Raman fiber laser system to improve high-order Raman threshold

Abstract

The present invention provides a Raman fiber laser system for improving the high-order Raman threshold, comprising: a backward multi-mode pump signal combiner, a Raman low-reflection grating, a Raman high-reflection grating and a Raman gain fiber. Based on the experimental structure of the backward multi-mode pump signal combiner, the present invention can integrate the output end of the signal light with the injection end of the pump light, thereby achieving the effect of rapid amplification of the signal light at the pump light injection end, while reducing the effective action distance of the signal light and the high-order Raman, thereby effectively improving the high-order Raman threshold and achieving higher-power signal light output. In the present invention, the signal light output and the pump light injection are in opposite directions, and the remaining pump light and the generated signal light can be effectively separated, reducing the use of splitter components and simplifying the experimental operation.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber laser technology, specifically relates to the field of Raman optical fiber laser technology, and in particular relates to a Raman optical fiber laser system for improving a high-order Raman threshold. Background Art

[0002] Raman fiber laser is a highly efficient, flexible and simple laser. It uses the stimulated Raman scattering effect in optical fiber to generate high-power lasers of different wavelengths. It has the advantages of strong wavelength conversion capability, high output beam quality, compact structure, good stability and high spectral purity. This laser is widely used in optical communications, biomedicine, laser processing, optical particle manipulation, super-resolution imaging and special band light sources, especially in the generation of special beam modes such as vortex beams and as a high-power pump source. It shows great potential and application value.

[0003] At present, there are three main mainstream technologies for suppressing high-order Raman generation in Raman gain fiber lasers: the first is to use Raman gain fiber with wavelength selectivity, mainly based on bending loss, such as W-type fiber. However, the fiber drawing process is complex, the cost is high, the transmission loss is large (~7.5dB / km@λ=1μm), and the high-order Raman gain is sensitive to the bending radius, which requires strict control of the bending radius. The second is to use spectral filtering optical devices, such as low-pass filters, long-period gratings and tilted gratings to couple the core mode to the cladding mode, polarization-maintaining fiber 45-degree offset welding technology, etc. The defects are: the low-pass filter is not an all-fiber structure, there is coupling loss, and the system stability is low. The power carrying capacity of long-period gratings and tilted gratings is limited. The 45-degree offset welding technology of polarization-maintaining fiber is only applicable to linearly polarized Raman fiber lasers. The third is to use a low-noise pump source with stable timing, such as a super-fluorescent fiber light source, an ultra-long cavity length laser, a single-frequency fiber laser, etc. However, the output power of such pump sources is low and cannot be used for high-order Raman suppression above the kilowatt level. Summary of the invention

[0004] In view of the defects and shortcomings in the prior art, the present invention discloses a Raman fiber laser system for improving the high-order Raman threshold. The pump light is injected into the Raman fiber resonant cavity by using a backward multi-mode pump signal combiner, and the signal light is laser oscillated by the resonant cavity, and finally the Raman fiber laser output is obtained in the signal arm of the backward multi-mode pump combiner.

[0005] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0006] A Raman fiber laser system for improving high-order Raman threshold comprises: a backward multi-mode pump signal combiner, a Raman low-reflection grating, a Raman high-reflection grating and a Raman gain fiber; the first end of the backward multi-mode pump signal combiner has a plurality of pump arms and a signal arm, wherein each pump arm is respectively connected to a pump source, the signal arm serves as an output fiber of the signal light, the second end of the backward multi-mode pump signal combiner is connected to the Raman low-reflection grating, the Raman low-reflection grating is connected to one end of the Raman gain fiber, and the other end of the Raman gain fiber is connected to the Raman high-reflection grating;

[0007] Based on the backward multimode pump signal combiner, the output fiber of the signal light is integrated with the pump arm, which can quickly amplify the signal light at the pump light injection end, shorten the action distance of the high-order Raman light in the optical fiber, reduce the gain accumulation of the signal light on the high-order Raman, and effectively improve the threshold of the high-order Raman.

[0008] Furthermore, a backward return light filter is connected between the pump source and the pump arm of the backward multi-mode pump signal combiner.

[0009] Furthermore, the output optical fiber of the signal light is connected to a first output end cap for outputting the signal light, and the first output end cap is coated with an anti-reflection film covering the wavelength range of the signal light.

[0010] Furthermore, the Raman high-reflection grating is connected to a second output end cap for outputting unconverted residual pump light, and the second output end cap is coated with an anti-reflection film covering the wavelength range of the residual pump light.

[0011] Furthermore, the Raman low-reflection grating includes a first-order Raman low-reflection grating, the Raman high-reflection grating includes a first-order Raman high-reflection grating, the second end of the backward multi-mode pump signal combiner is connected to the first-order Raman low-reflection grating, the first-order Raman low-reflection grating is connected to one end of the Raman gain fiber, and the other end of the Raman gain fiber is connected to the first-order Raman high-reflection grating.

[0012] Furthermore, the central wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain optical fiber.

[0013] Furthermore, the Raman low-reflection grating includes a first-order Raman low-reflection grating and a second-order Raman low-reflection grating, the Raman high-reflection grating includes a first-order Raman high-reflection grating and a second-order Raman high-reflection grating, the second end of the backward multi-mode pump signal combiner is connected to the second-order Raman low-reflection grating, the second-order Raman low-reflection grating is connected to the first-order Raman low-reflection grating, the first-order Raman low-reflection grating is connected to one end of the Raman gain fiber, the other end of the Raman gain fiber is connected to the first-order Raman high-reflection grating, and the first-order Raman high-reflection grating is connected to the second-order Raman high-reflection grating.

[0014] Furthermore, the center wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain fiber, the center wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are located near the Raman gain peak of the first-order Raman grating in the Raman gain fiber, and the output signal light is at the center wavelength corresponding to the second-order Raman grating.

[0015] Furthermore, the Raman low-reflection grating includes a first-order Raman low-reflection grating, a second-order Raman low-reflection grating, ..., and an n-order Raman low-reflection grating; the Raman high-reflection grating includes a first-order Raman high-reflection grating, a second-order Raman high-reflection grating, ..., and an n-order Raman high-reflection grating; the second end of the backward multi-mode pump signal combiner is sequentially connected to the n-order Raman low-reflection grating, ..., the second-order Raman low-reflection grating, and the first-order Raman low-reflection grating; the first-order Raman low-reflection grating is connected to one end of the Raman gain fiber; the other end of the Raman gain fiber is sequentially connected to the first-order Raman high-reflection grating, the second-order Raman high-reflection grating, ..., and the n-order Raman high-reflection grating, and the n-order is any order before the zero dispersion wavelength of the optical fiber.

[0016] Furthermore, the Raman gain fiber is a silicate fiber, a tellurite fiber or a fluorite fiber, and the Raman gain peak of the silicate fiber is located at 440 cm -1 , the Raman gain peak of tellurite fiber is located at 750cm -1 The Raman gain peak of the fluoride fiber is located at 580 cm -1 .

[0017] Furthermore, the pump source is a semiconductor laser, an ASE source or a fiber laser.

[0018] Compared with the prior art, the technical effects of the present invention are:

[0019] The present invention provides a Raman fiber laser system for improving the high-order Raman threshold. Based on the experimental structure of a backward multi-mode pump signal combiner, the output end of the signal light and the injection end of the pump light can be integrated together, thereby achieving the effect of rapid amplification of the signal light at the pump light injection end, while reducing the effective action distance of the signal light and the high-order Raman (equivalent to reducing the transmission distance of the high-order Raman in the optical fiber), thereby effectively improving the high-order Raman threshold and achieving higher-power signal light output.

[0020] In addition, since the signal light output and pump light injection are in opposite directions, the remaining pump light and the generated signal light can be effectively separated, reducing the use of splitter components and simplifying experimental operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0022] Figure 1 Schematic diagram of the optical path structure of the Raman fiber laser system for improving the high-order Raman threshold in Example 1 of the present invention;

[0023] Figure 2 Schematic diagram of the optical path structure of the Raman fiber laser system for improving the high-order Raman threshold in Example 2 of the present invention;

[0024] Figure 3 The power and spectrum evolution diagrams of Comparative Example 1, wherein (a) is the power evolution diagram of Comparative Example 1, and (b) is the spectrum evolution diagram of Comparative Example 1;

[0025] Figure 4 FIG. 1 is a light path structure diagram of the Raman fiber laser system used in Comparative Example 2;

[0026] Figure 5 The power and spectrum evolution diagrams obtained in Comparative Example 2, wherein (a) is the power evolution diagram obtained in Comparative Example 2, and (b) is the spectrum evolution diagram obtained in Comparative Example 2;

[0027] Figure 6 Schematic diagram of the optical path structure of the Raman fiber laser system for improving the high-order Raman threshold in Example 3 of the present invention;

[0028] Figure annotation:

[0029] 101. First output end cap; 102. Second output end cap; 2. Backward multimode pump signal combiner; 3. First-order Raman low-reflection grating; 4. First-order Raman high-reflection grating; 5. Raman gain fiber; 6. Backward return light filter; 7. Second-order Raman low-reflection grating; 8. Second-order Raman high-reflection grating; 9. Pump light. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] Since Raman gain is different from the energy level transition of rare earth ion-doped optical fiber, Raman fiber laser has obvious threshold characteristics. Raman laser will only be generated when the pump light power reaches a certain level. In order to reduce the light output threshold of Raman signal light, it is necessary to increase the length of Raman gain fiber. However, too long Raman gain fiber will reduce the threshold power of high-order Raman, which limits the further increase of Raman fiber laser power.

[0032] The present invention is based on the backward multi-mode pump signal combiner 2 to integrate the output end of the signal light with the incident end of the pump light 9, thereby realizing rapid amplification of the signal light at the pump injection end, which can reduce the action distance of high-order Raman in the optical fiber and thus improve the threshold of high-order Raman.

[0033] An embodiment provides a Raman fiber laser system for improving the high-order Raman threshold, comprising: a backward multi-mode pump signal combiner 2, a Raman low-reflection grating, a Raman high-reflection grating, and a Raman gain fiber 5;

[0034] The central wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain optical fiber.

[0035] The types of the Raman gain fiber include but are not limited to graded refractive index fiber and step refractive index fiber; the cladding form of the Raman gain fiber includes but is not limited to single cladding fiber and multi-cladding fiber; the matrix type of the Raman gain fiber includes but is not limited to silicate fiber, tellurite fiber and fluorite fiber. The Raman gain peak of silicate fiber is located at 440cm -1 , the Raman gain peak of tellurite fiber is located at 750cm -1 The Raman gain peak of the fluoride fiber is located at 580 cm -1 .

[0036] The writing methods of the Raman low-reflection grating and the Raman high-reflection grating include but are not limited to ultraviolet writing and femtosecond writing; the types of the Raman low-reflection grating and the Raman high-reflection grating include but are not limited to fiber gratings based on gradient refractive index fibers and fiber gratings based on step refractive index fibers.

[0037] The first output end cap 101 is coated with an anti-reflection film covering the wavelength range of the signal light, and the second output end cap 102 is coated with an anti-reflection film covering the wavelength range of the remaining pump light. The above anti-reflection film can reduce the end face feedback of the signal light and the remaining pump light, and avoid end face damage of the optical fiber.

[0038] The backward multi-mode pump signal combiner 2 is a (N+1)×1 backward pump signal combiner, wherein N can be any value, including 2, 6 and other commercial models.

[0039] The pump source includes but is not limited to a semiconductor laser, an ASE source or a fiber laser.

[0040] Specifically, embodiment 1 provides a Raman fiber laser system for improving the high-order Raman threshold, such as Figure 1 As shown, the first end of the backward multimode pump signal combiner 2 has multiple pump arms and a signal arm, wherein each pump arm is connected to a corresponding pump source, the signal arm is connected to the output pigtail of the first output end cap 101, and the output end of the first output end cap 101 outputs signal light, the second end of the backward multimode pump signal combiner 2 is connected to the input end of the first-order Raman low-reflection grating 3, the output end of the first-order Raman low-reflection grating 3 is connected to the input end of the Raman gain fiber 5, the output end of the Raman gain fiber 5 is connected to the input end of the first-order Raman high-reflection grating 4, and the output end of the first-order Raman high-reflection grating 4 is connected to the second output end cap 102 that outputs the remaining pump light.

[0041] After the pump source injects the pump light 9 from the pump arm of the backward multimode pump signal combiner 2, the Raman laser corresponding to the signal light wavelength is selected under the mode selection function of the Raman grating, and finally the Raman laser is coupled into the signal arm of the backward multimode pump signal combiner 2 and output through the first output end cap 101, and the remaining pump light that has not been converted is output through the second output end cap 102. This experimental structure can effectively separate the generated signal light and the remaining pump light.

[0042] Embodiment 2 provides a Raman fiber laser system for improving the high-order Raman threshold, such as Figure 2 As shown, it includes: a backward multi-mode pump signal combiner 2, a Raman low-reflection grating, a Raman high-reflection grating, a Raman gain fiber 5 and a backward return light filter 6;

[0043] The first end of the backward multimode pump signal combiner 2 has multiple pump arms and a signal arm, wherein each pump arm is connected to a corresponding pump source, a backward return light filter 6 is connected between the pump source and the pump arm of the backward multimode pump signal combiner 2, the signal arm is connected to the output pigtail of the first output end cap 101, and the output end of the first output end cap 101 outputs signal light, the second end of the backward multimode pump signal combiner 2 is connected to the input end of the first-order Raman low-reflection grating 3, the output end of the first-order Raman low-reflection grating 3 is connected to the input end of the Raman gain fiber 5, the output end of the Raman gain fiber 5 is connected to the input end of the first-order Raman high-reflection grating 4, and the output end of the first-order Raman high-reflection grating 4 is connected to the second output end cap 102 that outputs the remaining pump light.

[0044] The back-return light filter 6 is used to filter the backward transmitted signal light to prevent the backward transmitted signal light from returning to the pump source and damaging the pump source. The types of the back-return light filter 6 include but are not limited to tilted gratings, commercial cladding optical filters, and optical fiber devices based on special optical fibers.

[0045] After the pump source injects the pump light 9 from the pump arm of the backward multimode pump signal combiner 2, the Raman laser corresponding to the signal light wavelength is selected under the mode selection function of the Raman grating, and finally the Raman laser is coupled into the signal arm of the backward multimode pump signal combiner 2 and output through the first output end cap 101, and the remaining pump light that has not been converted is output through the second output end cap 102. This experimental structure can effectively separate the generated signal light and the remaining pump light.

[0046] In order to demonstrate the technical effect of the present invention, two comparative examples are designed, wherein comparative example 1 is based on Figure 2 The illustrated embodiment provides an optical path structure for constructing an optical path of a Raman fiber laser system for improving the high-order Raman threshold. The pump source in comparative example 1 is a 1080nm ytterbium-doped fiber laser, and the output pigtail is 20 / 400um. After the output pigtail of the pump source, a section of inclined grating engraved on the same 20 / 400um optical fiber is fused as a backward return light filter 6 to filter out the backward transmitted signal light, thereby achieving the purpose of protecting the 1080nm pump source. The specification of the backward multimode pump signal combiner 2 is (6+1)×1, wherein the pump arm optical fiber specification of the backward multimode pump signal combiner 2 is 20 / 130um, and the signal arm optical fiber specification is 50 / 400um. The pigtail of the first output end cap 101 adopts an optical fiber with the same specification as the signal arm optical fiber 50 / 400um of the backward multimode pump signal combiner 2. The specifications of the optical fiber output end of the backward multimode pump signal combiner 2 are 150 / 210um, the specifications of the first-order Raman low-reflection grating 3, the first-order Raman high-reflection grating 4, the Raman gain fiber 5 and the second output end cap 102 are 150 / 210um, the purpose is to match the specifications of the output end optical fiber of the backward multimode pump signal combiner 2 to minimize the welding loss. The length of the Raman gain fiber 5 used in Comparative Example 1 is 170 meters. Figure 3 The power and spectrum evolution diagrams of Comparative Example 1 are shown in FIG. (a) is the power evolution diagram of Comparative Example 1, and (b) is the spectrum evolution diagram of Comparative Example 1. Figure 3 As shown in (a), when the injected pump light power is 1890W, the first output end cap 101 outputs a signal light of 833W. Figure 3 (b) is the output spectrum at 833W, the intensity difference between the signal light and the high-order Raman light is 55dB. It is worth noting that Figure 3It can be observed in (b) that the residual pump light at 1080nm has a high signal-to-noise ratio, which can well separate the signal light and the residual pump light.

[0047] In order to compare with Comparative Example 1, Comparative Example 2 is provided. Figure 4 is the optical path structure diagram of the Raman fiber laser system used in Comparative Example 2. Comparative Example 2 is based on Figure 4 The optical path structure of the Raman fiber laser system shown in the figure is exactly the same as that of Comparison 1, with the following differences: Figure 4 The positions of the first-order Raman low-reflection grating 3 and the first-order Raman high-reflection grating 4 are relative to Figure 2 The positions of the first-order Raman low-reflection grating 3 and the first-order Raman high-reflection grating 4 are swapped, that is, Figure 4 The comparative example 2 of the optical path structure shown realizes the pumping of the forward structure, and the other conditions are exactly the same as those of the comparative example 1.

[0048] Reference Figure 5 , Figure 5 The power and spectrum evolution diagrams obtained in Comparative Example 2 are shown in FIG. (a) is the power evolution diagram obtained in Comparative Example 2, and (b) is the spectrum evolution diagram obtained in Comparative Example 2. Figure 5 As shown in (a), when the injected pump light power is 1304W, 584W of signal light can be obtained. Figure 5 (b) is the output spectrum at 584W, and the intensity difference between the signal light and the high-order Raman light is also maintained at 55dB. Figure 4 The residual pump light of Comparative Example 2 of the optical path structure shown needs to be split by a spectroscope, so the signal-to-noise ratio of the spectrum of the residual pump light at a wavelength of 1080 nm is relatively low.

[0049] In summary, according to the above comparative examples, it can be concluded that the backward multimode pump combiner can effectively suppress high-order Raman, thereby improving the level of signal light output power; at the same time, based on the optical path structure of the backward multimode pump signal combiner, the signal light and the remaining pump light can be effectively separated, the use of splitter components can be reduced, and the obtained spectral signal-to-noise ratio is better.

[0050] The physical length is In the Raman resonant cavity, when the pump laser reaches a certain power level, the gain of the first-order Stokes light (i.e., signal light) is greater than the loss, and its power continues to increase, while the corresponding pump light is continuously consumed and its power continues to decrease. Similarly, after the power of the first-order Stokes light reaches a certain power level, its energy will be continuously converted into high-order Raman light and amplified. In the backward pumped Raman oscillator, the expression of Raman gain is as follows:

[0051] ;

[0052] is the Raman gain;

[0053] is the Raman gain coefficient, which is not only related to the material of the gain medium, but also to the wavelength difference between the pump and signal light;

[0054] is the pump light power;

[0055] is the effective length of the fiber and is given by: ;

[0056] is the loss coefficient of the signal light;

[0057] is the physical length of the fiber;

[0058] is the effective mode field area of ​​the interaction between signal light and pump light.

[0059] In the backward pumping structure, the pump light and the signal light propagate in opposite directions. Due to the loss in the optical fiber, the pump light enters the optical fiber first and begins to experience attenuation, which reduces the pump light power that interacts with the signal light and reduces the effective action distance of the signal light and the high-order Raman (equivalent to reducing the effective transmission distance of the high-order Raman in the optical fiber), thereby reducing the probability of the occurrence of the high-order Raman effect.

[0060] Embodiment 3 provides a Raman fiber laser system for improving the high-order Raman threshold, such as Figure 6 As shown, it includes: a backward multi-mode pump signal combiner 2, a Raman low-reflection grating, a Raman high-reflection grating and a Raman gain fiber 5, and a backward return light filter 6;

[0061] The first end of the backward multimode pump signal combiner 2 has a plurality of pump arms and a signal arm, wherein each pump arm is respectively connected to a pump source, a backward return light filter 6 is connected between the pump source and the pump arm of the backward multimode pump signal combiner 2, the signal arm is connected to the output pigtail of the first output end cap 101, and the output end of the first output end cap 101 outputs signal light, and the second end of the backward multimode pump signal combiner 2 is connected to the output of the second-order Raman low-reflection grating 7. The output end of the second-order Raman low-reflection grating 7 is connected to the input end of the first-order Raman low-reflection grating 3, the output end of the first-order Raman low-reflection grating 3 is connected to the input end of the Raman gain fiber 5, the output end of the Raman gain fiber 5 is connected to the input end of the first-order Raman high-reflection grating 4, the output end of the first-order Raman high-reflection grating 4 is connected to the input end of the second-order Raman high-reflection grating 8, and the output end of the second-order Raman high-reflection grating 8 is connected to the second output end cap 102 that outputs the residual pump light.

[0062] The central wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain fiber, the central wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are located near the Raman gain peak of the first-order Raman grating in the Raman gain fiber, and the output signal light is at the central wavelength corresponding to the second-order Raman grating.

[0063] After the pump source injects the pump light 9 from the pump arm of the backward multimode pump signal combiner 2, the Raman laser corresponding to the signal light wavelength is selected under the mode selection function of the Raman grating, and finally the Raman laser is coupled into the signal arm of the backward multimode pump signal combiner 2 and output through the first output end cap 101, and the remaining pump light that has not been converted is output through the second output end cap 102. This experimental structure can effectively separate the generated signal light and the remaining pump light.

[0064] In addition, the Raman low-reflection grating and high-reflection grating in the present invention can be any order before the zero dispersion wavelength of the optical fiber in addition to the first order and the second order. Specifically, the Raman low-reflection grating includes a first-order Raman low-reflection grating, a second-order Raman low-reflection grating, ..., and an n-order Raman low-reflection grating. The Raman high-reflection grating includes a first-order Raman high-reflection grating, a second-order Raman high-reflection grating, ..., and an n-order Raman high-reflection grating. The second end of the backward multi-mode pump signal combiner is sequentially connected to the n-order Raman low-reflection grating, ..., the second-order Raman low-reflection grating, and the first-order Raman low-reflection grating. The first-order Raman low-reflection grating is connected to one end of the Raman gain optical fiber, and the other end of the Raman gain optical fiber is sequentially connected to the first-order Raman high-reflection grating, the second-order Raman high-reflection grating, ..., and the n-order Raman high-reflection grating. The n-order is any order before the zero dispersion wavelength of the optical fiber. The zero dispersion wavelength of conventional communication optical fiber is located near 1.3um and 1.5um. The central wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain fiber, and the central wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are located near the Raman gain peak of the first-order Raman grating in the Raman gain fiber, and so on. The output signal light is at the central wavelength corresponding to the n-order Raman grating.

[0065] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0066] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A Raman fiber laser system for improving the high-order Raman threshold, characterized in that: It includes a backward multimode pump signal combiner, a Raman low-reflection grating, a Raman high-reflection grating and a Raman gain fiber. The first end of the backward multimode pump signal combiner has multiple pump arms and a signal arm, wherein each pump arm is respectively connected to a pump source, and the signal arm serves as an output fiber of the signal light. The second end of the backward multimode pump signal combiner is connected to the Raman low-reflection grating, the Raman low-reflection grating is connected to one end of the Raman gain fiber, and the other end of the Raman gain fiber is connected to the Raman high-reflection grating. Based on the backward multi-mode pump signal combiner, the output fiber of the signal light is integrated with the pump arm, which can quickly amplify the signal light at the pump light injection end, shorten the action distance of the high-order Raman light in the optical fiber, reduce the gain accumulation of the signal light on the high-order Raman, and effectively improve the threshold of the high-order Raman. The Raman low-reflection grating includes a first-order Raman low-reflection grating, a second-order Raman low-reflection grating, ..., and an n-order Raman low-reflection grating; the Raman high-reflection grating includes a first-order Raman high-reflection grating, a second-order Raman high-reflection grating, ..., and an n-order Raman high-reflection grating; the second end of the backward multi-mode pump signal combiner is sequentially connected to the n-order Raman low-reflection grating, ..., the second-order Raman low-reflection grating, and the first-order Raman low-reflection grating; the first-order Raman low-reflection grating is connected to one end of the Raman gain fiber; the other end of the Raman gain fiber is sequentially connected to the first-order Raman high-reflection grating, the second-order Raman high-reflection grating, ..., and the n-order Raman high-reflection grating, wherein n is greater than or equal to 2; The central wavelengths of the first-order Raman low-reflection grating and the first-order Raman high-reflection grating are located near the Raman gain peak of the pump light in the Raman gain fiber, and the central wavelengths of the second-order Raman low-reflection grating and the second-order Raman high-reflection grating are located near the Raman gain peak of the first-order Raman grating in the Raman gain fiber, and so on. The output signal light is at the central wavelength corresponding to the n-order Raman grating.

2. The Raman fiber laser system for improving the high-order Raman threshold according to claim 1, characterized in that: A backward return light filter is also connected between the pump source and the pump arm of the backward multi-mode pump signal combiner.

3. The Raman fiber laser system for improving the high-order Raman threshold according to claim 1, characterized in that: The output optical fiber of the signal light is connected to a first output end cap for outputting the signal light, and the first output end cap is plated with an anti-reflection film covering the wavelength range of the signal light.

4. The Raman fiber laser system for improving the high-order Raman threshold according to claim 1, 2 or 3, characterized in that: The Raman high-reflection grating is connected to a second output end cap for outputting unconverted residual pump light, and the second output end cap is plated with an anti-reflection film covering the wavelength range of the residual pump light.

5. The Raman fiber laser system for improving the high-order Raman threshold according to claim 4, characterized in that: The Raman gain fiber is a silicate fiber, a tellurite fiber or a fluorite fiber. The Raman gain peak of the silicate fiber is located at 440 cm -1 , the Raman gain peak of tellurite fiber is located at 750cm -1 The Raman gain peak of fluoride fiber is located at 580 cm -1 .

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