Gain medium based on sapphire optical waveguide

Abstract

A method of exciting light emission includes using a single mode sapphire optical waveguide as a gain medium, and a sapphire optic including an optical gain medium, the optical gain medium including the single mode sapphire optical waveguide.

Description

Technical Field

[0001] The present invention relates to a method of stimulating light emission, and a sapphire optical device and method. Background Art

[0002] Doped optical fibers can be used to provide optical gain and lasing. However, optical fibers are typically made of silica, which has disadvantages in certain situations. First, silica absorbs at certain wavelengths, particularly in the mid-infrared, an important band for spectroscopy. Another disadvantage is that they degrade when exposed to radiation. This is a problem for applications such as use in space or in nuclear reactors.

[0003] An important class of lasers are titanium sapphire lasers (Ti:sapphire). These lasers allow the generation of ultrashort pulses and have very wide gain bandwidths, for example allowing access to a very wide range of lasing wavelengths and wide tunability (tunability). However, Ti:sapphire lasers can be bulky and expensive. Another problem is that because the light diverges quickly, its interaction length is relatively short, which limits the amount of optical gain available.

[0004] The pump absorption length of Ti:sapphire is relatively large, while the product of the emission cross section and the lifetime of the high lasing level is relatively small. The small high lasing level lifetime means that small lasing and pump mode radii are required to keep the threshold pump power of the Ti:sapphire laser reasonable. In a bulk Ti:sapphire laser, this means that the pump and the laser will move rapidly out of focus. That is, it is difficult to maintain a small mode radius over the length of the Ti:sapphire crystal required to absorb the pump efficiently. This often results in the need for high spatial brightness frequency-doubled Nd-based lasers as pumps for Ti:sapphire, which can be expensive, e.g. around £10,000 / W. Some blue diode lasers are available for much less than this (e.g. around £100 / W), but it is difficult to use such blue diode lasers to pump Ti:sapphire.

[0005] In general, improvements in gain media for lasers and optical amplifiers are desirable. For example, bulk Ti:sapphire needs to have its gain limitations, absorption length disadvantages, and cost improved. Summary of the invention

[0006] To overcome the above limitations, the present invention provides an apparatus and method for providing optical gain and lasing.

[0007] In a first aspect, the present invention provides a method of stimulating light emission, comprising using a single-mode sapphire optical waveguide as a gain medium.

[0008] Thus, the method comprises stimulating the emission of light in a single-mode sapphire optical waveguide, and thus the waveguide acts as an optical gain source. This may have various applications, for example, the method may comprise using the single-mode sapphire optical waveguide as part of an optical amplifier for amplifying an optical signal. The method may comprise using the single-mode sapphire optical waveguide as part of a laser system for generating laser light. In use, the gain medium (also called laser medium, laser working medium, active laser medium, etc.) thus transfers energy in the form of emitted electromagnetic radiation (i.e. light). Thus, the method may be a method of amplifying and / or generating light.

[0009] The single-mode sapphire optical waveguide may be within an optical device such as an optical fiber, or within a sapphire block or rod. The single-mode sapphire optical waveguide may be within a sapphire optical fiber. Thus, the single-mode sapphire optical waveguide may be part of an optical fiber. Thus, the method may include providing a sapphire optical fiber including a single-mode sapphire optical waveguide, and using the optical fiber (particularly the single-mode waveguide therein) as a gain medium, such as a gain medium for an optical amplifier and / or a laser. From another perspective, the single-mode sapphire optical waveguide may include the optical fiber core of a sapphire optical fiber (coordinated with suitable boundaries, such as provided by a laser-made cladding within the optical fiber).

[0010] By providing a single-mode sapphire optical waveguide within the sapphire optical fiber, the optical fiber itself may be considered a "single-mode optical fiber", but it will be appreciated that it is the single-mode waveguide that makes the optical fiber "single-mode" in this sense, since the light propagation is confined within the single-mode waveguide. Therefore, the remainder of the optical fiber (i.e., the portion of the optical fiber outside the single-mode waveguide) may not be single-mode. The remainder of the optical fiber may be multimode, and therefore may support multiple propagation modes therein, whereas the single-mode waveguide only supports a single propagation mode. The remainder of the optical fiber may include a cladding, a modified region (e.g., a laser modified region, an etched region, etc.), bulk sapphire, etc.

[0011] Patent application GB1712640.0 discloses a method for preparing a single-mode sapphire optical fiber, the contents of which are incorporated herein in their entirety. The inventors have discovered that the methods and features disclosed therein can be combined with features of the present invention. By laser modification of the optical fiber, a single-mode sapphire optical waveguide can be produced in a sapphire optical fiber. However, due to the relatively high refractive index of sapphire (e.g., about 1.75), focusing in such an optical fiber is subject to significant aberrations, and the precision and accuracy of the laser modification is limited by this aberration. However, as recognized in GB1712640.0, corrections can be determined and applied to the modified laser (e.g., using active optical elements to modify its wavefront properties) to offset the aberration effects of the optical fiber on the laser focus therein. By offsetting these aberration effects of the optical fiber, accurate and precise laser modification of the optical fiber can be performed on length scales and materials that were previously impossible to use. Therefore, the method can include engraving the structure with femtosecond laser direct writing, using adaptive beam shaping with a non-immersion objective. Alternatively, the method may include the use of an immersion objective and / or immersion oil, and may include engraving the structure, for example, by femtosecond laser direct writing (and) using adaptive beam shaping with an immersion objective and immersion oil. Other methods of preparing single-mode sapphire optical fibers are also possible, such as by etching, refractive index matching, etc.

[0012] The method may include laser modifying the optical fiber (e.g., sapphire optical fiber) to form a single-mode sapphire optical waveguide at a target location within the optical fiber. The laser modification may configure the waveguide and / or the optical fiber to prevent, inhibit, or reduce the propagation of multiple optical modes therein.

[0013] The method may include positioning at least a portion of the optical fiber in a laser system for modification by a laser; and laser modifying the optical fiber at a target location using the laser to produce a waveguide.

[0014] The method may include correcting active optical elements of a laser system to modify the wavefront characteristics of the laser to counteract the effects of fiber-induced aberrations on the laser focus. The method may include laser-modifying the optical fiber at a target location using a laser having the corrected wavefront characteristics to produce a waveguide.

[0015] A single-mode sapphire optical waveguide can be formed in a sapphire optical fiber by laser-modifying a region of the sapphire optical fiber. Thus, the optical fiber can be laser-modified to prevent or reduce the propagation of higher-order modes therein. The method may include preparing a single-mode waveguide by laser-modifying a sapphire optical fiber (e.g., a multimode sapphire optical fiber) to form a single-mode sapphire optical waveguide therein.

[0016] Laser modification of sapphire may change the physical properties of the modified region, for example by changing its refractive index, size, density, uniformity, homogeneity, etc. The method may include directly modifying a portion of a sapphire optical fiber to produce a waveguide, for example by modifying the properties of the optical fiber core to directly form a waveguide.

[0017] The method may comprise modifying a region around a portion of an optical fiber to produce, for example, a strain field in the portion, thereby changing its optical properties and thereby turning the portion into an optical core (e.g., a single-mode waveguide) having desired properties, even though the core itself is not directly modified using a laser.

[0018] The method may include modifying regions around portions of the optical fiber so that the modified regions have a different refractive index (e.g. a reduced refractive index) than portions surrounding them, thereby effectively forming a cladding around an unmodified optical core of the optical fiber. In practice, the method may include laser modifying a portion of the optical fiber to create a cladding around the unmodified core of the optical fiber, thereby providing the single mode waveguide. The method may include modifying (e.g. laser modifying) the optical fiber to create a depressed cladding waveguide therein.

[0019] A waveguide generally comprises an optical core in which waves can propagate and a boundary that suitably confines the wave propagation. The boundary can be provided by any suitable material having sufficiently different optical properties, for example by an air interface and / or a cladding interface. Thus, the core and cladding (or at least the core and the boundary) cooperate to provide a waveguide. By appropriate configuration of the core and cladding (or core and boundary), the waveguide can be configured as single mode.

[0020] The method may include forming a cladding within a sapphire optical fiber to indirectly form a single-mode waveguide (e.g., by not directly modifying the optical core). Thus, the method may include modifying a property within the optical fiber (e.g., bulk sapphire) to effectively form a cladding configured to cooperate with an unmodified portion of the sapphire optical fiber (e.g., the optical core) to provide a waveguide.

[0021] Single-mode optical fibers are known, although single-mode sapphire optical fibers had not been achieved prior to the method disclosed in GB1712640.0, at least in part due to the inherent difficulties of laser modification using sapphire and using sapphire optical fibers. Therefore, GB1712640.0 discloses the first laser-prepared single-mode sapphire optical waveguide in sapphire optical fiber. Single-mode optical fiber is an optical fiber designed to transmit only single-mode light. Single-mode optical fiber can meet the single-mode criteria, such as having a normalized frequency less than a predetermined threshold. For example, for a step-index optical fiber, the normalized frequency may be less than 2.4. For other types of optical fibers, such as photonic crystal fibers, depressed cladding fibers, microhole fibers, photonic-bandgap fibers, etc., the threshold may be different.

[0022] The method may include configuring the waveguide (e.g. by laser modification) to be single mode. The method may include configuring the waveguide to prevent all light except a single mode from propagating therein. The method may include modifying the optical fiber to ensure that a predetermined mode experiences increased losses during propagation. The method may include manufacturing (e.g. laser writing) a step index waveguide within the optical fiber. The method may include manufacturing (e.g. laser manufacturing) a periodic structure waveguide (e.g. having a periodic variation in refractive index across a cross section), a photonic crystal fiber, a microhole fiber, a photonic bandgap fiber or the like within the optical fiber. The single mode waveguide may restrict energy transmission to substantially one direction. The method may include modifying only the interior of the optical fiber. The method may include providing a laser manufactured waveguide entirely within a sapphire optical fiber. The method may include providing a laser manufactured waveguide embedded within a sapphire optical fiber.

[0023] The optical fiber may be a hollow core optical fiber, for example where the core is a void (e.g. air or another fluid / liquid / gas). The optical fiber may be a photonic crystal fiber with a hollow core. The optical fiber may be an anti-resonant fiber or a negative curvature fiber with a hollow core. The hollow core may be filled with a suitable material, such as a gas (e.g. helium, neon, carbon dioxide, rubidium, nitrogen, etc.), so that a gas laser may be provided. The hollow core optical fiber may be a non-sapphire optical fiber.

[0024] The method may include configuring a waveguide to increase the loss of a predetermined propagation mode, thereby configuring the waveguide as single mode. The method may include configuring the waveguide so that higher order modes experience increased losses during propagation. The method may include creating a waveguide within an optical fiber so that the loss of a predetermined mode is greater than about 1 dB (decibel) / meter, about 3 dB / meter, about 10 dB / meter, about 15 dB / meter, or about 20 dB / meter. The method may include modifying the optical fiber so that all modes except the single mode experience losses in the waveguide, such as losses greater than about 1 dB / meter, about 3 dB / meter, or about 10 dB / meter. The method may include manufacturing a sapphire optical fiber that supports a reduced number of modes in a waveguide therein.

[0025] Thus, a sapphire optical fiber may include a laser modified region configured to cause multiple modes to exhibit losses during propagation within a waveguide, and may cause all but a single mode to exhibit losses during propagation within the waveguide. A sapphire optical fiber may be configured to substantially support a single propagating mode in a waveguide.

[0026] Sapphire optical fiber may include a bulk material having a first refractive index and an optical core having a second refractive index different from the first refractive index. The bulk sapphire may have only the first refractive index, so that the optically functional part of the optical fiber consists only of the optical core for propagating a single mode and the homogeneous surrounding sapphire material.

[0027] A sapphire optical fiber may include an optical core (e.g. unmodified sapphire) having a first refractive index and a surrounding modified material (e.g. cladding and / or depressed cladding) having a second refractive index. Thus, there may be a boundary between regions having different refractive indices, which boundary may be arranged to provide a waveguide. Thus, a waveguide may be formed by the mating of a material having a first refractive index with a material having a second refractive index.

[0028] The waveguide may include a region of modified refractive index configured to guide light therein. The waveguide may include a modified region having a modified refractive index, which may be substantially solid. The modified region may include a modified material containing micropores therein. The modified region may be periodic and may provide a microstructure and / or a photonic crystal. The sapphire fiber may be a microstructure fiber and / or a photonic crystal fiber including an array of modified regions, thereby forming a waveguide. The modified region may be used to define the optical core as a light-guiding region of the sapphire fiber. Thus, the sapphire fiber may be configured such that the waveguide includes an unmodified region and / or a modified region. The waveguide may be formed by the cooperation of an optical core (e.g., an unmodified core) and a laser modified region (e.g., a laser-made microstructure, a laser-made periodic structure, a laser-made photonic crystal, a laser-made depressed cladding, etc.). Thus, the laser modified region may surround the optical core, thereby acting as a cladding of a single-mode waveguide (although completely within the optical fiber). The laser modified region may include voids or holes, which may be filled with air or another fluid (i.e., another gas or liquid).

[0029] The single-mode sapphire optical waveguide may have a normalized frequency V that is less than a predetermined threshold applicable to the optical fiber type. For example, for a step-index optical fiber, the single-mode sapphire optical waveguide may have a normalized frequency V that is less than 2.4, and may have a normalized frequency that is less than 2.405. The normalized frequency V may be defined as: .

[0030] in a is the waveguide radius, λ is the operating wavelength, n1 is the core refractive index, and n2 is the cladding refractive index. The refractive index of sapphire is about 1.75, so it is single-mode at 1550nm, the refractive index modification between the waveguide and the cladding is 0.005, and the waveguide radius should be less than 4.47μm (diameter less than 8.94μm).

[0031] A single-mode sapphire optical fiber may include an optical core and a cladding surrounding the optical core. Thus, a single mode propagating in a waveguide may be substantially confined to the optical core. Due to a change in refractive index between the core and the cladding, or at least due to a change in optical properties between the optical core and the cladding, light may be confined in (and therefore along) a waveguide (e.g., along the optical core). This change in refractive index between adjacent materials may provide a waveguide through a core-cladding interface. Thus, the waveguide may be provided by the cooperation of the core and the cladding or by a boundary between them. A single-mode sapphire optical waveguide may include a region in which a single mode of light is confined to propagate; the region being formed by a sapphire-based material. The waveguide may be the optical core of a sapphire optical fiber. A single-mode sapphire optical waveguide may include an optical core and a core-cladding interface.

[0032] Therefore, the light-guiding portion of the waveguide can be considered to be consistent with the optical core. Therefore, the optical core can provide a gain medium.

[0033] It should be noted that the term "cladding" as used herein includes modified materials within the optical fiber that behave similarly to the cladding of a typical optical fiber by providing a boundary. The cladding may be completely within the optical fiber. In fact, the cladding may be surrounded by unmodified optical fiber material, such as bulk sapphire.

[0034] The method may include generating light by stimulated emission. The method may include exciting a gain medium (eg, an optical core) using pump light. The method may include stimulating the excited gain medium to emit light using a signal or input light.

[0035] The gain medium in the waveguide may be doped sapphire. The doping of sapphire may make it a gain medium. The doping element may be any suitable doping element that enables the waveguide to act as a gain medium. In particular, the doping element may be titanium, or may be chromium, or may be any suitable combination of elements. The optical fiber may be a titanium-doped sapphire optical fiber, such as a Ti:sapphire optical fiber.

[0036] The single-mode sapphire optical waveguide can be configured to offset propagation losses. The method may include providing gain using the single-mode sapphire optical waveguide to overcome propagation losses in the waveguide. Thus, the method may use longer sapphire optical fibers. Commercial sapphire optical fibers are typically less than about 4 meters in length, and by providing a sapphire optical fiber including a single-mode sapphire optical waveguide to offset propagation losses, this limitation of the length of the sapphire optical fiber is not encountered. That is, by the method described in the present invention, longer optical fibers can be used. Therefore, the present invention may include providing a single-mode sapphire optical fiber longer than 2.5 centimeters (cm), longer than 5 centimeters, longer than 10 centimeters, longer than 15 centimeters, longer than 20 centimeters, longer than 30 centimeters, longer than 50 centimeters, longer than 1 meter (e.g., 1.00 meters), longer than 2 meters (e.g., 2.00 meters), or longer than 4 meters (e.g., 4.00 meters). The length of the single-mode sapphire optical fiber may be suitable for practical applications.

[0037] The method may include amplifying the signal light using a single-mode sapphire optical waveguide as a gain medium.

[0038] Thus, the method may include amplifying the signal light so that the single-mode sapphire optical waveguide can be used as an optical amplifier. An optical amplifier can be a device that directly amplifies the signal, for example without first converting it to an electrical signal. An optical amplifier can be considered to be a laser without an optical cavity, or a laser in which feedback from the optical cavity is suppressed. The method may include using sapphire optical fiber in the optical amplifier. The method may include providing an optical amplifier. The method may include providing the optical amplifier as an optical repeater, for example for use in communication systems such as telecommunications. In fact, viewed from another perspective, one aspect of the present invention provides a method for amplifying an optical signal, comprising using a single-mode sapphire optical waveguide as a gain medium. This aspect may include any features as described in the present invention in relation to any other aspect of the present invention.

[0039] The method (any aspect of the invention) may comprise exciting the gain medium, for example to produce amplification of the signal light. The method may comprise pumping the gain medium to excite it to produce amplification of the signal light.

[0040] The method may include generating laser light using a single-mode sapphire optical waveguide as a gain medium.

[0041] Thus, the method may include providing a single mode sapphire fiber laser. The method may include stimulating emission of light from a gain medium provided by a single mode sapphire optical waveguide. The method may include pumping the gain medium, for example using pump light. The method may include providing an optical cavity (e.g., a resonant cavity of a laser system), and may include demonstrating that the single mode sapphire fiber is at least a part of the optical cavity.

[0042] Indeed, viewed from another perspective, one aspect of the present invention provides a method for generating laser light using a single-mode sapphire optical waveguide as a gain medium. Thus, the method may include using a single-mode sapphire optical fiber as a gain medium, the optical fiber including a single-mode sapphire waveguide. This aspect may include any features of the present invention as described herein in relation to any other aspect of the present invention. Viewed from another perspective, one aspect of the present invention provides a single-mode sapphire fiber laser. This aspect may include any features of the present invention as described herein in relation to any other aspect of the present invention.

[0043] The method (any aspect of the invention) may include pumping a single-mode sapphire optical waveguide using pump light above a predetermined threshold power level. The method may include stimulating emission of light from the pumped single-mode waveguide. The method may include providing a pump light source, such as a pump laser, a diode laser, a semiconductor laser, etc., and configuring the pump light source to excite a single-mode sapphire optical fiber, thereby for subsequent stimulated emission. The method may include population inversion to achieve stimulated emission. The method may include using any suitable pumping device to pump the gain medium, that is, any suitable device to excite the gain medium, such as by an electric current, a chemical reaction, nuclear fission or a high-energy electron beam.

[0044] The method may include pumping the gain medium with a diode laser. Diode lasers are generally difficult to use effectively with, for example, titanium-doped sapphire, but by providing a waveguide, the pump may be coupled more efficiently into the gain medium. Thus, a smaller, cheaper, lighter system may be achieved.

[0045] The method may include mode locking a single mode sapphire fiber laser. The method may include using a single mode sapphire optical waveguide to generate laser pulses.

[0046] The method may include providing an optical cavity, for example using a feedback device. The method may include manufacturing a feedback device. The feedback device may be a reflector. The feedback device may be a grating, a Bragg grating, a diffraction grating, a mirror, a dielectric mirror, a sapphire-air interface of an optical fiber and / or an optical waveguide, etc. The reflector may be adjustable, for example, it may be a tunable grating, for example, it is rotatable and therefore tunable. Therefore, the method may include adjusting the output of the laser light emitted by the gain medium, for example, changing and / or selecting the wavelength of the light output by the laser. Therefore, a tunable laser may be achieved, and the method may include providing a tunable laser. The ability to change and select the wavelength emitted by the gain medium allows the laser to be used, for example, to study the spectrum. For example, absorption lines or peak wavelengths within the spectrum can be studied. The tunable system also allows the wavelength peak of the manufactured fiber Bragg grating (FBG) to be determined.

[0047] The grating may be external to the optical fiber, for example adjacent to it, and / or in optical communication with it. That is, the reflector and / or the grating may be a component external to the optical fiber.

[0048] The method may include forming an optical cavity (e.g., a laser cavity) including a single-mode sapphire optical waveguide. The method may include providing a reflector and either end of the waveguide to form the cavity. The method may include providing a reflector within the single-mode sapphire optical waveguide and / or within a sapphire optical fiber. The method may include fabricating a reflector within the single-mode sapphire optical waveguide. Thus, the method may include providing a single-mode sapphire optical fiber optical cavity for a laser system.

[0049] The method may include fabricating a Bragg grating within a single-mode sapphire optical waveguide.

[0050] The method may include using a Bragg grating to reflect light within a waveguide, and thus may include using a fabricated Bragg grating as a reflector, such as as part of an optical cavity, or as part of a sensor system. The method may include detecting reflected light from the Bragg grating, and may include using a single-mode sapphire optical fiber as part of a sensor system. For example, the method may include determining a physical parameter (e.g., temperature, pressure, strain, magnetic field, electric field, vibration, and / or shock) based on the reflected light from the Bragg grating. Since sapphire is an extremely durable material, sapphire optical fibers may be used in extreme environments, such as engines, etc. The method may include arranging at least a portion of the sapphire optical fiber in an extreme environment, such as an engine, to detect a physical parameter (e.g., temperature, pressure, strain, magnetic field, electric field, vibration, and / or shock) in the environment. For example, the method may include arranging at least a portion of the sapphire optical fiber in an aerospace engine. The method may include utilizing a sensor system including a sapphire optical fiber in an environment that experiences high levels of radiation. For example, the method may include utilizing a sensor system including a sapphire optical fiber in a nuclear reactor, particularly in a nuclear fission power reactor or a nuclear fusion power reactor, and / or in satellites and other devices suitable for use in space. Sapphire is also transparent in the mid-infrared (e.g. wavelengths > 2 μm). Therefore, sensor systems including sapphire optical fibers can be effectively used for spectroscopy and gas species detection.

[0051] The method may include configuring the Bragg grating (or each Bragg grating where a plurality of gratings are provided) to be tunable, for example by temperature or strain. For example, the structure of the grating may change when exposed to different conditions, for example the spacing may depend on the temperature of the optical fiber. Thus, the method may include reconfiguring and / or deforming the Bragg grating within the waveguide and detecting changes in reflected light due to the reconfiguration or deformation, thereby detecting changes in the parameter to be measured (e.g. temperature and / or strain).

[0052] The method may include providing a plurality of Bragg gratings within the optical fiber, for example to provide an optical cavity or for performing a plurality of measurements. The method may include providing a plurality of Bragg gratings, each Bragg grating within a respective optical core (i.e. each Bragg grating within a respective waveguide), and / or may include providing a plurality of Bragg gratings within the same optical core (e.g. in series or overlapping each other). The method may include using the optical fiber to perform a plurality of simultaneous measurements of a physical parameter, for example by detecting a plurality of reflected signals from a plurality of Bragg gratings.

[0053] The Bragg grating may have a narrow bandwidth, for example less than 5 nm (nanometers), less than 2 nm, less than 1 nm, less than 0.5 nm, or less than 0.1 nm.

[0054] The method may include providing a plurality of waveguides within the optical fiber, and may include providing a plurality of single mode waveguides within the optical fiber, for example in parallel or adjacently. The method may include providing overlapping waveguides within the optical fiber. Thus, the method may include manufacturing a plurality of waveguides within the optical fiber.

[0055] The method may include providing opposing Bragg gratings within a single mode sapphire optical waveguide to form an optical cavity.

[0056] The method may include fabricating an opposing Bragg grating in a single-mode sapphire optical waveguide by laser modification, for example using adaptive optics to correct for aberrations of the modified laser focus caused by the fiber. Thus, the method may include providing a sapphire optical fiber including a laser cavity, for example a laser cavity including a single-mode sapphire optical waveguide.

[0057] The method may include using a single mode sapphire optical waveguide as part of a sensor system.

[0058] Since sapphire is very durable and can withstand temperatures up to 1000°C, up to 1500°C, up to 1700°C, up to 1900°C, and up to 2000°C, optical fibers and single-mode waveguides can be used in extreme environments, such as engines or any other suitable application. Therefore, the method can include using a single-mode sapphire optical waveguide in extreme conditions, such as up to 1000°C, up to 1500°C, up to 1700°C, up to 1900°C, and up to 2000°C. The method can include using a sensor system to measure the temperature, such as up to 1000°C, up to 1500°C, up to 1700°C, up to 1900°C, and up to 2000°C. Bragg gratings may also be stable at such high temperatures, so the method can include using fiber Bragg gratings at temperatures up to 1000°C, up to 1500°C, up to 1700°C, up to 1900°C, and up to 2000°C.

[0059] The single-mode sapphire optical waveguide may be a depressed cladding waveguide.

[0060] The depressed cladding waveguide may be a laser modified sapphire optical fiber as described herein in connection with any aspect of the present invention. Thus, the depressed cladding may be sapphire. The depressed cladding may be laser modified to reduce its diffraction index, e.g. compared to unmodified material within the optical fiber. Thus, the depressed cladding may have a lower refractive index than the unmodified portion and may cooperate with the unmodified portion to form a waveguide. In the depressed cladding waveguide, the refractive index of the sapphire may be reduced (e.g. uniformly reduced) around the (optical) core. The reduced refractive index may not extend all the way to the edge of the sapphire optical fiber, so the cladding may be surrounded by unmodified optical fiber.

[0061] The depressed cladding waveguide may be a multi-layer depressed cladding waveguide comprising multiple layers of modified optical fiber material providing multiple claddings.

[0062] The single mode sapphire optical waveguide may be a photonic crystal waveguide (comprising a cladding comprising a periodic array of regions having a modified refractive index within the cladding).

[0063] The cladding may include a periodic arrangement of "holes", i.e. areas where material has been removed. This may be air (refractive index is about ~1) or another fluid (i.e. liquid or gas).

[0064] The method may include fabricating a single-mode sapphire optical waveguide using laser modification and adaptive optics aberration compensation.

[0065] The method may include direct writing using a femtosecond laser, for example using the method disclosed in GB1712640.0, the contents of which are incorporated herein in their entirety. The inventors have found that the methods and features disclosed therein can be combined with features of the present invention.

[0066] The method may include using non-immersion lenses, such as focusing with optical fibers. The method may include counteracting the effects of sapphire-induced aberrations on laser focusing, such as sapphire-air interfaces, sapphire-cladding interfaces, sapphire-modified sapphire interfaces, and the like.

[0067] The method may include fabricating a single-mode sapphire optical waveguide using an immersion oil technique and an immersion objective. The method may include fabricating a single-mode sapphire optical waveguide by embedding a sapphire optical fiber within a substrate of similar refractive index (e.g., a block of sapphire having holes along its length to bind the sapphire). The fabrication step may include counteracting the aberration effects of the optical fiber on the focus of the modified laser.

[0068] The method may include providing a single-mode sapphire optical waveguide within a sapphire optical fiber and using the sapphire optical fiber as a multimode waveguide while using the single-mode sapphire optical waveguide as a single-mode waveguide.

[0069] Thus, the method may include providing a single-mode waveguide within a multimode waveguide. The method may include propagating multiple modes of light within a sapphire optical fiber as a pump for the single-mode sapphire optical waveguide. Sapphire optical fibers are naturally multimode, and thus multiple modes of light can propagate along the optical fiber, for example through the sapphire-air boundary. Therefore, the optical fiber can be considered as a combination of waveguides, such as a first single-mode sapphire optical waveguide and a second multimode waveguide provided by the remainder of the optical fiber. Therefore, the propagating mode in the multimode waveguide of the optical fiber can excite the gain medium of the single-mode waveguide for subsequent stimulated emission, for example by single-mode propagation within the waveguide. The method may include propagating pump light within the optical fiber, and thereby overlapping the pump light with the single-mode waveguide to excite ions within the single-mode waveguide. Therefore, the method may include using multiple modes of light propagating within the optical fiber to pump the single-mode waveguide.

[0070] The input signal may then excite emission from the pump single mode waveguide. Thus, the method may include using a single light source for both the pump light and the input signal light. Thus, the input signal may be a fundamental mode of the source laser and may propagate within the single mode sapphire optical waveguide. Higher order modes from the source laser may propagate within the remainder of the optical fiber and may thus pump the single mode waveguide.

[0071] The method may include fabricating a cladding within a sapphire optical fiber to form a laser-modified cladding around an optical core, whereby the optical core and cladding cooperate to provide a single-mode sapphire optical waveguide.

[0072] The method may include fabricating a depressed cladding waveguide by reducing the refractive index of a material surrounding the optical core, such as to form a material with a substantially uniform refractive index. The method may include fabricating a periodic structure around the optical core using a laser to form the cladding, such as to form a material with a periodic variation in refractive index. The periodic structure may be a microstructure (e.g., a structure on a microscopic length scale), a microhole, a photonic crystal structure, a photonic bandgap structure, etc. The method may include fabricating a non-periodic structure.

[0073] The method may include modifying material only within a sapphire fiber, and may include, for example, not changing the overall shape of the cross-section of the fiber. That is, the surface of the fiber may not be modified. The fiber may have a cylindrical shape, or, as is more common with sapphire fibers, may have a substantially hexagonal cross-section. The cross-section of the fiber may be hexagonal or rounded hexagonal. Since the overall shape of the cross-section of the fiber is substantially the same as before modification, the fiber may retain substantially the same mechanical properties. For example, it may not be necessary to remove material from the fiber to provide the present invention, and therefore it may not be necessary to weaken the fiber and make it more mechanically fragile. Thus, a waveguide (or waveguides) may consist of modified and unmodified sapphire material.

[0074] The cladding may be a first cladding, and the method may include preparing a second cladding around the first cladding.

[0075] The method may include making nested claddings within the optical fiber. The method may include making double-clad optical fibers, multiple claddings within the optical fiber. The second cladding may have a different refractive index than the first cladding (e.g., lower). The second cladding may be any type of cladding as described herein in connection with any aspect of the present invention. The second cladding may have different optical properties than the first cladding, such as a different refractive index than the first cladding (e.g., lower). It may have a different microstructure, a different period, etc. The second cladding may be configured to act as a cladding for a second optical core provided by the first cladding. That is, the first cladding may surround the optical core of a single-mode waveguide, and the second cladding may surround the first cladding. Thus, the first cladding may serve as the (second) optical core of a multimode waveguide defined by the second cladding, and the (first) optical core may serve as a single-mode waveguide defined by the first cladding. The first and second claddings may be formed of a sapphire-based material.

[0076] The optical fiber may be configured to include a plurality of waveguides. The optical fiber may be configured to include a plurality of overlapping waveguides. The optical fiber may include a plurality of nested waveguides. The cross-sections of the waveguides may overlap, for example, one waveguide may be within another waveguide. A plurality of waveguides may overlap in a radial direction, thereby forming an inner waveguide and an outer waveguide. The central longitudinal axis of the outer waveguide may be located within the region of the corresponding inner waveguide, for example, the waveguides may be concentric (although the waveguides do not necessarily have circular symmetry). The cladding of the inner waveguide may serve as the optical core of the outer waveguide. Thus, the optical fiber may be configured to provide a multimode waveguide surrounding a single-mode waveguide, and the method may include propagating multiple modes of light in the multimode waveguide, thereby pumping the single-mode waveguide. The method may include using the cladding as a pump waveguide. Thus, the method may include providing a cladding-pumped single-mode sapphire optical waveguide, i.e., a cladding-pumped gain medium. The second (or outer) cladding may be surrounded by an unmodified sapphire optical fiber. That is, the single-mode waveguide and the multimode waveguide may be completely located within the optical fiber, both of which are manufactured therein.

[0077] The method may include doping a sapphire optical fiber to form a doped region therein, and fabricating an optical core of a single-mode sapphire optical waveguide within the doped region.

[0078] The doped region may include the entire cross-section of the optical fiber. Alternatively, the doped region may include only a portion of the sapphire optical fiber, such as a portion of the cross-section of the optical fiber. For example, only the inner region of the optical fiber may be doped, which region is intended to provide the optical core of a single-mode waveguide. Thus, the doped portion may be surrounded by undoped sapphire material. The method may include doping only the inner portion of the optical fiber so that the cross-section of the optical fiber includes an inner doped portion (e.g., a central portion) surrounded by an undoped portion. Thus, a sapphire optical fiber (e.g., a cross-section thereof) may include a doped region and an undoped region. Only a portion of the cross-section of the sapphire optical fiber may be doped, and a single-mode sapphire optical waveguide may be formed within the doped portion. The doped portion may be eccentric within the optical fiber to provide an eccentric core as described in the present invention.

[0079] By doping only a portion of the optical fiber (e.g., only a portion of the optical fiber cross section), the risk of the dopant material absorbing energy in higher-order transmission modes that propagate in the optical fiber but are outside the single-mode waveguide is reduced. If the cladding of the single-mode waveguide (e.g., the second core of a cladding-pumped single-mode waveguide) is doped, it can absorb the pump power without affecting the optical gain, so the overlap between the core of the pump waveguide and the doped portion should preferably be minimized. In a cladding-pumped single-mode waveguide, the core of the single-mode waveguide may be arranged within the core of the pump waveguide. By providing the doping within the core of the single-mode waveguide, the core of the pump waveguide may include the doping material at least in the overlap between the single-mode waveguide core and the pump-waveguide core. Any additional overlap between the doped region and the pump waveguide core should preferably be minimized.

[0080] Therefore, it is preferred to dope only the amount of optical fiber required to provide the core of a single-mode waveguide. However, precise control over the extent of the doped portion is difficult to achieve, and therefore it may be necessary to dope a larger area than required for the core of a single-mode waveguide.

[0081] The method may include fabricating a single-mode sapphire optical waveguide eccentricity within a sapphire optical fiber.

[0082] The method may include fabricating a single-mode sapphire optical waveguide near the periphery of the doped portion, for example off-center relative to and within the doped portion of the optical fiber, for example at the edge of the doped portion (the doped portion itself may be off-center within the optical fiber). The single-mode waveguide may be arranged at the periphery of the doped region, which in turn allows the overlap of the fabricated cladding surrounding the single-mode waveguide optical core with the doped portion to be reduced (and ideally minimized). Thus, when the cladding of the single-mode waveguide is used as a pump waveguide, this arrangement may reduce pump energy losses of the doped material in the pump waveguide / first cladding. That is, by providing the core of the single-mode waveguide at the periphery of the doped portion, the risk of pump power being lost in the doped cladding is reduced.

[0083] In addition to the above advantages, the method may also include providing a gain medium / core that is off-center for the single-mode waveguide to increase the overlap of the pump mode with the gain medium of the single-mode waveguide. If the core of the single-mode waveguide (e.g., the core that guides the laser mode and is doped with active ions) is located exactly (exactly) in the middle of the multi-mode pump waveguide, there may be modes of the multi-mode waveguide that do not significantly overlap with the gain medium of the core of the single-mode waveguide, and therefore light in these modes coupled to the multi-mode waveguide may not be absorbed by the gain medium of the core of the single-mode waveguide, but may exit the optical fiber unused. By providing an off-center core for the single-mode waveguide, the energy absorbed by the gain medium from the pump mode may be increased. Therefore, the core of the single-mode waveguide may be off-centered within the optical fiber, and may also be off-centered within any pump waveguide that may be provided.

[0084] Another way to increase the energy absorbed by the gain medium from the pump mode is to reduce the cross-sectional symmetry of the multimode waveguide, for example by making its cross section elliptical or D-shaped. Thus, the method may include providing a pump waveguide having asymmetric features. The method may include providing such asymmetric features to increase the energy absorbed by the gain medium from the pump mode.

[0085] The method may include fabricating a single-mode sapphire optical waveguide including asymmetric features.

[0086] For example, the method may include fabricating a single-mode sapphire waveguide to have an elliptical cross-section (e.g., in a plane perpendicular to the length of the optical fiber). The resulting waveguide may maintain polarization, thereby maintaining the polarization state of, for example, a single mode when the optical fiber is bent. Thus, the method may include providing a polarization-maintaining single-mode sapphire optical waveguide.

[0087] The method may further comprise fabricating the cladding of the single mode waveguide to include asymmetric features, such as having an elliptical cross-section.

[0088] The method may include coupling a pump laser to a single-mode sapphire optical waveguide.

[0089] The method may include providing a pump laser for pumping a gain medium, i.e., for pumping a single-mode sapphire optical waveguide. The method may include exciting the gain medium using the pump laser. The method may include exciting emission from the gain medium using the pump laser. The pump laser may be any suitable type of laser, such as an argon ion laser. The pump laser may be a diode laser. The method may include providing an optical amplifier and / or a laser system. The method may include providing all relevant components for the optical amplifier and / or the laser system.

[0090] Combinations of the above features may have specific advantages. For example, a cladding-pumped single-mode sapphire optical waveguide combined with asymmetric features of the pump waveguide can improve coupling of the pump mode into the gain medium. An optical core with an eccentricity of the single-mode waveguide within the pump waveguide can further improve coupling of the mode into the gain medium and improve energy transfer. Selective doping to reduce overlap of the pump waveguide with the doped material can further improve energy transfer by reducing losses into the pump waveguide.

[0091] According to a second aspect of the present invention, there is provided a sapphire optical device comprising an optical gain medium including a single-mode sapphire optical waveguide.

[0092] The sapphire optical device may comprise any of the features discussed according to the first aspect of the invention and / or according to any other aspect of the invention.

[0093] The sapphire optical device may be a sapphire optical fiber. The sapphire optical device may be a bulk sapphire (e.g., a block or a rod) having a single-mode sapphire optical waveguide therein. The diameter of the optical core of the single-mode waveguide may be less than 20 micrometers (μm), less than 15 μm, less than 10 μm, or may be 10 μm or less.

[0094] The diameter of the sapphire fiber may be less than 1000 μm, less than 425 μm, less than 250 μm, less than 125 μm, less than 100 μm, less than 75 μm or less, or may be 50 μm or less. The diameter of the cladding surrounding the optical core may be the same as the diameter of the sapphire fiber (e.g., if a single waveguide is provided in the sapphire fiber). The diameter of the cladding surrounding the optical core may be different from the diameter of the optical fiber (e.g., in the case of providing additional material around the cladding, or in the case of providing multiple waveguides in the sapphire fiber, or in the case of providing a cladding pump optical waveguide as further discussed below), and the size of the cladding may be less than 425 μm, less than 250 μm, less than 125 μm, less than 100 μm, less than 75 μm or less, or may be less than 50 μm or less. The diameter of the cladding surrounding the optical core may be more than 2.5 times the diameter of the optical core, or may be more than 5 times the diameter of the optical core, or may be more than 10 times, or may be more than 20 times. A cladding diameter of 5 times or more of the core diameter is advantageous for achieving low losses.

[0095] The length of the single-mode sapphire optical waveguide can be the same as the length of the optical fiber. For example, the single-mode sapphire optical waveguide can extend along the entire length of the sapphire optical fiber. The length of the sapphire optical fiber can be, for example, 5 cm or more, 10 cm or more, 50 cm or more, 100 cm or more, or 200 cm or more.

[0096] The sapphire optics may include a Bragg grating.

[0097] The Bragg grating may be within a single mode sapphire optical waveguide. It may be suitable for use as part of a sensor system, and / or it may be provided to reflect a predetermined wavelength, thereby providing feedback to form a laser system. The device may include multiple Bragg gratings, for example in series and / or overlapping.

[0098] The sapphire optical device may include opposing Bragg gratings that provide an optical cavity for the laser system. The opposing Bragg gratings may be at substantially the same wavelength.

[0099] The device may include two opposing Bragg gratings, each grating being within a single-mode sapphire optical waveguide. The Bragg gratings may be laser-fabricated within the waveguide. The two opposing Bragg gratings may be at substantially the same wavelength.

[0100] The single-mode sapphire optical waveguide may be a depressed cladding waveguide.

[0101] The waveguide may be a periodic structure waveguide and / or a microstructure waveguide. The waveguide may be a photonic crystal waveguide, a microhole waveguide, a photonic bandgap waveguide, etc.

[0102] The sapphire optics may be laser modified sapphire optical fibers.

[0103] Sapphire optical fiber can be a step-index optical fiber, a photonic crystal optical fiber, a depressed cladding optical fiber, a microhole optical fiber, a photonic bandgap optical fiber, etc. Optical devices can be formed by etching, refractive index matching, etc.

[0104] The sapphire optical device may be a multimode sapphire optical fiber, and the single-mode sapphire optical waveguide may be disposed within the multimode optical fiber.

[0105] Thus, the sapphire optical device may include overlapping waveguides. The sapphire optical device may include nested waveguides, such as an inner waveguide (e.g., a single-mode waveguide) disposed within an outer waveguide (e.g., a pump waveguide having an optical core as a cladding of the inner waveguide). The optical device may be configured such that a propagating mode in a multimode optical fiber acts as a pump light for a single-mode waveguide. Thus, the optical device may include a plurality of waveguides, at least one of which is a single-mode waveguide and another of which is a multimode waveguide.

[0106] The sapphire optical device may include a doped region, wherein an optical core of the single-mode sapphire optical waveguide is within the doped region.

[0107] A single-mode sapphire optical waveguide (e.g., its optical core) may be off-centered relative to and located within a doped region. The optical core of a single-mode waveguide may be located at the edge of a doped region. A doped region may be off-centered within an optical device. A single-mode waveguide may be off-centered relative to and located within an optical device.

[0108] The single-mode sapphire optical waveguide may be eccentric relative to the multimode waveguide and located within the multimode waveguide, for example, the multimode waveguide is provided by the remainder of the optical fiber other than the single-mode waveguide, or by the cladding of the single-mode waveguide. Thus, the optical device may include a pump waveguide for propagating multimode light therein to pump the single-mode waveguide. The optical device may include a cladding-pumped single-mode sapphire optical waveguide. The pump waveguide may have unmodified optical fiber material inside (e.g., surrounded by it).

[0109] A single-mode sapphire optical waveguide may include a laser-fabricated cladding surrounding an optical core.

[0110] Thus, the laser fabricated cladding may include different optical properties than the optical core and may cooperate therewith to provide a single mode sapphire optical waveguide. The cladding may be substantially uniform, having a substantially uniform refractive index. Alternatively, the cladding may be a periodic structure as described herein, for example including periodically varying microstructures and optical properties in its cross section.

[0111] The laser production cladding may be a first cladding, and the optical device may include a second laser production cladding surrounding the first cladding.

[0112] The second cladding may have a different (eg, lower) refractive index than the first cladding. The first cladding may serve as a second optical core that cooperates with the second cladding to provide a multimode waveguide for pumping the gain medium of the single-mode waveguide.

[0113] According to a third aspect of the present invention, there is provided a laser system for generating laser light, comprising an optical device as described in the present invention in relation to the second aspect of the present invention. The optical device may provide a gain medium for the laser system.

[0114] The laser may be a mode-locked laser as described in the present invention. The laser system may include a feedback device as described in the present invention. The laser system may include a rotatable grating, which is therefore tunable. The laser system may include a diode pump laser or any suitable pump laser. The laser system may be configured to perform the method as described in the present invention in relation to the first aspect or any other aspect of the present invention.

[0115] According to a fourth aspect of the present invention, an optical amplifier is provided, comprising the optical device described in the present invention in relation to the second aspect of the present invention. The optical device may provide a gain medium of the optical amplifier.

[0116] The optical device may be an optical amplifier of the sensor system. Thus, the waveguide itself may be active and the losses of the waveguide may be compensated by optical gain, rather than having a separate amplifier (this also applies to use as a laser). Thus, the waveguide may be considered a distributed amplifier. Therefore, according to another aspect of the present invention, a sensor system is provided, comprising an optical amplifier as described herein in relation to any aspect of the present invention. The sensor system may be configured to perform a method as described herein in relation to any aspect of the present invention and may include any features as described herein in relation to the present invention.

[0117] By providing an optical fiber (e.g., a fiber laser or optical amplifier) ​​with collinear waveguides for multimode pumping and single-mode propagation, the present invention can reduce gain limitations and can also improve (or maximize) the worthwhile absorption of pump light, which in turn can use cheap, low-brightness pumps, such as diode lasers. Thus, the present invention can reduce the cost of, for example, Ti:sapphire lasers. The present invention can also provide built-in engineering advantages, such as pump redundancy, and fast, relatively easy pump modulation for controlling laser operation (e.g., by feedback stabilization of the power of the diode current). The present invention can provide an all-fiberized Ti:sapphire laser, for example, with an output power of 10 W, from a rugged package with built-in pump redundancy and, for example, fiber Bragg grating control of the spectral output.

[0118] According to a fifth aspect of the present invention, there is provided a sensor system comprising an optical device as described in the present invention in relation to the second aspect of the present invention. The sensor system may comprise an optical amplifier as described in the present invention in relation to the fourth aspect of the present invention. The sensor system may comprise any features as described in the present invention in relation to any aspect of the present invention.

[0119] According to another aspect of the present invention, an optical gain element is provided, the optical gain element comprising: a single-mode sapphire optical waveguide with optical gain. The advantage of this design is that sapphire can be used as a gain medium while maintaining the characteristics of a single transverse mode. This aspect may include any features of the present invention as described herein in relation to any other aspect of the present invention.

[0120] The sapphire optical waveguide may be a sapphire optical fiber. The sapphire optical waveguide may be within a sapphire optical fiber. An advantage of these designs is that the optical fiber may provide a longer interaction length with the gain medium.

[0121] The single mode waveguide may have an active gain medium. The single mode sapphire waveguide may be doped. The single mode sapphire waveguide may be doped with titanium. The single mode sapphire waveguide may be doped with rare earth elements. The single mode sapphire waveguide may be co-doped with two or more elements. The single mode sapphire waveguide may be doped with any one or more of ytterbium, erbium, neodymium, erbium, thulium, praseodymium, palladium, holmium, chromium, cobalt, iron, magnesium, manganese, nickel or carbon. The optical gain element may provide gain at a different wavelength than the pump laser. These designs have the advantage of allowing optical gain.

[0122] There may be a pump laser coupled to a sapphire optical waveguide. This design has the advantage of allowing the gain medium to be excited.

[0123] The sapphire waveguide can be single mode for the signal light and multimode for the pump light. This has the advantage of allowing the pump laser to be efficiently coupled to a divergent beam. High power pump lasers are typically divergent, so it is difficult to couple much light into a single mode fiber. However, this arrangement allows the pump light to excite ions in a single mode waveguide. Double clad fibers are known in fiber amplifier design, but typically include an outer sheath around the fiber to form an additional cladding. The fiber of the present invention may include an outer cladding around the sapphire fiber, or the pump light may be guided by a sapphire-air interface. Alternatively, a femtosecond laser can be used to write a multimode waveguide around a single mode waveguide, so that the fiber itself provides a double clad waveguide arrangement.

[0124] A single-mode sapphire optical fiber may include a single-mode waveguide including a core of a gain medium and a cladding surrounding the core. The single-mode waveguide may be single-mode for signal light. The single-mode sapphire optical fiber may include a multi-mode waveguide. The cladding of the single-mode waveguide may provide a core of the multi-mode waveguide, and a second cladding may be provided surrounding the core of the multi-mode waveguide. The second cladding may be formed of air, such that a boundary between the core of the multi-mode waveguide and the second cladding is provided by a sapphire-air boundary. Alternatively, the second cladding may be disposed within the sapphire optical fiber. The multi-mode waveguide may be multi-mode for pump light. This has the advantage of allowing a pump laser to be efficiently coupled with a divergent light beam into the core / gain medium of the single-mode waveguide.

[0125] Thus, according to another aspect of the present invention, there is provided an optical fiber including a nested waveguide. The optical fiber may be a sapphire fiber or any other suitable material. The nested waveguide may include an outer waveguide and an inner waveguide within the outer waveguide. The nested waveguide may include a pre-made cladding (e.g., a laser pre-made cladding). The cladding of the inner waveguide may be the core of the outer waveguide, and the outer waveguide may be used to pump the inner waveguide. The optical fiber may include any of the features described herein in relation to any aspect of the present invention.

[0126] A single-mode sapphire fiber may include a sapphire fiber having an optical waveguide along its length. The optical waveguide may be a depressed cladding waveguide or a microstructured waveguide or a photonic crystal waveguide. The waveguide may be etched. The waveguide may have air holes. The waveguide may be formed by irradiation. The waveguide may be eccentric within the fiber cross section. The sapphire fiber may have more than one waveguide. The fiber may be a double cladding waveguide. There may be a single-mode waveguide within a multimode waveguide. The single-mode waveguide may be offset within a multimode waveguide. The multimode waveguide cross section may be substantially circular, square, rectangular, hexagonal, triangular, pentagonal, heptagonal, octagonal, or circular in shape. Single or multiple waveguides may be written with a femtosecond laser. The signal wavelength may be more strictly restricted than the pump wavelength. The pump wavelength may be restricted at the sapphire-air interface. A combiner may be used to combine the signal wavelength and the pump wavelength. The combiner may be a dichroic mirror. The coupler may be a waveguide coupler within the sapphire fiber.

[0127] The signal wavelength may be coupled into a single mode waveguide and the pump wavelength may be coupled into a multimode waveguide. There may be more than one pump laser. The signal wavelength and the pump wavelength may be co-propagating, counter-propagating or both. The signal or laser wavelength may be 1064 nm or 1550 nm. The signal, laser or pump wavelength may be in the O-, E-, S-, C- or L-band. The signal, laser or pump wavelength may be in the near infrared, mid-infrared, visible or ultraviolet. The length of the single mode fiber may be greater than one of 1 cm, 2 cm, 5 cm, 10 cm, 50 cm, 1 m, 1.5 m or 2 m. The pump laser may be an argon ion laser, a Nd:YAG laser, a semiconductor laser or a diode laser. There may be tilted and / or chirped Bragg gratings.

[0128] A “double-cladding” type approach of separately but co-linearly guiding the pump and signal lasers helps decouple the achievable output power of the Ti:sapphire from the spatial brightness of the pump.

[0129] According to another aspect of the invention, there is provided a master-oscillator power amplifier (MOPA) in which a sapphire fiber is used as an amplifier (e.g., without feedback) to enhance a high quality seed (light) from a diode laser or semiconductor laser (e.g., a distributed feedback laser (DFB), a distributed Bragg reflector (DBR) laser, a Fabry-Perot laser, or an external cavity diode laser). The MOPA may include an amplifier chain. This aspect may include any features as described herein in relation to any other aspect of the invention.

[0130] According to another aspect of the invention, a laser is provided, the laser comprising: an optical gain element according to another aspect of the invention and a feedback element. This aspect may include any features of the invention as described herein in relation to any other aspect of the invention. The feedback element may include one or more reflectors. The feedback element may include one or more reflectors formed at a sapphire-air interface. The feedback element may include one or more reflectors that are thin film coatings. The feedback element may include one or more Bragg gratings. The Bragg gratings may be in a single mode waveguide. The Bragg gratings may be single mode fiber Bragg gratings. There may be one or more tunable Bragg gratings. The tunable Bragg gratings may be tunable by temperature, strain, voltage or current. The feedback element may include a highly reflective reflector and a partially reflective reflector. The feedback element may include a highly reflective Bragg grating and a partially reflective Bragg grating. The feedback element may include a diffraction grating. The angle of the diffraction grating may be adjusted so that the output of the laser may be tunable. The laser may include a seed laser to seed the optical gain element. The seed laser may be tunable. The laser may emit light of a substantially single wavelength. The laser might be located inside a spacecraft or a nuclear reactor.

[0131] The laser may be mode locked. The laser may be actively mode locked. The laser may be passively mode locked. The laser may include an optical modulator. The laser may include an acousto-optic modulator. The laser may include a saturable absorber. These features have the advantage of allowing very short optical pulses to be generated.

[0132] According to another aspect of the invention, there is provided a sensor system comprising an optical gain element according to any aspect of the invention, wherein a single mode sapphire optical fiber has at least one Bragg grating sensor. The Bragg grating may be tuned, for example by adjustment or stretching. Any suitable type of tuning may be used. This aspect may include any feature of the invention as described herein in relation to any other aspect of the invention.

[0133] According to another aspect of the present invention, there is provided a doped single-mode sapphire optical fiber. The doped optical fiber may provide an optical gain element according to the first aspect of the present invention. This aspect may include any feature of the present invention as described herein in relation to any other aspect of the present invention.

[0134] According to another aspect of the present invention, a method for amplifying an optical signal is provided, the method comprising: providing a single-mode sapphire optical waveguide; injecting pump light into the optical waveguide. This aspect may include any feature of the present invention as described in the present invention in relation to any other aspect of the present invention.

[0135] According to another aspect of the invention, there is provided a method of providing laser light, comprising the method according to any aspect of the invention, the method further comprising: providing a feedback element and injecting a pump light level above a threshold power level. This aspect may include any feature of the invention as described herein in relation to any other aspect of the invention.

[0136] According to another aspect of the invention, there is provided an optical gain element comprising a multimode sapphire optical fiber coupled to a pump laser, wherein the multimode sapphire optical fiber comprises a single mode waveguide. This aspect may include any feature of the invention as described herein in relation to any other aspect of the invention.

[0137] According to another aspect of the present invention, a polarization-maintaining single-mode sapphire optical fiber is provided. A single-mode waveguide may be present in the sapphire optical fiber. The cross-section of the waveguide may have asymmetry. For example, the core may be elliptical or other asymmetric shapes. A stress-inducing structure may be present in the cladding, for example, surrounding the single-mode waveguide. This aspect may include any features of the present invention as described herein in relation to any other aspect of the present invention.

[0138] A single-mode waveguide may support mainly (dominantly) a single transverse mode. However, there may be other modes that, although they do exist, exhibit higher losses. This may be due, for example, to the presence of a depressed cladding of finite diameter. For example, the loss of the other modes may be 10 times that of the dominant single-mode mode.

[0139] According to another aspect of the present invention, instead of a single-mode sapphire optical waveguide, a few-mode, reduced-mode or restricted-mode sapphire waveguide can be used instead. That is, the present invention also provides any aspect of the present invention, but includes replacing the single-mode waveguide with a reduced-mode waveguide. Although it is generally preferred to use a single-mode waveguide, many advantages also extend to waveguides that allow more than a single mode. The few-mode sapphire waveguide may have less than 1000 modes or less than 100 modes or less than 20 modes or less than 10 modes or less than 5 modes or more than 3 modes. This aspect may include any feature of the present invention as described in the present invention in relation to any other aspect of the present invention, except that a reduced-mode (i.e., digital-mode) waveguide may be used instead of the single-mode waveguide described in relation to these aspects. For example, the waveguide may be configured to suppress the propagation of certain modes (e.g., higher-order modes), but may still allow more than one mode, such as several modes, to propagate therein.

[0140] The single mode mentioned in the present invention refers to a single mode including multiple polarization states. The mode mentioned in the present invention refers to a mode including a predetermined wavelength or wavelength range. The term "sapphire" is considered to include doped sapphire, pure sapphire with impurities and Al2O3.

[0141] To achieve optimal efficiency for stimulated emission of light using single-mode sapphire optical waveguides (and, in fact, for optimal efficiency for stimulated emission of light using optical waveguides of any composition, such as within single crystal optical fibers according to another aspect of the present invention), the pump light will be absorbed only in the active waveguide and not wasted by absorption in the active material surrounding the waveguide. However, cheap pumps are divergent and couple into a wide area surrounding the waveguide. Since it is difficult to dope only the core of the waveguide, the light from the cheap pump may couple into the active (i.e., doped) material surrounding the waveguide. By providing multiple cores within the doped region, the absorbed pump power can be used more efficiently. The light emitted by the individual cores can then be coherently combined to form a single output of higher power.

[0142] The optical fiber may include a plurality of single-mode sapphire optical waveguides. Each waveguide of the plurality of single-mode sapphire optical waveguides may include any aspect of the single-mode sapphire optical waveguides described herein. In particular, each waveguide of the plurality of single-mode sapphire optical waveguides may be a depressed cladding waveguide, may be a multi-layer depressed cladding waveguide (e.g., including multiple layers of modified optical fiber material providing multiple layers of cladding), or may be a photonic crystal waveguide (e.g., including a cladding including a periodic array of regions within the cladding having a modified refractive index).

[0143] Multiple single-mode sapphire optical waveguides can be parallel, can be adjacent to each other, and / or can be arranged to include most of the cross-section of the optical fiber. For example, three waveguides can be arranged in a triangle in the cross-section of the optical fiber. Seven waveguides can be arranged in the cross-section of the optical fiber so that six of the waveguides are distributed in a hexagonal pattern around the central waveguide. Multiple waveguides can be arranged in a closely packed hexagonal manner in the cross-section of the optical fiber. This arrangement may be particularly useful in optical fibers with hexagonal cross-sections. In general, any suitable number of parallel waveguides can be used, and the waveguides can be arranged to include as much cross-section of the optical fiber as possible. The waveguides can be arranged to contain as much doping material as possible in the optical fiber.

[0144] Each waveguide may have its own distinct cladding, or they may share a common cladding.Each waveguide may comprise an unmodified portion of optical fiber surrounded by a cladding.

[0145] Each of the plurality of single-mode sapphire optical waveguides may include a Bragg grating or a plurality of Bragg gratings. Each of the single-mode sapphire optical waveguides in the plurality of single-mode sapphire optical waveguides may include an optical cavity by providing a feedback device within the single-mode sapphire optical waveguide. The feedback device may be a pair of opposing Bragg gratings or any suitable device.

[0146] The Bragg grating pairs within each single-mode sapphire optical waveguide may be at the same wavelength or overlapping wavelengths as the Bragg grating pairs within other waveguides of the plurality of waveguides. Thus, the optical fiber may be configured to provide a single coherent (coherent) output from the plurality of waveguides. The optical fiber may include an optical coupler (e.g., a directional coupler or an evanescent field coupler) for combining the output of each waveguide of the plurality of single-mode sapphire optical waveguides. The optical fiber may be equipped with one or more phase shifters operably connected to one or more waveguides. For example, each additional waveguide may have a corresponding phase shifter associated therewith and operable to help combine the outputs of the plurality of waveguides into a single optical fiber.

[0147] Alternatively, the Bragg grating of each waveguide of the plurality of single-mode sapphire optical waveguides can be at a different wavelength than the Bragg gratings in the other waveguides of the plurality of single-mode sapphire optical waveguides within the optical fiber. The optical fiber can then generate a range of different wavelengths of light.

[0148] Light from each waveguide may exit the sapphire fiber and enter one of the respective plurality of auxiliary fibers. Some or each of the auxiliary fibers may include, or be in operative communication with, a phase shifter before the coupler couples the light from each of the auxiliary fibers into a single output fiber. Thus, the phase shifter may alter the phase of the light in the respective auxiliary fibers such that constructive interference of the light produces a single output beam of increased power.

[0149] In some embodiments, including those providing a plurality of single-mode sapphire optical waveguides, bulk sapphire may be used instead of optical fibers. For example, a planar sapphire substrate may be used that includes a plurality of single-mode sapphire optical waveguides.

[0150] According to another aspect of the invention, another hard crystal may be used in place of sapphire. For example, diamond may be used in place of sapphire. That is, the invention also provides any aspect of the invention described herein, but includes a different fiber material in place of the sapphire fiber. Although the use of sapphire fiber may be preferred, many of the advantages of the invention also extend to fibers including other materials. Therefore, this aspect may include any of the features of the invention as described herein in relation to any other aspect of the invention, except that another fiber may be used (e.g., a fiber including a material with a high sapphire (Al2O3) content, diamond, yttrium aluminum garnet (YAG, Y3Al5O 12 ), germanosilicate, borosilicate, or lithium niobate (LiNbO3)) to replace the sapphire optical fiber described in these aspects.

[0151] According to another aspect of the invention, there is provided a method of exciting the emission of light, comprising using a single-mode crystal optical waveguide as a gain medium. The crystal may be in an optical fiber, and an optical fiber may be provided. The crystal may be a single crystal. Thus, the method may include the emission of exciting light, comprising using a single-mode single crystal waveguide as a gain medium, wherein the single-mode single crystal waveguide is disposed within the optical fiber. Viewed from another aspect, the invention provides an optical device, comprising an optical gain medium, the optical gain medium comprising a single-mode crystal optical waveguide, and may comprise a single-mode single crystal optical waveguide. Viewed from another aspect, the invention provides a single-mode crystal optical waveguide, and may provide a single-mode single crystal optical waveguide. Viewed from another aspect, the invention provides an optical fiber comprising a single-mode crystal optical waveguide. The optical fiber may comprise a single-mode, single-crystal optical waveguide, i.e., a single-mode waveguide within a single crystal optical fiber. Thus, according to these aspects of the invention, the optical fiber is a crystal optical fiber (e.g., a single crystal optical fiber) having a waveguide therein. The optical fiber may comprise other materials, such as a cladding, but the portion of the optical fiber comprising the waveguide may be crystalline and / or single crystal.

[0152] A single crystal material may be a single crystal. It may be a material in which the crystal lattice is continuous throughout the sample (such as an optical fiber) and is uninterrupted to the edge of the sample, i.e., no part of the sample (such as an optical fiber) is separated from another part of the sample material by a grain boundary.

[0153] The single-mode crystal optical waveguide may be used in place of the single-mode sapphire optical waveguide described in any aspect of the present invention and described in conjunction with the following figure. Thus, the single-mode crystal waveguide may include any of the features described in connection with any aspect of the present invention. The optical fiber and / or waveguide may be replaced with another suitable material in place of sapphire as described in any other aspect of the present invention, such as any material suitable for use as an optical fiber and / or gain medium. In particular, any single crystal material may be used in place of sapphire.

[0154] The crystal may comprise diamond, a silicate mineral, an oxide mineral, a phosphate, a carbonate or a halide. The crystal may comprise an aluminum silicate, such as a garnet, for example, yttrium aluminum garnet (YAG), lutetium aluminum garnet (LuAG). The crystal may comprise an aluminum oxide, such as corundum (e.g., sapphire or ruby) or CaGdAlO4 (CAlGO). The crystal may comprise zirconium dioxide (zirconia). The crystal may comprise any material suitable for use as an optical fiber.

[0155] The single mode crystal optical waveguide may be doped. The single mode crystal optical waveguide may be doped with titanium. The single mode crystal optical waveguide may be doped with rare earth elements. The single mode crystal optical waveguide may be co-doped with two or more elements. The single mode crystal optical waveguide may be doped with any one or more of titanium, ytterbium, erbium, neodymium, thulium, praseodymium, palladium, holmium, chromium, cobalt, iron, magnesium, manganese, nickel, carbon or zinc. These elements have the advantage of allowing optical gain within the crystal. Therefore, the single mode crystal optical waveguide may be doped to provide a gain medium.

[0156] In particular, the single mode crystal optical waveguide may include ytterbium doped yttrium aluminum garnet.

[0157] According to another aspect of the invention, there is provided an optical device comprising a crystal fiber, wherein a plurality of waveguides are included in the crystal fiber. The crystal fiber may be a single crystal fiber. The invention may include using the optical device as a gain medium. The waveguides may be adjacent to each other and may be parallel, for example guiding light within the same length of the fiber. Thus, the fiber may be a multi-core, single crystal fiber. One or more (or all) of the waveguides may be single mode waveguides. The crystal may be sapphire or yttrium aluminum garnet (YAG) or any other suitable material. The crystal may be doped. The dopant may be a rare earth element. The dopant may be chromium and / or ytterbium, and / or any suitable dopant for use as a gain medium, for example. The waveguide may be a depressed cladding waveguide, for example as described herein in connection with any aspect of the invention. The waveguide may be a microstructure waveguide, for example as described herein in connection with any aspect of the invention.

[0158] The waveguide may be a photonic crystal or photonic bandgap waveguide, such as described herein in relation to any aspect of the invention. The waveguide may be laser written. There may be a Bragg grating within the waveguide. The optical device and / or crystal fiber may include any of the features described herein in relation to any other aspect of the invention.

[0159] Although individual aspects of the invention are presented above, it will be appreciated that features described with respect to a particular aspect may be used in combination with any other aspect as desired. BRIEF DESCRIPTION OF THE DRAWINGS

[0160] Exemplary embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 A schematic diagram of the optics is shown; Figure 2 Another schematic diagram of the optics is shown; Figure 3 Another schematic diagram of the optics is shown; Figure 4 Another schematic diagram of the optics is shown; Figure 5a and5b A schematic cross-sectional view showing an alternative optical waveguide design within an optical device; Figure 6a and 6b A cross-sectional schematic diagram showing an alternative optical waveguide design; Figure 7 A schematic diagram of the sensor system is shown; Figure 8a and 8b shows a schematic cross-sectional view of an optical device including a plurality of optical waveguides; Fig. 9 shows a schematic cross-sectional view of an optical device including a plurality of optical waveguides; Fig.10a A cross section of the optic perpendicular to its longitudinal length is shown; Fig.10b Shows the length of the Fig.10a Cross section of an optical device; Fig.11 shows a schematic diagram of an optical device including a plurality of waveguides; Fig.12 A schematic diagram of an alternative optical device including multiple waveguides is shown; Fig.13a A cross section of the optic perpendicular to its longitudinal length is shown; Fig.13b Shows the length of the Fig.13a Cross section of an optical device. DETAILED DESCRIPTION

[0161] Figure 1 A schematic diagram of an apparatus comprising an optical device 100 arranged as a gain element is shown. This figure and subsequent figures are schematic diagrams only and are not drawn to scale. The optical device comprises a sapphire optical fiber 100. Within the sapphire 100, a single-mode optical waveguide 101 is arranged along the length of the optical fiber 100. The single-mode sapphire optical waveguide 101 comprises an optical core 101'. The sapphire waveguide 101 is doped with titanium or an alternative dopant, such as chromium or any other doping element that enables the waveguide 101 to act as a gain medium. For example, the waveguide 101 can be written into the sapphire optical fiber 100 using a femtosecond laser. Such laser modification may result in a reduction in the refractive index of the sapphire in the area exposed to the femtosecond laser. Alternatively, the waveguide 101 may be formed in another way, such as by modifying the exposed area by irradiation, such as to form an inner cladding. The waveguide 101 may be provided by laser modification of an area of ​​the optical fiber 100 surrounding the core 101', thereby forming a cladding within the optical fiber 100 (see, for example, Figure 5a The unmodified optical core 101 ′ and the cladding 402 may then cooperate to provide the waveguide 101 .

[0162] When the optical device 100 is used to emit light, the pump laser 102 is used to excite the ions in the waveguide 101 to a higher energy state. For example, the pump laser 102 can be a 514.5 nm argon ion laser or a 532 nm frequency-doubled Nd:YAG laser. The pump laser can also be a diode laser. Any suitable way of pumping the gain medium can be used. Light from the pump laser 102 is injected into the waveguide 101 via a dichroic mirror 103 and a lens 104. Input light 105 enters through an input light isolator 106 (e.g., a Faraday isolator) and is reflected on the dichroic mirror 103. The input light 105 reflected from the dichroic mirror passes through the lens 104 and is coupled into the waveguide 101. The input light 105 causes stimulated emission of light from the excited ions in the waveguide 101, particularly in the optical core 101'. The photons emitted in this way have the same wavelength as the input light 105. The ions are excited to emit photons of the same wavelength as the input light 105, and thus the emitted light is amplified within the waveguide 101. The amplified light exits the waveguide 101 at one end of the optical fiber 100 and passes through an output optical isolator 107 (which can be the same type of isolator as the input isolator 106). The optical isolator 107 is configured to prevent the emitted light from being reflected back into the waveguide 101. Finally, any remaining pump light is filtered by a filter 108. Thus, in some uses of the optical device 100, the input light 105 is amplified within the waveguide 101.

[0163] In an alternative embodiment of the present invention, the optical device 100 may include bulk sapphire as Figure 1 An alternative to the sapphire fiber shown. A single-mode optical waveguide can then be provided within bulk sapphire.

[0164] Figure 2 A configuration of the optical device 100 is shown in which the waveguide 101 is pumped in both the forward and reverse propagation directions. That is, light from a first pump laser 102a enters the waveguide 101 at a first end 101a of the waveguide 101, and light from a second pump laser 102b enters the waveguide 101 at another end 101b of the waveguide 101 opposite the first end 101a. Light from the second pump laser 102b is coupled into the waveguide 101 from the end 101b via a dichroic mirror 103b and a lens 104b. The waveguide 101 may alternatively be pumped only from the backward direction, i.e., using only the second pump laser 102b. That is, the waveguide 101 may be used in either direction.

[0165] Figure 3 The use of an optical device 100 (such as Figure 11 ) as the laser gain medium. Therefore, the laser utilizes a doped sapphire single-mode waveguide 101 as the gain medium. The waveguide 101 includes Bragg grating reflectors 201, 202 located within the optical core 101' of the waveguide 101, forming a cavity in the waveguide 101 between them. The waveguide 101 is optically pumped by a pump laser 102. The Bragg gratings 201, 202 have a periodic variation in the effective refractive index so that the gratings reflect within a narrow wavelength range. This wavelength range falls within the gain window of the doped sapphire within the waveguide 101 and the optical core 101'. The Bragg gratings 201, 202 provide feedback to achieve laser operation. The Bragg grating 201 has a very high reflectivity, while the Bragg grating 202 can have a lower reflectivity to allow light to be emitted from the waveguide 101 and the end of the optical fiber 100. The optical isolator 107 prevents light from being reflected back to the waveguide 101, and the filter 108 removes any pump light emitted. Alternatively, the laser can be pumped in a counter-propagating or co-propagating direction, such as Figure 2 shown.

[0166] The laser system may include one or more gratings (e.g., Bragg gratings) external to the optical fiber 100, and the angles of these gratings may be adjusted so that the output of the laser system is tunable. The optical fiber 100 may include a single Bragg grating in the waveguide 101 as part of the optical cavity for laser generation, or a reflector may be provided external to the optical fiber 100.

[0167] In alternative embodiments of the present invention, reflection of light within the waveguide 101 may be provided by a mirror external to the waveguide, or by a waveguide / air boundary at the waveguide entrance and / or exit. That is, any suitable optical cavity may be used, and the optical fiber 100 and waveguide 101 may include at least a portion of the optical cavity.

[0168] The Bragg gratings 201, 202 may also be used as part of a sensor system, because the optical fiber 100 is sapphire and therefore very durable, and the optical fiber may be placed in extreme environments, such as engines suitable for aerospace use. The optical fiber may be placed in an environment where it is exposed to high levels of radiation, such as a nuclear reactor. The conditions to which the optical fiber is exposed may change the configuration of the optical fiber, in particular the configuration of the Bragg grating (or gratings, if multiple gratings are present), thereby changing its optical properties. Therefore, the configuration of the grating can be used to measure the properties of the environment in which it is placed.

[0169] Figure 4Another example of a laser is shown. It is difficult to efficiently couple light from a pump laser into a single-mode waveguide because the divergence angle of the pump laser is typically wide. However, sapphire fiber 100 is naturally a multimode waveguide, with the guiding occurring at the interface of sapphire and air (so the air is actually the cladding). A divergent pump laser 300 is used to launch pump light 301 into a multimode waveguide, i.e., fiber 100. The higher-order propagation modes of the pump light 301 can then be guided within the larger fiber 100, while the single mode of the signal pump light 301 is guided along the core 101' of the single-mode waveguide 101. The pump light 301 then overlaps the single-mode waveguide 101 and excites the ions in the single-mode waveguide 101 to a higher energy state. Some of the excited ions within the single-mode waveguide will spontaneously decay back to a lower energy state, emitting photons. These photons will excite other excited ions within the single-mode waveguide to emit more (phase-coherent) photons. Feedback is provided by two Bragg gratings 201, 202, which reflect light of a specific wavelength, thereby producing a narrow spectrum.

[0170] Providing a single-mode waveguide in a multimode waveguide has many advantages, such as Figure 4 As shown. These advantages include that low spatial mass pumps can be efficiently coupled into single-mode waveguides. In addition, power scaling can be achieved by coupling multiple pumps into the fiber. In addition, the pump light mode and the laser mode can have a small mode radius, which can be maintained over longer distances than can be achieved when relying on diffraction, which in turn reduces the threshold pump power (for example, Ti:sapphire has an "intrinsic" threshold that is about 50 times that of Nd:YAG, a commonly used laser gain medium). Optically pumped lasers require a certain amount of pump power before any lasing occurs. Therefore, it is desirable to have a low threshold pump power. In addition, the pump light can be directed so that pump absorption occurs over distances equivalent to several absorption lengths in Ti:sapphire, but without the adverse effects of diffraction (which causes the mode radius to become too large to be practical when integrated over this distance). These advantages help overcome the common shortcomings associated with doping level, pump beam quality, and minimum pump mode size that occur in bulk Ti:sapphire lasers.

[0171] FIG5 shows different embodiments of forming a single-mode waveguide in sapphire. FIG5(a) shows a cross section of an optical device 100 including a microstructure waveguide 401 (e.g., a periodic structure waveguide, a photonic crystal fiber, a microhole fiber, a photonic bandgap fiber, etc.). The optical device 100 includes an unmodified sapphire core 101' and a microstructure cladding 402 in the waveguide 401, so that there is a periodic change in the refractive index on the cross section of the optical device 100. The waveguide 401 is configured to suppress the propagation of all modes except the single mode in the core 101' by the cladding 402, and the optical device 100 is a single-mode sapphire fiber 100. These fibers may be referred to as photonic crystal fibers or photonic bandgap fibers. The periodic structure may be formed by changing (lowering) the refractive index in the exposed region 403 using a femtosecond laser. They can also be prepared by using, for example, an etching process or as described in A. Rodenas et al. "Three-dimensional femtosecond laser nanolithography of crystals," Nature Photonics 13, 105-109 (2019) https: / / doi.org / 10.1038 / s41566-018-0327-9, the contents of which are incorporated herein in their entirety. The inventors have discovered that the methods and features disclosed therein can be combined with the features of the present invention.

[0172] FIG5( b ) shows a cross section of an optical device 100 including a depressed cladding single mode waveguide 410 . The optical device 100 includes a core 101 ′ of unmodified sapphire surrounded by a cladding 411 . The cladding 411 has a reduced refractive index compared to unmodified sapphire and is formed by exposing sapphire in the cladding region to femtosecond laser pulses, such as described in GB1712640.0 . Thus, the core 101 ′ has a higher refractive index than the cladding 411 (e.g., the refractive index of the core 101 ′ may be 5×10 greater than the refractive index of the cladding 411 ). -4 Up to 5×10 -2 ). Thus, the cladding 411 cooperates with the core 101' to provide the waveguide 410. The cladding 402, 411 need not extend to the edge of the optical fiber 100 and therefore may be surrounded by unmodified sapphire.

[0173] It may be advantageous to make the diameter of the cladding 411 as large as possible to reduce radiation losses. In this example, the core 101' has an elliptical cross-section to form a polarization-maintaining optical waveguide 410. This is particularly useful for maintaining the polarization state when the optical fiber 100 is bent. The depressed cladding waveguide 410 can also be used, for example, Wang et al., "Single-mode sapphire fiber Bragg grating," Optics Express 30, 15482-15494 (2022) https: / / doi.org / 10.1016 / 0 ... . org / 10 . 1364 / OE ; The inventors have discovered that the methods and features disclosed therein can be combined with the features of the present invention.

[0174] In the optical devices of Figures 5(a) and 5(b), each core 101' (and therefore each waveguide 401, 410) is off-center compared to the center of the corresponding fiber cross section shown. The sapphire surrounding the core 101' can act as a multimode optical waveguide. Off-centering the single-mode waveguide within the multimode optical waveguide has an advantage because an increased proportion of the pump light provided to the multimode optical waveguide can be coupled into the single-mode optical waveguide 401, 410.

[0175] In alternative embodiments, the waveguide may have reduced cross-sectional symmetry to achieve similar advantages, providing increased coupling of pump light into the gain medium of the core. For example, the cross-section of the multimode waveguide may be D-shaped.

[0176] Furthermore, while the entire sapphire fiber 100 may be used as a multimode fiber surrounding the single mode fibers 101 , 401 , 410 , the concept of having one waveguide within another waveguide may be taken further to provide nested fabricated waveguides within the fiber 100 .

[0177] FIG6 shows a single-mode waveguide 410 in a sapphire optical fiber 100 formed using an alternative method. FIG6(a) shows a cross section of an optical device 100 formed from a sapphire optical fiber 100, wherein there is an inner single-mode waveguide 410 including a core 101', a first cladding 411 surrounding and adjacent to the core 101' of the waveguide 410, and a second outer cladding 501 surrounding and adjacent to the first cladding 410. The refractive index of the first cladding 411 is lower than that of the core 101', while the refractive index of the second cladding 501 is lower than that of the first cladding 411. Thus, two nested waveguides are provided, a first single-mode waveguide provided by the core 101' cooperates with the first cladding 411, and a second multimode waveguide provided by the first cladding 411 cooperates with the second cladding 501. In this example, the multimode pump light is confined within the boundaries of the second cladding 501 and propagates within the first cladding 411. That is, the first cladding 411 actually provides the optical core of the second waveguide.

[0178] Thus, the first cladding 411 provides a cladding to form the first single-mode optical waveguide 410. The first cladding 411 also forms the "core" of the second multimode optical waveguide by cooperating with the second cladding 501. The cladding of the second multimode optical waveguide is provided by the second cladding 501. The laser-modified sapphire forming the second cladding 501 can be formed by exposing the sapphire in this region to a higher dose of femtosecond laser (e.g., at a higher pulse energy and a faster repetition rate) than the sapphire in the region of the first cladding 411, so that the refractive index of the second cladding 501 is lower than that of the first cladding 411.

[0179] The above and Figure 4 The advantages associated with the illustrated arrangement also apply to the nested waveguide arrangement shown in Figure 6. Multimode light propagating in the outer waveguide can be used to pump the gain medium of the core 101' of the inner (single mode) waveguide.

[0180] As described above, the optical device 100 including a multimode waveguide and a nested single-mode waveguide may be referred to as a cladding-pumped optical device 100 or a cladding-pumped optical waveguide 101. It is best to ensure that only the core is doped in the cladding-pumped optical waveguide, because if the first cladding (i.e., the fiber core or the cladding-pumped waveguide) is doped, it may absorb the pump power without contributing to the optical gain. A method for selectively doping sapphire optical fibers to form an inner core of doped material is described in V, N, Kurlov et al., "Growth of sapphire core-doped fibers", Journal of Crystal Growth 191 (3) 520-524 (1998), the contents of which are incorporated herein in their entirety. The inventors have found that the methods and features disclosed therein can be combined with the features of the present invention. However, it is challenging to confine the doping region to the single-mode optical waveguide core 101', at least because the core is very small.

[0181] FIG6( b ) shows a cross section of an optical device 100 formed of a sapphire optical fiber 100 having a doped region 510 that has been selectively doped with Ti 3+ . Therefore, the doped region 510 has the ability to serve as a gain medium. A core 101', a first cladding 411 surrounding and adjacent to the core 101', to thereby provide a waveguide 410, and a second cladding 501 surrounding and adjacent to the first cladding 411. The core 101' is eccentric within the cladding 411. The regions of the core 101', the first cladding 411, the second cladding 501, and the doped region 510 are designed to minimize the overlap of the gain region (i.e., the doped region 510) with the first cladding 411 forming the core of the multimode waveguide. Therefore, the overlap of multiple propagation modes in the pump waveguide (i.e., the first cladding 411) with the doped region 510 is reduced (even minimized). Therefore, the energy loss of the pump mode outside the core 101' of the single-mode waveguide to excite the gain material is reduced. The shape of the doped region 510 can be configured to maximize the overlap with the optical core 101' of the single-mode waveguide while minimizing the overlap with the first cladding 411. For example, the doped region 510 of the optical fiber may have an elliptical cross-section, a rectangular cross-section, or any suitable shape.

[0182] Thus, the cladding of a single-mode sapphire optical fiber may include doped and undoped portions. Most of the cladding of a single-mode waveguide may be undoped. The optical fiber may include a doped region outside the cladding.

[0183] The method may include modifying the optical fiber 100 so that the core 101 ′ is located at the edge of the doped region 510 , thereby improving efficiency by reducing the loss of pump power in the region outside the core 101 ′ of the single-mode waveguide 101 , 410 .

[0184] Many types of lasers can be made or provided using the doped single-mode sapphire fiber described in the present invention. The laser can be passively mode-locked (e.g. using a saturable absorber) or actively mode-locked (e.g. using a modulator). The laser can be a narrow bandwidth laser or a single frequency laser. The laser can be diode pumped (e.g. using a DFB or DBR semiconductor laser). The laser can be a master oscillator power amplifier (laser followed by an amplifier). The laser may be locked by light injection from a seed laser. The laser can be used for applications such as gas sensing and quantum technology. The laser can be a wavelength swept laser and can be used for applications such as optical coherence tomography. There are many possible configurations (e.g. ring laser, figure eight). It would be beneficial to keep the single-mode sapphire fiber polarization-preserving to avoid the need for polarization control.

[0185] Figure 7A sensor system using a single-mode sapphire optical fiber 100 is shown, where a single-mode waveguide is used as a gain medium to mitigate propagation losses. The system has a length of sapphire optical fiber 100, which includes a waveguide 110 along its length, and a series of fiber Bragg gratings (601a, 601b, etc.) arranged along the length of the waveguide 110. The sapphire waveguide 110 is doped with Ti 3+ , so that the sapphire waveguide 110 can act as a gain medium (thereby playing a role in reducing waveguide losses). The sapphire fiber 100 is excited by the pump laser 102. Each of the Bragg gratings 601a, 602b is at a different wavelength and is sensitive to temperature. The wavelength of each Bragg grating 601a, 601b (e.g., the center wavelength of the Bragg reflection spectrum) is determined by scanning the tunable laser 610. The light reflected from the Bragg gratings 601a, 601b is guided to the photodetector 611 via the beam splitter 612. The splitter can be a separate quartz coupler (e.g., a fused silica coupler) or a waveguide splitter in sapphire (e.g., a Y splitter, a multimode interference coupler) or a separate optical circulator. The Bragg grating is a sensor. For example, strain or temperature near each Bragg grating will change its wavelength. The laser 610 can be tuned (scanned) in wavelength to measure the center wavelength of each Bragg grating, so the device shown is used as a sensor system.

[0186] Instead of using FBGs, distributed sensing, such as sensing based on backscattered light, combined with sapphire optics according to any aspect of the invention may be used.The sensor may be based on Rayleigh scattering, Brillouin scattering or Raman scattering.

[0187] Figure 8a A cross section of an optical device 100, 800 including a plurality of depressed cladding single mode waveguides 810 is shown. Thus, the waveguides are arranged in parallel within the optical fiber. The optical device 100, 800 includes a plurality of unmodified sapphire cores 101', 801', each surrounded by a respective cladding 411, 811, which is modified to include a reduced refractive index compared to the unmodified sapphire cores 101' and 801'. In this example, the cores 101', 801' are elliptical so that the waveguides maintain polarization. In this example, most of the sapphire has been doped, as shown by the points on the cross section in the figure. Selective doping of sapphire is difficult, and it may be easier or more desirable to dope a larger area of ​​the optical device 100, 800 or even the entire optical fiber. By providing multiple waveguides within the optical device 100, 800, efficiency losses due to coupling of pump light into the gain medium (i.e., doped sapphire) outside the waveguide can be minimized. The waveguides are arranged in a closely packed configuration to maximize coverage within the optical fiber. In this example, seven waveguides 101, 810 are provided in a closely packed configuration to form a hexagon.

[0188] Figure 8b Another example of an optical device 100, 820 is shown that includes a plurality of depressed cladding single mode waveguides 101, 830. However, in this example, the majority of the sapphire between the cores 101', 831' has been modified to form a cladding 411, 831 of each waveguide 101, 830 that has a reduced refractive index compared to the unmodified sapphire of the cores 101', 831'. Thus, the waveguides share a common depressed cladding.

[0189] Figure 5b The description of the depressed cladding waveguide 410 applies to each waveguide 101 , 810 , 830 .

[0190] Fig. 9 A cross section of an optical device 100, 900 including a plurality of microstructured waveguides 101, 910 (e.g., periodic structured waveguides, photonic crystal fibers, microhole fibers, photonic bandgap fibers, etc.) is shown. The optical device 100, 900 includes a plurality of cores 101', 901' of unmodified sapphire and a microstructured cladding 402, 902 (e.g., a periodic array of laser modified regions), so that there is a periodic variation in the refractive index across the cross section of the optical device 100, 900. In this example, most of the sapphire between the cores 101', 901' has been modified to form the cladding 402, 902 of each waveguide 101, 910, and the core is formed where there is a gap in the microstructure.

[0191] Figure 5a The description of the microstructured waveguide 401 applies to each waveguide 101 , 910 .

[0192] FIG10( a ) shows an optical device 100 , 1000 including an optical fiber 100 , 1002 shown in a cross section perpendicular to the longitudinal length of the optical fiber. The optical device 100 , 1000 includes a plurality of waveguides 101 , 1010 , such as those shown in FIG8( a ), FIG8( b ) and / or Fig. 9 The waveguide in.

[0193] Figure 10(b) shows a cross section of the optical device 100, 1000 parallel to the longitudinal length of the optical fibre. Each waveguide 101, 1010 comprises a pair of Bragg gratings 1011, one at either end of the waveguide, to provide feedback.

[0194] Each pair of Bragg gratings 1011 can be at the same Bragg wavelength as the other pairs of Bragg gratings 1011 to provide a single coherent output from the optical device 100, 1000. The output of each waveguide 101, 1010 is coherently combined with an optical coupler 1003 (either a directional coupler or an evanescent field coupler). In this example, the optical coupler is formed within the optical fiber 1002.

[0195] Preferably, the light from each waveguide should be in phase so that when the light is combined (merged), constructive interference occurs. In some examples, one or more waveguides can be operably connected to a phase shifter (e.g., a thermal phase shifter, etc.). They can also be passive or passively trimmed. The optical fiber can be fixed in place, for example, so that it does not bend, for example, to avoid changes in phase between waveguides. The phase of the light within the waveguide can be controlled by modifying the length of the waveguide or modifying the "effective refractive index" of the waveguide. The effective refractive index depends on the core size and the refractive index difference between the core and the cladding. For example, the effective refractive index of one or more waveguides can be modified by: laser writing lines through the (initially unmodified) core to reduce the refractive index of the core; laser writing a ring structure around the core; and / or laser modifying the cladding (e.g., "rewriting the cladding") to further reduce the refractive index of the cladding. Therefore, the optical device includes an optical coupler configured to combine (merge) light from multiple waveguides into a reduced number of waveguides, such as into a single waveguide.

[0196] Instead of each pair of Bragg gratings having the same wavelength, each waveguide may have a pair of Bragg gratings of different wavelengths to allow a range of different wavelengths to be generated.Any suitable configuration of Bragg gratings may be provided as required.

[0197] Fig.11 An optical device 100, 1000 as a laser is shown in Figures 10(a) and 10(b). The optical device 100, 1000 includes multiple parallel waveguides within a single optical fiber. Light from at least one pump laser 102, 1100 is injected into each waveguide 101, 1010 to excite ions in the waveguides 101, 1010. The same pump laser 102, 1100 can be used to pump all parallel waveguides so that the power of the pump lasers 102 and 1100 is used as much as possible. The input light causes stimulated emission of excited ions in the waveguides 101, 1010. The light emitted from the waveguides is coupled into a single waveguide within the optical fiber 100, 1000 to form a laser output, which is provided to an isolator 106, 1120 and a filter 108, 1130 to filter the pump light from the laser output.

[0198] Fig.12Another optical device 100, 1200 is shown that includes multiple waveguides 101, 1210 in a single optical fiber as a laser. Light from at least one pump laser 102, 1220 is injected into each waveguide 101, 1210 to excite ions in the waveguide 1210. The same pump laser 102, 1220 can be used to pump all parallel waveguides. The output of each waveguide 101, 1210 is provided to an auxiliary optical fiber 1230. One or more auxiliary optical fibers 1230 are in operative communication with a phase shifter 1232. Multiple phase shifters 1232 can be provided so that each phase shifter 1232 is dedicated to only one auxiliary optical fiber 1232. Because it may be difficult to maintain phase control within and between waveguides of an optical fiber, the phase of each phase shifter 1232 can be independently adjusted to ensure that the light output from each waveguide 101, 1210 is in phase and constructively interferes to produce a single output beam with higher power (e.g., increased power compared to an optical fiber with only a single waveguide therein).

[0199] Figure 10 (b) Fig.11 and Fig.12 Jagged breaks in the fibers 1002 and 1202 are included to truncate the image, but the fibers are continuous between the Bragg gratings and may be longer than shown. Any suitable length of fiber may be used. Of course, these figures are merely schematic diagrams for illustrating the concepts.

[0200] Figures 13(a) and 13(b) show an optical device 100, 1300 comprising a planar substrate 1302 extending in a longitudinal direction. Figure 13(a) shows a cross section of the optical device 100, 1300 perpendicular to its longitudinal length, and Figure 13(b) shows a cross section of the optical device 100, 1300 parallel to the longitudinal length. The planar substrate comprises a plurality of waveguides 101, 1310, each waveguide comprising a pair of Bragg gratings 1320 and a coherent coupler 1330. Any suitable shape of material may be used, and although optical fiber has associated advantages, other shapes of material may also be used and have associated advantages.

[0201] The optical device 100, 1300 may act as a laser, in which case pump light is launched into the substrate and the output of the waveguides 101, 1310 is combined with a coherent coupler 1330 to form a single coherent combined output. By using a planar substrate, it is easier to ensure that the outputs remain in phase.

[0202] Although the figures and embodiments of the present invention are described with reference to optical fibers including sapphire, the optical fibers may include any suitable material in place of sapphire, and the present invention extends to such embodiments. For example, crystalline materials may be used in place of sapphire, such as crystal fibers, such as single crystal fibers. As described herein, any suitable material may be used.

[0203] Various changes and modifications may be made without departing from the broader aspects and spirit of the present invention. Any value or range provided may be replaced with another value or range to achieve the desired result. When the singular is used (e.g., "an," "a," "the," "this"), it is considered to be one or more terms. When the word "comprising" is used, it is considered to include subsequent method steps and / or elements, but may also include additional method steps or elements. The steps described in the method of the present invention may be performed in any order or simultaneously. A single step or combination of steps may be deleted from any method without losing the desired effect. A single element or combination of elements may be removed from any device without losing the desired effect. In order to gain advantage, parts of any example may be combined with parts of any other example. If an element or step is stated as optional, it should not be considered to imply that other elements or steps are essential. Those skilled in the art will understand that any combination of features is possible within the scope of the present invention.

Claims

1. A method for stimulating light emission, characterized in that: This includes using a single-mode sapphire optical waveguide as a gain medium.

2. The method according to claim 1, characterized in that The method comprises using the single-mode sapphire optical waveguide as a gain medium to amplify signal light.

3. The method according to claim 1 or 2, characterized in that: The invention comprises using the single-mode sapphire optical waveguide as a gain medium to generate laser.

4. The method according to any one of claims 1 to 3, characterized in that This includes fabricating a Bragg grating within the single-mode sapphire optical waveguide.

5. The method according to any one of claims 1 to 4, characterized in that This includes providing opposing Bragg gratings within the single-mode sapphire optical waveguide to form an optical cavity.

6. The method according to any one of claims 1 to 5, characterized in that Includes using the single-mode sapphire optical waveguide as part of a sensor system.

7. The method according to any one of claims 1 to 6, characterized in that The single-mode sapphire optical waveguide is a depressed cladding waveguide.

8. The method according to any one of claims 1 to 7, characterized in that The invention comprises using laser modification and adaptive optical aberration compensation to manufacture the single-mode sapphire optical waveguide.

9. The method according to any one of claims 1 to 8, characterized in that The method comprises providing the single-mode sapphire optical waveguide in a sapphire optical fiber and using the sapphire optical fiber as a multimode waveguide while using the single-mode sapphire optical waveguide as a single-mode waveguide.

10. The method according to any one of claims 1 to 9, characterized in that This includes fabricating a cladding within a sapphire optical fiber to form a laser-modified cladding around an optical core, the optical core and the cladding thereby cooperating to provide the single-mode sapphire optical waveguide.

11. The method according to claim 10, characterized in that The cladding is a first cladding and the method includes fabricating a second cladding around the first cladding.

12. The method according to any one of claims 1 to 11, characterized in that The invention comprises doping a sapphire optical fiber to form a doped region therein, and manufacturing an optical core of a single-mode sapphire optical waveguide in the doped region.

13. The method according to any one of claims 1 to 12, characterized in that Including the production of single-mode sapphire optical waveguide eccentricity in sapphire optical fiber.

14. The method according to any one of claims 1 to 13, characterized in that Included is the fabrication of a single-mode sapphire optical waveguide including asymmetric features.

15. The method according to any one of claims 1 to 14, characterized in that This involves coupling a pump laser to a single-mode sapphire optical waveguide.

16. A sapphire optical device, characterized in that: An optical gain medium is included, the optical gain medium comprising a single-mode sapphire optical waveguide.

17. The sapphire optical device according to claim 16, characterized in that: Includes Bragg grating.

18. The sapphire optical device according to claim 17, characterized in that: Includes opposing Bragg gratings that provide an optical cavity for a laser system.

19. The device according to any one of claims 16 to 18, characterized in that The single-mode sapphire optical waveguide is a depressed cladding waveguide.

20. The device according to any one of claims 16 to 19, characterized in that The sapphire optical device is a laser-modified sapphire optical fiber.

21. The sapphire optical device according to any one of claims 16 to 20, characterized in that: The sapphire optical device is a multi-mode sapphire optical fiber, and the single-mode sapphire optical waveguide is disposed within the multi-mode optical fiber.

22. The sapphire optical device according to any one of claims 16 to 21, characterized in that: A doped region is included, and wherein the optical core of the single-mode sapphire optical waveguide is within the doped region.

23. The sapphire optical device according to any one of claims 16 to 22, characterized in that: The single mode waveguide is off-centered within the optical device.

24. The sapphire optical device according to any one of claims 16 to 23, characterized in that: The single-mode sapphire optical waveguide includes a laser-fabricated cladding surrounding an optical core.

25. The sapphire optical device according to claim 24, characterized in that: The laser-fabricated cladding is a first cladding, and the optical device includes a second laser-fabricated cladding surrounding the first cladding.

26. A laser system for generating laser light, comprising an optical device according to any one of claims 16 to 25, characterized in that: The optical device is the gain medium of the laser system.

27. An optical amplifier, characterized in that: Comprising an optical device according to any one of claims 16-25.

28. A sensor system, characterized in that: Comprising an optical device according to any one of claims 16-25.

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