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

Abstract

The application discloses a few-mode fiber double-cascaded mode converter and a high-power few-mode fiber laser. A core base mode and a core high-order mode converter and a core high-order mode and a cladding mode converter are cascaded on the few-mode fiber. The refractive index modulation distribution of the core base mode and the core high-order mode converter is uniformly distributed in the core center, and is used for coupling the core base mode to the core high-order mode. The refractive index modulation distribution 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 for coupling the core high-order mode to the cladding to form loss. The few-mode fiber double-cascaded mode converter is connected before an output end cap of the high-power few-mode fiber laser, and can inhibit stimulated Raman scattering in the large-mode-field few-mode fiber laser, so as to improve the output performance of the fiber laser.

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:

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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, and therefore cross-coupling occurs, resulting in a significant reduction in 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

[0012] In view of the defects of the prior art, the application provides a few-mode fiber double-cascade mode converter and a high-power few-mode fiber laser.

[0013] To achieve the above technical purposes, the technical scheme adopted by the application is as follows:

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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 the laser on the surface of the fiber cladding to form ablation, release the stress at the center of the core to realize uniform modulation of the refractive index at the center of the core; the femtosecond laser point-by-point writing method is to focus the laser pulse directly on the center of the core to modify the core material at the center of the core by subtractive machining to realize uniform modulation of the refractive index at the center of the core.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] Further, the structure and type of the fiber laser are not limited, and the fiber laser can be a fiber laser amplifier.

[0026] 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.

[0027] 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.

[0028] The present application can achieve the following beneficial effects:

[0029] 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 loss. 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.

[0030] 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.

[0031] 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.

[0032] The double-cascaded mode converter of the few-mode fiber has the advantages of convenient preparation, small loss and fiber compatibility in suppressing stimulated Raman scattering in the high-power fiber laser.

[0033] The double-cascaded mode converter of the few-mode fiber has the advantages of convenient preparation, small loss and fiber compatibility in suppressing stimulated Raman scattering in the high-power fiber laser.

[0034] The double-cascaded mode converter of the few-mode fiber has the advantages of convenient preparation, small loss and fiber compatibility in suppressing stimulated Raman scattering in the high-power fiber laser. BRIEF DESCRIPTION OF DRAWINGS

[0035] 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 be obtained by those skilled in the art without creative labor based on the drawings shown.

[0036] 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.

[0037] Figure 2 It is a schematic diagram of the structure of the double-cascaded mode converter of the few-mode fiber.

[0038] Figure 3 It is a schematic diagram of the phase matching condition of the double-cascaded mode coupling.

[0039] Figure 4 It is a schematic diagram of the refractive index modulation distribution of the core fundamental mode and the core high-order mode converter.

[0040] Figure 5 It is a schematic diagram of the refractive index modulation distribution of the core high-order mode and the cladding mode converter.

[0041] Figure 6Structure diagram of high-power few-mode fiber laser in an embodiment;

[0042] Figure 7 Structure diagram of high-power few-mode fiber laser in an embodiment;

[0043] Reference numerals in the drawings:

[0044] 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.

[0045] The implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly and completely described below 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, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0047] Reference Figure 2 , Figure 4 and Figure 5 , wherein Figure 2 is a structure diagram of few-mode fiber double-cascaded mode converter; Figure 4 is a refractive index modulation distribution diagram of core fundamental mode and core high-order mode converter; Figure 5 is a refractive index modulation distribution diagram of 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;

[0048] 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;

[0049] The core high-order mode and cladding mode converter 62 is used to couple the core high-order mode to the cladding mode and introduce loss.

[0050] As shown in Figure 4 The few-mode fiber 10 includes a core 101 and a cladding 102, and the core high-order mode and cladding mode converter 61 is 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 high-order mode and cladding mode converter 61 is not limited. The refractive index modulation region of the core high-order mode and cladding mode converter 61 is prepared by a carbon dioxide laser point-by-point writing method or a femtosecond laser point-by-point writing method. Both the carbon dioxide laser point-by-point writing method and the femtosecond laser point-by-point writing method are very mature writing technologies in the art. In the present application, if the carbon dioxide laser point-by-point writing method is used to write the refractive index modulation region of the core high-order mode and cladding mode converter 61, the carbon dioxide laser point-by-point writing method is used to form an ablation by focusing the laser on the surface of the fiber cladding, release the stress in the core center to achieve the uniform modulation of the refractive index at the center of the core. If the femtosecond laser point-by-point writing method is used to write the refractive index modulation region of the core high-order mode and cladding mode converter 61, the femtosecond laser point-by-point writing method is used to directly focus the laser pulse on the core center to modify the core material in the core center by subtractive machining to achieve the uniform modulation of the refractive index at the center of the core.

[0051] As shown in Figure 4 , Figure 5 The few-mode fiber 10 includes a core 101 and a cladding 102, and the refractive index modulation region 64 of the core high-order mode and cladding mode converter is 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 cladding mode converter includes a plurality of sub-refractive index modulation regions that are distributed at the interface between the core 101 and the cladding 102 and are 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 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 includes 4 sub-refractive index modulation regions that are distributed at the interface between the core 101 and the cladding 102 and are 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.

[0052] Further, the center wavelength of the core fundamental mode and the core high-order mode converter 61, the core high-order mode and the cladding mode converter 62 are consistent, and are in the range of 1130-1140 nm, which is exactly the excitation wavelength of the stimulated Raman scattering of quartz optical fiber.

[0053] Figure 3 The figure is a schematic diagram of the phase matching condition of double cascade mode coupling, and 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 represent different mode couplings. Therefore, the double cascade mode converter of the few-mode fiber provided by the application first utilizes the refractive index modulation region of the core fundamental mode and the 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 the cladding mode converter to realize the coupling of another core high-order mode and cladding mode.

[0054] The carrier optical fiber of the core fundamental mode and the core high-order mode converter 61 and the core high-order mode and the cladding mode converter 62 is a large-mode-area few-mode fiber. 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., and is adapted to the size of the large-mode-area few-mode fiber in the actual optical fiber laser system in the specific application.

[0055] 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, and between the seed light and the amplification stage of the fiber laser amplifier, for filtering the stimulated Raman scattering components in the seed laser.

[0056] The structure and type of the fiber laser are not limited, and 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.

[0057] Reference Figure 6One 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-cascaded 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 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 inscribed on the large-mode-area few-mode fiber.

[0058] The few-mode fiber double-cascaded mode converter 6 in the embodiment is a few-mode fiber double-cascaded mode converter provided in the embodiment shown in Figure 2 、 Figure 4 and Figure 5 The few-mode fiber double-cascaded mode converter provided in the embodiment shown in Figure 4 The refractive index modulation distribution of 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 5 The refractive index modulation distribution of the core high-order mode and cladding mode converter 62 is distributed at the junction of the core and the cladding, so as to increase the mode coupling efficiency, as shown in the refractive index modulation distribution in

[0059] Referring to Figure 7In an embodiment, a high-power few-mode fiber laser is provided, which comprises 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 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, the few-mode fiber double-cascaded mode converter 6, the 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 sequentially connected to form a resonant cavity, and one or more pump sources 1 are respectively connected to 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 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, and 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 all 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 the few-mode fiber double-cascaded mode converter provided in the embodiments shown in Figure 2 、 Figure 4 and Figure 5 The few-mode fiber double-cascaded mode converter provided in the embodiments shown in the above.

[0060] In another embodiment, a fiber laser system is provided, which comprises a seed laser and a laser power amplification optical path, and the few-mode fiber double-cascaded mode converter 6 is connected between the seed laser and the laser power amplification optical path.

[0061] The remaining matters of the present application are known technologies.

[0062] The technical features of the above embodiments can be combined in any manner, and to make the description concise, not all possible combinations of the technical features in the above embodiments are 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.

[0063] The above-described embodiments are merely illustrative for several embodiments of the present application, and the description is relatively specific and detailed, but should not be understood as limiting the scope of the patent. It should be noted that, for those skilled 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.

[0064] The above-described embodiments are merely preferred embodiments of the present application, and are 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. A few-mode fiber dual-cascaded mode converter, characterized in that, It includes a fiber core fundamental mode to fiber core higher-order mode converter and a fiber core higher-order mode to cladding mode converter; the fiber core fundamental mode to fiber core higher-order mode converter and the fiber core higher-order mode to cladding mode converter are cascaded on the same few-mode fiber, and the fiber core fundamental mode to fiber core higher-order mode converter and the fiber core higher-order mode to cladding mode converter are separated by a certain distance; The fiber core fundamental mode to fiber core higher-order mode converter is used to couple the fiber core fundamental mode to the fiber core higher-order mode. The core higher-order mode to cladding mode converter is used to couple the core higher-order mode to the cladding mode and introduce loss. The refractive index modulation region of the core fundamental mode and core higher-order mode converter is located at the center of the core and has a uniform refractive index distribution. The refractive index modulation region of the core higher-order mode and cladding mode converter is distributed at the junction of the core and cladding. The refractive index modulation region of the core higher-order mode and cladding mode converter includes multiple sub-refractive index modulation regions distributed at the junction of the core and cladding and symmetrically distributed in a circle with the core center as the center. The refractive index of each sub-refractive index modulation region is uniformly distributed. The multiple sub-refractive index modulation regions are used to simultaneously perturb the core higher-order mode and the cladding mode to achieve energy coupling between the two.

2. The few-mode fiber dual-cascaded mode converter according to claim 1, characterized in that, The refractive index modulation region of the fiber core fundamental mode and fiber core higher-order mode converter is prepared by carbon dioxide laser point-by-point writing or femtosecond laser point-by-point writing. The carbon dioxide laser point-by-point writing method is to focus the laser on the surface of the fiber cladding to form ablation, thereby releasing the stress in the center of the fiber core and achieving uniform modulation of the refractive index in the center of the fiber core. The femtosecond laser point-by-point writing method is to focus the laser pulse directly on the center of the fiber core and perform subtractive modification processing on the fiber core material in the center of the fiber core to achieve uniform modulation of the refractive index in the center of the fiber core.

3. The few-mode fiber dual-cascaded mode converter according to claim 1, characterized in that, Multiple sub-refractive index modulation regions in the core-to-cladding mode converter are prepared by femtosecond laser point-by-point exposure method. The laser action area is located at the junction of the core and the cladding, and refractive index modulation is performed at the junction of the core and the cladding.

4. The few-mode fiber dual-cascaded mode converter according to claim 1, 2, or 3, characterized in that, The center wavelengths of the fiber core fundamental mode and fiber core higher-order mode converter, and the fiber core higher-order mode and cladding mode converter are the same and are all in the range of 1130-1140nm.

5. A high-power few-mode fiber laser, characterized in that, A dual-cascaded mode converter of a few-mode fiber as described in claim 1, 2, or 3 is connected between the output fiber and the output cap of a high-power few-mode fiber laser to suppress stimulated Raman scattering in the high-power few-mode fiber laser.

6. The high-power few-mode fiber laser according to claim 5, characterized in that, The high-power few-mode fiber laser is a fiber laser oscillator or a fiber laser amplifier.

7. The high-power few-mode fiber laser according to claim 5, characterized in that, 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 a few-mode fiber dual-cascaded mode converter is connected between the output end of the laser power amplification optical path and the output cap.

8. A high-power few-mode fiber laser, characterized in that, It includes a seed laser and a laser power amplification optical path, with a few-mode fiber dual-cascade mode converter as described in claim 1, 2 or 3 connected between the seed laser and the laser power amplification optical path.

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