High-order mode fiber laser based on few-mode fiber grating

Abstract

The invention discloses a high-order mode fiber laser based on a few-mode fiber grating, and belongs to the technical field of optical communication. The fiber laser is composed of a pumping 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. Mode conversion is achieved through the few-mode long-period fiber bragg grating, mode reflection and mode separation are achieved through the few-mode fiber bragg grating, and simultaneous output of LP01 mode lasing, LP11 mode lasing and LP21 mode lasing is achieved under a single resonant cavity. The method for outputting the laser by taking the optical fiber as the medium is one of effective methods for realizing optical tweezers, and the high-order mode optical fiber laser based on the few-mode optical fiber grating can output a low-order mode light beam and a high-order mode light beam at the same time, realizes multi-dimensional control of particles, and has a better application prospect in the fields of material science and life science.

Description

Technical Field

[0001] The invention belongs to the technical field of optical communication, 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 beam, Laguerre-Gaussian beam, Bessel beam, 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 a fiber laser that outputs high-order mode beams.

[0003] At present, fiber lasers that output high-order mode beams can be divided into two categories based on the output mode mechanism. One category is fiber lasers that output a single mode, which achieve 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 achieve mode conversion through the interface of the switching mode selective photon lantern, the phase diagram of the spatial light modulator, etc., thereby switching and outputting different mode beams. Low-order mode beams can realize one-dimensional and two-dimensional manipulation of particles, and high-order mode beams can realize three-dimensional manipulation of particles. Therefore, to realize multi-dimensional manipulation of particles, fiber lasers that output different mode beams at the same time are the 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 gratings to achieve mode conversion, and uses few-mode fiber Bragg gratings to achieve 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 to realize optical tweezers. This high-order mode fiber laser based on few-mode fiber gratings 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.

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

[0006] A high-order mode fiber laser based on few-mode fiber 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 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;

[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 optical 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 to LP 11 Mode switched to LP 11 Mode 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 mode is output through port A 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 turn. 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 beam01 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] Further, 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 is a 2×2 structure with two inputs and two outputs, and the splitting ratio is 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 category is the conversion of the forward-transmitted fundamental mode into the forward-transmitted high-order mode, and the other category 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 represented by LP 01 Mode conversion to LP 11 mode, the second few-mode long-period fiber grating 5 exhibits LP11 Mode conversion to LP 11 Mode 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 depressed peak will appear at the resonant wavelength of its transmission spectrum.

[0025] λ=(n 1 -n 2 )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 n 1 and n 2 is the effective refractive index of the mode at the resonant wavelength in the few-mode long-period fiber grating.

[0027] The present invention adopts a few-mode fiber Bragg grating to realize mode reflection and mode separation. According to the present invention, the mode reflection and mode separation are manifested as 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 realized, and the other high-order mode continues to be transmitted forward, thereby realizing mode separation. The few-mode fiber Bragg grating realizes 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 depressed peak will appear at the resonant wavelength of its reflection spectrum.

[0028] λ=(n 1 +n 2 )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 n 1 and n 2 is the effective refractive index of the mode at the resonant wavelength in the few-mode fiber Bragg grating.

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

[0031] The invention discloses a high-order mode fiber laser based on few-mode fiber grating and a mode conversion method thereof, which realizes mode conversion by using few-mode long-period fiber 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 21Simultaneous 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] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

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

[0034] Figure 2 :Schematic diagram of mode conversion in few-mode long-period fiber 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 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 different mode wavelength lasing and light field distribution;

[0042] Among them, (a) is LP 01 The wavelength lasing and light field distribution in the mode; (b) LP 11 The wavelength lasing and light field distribution in the mode; (c) is LP 21 The wavelength lasing and light field distribution in the mode; DETAILED DESCRIPTION

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

[0044] Example 1

[0045] like Figure 1 As shown, this embodiment provides a high-order mode fiber laser based on few-mode fiber gratings, and its system block diagram is shown in Figure 1 As shown, it is composed 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 optical 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 to LP 11 Mode switched to LP 11 Mode 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 mode is output through port A 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 turn. The LP of the light beam 11Part 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. 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, therefore, some 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 is used 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, the 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 grating 5 will LP 11 Mode conversion to LP 11 Mode and LP 21 The few-mode long period fiber grating is formed by engraving on the few-mode fiber by using a carbon dioxide laser, and the periods of the first few-mode long period fiber grating 4 and the second few-mode long period fiber grating 5 are 1100 μm and 960 μm respectively.

[0055] Use Figure 4The system shown measures the transmission spectrum of a few-mode long-period fiber grating, and 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; wherein the broadband light source has an operating wavelength of 1535-1565nm, 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. 01 mode, 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 9 dB. Therefore, the first few-mode long-period fiber grating 4 can 01 Mode conversion to LP 11 mode. When the mode-selective photon lantern is used to excite the 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 3 dB. Therefore, the LP 11 Mode 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 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 the few-mode fiber Bragg grating, and 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; wherein the broadband light source has an operating wavelength of 1535-1565nm, the single-mode fiber coupler is a one-input and two-output structure, which divides a beam of light into two beams, and the mode-selective photon lantern is used to simultaneously excite the LP 11 Mode and LP 21After the mode reflection and mode separation are realized by the few-mode fiber Bragg grating, the reflection spectrum of the few-mode fiber Bragg grating is measured by 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 as above are applied to the high-order mode fiber laser based on 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, respectively. 01 LP 11 LP 21 Mode lasing conditions, such as Figure 8 As shown, LP 01 LP 11 LP 21 The mode lasing 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 variation was less than 0.02 nm, and the power variation was less than 0.72 dB. Table 1 Variation of wavelength and power of different laser modes In this embodiment 1, a high-order mode fiber laser based on a few-mode fiber grating is established, which uses a few-mode long-period fiber grating to realize mode conversion, adopts a 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 to realize optical tweezers. This high-order mode fiber laser based on few-mode fiber gratings 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.

[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, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope 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 suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0060] In addition, 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 grating, characterized in that: It is composed of a pump source (1), a wavelength division multiplexer (2), a single-mode erbium-doped optical fiber (3), a first few-mode long-period optical fiber grating (4), a second few-mode long-period optical fiber grating (5), a few-mode optical fiber coupler (6), a few-mode optical fiber circulator (7), a few-mode optical fiber Bragg grating (8), a few-mode optical fiber circulator (9) and a single-mode optical fiber coupler (10); The wavelength division multiplexer (2) comprises an A port and a B port; The few-mode optical 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) comprises 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 output by the pump source (1) is sequentially input into the single-mode erbium-doped optical fiber (3), the first few-mode long-period optical fiber grating (4) and the second few-mode long-period optical fiber grating (5) through the A port of the wavelength division multiplexer (2). The first few-mode long-period optical fiber grating (4) converts the LP of the optical 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 Mode 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 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 optical 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 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. A high-order mode fiber laser based on few-mode fiber 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 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 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 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 grating according to claim 1, characterized in that: The few-mode optical fiber coupler (6) is a 2×2 structure with two inputs and two outputs, and the splitting ratio is 1:

1.

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

1.

9. The high-order mode fiber laser based on few-mode fiber 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 core and cladding of the few-mode optical fibers are 18.5 μm and 125 μm, respectively.

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