A narrow linewidth all-fiber laser

Abstract

The invention discloses a narrow line width all-fiber laser, comprising a seed source, at least two-stage amplification modules, a multi-way circulator, an optical coupler and a pump drive module; the seed source and the gain optical fiber in each stage of the amplification module are sequentially connected to the port of the multi-way circulator to form a multi-stage double-pass amplification structure; the narrow line width high reflectivity grating in each stage of the amplification module is respectively connected to the pump drive module through an optical coupler, and the optical couplers corresponding to the adjacent amplification modules are connected to each other, so that the pulse light amplified by the previous stage of the amplification module is reversely injected into the next stage of the amplification module through the optical coupler, and a two-way three-pass amplification is formed in the next stage of the amplification module; the pulse light emitted by the seed source is amplified step by step through the amplification modules of each stage in turn, and is output from the last port of the multi-way circulator. The invention reduces the use of devices such as couplers, mode adapters, and online isolators, greatly simplifies the optical path structure and control system, and greatly improves the stability and reliability of the laser.

Description

Technical Field

[0001] The invention relates to the field of optical fiber lasers, and in particular to a narrow-linewidth all-optical fiber laser. Background Art

[0002] At present, domestic pulse fiber laser products and markets are developing rapidly. Major laser manufacturers are competing to launch higher power, smaller size, and higher performance lasers. The optical path structure and control system of traditional hierarchical lasers are complex, and the system contains a large number of device types. The yield and long-term stability of some core devices are difficult to guarantee, resulting in the overall size and cost of the laser being difficult to further reduce. In order to meet market demand and expand the application scenarios of fiber lasers, it is urgent to reduce the number of devices and circuit controls, while reducing costs and improving system performance indicators.

[0003] At present, the amplification structure of fiber lasers on the market mostly adopts multi-stage coupling. Each stage of pumping enters the stage-by-stage single-pass amplification through each stage of coupler. Since the amplification factor is limited by factors such as spectrum broadening and ASE excitation, the number of devices in the traditional multi-stage structure is large. At the same time, high-power amplification is limited by coupler manufacturing and gain fiber heat accumulation, which easily leads to problems such as beam quality and spectrum degradation, low efficiency, and fiber damage. Therefore, it is particularly important to optimize the amplification structure of laser products.

[0004] At present, nanosecond pulse high-power lasers are limited by problems such as heat dissipation and spectrum broadening. There are bidirectional coupler pump structures that can solve the heat dissipation problem of the front section of the optical fiber, but they are limited by device problems such as reverse coupler insertion loss and spot degradation. In addition, there are technical solutions that use a few-way circulator + high-reflection grating to increase the number of single-way amplification. Most of the pump energy still relies on the coupler to enter the gain fiber. The amplification method is similar to the traditional method, and the overall efficiency and spectrum improvement are limited. Summary of the invention

[0005] The main purpose of the present invention is to provide a narrow linewidth all-fiber laser, optimize the amplification structure, and improve the laser performance.

[0006] The technical solution adopted by the present invention is: a narrow linewidth all-fiber laser, including a seed source and an optical power amplification module, and also including a multi-channel circulator, an optical coupler and a pump driving module;

[0007] The optical power amplification module comprises at least two stages of amplification modules, each stage of the amplification module comprises a gain optical fiber and a narrow line width high reflectivity grating, the seed source and the gain optical fiber in each stage of the amplification module are sequentially connected to the port of the multi-way circulator, and the multi-way circulator and the narrow line width high reflectivity grating of each stage of the amplification module form a multi-stage double-pass amplification structure;

[0008] The narrow line width and high reflectivity grating in each stage of the amplification module is connected to the pump driving module through an optical coupler, and the optical couplers corresponding to adjacent amplification modules are connected to each other through signal terminals, so that the pulse light amplified by the previous stage of the amplification module is reversely injected into the next stage of the amplification module through the optical coupler, forming a bidirectional three-pass amplification in the next stage of the amplification module;

[0009] In each stage of the amplification module, the gain fiber absorbs the pump energy to form an excited state population inversion to gather energy. After the pulse light entering each stage of the amplification module is reversely amplified and absorbs most of the pump energy, it is returned to the forward direction by the narrow line width and high reflectivity grating to amplify and absorb the remaining pump energy.

[0010] The pulse light emitted by the seed source is amplified step by step through each level of amplification modules in sequence and then output from the last port of the multi-way circulator.

[0011] According to the above scheme, the at least two-stage amplification module includes a primary main amplification stage optical power amplification module and a secondary main amplification stage optical power amplification module; the optical coupler includes a first optical coupler and a second optical coupler; wherein,

[0012] The first-stage main amplifier optical power amplification module includes a first-stage main amplifier first gain optical fiber and a first-stage main amplifier narrow linewidth high reflectivity grating; the second-stage main amplifier optical power amplification module includes a second-stage main amplifier gain optical fiber and a second-stage main amplifier narrow linewidth high reflectivity grating; the pump drive module is connected to the first-stage main amplifier narrow linewidth high reflectivity grating and the second-stage main amplifier narrow linewidth high reflectivity grating through a first optical coupler and a second optical coupler, respectively, and the first optical coupler and the second optical coupler are connected to each other through a signal end; the first-stage main amplifier first gain optical fiber is connected to the port of the multi-way circulator, and the second-stage main amplifier gain optical fiber is connected to the port of the multi-way circulator after being connected to a cladding light stripper.

[0013] According to the above scheme, the first-stage main amplifier optical power amplification module also includes a first-stage main amplifier second gain optical fiber, which is connected in series between the first-stage main amplifier narrow linewidth high reflectivity grating and the first optical coupler, and is used to re-amplify the pulse light after amplification by the first-stage main amplifier optical power amplification module.

[0014] According to the above scheme, the optical power amplification module also includes a pre-amplification stage optical power amplification module, which is connected to the port of the multi-way circulator before the at least two-stage amplification module;

[0015] The pre-amplification stage optical power amplification module includes a pre-amplification gain fiber and a pre-amplification narrow linewidth high reflectivity grating, wherein the pre-amplification gain fiber is connected to the port of the multi-way circulator, and the pre-amplification narrow linewidth high reflectivity grating is connected to the pump driving module.

[0016] According to the above scheme, the pump driving module includes a pump source and a pump beam splitter. The pump light emitted by the pump source is split after passing through the pump beam splitter and connected to each level of amplification modules through corresponding optical couplers.

[0017] According to the above scheme, the seed source is a nanosecond, picosecond, femtosecond seed source or a continuous seed source.

[0018] According to the above scheme, the central wavelength of the narrow line width and high reflectivity grating is 1064nm or 1550nm.

[0019] According to the above scheme, the 3dB bandwidth of the reflection spectrum of the narrow line width and high reflectivity grating is 2nm or 5nm.

[0020] According to the above solution, the gain optical fiber is a polarization-maintaining optical fiber, a non-polarization-maintaining optical fiber or a photonic crystal optical fiber.

[0021] According to the above scheme, the central wavelength of the pump source is 915nm or 976nm; the beam splitting ratio of the pump beam splitter is set according to the output power requirements of each level of amplification modules.

[0022] The beneficial effects produced by the present invention are:

[0023] 1. The use of multi-way circulators and narrow linewidth high reflectivity gratings to form multi-stage double-pass amplification reduces the use of devices such as couplers, mode adapters, and online isolators, greatly simplifies the optical path structure and control system, and the multi-way circulators have high isolation at each level, which improves the system's ability to resist high-reflection. The output port is isolated from the amplifier module, and after connecting the output optical isolation, it has multi-stage anti-high-reflection capabilities, thereby greatly improving the stability and reliability of the laser; the double-pass amplification structure can shorten the length of the optical fiber at the same amplification factor. Reverse amplification and narrow linewidth high reflectivity gratings can greatly optimize the spectrum, suppress the generation of nonlinearity and high-order modes, and improve the power adjustable range of each level.

[0024] 2. Use a pump beam splitter to split the pump light and then send it to each level of amplification modules. You only need to control the splitting ratio of the pump beam splitter, which reduces the number of pump sources, simplifies the difficulty of circuit control, and saves the cost and volume of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0026] Figure 1 It is a schematic diagram of the optical path structure of an embodiment of the present invention.

[0027] In the figure: 1-seed source, 2-multi-way circulator, 3-pre-amplifier stage optical power amplifier module, 4-first-level main amplifier stage optical power amplifier module, 5-second-level main amplifier stage optical power amplifier module, 6-pump beam splitter, 7-pump source, 8-first optical coupler, 9-second optical coupler, 3-1-pre-amplifier gain fiber, 3-2-pre-amplifier narrow linewidth high reflectivity grating, 4-1-first-level main amplifier first gain fiber, 4-2-first-level main amplifier narrow linewidth high reflectivity grating, 4-3-first-level main amplifier second gain fiber, 5-1-second-level main amplifier gain fiber, 5-2-second-level main amplifier narrow linewidth high reflectivity grating, 5-4-cladding light stripper. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] like Figure 1 As shown, the present invention provides a narrow line width all-fiber laser, comprising a seed source 1, a multi-way circulator 2, an optical power amplification module, an optical coupler and a pump drive module. The optical power amplification module comprises at least two stages of amplification modules, each stage of the amplification module comprises a gain fiber and a narrow line width high reflectivity grating, the seed source 1 and the gain fiber in each amplification module are sequentially connected to the port of the multi-way circulator 2, and the multi-way circulator and the narrow line width high reflectivity grating of each stage of the amplification module form a multi-stage double-pass amplification structure.

[0030] The narrow line width and high reflectivity grating in each stage of the amplification module is connected to the pump driving module through an optical coupler, and the optical couplers corresponding to adjacent amplification modules are connected to each other through signal terminals, so that the pulse light amplified by the previous stage of the amplification module is reversely injected into the next stage of the amplification module through the optical coupler, forming a bidirectional three-pass amplification in the next stage of the amplification module.

[0031] In this embodiment, the optical power amplification module comprises a pre-amplification stage optical power amplification module 3 , a primary main amplification stage optical power amplification module 4 and a secondary main amplification stage optical power amplification module 5 .

[0032] The pre-amplifier stage optical power amplification module 3 includes a pre-amplifier gain fiber 3-1 and a pre-amplifier narrow linewidth high reflectivity grating 3-2, wherein the pre-amplifier gain fiber 3-1 is connected to the port of the multi-way circulator 2, and the pre-amplifier narrow linewidth high reflectivity grating 3-2 is connected to the pump drive module.

[0033] The first-stage main amplifier optical power amplification module 4 includes a first-stage main amplifier first gain fiber 4-1 and a first-stage main amplifier narrow linewidth high reflectivity grating 4-2; the second-stage main amplifier optical power amplification module 5 includes a second-stage main amplifier gain fiber 5-1 and a second-stage main amplifier narrow linewidth high reflectivity grating 5-2.

[0034] The pump driving module is connected to the first-level main amplifier narrow linewidth high reflectivity grating 4-2 and the second-level main amplifier narrow linewidth high reflectivity grating 5-2 through the first optical coupler 8 and the second optical coupler 9 respectively. The first optical coupler 8 and the second optical coupler 9 are connected to each other through the signal end for reverse coupling, and provide the pump source input while reversely injecting the pre-amplified pulse light in the first-level main amplifier optical power amplifier module 4 into the second-level main amplifier optical power amplifier module 5; the first-level main amplifier first gain optical fiber 4-1 is connected to the port of the multi-way circulator 2, and the second-level main amplifier gain optical fiber 5-1 is connected to the cladding light stripper and then 5-4 is connected to the port of the multi-way circulator 2.

[0035] In each stage of the amplification module, the gain fiber absorbs the pump energy to form an excited state population inversion to concentrate the energy. The pulse light entering each stage of the amplification module is reversely amplified and absorbs most of the pump energy. It is then returned to the forward direction by the narrow linewidth and high reflectivity grating to absorb the remaining pump energy, thereby improving the amplification efficiency while suppressing the nonlinear effect.

[0036] The pulse light emitted by the seed source 1 is input from the I port of the multi-way circulator, and is connected to the pre-amplifier optical power amplifier module through the II port for power pre-amplification. The pulse light and the back-coupled pump light form reverse amplification. After the pre-amplification gain fiber 3-1 absorbs the pump, the excited state particle number is reversed and the energy is gathered. After the pulse light is reversely amplified and absorbs most of the energy, it will be returned to the pre-amplified narrow linewidth high reflectivity grating 3-2 for forward amplification to absorb the remaining pump energy. Then, the first-level main amplifier optical power amplifier module 4 and the second-level main amplifier optical power amplifier module 5 are connected through the III and IV ports for step-by-step main power amplification, adjusting the output power at each level, optimizing the output spectrum, and maximizing the output index by taking advantage of the structure. Finally, it is output from the V port of the multi-way circulator 2, and can be directly connected to the end cap as the output of the whole machine.

[0037] The seed source can be a nanosecond, picosecond, femtosecond seed source or a continuous seed source. According to the characteristics of sequential light output and high isolation of the multi-channel optical fiber circulator, the seed light will enter the pre-amplification stage optical power amplifier module 3, the first-level main amplifier stage optical power amplifier module 4 and the second-level main amplifier stage optical power amplifier module 5 for amplification in sequence, replacing the isolator and online optical isolation required at each level of the traditional structure. It can be directly connected to the end cap output, or the output optical isolation can be linked to form multi-level isolation. The number of circulator ports can be customized to support high-power multi-level amplification, and each port can match 10 / 130μm, 30 / 250μm, 100 / 400μm and other double-clad optical fibers, which are sufficient to adapt to various single-mode small fiber core and multi-mode large fiber core uses.

[0038] Each stage of the amplification module of the present invention uses the return light of the high reflectivity grating to form a single-stage double-pass amplification structure, the pump is back-coupled into the gain fiber, and the signal light direction and the pump light form reverse amplification. The reverse amplification improves the amplification efficiency while suppressing the nonlinear effect. Compared with the traditional forward amplification spectrum, it has obvious advantages and solves the problems of low efficiency and beam quality of the traditional reverse coupler. The narrow linewidth grating can also further constrain the spectrum broadening, prevent other wavelengths from self-excitation and ASE due to excessive amplification, and at the same time increase the adjustable power range of each stage.

[0039] Preferably, the first-stage main amplifier optical power amplification module 4 also includes a first-stage main amplifier second gain optical fiber 4-3, which is connected in series between the first-stage main amplifier narrow linewidth high reflectivity grating 4-2 and the first optical coupler 8, and is used to amplify the pulse light after amplification by the first-stage main amplifier optical power amplification module 4, thereby increasing the injected seed power of the first-stage main amplifier optical power amplification module 4.

[0040] Further preferably, the pump driving module includes a pump source 7 and a pump beam splitter 6, and the pump light emitted by the pump source 7 is split after passing through the pump beam splitter 6 and connected to each level of amplification modules. The central wavelength of the pump source 7 is 915nm or 976nm; the splitting ratio of the pump beam splitter 6 is set according to the output power requirements of each level of amplification modules. The present invention replaces the multi-stage pump of the traditional structure with a single pump source, and uses a beam splitter to split the pump source into pump lights of each amplification module. The splitting ratio is based on the output power requirements of each level, and the splitting ratio can be 1:1:8, 1:3:6 or other arbitrary ratios. Usually, a single pump will have different power outputs at different percentages, and the front-stage power cannot meet the pre-amplification, so the output power percentage cannot be adjusted. The output power of the pre-amplification stage of the structure of the present invention has a large adjustable range, and the pre-amplification requirements can be met above a certain power, and the output power can be adjusted with the percentage. Similarly, variable power output can be achieved below the inflection point frequency. The number of pump sources is selected according to the power of the whole machine. When the output power is high, multiple pumps can be combined and then split in proportion. At present, the power of a single semiconductor pump tube is gradually increasing. Generally, a pulse laser uses a single or a small amount of pump to provide enough energy, which will greatly save costs, simplify circuit control, and compress the system volume, which can greatly promote the development of miniaturization and industrialization.

[0041] In this embodiment, the reflectivity of the pre-amplification narrow line width high reflectivity grating 3-2 and the secondary main amplifier narrow line width high reflectivity grating 5-2 is 99.99%, and the reflectivity and transmittance of the primary main amplifier narrow line width high reflectivity grating are 50% or other transmittance ratios. The pump light is injected into the primary main amplifier stage optical power amplification module 4 and the secondary main amplifier stage optical power amplification module 5 respectively through the first optical coupler 8 and the second optical coupler 9, and at the same time, the transmitted light of the primary main amplifier stage optical power amplification module 4 will be amplified by the primary main amplifier second gain fiber 4-3 and then reversely output and injected into the secondary main amplifier stage optical power amplification module 5. The bidirectional power of the bidirectional output structure of the primary main amplifier stage optical power amplification module 4 can reach the maximum respectively, which improves the threshold limit of the unidirectional output, and also improves the pump absorption efficiency, and simplifies the general form of the bidirectional output optical path.

[0042] The central wavelength of the narrow line width high reflectivity grating is 1064nm, 1550nm or other wavelengths, and the grating reflection spectrum 3dB bandwidth is above 0.5nm, preferably 2nm or 5nm. The selection of the grating can affect the composition ratio of the fundamental mode and reduce the generation and transmission of high-order modes.

[0043] The gain fiber is a polarization-maintaining fiber, a non-polarization-maintaining fiber or a photonic crystal fiber. The length of the gain fiber of the double-pass amplification structure can be shortened by more than half, further improving the nonlinear threshold. Especially for the main amplifier stage, the shorter fiber still maintains a higher absorption efficiency, and the output spectrum is significantly optimized compared with the traditional structure.

[0044] In the two-stage amplification module, the first-stage main amplification stage optical power amplification module does not reach the fiber threshold power, but due to the limitation of spectrum broadening and amplification multiple, the pre-stage low-power amplification is performed. In the traditional technology, it is usually limited by the low seed light power, the pre-stage low-power amplification spectrum deteriorates rapidly and the output power is low. Therefore, the pre-stage low-power amplification generally adopts a two-stage pre-amplification structure to reach the main amplification stage input power, and then the main amplification is performed. The present invention improves the pre-amplification stage power through narrow-band gratings, shortening the gain fiber length, reverse amplification and other measures. Only one-stage pre-stage low-power amplification (i.e., the first-stage main amplification stage optical power amplification module) is needed to directly meet the input power requirements of the main power amplification, shortening the number of amplification stages. The first-stage main amplification stage optical power amplification module of the double-pass amplification structure not only improves the efficiency, but also reduces the use of the large-stage stripper. The pre-amplification uses a partial reflection grating to form a bidirectional amplification output, which is adapted to the bidirectional injection of the subsequent main amplification stage optical power amplification module (i.e., the second-stage main amplification stage optical power amplification module), which is equivalent to improving the pre-amplification stage power, and is more conducive to improving the amplification efficiency of the main amplification stage.

[0045] The main amplifier stage optical power amplifier module (i.e., the second-stage main amplifier stage optical power amplifier module) with a bidirectional injection structure will absorb the pump light from both ends to form a bidirectional three-pass amplification through the pre-release pulse, which can greatly reduce the heating problems of the single-ended pump amplification fusion point and the front-end optical fiber. At the same time, the second-stage main amplifier narrow linewidth and high reflectivity grating can optimize the output spectrum.

[0046] The present invention provides a narrow linewidth all-fiber laser, which uses a multi-way circulator to emit light sequentially, combined with a high-reflection grating back-coupled pump amplification, and the efficiency of the two-way gain fiber is increased. The combination of a narrow linewidth high-reflection grating can greatly optimize the spectrum and improve the amplification factor. At the same time, the main amplifier stage (i.e., the secondary main amplifier stage optical power amplification module) adopts a bidirectional injection method to form a bidirectional three-way amplification structure, further improving the absorption efficiency, solving the problems of fusion points and optical fiber thermal effects caused by the main amplifier unidirectional amplification, and the main amplifier stage temperature reduction system will be more stable and can achieve higher average power amplification. Obviously, the multi-way circulator will replace multiple devices instead of just increasing the amplification factor. The entire system will reduce the use of devices such as mode adapters, isolators, and filters. Multi-stage amplification only requires a single pump tube, which greatly saves costs and simplifies circuit control.

[0047] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A narrow linewidth all-fiber laser, comprising a seed source and an optical power amplification module, characterized in that: It also includes a multi-circulator, an optical coupler, and a pump driver module; The optical power amplification module comprises at least two stages of amplification modules, each stage of the amplification module comprises a gain optical fiber and a narrow line width high reflectivity grating, the seed source and the gain optical fiber in each stage of the amplification module are sequentially connected to the port of the multi-way circulator, and the multi-way circulator and the narrow line width high reflectivity grating of each stage of the amplification module form a multi-stage double-pass amplification structure; The narrow line width and high reflectivity grating in each stage of the amplification module is connected to the pump driving module through an optical coupler, and the optical couplers corresponding to adjacent amplification modules are connected to each other through signal terminals, so that the pulse light amplified by the previous stage of the amplification module is reversely injected into the next stage of the amplification module through the optical coupler, forming a bidirectional three-pass amplification in the next stage of the amplification module; In each stage of the amplification module, the gain fiber absorbs the pump energy to form an excited state population inversion to gather energy. After the pulse light entering each stage of the amplification module is reversely amplified and absorbs most of the pump energy, it is returned to the forward direction by the narrow line width and high reflectivity grating to amplify and absorb the remaining pump energy. The pulse light emitted by the seed source is amplified step by step through each level of amplification modules and then output from the last port of the multi-way circulator. The optical power amplification module also includes a pre-amplification stage optical power amplification module, which is connected to the port of the multi-way circulator before the at least two-stage amplification module; The pre-amplification stage optical power amplification module comprises a pre-amplification gain optical fiber and a pre-amplification narrow line width high reflectivity grating, wherein the pre-amplification gain optical fiber is connected to the port of the multi-way circulator, and the pre-amplification narrow line width high reflectivity grating is connected to the pump driving module; The pump driving module comprises a pump source and a pump beam splitter. The pump light emitted by the pump source is split after passing through the pump beam splitter and connected to each level of amplification modules through corresponding optical couplers.

2. The narrow linewidth all-fiber laser according to claim 1, characterized in that: The at least two-stage amplification module includes a primary main amplification stage optical power amplification module and a secondary main amplification stage optical power amplification module; the optical coupler includes a first optical coupler and a second optical coupler; wherein, The first-stage main amplifier optical power amplification module includes a first-stage main amplifier first gain optical fiber and a first-stage main amplifier narrow linewidth high reflectivity grating; the second-stage main amplifier optical power amplification module includes a second-stage main amplifier gain optical fiber and a second-stage main amplifier narrow linewidth high reflectivity grating; the pump drive module is connected to the first-stage main amplifier narrow linewidth high reflectivity grating and the second-stage main amplifier narrow linewidth high reflectivity grating through a first optical coupler and a second optical coupler, respectively, and the first optical coupler and the second optical coupler are connected to each other through a signal end; the first-stage main amplifier first gain optical fiber is connected to the port of the multi-way circulator, and the second-stage main amplifier gain optical fiber is connected to the port of the multi-way circulator after being connected to a cladding light stripper.

3. The narrow linewidth all-fiber laser according to claim 2, characterized in that: The first-stage main amplifier optical power amplification module also includes a first-stage main amplifier second gain optical fiber, which is serially connected between the first-stage main amplifier narrow linewidth high reflectivity grating and the first optical coupler, and is used to re-amplify the pulse light after amplification by the first-stage main amplifier optical power amplification module.

4. The narrow linewidth all-fiber laser according to claim 1, characterized in that: The seed source is a nanosecond, picosecond, femtosecond seed source or a continuous seed source.

5. The narrow linewidth all-fiber laser according to claim 1, characterized in that: The central wavelength of the narrow line width high reflectivity grating is 1064nm or 1550nm; the 3dB bandwidth of the reflection spectrum of the narrow line width high reflectivity grating is above 0.5nm.

6. The narrow linewidth all-fiber laser according to claim 2, characterized in that: The reflectivity and transmittance of the first-stage main amplification narrow line width high reflectivity grating are 50% each.

7. The narrow linewidth all-fiber laser according to claim 1, characterized in that: The gain optical fiber is a polarization-maintaining optical fiber, a non-polarization-maintaining optical fiber or a photonic crystal optical fiber.

8. The narrow linewidth all-fiber laser according to claim 1, characterized in that: The central wavelength of the pump source is 915nm or 976nm; the beam splitting ratio of the pump beam splitter is set according to the output power requirements of each level of amplification modules.

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