Optical Device and Method

Abstract

A method of stimulating light emission including using a single-mode sapphire optical waveguide as a gain medium, and a sapphire optical device comprising an optical gain medium including a single-mode sapphire optical waveguide.

Description

Technical Field

[0001] The present invention relates to a method for stimulating light emission, as well as sapphire optical devices and methods.

Background Art

[0002] Doped optical fibers can be used to provide optical gain and also laser light. However, typically optical fibers are made of silica and have disadvantages in certain situations. First, silica is absorptive to mid-infrared, a specific wavelength, particularly an important wavelength band for spectroscopy. A further disadvantage is that it may degrade when exposed to radiation. This is a problem for applications such as use in space or use in a nuclear reactor.

[0003] An important class of lasers is the titanium-doped sapphire laser (Ti:sapphire). These lasers enable the generation of ultrashort pulses and have a very wide gain bandwidth, for example, allowing access to a very wide range of laser wavelengths and enabling a wide tuning range. However, Ti:sapphire lasers are bulky and can be expensive. Another problem is that the interaction length is relatively short because the light diverges rapidly, limiting the amount of available optical gain.

[0004] The pump absorption length of titanium-doped sapphire is relatively large, but the product of the emission cross-sectional area and the lifetime of the upper laser level is relatively small. The short lifetime of the upper laser level means that a small laser and pump mode radius are required to keep the threshold pump power for a titanium-doped sapphire laser reasonable. In a bulk titanium-doped sapphire laser, this means that the pump and laser diverge rapidly from the focus. This means that it is difficult to keep the mode radius small over the length of the titanium-doped sapphire crystal, which is necessary to efficiently absorb the pump. This typically requires a frequency-doubled Nd-based laser with high spatial brightness as the pump for titanium-doped sapphire, which can be expensive, for example, about £10,000 / W. Some blue diode lasers are significantly cheaper than this (e.g., about £100 / W), but it is difficult to use such blue diode lasers to pump titanium-doped sapphire.

[0005] Generally, improvement of gain media for lasers and optical amplifiers is desired. For example, it is necessary to improve the gain limit, the drawbacks of absorption length, and the cost of bulk titanium-doped sapphire.

[0006] The present invention provides an apparatus and method for providing optical gain and laser light that overcome the limitations as described above.

Summary of the Invention

[0007] According to a first aspect of the present invention, there is provided a method of stimulating the emission of light, including using a single-mode sapphire optical waveguide as a gain medium.

[0008] That is, this method involves stimulating the emission of light in a single-mode sapphire optical waveguide, and thus the waveguide acts as a source of optical gain. This can have various applications, for example, this method can include using a single-mode sapphire optical waveguide as part of an optical amplifier for amplifying optical signals. This method can include using a single-mode sapphire optical waveguide as part of a laser system for generating laser light. During use, the gain medium (also called the lasing medium, laser medium, active laser medium, etc.) transfers energy as emitted electromagnetic radiation, i.e., light. Thus, the present method can be a method for amplifying and / or generating light.

[0009] The single-mode sapphire optical waveguide may be within an optical device such as an optical fiber, or may be within a block or rod of sapphire. 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 present method can include providing a sapphire optical fiber including a single-mode sapphire optical waveguide, and using this fiber, particularly the single-mode waveguide therein, as, for example, the gain medium of an optical amplifier and / or a laser. Alternatively, the single-mode sapphire optical waveguide can include the optical core of a sapphire optical fiber (e.g., provided by a laser-machined cladding within the fiber, cooperating with an appropriate boundary).

[0010] By providing a single-mode sapphire optical waveguide within a sapphire optical fiber, the fiber itself can be regarded as a "single-mode fiber". However, since the propagation of light is restricted to single mode within the single-mode waveguide, it will be understood that it is the single-mode waveguide that makes the fiber "single-mode" in that sense. Therefore, the remaining part of the fiber (i.e., the part of the fiber other than the single-mode waveguide) does not have to be single-mode. While the single-mode waveguide supports only a single propagation mode, the remaining part of the fiber may be multimode and thus may support multiple propagation modes within that fiber. The remaining part of the fiber can include a cladding, a modified region (e.g., a laser-modified region, an etching region, etc.), bulk sapphire, and the like.

[0011] A method for manufacturing a single-mode sapphire optical fiber is disclosed in patent application GB1712640.0, the content of which is incorporated herein in its entirety. The inventor has discovered that the methods and features disclosed therein can be combined with the features of the present invention. A single-mode sapphire optical waveguide can be created within a sapphire optical fiber by laser modifying the fiber. However, since sapphire has a relatively high refractive index (e.g., about 1.75), focusing within such a fiber is susceptible to significant aberrations, and thus the accuracy and precision of laser modification are limited by those aberrations. However, as recognized in GB1712640.0, corrections can be determined and applied to the modification laser (e.g., using an active optical element to correct its wavefront characteristics) to cancel out the effect of the fiber's aberrations on the internal laser focus. By canceling out these aberration effects of the fiber, accurate and precise laser modification of the fiber becomes possible on length scales and materials that were previously impossible. This method can thus include structuring by femtosecond laser direct writing using adaptive beam shaping with a non-immersion objective lens. Alternatively, the method can include using an immersion objective lens and / or immersion oil, for example, structuring by femtosecond laser direct writing using adaptive beam shaping with an immersion objective lens and immersion oil. Other methods for manufacturing single-mode sapphire optical fibers, such as etching, index matching, etc., are also currently possible.

[0012] This method can include laser modifying an optical fiber (e.g., a sapphire optical fiber) to form a single-mode sapphire optical waveguide at a target location within the fiber. The laser modification can configure the waveguide and / or the fiber to prevent, suppress, or reduce the propagation of multiple optical modes therein. This method can include positioning at least a portion of the optical fiber in a laser system for laser modification and laser modifying the optical fiber at the target location to form the waveguide.

[0013] This method can include applying corrections to active optical elements of the laser system to modify the wavefront characteristics of the laser in order to cancel the effects of aberrations on the laser focus caused by the fiber. This method can include forming the waveguide by laser modifying the optical fiber at the target location using a laser having corrected wavefront characteristics.

[0014] The single-mode sapphire optical waveguide may be formed within the sapphire optical fiber by a laser-modified region of the sapphire optical fiber. Thus, the optical fiber may be laser modified to prevent or reduce the propagation of higher-order modes therein. This method can include fabricating 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.

[0015] The laser modification of sapphire can change the physical properties of the modified region, for example, by changing its refractive index, size, density, uniformity, homogeneity, etc. This method can include directly changing a portion of the sapphire optical fiber to form a waveguide, for example, by changing the characteristics of the optical core of the fiber to directly form the waveguide.

[0016] This method can include modifying a region surrounding a portion of the fiber to create, for example, a strain field in that portion, thereby changing the optical properties of that portion and, as a result, converting that portion into an optical core (e.g., a single-mode waveguide) that has desired properties but is not itself modified using a laser while surrounded by an unmodified core.

[0017] This method can include modifying a region surrounding a part of the fiber such that the modified regions have a different refractive index (e.g., a reduced refractive index) compared to the portions they surround, thereby effectively forming a cladding surrounding the unmodified optical core of the fiber. In fact, the method can include laser-modifying a part of the optical fiber to form a cladding around the unmodified core of the fiber, thereby providing a single-mode waveguide. This method can include modifying (e.g., laser-modifying) the fiber to create a depressed cladding waveguide therein.

[0018] A waveguide typically comprises an optical core through which waves can propagate internally and a boundary that appropriately restricts the propagation of the waves. 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 the cladding (or at least the core and the boundary) cooperate to provide the waveguide. With an appropriate configuration of the core and the cladding (or the core and the boundary), the waveguide can be configured to be single-mode.

[0019] This method can include indirectly forming a single-mode waveguide (e.g., by not directly modifying the optical core) by forming a cladding within a sapphire optical fiber. Thus, this method can change the properties within the optical fiber (e.g., the properties of bulk sapphire) to thereby form a cladding that is configured to cooperate with an unmodified portion (e.g., the optical core) of the sapphire optical fiber to thereby provide a waveguide.

[0020] Single-mode optical fibers are known, but single-mode sapphire optical fibers have not been realized prior to the method disclosed in GB1712640.0 due to, at least in part, the handling of sapphire for laser modification and the inherent difficulties in handling sapphire fibers. Thus, GB1712640.0 discloses the first laser-modified single-mode sapphire optical waveguide in a sapphire optical fiber. A single-mode optical fiber is an optical fiber designed to carry only single-mode light. A single-mode optical fiber can meet single-mode criteria, for example, by having a normalized frequency below a given threshold. For example, in the case of a step-index fiber, the normalized frequency may be less than 2.4. For example, for photonic crystal fibers, depressed-clad fibers, microvoid fibers, photonic bandgap fibers, etc., the thresholds may vary.

[0021] This method can include configuring the waveguide to be single-mode (e.g., by laser modification). This method can include configuring the waveguide to prevent the propagation of all light other than the single mode therein. The method can include modifying the fiber to ensure that the loss increases during the propagation of a predetermined mode. This method can include fabricating a step-index waveguide (e.g., by laser writing) in the optical fiber. This method can include manufacturing (e.g., by laser processing) a periodic structure waveguide (e.g., one with a periodically varying refractive index across the cross-section), a photonic crystal fiber, a microvoid fiber, a photonic bandgap fiber, etc. in the optical fiber. The single-mode waveguide can substantially limit the transmission of energy in one direction. This method can include modifying only the interior of the fiber. This method can include completely providing the laser-processed waveguide within a sapphire optical fiber. This method can include providing a laser-processed waveguide embedded within a sapphire optical fiber.

[0022] The fiber can be, for example, a hollow-core fiber where the core is a void (e.g., air or another fluid / liquid / gas). The fiber can be a photonic crystal fiber having a hollow core. The fiber can be an anti-resonant fiber or a negative curvature fiber having a hollow core. The hollow core can be filled with a suitable material, such as a gas (e.g., helium, neon, carbon dioxide, rubidium, nitrogen, etc.), and thus a gas laser can be provided. The hollow-core fiber can be a non-sapphire fiber.

[0023] This method can include configuring a waveguide to increase the loss for a given propagation mode, whereby the waveguide can be configured to be single-mode. This method can include configuring the waveguide such that the loss of higher-order modes increases during propagation. This method can include forming a waveguide in a fiber to cause a loss of about 1 dB (decibel) per meter, about 3 dB per meter, about 10 dB per meter, about 15 dB per meter, or more than about 20 dB per meter for a given mode. This method can include modifying an optical fiber such that all modes other than the single mode experience a loss of more than about 1 dB per meter, about 3 dB per meter, or about 10 dB per meter in the waveguide. This method can include manufacturing a sapphire optical fiber that supports a reduced number of modes in the waveguide.

[0024] Accordingly, a sapphire fiber can include a laser-modified region configured such that multiple modes exhibit loss during propagation in the waveguide, and such that all modes other than the single mode exhibit loss during propagation in the waveguide. A sapphire optical fiber can be configured to support substantially a single propagation mode in the waveguide.

[0025] A sapphire optical fiber can include a bulk material having a first refractive index and an optical core having a second refractive index different from the first refractive index. Since the bulk sapphire can have only the first refractive index, the optically functional part of the fiber is composed only of an optical core for single-mode propagation and a homogeneous surrounding sapphire material.

[0026] A sapphire optical fiber can comprise an optical core having a first refractive index (e.g., unmodified sapphire) and a surrounding modified material having a second refractive index (e.g., a cladding and / or a depressed cladding). Thus, a boundary can exist between regions having different refractive indices, and the boundary can be arranged to provide a waveguide. As a result, the waveguide can be formed by the cooperation of a material having a first refractive index and a material having a second refractive index.

[0027] The waveguide can include a region of modified refractive index configured to guide light therein. The waveguide can include a modified region having a modified refractive index that can be substantially solid. The modified region can comprise a modified material that includes microvoids therein. The modified region can be periodic and can provide a microstructure and / or a photonic crystal. The sapphire fiber is a microstructure fiber and / or a photonic crystal fiber that includes an array of modified regions, thereby forming a waveguide. The modified region can function to define an optical core as a light guiding region of the sapphire fiber. Thus, the sapphire fiber can be configured to include regions that are unmodified and / or modified by the waveguide. The waveguide can be formed by the cooperation of an optical core (e.g., an unmodified core) and a laser modified region (e.g., a laser processed microstructure, a laser processed periodic structure, a laser processed photonic crystal, a laser processed depressed cladding, etc.). The laser modified region can thus surround the optical core and thereby function as a cladding of a single mode waveguide (however, completely within the optical fiber). The laser modified region can comprise voids or pores that can be filled with air or another fluid (i.e., another gas or liquid).

[0028] The single-mode sapphire optical waveguide can have a normalized frequency V that is smaller than a predetermined threshold applicable to the type of fiber. For example, the single-mode sapphire optical waveguide may have a normalized frequency V of less than 2.4 for a step-index fiber, and may have a normalized frequency of less than 2.405. The normalized frequency V can be defined as follows. [Number] Here, a is the waveguide radius, λ is the operating wavelength, n1 is the refractive index of the core, and n2 is the refractive index of the cladding. Since sapphire has a refractive index of about 1.75, for a refractive index change between the waveguide and the cladding of 0.005 at 1550 nm to be single-mode, the waveguide radius needs to be less than 4.47 μm (diameter less than 8.94 μm).

[0029] The single-mode sapphire optical fiber can include an optical core and a cladding surrounding this core. Therefore, the single mode propagating in the waveguide can be substantially confined to the optical core. As a result of the refractive index change between the core and the cladding, or at least the change in optical properties between the core and the cladding, light can be confined (and thus guided) to the waveguide (e.g., along the optical core). This refractive index change between adjacent materials can provide a waveguide by the core-cladding interface. Therefore, a waveguide can be provided by the cooperation of the core and the cladding, or by the boundary between them. The single-mode sapphire optical waveguide can include a region where single-mode light is restricted to propagate, and this region is formed from a sapphire-based material. The waveguide can be the core of the sapphire optical fiber. The single-mode sapphire optical waveguide can include a core and a core-cladding interface. Therefore, the light-guiding part of the waveguide can be considered to coincide with the core. Therefore, the core can provide a gain medium.

[0030] The term "clad" as used herein includes a modified material within the fiber, and it should be noted that this modified material operates in a manner similar to the cladding of a typical optical fiber by providing a boundary. The clad may be entirely contained within the optical fiber. In fact, the clad may be surrounded by unmodified fiber material, such as bulk sapphire.

[0031] This method can include generating light by stimulated emission. This method can include exciting a gain medium (such as an optical core) using pump light. This method can include stimulating the excited gain medium using a signal or input light to emit light.

[0032] The gain medium in the waveguide may be doped sapphire. Doping of the sapphire can enable the sapphire to function as a gain medium. The doping component can be any suitable doping component that enables the waveguide to function as a gain medium. In particular, the doping component may be titanium, may be chromium, or may be any suitable combination of components. The optical fiber may be a titanium-doped sapphire fiber, for example, a Ti:sapphire fiber.

[0033] The single-mode sapphire optical waveguide can be configured to cancel out propagation loss. This method can include overcoming the propagation loss in the waveguide by providing gain using the single-mode sapphire optical waveguide. Thus, this method can enable the use of longer sapphire optical fibers. Commercially available sapphire fibers are typically less than about 4 meters in length, and by providing a sapphire optical fiber with a single-mode sapphire optical waveguide to cancel out the propagation loss, one no longer faces such a limitation on the length of the sapphire fiber. That is, the method described herein can potentially enable the use of longer fibers. Accordingly, the present invention can include providing a single-mode sapphire optical fiber that is longer than 2.5 centimeters (cm), longer than 5 cm, longer than 10 cm, longer than 15 cm, longer than 20 cm, longer than 30 cm, longer than 50 cm, longer than 1 meter (e.g., 1.00 m), longer than 2 meters (e.g., 2.00 m), or longer than 4 meters (e.g., 4.00 m). The length of the single-mode sapphire optical fiber may be suitable for practical applications.

[0034] This method can include using a single-mode sapphire optical waveguide as a gain medium for amplifying the signal light.

[0035] That is, this method can include amplifying the signal light, and thus, a single-mode sapphire optical waveguide can be used as an optical amplifier. The optical amplifier may be, for example, a device that directly amplifies the signal without the need to first convert it into an electrical signal. The optical amplifier can be considered as a laser without an optical cavity or a laser with suppressed feedback from the cavity. This method can include using a sapphire optical fiber as the optical amplifier. This method can include providing an optical amplifier. This method can include providing an optical amplifier as an optical repeater for use in a communication system such as, for example, telecommunications. In fact, from another perspective, one aspect of the present invention provides a method for amplifying an optical signal including using a single-mode sapphire optical waveguide as a gain medium. This aspect can include any of the features described herein with reference to any other aspect of the present invention.

[0036] This method (of any aspect of the present invention) can include, for example, stimulating the gain medium, thereby generating amplification of the signal light. This method can include pumping the gain medium to excite the gain medium, thereby enabling amplification of the signal light.

[0037] This method can include using a single-mode sapphire optical waveguide as the gain medium to generate laser light.

[0038] That is, this method can include providing a single-mode sapphire fiber laser. This method can include stimulating the emission of light from the gain medium provided by the single-mode sapphire optical waveguide. This method can include pumping the gain medium, for example, using pump light. This method can include providing an optical cavity (for example, the resonator cavity of a laser system), and can include providing a single-mode sapphire optical fiber as at least a part of the optical cavity.

[0039] In fact, 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. This method can thus include using a single-mode sapphire optical fiber as the gain medium, which includes a single-mode sapphire waveguide. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention. From another perspective, one aspect of the present invention provides a single-mode sapphire optical fiber laser. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0040] This (any aspect of the present invention's) method can include pumping a single-mode sapphire optical waveguide using pump light that exceeds a predetermined threshold power level. This method can include stimulating the emission of light from the excited single-mode waveguide. This method can include providing a light source for the pump light, such as a pump laser, diode laser, semiconductor laser, etc., and configuring the light source of the pump light to excite a single-mode sapphire optical fiber for subsequent stimulated emission. This method can include including an inverted distribution to enable stimulated emission. This method can include using any suitable pumping means for pumping the gain medium, i.e., any suitable means for exciting the gain medium via, for example, an electric current, a chemical reaction, nuclear fission, or a high-energy electron beam.

[0041] This method can include pumping the gain medium with a diode laser. Diode lasers are usually difficult to use efficiently, for example, with titanium-doped sapphire, but by providing a waveguide, more efficient coupling of the pump to the gain medium becomes possible. Thus, a smaller, less expensive, and lighter system can be realized.

[0042] This method can include mode-locking a single-mode sapphire fiber laser. This method can include using a single-mode sapphire optical waveguide to generate laser pulses.

[0043] This method can include providing an optical cavity, for example, using a feedback device. This method can include fabricating a feedback device. The feedback device may be a reflector. The feedback device may be, for example, 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. The reflector may be rotatable, for example, and thus adjustable, for example, an adjustable grating. Thus, this method can include, for example, adjusting the output of the laser light emitted by the gain medium, for example, changing and / or selecting the wavelength of the light output from the laser. In this way, an adjustable laser can be realized, and this method can include providing an adjustable laser. Due to the ability to change and select the wavelength emitted by the gain medium, the laser can be used, for example, to examine the optical spectrum. For example, absorption lines or peak wavelengths in the spectrum can be examined. Also, with an adjustable system, the wavelength peak of a fabricated fiber Bragg grating (FBG) can be determined.

[0044] The grating can be located, for example, adjacent to the outside of the optical fiber and / or communicate optically with the optical fiber. That is, the reflector and / or the grating may be components added to the optical fiber.

[0045] This method can include forming an optical cavity (e.g., a laser cavity) that includes a single-mode sapphire optical waveguide. This method can include providing a reflector and either end of the waveguide, thereby forming a cavity. This method can include providing a reflector within the single-mode sapphire optical waveguide and / or within a sapphire optical fiber. This method can include fabricating a reflector within the single-mode sapphire optical waveguide. Thus, this method can include providing an optical cavity of a single-mode sapphire optical fiber for a laser system.

[0046] This method can include fabricating a Bragg grating within the single-mode sapphire optical waveguide.

[0047] This method can include using a Bragg grating to reflect light within a waveguide, and thus can include using the fabricated Bragg grating as a reflector, for example, as part of an optical cavity or as part of a sensor system. This method can include detecting the reflected light from the Bragg grating and can include using a single-mode sapphire optical fiber as part of a sensor system. For example, this method can include determining physical parameters (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 can be used in harsh environments, for example, together with engines and the like. This method can include disposing at least a portion of the sapphire optical fiber in an extreme environment within an engine, for example, to detect physical parameters (e.g., temperature, pressure, strain, magnetic field, electric field, vibration, and / or shock) within the environment. For example, this method can include disposing at least a portion of the sapphire optical fiber within an aerospace engine. This method can include using a sensor system including a sapphire optical fiber in an environment exposed to high levels of radiation. For example, this method can include using a sensor system including a sapphire optical fiber in a nuclear reactor, particularly a fission nuclear reactor or a fusion nuclear reactor, and / or in satellites and other equipment suitable for use in space. Sapphire is also transparent in the mid-infrared (e.g., wavelength > 2 μm). Thus, a sensor system including a sapphire optical fiber can be effectively used for spectroscopy and detection of gas species.

[0048] This method can include configuring a Bragg grating (or each Bragg grating if a plurality of gratings are provided) to be adjustable, for example, via temperature or strain. For example, the structure of the grating can be subject to change when exposed to different conditions. For example, the spacing may depend on the temperature of the optical fiber. Thus, this method can include reconfiguring and / or deforming the Bragg grating in the waveguide and detecting a change in the reflected light as a result of the reconfiguration or deformation, thereby detecting a change in a parameter to be measured, such as temperature and / or strain.

[0049] This method can include providing a plurality of Bragg gratings in an optical fiber, for example, to provide an optical cavity or to perform a plurality of measurements. This method can include providing each of the plurality of Bragg gratings within a respective optical core (i.e., within a respective waveguide), and / or can include providing a plurality of Bragg gratings within the same optical core, for example, in series or overlapping with each other. This method can include performing a plurality of simultaneous measurements of physical parameters using the optical fiber, for example, by detecting a plurality of reflected signals from the plurality of Bragg gratings.

[0050] The Bragg grating can 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.

[0051] This method can include providing a plurality of waveguides in the optical fiber, which can include providing a plurality of single-mode waveguides in the optical fiber, for example, in parallel or adjacent to each other. This method can include providing overlapping waveguides in the optical fiber. Thus, this method can include fabricating a plurality of waveguides in the optical fiber.

[0052] This method can include providing opposing Bragg gratings in a single-mode sapphire optical waveguide to form an optical cavity.

[0053] This method can include fabricating opposing Bragg gratings in a single-mode sapphire optical waveguide, for example, by laser modification such as using an adaptive optical element to correct the aberration of the changed laser focus caused by the fiber. Thereby, this method can include providing a sapphire optical fiber having a laser cavity, for example, a laser cavity including a single-mode sapphire optical waveguide.

[0054] This method can include using a single-mode sapphire optical waveguide as part of a sensor system.

[0055] Sapphire is very durable and can withstand high temperatures up to, for example, 1000 °C, 1500 °C, 1700 °C, 1900 °C, and 2000 °C. Therefore, the optical fiber and the single-mode waveguide can be used in harsh environments such as inside an engine or any other suitable application. Thus, this method can include using a single-mode sapphire optical waveguide under harsh conditions up to, for example, 1000 °C, 1500 °C, 1700 °C, 1900 °C, and 2000 °C. This method can include using a sensor system to measure temperatures up to, for example, 1000 °C, 1500 °C, 1700 °C, 1900 °C, and 2000 °C. The Bragg grating can also be stable up to such high temperatures. Therefore, this method can include using a fiber Bragg grating at temperatures up to 1000 °C, 1500 °C, 1700 °C, 1900 °C, and 2000 °C.

[0056] The single-mode sapphire optical waveguide may be a depressed-clad waveguide.

[0057] The depressed cladding waveguide can be the laser-modified sapphire optical fiber described herein with reference to any aspect of the present invention. Thus, the depressed cladding can be sapphire. The depressed cladding may be laser-modified to reduce its refractive index, for example, compared to the unmodified material within the optical fiber. Thus, the depressed cladding can have a lower refractive index than the unmodified portion and can cooperate with the unmodified portion to thereby form a waveguide. In the depressed cladding waveguide, the refractive index of the sapphire may decrease (e.g., uniformly) around the core. The decreased refractive index need not extend to the ends of the sapphire fiber, and thus the cladding may be surrounded by unmodified optical fiber.

[0058] This depressed cladding waveguide may be a multi-layer depressed cladding waveguide and includes a plurality of layers of a modified fiber material that provides a plurality of layers of cladding.

[0059] The single-mode sapphire optical waveguide may be a photonic crystal waveguide (comprising a cladding that includes a periodic array of regions having a modified refractive index within the cladding).

[0060] The cladding may include a periodic array of "holes", such as regions where material has been removed. This may be air (having a refractive index of approximately 1) or another fluid (i.e., a liquid or a gas).

[0061] The method can include manufacturing a single-mode sapphire optical waveguide using laser modification and adaptive optical aberration compensation.

[0062] This method can include using femtosecond laser direct writing, for example, using a method as disclosed in GB712640.0, the content of which is incorporated herein in its entirety. The inventor has discovered that the methods and features disclosed therein can be combined with the features of the present invention.

[0063] This method can include using a non-immersion lens, for example, to focus on the fiber. This method can include counteracting the influence of aberrations on the laser focus caused by sapphire, such as at the sapphire-air interface, sapphire-clad interface, sapphire-modified sapphire interface, etc.

[0064] This method can include using oil immersion technology and an immersion objective lens to fabricate a single-mode sapphire optical waveguide. This method can include fabricating a single-mode sapphire optical waveguide by embedding a sapphire fiber in a substrate with a similar refractive index (for example, a block of sapphire having holes along its length for incorporating sapphire). This fabrication step can include counteracting the aberration effect of the fiber at the focus of the modified laser.

[0065] This method can include providing a single-mode sapphire optical waveguide in a sapphire optical fiber, using the sapphire optical fiber as a multimode waveguide, and simultaneously using the single-mode sapphire optical waveguide as a single-mode waveguide.

[0066] Therefore, this method can include providing a single-mode waveguide within a multi-mode waveguide. This method can include propagating multi-mode light within a sapphire optical fiber as a pump for a single-mode sapphire optical waveguide. The sapphire fiber is inherently multi-mode, and thus, light of multiple modes can propagate along the fiber, for example, due to a sapphire-air boundary. Thus, the optical fiber can be considered as a combination of a plurality of waveguides, for example, a first single-mode sapphire optical waveguide and a second multi-mode waveguide provided by the remainder of the fiber. Thus, the propagation modes within the multi-mode waveguide of the fiber can excite the gain medium of the single-mode waveguide, for example, for subsequent stimulated emission by a single mode propagating within the waveguide. This method can 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. Thus, the method of the present invention can include pumping a single-mode waveguide using light of multiple modes propagating within the optical fiber.

[0067] The input signal can then stimulate emission from the pumped single-mode waveguide. Thus, this method can include using a single light source for the pump light and the input signal light. Thus, the input signal can be the fundamental mode of a laser light source and can propagate within the single-mode sapphire optical waveguide. Higher-order modes from the laser light source can propagate within the remainder of the optical fiber and thus can pump the single-mode waveguide.

[0068] This method can include fabricating a cladding within the sapphire optical fiber to form a laser-modified cladding around the optical core, with the optical core and the cladding cooperating to provide a single-mode sapphire optical waveguide.

[0069] This method can include fabricating a depressed clad waveguide by reducing the refractive index of the material surrounding the optical core, for example, by forming a material having a substantially uniform refractive index. This method can include laser machining a periodic structure around the core, thereby forming a clad, for example, forming a material having a periodically varying refractive index. The periodic structure can be a microstructure (e.g., a structure on a microscopic length scale), a microvoid, a photonic crystal structure, a photonic bandgap structure, etc. This method can include manufacturing a non-periodic structure.

[0070] This method can include only modifying the material within the sapphire optical fiber, for example, including not changing the overall shape of the cross-section of the fiber. That is, the surface of the optical fiber may not be changed. The fiber may have a cylindrical shape, or more generally in the case of a sapphire fiber, may have a substantially hexagonal cross-section. The cross-section of the optical fiber can be hexagonal or a rounded hexagon. Since the overall shape of the cross-section of the optical fiber is substantially the same as before the modification, the fiber can maintain substantially the same mechanical properties. For example, it can be made such that there is no need to remove material from the fiber to provide the present invention, and thus it can be made such that there is no need to weaken the fiber and make it mechanically fragile. Thus, the waveguide (or waveguides) can be composed of both the modified sapphire material and the unmodified sapphire material.

[0071] The clad can be a first clad, and the method can include fabricating a second clad around the first clad.

[0072] This method can include fabricating a cladding nested within an optical fiber. This method can include a dual-clad fiber having a multilayer cladding fabricated within the fiber. The second cladding can have a refractive index different from (e.g., lower than) that of the first cladding. The second cladding can be any type of cladding as described herein with reference to any aspect of the present invention. The second cladding can have different optical properties from the first cladding, e.g., a different refractive index (e.g., a lower refractive index) from the first cladding. It can have a different microstructure, a different period, etc. The second cladding can be configured to function as a cladding for a second optical core provided by the first cladding. That is, the first cladding can surround the optical core of a single-mode waveguide, and the second cladding can surround the first cladding. Thus, the first cladding can be used as the (second) optical core for a multimode waveguide bounded by the second cladding, and the (first) optical core can be used as a single-mode waveguide bounded by the first cladding. The first and second claddings can be formed from a sapphire-based material.

[0073] An optical fiber can be configured to include a plurality of waveguides. The optical fiber can be configured to include a plurality of overlapping waveguides. The optical fiber can include a plurality of nested waveguides. The cross-sections of the waveguides can overlap, for example, one waveguide can be within the other waveguide. The plurality of waveguides can overlap radially such that an inner waveguide and an outer waveguide are formed. The longitudinal central axis of the outer waveguide can be located within the region of each inner waveguide, for example, the waveguides can be concentric (however, the waveguides do not necessarily have circular symmetry). The cladding of the inner waveguide can function as the optical core of the outer waveguide. Thus, the optical fiber can be configured to provide a multi-mode waveguide surrounding a single-mode waveguide, and this method can include propagating light of a plurality of modes within the multi-mode waveguide, thereby pumping the single-mode waveguide. This method can include using the cladding as a pump waveguide. Thus, this method can include providing a cladding-pumped single-mode sapphire optical waveguide, i.e., a cladding-pumped gain medium. The second (or outer) cladding can be surrounded by an unmodified sapphire optical fiber. That is, the single-mode waveguide and the multi-mode waveguide are completely contained within the optical fiber and both can be fabricated therein.

[0074] This method can 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.

[0075] The doped region can constitute the entire cross-section of the optical fiber. Alternatively, the doped region may only include a part of the sapphire optical fiber, for example, only a part of the cross-section of the fiber. For example, only the inner region of the fiber intended to provide the optical core of a single-mode waveguide can be doped. Thus, the doped portion may be surrounded by undoped sapphire material. This method can include doping only the inner part of the optical fiber, so that the cross-section of the fiber comprises an inner doped portion (e.g., the central portion) surrounded by an undoped portion. Thus, a sapphire optical fiber (e.g., its cross-section) can comprise both a doped region and an undoped region. Only a part 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. As described herein, the doped portion may be offset from the center within the optical fiber to provide an off-center optical core.

[0076] By doping only a part of the fiber (e.g., only a part of the cross-section of the fiber), the risk that energy from higher-order propagation modes that propagate within the optical fiber but outside the single-mode waveguide is absorbed by the doped material is reduced. When doping the cladding of a single-mode waveguide (e.g., the second optical core of a cladding-pumped single-mode waveguide), there is a possibility of absorbing pump power without contributing to optical gain, so the overlap between the core of the pump waveguide and the doped portion should preferably be minimized.

[0077] In a cladding-pumped single-mode waveguide, the optical core of the single-mode waveguide can be provided within the core of the pump waveguide. By doping within the optical core of the single-mode waveguide, the core of the pump waveguide can include the doped material at least at the location where this overlap between the core of the single-mode waveguide and the core of the pump waveguide occurs. The additional overlap between the doped region and the core of the pump waveguide should preferably be minimized.

[0078] Therefore, it is preferable to dope only as much fiber as necessary to provide the core of the single-mode waveguide. However, it is difficult to achieve such precise control of the extent of the doped portion, and thus it may be necessary to dope a larger area than required for the core of the single-mode waveguide.

[0079] This method can include fabricating a single-mode sapphire optical waveguide at a position offset from the center within the sapphire optical fiber.

[0080] This method can include manufacturing a single-mode sapphire optical waveguide near the periphery of the doped portion, for example, at a position offset from the center with respect to the doped portion of the fiber, such as at the end of the doped portion (the doped portion itself may be offset from the center within the fiber). The single-mode waveguide can be provided at the periphery of the doped region, thereby reducing (ideally minimizing) the overlap between the fabricated cladding surrounding the optical core of the single-mode waveguide and the doped portion. Therefore, when the cladding of the single-mode waveguide is used as the pump waveguide, this arrangement can reduce the pump energy lost in the doped material in the pump waveguide / first cladding. That is, by providing the optical core of the single-mode waveguide at the periphery of the doped portion, the risk of pump power loss in the doped cladding is reduced.

[0081] In addition to the above advantages, this method can include providing the gain medium of the single-mode waveguide offset from the center in order to increase the overlap between the gain medium of the single-mode waveguide and the pump mode. If the optical core of the single-mode waveguide (e.g., one that guides a laser mode and is doped with active ions) is exactly at the center of the multimode pump waveguide, there may be modes of the multimode guide that do not significantly overlap with the gain medium of the optical core of the multimode waveguide, and thus the light coupled to those modes of the multimode waveguide may not be absorbed by the gain medium of the optical core of the single-mode waveguide and instead may exit the fiber unused. By providing an offset optical core for the single-mode waveguide, the energy absorbed from the pump mode by the gain medium can be increased. Thus, the optical core of the single-mode waveguide may be offset from the center within the optical fiber and, similarly, may be offset from the center within the pump waveguide, if provided.

[0082] Another approach for increasing the amount of energy absorbed from the pump mode by the gain medium is, for example, to reduce the cross-sectional symmetry of the multimode waveguide by making the cross-section elliptical or D-shaped. Thus, this method can include providing a pump waveguide having an asymmetric feature. This method can include providing such an asymmetric feature in order to increase the energy absorbed from the pump mode by the gain medium.

[0083] This method can include fabricating a single-mode sapphire optical waveguide having an asymmetric feature.

[0084] For example, this method can 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 can maintain polarization, such that, for example, the single-mode polarization state is maintained when the optical fiber is bent. Thus, this method can include providing a single-mode sapphire optical waveguide that maintains polarization.

[0085] This method can include fabricating the cladding of the single-mode waveguide to include an asymmetric feature having an elliptical cross-section.

[0086] This method can include coupling a pump laser to the single-mode sapphire optical waveguide.

[0087] This method can include providing a pump laser for pumping the gain medium, i.e., for pumping the single-mode sapphire optical waveguide. This method can include using the pump laser to excite the gain medium. This method can include using the pump laser to stimulate emission from the gain medium. The pump laser can be any suitable type of laser, such as an argon-ion laser. The pump laser can also be a diode laser. This method can include providing an optical amplifier and / or a laser system. This method can include providing any component for the optical amplifier and / or the laser system.

[0088] The combination of the above features may have special advantages. For example, the combination of a clad-pumped single-mode sapphire optical waveguide and the asymmetric feature of the pump waveguide can improve the coupling of the pump mode into the gain medium. Having a single-mode waveguide optical core offset from the center in the pump waveguide can further improve the mode coupling into the gain medium and improve the energy transfer. Selective doping to reduce the overlap between the doped material and the pump waveguide can further improve the energy transfer by reducing the loss to the pump waveguide.

[0089] According to a second aspect of the present invention, a sapphire optical device is provided that includes an optical gain medium having a single-mode sapphire optical waveguide.

[0090] The sapphire optical device can include any of the features described herein with reference to the first aspect of the present invention and / or with reference to any other aspect of the present invention.

[0091] 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 inside. The diameter of the optical core of the single-mode waveguide can be less than 20 micrometers (μm), less than 15 μm, less than 10 μm, or 10 μm or less.

[0092] The diameter of the sapphire optical fiber can 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 50 μm or less. The diameter of the cladding surrounding the core can be the same as the diameter of the sapphire optical fiber (for example, when a single waveguide is provided within the sapphire optical fiber). The diameter of the cladding surrounding the core may be different from the diameter of the fiber (for example, when additional material surrounding the cladding is provided, or when multiple waveguides are provided within the sapphire optical fiber, or when a cladding pump optical waveguide is provided as further described below), and the diameter of the cladding can be less than 425 μm, less than 250 μm, less than 125 μm, less than 100 μm, less than 75 μm, or 50 μm or less. The diameter of the cladding surrounding the core can be 2.5 times or more, 5 times or more, 10 times or more, or 20 times or more the diameter of the core. A cladding diameter of 5 times or more the diameter of the core is advantageous in achieving low loss.

[0093] The length of the single-mode sapphire optical waveguide may be the same as the length of the fiber. For example, the single-mode sapphire optical waveguide may be continuous 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.

[0094] The sapphire optical device can include a Bragg grating.

[0095] The Bragg grating may be within the single-mode sapphire optical waveguide. This may be suitable for use as part of a sensor system and / or may be provided to reflect a predetermined wavelength to provide feedback and thereby form a laser system. This device can include, for example, a plurality of Bragg gratings in series and / or overlapping.

[0096] The sapphire optical device can include opposing Bragg gratings that provide an optical cavity for a laser system. The opposing Bragg gratings may have substantially the same wavelength.

[0097] This device can include two opposing Bragg gratings, each located within a single-mode sapphire optical waveguide. The Bragg gratings can be laser processed within the waveguide. The two opposing Bragg gratings may have substantially the same wavelength.

[0098] The single-mode sapphire optical waveguide can be a depressed-clad waveguide.

[0099] The waveguide can be a periodic structure waveguide and / or a microstructured waveguide. The waveguide can be a photonic crystal waveguide, a microvoid waveguide, a photonic bandgap waveguide, etc.

[0100] The sapphire optical device can be a laser-modified sapphire optical fiber.

[0101] This sapphire optical fiber can be a step-index fiber, a photonic crystal fiber, a depressed-clad fiber, a microvoid fiber, a photonic bandgap fiber, etc. This optical device can be formed by etching, index matching, etc.

[0102] The sapphire optical device can be a multimode sapphire optical fiber, and the single-mode sapphire optical waveguide can be disposed within the multimode fiber.

[0103] Therefore, a sapphire optical device can comprise overlapping waveguides. The sapphire optical device can comprise nested waveguides, for example an inner waveguide (e.g., a single-mode waveguide) disposed within an outer waveguide (e.g., a pump waveguide having an optical core that is the cladding of the inner waveguide). The optical device can be configured such that propagation modes within a multimode fiber function as pump light for the single-mode waveguide. Thus, the optical device can comprise a plurality of waveguides, at least one of which is a single-mode waveguide and another of which is a multimode waveguide.

[0104] The sapphire optical device can comprise a doped region, and the optical core of the single-mode sapphire optical waveguide is within the doped region.

[0105] The single-mode sapphire optical waveguide (e.g., its optical core) can be offset from the center within the doped region with respect to the doped region. The optical core of the single-mode waveguide can be at the edge of the doped region. The doped region can be offset from the center within the optical device. The single-mode waveguide can be offset from the center with respect to the optical device within the optical device.

[0106] The single-mode sapphire optical waveguide can be offset from the center with respect to the multimode waveguide within the multimode waveguide. For example, the multimode waveguide can be provided by the remainder of the optical fiber other than the single-mode waveguide or can be provided by the cladding of the single-mode waveguide. Thus, the optical device can comprise a pump waveguide for propagating multimode light therein to pump the single-mode waveguide. The optical device can comprise a cladding-pumped single-mode sapphire optical waveguide. The pump waveguide can be inside (e.g., surrounded by) an unmodified optical fiber material.

[0107] The single-mode sapphire optical waveguide can include a laser-processed cladding surrounding the optical core.

[0108] The laser-processed cladding can thus include optical properties different from those of the optical core and, in cooperation therewith, can provide a single-mode sapphire optical waveguide. The cladding may be substantially homogeneous, having a substantially uniform refractive index. Alternatively, the cladding may be a periodic structure having, for example, a microstructure and optical properties that vary periodically across its cross-section, as described herein.

[0109] The laser-processed cladding may be a first cladding, and the optical device can include a second laser-processed cladding surrounding the first cladding.

[0110] The second cladding can have a refractive index different from (e.g., lower than) that of the first cladding. The first cladding can function 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.

[0111] According to a third aspect of the present invention, a laser system for generating laser light can be provided that includes the optical device described herein with reference to the second aspect of the present invention. The optical device can provide the gain medium of the laser system.

[0112] The laser may be a mode-locked laser as described herein. The laser system may include a feedback device as described herein. The laser system may include a rotatable grating and thereby be tunable. The laser system can include a diode-pumped laser or any suitable pump laser. The laser system can be configured to perform the method of the present invention as described herein with reference to the first aspect or any other aspect.

[0113] According to a fourth aspect of the present invention, there is provided an optical amplifier comprising the optical device described herein with reference to the second aspect of the present invention. The optical device can provide a gain medium for the optical amplifier.

[0114] The optical device may be an optical amplifier of a sensor system. Thus, instead of having a separate amplifier, the waveguide itself can be active and the losses of the waveguide can be compensated by optical gain (this can also be applied when used as a laser). Thus, the waveguide can be considered a distributed amplifier. Thus, according to another aspect of the present invention, there is provided a sensor system comprising the optical amplifier described herein with reference to any aspect of the present invention. The sensor system can be configured to perform the methods described herein with reference to any aspect of the present invention and can comprise any of the features described herein with reference to any aspect of the present invention.

[0115] By providing a fiber (e.g., a fiber laser or an optical amplifier) having a waveguide on the same line for multimode pumping and single mode propagation, the present invention can reduce gain limitations and improve (or maximize) the useful absorption of pump light, which can result in enabling the use of inexpensive and 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 advantages of embedded engineering such as pump redundancy and fast and relatively easy pump modulation for the control of laser operation (e.g., by power feedback stabilization via diode current). The present invention can provide, for example, an all-fiberized Ti:sapphire laser having 10 W of output power from a rugged package having, for example, built-in pump redundancy and fiber Bragg grating control of spectral output.

[0116] According to a fifth aspect of the present invention, there is provided a sensor system comprising an optical device described herein with reference to the second aspect of the present invention. The sensor system can comprise an optical amplifier described herein with reference to the fourth aspect of the present invention. The sensor system can comprise any of the features described herein with reference to any aspect of the present invention.

[0117] According to a further aspect of the present invention, there is provided an optical gain element, which comprises a single-mode sapphire optical waveguide having optical gain. This has the advantage that sapphire can be used as a gain medium while maintaining a single transverse mode. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0118] The sapphire optical waveguide may be a sapphire optical fiber. The sapphire optical waveguide may be within a sapphire optical fiber. These have the advantage that the fiber can provide a longer interaction length with the gain medium.

[0119] The single-mode waveguide can 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 a rare earth element. 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 can provide gain at a wavelength different from that of the pump laser. These have the advantage of enabling optical gain.

[0120] It is possible to have a pump laser coupled to the sapphire optical waveguide. This has the advantage of enabling the gain medium to be excited.

[0121] The sapphire waveguide can be single-mode for the signal light and multi-mode for the pump light. This has the advantage of enabling an efficient coupling between the pump laser and the diverging beam. Since high-power pump lasers generally diverge, it is difficult to couple a lot of light into a single-mode fiber. However, this configuration enables the pump light to excite the ions within the single-mode waveguide. Double-clad fibers are known in fiber amplifier design and usually have an outer jacket around the fiber to form an additional cladding. The optical fiber of the present invention may comprise an external cladding around the sapphire optical fiber, or the pump light may be guided by the sapphire-air interface. Alternatively, a femtosecond laser can be used to write a multi-mode waveguide around the single-mode waveguide, such that the optical fiber itself provides a double-clad waveguide configuration.

[0122] The single-mode sapphire fiber can comprise a single-mode waveguide including a core of the gain medium and a cladding surrounding the core. This single-mode waveguide can be single-mode for the signal light. The single-mode sapphire fiber can comprise a multi-mode waveguide. The cladding of the single-mode waveguide can provide the core of the multi-mode waveguide, and a second cladding can be provided surrounding the core of the multi-mode waveguide. The second cladding may be formed from air such that the boundary between the core of the multi-mode waveguide and the second cladding is provided by the sapphire-air boundary. Alternatively, the second cladding may be provided within the sapphire fiber. The multi-mode waveguide may be multi-mode for the pump light. This has the advantage of enabling an efficient coupling of a pump laser having a diverging beam into the core / gain medium of the single-mode waveguide.

[0123] Accordingly, according to another aspect of the present invention, an optical fiber having a waveguide nested therein is provided. The optical fiber may be a sapphire optical fiber or any other suitable material. The nested waveguide can include an outer waveguide and an inner waveguide within the outer waveguide. The nested waveguide can include a machined cladding (e.g., a laser machined cladding). The cladding of the inner waveguide can be the optical core of the outer waveguide, and the outer waveguide can pump the inner waveguide. The optical fiber can include any of the features described herein with reference to any aspect of the present invention.

[0124] The single-mode sapphire fiber can include a sapphire optical fiber having an optical waveguide along its length. The optical waveguide can be a depressed cladding waveguide, a microstructure 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 offset from the center within the fiber cross-section. The sapphire fiber may have two or more waveguides. The fiber may be a double-clad waveguide. There may be a single-mode waveguide within a multimode waveguide. The single-mode waveguide may be offset within the multimode waveguide. The multimode waveguide cross-section may be substantially circular, square, rectangular, hexagonal, triangular, pentagonal, heptagonal, octagonal, or a rounded version thereof. One or more waveguides may be written with a femtosecond laser. The signal wavelength may be more tightly confined than the pump wavelength. The pump wavelength may be confined to 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.

[0125] The signal wavelength can be coupled to a single-mode waveguide, and the pump wavelength can be coupled to a multi-mode waveguide. There may be two or more pump lasers. 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-band, E-band, S-band, C-band or L-band. The signal, laser or pump wavelength may be near-infrared, mid-infrared, visible light or ultraviolet light. The length of the single-mode fiber can be made longer than any of 1 cm, 2 cm, 5 cm, 10 cm, 50 cm, 1 m, 1.5 m, or 2 m. The pump laser can be an argon ion laser, an Nd:YAG laser, a semiconductor laser, or a diode laser. There may be tilted and / or chirped Bragg gratings.

[0126] The "double-clad" type approach of guiding the pump laser light and the signal laser light separately but almost collinearly can contribute to decoupling the output power achievable from a Ti:sapphire laser from the spatial brightness of the pump.

[0127] According to another aspect of the present invention, a master oscillator power amplifier (MOPA) is provided, where the sapphire fiber is an amplifier (e.g., without feedback) used to boost a high-quality seed from a diode laser or a semiconductor laser (such as a distributed feedback laser (DFB), a distributed Bragg reflector (DBR) laser, a Fabry-Perot laser, or an external cavity diode laser). The MOPA can comprise a chain of amplifiers. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0128] According to another aspect of the present invention, a laser is provided, the laser comprising an optical gain element according to a further aspect of the present invention and a feedback element. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0129] The feedback element can comprise one or more reflectors. The feedback element can include one or more reflectors formed at the sapphire-air interface. The feedback element can include one or more reflectors that are thin film coatings. The feedback element can include one or more Bragg gratings. The Bragg grating can be in a single-mode waveguide. The Bragg grating can be a single-mode fiber Bragg grating. One or more adjustable Bragg gratings may be present. One or more adjustable Bragg gratings may be adjusted by temperature, strain, voltage, or current. The feedback element can include one high-reflectivity reflector and one partial-reflection reflector. The feedback element can include one high-reflectivity Bragg grating and one partial-reflection Bragg grating. The feedback element can include a diffraction grating. The angle of the diffraction grating is adjustable, and thus the output of the laser can be made adjustable. The laser can include a seed laser that seeds an optical gain element. The seed laser may be adjustable. The laser may emit light at a substantially single wavelength. The laser may be in a spacecraft or a nuclear reactor.

[0130] The laser may be mode-locked. The laser may be actively mode-locked. The laser may be passively mode-locked. The laser can comprise an optical modulator. The laser can comprise an acousto-optic modulator. The laser can comprise a saturable absorber. These features have the advantage of making it possible to generate very short optical pulses.

[0131] According to another aspect of the present invention, a sensor system is provided, which includes a optical gain element according to any aspect of the present invention, and the single-mode sapphire fiber has at least one Bragg grating sensor. The Bragg grating can be adjusted, for example, by temperature or strain. Any suitable type of adjustment can be used. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0132] According to another aspect of the present invention, a doped single-mode sapphire optical fiber is provided. The doped fiber can provide an optical gain element according to the first aspect of the present invention. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0133] According to another aspect of the present invention, a method for amplifying an optical signal is provided, which includes providing a single-mode sapphire optical waveguide and injecting pump light into the optical waveguide. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0134] According to another aspect of the present invention, a method for providing laser light including a method according to any aspect of the present invention is provided, which further includes providing a feedback element and injecting pump light at a level exceeding a threshold power level. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0135] According to another aspect of the present invention, an optical gain element including a multimode sapphire fiber coupled to a pump laser is provided, and the multimode sapphire fiber includes a single-mode waveguide. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0136] 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 within the sapphire optical fiber. The waveguide can have asymmetry in its cross-section. For example, the core may be elliptical or asymmetric. For example, there may be a structure that induces stress in the cladding so as to surround the single-mode waveguide. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention.

[0137] The single-mode waveguide can mainly support a single transverse mode. However, there may be other modes that exist but exhibit high losses. For example, these may occur due to having a depressed cladding of finite diameter. For instance, other modes may have a loss 10 times that of the dominant single mode.

[0138] 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 in its placement state. That is, the present invention also provides any of the aspects described herein, but with a reduced-mode waveguide instead of a single-mode waveguide. Although the use of a single-mode waveguide is usually preferred, many of the advantages extend to waveguides that allow more than single mode. The few-mode sapphire waveguide can have modes 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 less than 3 modes. This aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention, provided that a reduced-mode (i.e., few-mode) waveguide can be used instead of the single-mode waveguide described in relation to these aspects. For example, the waveguide can be configured to suppress the propagation of some modes (e.g., higher-order modes), yet still allow the propagation of more than a single mode, for example, some modes.

[0139] References herein to single mode are intended to include single mode including multiple polarization states. References herein to mode are intended to include mode including a given wavelength or wavelength range. The term "sapphire" is taken to include doped sapphire, impure sapphire and Al2О3.

[0140] For optimal efficiency of optically induced emission using a single mode sapphire optical waveguide (and, according to another aspect of the invention, for optimal efficiency of optically induced emission using an optical waveguide of any composition within a single crystal fiber, for example), the pump light is absorbed only within the active waveguide and not wasted by being absorbed within the active material surrounding the waveguide. However, inexpensive pumps diverge and couple to a wide area surrounding the waveguide. Since it is difficult to dope only the core of the waveguide, light from an inexpensive pump may couple to the active (i.e., doped) material surrounding the waveguide. By providing multiple cores within the doped region, more efficient use of the absorbed pump power can be achieved. And the light emitted by the individual cores can be coherently combined to form a single output with higher power.

[0141] An optical fiber can comprise a plurality of single mode sapphire optical waveguides. Each waveguide of the plurality of single mode sapphire optical waveguides can include any of the aspects of the single mode sapphire optical waveguide discussed herein. In particular, each waveguide of the plurality of single mode sapphire optical waveguides can be a depressed clad waveguide, can be a multi-layer depressed clad waveguide (e.g., including layers of a plurality of modified fiber materials providing multiple layers of cladding), or can be a photonic crystal waveguide (e.g., a cladding including a periodic array of regions having a modified refractive index within the cladding).

[0142] A plurality of single-mode sapphire optical waveguides may be arranged in parallel, adjacent to each other, and / or arranged to occupy most of the cross-section of the optical fiber. For example, three waveguides may be arranged in a triangle within the cross-section of the fiber. Seven waveguides may be arranged within the cross-section of the fiber, with six of them distributed in a hexagonal pattern around the central waveguide. The plurality of waveguides may be arranged in a hexagonal pattern of closest packing within the cross-section of the optical fiber. Such a configuration can be particularly useful in fibers having a hexagonal cross-section. In general, any suitable number of parallel waveguides can be used, and the waveguides can be arranged to occupy as much of the cross-section of the optical fiber as possible. The waveguides may be arranged to include as much of the doped material within the fiber as possible.

[0143] Each waveguide may have an individual cladding or may share a common cladding. Each waveguide can include an unmodified portion of the fiber surrounded by the cladding.

[0144] Each of the plurality of single-mode sapphire optical waveguides can be provided with a Bragg grating or a plurality of Bragg gratings. Each of the single-mode sapphire optical waveguides among 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.

[0145] A pair of Bragg gratings in each single-mode sapphire optical waveguide can have the same wavelength or overlapping wavelengths as a pair of Bragg gratings in other waveguides among the plurality of waveguides. Thus, the optical fiber can be configured to provide a single coherent output from the plurality of waveguides. The optical fiber can include an optical coupler (such as a directional coupler or an evanescent coupler) operable to couple the outputs of the respective waveguides of the plurality of single-mode sapphire optical waveguides. One or more phase shifters can be provided in the optical fiber and operably coupled to one or more waveguides. For example, each additional waveguide can have a respective phase shifter associated therewith and operable to assist in coupling the outputs of the plurality of waveguides to a single fiber.

[0146] Alternatively, the Bragg gratings of the respective waveguides of the plurality of single-mode sapphire optical waveguides may have wavelengths different from those of the Bragg gratings in other waveguides of the plurality of single-mode sapphire optical waveguides in the optical fiber. Thus, this optical fiber can generate light of a series of different wavelengths.

[0147] Light from each waveguide can exit the sapphire optical fiber and enter one of the corresponding plurality of auxiliary fibers. Some or each of the auxiliary fibers can include a phase shifter or communicate operably with a phase shifter such that a coupler can couple the light from each of the auxiliary fibers to a single output fiber. Thus, the phase shifter can change the phase of the light in each supplementary fiber to produce a single output beam of increased output by constructive interference of the light.

[0148] In some embodiments, including where a plurality of single-mode sapphire optical waveguides are provided, bulk sapphire can be used instead of an optical fiber. For example, a planar sapphire substrate can be used and can include a plurality of single-mode sapphire optical waveguides.

[0149] According to another aspect of the present invention, instead of sapphire, another hard crystal can be used. For example, diamond may be used instead of sapphire. That is, the present invention also provides any of the aspects described herein that include a different optical fiber material instead of a sapphire optical fiber. Although the use of sapphire optical fibers is preferred, many of the advantages of the present invention extend to optical fibers containing other materials. Accordingly, this aspect can include any of the features of the present invention described herein with reference to any other aspect of the present invention, provided that another fiber (e.g., a material containing a high sapphire (Al2O3) content, diamond, yttrium aluminum garnet (YAG, Y3Al5O 12 )), germanium silicate, borosilicate, or a fiber containing lithium niobate (LiNbO3)) can be used in place of the sapphire fiber described with respect to these aspects.

[0150] According to another aspect of the present invention, there is provided a method of stimulating light emission, which includes using a single-mode crystalline optical waveguide as a gain medium. The crystal may be within an optical fiber or may provide the optical fiber. The crystal may be a single crystal. Thus, the method can include stimulating light emission, which includes using a single-mode single-crystal waveguide as a gain medium, where the single-mode single-crystal waveguide is provided within an optical fiber. Viewed from another aspect, the present invention provides an optical device comprising an optical gain medium including a single-mode crystalline optical waveguide, which can include a single-mode single-crystal optical waveguide. Viewed from another aspect, the present invention provides a single-mode crystalline optical waveguide, which can provide a single-mode single-crystal optical waveguide. Viewed from another aspect, the present invention provides an optical fiber including a single-mode crystalline optical waveguide. The optical fiber can comprise a single-mode, single-crystal optical waveguide, i.e., a single-mode waveguide within a single-crystal fiber. Thus, according to these aspects of the present invention, the fiber is a crystalline fiber (e.g., a single-crystal fiber) having a waveguide therein. The fiber may include other materials such as a cladding, but the portion of the fiber including the waveguide may be crystalline and / or single-crystalline.

[0151] The single-crystalline material may be a single crystal. This may be a material in which the crystal lattice of the entire sample (e.g., fiber) is continuous and not broken to the ends of the sample, i.e., no portion of the material of the sample (e.g., fiber) is separated from another portion of the material of the sample by a grain boundary.

[0152] The single-mode crystalline optical waveguide can be used in place of the single-mode sapphire optical waveguide described in any of the aspects discussed herein and described in connection with the following figures. Thus, the single-mode crystalline waveguide can include any of the features described herein with reference to any aspect of the present invention. The optical fiber and / or waveguide can be made in accordance with any other aspect described herein, except that a different suitable material can be used instead of sapphire, for example, any material suitable for use as an optical fiber and / or gain medium. In particular, any single-crystalline material can be used instead of sapphire.

[0153] The crystal can include diamond, or a silicate mineral, or an oxide mineral, or a phosphate, or a carbonate, or a halide. The crystal can include garnet, for example, aluminum silicate such as yttrium aluminum garnet (YAG), lutetium aluminum garnet (LuAG). The crystal can include aluminum oxide such as corundum (e.g., sapphire or ruby) or CaGdAlO4 (CAIGO). The crystal can include zirconium dioxide (zirconia). The crystal can include any material suitable for use as an optical fiber.

[0154] The single-mode crystalline optical waveguide may be doped. The single-mode crystalline optical waveguide may be doped with titanium. The single-mode crystalline optical waveguide may be doped with a rare earth element. The single-mode crystalline optical waveguide may be co-doped with two or more elements. The single-mode crystalline 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 have the advantage of enabling optical gain within the crystal. Thus, the single-mode crystalline optical waveguide may be doped to provide a gain medium.

[0155] In particular, the single-mode crystalline optical waveguide may include yttrium aluminum garnet doped with ytterbium.

[0156] According to another aspect of the present invention, an optical device including a crystalline fiber is provided, the crystalline fiber including a plurality of waveguides therein. The crystalline fiber may be a single-crystalline fiber. The present invention can 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-crystalline 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, for example, a depressed-clad waveguide as described herein with reference to any aspect of the present invention. The waveguide may be, for example, a microstructure waveguide as described herein with reference to any aspect of the present invention. The waveguide may be, for example, a photonic crystal or photonic bandgap waveguide as described herein with reference to any aspect of the present invention. The waveguide may be laser-written. The waveguide may have a Bragg grating therein. The optical device and / or the crystalline fiber can include any of the features described herein with reference to any other aspect of the present invention.

[0157] Although separate aspects of the present invention are shown above, it will be understood that features described with reference to a particular aspect can be used in combination with any aspect as needed. BRIEF DESCRIPTION OF THE DRAWINGS

[0158] Here, exemplary embodiments of the present invention will be described by way of example only, with reference to the accompanying drawings.

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[0159] FIG. 1 shows a schematic diagram of an apparatus comprising an optical device 100 arranged as a gain element. This and subsequent figures are merely schematic and not drawn to scale. The optical device comprises a sapphire optical fiber 100. Within the sapphire 100, there is a single-mode optical waveguide 101 provided along the length of the fiber 100. The single-mode sapphire optical waveguide 101 comprises an optical core 101'. The sapphire waveguide 101 is doped with an alternative dopant such as titanium or a dopant element such as chromium that enables the waveguide 101 to function as a gain medium. This waveguide 101 can be written into the sapphire optical fiber 100, for example, using a femtosecond laser. This laser modification can cause a decrease in the refractive index of the sapphire within the region exposed to the femtosecond laser. Alternatively, the waveguide 101 may be formed by other means, such as causing modification in the exposed region by irradiation to form an inner cladding, for example. The waveguide 101 can be provided by forming a cladding within the fiber 100 by laser modification of the region surrounding the core 101' of the optical fiber 100 (see, for example, cladding 402 in FIG. 5a), and the unmodified core 101' and the cladding cooperate to provide the waveguide 101.

[0160] When the optical device 100 is used to emit light, a pump laser 102 is used to excite the ions in the waveguide 101 to a higher energy state. The pump laser 102 may be, for example, an argon ion laser at 514.5 nm or a frequency-doubled Nd:YAG at 532 nm. The pump laser may also be a diode laser. Any suitable means for pumping the gain medium can be used. The light from the pump laser 102 is coupled into the waveguide 101 via a dichroic mirror 103 and a lens 104. The input light 105 is incident through an input optical isolator 106 (e.g., a Faraday isolator) and is reflected by 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, specifically within the core 101'. The photons emitted in this way have the same wavelength as the input light 105. As a result, the emitted light is amplified within the waveguide 101 by the stimulation of the excited ions and emits photons of the same wavelength as the input light 105. The amplified light exits the waveguide 101 at the end of the fiber 100 and passes through an output optical isolator 107 (which may be an isolator of the same type 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, the residual pump light is removed by a filter 108. Thus, in some uses of the optical device 100, the input light 105 is amplified within the waveguide 101.

[0161] In an alternative embodiment of the present invention, the optical device 100 can comprise bulk sapphire instead of the sapphire optical fiber shown in FIG. 1. In this case, a single-mode optical waveguide can be provided within the bulk sapphire.

[0162] Figure 2 shows the configuration of an optical device 100 in which the waveguide 101 is pumped in both forward and backward propagation directions. That is, the light from the first pump laser 102a is incident on the waveguide 101 at the first end 101a of the waveguide 101, and the light from the second pump laser 102b is incident on the waveguide 101 at the second end 101b of the waveguide 101 facing the first end 101a. The light from the second pump laser 102b is coupled into the waveguide 101 from the end 101b via the dichroic mirror 103b and the lens 104b. Alternatively, the waveguide 101 can be pumped only from the rear, that is, only the second pump laser 102b can be used. That is, the waveguide 101 can be used in either direction.

[0163] Figure 3 shows a laser that uses the optical device 100 (for example, as shown in FIG. 1) as a gain medium of the laser. That is, the laser uses the doped sapphire single-mode waveguide 101 as a gain medium. The waveguide 101 includes Bragg grating reflectors 201 and 202 in the core 101' of the waveguide 101, and forms a cavity therebetween within the waveguide 101. The waveguide 101 is optically pumped by the pump laser 102. Since the effective refractive index of the Bragg gratings 201 and 202 varies periodically, the gratings reflect within a narrow wavelength range. This wavelength range falls within the gain window of the doped sapphire in the waveguide 101 and the optical core 101'. The Bragg gratings 201 and 202 provide the feedback that enables laser operation. The Bragg grating 201 has a very high reflectivity, but the Bragg grating 202 can have a lower reflectivity in order to allow light to be emitted from the ends of the waveguide 101 and the fiber 100. The optical isolator 107 prevents light from being reflected back toward the waveguide 101, and the optical filter 108 removes the emitted pump light. Alternatively, as shown in FIG. 2, the laser may use backward-propagating pump or counter-propagating pump laser light.

[0164] The laser system can include one or more gratings (e.g., Bragg gratings) outside the optical fiber 100, and the angles of these gratings can be adjustable so as to be able to adjust the output of the laser system. The optical fiber 100 may be provided with a single Bragg grating in the waveguide 101 as part of the optical cavity for laser generation, or may be provided with a reflector outside the optical fiber 100.

[0165] In other embodiments of the present invention, the reflection of light in the waveguide 101 can be provided by a mirror outside the waveguide or by the waveguide / air boundary at the inlet and / or outlet of the waveguide. That is, any suitable optical cavity can be used, and the optical fiber 100 and the waveguide 101 can include at least a part of the optical cavity.

[0166] The Bragg gratings 201, 202 can also be used as part of a sensor system. The optical fiber 100 is made of sapphire, and thus has high durability, so the fiber can be placed in a harsh environment such as an engine suitable for aerospace use. The fiber can be placed in an environment exposed to high levels of radiation such as a nuclear reactor. The conditions to which the fiber is exposed change the configuration of the fiber, particularly the configuration of the Bragg grating (or gratings if there are multiple), thereby changing the optical properties. Therefore, the configuration of the grating can be used to measure the characteristics of the environment in which it is placed.

[0167] Figure 4 shows another example of a laser. Since the divergence angle of the pump laser is generally wide, it may be difficult to efficiently couple the light from the pump laser into a single-mode waveguide. However, the sapphire fiber 100 is inherently a multimode waveguide, and the waveguide is formed at the interface between sapphire and air (where air substantially serves as the cladding). The divergent pump laser 300 is used to emit the pump light 301 into the multimode waveguide, i.e., into the optical fiber 100. Then, the higher-order propagation modes of the pump light 301 are guided within the larger optical 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 with the single-mode waveguide 101 and excites the ions within the single-mode waveguide 101 to a higher energy state. Some of the excited ions within the single-mode waveguide naturally decay back to the low energy state and emit photons. These photons stimulate additional (phase-coherent) photons emitted from other excited ions within the single-mode waveguide. Feedback is provided by two Bragg gratings 201, 202 that reflect light at a specific wavelength, resulting in a narrow spectrum.

[0168] For example, as shown in FIG. 4, there are a number of advantages associated with providing a single-mode waveguide within a multimode waveguide. These include being able to efficiently couple a pump with low spatial quality into the single-mode waveguide. Also, power scaling can be achieved by coupling multiple pumps into an optical fiber. Further, the pump light mode and the laser light mode can have a small mode radius that can be maintained over a longer distance compared to the distance achieved by diffraction, and as a result, this results in a reduction in the threshold pump power (for example, titanium-doped sapphire has an “intrinsic” threshold that is approximately 50 times that of neodymium-doped yttrium aluminum garnet, a commonly used laser gain medium). An optical pumping laser requires a certain amount of pump power before laser light is emitted. Therefore, it is desirable to have a lower threshold pump power. Further, the pump light can be guided such that pump absorption occurs over a distance corresponding to several times the absorption length in titanium-doped sapphire, and integrating over this distance does not involve the adverse effects of diffraction that would cause the mode radius to become impractically large. These advantages help to overcome common drawbacks regarding the doping level, pump beam quality, and minimum pump mode size that occur in bulk titanium-doped sapphire lasers.

[0169] Figure 5 shows various embodiments of forming a single-mode waveguide in sapphire. Figure 5(a) shows a cross-section of an optical device 100 (e.g., a periodic structure waveguide, a photonic crystal fiber, a microvoid fiber, a photonic bandgap fiber, etc.) comprising a microstructured waveguide 401. The optical device 100 comprises an unmodified sapphire core 101' within the waveguide 401 and a microstructured cladding 402, and thus there are periodic variations in the refractive index across the cross-section of the optical device 100. The waveguide 401 is configured to suppress all propagation other than the single mode within the core 101' by the cladding 402, and the optical device 100 is a single-mode optical sapphire fiber 100. These fibers are sometimes referred to as photonic crystal fibers or photonic bandgap fibers. The periodic structure can be formed by changing (lowering) the refractive index of the exposed region 403 using a femtosecond laser. These can also be formed, for example, by using 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 content of which is incorporated herein by reference in its entirety. The inventors have discovered that the methods and features disclosed therein can be combined with the features of the present invention.

[0170] Figure 5(b) shows a cross-section of an optical device 100 comprising a depressed cladding single-mode waveguide 410. The optical device 100 includes an unmodified sapphire core 101' surrounded by a cladding 411. The cladding 411 has a lower refractive index compared to the unmodified sapphire and is formed, for example, as described in GB1712640.0, by exposing the sapphire in the cladding region to femtosecond laser light pulses. Thus, the core 101' has a higher refractive index compared to the cladding 411 (e.g., the refractive index of the core 101' is 5×10 -4 to 5×10-2 (which may be large). Thus, the cladding 411 cooperates with the core 101’ to provide the waveguide 410. The claddings 402, 411 need not extend to the edge portions of the fiber 100 and may thus be surrounded by unmodified sapphire.

[0171] In order to reduce radiation loss, it may be advantageous to make the diameter of the cladding 411 as large as possible. In this example, the core 101’ has an elliptical cross-section and forms 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 be fabricated, for example, using the techniques described in Wang et al., “Single-Mode Sapphire Fiber Bragg Gratings” Opt. Express 30, 15482-15494 (2022) https: / / doi.org / 10.1364 / OE.446664, the content of which is hereby incorporated by reference in its entirety. The inventors have found that the methods and features disclosed therein can be combined with the features of the present invention.

[0172] In the optical devices of FIGS. 5(a) and (b), each of the cores 101’ (and thus each of the waveguides 401, 410) is offset from the center of the respective fiber cross-section shown. The sapphire surrounding the core 101’ can function as a multimode optical waveguide. Having a single-mode waveguide with an off-center core within a multimode optical waveguide has the advantage that an increased proportion of the pump light provided to the multimode optical waveguide can be coupled into the single-mode optical waveguides 401, 410.

[0173] In an alternative embodiment, to achieve similar advantages, the cross-sectional symmetry of the waveguide can be reduced to increase the pump light coupled to the gain medium of the core. For example, the cross-section of the multimode waveguide may be D-shaped.

[0174] Furthermore, while it is possible to use the entire sapphire optical fiber 100 as a multimode fiber surrounding the single mode fibers 101, 401, 410, one can take the concept of having one waveguide within another waveguide a step further and provide a fabricated waveguide nested within the optical fiber 100.

[0175] FIG. 6 shows a single mode waveguide 410 within a sapphire optical fiber 100 formed using an alternative method. FIG. 6(a) shows a cross-section of the optical device 100 formed from the sapphire optical fiber 100, which includes a core 101', a first cladding 411 surrounding and adjacent to the core 101' of the waveguide 410, and an inner single mode waveguide 410 including a second outer cladding 501 surrounding and adjacent to the first cladding 411. The first cladding 411 has a lower refractive index than the core 101', and the second cladding 501 has a lower refractive index than the first cladding 411. Thus, two nested waveguides are provided, with the first single mode waveguide provided by the core 101' cooperating with the first cladding 411, and the second multimode waveguide provided by the first cladding 411 cooperating with the second cladding 501. In this example, the multimode pump light is confined within the boundaries of the second cladding 501 and propagates through the first cladding 411. That is, the first cladding 411 substantially provides the optical core of the second waveguide.

[0176] Accordingly, the first cladding 411 provides a cladding that forms the first single-mode optical waveguide 410. The first cladding 411 also, in cooperation with the second cladding 501, forms the "core" of the second multimode optical waveguide. The cladding for 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 within the region to a larger dose of femtosecond laser (e.g., higher pulse energy and higher repetition rate) compared to the sapphire within the region of the first cladding 411, and the second cladding 501 has a lower refractive index than the first cladding 411.

[0177] The same advantages described above with respect to the arrangement depicted in FIG. 4 can also be applied to the nested waveguide arrangement shown in FIG. 6. The multimode light propagating within the outer waveguide can be used to pump the gain medium of the core 101' of the inner (single-mode) waveguide.

[0178] The optical device 100 comprising a multimode waveguide and a nested single-mode waveguide as described above can be referred to as a cladding-pumped optical device 100 or a cladding-pumped optical waveguide 101. In a cladding-pumped optical waveguide, it is desirable to ensure that only the core is doped. This is because if the first cladding (i.e., the optical core or the cladding-pumped waveguide) is doped, it may absorb pump power without contributing to optical gain. A method of selectively doping a sapphire fiber to form an internal core of a doped material is described in V, N, Kurlov et al., "Growth of Sapphire Core-Doped Fibers", Journal of Crystal Growth 91(3) 520-524 (1998), the content of which is incorporated herein by reference in its entirety. The inventor has discovered that the methods and features disclosed therein can be combined with the features of the present invention. However, it is difficult to limit the doped region to be within the single-mode optical waveguide core 101' because at least its core is very small.

[0179] Figure 6(b) shows a cross-section of the optical device 100 formed of a sapphire optical fiber 100 having a doped region 510 selectively doped with Ti 3+ has a doped region 510 formed of a sapphire optical fiber 100 having a doped region 510 selectively doped with Ti. Thus, this doped region 510 has the ability to function as a gain medium. There are a core 101', a first cladding 411 that surrounds and is adjacent to the core 101' to thereby provide a waveguide 410, and a second cladding 501 that surrounds and is adjacent to the first cladding 411. The core 101' is off-center 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 between the gain region (i.e., the doped region 510) and the first cladding 411 that forms the core of the multimode waveguide. Thus, the overlap between the plurality of propagation modes in the pump waveguide (i.e., within the first cladding 411) and the doped region 510 is reduced (or minimized). Thus, the energy loss due to the pump mode that excites the gain material outside the core 101' of the single-mode waveguide 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 fiber can have an elliptical cross-section, a rectangular cross-section, or any suitable shape.

[0180] Therefore, the cladding of the single-mode sapphire optical fiber can include a doped portion and an undoped portion. Most of the cladding of the single-mode waveguide can be undoped. The fiber can include a doped region outside the cladding.

[0181] This method can include modifying the optical fiber 100 such that the core 101' is located at the end 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 waveguides 101, 410.

[0182] There are a number of types of lasers that can be manufactured or provided using the doped single-mode sapphire fiber described herein. The laser may be passively mode-locked (e.g., with a saturable absorber) or actively mode-locked (e.g., with a modulator). The laser may be a narrow bandwidth laser or a single frequency laser. The laser can be diode pumped (e.g., with a DFB or DBR semiconductor laser). The laser can be a master oscillator power amplifier (with the amplifier following the laser). The laser can be injection locked with light 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. Many configurations are possible (e.g., ring laser, figure-eight). To avoid the need for polarization control, it is beneficial to maintain the polarization of the single-mode sapphire optical fiber.

[0183] Figure 7 shows a sensor system using a single-mode sapphire optical fiber 100, in which a single-mode waveguide is used as a gain medium and the propagation loss is reduced. This system has the length of the sapphire fiber 100, which includes along its length a waveguide 110 having a series of fiber Bragg gratings (601a, 601b, etc.) provided along the length of the waveguide 110. The sapphire waveguide 110 is Ti 3+Since it is doped, the sapphire waveguide 110 can function as a gain medium (which acts to reduce waveguide loss). The sapphire fiber 100 is excited by the pump laser 102. The Bragg gratings 601a, 602b have different wavelengths and are temperature sensitive. The wavelengths of each Bragg grating 601a, 601b (for example, the central wavelength of the Bragg reflection spectrum) are determined by scanning the adjustable laser 610. The light reflected from the Bragg gratings 601a, 601b is directed to the photodetector 611 via the splitter 612. The splitter may be a separate silica coupler (for example, a fused silica coupler) or a waveguide splitter in the sapphire (for example, a Y splitter, a multimode interference coupler) or a separate optical circulator. The Bragg grating is a sensor. For example, the strain or temperature near each Bragg grating changes its wavelength. The laser 610 can adjust (sweep) the wavelength to measure what value the central wavelength of each Bragg grating is, and thus the illustrated configuration can be used as a sensor system.

[0184] Instead of using an FBG, distributed sensing incorporating a sapphire optical device according to any aspect of the present invention can be used, for example, sensing based on backscattered light. The sensor may be based on Rayleigh scattering, Brillouin scattering, or Raman scattering.

[0185] FIG. 8a shows a cross-section of optical devices 100, 800 comprising a plurality of depressed-clad single-mode waveguides 810. That is, the waveguides are arranged in parallel within an optical fiber. The optical devices 100, 800 each comprise a plurality of cores 101’, 801' of unmodified sapphire, each core being surrounded by respective clads 411, 811 that are modified to have a lower refractive index compared to the unmodified sapphire cores 101’, 801'. In this example, the cores 101’, 801' are elliptical so that the waveguides maintain polarization. In this example, most of the sapphire is doped as indicated by dots throughout the cross-section of the figure. Selective doping of sapphire is difficult, and it may be easier or preferable to dope a larger area of the optical devices 100, 800, or the entire fiber. By providing a plurality of waveguides within such optical devices 100, 800, the efficiency loss due to pump light coupling to the gain medium outside the waveguides, i.e., the doped sapphire, can be minimized. The waveguides are arranged in a close-packed configuration to maximize coverage within the fiber. In this example, seven waveguides 101, 810 are provided in a close-packed configuration, forming a hexagon.

[0186] FIG. 8b shows another example of optical devices 100, 820 comprising a plurality of depressed-clad single-mode waveguides 101, 830. However, in this example, most of the sapphire between the cores 101’, 831' is modified to form clads 411, 831 for each waveguide 101, 830 that have a lower refractive index compared to the unmodified sapphire of the cores 101’, 831'. Thus, the waveguides share a common depressed-clad.

[0187] The description of the depressed-clad waveguide 410 in FIG. 5b is applicable to each of the waveguides 101, 810, 830.

[0188] FIG. 9 shows cross-sections of optical devices 100, 900 (e.g., periodic structure waveguides, photonic crystal fibers, microvoid fibers, photonic bandgap fibers, etc.) comprising a plurality of microstructured waveguides 101, 910. The optical devices 100, 900 include a plurality of cores 101’, 901' of unmodified sapphire and microstructured claddings 402, 902 (e.g., a periodic array of laser-modified regions), and thus there are periodic variations in refractive index across the cross-sections of the optical devices 100, 900. In this example, most of the sapphire between the cores 101’, 901' has been modified to form the claddings 402, 902 of each waveguide 101, 910, and the cores are formed where there are gaps within the microstructures.

[0189] The description of the microstructured waveguide 401 in FIG. 5a is applicable to each of the waveguides 101, 910.

[0190] FIG. 10(a) shows optical devices 100, 1000 comprising fibers 100, 1002 shown in cross-section perpendicular to the longitudinal length of the fibers. The optical devices 100, 1000 include a plurality of waveguides 101, 1010, e.g., the waveguides of FIGS. 8(a), 8(b), and / or FIG. 9.

[0191] FIG. 10(b) shows the optical devices 100, 1000 in cross-section parallel to the longitudinal length of the fibers. Each waveguide 101, 1010 includes a pair of Bragg gratings 1011 disposed one at each end of the waveguide to provide feedback. Each pair of Bragg gratings 1011 can be made to have the same Bragg wavelength as the other pairs of Bragg gratings 1011 to provide a single coherent output from the optical devices 100, 1000. The output from each waveguide 101, 1010 is coherently coupled to an optical coupler 1003 (directional coupler or evanescent coupler). In this example, the optical coupler is formed within the fiber 1002.

[0192] Preferably, the light from each waveguide should be in phase so that constructive interference occurs when the light is combined. In some examples, one or more of the waveguides can be operably connected to a phase shifter (such as a thermal phase shifter, etc.). Alternatively, they may be passive or trimmed passively. The fiber may be fixed in place, for example, so as not to bend in order to eliminate changes in the phase between waveguides. The phase of the light in the waveguide can be controlled by changing the length of the waveguide or by changing the "effective refractive index" of the waveguide. The effective refractive index depends on the core size and the difference in refractive index between the core and the cladding. For example, the effective refractive index of one or more waveguides can be changed by writing a line with a laser in the core (which is not originally modified) to reduce the refractive index of the core, writing a ring with a laser around the core, and / or modifying the cladding with a laser (e.g., "overwriting the cladding") to further reduce the refractive index of the cladding. Thus, the optical device includes an optical coupler configured to couple light from a plurality of waveguides to a smaller number of waveguides, such as a single waveguide.

[0193] Instead of having all pairs of Bragg gratings at the same wavelength, each waveguide can have a pair of Bragg gratings at different wavelengths to generate a series of different wavelengths. If desired, an appropriate configuration of the Bragg gratings can be provided.

[0194] Figure 11 shows the state in which the optical devices 100 and 1000 of FIGS. 10(a) and (b) are used as lasers. The optical devices 100 and 1000 include a plurality of parallel waveguides in a single optical fiber. In order to excite the ions in the waveguides 101 and 1010, light from at least one pump laser 102 and 1100 is injected into each of the waveguides 101 and 1010. In order to utilize the power from the pump lasers 102 and 1100 as much as possible, all of the parallel waveguides can be pumped using the same pump lasers 102 and 1100. The input light causes stimulated emission of light from the excited ions in the waveguides 101 and 1010. The light emitted from the waveguides is coupled to a single waveguide in the fibers 100 and 1000 to form a laser output, and this output is supplied to the isolators 106 and 1120 and the filters 108 and 1130 to filter the pump light from the laser output.

[0195] Figure 12 shows other optical devices 100 and 1200 that include a plurality of waveguides 101 and 1210 in a single fiber used as a laser. Light from at least one pump laser 102 and 1220 is injected into each of the waveguides 101 and 1210 to excite the ions in the waveguide 1210. All of the parallel waveguides can be pumped using the same pump lasers 102 and 1220. The output of each waveguide 101 and 1210 is supplied to an auxiliary optical fiber 1230. One or more of the auxiliary fibers 1230 are provided in operable communication with a phase shifter 1232. A plurality of phase shifters 1232 can be provided, and each phase shifter 1232 can be dedicated to only one auxiliary fiber 1232. Since it may be difficult to maintain phase control within and between the waveguides of the optical fiber, the phase of each phase shifter 1232 is adjusted independently to ensure that the optical outputs from each waveguide 101 and 1210 are in phase and constructively interfere to produce a single output beam having a higher power (e.g., increased power compared to the fiber in a single waveguide therein).

[0196] Figures 10(b), 11, and 12 show that there are jagged cuts in fibers 1002 and 1202 and the images are cropped, but the fibers are continuous between the Bragg gratings and may extend longer than shown in the images. Fibers of any suitable length can be used. Of course, these images are only schematic diagrams for explaining the concept.

[0197] Figures 13(a) and 13(b) show optical devices 100, 1300 comprising a planar substrate 1302 extending in the longitudinal direction. Figure 13(a) shows a cross-section perpendicular to the longitudinal length of the optical devices 100, 1300, and Figure 13(b) shows a cross-section parallel to the longitudinal length of the optical devices 100, 1300. The planar substrate comprises a plurality of waveguides 101, 1310 each having a pair of Bragg gratings 1320 and a coherent coupler 1330. Materials of any suitable shape can be used, and optical fibers have related advantages, but materials having other shapes can also be used and have related advantages.

[0198] The optical devices 100, 1300 can be used as lasers, in which case pump light is emitted onto the substrate, the outputs of the waveguides 101, 1310 are coupled to the coherent coupler 1330, and a single coherent combined output is formed. Using a planar substrate may make it easier to ensure that the outputs are maintained in phase.

[0199] The figures and embodiments in this specification are described with reference to optical fibers including sapphire, but the optical fibers can include any suitable material instead of sapphire, and the present invention extends to such embodiments. For example, instead of sapphire, crystalline materials, such as crystalline fibers like single-crystalline fibers, can be used. As described in this specification, any suitable material can be used.

[0200] Without departing from the broader aspects and spirit of the present invention, various changes and modifications can be made. The provided values or ranges can be replaced with other values or ranges to achieve the desired result. When a singular form is used (e.g., "an", "a", "the", "this"), it is to be interpreted as one or more items. When the word "comprise" is used, it is to be interpreted as including the subsequent method steps and / or elements, but can also include additional method steps and / or elements. The steps described in the methods herein can be performed in any order or simultaneously. Individual steps or groups of steps can be deleted from any method without losing the desired effect. Individual elements or groups of elements can be deleted from any device without losing the desired effect. Parts of any example can be combined with parts of any other example to obtain advantages. When an element or step is described as optional, it should not be construed as meaning that other elements or steps are essential. Those skilled in the art will understand that, as defined by the claims, any combination of features is possible within the scope of the present invention.

Claims

1. A method for stimulating the emission of light, including using a single-mode sapphire optical waveguide as a gain medium.

2. The method according to claim 1, wherein the single-mode sapphire optical waveguide is a depressed-clad waveguide.

3. The method according to claim 1 or 2, comprising amplifying signal light using the single-mode sapphire optical waveguide as a gain medium.

4. The method according to claim 1 or 2, comprising generating laser light using the single-mode sapphire optical waveguide as a gain medium.

5. The method according to claim 1 or 2, comprising fabricating a Bragg grating within the single-mode sapphire optical waveguide.

6. The method according to claim 1 or 2, comprising forming an optical cavity by providing opposing Bragg gratings within the single-mode sapphire optical waveguide.

7. The method according to claim 1 or 2, comprising using the single-mode sapphire optical waveguide as part of a sensor system.

8. The method according to claim 1 or 2, comprising fabricating the single-mode sapphire optical waveguide using laser modification and adaptive optical aberration compensation.

9. The method according to claim 1 or 2, comprising providing the single-mode sapphire optical waveguide within a sapphire optical fiber, and using the sapphire optical fiber as a multimode waveguide while simultaneously using the single-mode sapphire optical waveguide as a single-mode waveguide.

10. The method according to claim 1 or 2, comprising fabricating a cladding within a sapphire optical fiber to form a laser-modified cladding around an optical core, and the optical core and the cladding cooperating to provide a single-mode sapphire optical waveguide.

11. The method according to claim 10, wherein the cladding is a first cladding, and the method comprises creating a second cladding around the first cladding.

12. The method according to claim 1 or 2, comprising doping a sapphire optical fiber to form a doped region therein, and fabricating the optical core of the single-mode sapphire optical waveguide within the doped region.

13. The method according to claim 1 or 2, comprising fabricating the single-mode sapphire optical waveguide at an off-center position within a sapphire optical fiber.

14. The method according to claim 1 or 2, comprising fabricating the single-mode sapphire optical waveguide having asymmetrical characteristics.

15. The method according to claim 1 or 2, comprising coupling a pump laser to the single-mode sapphire optical waveguide.

16. A sapphire optical device comprising an optical gain medium including a single-mode sapphire optical waveguide.

17. The sapphire optical device according to claim 16, wherein the single-mode sapphire optical waveguide is a depressed-clad waveguide.

18. A sapphire optical device according to claim 16 or 17, comprising a Bragg grating.

19. The sapphire optical device according to claim 18, comprising opposing Bragg gratings providing an optical cavity for a laser system.

20. The sapphire optical device according to claim 16 or 17, wherein the sapphire optical device is a laser-modified sapphire optical fiber.

21. The sapphire optical device according to claim 16 or 17, wherein the sapphire optical device is a multimode sapphire optical fiber, and the single-mode sapphire optical waveguide is arranged within the multimode fiber.

22. The sapphire optical device according to claim 16 or 17, comprising a doped region, wherein the optical core of the single-mode sapphire optical waveguide is located within the doped region.

23. The sapphire optical device according to claim 16 or 17, wherein the single-mode waveguide is off-center within the optical device.

24. The sapphire optical device according to claim 16 or 17, wherein the single-mode sapphire optical waveguide comprises a laser-machined cladding surrounding the optical core.

25. The sapphire optical device according to claim 24, wherein the laser-processed cladding is a first cladding, and the optical device comprises a second laser-processed cladding surrounding the first cladding.

26. A laser system that generates laser light comprising the optical device described in claim 16 or 17, wherein the optical device is the gain medium of the laser system.

27. An optical amplifier comprising the optical device according to claim 16 or 17.

28. A sensor system comprising the optical device according to claim 16 or 17.

Images