High-brightness high-power raman photonic crystal fiber laser

Abstract

The application discloses a high-brightness and high-power Raman photonic crystal fiber laser, which comprises a pump source and a Raman gain medium connected with the pump source; the Raman gain medium is a full-solid photonic crystal fiber in a long cone shape; the full-solid long cone photonic crystal fiber is continuously and proportionally reduced in the whole cone area, and constantly satisfies the double-mode operation condition in the working wavelength range; the thick end is the input end, and the thin end is the output end; the full-solid photonic crystal fiber comprises a core and a cladding from inside to outside; the cladding microstructure comprises a high-refractive-index matrix material and a low-refractive-index filling material; the filling material is filled in the matrix material and is arranged in a two-dimensional closest equilateral triangle lattice periodically; the microstructure of the filling material is arranged in a concentric circle along the radial direction of the center of the core, and the number of the microstructure is 3-5 circles. The laser can effectively improve the laser beam quality of the output end and realize single-mode laser output.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and more specifically to a high-brightness, high-power Raman photonic crystal fiber laser. Background Technology

[0002] Quartz optical fiber has the characteristics of bendability, extremely large specific surface area, and waveguide mode with near-diffraction-limited beam quality. Rare-earth-doped optical fibers based on quartz glass optical fiber have achieved near-single-mode laser output with continuous power of kilowatts or even kilowatts in the near-infrared 1-2 micrometer range.

[0003] Regarding laser output wavelength, due to the characteristics of rare-earth energy level electronic transitions, the output wavelength of rare-earth-doped fiber lasers is discrete, and their gain bandwidth is limited. Even within the 1-2 micrometer band, where silica fiber loss is lowest, it only covers a few regions such as 1-1.1 micrometers, 1.3-1.4 micrometers, 1.5-1.6 micrometers, and 1.7-2.1 micrometers. Conversely, Raman silica fiber lasers do not rely on rare-earth doping and, theoretically, can achieve laser output of any wavelength within the low-loss band of silica fiber. Therefore, they are an irreplaceable high-power fiber laser type compared to rare-earth-doped fiber lasers. High-power rare-earth-doped silica fiber lasers or high-power semiconductor lasers are used as pump sources, combined with the Raman frequency shift of silica fiber (approximately 440 cm⁻¹). -1 It can achieve quartz fiber laser output at special wavelengths such as 1.1-1.3μm, 1.6-1.7μm and 2.1-2.5μm.

[0004] The method of using core-pumped graded refractive index large core diameter (typical core diameter: 50-100 micrometers) multimode passive fiber is one of the important ways to achieve kilowatt-level high power, high brightness and high beam quality Raman fiber laser output in pure Raman gain passive fiber.

[0005] According to optical waveguide theory, in graded-index large-core-diameter multimode Raman gain fiber, the spatial overlap between different transverse modes of the pump field and the generated Raman laser field is significantly different, leading to a large difference in the pump energy distribution obtained by different transverse modes. This mode competition results in the fundamental mode in the Raman laser field obtaining significantly greater gain than other higher-order transverse modes. Specifically, simulation results by Terry et al. (Nathan B. Terry, Thomas G. Alley, and Timothy H. Russell, "An explanation of SRS beam cleanup in graded-index fibers and the absence of SRS beam cleanup in step-index fibers," Opt. Express 15, 17509-17519 (2007)) show that under random coupling conditions, in graded-index large-core-diameter multimode Raman gain fiber, the fundamental mode LP... 01 The relative gain coefficient is higher than that of other higher-order transverse modes; correspondingly, in step-index large-core-diameter multimode Raman gain fibers, the fundamental mode LP 01 The relative gain coefficient is only higher than that of a few of the lowest-order higher-order transverse modes, while this competitive advantage is uncertain relative to higher-order transverse modes. Due to this "mode cleaning" effect, in graded-index large-core-diameter multimode Raman gain fibers, Raman laser energy is more concentrated in the fundamental mode and other lower-order modes, thus effectively improving the quality of the laser output beam.

[0006] In current work using this method to achieve kilowatt-level high-power Raman laser output, the laser output beam quality factor M... 2 Not less than 1.5, not achieving the ideal near-diffraction-limited single-mode high beam quality (i.e., M). 2 ≤1.1).

[0007] To achieve controllable high-power, high-brightness near-diffraction-limited single-mode Raman laser output in large-core non-single-mode optical fibers, a transverse mode stabilization mechanism needs to be introduced into the fiber resonator. This mechanism ensures that the fundamental mode always maintains a dominant position in the competition among several foreseeable transverse modes. This stabilization mechanism can ensure that mode cleaning can work stably under high laser power conditions of kilowatts or more.

[0008] For the technique of core-pumped graded-index large-core-diameter multimode passive Raman gain fiber, the problem can be solved by using few-mode passive Raman gain fiber with fewer transverse modes and whose mode number can be stably controlled; preferably, dual-mode passive Raman gain fiber is an acceptable choice. However, in traditional multimode passive fiber structures, each mode has its own unique cutoff wavelength and critical core diameter, making it impossible to simultaneously maintain a large core diameter to receive higher-power pump lasers while maintaining a small number of modes. Therefore, the goals of increasing Raman laser output power and significantly increasing the brightness difference between pump lasers and Raman lasers are contradictory. This restricts the simultaneous achievement of increased power and brightness in Raman fiber lasers based on traditional dual-mode passive Raman gain fiber structures at high power levels (especially at kilowatt-level and above). Summary of the Invention

[0009] The purpose of this invention is to disclose a high-brightness, high-power Raman photonic crystal fiber laser that solves the aforementioned technical problems by utilizing a fundamental mode LP supported by a two-mode fiber. 01 and the lowest order higher order mode LP 11 The difference in Raman gain leads to active mode cleaning, ultimately achieving high-power, high-brightness, near-diffraction-limited single-mode laser output.

[0010] To achieve the above objectives, the present invention provides a high-brightness, high-power Raman photonic crystal fiber laser, comprising: a pump source and a Raman gain medium connected to the pump source;

[0011] The Raman gain medium is a solid photonic crystal fiber, which is long and tapered, with the thicker end being the input end and the thinner end being the output end. The solid photonic crystal fiber comprises, from the inside out, a core and a cladding located outside the core. The microstructure of the cladding comprises a high-refractive-index matrix material and a low-refractive-index filling material. The filling material is filled in the matrix material and arranged periodically in a two-dimensional closest equilateral triangular lattice. The microstructure of the filling material is arranged in concentric circles radially around the center of the core, and the number of circles of the microstructure of the filling material is 3-5.

[0012] The refractive index difference between the filler material and the matrix material is 0.5%-5.0%;

[0013] The ratio of the cross-sectional diameter d of the microstructure of the filling material to the spacing Λ between adjacent microstructures is 0.50-0.60;

[0014] The operating wavelength λ of the fiber laser is in the range of 0.4-2.5μm, and the solid photonic crystal fiber only supports dual transverse mode operation within the operating wavelength range.

[0015] The microstructure parameters of the long tapered solid photonic crystal fiber from the input end to the output end satisfy D core =2Λ-d, and Λ, d and D core Continuously scaled down proportionally, with a scaling-down ratio ranging from 1.05 to 10.0;

[0016] The core diameter D of the input fiber core core 20-200μm;

[0017] The core diameter D of the output fiber core core '7-150μm;

[0018] The taper of the solid photonic crystal fiber is smaller than the numerical aperture of the fiber core; ideally, the taper range is 10. -7 -10 -5 / rice;

[0019] By elongating and tapering the solid microstructure photonic crystal fiber, the fiber remains within the microstructure parameter region that satisfies constant dual-mode operation within the range of tapered region variation. Utilizing the active mode cleaning effect brought by the Raman gain of the solid microstructure photonic crystal fiber, which only supports dual transverse mode operation, the Raman laser energy in the solid microstructure photonic crystal fiber is ultimately completely concentrated on the fundamental mode, thereby achieving single-mode laser output.

[0020] As a further improvement of the present invention, a first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at both ends of the Raman gain medium. The first resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity of less than 50% and is arranged at the output end; the second resonant cavity mirror is a high-reflectivity Bragg fiber grating with a reflectivity of greater than 70% and is arranged at the input end.

[0021] As a further improvement of the present invention, the Raman gain medium adopts a semi-open resonant cavity structure at both ends. A first resonant cavity mirror is arranged at the input end. The first resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity of less than 10%. The output end is an open structure resonant cavity, which only utilizes the distributed Rayleigh scattering present in the low-loss passive silica Raman gain fiber as weak light feedback at the output end of the resonant cavity, thereby forming a semi-open random distributed fiber resonant cavity.

[0022] As a further improvement of the present invention, the two ends of the Raman gain medium adopt a fully open resonant cavity structure, and the distributed Rayleigh scattering present in the low-loss passive quartz Raman gain fiber is used as the weak light feedback of the entire resonant cavity to form a fully open random distributed fiber resonant cavity.

[0023] As a further improvement of the present invention, the matrix material is pure quartz glass or quartz glass doped with a high molar refractive index component;

[0024] The filling material is quartz glass with a lower refractive index than the matrix material.

[0025] As a further improvement of the present invention, the filling material is quartz glass doped with a low molar refractive index component, or highly fluorine-doped quartz glass, or pure quartz glass.

[0026] As a further improvement of the present invention, the pump source and the Raman gain medium are connected in a low-loss manner.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] (1) A high-brightness, high-power Raman photonic crystal fiber laser uses a solid photonic crystal fiber as the Raman gain medium and a tapered design to ensure that, during the proportional reduction of the tapered fiber, any core size at any position of the tapered fiber can meet the dual-mode operation conditions within the required operating wavelength range. Since the fiber is always located within the microstructure parameter region that satisfies constant dual-mode operation within the tapered region variation range, the active mode cleaning effect brought by the Raman gain of the solid photonic crystal fiber that only supports dual transverse mode operation is used to completely concentrate the Raman laser energy in the solid photonic crystal fiber onto the fundamental mode, thereby effectively improving the laser beam quality at the output end and realizing single-mode laser output. Even when the temperature field inside the core changes during high-power laser operation, the constant dual-mode operation conditions of the fiber will not be destroyed, thus ensuring the stable existence of the mode cleaning conditions of the multimode Raman fiber laser.

[0029] (2) The solid microstructure of the fiber ensures that the ratio of the cross-sectional diameter d of the microstructure to the spacing Λ of the adjacent microstructures does not change in a wide range of lengths from meters to kilometers during the fabrication of the long tapered photonic crystal fiber. The input end of the long tapered fiber has a large core diameter and a high numerical aperture, which allows the fiber input end to receive high-power multimode laser with low beam quality as pump. At the same time, the large core diameter of the input end reduces the power density of the pump laser and increases the damage power of the input end. There is a large reduction ratio between the input end and the output end. At the same time, the mode cleaning effect generated by the mode competition between the fundamental mode and the lowest and highest order modes, the combined result of the two effects, enables the output end of the long tapered photonic crystal fiber to achieve high power, high brightness, and near diffraction limit single-mode Raman laser output. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the cross-sectional structure of a solid photonic crystal fiber in a high-brightness, high-power Raman photonic crystal fiber laser according to the present invention.

[0031] Figure 2This is a schematic diagram of the long tapered variation structure of a solid photonic crystal fiber in a high-brightness, high-power Raman photonic crystal fiber laser according to the present invention;

[0032] Figure 3 This is a schematic diagram of the selected range of the dual-mode region in a high-brightness, high-power Raman photonic crystal fiber laser of the present invention.

[0033] In the diagram: 1. Fiber core; 2. Cladding; 3. Input terminal; 4. Output terminal. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings. However, it should be noted that these embodiments are not intended to limit the present invention. Equivalent changes or substitutions in function, method, or structure made by those skilled in the art based on these embodiments are all within the scope of protection of the present invention.

[0035] Please refer to Figures 1 to 3 The present invention illustrates a specific embodiment of a high-brightness, high-power Raman photonic crystal fiber laser.

[0036] A high-brightness, high-power Raman photonic crystal fiber laser includes: a pump source and a Raman gain medium connected to the pump source; the pump source is a high-power single-mode fiber laser pump source or a high-power multimode semiconductor laser pump source; the Raman gain medium is a solid photonic crystal fiber, which is long and tapered, with a length of 1 meter to 10 kilometers; the thick end is the input end 3, and the thin end is the output end 4. The solid photonic crystal fiber includes, from the inside out, a core 1 and a cladding 2. The microstructure of the cladding 2 includes a high-refractive-index matrix material and a low-refractive-index filler material. The matrix material is pure silica glass or silica glass doped with a high molar refractive index component; the filler material is a silica glass material with a lower refractive index than the matrix material; the filler material is silica glass doped with a low molar refractive index component, or highly fluorine-doped silica glass, or pure silica glass.

[0037] The filler material is incorporated into the matrix material and arranged periodically in a two-dimensional tightest equilateral triangular lattice. Furthermore, the description of the microstructure parameters of the photonic crystal fiber in this invention is equally effective for other two-dimensional periodically arranged microstructure photonic crystal fibers, such as square, rectangular, and other two-dimensional periodic lattice arrangements. The microstructure of the filler material is arranged concentrically in radial circles around the center of the fiber core 1, with 3-5 circles. 3-5 circles ensure that the fiber outer diameter is not too large when the core diameter is large, and the constraint loss is not too small when the core diameter is small. Ideally, the cross-section of the microstructure of the filler material is circular; the refractive index difference between the filler material and the matrix material is 0.5%-5.0%; the ratio of the cross-sectional diameter d of the microstructure of the filler material to the spacing Λ of adjacent microstructures is 0.50-0.60; the operating wavelength λ of the fiber laser is in the range of 0.4-2.5 μm, and the solid photonic crystal fiber only supports dual transverse mode operation within the operating wavelength range; the microstructure of the long tapered solid photonic crystal fiber from input end 3 to output end 4 satisfies D… core =2Λ-d, and continuously reduced proportionally, with a reduction ratio ranging from 1.05 to 10.0; the core diameter D of fiber core 1 at input terminal 3. core The diameter is 20-200μm; the core diameter D of the 4-core fiber at the output end is 1. core The taper of the solid photonic crystal fiber is 17-150 μm; the taper of the fiber core is smaller than the numerical aperture of core 1, ideally 10 μm. -7 -10 -5 / meter; The microstructure design of the filling material in the solid photonic crystal fiber ensures that the ratio of the microstructure parameter d / Λ of the filling material does not change in the taper length range from meters to kilometers during the fabrication of the long tapered photonic crystal fiber; The range of microstructure parameters of the filling material in the solid photonic crystal fiber ensures that the photonic crystal fiber that generates Raman gain always meets the dual-mode operation conditions, thereby ensuring that the core 1 size of the long tapered fiber at any cross-section of the long tapered fiber is within the required operating wavelength range, that is, within the 0.4-2.5 micrometer band, and can meet the dual transverse mode operation conditions for both pump wavelength and Raman laser wavelength during the proportional reduction process;

[0038] The larger core diameter and higher numerical aperture of the fiber input end 3 enable the fiber to collect more pump power. At the same time, the few-mode operation of the fiber allows the fiber input end 3 to receive high-power multimode laser with low beam quality as pump. Finally, the larger core diameter of the input end 3 reduces the pump laser power density and effectively increases the power value at which damage occurs at the fiber input end 3. By elongating and taperting the solid photonic crystal fiber and selecting a constant dual-mode region, the active mode cleaning effect of the solid photonic crystal fiber is used to completely concentrate the Raman laser energy in the solid photonic crystal fiber onto the fundamental mode, thereby effectively improving the laser beam quality of the output end 4 and achieving single-mode laser output.

[0039] A first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at both ends of the Raman gain medium. The first resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity of less than 50% and is arranged at the output end 4. The second resonant cavity mirror is a high-reflectivity Bragg fiber grating with a reflectivity of greater than 70% and is arranged at the input end 3.

[0040] Alternatively, the Raman gain medium adopts a semi-open resonant cavity structure at both ends. A first resonant cavity mirror is arranged at the input end 3. The first resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity of less than 10%. The output end 4 is an open structure resonant cavity, which only utilizes the distributed Rayleigh scattering present in the low-loss passive silica Raman gain fiber as the weak light feedback of the resonant cavity output end 4, thus forming a semi-open random distributed fiber resonant cavity.

[0041] Alternatively, a fully open resonant cavity structure can be used at both ends of the Raman gain medium, utilizing the distributed Rayleigh scattering present in low-loss passive silica Raman gain fiber as weak optical feedback for the entire resonant cavity, thus forming a fully open, randomly distributed fiber resonant cavity. The pump source and the Raman gain medium are connected via low-loss fusion splicing. Ideally, a low-loss all-fiber structure can be constructed using fiber fusion splicing.

[0042] This invention achieves the fabrication of a long tapered fiber with a continuously decreasing outer diameter (i.e., core diameter) by controlling the relative acceleration of the preform feed speed and the fiber drawing speed on a high-temperature fiber drawing tower. Secondly, to ensure that laser transmission within the entire long tapered photonic crystal fiber meets the total internal reflection condition, the tapered aspect ratio of the fabricated long tapered photonic crystal fiber is smaller than the numerical aperture of the fiber core 1. This ensures that the laser field energy generated and transmitted along the long tapered photonic crystal fiber laser can only switch between higher-order modes and the fundamental mode without leakage into the cladding 2. Due to the different orders of transverse modes of the pump field and the Raman laser field in the photonic crystal fiber... The different spatial overlap of transverse modes of different orders leads to mode competition among different transverse modes within the laser cavity, resulting in a selective increase in the gain of the fundamental mode compared to higher-order transverse modes, i.e., a significant cleaning effect on higher-order modes. Consequently, lasers based on solid photonic crystal fibers can ultimately achieve single-mode high-power Raman laser output with beam quality approaching the diffraction limit. While achieving high-power Raman laser output, the significant reduction ratio at the input end 3 and output end 4 of the long tapered solid photonic crystal fiber significantly improves the mode quality of the pump light and Raman laser, resulting in a brightness increase of 1-2 orders of magnitude.

[0043] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A high-brightness, high-power Raman photonic crystal fiber laser, characterized in that, include: A pump source and a Raman gain medium connected to the pump source; The Raman gain medium is a solid photonic crystal fiber, which is long and tapered, with the thicker end being the input end and the thinner end being the output end. The solid photonic crystal fiber comprises, from the inside out, a core and a cladding located outside the core. The microstructure of the cladding comprises a high-refractive-index matrix material and a low-refractive-index filling material. The filling material is filled in the matrix material and arranged periodically in a two-dimensional closest equilateral triangular lattice. The microstructure of the filling material is arranged in concentric circles radially around the center of the core, and the number of microstructure circles is 3-5. The refractive index difference between the filler material and the matrix material is 0.5%-5.0%; The ratio of the cross-sectional diameter d of the microstructure of the filling material to the spacing Λ between adjacent microstructures is 0.50-0.60; The operating wavelength λ of the fiber laser is in the range of 0.4-2.5μm, and the solid photonic crystal fiber only supports dual transverse mode operation within the operating wavelength range. The microstructure parameters of the long tapered solid photonic crystal fiber from the input end to the output end satisfy D core =2Λ-d, and Λ, d and D core Continuously scaled down proportionally, with a scaling-down ratio ranging from 1.05 to 10.0; The core diameter D of the input fiber core core 20-200μm; The core diameter D of the output fiber core core '7-150μm; The taper of the solid photonic crystal fiber is smaller than the numerical aperture of the fiber core, and the taper range is 10. -7 -10 -5 / rice; By elongating and tapering the solid microstructured photonic crystal fiber, the fiber remains within the microstructure parameter region that satisfies constant dual-mode operation within the range of tapered region variation. Utilizing the active mode cleaning effect brought by the Raman gain of the solid microstructured photonic crystal fiber, which only supports dual transverse mode operation, the Raman laser energy in the solid microstructured photonic crystal fiber is ultimately concentrated entirely on the fundamental mode, thereby achieving single-mode laser output.

2. The high-brightness, high-power Raman photonic crystal fiber laser according to claim 1, characterized in that, A first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at both ends of the Raman gain medium. The first resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity of less than 50% and is arranged at the output end. The second resonant cavity mirror is a high-reflectivity Bragg fiber grating with a reflectivity of greater than 70% and is arranged at the input end.

3. A high-brightness, high-power Raman photonic crystal fiber laser according to claim 1, characterized in that, The Raman gain medium adopts a semi-open resonant cavity structure at both ends. A first resonant cavity mirror is arranged at the input end. The first resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity of less than 10%. The output end is an open resonant cavity structure. The distributed Rayleigh scattering present in the low-loss passive silica Raman gain fiber is used as the weak light feedback at the output end of the resonant cavity, thus forming a semi-open random distributed fiber resonant cavity.

4. A high-brightness, high-power Raman photonic crystal fiber laser according to claim 1, characterized in that, The Raman gain medium adopts a fully open resonant cavity structure at both ends, and utilizes the distributed Rayleigh scattering present in the low-loss passive quartz Raman gain fiber as the weak light feedback of the entire resonant cavity, thus forming a fully open random distributed fiber resonant cavity.

5. A high-brightness, high-power Raman photonic crystal fiber laser according to claim 1, characterized in that, The matrix material is pure quartz glass or quartz glass doped with a high molar refractive index component; The filling material is quartz glass with a lower refractive index than the matrix material.

6. A high-brightness, high-power Raman photonic crystal fiber laser according to claim 5, characterized in that, The filling material is quartz glass doped with a low molar refractive index component, or highly fluorine-doped quartz glass, or pure quartz glass.

7. A high-brightness, high-power Raman photonic crystal fiber laser according to claim 1, characterized in that, The pump source and the Raman gain medium are connected in a low-loss manner.

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