Online monitoring method and control system for transverse mode instability of fiber lasers

Abstract

This invention provides an online monitoring method and control system for transverse mode instability of fiber lasers, comprising the following steps: collecting cladding scattered light using a photodetector; converting the collected optical signal into a corresponding time-domain electrical signal, wherein the amplitude of the time-domain electrical signal is linearly related to the light intensity of the passive fiber cladding; transmitting the obtained time-domain electrical signal to a control board; performing low-pass filtering on the time-domain electrical signal through the control board; calculating the relative standard deviation P of the filtered time-domain electrical signal; and simultaneously retrieving the reference relative standard deviation K under normal and stable operating conditions of the fiber laser; calculating and determining eigenvalues; and performing closed-loop control for online monitoring and protection. This invention achieves early warning, accurate identification, and rapid protection of transverse mode instability in high-power fiber lasers without optical insertion loss, with precise point selection and anti-interference capabilities, high identification sensitivity, and fast response speed, ensuring the safe operation of the laser.

Description

Technical Field

[0001] This invention relates to the field of high-power fiber laser technology, and in particular to an online monitoring and control system for transverse mode instability of fiber lasers. Background Technology

[0002] With the widespread application of high-power fiber lasers in industrial processing, aerospace, scientific research, and other fields, the continuous improvement of single-fiber output power has become a core development direction for the industry. However, transverse mode instability is the core bottleneck limiting further improvements in the output power of single-mode fiber lasers.

[0003] The core cause of transverse mode instability is the thermo-induced refractive index grating formed by the interference between the fundamental mode and higher-order modes, which triggers rapid energy coupling and transfer between modes. When this effect occurs, the laser output will experience rapid mode coupling between the fundamental mode and higher-order modes. Accompanied by inter-mode interference, this not only leads to a sharp deterioration in the output beam quality, making it unsuitable for industrial processing and other applications, but can also cause thermal damage to fiber optic devices and even irreversible damage to the laser in severe cases. Therefore, online monitoring and closed-loop control of transverse mode instability are crucial.

[0004] Existing monitoring schemes for lateral mode instability have the following main drawbacks: 1. The response speed is slow, making it impossible to achieve early warning of unstable horizontal modes. In addition, the equipment cost is high, making it difficult to integrate into the laser unit for online monitoring. 2. The scheme based on fiber core output light splitting monitoring involves setting a beam splitter at the laser output end to collect part of the fiber core output light for time-domain electrical signal analysis. This introduces insertion loss, which affects the output efficiency of the laser. At the same time, the beam splitter increases the complexity of the overall structure, and the early signal characteristics of unstable transverse modes in the fiber core light are weak, resulting in insufficient monitoring sensitivity. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide an online monitoring method and control system for transverse mode instability of fiber lasers. The monitoring and control method of the present invention has no optical insertion loss, precise point selection and anti-interference, high identification sensitivity and fast response speed, and can realize early warning, accurate identification and rapid protection of transverse mode instability of high-power fiber lasers, ensuring the safe operation of lasers, while adapting to the needs of mass production and system integration.

[0006] The embodiments of the present invention are achieved through the following technical solutions: The online monitoring and control method for transverse mode instability of fiber lasers includes the following steps: Step S100: A photodetector is set on the passive fiber segment between the mode stripper and the laser output head of the fiber laser under test. The scattered light signal of the cladding of the passive fiber segment is collected by the photodetector. The collected scattered light signal is converted into a corresponding time-domain electrical signal. The amplitude of the time-domain electrical signal is linearly related to the light intensity of the passive fiber cladding, thus completing the signal acquisition. Step S200: The time-domain electrical signal obtained in step S100 is transmitted to the control board. The control board performs low-pass filtering on the time-domain electrical signal to filter out high-frequency noise and obtain the filtered time-domain electrical signal, thus completing the signal preprocessing. Step S300: Calculate the relative standard deviation P of the filtered time-domain electrical signal obtained in step S200, and simultaneously retrieve the reference relative standard deviation K of the fiber laser under normal and stable operating conditions. When the measured relative standard deviation P is 4 times or more higher than the benchmark relative standard deviation K, it is determined that the fiber laser has a transverse mode instability; otherwise, it is determined that the fiber laser is operating normally, and the characteristic value calculation and determination are completed. In step S400, when step S300 determines that the laser has experienced transverse mode instability, the laser's alarm module is immediately triggered to issue an alarm signal. At the same time, the control board sends a power reduction command to the laser's pump source, automatically reducing the output power of the pump source until the laser's operating state returns to stability, thus forming a closed-loop control for online monitoring and protection of transverse mode instability.

[0007] According to a preferred embodiment, in step S100, the photosensitive surface of the photodetector is directly opposite the cladding of the passive optical fiber segment, and the coating of the passive optical fiber segment is stripped off, with the stripping length matching the size of the photosensitive surface of the photodetector, to ensure efficient collection of scattered light.

[0008] According to a preferred embodiment, the mode stripper is a cladding light stripper at the output end of the fiber laser resonator, and the mode stripper has a stripping rate of not less than 30dB for the inner cladding light of the fiber. The passive fiber segment is a passive double-clad fiber that matches the mode field of the laser gain fiber to ensure mode matching and transmission efficiency of laser transmission.

[0009] According to a preferred embodiment, in step S200, the low-pass filtering process uses a Butterworth low-pass filter with a cutoff frequency of 10kHz.

[0010] According to a preferred embodiment, in step S300, the formula for calculating the relative standard deviation P is: RSTD = (STD / Mean) × 100% Where RSTD is the relative standard deviation P, STD is the standard deviation of the filtered time-domain electrical signal within a preset time window, and Mean is the average value of the signal within the same time-domain window.

[0011] According to a preferred embodiment, the duration of the preset time window is 10ms to 100ms, and the control board continuously samples the time-domain electrical signal at a fixed sampling frequency, updates the signal data in real time through a sliding time window, and continuously calculates the relative standard deviation P to achieve a fast response at the microsecond to millisecond level, thus meeting the real-time requirements of early warning of horizontal mode instability.

[0012] According to a preferred embodiment, the range of the reference relative standard deviation K is selected within the interval of 0%-X%; 4 times or more of the maximum reference value X% under normal operating conditions, i.e., 4X%, is taken as the threshold for judging the instability of the transverse mode; the filtered time-domain electrical signal is used with a 100ms sliding time window, and the relative standard deviation is calculated in real time and continuously using the formula RSTD=(STD / Mean)×100%, where STD is the standard deviation of the filtered time-domain electrical signal within the 100ms sliding time window, and Mean is the arithmetic mean of the signal within the 100ms sliding time window; The main control board compares the measured RSTD with the 4X% judgment threshold in real time. When the measured RSTD ≥ 4X%, it immediately determines that the laser has a transverse mode instability.

[0013] According to a preferred embodiment, the automatic reduction of the pump source output power includes the following modes: The continuous step adjustment mode continuously reduces the drive current of the pump source by a preset step value, and simultaneously monitors the relative standard deviation in real time. When the relative standard deviation falls back to within 3 times the reference relative standard deviation, the pump power reduction is stopped, and the current power is maintained to ensure stable operation. This ensures equipment safety while maximizing the continuity of processing. The threshold protection mode directly reduces the pump power to a preset safe power threshold, which is below 80% of the laser's transverse mode instability threshold power, in order to achieve rapid protection after transverse mode instability occurs and minimize the risk of equipment damage.

[0014] According to a preferred embodiment, the passive double-clad fiber is a passive double-clad fiber that matches the mode field of the gain fiber. The coating layer of the passive double-clad fiber is stripped off for a length of 6 mm to expose the inner cladding of the passive double-clad fiber as a scattered light acquisition window. The photodetector is a silicon-based photodetector. A narrow bandpass filter with a pump light cutoff depth ≥40dB is placed between the passive optical fiber under test and the photosensitive surface of the photodetector to filter out the scattering interference of the residual 976nm pump light and allow only the cladding scattered light of the 1080nm signal light to be incident on the photosensitive surface. The photosensitive surface of the photodetector is positioned directly opposite the scattered light acquisition window, with the distance between them controlled at 1.5 mm. The photodetector linearly converts the acquired scattered light signal into a time-domain electrical signal corresponding to the intensity of the 1080 nm signal light within the cladding.

[0015] An online monitoring and control system for transverse mode instability of fiber lasers includes: The fiber laser under test can be any of the following: 976nm pump-source fiber laser, 915nm pump-source fiber laser, 1018nm pump-source fiber laser, MOPA fiber laser, or oscillation-amplification integrated fiber laser.

[0016] According to a preferred embodiment, the fiber laser under test is a 976nm pump-source fiber laser; The fiber laser under test includes a 976nm pump source and, along with a forward pump combiner, a high-reflection grating, a ytterbium-doped active gain fiber, a low-reflection grating, a cladding stripper, a passive fiber segment, and a laser output head, arranged sequentially along the laser output direction; wherein, a photodetector is disposed on the passive fiber segment, and a filter is disposed between the passive fiber segment and the photodetector.

[0017] The technical solutions of the embodiments of the present invention have at least the following advantages and beneficial effects: The monitoring and control method of the present invention is equipped with a cladding stripper and a filter for stripping the residual cladding light from the previous stage, and collects cladding scattered light in the passive fiber segment, thereby eliminating the interference of residual pump light and stray light from the previous stage, and greatly improving the signal-to-noise ratio and recognition accuracy of the monitoring signal. Signal processing using low-pass filtering and relative standard deviation calculation can effectively eliminate the effects of power baseline drift and environmental interference, and has extremely high recognition sensitivity to the weak periodic fluctuations in the early stage of transverse mode instability; at the same time, setting the judgment threshold to more than 4 times the normal benchmark value can significantly reduce the probability of false judgment while ensuring high sensitivity. This invention requires no beam splitting modification to the main laser path, has no optical insertion loss, does not affect the output efficiency and beam quality of the laser, and features precise point selection, high identification sensitivity, and fast response speed. It enables early warning, accurate identification, and rapid protection of transverse mode instability in high-power fiber lasers, ensuring the safe operation of the laser, while also meeting the requirements of mass production and system integration. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart illustrating the online monitoring and control method for transverse mode instability of a fiber laser provided in an embodiment of the present invention; Figure 2 This is a time-domain schematic diagram of the electrical signal before the appearance of lateral mode instability, provided in an embodiment of the present invention. Figure 3 This is a time-domain schematic diagram of the electrical signal after the occurrence of lateral mode instability, provided in an embodiment of the present invention. Figure 4 This is a schematic diagram of the result data of a time-domain electrical signal after low-pass filtering, provided in an embodiment of the present invention. Figure 5 The frequency domain diagram is generated by performing a fast Fourier transform on the acquired time-domain electrical signal provided in the embodiments of the present invention. Detailed Implementation

[0020] To better understand and implement this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings.

[0021] In the description of this invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. Example

[0023] Please refer to Figures 1 to 5 A method for online monitoring and control of transverse mode instability in fiber lasers, specifically including the following steps: Step S100: A photodetector is set on the passive fiber segment between the mode stripper and the laser output head of the fiber laser under test. The scattered light signal of the passive fiber segment cladding is collected by the photodetector. The collected scattered light signal is converted into a corresponding time-domain electrical signal. The amplitude of the time-domain electrical signal is linearly related to the light intensity of the passive fiber cladding, thus completing the signal acquisition. Step S200: The time-domain electrical signal obtained in step S100 is transmitted to the control board. The control board performs low-pass filtering on the time-domain electrical signal to filter out high-frequency noise and obtain the filtered time-domain electrical signal, thus completing the signal preprocessing. Step S300: Calculate the relative standard deviation P of the filtered time-domain electrical signal obtained in step S200, and simultaneously retrieve the reference relative standard deviation K under the normal and stable operating state of the fiber laser. When the measured relative standard deviation P is 4 times or more higher than the benchmark relative standard deviation K, the fiber laser generation mode is determined to be unstable; otherwise, the fiber laser generation mode is determined to be normal operation, thus completing the characteristic value calculation and determination. In step S400, when step S300 determines that the laser has experienced transverse mode instability, the laser's alarm module is immediately triggered to issue an alarm signal. At the same time, the control board sends a power reduction command to the laser's pump source, automatically reducing the output power of the pump source until the laser's operating state returns to stability, thus forming a closed-loop control for online monitoring and protection of transverse mode instability.

[0024] Preferably, in step S100, the photosensitive surface of the photodetector is directly opposite the cladding of the passive optical fiber segment, and the coating of the passive optical fiber segment is stripped off, with the stripping length matching the size of the photosensitive surface of the photodetector, to ensure efficient collection of scattered light.

[0025] Preferably, the mode stripper is a cladding light stripper at the output end of the fiber laser resonator. The mode stripper has a stripping rate of not less than 30dB for the cladding light inside the fiber. It can strip the residual cladding light from the front stage output of the resonator, ensuring that the cladding light entering the subsequent passive fiber segment is generated only by the core-cladding light coupling caused by the transverse mode instability effect. The passive fiber segment is a passive double-clad fiber that matches the mode field of the laser gain fiber to ensure mode matching and transmission efficiency of laser transmission.

[0026] Preferably, in step S200, the low-pass filtering process uses a Butterworth low-pass filter with a cutoff frequency of 10kHz.

[0027] Preferably, in step S300, the formula for calculating the relative standard deviation P is: RSTD = (STD / Mean) × 100% Where RSTD is the relative standard deviation P, STD is the standard deviation of the filtered time-domain electrical signal within a preset time window, and Mean is the average value of the signal within the same time-domain window.

[0028] Preferably, the preset time window duration is 10ms to 100ms, and the control board continuously samples the time-domain electrical signal at a fixed sampling frequency. The signal data is updated in real time through a sliding time window, and the relative standard deviation P is continuously calculated to achieve a fast response at the microsecond to millisecond level, meeting the real-time requirements of early warning for horizontal mode instability.

[0029] Preferably, in this embodiment, the detected value of the relative standard deviation K is within the range of 0.10%-0.15%; four times the maximum reference value of 0.15% under normal operating conditions, i.e., 0.6%, is taken as the threshold for judging the instability of the lateral mode; the filtered time-domain electrical signal is used with a 100ms sliding time window, and the relative standard deviation is calculated continuously in real time using the formula RSTD=(STD / Mean)×100%, where STD is the standard deviation of the filtered time-domain electrical signal within the 100ms sliding time window, and Mean is the arithmetic mean of the signal within the 100ms sliding time window.

[0030] The main control board compares the measured RSTD with the 0.6% judgment threshold in real time. When the measured RSTD ≥ 0.6%, it immediately determines that the laser has experienced transverse mode instability.

[0031] Preferably, automatically reducing the output power of the pump source includes the following modes: The continuous step adjustment mode continuously reduces the drive current of the pump source by a preset step value, and simultaneously monitors the relative standard deviation in real time. When the relative standard deviation falls back to within 3 times the reference relative standard deviation, the pump power reduction is stopped, and the current power is maintained to ensure stable operation. This ensures equipment safety while maximizing the continuity of processing. The threshold protection mode directly reduces the pump power to a preset safe power threshold, which is below 80% of the laser's transverse mode instability threshold power. This enables rapid protection after transverse mode instability occurs, minimizing the risk of equipment damage.

[0032] Preferably, the passive double-clad fiber is a 20 / 400μm passive double-clad fiber that matches the mode field of the gain fiber. A 6mm length of the coating layer of the passive double-clad fiber is stripped off to expose the inner cladding of the passive double-clad fiber as a window for collecting scattered light. The photodetector uses a silicon-based photodetector with a response band of 400-1100nm. A narrow bandpass filter with a center wavelength of 1080nm, a full width at half maximum (FWHM) of 20nm, and a cutoff depth of ≥40dB for 976nm pump light is placed between the passive optical fiber under test and the photosensitive surface of the photodetector to filter out the scattering interference of residual 976nm pump light and allow only the cladding scattered light of the 1080nm signal light to be incident on the photosensitive surface. With the photosensitive surface of the photodetector facing the scattered light acquisition window and the distance between them controlled at 1.5 mm, the photodetector linearly converts the acquired scattered light signal into a time-domain electrical signal corresponding to the intensity of the 1080 nm signal light in the cladding.

[0033] An online monitoring and control system for transverse mode instability of fiber lasers includes: The fiber laser under test includes a 976nm pump source and, along with the laser output direction, a forward pump combiner, a high-reflection grating, a ytterbium-doped active gain fiber, a low-reflection grating, a cladding stripper, a passive fiber segment, and a laser output head, arranged sequentially. A photodetector is mounted on the passive fiber segment. The laser output head is a QBH output head, and a filter is placed between the passive fiber segment and the photodetector.

[0034] Working principle of the invention: This embodiment uses a 2200W-level pure forward-pumped ytterbium-doped fiber laser as the application object, and matches the method of this invention. The complete optical path sequentially passes through a 976nm pump source, a forward-pumped combiner, a high-reflectivity grating (center wavelength 1080nm, reflectivity ≥99.8%), a 20 / 400μm ytterbium-doped active gain fiber, a low-reflectivity grating (center wavelength 1080nm, reflectivity 10%), a high-power cladding stripper (mode stripper), a passive fiber segment, and a QBH output head.

[0035] After completing the laser optical path setup and core component selection, the hardware deployment of the monitoring system was completed first: a high-power cladding stripper with a cladding stripping rate ≥30dB for both 976nm pump light and 1080nm signal light was selected to ensure the removal of residual pump light and stray light from the resonant cavity output; between the mode stripper and the QBH output head, a 20 / 400μm passive double-clad fiber matching the gain fiber mode field was used, and the coating layer of this fiber section was stripped for 6mm to expose the inner cladding as a scattered light acquisition window; the photodetector used was a silicon photodetector with a response wavelength of 400-1100nm. A narrowband bandpass filter with a center wavelength of 1080 nm, a full width at half maximum (FWHM) of 20 nm, and a cutoff depth ≥40 dB for 976 nm pump light is placed between the passive optical fiber under test and the photosensitive surface of the detector. This filter can remove the scattering interference of residual 976 nm pump light, allowing only the cladding scattered light of the 1080 nm signal light to be incident on the photosensitive surface. The photosensitive surface of the photodetector is positioned directly opposite the scattered light acquisition window, with the distance between them controlled at 1.5 mm. The photodetector linearly converts the acquired scattered light signal into a time-domain electrical signal corresponding to the intensity of the 1080 nm signal light within the cladding.

[0036] The time-domain electrical signal output from the photodetector is transmitted to the laser main control board via a coaxial shielded cable. The time-domain electrical signal is continuously converted from analog to digital by the 16-bit ADC sampling module built into the main control board at a sampling frequency of 200kHz, satisfying the Nyquist sampling theorem and fully preserving the characteristics of the target signal. The sampled digital signal is preprocessed using a fourth-order Butterworth low-pass filter with a cutoff frequency of 10kHz to filter out high-frequency noise such as pump source switching ripple and environmental electromagnetic interference, while fully preserving the low-frequency periodic fluctuation signal caused by transverse mode instability, resulting in a time-domain electrical signal with excellent signal-to-noise ratio.

[0037] The laser is powered on and the baseline relative standard deviation is calibrated: within the safe power range of 0-2000W where no transverse mode instability occurs, 10 power gradients are set with a step size of 200W. The laser is stably operated for 3 seconds at each power point and the corresponding time-domain electrical signal is collected. The relative standard deviation (RSTD) at each power point is calculated. Finally, when the laser is operating stably without transverse mode instability, the relative standard deviation is stable within the range of 0.10%-0.15%. Four times the maximum baseline value of 0.15% under normal operating conditions, i.e., 0.6%, is taken as the threshold for judging transverse mode instability. During equipment operation, a 100ms sliding time window is used for the filtered time-domain electrical signal. The relative standard deviation is calculated in real time using the formula RSTD=(STD / Mean)×100%, where STD is the standard deviation of the filtered time-domain electrical signal within the 100ms sliding time window, and Mean is the arithmetic mean of the signals within the same time window. The main control board compares the measured RSTD with the 0.6% judgment threshold in real time. When the measured RSTD≥0.6%, it is immediately determined that the laser has experienced transverse mode instability.

[0038] This embodiment verifies through actual testing that when the laser output power is increased to 2200W, the cladding optical time-domain electrical signal exhibits significant periodic fluctuations, with a measured relative standard deviation of 0.81%, exceeding the 0.6% judgment threshold. The system then accurately triggers the determination of transverse mode instability.

[0039] In this embodiment, as Figure 5 As shown, to further verify the accuracy of the judgment, the time-domain electrical signals collected during this period were simultaneously subjected to Fast Fourier Transform to generate frequency domain graphs. During normal and stable operation, the frequency domain graphs showed no obvious discrete characteristic peaks, only a continuous low-frequency noise floor. However, when transverse mode instability occurred, multiple clear discrete frequency peaks appeared in the frequency domain graphs, with the frequency distribution concentrated in the 1kHz-6kHz range. This is consistent with the frequency domain characteristics of periodic fluctuations caused by the interference between the fundamental mode and higher-order modes of transverse mode instability, which further verifies the accuracy of the system's judgment.

[0040] When the system determines that lateral mode instability has occurred, the main control board immediately triggers the audible and visual alarm module and simultaneously reports a "lateral mode instability fault" on the laser's human-machine interface. Information such as the fault occurrence time, output power at the time of the fault, measured RSTD value, and operating parameters are synchronously stored in the main control board's storage module, supporting subsequent fault tracing and debugging. Simultaneously, an automatic pump power reduction operation is executed. This embodiment employs two protection modes, both based on the reference relative standard deviation calibrated in this scheme and the real-time RSTD monitoring results: Continuous step adjustment mode: Suitable for conventional industrial processing scenarios, ensuring equipment safety while maximizing processing continuity. The execution logic is as follows: the pump source drive current is continuously reduced by a preset step value, while the relative standard deviation is monitored in real time using a sliding time window. When the measured relative standard deviation falls to within three times the baseline relative standard deviation (i.e., ≤0.45%, corresponding to three times the maximum baseline value of 0.15% specified in this solution), the pump power reduction is immediately stopped, maintaining stable operation at the current power. In this embodiment, the preset step value is 20W / 10ms. The drive current of the 976nm pump source is continuously reduced by this step value, while the RSTD value is monitored in real time. When the pump power is reduced to 2000W, the measured RSTD falls to 0.13%, meeting the safety regression threshold requirement. The system immediately stops power reduction and locks the current power for stable operation.

[0041] Threshold Protection Mode: Suitable for applications with extremely high equipment safety requirements, this mode enables rapid protection after lateral mode instability occurs, minimizing the risk of thermal damage to fiber optic devices. The execution logic is as follows: Upon determining that lateral mode instability has occurred, the pump power is immediately reduced to a preset safe power threshold, which is below 80% of the laser's lateral mode instability threshold power. In this embodiment, the laser's lateral mode instability threshold power is 2200W, and the preset safe power threshold is 1760W (80% of the threshold power). Upon determining that lateral mode instability has occurred, the pump power is immediately reduced to 1760W and locked, while the RSTD value is continuously monitored using a sliding time window to ensure that the laser always operates within a safe range free of lateral mode instability.

[0042] The monitoring location selection in this embodiment is highly targeted and has strong anti-interference capabilities. This application selects the passive fiber segment between the mode stripper and the laser output head to collect cladding scattered light. The mode stripper has already stripped the residual cladding light from the previous stage. The change in cladding light at this location is only caused by core-cladding mode coupling due to transverse mode instability, eliminating the interference of residual light and stray light from the previous stage pump. The signal-to-noise ratio and recognition accuracy of the monitoring signal are greatly improved. At the same time, this monitoring method does not require beam splitting modification of the laser main optical path, has no optical insertion loss, does not affect the output efficiency and beam quality of the laser, has a simple structure, and is easy to integrate into the laser system.

[0043] The signal processing scheme is highly adaptable, combining high sensitivity and reliability. This invention addresses the time-domain periodic fluctuations caused by interference between the fundamental and higher-order modes during lateral mode instability. It employs a low-pass filtering and relative standard deviation calculation scheme. The relative standard deviation effectively eliminates the effects of power baseline drift and environmental interference, providing extremely high sensitivity to the subtle periodic fluctuations in the early stages of lateral mode instability. Simultaneously, by setting the judgment threshold to more than four times the normal benchmark value, it significantly reduces the probability of false positives while maintaining high sensitivity, thus balancing the real-time nature and reliability of the early warning system.

[0044] Full closed-loop protection control enhances equipment operational safety. This invention integrates closed-loop protection control logic, which immediately triggers an alarm and automatically reduces pump power upon detecting transverse mode instability. This rapidly suppresses further deterioration of transverse mode instability, preventing thermal damage to laser devices. It achieves full closed-loop control of "monitoring-determination-protection," effectively improving the operational safety and lifespan of high-power fiber lasers. It can be widely applied in various industrial and research-grade high-power continuous fiber lasers.

[0045] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

Claims

1. A method for online monitoring and control of transverse mode instability in fiber lasers, characterized in that, Specifically, the following steps are included: Step S100: A photodetector is set on the passive fiber segment between the mode stripper and the laser output head of the fiber laser under test. The scattered light signal of the cladding of the passive fiber segment is collected by the photodetector. The collected scattered light signal is converted into a corresponding time-domain electrical signal. The amplitude of the time-domain electrical signal is linearly related to the light intensity of the passive fiber cladding, thus completing the signal acquisition. Step S200: The time-domain electrical signal obtained in step S100 is transmitted to the control board. The control board performs low-pass filtering on the time-domain electrical signal to filter out high-frequency noise and obtain the filtered time-domain electrical signal, thus completing the signal preprocessing. Step S300: Calculate the relative standard deviation P of the filtered time-domain electrical signal obtained in step S200, and simultaneously retrieve the reference relative standard deviation K of the fiber laser under normal and stable operating conditions. When the measured relative standard deviation P is 4 times or more higher than the benchmark relative standard deviation K, it is determined that the fiber laser has a transverse mode instability; otherwise, it is determined that the fiber laser is operating normally, and the characteristic value calculation and determination are completed. In step S400, when step S300 determines that the laser has experienced transverse mode instability, the laser's alarm module is immediately triggered to issue an alarm signal. At the same time, the control board sends a power reduction command to the laser's pump source, automatically reducing the output power of the pump source until the laser's operating state returns to stability, thus forming a closed-loop control for online monitoring and protection of transverse mode instability.

2. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 1, characterized in that, In step S100, the photosensitive surface of the photodetector is facing the cladding of the passive optical fiber segment, and the coating of the passive optical fiber segment is stripped off. The stripping length matches the size of the photosensitive surface of the photodetector to ensure efficient collection of scattered light.

3. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 2, characterized in that, The mode stripper is a cladding light stripper at the output end of the fiber laser resonator, and the mode stripper has a stripping rate of not less than 30dB for the inner cladding light of the fiber. The passive fiber segment is a passive double-clad fiber that matches the mode field of the laser gain fiber to ensure mode matching and transmission efficiency of laser transmission.

4. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 1, characterized in that, In step S200, the low-pass filtering process uses a Butterworth low-pass filter with a cutoff frequency of 10kHz.

5. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 1, characterized in that, In step S300, the formula for calculating the relative standard deviation P is: RSTD = (STD / Mean) × 100% Where RSTD is the relative standard deviation P, STD is the standard deviation of the filtered time-domain electrical signal within a preset time window, and Mean is the average value of the signal within the same time-domain window.

6. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 5, characterized in that, The preset time window has a duration of 10ms to 100ms, and the control board continuously samples the time-domain electrical signal at a fixed sampling frequency. It updates the signal data in real time through a sliding time window and continuously calculates the relative standard deviation P to meet the real-time requirements of early warning for lateral mode instability.

7. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 6, characterized in that, The relative standard deviation K of the reference is selected within the range of 0%-X%. Four times or more of the maximum reference value X% under normal operating conditions, i.e., 4X%, is taken as the threshold for judging the instability of the transverse mode. The filtered time-domain electrical signal is used with a 100ms sliding time window, and the relative standard deviation is calculated in real time and continuously using the formula RSTD=(STD / Mean)×100%, where STD is the standard deviation of the filtered time-domain electrical signal within the 100ms sliding time window, and Mean is the arithmetic mean of the signal within the 100ms sliding time window. The main control board compares the measured RSTD with the 4X% judgment threshold in real time. When the measured RSTD ≥ 4X%, it immediately determines that the laser has a transverse mode instability.

8. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 1, characterized in that, The automatic reduction of pump source output power includes the following modes: The continuous step adjustment mode continuously reduces the drive current of the pump source by a preset step value, and simultaneously monitors the relative standard deviation in real time. When the relative standard deviation falls back to within 3 times the reference relative standard deviation, the pump power reduction is stopped, and the current power is maintained to ensure stable operation. This ensures equipment safety while maximizing the continuity of processing. The threshold protection mode directly reduces the pump power to a preset safe power threshold, which is below 80% of the laser's transverse mode instability threshold power, in order to achieve rapid protection after transverse mode instability occurs and minimize the risk of equipment damage.

9. The online monitoring and control method for transverse mode instability of a fiber laser according to claim 3, characterized in that, The passive double-clad fiber is a passive double-clad fiber that matches the mode field of the gain fiber. The coating layer of the passive double-clad fiber is stripped off to expose the inner cladding of the passive double-clad fiber as a scattered light acquisition window. The photodetector is a silicon-based photodetector. A narrow bandpass filter with a pump light cutoff depth ≥40dB is placed between the passive optical fiber under test and the photosensitive surface of the photodetector to filter out the scattering interference of residual pump light and allow only the cladding scattered light of the 1080nm signal light to be incident on the photosensitive surface. The photosensitive surface of the photodetector is positioned directly opposite the scattered light acquisition window, with the distance between them controlled at 1.5 mm. The photodetector linearly converts the acquired scattered light signal into a time-domain electrical signal corresponding to the intensity of the 1080 nm signal light within the cladding.

10. An online monitoring and control system for transverse mode instability of a fiber laser, characterized in that, The method for online monitoring and control of transverse mode instability in fiber lasers as described in any one of claims 1-9 includes: The fiber laser under test is any one of the following: 976nm pump-source fiber laser, 915nm pump-source fiber laser, 1018nm pump-source fiber laser, MOPA fiber laser, or oscillation-amplification integrated fiber laser.

11. The online monitoring and control system for transverse mode instability of a fiber laser according to claim 10, characterized in that, The fiber laser under test is a 976nm pump-source fiber laser; The fiber laser under test includes a 976nm pump source and, along with a forward pump combiner, a high-reflection grating, a ytterbium-doped active gain fiber, a low-reflection grating, a cladding stripper, a passive fiber segment, and a laser output head, arranged sequentially along the laser output direction; wherein, a photodetector is disposed on the passive fiber segment, and a filter is disposed between the passive fiber segment and the photodetector.

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