Method and device for evaluating transverse mode instability effect of single-frequency fiber laser

Abstract

The present invention proposes a method and device for evaluating the transverse mode instability effect of a single-frequency fiber laser. During the process of increasing the output power of the single-frequency fiber laser, the frequency noise or phase noise of the single-frequency laser is output in real time. If the frequency noise or phase noise of the single-frequency laser is degraded in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, the single-frequency fiber laser has a transverse mode instability effect; the output power when the frequency noise or phase noise of the single-frequency laser is first degraded in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and the average difference is greater than 3dB is the TMI threshold. The present invention has a high sensitivity to the TMI effect, does not require the single-frequency amplifier to work under a strong transverse mode instability effect, and can better protect the system safety and stable operation.

Description

Technical Field

[0001] The present invention mainly relates to the field of optical fiber laser technology, in particular to a method and a device for evaluating the instability effect of the transverse mode of a single-frequency optical fiber laser. Background Art

[0002] Single-frequency laser has only one longitudinal mode, that is, only a single frequency, so it has excellent temporal coherence. Fiber lasers have the advantages of low noise, high beam quality and high stability. Therefore, using optical fiber as a carrier to achieve single-frequency laser output is considered to be an excellent solution for obtaining high-power and high-performance single-frequency lasers. In recent years, single-frequency fiber lasers have developed rapidly and have been deeply applied in cutting-edge scientific fields such as high-power beam synthesis, gravitational wave detection, lidar, frequency conversion, and precision spectral detection. However, in high-power single-frequency fiber lasers, with the increase in output power, the influence of thermal effects in optical fibers has become more and more significant. The influence of thermal effects is mainly manifested in the thermally induced transverse mode instability (TMI) effect, which will cause energy coupling between the fundamental mode and the higher-order mode after the fiber laser system reaches a certain power, thereby reducing the beam quality of the output laser. When the system has high-order mode filtering, such as bending loss of high-order modes, severe TMI effects will also limit the further increase of the output power of single-frequency fiber lasers, and even damage the laser amplification system. Therefore, in order to ensure that the system operates stably in a high beam quality state, it is necessary to ensure that the system operates at a certain power level. This power level is called the transverse mode instability effect threshold.

[0003] Traditional methods for evaluating the threshold of transverse mode instability effects mainly include monitoring the timing fluctuation of the output laser, monitoring the Fourier transform spectrum of the time domain signal of the output laser, monitoring the beam quality of the output laser, etc. Among them, the method of monitoring the timing fluctuation of the output laser mainly collects the single-point timing intensity fluctuation of the output laser through a small hole, and uses the normalized standard deviation of the intensity fluctuation to determine the TMI threshold.

[0004] like Figure 1 As shown, Figure 1 This is a typical example of using the timing fluctuation of the output laser to determine the TMI threshold. When the output power is 500 W, the normalized standard deviation (NSD) of the output laser time domain fluctuation is 0.061%. When the output power is increased to 523 W, the TMI effect appears in the laser system, and the normalized standard deviation (NSD) of the output laser time domain fluctuation increases to 1.61%; the laser power when the NSD of the output laser time domain shows obvious degradation is defined as the system's TMI threshold. Based on this, it is judged that the TMI threshold of this amplifier is 523 W. Before and after the TMI effect occurs, the normalized standard deviation (NSD) of the timing intensity fluctuation increases significantly, indicating that the TMI effect appears in the system.

[0005] The method of monitoring the Fourier transform spectrum of the time domain signal of the output laser is mainly to observe whether the characteristic frequency peak appears in the Fourier transform spectrum of the time domain signal within the frequency range of 0-10 kHz to determine whether the TMI effect occurs. Figure 2 As shown, Figure 2 This is a typical example of using the Fourier change spectrum of the output laser sequence to determine the TMI threshold. When the TMI effect does not occur, the Fourier change spectrum of the output laser is flat and has no characteristic peaks; when the TMI effect occurs, the Fourier change spectrum of the output laser has obvious characteristic peaks at 0-10 kHz, which indicates that the TMI threshold of this amplifier is 523 W.

[0006] The method of monitoring the output laser beam quality is mainly to determine the TMI effect by detecting the degradation of the output laser beam quality, such as Figure 3 As shown, Figure 3 This is a typical example diagram of using the output laser beam quality degradation to determine the TMI threshold, where Figure 3 (a) is an example of the focal spot of the output laser when the TMI effect does not occur. At this time, the output laser power is 500W and the beam quality is stable at 1.23; Figure 3 (b) is an example of the focal spot of the output laser when the TMI effect occurs. When the TMI effect occurs, a large number of high-order mode components appear in the focal spot of the output laser, and the beam quality rapidly degrades to 1.82. It is judged that the TMI threshold of this amplifier is 523 W.

[0007] Although traditional methods define the TMI effect threshold of high-power fiber laser systems from multiple perspectives such as time domain, frequency domain and spatial domain, the occurrence of TMI effect also contains a large amount of unstable mode phase information and mode frequency information, which cannot be captured by traditional methods.

[0008] Based on the limitations of current detection methods, a method to define the threshold of transverse mode instability effect from the perspective of phase and frequency is provided, which is of great significance for a deep understanding of the physical process of transverse mode instability effect, and also has practical significance for enriching the threshold evaluation method of transverse mode instability effect. Summary of the invention

[0009] In view of the technical problems existing in the prior art, the present invention proposes a method and a device for evaluating the transverse mode instability effect of a single-frequency fiber laser.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] In one aspect, the present invention provides a method for evaluating the lateral mode instability effect of a single-frequency fiber laser, comprising:

[0012] During the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is monitored in real time, wherein the frequency noise Phase noise The relationship is ;

[0013] The situations in which the mode instability effect and the threshold of the mode instability effect of a single-frequency fiber laser are evaluated based on the change amplitude of the frequency noise or phase noise of the single-frequency laser include:

[0014] When the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, it indicates that the single-frequency fiber laser has a transverse mode instability effect;

[0015] The TMI threshold is the output power of the single-frequency laser corresponding to the first occurrence of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser being degraded relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser over the entire frequency range and the average difference is greater than 3 dB.

[0016] In the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser remains consistent in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and does not degrade, it means that the single-frequency fiber laser has not yet experienced transverse mode instability.

[0017] In another aspect, a device for evaluating the instability effect of the transverse mode of a single-frequency fiber laser is provided, comprising:

[0018] The monitoring device is used to monitor the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser in real time during the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser. Phase noise The relationship is ;

[0019] An evaluation unit, for evaluating a mode instability effect of a single-frequency fiber laser and a threshold of the mode instability effect based on a variation amplitude of a frequency noise or a phase noise of the single-frequency laser, comprising:

[0020] When the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, it indicates that the single-frequency fiber laser has a transverse mode instability effect;

[0021] The output power of the single-frequency laser corresponding to the first occurrence of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades over the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and the average difference is greater than 3dB is the TMI threshold.

[0022] Compared with the prior art, the technical effects of the present invention are:

[0023] The method for evaluating the transverse mode instability effect of a single-frequency fiber laser described in the present invention is compatible with the traditional transverse mode instability effect threshold evaluation method and can be simultaneously applied to high-power single-frequency fiber lasers.

[0024] The present invention has universality and can be widely applied to the evaluation of transverse mode instability effects and thresholds in various types of single-frequency fiber lasers (such as single-frequency fiber laser amplifiers).

[0025] The present invention evaluates the threshold of the transverse mode instability effect in a high-power single-frequency fiber laser from the dimension of frequency noise or phase noise information of the single-frequency laser, which has important reference significance for noise-sensitive application fields.

[0026] The evaluation method provided by the present invention has high sensitivity to the TMI effect, does not require the single-frequency amplifier to work under the strong transverse mode instability effect, and can better protect the system safety and stable operation.

[0027] In summary, the present invention has important application value in the field of high-power lasers, especially in the field of transverse mode instability effect threshold evaluation and overall system performance optimization for high-power single-frequency fiber laser amplifiers. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0029] Figure 1 This is a typical example diagram of using the timing fluctuation of the output laser to determine the TMI threshold;

[0030] Figure 2 This is a typical example diagram of using the Fourier change spectrum of the output laser timing to determine the TMI threshold;

[0031] Figure 3 This is a typical example diagram of using the output laser beam quality degradation to determine the TMI threshold, where Figure 3 (a) is an example of the focal spot of the output laser when the TMI effect does not occur. Figure 3 (b) is an example of the focal spot of the output laser when the TMI effect occurs;

[0032] Figure 4 It is a structural schematic diagram of a monitoring device for real-time monitoring of the frequency noise or phase noise of a single-frequency laser output by a single-frequency fiber laser;

[0033] Figure 5 The following is a graph showing the frequency noise test results of the output laser of an example single-frequency fiber laser at different power levels. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Since the transverse mode instability effect (TMI effect) in a single-frequency fiber laser contains a large amount of unstable mode phase information and mode frequency information, the phase / frequency noise of the single-frequency laser output by the single-frequency fiber laser system can be used to determine whether the TMI effect occurs in the single-frequency fiber laser system. Furthermore, the TMI effect threshold of the system can be defined by detecting the phase noise / frequency noise of the output single-frequency laser.

[0036] An embodiment of the present invention provides a method for evaluating the transverse mode instability effect of a single-frequency fiber laser, comprising:

[0037] During the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is monitored in real time, wherein the frequency noise Phase noise The relationship is ;

[0038] The situations in which the mode instability effect and the threshold of the mode instability effect of a single-frequency fiber laser are evaluated based on the change amplitude of the frequency noise or phase noise of the single-frequency laser include:

[0039] When the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, it indicates that the single-frequency fiber laser has a transverse mode instability effect;

[0040] The output power of the single-frequency laser corresponding to the first occurrence of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades over the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and the average difference is greater than 3dB is the TMI threshold.

[0041] Before the transverse mode instability effect occurs in the single-frequency fiber laser, that is, when the transverse mode instability effect has not yet occurred in the single-frequency fiber laser, the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser and the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser are consistent in the entire frequency range or the difference is within a very small range (the difference can be ignored). Once the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, the transverse mode instability effect occurs in the single-frequency fiber laser, and the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser will fluctuate significantly in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, that is, the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is significantly different from the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser in the entire frequency range. Furthermore, when the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser first appears, the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser degenerates in the entire frequency range and the average difference is greater than 3dB (that is, 10log(P output / Ps eed )>3dB) is the TMI threshold.

[0042] An embodiment of the present invention provides a device for evaluating the lateral mode instability effect of a single-frequency fiber laser, comprising:

[0043] The monitoring device is used to monitor the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser in real time during the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser. Phase noise The relationship is ;

[0044] An evaluation unit, for evaluating a mode instability effect of a single-frequency fiber laser and a threshold of the mode instability effect based on a variation amplitude of a frequency noise or a phase noise of the single-frequency laser, comprising:

[0045] In the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is consistent with the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser in the entire frequency range and no degradation occurs (strictly speaking, there is no absolute consistency, so the consistency here means that the difference between the frequency noise or phase noise of the single-frequency laser and the frequency noise or phase noise of the seed laser is very small and can be ignored, and technicians in this field can judge or select the threshold of the difference based on experience or common sense), it means that the single-frequency fiber laser has not yet experienced transverse mode instability effect.

[0046] When the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is no longer consistent with the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser in the entire frequency range but degenerates, it means that the single-frequency fiber laser has a transverse mode instability effect;

[0047] The output power of the single-frequency laser corresponding to the first occurrence of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades over the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and the average difference is greater than 3dB is the TMI threshold.

[0048] Reference Figure 4 A monitoring device for real-time monitoring of the frequency noise or phase noise of a single-frequency laser output by a single-frequency fiber laser comprises a single-frequency fiber laser 1, a high-power reflector 3, a power meter 4, a focusing lens 5, a single-mode fiber 6, a circulator 7, a 3×3 coupler 8, a delay fiber 9, two Faraday rotators 10, and three photodetectors 11.

[0049] The single-frequency laser 2 output by the single-frequency fiber laser 1 is incident on the high-power reflector 3, most of the power of the single-frequency laser is reflected to the power meter 4 for measuring the output power of the single-frequency laser, and the remaining small part of the power of the single-frequency laser is transmitted through the high-power reflector 3 and output to the focusing lens 5, and is coupled into the single-mode optical fiber 6 by the focusing lens 5. The single-mode optical fiber 6 is connected to the first port of the circulator 7, the second port of the circulator 7 is connected to the first port of the first side of the 3×3 coupler 8, the second port of the circulator 7 is connected to a photodetector 11, and the second port and the third port of the first side of the 3×3 coupler 8 are respectively connected to another two photodetectors 11; the first port on the second side of the 3×3 coupler 8 is connected to the delay optical fiber 9, and the other end of the delay optical fiber 9 is connected to a Faraday rotator 10; the second port on the second side of the 3×3 coupler 8 is vacant; the third port on the second side of the 3×3 coupler 8 is connected to another Faraday rotator 10.

[0050] use Figure 4 The monitoring device shown, the optical phase demodulator based on the unbalanced Michelson fiber interferometer combined with the phase generated carrier technology (PGC) is a benchmark test method for measuring the frequency noise of a single-frequency fiber laser. The test principle is that when the laser signal passes through the unbalanced interferometer, the fluctuation of the optical frequency causes a phase change, which is converted into a change in the intensity of the interferometer output light field. The output signal of the interferometer is converted into an electrical signal by a photodetector and then demodulated by a phase demodulator based on the PGC scheme. The demodulated interference phase term is the phase noise of the laser and the demodulation interference system. The demodulation result is analyzed by a spectrum analyzer, and the frequency noise can be obtained by calculating the demodulated interference phase term.

[0051] In order to verify the effectiveness of the present invention, the evaluation method proposed in the present invention is described by taking the experimental test results of a single-frequency fiber laser with a known TMI threshold of 202 W as an example.

[0052] Specifically, it includes:

[0053] Real-time monitoring of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser.

[0054] based on Figure 4 The monitoring device shown is used to monitor the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser in real time. Since the frequency noise and phase noise of the system differ by only 2π, the frequency noise Phase noise The relationship is .

[0055] Reference Figure 5 , Figure 5This is a test result diagram of the frequency noise of the output laser of an example single-frequency fiber laser at different power levels. When the output power of the single-frequency laser is 140 W and 177 W, the TMI effect does not occur in the single-frequency fiber laser, and the frequency noise of the output laser is almost consistent with the frequency noise of the seed laser. However, when the output power is greater than 202 W, the TMI effect in the system will cause the frequency noise of the output single-frequency laser to rapidly increase. It can be seen that when the single-frequency fiber laser does not have the TMI effect, the frequency noise of the output single-frequency laser is almost consistent with the seed laser. After the TMI effect occurs, due to the excitation of high-order modes accompanied by a large number of high-order mode random phase generation, inter-mode phase matching, inter-mode phase coupling, inter-mode frequency crosstalk and other physical processes, the frequency noise of the output single-frequency laser will be significantly degraded. When the degradation amplitude of the frequency noise of the output single-frequency laser relative to the frequency noise of the seed laser is greater than the set threshold (that is, significant degradation occurs). It can be seen that the TMI threshold of the single-frequency fiber laser can be determined by real-time monitoring of the frequency noise degradation (or phase noise change) of the output single-frequency laser during the process of increasing the output power of the single-frequency laser, that is, when the average degradation amplitude of the frequency noise of the output single-frequency laser relative to the frequency noise of the seed laser in the entire frequency range is greater than 3 dB, the output power at this time is defined as the TMI threshold of the single-frequency fiber laser. For this example, based on the evaluation method proposed in the present invention, the TMI threshold of the single-frequency fiber laser can be obtained to be 202 W, which is consistent with the actual situation, proving the effectiveness of the present invention in evaluating the TMI threshold of the single-frequency fiber laser based on the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser.

[0056] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0057] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for evaluating the instability effect of the transverse mode of a single-frequency fiber laser, characterized in that: include: During the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is monitored in real time, wherein the frequency noise Phase Noise The relationship is ; The situations in which the mode instability effect and the threshold of the mode instability effect of a single-frequency fiber laser are evaluated based on the change amplitude of the frequency noise or phase noise of the single-frequency laser include: When the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, it indicates that the single-frequency fiber laser has a transverse mode instability effect; The output power of the single-frequency laser corresponding to the first occurrence of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades over the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and the average difference is greater than 3dB is the TMI threshold.

2. The method for evaluating the transverse mode instability effect of a single-frequency fiber laser according to claim 1, characterized in that: In the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser remains consistent in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and does not degrade, it means that the single-frequency fiber laser has not yet experienced transverse mode instability.

3. A device for evaluating the instability effect of the transverse mode of a single-frequency fiber laser, characterized in that include: The monitoring device is used to monitor the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser in real time during the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser. Phase Noise The relationship is ; An evaluation unit, for evaluating a mode instability effect of a single-frequency fiber laser and a threshold of the mode instability effect based on a variation amplitude of a frequency noise or a phase noise of the single-frequency laser, comprising: When the output power of the single-frequency laser output by the single-frequency fiber laser is increased to a certain level, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades in the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser, it indicates that the single-frequency fiber laser has a transverse mode instability effect; The output power of the single-frequency laser corresponding to the first occurrence of the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser degrades over the entire frequency range relative to the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser and the average difference is greater than 3dB is the TMI threshold.

4. The device for evaluating the transverse mode instability effect of a single-frequency fiber laser according to claim 3, characterized in that: A monitoring device for real-time monitoring of frequency noise or phase noise of a single-frequency laser output by a single-frequency fiber laser, comprising a single-frequency fiber laser, a high-power reflector, a power meter, a focusing lens, a single-mode fiber, a circulator, a 3×3 coupler, a delay fiber, two Faraday rotators, and three photodetectors; The single-frequency laser output by the single-frequency fiber laser is incident on a high-power reflector, and most of the power of the single-frequency laser is reflected to a power meter for measuring the output power of the single-frequency laser. The remaining small power of the single-frequency laser is transmitted through the high-power reflector and output to a focusing lens, which is coupled into a single-mode optical fiber by the focusing lens. The single-mode optical fiber is connected to the first port of the circulator, the second port of the circulator is connected to the first port of the first side of the 3×3 coupler, the second port of the circulator is connected to a photodetector, and the second port and the third port on the first side of the 3×3 coupler are respectively connected to two other photodetectors; the first port on the second side of the 3×3 coupler is connected to a delay optical fiber, and the other end of the delay optical fiber is connected to a Faraday rotator; the second port on the second side of the 3×3 coupler is vacant; and the third port on the second side of the 3×3 coupler is connected to another Faraday rotator.

5. The device for evaluating the transverse mode instability effect of a single-frequency fiber laser according to claim 3 or 4, characterized in that: The evaluation unit is used to evaluate the mode instability effect and the threshold of the mode instability effect of the single-frequency fiber laser based on the change amplitude of the frequency noise or phase noise of the single-frequency laser, and also includes: during the output power increase process of the single-frequency laser output by the single-frequency fiber laser, if the frequency noise or phase noise of the single-frequency laser output by the single-frequency fiber laser is consistent with the frequency noise or phase noise of the seed laser injected into the single-frequency fiber laser in the entire frequency range and no degradation occurs, it means that the single-frequency fiber laser has not yet suffered from the transverse mode instability effect.

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