A near single-mode linearly polarized Raman laser based on step-index multimode fiber

Abstract

The application discloses a high-power single-mode linear polarization Raman fiber laser based on a multimode optical fiber, and is characterized in that: a Raman gain medium of the laser is a single-clad circularly symmetric step-index distribution large-core multimode optical fiber; a base mode diameter of the step-index large-core multimode optical fiber is more than 65% of a core diameter; a Bragg fiber grating is inscribed in the core of the multimode optical fiber by using a femtosecond laser direct writing technology; a long axis of the grating region is coincided with a base mode field supported by the multimode optical fiber in a radial direction in space and length; the grating forms selective optical feedback only to the base mode in the cavity; a grating region with a difference between the long axis and the short axis forms selective feedback to one polarization state of the base mode; thus, the multimode optical fiber Raman laser effectively suppresses generation of mode instability at a high power level through active mode cleaning, and finally realizes near-diffraction-limited single-mode, high linear polarization and high-power Raman fiber laser output.

Description

Technical Field

[0001] This invention relates to fiber lasers, and more particularly to a near-single-mode linearly polarized Raman laser based on a step-index multimode fiber. Background Technology

[0002] Quartz optical fiber has the characteristics of bendability, extremely large specific surface area, and waveguide mode with near-diffraction-limited beam quality. Rare-earth-doped optical fibers based on quartz glass optical fiber have achieved near-single-mode laser output with continuous power of kilowatts or even kilowatts in the near-infrared 1-2 micrometer range.

[0003] In addition to single-mode large-mode-field gain fibers, using graded-index large-core-diameter (typical core diameter: 50-100 micrometers) multimode passive silica fiber Raman gain medium, and utilizing randomly distributed Rayleigh scattering within the fiber as optical feedback for the laser resonator, is an important way to achieve kilowatt-level high-power, high-brightness Raman fiber laser output.

[0004] The refractive index distribution within the core of graded-index large-core-diameter multimode fiber exhibits a parabolic distribution: the refractive index is highest at the center of the circularly symmetrical core and gradually decreases along the radial direction of the cladding. Although Raman gain is more easily concentrated at the core center, i.e., on a few lower-order transverse modes, within a random Raman fiber resonator constructed from this type of fiber, it can achieve Raman laser output with high beam quality. Specifically, according to optical waveguide theory, in graded-index large-core-diameter multimode Raman gain fibers, the spatial overlap between different transverse modes of the pump field and the generated Raman laser field is significantly different, leading to substantial differences in the pump energy distribution obtained by different transverse modes. This mode competition results in the fundamental mode in the Raman laser field obtaining significantly greater gain than other higher-order transverse modes. Specifically, the simulation results of Terry et al. (Nathan B. Terry, Thomas G. Alley, and Timothy H. Russell, "An explanation of SRS beam cleanup in graded-index fibers and the absence of SRS beam cleanup in step-index fibers," Opt. Express 15, 17509-17519 (2007)) show that under random coupling conditions, in graded-index large-core-diameter multimode Raman gain fibers, the fundamental mode LP 01 The relative gain coefficient is higher than that of other higher-order transverse modes. Correspondingly, in step-index large-core-diameter multimode Raman gain fibers, the fundamental mode LP... 01The relative gain coefficient is only higher than that of a few of the lowest-order higher transverse modes, while this competitive advantage is uncertain relative to higher-order transverse modes. Based on this "mode cleaning" effect, in graded-index large-core-diameter multimode Raman gain fibers, Raman laser energy can be concentrated on a few low-order transverse modes, including the fundamental mode, thereby effectively improving the brightness from pump to output laser and obtaining higher output laser beam quality.

[0005] However, the reported beam quality factor M of kilowatt-level high-power Raman lasers (including fully open random laser resonators, semi-open random laser resonators, or resonators with Bragg fiber grating mirrors) based on graded-index large-core-diameter multimode fibers as Raman gain media is currently low. 2 Not less than 1.5, and ideal near-diffraction-limited single-mode high beam quality (i.e., M 2 ≤1.1) There is still a gap. This is because the random Raman fiber laser based on graded-index large-core-diameter multimode Raman gain fiber mentioned above relies entirely on the mode cleaning effect of the "passive mode selection" of the graded-index large-core-diameter multimode gain fiber. There is fierce competition for pump energy among the many low-order transverse modes supported in its laser cavity, and it inevitably contains a lot of low-order transverse mode components. Therefore, the final output Raman laser energy cannot be completely concentrated on the fundamental mode.

[0006] Researchers have already introduced Bragg fiber gratings (FBGs) that provide optical feedback only for low-order transverse modes in Raman fiber lasers based on large-core multimode fibers with graded refractive index distributions to achieve active control of the mode quality of Raman laser output. Specifically, in Dostovalov's work (AVDostovalov, AAWolf, MISkvortsov, SRAbdullina, AGKuznetsov, SIKablukov, SABabin, 'Femtosecond-pulseinscribed FBGs for mode selection in multimode fiber lasers', Optical Fiber Technology, Volume 52, 2019, 101988.), they inscribed Bragg fiber gratings at the center of the fiber core in a large-core multimode fiber (62.5 μm core diameter) with a graded refractive index distribution using a femtosecond laser direct-writing method. The grating surface has a radial length of 1 x 8 μm in the fiber cross-section, and its major axis dimension is close to the diameter of the fundamental mode field supported by the graded refractive index multimode fiber core. In their work on a Raman fiber laser constructed from fiber gratings (EAZlobina, SIKablukov, AAWolf, AVDostovalov, and SABabin, "Nearly single-mode Raman lasing at 954 nm in a graded-index fiber directly pumped by a multimode laser diode," Opt. Lett. 42, 9-12 (2017)), the team achieved beam quality M 2 A fiber Raman laser with a power output of less than 1.27 and approximately 10 watts was achieved. However, in this work, the fundamental mode field diameter (MFD) of the large-core multimode fiber with graded refractive index distribution used as the Raman gain medium is only about 1 / 8 of the core diameter of the large-core multimode fiber, comparable only to the core diameter of a common single-mode quartz fiber. Therefore, it is impossible to achieve kilowatt-level high-power fiber Raman laser output in a large-core multimode fiber. It should also be noted that the fundamental mode LP supported by this 62.5-micron core diameter graded refractive index distribution multimode fiber... 01 The calculated value of the mode field diameter is approximately 10 micrometers. Therefore, the cross-sectional size of the grating surface fabricated in the multimode fiber in this work is about 20% smaller than the diameter of its fundamental mode field. This results in a significant mismatch between the spatial position and size of the fundamental mode field on the fiber cross-section, which is why the beam quality M of its Raman laser output is low. 2 The reason why it is still greater than 1.1.

[0007] Therefore, although Raman fiber lasers based on graded-index large-core-diameter multimode fibers allow for a greater concentration of Raman gain at the core center and a more concentrated spatial distribution of the Raman laser across a few low-order transverse modes due to the graded-index distribution of the fiber core, the diameter of the supported fundamental mode field is much smaller than the core diameter. Inevitably, it is impossible to simultaneously achieve high power and near-single-mode high beam quality (M) in Raman fiber lasers based on graded-index large-core-diameter multimode fibers. 2 Raman laser output ≤1.1). Summary of the Invention

[0008] Purpose of the Invention: The purpose of this invention is to provide a technical approach for achieving high-power, near-single-mode Raman lasers with pure Raman gain based on multimode large-core fiber, in order to solve the problem that Raman fiber lasers based on graded-index large-core multimode fiber cannot simultaneously achieve high power and near-single-mode high beam quality (M). 2 Technical issues related to laser output (≤1.1).

[0009] Technical solution: A near-single-mode linearly polarized Raman laser based on step-index multimode fiber, comprising: a pump source, a large-core-diameter multimode fiber Raman gain medium with a single-cladding circularly symmetric step-index distribution connected to the pump source, and a femtosecond direct-written Bragg fiber grating with high fundamental mode selectivity inscribed in the core of the multimode Raman fiber.

[0010] The input and output ends of the fiber Raman gain medium are respectively provided with a first Bragg fiber grating and a second Bragg fiber grating, which serve as cavity mirrors of the laser resonant cavity; the pump source is connected to the first Bragg fiber grating.

[0011] Furthermore, the Bragg fiber grating is inscribed into the core of a large-diameter multimode fiber with a single-clad circularly symmetric step refractive index distribution using femtosecond direct-writing technology. On the cross-section of the fiber, the grating surface is elongated, with its major axis penetrating the center of the circularly symmetric fiber core and symmetrically distributed on both sides of the core center. The length of the major axis of the grating surface is 65-70% of the diameter of the multimode fiber core. The minor axis of the grating surface is orthogonal to the major axis, also penetrating the center of the circularly symmetric fiber core and symmetrically distributed on both sides of the core center. The length of the minor axis of the grating surface does not exceed one-quarter of the length of the major axis of the grating surface.

[0012] Furthermore, the fiber Raman gain medium is a single-clad, large-core multimode fiber with a circularly symmetric step refractive index distribution. The ratio of the fiber core non-circularity to the fiber core diameter does not exceed 10%. The fiber numerical aperture (NA) ranges from 0.1 to 0.5, the fiber core diameter is greater than 15 micrometers, and it supports more than 40 transverse modes. The length of the multimode Raman gain fiber ranges from 1 meter to 10 kilometers.

[0013] The numerical aperture NA of the optical fiber is calculated as follows:

[0014]

[0015] In the formula, n core Here, n is the refractive index of the fiber core and n is the refractive index of the fiber core. clad denoted as ...α, where α is the refractive index of the cladding.

[0016] Furthermore, the large-core diameter multimode fiber with a single-clad circular symmetric step refractive index distribution uses low-loss quartz glass, low-loss fluoride glass, or chalcogenide glass as the matrix for the large-core diameter circular symmetric step refractive index multimode fiber.

[0017] Furthermore, the pump source, fiber Raman gain medium, and other fiber components are connected by low-loss fusion splicing to form an all-fiber laser structure.

[0018] A near-single-mode linearly polarized Raman laser based on step-index multimode fiber includes: a pump source, a large-core-diameter multimode fiber Raman gain medium with a single-cladding circularly symmetric step-index distribution connected to the pump source, and a femtosecond direct-written Bragg fiber grating with high fundamental mode selectivity inscribed in the core of the multimode Raman fiber.

[0019] The fiber Raman gain medium has a first resonant cavity mirror and a second resonant cavity mirror at its two ends, respectively; the first resonant cavity mirror is a high-reflectivity Bragg fiber grating with a reflectivity greater than 70%, and is arranged at the input end of the fiber Raman gain medium; the second resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity less than 50%, and is arranged at the output end of the fiber Raman gain medium.

[0020] A near-single-mode linearly polarized Raman laser based on step-index multimode fiber includes: a pump source, a large-core-diameter multimode fiber Raman gain medium with a single-cladding circularly symmetric step-index distribution connected to the pump source, and a femtosecond direct-written Bragg fiber grating with high fundamental mode selectivity inscribed in the core of the multimode Raman fiber.

[0021] The fiber Raman laser adopts a semi-open resonant cavity structure, and a third Bragg fiber grating with a reflectivity greater than 5% is arranged at the input end of the fiber Raman gain medium; the output end of the fiber Raman gain medium does not contain a Bragg fiber grating.

[0022] Compared with the prior art, the significant advantages of this invention are as follows:

[0023] 1. A large-core multimode fiber with a single-clad circular symmetric step refractive index distribution is used as the Raman gain medium. The fundamental mode field diameter of the large-core multimode fiber with a step refractive index distribution exceeds 65% of the core diameter of the large-core multimode fiber with a step refractive index distribution. At the same time, a Bragg fiber grating surface that is perfectly matched in one direction with the fundamental mode field supported by the large-core fiber is etched in the fiber core using femtosecond laser direct writing technology. This allows selective optical feedback only to the fundamental mode in the multimode Raman fiber laser resonator, ultimately achieving near-single-mode Raman laser output. Additionally, the geometric difference of the Bragg fiber grating surface etched by femtosecond laser direct writing technology on the fiber cross-section enables highly linearly polarized single transverse mode Raman laser output.

[0024] 2. Replacing the traditional graded-index multimode fiber with a large-core multimode fiber using a step-index distribution as the Raman gain medium can significantly increase the diameter of the fundamental mode field supported by the multimode fiber; the step-index multimode fiber supports a larger LP (Laser Field Size) fundamental mode. 01 The mode field diameter is approximately 2 / 3 of the multimode fiber core diameter. This ratio is significantly larger than that of the fundamental mode LP supported by the large-core-diameter multimode fiber with graded refractive index distribution typically used in multimode fiber Raman lasers. 01 The proportion of the mode field diameter to the core diameter; at the same time, calculations show that the fundamental mode field diameter supported by a step-index multimode large-core fiber increases linearly with the increase of its core diameter, while the fundamental mode field diameter supported by a graded-index multimode large-core fiber used in commonly used multimode Raman fiber lasers gradually decreases with the increase of the core diameter. Compared with existing multimode large-core fibers, the multimode large-core fiber of this invention is a more ideal Raman gain medium that can achieve power enhancement. Under the condition of laser damage power density determined by the fiber material, it can significantly increase the output laser power while ensuring the quality of the high-power single-mode laser beam.

[0025] 3. Because the grating region of the Bragg fiber grating written by femtosecond laser is highly matched with the size and spatial position of the fundamental mode field on the cross-section of the fiber core, there is a high degree of mode and polarization selectivity feedback for the specific polarization state of the Raman laser fundamental mode. This transforms the passive random mode cleaning effect in multimode Raman lasers into active control and effectively suppresses the generation of intracavity mode instability under high power levels. The output Raman laser energy is completely concentrated on the specific polarization state of the fundamental mode, ultimately achieving near-diffraction-limited single-mode, highly linearly polarized, high-power Raman laser output. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the present invention;

[0027] Figure 2The present invention relates to the structure of a fiber Bragg grating with high fundamental mode selectivity and polarization selectivity, etched by femtosecond laser direct writing technology within the core of a large-diameter multimode fiber with a single-cladding circularly symmetric step refractive index distribution, on the cross-section of the fiber, and the fundamental mode LP supported by the grating and the fiber core. 01 A schematic diagram showing the relationship between the spatial location and geometric dimensions of the mold field diameter;

[0028] Figure 3 This is a comparison diagram of the fundamental mode field diameter (MFD) supported by the step-index multimode large-core-diameter Raman gain fiber used in this invention and the fundamental mode field diameter (MFD) supported by the graded-index multimode large-core-diameter Raman gain fiber used in traditional multimode fiber Raman lasers. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0030] like Figure 1 The high-power single-mode linearly polarized Raman fiber laser shown includes: a pump source 1, a large-core multimode fiber Raman gain medium 2 with a single-cladding circularly symmetric step refractive index distribution connected to the pump source 1, and a femtosecond direct-written first Bragg fiber grating 3a and a second Bragg fiber grating 3b located at the input and output ends of the multimode Raman fiber 2, respectively.

[0031] like Figure 2 As shown, a Bragg fiber grating is inscribed within the core of a large-core multimode fiber with a single-clad circularly symmetric step refractive index distribution using femtosecond direct writing technology. The grating surface is elongated, with its major axis penetrating the center of the circularly symmetric fiber core and symmetrically distributed on both sides of the core center. The length of the major axis of the grating surface is 65-70% of the fiber core diameter. This ensures that the grating region, in terms of spatial location and geometric dimensions, corresponds to the fundamental mode LP supported by the large-core multimode fiber with a single-clad circularly symmetric step refractive index distribution in the fiber cross-section. 01 The polarization directions of the mode fields are highly coincident; the short axis (i.e., the height of the grating) is orthogonal to the long axis, and also passes through the center of the circularly symmetrical fiber core and is symmetrically distributed on both sides of the core center. The length of the short axis of the grating does not exceed one-quarter of the length of the long axis.

[0032] The fiber Raman gain medium is a single-clad, large-core multimode fiber with a circularly symmetric step refractive index distribution. The ratio of the core non-circularity (the difference between the longest and shortest chord lengths passing through the core center) to the core diameter (the average of the longest and shortest chord lengths) does not exceed 10%. The fiber numerical aperture (NA) ranges from 0.1 to 0.5, the core diameter is greater than 15 micrometers, and it supports more than 40 transverse modes. The length of the multimode Raman gain fiber ranges from 1 meter to 10 kilometers.

[0033]

[0034] Where, n core n clad These are the refractive indices of the fiber core and the cladding, respectively.

[0035] The Bragg fiber grating is written using a femtosecond laser direct writing method within the core of a large-core multimode fiber with a single-clad circularly symmetric step refractive index distribution. First, a femtosecond laser is focused into the core. Then, the selected grating surface is scanned point-by-point along its major axis to a predetermined length. The scanning along the minor axis is then repeated until the first elongated grating surface reaches the predetermined length along both the major and minor axes. Next, the Bragg fiber grating surface is translated periodically along the fiber axis, and the writing of the next grating surface is repeated until the number of grating surfaces meets the required reflectivity for the Bragg fiber grating design.

[0036] As a further improvement of the present invention, a first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at both ends of the fiber Raman gain medium 2. The first resonant cavity mirror is a high-reflectivity Bragg fiber grating with a reflectivity greater than 70%, and is arranged at the laser input end; the second resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity less than 50%, and is arranged at the laser output end.

[0037] As a further improvement of the present invention, the fiber Raman gain medium 2 adopts a semi-open resonant cavity structure at both ends. That is, a third resonant cavity mirror with a reflectivity greater than 5% is arranged at the input end of the fiber Raman gain medium 2. The output end of the fiber Raman gain medium 2 does not contain a Bragg fiber grating. It only uses the distributed Rayleigh scattering of the low-loss passive Raman gain fiber as the weak light feedback at the output end of the resonant cavity. It and the single Bragg fiber grating at the laser input end provide selective light feedback for one polarization state of the fundamental mode, thereby realizing near-single-mode, high linear polarization, and high-power Raman laser output.

[0038] As a further improvement of the present invention, the fiber Raman gain medium used can be a large-core multimode fiber with a single-clad circularly symmetric step refractive index distribution based on low-loss quartz glass, low-loss fluoride glass, or low-loss chalcogenide glass.

[0039] As a further improvement of the present invention, the pump source, fiber Raman gain medium, wavelength division multiplexer, fiber combiner and other fiber components are spliced ​​together with low loss to form an all-fiber laser structure.

[0040] like Figure 3The figure shows a comparison diagram of the fundamental mode field diameter (MFD) supported by a multimode large-core Raman gain fiber with a step-index profile used in a high-power single-mode linearly polarized Raman fiber laser based on multimode silica fiber and the fundamental mode field diameter (MFD) supported by a multimode large-core Raman gain fiber with a graded-index profile used in a conventional multimode fiber Raman laser. The calculation formula for the fundamental mode field area is as follows:

[0041] A = π × MFD 2 / 4 (2)

[0042] where MFD is the fundamental mode field diameter.

[0043] In this embodiment, the numerical aperture of the fiber is fixed at 0.20, and the laser operating wavelength is 1 μm; assuming the radius of the fiber core is a, and the refractive index distribution function at a distance r from the center of the core radially outward is n(r); the refractive index distribution functions of different multimode fibers are as follows:

[0044] (1) For a multimode fiber with a step-index profile:

[0045] When r < a, n(r) = n core ;

[0046] When r ≥ a, n(r) = n clad .

[0047] (2) For a multimode fiber with a graded-index profile:

[0048] When r < a,

[0049]

[0050] When r ≥ a, n(r) = n clad .

[0051] It can be seen from Figure 3 that the fundamental mode field diameter supported by a multimode large-core Raman gain fiber with a step-index profile increases linearly with the increase of the core diameter, and the ratio of its fundamental mode field diameter to the core diameter is 65 - 70%; while the fundamental mode field diameter supported by a multimode large-core Raman gain fiber with a graded-index profile increases non-linearly and slowly and tends to saturate with the increase of the core diameter. When the core diameter is 62.5 μm, the ratio of its fundamental mode field diameter to the core diameter is about 20%, and this ratio becomes smaller with the continuous increase of the core diameter and tends to about 10%; when the core diameter is 100 μm, the fundamental mode field area supported by a multimode large-core Raman gain fiber with a step-index profile is nearly 14 times that of the fundamental mode field area supported by a multimode large-core Raman gain fiber with a graded-index profile. That is, while achieving the beam quality of nearly single-mode fiber Raman laser, the output power can be increased by one order of magnitude.

[0052] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A near single-mode linearly polarized Raman laser based on a step-index multimode optical fiber, characterized in that, It comprises: a pump source, a large-core multimode fiber Raman gain medium with single-clad circularly symmetric step-index distribution connected with the pump source, and a femtosecond direct-written Bragg fiber grating with high mode selectivity engraved in the core of the multimode fiber Raman gain medium; the input end and the output end of the multimode fiber Raman gain medium are respectively provided with a first Bragg fiber grating and a second Bragg fiber grating as laser resonant cavity mirrors; the pump source is connected with the first Bragg fiber grating; the first Bragg fiber grating and the second Bragg fiber grating are respectively engraved in the core of the single-clad circularly symmetric step-index distribution large-core multimode fiber by a femtosecond direct-writing method; on the fiber cross section, the grating surface is in a long strip shape, the long axis direction penetrates through the center of the circularly symmetric fiber core and is symmetrically distributed on both sides of the fiber core center, the length of the grating surface long axis is 65-70% of the diameter of the multimode fiber core; the short axis of the grating surface is orthogonal to the long axis direction, also penetrates through the center of the circularly symmetric fiber core and is symmetrically distributed on both sides of the fiber core center, and the length of the grating surface short axis is not more than one fourth of the length of the grating surface long axis.

2. The near single-mode linearly polarized Raman laser based on step-index multimode fiber according to claim 1, characterized in that, The multimode fiber Raman gain medium is a single-clad circularly symmetric step-index distribution large-core multimode fiber, the ratio of the fiber core non-circularity to the fiber core diameter is not more than 10%, the numerical aperture NA of the fiber ranges from 0.1 to 0.5, the fiber core diameter is greater than 15 microns, and the number of supported transverse modes is greater than 40; the length of the multimode fiber Raman gain medium ranges from 1 meter to 10 kilometers; In the formula, the calculation expression of the numerical aperture NA of the optical fiber is as follows: , where n core is the refractive index of the optical fiber core, and n clad is the refractive index of the cladding.

3. The near single-mode linearly polarized Raman laser based on step-index multimode fiber according to claim 2, characterized in that, The single-clad circularly symmetric step-index distribution large-core multimode fiber selects low-loss quartz glass, low-loss fluoride glass or sulfur-based glass as the large-core circularly symmetric step-index multimode fiber substrate.

4. A near single-mode linearly polarized Raman laser based on a step-index multimode optical fiber, characterized in that, It comprises: a pump source, a large-core multimode fiber Raman gain medium with single-clad circularly symmetric step-index distribution connected with the pump source, and a femtosecond direct-written Bragg fiber grating with high mode selectivity engraved in the core of the multimode fiber Raman gain medium; the two ends of the multimode fiber Raman gain medium are respectively provided with a first resonant cavity mirror and a second resonant cavity mirror; the first resonant cavity mirror is a high-reflectivity Bragg fiber grating with a reflectivity greater than 70%, and is arranged at the input end of the multimode fiber Raman gain medium; the second resonant cavity mirror is a low-reflectivity Bragg fiber grating with a reflectivity less than 50%, and is arranged at the output end of the multimode fiber Raman gain medium; the femtosecond direct-written Bragg fiber grating with high mode selectivity is engraved in the core of the single-clad circularly symmetric step-index distribution large-core multimode fiber by a femtosecond direct-writing method; on the fiber cross section, the grating surface is in a long strip shape, the long axis direction penetrates through the center of the circularly symmetric fiber core and is symmetrically distributed on both sides of the fiber core center, the length of the grating surface long axis is 65-70% of the diameter of the multimode fiber core; the short axis of the grating surface is orthogonal to the long axis direction, also penetrates through the center of the circularly symmetric fiber core and is symmetrically distributed on both sides of the fiber core center, and the length of the grating surface short axis is not more than one fourth of the length of the grating surface long axis.

5. A near single-mode linearly polarized Raman laser based on a step-index multimode optical fiber, characterized in that, It comprises: The pump source, the single-clad circularly symmetric step-index large-core multimode fiber Raman gain medium connected with the pump source, and the femtosecond direct-written Bragg fiber grating with high mode selectivity inscribed in the core of the multimode fiber Raman gain medium; The two ends of the multimode fiber Raman gain medium adopt a semi-open resonant cavity structure, a third Bragg fiber grating is arranged at the input end of the multimode fiber Raman gain medium, and the reflectivity of the third Bragg fiber grating is greater than 5%; and the output end of the multimode fiber Raman gain medium does not contain a Bragg fiber grating; The femtosecond direct-written Bragg fiber grating with high mode selectivity is inscribed in the core of the single-clad circularly symmetric step-index large-core multimode fiber by a femtosecond direct-writing method; in the fiber cross section, the grating surface is in a strip shape, the long axis direction of the grating surface penetrates the center of the circularly symmetric fiber core and is symmetrically distributed on both sides of the center of the fiber core, the length of the long axis of the grating surface is 65-70% of the diameter of the multimode fiber core; the short axis of the grating surface is orthogonal to the long axis direction, also penetrates the center of the circularly symmetric fiber core and is symmetrically distributed on both sides of the center of the fiber core, and the length of the short axis of the grating surface is not more than one fourth of the length of the long axis of the grating surface.

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