CN116316003A - Near single-mode linear polarization Raman laser based on step-index multimode fiber - Google Patents

Near single-mode linear polarization Raman laser based on step-index multimode fiber Download PDF

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CN116316003A
CN116316003A CN202310070742.0A CN202310070742A CN116316003A CN 116316003 A CN116316003 A CN 116316003A CN 202310070742 A CN202310070742 A CN 202310070742A CN 116316003 A CN116316003 A CN 116316003A
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fiber
core
multimode
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施进丹
冯宪
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Jiangsu Normal University
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Abstract

The invention discloses a high-power single-mode linear polarization Raman fiber laser based on multimode fibers, which is characterized in that: the Raman gain medium of the laser is a section of single-cladding circularly symmetric multi-mode optical fiber with large core diameter and distributed in step index; the fundamental mode diameter of the multimode fiber with the large step-index core diameter exceeds 65% of the core diameter; writing a Bragg fiber grating in a fiber core of a multimode fiber by using a femtosecond laser direct writing technology, wherein a long axis of a grating region and a fundamental mode field supported by the multimode fiber are overlapped in space and length in one radial direction, and the grating only forms selective optical feedback to a fundamental mode in a cavity; the grid region with difference of long and short axes forms selective feedback to one polarization state of the fundamental mode; therefore, the multimode fiber Raman laser effectively inhibits the generation of mode instability at a high power level through active mode cleaning, and finally realizes single-mode, high linear polarization and high-power Raman fiber laser output near the diffraction limit.

Description

Near single-mode linear polarization Raman laser based on step-index multimode fiber
Technical Field
The invention relates to an optical fiber laser, in particular to a near single-mode linear polarization Raman laser based on a step-index multimode optical fiber.
Background
The quartz fiber has the characteristics of bendable property, extremely large specific surface area, waveguide mode of near diffraction limit beam quality and the like, and the rare earth doped fiber based on the quartz glass fiber has realized near single-mode laser output of continuous operation with kilowatt or even kilowatt power in near infrared 1-2 micrometers.
Besides the single-mode large-mode-field gain fiber, a multimode passive quartz fiber Raman gain medium with a large graded refractive index (typical core diameter: 50-100 microns) is adopted, and Rayleigh scattering randomly distributed in the fiber is used as optical feedback of a laser resonant cavity, so that the method is an important way for realizing the laser output of the kilowatt-level high-power and high-brightness Raman fiber.
The refractive index distribution in the fiber core of the graded-index large-core-diameter multimode fiber is parabolic: i.e. the refractive index is greatest at the centre of the circularly symmetric core and gradually decreases in the radial cladding direction of the fiber core. In spite of the random Raman fiber resonant cavity formed by the fiber, raman gain is more easily concentrated at the center of the fiber core, namely on a few low-order transverse modes, so that Raman laser output with higher beam quality can be realized. Specifically, according to the optical waveguide theory, in the graded-index large-core multimode raman gain fiber, the coincidence ratios of different transverse modes of the pump optical field and different transverse modes of the generated raman laser optical field are obviously different in space, so that the pump energy distribution obtained by different transverse modes is greatly different, and as a result of mode competition, the fundamental mode in the raman laser optical field obtains obviously larger gain than other higher-order transverse modes. Specifically, 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)) indicate that: under random coupling condition, in graded-index large-core multimode Raman gain fiber, fundamental mode LP 01 The relative gain coefficient of (a) is higher than that of other higher-order transverse modes. Correspondingly, in a step-index large-core multimode raman gain fiber, the fundamental mode LP 01 The relative gain coefficient of (2) is only higher than the lowest of the several higher order transverse modes, and the bidding is relative to the transverse modes of higher orderThe contention advantage is then uncertain. According to the mode cleaning function, in the graded-index large-core multimode Raman gain fiber, raman laser energy can be concentrated on a few low-order transverse modes including a fundamental mode, so that the brightness from pumping to output laser is effectively improved, and higher output laser beam quality is obtained.
However, currently reported laser output beam quality factor M of kilowatt-level high-power Raman laser (comprising full-open random laser resonant cavity, half-open random laser resonant cavity or resonant cavity with Bragg fiber grating cavity mirror) based on graded-index large-core multimode fiber as Raman gain medium 2 Not less than 1.5, and desirably near diffraction limited single mode high beam quality (i.e., M 2 A distance of less than or equal to 1.1). This is because the random raman fiber laser based on the graded-index large-core multimode raman gain fiber is completely dependent on the mode cleaning effect of "passive mode selection" existing in the graded-index large-core multimode gain fiber, and there is a strong competition for pumping energy between many low-order transverse modes supported in the laser cavity, and it is unavoidable that many low-order transverse mode components are contained, so that the raman laser energy finally output cannot be completely concentrated on the fundamental mode.
Researchers have introduced bragg fiber gratings that perform optical feedback only on low-order transverse modes into raman fiber lasers based on large-core multimode fibers with graded refractive index profiles to realize active control of the mode quality of raman laser output. Specifically, in the Dostovalov work (A.V.Dostovalov, A.A.Wolf, M.I.Skvortsov, S.R.Abdullina, A.G.Kuznetsov, S.I.Kablukov, S.A.Babin,' "Femtosecond-pulse inscribed FBGs for mode selection in multimode fiber lasers," Optical Fiber Technology, volume52,2019, 101988.) it is in a large core multimode fiber with graded-index profile (core diameter 62.5 μm) that a Bragg fiber grating is inscribed in the center of the fiber core by a Femtosecond laser direct writing method, its grating surface has a radial length of 1x8 μm in the fiber cross section, and its long axis dimension is close to the fundamental mode field diameter supported by the graded-index profile multimode fiber core. Is formed by the fiber gratingIn the operation of the raman fiber laser of (E.A.Zlobina, S.I.Kablukov, A.A.Wolf, A.V.Dostovalov, and s.a. babin, "near single-mode Raman lasing at nm in a graded-index fiber directly pumped by a multimode laser diode," opt. Lett.42,9-12 (2017)), the team achieves a beam quality M 2 Less than 1.27, and a fiber raman laser output of about 10 watts. However, in this work, the fundamental mode field diameter MFD of the large-diameter multimode fiber having a graded-index profile as a raman gain medium is only about 1/8 of the diameter of the large-diameter multimode fiber, and is only equivalent to the diameter of a normal quartz single-mode fiber, and the purpose of outputting a kilowatt-level high-power fiber raman laser in the large-diameter multimode fiber cannot be achieved. Meanwhile, the graded index distributed multimode fiber with 62.5 microns core diameter supports the fundamental mode LP 01 The calculated mode field diameter of (2) is about 10 microns, so that the cross-sectional dimension of the grating surface prepared in the multimode fiber in this operation is less than about 20% of its fundamental mode field diameter, and there is a large mismatch in the spatial position and spatial dimension of the fundamental mode field over the fiber cross-section, which is the beam quality M of its Raman laser output 2 Still greater than 1.1.
Therefore, a raman fiber laser based on a graded-index large-core multimode fiber, although the raman gain is more easily concentrated in the center of the fiber core due to the graded-index distribution of the fiber core, so that the spatial distribution of the raman laser is more concentrated on a few low-order transverse modes; but it supports a fundamental mode field diameter that is much smaller than the core diameter. Inevitably, high power and near single mode high beam quality (M) cannot be achieved simultaneously in raman fiber lasers based on graded index large core multimode fibers 2 Less than or equal to 1.1).
Disclosure of Invention
The invention aims to: the invention aims to provide a technical approach for realizing high-power near-single-mode Raman laser of pure Raman gain based on a multimode large-core-diameter optical fiber, so as to solve the problem that a Raman fiber laser based on a graded-index large-core-diameter multimode optical fiber cannot realize high-power near-single-mode high-beam quality (M) 2 A laser output of less than or equal to 1.1).
The technical scheme is as follows: a step-index multimode fiber-based near single-mode linearly polarized raman laser comprising: the system comprises a pumping source, a large-core-diameter multimode fiber Raman gain medium with single-cladding circularly symmetric step-index distribution, and a femtosecond direct-writing Bragg fiber grating which is inscribed in a multimode Raman fiber core and has high fundamental mode selectivity, wherein the large-core-diameter multimode fiber Raman gain medium is connected with the pumping source;
the input end and the output end of the fiber Raman gain medium are respectively provided with a first Bragg fiber bragg grating and a second Bragg fiber bragg grating which are used as laser resonant cavity mirrors; the pump source is connected with the first Bragg fiber grating.
Further, the Bragg fiber grating is inscribed in a large-core-diameter multimode fiber core with single-cladding circularly symmetric step-index distribution by a femtosecond direct writing technology; on the cross section of the optical fiber, the grating surface is in a long strip shape, the long axis direction of the grating surface penetrates through the center of the circularly symmetric optical fiber core and is symmetrically distributed on two sides of the center of the optical fiber core, and the length of the long axis of the grating surface is 65-70% of the diameter of the multimode optical fiber core; the grating surface short axis is orthogonal to the long axis direction, also penetrates through the center of the circularly symmetric optical fiber core and is symmetrically distributed on two sides of the center of the fiber core, and the length of the grating surface short axis is not more than one quarter of the length of the grating surface long axis.
Further, the fiber Raman gain medium is a section of single-clad circularly symmetric multimode fiber with large core diameter and step index distribution, the ratio of the out-of-roundness of the fiber core divided by the diameter of the fiber core is not more than 10%, the numerical aperture NA range of the fiber is 0.1-0.5, the diameter of the fiber core is more than 15 microns, and the number of supported transverse modes is more than 40; the length of the multimode Raman gain fiber ranges from 1 meter to 10 kilometers;
wherein, the calculation expression of the numerical aperture NA of the optical fiber is as follows:
Figure BDA0004064685630000031
wherein n is core Is the refractive index of the fiber core, and n clad Is the refractive index of the cladding.
Further, the single-cladding circularly symmetric step-index distributed large-core multimode fiber is selected from low-loss quartz glass, low-loss fluoride glass or chalcogenide glass as a matrix of the large-core circularly symmetric step-index multimode fiber.
Furthermore, the pump source, the optical fiber Raman gain medium and other optical fiber components form an all-fiber laser structure through low-loss fusion welding.
A step-index multimode fiber-based near single-mode linearly polarized raman laser comprising: the system comprises a pumping source, a large-core-diameter multimode fiber Raman gain medium with single-cladding circularly symmetric step-index distribution, and a femtosecond direct-writing Bragg fiber grating which is inscribed in a multimode Raman fiber core and has high fundamental mode selectivity, wherein the large-core-diameter multimode fiber Raman gain medium is connected with the pumping source;
a first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at two ends of the optical fiber Raman gain medium; the first resonant cavity mirror is a high-reflectivity Bragg fiber grating, the reflectivity of the first resonant cavity mirror is more than 70%, and the first resonant cavity mirror is arranged at the input end of the fiber Raman gain medium; the second resonant cavity mirror is a low-reflectivity Bragg fiber grating, the reflectivity of the second resonant cavity mirror is smaller than 50%, and the second resonant cavity mirror is arranged at the output end of the fiber Raman gain medium.
A step-index multimode fiber-based near single-mode linearly polarized raman laser comprising: the system comprises a pumping source, a large-core-diameter multimode fiber Raman gain medium with single-cladding circularly symmetric step-index distribution, and a femtosecond direct-writing Bragg fiber grating which is inscribed in a multimode Raman fiber core and has high fundamental mode selectivity, wherein the large-core-diameter multimode fiber Raman gain medium is connected with the pumping source;
the fiber Raman laser adopts a semi-open resonant cavity structure, and a third Bragg fiber grating with reflectivity more 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 Bragg fiber gratings.
Compared with the prior art, the invention has the following remarkable effects:
1. the large-core multimode fiber with single-cladding circularly symmetric step-index distribution is used as a Raman gain medium, and the fundamental mode field diameter of the large-core multimode fiber with the step-index distribution exceeds 65% of the core diameter of the large-core multimode fiber with the step-index distribution; meanwhile, a Bragg fiber grating surface which is completely matched with a basic mode field supported by the large-core fiber in one direction is inscribed in the fiber core through a femtosecond laser direct writing technology, so that only the basic mode of the multi-mode Raman fiber laser resonant cavity is subjected to selective optical feedback, and near single-mode Raman laser output is finally realized; incidentally, the geometrical difference of the grating surface of the Bragg fiber grating inscribed by the femtosecond laser direct writing technology on the fiber cross section is utilized to realize the single transverse mode Raman laser output of high linear polarization;
2. the large-core-diameter multimode fiber with step-index distribution is adopted to replace the conventionally used multimode large-core-diameter fiber with graded-index distribution as a Raman gain medium, so that the diameter size of a fundamental mode field supported by the multimode fiber can be greatly improved; step-index distributed multimode large-core fiber supported fundamental mode LP 01 The mode field diameter of the fiber is about 2/3 of the core diameter of the multimode fiber, and the ratio is significantly larger than the fundamental mode LP supported by large core multimode fibers with graded index profile used in conventional multimode fiber Raman lasers 01 The ratio of the mode field diameter to the core diameter; meanwhile, the calculation shows that: the size of the fundamental mode field diameter supported by the multimode large-core-diameter optical fiber with step-type refractive index distribution is linearly increased along with the increase of the fiber core diameter, while the size of the fundamental mode field diameter supported by the multimode large-core-diameter optical fiber with graded refractive index distribution adopted in a common multimode Raman fiber laser is gradually reduced along with the increase of the fiber core diameter;
3. the grating region of the Bragg fiber grating written by femtosecond laser direct writing is highly matched with the size and the spatial position of the mode field of the fundamental mode on the cross section of the fiber core, and the high-mode and polarization selective feedback exists on the specific polarization state of the Raman laser fundamental mode, so that the random mode cleaning effect passively generated in the multimode Raman laser is improved to be actively regulated and controlled, the unstable generation of the mode in the laser cavity under the high power level is effectively inhibited, the output Raman laser energy is completely concentrated on the specific polarization state of the fundamental mode, and finally the single-mode and high-linear polarization high-power Raman laser output near the diffraction limit is realized.
Drawings
FIG. 1 is a schematic diagram of the structure of the present invention;
FIG. 2 shows the structure of a grating surface of a fiber Bragg grating with high fundamental mode selectivity and polarization selectivity on a fiber cross section, and the fundamental mode LP supported by the grating surface and the fiber core, in a large core multimode fiber core with a single cladding circularly symmetric step index profile, inscribed by a femtosecond laser direct write technique, according to the present invention 01 Schematic representation of the relationship between the spatial location and geometry of the mode field diameter;
fig. 3 is a graph showing the comparison of the fundamental mode field diameter MFD supported by the multimode large-core raman gain fiber with step-type refractive index profile used in the present invention and the fundamental mode field diameter MFD supported by the multimode large-core raman gain fiber with graded refractive index profile used in the conventional multimode fiber raman laser.
Detailed Description
The invention is described in further detail below with reference to the drawings and the detailed description.
The high power single mode linear polarization raman fiber laser shown in fig. 1 comprises: the optical fiber comprises a pumping source 1, a single-cladding circularly symmetric step-index distributed large-core multimode fiber Raman gain medium 2 connected with the pumping source 1, a first Bragg fiber grating 3a and a second Bragg fiber grating 3b which are respectively positioned at the input end and the output end of the multimode Raman fiber 2 and written in the fiber core and are directly written in femtosecond;
as shown in fig. 2, a bragg fiber grating is inscribed in a large-core-diameter multimode fiber core with single-cladding circularly symmetric step-index distribution by a femtosecond direct writing technology; the grating surface is long and strip-shaped, and its long axis direction is passed through the centre of circularly symmetrical optical fiber core and symmetrically distributed on two sides of said core centre, and the long axis length of grating surface is 65-70% of optical fiber core diameter so as to implement optical transmissionFundamental mode LP supported by large core multimode fiber with fiber cross section such that the grating region is spatially and geometrically matched to the circularly symmetric step index profile of the single cladding 01 Highly coincident with one polarization direction of the mode field of (2); the grating surface short axis (namely the grating surface height) is orthogonal to the long axis direction, also penetrates through the center of the circularly symmetric optical fiber core and is symmetrically distributed on two sides of the center of the fiber core, and the length of the grating surface short axis is not more than one quarter of the length of the long axis;
the fiber Raman gain medium is a section of single-clad circularly symmetric multi-mode fiber with large core diameter and step index distribution, the ratio of the out-of-roundness of the fiber core (the difference between the longest chord length passing through the center of the fiber core and the shortest chord length passing through the center of the fiber core) divided by the diameter of the fiber core (namely the average value of the longest chord length and the shortest chord length) is not more than 10 percent, the numerical aperture NA of the fiber is in the range of 0.1 to 0.5, the diameter of the fiber core is larger than 15 microns, and the number of supported transverse modes is larger than 40; the multimode Raman gain fiber is adopted, and the length range is 1 meter to 10 kilometers;
Figure BDA0004064685630000051
wherein n is core 、n clad The refractive index of the fiber core and the refractive index of the cladding, respectively.
The Bragg fiber grating adopts a femtosecond laser direct writing method, is inscribed in the fiber core of the single-cladding circularly symmetric step-index distributed large-core multimode fiber, firstly focuses the femtosecond laser in the fiber core, scans a set length point by point along the long axis direction of a selected grating surface, then moves upwards along a short axis, and repeats the scanning along the long axis direction until the dimension of a first long-strip grating surface on the long axis and the short axis reaches the set length; then translating the grating surface period of the Bragg fiber grating along the axial direction of the fiber, and repeating the writing of the next grating surface; until the number of the grating surfaces meets the required reflectivity of the Bragg fiber grating design.
As a further improvement of the invention, a first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at two ends of the optical fiber Raman gain medium 2, the first resonant cavity mirror is a high-reflectivity Bragg fiber bragg grating, the reflectivity of the first resonant cavity mirror is more than 70 percent, and the first resonant cavity mirror is arranged at the input end of the laser; the second resonant cavity mirror is a low-reflectivity Bragg fiber grating, the reflectivity of the second resonant cavity mirror is less than 50%, and the second resonant cavity mirror is arranged at the output end of the laser;
as a further improvement of the invention, the two ends of the optical fiber Raman gain medium 2 adopt a semi-open resonant cavity structure, namely a third resonant cavity mirror is arranged at the input end of the optical fiber Raman gain medium 2, and the reflectivity is more than 5%; the output end of the fiber Raman gain medium 2 does not contain Bragg fiber gratings, only uses the distributed Rayleigh scattering existing in the low-loss passive Raman gain fiber as weak light feedback of the output end of the resonant cavity, and realizes near single-mode, high-linear polarization and high-power Raman laser output by the fiber Bragg fiber gratings and the selective light feedback of the single Bragg fiber gratings at the input end of the laser on one polarization state of the fundamental mode.
As a further improvement of the present invention, the optical fiber raman gain medium employed may be a single-clad circularly symmetric step index distributed large core multimode optical fiber based on low-loss quartz glass, low-loss fluoride glass, or low-loss chalcogenide glass.
As a further improvement of the invention, the pumping source, the optical fiber Raman gain medium, the wavelength division multiplexer, the optical fiber combiner and other optical fiber elements are welded with each other in a low-loss manner to form an all-fiber laser structure.
Fig. 3 is a graph showing the comparison of the fundamental mode field diameter MFD supported by a multimode large-core raman gain fiber with step-type refractive index distribution used in a high-power single-mode linear polarization raman fiber laser based on a multimode quartz fiber and the fundamental mode field diameter MFD supported by a multimode large-core raman gain fiber with graded refractive index distribution used in a conventional multimode fiber raman laser. The calculation formula of the fundamental mode field area is as follows:
A ＝ π×MFD 2 /4 (2)
wherein MFD is the fundamental mode field diameter.
In this embodiment, the numerical aperture of the optical fiber is fixed to 0.20, and the working wavelength of the laser is 1 micron; setting the radius of the fiber core as a, setting the radial outward distance from the center of the fiber core as r, and setting the refractive index distribution function as n (r); the refractive index profile of the different multimode fibers is as follows:
(1) For a multimode fiber with a step index profile:
at r<at a, n (r) =n core ；
At r.gtoreq.a, n (r) =n clad 。
(2) For graded-index distributed multimode fibers:
in the case where r < a is chosen,
Figure BDA0004064685630000071
when r is equal to or greater than a, n (r) =n clad 。
As can be seen from fig. 3, the fundamental mode field diameter supported by the multimode raman gain fiber with large core diameter with step-type refractive index distribution increases linearly with the increase of the core diameter, and the ratio of the fundamental mode field diameter to the core diameter is 65-70%; the fundamental mode field diameter supported by the multimode large-core-diameter Raman gain fiber with graded refractive index distribution shows nonlinear slow growth and tends to be saturated along with the increase of the core diameter, and the ratio of the fundamental mode field diameter to the core diameter is about 20% when the core diameter is 62.5 microns, and becomes smaller along with the continuous increase of the core diameter and tends to be about 10%; when the core diameter is 100 micrometers, the area of the fundamental mode field supported by the multimode large-core-diameter Raman gain optical fiber with step-type refractive index distribution is approximately 14 times that of the fundamental mode field supported by the multimode large-core-diameter Raman gain optical fiber with graded refractive index distribution, namely, the output power can be increased by one order of magnitude while the quality of the near single-mode fiber Raman laser beam is realized.
Furthermore, it should be understood that although the present disclosure describes embodiments, not every embodiment is provided with a separate embodiment, and that this description is provided for clarity only, and that the disclosure is not limited to the embodiments described in detail below, and that the embodiments described in the examples may be combined as appropriate to form other embodiments that will be apparent to those skilled in the art.

Claims (7)

1. A near single mode linearly polarized raman laser based on a step index multimode fiber, comprising: the system comprises a pumping source, a large-core-diameter multimode fiber Raman gain medium with single-cladding circularly symmetric step-index distribution, and a femtosecond direct-writing Bragg fiber grating which is inscribed in a multimode Raman fiber core and has high fundamental mode selectivity, wherein the large-core-diameter multimode fiber Raman gain medium is connected with the pumping source;
the input end and the output end of the fiber Raman gain medium are respectively provided with a first Bragg fiber bragg grating and a second Bragg fiber bragg grating which are used as laser resonant cavity mirrors; the pump source is connected with the first Bragg fiber grating.
2. The near single-mode linear polarization Raman laser based on the step-index multimode fiber according to claim 1, wherein the Bragg fiber grating is inscribed in a large-core multimode fiber core with single-cladding circularly symmetric step-index distribution by a femtosecond direct writing method; on the cross section of the optical fiber, the grating surface is in a long strip shape, the long axis direction of the grating surface penetrates through the center of the circularly symmetric optical fiber core and is symmetrically distributed on two sides of the center of the optical fiber core, and the length of the long axis of the grating surface is 65-70% of the diameter of the multimode optical fiber core; the grating surface short axis is orthogonal to the long axis direction, also penetrates through the center of the circularly symmetric optical fiber core and is symmetrically distributed on two sides of the center of the fiber core, and the length of the grating surface short axis is not more than one quarter of the length of the grating surface long axis.
3. The step-index multimode fiber-based near single-mode linear polarization raman laser of claim 1, wherein the fiber raman gain medium is a section of single-clad circularly symmetric step-index distributed large-core multimode fiber, the ratio of the out-of-roundness of the fiber core divided by the diameter of the fiber core is not more than 10%, the numerical aperture NA of the fiber is in the range of 0.1-0.5, the diameter of the fiber core is more than 15 microns, and the number of supported transverse modes is more than 40; the length of the multimode Raman gain fiber ranges from 1 meter to 10 kilometers;
wherein, the calculation expression of the numerical aperture NA of the optical fiber is as follows:
Figure FDA0004064685610000011
wherein n is core Is the refractive index of the fiber core, and n clad Is the refractive index of the cladding.
4. A near single-mode linear polarization raman laser based on a step-index multimode fiber according to claim 3, wherein said single-clad circularly symmetric step-index distributed large-core multimode fiber is selected from low-loss quartz glass, low-loss fluoride glass, or chalcogenide glass as a matrix of large-core circularly symmetric step-index multimode fiber.
5. The near single-mode linear polarization raman laser based on step-index multimode fiber according to claim 1, wherein the pump source, the fiber raman gain medium and other fiber elements are fused by low loss to form an all-fiber laser structure.
6. A near single mode linearly polarized raman laser based on a step index multimode fiber, comprising: the system comprises a pumping source, a large-core-diameter multimode fiber Raman gain medium with single-cladding circularly symmetric step-index distribution, and a femtosecond direct-writing Bragg fiber grating which is inscribed in a multimode Raman fiber core and has high fundamental mode selectivity, wherein the large-core-diameter multimode fiber Raman gain medium is connected with the pumping source;
a first resonant cavity mirror and a second resonant cavity mirror are respectively arranged at two ends of the optical fiber Raman gain medium; the first resonant cavity mirror is a high-reflectivity Bragg fiber grating, the reflectivity of the first resonant cavity mirror is more than 70%, and the first resonant cavity mirror is arranged at the input end of the fiber Raman gain medium; the second resonant cavity mirror is a low-reflectivity Bragg fiber grating, the reflectivity of the second resonant cavity mirror is smaller than 50%, and the second resonant cavity mirror is arranged at the output end of the fiber Raman gain medium.
7. A near single mode linearly polarized raman laser based on a step index multimode fiber, comprising: the system comprises a pumping source, a large-core-diameter multimode fiber Raman gain medium with single-cladding circularly symmetric step-index distribution, and a femtosecond direct-writing Bragg fiber grating which is inscribed in a multimode Raman fiber core and has high fundamental mode selectivity, wherein the large-core-diameter multimode fiber Raman gain medium is connected with the pumping source;
the fiber Raman laser adopts a semi-open resonant cavity structure, and a third Bragg fiber grating with reflectivity more 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 Bragg fiber gratings.
CN202310070742.0A 2023-02-07 2023-02-07 Near single-mode linear polarization Raman laser based on step-index multimode fiber Pending CN116316003A (en)

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