Method and device for evaluating TMI effect of single-frequency fiber laser based on line width

Abstract

The present invention proposes a method and device for evaluating the TMI effect of a single-frequency fiber laser based on line width, including: in the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, real-time monitoring of the line width of the single-frequency laser output by the single-frequency fiber laser; when the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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, and the output power of the single-frequency laser corresponding to the first occurrence of the line width of the single-frequency laser output by the single-frequency fiber laser fluctuating greater than or equal to 10% relative to the line width of the seed laser injected into the single-frequency fiber laser is the TMI threshold. The present invention has a high sensitivity to the TMI effect, does not require the single-frequency fiber laser 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 TMI effect of a single-frequency optical fiber laser based on line width. 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 degradation of the output laser beam quality to determine the TMI threshold. Figure 3 (a) is a schematic diagram showing that the beam quality of the output laser is stable at 1.25 when the TMI effect does not occur, and the corresponding output power is 500 W; Figure 3 (b) is a schematic diagram showing that after 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 thus determined 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 the TMI effect also contains a large amount of unstable mode phase information and mode frequency information, which cannot be captured by traditional methods. The interference spectrum measured by the delayed self-heterodyne method can reflect the short-term phase / frequency noise integral information, which will eventually be reflected as the line width change of the single-frequency laser.

[0008] Based on the limitations of current detection methods, a method to define the threshold of transverse mode instability effect from the perspective of line width 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 methods 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 device for evaluating the TMI effect of a single-frequency fiber laser based on linewidth.

[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 TMI effect of a single-frequency fiber laser based on line width, comprising:

[0012] In the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, the line width of the single-frequency laser output by the single-frequency fiber laser is monitored in real time;

[0013] The fluctuation amplitude of the line width of the single-frequency laser is used to evaluate the mode instability effect of the single-frequency fiber laser and the threshold of the mode instability effect, including:

[0014] When the output power of the single-frequency laser output by the single-frequency fiber laser increases to a certain level, if the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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 output power of the single-frequency laser corresponding to the first time when the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates by 10% or more relative to the line width of the seed laser injected into the single-frequency fiber laser is the TMI threshold.

[0016] During the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, if the line width of the single-frequency laser output by the single-frequency fiber laser remains stable and consistent with the line width of the seed laser injected into the single-frequency fiber laser, it means that the single-frequency fiber laser has not yet experienced the transverse mode instability effect.

[0017] On the other hand, a device for evaluating the TMI effect of a single-frequency fiber laser based on line width is provided, comprising:

[0018] A monitoring device, used for real-time monitoring of the line width of the single-frequency laser output by the single-frequency fiber laser during the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser;

[0019] An evaluation unit is used to evaluate the mode instability effect of a single-frequency fiber laser and the threshold of the mode instability effect based on the fluctuation amplitude of the line width of the single-frequency laser, comprising:

[0020] When the output power of the single-frequency laser output by the single-frequency fiber laser increases to a certain level, if the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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 time when the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates by 10% or more relative to the line width of the seed laser injected into the single-frequency fiber laser is the TMI threshold.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The method for evaluating the TMI effect of a single-frequency fiber laser based on line width described in the present invention is compatible with the traditional method for evaluating the threshold value of the transverse mode instability effect, 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 method for evaluating the threshold value of the transverse mode instability effect in a high-power single-frequency fiber laser described in the present invention can take into account the evaluation of the temporal coherence of the single-frequency laser output by the single-frequency fiber laser, which is of great significance for evaluating the overall performance of the system.

[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 a schematic diagram showing that the beam quality of the output laser is stable at 1.25 when the TMI effect does not occur; Figure 3 (b) is a schematic diagram showing that after 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;

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

[0033] Figure 5 This is a graph of the linewidth results of an example single-frequency fiber laser measured at different power levels based on the delayed self-heterodyne method. 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 mode phase information and mode frequency information, and the integral result of phase / frequency noise degradation will be reflected in the line width of the single-frequency laser, the line width of the single-frequency laser can be used to determine whether the TMI effect occurs in the single-frequency fiber laser system. Furthermore, from the perspective of the line width change of the output single-frequency laser, the TMI effect threshold of the single-frequency laser system can also be defined.

[0036] An embodiment of the present invention provides a method for evaluating the TMI effect of a single-frequency fiber laser based on line width, comprising:

[0037] In the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, the line width of the single-frequency laser output by the single-frequency fiber laser is monitored in real time;

[0038] The fluctuation amplitude of the line width of the single-frequency laser is used to evaluate the mode instability effect of the single-frequency fiber laser and the threshold of the mode instability effect, including:

[0039] When the output power of the single-frequency laser output by the single-frequency fiber laser increases to a certain level, if the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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 time when the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates by 10% or more relative to the line width of the seed laser injected into the single-frequency fiber laser 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 line width of the single-frequency laser output by the single-frequency fiber laser is consistent with the line width of the seed laser injected into the single-frequency fiber laser, 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 line width of the single-frequency laser output by the single-frequency fiber laser will fluctuate significantly relative to the line width of the seed laser injected into the single-frequency fiber laser. When the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates by more than or equal to 10% relative to the line width of the seed laser injected into the single-frequency fiber laser for the first time, the output power of the single-frequency laser corresponding to the TMI threshold is.

[0042] An embodiment of the present invention provides a device for evaluating the TMI effect of a single-frequency fiber laser based on line width, comprising:

[0043] A monitoring device, used for real-time monitoring of the line width of the single-frequency laser output by the single-frequency fiber laser during the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser;

[0044] An evaluation unit is used to evaluate the mode instability effect of a single-frequency fiber laser and the threshold of the mode instability effect based on the fluctuation amplitude of the line width 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 line width of the single-frequency laser output by the single-frequency fiber laser remains stable and consistent with the line width of the seed laser injected into the single-frequency fiber laser (strictly speaking, there is no absolute consistency, so the consistency here means that the difference between the line width of the single-frequency laser and the line width 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 increases to a certain level, if the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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;

[0047] The output power of the single-frequency laser corresponding to the first time when the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates by 10% or more relative to the line width of the seed laser injected into the single-frequency fiber laser is the TMI threshold.

[0048] Reference Figure 4A monitoring device for real-time monitoring of the line width of a single-frequency laser output by a single-frequency fiber laser is provided, comprising 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 Mach-Zehnder interferometer and a spectrum analyzer 11. 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 is reflected to the power meter 4, which is used to measure the output power of the single-frequency laser, and the remaining small part of the power is transmitted through the high-power reflector 3 and output to the focusing lens 5, which is coupled into the single-mode fiber 6 by the focusing lens 5. The single-mode fiber 6 is connected to the Mach-Zehnder interferometer, and the Mach-Zehnder interferometer outputs a beat frequency spectrum to the spectrum analyzer 11. The line width of the single-frequency laser is obtained based on the spectrum analyzer 11.

[0049] The Mach-Zehnder interferometer is composed of an input fiber coupler 7, an acousto-optic modulator 8, a delay fiber 9 and an output fiber coupler 10. One end of the input fiber coupler 7 and the output fiber coupler 10 is a beam combining end, and the other end has two beam splitting arms. The single-mode optical fiber 6 is connected to the beam combining end of the input fiber coupler 7, one beam splitting arm of the input fiber coupler 7 is connected to the acousto-optic modulator 8, and the other beam splitting arm is connected to a certain length of delay fiber 9 (such as about 2 km long). The other end of the acousto-optic modulator 8 and the other end of the delay fiber 9 are respectively connected to the two beam splitting arms of the output fiber coupler 10, and the beam combining end of the output fiber coupler 10 is connected to the spectrum analyzer 11. The Mach-Zehnder interferometer forms a beat frequency spectrum signal and injects the beat frequency spectrum signal into the spectrum analyzer for measurement and recording, and the line width of the single-frequency laser is obtained by analysis.

[0050] 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.

[0051] Specifically, it includes:

[0052] Real-time monitoring of the line width of the single-frequency laser output by the single-frequency fiber laser.

[0053] based on Figure 4 The monitoring device shown is used for real-time monitoring of the line width of the single-frequency laser output by the single-frequency fiber laser, and real-time monitoring of the line width change of the single-frequency laser output by the single-frequency fiber laser. The delayed self-heterodyne method is used here to measure the interference envelope of the output laser, that is, the interference spectrum is obtained by interfering the two beams of laser light that have passed through the delayed fiber and the two beams of laser light that have not passed through the delayed fiber. The line width of the interference spectrum can represent the laser line width.

[0054] The light field function of the output laser can be expressed as:

[0055] ;

[0056] If the output laser contains only the fundamental mode, the interference light intensity can be expressed as:

[0057] ;

[0058] If the output laser contains both fundamental mode and high-order mode (here LP 11 mode as an example), the interference light intensity can be expressed as:

[0059] ;

[0060] in, represents the amplitude of the light field, is any non-negative integer, representing The first order Bessel function The light field distribution of each mode is corresponds to a solution of a Bessel function, , Respectively time, The light field distribution of the fundamental mode at time , Respectively time, The light field distribution of the high-order mode with the strongest energy coupling in the TMI effect at the moment, Represents the plural, , Respectively time, The light field phase of the corresponding mode at the moment, Indicates light intensity, represents the modulo budget, represents taking the complex conjugate, Indicates the delay time. Represents the conjugate of all listed terms on the same side of an equation.

[0061] The amplitude of the higher-order modes excited by the TMI effect ( A 11 ) and phase ( Φ 11 ) are all random, so the interference light intensity also changes in real time, which will cause the measured interference light intensity to change randomly. Therefore, the interference spectrum measurement results obtained by the time Fourier transform of the light intensity will also be unstable, and the measured single-frequency laser line width will inevitably fluctuate.

[0062] Reference Figure 5 , Figure 5This is a graph of the linewidth results of an example single-frequency fiber laser measured based on the delayed self-heterodyne method at different power levels. When the power of the output single-frequency laser is greater than 202 W, that is, after the TMI effect appears in the single-frequency fiber laser, the linewidth test result of the output laser will fluctuate significantly. Specifically, when the single-frequency fiber laser does not have the TMI effect, the linewidth of the output single-frequency laser remains basically consistent with the linewidth of the injected seed laser, which is about 0.8 kHz; after the TMI effect appears in the single-frequency fiber laser, due to the random phase / frequency / amplitude changes brought by the high-order mode, the linewidth of the output single-frequency laser fluctuates significantly, and as the TMI effect strengthens, the amplitude of the linewidth fluctuation will also increase. The fluctuation of the linewidth also indicates that the temporal coherence of the output single-frequency laser has deteriorated. It can be seen that in the process of increasing the output power of a single-frequency laser, the TMI threshold of a single-frequency fiber laser can be determined by using the delayed self-heterodyne method to monitor the line width change of the output single-frequency laser in real time, that is, when the line width of the first output single-frequency laser fluctuates by more than 10% relative to the line width of the seed laser, the output power of the single-frequency fiber laser 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 a single-frequency fiber laser based on the line width of the single-frequency laser output by the single-frequency fiber laser.

[0063] 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.

[0064] 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.

[0065] 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 transverse mode instability effect of a single-frequency fiber laser based on line width, characterized in that: include: In the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, the line width of the single-frequency laser output by the single-frequency fiber laser is monitored in real time; The fluctuation amplitude of the line width of the single-frequency laser is used to evaluate the mode instability effect of the single-frequency fiber laser and the threshold of the mode instability effect, including: When the output power of the single-frequency laser output by the single-frequency fiber laser increases to a certain level, if the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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; When the line width of the single-frequency laser output by the single-frequency fiber laser first appears to fluctuate by 10% or more relative to the line width of the seed laser injected into the single-frequency fiber laser, the output power of the single-frequency laser corresponding to the fluctuation is the transverse mode instability effect threshold.

2. The method for evaluating the transverse mode instability effect of a single-frequency fiber laser based on line width according to claim 1, characterized in that: During the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser, if the line width of the single-frequency laser output by the single-frequency fiber laser remains stable and consistent with the line width of the seed laser injected into the single-frequency fiber laser, it means that the single-frequency fiber laser has not yet experienced the transverse mode instability effect.

3. A device for evaluating the transverse mode instability effect of a single-frequency fiber laser based on line width, characterized in that: include: A monitoring device, used for real-time monitoring of the line width of the single-frequency laser output by the single-frequency fiber laser during the process of increasing the output power of the single-frequency laser output by the single-frequency fiber laser; An evaluation unit is used to evaluate the mode instability effect of a single-frequency fiber laser and the threshold of the mode instability effect based on the fluctuation amplitude of the line width of the single-frequency laser, comprising: When the output power of the single-frequency laser output by the single-frequency fiber laser increases to a certain level, if the line width of the single-frequency laser output by the single-frequency fiber laser fluctuates relative to the line width 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; When the line width of the single-frequency laser output by the single-frequency fiber laser first appears to fluctuate by 10% or more relative to the line width of the seed laser injected into the single-frequency fiber laser, the output power of the single-frequency laser corresponding to the fluctuation is the transverse mode instability effect threshold.

4. The device for evaluating the lateral mode instability effect of a single-frequency fiber laser based on line width according to claim 3, characterized in that: The monitoring device for real-time monitoring of the line width of a single-frequency laser output by a single-frequency fiber laser comprises a single-frequency fiber laser, a high-power reflector, a power meter, a focusing lens, a single-mode fiber, a Mach-Zehnder interferometer and a spectrum analyzer. The single-frequency laser output by the single-frequency fiber laser is incident on the high-power reflector, most of the power is reflected to the power meter for measuring the output power of the single-frequency laser, and the remaining small part of the power is transmitted through the high-power reflector and output to the focusing lens, which is coupled into the single-mode fiber by the focusing lens. The single-mode fiber is connected to the Mach-Zehnder interferometer, and the Mach-Zehnder interferometer outputs a beat frequency spectrum to the spectrum analyzer. The line width of the single-frequency laser is obtained based on the spectrum analyzer.

5. The device for evaluating the transverse mode instability effect of a single-frequency fiber laser based on line width according to claim 4, characterized in that: The Mach-Zehnder interferometer is composed of an input end optical fiber coupler, an acousto-optic modulator, a delay optical fiber and an output end optical fiber coupler.

6. The device for evaluating the lateral mode instability effect of a single-frequency fiber laser based on line width according to claim 3, 4 or 5, characterized in that: The evaluation unit evaluates the mode instability effect and the threshold of the mode instability effect of the single-frequency fiber laser based on the fluctuation amplitude of the line width 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 line width of the single-frequency laser output by the single-frequency fiber laser remains stable and consistent with the line width of the seed laser injected into the single-frequency fiber laser, it means that the single-frequency fiber laser has not yet suffered from the transverse mode instability effect.

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