser beam have durations of up to 1 ps, up to 500 fs, up to 200 fs, and up to 100 fs. This can provide more precise control over the laser characteristics.

[0112] Optionally, one or more exposure regions may have both high-exposure and low-exposure regions, with the high-exposure regions being exposed to higher doses of laser irradiation than the low-exposure regions. Even higher doses of laser irradiation are achieved by varying either or both the laser pulse energy and / or pulse repetition rate. Controlling these characteristics allows for precise control of the laser dose applied to optical fiber materials.

[0113] Optionally, selectively exposing an optical fiber to laser irradiation involves applying corrections to the active optical elements of the laser system to modify the wavefront characteristics of the laser beam and counteract the effects of aberrations on the laser focus. This allows for precise control of the region within the fiber exposed to laser irradiation by enabling the laser to be precisely directed within the fiber material.

[0114] Optionally, this method further involves forming a Bragg grating on the optical fiber. As mentioned above, Bragg gratings have various applications in sensing using optical fibers.

[0115] Optical fibers manufactured using the method of a sixth aspect of the present invention are also provided.

[0116] According to a seventh aspect of the present invention, an optical fiber is provided having a first portion and a second portion: the second portion is a void portion having one or more voids, the one or more voids extending longitudinally along the length of the optical fiber within the void portion; the first portion and the second portion are substantially mode-matched; and the one or more voids are at least partially filled with a filler having material properties different from those of the material of the optical fiber.

[0117] Advantageously, providing a void containing a filler material allows for tuning the properties of the optical fiber by varying the properties of the filler material and the arrangement of the void, and enables loss reduction in the substantially mode-matched portion of the optical fiber.

[0118] Optionally, the mode field diameter of the first part for a given wavelength and temperature is substantially the same as the mode field diameter of the second part for the same wavelength and temperature. This minimizes the loss of light traveling between the first and second parts.

[0119] Optionally, the first and second parts each have a core and a cladding, with each cladding surrounding its respective core.

[0120] Optionally, the refractive index of the core of the first part is substantially the same as that of the core of the second part; optionally, the refractive index of the core of the first part is 0.002 to 0.007 greater than the refractive index of the cladding surrounding the core of the first part; and optionally, the refractive index of the core of the first part is approximately 0.005 greater than the refractive index of the cladding surrounding the core of the first part. Matching the refractive indices of the cores of the first and second parts results in a low-loss transition between the first and second parts.

[0121] Optionally, the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fiber is substantially the same as the maximum lateral dimension of the core of the second portion in a direction substantially perpendicular to the longitudinal axis of the optical fiber at the interface between the first and second portions. Matching the dimensions of the cores of the first and second portions results in a low-loss transition between the first and second portions.

[0122] Optionally, the dopant concentration of the core in the first part is substantially the same as that of the core in the second part. Beneficially, the use of substantially the same dopant concentration means that the core size in the void can be better matched to that of the first part.

[0123] Optionally, the first portion is a sealed portion, and one or more voids are at least partially sealed by the sealed portion. Optionally, one or more voids are at least partially sealed from the outside of the optical fiber at the interface between the end face of the sealed portion and the end face of the void portion. Optionally, one or more voids are sealed from the outside of the optical fiber at a further interface between the other end face of the void portion and the end face of a further sealed portion. Optionally, the sealed portion has a standard single-mode fiber with a core surrounded by cladding, and the cladding has a substantially solid material that at least partially seals one or more voids in the void portion. Optionally, a further sealed portion has a standard single-mode fiber with a core surrounded by cladding, and the cladding has a substantially solid material that at least partially seals one or more voids in the void portion.

[0124] Beneficial inclusions ensure that the voids remain filled after manufacturing and that the effect of the voids on light in the fiber is consistent throughout the fiber's use.

[0125] Optionally, the optical fiber is a single-mode optical fiber. It provides preferred communication for specific applications.

[0126] Optionally, the mode field diameter is 9–12 micrometers, optionally, 9.6–11.2 micrometers, optionally, approximately 10.5 micrometers, and optionally, the given wavelength is within the C-band, L-band, and / or S-band. Advantageously, such mode matching enables single-mode communication with reduced loss.

[0127] Optionally, the core of the first part and the core of the second part each have a maximum lateral dimension greater than 5.5 micrometers and less than 11 micrometers at the interface between the first part and the second part in a direction substantially perpendicular to the longitudinal axis of the optical fiber, and optionally, the core of the first part and the core of the second part each have a maximum lateral dimension of 6 to 10.5 micrometers at the interface between the first part and the second part in a direction substantially perpendicular to the longitudinal axis of the optical fiber, and optionally, the core of the first part and the core of the second part each have a maximum lateral dimension of 6 to 10.5 micrometers at the interface between the first part and the second part At the interface between the two parts, the maximum lateral dimension is 7–10 micrometers in a direction substantially perpendicular to the longitudinal axis of the optical fiber, and optionally, the core of the first part and the core of the second part each have a maximum lateral dimension of 8–9 micrometers at the interface between the first and second parts in a direction substantially perpendicular to the longitudinal axis of the optical fiber, and optionally, the core of the first part and the core of the second part each have a maximum lateral dimension of approximately 8.2 micrometers at the interface between the first and second parts in a direction substantially perpendicular to the longitudinal axis of the optical fiber. Advantageously, such an optical fiber provides reduced loss in a standard single-mode optical fiber system.

[0128] Optionally, the first portion has a bridging portion configured to progressively convert the mode field diameter along the longitudinal direction of the bridging portion from a first mode field diameter to a second mode field diameter, wherein the second mode field diameter is substantially the same as the mode field diameter of the gap portion for a given wavelength and temperature. Optionally, the first portion has a bridging portion having a core having a cross-sectional area that varies along the longitudinal axis of the bridging portion from a first cross-sectional area to a second cross-sectional area, wherein the second cross-sectional area has a maximum lateral dimension substantially the same as the maximum lateral dimension of the core of the gap portion. Beneficially, the bridging portion has the effect of sealing the packed fiber and also acts as an adiabatic mode converter for converting from one mode size of the packed fiber to another mode size of a further fiber. Thus, it is possible to use packed fibers with mode field diameters that are very different from those of further optical fibers.

[0129] Optionally, the first component has a lens portion having a cross-sectional area that varies along the longitudinal axis of the lens portion, thereby progressively converting the mode field diameter along the longitudinal axis of the lens portion from a first mode field diameter to a second mode field diameter, the second mode field diameter being substantially the same as the mode field diameter of the gap portion for a given wavelength and temperature. Beneficially, such a conversion helps minimize coupling losses in optical systems with many components.

[0130] Optionally, optical fibers have a Bragg grating. The Bragg grating causes reflection of light at specific wavelengths, which can be affected by the fiber's characteristics. This allows the fiber to be used for a wide range of environmental sensing applications.

[0131] Optionally, the optical fiber has a core and cladding surrounding the core, and the Bragg grating is located at least partially on the core. Optionally, the Bragg grating is provided by periodic modification of the core material. This allows the Bragg grating to strongly interact with light in the fiber, typically most localized around the core. Optionally, the Bragg grating is provided by periodic modification of the cladding material, and optionally, the periodic modification is adjacent to the core. This may be preferable when modifying the fiber core is difficult or undesirable.

[0132] Optionally, the material properties are optical properties, and one or more voids are configured such that the optical properties of the filler affect the Bragg wavelength of the Bragg grating. Optionally, one or more voids at least partially surround the Bragg grating. This may allow the filler to modify the optical behavior of the Bragg grating.

[0133] Optionally, the longitudinal extent of one or more voids is at least the same length as the longitudinal extent of the Bragg grating. Optionally, one or more voids extend up to 1 mm beyond the ends of the Bragg grating, and optionally up to 0.5 mm beyond those ends. This ensures a uniform effect of the filler along the entire length of the grating.

[0134] Optionally, one or more voids are configured such that the temperature-dependent change in the material properties of the filler at least partially compensates for the effect of the temperature-dependent change in the material properties of the optical fiber material on the Bragg wavelength of the Bragg grating, where optionally the material properties are optical properties, and optionally, one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at a maximum of 5 pm / °C, optionally at a maximum of 2 pm / °C, and optionally at a maximum of 1 pm / °C over a temperature range of at least 10°C, optionally at least 20°C, and optionally at least 40°C. This enables more accurate sensing or more stable filtering of quantities such as pressure or strain without the confounding effects of temperature changes using the grating.

[0135] Optionally, one or more voids are configured such that the temperature-dependent change in the material properties of the filler increases the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating compared to optical fibers without one or more voids, and optionally, the material properties are optical properties. Optionally, one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 20 pm / °C, optionally at least 30 pm / °C, optionally at least 40 pm / °C, and optionally at least 50 pm / °C over a temperature range of at least 20°C, optionally at least 40°C, and optionally at least 50 pm / °C. Furthermore, optionally, the Bragg wavelength of the Bragg grating decreases with increasing temperature. This makes the effect of temperature on the grating more pronounced and may enable more sensitive detection of temperature shifts using the grating.

[0136] Optionally, the optical fiber has two separate Bragg gratings. This allows for greater design flexibility in the properties of the optical fiber.

[0137] Optionally, the material properties are optical properties, and one or more voids are configured such that the optical properties of the filler have different effects on the Bragg wavelengths of the two Bragg gratings, and optionally, one or more voids are configured such that the optical properties of the filler affect the Bragg wavelength of one of the two Bragg gratings but not the Bragg wavelength of the other Bragg grating. This can enable differential sensing or other applications by observing how changes in conditions affect the two Bragg gratings differently, for example, one grating being stabilized against the effects of temperature while the other is not. Another example is when one or more voids are configured such that the temperature-dependent change in the material properties of the filler increases the magnitude of the temperature-dependent change in the Bragg wavelength for only one of the two separate Bragg gratings, and optionally, the temperature-dependent change in the material properties of the filler is configured to at least partially compensate for the effect of the temperature-dependent change in the material properties of the optical fiber material on the Bragg wavelength of the other of the two separate Bragg gratings. This latter example can be achieved by using different fillers in different voids.

[0138] Optionally, the optical fiber has a core and cladding surrounding the core, and two Bragg gratings are spaced apart longitudinally along the core. This may be a simple method for manufacturing multiple Bragg gratings in an optical fiber.

[0139] Optionally, the optical fiber has a core and cladding surrounding the core; one of the two Bragg gratings is placed in the cladding; and the other of the two Bragg gratings is placed in the core. This reduces the longitudinal spread of the two Bragg gratings as a whole, making the assembly more compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fiber and that both experience environmental conditions at the same point along the fiber. This provides improved consistency in sensing applications.

[0140] Optionally, one of the two Bragg gratings is placed in a waveguide optically coupled to the core, so that some of the light guided by the core is transferred into the waveguide. This allows both Bragg gratings to interact with the light in the fiber at equivalent intensity levels.

[0141] Optionally, the material properties are optical properties, and one or more voids have at least one void configured such that the transmission of light in the waveguide is affected by the optical properties of the filler within the void, and optionally, at least one void is adjacent to the waveguide. This allows both gratings to be affected by the filler.

[0142] Optionally, one or more voids may have cladding voids configured to provide waveguides within the cladding; the cladding voids may be optically coupled to the core so that some of the light guided by the core is transferred into the cladding voids; and one of two Bragg gratings may be provided by periodic modification of the optical fiber material adjacent to the cladding void. This may allow the light to interact more directly with the filler in the waveguide.

[0143] Optionally, one or more air gaps are configured to isolate one of the Bragg gratings from strain within the optical fiber. Optionally, the isolation air gap surrounds one end of the Bragg grating, and optionally, the isolation air gap surrounds at least 50%, optionally at least 75%, optionally at least 90%, and optionally 100% of the length of one of the Bragg gratings. This enables differential strain sensing between two Bragg gratings by isolating one grating from the effects of strain. Differential sensing can then be performed to account for the effects of other factors, such as temperature.

[0144] Optionally, the optical fiber has a core and cladding surrounding the core; one of the two Bragg gratings is placed in the cladding; the other of the two Bragg gratings is placed in the core; and one of the two Bragg gratings is placed in a waveguide optically coupled to the core, so that some of the light guided by the core is transferred into the waveguide. This allows both gratings to be placed in more similar longitudinal positions, making the assembly more longitudinally compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fiber, and that both experience the same environmental conditions at the same point along the fiber. This provides improved consistency in sensing applications.

[0145] Optionally, optical fibers are configured to exhibit birefringence. Birefringence means that light with different polarizations is affected differently as it propagates through the fiber. This enables differential sensing because external conditions can affect the two propagation modes differently, and these conditions can be determined from the difference in polarization behavior.

[0146] Optionally, the optical fiber has a core and cladding surrounding the core, and the optical fiber has one or more stress-inducing regions positioned around the core to contribute to birefringence, and optionally, the stress-inducing regions have laser-exposed regions. This makes it possible to adjust the birefringence characteristics.

[0147] Optionally, the optical fiber has a core and cladding surrounding the core, and one or more voids are arranged symmetrically around the core to contribute to birefringence, etc. This allows for further methods of tuning the birefringence characteristics.

[0148] Optionally, the configuration may have two voids on either side of the core, aligned along the first diameter of the optical fiber, and further optionally, no voids may be provided along the second diameter of the optical fiber perpendicular to the first diameter. This is a convenient arrangement for generating a birefringence effect.

[0149] Optionally, the optical fiber has a Bragg grating, and the birefringence is different for light with different polarizations and Bragg wavelengths of the Bragg grating; optionally, the polarizations are orthogonal. Using a Bragg grating generates easily distinguishable peaks that can be used in sensing applications.

[0150] Optionally, birefringence is such that when the pressure in one or more gaps is substantially equal to the pressure outside the optical fiber, the reflected peaks of light with different polarizations around the corresponding Bragg wavelength of the Bragg grating are resolvable, and optionally, the reflected peaks are separated by the total width of at least half of the reflected peak. This ensures that the peaks are resolvable even without an external pressure difference, making it easier to calibrate and measure the pressure using sensors.

[0151] Optionally, the material properties are optical properties, and one or more voids are configured such that the effect of the filler's optical properties on light guided by the optical fiber differs for light with different polarizations, and optionally the polarizations are orthogonal. This allows for differential tuning of the properties of the two birefringence modes by the filler.

[0152] Optionally, an optical fiber may have a waveguide separate from its core. This may allow for the placement of additional features in the fiber at the same longitudinal positions as features in the core.

[0153] Optionally, the waveguide is formed by one or more modified regions of the optical fiber, in which the optical properties of the optical fiber differ from those of the optical fiber material surrounding the modified regions, optionally being the refractive index, and optionally each modified region having one or more laser-exposed regions. This creates a feature in the fiber that can guide light.

[0154] Optionally, one or more modified regions may have one or more voids extending longitudinally along the length of the optical fiber. Both of these are convenient methods for creating regions that can be integrated into the void manufacturing process.

[0155] Optionally, the optical fiber is configured to exhibit birefringence, and one or more modified regions are configured to contribute to birefringence. This enables the birefringence effect described above.

[0156] A strain sensor is also provided, which has: an optical fiber as described herein; and a controller configured to determine the strain applied to the optical fiber based on the Bragg wavelength of the Bragg grating. By measuring the wavelength of the Bragg grating, it is possible to determine the strain applied to the optical fiber. Fillers in the voids allow for tuning of the fiber properties to enhance or enable strain sensing for specific environments and applications.

[0157] Optionally, the optical fiber is configured to exhibit birefringence; and the controller is configured to determine strain based on the difference between Bragg wavelengths of the Bragg grating for light with different polarizations. This enables sensor readout.

[0158] A system for sensing strain and / or temperature is also provided, which has: an optical fiber with two separate Bragg gratings, the Bragg wavelengths of the two Bragg gratings changing with temperature differently; and a controller configured to determine the strain applied to the optical fiber and the temperature of the optical fiber based on the Bragg wavelengths of the two Bragg gratings. This allows for the separation of the effects of strain and temperature due to the different effects on the two gratings, so that both temperature and strain can be accurately determined. This enables readout of strain and temperature.

[0159] A pressure sensor is also provided, which has: an optical fiber with a Bragg grating; and a controller configured to determine the external pressure applied to the optical fiber based on the Bragg wavelength of the Bragg grating, and the pressure sensor is configured such that the pressure difference between the pressure of the filler and the external pressure applied to the optical fiber affects the Bragg wavelength of the Bragg grating. The external pressure shifts the Bragg wavelength. Optical fibers can be particularly useful as pressure sensors in constrained or extreme environments.

[0160] Optionally, the pressure sensor is configured such that the pressure difference between the pressure of the filler and the external pressure applied to the optical fiber affects the Bragg wavelength of the Bragg grating. This allows for control of pressure sensing based on the properties of the filler, for example, to increase sensitivity.

[0161] Optionally, the pressure sensor may further have a controller configured to determine the external pressure applied to the optical fiber based on the Bragg wavelength of the Bragg grating. This allows for pressure readout.

[0162] Optionally, the optical fiber is configured to exhibit birefringence; the pressure difference affects birefringence; and the controller is configured to determine the external pressure applied to the optical fiber based on the difference between Bragg wavelengths of the Bragg grating for different polarizations of light. Differential sensing can enable more accurate and sensitive sensing than determining the absolute value of the wavelength. Birefringence makes this possible using different polarizations of light.

[0163] Optionally, if two Bragg gratings are present, one of them may have a Bragg wavelength that decreases with increasing temperature, while the other has a Bragg wavelength that responds differently to increasing temperature. Advantageously, the two Bragg reflection wavelengths shift in opposite directions with increasing temperature, and the separation indicates temperature. This means that a greater distinction can be made between strain and temperature.

[0164] Optionally, the refractive index of the filler is greater than that of the optical fiber material. Beneficially, a fiber Bragg grating associated with such a filler operates with a steeper gradient as part of its Bragg wavelength-to-temperature characteristics, thereby providing greater temperature sensitivity.

[0165] Optionally, one or more voids may have two or more voids filled with different fillers, and optionally, the material properties of the fillers may differ between the different fillers. This provides further flexibility in tuning the properties of the fiber.

[0166] Optionally, the material properties are optical properties, and optionally, the refractive index. Optionally, one or more voids are configured such that the optical properties of the filler affect the light transmitted through the optical fiber. This can enable a stronger, more direct effect on light in the fiber, rather than relying on indirect effects such as changing the expansion properties of the fiber material.

[0167] Optionally, the temperature-dependent changes in the material properties of the filler are different from, and optionally inversely, those of the optical fiber material. Optionally, one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the temperature-dependent changes in the material properties of the optical fiber material. This can allow the voids to reduce the effect of temperature on the fiber, thereby further stabilizing its properties over a wider range of conditions.

[0168] Optionally, the cross-sectional area of ​​at least one of the one or more voids varies along the length of the void, and optionally, the cross-sectional area decreases along the length of the optical fiber, away from the center of the void, for at least a portion of the void's length. Optionally, the central axis of one or more voids extends along a direction inclined with respect to the longitudinal axis of the optical fiber for at least a portion of the voids. This allows for variations in properties along the length of the fiber and across the cross-section of the fiber, achieving more localized effects.

[0169] Optionally, at least one of the voids has a non-circular cross-section. This can offer greater flexibility in how the filler affects light in the fiber, and how the void and filler respond to external conditions such as pressure, compared to existing fibers stretched from preforms with substantially circular drilled holes.

[0170] Optionally, the material properties are optical properties, and optionally, refractive index; one or more voids are configured such that the optical properties of the filler affect the light transmitted through the optical fiber; and variations in the cross-sectional area of ​​one or more voids and / or the orientation of the central axis of one or more voids are such that the light transmitted through the optical fiber experiences substantially continuous variations in the effect of the optical properties on the transmission of light along the length of the optical fiber. Continuous changes in optical properties reduce reflections caused by discontinuous interfaces within the fiber, thereby improving transmission performance.

[0171] Optionally, one or more voids have at least one cladding void within the cladding. Optionally, the cladding voids are configured such that the transmission of light within the optical fiber is influenced by the filler within the cladding voids, and optionally, at least one cladding void is adjacent to the core. The cladding voids may allow for weaker interactions between light and filler in the fiber, which is primarily guided by the core, and enable a more controlled effect on light in the fiber.

[0172] Optionally, the cladding voids may extend either a) so that the filler material is in contact with the core, or b) so that the filler material is not in contact with the core. Contact between the filler material and the core may be desirable or undesirable in different applications, depending on the desired strength of the interaction between the filler material and the light guided by the fiber core.

[0173] Optionally, one or more voids may have multiple cladding voids within the cladding, optionally, at least two cladding voids, optionally, at least four cladding voids, optionally, at least six cladding voids, and more cladding voids may allow for greater control over the effect of the filler on light propagating in the fiber.

[0174] Optionally, multiple cladding voids are arranged symmetrically around the core. Optionally, the distance between the cladding voids and the core varies along the length of the cladding voids. This also allows for greater control over the effect of the filler on light in the fiber, and variation of the effect along the length of the fiber or around the outer circumference of the fiber.

[0175] Optionally, the optical fiber is formed by stretching a preform, and one or more voids are formed after the optical fiber has been stretched.

[0176] Optionally, one or more voids extend entirely within the void portion, and / or the boundaries of one or more voids are defined entirely within the void portion.

[0177] Optionally, one or more voids are filled with a filler material. This provides uniform properties throughout the void. Optionally, the filler material may be a gas, and optionally nitrogen or air. Optionally, the filler material may be a non-gaseous material, and optionally a liquid. Different fillers may be suitable for different applications, for example, based on the required refractive index or other properties.

[0178] Optionally, the filler material may contain liquid crystals. This could be useful in applications where changing the properties of the filler material is advantageous.

[0179] Optionally, the refractive index of the filler material is within 0.1 of the refractive index of the optical fiber material, optionally within 0.05, optionally within 0.01, optionally within 0.005, and optionally within 0.001.

[0180] Optionally, an optical fiber has a core and cladding surrounding the core, and the refractive index of the cladding material is approximately equal to the refractive index of the cladding material at a reference temperature. Strictly matching the refractive index of the cladding material to that of the optical fiber material reduces disturbances to the waveguide and mode characteristics of the optical fiber.

[0181] Optionally, optical fibers have outer diameters of 25 μm to 300 μm, optionally 100 μm to 300 μm, and optionally approximately 125 μm or 250 μm. These diameters are useful for typical optical fiber applications.

[0182] Optionally, optical fibers can be anti-resonant fibers or negative curvature fibers. These types of fibers may be suitable for specific applications.

[0183] Optionally, optical fibers contain silica. This is a common material for optical fibers because it is inexpensive to manufacture.

[0184] Optionally, an optical fiber may have a crystalline material, such as a single-crystal material; optionally, an optical fiber may be a crystalline fiber; optionally, a sapphire-derived fiber. Including at least a portion of a crystalline material in an optical fiber makes it possible to utilize some of the advantageous properties of the crystalline material in the optical fiber.

[0185] Optionally, optical fibers are crystalline optical fibers, and optionally, single-crystal optical fibers. Optionally, optical fibers may contain sapphire, diamond, or yttrium aluminum garnet (YAG) crystals. Crystalline optical fibers may be more resistant to extreme temperature and pressure conditions than other common optical fiber materials. Optionally, crystals are doped, and optionally, doped with rare earth elements. This may provide further control over the optical properties of the optical fiber.

[0186] Optionally, the optical fiber may have a coating; temperature-dependent changes in the material properties of the coating affect the light transmitted through the optical fiber; and one or more voids are configured such that temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of temperature-dependent changes in the material properties of the optical fiber and the coating on the light transmitted through the optical fiber. Coatings are commonly used to protect optical fibers, but because the coating has a different coefficient of thermal expansion than the optical fiber material, for example, it can affect how the optical fiber responds to external conditions such as temperature. Compensating for this allows the coating to be used while stabilizing the fiber properties.

[0187] Optionally, the coating may be: a) polyacrylate or polyimide; or b) metallic. These are common coating types used for many optical fiber applications.

[0188] Optionally, optical fibers may have Bragg gratings. This generates clearly detectable peaks in the fiber spectrum, which can be used in various sensing applications.

[0189] Optionally, the temperature-dependent changes in the material properties of the coating affect the Bragg wavelength of the Bragg grating; and one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of the temperature-dependent changes in the material properties of the optical fiber and the coating on the Bragg wavelength of the Bragg grating; and optionally, one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 5 pm / °C, optionally at least 20°C, and optionally at least 40°C, over a temperature range of at least 10°C, optionally at least 20°C, and optionally at least 40°C. Compensating for the effect on the Bragg wavelength stabilizes the peak for sensing applications.

[0190] Optionally, an optical fiber has a core and cladding surrounding the core, and one or more voids within the core are core voids. Core voids allow for very strong interactions between the filler material and light in the fiber, which are usually most strongly localized in the core.

[0191] Optionally, the core consists substantially of core voids. This means that the propagation of light is primarily determined by the filler material.

[0192] Optionally, Bragg gratings are provided by periodic modification of the cladding. This allows for Bragg gratings with several types of packing materials, such as liquid or gaseous packing materials, which may not exhibit periodic modification.

[0193] Optionally, the core is aligned with the central axis of the optical fiber. This creates symmetrical cladding around the core.

[0194] Optionally, Bragg grating is provided by periodic modification of the filler in the core void, and optionally the filler is liquid crystal. Optionally, the filler is polymerizable, and the periodic modification involves periodic polymerization, and optionally, the filler contains monomers and photoinitiators, and the periodic polymerization is performed using a laser. This allows for Bragg grating in the core where the interaction with light is strongest, even when using liquid fillers.

[0195] Optionally, the optical fiber may further have one or more electrodes configured to apply an electric field to the filler material, thereby influencing the material properties of the filler. This may allow for adjustment of the filler material's properties even after manufacturing.

[0196] Optionally, the filler material may be liquid crystal. This is a class of material that is well understood to have properties that can be tuned by an electric field.

[0197] Optionally, the electric field is configured to affect one or more of the refractive index, absorption, and scattering losses of the filler. Optionally, the filler exhibits birefringence, and the electric field is configured to affect one or more of the magnitude of the birefringence and the angle of the optical axis of the birefringence. These properties would allow for flexibility in influencing the propagation of light in the fiber.

[0198] Optionally, the optical fiber may have a Bragg grating, and the electric field may be configured to affect the Bragg wavelength of the Bragg grating. The Bragg grating generates an easily detectable peak, which may be advantageous for sensing applications.

[0199] According to an eighth aspect of the present invention, an optical fiber is provided having a first portion and a second portion: the second portion is a void portion having one or more voids, the one or more voids extending longitudinally within the void portion along the length of the optical fiber; and the one or more voids are at least partially filled with a filler having material properties different from those of the material of the optical fiber, and the first portion and the second portion each have: a core; and cladding, each cladding surrounding each core, and the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fiber is substantially the same as the maximum lateral dimension of the core of the second portion in a direction substantially perpendicular to the longitudinal axis of the optical fiber at the interface between the first portion and the second portion.

[0200] Advantageously, matching the core dimensions of a portion of an optical fiber containing air gaps with those of another portion of the optical fiber reduces optical loss during transitions between portions.

[0201] According to a ninth aspect of the present invention, an optical fiber is provided having a void portion having one or more voids, the one or more voids extending longitudinally within the void portion along the length of the optical fiber; and the one or more voids are at least partially filled with a filler having material properties different from those of the material of the optical fiber, the void portion having: a core; and cladding, each cladding surrounding each core, and the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fiber is greater than 5.5 micrometers and less than 11 micrometers, optionally In the first part, the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fiber is 6–10.5 micrometers, optionally, the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fiber is 7–10 micrometers, optionally, the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fiber is 8–9 micrometers, and optionally, the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fiber is approximately 8.2 micrometers.

[0202] Beneficially, optical fibers having gaps of such dimensions can be joined with standard single-mode optical fibers to offer the advantage of being associated with a filled gap, while ensuring single-mode transmission with minimal loss.

[0203] According to a tenth aspect of the present invention, a device having two Bragg gratings is provided; one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature, and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature. Advantageously, such a device provides an improved distinction between temperature and strain and can be used in combination with or independently of further features described herein.

[0204] Embodiments of the present invention are described herein by reference only, by reference to the accompanying drawings, where corresponding reference symbols indicate corresponding parts. [Brief explanation of the drawing]

[0205] [Figure 1] Figures 1A and 1B show optical fibers with voids in the void portions. [Figure 2] Figure 2 is a flowchart of the method for forming the optical fibers shown in Figures 1A and 1B. [Figure 3] Figures 3A and 3B show unmodified optical fibers. [Figure 4] Figures 4A and 4B show the optical fibers from Figures 3A and 3B after exposure to laser irradiation. [Figure 5] Figures 5A and 5B show the optical fibers from Figures 4A and 4B after they have been brought into contact with the etching solution. [Figure 6] Figures 6A and 6B show optical fibers exposed to laser irradiation of different intensities. [Figure 7] Figures 7A and 7B show the optical fibers of Figures 6A and 6B after they have been brought into contact with the etching solution. [Figure 8] Figures 8A and 8B show the optical fibers from Figures 5A and 5B after the voids have been filled with etching solution. [Figure 9] Figures 9A and 9B show the optical fibers from Figures 8A and 8B after the gap has been sealed. [Figure 10] Figures 10A and 10B show optical fibers in which the access gap does not extend radially. [Figure 11] Figure 11A shows an unmodified optical fiber with a coating. Figure 11B shows the same optical fiber as in Figure 11A after some of the coating has been removed and the fiber has been exposed to laser irradiation. [Figure 12] Figures 12A to 12F show cross-sectional views of optical fibers with different voids containing filler material. [Figure 13] Figures 13A to 13D show the change in refractive index of an optical fiber with respect to temperature. [Figure 14] Figure 14 shows the optical fiber of Figure 11B, which has a Bragg grating in its core. [Figure 15] Figure 15 shows the optical fibers of Figures 1A and 1B, which have a Bragg grating in the core. [Figure 16] Figure 16 shows the optical fibers of Figures 10A and 10B, which have a Bragg grating in the core. [Figure 17] Figure 17 shows the optical fibers of Figures 7A and 7B, which have a Bragg grating in the core. [Figure 18] Figure 18 shows the change in Bragg wavelength with temperature for three different optical fibers with Bragg gratings. [Figure 19] Figures 19A to 19C show the reflection spectra of the three optical fibers in Figure 18 at different temperatures. [Figure 20] Figure 20 shows the change in Bragg wavelength for a coated optical fiber compensated using a void containing filler material. [Figure 21] Figure 21 shows an optical fiber with two Bragg gratings in its core. [Figure 22] Figure 22 shows a sampled Bragg grating. [Figure 23] Figures 23A to 23C show the spectra of the sampled Bragg grating from Figure 22. [Figure 24] Figures 24A and 24B show optical fibers having waveguides that include Bragg gratings in the cladding. [Figure 25]Figures 25A and 25B show optical fibers in which the waveguide in the cladding has gaps filled with filler material. Figures 25C and 25D show multicore optical fibers in which the central core has gaps filled with filler material. [Figure 26] Figures 26A and 26B show optical fibers with strain-isolated Bragg gratings in the cladding. [Figure 27] Figures 27A to 27D show birefringent optical fibers. [Figure 28] Figures 28A and 28B show optical fiber pressure sensors designed to have non-zero birefringence at an external pressure difference of zero. [Figure 29] Figures 29A and 29B show alternative designs for an optical fiber pressure sensor designed to have non-zero birefringence at a zero external pressure difference. [Figure 30] Figures 30A and 30B show the side-hole fiber 110 that has been birefred by laser irradiation. [Figure 31] Figures 31A and 31B show examples of pressure sensors fabricated in the form of polarization-maintaining optical fibers. [Figure 32] Figure 32 shows a system for measuring environmental variables such as strain or temperature using optical fibers. [Figure 33] Figures 33A to 33C show exemplary strain and temperature measurements using the system shown in Figure 32. [Figure 34] Figures 34A to 34E show exemplary pressure measurements at different temperatures using the system shown in Figure 32. [Figure 35] Figures 35A and 35B show exemplary pressure measurements from a pressure sensor having birefringence that is non-zero at low pressure differences. [Figure 36] Figure 36A shows a thermally tuned optical filter. Figure 36B shows the spectrum of the filter in Figure 36A. [Figure 37]Figure 37 shows an add / drop multiplexer. [Figure 38] Figure 38A shows a temperature-stable laser. Figure 38B shows the spectrum of the laser in Figure 38A. [Figure 39] Figure 39A shows an adjustable laser. Figure 39B shows the spectrum of the laser in Figure 39A. [Figure 40] Figure 40 shows an adjustable fiber laser. [Figure 41] Figures 41 to 41D show optical fibers with air gaps in the core. [Figure 42] Figures 42A and 42B show optical fibers containing electrical adjustment electrodes. [Figure 43] Figures 43A and 43B show electrically tunable optical fibers with a core void. [Figure 44] Figures 44A to 44D show various exemplary arrangements of electrodes and voids within the cross-section of an optical fiber. [Figure 45] Figure 45A shows an electrically adjustable optical filter. Figure 45B shows the spectrum of the filter in Figure 45A. [Figure 46] Figures 46A and 46B show unmodified crystalline optical fibers. [Figure 47] Figures 47A and 47B show the crystalline optical fibers of Figures 46A and 46B after exposure to laser irradiation. [Figure 48] Figures 48A and 48B show the crystalline optical fibers of Figures 47A and 47B after contact with the etching solution. [Figure 49] Figures 49A and 49B show crystalline optical fibers that also have a core void. [Figure 50] Figure 50 shows a pressure sensor formed using a crystalline optical fiber. [Figure 51] Figure 51 shows an alternative pressure sensor design using crystalline optical fiber. [Figure 52]Figures 52A and 52B show the substrate after exposure to laser irradiation. [Figure 53] Figures 53A and 53B show the substrates of Figures 52A and 52B after contact with the etching solution. [Figure 54] Figures 54A and 54B show the substrates of Figures 53A and 53B after the voids have been filled with filler material. [Figure 55] Figure 55 shows an exemplary device containing a Bragg grating. [Figure 56] Figures 56A and 56B show the simulated shift at the Bragg wavelength. [Figure 57] Figure 57A shows a design of a Bragg grating and an optical fiber with high thermal sensitivity at the Bragg wavelength. Figure 57B shows the measured reflectance spectra of the fiber from Figure 57A at different temperatures. [Figure 58] Figure 58A shows a design of an optical fiber with a Bragg grating and compensated thermal sensitivity at the Bragg wavelength. Figure 58B shows the measured reflectance spectra of the fiber from Figure 58A at different temperatures. [Figure 59] Figure 59 shows the Bragg wavelength peaks of the optical fibers in Figures 57A and 58A. [Figure 60] Figure 60 shows a flowchart illustrating the method for forming the optical fibers shown in Figures 61A, 61B, 62, and 63. [Figure 61] Figures 61A and 61B show mode-matched optical fibers. [Figure 62] Figure 62 shows an optical fiber with a bridging portion. [Figure 63] Figure 63 shows an optical fiber with a lens portion. [Figure 64] Figure 64 shows the change in wavelength as a function of temperature for two optical fibers with Bragg gratings. [Modes for carrying out the invention]

[0206] Optical fibers can be used in a variety of applications, including sensing applications. Environmental conditions affecting optical fibers, such as strain, pressure, or temperature, influence the material properties of the optical fiber. This then affects the propagation of light in the fiber, which can be detected and used to derive conditions around or within the fiber.

[0207] For example, an optical fiber pressure sensor can be made from a side-hole optical fiber. This optical fiber has a core surrounded by cladding of a lower refractive index material along its length. Within the cladding, there are two air holes running longitudinally along the length of the fiber, so that they are on either side of the core in cross-section.

[0208] Within the core of an optical fiber, there exists a Bragg grating consisting of periodic modulation of the core refractive index along its length. When placed within an optical fiber, the Bragg grating may also be called a fiber Bragg grating (FBG). The FBG is mλ b =2n eff The pitch Λ of the Bragg grating and the effective refractive index n of the optical waveguide are given by Λ (where m is an integer) in the equation. eff The Bragg wavelength λ is determined by b It reflects light.

[0209] When a pressure difference exists between the air inside a fiber and the outside of the fiber, the fiber deforms, resulting in stress-induced birefringence. This means that the fiber has two orthogonal polarization axes (called the fast axis and the slow axis) within its cross-section. These orthogonal polarization axes have different effective refractive indices. Because the two polarizations have different effective refractive indices, a Bragg grating will reflect light from one polarization axis at one wavelength and from the other polarization axis at a different wavelength.

[0210] The difference between the effective refractive indices of the two polarization axes, and consequently the difference in Bragg wavelengths, is determined by the pressure difference between the inside and outside of the optical fiber. Therefore, the pressure outside the fiber can be determined by wavelength separation between the two Bragg wavelength peaks.

[0211] Existing fiber Bragg grating pressure sensors have many limitations. There are limitations on the size and shape of the holes that can be created within the optical fiber. Air-hole fibers need to be spliced ​​into conventional fibers to couple light, which can result in significant losses due to mode mismatch between fibers. This coupling loss can be significant when multiple sensors are linked together in a chain, because each sensor operates in reflection, and each sensor has two splices, resulting in four splice losses for each preceding sensor in the chain. Furthermore, there are no suitable optical fibers that will operate at extremely high temperatures exceeding 1000°C. A further problem is the difficulty in measuring low pressures where separation at wavelength peaks is extremely small.

[0212] The present invention includes a manufacturing method that enables the fabrication of FBG pressure sensors within standard optical fibers, thereby avoiding the need to splice the fiber and enabling low coupling loss. This method allows for the creation of gaps of arbitrary shapes, enabling the achievement of even greater pressure sensitivity. This also makes it possible to fabricate pressure sensors in single-crystal fibers to withstand ultra-high temperatures exceeding 1000°C.

[0213] The optical fiber 1 containing the Bragg grating 31 can also function as a strain sensor, because any strain applied will result in a change in pitch, which in turn will affect the Bragg wavelength. However, the Bragg grating 31 is also sensitive to temperature, because temperature will change the effective refractive index through thermo-optic effects and will also change the pitch due to thermal expansion. The fact that the Bragg grating 31 is sensitive to both strain and temperature is problematic because it is impossible to determine whether the change in Bragg wavelength is due to a change in temperature, a change in strain, or a combination of both.

[0214] Existing solutions to this problem include placing fiberglass generators (FBGs) in housings that strain the fiber and mitigate the strain under thermal expansion. This improves temperature performance but strains the fiber, preventing the device from being used as a strain sensor. Alternatively, "strain-relieving jackets" can be used to isolate the FBGs so that they are not affected by strain. However, these increase costs and the extra bulk limits the scenarios in which they can be deployed. Other techniques for distinguishing between temperature and strain include dual-wavelength or dual-polarization sensing and tapered fibers. However, none of these techniques provide a significant change in response to enable good distinction between the effects of temperature and strain with a good signal-to-noise ratio.

[0215] In addition to sensing, FBGs are also used for other functions such as optical filtering and laser stabilization. These applications often require the Bragg wavelength to remain constant, and changes in wavelength with temperature can be problematic. Sometimes, it is necessary to be able to adjust the wavelength by incorporating a heating element, and in these situations, greater temperature sensitivity would be desirable.

[0216] The invention's ability to create arbitrary void shapes also opens up a variety of other possibilities that enable improved performance in applications such as strain and temperature sensing. Applications and advantages of embodiments of the invention include strain measurement with low temperature cross-sensitivity, independent strain and temperature measurement, low-cost temperature-insensitive lasers and optical filters, low-cost adjustable add-drop multiplexers, and adjustable fiber lasers. Other applications include the fabrication of "perforated" single-crystal single-mode fibers and refractive index sensors. The invention enables both temperature insensitivity and high temperature sensitivity without compromising the signal-to-noise ratio and without increasing cost and bulk.

[0217] The present invention relates to an optical fiber 1 as shown in Figures 1A and 1B. The optical fiber 1 may have silica. The optical fiber 1 may have an outer diameter of 25 μm to 300 μm, optionally 100 μm to 300 μm, and optionally approximately 125 μm or 250 μm. The optical fiber 1 is typically made of silica and has a diameter of 125 μm. Other common diameters include 50 μm, 80 μm, 250 μm, and 425 μm. Larger diameters may be preferable for pressure sensing applications because larger diameter fibers are more sensitive to changes in pressure. The optical fiber 1 has a core 3 and cladding 5 surrounding the core 3.

[0218] Optical fiber 1 may be a single-mode optical fiber. However, many other types of fibers are possible. For example, optical fiber 1 may be an anti-resonant fiber or a negative curvature fiber.

[0219] The optical fiber 1 may have a crystalline material, and optionally a single-crystal material. In such a case, the optical fiber 1 may have some crystalline (or single-crystal) material portion and also portions of other materials such as silica. For example, the optical fiber 1 may be a crystal-derived fiber, and optionally a sapphire-derived fiber. An example of a crystal-derived fiber is described in “Sapphire-derived all-glass optical fibers” by Dragic, P., Hawkins, T., Foy, P. et al., Nature Photonics 6, 627-633 (2012).

[0220] Furthermore, optical fiber 1 may be a crystalline optical fiber, and optionally a single-crystal optical fiber. In this case, the material of optical fiber 1 consists entirely or substantially entirely of a crystalline material. For example, optical fiber 1 may have a sapphire, diamond, or yttrium aluminum garnet (YAG) crystal. The crystalline material in optical fiber 1 may be doped, whether it provides all or part of the optical fiber 1, and optionally doped with a rare earth element. Other fiber types include fibers having a pure silica core, a photonic crystal, a polymer, a hydrogel, and the like.

[0221] Single-mode silica fibers typically have a diameter of approximately 9 μm and an even higher degree of flexure (e.g., about 10 μm). -3 It has a germanium-doped core 3 to have a high refractive index (n). Around the core 3 is an undoped cladding 5. In a single-mode fiber, only a single transverse mode can propagate. This mode is primarily within the core 3 but has an evanescent field extending into the cladding 5. Its effective refractive index n eff It can be characterized by having a value at a specific wavelength, which is determined by the core diameter and the refractive index of the core and cladding at that wavelength.

[0222] The optical fiber 1 has one or more voids 9. The one or more voids 9 extend longitudinally along the length of the optical fiber 1 within the void portion 10.

[0223] The void portion 10 is a continuous and integral part of the optical fiber 1. The void portion 10 has no interfaces between adjacent solid materials in the longitudinal direction. There are no longitudinal joints or splices in the material of the optical fiber 1 within the void portion 10. Apart from the voids 9 and any other structures defined therein, the void portion 10 is formed from a single material. The voids 9 are formed by starting from a single, continuous and integral material and removing a part of the material of the void portion 10 from the inside, as further described below.

[0224] The one or more voids 9 may extend entirely within the void portion 10. The boundaries of the one or more voids 9 may be defined entirely within the void portion 10. The longitudinal extent of each of the one or more voids 9 may be smaller than the longitudinal extent of the void portion 10. The longitudinal extent of each void 9 within the optical fiber 1 may be smaller than the longitudinal extent of the void portion 10, that is, there are no voids 9 within the optical fiber 1 having a longitudinal extent larger than the longitudinal extent of the void portion 10.

[0225] Forming the voids 9 within the void portion 10 in this manner means that it is not necessary to splice two sections of the fiber together to form the optical fiber 1. Splicing different types of fibers results in mode mismatch and additional losses. It can also introduce structural weaknesses at the splice points. This problem is particularly severe in applications using FBGs. When a chain of FBGs is connected using splicing, each FBG will have two splice losses at its ends. If light is reflected from the last FBG in the chain, four times this splice loss will be seen for each previous FBG, and thus the losses accumulate rapidly.

[0226] The process of defining the gap 9 within the gap portion 10 is also much more expandable during manufacturing. Multiple optical fibers 1 can be mounted on a tray and processed together. This eliminates the need to separately splice the ends of each individual fiber, which would require time-consuming alignment of the fiber core with micrometer precision.

[0227] One or more voids 9 are sealed from the outside of the optical fiber 1 and are at least partially filled with a filler material 15. The filler material 15 has material properties different from those of the material of the optical fiber 1. One or more voids 9 may be entirely filled with the filler material 15. The filler material 15 may be a fluid. The filler material may contain a gas, and optionally contain nitrogen or air. The filler material 15 may contain a non-gaseous material, and optionally contain a liquid. The filler material 15 may contain glycerol or a glycerol-water mixture. The filler material 15 may contain liquid crystals. The filler material 15 may contain monomers and photoinitiators, and a laser may be used to selectively polymerize the filler material. The filler material 15 may be glass. The material of the filler material 15 or the optical fiber 1 may be chalcogenide glass or borosilicate glass. The filler material 15 may have a lower melting temperature than the material of the optical fiber 1. One or more voids may be filled by heating the optical fiber 1 and filler 15 to a temperature between the melting point of the filler and the melting point of the optical fiber 1 material. The filler 15 may then solidify after filling one or more voids by allowing the filler 15 to cool below its melting point. The filler 15 may have a polymer. The filler 15 may be supplied as a liquid polymer resin. One or more voids 9 may have two or more voids 9 filled with different fillers 15. The material properties of the fillers 15 may differ among the different fillers 15.

[0228] The void 9 containing the fluid filler 15 is extremely difficult to create using existing manufacturing techniques involving splicing. The fluid may leak from the fiber ends during splicing and may evaporate during the splicing process, leading to uneven filling of the void 9. Furthermore, it is difficult to efficiently fill the void 9 from both ends across long sections of the fiber 1. This hinders scalable manufacturing processes because it requires splicing very short fiber lengths with liquid inside.

[0229] As described above, the present invention also relates to a method for forming one or more voids 9 in an optical fiber 1 in order to manufacture an optical fiber 1 as shown in Figures 1A and 1B. Figure 2 is a flowchart of this method.

[0230] Figure 3A shows a side view of an unmodified optical fiber 1, and Figure 3B shows a cross-section of an unmodified optical fiber 1. Typically, the optical fiber 1 may be silica and may have a diameter of approximately 125 μm and may have a germanium-doped core with a diameter of approximately 9 μm. However, as mentioned above, other sizes and materials are possible. The optical fiber 1 may be formed by stretching a preform, and one or more voids 9 are formed after the optical fiber 1 has been stretched.

[0231] The optical fiber 1 may initially have a coating 23 of a polymer material, such as polyacrylate with a diameter of 250 μm or polyimide with a diameter of 150 μm. The coating 23 may be removed before carrying out the remaining steps of this method (S5). The coating may be removed by any suitable method such as chemical removal or ablation (S5). The coating 23 may be removed substantially along the entire longitudinal extent of the void portion 10, and may even be removed from the entire length of the optical fiber 1.

[0232] This method involves selectively exposing the optical fiber 1 to laser irradiation (S10) to define one or more exposed regions 7 within the optical fiber 1. Figure 4A shows the case where the optical fiber 1 has been modified by exposure to laser irradiation. Figure 4B shows a cross-section of the optical fiber 1 in Figure 4A.

[0233] The exposed region 7 is formed at a location where the energy delivered to the material of the optical fiber 1 by laser irradiation exceeds a predetermined threshold, which causes modification of the material of the optical fiber 1. The laser irradiation is concentrated within the material of the optical fiber 1 at the focal point. The energy delivered to the material by laser irradiation exceeds a predetermined threshold at the focal point, but falls below the predetermined threshold outside the focal point. In this way, only the portion of the material of the optical fiber 1 exposed at the focal point is modified, and the exposed region is formed. The laser irradiation may pass through other parts of the material of the optical fiber without forming the exposed region 7. This allows the entire exposed region 7 to be formed within the material of the optical fiber 1.

[0234] The focal point is moved through the material of optical fiber 1 to define an exposure area larger than the focal point. This can be achieved by moving optical fiber 1 while fixing the position of the focal point, moving the focal point while fixing the position of optical fiber 1, or moving optical fiber 1 and the focal point simultaneously.

[0235] The exposed region 7 defines the area of ​​optical fiber 1 that will be selectively removed later. When silica or other common optical fiber materials such as sapphire are exposed to high-energy laser pulses, nanogratings are formed. When such exposed material is placed in an etching solution, the etching solution will preferentially remove the exposed region compared to the original unmodified material.

[0236] The laser irradiation may be provided by a laser beam generated using a laser system. Selectively exposing the optical fiber 1 to laser irradiation (S10) may involve applying corrections to the active optical elements of the laser system. These corrections modify the wavefront characteristics of the laser beam to counteract the effect of aberrations on the laser focus. Aberrations are caused, among other things, by refraction at the outer surface of the optical fiber 1 and at the interface between the core 3 and the cladding 5. This distorts the focus where the laser beam converges, leading to inconsistent results when exposing the material of the optical fiber 1. Counteracting the effect of aberrations allows for much more precise control of the position, size, and shape of the laser beam focus within the internal capacitance of the optical fiber 1. This allows any region within the optical fiber 1 to be precisely exposed to form an exposed region 7.

[0237] The laser irradiation may have infrared or visible light. For example, the laser irradiation may have wavelengths of 700 nm to 900 nm, optionally 750 nm to 850 nm, and optionally approximately 790 nm. The laser irradiation may have wavelengths of 450 nm to 650 nm, optionally 500 nm to 600 nm, and optionally approximately 530 nm.

[0238] The laser beam may be a pulsed laser beam. For example, the laser system may be a femtosecond laser system. The pulses of the pulsed laser beam may have a duration of up to 1 ps, optionally up to 500 fs, optionally up to 200 fs, and optionally up to 100 fs. The pulses of the pulsed laser beam may have an energy of at least 0.1 μJ, optionally at least 0.15 μJ, optionally at least 0.2 μJ, and optionally at least 0.25 μJ.

[0239] Selectively exposing the optical fiber 1 to laser irradiation (S10) may further involve defining one or more access regions 17. Similar to the exposure region 7, the access region 17 is a region where the energy delivered to the material of the optical fiber by laser irradiation exceeds a predetermined threshold. This means that the material of the optical fiber 1 is modified in the access region 17 in the same manner as in the exposure region 7. The access region 17 can be defined in a similar manner to the exposure region 7 by moving the focal point through the material of the optical fiber 1.

[0240] Each access region 17 is either continuous with or adjacent to at least one of the exposed regions 7. The access regions 17 extend to the outer surface of the optical fiber 1, so that each of the one or more exposed regions 7 is connected to the outer surface by at least one of the one or more access regions 17. The access regions 17 create a continuous path of laser-modified material entering from the outside of the optical fiber 1 into the exposed regions 7. Preferably, every point within the exposed regions 7 is connected to the outer surface of the optical fiber 1 by a continuous path through the laser-modified material.

[0241] The outer surface on which the access region 17 extends is preferably the outermost surface in the radial direction of the optical fiber 1, rather than the flat surface at the longitudinal end of the optical fiber 1. This provides a shorter path from the outside of the optical fiber 1 to the exposed region 7.

[0242] This method further comprises bringing the optical fiber 1 into contact with an etching solution (S20). The etching solution etches the exposed region 7 faster than the region of the optical fiber 1 not exposed to laser irradiation, so that one or more voids 9 are formed by the etching of the exposed region 7. That is, the etching solution preferentially etches the exposed region 7. The optical fiber 1 may be in contact with the etching solution for a period of time long enough to remove substantially all of the exposed region 7 and access region 17 (S20). This period may be at least 2 hours, optionally at least 5 hours, optionally at least 10 hours, optionally at least 15 hours, or optionally at least 24 hours.

[0243] As described above, the exposed region 7 is a region in the optical fiber 1 where the material is exposed to laser irradiation and the energy delivered to the material exceeds a predetermined threshold. This causes modification of the material, and the etching solution etches the exposed region 7 faster than the unexposed region. Some of the material in the optical fiber 1 may be exposed to laser irradiation during step S10, which selectively exposes the optical fiber 1, but without the delivered energy exceeding a predetermined threshold. Such a portion will be etched by the etching solution at substantially the same rate as the etching rate for the region of the optical fiber 1 material that is not exposed to laser irradiation at all.

[0244] When the access region 17 is formed, during the step of bringing the optical fiber 1 into contact with the etching solution, the access region 17 is first etched, and one or more access voids 19 are formed. As a result, the optical fiber 1 has one or more access voids 19 extending from one or more voids 9 toward the outer surface of the optical fiber 1. Since the access region 17 is adjacent to the exposed region 7, the access voids 19 are connected to the voids 9. The access voids 19 thus create a continuous flow path through the access voids 19 between the outside of the optical fiber 1 and the voids 9. The access voids 19 allow the etching solution to contact the exposed region 7 within the optical fiber 1. This means that one or more voids 9 are formed by etching the material of the optical fiber 1 through one or more access voids 19 before the sealing S40 (sealing is further described below) of one or more access voids 19.

[0245] Having at least two access voids 19 allows the voids 9 to be more efficiently filled with the etching solution during the etching process. The two access voids 19 also allow capillary filling of the voids 9 when using the liquid filler 15 as described below. However, having more than two access voids 19 along the length of the optical fiber 1 can be advantageous as it allows longer voids 9 along the length of the optical fiber 1 to be formed and filled with an overly long etching time and filling time. For example, the optical fiber 1 may have regularly spaced access voids 19 along its length, particularly along the length of the void portion 10. The maximum longitudinal distance between successive access voids 19 along the length of the void portion 10 may be up to 10 mm, optionally up to 5 mm, and optionally up to 2 mm. The maximum longitudinal distance between successive access voids 19 along the length of the void portion 10 may be up to 50% of the longitudinal spread of the voids 9 to which the access voids 19 are connected, optionally up to 25%.

[0246] Figures 5A and 5B show the optical fiber 1 after it has been placed in the etching solution and the etching solution has come into contact with the optical fiber 1. The etching solution preferentially removes the exposed region 7 of the optical fiber 1, which has been modified to form a nanograting. The etching solution may contain potassium hydroxide, optionally in a concentration of at least 5 moles and optionally at least 8 moles. The etching solution may be heated above room temperature, for example to at least 50°C and optionally at least 70°C. A suitable etching solution is 8 mol / l KOH heated to 85°C using a hot plate. An ultrasonic bath or water bath filled with the etching solution may also be used. Any other suitable etching solution may be used instead of potassium hydroxide, for example hydrofluoric acid (HF).

[0247] As shown in Figures 6A and 6B, one or more exposure regions 7 may have high-exposure regions 11 and low-exposure regions 13. The high-exposure regions 11 are exposed to a higher dose of laser irradiation than the low-exposure regions 13 during step S10, in which the optical fiber is selectively exposed. In other words, more energy is delivered to the material of the optical fiber 1 by laser irradiation in the high-exposure regions 11 than in the low-exposure regions 13. This causes more significant modification of the material of the optical fiber 1 in the high-exposure regions 11 than in the low-exposure regions 13. Due to the increased modification of the material in the high-exposure regions 11, the etching solution etches the high-exposure regions 11 faster than the low-exposure regions 13 during step S20, in which the optical fiber is brought into contact with the etching solution.

[0248] Higher dose laser irradiation in the high-exposure region 11 may be achieved by varying one or more of the laser power, laser scanning speed, and laser beam profile. When a pulsed laser beam is used, higher dose laser irradiation in the high-exposure region 11 may be achieved by varying one or both of the laser pulse energy and / or pulse repetition rate.

[0249] In Figures 6A and 6B, the center of the exposed region 7 is exposed to a higher dose of laser irradiation than the region at the edge of the exposed region 7. This allows the etching solution to flow initially along the center of the exposed region 7 and then toward the edge. This avoids the problem of the portion of the exposed region 7 closest to the point of incidence on the surface of the optical fiber 1 being over-etched due to longer contact with the etching solution. Figures 7A and 7B show the fibers from Figures 6A and 6B after etching.

[0250] Figures 6A to 7B also show embodiments in which the cross-sectional area of ​​at least one of the one or more voids 9 varies along the length of the optical fiber 1. In this specification, cross-sectional area refers to a cross-section taken in a plane perpendicular to the length of the optical fiber 1. The shape, size, and orientation of the void 9 are determined by exposing the optical fiber 10 to laser irradiation (S10). The cross-sectional area may decrease along the length of the optical fiber 1, away from the center of the void 9, for at least a portion of the length of the void 9. At least one cross-section of the one or more voids 9 may be non-circular. For example, the cross-section of the void 9 may be rectangular, elliptical, or have the shape of a circular segment. The central axis of one or more voids 9 may extend along a direction inclined with respect to the longitudinal axis of the optical fiber 1, for at least a portion of the one or more voids 9. In this specification, the central axis of a void 9 refers to an axis substantially aligned with the maximum dimension of the void 9. This is typically the same as the longitudinal axis of the optical fiber 1.

[0251] This method further involves, once one or more voids 9 are formed, at least partially filling one or more voids 9 with a filler 15 having different material properties than that of the optical fiber 1. Figures 8A and 8B show the optical fiber 1 of Figures 5A and 5B after the microchannels have been filled with the filler 15.

[0252] This method may involve washing out the etching solution from one or more voids 9 before filling one or more voids 9 with the filler material 15. This can be achieved using any suitable solvent to remove the residual etching solution. Removing the residual etching solution prevents further etching that could distort the shape of the voids 9. It also prevents contamination that could alter the material properties of the filler material 15.

[0253] The void 9 may be filled using any suitable technique based on the properties of the filler 15. For example, if the filler 15 is liquid, the void 9 may be filled via capillary action by dripping the filler into one of the access voids 19, leaving another access void 19 open. Alternatively, the void 9 may be filled by pressure. Filling one or more voids 9 also includes the possibility of simply allowing ambient fluid, such as air, to enter the void 9. For example, the optical fiber 1 may be removed from the etching solution and then, after a possible rinsing step, left in a chamber filled with, for example, the ambient environment or an inert gas such as nitrogen, before the void 9 is sealed. This would allow ambient fluid to fill the void 9.

[0254] As described above, the filler 15 may be glass. The filler 15 may have a lower melting point than the material of the optical fiber 1. One or more voids may be filled by heating the optical fiber 1 and the filler 15 to a temperature between the melting point of the filler and the melting point of the material of the optical fiber 1. The filler 15 may then solidify after filling one or more voids by allowing the filler 15 to cool below its melting point.

[0255] Filling the void using the access void 19 from the side (i.e., via the outermost radial surface of the optical fiber 1, rather than the longitudinal end of the optical fiber 1) avoids the need to attempt splicing the fiber with liquid coming out of the end, as required in prior art methods. The arc temperature of the fiber splicer is high enough to melt the material of the optical fiber 1 and thus evaporate many common liquid fillers 15. This method avoids the need to splice the fiber to define the void 9 and thus avoids this problem.

[0256] This method further includes sealing one or more voids 9 from the outside of the optical fiber 1 after filling the voids 9 (S30) (S40). Figures 9A and 9B show the optical fiber 1 of Figures 8A and 8B after the voids 9 have been sealed.

[0257] Sealing (S40) can be achieved using an adhesive (e.g., a UV-curing adhesive). Alternatively, the material of the optical fiber 1 can be melted. Melting can be achieved using an arc, which is used in typical splicing machines, or by selectively melting the material on the outer surface of the optical fiber 1 using a laser.

[0258] If the optical fiber 1 has access gaps 19, sealing one or more gaps 9 (S40) means sealing one or more access gaps 19 on the outer surface of the optical fiber 1. As a result, in the completed optical fiber 1, one or more access gaps 19 are sealed on the outer surface of the optical fiber 1.

[0259] The access gap 19 may be sealed by inserting a block member 21 into the access gap 19. As a result, the completed optical fiber 1 has a block member 21 in the access gap 19. The block member 21 may be entirely within the access gap 19. The block member may have an adhesive as described above. Alternatively, the access gap 19 may be sealed by melting the material of the optical fiber 1 on the outer surface of the optical fiber 1. Melting may be performed using a laser or an electric arc.

[0260] In the examples shown so far, the access gap 19 has generally extended along the radius of the optical fiber 1. However, this is not generally necessary. Figures 10A and 10B show examples where the access gap 19 extends in a non-radial direction.

[0261] As described above, the optical fiber 1 may have initially had a coating that was removed before processing was performed to form and fill the void 9 (S5). Once the void 9 was filled and sealed, the method may further involve recoating the optical fiber 1 (S45). This results in the final optical fiber 1 having a coating 23, as shown in Figures 1A and 1B. The coating 23 may have a plastic material or a polymer material. Polyacrylate is a common optical fiber coating, but polyimide coatings may be used instead in high-temperature applications. The recoating (S45) may be performed using standard recoating machines and processes.

[0262] Instead of removing the optical fiber coating 23 before processing (S5) substantially along the entire longitudinal extent of the gap portion 10 or from the entire length of the optical fiber 1, it is also possible to leave the coating 23 if it is resistant to etching. In this case, removing the coating 23 (S5) involves selectively removing the coating 23 in order to form a gap 25 in the area where the access gap 19 will be in contact with the outer surface of the optical fiber 1. The coating 23 may be removed by any suitable process (e.g., laser ablation) (S5). Laser ablation may be performed using the same laser system used for selective exposure. Exposing the optical fiber 1 to laser irradiation (S10) involves concentrating the laser irradiation so that it passes through the coating 23.

[0263] Figure 11A shows the coated optical fiber 1 before performing this method. Figure 11B shows the optical fiber 1 after selectively removing the coating 23 (S5) and exposing the optical fiber 1 to form exposed areas 7 (S10). The optical fiber 1 may then be brought into contact with an etching solution (S20) so that the etching solution enters through the gaps 25 in the coating 23. The gaps 9 may then be filled (S30) and sealed (S40) as described above. After this, the coating 23 may be restored to the gaps 25 so that a uniform coating 23 can be formed again in the completed optical fiber 1.

[0264] The method of the present invention offers far greater flexibility than existing methods to manufacture air gaps 9 in optical fibers customized to individual requirements. The precise dimensions and location of the air gaps 9 can be determined for each individual optical fiber 1 and can vary along its length without requiring the manufacture of a new fiber preform each time and the splicing of fibers together. This significantly reduces manufacturing time, complexity, and cost.

[0265] A method for forming an optical fiber 1 according to the present invention has been described, and embodiments of the optical fiber 1 manufactured using this method, which can be used in various different applications, will now be described.

[0266] One or more voids 9 are configured such that the optical properties of the filler 15 affect the light transmitted through the optical fiber 1. As described above, the filler 15 has different material properties than the material properties of the optical fiber 1. This makes it possible to use the filler 15 to adjust the optical properties of the optical fiber 1 in the void portion 10. Importantly, the filler 15 may also have material properties that change with environmental variables (such as temperature, pressure, or strain), which are different from the material properties that change with environmental variables (such as temperature, pressure, or strain) of the material of the optical fiber 1. This makes it possible that changes in the optical properties of the optical fiber 1 with respect to environmental variables can also be controlled by the appropriate selection of the filler 15.

[0267] The material properties of the optical fiber 1 and the filler 15 may be optical properties, or optionally, refractive index. The filler 15 may have a refractive index different from that of the optical fiber 1. However, the refractive index of the filler 15 should preferably not differ excessively from that of the optical fiber 1. The refractive index of the filler may be within 0.1 of the refractive index of the optical fiber, optionally within 0.05, optionally within 0.01, optionally within 0.005, or optionally within 0.001. If the optical fiber 1 has a core 3 and cladding 5 surrounding the core 3, the refractive index of the filler 15 may be approximately equal to that of the cladding 5 at a reference temperature. Approximately matching the refractive index of the filler 15 can reduce reflection and loss in the optical fiber 1, but is also important to avoid impairing the waveguide and mode characteristics of the optical fiber 1. Ideally, optical fiber 1 forms a "weakly guiding" waveguide so that optical modes extend into the cladding 5. This requires a relatively small refractive index difference between core 3 and cladding 5. If the refractive index of cladding 5 (which is influenced by the refractive index of filler 15) is greater than that of core 3, then no waveguide exists. If the refractive index of cladding 5 is too large, then optical fiber 1 becomes multimode, and there will be more than one lateral spatial mode.

[0268] The filler material 15 may have a different change in refractive index dn / dT with temperature than that of the optical fiber 1. The change in refractive index of the filler material 15 with temperature may be negative.

[0269] In general, it is preferable to avoid abrupt transitions in the effective optical properties experienced by the light transmitted in the optical fiber 1. Abrupt transitions can lead to reflections and losses that have detrimental effects on light transmission. To mitigate this effect, variations in the cross-sectional area of ​​one or more voids 9 and / or the orientation of the central axis of one or more voids 9 may be such that the light transmitted through the optical fiber experiences substantially continuous variations in the effect of the optical properties on light transmission along the length of the optical fiber 1. For example, as shown in Figures 7A and 7B or Figures 10A and 10B, the cross-sectional area of ​​the void 9 may increase smoothly from zero to its maximum value at both longitudinal ends of the void 9. Since the light transmitted in the optical fiber 1 is most strongly localized at the center of the optical fiber 1 around the core 3, the effect of the filler 15 on light propagating in any direction along the optical fiber 1 gradually increases toward the center of the void 9 as the cross-sectional area of ​​the void 9 increases. Alternatively or additionally, the cross-sectional area of ​​the void 9 may be substantially constant, but the distance between the void 9 and the core 3 may increase toward both ends of the void 9. This also means that the effect of the filler 15 on the light transmitted in the fiber decreases toward both ends of the void 9.

[0270] As described above, the optical fiber 1 may have a core 3 and cladding 5 surrounding the core 3. In this case, one or more voids 9 may have at least one cladding void within the cladding 5.

[0271] Figures 12A to 12F show cross-sectional views of different optical fibers 1 having a void 9 containing a filler material 15. The light transmitted in the optical fiber 1 is primarily confined and guided by the core 3. However, the evanescent field of the light extends into the surrounding cladding 5 (and consequently into the filler material 15 in the void 9). The material properties of the cladding 5, such as the refractive index, are modified by the presence of the filler material 15. This means that the cladding void is configured such that the transmission of light within the optical fiber 1 is influenced by the filler material 15 within the cladding void.

[0272] If the filler 15 has material properties that change based on environmental variables such as temperature or electric field, then the effective material properties of the cladding 5 experienced by the light transmitted in the optical fiber 1 will also change. By adjusting the size, shape, and arrangement of the gaps 9, the region where the filler 15 exists can be adjusted. This allows control of the amount of overlap between the optical fiber modes and the filler 15 and the intensity of the interaction with light in the fiber.

[0273] At least one cladding void may be adjacent to the core 3. As shown in Figure 12C, the cladding void may extend so that the filler 15 is in contact with the core 3. Alternatively, as shown in Figures 12A, 12B and 12D-12F, the cladding void may extend so that the filler 15 is not in contact with the core 3.

[0274] One or more voids may have exactly one void 9. Figure 12B shows such an example where there is a relatively small gap between the core 3 and the filler 15 in the void 9. In this example, the core 3 is completely enclosed and suspended within the void 9 in the void portion 10. This achieves a very high overlap of optical modes with the filler 15.

[0275] One or more voids 9 may have multiple cladding voids within the cladding 5. As shown in Figure 12A, multiple cladding voids may have at least two cladding voids. Figure 12A shows two voids 9, each covering approximately half of the circumference around the core 3, with gaps between the voids 9.

[0276] As shown in Figures 12E and 12F, the multiple cladding voids may have at least four cladding voids. Figure 12E shows a configuration with four elliptical voids 9 very close to the core 3, giving a high overlap between the light modes and the filler 15. Figure 12F shows a configuration with four circular voids 9 far from the core 3 to reduce overlap as a design parameter.

[0277] As shown in Figures 12C and 12D, the multiple cladding voids may have at least six cladding voids. Figure 12C shows a spoke arrangement configuration in which the overlap between the optical modes localized in the core 3 and the material is reduced compared to the example in Figure 12B. Figure 12D shows a spoke arrangement configuration with an even larger gap between the core 3 and the filler 15, further reducing the overlap between the optical modes and the filler 15.

[0278] Multiple cladding voids may be arranged symmetrically around the core 3. Radial symmetry around the core 3 reduces polarization-dependent effects and leads to a more uniform effect of the filler 15 on different polarization modes within the optical fiber 1. The distance between the cladding voids and the core 3 may vary along the length of the cladding voids. This can contribute to the stepwise transition of material properties experienced by light in the optical fiber 1, as described above.

[0279] As described above, the filler 15 may have material properties that change in response to environmental variables different from those of the optical fiber 1. This can be used to compensate for changes in the material properties of the optical fiber 1 and thereby stabilize its behavior in response to environmental variables. Alternatively, the changes in the material properties of the optical fiber 1 may be improved. The latter may increase sensitivity for sensing applications.

[0280] In particular, the temperature-dependent changes in the material properties of the filler 15 may differ from the temperature-dependent changes in the material properties of the optical fiber 1, and may even be inversely related.

[0281] One or more voids 9 may be configured such that a change in the material properties of the filler 15 with temperature at least partially compensates for a change in the material properties of the material of the optical fiber 1 with temperature.

[0282] Figures 13A to 13D show exemplary refractive index profiles across an optical fiber cross-section demonstrating this effect.

[0283] Figure 13A shows the refractive index profile of an unmodified step index single mode silica optical fiber 1. The core refractive index is higher than the cladding refractive index. The optical mode is mainly localized within the core 3 but also extends into the cladding 5.

[0284] Figure 13B shows the refractive index profile of the same unmodified optical fiber but at a higher temperature. The refractive indices of both the core 3 and the cladding 5 increase, and thus the effective refractive index n eff experienced by the optical mode also increases. A standard optical fiber has a refractive index of 1.4440 at 1550 nm and a refractive index variation with temperature of dn / dT = 7.97×10 -6 °C -1 and is made of silica.

[0285] Figure 13C shows the refractive index profile of the optical fiber 1 according to the present invention having a void 9 filled with a filler 15. The refractive index of the filler is selected to be lower than the refractive index of the core 3 and higher than the refractive index of the material of the cladding 5 in this example. However, it would equally be possible to select the refractive index of the filler to be lower than the refractive index of the original silica cladding.

[0286] Figure 13D shows the refractive index profile of the same optical fiber as in Figure 13C at an even higher temperature. The refractive indices of the core 3 and the cladding 5 have increased, but the refractive index of the filler 15 has decreased. The effective refractive index n effThis will be somewhere between the refractive index of the core and the refractive index of the cladding. The effect of the filler 15 on the effective refractive index is therefore opposite to the change caused by the refractive indices of the core and cladding.

[0287] One example of a suitable filler 15 with a large difference in dn / dT is glycerol. Glycerol has a refractive index of 1.4473 at 1550 nm and dn / dT = -225 × 10⁻¹⁵ -6 ℃ -1 Glycerol can be appropriately diluted with distilled water to reduce its refractive index, forming an aqueous glycerol solution that is well matched with cladding 5 over the required operating temperature range. As an alternative to glycerol, specialized refractive index oils, such as those from Cargille Laboratories (www.cargille.com), can be used. For example, Cargille Series AA liquid 1.44000 refractive index (specified at 589.3 nm, 25°C) has a refractive index of 1.43 at 1550 nm and -395 × 10⁻⁶. -6 ℃ -1 It has a temperature coefficient of . Because the introduced selected filler 15 has a much larger dn / dT than silica, even relatively low interactions with the optical mode can have a significant effect on the effective refractive index.

[0288] Net change in effective refractive index dn with temperature eff / dT may be positive or negative depending on the application. The net change may also be designed to be nearly zero over certain temperature ranges. eff The value of / dT is (i) the refractive index n of the filler 15. fill (ii) Change in refractive index dn of filler 15 with temperature fill / dT; (iii) the amount of filler 15 in the cladding 5; and (iv) the proximity of the filler 15 to the core 3. Diluting the liquid filler 15 with a solvent has the effect of reducing (i) and (ii). By appropriately adjusting these parameters, dn effBy manipulating / dT, it is possible to achieve a desired value and change in the effective refractive index.

[0289] As described above, in many situations, the optical fiber 1 may have a coating 23. This is desirable for several reasons, including protecting the optical fiber 1 from damage, as uncoated optical fibers are brittle and easily broken. However, the coating 23 can also affect the behavior of light transmitted through the optical fiber 1. In particular, temperature-dependent changes in the material properties of the coating 23 can affect the light transmitted through the optical fiber 1.

[0290] The coating 23 may be too far from the core to significantly interact with the light localized in the core 3, but it can affect the light in other ways. For example, the coefficient of thermal expansion of the coating 23 with respect to temperature may differ from that of the optical fiber 1 material. This can cause strain in the optical fiber 1 as the temperature changes, which in turn can affect the refractive index of the optical fiber 1 material. This can stretch the optical fiber 1 through the thermal expansion of the coating 23. For optical fiber 1 containing a Bragg grating (described further below), this can increase the pitch of the Bragg grating, thereby changing the Bragg wavelength.

[0291] For this reason, in some embodiments, one or more voids 9 may be configured so that the temperature-dependent changes in the material properties of the filler 15 at least partially compensate for the combined effect of the temperature-dependent changes in the material properties of the optical fiber 1 and the material properties of the coating 23 on the light transmitted through the optical fiber 1. This means that the coated optical fibers can be thermally compensated, and they are better protected against damage. It also means that they can be embedded in a structure without temperature affecting their properties.

[0292] For some applications, it may be advantageous to split the light transmitted in the optical fiber 1 along multiple paths within the optical fiber 1. Examples of such applications will be further described below. In such cases, the optical fiber 1 may have waveguides 41 separate from the core 3 of the optical fiber 1.

[0293] Figures 24A and 24B show examples of such arrangement configurations. Waveguides 41 may be placed in the cladding 5 of the optical fiber 1. Waveguides 41 may be substantially parallel to the core 3 of the optical fiber 1 for at least a portion of the length of waveguide 41. For example, at least 50% of the waveguide length, and optionally at least 75% of the waveguide length.

[0294] Waveguide 41 may be arranged so that light is coupled from the core 3 of the optical fiber 1 to waveguide 41. This can be achieved using waveguide coupler 45. Waveguide coupler 45 may be part of the length of waveguide 41 when waveguide 41 is within a predetermined coupling distance of core 3, so that light is briefly coupled from core 3 to waveguide 41.

[0295] The waveguide 41 may be formed by one or more modified regions of the optical fiber 1, in which the optical properties of the optical fiber 1 differ from the optical properties of the material of the optical fiber 1 surrounding the one or more modified regions. The optical properties may be refractive index. The one or more modified regions may have one or more laser-exposed regions. This allows the waveguide 41 to be formed by exposing it to laser irradiation using the same laser mechanism used to form the air gap 9, thereby reducing the complexity of manufacturing.

[0296] The waveguide 41 may be configured such that the transmission of light in the waveguide 41 is affected by the filler material 15 in at least one of the one or more voids 9. The one or more voids 9 may have at least one void adjacent to the waveguide 41. Alternatively, the one or more modified regions forming the waveguide 41 may have one or more voids 9 extending longitudinally along the length of the optical fiber 1. In this case, the filler material 15 in the voids 9 essentially forms the waveguide 41, enabling a strong interaction between the filler material 15 and light in the waveguide 41.

[0297] The temperature-dependent change in the refractive index of waveguide 41 may differ from that of core 3. This can be achieved by forming the optical fiber 1 such that the material of waveguide 41 is different from the material of core 3. In particular, the materials of waveguide 41 and core 3 may include glass. If the optical fiber is a multicore fiber, the waveguide may be provided by a second core (e.g., a core other than the central core running along the longitudinal axis of the fiber). The second core may have a material with a different refractive index than core 3 of the optical waveguide. The materials of both core 3 and the second core may include glass. As will be further described below, both waveguide 41 and core 3 may have Bragg gratings.

[0298] In a further embodiment, the optical fiber may have at least two waveguides at different locations within the cross-section of the optical fiber. The temperature-dependent change in the refractive index of the first waveguide among the at least two waveguides may differ from the temperature-dependent change in the refractive index of the second waveguide among the at least two waveguides. One or both of the waveguides may be provided by the core of the optical fiber, for example, if the optical fiber has one or two cores. If the optical fiber is a multicore fiber, the two waveguides may be provided by different cores of the optical fiber. However, in some embodiments, if the waveguides are formed by other changes in the material of the optical fiber, such as stress waveguides, neither waveguide may be provided by a core. One or both of the waveguides may be formed by laser modification of the material of the optical fiber. The material of the first waveguide may be different from the material of the second waveguide, and optionally, the material of the first waveguide and the material of the second waveguide may be glass.

[0299] In some applications described further below, the optical fiber 1 is configured to exhibit birefringence. One or more modified regions may be configured to contribute to birefringence.

[0300] As described above, various sensors can be constructed from optical fibers containing Bragg gratings. The present invention is also advantageous in embodiments in which the optical fiber 1 has a Bragg grating 31.

[0301] Figures 14 to 17 show various embodiments containing the Bragg grating 31, similar to those already described. Figure 14 corresponds to Figure 11B, Figure 15 to Figure 1A, Figure 16 to Figure 10A, and Figure 17 to Figure 7A. In an optical fiber 1 already containing the Bragg grating 31, the void 9 may be formed as described above. Alternatively, the Bragg grating may be formed during this method, and the method further comprises forming the Bragg grating 31 in the optical fiber 1. This can be achieved by modifying the refractive index of the material of the optical fiber 1.

[0302] One method used to form a Bragg grating is to make the material of the optical fiber 1 photosensitive (for example, by exposing it to hydrogen or by doping the core with a dopant such as boron) and then expose it to an ultraviolet (UV) laser. Periodicity can be imparted by splitting the output of a high-power UV laser into two beams and creating an interference pattern on the optical fiber 1. Alternatively, a phase mask may be used to create an interference pattern on the optical fiber 1.

[0303] Another method involves directly modifying the refractive index at points along the length of the optical fiber 1 using a laser (e.g., with a wavelength of 790 nm or 532 nm). The laser irradiation is focused at a focal point in the optical fiber 1, creating a modified region where the refractive index periodically increases along the length of the optical fiber 1. This can be achieved, for example, by using pulsed laser irradiation and moving the optical fiber 1 and / or the laser focal point at a constant speed to obtain equally spaced modified regions. Suitable parameters for a typical optical fiber are a pulse duration of ~190 fs and a pulse energy of approximately 0.15 μJ. Alternative types of FBG include: a) chirpped FBG (where the pitch of the modified region varies along the length of the optical fiber 1); b) long-term FBG (where the grating has a much longer duration, resulting in coupling from the core to lossy cladding modes); c) sampled FBG (further described below in relation to Figure 22); or d) π-phase-shifted FBG (where it has a narrow optical transmission bandwidth within the Bragg reflection bandwidth).

[0304] The Bragg grating 31 may be formed after the void 9, but is preferably formed before the void 9 if possible. This is because the additional structure of the void 9 within the optical fiber 1 can make it more difficult to accurately focus the laser within the optical fiber 1 to form the Bragg grating 31.

[0305] If the optical fiber 1 has a core 3 and cladding 5 surrounding the core 3, the Bragg grating 31 may be at least partially located in the core 3, and optionally entirely located within the core 3. The Bragg grating 31 may be provided by periodic modification of the material of the core 3. Alternatively or additionally, the Bragg grating 31 may be provided by periodic modification of the material of the cladding 5. The periodic modification may be adjacent to the core 3, which creates a stronger interaction between the periodic modification and the light in the core 3. The Bragg grating 31 in the cladding 5 may also be provided using a void 9 filled with a filler 15, in which case the filler 15 can be periodically modified. For example, the filler 15 may be a liquid crystal. The filler 15 may have monomers and photoinitiators, and a laser may be used to periodically and selectively polymerize the filler along the length of the void 9 to form the Bragg grating.

[0306] As described above, one or more voids 9 are configured such that the material properties of the filler 15 affect the light transmitted in the optical fiber 1. If the material properties of the optical fiber 1 are optical properties, one or more voids 9 may be configured such that the optical properties of the filler 15 affect the Bragg wavelength of the Bragg grating 31. This allows for advantageous modification of its changes with respect to environmental variables such as Bragg wavelength and temperature, according to the needs of a particular application.

[0307] The voids 9 and filler material 15 may be localized to the length of the optical fiber 1 on which the Bragg grating 31 is placed. This reduces the overall optical loss along the optical fiber 1. As shown in Figures 15 and 17, one or more voids 9 may at least partially surround the Bragg grating 31. The longitudinal extent of one or more voids 9 may be at least the same as the longitudinal extent of the Bragg grating 31. One or more voids 9 may extend beyond both ends of the Bragg grating 31 by up to 1 mm, and optionally up to 0.5 mm. This ensures a uniform effect of the filler material 15 along the entire length of the Bragg grating 31.

[0308] One or more voids 9 may be configured such that the temperature-dependent change in the optical properties of the filler 15 at least partially compensates for the effect of the temperature-dependent change in the optical properties of the optical fiber 1 material on the Bragg wavelength of the Bragg grating 31. This can be achieved by appropriately selecting the composition of the filler 15 and the size, shape, and arrangement of the voids 9 within the optical fiber 1. The temperature-dependent change in the Bragg wavelength may be substantially offset at a predetermined wavelength over a given temperature range.

[0309] Net change in effective refractive index dn with temperature eff / dT may be positive or negative depending on the application. The net change may also be designed to be approximately zero over certain temperature ranges. eff The value of / dT is determined by the factors mentioned above. By appropriately adjusting these factors, dn eff It is possible to manipulate / dT. This can then be used to correct the change in Bragg wavelength △λb / △T with respect to temperature.

[0310] The net change in Bragg wavelength shift is also determined by the thermal expansion of the optical fiber. The change in Bragg wavelength Δλb is:

[0311]

number

[0312]

number

[0313] It is given by, where ξ is the thermo-optic coefficient and P e is the photoelastic coefficient, α is the thermal expansion coefficient of the optical fiber 1 material, ε is the strain, and ΔT is the temperature change. In reality, for silica fibers, α is about an order of magnitude smaller than the thermo-optic effect, and the Bragg wavelength shift is dominated by the thermo-optic effect. Considering α, n eff and dn eff / dT can be adjusted as described above to flatten the change in Bragg wavelength over the required temperature range. For example, one or more voids 9 may be configured such that the magnitude of the change in the Bragg wavelength of the Bragg grating 31 with respect to temperature is at least 5 pm / °C, optionally at least 20°C, optionally at least 40°C, over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. To flatten the temperature response over an even wider temperature range, multiple voids 9 containing optionally different fillers 15 may be used.

[0314] In addition to flattening the temperature response, one or more voids 9 may be configured to improve the temperature response. One or more voids 9 may be configured such that the temperature-dependent change in the optical properties of the filler 15 increases the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating 31 compared to the optical fiber 1 without one or more voids 9. This may be advantageous in providing improved sensitivity in temperature sensing applications. The temperature-dependent change in the Bragg wavelength may be negative, such that the Bragg wavelength of the Bragg grating 31 decreases with increasing temperature. A negative change in the Bragg wavelength may be greater than a positive change in the Bragg wavelength for the optical fiber 1 without one or more voids 9. For example, one or more voids 9 may be configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating 31 is at least 20 pm / °C, optionally at least 30 pm / °C, optionally at least 40 pm / °C, and optionally at least 50 pm / °C over a temperature range of at least 20°C, optionally at least 40°C.

[0315] Figure 18 shows a graph of the change in Bragg wavelength versus the change in temperature for a descriptive example of an optical fiber. The graph for an unmodified silica optical fiber (FBG#1) containing FBG31 but without voids 9 shows a linear positive change in Bragg wavelength with temperature (+10pmC). -1 The optical fiber 1 (FBG♯2) having FBG31 and a gap 9 configured for high-temperature sensitivity shows a large negative gradient. The optical fiber 1 (FBG♯3) having FBG31 and a gap 9 configured to be thermally compensated shows a flat curve with a minimum point and negative and positive gradients on both sides of the minimum point.

[0316] Figures 19A and 19C show the spectra for the same three Bragg gratings as in Figure 18 at two temperatures, T0 and T1 (T1 > T0). Figure 19A shows the spectrum of an unmodified silica FBG (FBG #1), where a shift to longer wavelengths is observed with increasing temperature. Figure 19B shows the spectrum of a high-sensitivity FBG (FBG #2), where an even larger shift is observed at higher temperatures, but this time it is a shift to shorter wavelengths. Figure 19C shows the spectrum of a temperature-compensated FBG (FBG #3), where the wavelength shift is much smaller than that in Figure 19A.

[0317] If the optical fiber 1 has a coating 23, the temperature-dependent changes in the material properties of the coating 23 may affect the Bragg wavelength of the Bragg grating 31, as described above. This may also be the case if the optical fiber 1 is embedded in or bonded to a substrate. In this situation, the thermal expansion of the material may also be considered when determining the Bragg wavelength. In this case, the Bragg wavelength shift is,

[0318]

number

[0319] Given by, in the formula α c is the thermal expansion coefficient of coating 23. In this case, (1-P e )α c It is possible that this may become more dominant than ξ. The fiber may be modified, and ξ may be substantially -(1-P e )α c If it becomes equal to this, the temperature dependence can be compensated for.

[0320] In this case, one or more voids 9 may be configured such that the temperature-dependent change in the material properties of the filler 15 at least partially compensates for the combined effect of the temperature-dependent change in the material properties of the optical fiber 1 with respect to the Bragg wavelength of the Bragg grating 31 and the temperature-dependent change in the material properties of the coating 23. For example, one or more voids 9 may be configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating 31 is at least 5 pm / °C, at least 20°C, and at least 40°C, over a temperature range of at least 10°C, optionally at least 20°C, and optionally at least 40°C, with a maximum of 5 pm / °C, optionally at least 2 pm / °C, and optionally at least 1 pm / °C. The combined optical fiber 1 and coating 23 may have a temperature-dependent Bragg wavelength shift with a steady-state point. The temperature-dependent change in Bragg wavelength may be negative at temperatures below the steady-state point and positive at temperatures above the steady-state point.

[0321] Figure 20 shows the compensation for a coated optical fiber, or an optical fiber 1 embedded in or bonded to a substrate. Figure 20 shows (i) the Bragg wavelength shift associated with thermal expansion with a large positive gradient (1-P e )α c (ii) a Bragg wavelength shift (ξ) associated with a thermo-optic effect having a corresponding negative gradient, and (iii) a flattened and near-zero resulting Bragg grating shift. This response has a minimum value as well as a steady point where negative and positive gradients exist on both sides.

[0322] The optical fiber 1 may have two separate Bragg gratings 31. This may enable differential sensing applications. One or more voids 9 may be configured such that the optical properties of the filler material have different effects on the Bragg wavelengths of the two Bragg gratings 31. For example, one or more voids 9 may be configured such that the optical properties of the filler material 15 affect the Bragg wavelength of one of the two Bragg gratings 31, but not the Bragg wavelength of the other Bragg grating 31.

[0323] The two Bragg gratings 31 may be provided in various different configurations. If the optical fiber 1 has a core 3 and cladding 5 surrounding the core 3, both Bragg gratings 31 may be provided at least partially to the core 3 and spaced apart longitudinally along the core 3.

[0324] Figure 21 shows an optical fiber 1 having two adjacent FBGs 31. The optical fiber 1 in Figure 21 can be used to measure both strain and temperature. The first Bragg grating 31-1 is an unmodified silica FBG. The second Bragg grating 31-2 is a high-sensitivity FBG, and the void 9 is configured to increase the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating 31-2. Since FBG 31-1 has a low positive temperature coefficient (change in Bragg wavelength with temperature) and FBG 31-2 has a high negative change with temperature, it is possible to measure the change in Bragg wavelength of both Bragg gratings and solve a system of equations to obtain independent measurements of strain and temperature. For example, matrix linear algebra may be used.

[0325] Figure 22 shows a sampled Bragg grating with periodically missing sections. These can be used to achieve a wider adjustment range. FBG31-A has gaps with different durations than FBG31-B. In Figure 21, FBG31-A can be used in place of FBG31-1, and FBG31-B can be used in place of FBG31-2.

[0326] Figures 23A and 23C show the spectra for sampled FBGs. Figure 23A shows FBG31-A, and Figure 23B shows FBG31-B. Each FBG has a comb of spectral peaks, but FBG31-A has wider spacing than FBG31-B. Only one peak overlaps from each of FBG31-A and FBG31-B (the central one in this example), and this will therefore be used as the wavelength of light emitted from the laser into the optical fiber during sensing applications. Figure 23C shows the spectrum of FBG31-B after adjustment relative to FBG31-A by changes in temperature or strain. Here, different peaks from FBG31-B are aligned with the peaks of FBG31-A, and therefore the wavelength of emission has changed. However, the shift in the wavelength of emission is greater than the amount that FBG31-B was adjusted for. Further relative shifts in FBG31-B result in further jumps in wavelength. To access intermediate wavelengths, both FBGs can be tuned together.

[0327] Instead of both Bragg gratings 31 being placed on the core 3, one of the two Bragg gratings may be placed on the cladding 5 and the other Bragg grating on the core 3.

[0328] One of the two Bragg gratings placed in the cladding 5 may be placed in the waveguide 41 as described above. The waveguide 41 is optically coupled to the core 3 via a waveguide coupler 45 so that some of the light guided by the core 3 is transferred into the waveguide 41. One or more voids 9 may then have at least one void 9 configured such that the transmission of light in the waveguide 41 is influenced by the optical properties of the filler 15 in the void 9. At least one void 9 may be adjacent to the waveguide 41.

[0329] Figures 24A and 24B show an optical fiber 1 having two FBGs in close proximity at the same point along the length of the optical fiber. FBG 31-1 is located in the core 3 of the optical fiber 1. Within the cladding 5 of the optical fiber 1 is a waveguide 41. Waveguides 41 may be formed by modifying the cladding material 5 by exposing it to laser irradiation. A first portion of waveguide 41 is adjacent to the core 3 to form an evanescent waveguide coupler 45, so that light is coupled from the core 3 into waveguide 41. Subsequently, a Bragg grating 31-3 is formed within this additional waveguide by creating an additional periodic modulation of the refractive index along the length of waveguide 41, for example by exposure to laser irradiation as described above. A void 9 is formed around waveguide 41 and filled with filler material 15. This allows strain and temperature to be determined independently, as shown in Figure 23, but has the advantage that the two FBGs are located at the same point along the length of the optical fiber 1. This can reduce confounding effects such as temperature differences along the length of the optical fiber 1.

[0330] A second Bragg grating may also be provided in an alternative manner within the cladding. One or more voids 9 may have cladding voids configured to provide a waveguide 41 within the cladding 5. The cladding void acting as a waveguide 41 is optically coupled to the core 3 via a waveguide coupler 45 so that some of the light guided by the core 3 is transferred into the cladding void. One of the two Bragg gratings within the cladding 5 is then provided by periodic modification of the material of the optical fiber 1 adjacent to the cladding void acting as a waveguide 41.

[0331] Figures 25A and 25B show an example of this arrangement configuration. The high-sensitivity FBG31-4 is provided adjacent to the silica FBG31-1 at the same point along the length of the optical fiber 1. The FBG31-4 is formed within the waveguide 41, which is formed by the gap 9 in the cladding 5 of the optical fiber 1, which is filled with filler 15. Light enters the waveguide 41, and is coupled by shaping it so that it is adjacent to the core 3 at one end to form an evanescent waveguide coupler 45, coupling light from the core 3 into the waveguide 41. The Bragg grating 31-4 is formed by inscribed a ring of a material with a higher refractive index in the silica around the gap 9 forming the waveguide 41. Having the filler 15 in the waveguide 41 rather than surrounding the waveguide 41 in the cladding 5 allows for a much stronger interaction between the filler and the light, resulting in a greater change in effectiveness with temperature.

[0332] Another method for compensating for cross-sensitivity to strain and temperature may be to isolate one of the Bragg gratings from the strain. To achieve this, one or more voids 9 may have isolation voids 47 configured to at least partially isolate one of the Bragg gratings 31 from the strain in the optical fiber 1.

[0333] Examples are shown in Figures 26A and 26B, which illustrate strain-insensitive FBG31-5 in optical fiber 1. In these examples, the isolation gap 47 removes a portion of the cladding 5 of optical fiber 1, mechanically isolating the Bragg grating 31-5 from strain in optical fiber 1. This means that any strain in optical fiber 1 will not result in strain being applied to the FBG31-5. This makes the Bragg grating 31-5 strain-insensitive, allowing it to act as a temperature sensor independent of strain. The isolation gap 47 may surround one end of the Bragg grating 31-5. The isolation gap 47 may surround at least 50%, optionally at least 75%, optionally at least 90%, and optionally 100% of the length of one end of the Bragg grating 31-5. The isolation void 47 encloses the waveguide 41 in a cylindrical shape, and some cladding material may be left between the waveguide 41 and the isolation void 47.

[0334] In Figures 26A and 26B, the optical fiber 1 has a core 3 and cladding 5 surrounding the core 3. One of two Bragg gratings 31-5 to be strain-isolated is located in cladding 5. The other of two Bragg gratings 31-1 is located in core 3. One of the two Bragg gratings 31-5 in cladding 5 is located in waveguide 41. Waveguide 41 is optically coupled to core 3 via waveguide coupler 45 so that some of the light guided by core 3 is transferred into waveguide 41.

[0335] Waveguides 41 may be formed as described above. In particular, as explained in relation to Figures 25A and 25B, FBG31-5 can be made more sensitive to temperature by forming waveguides 41 using gaps 9 filled with filler 15. In this case, using a filler with a high dn / dT increases the temperature sensitivity. Alternatively or additionally, gaps 9 with a high dn / dT filler may be incorporated into the cladding 5, which will affect waveguides 41. In Figures 26A and 26B, the conventional FBG31-1 in the core 3 of the optical fiber may be additionally used for strain measurement by taking into account the temperature measured by 31-5. However, it is also not essential to include a second Bragg grating 31-1 and a strain-isolated Bragg grating 31-5. Multiple waveguides 41 having Bragg gratings 31 may be provided along the length of the optical fiber 1, each individually coupled to the core.

[0336] Multicore fibers are used for 3D shape sensing. They consist of a central core and a series of cores surrounding the central core (typically equally spaced in cross-section). Four-core and seven-core fibers are common. For example, an optical fiber may have at least four cores. The refractive index of the central core among the at least four cores may differ from the refractive index of the other cores among the at least four cores, and / or may have different temperature dependencies. Bragg gratings may be written on each core at intervals along the fiber. By determining the strain on each Bragg grating, it is possible to determine the bent shape of the fiber.

[0337] Figure 25C shows a custom fiber, which is depicted with a void 9 along its central axis. Around the void 9 in the cross-section is a doped core 3a (e.g., a germanium-doped core). This fiber can be stretched using conventional "laminated stretching" techniques. The optical fiber 1 may also be stretched from a preform having a doped core and a void 9.

[0338] After the fiber is stretched, the void 9 may be filled with glass having a lower melting point than the optical fiber material, as shown in Figure 25D. For example, the length of the fiber is taken, and the low-melting-point glass is injected into the void 9 along the central axis at a temperature raised in the furnace. The fiber can then be cooled so that the glass solidifies and fills the void 9a.

[0339] In this context, low-melting-point glass refers to glass having a melting point significantly lower than the melting temperature of the optical fiber 1 material (for example, at least 200°C lower, optionally at least 500°C lower, optionally at least 700°C lower, optionally at least 1000°C lower). For example, the material of optical fiber 1 may typically be silica glass having a melting point around 1700°C. Low-melting-point glass may also be glass having a melting point below 1000°C, optionally below 800°C. Low-melting-point glass may also be glass having a melting point in the range of 400 to 600°C.

[0340] Low melting point glasses are selected to have a refractive index slightly above the fiber refractive index (e.g., 0.005 above), but the change in refractive index with temperature (dn / dT) differs from that of standard silica. A wide range of optical glasses with low melting points and fluctuating refractive index parameters are commercially available from Schott, such as FK5HTi, F2, FK, and SKF.

[0341] The central core, together with the silica cladding, forms a waveguide, but the change in the effective refractive index of the waveguide with temperature differs here from that of other doped cores. In other words, the doped core provides either the waveguide or the core, while the filled void provides the other. This allows the fiber to be used as a shape sensor, but with the added benefit of temperature compensation. The temperature can be determined by comparing the mean wavelength shift of the outer core with the wavelength shift of the central core relative to the linear position. This can then be used as a correction factor for the shape sensor. For even simpler applications, it may also be possible to use only one of the outer cores in combination with the central core.

[0342] The optical fiber 1 may be configured to exhibit birefringence. This can be achieved in various ways. For example, if the optical fiber 1 has a core 3 and cladding 5 surrounding the core 3, the optical fiber 1 may have one or more stress induction regions 49 arranged around the core 3 to contribute to birefringence.

[0343] The fiber may exhibit birefringence before the formation of one or more voids 9. Alternatively or additionally, stress induction regions may be formed during the manufacturing of the optical fiber 1, in which case the method further includes the step of forming one or more stress induction regions 49 in the optical fiber 1. The one or more stress induction regions 49 are configured to contribute to the birefringence of the optical fiber 1. The stress induction regions 49 may have laser exposure regions, and forming one or more stress induction regions 49 may involve selectively exposing the optical fiber 1 to laser irradiation.

[0344] The step of forming one or more stress induction regions 49 may be performed before the step of bringing the optical fiber 1 into contact with the etching solution (S20). In this case, the stress induction regions 49 should be separated from the exposed region 7 so as not to be etched by the etching solution. Alternatively, the step of forming one or more stress induction regions 49 may be performed after the step of bringing the optical fiber 1 into contact with the etching solution (S20).

[0345] Instead of including the stress induction region 49, or in addition, one or more voids 9 may have multiple voids 9 arranged symmetrically around the core 3 to contribute to birefringence. For example, the multiple voids 9 may have two voids 9 arranged on either side of the core 3 along the first diameter of the optical fiber 1 (i.e., exhibiting two-fold rotational symmetry about the longitudinal axis). No voids 9 may be provided along a second diameter perpendicular to the first diameter of the optical fiber 1. Alternatively, voids 9 along different diameters of the optical fiber 1 may be filled with fillers 15 having different material properties.

[0346] Figures 27A to 27D show examples of configurations in which optical fibers have birefringence and have different effective refractive indices for orthogonal polarizations of light. Figure 27A shows an elliptic core birefringent fiber with a liquid-filled channel in one radial direction. Figure 27B shows a similar configuration but with a rectangular core.

[0347] Figure 27C shows a PANDA birefringent fiber having a void 9 containing a filler 15 in one radial direction and a stress-inducing region 49 that gives stress-induced birefringence. Here, the filler 15 is introduced into the void 9 along an access void 19 in an axis perpendicular to the stress rod. Figure 27D shows a standard optical fiber that has been birefred by creating a stress-inducing region 49 by laser exposure. This generates high stress within these regions, resulting in stress-induced birefringence. A void 9 filled with filler 15 also exists along the orthogonal axis. The void 9 is located along only one diameter of the optical fiber 1, and there is no void 9 along the perpendicular diameter. This means that only one of the two orthogonal polarizations of light interacts significantly with the filler 15. This allows for strain-temperature discrimination because the temperature-dependent change in refractive index is different for the two orthogonal polarizations of light.

[0348] The optical fiber 1 may have a Bragg grating 31. The birefringence of the optical fiber 1 may differ for light with different polarizations and Bragg wavelengths of the Bragg grating 31, and optionally the polarizations may be orthogonal.

[0349] A Bragg grating in a birefringent fiber will have different Bragg wavelengths for two orthogonal polarizations. If one polarization is sensitive to a different environmental variable than the other, then that variable may be determined by wavelength separation of the Bragg reflection peaks. For example, temperature may be measured. Wavelength separation may also be an indicator of external pressure. If both polarizations are equally sensitive to strain, then the strain may be measured by the absolute wavelength of the peaks.

[0350] For example, in the configurations shown in Figures 27A to 27D, the stress induction region 49 induces birefringence such that there are two Bragg reflection peaks. One of the peaks is affected by the filler 15, and for example, has a greater change in refractive index with temperature, while the other peak does not. This means that the temperature measurement can be derived from the difference between the two peaks, while the strain measurement can be determined by the position of the peak (which should similarly be affected by strain).

[0351] Birefringence may be such that, when an environmental variable such as the pressure within one or more gaps 9 is substantially equal to an environmental variable outside the optical fiber 1 (i.e., at a pressure difference of zero), the reflection peaks for light having different polarizations around the corresponding Bragg wavelength of the Bragg grating 31 are resolvable. This ensures that the two peaks are completely separated. Preferably, the reflection peaks are separated by at least the full width at half maximum of the reflection peak at a pressure difference of zero. The separation of the two peaks may also be temperature-dependent.

[0352] Figures 28A and 28B show a pressure sensor designed to be birefringent even with a pressure difference of zero. Figure 28A shows a cross-section of an optical fiber 1 having two sets of laser-inscribed regions. One set is an exposed region 7 used to define the region to be removed by bringing the optical fiber 1 into contact with an etching solution (S20). This set has an access region 17 that connects the exposed region 7 to the outer surface of the optical fiber 1. The other set of laser-inscribed regions is a stress-inducing region 49 used to apply stress to the optical fiber 1. These are not connected to the access region 17. Figure 28B shows the optical fiber 1 after etching, with voids 9 not exposed to the etching solution and additional stress-inducing regions 49 present. For pressure sensing applications, the filler 15 is preferably an inert gas such as air or nitrogen.

[0353] The stress-inducing region 49 causes stress-induced birefringence in the optical fiber 1 even when there is no pressure difference between the internal pressure in the gap 9 and the external pressure on the optical fiber 1. The additional birefringence caused by the stress-inducing region 49 results in the wavelength peaks of the Bragg grating 31 for two orthogonal polarizations being separated even at a pressure difference of zero. This facilitates the measurement of low pressures because the peaks in the spectra from the two polarizations will be distinguishable from each other. This allows the side-hole optical fiber pressure sensor to operate at low pressures without the need for polarization management. This means that the sensor can be queryed by an incoherent or polarization-scrambled light source, and the wavelength peaks of the orthogonal polarizations will be distinguishable.

[0354] One or more voids 9 may also be configured such that the effect of the optical properties of the filler 15 on the light guided by the optical fiber 1 differs for light with different polarizations, and optionally the polarizations are orthogonal. Figures 29A and 29B show a modified version of the design in Figures 28A and 28B, in which, in addition to the bowtie-shaped voids 9, the voids 9 have additional portions that increase pressure-induced birefringence in the optical fiber 1. Figure 29A shows the device after laser exposure, and Figure 29B shows the device after etching.

[0355] By using birefringence, a single FBG can simultaneously provide information about both strain and temperature. An advantage over having two separate FBGs is that the FBGs are physically located in the same place and therefore provide information from precisely the same physical location. Another advantage is that the two FBG peaks are close to each other in wavelength and track each other, and therefore the number of sensors for a given source bandwidth is not significantly reduced. Using two separate FBGs, it may be possible to sense only half of the locations.

[0356] Figures 30A and 30B show a side-hole fiber 110 that has been made birefringent by laser exposure. Figure 30A shows the fiber stretched to have side holes. Figure 30B shows the fiber after exposure to laser irradiation has created a stress-inducing region 49. This makes it possible to make a conventional air-hole fiber birefringent, so that even pressure differences where the internally written FBG is zero are separated at wavelength.

[0357] Figures 31A and 31B show examples of pressure sensors fabricated in the form of polarization-maintaining optical fibers. Instead of fabricating a void 9 within a standard optical fiber, the void 9 is fabricated within the polarization-maintaining fiber. Since the polarization-maintaining fiber is already birefringent, this provides the desired birefringence at a pressure difference of zero. Figure 31A shows a void 9 fabricated in a "bowtie" polarization-maintaining fiber having a bowtie-shaped stress-inducing region 49. Figure 31B shows a void fabricated in a PANDA polarization-maintaining optical fiber having a circular stress-inducing region 49. The void 9 can be fabricated by exposure to laser irradiation and subsequent etching, as described above.

[0358] The optical fiber 1 described above may be incorporated into a system for use in various applications such as environmental sensing or optical filtering and control.

[0359] Figure 32 shows a system for measuring environmental variables such as strain or temperature using an optical fiber 1 according to the present invention, which includes a Bragg grating 31. Light from an adjustable laser 61 travels through an optical circulator 65 and into the optical fiber 1. In this system, a polarization scrambler 63 is also present to scramble the polarization of the laser light. This is useful when using a birefringent fiber where information should be obtained from multiple orthogonal polarizations of the light. However, it is generally not necessary and may be omitted, in particular when the system will not use information from multiple different polarizations. Alternatively, the scrambler 63 can be replaced by polarization control to allow measurement of the response of the optical fiber 1 to different polarizations. A polarization controller may be used to measure the response of different polarization modes by individually adjusting the polarization of the laser 61 to each polarization axis for each FBG.

[0360] In this example, the optical fiber 1 contains multiple Bragg gratings 31a, 31b, and 31c, although this is not generally required for all applications, and in some situations one or two Bragg gratings may suffice. Each FBG 31a, 31b, and 31c has a different Bragg wavelength, and the wavelength separation is large enough to accommodate the change in Bragg wavelength associated with the variable being measured. The light reflected from each FBG travels through the optical circulator 65 and returns to the detector 69. The controller 67 measures the spectrum of the FBG array in the optical fiber 1 by sweeping the wavelength of the laser 61 by applying an electrical signal and recording the response of the detector 69.

[0361] An exemplary application is a strain sensor having an optical fiber with a Bragg grating. The gap 9 may be configured to provide reduced temperature sensitivity of the strain sensor, as described above. The strain sensor may be configured to measure the Bragg wavelength of the Bragg grating. The controller 67 may be configured to determine the strain applied to the optical fiber 1 based on the Bragg wavelength of the Bragg grating. If the optical fiber 1 is configured to exhibit birefringence, the controller 67 may be configured to determine the strain based on the difference between the Bragg wavelengths of the Bragg grating 31 for light with different polarizations.

[0362] Another application is a system for sensing strain and / or temperature, having an optical fiber 1 with two Bragg gratings 31. The temperature-dependent changes in the Bragg wavelengths of the two Bragg gratings 31 are different. For example, one FBG may be a standard FBG in a standard silica fiber, and the void 9 is configured to affect the Bragg wavelength of the other FBG. Alternatively, both FBGs may be affected by the void 9, but the void 9 affecting each FBG may have different fillers 15 and / or have different shapes and sizes and / or be located in different positions relative to the Bragg grating. The system may have a controller configured to determine the strain applied to the optical fiber 1 and the temperature of the optical fiber 1 based on the Bragg wavelengths of the two Bragg gratings 31. As described above, this can be achieved by using linear algebraic techniques to isolate the effects of temperature and strain on the two Bragg gratings 31.

[0363] Instead of having two separate Bragg gratings, strain and temperature can also be sensed simultaneously using birefringent fibers with two orthogonal polarizations that respond differently to temperature and / or strain. Figures 33A–33C show an example of the operation of a system for sensing strain and temperature with birefringent optical fiber 1.

[0364] Figure 33A shows the sensor at temperature T0 and strain S0. S0 is a non-zero reference strain, so that the optical fiber 1 is still under some tension and not slack. Since each polarization has a different effective refractive index, there are two reflection peaks (one for each polarization). Figure 33B shows the sensor at the same temperature T0 when the fiber is subjected to strain S1 (>S0). Under the applied strain, both reflection peaks shift together to longer wavelengths. Figure 33C shows the sensor at temperature T1 (>T0) with the same strain S0 applied. Here the two reflection peaks are separated in wavelength, one shifting to a longer wavelength and the other shifting to a shorter wavelength. Thus, the sensor can be used to determine strain via the absolute shift of the peaks and temperature via the separation of the reflection peaks.

[0365] A pressure sensor having an optical fiber 1 including a Bragg grating 31 may also be fabricated. The pressure sensor may be configured such that the pressure difference between the pressure of the filler 15 in one or more voids 9 and the external pressure applied to the optical fiber 1 affects the Bragg wavelength of the Bragg grating. The pressure sensor may have a controller 67, as shown in Figure 32, configured to determine the external pressure applied to the optical fiber 1 based on the Bragg wavelength of the Bragg grating 31. If the optical fiber is configured to exhibit birefringence, the pressure difference may affect the birefringence. In this case, the controller may be configured to determine the external pressure applied to the optical fiber based on the difference between the Bragg wavelengths of the Bragg grating for different polarizations of light.

[0366] A system like the one shown in Figure 32 can also be used to provide a pressure sensor. Light reflected from Bragg gratings 31a, 31b, and 31c is detected by detector 69. The controller 67 unit sweeps the wavelength of the adjustable laser 61 and reads the signal from detector 69, for example by sampling with an analog-to-digital converter. The controller 67 then records the reflection spectrum of the optical fiber 1, determines the wavelength of each peak, and converts this information into pressure and temperature measurements.

[0367] Instead of using a swept, adjustable laser 61 and a polarization scrambler 63 or polarization control, it is possible to generate amplified spontaneous illumination using non-coherent light sources such as light-emitting diodes (LEDs), superluminescent diodes or semiconductor lasers, or lengths of rare-earth doped optical fibers optically pumped with any optical amplifier.

[0368] Figures 34A to 34E show the reflection spectra from an optical fiber pressure sensor, similar to those shown in Figures 7A and 7B. Figure 34A shows the spectrum when the temperature (T) is T0 and there is virtually no pressure difference between the gap 9 and the outside of the fiber (ΔP=0). Here, there is no induced birefringence, and the wavelength spectra for the two orthogonal polarizations overlap. Only a single reflection peak is observed. Figure 34B shows the spectrum at the same temperature (T=T0) but with a slightly increased pressure difference (ΔP=P1>0). Here, there are two overlapping but partially separated spectra for the two orthogonal polarizations. Figure 34C shows the spectrum at the same temperature (T=T0) but with an even higher pressure difference (ΔP=P2>P1). Here, the spectra for the two polarizations are completely separated. Figure 34D shows the spectrum when the temperature (T) is T0 and the pressure difference ΔP is P3 (>P2). Here, the spectra for two orthogonal polarizations are still separated. Figure 34E shows the spectrum for a higher temperature (T=T1>T0) but with the same pressure difference as in Figure 34C (△P=P2). Here, the wavelength separation is substantially the same as in Figure 34C, but the entire spectrum is shifted to longer wavelengths. Therefore, using this system, it is possible to determine the pressure from the wavelength separation and the temperature from the absolute wavelength. This allows for independent measurements of pressure and temperature.

[0369] Figures 35A and 35B show spectra from a pressure sensor that has additional birefringence at low pressure differences, for example, through the use of the stress-induced region 49 described above. Figure 35A shows the pressure sensor at a pressure difference of zero, where T=T0 and △P=0. Here, the spectrum shows two peaks for two separated and distinguishable orthogonal polarizations. Figure 35B shows the spectrum at an even higher pressure (△P=P1), where the wavelength separation of the two polarizations is increased.

[0370] Other applications of the present invention include temperature-stable Bragg grating optical filters. Bragg gratings can be used as optical filters, and compensating for their temperature dependence of the Bragg wavelength allows the filter to remain stable over a certain temperature range. This can also be applied to the fabrication of Bragg grating-stabilized lasers in which at least one of the reflectors is a Bragg grating having a gap 9 that at least partially compensates for the temperature dependence of the Bragg wavelength. Semiconductor-stabilized lasers or fiber lasers having temperature-insensitive Bragg gratings may also be fabricated.

[0371] In addition to highly stable optical filters and lasers, the optical fiber 1 of the present invention can also be used to create highly tunable optical filters and lasers. If the void 9 and filler 15 are configured to increase the magnitude of the temperature-dependent change in the Bragg wavelength, this can be used to create an optical fiber 1 containing a highly temperature-sensitive Bragg grating for tuning. Using such an optical fiber can create a Bragg grating optical filter whose temperature is tunable. A tunable laser can be provided in which at least one of the reflectors is provided by an optical fiber having a highly temperature-sensitive Bragg grating. Using two highly temperature-sensitive Bragg gratings, a tunable fiber laser or a fiber laser tunable with sampled gratings can be created.

[0372] Figure 36A shows a thermally adjustable FBG optical filter. An optical fiber with a highly temperature-sensitive Bragg grating 31 is coupled to a Peltier element 71 that provides a thermoelectric heat pump device. The Peltier element 71 is electrically controlled by a controller 67 to provide a set temperature. By changing the set temperature, the wavelength of the optical filter is adjusted. Because the temperature sensitivity of the highly temperature-sensitive Bragg grating 31 is significantly higher than that of a standard FBG, a much larger adjustment range is achievable. Figure 36B shows the spectrum of the filter in Figure 36A at temperatures T0 and T1 (>T0).

[0373] Another application is an add-drop multiplexer, as shown in Figure 37. This is used to "drop" wavelength channels on a particular optical fiber and replace them with wavelength channels from another source. The dropped channels can be routed to a different destination. A thermally tunable FBG31 is used to select which wavelengths should be dropped. In Figure 37, the dropped wavelength is set to a third wavelength channel, but generally any channel can be dropped. The input channels enter the first optical circulator 65 and are transmitted to the tunable FBG31. The third wavelength channel is reflected back from the FBG31 via the first optical circulator 65 and is dropped out. The remaining channels pass through the FBG31 and are transmitted to the second circulator 66, where they are combined with the "added" channels before being sent to the required destination. The wavelength of the FBG31 can be tuned to other wavelengths so that different wavelength channels can be added / dropped. FBG31 can be tuned outside the wavelength range of all channels so that all channels pass through.

[0374] Figure 38A illustrates an application for providing a temperature-stabilized laser. A laser cavity is formed in a semiconductor gain medium 75 having a highly reflective back facet 79 and an anti-reflective coated front facet 77. Light exits from the front facet 77 and is coupled to a lensed fiber 73 (provided by a tapered fiber). Within the optical fiber 1, there is a temperature-insensitive FBG31 that acts as a narrowband front reflector for the laser. The wavelength of the FBG31 determines the wavelength of emission from the laser. By having a thermally insensitive FBG31, the laser wavelength will experience less wavelength change with changes in ambient temperature. Figure 38B shows that the emission spectrum remains substantially unchanged with temperature changes.

[0375] Figure 39A illustrates an application for providing a tunable laser. A laser cavity is formed in a semiconductor gain medium 75 having a high reflectivity back facet 79 and a tunable FBG 31 as a front reflector. The tunable FBG 31 can be thermally tuned as described above. The wavelength of emission is determined by the wavelength of the FBG 31. Therefore, tuning the FBG wavelength via the controller 67 has the effect of tuning the laser wavelength. Figure 39B shows an exemplary spectrum when the laser wavelength is tuned.

[0376] Figure 40 shows an application for providing a tunable fiber laser. The gain medium is a doped optical fiber 120 (e.g., doped with erbium, or any other suitable dopant including rare earth dopants such as erbium-yttrium). The doped optical fiber 120 is spliced ​​at splice 85 into optical fiber 1 containing tunable Bragg gratings 31-1 and 31-2. The doped fiber 120 is optically pumped through a coupler 81 using a pump laser 83 (e.g., 980 nm or 1480 nm). There may be pump light propagating in the same direction and / or in the opposite direction. The tunable FBGs 31-1 and 31-2 provide reflectors and are tuned via a controller 67.

[0377] As described above, the void 9 may include one or more cladding voids located in the cladding 5 of the optical fiber 1, which has a core 3 and cladding 5 surrounding the core 3. Additionally or alternatively, one or more voids 9 may have core voids within the core 3. Core voids allow for much stronger interaction between the light transmitted in the fiber and the filler 15, as the optical modes are mainly localized within the core 3. This can enable sensors with much higher temperature sensitivity and adjustable optical devices with a much larger adjustment range. The core 3 may be aligned with the central axis of the optical fiber 1.

[0378] The core voids may be completely or partially filled with the filler 15. The core voids are located within the core 3, but do not have to completely replace the core 3. In other examples, the core 3 may consist substantially of core voids. If both core voids and cladding voids are present, the cladding voids may be filled with a filler 15 having different material properties than the filler 15 in the core voids. Multiple core voids may exist within the core 3. Multiple core voids may be filled with the same filler 15, or they may be filled with different fillers 15 having different material properties.

[0379] If the fiber has a core void, it may also have a Bragg grating 31. The Bragg grating 31 may be provided, for example, by periodic modification of the cladding 5 adjacent to and / or surrounding the core void. The Bragg grating may be formed by a ring of modified material around the core 3. Alternatively or additionally, if the core void does not completely replace the core 3, the Bragg grating 31 may be created by periodic modification of the remaining solid portion of the core 3.

[0380] Figures 41A to 41D illustrate the fabrication of an optical fiber 1 having a core void containing filler material 15 in the core 3 and a cladding void containing filler material 15 in the cladding 5. Figure 41A shows the optical fiber 1 before filling the void 9. The void 9 may be etched from a suitably defined exposure region 7 as described above. Alternatively, the void 9 may be formed by the prior art of manufacturing an optical fiber 1 containing the void 9 by stretching a suitably defined preform.

[0381] As can be seen, the void 9 has a core void in the core 3 of the optical fiber 1. Figure 41B shows an optical fiber 1 in which filler 15-A is filled in the cladding void and filler 15-B, which has a higher refractive index, is filled in the core void. Figure 41C shows that a Bragg grating 31 has been inscribed on the remaining solid region of the optical fiber 1 around the core void by modifying the material of the optical fiber to create an exposed region 7 around the core void. Figure 41C shows a cross-section in which the exposed region 7 exists, but in order to create a Bragg grating, the exposed region 7 would only exist periodically along the length of the optical fiber 1. Figure 141D shows an optical fiber 1 having two different fillers 15-A and 15-C in the cladding void in addition to filler 15-B in the core void. The three different fillers 15-A, 15-B and 15-C may be different materials having different refractive indices and / or different refractive index changes dn / dT with different temperatures. The fillers 15-A and 15-B in the cladding voids may also have different proximity to the core voids or may be filled in different amounts. Using more than one filler allows the thermal response of the Bragg grating 31 to be flattened over a wider temperature range.

[0382] Rather than providing the Bragg grating 31 by periodic modification of the material surrounding the core void, the Bragg grating 31 may be provided by periodic modification of the filler 15 in the core void, similar to what is described above for the cladding void. For example, the filler 15 may have liquid crystals. The filler 15 may be polymerizable, and the periodic modification may involve periodic polymerization. For example, the filler 15 may have monomers and photoinitiators, and the periodic polymerization may be carried out by selectively polymerizing the filler 15 periodically along the length of the core void using a laser.

[0383] In addition to the thermal adjustment of the properties of the optical fiber 1 described above, it is possible to electrically adjust the optical fiber containing the void 9 filled with the filler material 15. To achieve this, the optical fiber may have one or more electrodes 93 configured to apply an electric field to the filler material 15 and affect the material properties of the filler material 15.

[0384] Figures 42A and 42B show electrically adjustable optical fibers 1. The filler material 15 used in this case is a liquid crystal or another material having material properties that are altered by the presence of an electric field. In addition to the voids 9 in the cladding 5 containing the liquid crystal filler material, electrodes 93 are also present in the cladding 5. The electrodes 93 are formed using voids 9 created in the same way as the voids 9 filled with the filler material 15, but the voids 9 may then be formed by filling them with a conductive material. Filling the voids 9 with a conductive material can be done, for example, by pouring a conductive material such as indium tin solder into the voids 9 while it is above its melting point (approximately 125°C for indium tin solder).

[0385] An electric field generator 91 is used to apply an electric field across the liquid crystal filler 15 to change its material properties (e.g., its optical properties). The electric field generator 91 may be any suitable device, such as a bipolar square wave generator coupled to a high-voltage amplifier. By changing the magnitude of the applied electric field, material properties such as the refractive index of the filler 15 are changed. This affects the effective refractive index experienced by the light transmitted in the optical fiber 1. This can also be used to change the Bragg wavelength of FBG31 in the optical fiber 1, as shown in Figures 42A and 42B. Other material properties that may be changed may include the magnitude of birefringence, the angle of the optical axis of the birefringent material, and light absorption or scattering loss. The electric field may change the effective refractive index of at least one polarization axis of the optical fiber. The electric field may change the effective refractive index of both polarizations unevenly or substantially uniformly.

[0386] Figures 43A and 43B show an optical fiber 1 containing an electrically adjustable FBG31, having a core void in which one or more voids 9 are filled with liquid crystal. Instead of liquid crystal, any other liquid whose material properties change depending on the applied electric field may also be used. Access voids 19 allow the core void to be filled with liquid crystal or other suitable filler 15. The voids 9 in the cladding are filled with a conductive material to form electrodes 93 as described above.

[0387] Figures 44A to 44D show various different exemplary arrangements of materials within a cross-section of an optical fiber 1. Figure 44A shows rectangular voids containing liquid crystal 95 on both sides of an elliptical core 3, with electrodes 93 on both sides of the liquid crystal 95. The inner surface of the voids 9 containing the liquid crystal 95 has alignment surfaces 97 having a structure that assists in the alignment of the liquid crystal. These alignment surfaces 97 may be created by exposure to laser irradiation. Figure 44B shows an elliptical core void filled with liquid crystal 95, with rectangular electrodes 93 on both sides. Figure 44C shows a rectangular core 3 with rectangular voids 9 containing liquid crystal 95 and rectangular electrodes 93. Figure 44D shows a rectangular core void containing liquid crystal 95 with rectangular electrodes 93. In all of these examples, the voids do not necessarily have to contain liquid crystal 95, and any other suitable filler 15 that changes its material properties in response to exposure to an electric field may be used instead.

[0388] These electrically adjustable embodiments enable low-cost adjustable optical components, such as optical filters. One of the biggest costs associated with active optical devices is the packaging cost associated with aligning the optical fiber to the active device, rather than the device itself. By fabricating an electrically active component inside the optical fiber itself, the need for the fiber alignment process is eliminated, thereby eliminating cost and manufacturing complexity.

[0389] Using these embodiments, Bragg grating optical filters can be created, similar to the temperature-adjustable optical filters described above. Figure 45A shows an electrically adjustable FBG filter. The electrically adjustable FBG 31 is coupled to a controller 67 to set a specific Bragg wavelength for the Bragg grating 31 by adjusting the material properties of the filler 15 in one or more voids 9 that affect the Bragg grating 31. Figure 45B shows the corresponding spectra at two different applied voltages.

[0390] An electrically tuned fiber grating (FBG) may also be used instead of a thermally tuned FBG to provide an add-drop multiplexer as shown in Figure 40. A tuneable laser may be provided in which at least one of the reflectors is an electrically tuneable Bragg grating. Using two electrically tuneable Bragg gratings, a tuneable fiber laser or a tuneable fiber laser with sampled gratings may be created. An electrically tuned FBG may be used instead of a thermally tuned FBG to provide a tuneable laser as shown in Figure 39A.

[0391] Crystalline optical fibers may be useful in certain applications where conventional silica optical fibers are unsuitable. This may include extreme temperature environments where the higher melting point of crystalline optical fibers is relevant. The present invention enables crystalline optical fibers to be formed with one or more voids, one or more voids extending longitudinally along the length of the optical fiber. This was historically impossible using prior art because crystalline optical fibers cannot be stretched from preforms and cannot retain their crystalline structure. However, this method, using exposure to laser irradiation and subsequent etching of the exposed area 7, makes it possible to form voids 9 even in crystalline optical fibers. This makes it possible to fabricate optical fiber sensors, such as those described above, within crystalline optical fibers, allowing the sensors to be used under extreme conditions such as high temperatures exceeding 1000°C or extreme pressures.

[0392] Any suitable crystalline optical fiber may be used. The optical fiber may be a microstructured fiber. The optical fiber may be an anti-resonant fiber or a negative curvature fiber. The optical fiber may be a single-crystal optical fiber, and optionally may be a sapphire, diamond, or yttrium aluminum garnet (YAG) crystal. The optical fiber may be doped, and optionally may be doped with a rare earth element. The void may be formed in the optical fiber using the method described above.

[0393] Figures 46A to 49B illustrate the fabrication process for crystalline fibers. The crystalline fiber may be a single-crystal fiber. Figures 46A and 46B show pieces of an unmodified single-crystal (hard crystal) optical fiber 100 (e.g., sapphire fiber). Figures 47A and 47B show the crystalline optical fiber 100 of Figures 46A and 46B, which is selectively exposed to laser irradiation (S10) and has defined exposed regions 7 to be removed by selective etching and access regions 17 that allow the etching solution to reach the exposed regions 7. The Bragg grating 31 is also inscribed in the crystalline optical fiber 100 by periodic modification of the material of the crystalline optical fiber 100 using laser irradiation. However, it is not essential for the crystalline optical fiber 100 to have the Bragg grating 31.

[0394] Figures 48A and 48B show the crystalline optical fiber 100 of Figures 47A and 47B, after contact with an etching solution (S20) to etch the access region 17 and exposed region 7, thereby forming access gaps 19 and 19. In this example, the crystalline optical fiber 100 was etched with 8 mol / l KOH at 85°C, but any suitable etching process may be used. In this example, the access gaps 19 are used, which allows long fibers to be etched without excessively long etching times. However, it is also possible to avoid using the access gaps 19 by etching from the ends of the crystalline optical fiber 100.

[0395] As shown in Figures 48A and 48B, the air gap 9 is defined so that the crystalline optical fiber 100 forms a microstructured optical fiber. The periodic array of air gap 9 forms a guide structure, and the light is guided within the central region. The microstructured optical fiber may be used to form a photonic crystalline waveguide or a photonic bandgap waveguide.

[0396] One or more voids 9 may be at least partially filled with a filler 15 having material properties different from those of the crystalline optical fiber 100 material. The material properties may be optical properties such as refractive index. In the examples in Figures 48A and 48B, the filler is simply air, but other fillers 15 may be used depending on the application. For example, another inert gas such as nitrogen may be used. Figures 49A and 49B show a hollow core microstructure crystalline optical fiber 100, where the core 3 itself is also a void 9, and light is guided within the core void.

[0397] The void 9 may be sealed from the outside of the optical fiber 100 as described above. This can be achieved, if used, by sealing the access void 19 after filling the void 9 with the filler material 15. However, in the case of a single-crystal optical fiber 100, it is not always necessary to seal the void 9 from the outside. Single-crystal materials are very hard and robust, and if the filler material 15 is not used, there is no need to seal the void to contain the filler material 15. The void 9 may be sealed if there is a fluid expected to be present around the optical fiber 1 (which changes the material properties within the void and adversely affects the behavior of the optical fiber 1). For example, if the void 9 is not sealed, the gas concentration within the void may change with temperature, which may adversely affect the performance of a temperature sensor using a single-crystal optical fiber 100. Additionally, if the crystalline optical fiber 100 is used for a pressure sensor, the void 9 should be sealed. This is because this application relies on the pressure difference between the air gap 9 and the external environment, and this pressure difference cannot exist if the air gap 9 is not sealed from the outside of the optical fiber.

[0398] Regarding the optical fiber described above, the longitudinal extent of one or more voids 9 may be less than the length of the crystalline optical fiber 100. One or more voids 9 may extend within the void portion. The void portion is a continuous and integral part of the crystalline optical fiber 100. The longitudinal extent of one or more voids 9 may be less than the longitudinal extent of the void portion.

[0399] The central axis of one or more voids 9 may extend parallel to the longitudinal axis of the crystalline optical fiber. Alternatively, the central axis of one or more voids 9 may extend in a direction inclined with respect to the longitudinal axis of the crystalline optical fiber 100 for at least a portion of the voids 9. Any of the features of the optical fiber 1 and voids 9 described above for other embodiments may be applied to the voids 9 of the crystalline optical fiber 100. For example, the crystalline optical fiber 100 may have a Bragg grating 31. This may be particularly useful in sensing applications.

[0400] The crystalline optical fiber 100 may be configured to exhibit birefringence. This can be achieved using a gap 9, which can result in stress-induced birefringence in the crystalline optical fiber 100 when it is subjected to pressure. A stress-inducing region 49 may also be provided to correct the birefringence. The crystalline optical fiber 100 may be configured such that two orthogonal polarization modes of light are reflected by the Bragg grating 31 at different wavelengths. Wavelength separation of Bragg wavelengths for different polarizations may be used as an indicator of external pressure, as described above.

[0401] The single-crystal optical fiber 100 may have a waveguide 41 inside it. The waveguide 41 may be a single-mode waveguide. A single-mode waveguide may have two orthogonal polarization modes. The waveguide 41 may be any suitable waveguide, such as a concave cladding waveguide, a microstructure waveguide, a photonic crystal waveguide, or an anti-resonant waveguide. The waveguide 41 may be formed by modifying the refractive index of the crystalline optical fiber 100. The waveguide may also be formed by modifying the material of the crystalline optical fiber 100 by exposing the crystalline optical fiber 100 to laser irradiation. The waveguide 41 may be formed using a void 9 which may be filled with a filler 15. For example, the crystalline optical fiber 100 may be selectively etched to form the waveguide 41 by selectively removing material from the crystalline optical fiber 100. This potentially enables a single-mode single-crystal optical fiber that has lower loss and can withstand higher temperatures than conventional silica optical fibers.

[0402] Single-crystal fibers, such as sapphire, are often used for ultra-high temperature applications. Waveguides can be formed in crystalline optical fibers by exposure to laser irradiation. However, at high temperatures, the laser-induced modification of the crystalline optical fiber material may be attenuated by effective annealing or completely removed. By forming waveguides within the crystalline optical fiber using voids 9, the structural changes to the optical fiber material become irreversible because the voids 9 are filled with filler material 15, and therefore the structure will not change when subjected to extreme temperatures. Single-crystal optical fibers can have higher optical losses than conventional silica optical fibers, but the losses of crystalline optical fibers can be reduced by using voids 9 to change the majority of the waveguide material into a low-loss filler such as air.

[0403] Figure 50 illustrates an application of the present invention for forming a crystalline optical fiber pressure sensor. A waveguide is created by exposing a region to laser irradiation to form an exposed region 7 in which the refractive index is reduced. In the central part of the exposed region 7, which is coaxial with the longitudinal axis of the crystalline optical fiber 100, the unexposed portion of the crystal is unmodified and therefore has a higher refractive index than the exposed region 7. This structure effectively creates a "cladding" of the exposed region 7 around a "core" of unexposed material. Light injected into the core is guided by the waveguide formed by the core / cladding interface.

[0404] In addition to the cladding / core structure, two voids 9 are formed by the method described above. The voids are sealed in any suitable manner as described above. For example, a heat-resistant ceramic adhesive may be used. The voids 9 cause the crystalline optical fiber 100 to deform under external pressure, resulting in stress-induced birefringence. A Bragg grating is also written on the core 3. The crystalline optical fiber 100 then acts as a pressure sensor, and the wavelength separation of the Bragg reflection peaks of two polarizations indicates the pressure. The use of crystalline optical fiber allows for pressure measurement at much higher temperatures (e.g., over 2000°C for sapphire fiber).

[0405] Figure 51 shows a microstructured crystalline optical fiber pressure sensor. This crystalline optical fiber 100 also has a waveguide written therein. In this case, the waveguide is a microstructured waveguide. The waveguide consists of an unmodified core surrounded by a periodic array of exposed regions 7. Some of the exposed regions 7 are etched to form voids 9. The voids 9 are sealed from the outside of the optical fiber. A Bragg grating is provided inside the core. Under pressure, stress-induced birefringence will exist in the optical fiber created by the array of voids 9. The pressure can therefore be measured by the separation at wavelength between the Bragg reflection peaks of two orthogonal polarization modes.

[0406] Figures 52A to 54B show devices fabricated within a substrate 130 such as silica, quartz, borosilicate glass, or other transparent material. This can be considered as a coreless fiber, and generally, any of the embodiments described above in relation to the optical fiber 1 can be similarly applied to a substrate (e.g., a planar substrate) as shown in Figures 52A to 54B. This process is similar to the fabrication in conventional optical fibers, except that the waveguide does not have a core. Instead, a waveguide 41 that serves a similar purpose to the core of the optical fiber is created within the substrate 130 by modifying its material properties (e.g., increasing its refractive index) by exposing it to laser irradiation. The waveguide 41 that provides an effective core can also be provided in any other suitable form.

[0407] Figures 52A and 52B show the substrate 130 after it has been exposed to laser irradiation (S10) to create a waveguide 41, a Bragg grating 31 has been inscribed within the waveguide 41, and the exposed area 7 has been defined. Figures 53A and 53B show the substrate 130 after it has been in contact with the etching solution (S20) to remove the exposed area 7 and form a void 9. Figures 54A and 54B show the substrate 130 after the void 9 has been filled with filler material 15.

[0408] Several specific examples of optical fibers manufactured according to the above methods and embodiments will be described here.

[0409] As described above, FBG is a periodic modulation of the refractive index in an optical fiber. When light is guided, a narrow band centered on the Bragg wavelength is reflected. The Bragg wavelength depends on the effective refractive index of the fiber and the pitch of the periodic grating, and is given by the following equation:

[0410]

number

[0411] In the formula, λ BThis is the Bragg wavelength, and n eff is the effective refractive index of the fiber, m is an integer, and Λ is the pitch of the periodic structure. When temperature or strain fluctuations exist, n eff Both and Λ will fluctuate. As a result, the Bragg wavelength shift due to changes in temperature and strain is given by the following equation:

[0412]

number

[0413] In the formula, △λ B This is a shift in the Bragg wavelength caused by strain ε or temperature variation ΔT, and P ij ν is the Pockels coefficient of the stress optics tensor, ν is Poisson's ratio, α is the coefficient of thermal expansion, and ξ is the thermal response of the fiber.

[0414] To distinguish between the strain response and the temperature response, FBG sensor devices with significantly different thermal responses are expected. α mainly depends on the fiber material, while ξ depends on the effective refractive index of the FBG section and is given by the following equation:

[0415]

number

[0416] In conventional FBG fiber devices, dn eff / dT is the thermo-optic coefficient of the fiber material (typically 7.97 × 10⁻¹⁰ in fused silica). -6 ℃ -1 It is a constant equal to ). The Bragg wavelength shift due to the rise in temperature is always positive over the entire temperature range. To improve the thermal response, a more significant dn effA dT of 1 / dT is required. Therefore, either the core or cladding of the fiber needs to be made of a material with a different thermo-optic coefficient (because the evanescent field of the fiber waveguide extends into the cladding region). This can be achieved using microchannels formed by voids, as described above. These may be filled with a filler such as a glycerol solution.

[0417] In fact, there are several advantages to using a microfluidic glycerol solution in the cladding rather than the core. First, the microchannels in the cladding allow for the inscribement of FBGs in the fiber core. Furthermore, waveguides in liquid solutions are prone to losses and may be potentially susceptible to bending. Additionally, because the cladding occupies a larger area than the core, there is much greater flexibility when designing devices with different thermal responses. Glycerol is non-toxic and non-flammable, and has a much larger thermo-optic coefficient (-225 × 10⁻¹⁵) than fused silica. -6 ℃ -1 It is an optimal choice because it possesses the following properties. Furthermore, pure glycerol has a refractive index of 1.4473 at 1550 nm at room temperature, and can be easily adjusted to an even lower refractive index by dilution with water to match the refractive index of fused silica.

[0418] The cross-section of the FBG device was designed to be a "double bowtie" pattern consisting of four identical trapezoidal sectors (sector angle φ), as shown in Figure 55. The FBG is written within the core region, and the glycerol-filled microchannels are exposed from the fiber side without touching the fiber core, so that the proportion of glycerol solution in the cladding can be parameterized by the sector angle. In such a device, as the temperature increases, the refractive index n of the core co However, it increases at a constant rate equal to the thermo-optic coefficient of molten silica. Cladding rate n cl The total variation is due to the two components of the cladding rate that vary in opposite directions (silica cladding n). cl,siand glycerol cladding n cl,gl ) is affected by n cl The net thermal response of / dT is between the thermo-optic coefficient of molten silica and the thermo-optic coefficient of pure glycerol, while the average cladding refractive index n cl is n cl,si and n cl,gl It is between these two. Both can be adjusted by varying the glycerol solution concentration and / or the proportion of glycerol. Different glycerol solution concentrations result in very different n cl It brings about dn cl It does not have much effect on / dT because the thermo-optic coefficients for both glycerol and water are much larger than those for molten silica. The proportion of glycerol substantially affects both parameters. For example, increasing the proportion of glycerol has little effect on dn cl Effectively shift / dT to the negative side, and n cl to n cl,gl To approach this. Therefore, the two parameters of the evanescent field in the cladding region (the resulting cladding refractive index n) cl and the resulting thermal response dn of the fiber cladding cl To control the glycerol content (dT), two variables of the microfluidic are used: the proportion of glycerol and the mass concentration of glycerol in the glycerol-aqueous solution (C). m,gl ) exists.

[0419] Cladding parameters can be used to control desired performance metrics of FBG sensor devices, with thermal response being the most important. The Bragg wavelength is the refractive index n of the FBG. eff It is proportional to n. Therefore, the cladding refractive index n cl The relationship between and the thermal response is n in the waveguide. eff This should be revealed by calculating n in such a complex geometry. effAnalytical solving this is difficult. However, it is at least possible to identify the factors that affect the efficiency and perform numerical simulations based on them. Because the glycerol channel is located between the core and the cladding, the mode width depends on the refractive index of the glycerol. cl,gl ≒ n co In this case, the mode width expands, and therefore variations in glycerol content significantly affect the efficacy rate. cl,gl < <n co In this case, the mode is restricted to the core region. The efficiency does not depend much on the glycerol ratio in this case. cl By adjusting itself, the DN of the fiber eff This shows that it is possible to modify / dT, and consequently the thermal response. On the other hand, dn cl / dT is n cl Since it is derived from, dn cl / dT caused dn eff The effect on / dT becomes simpler.

[0420] Another important performance indicator for FBG sensor devices is operating temperature. Compared to conventional FBG sensors whose thermal response is constant across the entire temperature range, the thermal response of the FBG devices of the present invention varies with temperature. Therefore, the operating temperature is characterized as the temperature at which the device exhibits a desired thermal response. Generally, the operating temperature is n cl It is shifted by and the operating temperature range is dn cl This can be expanded or contracted by varying / dT. The fundamental limit of the operating temperature is that, since a glycerol aqueous solution is used, the temperature should not fall below the freezing point of any of the liquids or rise above the boiling point.

[0421] The process for designing the device is important. A general example is provided below. First, the proportion of glycerol is selected to meet the requirements for the device's thermal response. Generally, a lower proportion of glycerol results in a more positive thermal response, and a higher proportion of glycerol results in a more negative thermal response. Then, the concentration of the glycerol solution is adjusted to set the operating temperature, because the concentration of the glycerol solution is dn cl It has a slight effect on / dT, but mainly on n cl This is because it affects the absolute value of [the variable].

[0422] To achieve optimized parameters, the FIMMWAVE (Photon Design Limited) mode solver software was used. The fiber used in the simulation was SMF28e+ with a core diameter of 9.2 μm and a cladding diameter of 125 μm. Convergence tests were performed to select an optimal effective cladding diameter of 50 μm to increase the simulation speed. The core and cladding ratios of the fiber were 1.451 and 1.445, respectively. In the simulation, it was assumed that the thermo-optic coefficient of the glycerol aqueous solution varied according to a simple sum with respect to concentration:

[0423]

number

[0424] In the formula, dn / dT is the thermo-optic response of the material, and C m The values ​​represent the mass concentration of the liquid, labeled with the subscript "gl" for glycerol and "wa" for water.

[0425] Each trapezoidal cross-sectional microchannel had a height of 10 μm, and the axial length of the FBG and microchannel was 6 mm. The pitch was finely tuned so that all Bragg wavelengths did not deviate significantly from 1550 nm. The simulated shifts of Bragg wavelengths are plotted against temperature at different glycerol percentages and glycerol concentrations in Figures 56A and 56B. Figure 56A shows the shift for the glycerol percentage from 0 (pure silica cladding) to 1 (pure glycerol cladding) at a glycerol solution concentration of 85%. Figure 56B shows the shift for glycerol weight concentrations of 80% to 90%, where the glycerol percentage is 1. Raw data labeled with blue stars are fitted with a quadratic polynomial plotted by a red solid line.

[0426] It should be noted that the thermal response at the Bragg wavelength generally increases as the temperature rises. This is because n cl,gl ga n co This is because, as the distance from the temperature increases, the effect of glycerol cladding becomes less significant than that of silica cladding. Ultimately, at higher temperatures, silica cladding becomes dominant in the cladding region, resulting in a positive thermal response for the device. In contrast, at lower temperatures where the refractive index of glycerol is higher, glycerol cladding becomes dominant, and therefore the thermal response is negative. A similar trend is observed from the concentrations of the glycerol solution shown in Figure 56B, where the thermal response is more negative at higher glycerol concentrations and more positive at lower glycerol concentrations.

[0427] C m,gl When C reaches 1, the device has the most negative thermal response, and m,glWhen the glycerol ratio is zero, the microfluidic is water only, and a positive but reduced thermal response is expected. Different glycerol ratios not only shift the thermal response but also change the operating temperature range. One extreme case is when the glycerol ratio is equal to 1, and the device has cladding made of a glycerol solution. In this case, the variation in the thermal response due to temperature is high and can be approximately fitted to a quadratic function. When the glycerol ratio is zero, the device is a conventional FBG with a constant thermal response. Therefore, by varying the glycerol ratio between the two extreme cases, it can be determined whether to significantly change the thermal response and, subsequently, whether to manipulate the operating temperature range.

[0428] An optical fiber was fabricated according to the embodiment of the method described above. A regenerative femtosecond laser system (Light Conversion Pharos SP-06-1000-PP) was used to fabricate the required pattern inside the fiber. For fabrication, a pulse duration of 170 fs and a second harmonic generation wavelength of 515 nm were used. Half-wave plates were used to control the pulse energy between 10 and 300 nJ. An SLM (Hamamatsu X10468) was used to compensate for aberrations generated in the optical path of the entire system. The sample fiber was fixed to a microscope slide with tape and placed on a motion stage (x,y: Aerotech ABL10100L and z: ANT95-3-V) to provide different focal positions for the laser. The objective lens used had a magnification of 20 and an NA of 0.5.

[0429] The secondary fiberglass generator (FBG) is written to a single-mode fused silica fiber (Corning SMF28e+) by scanning the laser focus along the center of the fiber. Microchannels are written and exposed to the fiber surface with slightly higher pulse energy. The device was fabricated without oil immersion, and therefore aberration correction for the fiber surface was required in air.

[0430] The device was placed in a KOH solution for etching so that the laser-modified regions would be removed. Potassium hydroxide (KOH) solution was chosen as the etching solution due to its high selectivity of up to 300 and its relatively fast etching rate of up to 300 μm / h. The etched microchannels were then filled with a glycerol solution by capillary effect.

[0431] Highly thermally sensitive FBG devices can be used for temperature and strain identification. Ideally, the highest sensitivity is obtained when the entire evanescent field is occupied by glycerol cladding. However, a core completely surrounded by fluid is mechanically unstable. Therefore, several connections are left between the glycerol channels, as shown in Figure 57A.

[0432] Each of the four channels is approximately 7 μm wide and 10 μm long, symmetrically arranged, with a gap of approximately 0.5 μm between them, and in close contact with the core. The fiber grain graph (FBG) is 2 mm long, and the channels are 3 mm long, with surface exposure every 1.5 mm along the length of the fiber to achieve approximately uniform etching. The FBG was written into the fiber core with a pulsed energy of 0.15 μJ and a repetition rate of 100 Hz controlled by a pulse picker. The writing speed was 1.071 μm, and therefore the Bragg wavelength after the strain induced by the tape had relaxed was approximately 1550 nm. The microchannels were written with a pulsed energy of 0.26 μJ, a repetition rate of 250 kHz, and a speed of 0.1 mm / s. The microchannels were exposed to the surface without passing through the fiber core. The device was then placed in an 8 mol / L KOH 210 solution and etched at 75°C on a hot plate in a water bath for approximately 15 hours.

[0433] While a glycerol concentration as high as possible is desirable to improve thermal sensitivity, it is also important that the glycerol concentration is controlled to achieve high sensitivity over the desired temperature range. A relatively high glycerol concentration of 85% was selected.

[0434] A tunable laser (ID Photonics, CoBrite) was used to generate a broadband spectrum near the Bragg wavelength. The measured reflectance spectra of this device from 22°C to 70°C are plotted in Figure 57B, and its peaks are plotted in Figure 59. The thermal response is determined by the difference in Bragg wavelength across each temperature point. In the temperature range of 20°C to 40°C, the average thermal response was measured to be -35 pm / °C. In the range of 40°C to 70°C, the average thermal response was measured to be -11 pm / °C. The Bragg wavelength of a typical FBG was also plotted, showing a thermal sensitivity of 9.98 pm / °C. The peak intensity depends on the coupling coefficient given by the following equation: R max = tanh 2 κL g , where R max ∫ represents the maximum reflectance at the Bragg wavelength, κ is the coupling coefficient, and L is the length of the Bragg grating.

[0435] The peak reflectance at 24°C was reduced to one-quarter of that at 70°C. At even lower temperatures, the glycerol refractive index is closer to the core fraction. The model is extended to the glycerol region, and therefore the bonding coefficient is even lower. Consequently, a high thermal response but low peak reflectance is predicted. As the temperature increases, the bonding coefficient gradually decreases, and therefore the thermal response decreases while the peak reflectance increases. The bandwidth for the measured spectra at different temperatures is nearly uniform at 5 nm.

[0436] An FBG sensor device with a compensated thermal response, potentially usable for laser stabilization, was fabricated. The temperature-dependent nonlinear thermal response allows for the existence of a transition point where the thermal response crosses zero. To maintain a low thermal response near the transition point, the proportion of glycerol in the evanescent field was reduced by moving the microchannel further away from the core, as shown in Figure 58A. The proportion of glycerol in the evanescent field is approximately 0.35 at room temperature.

[0437] The four channels are circular, with a diameter of approximately 7 μm, allowing for some over-etching by leaving a larger gap 1 μm away from the core. The same parameters as those used for the high-sensitivity device were employed for FBG writing. The FBG is 2 mm long, and the channels are 3 mm long, with surface exposure every 1.5 mm. The microchannels were fabricated at an even higher pulse energy of 0.290 μJ to overcome aberrations caused by the asymmetric fiber top surface when the focal point is far from the center.

[0438] The glycerol concentration was selected to be 85%, allowing the device to operate at room temperature. Measured reflectance spectra over the temperature range of 10°C to 70°C are plotted in Figure 58B, showing a bandwidth of approximately 0.5 nm, albeit slightly dependent on temperature conditions. The relative intensity at 24°C is approximately one-third of that at 70°C. This represents a slight improvement compared to highly sensitive devices, as the modes are more restricted within the core. Bragg wavelengths are plotted in Figure 59, showing a compensated thermal response of 0.97 pm / °C on average over the room temperature range of 14°C to 50°C.

[0439] Figure 59 is a plot of the measured reflected Bragg wavelength against temperature for a high negative thermal response FBG device shown in Figure 57A, a compensated thermal response FBG device shown in Figure 58A, and a normal FBG.

[0440] Figure 60 shows a flowchart of a method for forming an optical fiber having a gap, such as optical fiber 1001, as described with reference to Figures 61A to 63. This method provides an alternative method for forming an optical fiber having a gap (such as a microchannel) compared to the method described with reference to Figure 2, and advantageously reduces loss in the light guided through the optical fiber, while allowing the advantages associated with the gap described herein to be applied to the optical fiber, and enabling customized portions of the gap-containing optical fiber to be spliced ​​into conventional optical fibers.

[0441] To form an optical fiber with a gap and provide the effects described herein, the method shown in Figure 60 may be used. This method involves splicing a customized portion of the optical fiber with another portion of the optical fiber (optionally, a conventional optical fiber such as a standard single-mode optical fiber having properties as shown in Table 1 below). A standard single-mode optical fiber typically has a core extending along the longitudinal axis of the standard single-mode optical fiber, surrounded by cladding material. Typically, the core of a standard single-mode optical fiber has a circular cross-section along the longitudinal length of the standard single-mode optical fiber, and the core diameter is 8.2 micrometers. In further examples, the core diameter of a standard single-mode optical fiber is greater than 5.5 micrometers and less than 11 micrometers. In further examples, the core diameter of a standard single-mode optical fiber is 6 to 10.5 micrometers. In further examples, the core diameter of a standard single-mode optical fiber is 8 to 9 micrometers. Typically, the cladding of a standard single-mode optical fiber has a material that is substantially solid, in contrast to the customized gap portion described herein. In one example, the core of a standard single-mode optical fiber has a refractive index 0.002–0.007 greater than that of the cladding material of the standard single-mode optical fiber. In a further example, the core of a standard single-mode optical fiber has a refractive index approximately 0.005 greater than that of the cladding material of the standard single-mode optical fiber. In one example, the refractive index of the cladding of a standard single-mode optical fiber under typical conditions is approximately 1.441. Optionally, a standard single-mode optical fiber is configured to operate at wavelengths of electromagnetic irradiation within the C-band (1530 nm–1565 nm), L-band (1565 nm–1625 nm), and / or S-band (1460 nm–1530 nm). Advantageously, such a standard single-mode optical fiber can be used to at least partially seal one or more voids in a void portion, as described herein.

[0442] Examples of optical fibers formed according to the process shown in Figure 60 are shown in Figures 61A and 61B. Figures 61A and 61B show a mode-matched optical fiber 1001 in a side view and a cross-sectional view, respectively. A customized gap portion 1004 of the optical fiber is shown between two conventional portions 1002 and 1006 of the optical fiber. Optionally, the two conventional portions 1002 and 1006 of the optical fiber are standard single-mode optical fibers.

[0443] The portions 1002 and 1006 of the conventional optical fiber, which are spliced ​​into the void portion 1004 of the optical fiber 1001, may have initially had a coating of a polymer material (not shown), such as a polyacrylate with a diameter of 250 μm or a polyimide with a diameter of 150 μm. The coating may be removed in step S105 of Figure 60 before splicing portions 1002, 1004, and 1006 of the optical fiber 1001. The coating may be removed by any suitable method, such as chemical removal or ablation. The coating may be removed along substantially the entire longitudinal extent of portions 1002 and 1006 of the conventional optical fiber, or even from the entire length of the conventional optical fiber.

[0444] In step S110, a custom portion of the optical fiber containing one or more voids is formed. The optical fiber may have voids such as the void 1009 in the void portion 1004 of the optical fiber 1001 shown in Figures 61A and 61B, by extending the custom fiber with holes longitudinally along its length. The custom fiber may have a core 1003. The core 1003 may have a circular cross-section along the length of the custom fiber. In an example, the core 1003 has the same shape and maximum lateral dimension as the core of the adjacent fiber at the interface between the fibers. For example, if the core has a circular cross-section extending along the longitudinal axis of the fiber, it may have the same diameter and the same core refractive index as the later adjacent fiber at the interface to provide mode matching between the custom fiber and the conventional fiber. In a further example, mode matching is achieved by selecting a combination of core diameter and core refractive index to provide mode matching such that the mode field diameter of the custom portion of the optical fiber matches the mode field diameter of the conventional optical fiber at the interface between the custom portion of the optical fiber and the conventional optical fiber.

[0445] The core 1003 is surrounded by undoped cladding 1005. The custom portion of the optical fiber is a void portion 1004 having one or more voids 1009. The one or more voids 1009 extend longitudinally within the void portion 1004 along the length of the optical fiber 1001.

[0446] As described above, the optical fiber 1001 may have a core 1003 and cladding 1005 surrounding the core 1003. In this case, one or more voids 1009 may have at least one cladding void within the cladding 1005.

[0447] In a manner similar to the optical fiber 1 described with reference to Figures 12A to 12F, the optical fiber 1001 may have voids 1009 with different arrangement configurations to provide different effects. For example, Figures 12A to 12F show cross-sectional views of different optical fibers 1 having voids 9 containing a filler 15, and equivalent cross-sectional voids 1009 may be formed in the void portion 1004 of the optical fiber 1001 shown in Figures 61A and 61B.

[0448] After the formation of the custom fiber having the void 1009, in step S115, the void 1009 is at least partially filled with a filler 1015, which may be a polymerizable liquid. The polymerizable liquid is then optionally cured within the custom optical fiber. The filler 1015 has material properties different from those of the optical fiber 1001 (e.g., different from the materials of the core 1003 and cladding 1005).

[0449] The void 1009 may be filled using any suitable technique depending on the properties of the filler 1015. For example, if the filler 1015 is liquid, the void 1009 may be filled via capillary action by dripping the filler 1015 into one end of the void 1009 while leaving the other end empty. Alternatively, the void 1009 may be filled by pressure. Filling one or more voids 1009 also involves the possibility of simply allowing ambient fluid, such as air, to flow into the void 1009. For example, the void portion 1004 may be left in a chamber filled with, for example, the ambient environment or an inert gas such as nitrogen, after a possible flushing step, before the void 1009 is sealed. This allows ambient fluid to fill the void 1009.

[0450] One or more voids 1009 are configured such that the optical properties of the filler 1015 affect the light transmitted through the optical fiber 1001. As described above, the filler 1015 has different material properties from those of the material of the optical fiber 1001. This makes it possible to use the filler 1015 to adjust the optical properties of the optical fiber 1001 in the void portion 1004. Importantly, the filler 1015 may also have material properties that change with environmental variables (such as temperature, pressure, or strain), which are different from the material properties that change with environmental variables (such as temperature, pressure, or strain) of the material of the optical fiber 1001. This also makes it possible to control the changes in the optical properties of the optical fiber 1001 with respect to environmental variables by appropriate selection of the filler 1015.

[0451] The material properties of the optical fiber 1001 and the filler 1015 may be optical properties, or optionally, refractive index. The filler 1015 may have a refractive index different from that of the optical fiber 1001. However, the refractive index of the filler 1015 will preferably not differ significantly from that of the optical fiber 1001. The refractive index of the filler may be within 0.1 of the refractive index of the optical fiber, optionally within 0.05, optionally within 0.01, optionally within 0.005, or optionally within 0.001. If the optical fiber 1001 has a core 1003 and cladding 1005 surrounding the core 1003, the refractive index of the filler 1015 may be approximately equal to the refractive index of the cladding 1005 at a reference temperature. Approximately matching the refractive index of the filler 1015 can reduce reflection and loss in the optical fiber 1001, but it is also important to ensure that the waveguide and modal characteristics of the optical fiber 1001 are not impaired. Ideally, the optical fiber 1001 forms a waveguide that "weakly guides" the optical modes into the cladding 1005. This requires a relatively small refractive index difference between the core 1003 and the cladding 1005. If the refractive index of the cladding 1005 (which is influenced by the refractive index of the filler 1015) is greater than that of the core 1003, then no waveguide exists. If the refractive index of the cladding 1005 is too large, then the optical fiber 1001 will become a multimode optical fiber with more than one transverse spatial mode.

[0452] Optionally, the refractive index of the filler 1015 is equal to or greater than that of the cladding 1005. Beneficially, such a relationship between refractive indices can be used to control the dependence of the Bragg wavelength on temperature for the Bragg grating associated with the void portion 1004, as described herein.

[0453] The filler material 1015 may have a different change in refractive index dn / dT with temperature than that of the optical fiber 1001 material. The change in refractive index of the filler material 1015 with temperature may be negative.

[0454] Once the void 1009 is filled with the filler 1015, the custom-filled fiber and the conventional fiber may be cut to provide suitable end faces for splicing the custom fiber and the conventional fiber in step S120. The fiber may be cut using a diamond cutter, a laser, or any suitable method to provide end faces that are substantially perpendicular to the longitudinal axis of the fiber. The void portion 1004 is then spliced ​​with the conventional fiber in step S125, such that the first portion 1002 of the conventional fiber is adjacent to the void portion 1004 at a splice at the interface 1008 between the end face of the first portion 1002 and the end face of the void portion 1004. The first portion 1002 of the conventional fiber is spliced ​​with the void portion 1004 of the custom fiber by fusing portions 1002,1004 using any suitable method such as an electric arc or a laser.

[0455] Once the first portion 1002 of the conventional optical fiber is adjacent to the gap portion 1004 of the custom optical fiber, the custom optical fiber is cut again in step S120 to provide an appropriate length for the gap portion 1004. In an example, the gap portion 1004 is cut such that it is less than 20 mm in length along its longitudinal axis, optionally less than 10 mm, and optionally between 5 and 10 mm. In a further example, the gap portion 1004 may have any length suitable for its application. Typically, the longitudinal length of the gap portion 1004 is much shorter than the longitudinal length of the portion of the conventional optical fiber from which the gap portion 1004 is spliced. The gap portion 1004 of the custom optical fiber may be cut using a diamond cutter, a laser, or any suitable method.

[0456] Once the adjacent gap portion 1004 is cut in step 120, the other end face of the gap portion 1004 adjacent to the first portion 1002 of the conventional optical fiber is adjacent to a further portion 1006 of the conventional optical fiber at the splice interface 1010. The further portion 1006 of the conventional optical fiber is spliced ​​with the gap portion 1004 of the custom optical fiber by fusing portions 1004,1006 using any suitable method such as an electric arc or a laser.

[0457] After the introduction of the custom void portion 1004 into the conventional optical fiber, the Bragg grating 1031 may be written into the void portion 1004 in step S130, or optionally, at least partially written into the core 1003. Optionally, the Bragg grating 1031 is formed using a femtosecond laser system, as described, for example, with respect to the Bragg grating 31 in Figures 14-17. Beneficially, the use of a femtosecond laser system allows for the provision of a Bragg grating into an optical fiber having the same core diameter and dopant level as used in conventional standard single-mode fibers.

[0458] The optical fiber 1001 is then optionally coated in step S135 in any suitable manner, including, as described herein with respect to the coating 23 of the optical fiber 1 shown in Figure 1A.

[0459] While the process described with reference to Figure 60 involves many steps in a specific order, in further examples, there are additional, fewer, and / or alternative steps that can be implemented to provide an optical fiber having a void 1009 having the functions described herein. For example, the void 1009 of void portion 1004 is optionally at least partially filled with filler material 1015 after splicing void portion 1004 with a first portion 1002 of a conventional optical fiber and before splicing the other end of void portion 1004 with a further portion 1006 of a conventional optical fiber. Conveniently, the filler material 1005 is sealed in the void 1009 of void portion 1004 by portions 1002 and 1006 of the conventional fiber. Furthermore, although this method does not involve fiber stretching, a stretched fiber on which the other steps of this method are performed may be provided.

[0460] A method for manufacturing a device having optical fibers may be provided, similar to the method described in relation to Figure 60. This method is particularly suitable for mass production of the device.

[0461] This method involves providing an optical fiber having one or more voids extending longitudinally along the length of the optical fiber. The optical fiber may be a custom-stretched optical fiber. The optical fiber has one or more voids along its length. This method involves filling one or more voids with a liquid filler. The liquid filler is then solidified.

[0462] If the liquid filler contains a polymer that can be solidified by exposure to ultraviolet (UV) light or by thermal curing of the polymer, solidification may be carried out by curing the polymer by UV irradiation or heating. Alternatively, the filler may have a melting point lower than the melting point of the optical fiber material, for example, the filler may contain a glass with a low melting point as described above. The step of filling one or more voids with the filler may then be carried out at a temperature above the melting point of the filler but below the melting point of the optical fiber material.

[0463] This method further comprises splicing an optical fiber into a transmission fiber. The transmission fiber (which may be a solid step-index fiber) is cut to length. The optical fiber, which has a filled void, is cut and spliced ​​into a transmission fiber.

[0464] Splicing is performed, for example, by laser splicing, which selectively melts the material (e.g., glass) of the optical fiber and / or transmission fiber using a focused laser. This allows laser splicing to selectively target portions of the optical fiber outside one or more voids, avoiding heating the filler material to a temperature that could damage or carbonize it.

[0465] The filled optical fiber is then cut into lengths of several millimeters (e.g., 3, 5, 10 mm) to be attached to the transmission fiber. Another length of transmission fiber is then spliced ​​onto the cut end.

[0466] The Bragg grating may then be written into the core of the packed fiber, for example, using a femtosecond laser. The packed fiber and the removed sections of the surrounding transmission fiber may then be recoated (for example, with polyimide or polyacrylate). This process may be repeated multiple times to form an array of fiber Bragg grating devices along the length of the fiber.

[0467] This method may further include providing a bridging fiber between the optical fiber and the transmission fiber. The bridging fiber may have one or more voids filled with a liquid having a melting point below the melting point of the transmission fiber material.

[0468] An alternative method for manufacturing devices with optical fibers is to provide optical fibers having one or more voids extending longitudinally along the length of the optical fiber, for example, by stretching a custom optical fiber having one or more voids adjacent to the core along its length.

[0469] The optical fiber is spliced ​​with solid-state transmission fiber at one or both ends. The optical fiber is then cut into lengths of a few millimeters and then spliced ​​with transmission fiber of another length.

[0470] This method then involves forming a hole on the side of the optical fiber. The hole or access channel is made through the side of the fiber to allow access to the air gap. The hole provides access from outside the optical fiber to at least one of one or more air gaps. The access channel may be directed through to one air gap, and then further channels may be formed to connect all the air gaps. Alternatively, there may be an access channel formed for each air gap.

[0471] Hole formation may involve the use of a laser. For example, the access channel may be made by laser ablation, as described in the other manufacturing methods described above, or alternatively, by defining an exposure region with a femtosecond laser and bringing this region into contact with an etching solution. In the latter case, hole formation involves: selectively exposing the optical fiber to laser irradiation to define an exposure region within the optical fiber; and bringing the optical fiber into contact with an etching solution, the etching solution etching the exposure region faster than the areas of the optical fiber not exposed to laser irradiation, and the hole is formed by etching the exposure region.

[0472] The void may then be filled with, for example, a polymerizable liquid through the access channel. Preferably, the entire void, including the access channel, is filled. Excess liquid is removed from the fiber surface.

[0473] This method further includes sealing the hole. Sealing the hole may include solidifying the filler. For example, the optical fiber may be placed in a UV chamber and the polymer may be cured. This method has the effect of sealing the access channel while solidifying the polymer. Otherwise, it is difficult to seal a channel that has liquid inside, because if an adhesive is used, it will mix with the liquid filler and tend to dissolve the fiber. The area of ​​the fiber from which the fiber was removed can then be recoated with polyimide or polyacrylate.

[0474] As a result of the process described with reference to Figure 60, an optical fiber 1001 as shown in Figures 61A and 61B may be provided. The void portion 1004 is positioned longitudinally along the length of the optical fiber 1001 between two further portions 1002,1006 of the conventional optical fiber 1001. In the example, the two further portions 1002,1006 of the conventional optical fiber are sealing portions 1002,1006 that have the effect of sealing the open end of the void 1009 of the void portion 1004.

[0475] A conventional fiber used to provide a first portion 1002 of an optical fiber 1001 has a core 1003' surrounded by cladding 1005' and is spliced ​​into a second portion of the optical fiber 1001, which is a void portion 1004, at the interface 1008 between the end face of the first portion 1002 of the optical fiber 1001 and the end face of the second void portion 1004 of the optical fiber 1001. A third portion 1006 has a core 1003' surrounded by cladding 1005. The third portion 1006 of the optical fiber 1001 is spliced ​​into the second void portion 1004 of the optical fiber at the interface 1010 between the second end face of the second void portion 1004 of the optical fiber and the end face of the third portion 1006 of the optical fiber 1001. The open ends of the gaps 1009 at both ends of the gap portion 1004 are sealed at interfaces 1008 and 1010 between the end faces of the gap portion 1004 and the end faces of two further portions 1002 and 1006 of the optical fiber 1001.

[0476] The optical fiber 1001 may contain silica. This optical fiber 1001 may have an outer diameter of 25 μm to 300 μm, optionally 100 μm to 300 μm, and optionally approximately 125 μm or 250 μm. Typically, the optical fiber 1001 is made of silica and has a diameter of 125 μm. Other common diameters include 50 μm, 80 μm, 250 μm, and 425 μm. Larger diameters may be preferable for pressure sensing applications because larger diameter fibers are more sensitive to pressure changes. The optical fiber 1001 has a core 1003 and cladding 1005 surrounding the core 1003.

[0477] Optical fiber 1001 may be a single-mode optical fiber. However, there are many other possible fiber types. For example, optical fiber 1003 may be an anti-resonant fiber or a negative curvature fiber.

[0478] The optical fiber 1003 may have a crystalline material, and optionally a single-crystal material. In such a case, the optical fiber 1003 may have some crystalline (or single-crystal) material portion, and may also have a portion of other material such as silica. For example, the optical fiber 1001 may be a crystal-derived fiber, and optionally a sapphire-derived fiber. An example of a crystal-derived fiber is described in Dragic, P., Hawkins, T., Foy, P. et al. “Sapphire-derived all-glass optical fiber,” Nature Photonics 6, 627-633 (2012).

[0479] Furthermore, the optical fiber 1001 may be a crystalline optical fiber, and optionally a single-crystal optical fiber. In this case, the material of the optical fiber 1001 consists entirely or substantially entirely of a crystalline material. For example, the optical fiber 1001 may have a sapphire, diamond, or yttrium aluminum garnet (YAG) crystal. The crystalline material in the optical fiber 1001 may be doped, whether this provides all or part of the optical fiber 1001, and optionally doped with a rare earth element. Other fiber types include fibers having a pure silica core, a photonic crystal, a polymer, a hydrogel, and the like.

[0480] The optical fiber 1001 is arranged such that the gap portion 1004 is substantially mode-matched with the other adjacent portions 1002, 1006. In order to mode-match the gap portion 1004 with one or both of the other portions 1002, 1006, the gap portion 1004 is configured to match one or more characteristics of the other adjacent portions 1002, 1006. For example, the gap portion 1004 and the other portions 1002, 1006 may have similar core sizes, similar core refractive indices, and / or similar mode field diameters.

[0481] For example, the cross-sectional shape and area of ​​the core 1003 of the void portion 1004 are matched with the cross-sectional shape and area of ​​the core 1003' of the first portion 1002 of a conventional optical fiber at the interface 1008 between the end faces of each portion of the optical fiber 1001. Optionally, matching the cross-sectional shape and area of ​​the core 1003' of the first portion 1002 of a conventional optical fiber at the interface 1008 between the end faces of each portion of the optical fiber 1001 involves matching the maximum lateral dimension of the core 1003 of the void portion 1004, such that it is substantially the same as the maximum lateral dimension of the core 1003' of the first portion 1002 at the interface 1008 which is substantially perpendicular to the longitudinal axis of the optical fiber 1001. If the cores 1003, 1003' of the fiber are circular in cross-section, the core diameters may be matched. In a further example, the cross-sectional shapes of the cores are matched such that they are the same at the interface between the first portion 1002 and the void portion 1004.

[0482] Similarly, the cross-sectional shape and area of ​​the core 1003 of the void portion 1004 are matched with the cross-sectional shape and area of ​​the core 1003' of the further portion 1006 of the conventional optical fiber at the interface 1010 between the end faces of each portion of the optical fiber 1001. Optionally, matching the cross-sectional shape and area of ​​the core 1003' of the further portion 1006 of the conventional optical fiber at the interface 1010 between the end faces of each portion of the optical fiber 1001 involves matching the maximum lateral dimension of the core 1003 of the void portion 1004, such that it is substantially the same as the maximum lateral dimension of the core 1003' of the further portion 1006 at the interface 1010 which is substantially perpendicular to the longitudinal axis of the optical fiber 1001. If the cores 1003, 1003' of the fiber are circular in cross-section, the core diameters may be matched. In a further example, the cross-sectional shapes of the cores are matched such that they are the same at the interface between the further portion 1006 and the void portion 1004.

[0483] In one example, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fiber 1001, with a diameter of approximately 8.2 micrometers, which matches that of a conventional optical fiber at interface 1008,1100. In a further example, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fiber 1001, with a diameter greater than 5.5 micrometers and less than 11 micrometers, which matches that of a conventional optical fiber at interface 1008,1100. In a further example, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fiber 1001, with a diameter of 6 to 10.5 micrometers, which matches that of a conventional optical fiber at interface 1008,1100. In a further example, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fiber 1001, with a diameter of 7–10 micrometers, matching that of a conventional optical fiber at interface 1008,1100. In a further example, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fiber 1001, with a diameter of 8–9 micrometers, matching that of a conventional optical fiber at interface 1008,1100. Beneficially, matching the core size of the void portion 1004 to that of a conventional optical fiber having a core size of such dimensions provides improved single-mode transmission with reduced loss and benefits from improved control through the use of the void.

[0484] In a further example, to provide a mode-matched optical fiber, the core refractive index of the core 1003 of the void portion 1004 of the optical fiber 1001 is matched to the core refractive index of the core 1003' of the conventional first portion 1002 of the optical fiber 1001, and optionally, to the core refractive index of the core 1003' of the conventional further portion 1006 of the optical fiber 1001. In the example, the core 1003' of the first portion 1002 and / or further portion 1006 is made of a material having a refractive index 0.002 to 0.007 greater than the refractive index of the cladding material surrounding each of the cores 1003' of the first portion 1002 and / or further portion 1006. In a further example, the core 1003' of the first portion 1002 and / or further portion 1006 has a material having a refractive index approximately 0.005 greater than the refractive index of the cladding material surrounding each of the cores 1003' of the first portion 1002 and / or further portion 1006.

[0485] Mode matching of the first portion 1002, the void portion 1004 and / or further portions may be achieved by changing the core dimensions and / or refractive index of the portions. Advantageously, the physical properties of portions 1002, 1004, and 1006 of the optical fiber 1001 are controlled to minimize losses associated with light passing through interfaces 1008 and 10101 between portions 1002, 1004, and 1006 of the optical fiber 1001.

[0486] In a further example, the void portion 1004 is mode-matched with the conventional optical fiber portions 1002 and 1006, such that the mode field diameter of the void portion 1004 is substantially the same as that of the adjacent conventional optical portions 1002 and 1006 at interfaces 1008 and 1100 with each of the conventional optical fiber portions 1002 and 1004.

[0487] The mode field diameter is 1 / e of the peak intensity. 2 It is determined at the point where it falls. An approximate value for the mode field diameter (MFD) is given by the following formula:

[0488]

number

[0489] In the formula, a is the core radius, and V is the normalized frequency given by the following formula:

[0490]

number

[0491] In the formula, a is the core radius, λ is the wavelength, and n1 and n2 are the refractive indices of the core and cladding, respectively. Thus, the mode field diameter of the void portion 1004 can be changed with respect to a given wavelength of light by changing the refractive indices of the core 1003 and cladding 1005 of the void portion 1004 and / or the physical diameter of the substantially cylindrical core 1003.

[0492] Table 1 below shows an example of a conventional optical fiber in which the void portion 1004 may be mode-matched. This table provides a comparison of the physical core diameter and the corresponding mode field diameter at wavelengths of 1550 nm and 1300 nm for commercially available step-index single-mode fibers.

[0493] [Table 1]

[0494] In the example, the mode field diameter of the first portion 1002 of the conventional optical fiber is substantially the same as the mode field diameter of the air gap portion 1004 of the optical fiber 1001. Optionally, the mode field diameter is 9–12 micrometers, optionally 9.6–11.2 micrometers, and optionally approximately 10.5 micrometers. Optionally, the mode field diameter of the first portion 1002 of the conventional optical fiber is substantially the same as the mode field diameter of the air gap portion 1004 of the optical fiber 1001 at a given temperature and wavelength. For example, under standard operating temperatures, the mode field diameter of the first portion 1002 of a conventional optical fiber is substantially the same as the mode field diameter of the air gap portion 1004 of the optical fiber for wavelengths in the C-band, L-band, and / or S-band (C-band communication has a wavelength range of 1530 nm to 1565 nm, L-band communication has a wavelength range of 1565 nm to 1625 nm, and S-band communication has a wavelength range of 1460 nm to 1530 nm).

[0495] The optical fiber 1001 optionally has at least partially a Bragg grating 1031 within the core 1003. Optionally, the optical fiber 1001 is a single-mode optical fiber. Optionally, the Bragg grating 1031 is formed using a femtosecond laser system, for example, as described with respect to the Bragg grating 31 in Figures 14-17. Beneficially, the use of a femtosecond laser system allows for the provision of a Bragg grating into an optical fiber having the same core diameter and dopant level as used in conventional single-mode fibers.

[0496] In the example, the optical fiber 1001 typically has a diameter of approximately 9 μm and an even higher refractive index (e.g., approximately 10 μm). -3A single-mode silica fiber having a core 1003 doped with germanium in order to have a high (value). In a single-mode fiber, only a single transverse mode can propagate. This mode exists mainly within the core 1003 but has an evanescent field that extends into the cladding 1005. It can be characterized by an effective refractive index n eff which can be characterized by a value at a specific wavelength, said value being determined by the core diameter and the refractive indices of the core and the cladding at that wavelength.

[0497] Optionally, the refractive index of the filler 1015 has a change with temperature dn / dT that is different from the change in refractive index with the temperature of the surrounding material.

[0498] The optical fibers 1001 of FIGS. 61A and 61B are shown in a state where the customized void portion 1004 is spliced with the portions 1002, 1006 of a conventional optical fiber. In a further example, the void portion 1004 is spliced to the other portions 1002, 1006 of the optical fiber using one or more intermediate components. FIG. 62 shows an optical fiber 1001' having a bridging portion used to connect a lead-out / lead-in fiber having a significantly different core diameter to the void portion 1004, in contrast to the optical fiber 1001 described with reference to FIGS. 61A and 61B.

[0499] A first portion 1002' of the optical fiber 1001' is shown, which is a lead-out portion having a conventional optical fiber portion 1012 and a bridging portion 1022. A further portion 1006' of the optical fiber 1001' is also shown, which is a lead-in portion having a conventional optical fiber portion 1016 and a further bridging portion 1026. The bridging portions 1022,1026 are configured such that the cross-sectional area of ​​the core 1003' within the bridging portions 1022,1026 varies over the longitudinal length of the bridging portions 1022,1026. The cross-sectional area of ​​core 1003' varies to compensate for the difference between the core diameter of core 1003 in the void portion 1004 and the core diameter of core 1003' in the conventional optical fiber portions 1012 and 1016, so that the maximum lateral dimension of core 1003 in void portion 1004 is substantially the same as the maximum lateral dimension of core 1003' in bridging portions 1022 and 1026 at each interface 1008 and 1010 between void portion 1004 and each bridging portion 1022 and 1026. The bridging portions 1022 and 1026 are configured to progressively change the mode field diameter for a given wavelength and temperature along the longitudinal axis of the optical fiber 1001' between void portion 1004 and each conventional optical fiber portion 1012 and 1016.

[0500] Advantageously, the bridging portions 1022,1026 not only seal the void 1009 of the void portion 1004 in a manner similar to that described with reference to Figures 61A and 61B, but they also effectively act as adiabatic mode converters for converting from the first mode field diameter of the void portion 1004 of the optical fiber 1001' to the second and / or third mode field diameters of the conventional optical fiber portions 1012,1016, which may be significantly different. Advantageously, the benefits of the customized void portion 1004 are implemented in combination with optical fibers having very different mode field diameters. While the configuration in Figure 62 shows distinct portions, in further examples, the bridging portions 1022,1026 are continuous and integral portions, forming parts of each of the conventional optical fiber portions 1012,1016.

[0501] In a further example, Figure 63 shows an optical fiber having a lens portion. The optical fiber 1001'' has a gap portion 1004 as described with reference to Figures 61A, 61B and 62. However, in contrast to a conventional first portion 1002 of an optical fiber as shown with reference to optical fiber 1001 in Figures 61A and 61B, a first portion 1002'' of an optical fiber having a lens portion 1032 and a bridging portion 1042 is shown. In a further example, the lens portion 1032 and the bridging portion 1042 are formed as a single, continuous entity. The lens portion 1032 is tapered, thereby converting the mode field diameter of the optical fiber 1001'' to match a semiconductor laser. Thus, the lens portion 1032 has a cross-sectional area that varies along the longitudinal axis of the lens portion and is configured to progressively change the mode field diameter for a given wavelength and temperature along the length of its longitudinal axis shared with the optical fiber 1001''. The optical fiber 1001'' in Figure 63 can be used in a grating-stabilized laser similar to that described with reference to Figure 38A, where the laser has a high-reflectivity back facet, and the Bragg grating 1031 acts as an additional reflector. Beneficially, the lens portion 1032 converts the mode profile of the laser output to match that of the optical fiber 1001'' in order to minimize coupling loss.

[0502] Figure 64 shows the change in wavelength as a function of temperature change for two optical fibers having Bragg gratings. For example, as described with reference to Figure 21, optical fiber 1 may have two separate Bragg gratings 31, and one or more voids 9 are configured such that the optical properties of the filler material have different effects on the Bragg wavelengths of the two Bragg gratings 31. Similarly, optical fiber 1001 described with reference to Figures 61A and 60B may have two similar separate Bragg gratings 1031, which can be provided in a variety of different configurations. Advantageously, providing two Bragg gratings 31, 1031 with specific properties allows for improved determination of both strain and temperature. The combination of a fiber Bragg grating 31,1031 having a Bragg reflection wavelength with a negative temperature gradient (the Bragg wavelength decreases as the temperature rises) and a Bragg grating with a different temperature gradient (optionally a positive temperature gradient or a substantially zero temperature gradient) means that the two Bragg reflection wavelengths shift in opposite directions as the temperature rises, and the separation indicates temperature. This, therefore, allows for improved discrimination between strain and temperature.

[0503] While the properties of two Bragg gratings are described with reference to optical fiber 1,1001 in Figures 1 to 63, the advantageous properties provided by a combination of two Bragg gratings with opposite signs of temperature coefficients are more broadly applicable. For example, a device having a Bragg grating with a Bragg reflection wavelength having a negative gradient with temperature (the Bragg wavelength decreases with increasing temperature) and a Bragg grating with a Bragg reflection wavelength having a different temperature gradient (optionally a positive temperature gradient or a substantially zero temperature gradient) can be used simultaneously to provide improved strain and temperature identification. For example, if two Bragg gratings are in thermal equilibrium, their respective responses to the same temperature change will be different. For example, a device having two Bragg gratings in thermal equilibrium may be a strain, pressure, and / or temperature sensor. Optionally, the Bragg gratings in such a device form part of one or more waveguides, such as one or more optical fibers. In further examples, the Bragg gratings are implemented using any suitable medium to provide the advantages described herein.

[0504] Exemplary characteristics are shown in Figure 64. In particular, there are two plots showing the change in Bragg wavelength as a function of temperature. One plot is for the response of a standard fiber Bragg grating, with a temperature coefficient of +10 pm / °C. The other plot is for a modified fiber Bragg grating, with a temperature coefficient of approximately -50 pm / °C at the steepest point of the characteristic. The modified fiber Bragg grating may be the modified Bragg gratings 31, 1031 described with reference to Figures 1 to 63.

[0505] Advantageously, by measuring the Bragg wavelength of each fiber Bragg grating, the strain and temperature may be independently determined by linear algebra (e.g., using the matrix method). Each measurement is 、Two comparable Bragg wavelengths λ1 and λ2 are given from each of the two devices. Therefore, the identification of temperature and strain can be achieved by solving the following matrix:

[0506]

number

[0507] In the formula, K ij is the sensitivity of two fiber Bragg grating devices to strain ε and temperature change ΔT.

[0508] As described above for optical fibers 1,1001 having gaps 9,1009 along their lengths, the filler material 15,1015 used to at least partially fill the gaps 9,1009 may have a different temperature-dependent refractive index change dn / DT than that of the material of optical fiber 1,1001. At a specific temperature within the operating range of optical fiber 1,1001, the refractive index of the filler material 15,1015 is greater than that of the cladding 5,1005. Advantageously, if the refractive index of the filler material 15,1015 exceeds that of the cladding 5,1005, the fiber Bragg grating 31,1031 affected by the use of such filler material 15,1015 will operate with a larger Bragg wavelength change for a given temperature change—for example, on the steeper portion of the modified FBG curve in Figure 64. Beneficial in this regard, it provides even greater temperature sensitivity compared to the use of material filler 15,1015, which has a lower refractive index than cladding 5,1005.

[0509] It will be understood that the features and characteristics detailed with reference to Figures 1A to 59 may be implemented in combination with the devices and features described with reference to Figures 60 to 64, and vice versa.

[0510] Aspects of the present invention may also be described by the following numbered clauses, which are not part of the claims of this application that follow the headings of the claims below. A1. An optical fiber having one or more voids: the one or more voids extend longitudinally along the length of the optical fiber within the void portion, the void portion is a continuous and integral part of the optical fiber; the longitudinal extent of each of the one or more voids is smaller than the longitudinal extent of the void portion; and the one or more voids are sealed from the outside of the optical fiber and are at least partially filled with a filler having material properties different from those of the material of the optical fiber. A2. An optical fiber according to clause A1, having one or more voids, each filled with a different filler material. A3. Optical fibers under clause A2, where the material properties of the filler differ between fillers. A4. An optical fiber whose material properties are optical properties, and optionally refractive index, according to any of clauses A1 to A3. A5. An optical fiber according to clause A4, wherein one or more voids are configured such that the optical properties of the filler affect the light transmitted through the optical fiber. A6. An optical fiber according to any of clauses A1 to A5, wherein the temperature-dependent changes in the material properties of the filler differ from, and optionally inversely, from, the temperature-dependent changes in the material properties of the optical fiber material. A7. An optical fiber according to clause A6, wherein one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the temperature-dependent changes in the material properties of the optical fiber material. A8. An optical fiber according to any of the clauses A1 to A7, wherein the cross-sectional area of ​​at least one of the gaps varies along the length of the optical fiber. A9. An optical fiber according to clause A8, wherein the cross-sectional area decreases along the length of the optical fiber, away from the center of the void, for at least a portion of the length of the void. A10. An optical fiber of any of the clauses A1 to A9, wherein at least one of the voids has a non-circular cross-section. A11. An optical fiber according to any of the clauses A1 to A10, wherein the central axis of one or more voids extends in a direction inclined with respect to the longitudinal axis of the optical fiber for at least a portion of one or more voids. A11a. An optical fiber according to any of the clauses A8 to A11, wherein the material property is an optical property, optionally the refractive index; one or more voids are configured such that the optical property of the filler affects the light transmitted through the optical fiber; and variations in the cross-sectional area of ​​one or more voids and / or the orientation of the central axis of one or more voids cause the light transmitted through the optical fiber to experience substantially continuous variations in the effect of the optical property on the transmission of light along the length of the optical fiber. A12. An optical fiber according to any of the clauses A1 to A11, wherein the optical fiber has a core and cladding surrounding the core, and one or more voids have at least one cladding void within the cladding. A13. The optical fiber according to clause A12, wherein the cladding voids are configured such that the transmission of light within the optical fiber is affected by the filler material within the cladding voids, and optionally, at least one cladding void is adjacent to the core. A14. A cladding void extending so that the filler material is in contact with the core, as per clause A12 or A13. A15. An optical fiber of clause A12 or A13, in which the cladding void extends so that the filler does not come into contact with the core. A16. An optical fiber according to any of the clauses A12 to A15, wherein one or more voids have multiple cladding voids within the cladding, optionally having at least two cladding voids, optionally having at least four cladding voids, and optionally having at least six cladding voids. A17. An optical fiber according to clause A16, wherein multiple cladding voids are symmetrically arranged around the core. A18. An optical fiber according to any of the clauses A12-A17, in which the distance between the cladding void and the core varies along the length of the cladding void. A19. An optical fiber according to any of the clauses A1 to A18, wherein the optical fiber further has one or more access gaps extending from one or more gaps toward the outer surface of the optical fiber. A20. An optical fiber according to clause A19, wherein one or more access gaps are sealed on the outer surface of the optical fiber. A20a. An optical fiber according to clause A20, wherein the access gap is sealed by a block member within the access gap, optionally the entire block member being located within the access gap; or the optical fiber is sealed by melting the optical fiber material on the outer surface of the optical fiber. A21. An optical fiber according to clause A20, wherein one or more voids are formed by etching the optical fiber material through one or more access voids before sealing one or more access voids. A22. An optical fiber according to any of the clauses A1 to A21, wherein the optical fiber is formed by stretching a preform, and one or more voids are formed after the optical fiber has been stretched. A23. An optical fiber of any of the terms A1 to A22, wherein one or more voids extend entirely within the void portion, and / or the boundaries of one or more voids are entirely defined within the void portion. A24. An optical fiber according to any of the clauses A1 to A23, wherein the void portion has no interface with any adjacent solid material in the longitudinal direction. A25. An optical fiber conforming to any of the clauses A1 to A24, in which one or more voids are completely filled with filler material. A26. An optical fiber according to any of the clauses A1 to A25, wherein the filler material contains gas, optionally nitrogen or air. A27. An optical fiber according to any of the clauses A1 to A26, wherein the filler material is a non-gaseous material and optionally a liquid. A28. Optical fiber of clause A27, wherein the filler material is glycerol or a glycerol-water mixture. A29. An optical fiber having liquid crystal as the filler, according to any of the clauses A27 to A28. A30. An optical fiber according to any of the clauses A1 to A29, wherein the refractive index of the filler is within 0.1 of the refractive index of the optical fiber material, optionally within 0.05, optionally within 0.01, optionally within 0.005, and optionally within 0.001. A31. An optical fiber according to clause A30, wherein the optical fiber has a core and cladding surrounding the core, and the refractive index of the filler is approximately equal to the refractive index of the cladding material at a reference temperature. A32. An optical fiber having an outer diameter of 25 μm to 300 μm, optionally 100 μm to 300 μm, and optionally approximately 125 μm or 250 μm, according to any of the clauses A1 to A31. A33. An optical fiber that is a single-mode optical fiber, as specified in any of clauses A1 to A32. A34. An optical fiber that is an anti-resonant fiber or a negative curvature fiber, as specified in any of clauses A1 to A33. A35. An optical fiber having silica, as specified in any of clauses A1 to A34. A36. An optical fiber according to any of the clauses A1 to A35, wherein the optical fiber has a crystalline material and optionally a single-crystal material. A37. An optical fiber according to Clause A36, wherein the optical fiber is a crystalline fiber, and optionally a sapphire-derived fiber. A38. An optical fiber that is a crystalline optical fiber, and optionally a single-crystal optical fiber, according to any of the clauses A1 to A34. A39. An optical fiber according to any of the clauses A36-A38, which is crystal-doped and optionally doped with rare earth elements. A40. Optical fiber having sapphire, diamond, or yttrium aluminum garnet (YAG) crystals, as specified in any of clauses A36 to A39. A41. Optical fiber having a Bragg grating, as specified in any of clauses A1 to A40. A42. An optical fiber according to clause A41, wherein the optical fiber has a core and cladding surrounding the core, and a Bragg grating is at least partially located on the core. A43. Optical fiber of either clause A41 or A42, wherein the Bragg grating is provided by periodic modification of the core material. A44. The Bragg grating is provided by periodic modification of the cladding material, and optionally, the periodic modification is adjacent to the core, in either optical fiber of clause A41 or A42. A45. An optical fiber according to any of the clauses A41 to A44, wherein the material properties are optical properties, and one or more voids are configured such that the optical properties of the filler affect the Bragg wavelength of the Bragg grating. A46. One or more voids enclose the Bragg grating at least partially, as per any of the clauses A41-A45. A47. Optical fibers of any of the clauses A41 to A46, where the longitudinal extent of one or more voids is at least the same length as the longitudinal extent of the Bragg grating. A48. Optical fiber of any of clauses A41 to A47, in which one or more voids extend up to 1 mm, optionally up to 0.5 mm, beyond both ends of the Bragg grating. A49. One or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the effect of the temperature-dependent changes in the material properties of the optical fiber with respect to the Bragg wavelength of the Bragg grating, and optionally, the material properties are optical properties, an optical fiber of any of the clauses A41 to A48. A50. Optical fiber of Clause A49, wherein one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 5 pm / °C, optionally at least 20°C, optionally at least 40°C, and optionally at least 1 pm / °C. A51. An optical fiber according to any of the clauses A41 to A48, wherein one or more voids are configured such that the temperature-dependent change in the material properties of the filler increases the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating compared to an optical fiber without one or more voids, and optionally the material properties are optical properties. A52. An optical fiber according to Clause A51, wherein one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 20 pm / °C, optionally at least 30 pm / °C, optionally at least 40 pm / °C, and optionally at least 50 pm / °C over a temperature range where the magnitude is at least 20°C and optionally at least 40°C. A53. Optical fiber of clause A52 in which the Bragg wavelength of the Bragg grating decreases with increasing temperature. A54. An optical fiber having two separate Bragg gratings, as specified in any of clauses A41 to A53. A55. An optical fiber according to clause A54, wherein the material properties are optical properties, and one or more voids are configured such that the optical properties of the filler have different effects on the Bragg wavelengths of the two Bragg gratings. A56. An optical fiber according to clause A55, wherein one or more voids are configured such that the optical properties of the filler affect the Bragg wavelength of one of the two Bragg gratings, but do not affect the Bragg wavelength of the other of the two Bragg gratings. A57. An optical fiber according to any of the clauses A54 to A56, wherein the optical fiber has a core and cladding surrounding the core, and two Bragg gratings are spaced apart longitudinally along the core. A58. An optical fiber according to any of the provisions of A54 to A56, wherein the optical fiber has a core and cladding surrounding the core; one of two Bragg gratings is placed in the cladding; and the other of two Bragg gratings is placed in the core. A59. An optical fiber according to Clause A58, wherein one of two Bragg gratings is placed in a waveguide optically coupled to the core, so that a portion of the light guided by the core is transferred into the waveguide. A60. An optical fiber according to clause A59, wherein the material properties are optical properties, and one or more voids have at least one void configured such that the transmission of light in the waveguide is affected by the optical properties of the filler in the void, and optionally, at least one void is adjacent to the waveguide. A61. An optical fiber according to clause A59 or A60, having one or more voids configured to provide waveguides within the cladding; the cladding voids are optically coupled to the core so that a portion of the light guided by the core is transferred into the cladding voids; and one of two Bragg gratings is provided by periodic modification of the material of the optical fiber adjacent to the cladding void. A62. An optical fiber according to any of the clauses A54 to A61, having an isolation void configured such that one or more voids isolate one of the Bragg gratings from strain within the optical fiber at least partially. A63. An optical fiber according to clause A62, with an isolation void surrounding one end of the Bragg grating. A64. An optical fiber as per clause A63, wherein the isolation void encloses at least 50%, optionally at least 75%, optionally at least 90%, and optionally 100% of the length of one of the Bragg gratings. A65. An optical fiber according to any of the clauses A62 to A64, wherein the optical fiber has a core and cladding surrounding the core; one of two Bragg gratings is placed in the cladding; the other of two Bragg gratings is placed in the core; and one of the two Bragg gratings is placed in a waveguide optically coupled to the core, such that a portion of the light guided by the core is transferred into the waveguide. A66. An optical fiber according to any of the clauses A1 to A65, configured to exhibit birefringence. A67. An optical fiber according to clause A66, wherein the optical fiber has a core and cladding surrounding the core, and the optical fiber has one or more stress induction regions arranged around the core that contribute to birefringence. A68. An optical fiber according to clause A67, wherein the stress induction region has a laser exposure region. A69. An optical fiber according to any of the clauses A66 to A68, wherein the optical fiber has a core and cladding surrounding the core, and one or more voids are arranged symmetrically around the core and contribute to birefringence. A70. An optical fiber according to clause A69, having multiple voids, with two voids along the first diameter of the optical fiber on the opposite side of the core. A71. An optical fiber of Clause 70, in which no void is provided along the second diameter which is perpendicular to the first diameter. A72 An optical fiber having a Bragg grating, wherein the birefringence is configured to be different for light having different polarizations for Bragg wavelengths of the Bragg grating, and optionally the polarizations are orthogonal, according to any optical fiber of clauses A66 to A71. A73. The optical fiber of Clause A72, wherein the birefringence is such that when the pressure in one or more gaps is substantially equal to the external pressure of the optical fiber, the reflection peaks for light with different polarizations around the corresponding Bragg wavelength of the Bragg grating are resolvable, and optionally the reflection peaks are separated by at least the full width at half maximum of the reflection peaks. A74. An optical fiber according to any of the clauses A66 to A73, wherein the material properties are optical properties, and one or more voids are configured such that the effect of the optical properties of the filler on light guided by the optical fiber differs for light with different polarizations, and optionally the polarizations are orthogonal. A75. An optical fiber having a waveguide separate from the optical fiber core, as specified in any of the clauses A1 to A74. A76. An optical fiber according to clause A75, wherein the waveguide is formed by one or more modified regions of the optical fiber, and the optical properties of the optical fiber in the regions differ from the optical properties of the optical fiber material surrounding the one or more modified regions, optionally the optical property being the refractive index. A77. An optical fiber according to clause A76, wherein one or more modified regions have one or more laser-exposed regions. A78. An optical fiber according to clause A76 or A77, wherein one or more modified regions have one or more voids extending longitudinally along the length of the optical fiber. A79. An optical fiber according to any of the provisions of A76 to A78, wherein the optical fiber is configured to exhibit birefringence, and one or more modified regions are configured to contribute to the birefringence. A80. A strain sensor having an optical fiber as specified in clause A41 or any preceding clause subordinate thereto. A81. A strain sensor according to clause A80, further comprising a controller configured to determine the strain applied to an optical fiber based on the Bragg wavelength of a Bragg grating. A82. A strain sensor according to clause A81, wherein the optical fiber is configured to exhibit birefringence, and the controller is configured to determine strain based on the difference between Bragg wavelengths of a Bragg grating for light of different polarizations. A83. A system for sensing strain and / or temperature, having optical fibers of clause A54 or any preceding clause subordinate thereto, wherein the temperature-dependent changes of the Bragg wavelengths of two Bragg gratings are different. A84. The system of clause A83, further comprising a controller configured to determine the strain applied to the optical fiber and the temperature of the optical fiber based on the Bragg wavelengths of two Bragg gratings. A85. A pressure sensor having an optical fiber as per clause A41 or any preceding clause subordinate thereto. A86. The pressure sensor of clause A85, configured such that the pressure difference between the pressure of the filler and the external pressure applied to the optical fiber affects the Bragg wavelength of the Bragg grating. A87. The pressure sensor of clause A86, further comprising a controller configured to determine the external pressure applied to the optical fiber based on the Bragg wavelength of the Bragg grating. A88. A pressure sensor according to clause A86 or A87, wherein the optical fiber is configured to exhibit birefringence; a pressure difference affects the birefringence; and the controller is configured to determine the external pressure applied to the optical fiber based on the difference between Bragg wavelengths of a Bragg grating for different polarizations of light. B1. An optical fiber of any preceding clause, wherein the optical fiber has a coating; temperature-dependent changes in the material properties of the coating affect the light transmitted through the optical fiber; and one or more voids are configured such that temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of temperature-dependent changes in the material properties of the optical fiber and the coating on the light transmitted through the optical fiber. B2. An optical fiber having one or more voids at least partially filled with a filler, wherein the filler has material properties different from those of the optical fiber material; and having a coating: the temperature-dependent changes in the material properties of the coating affect the light transmitted through the optical fiber; and the one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of the temperature-dependent changes in the material properties of the optical fiber material and the temperature-dependent changes in the material properties of the coating on the light transmitted through the optical fiber. B3. An optical fiber according to clause B2, wherein one or more voids extend longitudinally within the void portion along the length of the optical fiber, and the void portion is a continuous and integral part of the optical fiber. B4. An optical fiber of clause B2 or B3, wherein the longitudinal extent of each of the one or more voids is less than the longitudinal extent of the void portion. B5. An optical fiber conforming to any of the clauses B2-B4, in which one or more voids are sealed from the outside of the optical fiber. B6. An optical fiber having a coating of polyacrylate or polyimide, as specified in any of clauses B1 to B5. B7. An optical fiber having a metallic coating, as specified in any of clauses B1-B5. B8. An optical fiber according to any of the clauses B1 to B7, wherein the material property is an optical property, and optionally, the refractive index. B9. Optical fiber having a Bragg grating, as specified in any of clauses B1 to B8. B10. An optical fiber according to clause B9, wherein the temperature-dependent changes in the material properties of the coating affect the Bragg wavelength of the Bragg grating; and one or more voids are configured such that the temperature-dependent changes in the material properties of the filler at least partially compensate for the combined effect of the temperature-dependent changes in the material properties of the optical fiber and the coating with respect to the Bragg wavelength of the Bragg grating. B11. An optical fiber of clause B10, wherein one or more voids are configured such that the magnitude of the temperature-dependent change in the Bragg wavelength of the Bragg grating is at least 5 pm / °C, optionally at least 20°C, optionally at least 40°C, and optionally at least 1 pm / °C. C1. An optical fiber of any preceding clause, wherein the optical fiber has a core and cladding surrounding the core, and one or more voids ...

Claims

1. An optical fiber having a first part and a second part: The second portion is a void portion having one or more voids, the one or more voids extending longitudinally within the void portion along the length of the optical fiber; The first and second portions are substantially mode-matched; and, The aforementioned one or more voids are at least partially filled with a filler having material properties different from those of the optical fiber material. The aforementioned optical fiber.

2. The optical fiber according to claim 1, wherein the mode field diameter of the first portion for a predetermined wavelength and temperature is substantially the same as the mode field diameter of the second portion for the predetermined wavelength and temperature.

3. The optical fiber according to claim 1 or 2, wherein the first portion and the second portion each have: a core; and cladding, each cladding surrounding each core.

4. The optical fiber according to claim 3, wherein the refractive index of the core in the first portion is substantially the same as the refractive index of the core in the second portion, optionally, the refractive index of the core in the first portion is 0.002 to 0.007 greater than the refractive index of the cladding surrounding the core in the first portion, and optionally, the refractive index of the core in the first portion is approximately 0.005 greater than the refractive index of the cladding surrounding the core in the first portion.

5. The optical fiber according to claim 3 or 4, wherein the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fiber is substantially the same as the maximum lateral dimension of the core of the second portion in a direction substantially perpendicular to the longitudinal axis of the optical fiber at the interface between the first portion and the second portion.

6. The optical fiber according to any preceding claim, wherein the first portion is a sealed portion, and one or more voids are at least partially sealed by the sealed portion.

7. The optical fiber according to claim 6, wherein one or more of the aforementioned voids are at least partially sealed from the outside of the optical fiber at the interface between the end face of the sealed portion and the end face of the void portion.

8. The optical fiber according to claim 7, wherein one or more of the aforementioned voids are sealed from the outside of the optical fiber at a further interface between the other end face of the void portion and the end face of the further sealed portion.

9. The optical fiber according to any prior claim, wherein the optical fiber is a single-mode optical fiber.

10. The optical fiber according to any prior claim, wherein the first portion has a bridging portion, the bridging portion has a core having a cross-sectional area that varies along the longitudinal axis of the bridging portion from a first cross-sectional area to a second cross-sectional area, the second cross-sectional area having a maximum lateral dimension that is substantially the same as the maximum lateral dimension of the core of the void portion.

11. The optical fiber according to any prior claim, wherein the optical fiber has a Bragg grating.

12. The optical fiber according to claim 11, wherein the material property is an optical property, and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of the Bragg grating.

13. The one or more voids are configured such that the temperature-dependent change in the material properties of the filler at least partially compensates for the effect of the temperature-dependent change in the material properties of the optical fiber with respect to the Bragg wavelength of the Bragg grating, and optionally, the material properties are optical properties. Optionally, one or more of the voids are configured such that the magnitude of the change in the Bragg wavelength of the Bragg grating with respect to temperature is at least 5 pm / °C, optionally at least 20°C, optionally at least 40°C, and optionally at least 1 pm / °C. The optical fiber according to claim 11 or 12.

14. The aforementioned gap portion is a continuous and integral part of the optical fiber; The longitudinal extent of each of the one or more voids is smaller than the longitudinal extent of the void portion; and, The aforementioned one or more voids are sealed from the outside of the optical fiber. An optical fiber according to any prior claim.

15. The optical fiber according to any one of claims 11 to 14, wherein the optical fiber has two separate Bragg gratings.

16. A system for sensing strain and / or temperature: The optical fiber is as described in claim 15, wherein the temperature-dependent changes in the Bragg wavelengths of the two Bragg gratings are different; and, A controller configured to determine the strain applied to the optical fiber and / or the temperature of the optical fiber based on the Bragg wavelengths of the two Bragg gratings, The aforementioned system.

17. A method for manufacturing a device having optical fibers: The invention provides an optical fiber having one or more voids extending longitudinally along the length of the optical fiber; This includes filling one or more of the aforementioned voids with a filler material; Having the means of solidifying the filler; and, The process involves splicing the optical fiber having a solidified filler into a transmission fiber. The aforementioned method.

18. The method according to claim 17, wherein the splicing is performed by laser splicing, and optionally, the laser splicing selectively targets the portion of the optical fiber outside the one or more gaps.

19. The method according to claim 17 or 18, wherein the filler is a polymer that can be solidified by exposure to ultraviolet light or by thermal curing.

20. The method according to any one of claims 17 to 19, wherein the filler has a melting point lower than the melting point of the optical fiber material, and the step of filling one or more voids with the filler is performed at a temperature above the melting point of the filler and below the melting point of the optical fiber material, and optionally the filler is a glass with a low melting point.

21. The method according to any one of claims 17 to 20, further comprising providing a bridging fiber between the optical fiber and the transmission fiber, wherein the bridging fiber has one or more voids filled with a liquid having a melting point below the melting point of the material of the transmission fiber.

22. A method for manufacturing a device having an optical fiber, the method being: The invention provides an optical fiber having one or more voids extending longitudinally along the length of the optical fiber; The optical fiber is spliced ​​into a transmission fiber at one or both ends; This includes forming a hole on the side of the optical fiber, the hole providing access from the outside of the optical fiber to a first of the one or more voids; The first void is filled with a filler; and, Having to seal the aforementioned hole, The aforementioned method.

23. The method according to claim 22, wherein sealing the hole causes the filler to solidify.

24. The method according to claim 22 or 23, wherein forming the hole involves using a laser.

25. The method according to claim 24, wherein forming the hole involves using laser ablation.

26. To form the aforementioned hole: The method comprises selectively exposing the optical fiber to laser irradiation to define an exposure region within the optical fiber; and, The optical fiber is brought into contact with an etching solution. The etching solution etches the exposed region at a faster rate than the region of the optical fiber that is not exposed to laser irradiation, so that the holes are formed by etching the exposed region. The method according to claim 24.

27. A method for forming one or more voids in an optical fiber, wherein: The method involves selectively exposing the optical fiber to laser irradiation to define one or more exposure regions within the optical fiber; The optical fiber is brought into contact with an etching solution, the etching solution etchings the exposed region at a faster rate than the region of the optical fiber not exposed to laser irradiation, and one or more voids are formed by etching the exposed region; The above-mentioned one or more voids are at least partially filled with a filler having material properties different from those of the optical fiber material; and, The method involves sealing one or more of the aforementioned voids from the outside of the optical fiber. The aforementioned method.

28. An optical fiber having at least two waveguides at different positions within the cross-section of the optical fiber, wherein the temperature-dependent change in the refractive index of the first waveguide among the at least two waveguides is different from the temperature-dependent change in the refractive index of the second waveguide among the at least two waveguides.

29. A crystalline optical fiber having one or more voids, wherein the one or more voids extend longitudinally along the length of the crystalline optical fiber.

30. A device having two Bragg gratings, wherein one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature, and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature.

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