A polarization-maintaining single-frequency broadening and amplification system

Abstract

A polarization-maintaining single-frequency broadening amplification system, a polarization-controlled single-frequency broadening fiber amplifier system using phase modulation technology based on a white noise source. It effectively suppresses stimulated Brillouin scattering and stimulated Raman scattering, improves the nonlinear threshold, and reduces the production cost. Finally, the amplified output of a linearly polarized 3kW narrow-linewidth fiber laser was successfully achieved. The light-to-light conversion efficiency is 78%, the final output laser beam quality M2≈1.2, and the polarization extinction ratio is 15dB.

Description

Technical Field

[0001] The invention relates to a polarization-maintaining single-frequency broadening amplification system, and in particular to a polarization-controlled single-frequency broadening optical fiber amplification system utilizing white noise phase modulation technology. Background Art

[0002] High-power narrow-linewidth fiber lasers have outstanding advantages such as good beam quality, high efficiency, compact structure, and good coherence. They are widely used in beam synthesis, earth exploration, scientific research and other fields. In addition, coherent synthesis and spectral synthesis are effective solutions to obtain higher power output while maintaining good beam quality. The coherent synthesis / spectral synthesis system requires its basic unit module to have a single-channel fiber laser narrow linewidth output. Mode instability effect and nonlinear effect are the two major bottleneck scientific problems that limit the power increase of high-power narrow-linewidth fiber lasers, especially the mode instability effect of large mode field fiber, which not only severely limits the power increase but also causes the output laser beam quality to degrade sharply. It is a technical problem that needs to be solved urgently in the development of high-power fiber lasers. In this regard, domestic and foreign scholars have proposed a variety of technical solutions such as pulse pumping, dynamic mode excitation, and multi-core optical fiber to alleviate the mode instability effect.

[0003] At present, there are several solutions for single-frequency stretch amplifier systems: (1) Single-cavity 3000W fiber laser, which has the advantages of high integration, easy implementation and low cost; however, its final output laser is randomly polarized, the spectral width is wider, usually greater than about 6nm, and the beam quality is poor M2≈1.3. (2) Non-polarization-maintaining single-frequency stretch amplifier system, which can achieve relatively good beam quality, such as M2≈1.2, and the spectral linewidth is relatively narrow, usually around 30GHz, but its final output laser is randomly polarized and has poor stability. (3) Full polarization-maintaining single-frequency stretch amplifier system based on white noise source phase modulation technology, which has relatively better beam quality M2≈1.2, relatively narrow spectral linewidth, usually around 30GHz, and the final output laser is linearly polarized, but its device parameter requirements are high, it is difficult to implement and the cost is high. In recent years of technological development, it has been revealed that it is possible to use white noise source phase modulation technology to manufacture polarization-controlled single-frequency stretch amplifier systems, which can theoretically achieve linear polarization output with high beam quality and narrow spectral linewidth. At the same time, the nonlinear threshold is improved and the manufacturing difficulty and cost are reduced. However, based on the current theory, it is difficult to implement, and there are few documents on how to manufacture an easily implementable amplification system. At the same time, there is no document that discloses how to reasonably design the device structure layout and parameters of the polarization-controlled single-frequency broadening amplification system to achieve the ultimate polarization-maintaining single-frequency broadening amplification. Summary of the invention

[0004] In view of the deficiencies of the prior art, the present invention provides a polarization-maintaining single-frequency broadening and amplification system, which overcomes the deficiencies of the prior art and has a reasonable design.

[0005] The phase modulation technology based on white noise source is used to effectively suppress stimulated Brillouin scattering and stimulated Raman scattering. The single-frequency broadening amplification system using polarization control technology improves the nonlinear threshold and reduces the production cost. Finally, the amplified output of linearly polarized 3kW narrow-linewidth fiber laser is successfully achieved. The light-to-light conversion efficiency is 78%, the beam quality of the final output laser is M2≈1.2, and the polarization extinction ratio is 15dB.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0007] The invention provides a polarization-maintaining single-frequency-broadened optical fiber amplifier system, which is a polarization-controlled single-frequency-broadened optical fiber amplifier system using a phase modulation technology based on a white noise source.

[0008] The system includes an input seed light source seed, which can output seed light with a narrow line width; a stretcher, which is used to stretch the narrow line width seed light output by the seed light source and then output it; a single cladding amplifier, which is used to perform primary amplification on the stretched seed light; a polarization controller (PC), which is used to perform polarization control on the input seed light according to the input control signal, so that the polarization of the output light meets the requirements of the input control signal; an isolator (ISO), which is used to isolate the optical signal output by the polarization controller; a first-stage double-cladding amplifier, in which the seed light output after passing through the polarization controller can pass through the isolator (ISO) and then enter the first-stage double-cladding amplifier through the first combiner COM1 for amplification; a first cladding pump stripper, in which the amplified light passes through the first cladding pump stripper CPS 1After filtering out the excess pump light, it is output through the circulator CIR; the mode field adapter is used to change the mode field of the light, couple the seed light signal output by the circulator CIR into the optical fiber with a large mode field, and then input it into the second-stage double-clad amplifier; the second-stage double-clad amplifier adopts a reverse pump amplification structure, couples the pump light into the double-clad gain optical fiber through a reverse combiner (reverse coupler), and amplifies the seed signal light; the second cladding pump stripper is located between the mode field adapter and the second-stage double-clad amplifier, and is used to filter out most of the excess pump light; the third cladding pump stripper is located between the second combiner and the final laser output head (laser output part), and is used to filter out the excess pump light output in the forward direction; the amplified seed light output by the second-stage double-clad amplifier, after passing through the reverse coupler COM2, passes through the third cladding pump stripper CPS 3 is output to the final optical fiber output head, and the light is output through the final optical fiber output head; the first coupling structure couples a small part of the laser at the final laser output head, first passes through the PBS beam splitter to filter out the slow axis light, and then uses a photodetector (PD) to convert the optical signal into an electrical signal and input it into the polarization controller to control the polarization of the light input to the polarization controller, so that the polarization extinction ratio of the final output reaches 15dB.

[0009] Preferably, the light source (Seed) has an adjustable central wavelength of 1064±0.4 nm, an average output light power of 50 mW-150 mW, for example, 100 mW, and a line width of 10 kHz.

[0010] Preferably, the optical stretcher is composed of two identical phase modulators and two different white noise sources, wherein the frequency of one white noise source is 0.01 GHz to 2.5 GHz, and the frequency of the other white noise source is 2 GHz to 9 GHz, so that the line width of the seed light after passing through the optical stretcher becomes a line width in the range of 20 GHz to 50 GHz, preferably a line width of 32 GHz can be achieved. In other cases, a line width in the range of 10 GHz to 80 GHz can also be selected.

[0011] The polarization controller is used to control the polarization of the input seed light according to the input control signal, so that the polarization of the output light meets the requirements of the input control signal; a small part of the spatial light is coupled from the final laser output head, and the slow axis light is first filtered out by a glass slide PBS beam splitter, and then a photodetector (PD) is used to convert the optical signal into a control (electrical) signal.

[0012] Preferably, the single-clad amplifier uses a semiconductor laser LD 1 with a maximum output power of 500mW-1000mW (for example, a maximum output power of 800mW) and a central wavelength of 976nm as a pump source, and couples the pump light and the seed light into the gain fiber PM-YSF-HI (YDF 1) simultaneously through a wavelength division multiplexer (WDM), and the gain medium is an ytterbium-doped fiber with a length of about 6m-10m, preferably an ytterbium-doped fiber with a length of 8m. The output power of the single-clad amplifier can be greater than or equal to 180mW, for example, 200mW.

[0013] Preferably, the second-stage double-cladding amplifier preferably uses a cladding diameter of 400 microns. The light-to-light conversion efficiency of the second-stage double-cladding amplifier is 78%, and the beam quality of the final output laser M2≈1.2. The first-stage double-cladding amplifier preferably uses a cladding diameter of 125 microns. Preferably, the output power of the seed light output through the CIR is 30W-80W, for example, 50W. Preferably, the mode field adapter (MFA) is capable of converting light from LMA-GDF-10 / 125 to LMA-GDF-20 / 400. Preferably, the circulator can use a three-port circulator.

[0014] Preferably, the first-stage double-clad amplifier uses two semiconductor lasers LD2 with a maximum output power of 40W-80W (for example, 60W) and a central wavelength of 976nm as pump sources, and the pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-10 / 125 (YDF 2) through a beam combiner. The gain medium is a 3m-5m long double-clad ytterbium-doped fiber, for example, a 4m long double-clad ytterbium-doped fiber.

[0015] Preferably, the second-stage double-clad amplifier uses six semiconductor lasers LD 3 with a maximum output power of 500W-800W (for example, 650W) and a central wavelength of 976nm as pump sources, and the pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-25 / 400 (YDF 3) through a reverse combiner. The gain medium is a double-clad ytterbium-doped fiber with a length of 8m-15m, for example, a double-clad ytterbium-doped fiber with a length of 11m.

[0016] Preferably, the final output laser can reach an average output power of 3000 W, a line width of 20 GHz-50 GHz, for example, 32 GHz, and a polarization extinction ratio of 15 dB.

[0017] The beneficial effects of the present invention are:

[0018] 1. The present invention provides a polarization-maintaining single-frequency stretched fiber amplifier system, which uses a polarization-controlled single-frequency stretched fiber amplifier system based on a phase modulation technology of a white noise source. An all-fiber gain amplification structure can be used to achieve high-power polarization-maintaining output. Through a stretcher composed of two identical phase modulators and two different white noise sources, the seed light is stably stretched to improve the power output threshold and polarization-maintaining performance of the entire system.

[0019] 2. By obtaining part of the laser from the final laser output head, the slow axis light is first filtered out through a PBS beam splitter, and then a photodetector (PD) is used to convert the optical signal into an electrical signal and input it into a polarization controller on the front side of the double-cladding amplifier to control the polarization of the light input to the polarization controller, so as to obtain an extremely high polarization extinction ratio, with a simple structure; at the same time, polarization control is performed after the seed light is amplified by the single-cladding amplifier, so that the polarization control is more stable and accurate, and the subsequent gain amplification is more balanced, so that high average power output can be stably achieved, so that an average output power of up to 3000W can be obtained while also obtaining an excellent polarization control effect.

[0020] 3. The second-stage double-cladding amplifier adopts a reverse pumping structure to prevent the pump light from directly affecting the final output light. At the same time, a second cladding pump stripper is set between the mode field adapter and the second-stage double-cladding amplifier to filter out most of the redundant pump light in the first direction; the redundant pump light in the second direction is stripped by the third cladding pump stripper CPS 3 between the second combiner and the final laser output head; a circulator CIR is set after the pump stripper after the first-stage double-cladding amplifier to export the remaining redundant pump light; that is, the reverse coupling setting and the cooperation of two pump isolators and a circulator are used to perfectly filter out the redundant pump light, preventing the pump light from affecting the output signal, such as the optical polarization signal collection, and other possible hazards. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the present invention or the technical solutions in the prior art, the drawings required for describing the prior art are briefly introduced below.

[0022] Figure 1 This is the schematic diagram of the structure of the polarization controlled single-frequency broadening amplifier system; DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention.

[0024] The embodiment of the present invention provides a polarization-maintaining single-frequency-broadened optical fiber amplification system.

[0025] Polarization-controlled single-frequency spread-width fiber amplifier system based on white noise source phase modulation technology:

[0026] An input seed light source seed is provided, which can output a seed light with a narrow line width, in some embodiments, the line width is 10-50kHz, and is connected to a stretcher through an optical fiber;

[0027] The optical stretcher stretches the seed light and then outputs it, so that the whole system can achieve a higher output power. The optical stretcher uses a phase modulator to perform stretching. In an embodiment of the present invention, it can use a white noise source to achieve stretching, changing the line width to a line width in the range of 10GHz-80GHz. In some embodiments, a line width in the range of 20GHz-50GHz can be achieved.

[0028] The inventors realized that for polarization-maintaining single-frequency stretching and amplification, when the stretcher adopts a single white noise source, it is easy to cause problems such as unstable line width, uneven broadening and difficulty in control. For this reason, the inventors adopted a designed stretcher, which is composed of two identical phase modulators and two different white noise sources. The two different white noise sources are divided into a first white noise source and a second white noise source. The frequency of the first white noise source is F1, and the frequency of the second white noise source is F2. In some embodiments, in order to obtain a better polarization-maintaining effect, the frequency F1 of the first white noise is 0.01GHz to 2.5GHz, and the frequency F2 of the second white noise source is 2GHz to 9GHz. At this time, the seed light can be broadened to 10GHz-50GHz and then output, for example, 32GHz.

[0029] The polarization controller is used to control the polarization of the input seed light according to the input control signal, so that the polarization of the output light meets the requirements of the input signal; a small part of the laser is coupled from the final laser output head, the slow axis light is first filtered out by the PBS beam splitter, and then the optical signal is converted into a control (electrical) signal using a photodetector (PD).

[0030] The seed light output after the polarization controller can enter the first double-clad amplifier through the first beam combiner COM1 after passing through the isolator (ISO) for amplification. The amplified light passes through the first cladding pump stripper CPS 1 to filter out the excess pump light, and then passes through the circulator CIR (the circulator can use a three-port circulator); the output seed light passes through the mode field adapter (MFA) and enters the second double-clad amplifier. The amplified seed light passes through the second beam combiner COM2 and is output through the third cladding pump stripper CPS 3. The final output laser beam quality M2≈1.2. It includes a control light coupling structure for coupling a small part of the laser at the final laser output head, first filtering out the slow axis light through the PBS beam splitter, and then using a photodetector (PD) to convert the optical signal into an electrical signal and input it into the polarization controller.

[0031] The isolator is used to prevent the return light generated by the rear amplification part from being transmitted to the polarization controller, which may cause interference and damage.

[0032] The final output laser can reach an average output power of 3000W, a line width of 20GHz-50GHz, for example, 32GHz, and a polarization extinction ratio of 15dB.

[0033] The optical fiber, combiner, converter and gain amplifier structure after the polarization controller (PC) will affect the polarization maintenance, especially the double-clad amplifier structure will have a greater impact on the polarization and affect the final polarization extinction ratio. Therefore, it is not advisable to have multiple stages of (double-clad) amplifier structures. In the two-stage double-clad amplifier structure, if high-power polarization-maintaining output is to be achieved, then the laser power input to the polarization controller and the amplification capacity of the subsequent two-stage amplifier structure must be considered comprehensively. Since the gain fiber length of the double-clad amplifier has a greater impact on the polarization, the various parameters of the first-stage double-clad amplifier structure and the second-stage double-clad amplifier structure need to be balanced and scientifically designed.

[0034] The second-stage double-clad amplifier preferably uses a cladding diameter of 400 microns (the second-stage double-clad amplifier requires a larger optical fiber diameter).

[0035] The first-stage double-cladding amplifier preferably uses a cladding diameter of 125 microns. In some embodiments, the output power of the seed light output through the CIR is 30W-80W, for example, 50W.

[0036] The Mode Field Adapter (MFA) is used to convert the light from LMA-GDF-10 / 125 to LMA-GDF-20 / 400.

[0037] Therefore, in some embodiments, the first-stage double-clad amplifier uses 2-4 (for example, 2) semiconductor lasers LD2 with a maximum output power of 40W-80W (for example, 60W) and a central wavelength of 976nm as pump sources, and the pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-10 / 125 (YDF 2) through a beam combiner. The gain medium is a 3m-5m long double-clad ytterbium-doped fiber, for example, a 4m long double-clad ytterbium-doped fiber (when two semiconductor lasers are used as pump sources, the output powers of the two pump sources can be set to be different. It is unexpectedly found that the power ratio between the two can affect the final polarization extinction ratio. When the output power of one of the pump sources is set to 60W, the output power of the other pump source is gradually increased from 60W to 80W. It is found that in this process, the polarization extinction ratio is improved by 28%, that is, when the number of pump sources is preferably two, the output power ratio of the two pump sources is 1:1.2 to 1:1.4).

[0038] In some embodiments, the second-stage double-clad amplifier uses 4-8 (for example, 6) semiconductor lasers LD 3 with a maximum output power of 500W-800W (for example, 650W) and a central wavelength of 976nm as pump sources, and the pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-25 / 400 (YDF 3) through a beam combiner. The gain medium is a double-clad ytterbium-doped fiber with a length of 8m-15m, for example, a double-clad ytterbium-doped fiber with a length of 11m.

[0039] Usually, the average output power of the input seed light source seed is tens to one or two hundred milliwatts. Referring to the amplification requirements of the first-stage double-clad amplifier, it can be seen that the input seed light source is not conducive to being directly polarization-controlled and then output to the first double-clad amplifier. The low optical power output from the polarization controller is also not conducive to polarization control. For subsequent stable amplification and effective polarization control, the output signal of the seed light source seed is widened and then input into the single-clad amplifier through a wavelength division multiplexer (WDM), and then input into the polarization controller after being amplified by the single-clad amplifier.

[0040] In some embodiments, the single-clad amplifier uses a semiconductor laser LD 1 with a maximum output power of 500mW-1000mW (e.g., 800mW) and a central wavelength of 976nm as a pump source, and couples the pump light and the seed light into the gain fiber PM-YSF-HI (YDF 1) simultaneously through a wavelength division multiplexer (WDM), and the gain medium is an ytterbium-doped fiber with a length of about 6m-10m, preferably an ytterbium-doped fiber with a length of 8m. The output power of the single-clad amplifier can be greater than or equal to 180mW, for example, 200mW.

[0041] Since the pump power of the second-stage double-clad amplifier is extremely high, its excess pump light can easily damage the device and affect the output. Most of the excess pump light is removed by using the second-clad pump stripper (CPS) made by etching process. 2) Filtering. Since the pump light power of the second-stage double-cladding amplifier is extremely large, even after passing through the pump stripper, there is still a considerable portion of pump light that is difficult to strip (which may cause inaccurate collection of optical polarization signals). In order to solve this problem, the present invention sets the second-stage double-cladding amplifier as a reverse amplification structure with reverse pumping, and adopts a reverse coupling device, that is, a reverse combiner (second combiner) to couple the pump light into the gain fiber. A second cladding pump stripper is set between the mode field adapter and the second-stage double-cladding amplifier to filter out most of the excess pump light in the first direction, and then the remaining excess pump light in the first direction is exported through the circulator between the first cladding pump stripper and the mode field adapter; then, due to the pump light of the reverse amplification structure being reflected by the front-end device, a portion of excess pump light in the second direction will also be generated. Since this portion of light is already very small, it can be stripped by the third cladding pump stripper CPS 3 between the second combiner and the final laser output head.

[0042] In some embodiments, the circulator may be a three-port circulator.

[0043] As common knowledge in the field of optical fiber amplification systems, the functional components of a polarization-maintaining single-frequency stretched optical fiber amplification system are connected in sequence through optical fibers. In some embodiments, a seed light source is connected to a stretcher through an optical fiber, a stretcher is connected to a single-clad amplifier through an optical fiber, a single-clad amplifier is connected to a polarization control device through an optical fiber, a polarization control device is connected to an isolator through an optical fiber, an isolator is connected to a first-stage double-clad amplifier through an optical fiber, a first-stage double-clad amplifier is connected to a circulator through an optical fiber, a circulator is connected to a mode field adapter through an optical fiber, a mode field adapter is connected to a second-stage double-clad amplifier through an optical fiber, and a second-stage double-clad amplifier is connected to a laser output head through an optical fiber; the first, second, and third cladding pump strippers are also made on the optical fiber structure through an etching process.

[0044] Reference Figure 1 , the specific embodiments are described as follows:

[0045] In some specific embodiments, the light source (Seed) has an adjustable central wavelength of 1064±0.4 nm, an average output light power of 50 mW-150 mW, such as 100 mW, and a line width of 10-50 kHz, such as 10 kHz.

[0046] The optical stretcher is composed of two identical phase modulators and two different white noise sources. The frequency of one white noise source is 0.01GHz to 2.5GHz, and the frequency of the other white noise source is 2GHz to 9GHz. After the seed light passes through the optical stretcher, the line width becomes 32GHz, allowing the entire system to achieve a higher output power.

[0047] The single-clad amplifier uses a semiconductor laser LD1 with a maximum output power of 800mW and a central wavelength of 976nm as the pump source. The pump light and the seed light are simultaneously coupled into the gain fiber PM-YSF-HI (YDF 1) through a wavelength division multiplexer (WDM). The gain medium is an 8m long ytterbium-doped fiber. The output power of the single-clad amplifier is 200mW. The seed light first passes through the polarization controller (PC) and then enters the first-stage double-clad amplifier through ISO. The first-stage double-clad amplifier uses two semiconductor lasers LD2 with a maximum output power of 60W and a central wavelength of 976nm as the pump source. The pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-10 / 125 (YDF 2) through a beam combiner. The gain medium is a 4m long double-clad ytterbium-doped fiber. After filtering out the excess pump light through CPS 1, it is output through CIR, and the output power is 50W. The seed light is input into the second-stage double-clad amplifier (i.e., the seed light is input into the gain fiber YDF 3) after passing through the mode field adapter (MFA) of LMA-GDF-10 / 125 to LMA-GDF-20 / 400. The second-stage double-clad amplifier uses six semiconductor lasers LD 3 with a maximum output power of 650W and a central wavelength of 976nm as pump sources. The pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-25 / 400 (YDF 3) through a reverse beam combiner. The gain medium is a 11m long double-clad ytterbium-doped fiber. In addition, the stress of the gain fiber is kept as small as possible during the winding process of YDF 3, thereby ensuring the polarization extinction ratio of the final output laser. The second-stage double-clad amplifier is a reverse amplification structure. The excess pump light is filtered out by a cladding pump stripper (CPS 2) made using an etching process. The amplified seed light passes through COM2 and is output through CPS 3. The average output power is 3000W and the line width is 32GHz. The optical-optical conversion efficiency of the second-stage double-cladding amplifier is 78%, and the beam quality of the final output laser is M2≈1.2. A small part of the laser is coupled at the final laser output head, and the slow axis light is first filtered out by the PBS beam splitter, and then the optical signal is converted into an electrical signal using a photodetector (PD) and input into the polarization controller, so that the polarization extinction ratio of the final output reaches 15dB. Compared with the full polarization-maintaining structure, the nonlinear threshold is improved and the production cost is reduced.

[0048] 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 may still be modified, or some of the technical features may 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 embodiments of the present invention.

Claims

1. A single-frequency broadening optical fiber amplification system, include, A seed light source, used for outputting seed light; A stretcher, used for stretching the seed light output by the seed light source and then outputting it; The stretcher uses a phase modulator to perform the stretching; A polarization controller, used for performing polarization control on the input seed light according to an input control signal; The seed light outputted from the first-stage double-cladding amplifier after passing through the polarization controller enters the first-stage double-cladding amplifier for amplification and then output; The second-stage double-cladding amplifier amplifies the light amplified by the first-stage double-cladding amplifier and then outputs it; Acquire part of the output light of the second-stage double-cladding amplifier, then filter out the slow-axis light of the part of the light through a PBS beam splitter and input it into a photodetector, which converts the acquired optical signal into a control signal and inputs it into a polarization controller; After the seed light is outputted from the seed light source and broadened, it is inputted into the single cladding amplifier through the wavelength division multiplexer, and then inputted into the polarization controller after being amplified by the single cladding amplifier; The invention is characterized in that the line width of the seed light output by the seed light source is 10-50kHz; the stretcher includes two identical phase modulators and two different white noise sources, wherein the frequency of the first white noise source is 0.01 GHz to 2.5 GHz, and the frequency of the second white noise source is 2 GHz to 9 GHz; It includes a mode field adapter, which changes the mode field of the signal amplified by the first-stage double-clad amplifier and inputs it into the second-stage double-clad amplifier, the core diameter of the gain fiber of the first-stage double-clad amplifier is smaller than the core diameter of the gain fiber of the second-stage double-clad amplifier, and the cladding diameter of the gain fiber of the first-stage double-clad amplifier is smaller than the cladding diameter of the gain fiber of the second-stage double-clad amplifier; The second-stage double-clad amplifier is a reverse amplification structure of reverse pumping, and a reverse coupler is used to couple the pump light into the gain fiber. A second cladding pump stripper is arranged at the front end of the second-stage double-clad amplifier to filter out the redundant pump light in the first direction; a circulator is arranged between the first-stage double-clad amplifier and the second-stage double-clad amplifier, and the circulator can derive the return light from the subsequent amplifier; The amplified seed light outputted by the second-stage double-cladding amplifier passes through a reverse coupler and then through a third-cladding pump stripper and is outputted to a laser output head; The pump stripper is made on the optical fiber structure through an etching process; The isolator is located after the polarization controller and is used to isolate the optical signal output by the polarization controller; the seed light output after the polarization controller passes through the isolator and then enters the first-stage double-clad amplifier through the first beam combiner for amplification; The first cladding pump stripper is located after the first-stage double-cladding amplifier. After the amplified light passes through the first cladding pump stripper to filter out the redundant pump light, it is output through the circulator; the circulator adopts a three-port circulator.

2. The single-frequency-broadened optical fiber amplification system according to claim 1, It is characterized in that The seed light becomes a line width in the range of 20GHz-50GHz after passing through the stretcher. The first-stage double-clad amplifier uses 2-4 semiconductor laser LD2s with a maximum output power of 40W-80W as pump sources. The pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-10 / 125 through a beam combiner. The gain medium is a 3m-5m long double-clad ytterbium-doped fiber. The second-stage double-clad amplifier uses 4-8 semiconductor laser LD3s with a maximum output power of 500W-800W as pump sources. The pump light and the seed light are simultaneously coupled into the gain fiber LMA-YDF-25 / 400 through a beam combiner. The gain medium is a 8m-15m long double-clad ytterbium-doped fiber.

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