A high-order mode fiber laser based on few-mode fiber Bragg gratings

Abstract

The present invention discloses a high-order mode fiber laser based on few-mode fiber gratings, which belongs to the field of optical communication technology. The high-order mode fiber laser consists of a pump source, a wavelength division multiplexer, a single-mode erbium-doped fiber, a first few-mode long-period fiber grating, a second few-mode long-period fiber grating, a few-mode fiber coupler, a few-mode fiber circulator, a few-mode fiber Bragg grating, a few-mode fiber circulator and a single-mode fiber coupler. The present invention uses the few-mode long-period fiber grating to achieve mode conversion, and uses the few-mode fiber Bragg grating to achieve mode reflection and mode separation, so as to achieve LP under a single resonant cavity. 01 LP 11 LP 21 Simultaneous output of mode lasing. Using optical fiber as a medium to output lasers is one of the effective methods for realizing optical tweezers. This high-order mode fiber laser based on few-mode fiber Bragg gratings can simultaneously output low-order mode beams and high-order mode beams, achieving multi-dimensional manipulation of particles and has good application prospects in the fields of materials science and life sciences.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical communications, and in particular relates to a high-order mode fiber laser based on few-mode fiber gratings. Background Art

[0002] Optical tweezers are widely used in the fields of materials science and life sciences. They use lasers to capture and arrange tiny particles, among which the realization of multi-dimensional manipulation of particles is an important issue. Among the methods of using lasers as optical tweezers, using optical fiber as a transmission medium to output laser is one of the effective methods. This method generally outputs lasers through multiple single-mode optical fibers, single-mode multi-core optical fibers, photonic crystal fibers, etc., and has the advantages of flexible operation, simple structure, and low cost. However, the Gaussian beams, Laguerre-Gaussian beams, Bessel beams, etc. output by the above-mentioned optical fibers can only realize one-dimensional and two-dimensional manipulation of particles, while high-order mode beams can perform three-dimensional manipulation of particles. Therefore, the key is to realize the fiber laser that can output high-order mode beams.

[0003] Currently, fiber lasers that output high-order mode beams can be divided into two categories based on their output mode mechanism. One category is fiber lasers that output a single mode, which achieves mode conversion through off-core fusion of different types of optical fibers, mode-selective couplers, etc., thereby outputting a single high-order mode beam. The other category is fiber lasers that output different modes in a switching manner, which achieves mode conversion through switching the interface of the mode-selective photon lantern and the phase diagram of the spatial light modulator, thereby switching the output of different mode beams. Low-order mode beams can achieve one-dimensional and two-dimensional manipulation of particles, while high-order mode beams can achieve three-dimensional manipulation of particles. Therefore, to achieve multi-dimensional manipulation of particles, fiber lasers that can simultaneously output different mode beams are key. Summary of the Invention

[0004] Based on the above-mentioned deficiencies in the prior art, the present invention provides a high-order mode fiber laser based on few-mode fiber Bragg gratings. The present invention uses few-mode long-period fiber Bragg gratings to achieve mode conversion, and realizes mode reflection and mode separation through few-mode fiber Bragg gratings, and realizes LP under a single resonant cavity. 01 LP 11 LP 21 Simultaneous output of mode lasing. Using optical fiber as a medium to output lasers is one of the effective methods for realizing optical tweezers. This high-order mode fiber laser based on few-mode fiber Bragg gratings can simultaneously output low-order mode beams and high-order mode beams, achieving multi-dimensional manipulation of particles and has good application prospects in the fields of materials science and life sciences.

[0005] The present invention is achieved through the following technical solutions:

[0006] A high-order mode fiber laser based on few-mode fiber Bragg gratings, comprising a pump source 1, a wavelength division multiplexer 2, a single-mode erbium-doped fiber 3, a first few-mode long-period fiber Bragg grating 4, a second few-mode long-period fiber Bragg grating 5, a few-mode fiber coupler 6, a few-mode fiber circulator 7, a few-mode fiber Bragg grating 8, a few-mode fiber circulator 9, and a single-mode fiber coupler 10;

[0007] The wavelength division multiplexer 2 includes an A port and a B port;

[0008] The few-mode fiber coupler 6 includes an A port, a B port, a C port and a D port;

[0009] The few-mode fiber circulator 7 includes an A port, a B port and a C port;

[0010] The few-mode fiber Bragg grating 8 includes an A port and a B port;

[0011] The few-mode fiber circulator 9 includes an A port, a B port and a C port;

[0012] The single-mode fiber coupler 10 includes an A port, a B port, a C port, and a D port;

[0013] The pump light output by the pump source 1 is sequentially input to the single-mode erbium-doped fiber 3, the first few-mode long-period fiber grating 4 and the second few-mode long-period fiber grating 5 through the A port of the wavelength division multiplexer 2. The first few-mode long-period fiber grating 4 converts the LP of the light beam into 01 Mode conversion to LP 11 mode, the second few-mode long-period fiber grating 5 converts the light beam into LP 11 Mode switched to LP 11 Model and LP 21 mode, and then enters the C port of the few-mode fiber coupler 6, the A port of the few-mode fiber circulator 7 and the B port of the few-mode fiber circulator 7 in sequence through the A port of the few-mode fiber coupler 6, wherein the LP of the light beam 21 The mode is output through the A port of the few-mode fiber Bragg grating 8, and the LP beam 11 The mode enters the C port of the few-mode fiber circulator 7 from the B port of the few-mode fiber circulator 7, and then enters the B port of the few-mode fiber circulator 9, the C port of the few-mode fiber circulator 9 and the D port of the few-mode fiber coupler 6 in sequence. The LP of the light beam 11 Part of the mode is output from the B port of the few-mode fiber coupler 6, and the other part enters the A port of the few-mode fiber coupler 6, and passes through the second few-mode long period fiber grating 5, the first few-mode long period fiber grating 4, the single-mode erbium-doped fiber 3, the B port of the wavelength division multiplexer 2 and the A port of the single-mode fiber coupler 10 in sequence; when the LP of the light beam 11 When the mode enters the single-mode erbium-doped fiber 3 from the first few-mode long-period fiber grating 4, it is recoupled into the LP beam.01 mode, part of the rear beam LP 01 The mode is output from the B port of the single-mode fiber coupler 10, and the other part is transmitted from the C port and the D port of the single-mode fiber coupler 10 and then re-enters the A port of the single-mode fiber coupler 10, thereby forming oscillation in the resonant cavity.

[0014] Furthermore, the pump source 1 outputs 980nm pump light.

[0015] Furthermore, the gain range of the single-mode erbium-doped optical fiber 3 is 1530-1570 nm.

[0016] Furthermore, the period of the first few-mode long-period fiber grating 4 is 1100 μm;

[0017] The period of the second few-mode long-period fiber grating 5 is 960 μm.

[0018] Furthermore, the period of the few-mode fiber Bragg grating 8 is 535.4 nm.

[0019] Furthermore, the few-mode fiber coupler 6 is a 2×2 structure with two inputs and two outputs, and the splitting ratio is 1:1.

[0020] Furthermore, the few-mode fiber circulator 7 and the few-mode fiber circulator 9 both exhibit unidirectional transmission in terms of the light beam transmission path, that is, the light beam enters from port A and exits from port B; the light beam enters from port B and exits from port C.

[0021] Furthermore, the single-mode fiber coupler 10 has a 2×2 structure with two inputs and two outputs, and a splitting ratio of 1:1.

[0022] Furthermore, the first few-mode long-period fiber grating 4, the second few-mode long-period fiber grating 5, the few-mode fiber coupler 6, the few-mode fiber circulator 7, the few-mode fiber Bragg grating 8, and the few-mode fiber circulator 9 are all made of few-mode optical fiber, and the diameters of the few-mode optical fiber core and cladding are 18.5μm and 125μm, respectively.

[0023] The working principle of the present invention is as follows:

[0024] The present invention uses a few-mode long-period fiber grating to achieve mode conversion. The mode conversion of the present invention can be divided into two categories: one is the conversion of the forward-transmitted fundamental mode into the forward-transmitted high-order mode, and the other is the conversion of one forward-transmitted high-order mode into two forward-transmitted high-order modes. The mode conversion method of the few-mode long-period fiber grating is as follows: Figure 2 As shown; the first few-mode long period fiber grating 4 in the present invention is shown as LP 01 Mode conversion to LP 11 mode, the second few-mode long-period fiber Bragg grating 5 shows LP11 Mode conversion to LP 11 Model and LP 21 The phase matching condition of the few-mode long-period fiber grating is shown in formula (1). When the phase matching condition is met, a dip peak will appear at the resonant wavelength of its transmission spectrum.

[0025] λ=(n1-n2)Λ (1)

[0026] Where λ is the resonant wavelength of the few-mode long-period fiber grating, Λ is the period of the few-mode long-period fiber grating, and n1 and n2 are the effective refractive indices of the modes at the resonant wavelengths in the few-mode long-period fiber grating.

[0027] The present invention uses a few-mode fiber Bragg grating to achieve mode reflection and mode separation. For the present invention, its mode reflection and mode separation are as follows: two high-order modes of forward transmission, one of which is reflected as a high-order mode of reverse transmission, that is, mode reflection is achieved, and the other high-order mode continues to transmit forward, thereby achieving mode separation. The few-mode fiber Bragg grating achieves mode reflection and mode separation in the following manner: Figure 3 As shown. In the present invention, the few-mode fiber Bragg grating 8 is represented by LP 11 Mode reflection and LP 21 Mode transmission. The phase matching condition of the few-mode fiber Bragg grating is shown in formula (2). When the phase matching condition is met, a dip peak will appear at the resonant wavelength of its reflection spectrum.

[0028] λ=(n1+n2)Λ (2)

[0029] Where λ is the resonant wavelength of the few-mode fiber Bragg grating, Λ is the period of the few-mode fiber Bragg grating, and n1 and n2 are the effective refractive indices of the modes at the resonant wavelengths in the few-mode fiber Bragg grating.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] The present invention discloses a high-order mode fiber laser based on few-mode fiber Bragg grating and a mode conversion method thereof, which realizes mode conversion by using few-mode long-period fiber Bragg grating, realizes mode reflection and mode separation by using few-mode fiber Bragg grating, and realizes LP under a single resonant cavity. 01 LP 11 LP 21 Simultaneous output of mode lasing; outputting laser using optical fiber as the medium is one of the effective methods to realize optical tweezers. This high-order mode fiber laser based on few-mode fiber grating can simultaneously output low-order mode beams and high-order mode beams, realizing multi-dimensional manipulation of particles, and has good application prospects in the fields of materials science and life sciences. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly describes the drawings required for the specific embodiments or the description of the prior art. Similar elements or parts are generally identified by similar reference numerals throughout the drawings. Elements or parts in the drawings are not necessarily drawn to scale.

[0033] Figure 1 : A schematic structural diagram of a high-order mode fiber laser based on few-mode fiber Bragg grating according to the present invention;

[0034] Figure 2 :Schematic diagram of mode conversion in few-mode long-period fiber Bragg grating;

[0035] Figure 3 :Schematic diagram of mode reflection and mode separation of few-mode fiber Bragg grating;

[0036] Figure 4 :Block diagram of the transmission spectrum measurement system of few-mode long-period fiber Bragg grating;

[0037] Figure 5 : Transmission spectrum of few-mode long-period fiber grating;

[0038] Wherein, (a) is the transmission spectrum of the first few-mode fiber long-period fiber grating; (b) is the transmission spectrum of the second few-mode fiber long-period fiber grating;

[0039] Figure 6 :Block diagram of the reflection spectrum measurement system of few-mode fiber Bragg grating;

[0040] Figure 7 : Reflection spectrum of few-mode fiber Bragg grating;

[0041] Figure 8 : Schematic diagram of lasing conditions and light field distribution of different mode wavelengths;

[0042] Where (a) is LP 01 Wavelength lasing and light field distribution in the mode; (b) is LP 11 Wavelength lasing and light field distribution in the mode; (c) is LP 21 Wavelength lasing and light field distribution under the mode; DETAILED DESCRIPTION

[0043] In order to clearly and completely describe the technical solution and specific working process of the present invention, the specific implementation methods of the present invention are as follows in conjunction with the accompanying drawings:

[0044] Example 1

[0045] like Figure 1As shown, this embodiment provides a high-order mode fiber laser based on few-mode fiber Bragg gratings, and its system block diagram is shown in FIG. Figure 1 As shown, it consists of a pump source 1, a wavelength division multiplexer 2, a single-mode erbium-doped fiber 3, a first few-mode long-period fiber grating 4, a second few-mode long-period fiber grating 5, a few-mode fiber coupler 6, a few-mode fiber circulator 7, a few-mode fiber Bragg grating 8, a few-mode fiber circulator 9 and a single-mode fiber coupler 10;

[0046] The wavelength division multiplexer 2 includes an A port and a B port;

[0047] The few-mode fiber coupler 6 includes an A port, a B port, a C port and a D port;

[0048] The few-mode fiber circulator 7 includes an A port, a B port and a C port;

[0049] The few-mode fiber Bragg grating 8 includes an A port and a B port;

[0050] The few-mode fiber circulator 9 includes an A port, a B port and a C port;

[0051] The single-mode fiber coupler 10 includes an A port, a B port, a C port, and a D port;

[0052] The pump light output by the pump source 1 is sequentially input to the single-mode erbium-doped fiber 3, the first few-mode long-period fiber grating 4 and the second few-mode long-period fiber grating 5 through the A port of the wavelength division multiplexer 2. The first few-mode long-period fiber grating 4 converts the LP of the light beam into 01 Mode conversion to LP 11 mode, the second few-mode long-period fiber grating 5 converts the light beam into LP 11 Mode switched to LP 11 Model and LP 21 mode, and then enters the C port of the few-mode fiber coupler 6, the A port of the few-mode fiber circulator 7 and the B port of the few-mode fiber circulator 7 in sequence through the A port of the few-mode fiber coupler 6, wherein the LP of the light beam 21 The mode is output through the A port of the few-mode fiber Bragg grating 8, and the LP beam 11 The mode enters the C port of the few-mode fiber circulator 7 from the B port of the few-mode fiber circulator 7, and then enters the B port of the few-mode fiber circulator 9, the C port of the few-mode fiber circulator 9 and the D port of the few-mode fiber coupler 6 in sequence. The LP of the light beam 11 Part of the mode is output from the B port of the few-mode fiber coupler 6, and the other part enters the A port of the few-mode fiber coupler 6, and passes through the second few-mode long period fiber grating 5, the first few-mode long period fiber grating 4, the single-mode erbium-doped fiber 3, the B port of the wavelength division multiplexer 2 and the A port of the single-mode fiber coupler 10 in sequence. 11When the mode enters the single-mode erbium-doped fiber 3 from the first few-mode long-period fiber grating 4, it is recoupled into the LP beam. 01 mode, therefore, a portion of LP 01 The mode laser is output from the B port of the single-mode fiber coupler 10, and a part of it is transmitted from the C port and the D port of the single-mode fiber coupler 10 and then re-enters the A port of the single-mode fiber coupler 10, thereby forming oscillation in the resonant cavity.

[0053] The control principle of the present invention is as follows: the laser is composed of a pump source, a wavelength division multiplexer, a single-mode erbium-doped fiber, a few-mode long-period fiber grating, a few-mode fiber coupler, a few-mode fiber circulator, a few-mode fiber Bragg grating, and a single-mode fiber coupler. Among them, the pump source provides 980nm pump light, the wavelength division multiplexer multiplexes and demultiplexes light beams of different wavelengths, the single-mode erbium-doped fiber serves as the gain medium in the resonant cavity, the few-mode long-period fiber grating realizes mode conversion, the few-mode fiber coupler realizes the merging and separation of light beams of different modes, the few-mode fiber circulator realizes unidirectional transmission of light beams, the few-mode fiber Bragg grating realizes mode reflection and mode separation, and the single-mode fiber coupler realizes LP 01 The mode beams are combined and separated, and serve as one of the cavity facets of the laser resonator.

[0054] In a high-order mode fiber laser based on a few-mode fiber grating established in this embodiment, a few-mode long-period fiber grating is used to realize mode conversion, wherein the first few-mode long-period fiber grating 4 converts the LP 01 Mode conversion to LP 11 mode, the second few-mode long-period fiber Bragg grating 5 will LP 11 Mode conversion to LP 11 Mode and LP 21 The few-mode long-period fiber grating (FMPFBG) is formed by engraving the FMPFBG on the FMPFBG using a carbon dioxide laser. The periods of the first FMPFBG 4 and the second FMPFBG 5 are 1100 μm and 960 μm, respectively.

[0055] Use Figure 4 The system shown measures the transmission spectrum of a few-mode long-period fiber grating. The system consists of a broadband light source 11, a single-mode fiber circulator 12, a mode-selective photon lantern 13, a first few-mode long-period fiber grating 4, a second few-mode long-period fiber grating 5, and a spectrometer 14. The broadband light source has an operating wavelength of 1535-1565 nm, the single-mode fiber circulator realizes unidirectional transmission of the light beam, and the mode-selective photon lantern is used to excite the LP. 01 Mode or LP 11 After the mode conversion is realized by the few-mode long-period fiber grating, the transmission spectrum of the few-mode long-period fiber grating is measured by a spectrometer. 01mode, the transmission spectrum of the first few-mode long-period fiber grating 4 is measured, such as Figure 5 As shown in (a); from Figure 5 As can be seen from (a), the depression peak depth of the first few-mode long-period fiber grating 4 is greater than 9dB. Therefore, the first few-mode long-period fiber grating 4 can 01 Mode conversion to LP 11 mode. When a mode-selective photon lantern is used to excite LP 11 mode, the transmission spectrum of the second few-mode long-period fiber grating 5 is measured, such as Figure 5 As shown in (b), from Figure 5 As can be seen from (b), the depression peak depth of the second few-mode long-period fiber grating 5 is close to 3dB. Therefore, the LP 11 Model and LP 21 model.

[0056] In a high-order mode fiber laser based on a few-mode fiber Bragg grating established in this embodiment, the few-mode fiber Bragg grating is used to realize mode reflection and mode separation, and the few-mode fiber Bragg grating 8 realizes LP 21 Mode Transmission and LP 11 Mode reflection. The few-mode fiber Bragg grating is engraved on the few-mode fiber by femtosecond laser point-by-point writing method. The period of the few-mode fiber Bragg grating is 535.4nm. Figure 6 The system shown measures the reflection spectrum of a few-mode fiber Bragg grating. The system consists of a broadband light source 11, a single-mode fiber coupler 15, a mode-selective photon lantern 13, a few-mode fiber circulator 7, a few-mode fiber Bragg grating 8, and a spectrometer 14. The broadband light source has an operating wavelength of 1535-1565 nm, the single-mode fiber coupler has a one-input and two-output structure, which splits a beam of light into two beams, and the mode-selective photon lantern is used to simultaneously excite the LP 11 Model and LP 21 Mode, after mode reflection and mode separation are achieved by the few-mode fiber Bragg grating, the reflection spectrum of the few-mode fiber Bragg grating is measured using a spectrometer, such as Figure 7 As shown, from Figure 7 It can be seen that the few-mode fiber Bragg grating can reflect LP at 1551.61nm 11 model.

[0057] The few-mode long-period fiber grating and few-mode fiber Bragg grating prepared above are applied to the high-order mode fiber laser based on the few-mode fiber grating established in this embodiment, and the LP is measured at the B port of the single-mode fiber coupler 10, the B port of the few-mode fiber coupler 6, and the A port of the few-mode fiber Bragg grating 8. 01 LP 11 LP21 Mode lasing conditions, such as Figure 8 As shown, LP 01 LP 11 LP 21 The mode laser wavelengths are 1551.58nm, 1551.61nm, 1551.62nm, LP 01 LP 11 LP 21 The optical signal-to-noise ratios of the modes are 40dB, 40dB, and 40dB respectively. 01 LP 11 LP 21 The wavelength and power stability of the mode lasing were monitored. As shown in Table 1, the wavelength change was less than 0.02nm and the power change was less than 0.72dB. Table 1 Wavelength and power changes of different mode lasing In this embodiment 1, a high-order mode fiber laser based on few-mode fiber Bragg grating is established, which uses few-mode long-period fiber Bragg grating to realize mode conversion, adopts few-mode fiber Bragg grating to realize mode reflection and mode separation, and realizes LP under a single resonant cavity. 01 LP 11 LP 21 Simultaneous output of mode lasing. Using optical fiber as a medium to output lasers is one of the effective methods for realizing optical tweezers. This high-order mode fiber laser based on few-mode fiber Bragg gratings can simultaneously output low-order mode beams and high-order mode beams, achieving multi-dimensional manipulation of particles and has good application prospects in the fields of materials science and life sciences.

[0058] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0059] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0060] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A high-order mode fiber laser based on few-mode fiber Bragg grating, characterized in that: The invention comprises a pump source (1), a wavelength division multiplexer (2), a single-mode erbium-doped fiber (3), a first few-mode long-period fiber grating (4), a second few-mode long-period fiber grating (5), a few-mode fiber coupler (6), a few-mode fiber circulator (7), a few-mode fiber Bragg grating (8), a few-mode fiber circulator (9) and a single-mode fiber coupler (10); The wavelength division multiplexer (2) comprises an A port and a B port; The few-mode fiber coupler (6) comprises an A port, a B port, a C port and a D port; The few-mode optical fiber circulator (7) comprises an A port, a B port and a C port; The few-mode fiber Bragg grating (8) includes an A port and a B port; The few-mode optical fiber circulator (9) comprises an A port, a B port and a C port; The single-mode optical fiber coupler (10) comprises an A port, a B port, a C port and a D port; The pump light outputted by the pump source (1) is sequentially inputted into the single-mode erbium-doped fiber (3), the first few-mode long-period fiber grating (4) and the second few-mode long-period fiber grating (5) through the A port of the wavelength division multiplexer (2). The first few-mode long-period fiber grating (4) converts the LP of the light beam into 01 Mode conversion to LP 11 mode, the second few-mode long-period fiber grating (5) converts the LP 11 Mode switched to LP 11 Model and LP 21 mode, and then enters the C port of the few-mode fiber coupler (6), the A port of the few-mode fiber circulator (7), and the B port of the few-mode fiber circulator (7) in sequence through the A port of the few-mode fiber coupler (6), wherein the LP of the light beam 21 The mode is output through the A port of the few-mode fiber Bragg grating (8), and the LP of the light beam 11 The mode enters the C port of the few-mode fiber circulator (7) from the B port of the few-mode fiber circulator (7), and then enters the B port of the few-mode fiber circulator (9), the C port of the few-mode fiber circulator (9) and the D port of the few-mode fiber coupler (6) in sequence. The LP of the light beam 11 A part of the mode is output from the B port of the few-mode fiber coupler (6), and the other part enters the A port of the few-mode fiber coupler (6), and passes through the second few-mode long-period fiber grating (5), the first few-mode long-period fiber grating (4), the single-mode erbium-doped fiber (3), the B port of the wavelength division multiplexer (2) and the A port of the single-mode fiber coupler (10) in sequence; when the LP of the light beam 11 When the mode enters the single-mode erbium-doped fiber (3) from the first few-mode long-period fiber grating (4), it is recoupled into the LP of the light beam. 01 mode, part of the rear beam LP 01 The mode is output from the B port of the single-mode optical fiber coupler (10), and the other part is transmitted from the C port and the D port of the single-mode optical fiber coupler (10) and then re-enters the A port of the single-mode optical fiber coupler (10), thereby forming oscillation in the resonant cavity.

2. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The pump source (1) outputs 980nm pump light.

3. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The gain range of the single-mode erbium-doped optical fiber (3) is 1530-1570 nm.

4. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The period of the first few-mode long-period fiber grating (4) is 1100 μm; The period of the second few-mode long-period fiber grating (5) is 960 μm.

5. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The period of the few-mode fiber Bragg grating (8) is 535.4 nm.

6. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The few-mode fiber coupler (6) has a 2×2 structure with two inputs and two outputs, and a splitting ratio of 1:

1.

7. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The few-mode optical fiber circulator (7) and the few-mode optical fiber circulator (9) both exhibit unidirectional transmission in terms of the light beam transmission path, that is, the light beam enters from port A and exits from port B; the light beam enters from port B and exits from port C.

8. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The single-mode optical fiber coupler (10) has a 2×2 structure with two inputs and two outputs, and a splitting ratio of 1:

1.

9. The high-order mode fiber laser based on few-mode fiber Bragg grating according to claim 1, characterized in that: The first few-mode long-period fiber grating (4), the second few-mode long-period fiber grating (5), the few-mode fiber coupler (6), the few-mode fiber circulator (7), the few-mode fiber Bragg grating (8), and the few-mode fiber circulator (9) are all made of few-mode optical fibers, and the diameters of the few-mode optical fiber core and cladding are 18.5 μm and 125 μm, respectively.

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