Few-mode fiber double-cascade mode converter and high-power few-mode fiber laser

Abstract

The invention discloses a few-mode fiber double-cascade mode converter and a high-power few-mode fiber laser. A fiber core fundamental mode and fiber core high-order mode converter and a fiber core high-order mode and cladding mode converter are cascaded on a few-mode fiber; the refractive index modulation distribution of the fiber core fundamental mode and fiber core high-order mode converter is uniformly distributed in the center of the fiber core and is used for coupling the fiber core fundamental mode to a fiber core high-order mode; the refractive index modulation of the fiber core high-order mode and cladding mode converter is distributed at the junction of the fiber core and the cladding, and is used for coupling the fiber core high-order mode into the cladding to form loss. The few-mode fiber double-cascade mode converter is connected in front of an output end cap of a high-power few-mode fiber laser, so that stimulated Raman scattering in the large-mode-field few-mode fiber laser can be inhibited, and the output performance of the fiber laser is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high-power fiber lasers, and more particularly, to a few-mode fiber double cascade mode converter and a high-power few-mode fiber laser. BACKGROUND

[0002] Laser has evolved into a research field with a large branch after decades of rapid development. Transversely, the development of high-power laser represents one of the highest levels of human energy utilization technology. Among all lasers, fiber lasers are widely used in many technical fields due to their good beam quality, high efficiency, excellent heat dissipation performance, compact structure, high reliability, waveguide transmission and other advantages.

[0003] For high-power fiber lasers, when the output power of a single fiber reaches kilowatts, the power density in the core will be high, and at this time, nonlinear effects are easy to occur, causing the monochromaticity of the output light to decrease. Usually, industrial applications such as cutting, welding, and cladding often use fiber lasers with relatively wide spectra, and stimulated Raman scattering is the most important limiting factor. 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 in a large amount, which has the following main effects on the high-power fiber laser system: a. 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.

[0004] b. Stimulated Raman scattering leads to a decrease in beam quality. In recent years, multiple reports have shown that stimulated Raman scattering not only causes the output signal light power to increase, but also the heat generated by quantum loss during stimulated Raman scattering causes a grating effect, which is an important factor causing beam quality degradation and even transverse mode instability. The latest characterization experimental research shows that even in the case of low stimulated Raman scattering power, it will still cause quasi-static mode degradation, affecting the beam quality.

[0005] c. Stimulated Raman scattering reduces the reliability of the laser system. The Stokes light generated by stimulated Raman scattering is bidirectional, and the backward transmission of the Stokes light will undoubtedly pose a risk to the fiber device if the power is too high. On the other hand, since the transmission wavelength of the finished laser system is not completely matched with the Stokes light wavelength, 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.

[0006] d.The output armor length of stimulated Raman scattering laser. The inverse relationship between the stimulated Raman scattering threshold and the length of the effective optical fiber has a certain impact on the application of the current high-power fiber laser machine. The so-called 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 very 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.

[0007] In summary, in practical applications, 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.

[0008] 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. Among them, long-period gratings couple the forward transmission core mode into the cladding to form loss. Since there is no Bragg reflection in the formation mechanism, it can avoid the damage to the fiber device caused by backward light in high-power applications. In addition, the preparation method of long-period gratings is diverse, and there is no hard requirement for the photosensitivity of the optical fiber. Therefore, it does not need a long hydrogenation annealing process, and also avoids the thermal effect caused by residual hydrogen during laser transmission. Some preparation methods can form long-period gratings by refractive index modulation in the cladding, so the insertion loss is theoretically very low. The above characteristics show that long-period gratings are a kind of passive optical fiber devices that are naturally compatible with high-power fiber lasers.

[0009] Although in recent years, multiple reports have shown that long-period gratings are more suitable for suppressing stimulated Raman scattering in high-power fiber lasers under normal working conditions, the suppression of stimulated Raman scattering by long-period gratings is currently limited to single-mode / quasi-single-mode kilowatt-level fiber lasers, which still has a certain gap with actual high-power applications. The reason is that the mode coupling of long-period gratings in large-mode-area few-mode fibers with large power carrying capacity is unstable, leading to the degradation of stimulated Raman scattering suppression effect.

[0010] Figure 1 The schematic diagram of the mode field distribution and effective refractive index distribution in a typical large-mode-area few-mode fiber. From the left to the right, the core mode, the first-order cladding mode, the second-order cladding mode, and the third-order cladding mode are shown. Figure 1It can be seen that the optical fiber core can carry four transmission modes, and the effective refractive indexes of different modes are different. Generally, the preparation and formation of the long-period fiber grating depend on the refractive index modulation period, and each refractive index modulation period corresponds to the energy coupling between two different effective refractive indexes. Once the core mode is more, one refractive index modulation period can simultaneously meet the coupling conditions between multiple modes, so cross-coupling occurs, which greatly reduces the coupling efficiency between target modes. Therefore, how to effectively avoid mode cross-coupling is a technical problem that needs to be solved in the field. SUMMARY

[0011] In view of the defects in the prior art, the application provides a few-mode fiber double-cascade mode converter and a high-power few-mode fiber laser.

[0012] To achieve the above technical purposes, the technical scheme adopted by the application is as follows: The application provides a few-mode fiber double-cascade mode converter, which comprises a core base mode and a core high-order mode converter and a core high-order mode and cladding mode converter; the core base mode and the core high-order mode converter and the core high-order mode and cladding mode converter are cascaded on the same few-mode fiber, and the core base mode and the core high-order mode converter and the core high-order mode and cladding mode converter are separated by a certain distance. The core base mode and the core high-order mode converter are used for coupling the core base mode to the core high-order mode. The core high-order mode and cladding mode converter is used for coupling the core high-order mode to the cladding mode and introducing loss.

[0013] Further, the refractive index modulation region of the core base mode and the core high-order mode converter is located at the center of the core and has a uniform distribution of refractive index.

[0014] Further, the refractive index modulation region of the core base mode and the core high-order mode converter is prepared by a carbon dioxide laser point-by-point writing method or a femtosecond laser point-by-point writing method; the carbon dioxide laser point-by-point writing method is to focus laser on the surface of the fiber cladding to form ablation, release the stress in the center of the core to realize the uniform modulation of the refractive index at the center of the core; the femtosecond laser point-by-point writing method is to focus laser pulses directly on the center of the core to modify the core material in the center of the core for subtractive machining to realize the uniform modulation of the refractive index at the center of the core.

[0015] Further, the refractive index modulation region of the core high-order mode and cladding mode converter is distributed at the junction of the core and the cladding, and the refractive index modulation region of the core high-order mode and cladding mode converter comprises a plurality of sub-refractive index modulation regions which are distributed at the junction of the core and the cladding and are circularly symmetrically distributed with the center of the core as the center, and the refractive index of each sub-refractive index modulation region is uniformly distributed, and the plurality of sub-refractive index modulation regions are used to simultaneously disturb the core high-order mode and the cladding mode to realize energy coupling between the two.

[0016] Further, the plurality of sub-refractive index modulation regions in the core high-order mode and cladding mode converter are prepared by a femtosecond laser point-by-point exposure method, and the laser action region is located at the junction of the core and the cladding, and the refractive index is modulated at the junction of the core and the cladding.

[0017] Further, the center wavelengths of the core fundamental mode and core high-order mode converter and the core high-order mode and cladding mode converter are consistent and are in the range of 1130-1140 nm, which exactly corresponds to the excitation wavelength of stimulated Raman scattering of quartz optical fiber.

[0018] On the other hand, the application provides a high-power few-mode fiber laser, and the above-mentioned few-mode fiber double-cascaded mode converter is connected between the output fiber and the output end cap of the high-power few-mode fiber laser, and stimulated Raman scattering in the high-power few-mode fiber laser is suppressed by the double-cascaded mode converter.

[0019] Further, the structure and type of the high-power few-mode fiber laser are not limited, and the high-power few-mode fiber laser can be a fiber laser oscillator. 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.

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

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

[0022] Further, the structure and type of the fiber laser are not limited, and 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 above-mentioned few-mode fiber double-cascaded mode converter is connected between the output end of the laser power amplification optical path and the input end of the output end cap.

[0023] In another aspect, provided is an optical fiber laser system, comprising a seed laser and a laser power amplification optical path, and a double-cascaded mode converter as described above is connected between the seed laser and the laser power amplification optical path.

[0024] The present application can achieve the following beneficial effects: Directly inscribing a long-period fiber grating in a large-mode-area fiber will cause core mode conversion and core-cladding mode cross-coupling effect, which seriously affects the application effect of the long-period fiber grating. The double-cascaded mode converter of the present application can avoid mode cross-coupling in a large-mode-area few-mode fiber through two-mode coupling and can couple the core fundamental mode into the cladding mode twice without damage. Specifically, the double-cascaded mode converter of the present application has a core fundamental mode and a core high-order mode converter and a core high-order mode and a cladding mode converter cascaded on a few-mode fiber. First, the core fundamental mode and the core high-order mode converter are arranged on the few-mode fiber, and the refractive index modulation is uniformly distributed in the center of the core, so as to couple the core fundamental mode into the core high-order mode. Then, the core high-order mode and the cladding mode converter are arranged on the few-mode fiber, and the refractive index modulation of the core high-order mode and the cladding mode converter is distributed at the junction of the core and the cladding (i.e., the refractive index modulation is formed at the junction of the core and the cladding), so as to couple the core high-order mode into the cladding to form loss.

[0025] The refractive index modulation region of the core fundamental mode and the core high-order mode converter can be prepared by a carbon dioxide laser point-by-point inscription method or a femtosecond laser point-by-point inscription method. When the carbon dioxide laser point-by-point inscription method is used, a carbon dioxide laser is used to form an ablation effect on the surface of the fiber cladding. When the cladding is ablated, the constraint on the internal core is reduced, and the inherent external expansion stress in the core is released to form uniform refractive index modulation. When the femtosecond laser point-by-point inscription method is used, the femtosecond laser is directly focused on the center of the core of the fiber to expose the quartz material in the center of the core to light processing to form uniform refractive index modulation.

[0026] The refractive index modulation of the core high-order mode and the cladding mode converter is distributed at the junction of the core and the cladding, and is used to couple the core high-order mode into the cladding to form loss. The optical field of the core high-order mode is mainly distributed in the outer circle of the core and the part of the cladding close to the core, and the cladding mode is mainly distributed in the part of the cladding close to the core and the outermost layer of the core. Therefore, the refractive index modulation at the junction of the core and the cladding can simultaneously disturb the core high-order mode and the cladding mode, so that energy coupling occurs between the two. The core high-order mode and the cladding mode converter directly acts on the core and the boundary of the core and the cladding through the core cladding by the femtosecond laser point-by-point exposure method, and the refractive index modulation is performed at the junction of the core and the cladding.

[0027] The double-cascaded mode converter of the few-mode fiber can be applied to the high-power fiber laser to filter out the nonlinear effect in the output laser, and provides important technical support for accelerating the performance improvement of the high-power fiber laser.

[0028] The double-cascaded mode converter of the few-mode fiber has the characteristics of no fusion point, low insertion loss, large Raman scattering suppression ratio and simple preparation process, and can be flexibly placed in a high-power fiber laser of various fiber sizes.

[0029] The double-cascaded mode converter of the few-mode fiber can be applied to the high-power fiber laser to filter out the nonlinear effect in the output laser, and provides important technical support for accelerating the performance improvement of the high-power fiber laser. Specifically, by connecting the double-cascaded mode converter to the output end cap of the high-power fiber laser oscillator or the output end cap of the high-power fiber laser amplifier, the resonant center wavelength and the stimulated Raman scattering center wavelength of the two mode converters are designed to be consistent. When the high-power laser is transmitted through the double-cascaded mode converter of the few-mode fiber, the signal light part in the laser can pass through the double-cascaded mode converter of the few-mode fiber without being affected, while the stimulated Raman scattering component in the laser is coupled from the core fundamental mode to the core high-order mode, and then coupled from the core high-order mode to the cladding to form loss, thereby improving the signal light purity in the output laser. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the present application or prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained according to the structures shown in the drawings without creative labor for those skilled in the art.

[0031] Figure 1 It is a schematic diagram of mode field distribution and effective refractive index distribution of different modes in a typical large-mode-field few-mode fiber. Figure 2 It is a structural schematic diagram of the double-cascaded mode converter of the few-mode fiber. Figure 3 It is a schematic diagram of phase matching condition of the double-cascaded mode coupling. Figure 4 It is a schematic diagram of refractive index modulation distribution of the core fundamental mode and the core high-order mode converter. Figure 5 It is a schematic diagram of refractive index modulation distribution of the core high-order mode and the cladding mode converter. Figure 6 It is a structural schematic diagram of the high-power few-mode fiber laser in an embodiment. Figure 7 It is a structural schematic diagram of the high-power few-mode fiber laser in an embodiment. Reference signs in the figures: 1, pump source; 2, first pump combiner; 3, high reflection grating; 4, low reflection grating; 5, first gain fiber; 6, few-mode fiber double-cascaded mode converter; 7, output end cap; 8, second pump combiner; 9, second gain fiber; 61, core fundamental mode and core high-order mode converter; 62, core high-order mode and cladding mode converter; 63, refractive index modulation region of core fundamental mode and core high-order mode converter; 64, refractive index modulation region of core high-order mode and cladding mode converter; 10, few-mode fiber; 101, core; 102, cladding.

[0032] The purposes, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0034] Reference Figure 2 , Figure 4 and Figure 5 , wherein Figure 2 is a structural schematic diagram of a few-mode fiber double-cascaded mode converter; Figure 4 is a refractive index modulation distribution schematic diagram of a core fundamental mode and core high-order mode converter; Figure 5 is a refractive index modulation distribution schematic diagram of a core high-order mode and cladding mode converter. In an embodiment, a few-mode fiber double-cascaded mode converter is provided, which comprises a core fundamental mode and core high-order mode converter 61 and a core high-order mode and cladding mode converter 62; the core fundamental mode and core high-order mode converter 61 and the core high-order mode and cladding mode converter 62 are cascaded on the same few-mode fiber, and the core fundamental mode and core high-order mode converter 61 and the core high-order mode and cladding mode converter 62 are separated by a certain distance; the core fundamental mode and core high-order mode converter 61 is used for coupling the core fundamental mode to the core high-order mode; the core high-order mode and cladding mode converter 62 is used for coupling the core high-order mode to the cladding mode and introducing loss.

[0035] As Figure 4As shown, the few-mode fiber 10 includes a core 101 and a cladding 102, and the core fundamental mode and the core high-order mode converter has a refractive index modulation region 63 located at the center of the core 101 and has a uniform refractive index. The cross-sectional shape of the refractive index modulation region of the core fundamental mode and the core high-order mode converter 61 is not limited. The refractive index modulation region of the core fundamental mode and the core high-order mode converter 61 is prepared by a carbon dioxide laser point-by-point inscription method or a femtosecond laser point-by-point inscription method. Both the carbon dioxide laser point-by-point inscription method and the femtosecond laser point-by-point inscription method are very mature inscription technologies in the art. In the present application, if the carbon dioxide laser point-by-point inscription method is used to inscribe the refractive index modulation region of the core fundamental mode and the core high-order mode converter 61, the carbon dioxide laser point-by-point inscription method is used to form an ablation by focusing the laser on the surface of the fiber cladding, release the stress in the center of the core to achieve the uniform modulation of the refractive index at the center of the core. If the femtosecond laser point-by-point inscription method is used to inscribe the refractive index modulation region of the core fundamental mode and the core high-order mode converter 61, the femtosecond laser point-by-point inscription method is used to directly focus the laser pulse on the center of the core to modify the core material in the center of the core by subtractive machining to achieve the uniform modulation of the refractive index at the center of the core.

[0036] As shown in Figure 4 , Figure 5 As shown, the few-mode fiber 10 includes a core 101 and a cladding 102, and the core high-order mode and the cladding mode converter has a refractive index modulation region 64 distributed at the interface between the core 101 and the cladding 102. The refractive index modulation region 64 of the core high-order mode and the cladding mode converter includes a plurality of sub-refractive index modulation regions distributed at the interface between the core 101 and the cladding 102 and circularly symmetrically distributed with the center of the core 101 as the center. The refractive index of each sub-refractive index modulation region is uniformly distributed, and the plurality of sub-refractive index modulation regions are used to simultaneously perturb the core high-order mode and the cladding mode to achieve energy coupling between the two. The plurality of sub-refractive index modulation regions in the core high-order mode and the cladding mode converter are prepared by a femtosecond laser point-by-point exposure method, and the laser action region is located at the interface between the core and the cladding to modulate the refractive index at the interface between the core and the cladding. Figure 5 The refractive index modulation region 64 of the core high-order mode and the cladding mode converter includes 4 sub-refractive index modulation regions distributed at the interface between the core 101 and the cladding 102 and circularly symmetrically distributed with the center of the core 101 as the center. In actual application, the cross-sectional shape of each sub-refractive index modulation region is not limited, and the number of sub-refractive index modulation regions is also not limited.

[0037] Further, the core fundamental mode and the core high-order mode converter 61 and the core high-order mode and the cladding mode converter 62 have the same center wavelength and are both in the range of 1130-1140 nm, which exactly corresponds to the excitation wavelength of the stimulated Raman scattering of silica optical fiber.

[0038] Figure 3 The figure is a schematic diagram of phase matching conditions of double cascade mode coupling, the horizontal line is the phase matching condition value of different refractive index modulation periods, the intersection of the horizontal line and the curve is the mode coupling of the wavelength, and the three curves respectively refer to different mode couplings. Therefore, the double cascade mode converter of the few-mode fiber provided by the present application firstly utilizes the refractive index modulation region of the core fundamental mode and core high-order mode converter 61 to couple the core fundamental mode to the core high-order mode, and then utilizes the refractive index modulation region of the core high-order mode and cladding mode converter to realize the coupling of another core high-order mode and cladding mode.

[0039] The carrier fiber of the core fundamental mode and core high-order mode converter 61 and the core high-order mode and cladding mode converter 62 is a large-mode-area few-mode fiber, and the diameter of the large-mode-area few-mode fiber is not limited and can be commonly used 250 microns, 400 microns, 600 microns, etc. The diameter of the large-mode-area few-mode fiber is adapted to the size of the large-mode-area few-mode fiber in the actual fiber laser system in the specific application.

[0040] The double cascade mode converter of the few-mode fiber provided in the above embodiment can be used before the output end cap of the fiber laser oscillator, before the output end cap of the fiber laser amplifier, or between the seed light and the amplification stage of the fiber laser amplifier, and is used to filter out the stimulated Raman scattering components in the seed laser.

[0041] 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-pumping structure fiber laser oscillator, a backward-pumping structure fiber laser oscillator, or a bidirectional-pumping structure fiber laser oscillator.

[0042] Reference Figure 6 An embodiment provides a high-power few-mode fiber laser, comprising 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 few-mode fiber double cascade mode converter 6, and an output end cap 7. The high-reflection grating 3, the first gain fiber 5, and the low-reflection grating 4 are sequentially connected to form a resonant cavity. 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. 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 end 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 capable of efficiently suppressing stimulated Raman scattering cannot be directly written on the large-mode-area few-mode fiber.

[0043] The few-mode fiber double-cascaded mode converter 6 in the embodiment is shown in Figure 2 、 Figure 4 and Figure 5 . The few-mode fiber double-cascaded mode converter provided by the embodiment. The few-mode fiber double-cascaded mode converter 6 includes a core fundamental mode and core high-order mode converter 61, and a core high-order mode and cladding mode converter 62. The refractive index modulation distribution of the core fundamental mode and core high-order mode converter 61 is uniformly distributed in the center of the core, as shown in the refractive index modulation distribution in Figure 4 , and the refractive index modulation distribution of the core high-order mode and cladding mode converter 62 is at the interface of the core and the cladding to increase the mode coupling efficiency, as shown in the refractive index modulation distribution in Figure 5 .

[0044] Referring to Figure 7 , an embodiment provides a high-power few-mode fiber laser, which includes a seed laser and a laser power amplification optical path, the seed laser is connected to the laser power amplification optical path, and a few-mode fiber double-cascaded mode converter 6 is connected between the output end of the laser power amplification optical path and the input end of the output end cap 7. Specifically, it 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 few-mode fiber double-cascaded mode converter 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 a specific wavelength laser. 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 few-mode fiber double-cascaded mode converter 6. The output end of the few-mode fiber double-cascaded mode converter 6 is connected to the input end of the output end cap. 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 few-mode fiber double-cascaded mode converter 6 in the embodiment is shown in Figure 2 、 Figure 4 and Figure 5 .The embodiment shown provides a few-mode fiber dual-cascaded mode converter. The core-to-core-higher-order mode converter 61 couples the laser of the corresponding wavelength core-to-core fundamental mode to the core-to-core-higher-order mode, and the core-to-core-higher-order mode converter 62 couples the higher-order mode into the cladding to form loss. Thus, the laser of the corresponding wavelength is coupled from the core-to-core fundamental mode to the cladding.

[0045] In another embodiment, a fiber laser system is provided, including a seed laser and a laser power amplification optical path, with the few-mode fiber dual-cascade mode converter 6 connected between the seed laser and the laser power amplification optical path.

[0046] Matters not covered in this invention are common knowledge.

[0047] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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.

[0048] The embodiments described above are merely examples of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A few-mode fiber dual-stage mode converter, characterized in that, The core fundamental mode and core high-order mode converter and the core high-order mode and cladding mode converter are cascaded on the same few-mode fiber, and the core fundamental mode and core high-order mode converter and the core high-order mode and cladding mode converter are separated by a certain distance. The core fundamental mode and core high-order mode converter is used for coupling the core fundamental mode to the core high-order mode. The core high-order mode and cladding mode converter is used for coupling the core high-order mode to the cladding mode and introducing loss.

2. The few-mode fiber dual-stage mode converter of claim 1, wherein, The refractive index modulation region of the core fundamental mode and core high-order mode converter is located at the center of the core and has a uniform distribution of refractive index.

3. The few-mode fiber dual-stage mode converter of claim 2, wherein, The refractive index modulation region of the core fundamental mode and core high-order mode converter is prepared by a carbon dioxide laser point-by-point inscription method or a femtosecond laser point-by-point inscription method; the carbon dioxide laser point-by-point inscription method is to focus the laser on the surface of the fiber cladding to form ablation, release the stress at the center of the core to achieve uniform modulation of the refractive index at the center of the core; the femtosecond laser point-by-point inscription method is to directly focus the laser pulse on the center of the core to modify the core material at the center of the core to achieve uniform modulation of the refractive index at the center of the core.

4. The few-mode fiber dual-stage mode converter according to claim 1 or 2 or 3, characterized in that, The refractive index modulation region of the core high-order mode and cladding mode converter is distributed at the junction of the core and the cladding, and the refractive index modulation region of the core high-order mode and cladding mode converter includes a plurality of sub-refractive index modulation regions which are distributed at the junction of the core and the cladding and are circularly symmetrically distributed with the center of the core as the center, and the refractive index of each sub-refractive index modulation region has a uniform distribution, and the plurality of sub-refractive index modulation regions are used to simultaneously disturb the core high-order mode and the cladding mode to achieve energy coupling between the two.

5. The few-mode fiber dual-stage mode converter of claim 4, wherein, The plurality of sub-refractive index modulation regions in the core high-order mode and cladding mode converter are prepared by a femtosecond laser point-by-point exposure method, and the laser action region is located at the junction of the core and the cladding to modulate the refractive index at the junction of the core and the cladding.

6. The few-mode fiber dual-stage mode converter of claim 1 or 2 or 3 or 5, wherein, The center wavelengths of the core fundamental mode and core high-order mode converter and the core high-order mode and cladding mode converter are consistent and are in the range of 1130-1140 nm.

7. A high power few-mode fiber laser characterized by, The few-mode fiber double-cascaded mode converter as claimed in claim 1 or 2 or 3 or 5 is connected between the output fiber of a high-power few-mode fiber laser and an output end cap, and stimulated Raman scattering in the high-power few-mode fiber laser is suppressed by the double-cascaded mode converter.

8. The high-power few-mode fiber laser of claim 7, wherein, The high-power few-mode fiber laser is a fiber laser oscillator or a fiber laser amplifier.

9. The high power few-mode fiber laser of claim 7, wherein, The fiber laser includes a seed laser and a laser power amplification optical path, the seed laser is connected to the laser power amplification optical path, and the few-mode fiber double-cascaded mode converter is connected between the output end of the laser power amplification optical path and the output end cap.

10. A high power few-mode fiber laser characterized in that, The fiber laser includes a seed laser and a laser power amplification optical path, and the few-mode fiber double-cascaded mode converter as claimed in claim 1 or 2 or 3 or 5 is connected between the seed laser and the laser power amplification optical path.

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