Optical fiber filter and optical fiber laser system

Abstract

The utility model discloses an optical fiber filter and an optical fiber laser system, the optical fiber filter is formed by connecting a first mode field matcher, a long-period fiber bragg grating and a second mode field matcher, and the long-period fiber bragg grating is a single-mode fiber long-period fiber bragg grating. An input end optical fiber of the first mode field matcher and an output end optical fiber of the second mode field matcher are both large-mode-field few-mode optical fibers. The optical fiber filter is applied to the optical fiber laser system, the output laser of the optical fiber laser system passes through the optical fiber filter, the nonlinear effect in the laser is filtered out through the single-mode optical fiber long-period fiber grating, and then the laser is output, so that the high-power laser output performance of the optical fiber laser system is improved.

Description

TECHNICAL FIELD

[0001] The utility model belongs to high -power fiber laser technical field, more specifically, it relates to a kind of fiber filter and fiber laser system. BACKGROUND

[0002] Laser has evolved into a research field with huge branches after decades of rapid development. From a horizontal point of view, the development of high-power lasers represents one of the highest levels of technology in the use of energy. Among the various lasers, fiber lasers are widely used in different fields due to their good beam quality, high efficiency, excellent heat dissipation performance, compact structure, high reliability, waveguide transmission and other advantages.

[0003] Stimulated Raman scattering is an important limiting factor for further improving the output power of fiber lasers. For high-power fiber lasers, when the output power of a single fiber reaches kilowatts, the power density in the core will be very high, and at this time, nonlinear effects are likely to occur, causing the monochromaticity of the output light to decrease. At this time, the nonlinear effects in the fiber are also the main source of the decline in the output signal light power characteristics, including stimulated Brillouin scattering and stimulated Raman scattering. Stimulated Brillouin scattering light frequency shift mainly occurs in narrow spectrum (usually spectral width less than 0.8 nm) fiber lasers, and usually industrial applications such as cutting, welding, cladding, etc. use fiber lasers with relatively wide spectrum, where stimulated Raman scattering is the most important limiting factor.

[0004] In a wide-spectrum high-power fiber laser, once the signal light output power reaches the stimulated Raman scattering threshold, the signal light power will be converted into Stokes light power, which will have a significant impact on the high-power fiber laser system, including the following aspects:

[0005] (1) Stimulated Raman scattering reduces the signal-to-noise ratio of the laser system. Due to stimulated Raman scattering, the Stokes light power grows exponentially. If the light outside the signal light is considered as noise, the signal-to-noise ratio of the laser output will decrease dramatically.

[0006] (2) Stimulated Raman scattering leads to a decrease in beam quality. In recent years, several reports have shown that stimulated Raman scattering not only causes the output signal light power to stagnate, but also the heat generated by quantum loss during stimulated Raman scattering process causes the grating effect, which is an important factor causing the degradation of beam quality and even transverse mode instability. Recent experimental studies have shown that even in the case of low stimulated Raman scattering power, it can still cause quasi-static mode degradation, affecting the beam quality.

[0007] (3) Stimulated Raman scattering reduces the reliability of the laser system. The Stokes light generated by stimulated Raman scattering is bidirectional, and the back transmission of the Stokes light will undoubtedly bring risks to the optical fiber device if the power is too high. On the other hand, due to the fact that the transmission wavelength of the high-transmission membrane of the finished laser system is not completely matched with the wavelength of the Stokes light, the thermal effect of the reflection site under high-power conditions will cause the system reliability to decrease, and even cause the laser to burn out.

[0008] (4) Stimulated Raman scattering limits the length of the laser output armor. The stimulated Raman scattering threshold and the length of the effective optical fiber are inversely proportional, which has a certain impact on the application of the current high-power fiber laser machine. The effective length of the optical fiber refers to the length of the optical fiber on the whole laser transmission route including the active optical fiber and the passive optical fiber. Therefore, if the stimulated Raman scattering cannot be effectively controlled, the length of the energy transmission cable cannot be made too long, and the length of the armor will inevitably limit the convenience and work efficiency of the super-large format processing machine tool in application.

[0009] In summary, in practical applications, in order to further improve the output performance of the existing high-power high-beam-quality fiber laser system, effective strategies must be taken to suppress stimulated Raman scattering in high-power fiber lasers. At present, researchers at home and abroad have proposed various methods to suppress stimulated Raman scattering in high-power fiber lasers from the aspects of fiber design and system optimization. Among them, fiber grating filters have attracted widespread attention due to their high Raman suppression ratio, low insertion loss, flexible application, relatively short preparation period, and other characteristics. It is a high-efficiency and simple method for suppressing stimulated Raman scattering in high-power fiber lasers, mainly including chirped tilted Bragg gratings and long-period gratings. Although chirped tilted Bragg gratings have achieved good results in the filtering and suppression of stimulated Raman scattering, the current chirped tilted Bragg gratings are formed by refractive index modulation in the fiber core to realize mode coupling, which inevitably causes insertion loss to the signal light transmitted in the fiber core during the preparation process, causing heating and reducing system stability. In addition, the residual Bragg reflection of the chirped tilted Bragg grating also has a great impact on its working performance, such as the easy excitation of random Raman laser under high power.

[0010] Long period grating is a kind of passive optical fiber device, which can avoid the back reflection of Bragg reflection in the high power application, because the mechanism of long period grating does not exist. In addition, the preparation method of long period grating is various, and the photosensitivity of optical fiber is not hard, so there is no need for long hydrogenation annealing process, and the heat effect caused by residual hydrogen in the laser transmission process is also avoided. Some preparation methods can form long period grating by refractive index modulation in the cladding, so the insertion loss is extremely low in theory. The above characteristics show that long period grating is a kind of passive optical fiber device which is naturally compatible with high power fiber laser. However, although many reports in recent years show that long period grating is more suitable for high power fiber laser stimulated Raman scattering suppression in normal working state, the long period grating stimulated Raman scattering suppression is still limited to single mode / quasi single mode kilowatt level fiber laser, and there is still a certain gap with actual high power application, which is due to the following reasons: the current long period fiber grating is a large mode field few mode fiber long period grating, that is, the long period fiber grating is directly written in the large mode field few mode fiber, which will produce core mode conversion and core-cladding mode cross coupling effect, and seriously affect the application effect of long period fiber grating. The mode coupling of large mode field few mode fiber long period grating with large power bearing capacity leads to the degradation of stimulated Raman scattering suppression effect. Content of the utility model

[0011] In view of the defects of prior art, the utility model provides a kind of optical fiber filter and optical fiber laser system, wherein the optical fiber filter is passive optical fiber device, and stimulated Raman scattering suppression can be realized without involving high order mode conversion and cross coupling.

[0012] To achieve the above technical purpose, the technical scheme adopted by the utility model is as follows:

[0013] The utility model provides a kind of optical fiber filter, by first mode field adapter, long period fiber grating and second mode field adapter are connected, long period fiber grating is single mode fiber long period fiber grating, the input end optical fiber of first mode field adapter, the output end optical fiber of second mode field adapter are all large mode field few mode fiber, the input end of first mode field adapter and the output end of second mode field adapter are big end, and the core diameter, cladding diameter and refractive index distribution of big end are matched with the large mode field few mode fiber, the output end of first mode field adapter and the input end of second mode field adapter are small end, and the core diameter, cladding diameter and refractive index distribution of small end are matched with the single mode fiber of long period fiber grating, and the big end and small end between mode field adapter are taper region, and the length of taper region satisfies the following relationship:

[0014] ;

[0015] Wherein , core diameter, maximum taper angle allowed by the adiabatic taper.

[0016] Further, the large-mode-area few-mode fiber has a core diameter of 15-30 μm, a relative refractive index difference between the core and the cladding of 0.1%-0.5%, and a mode number of 2-10.

[0017] Further, the long-period fiber grating has a resonance peak center wavelength of the transmission spectrum in the range of 1130-1140 nm.

[0018] In another aspect, provided is a fiber laser system, comprising a fiber laser and an output end cap, the fiber filter being connected between an output end of the fiber laser and an input end of the output end cap, the output end fiber of the fiber laser and the input end fiber of the output end cap both being a large-mode-area few-mode fiber, the output end fiber of the fiber laser being connected to an input end of a first mode field adapter of the fiber filter, and an output end of a second mode field adapter of the fiber filter being connected to the input end of the output end cap, wherein the input end fiber of the first mode field adapter and the output end fiber of the second mode field adapter are both a large-mode-area few-mode fiber corresponding to the output end fiber of the fiber laser and the input end fiber of the output end cap respectively.

[0019] Further, the fiber laser is not limited in structure and type, and can be a fiber laser oscillator. The fiber laser oscillator is not limited in structure and type, and can be a forward-pumped fiber laser oscillator, a backward-pumped fiber laser oscillator, or a bidirectional-pumped fiber laser oscillator.

[0020] Further, the fiber laser oscillator comprises a pump source, a pump combiner, a high-reflection grating, a gain fiber, and a low-reflection grating, the high-reflection grating, the gain fiber, and the low-reflection grating being connected in sequence to form a resonant cavity, and one or more pump sources being connected to corresponding pump arms of the pump combiner respectively, and the pump light output by the pump source being injected into the gain fiber of the resonant cavity through the pump combiner. The high-reflection grating and the low-reflection grating have the same center wavelength.

[0021] Further, the fiber laser is not limited in structure and type, and can be a fiber laser amplifier.

[0022] Further, the fiber laser is not limited in structure and type, and comprises a seed laser and a laser power amplification optical path, the seed laser being connected to the laser power amplification optical path, and the fiber filter being connected between an output end of the laser power amplification optical path and an input end of the output end cap.

[0023] In another aspect, a fiber laser system is provided, comprising a seed laser and a laser power amplification optical path, the fiber filter is connected between the seed laser and the laser power amplification optical path, the output end fiber of the seed laser and the input end fiber of the laser power amplification optical path are both large-mode-area few-mode fibers, the output end fiber of the seed laser is connected to the input end of the first mode field adapter, and the output end of the second mode field adapter is connected to the input end of the laser power amplification optical path, wherein the input end fiber of the first mode field adapter and the output end fiber of the second mode field adapter are large-mode-area few-mode fibers corresponding to the output end fiber of the seed laser and the input end fiber of the laser power amplification optical path respectively.

[0024] The utility model can achieve the following beneficial effects:

[0025] In view of the fact that directly inscribing long-period fiber grating in large-mode-area few-mode fiber will cause core mode conversion and core-cladding mode cross-coupling effect, which seriously affects the application effect of long-period fiber grating, the utility model provides a fiber filter, which couples the optical field in the large-mode-area few-mode fiber to the single-mode fiber long-period fiber grating by the first mode field adapter, the resonant wavelength of the single-mode fiber long-period fiber grating corresponds to the center wavelength of stimulated Raman scattering, and then the single-mode optical field is coupled to the large-mode-area few-mode fiber with greater power bearing by the second mode field adapter.

[0026] The utility model is based on passive fiber filter device to suppress stimulated Raman scattering of high-power fiber laser system, which has the advantages of convenient preparation, small loss and fiber compatibility.

[0027] The fiber filter provided by the utility model has the characteristics of low long-period grating insertion loss in large-mode-area few-mode fiber, large Raman scattering suppression ratio and simple preparation process, and can be flexibly placed in high-power fiber laser systems of various fiber sizes. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical scheme in the utility model or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or the prior art description, and obviously, the drawings in the following description are only some embodiments of the utility model, and for those skilled in the art, other drawings can be obtained according to the structure shown in the drawings without creative labor.

[0029] Figure 1 It is a structural schematic view of the fiber filter provided in an embodiment;

[0030] Figure 2 Transmission spectrum of the fiber filter in an embodiment;

[0031] Figure 3 Structure diagram of the fiber laser system provided in an embodiment;

[0032] Figure 4 Structure diagram of the fiber laser system provided in an embodiment;

[0033] Reference numerals in the drawings:

[0034] 1, pump source; 2, first pump combiner; 3, high reflection grating; 4, low reflection grating; 5, first gain fiber; 6, fiber filter; 7, output end cap; 8, second pump combiner; 9, second gain fiber; 61, input end fiber of the first mode field adapter; 62, first mode field adapter; 63, single mode long period fiber grating; 64, second mode field adapter; 65, output end fiber of the second mode field adapter.

[0035] The realization, functional features and advantages of the utility model will be further described in combination with embodiments and with reference to the drawings. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the utility model will be apparently and completely described in combination with the drawings in the embodiments of the utility model. Apparently, the described embodiments are only part of the embodiments of the utility model, rather than all the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by the ordinary skilled in the art without creative labor belong to the protection scope of the utility model.

[0037] Reference Figure 1 In an embodiment, a fiber filter is provided, which is connected by a first mode field adapter 62, a long period fiber grating and a second mode field adapter 64. The long period fiber grating is a single mode long period fiber grating 63. The input end fiber 61 of the first mode field adapter and the output end fiber 65 of the second mode field adapter are both large mode field few mode fibers. The input end of the first mode field adapter and the output end of the second mode field adapter are large ends. The core diameter, cladding diameter and refractive index distribution of the large end are matched with the large mode field few mode fiber. The output end of the first mode field adapter and the input end of the second mode field adapter are small ends. The core diameter, cladding diameter and refractive index distribution of the small end are matched with the single mode fiber of the long period fiber grating. The large end and the small end of the mode field adapter are a taper region. The length of the taper region satisfies the following relationship:

[0038] ;

[0039] wherein , These are the core diameters at the large and small ends, respectively. This is the maximum allowable cone angle for an adiabatic taper.

[0040] The cone length of the mode field matcher is designed based on the adiabatic conditions. These conditions require slow changes in the mode field to avoid mode coupling. ,in For the operating wavelength, The difference in refractive index between the core and the cladding. The average core diameter of the cone region.

[0041] The mode field diameter of large-mode-field few-mode fibers (e.g., >15μm) is significantly mismatched with that of single-mode fibers (approximately 10μm) and the mode field of long-period fiber gratings in single-mode fibers. Direct connection of these fibers results in extremely high insertion loss (potentially exceeding 3dB-5dB) and severe mode perturbations. By employing a tapered design between the large and small ends of the mode field matcher, a smooth, low-loss conversion of the mode field size is achieved.

[0042] The optical fibers at both ends of the fiber filter are large-mode-field few-mode fibers, enabling the entire device to withstand and transmit extremely high optical power while maintaining very low linear and nonlinear losses. The structural dimensions of the large-mode-field few-mode fiber can be determined based on the fiber dimensions in the actual fiber laser system used in the application. The middle part of the structure is a single-mode fiber long-period fiber grating 63. The large-mode-field few-mode fiber and the single-mode fiber are connected by a mode field matcher. This structure can couple the laser light of all core modes in the core of the large-mode-field few-mode fiber into the single-mode fiber. The center wavelength of the resonant peak of the transmission spectrum of the long-period fiber grating is in the range of 1130nm-1140nm, which corresponds precisely to the excitation wavelength of stimulated Raman scattering in silica fiber.

[0043] like Figure 2 As shown, in the transmission spectrum of an embodiment of the fiber optic filter, the center wavelength of the resonance peak of the single-mode fiber long-period fiber grating 63 coincides with the center wavelength of stimulated Raman scattering, thus suppressing stimulated Raman scattering. The single-mode fiber long-period fiber grating 63 is a long-period fiber grating inscribed on a single-mode fiber.

[0044] The optical fibers at both ends of the fiber filter are large-mode-field few-mode fibers. The diameter of the large-mode-field few-mode fiber is not limited and can be commonly used such as 250 micrometers, 400 micrometers, 600 micrometers, etc., to match the size of the large-mode-field few-mode fiber in the actual fiber laser system in the specific application.

[0045] The large-mode-field few-mode fiber has a core diameter between 15μm and 30μm, a relative refractive index difference between the core and cladding between 0.1% and 0.5%, and supports 2 to 10 modes.

[0046] The fiber filter provided by the above embodiments can be used before the output end cap of a fiber laser oscillator, before the output end cap of a fiber laser amplifier, or between the seed light and the amplification stage of a fiber laser amplifier, to filter out the stimulated Raman scattering components in the seed laser.

[0047] In an embodiment, a fiber laser system is provided, which includes a fiber laser and an output end cap, and the fiber filter is connected between the output end of the fiber laser and the input end of the output end cap. The output fiber of the fiber laser and the input fiber of the output end cap are both large-mode-area few-mode fibers. The output fiber of the fiber laser is connected to the input end of the first mode field adapter of the fiber filter, and the output end of the second mode field adapter of the fiber filter is connected to the input end of the output end cap. The input fiber of the first mode field adapter and the output fiber of the second mode field adapter are both large-mode-area few-mode fibers corresponding to the output fiber of the fiber laser and the input fiber of the output end cap respectively.

[0048] The structure and type of the fiber laser are not limited, and the fiber laser can be a fiber laser oscillator or a fiber laser amplifier. The structure and type of the fiber laser oscillator are not limited, and the fiber laser oscillator can be a forward-pumped fiber laser oscillator, a backward-pumped fiber laser oscillator, or a bidirectional-pumped fiber laser oscillator.

[0049] Reference Figure 3 In an embodiment, a fiber laser system is provided, which includes a pump source 1, a first pump combiner 2, a high-reflection grating 3, a first gain fiber 5, a low-reflection grating 4, a fiber filter 6, and an output end cap 7. The high-reflection grating 3, the first gain fiber 5, and the low-reflection grating 4 are connected in sequence to form a resonant cavity. One or more pump sources 1 are connected to the corresponding pump arms of the first pump combiner 2, and the pump light output by the pump source 1 is injected into the first gain fiber 5 of the resonant cavity through the first pump combiner 2. The center wavelengths of the high-reflection grating 3 and the low-reflection grating 4 are the same, and the reflectivities are different, to achieve the function of outputting laser of a specific wavelength. The high-reflection grating 3, the first gain fiber 5, and the low-reflection grating 4 are all large-mode-area few-mode fibers, and the input fiber of the output end cap 7 is also a large-mode-area few-mode fiber. The core of the large-mode-area few-mode fiber can accommodate more than one transmission mode, and a long-period fiber grating that can efficiently suppress stimulated Raman scattering cannot be directly written on the large-mode-area few-mode fiber. The fiber filter 6 in this embodiment is the fiber filter provided in the above embodiments. Figure 1 The fiber filter provided by the above embodiments. In the fiber filter 6, the input fiber 61 of the first mode field adapter is directly fused with the low-reflection grating 4 based on the large-mode-area few-mode fiber, and the output fiber 65 of the second mode field adapter is fused with the input fiber of the output end cap 7 to output high-power fiber laser.

[0050] The introduction of the optical fiber filter can suppress stimulated Raman scattering in the large-mode-area few-mode fiber laser by the single-mode fiber long-period fiber grating of the optical fiber filter without affecting the output power, thereby improving the output performance of the fiber laser. The resonant wavelength of the single-mode fiber long-period fiber grating is consistent with the center wavelength of the stimulated Raman scattering. When high-power laser is transmitted through the optical fiber filter, the signal light part in the laser can pass through the optical fiber filter without being affected, while the stimulated Raman scattering component in the laser is coupled into the cladding by the single-mode fiber long-period fiber grating and is lost, thereby improving the purity of the signal light in the output laser.

[0051] Reference Figure 4 In an embodiment, a fiber laser system is provided, the fiber laser comprising a seed laser and a laser power amplification optical path, the seed laser being connected to the laser power amplification optical path, and an optical fiber filter 6 being connected between the output end of the laser power amplification optical path and the input end of an output end cap 7. Specifically, it comprises a pump source 1, a first pump combiner 2, a high-reflection grating 3, a first gain fiber 5, a low-reflection grating 4, an optical fiber filter 6, an output end cap 7, a second pump combiner 8, and a second gain fiber 9. In the seed laser, the high-reflection grating 3, the first gain fiber 5, and the low-reflection grating 4 are connected in sequence to form a resonant cavity, and one or more pump sources 1 are respectively connected to the corresponding pump arms of the first pump combiner 2, and the pump light output by the pump source 1 is injected into the first gain fiber 5 of the resonant cavity through the first pump combiner 2. The center wavelengths of the high-reflection grating 3 and the low-reflection grating 4 are the same, and the reflectivities are different, so as to output laser of a specific wavelength. In the laser power amplification optical path, one or more pump sources 1 are respectively connected to the corresponding pump arms of the two second pump combiners 8, and the second gain fiber 9 is connected between the two second pump combiners 8, one of which is connected to the output end of the seed laser (i.e., the output end of the low-reflection grating 4), and the other is connected to the input end of the optical fiber filter 6, and the output end of the optical fiber filter 6 is connected to the input end of the output end cap 7. The high-reflection grating 3, the first gain fiber 5, the low-reflection grating 4, the second gain fiber 9, and each pump combiner are large-mode-area few-mode fibers or use large-mode-area few-mode fibers as carrier fibers, and the core of the large-mode-area few-mode fiber can accommodate more than one transmission mode. The optical fiber filter 6 in the embodiment is a single-mode fiber long-period fiber grating Figure 1The embodiment shown provides a fiber filter. In the fiber filter 6, the input end fiber 61 of the first mode field adapter is directly fused with the output end fiber (a large mode field few mode fiber) of the second pump combiner 8, the output end fiber 65 of the second mode field adapter is fused with the input end fiber (a large mode field few mode fiber) of the output end cap 7, and high-power fiber laser is output through the output end cap 7. The large mode field few mode fibers at both ends of the fiber filter can be directly fused with corresponding large mode field few mode fibers in the fiber laser system, the high-power laser input into the fiber filter 6 is injected into the single mode fiber long period fiber grating 63 through the first mode field adapter 62, and the stimulated Raman scattering in the high-power fiber laser is filtered out through the single mode fiber long period fiber grating 63 and then transmitted to the output end cap 7 through the second mode field adapter 64, that is, the laser amplified through the laser power amplification optical path is output by the output end cap 7 after the stimulated Raman scattering in the laser is suppressed through the fiber filter 6.

[0052] In another embodiment, a fiber laser system is provided, comprising a seed laser and a laser power amplification optical path, and the fiber filter 6 is connected between the seed laser and the laser power amplification optical path, the output end fiber of the seed laser and the input end fiber of the laser power amplification optical path are both large mode field few mode fibers, the output end fiber of the seed laser is connected to the input end of the first mode field adapter, and the output end of the second mode field adapter is connected to the input end of the laser power amplification optical path, wherein the input end fiber of the first mode field adapter and the output end fiber of the second mode field adapter are large mode field few mode fibers corresponding to the output end fiber of the seed laser and the input end fiber of the laser power amplification optical path respectively.

[0053] The remaining matters of the utility model are well-known technologies.

[0054] The technical features of the above embodiments can be combined arbitrarily, and to make the description concise, all possible combinations of the technical features in the above embodiments are not described, however, as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present application.

[0055] The above-described embodiments only express several implementation manners of the present application, the description is more specific and detailed, but it should not be understood as the limitation of the utility model. It should be pointed out that for ordinary skilled persons in the art, on the premise of not departing from the concept of the present application, a number of modifications and improvements can be made, which belong to the protection scope of the present application.

[0056] The above merely describes preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An optical fiber filter, characterized by, The first mode field adapter, the long period fiber grating and the second mode field adapter are connected, the long period fiber grating is a single-mode long period fiber grating, and the input end fiber of the first mode field adapter and the output end fiber of the second mode field adapter are both large-mode-area few-mode fibers; the input end of the first mode field adapter and the output end of the second mode field adapter are large ends, the core diameter, the cladding diameter and the refractive index distribution of the large ends are matched with the large-mode-area few-mode fibers, the output end of the first mode field adapter and the input end of the second mode field adapter are small ends, the core diameter, the cladding diameter and the refractive index distribution of the small ends are matched with the single-mode fiber of the long period fiber grating, and a taper region is between the large end and the small end of the mode field adapter, and the length of the taper region satisfies the following relationship: ； wherein , are the core diameters for the large end and the small end, respectively, is the maximum taper angle allowed for the adiabatic taper.

2. The optical fiber filter according to claim 1, characterized in that, The large-mode-area few-mode fiber has a core diameter of 15-30 mu m, a relative refractive index difference between the core and the cladding of 0.1-0.5%, and a mode number of 2-10.

3. The optical fiber filter according to claim 1 or 2, characterized in that, The long period fiber grating has a resonance peak center wavelength of the transmission spectrum in the range of 1130-1140 nm.

4. A fiber laser system, characterized by, The fiber laser and the output end cap are connected The fiber laser of the fiber filter of claim 1 or 2, the output end fiber of the fiber laser and the input end fiber of the output end cap are both large-mode-area few-mode fibers, the output end fiber of the fiber laser is connected to the input end of the first mode field adapter of the fiber filter, the output end of the second mode field adapter of the fiber filter is connected to the input end of the output end cap, and the input end fiber of the first mode field adapter and the output end fiber of the second mode field adapter are respectively large-mode-area few-mode fibers corresponding to the output end fiber of the fiber laser and the input end fiber of the output end cap.

5. The fiber laser system of claim 4, wherein, The fiber laser is a fiber laser oscillator.

6. The fiber laser system of claim 5, wherein, The fiber laser oscillator is a forward-pumping fiber laser oscillator, a backward-pumping fiber laser oscillator or a bidirectional-pumping fiber laser oscillator.

7. The fiber laser system of claim 6, wherein, The fiber laser oscillator comprises a pump source, a pump combiner, a high-reflection grating, a gain fiber and a low-reflection grating, the high-reflection grating, the gain fiber and the low-reflection grating are sequentially connected to form a resonant cavity, one or more pump sources are respectively connected to corresponding pump arms of the pump combiner, pump light output by the pump source is injected into the gain fiber of the resonant cavity through the pump combiner, and the center wavelengths of the high-reflection grating and the low-reflection grating are the same.

8. The fiber laser system of claim 4, wherein, The fiber laser is a fiber laser amplifier.

9. The fiber laser system of claim 4, wherein, The fiber laser comprises a seed laser and a laser power amplification optical path, the seed laser is connected to the laser power amplification optical path, and the fiber filter is connected between the output end of the laser power amplification optical path and the input end of the output end cap.

10. A fiber laser system, characterized by, The application also discloses a seed laser and a laser power amplification optical path, wherein the seed laser and the laser power amplification optical path are connected with the optical fiber filter as claimed in claim 1 or 2, the output end optical fiber of the seed laser and the input end optical fiber of the laser power amplification optical path are both large-mode-field few-mode optical fibers, the output end optical fiber of the seed laser is connected with the input end of the first mode field matcher, and the output end of the second mode field matcher is connected with the input end of the laser power amplification optical path, wherein the input end optical fiber of the first mode field matcher and the output end optical fiber of the second mode field matcher are respectively large-mode-field few-mode optical fibers corresponding to the output end optical fiber of the seed laser and the input end optical fiber of the laser power amplification optical path.

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