A space-time mode-locked multi-mode fiber laser based on mode division multiplexing

Abstract

The application discloses a kind of space-time mode-locked multimode fiber lasers based on mode division multiplexing, belong to fiber laser field.Laser resonator includes multimode fiber trunk and mode division multiplexing demultiplexing module, mode division multiplexing demultiplexing module includes mode demultiplexer and mode multiplexer, by mode demultiplexer the multiple transverse spatial modes in multimode fiber trunk are disentangled and output to multiple single-mode fiber branch, single-mode fiber branch independently controls the gain and time delay of corresponding transverse spatial mode, then by mode multiplexer multiple single-mode fiber branch is coupled input to multimode fiber trunk.Under the action of pump, saturable absorber and spatial filter, by adjusting optical time delay in single-mode branch, multiple single-mode fiber branch is simultaneously mode-locked, and single space-time mode-locked pulse sequence is obtained at the output end of the output coupler of multimode fiber trunk.The application breaks through the limitation of multimode fiber intermodal dispersion, and realizes space-time mode-locked laser with controllable mode content.

Description

Technical Field

[0001] This invention belongs to the field of fiber lasers, and more specifically, relates to a spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing. Background Technology

[0002] Ultrafast laser technology has wide applications in scientific research, industrial manufacturing, and precision measurement. Fiber lasers possess outstanding advantages such as high efficiency, good beam quality, compact structure, easy heat dissipation, and flexible operation, making them a rapidly developing ultrafast laser technology. Among them, spatiotemporally mode-locked ultrafast lasers based on multimode fibers have performance advantages compared to ultrafast lasers based on single-mode fibers. For example, multimode fibers have a larger mode field area than single-mode fibers, enabling them to overcome greater nonlinear accumulation phase shifts, thus allowing multimode fiber lasers to generate laser pulses with higher energy and peak power. Furthermore, multimode fiber lasers possess multiple transverse modes with lateral spatial degrees of freedom, enabling the realization of laser output with spatially dimensional control.

[0003] Single-mode fiber-locked ultrafast lasers support only a single transverse spatial mode transmission within their resonant cavity. Laser design must consider single-mode chromatic dispersion, gain, and loss. The mode-locking process only requires longitudinal time locking of the single transverse spatial mode. When a single-mode fiber-locked ultrafast laser operates in the anomalous dispersion region, the nonlinear interactions within the resonant cavity can balance the mode chromatic dispersion. Under effective pump gain and a saturable absorber mode-locker, conventional optical soliton mode-locking states are spontaneously generated and maintained within the cavity. When a single-mode fiber-locked ultrafast laser operates in the all-positive dispersion region, the nonlinear interactions within the resonant cavity cannot balance the mode chromatic dispersion. A spectral filter is required to limit the mode chromatic dispersion, and the laser typically operates in a dissipative soliton mode-locked state or a self-similar soliton mode-locked state.

[0004] Multimode fibers exhibit not only chromatic dispersion of transverse spatial modes but also intermode dispersion, namely the differences in group velocity and group velocity dispersion among different transverse spatial modes. Intermode dispersion easily leads to mode desynchronization and mode walk-off. Therefore, compared to single-mode fiber mode-locked ultrafast lasers, multimode fiber mode-locked ultrafast lasers support the operation of multiple transverse spatial modes within the cavity. The mode-locking process requires not only longitudinal time locking for each transverse spatial mode but also phase locking between different transverse spatial modes to achieve synchronization. According to current literature, multimode fiber mode-locked ultrafast lasers often employ spatial filters to suppress intermode dispersion and establish multimode-locked laser output. Graded-index multimode fibers have relatively small intermode dispersion, which is beneficial for multimode locking; therefore, most current multimode fiber mode-locked lasers use graded-index fibers. Compared to graded-index multimode fibers, step-index multimode fibers have larger intermode dispersion, and mode walk-off can lead to difficulties in spatiotemporal mode-locking.

[0005] Controlling laser output using the transverse spatial degrees of freedom of multimode fiber lasers is a research direction. Due to the presence of multiple transverse spatial modes, the output laser from multimode fiber mode-locked ultrafast lasers exhibits a speckle distribution and relatively low beam quality. By adjusting the dispersion of the laser resonator and enhancing the intracavity nonlinear interaction, the nonlinear multimode Kerr self-cleaning effect can be incorporated into the mode-locking process, resulting in a Gaussian-like ultrafast beam and improved multimode pulse quality. Using a genetic algorithm in conjunction with a spatial light modulator, wavefront shaping of the laser beam within the laser cavity can be performed to control output power, beam quality, and wavelength shift. If each transverse spatial mode in the multimode fiber laser resonator can be precisely controlled, the pulse width, pulse energy, and transverse spatial power distribution of the output laser beam can be precisely controlled, representing a performance breakthrough for multimode fiber mode-locked ultrafast lasers. Summary of the Invention

[0006] To address the limitations of existing technologies, the present invention aims to provide a spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing. This not only overcomes the limitation of traditional technical solutions in accurately controlling the lateral spatial modes, but also overcomes the limitation of spatiotemporal mode-locking caused by lateral spatial mode walk-off due to large intermode dispersion in multimode fibers. This enables the control of mode power distribution and beam quality of mode-locked pulse beams, filling a gap in related technologies.

[0007] To achieve the above objectives, the present invention provides a spatiotemporally mode-locked multimode fiber laser based on mode division multiplexing, comprising a multimode fiber trunk and a mode multiplexing / demultiplexing module disposed therein, wherein, The multimode fiber trunk includes a first multimode fiber, an output coupler, a second multimode fiber, a spatial filter, and a third multimode fiber connected in sequence. The output coupler is placed between the first and second multimode fibers for coupling the output laser. The spatial filter is placed between the second and third multimode fibers for adjusting the coupling efficiency of the lateral spatial mode of light at the connection between the two multimode fibers. The mode multiplexing / demultiplexing module includes a 1×N mode demultiplexer, N single-mode fiber branches, and an N×1 mode multiplexer connected in sequence. The 1×N mode demultiplexer is used to demultiplex multiple lateral modes in the multimode fiber trunk and output them to the N single-mode fiber branches. The N×1 mode multiplexer is used to couple the N single-mode fiber branches into the multimode fiber trunk. Each of the N single-mode fiber branches includes an optical delayer and a single-mode fiber mode-locking module, which are connected to each other by single-mode fiber fusion splicing. The single-mode fiber mode-locking module individually controls the locking of a single lateral spatial mode in the longitudinal time dimension of its branch, and obtains a single-mode locking pulse at the second output of the output coupler. The optical delayer is used to tune the laser repetition frequency of different single-mode fiber branches to achieve simultaneous locking of multiple lateral spatial modes in the time and spatial dimensions, and obtains a multimode spatiotemporal locking pulse at the second output of the output coupler. The connection method adopts fiber optic fusion splicing or spatial coupling.

[0008] Preferably, the first multimode fiber, the second multimode fiber, and the third multimode fiber are the same type of step-index or graded-index fiber, with a mode number of M. The number of modes in the multimode fiber is greater than or equal to the number of single-mode fiber branches N, i.e., M ≥ N. When M = N, the mode multiplexing / demultiplexing module completely controls the modes in the multimode fiber trunk. When M > N, the mode multiplexing / demultiplexing module partially controls the modes in the multimode fiber trunk. When M is small, the first multimode fiber and the second multimode fiber may be referred to as few-mode fibers.

[0009] Preferably, the spatial filter is placed between the second multimode fiber and the third multimode fiber to adjust the coupling efficiency of light through different lateral modes at the connection of the two multimode fibers. The spatial filter is formed by staggered fusion of the second and third multimode fibers, or by staggered fusion of a single-mode or few-mode fiber between the second and third multimode fibers, or by inserting a small aperture between the second and third multimode fibers, or by the second or third multimode fiber subjected to local mechanical stretching, bending, and torsional deformation.

[0010] Preferably, the 1×N mode demultiplexer and N×1 mode multiplexer are all-fiber directional couplers, all-fiber photonic lanterns, silicon-based optical waveguides, or femtosecond direct-write glass-based waveguides.

[0011] Preferably, the optical delay device is a manually adjustable optical delay device or an electrically adjustable optical delay device. The maximum adjustment range of the optical delay device is 600 picoseconds; the maximum length difference between each of the N single-mode fiber branches is 12 centimeters.

[0012] Preferably, the components of the single-mode fiber mode-locking module include a pump source, an input coupler, a single-mode active fiber, a saturable absorber, and a power controller, all of which are single-mode devices and are sequentially fused together by single-mode fibers. The pump source, input coupler, active fiber, and power controller are used to control the single-channel mode-locking gain. The saturable absorber is used to generate and maintain single-channel single-mode locking. The function of the single-mode fiber mode-locking module is to achieve mode-locking in a single mode. Therefore, as long as this function is met, the specific types and quantities of components in the single-mode fiber mode-locking module can be adjusted.

[0013] Preferably, the single-mode active optical fiber is a rare-earth-doped optical fiber in the core region, and the doped rare-earth ions include erbium, neodymium or thulium.

[0014] Preferably, the saturable absorber is a graphene saturable absorber, a carbon nanotube saturable absorber, or a polarized light rotation saturable absorber.

[0015] The principle of the mode-locking method for spatiotemporal mode-locked multimode fiber lasers based on mode division multiplexing is as follows: The single-mode fiber mode-locking module individually controls the locking of a single lateral spatial mode in the longitudinal time dimension of its branch, and obtains a single-mode locking pulse at the output of the output coupler. The spatial filter adjusts and controls the coupling efficiency of the transverse spatial modes of light at the junction of the two multimode fibers, thereby promoting linear and nonlinear interactions between different transverse spatial modes. The optical delayer is used to tune the laser repetition frequency of different single-mode fiber branches, enhance the linear and nonlinear interaction between pulses of different transverse spatial modes, realize the simultaneous locking of multiple transverse spatial modes in the time and space dimensions, and obtain multimode spatiotemporally locked pulses at the output of the output coupler.

[0016] The mode-locking steps of a spatiotemporally mode-locked multimode fiber laser based on mode division multiplexing include: The first step is to pump only one of the single-mode fiber branches, adjusting its loss to the minimum. At this point, the power losses of other branches are at their maximum. The single-mode fiber mode-locking system is then adjusted to achieve independent mode-locking for each branch, thus obtaining the laser's single-pulse repetition frequency f. n n is 1, 2 to N; The second step is to adjust the optical delay device in the single-mode fiber branch to make the branch's f... n They are the same value; The third step involves simultaneously pumping multiple or all single-mode fiber branches to further precisely adjust the optical delayer, thereby achieving spatiotemporal mode locking for multiple lateral spatial modes.

[0017] This invention employs a mode division multiplexing / demultiplexing scheme to first lock each lateral spatial mode in a multimode fiber within its corresponding single-mode fiber branch. Then, through an optical delay unit and a spatial filter, the lateral spatial modes controlled by multiple single-mode fiber branches are linked, thereby achieving spatiotemporal mode locking. The mode-locking method of this invention is not limited to the cavity shape of the laser, and is not limited to ring cavities, linear cavities, figure-eight cavities, or figure-nine cavities. All lasers involving the mode-locking method of this invention should fall within the protection scope of this patent.

[0018] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following beneficial effects: 1. The spacetime mode-locked laser proposed in this invention achieves precise control of transverse modes, which is groundbreaking and instructive. No method or device has ever been able to control the transverse spatial modes in a laser resonator so precisely before. This method realizes the adjustment of gain and time delay for different transverse spatial modes, thereby achieving control over the pulse width, energy and power distribution of the output pulse beam.

[0019] 2. The spacetime mode-locked laser proposed in this invention breaks through the limitations of intermode dispersion attraction and mode walk-off on spacetime mode-locking in multimode fibers. It is suitable for large intermode dispersion step-index multimode fibers and weakens the dependence of current spacetime mode-locked multimode fiber laser schemes on graded-index multimode fibers. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of the spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to the present invention; Figure 2 This is a schematic diagram of a specific spatiotemporal mode-locked laser based on mode division multiplexing in an embodiment of the present invention; Figure 3 These are simulation results of a mode-division multiplexing spatiotemporally mode-locked laser in this embodiment of the invention; (A) is a graph showing the change of the laser output spectrum with the laser operating cycle; (B) is the laser output spectrum after the spatiotemporal mode-locking of the laser is established; (C) is a time-domain pulse diagram with mode resolution after the spatiotemporal mode-locking of the laser is established. Figure 4 These are the experimental results of a mode-division multiplexing spatiotemporal mode-locked laser in this embodiment of the invention; (A) is the laser output spectrum after the spatiotemporal mode-locking of the laser is established; (B) is the pulse autocorrelation diagram after the spatiotemporal mode-locking of the laser is established; (C) is the laser pulse sequence after the spatiotemporal mode-locking of the laser is established. Figure 5 The laser output beam field intensity distribution (A) and beam mode power distribution (B) are shown in the embodiment of the present invention after the laser spatiotemporal mode-locking is established under the first pump setting. Figure 6The above are the beam intensity distribution (A) and beam mode power distribution (B) of the laser output under the second pump setting of the mode-division multiplexing spatiotemporal mode-locked laser in the embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0022] Please see Figure 1 . Figure 1 This is a schematic diagram of the spatiotemporal mode-locked laser based on mode division multiplexing (MDM). As shown in the figure, the laser resonator of this invention, based on MDM, comprises two parts: a multimode fiber trunk 1 and a mode division multiplexing / demultiplexing module 2. The multimode fiber trunk 1 includes a first multimode fiber 8, an output coupler 9, a second multimode fiber 10, a spatial filter 12, and a third multimode fiber 13 connected in sequence. The input end of the output coupler 9 is connected to the output end of the mode multiplexing / demultiplexing module 2 via the first multimode fiber 8. The first output end is connected to the spatial filter 12 via the second multimode fiber 10, and the second output end 11 is used to couple the output laser. The spatial filter 12 is connected to the input end of the mode multiplexing / demultiplexing module 2 via the third multimode fiber 13 and is used to adjust the coupling efficiency of the lateral modes of the laser at the junction of the two multimode fibers. The mode multiplexing / demultiplexing module 2 includes a 1×N mode demultiplexer 3, N single-mode fiber branches 4, and an N×1 mode multiplexer 7 connected in sequence. The 1×N mode demultiplexer 3 is used to demultiplex multiple lateral modes in the multimode fiber trunk 1 and output them to the N single-mode fiber branches 4. The N×1 mode multiplexer 7 is used to couple the N single-mode fiber branches into the multimode fiber trunk 1. Each of the N single-mode fiber branches 4 includes an optical delayer 5 and a single-mode fiber mode-locking module 6, which are connected to each other by single-mode fiber fusion splicing. The single-mode fiber mode-locking module 6 independently controls the locking of a single lateral spatial mode in the longitudinal time dimension of its branch, and obtains a single-mode locking pulse at the second output terminal 11 of the output coupler 9. The optical delayer 5 is used to tune the laser repetition frequency of different single-mode fiber branches 4 to achieve simultaneous locking of multiple lateral spatial modes in the time and spatial dimensions, and obtains a multimode spatiotemporal locking pulse at the second output terminal 11 of the output coupler 9. The connection method adopts fiber optic fusion splicing or spatial coupling.

[0023] Please see Figure 2 . Figure 2 This is a schematic diagram of a specific spatiotemporal mode-locked laser based on mode division multiplexing in an embodiment of the present invention. The laser resonant cavity consists of two parts: a multimode fiber trunk 1 and a mode division multiplexing / demultiplexing module 2. The multimode fiber trunk 1 includes a first section of two-mode fiber 8, a two-mode fiber output coupler 9, a second section of two-mode fiber, a spatial filter 12, and a third section of two-mode fiber 13; the spatial filter 12 is composed of a fiber clamp and a displacement stage, and its filtering effect is achieved by the misaligned coupling of the second section of two-mode fiber 10 and the third section of two-mode fiber 13. The mode division multiplexing module 2 includes a 1×2 mode demultiplexer 3, two single-mode fiber branches 4 and a 2×1 mode multiplexer 7. The multimode fiber trunk 1 includes an output coupler 9. The single-mode fiber branch 4 includes an optical delayer 5 and a single-mode fiber mode-locking module 6. The single-mode fiber mode-locking module 6-1 includes a single-pump light source 6-1a, a first wavelength division multiplexer 6-1b, an active erbium-doped single-mode fiber 6-1c, a second wavelength division multiplexer 6-1d, a power controller 6-1e, a single-mode fiber isolator 6-1f, a polarization controller 6-1g, a saturable absorber 6-1h, and a single-mode fiber coupler 6-1i. The devices are interconnected by single-mode fiber fusion splicing. The single-mode fiber mode-locking module 6-2 includes a single-pump light source 6-2a, a first wavelength division multiplexer 6-2b, an active erbium-doped single-mode fiber 6-2c, a second wavelength division multiplexer 6-2d, a power controller 6-2e, a single-mode fiber isolator 6-2f, a polarization controller 6-2g, a saturable absorber 6-2h, and a single-mode fiber coupler 6-2i. The devices are interconnected by single-mode fiber fusion splicing. The two-mode fiber supports fundamental mode and first-order mode transmission. The 1×2 mode demultiplexer 3 converts the fundamental mode in the two-mode fiber into a Gaussian mode and transmits it through the first single-mode fiber branch 4-1. The 1×2 mode demultiplexer converts the first-order mode in the two-mode fiber into a Gaussian mode and transmits it through the second single-mode fiber branch 4-2. The two fiber signals are controlled by time delay, power and mode-locking, and then coupled into the fundamental mode and first-order mode of the two-mode fiber respectively by the 2×1 mode multiplexer 7. The laser ring cavity is controlled by single-mode fiber isolators 6-1f and 6-2f to control the unidirectional propagation of the beam. The power output of the output port 6-1j of the single-mode fiber coupler 6-1i accounts for 1%, and is used to monitor the power of the first single-mode fiber branch 4-1 and the operating status of the laser. The power output of the output port 6-2j of the single-mode fiber coupler 6-2i accounts for 1%, and is used to monitor the power of the second single-mode fiber branch 4-2 and the laser operating status. The power output ratio of the two-mode fiber coupler 9 is 10%. The basic steps of spatiotemporal mode-locking based on a mode-division multiplexing spatiotemporal mode-locked laser include: The first step is to pump the first single-mode fiber branch 4-1 separately, adjusting its loss to the minimum value, thus creating a closed circuit. At this time, the power loss of the second single-mode fiber branch 4-2 is at its maximum value, thus creating an open circuit. The gain and polarization of the first single-mode fiber branch 4-1 are adjusted to achieve independent mode-locking of the first single-mode fiber branch 4-1, obtaining the laser's single-pulse repetition frequency f1. The mode field distribution of the output beam at the output end 11 of the two-mode fiber coupler 9 corresponds to the optical field distribution of the fundamental mode. The second step is to adjust the power loss of the first single-mode fiber branch 4-1 to the maximum value, while keeping the other devices unchanged. At this time, the power loss of the second single-mode fiber branch 4-2 is adjusted to the minimum value. The gain, spectral filtering and polarization of the second single-mode fiber branch 4-2 are adjusted to achieve independent mode-locking of the second single-mode fiber branch 4-2 and obtain the laser single pulse repetition frequency f2. The fiber delay unit 5-2 is adjusted so that f2 is equal to f1. At this time, the output beam mode field distribution of the two-mode fiber coupler 9 output terminal 11 corresponds to the optical field distribution of the first-order mode. The third step involves simultaneously pumping the first single-mode fiber branch 4-1 and the second single-mode fiber branch 4-2, and further precisely adjusting one of the optical delayer 5-1 and the fiber delayer 5-2 to achieve spatiotemporal mode locking of the fundamental mode and the first-order mode in the two-mode fiber. At this time, the mode field distribution of the output beam at the output end 11 of the two-mode fiber coupler 9 corresponds to the superimposed optical field distribution of the fundamental mode and the first-order mode.

[0024] Please see Figure 3 . Figure 3 These are simulation results of a spatiotemporally mode-locked laser based on a mode-division multiplexing two-mode fiber in an embodiment of the present invention. Figure 3 (A) is a graph showing the change of the laser output spectrum with the laser operating cycle, (B) is a graph showing the laser output spectrum after the laser has established spatiotemporal mode-locking, and (C) is a time-domain pulse diagram with mode resolution after the laser has established spatiotemporal mode-locking. During the simulation, the laser establishes a mode-locked state and operates stably after 330 operating cycles. After mode-locking is established, the pulse packets corresponding to the fundamental mode and the first-order mode are locked together, the two modes are synchronized, the phase lock of different frequency components remains unchanged, and the spectrum does not change.

[0025] Please see Figure 4 . Figure 4These are experimental results of a mode-division multiplexing two-mode fiber spatiotemporal mode-locked laser in this embodiment of the invention. The pump power of pump source 6-1a is 200mW, and the pump power of pump source 6-2a is 500mW. Figure 4 In the diagram, (A) is the laser output spectrum after the laser's spatiotemporal mode-locking is established, (B) is the pulse autocorrelation diagram corresponding to (A) after the laser's spatiotemporal mode-locking is established, and (C) is the laser single-pulse sequence corresponding to (A) and (B) after the laser's spatiotemporal mode-locking is established, indicating that the laser operates in single-pulse mode during the spatiotemporal mode-locking process.

[0026] Please see Figure 5 . Figure 5 (A) in the equation refers to the laser after spatiotemporal mode-locking is established and... Figure 4 The corresponding laser output beam field intensity distribution, (B) is the output beam mode power distribution, in which the fundamental mode power accounts for 28% and the first-order mode power accounts for 62%; Please see Figure 6 . Figure 6 The output beam characteristics of the mode-division multiplexing two-mode fiber spatiotemporally mode-locked laser in this embodiment of the invention are shown under the second pump setting. Under this setting, the pump power of pump source 6-1a is 500mW and the pump power of pump source 6-2a is 200mW. Figure 6 In the diagram, (A) represents the field intensity distribution of the laser output beam, and (B) represents the power distribution of the output beam modes. The fundamental mode power accounts for 61%, and the first-order mode power accounts for 31%.

[0027] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing, characterized in that, The resonant cavity of the laser includes a multimode fiber trunk (1) and a mode multiplexing / demultiplexing module (2) disposed therein, wherein, The multimode fiber trunk (1) includes a first multimode fiber (8), an output coupler (9), a second multimode fiber (10), a spatial filter (12), and a third multimode fiber (13) connected in sequence. The input end of the output coupler (9) is connected to the output end of the mode multiplexing / demultiplexing module (2) through the first multimode fiber (8), the first output end is connected to the spatial filter (12) through the second multimode fiber (10), and the second output end (11) is used to couple the output laser. The spatial filter (12) is connected to the input end of the mode multiplexing / demultiplexing module (2) through the third multimode fiber (13) and is used to adjust the coupling efficiency of the lateral mode of the laser at the connection of the two multimode fibers. The mode multiplexing and demultiplexing module (2) includes a 1×N mode demultiplexer (3), N single-mode fiber branches (4), and an N×1 mode multiplexer (7) connected in sequence. The 1×N mode demultiplexer (3) is used to demultiplex multiple lateral modes in the multimode fiber trunk (1) and output them to the N single-mode fiber branches (4). The N×1 mode multiplexer (7) is used to couple the N single-mode fiber branches into the multimode fiber trunk (1). Each of the N single-mode fiber branches (4) includes an optical delay unit (5) and a single-mode fiber mode-locking module (6), which are connected to each other by single-mode fiber fusion splicing. The single-mode fiber mode-locking module (6) controls the locking of a single transverse spatial mode in the longitudinal time dimension of its branch, and obtains a single-mode locking pulse at the second output end (11) of the output coupler (9). The optical delay unit (5) is used to tune the laser repetition frequency of different single-mode fiber branches (4) to realize the simultaneous locking of multiple transverse spatial modes in the time and spatial dimensions, and obtains a multimode spatiotemporal locking pulse at the second output end (11) of the output coupler (9). The connection method adopts fiber optic fusion splicing or spatial coupling.

2. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 1, characterized in that, The first multimode fiber (8), the second multimode fiber (10), and the third multimode fiber (13) are the same type of step-index or graded-index fiber, and the number of transverse spatial modes is M. The number of transverse spatial modes of the multimode fiber is greater than or equal to the number of single-mode fiber branches N, i.e., M≥N. When M=N, the mode multiplexing and demultiplexing module (2) completely controls the transverse spatial modes in the multimode fiber trunk (1). When M>N, the mode multiplexing and demultiplexing module (2) partially controls the transverse spatial modes in the multimode fiber trunk (1), and the number of transverse spatial modes controlled is equal to N.

3. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 1, characterized in that, The spatial filter (12) is formed by staggered fusion of the second multimode fiber (10) and the third multimode fiber (13), or by staggered fusion of a single-mode fiber or few-mode fiber between the second multimode fiber (10) and the third multimode fiber (13), or by inserting a small aperture between the second multimode fiber (10) and the third multimode fiber (13), or by the second multimode fiber (10) or the third multimode fiber (13) subjected to local mechanical stretching, bending, and torsional deformation.

4. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 1, characterized in that, The 1×N mode demultiplexer (3) and N×1 mode multiplexer (7) are of the type of all-fiber directional coupler, or all-fiber photonic lantern, or silicon-based optical waveguide, or femtosecond direct-write glass-based waveguide.

5. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 1, characterized in that, The optical delay device (5) is a manually adjustable optical delay device or an electrically adjustable optical delay device.

6. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 1, characterized in that, The single-mode fiber mode-locking module (6) includes a pump source, an input coupler, a single-mode active fiber, a saturable absorber, and a power controller, which are sequentially fused together by single-mode fibers; the pump source, input coupler, single-mode active fiber, and power controller are used to control the single-path mode-locking gain; the saturable absorber is used to generate and maintain single-path single-mode locking.

7. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 6, characterized in that, The single-mode active optical fiber is a rare-earth-doped fiber in the core region, and the doped rare-earth ions include erbium, neodymium or thulium.

8. The spatiotemporal mode-locked multimode fiber laser based on mode division multiplexing according to claim 6, characterized in that, The saturable absorber is a graphene saturable absorber, a carbon nanotube saturable absorber, or a polarized light rotation saturable absorber.

Images