Thulium-doped fiber laser based on composite cavity structure

Abstract

The utility model discloses a thulium-doped fiber laser based on a composite cavity structure. The thulium-doped fiber laser comprises a pumping source, a thulium-doped gain active fiber, a high-reflectivity fiber grating HR, a first low-reflectivity fiber grating OC1 and a second low-reflectivity fiber grating OC2, the self-pulse effect is inhibited by adopting a composite resonant structure, so that stable 2-micron fiber laser output is obtained; the overall optical path length is short, and self-pulse suppression can be more effectively achieved under the condition that the overall optical path length is short.

Description

Technical Field

[0001] The utility model relates to a thulium-doped fiber laser based on a composite cavity structure, and in particular to a 2μm fiber high-power laser based on a double low-reflection grating composite cavity structure. It can provide a solution for obtaining a highly reliable mid-infrared laser light source. Background Art

[0002] In recent years, the maturation of 2-micron fiber device fabrication processes and the continuous improvement of device reliability have significantly fueled the rapid development of 2-micron fiber laser research and applications. Many non-metallic materials and water molecules strongly absorb lasers in the 2-micron wavelength band. High absorption efficiency translates to high processing or efficiency, leading to important applications in non-metallic material processing and laser medical treatment. However, the achievement of high-power, high-efficiency 2-micron lasers is limited by physical factors. To improve optical-to-optical conversion efficiency, thulium-doped active fibers typically have high doping concentrations. However, high doping concentrations enhance excited-state absorption, cross-relaxation, and energy upconversion, leading to the formation of self-pulsations within the fiber resonator. Under high average power output conditions, the random pulse width and peak power intensity of self-pulsations can cause irreversible damage to fiber devices and reduce reliability. Furthermore, the presence of self-pulsations can also affect the linear power scaling. Currently, the commonly used techniques for suppressing self-pulsations include: 1. Increasing the OC reflectivity of the resonant cavity: On the one hand, this method can increase the laser energy density within the cavity. However, due to the reabsorption effect of thulium-doped fiber, it will repeatedly absorb the laser light within the resonant cavity and generate a certain amount of heat, causing heating of the active fiber. Therefore, this method will increase the thermal load of the thulium-doped fiber. On the other hand, the increased energy density within the cavity caused by the increased OC reflectivity is prone to the generation of nonlinear effects within the cavity, such as Raman and four-wave mixing, which will have a certain impact on the output spectral composition. Utility Model Content

[0003] In view of the deficiencies of the prior art, the utility model provides a thulium-doped fiber laser based on a composite cavity structure, which overcomes the deficiencies of the prior art and has a reasonable design.

[0004] In order to achieve the above objectives, the present invention is implemented through the following technical solutions:

[0005] A thulium-doped fiber laser based on a composite cavity structure includes a pump source, a thulium-doped gain active fiber, a high-reflection fiber Bragg grating (FBG) HR, a first low-reflection fiber Bragg grating (OC1), a second low-reflection fiber Bragg grating (OC2), and a pump combiner. Pump light output by the pump source is coupled into the thulium-doped gain active fiber through the pump combiner. The high-reflection fiber Bragg grating (HR) includes a first end on the front side and a second end on the rear side. The second end of the high-reflection fiber Bragg grating (HR) is connected to the first end of the thulium-doped gain active fiber through an optical fiber. The second end of the thulium-doped gain active fiber is connected to the first end of the first low-reflection fiber Bragg grating (OC1) through an optical fiber. The second end of the first low-reflection fiber Bragg grating (OC1) is connected to the first end of the second low-reflection fiber Bragg grating (OC2) through an optical fiber. The second end of the second low-reflection fiber Bragg grating (OC2) outputs the excitation light generated in the thulium-doped gain active fiber.

[0006] Preferably, the pump combiner includes a forward combiner, the forward combiner includes a branch end at a first end and a combining end at a second end, the branch end of the forward combiner includes a central fiber and multiple pump fibers, the pump source includes a plurality of semiconductor lasers, the multiple pump fibers at the branch end of the pump forward combiner are connected to the multiple semiconductor lasers of the pump source, and are used to input the pump light into the resonant cavity through the forward combiner; the combining end of the forward combiner includes a central fiber, and the central fiber of the combining end of the forward combiner is connected to the first end of the high-reflection fiber Bragg grating HR.

[0007] Preferably, the pump combiner includes a reverse combiner, the reverse combiner includes a combining end at a first end and a branch end at a second end, the branch end of the reverse combiner includes a central fiber and multiple pump fibers, the multiple pump fibers at the branch end of the reverse combiner are connected to multiple semiconductor lasers of the pump source, and are used to input the pump light into the resonant cavity through the reverse combiner, and the second end of the second low-reflection fiber grating OC2 is connected to the central fiber of the combining end of the reverse combiner through an optical fiber; the central fiber of the branch end of the reverse combiner outputs the excitation light generated in the thulium-doped gain active fiber to the rear side.

[0008] Preferably, the central fiber of the forward combiner is connected to the indicator light source, that is, the indicator light source is connected to the central fiber of the branch end of the forward combiner through an optical fiber, and the indicator light is input into the forward combiner.

[0009] Preferably, it also includes a stripper and an outputter, wherein the stripper is used to strip the cladding light from the forward transmission or the reverse transmission, and the outputter is used to couple out the output light of the laser.

[0010] Preferably, in the composite resonant cavity formed by the high-reflection fiber Bragg grating HR, the first low-reflection fiber Bragg grating OC1, and the second low-reflection fiber Bragg grating OC2, the high-reflection fiber Bragg grating HR and the first low-reflection fiber Bragg grating OC1 cooperate to form a first sub-cavity, the high-reflection fiber Bragg grating HR and the second low-reflection fiber Bragg grating OC2 are combined into a second sub-cavity, and the first low-reflection fiber Bragg grating OC1 and the second low-reflection fiber Bragg grating OC2 are combined into a third sub-cavity.

[0011] Preferably, the high-reflection fiber Bragg grating (HR) has a central wavelength in the range of 1900-2020 nm, a spectral bandwidth of 1-4 nm, and a reflectivity ≥ 99.5%; the first low-reflection fiber Bragg grating (OC1) has a central wavelength in the range of 1900-2020 nm, a spectral bandwidth of 0.5-2 nm, and a reflectivity of 15%-5%; the second low-reflection fiber Bragg grating (OC2) has a central wavelength in the range of 1900-2020 nm, a spectral bandwidth of 0.5-2 nm, and a reflectivity of 15%-5%.

[0012] Preferably, the output end of the output device is welded to a quartz end cap coated with an anti-reflection film, and there is a roughened area inside by corrosive abrasive corrosion or CO2 laser etching process to achieve forward cladding light stripping and reverse return cladding light stripping, and the fiber laser output is greater than 300W.

[0013] Preferably, the high-reflection fiber Bragg grating HR, the first low-reflection fiber Bragg grating OC1, and the second low-reflection fiber Bragg grating OC2 are obtained by a femtosecond laser etching process.

[0014] Preferably, the thulium-doped gain active optical fiber has a length greater than or equal to 10 m.

[0015] The utility model provides a 2μm optical fiber high-power laser based on a double low-reflection grating composite cavity structure.

[0016] The beneficial effects of the utility model are:

[0017] 1. The present invention provides a 2μm fiber laser that can achieve self-pulsation suppression using a composite cavity optical path structure. The mechanism by which the composite optical path structure suppresses self-pulsation lies in the fact that the oscillation longitudinal mode of a single oscillator cavity is significantly affected by the thermal effect of the gain fiber, which can induce continuous parasitic oscillations and exacerbate self-pulsation. After the composite oscillator cavity is formed, the oscillation longitudinal mode needs to simultaneously satisfy two cavity structures (HR and OC1, HR and OC2), which is conducive to stabilizing the oscillation frequency. The FP cavity formed by OC2 and OC1 is a passive fiber cavity and is less affected by the environment. From the perspective of the photon lifetime in the cavity, increasing the FP cavity also prolongs the photon lifetime in the cavity, which helps to suppress the self-pulsation effect.

[0018] 2. This utility model utilizes a composite resonant structure to suppress the self-pulsation effect, thereby achieving stable 2μm fiber laser output. Its short overall optical path length allows for more effective self-pulsation suppression. This patent utilizes a composite cavity structure with dual low-reflection gratings to suppress the self-pulsation effect of high-power thulium-doped fiber lasers, thereby achieving more stable and reliable high-power thulium-doped fiber lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the present invention or the prior art, the following briefly introduces the drawings required for describing the prior art.

[0020] Figure 1 This is a schematic diagram of the laser structure of the present utility model. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention.

[0022] The embodiment of the utility model provides a 2μm (thulium-doped) high-power fiber laser based on a composite cavity structure.

[0023] See also Figure 1 The high-power fiber laser includes an indicator light source 1, a pump source 2, a thulium-doped gain active fiber 3, a high-reflection fiber Bragg grating (HR), a first low-reflection fiber Bragg grating (OC1), a second low-reflection fiber Bragg grating (OC2) (a low-reflection fiber Bragg grating is also called an output coupler, i.e., OC: Output Coupler), a pump combiner, a stripper 9, an output device 10, and other components.

[0024] Preferably, the pump combiner includes a forward combiner 7 and a reverse combiner 8 .

[0025] Preferably, the forward beam combiner adopts an (N+1) beam combiner, and preferably, the forward beam combiner adopts a (6+1)*1 beam combiner. Preferably, the reverse beam combiner adopts an (N+1) beam combiner, and preferably, the reverse beam combiner adopts a (6+1)*1 beam combiner.

[0026] The forward combiner 7 includes a branch end at a first end and a combining end at a second end. The branch end of the forward combiner 7 includes a central fiber (i.e., a signal input fiber) and multiple pump fibers. The central fiber of the branch end of the forward combiner is connected to the indicator light source 1, i.e., the indicator light source 1 is connected to the forward combiner through an optical fiber to input the indicator light into the forward combiner. The pump source 2 includes a plurality of semiconductor lasers, and the multiple pump fibers at the branch end of the pump forward combiner 7 are connected to the multiple semiconductor lasers of the pump source 2, so as to input the pump light into the resonant cavity through the forward combiner; the combining end of the forward combiner includes a central fiber, and the central fiber of the combining end of the forward combiner is connected to the first end of the high-reflection fiber Bragg grating HR; the second end of the high-reflection fiber Bragg grating HR is connected to the first end of the thulium-doped gain active fiber through the optical fiber; the second end of the thulium-doped gain active fiber is connected to the first end of the first low-reflection fiber Bragg grating OC1 through the optical fiber; the second end of the first low-reflection fiber Bragg grating OC1 is connected to the first end of the second low-reflection fiber Bragg grating OC2 through the optical fiber; the second end of the second low-reflection fiber Bragg grating OC2 is connected to the central fiber of the combining end of the reverse combiner 8 through the optical fiber; the central fiber of the branch end of the reverse combiner is connected to the first end of the stripper through the optical fiber; the second end of the stripper is connected to the first end of the output device through the optical fiber; the second end of the output device serves as the output end of the laser and is used to output the excitation light generated by the active amplification part of the laser. Preferably, it is generally defined that in each structure of the optical path, the direction of the indicator light output emitted by the indicator light source and / or the direction of the laser output by the laser is defined as the forward direction of the optical path. Generally, an optical structure, along the forward direction, has its front side as the first end and its rear side as the second end.

[0027] Preferably, the reverse combiner 8 includes a combining end at a first end and a branch end at a second end, the branch end of the reverse combiner 8 includes a central fiber (i.e., output fiber) and multiple pump fibers, and the multiple pump fibers at the branch end of the reverse combiner 8 are connected to multiple semiconductor lasers of the pump source 2, so as to input the pump light into the resonant cavity through the reverse combiner.

[0028] Indicator light source 1: The output light of the indicator light source 1 is a red indicator light coupled with a semiconductor optical fiber. Preferably, the output optical fiber is 10 / 125 / 0.075.

[0029] Pump source 2: see Figure 1 The pump light is generated by a semiconductor and output through optical fiber coupling. Preferably, the number is 12. The pump light output wavelength is 793nm, the output power is ≥100W, and the output pigtail is 200 / 220 / 0.22.

[0030] Thulium-doped gain active fiber 3: Preferably, the fiber structure parameters are 20 / 400 / 0.11, the absorption coefficient is about 2 dB / m, and the fiber length is greater than or equal to 10 m.

[0031] For the high-reflection fiber Bragg grating HR, the first low-reflection fiber Bragg grating OC1, and the second low-reflection fiber Bragg grating OC2: wherein the high-reflection fiber Bragg grating HR: preferably, the central wavelength may be in the range of 1900-2020nm, the spectral bandwidth is 1-4nm, preferably, the spectral bandwidth is 4nm, and the reflectivity is ≥99.5%; the first low-reflection fiber Bragg grating OC1: preferably, the central wavelength may be in the range of 1900-2020nm, the spectral bandwidth is 0.5-2nm, preferably, the spectral bandwidth is 2nm, and the reflectivity is lower than 15%, preferably, the reflectivity is 15%-5%; preferably, it is 10%; the second low-reflection fiber Bragg grating OC2: preferably, the central wavelength may be in the range of 1900-2020nm, the spectral bandwidth is 0.5-2nm, preferably, the spectral bandwidth is 2nm, the reflectivity is less than 15%, preferably, the reflectivity is 15%-5%, preferably, it is 10%.

[0032] Preferably, the high-reflection fiber Bragg grating (FBG) HR, the first low-reflection fiber Bragg grating (OC1), and the second low-reflection fiber Bragg grating (OC2) are obtained through hydrogen loading or femtosecond mask lithography processes and combined with a thulium-doped active fiber to form a composite resonant cavity. In the composite resonant cavity, the high-reflection fiber Bragg grating (HR) and the first low-reflection fiber Bragg grating (OC1) cooperate to form a first sub-cavity (i.e., the first sub-cavity is formed between the high-reflection fiber Bragg grating (HR) and the first low-reflection fiber Bragg grating (OC1), the high-reflection fiber Bragg grating (HR) and the second low-reflection fiber Bragg grating (OC2) form a second sub-cavity (i.e., the second sub-cavity is formed between the high-reflection fiber Bragg grating (HR) and the second low-reflection fiber Bragg grating (OC2), and the first low-reflection fiber Bragg grating (OC1) and the second low-reflection fiber Bragg grating (OC2) form a third sub-cavity (i.e., the third sub-cavity is formed between the first low-reflection fiber Bragg grating (OC1) and the second low-reflection fiber Bragg grating (OC2). Under the joint action of the first, second, and third sub-cavities, effective suppression of self-pulsations can be achieved.

[0033] Forward combiner 7, preferably, the forward combiner is a (6+1)*1 combiner: the signal input fiber is 10 / 125 / 0.075, the output fiber is 20 / 400 / 0.11, the pump input fiber is 200 / 220 / 0.22, and there are 6 input arms.

[0034] Preferably, the reverse combiner 8 is a (6+1)*1 combiner: the signal input fiber is 20 / 400 / 0.11, the output fiber is 20 / 400 / 0.11, the pump input fiber is 200 / 220 / 0.22, and there are 6 input arms.

[0035] The forward combiner and the reverse combiner are used to inject the pump into the resonant cavity to provide pumping.

[0036] Stripper 9: Preferably, the stripper is obtained by CO2 laser etching from 20 / 400 / 0.11 passive optical fiber, and is used to efficiently strip the cladding light from forward transmission or reverse transmission, with a cladding light stripping rate of about 20dB.

[0037] Output 10: Preferably, the output fiber is a 20 / 400 / 0.11 passive fiber. The output end is fused to a quartz end cap coated with an antireflection coating. An internal roughening area is created using a frosted corroded surface or a CO2 laser etching process to remove both forward and reverse cladding light. (Our engineers realized that a stripper alone would be difficult to completely strip the cladding light near the fiber core, and that the stripper's ability to strip the return light was poor due to its distance from the output end. Therefore, an additional roughening area was provided inside the output.) This laser can achieve high-power fiber laser output exceeding 300W by modulating the injected pump light.

[0038] A composite resonant structure is used to suppress the self-pulsation effect, thereby achieving stable 2μm fiber laser output; under the condition of a short overall optical path length, more effective self-pulsation suppression can be achieved. The laser modulates the injected pump light.

[0039] The FP cavity formed by OC1 and OC2 is a passive fiber cavity that is less affected by the environment. From the perspective of the photon lifetime within the cavity, the addition of the FP cavity also extends the photon lifetime within the cavity, helping to suppress the self-pulsation effect.

[0040] Preferably, the high-reflection fiber Bragg grating HR, the first low-reflection fiber Bragg grating OC1, and the second low-reflection fiber Bragg grating OC2 can be obtained by femtosecond laser etching process, which can avoid the grating area heat problem caused by laser absorption brought by the hydrogen loading process and reduce the grating risk.

[0041] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments can still be modified, or some of the technical features thereof can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A thulium-doped fiber laser based on a composite cavity structure, comprising a pump source, a thulium-doped gain active fiber, a high-reflection fiber Bragg grating (HR), a first low-reflection fiber Bragg grating (OC1), a second low-reflection fiber Bragg grating (OC2), and a pump combiner; the pump light output by the pump source is coupled into the thulium-doped gain active fiber through the pump combiner. Its characteristics are: The high-reflection fiber Bragg grating HR includes a first end on the front side and a second end on the rear side. The second end of the high-reflection fiber Bragg grating HR is connected to the first end of the thulium-doped gain active fiber through an optical fiber; the second end of the thulium-doped gain active fiber is connected to the first end of the first low-reflection fiber Bragg grating OC1 through an optical fiber; the second end of the first low-reflection fiber Bragg grating OC1 is connected to the first end of the second low-reflection fiber Bragg grating OC2 through an optical fiber; the second end of the second low-reflection fiber Bragg grating OC2 outputs the excitation light generated in the thulium-doped gain active fiber.

2. The thulium-doped fiber laser based on a composite cavity structure according to claim 1, characterized in that: The pump combiner includes a forward combiner, which includes a branch end at a first end and a combining end at a second end. The branch end of the forward combiner includes a central fiber and multiple pump fibers. The pump source includes a plurality of semiconductor lasers. The multiple pump fibers at the branch end of the pump forward combiner are connected to the multiple semiconductor lasers of the pump source to input the pump light into the resonant cavity through the forward combiner. The combining end of the forward combiner includes a central fiber, and the central fiber of the combining end of the forward combiner is connected to the first end of the high-reflection fiber Bragg grating HR.

3. The thulium-doped fiber laser based on a composite cavity structure according to claim 1, characterized in that: The pump combiner includes a reverse combiner, which includes a combining end at a first end and a branch end at a second end. The branch end of the reverse combiner includes a central fiber and multiple pump fibers. The multiple pump fibers at the branch end of the reverse combiner are connected to multiple semiconductor lasers of a pump source and are used to input pump light into the resonant cavity through the reverse combiner. The second end of the second low-reflection fiber grating OC2 is connected to the central fiber of the combining end of the reverse combiner through an optical fiber; the central fiber of the branch end of the reverse combiner outputs the excitation light generated in the thulium-doped gain active fiber to the rear side.

4. The thulium-doped fiber laser based on a composite cavity structure according to claim 2, characterized in that: The central fiber of the forward combiner is connected to the indicator light source, that is, the indicator light source is connected to the central fiber of the branch end of the forward combiner through the optical fiber, and the indicator light is input into the forward combiner.

5. The thulium-doped fiber laser based on a composite cavity structure according to claim 1, characterized in that: It also includes a stripper and an output device. The stripper is used to strip the cladding light from the forward transmission or reverse transmission, and the output device is used to couple the output light of the laser.

6. The thulium-doped fiber laser based on a composite cavity structure according to claim 1, characterized in that: In the composite resonant cavity formed by the high-reflection fiber Bragg grating HR, the first low-reflection fiber Bragg grating OC1, and the second low-reflection fiber Bragg grating OC2, the high-reflection fiber Bragg grating HR and the first low-reflection fiber Bragg grating OC1 cooperate to form a first sub-cavity, the high-reflection fiber Bragg grating HR and the second low-reflection fiber Bragg grating OC2 are combined into a second sub-cavity, and the first low-reflection fiber Bragg grating OC1 and the second low-reflection fiber Bragg grating OC2 are combined into a third sub-cavity.

7. The thulium-doped fiber laser based on a composite cavity structure according to claim 6, characterized in that: The high-reflection fiber Bragg grating (HR) has a central wavelength in the range of 1900-2020nm, a spectral bandwidth of 1-4nm, and a reflectivity ≥99.5%; the first low-reflection fiber Bragg grating (OC1) has a central wavelength in the range of 1900-2020nm, a spectral bandwidth of 0.5-2nm, and a reflectivity of 15%-5%; the second low-reflection fiber Bragg grating (OC2) has a central wavelength in the range of 1900-2020nm, a spectral bandwidth of 0.5-2nm, and a reflectivity of 15%-5%.

8. The thulium-doped fiber laser based on a composite cavity structure according to claim 5, characterized in that: The output end of the output device is fused to a quartz end cap coated with an anti-reflection film. There is a roughened area inside the output device that is obtained by corrosive abrasive corrosion or CO2 laser etching process to achieve forward cladding light stripping and reverse return cladding light stripping. The fiber laser output is greater than 300W.

9. The thulium-doped fiber laser based on a composite cavity structure according to claim 1, characterized in that: The high-reflection fiber Bragg grating HR, the first low-reflection fiber Bragg grating OC1, and the second low-reflection fiber Bragg grating OC2 are obtained by adopting a femtosecond laser etching process.

10. The thulium-doped fiber laser based on a composite cavity structure according to claim 1, characterized in that: Thulium-doped gain active fiber, fiber length greater than or equal to 10m.

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