High-power all-fiber laser capable of inhibiting mode instability effect

Abstract

The invention discloses a high-power all-fiber laser capable of suppressing a mode instability effect, and the laser comprises a multimode coupling light field module which is used for splitting a single-mode light beam into N paths, the phase, amplitude and polarization parameters of the N paths of split single-mode laser are subjected to closed-loop adaptive adjustment and then are combined into one path of time-varying multi-mode coupling light field output by the N-in-one coupler; the time-varying multimode coupling light field is injected into the main amplifier; and the spatial light detection feedback system is used for collecting a part of output laser of the main amplifier for closed-loop feedback control of the light field phase, polarization and amplitude of the N paths of single-mode laser in the multimode coupling light field module. The time-varying multimode coupling optical field is injected into the main amplifier, the thermally induced refractive index grating in the gain optical fiber can be destroyed, mutual coupling between a fundamental mode and a high-order mode is avoided, the purpose of restraining mode instability is finally achieved, and stable output of high light beam quality is achieved through a space light detection feedback system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fiber lasers, and more particularly relates to a high-power all-fiber laser capable of suppressing mode instability effect. BACKGROUND

[0002] High-power all-fiber lasers have the advantages of high reliability, high beam quality, compact structure and the like. With its unique advantages and wide application prospect, high-power all-fiber amplifiers are more favored in practical applications. With the continuous improvement of industrial level requirements, the power improvement has been one of the research hotspots. However, due to the combined effects of nonlinear effects and mode instability and other physical factors, the improvement of the brightness of such fiber amplifiers is the main technical problem in the field of high-power fiber lasers. Increasing the mode field area of the gain fiber is one of the simplest methods to improve the nonlinear effect threshold of fiber lasers. However, with the increase of optical power, the high-order modes supported by the large-mode-area gain fiber are coupled severely at high power, forming a thermal-induced refractive grating, i.e., mode instability, which leads to a sharp degradation of the beam quality of the output laser.

[0003] At present, the methods for improving the mode instability threshold in high-power fiber laser systems mainly include passive suppression and active suppression. The passive suppression mainly includes optimizing the seed laser, the pumping method and developing new optical fibers. The first two methods balance the power and wavelength of the seed light and the pump light, but this method has limited effect on improving the TMI threshold, and at most can only improve by one time. More seriously, for narrow linewidth lasers, this type of amplification will introduce nonlinear effects such as stimulated Raman scattering and stimulated Brillouin scattering while suppressing mode instability. As for the third method, the new fiber structure is complex, difficult to manufacture, and difficult to mass-produce, and the optical fiber devices matched with it usually need to be customized, which limits the practical application of new large-mode-area fiber structures.

[0004] Generally, the active suppression scheme usually exists only in the spatial optical system, and the amplifier is difficult to realize full-fiber integration. Therefore, under the premise of not affecting the nonlinear effect suppression capability, further improving the mode instability threshold of the large-mode-area all-fiber amplifier has important practical significance for improving the performance of high-power fiber lasers and expanding the application of high-power fiber lasers. SUMMARY

[0005] In view of the defects in the prior art, the application provides a high-power all-fiber laser capable of suppressing mode instability effect.

[0006] To achieve the above technical purpose, the technical scheme adopted by the application is as follows: The application provides a high-power all-fiber laser capable of suppressing mode instability effect, comprising: A multimode coupling light field module is used to split a single mode light beam into N paths, and to perform closed-loop adaptive adjustment on the phase, amplitude and polarization parameters of the N single mode lasers after splitting, and then to combine the N single mode lasers into a time-varying multimode coupling light field output by an N-to-1 coupler. A main amplifier, the time-varying multimode coupling light field is injected into the main amplifier, which can achieve the purpose of suppressing mode instability. A spatial light detection feedback system is used to collect a part of the output laser of the main amplifier for closed-loop feedback control of the light field phase, polarization and amplitude of the N single mode lasers in the multimode coupling light field module.

[0007] Preferably, the spatial light detection feedback system collects a part of the output laser to input into a spot analyzer for observing the spot shape of the output laser, and collects another part of the output laser to input into a photodetector, the photodetector converts the received optical signal into an electrical signal and inputs the electrical signal into a controller, the controller obtains the voltage intensity of the time domain signal fluctuation of the output laser based on the electrical signal, and takes the maximum value of the voltage intensity of the time domain signal fluctuation of the output laser as an evaluation function, takes the maximum power in the bucket as the target, runs an optimization algorithm to generate control signals of the phase, amplitude and polarization parameters of the N single mode lasers in the multimode coupling light field module and loads the control signals into the corresponding phase modulators, polarization controllers and preamplifiers of the N single mode lasers, to realize closed-loop feedback control of the light field phase, polarization and amplitude of the N single mode lasers in the multimode coupling light field module, so that the output signal of the photodetector remains to the maximum value, and the light field information of the output laser of the main amplifier is locked to the fundamental mode.

[0008] Preferably, the multimode coupling light field module includes a single mode fiber laser, a 1-to-N laser beam splitter, N phase modulators, N polarization controllers, N preamplifiers, N fiber delay lines and an N-to-1 coupler. The single mode fiber laser is used to output a single mode light beam. The 1-to-N laser beam splitter is used to split the single mode light beam into N single mode lasers. Each single mode laser passes through a phase modulator in turn for phase control, then passes through a polarization controller for polarization control, then passes through a preamplifier for power amplification, and then passes through a fiber delay line to maintain coherence, and finally the N single mode lasers form a time-varying multimode coupling light field through an N-to-1 coupler and then inject the time-varying multimode coupling light field into the main amplifier.

[0009] Preferably, the input end of the N-to-1 coupler is N single mode input fibers, and the output end is a single large mode field output fiber, and the inherent structural characteristics require that a long enough and thin enough taper structure be introduced in the single mode input beam to destroy the waveguide, so as to form a coupling light field in the multimode fiber.

[0010] Preferably, the main amplifier comprises a pump light source, a pump-signal combiner, a gain fiber and an end cap collimator, the pump-signal combiner is connected with the pump light source, the signal arm of the pump-signal combiner is connected with the output end of the N+1 coupler, the output end of the pump-signal combiner is connected with one end of the gain fiber, and the other end of the gain fiber is connected with the end cap collimator.

[0011] Preferably, the spatial light detection feedback system comprises a high reflection mirror, a power receiver, a beam splitter, a spot analyzer, a photodetector and a controller, the output laser of the main amplifier is incident to the high reflection mirror, the laser reflected by the high reflection mirror is output to the power receiver, the laser transmitted by the high reflection mirror is transmitted to the beam splitter, the beam splitter is used to divide the incident light beam into two parts, one part is transmitted to the spot analyzer to observe the spot shape, and the other part is transmitted to the photodetector.

[0012] The method of the present application can achieve the following beneficial effects: The present application provides a high-power all-fiber laser capable of suppressing mode instability effect, which replaces the traditional single-mode beam input of the amplifier with a multi-mode coupled light field obtained by splitting the single-mode beam into N paths and then combining one path, and performs closed-loop adaptive adjustment on the phase, amplitude and polarization parameters of the N-path input single-mode signals. Without considering nonlinearity and cost, the present application can design a closed-loop feedback system with good mode instability suppression capability, providing a reliable technical means for further power improvement and performance optimization of high-power all-fiber amplifiers.

[0013] Specifically, the present application injects a time-varying multi-mode coupled light field into the main amplifier, the fundamental mode and the high-order mode are in a time-varying random phase interference superposition state, different multi-mode input beams will cause different overall temperature distributions of the active fiber core, thereby producing different mode interference patterns, further destroying the thermal refractive index grating in the gain fiber, avoiding the mutual coupling between the fundamental mode and the high-order mode, and finally achieving the purpose of suppressing mode instability, and realizing stable output of high beam quality through the spatial light detection feedback system.

[0014] Compared with the traditional mode instability suppression method, the present application adopts a full-fiber system structure design, has the advantages of high compactness and high reliability. Moreover, the present application is novel and has strong universality, simple structure and easy implementation, does not need to change the refractive index distribution of the gain fiber, does not need to consider the compatibility problem of the gain fiber and the energy transmission fiber, and has high universality.

[0015] The present application effectively improves the mode instability threshold of the large-mode-area fiber amplifier, and does not need to consider the nonlinear effects such as stimulated Raman scattering generated in the system with power improvement.

[0016] The application has universality, and the method can be used for amplification of any wavelength in the range of 1060nm-1150nm. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative effort on the basis of the drawings shown.

[0018] Figure 1 A schematic structural diagram of a high-power all-fiber laser capable of suppressing mode instability effect provided in an embodiment; Figure 2 A schematic diagram of the change of the optical field information received by the photodetector in an embodiment; Reference numerals in the drawings: 1-1, single-mode fiber laser, 1-2, 1-to-N laser beam splitter, 1-3, phase modulator, 1-4, polarization controller, 1-5, preamplifier, 1-6, fiber delay line, 1-7, N-to-1 coupler, 1-8, main amplifier, 1-9, end cap collimator, 1-10, high-reflectivity mirror, 1-11, power receiver, 1-12, beam splitter, 1-13, beam profiler, 1-14, photodetector, 1-15, controller.

[0019] The implementation, functional features and advantages of the application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the application. Obviously, the described embodiments are only some of the embodiments of the application, but not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative effort belong to the protection scope of the application.

[0021] In an embodiment, a high-power all-fiber laser capable of suppressing mode instability effect is provided, which comprises a multimode coupling optical field module, a main amplifier and a spatial light detection feedback system.

[0022] The multimode coupling optical field module is used for splitting a single-mode light beam into N paths, and performing closed-loop adaptive adjustment on the phase, amplitude and polarization parameters of the N paths of single-mode lasers after splitting, and then combining the N paths into one time-varying multimode coupling optical field output by an N-to-1 coupler; The main amplifier is injected with a time-varying multimode coupling light field, the fundamental mode and the high-order mode are in a time-varying random phase interference superposition state, different multimode input light beams will cause different overall temperature distributions of the active optical fiber core, thereby producing different modal interference patterns, and further destroying the thermal refractive index grating in the gain optical fiber, avoiding the mutual coupling between the fundamental mode and the high-order mode, and achieving the purpose of suppressing mode instability; The spatial light detection feedback system is used to collect a part of the output laser of the main amplifier for spot information detection, wherein a part of the collected output laser is input into a spot analyzer for observing the spot shape of the output laser, and another part of the collected output laser is input into a photodetector, the photodetector converts the received optical signal into an electrical signal and inputs the electrical signal into a controller, the controller obtains the voltage intensity of the time-domain signal fluctuation of the output laser based on the electrical signal, and takes the maximum value of the voltage intensity of the time-domain signal fluctuation of the output laser as an evaluation function, takes the maximum power in the bucket as a target, and runs an optimization algorithm to generate control signals of the phase, amplitude and polarization parameters of the N single-mode lasers in the multimode coupling light field module and load the control signals into the corresponding phase modulators, polarization controllers and preamplifiers of the N single-mode lasers respectively, so as to realize closed-loop feedback control of the light field phase, polarization and amplitude of the N single-mode lasers in the multimode coupling light field module, keep the output signal of the photodetector to the maximum, and realize the light field information locking of the output laser of the main amplifier to the fundamental mode.

[0023] The single-mode light beam is a single-mode narrow linewidth seed laser, the single-mode light beam is split into N paths, and the phase, amplitude and polarization parameters of the N single-mode lasers after splitting are closed-loop adaptively adjusted, and then the N single-mode lasers are combined by an N-to-1 coupler. Through the design of a special waveguide structure, the light field is coupled to form a rapidly modulated transverse mode field distribution. This continuously refreshed multimode coupling light field will have corresponding time-varying light field information in the gain optical fiber of the main amplifier, thereby avoiding the formation of the thermal refractive index grating of the gain optical fiber of the main amplifier. Under experimental conditions, it is proved that compared with the traditional single-mode seed laser, the multimode coupling light field as the seed source of the amplifier improves the TMI threshold by at least 4 times, and further threshold improvement is only limited by the lack of pump power. In order to ensure that the system still maintains high beam quality output, the phase, amplitude and polarization parameters of the N single-mode lasers obtained by splitting the single-mode light beam are closed-loop controlled, and the maximum power in the bucket is taken as the control target, so that the system can effectively suppress the thermal mode instability while still maintaining the high beam quality laser output of the fundamental mode beam profile. This scheme can effectively suppress mode instability in a large-mode-area fiber while maintaining good beam quality.

[0024] Reference Figure 1In one embodiment, a high power all fiber laser capable of suppressing mode instability is provided, comprising a single mode fiber laser 1-1, a 1-to-N laser beam splitter 1-2, N phase modulators 1-3, N polarization controllers 1-4, N preamplifiers 1-5, N fiber delay lines 1-6, a 1-to-N coupler 1-7, a main amplifier 1-8, an end cap collimator 1-9, a high reflectivity mirror 1-10, a power receiver 1-11, a beamsplitter 1-12, a beam profiler 1-13, a photodetector 1-14, and a controller 1-15.

[0025] The single mode fiber laser 1-1 is connected to the 1-to-N laser beam splitter 1-2 to provide a seed laser source for the system. The multimode coupling light field module is used to form a coherently coupled multimode light field.

[0026] The 1-to-N coupler 1-7 is a core device that forms a coupled multimode light field and inputs it as a seed source into the main amplifier 1-8. This multimode coupling input will exist in the gain fiber as a time-varying light field information. The fundamental mode and high-order modes are in an interference superposition state from the beginning. Different multimode input beams will cause changes in the thermal load along the core of the active fiber and different temperature distributions, resulting in different modal interference patterns. Therefore, the non-periodic fast-changing multimode coupling seed light continuously refreshes the state of the existence of the thermal refractive index grating. The temperature distribution in the core does not have enough time to adapt to the new multimode signal and is always in a transient state.

[0027] The main amplifier 1-8 is used to amplify the power of the seed light output by the 1-to-N coupler 1-7. The main amplifier 1-8 is composed of a pump source, a signal-pump beam combiner, a doped fiber, and a fiber end cap collimator. The core structure of the main amplifier 1-8 is to use a rare earth doped fiber doped with rare earth elements (such as erbium, ytterbium, thulium, etc.) as a gain fiber to amplify the signal light of a specific wavelength passing through it under the excitation of pump light. The pump light generated by the pump source is injected into the gain fiber through the signal-pump beam combiner. The amplified laser is output to free space through the end cap collimator 1-9.

[0028] In an embodiment, the main amplifier 1-8 comprises a pump light source, a pump-signal combiner, a gain fiber and an end cap collimator 1-9, the pump arm of the pump-signal combiner is connected with the pump light source, the signal arm of the pump-signal combiner is connected with the output end of the N-coupler, the output end of the pump-signal combiner is connected with one end of the gain fiber, and the other end of the gain fiber is connected with the end cap collimator 1-9.

[0029] The spatial light detection feedback system, the high reflector 1-10 is used for outputting most of the power laser in the fiber amplifier system to the power receiver 1-11, and transmitting a small part of the laser for spot information detection. The beam splitter 1-12 is used to divide the light beam into two parts. One part is transmitted to the spot analyzer 1-13 for observing the spot shape, and the other part is transmitted to the photodetector 1-14. The photodetector 1-14 converts the received optical signal into an electrical signal, which is then fed back to the controller 1-15. The optimization algorithm of the controller 1-15 is not limited, and can adopt a dithering algorithm, an SPGD algorithm, etc. The single iteration time of the algorithm is generally less than 1 µs, that is, the iteration rate is greater than 1 MHz. The maximum value of the voltage strength of the output laser time domain signal fluctuation is used as the evaluation function of the controller 1-15 loaded with the optimization algorithm, and is respectively loaded on the N phase modulators 1-3, the N polarization controllers 1-4 and the N preamplifiers 1-5, to realize closed-loop feedback control of the light field phase, polarization and amplitude. Finally, the output signal of the photodetector 1-14 is kept to the maximum value, and the output light field information is locked to the fundamental mode. The photodetector 1-14 converts the detected voltage signal into a digital signal and sends it to the controller 1-15. The controller 1-15 realizes a closed-loop control system by signal modulation-demodulation and digital-to-analog conversion, and applies the voltage control signal generated by the controller 1-15 to the piezoelectric driver voltage signal input end of the corresponding modulation device. The final goal of the whole system optimization is to detect and optimize the profile of the output spot, that is, to control the coupling degree of the fundamental mode and the high-order mode. As shown in the figure, the voltage strength value of the laser time domain signal fluctuation received by the photodetector 1-14 fluctuates obviously before the control is started, and the voltage strength value is constant after the control is started and the mode is not unstable. Therefore, the system can effectively suppress the thermal mode instability effect while still maintaining the high beam quality laser output of the fundamental mode beam profile. Figure 2

[0030] ​The type of the single-mode fiber laser 1-1 is not limited; it can be a polarization-maintaining continuous laser or a pulsed laser, as long as the fiber and device types of the overall system are matched. In terms of linewidth, it can be a narrow-linewidth fiber source or a single-frequency fiber source, as long as the laser in each beam-splitting path is coherent. The number of channels of the 1-to-N laser beam splitter 1-2 is not limited (N≥2), and the N single-mode channel outputs are connected to the single-mode inputs of N phase modulators.

[0031] Each single-mode laser beam obtained from the beam splitting is sequentially phase-modulated by phase modulators 1-3, then polarized by polarization controllers 1-4, followed by power amplification by preamplifiers 1-5, and coherence is maintained by fiber delay lines 1-6. Finally, after forming a multimode coupled optical field through N-to-1 couplers 1-7, it is injected into the main amplifier 1-8. The main amplifier 1-8 in this invention generally requires an electro-optic bandwidth greater than 150MHz. The polarization state generated by polarization controllers 1-4 in this invention is not limited. The preamplifier 1-5 in this invention only needs to meet the minimum power requirement injected by the main amplifier. The fiber delay lines 1-6 in this invention have no limit on the fiber path length delay range, as long as good coherence is maintained in each path.

[0032] In this invention, the N-to-1 coupler 1-7 is connected to the main amplifier 1-8. The input end of the N-to-1 coupler consists of N single-mode input fibers, and the output end consists of a single large-mode-field output fiber. Preferably, the N single-mode input fibers are bundled and tapered to form a single-mode fiber bundle tapered region, and the end of the single-mode fiber bundle tapered region is fused together with the large-mode-field output fiber to form the N-to-1 coupler. In order to ensure that the N-to-1 coupler used for mode control has low loss characteristics, linear mode evolution characteristics and convergence of control algorithm, its internal structural characteristics need to meet three design criteria, namely: (1) The single-mode fiber bundle tapered region needs to ensure that the tapered region is long enough and the tapered angle is small enough so that the proportion of guided mode in the final result of mode evolution is much greater than the proportion of radiation mode. The specially designed tapered region leads to the destruction of waveguide structure, thereby forming a coupled optical field in multimode fiber. Generally, the length of the single-mode fiber bundle taper region should not be less than 1.7cm and the taper angle should not be greater than 0.02rad, so that the optical field is coupled and thus a coupled optical field is formed in the multimode fiber; (2) The single-mode fiber bundle should be bundled according to the optimal arrangement scheme. Generally, N single-mode input fibers are arranged in the shape of equilateral triangle, square, regular pentagon, circle, etc. and then tapered to form the single-mode fiber bundle taper region, which can effectively reduce the loss of the device; (3) The lateral dimension of the end of the single-mode fiber bundle taper region should be smaller than the core size of the large mode field output fiber to ensure that the coupled optical field formed by the single-mode fiber bundle taper region can be propagated to the large mode field output fiber without loss, thereby satisfying the high-efficiency energy transfer.

[0033] The main amplifier 1-8 includes a pump light source, a pump-signal beam combiner, a gain fiber, and an end cap collimator. The pump source wavelength is not limited, and can be selected from special pump wavelengths such as 915 nm, 976 nm, and 981 nm. The pump mode is not limited, and can be selected from unidirectional pumping or bidirectional pumping. The gain fiber is a rare earth doped fiber, and the core diameter of the gain fiber can be greater than the output fiber core of the N+1 coupler 1-7. The end cap collimator 1-9 of the main amplifier 1-8 emits a light spot with a diameter not limited to 10-50 mm, which can effectively reduce the laser power density of the output end face.

[0034] The spatial light detection feedback system includes a high reflector 1-10, a power receiver 1-11, a beam splitter 1-12, a light spot analyzer 1-13, a photodetector 1-14, and a controller 1-15. The material of the high reflector 1-10 is not limited, and can be fused quartz, K9, ZnSe, CaF2, etc. The signal wavelength reflectivity is generally greater than 99%. The model and range of the power receiver 1-11 are not limited, and can be air-cooled or water-cooled. The maximum receiving power is generally greater than 500 W. The material of the beam splitter 1-12 is not limited, and the transmittance and reflectance are generally 5:5. The type of the light spot analyzer 1-13 is not limited, and can be a CCD camera or a light quality measuring instrument. In this embodiment, the photodetector is placed behind the pinhole diaphragm to obtain the bucket power at one time. A pinhole diaphragm is placed in front of the photodetector 1-14, and the light beam is incident on the photodetector 1-14 through the pinhole diaphragm. The size of the pinhole diaphragm is generally 20-100 μm, the spectral response range is generally 400-1100 nm, and the bandwidth is greater than 1 MHz under a 10 dB gain. The photodetector 1-14 collects the bucket power of the entire amplifier terminal output light beam, and transmits this as an evaluation function to the controller 1-15. The controller 1-15 generally requires an internal signal conditioning circuit to facilitate signal receiving range expansion and reduction, has a precision of more than 10 bits and a sampling rate of more than 125 MHz. The optimization algorithm loaded by the controller 1-15 is not limited, and generally adopts various optimization algorithms in the prior art, such as dithering algorithm, SPGD algorithm, etc. The single iteration time of the algorithm is generally less than 1 µs, that is, the iteration rate is greater than 1 MHz. The adaptive control process takes the maximum bucket power as the target, and the phase, polarization, and amplitude control algorithms generate corresponding electrical signals to be applied to the phase modulator, the polarization controller, and the preamplifier. Finally, the output signal of the photodetector 1-14 is kept to the maximum value, and the output light field information is locked to the fundamental mode.

[0035] The number of stages of the preamplifier 1-5 is not limited, and can be a single-stage optical fiber preamplifier or a multi-stage optical fiber preamplifier, as long as the minimum amplification power requirement is met. For a hundred-milliwatt-level optical fiber preamplifier, the single-stage amplification multiple is 30-50 times, and for a tens-of-watt-level preamplifier, the single-stage amplification multiple is greater than 100 times. In actual application, the minimum amplification multiple can also be met by amplifying the injection laser power of the 1 / N laser beam splitter 1-2.

[0036] The output line width of the single-mode fiber laser 1-1 determines the optical coherence of the entire system. With the widening of the spectral line width, the coherence length is also correspondingly reduced. The basis for the system to realize mode control is the coherence of the input beams of each channel, and therefore, the optical path difference between each channel needs to be eliminated at the beginning of system construction.

[0037] The size and position of the aperture seriously affect the global optimal solution and cannot match the mode content, and therefore, need to be optimized and selected according to the target.

[0038] In this embodiment, the maximum power in the bucket is taken as an evaluation function, and the power in the bucket is obtained at one time by placing a photodetector behind the aperture. The signal acquisition bandwidth is much higher than kHz. The control algorithm is not limited, and the random parallel gradient descent algorithm is taken as an example in this embodiment. The initial phase of each input end is set to a random phase in the range of 0-2π, and the random fluctuation of each power is not more than 10% based on the ideal value. Since the hardware is easy to generate a disturbance voltage subject to Bernoulli distribution, and this distribution is conducive to the rapid convergence of the evaluation function, the disturbance voltage generated by the SPGD algorithm in one step is set to subject to Bernoulli distribution; the disturbance voltage and the gain coefficient can meet the fastest average convergence speed of the evaluation function. Before 0 seconds, the controller is not working, and the system is in an open-loop state. After 0 seconds, the controller starts to work, and the controller is in a closed-loop state. Refer to Figure 2 A schematic view of the change of the light field information received by the photodetector.

[0039] The test results of this embodiment show that, compared with the traditional single-mode seed laser, the multi-mode coupled light field as the seed source of the amplifier improves the TMI threshold by at least 4 times, and the further threshold improvement is only limited by the insufficient pump power. The present application suppresses the thermal mode instability effect of the fiber laser based on the high refresh rate multi-mode coherent coupling seed input. The present application has the advantages of high practicability, all-fiber system, modular design, etc.

[0040] The above are only preferred embodiments of the present application, and the protection scope of the present application is not limited to the above-mentioned embodiments. Any technical scheme falling within the concept of the present application shall fall within the protection scope of the present application. It should be noted that, for ordinary skilled in the art, some improvements and refinements without departing from the principles of the present application shall be considered as the protection scope of the present application.

Claims

1. A high power all fiber laser that can suppress mode instability effects, characterized in that, The application relates to a multi-mode coupling light field module, a main amplifier and a spatial light detection feedback system. The multi-mode coupling light field module is used for splitting a single-mode light beam into N paths and performing closed-loop adaptive adjustment on the phase, amplitude and polarization parameters of the N single-mode lasers after splitting, and then performing beam combination by an N-to-1 coupler to form a time-varying multi-mode coupling light field output. The time-varying multi-mode coupling light field is injected into the main amplifier, so that mode instability can be suppressed. The spatial light detection feedback system is used for collecting part of the output laser of the main amplifier to perform closed-loop feedback control on the light field phase, polarization and amplitude of the N single-mode lasers in the multi-mode coupling light field module.

2. The high power all-fiber laser with suppressed mode instability effect according to claim 1, characterized in that, The multi-mode coupling light field module comprises a single-mode fiber laser, an N-to-1 laser beam splitter, N phase modulators, N polarization controllers, N preamplifiers, N fiber delay lines and an N-to-1 coupler. The single-mode fiber laser is used for outputting a single-mode light beam. The N-to-1 laser beam splitter is used for splitting the single-mode light beam into N single-mode lasers. Each single-mode laser passes through a phase modulator, a polarization controller, a preamplifier and a fiber delay line in sequence to perform phase control, polarization control, power amplification and coherence maintenance, and finally, the N single-mode lasers are injected into the main amplifier after forming a time-varying multi-mode coupling light field by the N-to-1 coupler.

3. The high power all-fiber laser with suppressed mode instability effect according to claim 2, characterized in that, The spatial light detection feedback system collects part of the output laser and inputs the output laser into a light spot analyzer to observe the light spot shape of the output laser, and inputs another part of the output laser into a photodetector, the photodetector converts the received optical signal into an electrical signal and inputs the electrical signal into a controller, the controller obtains the voltage intensity of the time-domain signal fluctuation of the output laser based on the electrical signal, takes the maximum value of the voltage intensity of the time-domain signal fluctuation of the output laser as an evaluation function, takes the maximum power in the bucket as a target, runs an optimization algorithm to generate control signals of the phase, amplitude and polarization parameters of the N single-mode lasers in the multi-mode coupling light field module and loads the control signals into the corresponding phase modulators, polarization controllers and preamplifiers of the N single-mode lasers, so that the closed-loop feedback control of the light field phase, polarization and amplitude of the N single-mode lasers in the multi-mode coupling light field module is realized, the output signal of the photodetector is kept to the maximum value, and the light field information of the output laser of the main amplifier is locked to the fundamental mode.

4. The high-power all-fiber laser with suppressed mode instability effect according to claim 1 or 2 or 3, characterized in that, The input end of the N-to-1 coupler is N single-mode input fibers, and the output end is a single large-mode-field output fiber.

5. The high power all-fiber laser with suppressed mode instability effect according to claim 4, characterized in that, In the N-to-1 coupler, the N single-mode input fibers are bundled and tapered to form a single-mode fiber bundle taper region, and the end of the single-mode fiber bundle taper region is fused together with the large-mode-field output fiber to form the N-to-1 coupler, wherein the length of the single-mode fiber bundle taper region is not less than 1.7 cm, and the taper angle is not greater than 0.02 rad.

6. The high power all-fiber laser with suppressed mode instability effect according to claim 5, characterized in that, In the N-to-1 coupler, the N single-mode input fibers are arranged in a regular triangle, square, regular pentagon or circle to form a single-mode fiber bundle taper region after being bundled and tapered.

7. The high power all-fiber laser with suppressed mode instability effect according to claim 5, characterized in that, The end of the single-mode fiber bundle taper region has a transverse size smaller than the core size of the large-mode-field output fiber.

8. The high power all-fiber laser capable of suppressing the effect of mode instability according to claim 1 or 2 or 3 or 5 or 6 or 7, characterized in that, The main amplifier comprises a pump light source, a pump-signal combiner, a gain fiber and an end cap collimator, the pump-signal combiner is connected with the pump light source, the signal arm of the pump-signal combiner is connected with the output end of the N combined 1 coupler, the output end of the pump-signal combiner is connected with one end of the gain fiber, and the other end of the gain fiber is connected with the end cap collimator.

9. The high power all-fiber laser with suppressed mode instability effect according to claim 3, characterized in that, The spatial light detection feedback system comprises a high reflection mirror, a power receiver, a beam splitter, a light spot analyzer, a photoelectric detector and a controller, the output laser of the main amplifier is incident to the high reflection mirror, wherein the reflected laser of the high reflection mirror is output to the power receiver, the transmitted laser of the high reflection mirror is transmitted to the beam splitter, the beam splitter is used to divide the incident light beam into two parts, one part is transmitted to the light spot analyzer to observe the light spot shape, and the other part is transmitted to the photoelectric detector.

10. The high power all-fiber laser with suppressed mode instability effect according to claim 9, characterized in that, A small aperture diaphragm is arranged in front of the photoelectric detector, and the light beam is incident to the photoelectric detector through the small aperture diaphragm.

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