Detection method of mode degradation in fiber laser transmission links

Abstract

The present invention provides a method for detecting mode degradation in an optical fiber laser transmission link, comprising determining the theoretical transmission delay of each eigenmode in the optical fiber laser transmission link, and obtaining theoretical transmission delay reference values ​​under all possible mode transmission and coupling paths; constructing a periodically varying multimode interference field by performing multi-wavelength injection at the input end of the optical fiber laser transmission link, obtaining characteristic frequencies of the multimode interference field in the frequency domain, each characteristic frequency strictly corresponding to a mode transmission and coupling path and having a corresponding transmission delay; determining mode coupling events of different output modes in the optical fiber laser transmission link that cause beam quality degradation based on each characteristic frequency; comparing the characteristic frequency corresponding to each mode coupling event with the theoretical transmission delay of each eigenmode, determining the transmission and coupling path of the mode coupling event of each different output mode one by one, and tracing the location where the mode degradation occurs.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber lasers, and in particular to a method for detecting mode degradation in an optical fiber laser transmission link. Background Art

[0002] Achieving high-quality beam output is a key goal in the development of high-power fiber laser systems. Higher beam quality means the light field is closer to the diffraction limit, which helps improve laser brightness and, in turn, enhances the effectiveness of laser applications, such as higher processing efficiency and speed, and better workpiece quality. To improve power handling capacity, current fiber laser systems often use large-core-diameter, large-mode-area fibers. These fibers have a high number of operating transverse modes, making the mode components at the system output difficult to control and easily leading to beam quality degradation.

[0003] Fiber laser systems typically involve a combination of optical fiber devices, gain fibers, passive fibers, and other components, with each module together forming a complete laser transmission link. When a near-single transverse mode beam with high beam quality is transmitted in a link, it may couple to higher-order modes at any or multiple nodes in the link, resulting in mode degradation, which ultimately manifests as degradation of the output beam quality. Therefore, without destroying the complete laser transmission link, in order to address the issue of output beam quality degradation and improve beam quality, and to implement an effective beam quality improvement strategy, the key is to identify the main "nodes" that cause beam quality degradation in the link. This means conducting a "traceable" analysis of beam quality degradation and then implementing targeted mode control methods.

[0004] The output light field of a fiber link is essentially the superposition of the fiber's intrinsic transverse modes. The modal content of this superposition determines the final beam quality. Therefore, tracing the source of beam quality degradation is essentially tracing the source of mode degradation within the fiber, requiring the use of mode analysis techniques.

[0005] Currently, existing pattern analysis technologies mainly fall into two categories: numerical analysis-based pattern analysis technology and wavefront measurement-based pattern analysis technology. Numerical analysis-based pattern analysis technology primarily uses different algorithms to reconstruct the intensity of the output light spot and fit the pattern component information in the light field. This type of method has achieved high-precision and high-speed computational reconstruction. However, numerical analysis-based pattern analysis technology can only determine the objective components of higher-order modes based on the output state of the current light field. It cannot establish a connection with the actual structure of the laser transmission system, nor can it determine the main causes of beam quality degradation in the system. Furthermore, it cannot perform a "traceable" analysis of beam quality degradation, and can only understand the phenomenon, not the reason. Wavefront measurement-based pattern analysis technology also has similar shortcomings. The results of its pattern analysis cannot yet provide researchers with feedback guidance for targeted pattern control and optimization.

[0006] In summary, in fiber laser transmission links, there is a lack of an analytical method that can simultaneously quantify the output light field mode components, determine the causes of system beam quality degradation, and evaluate the mode control effect. This is a technical problem that technical personnel in this field urgently need to solve. Summary of the Invention

[0007] In view of the defects of the prior art, the present invention provides a method for detecting mode degradation in a fiber laser transmission link.

[0008] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0009] The present invention provides a method for detecting mode degradation in a fiber laser transmission link, comprising:

[0010] Determine the fiber laser transmission link to be tested;

[0011] Based on the actual optical path structure of the fiber laser transmission link, the theoretical transmission delay of each eigenmode in the fiber laser transmission link is determined, and the theoretical transmission delay reference values ​​under all possible mode transmission and coupling paths in the fiber laser transmission link are obtained;

[0012] By implementing multi-wavelength injection at the input end of the fiber laser transmission link to construct a periodically changing multi-mode interference field, the multi-mode interference field is detected and collected frame by frame at the output end of the fiber laser transmission link. The characteristic frequencies of the multi-mode interference field are obtained in the frequency domain. Each characteristic frequency strictly corresponds to a mode transmission and coupling path and has a corresponding transmission delay. Based on each characteristic frequency, the mode coupling events of different output modes in the fiber laser transmission link that cause beam quality degradation are determined.

[0013] The characteristic frequency corresponding to each mode coupling event is compared with the theoretical transmission delay of each eigenmode, and the transmission and coupling paths of the mode coupling events of different output modes are determined one by one, and the location where the mode degradation occurs is traced.

[0014] Furthermore, each characteristic frequency of the multimode interference field corresponds to a mode coupling event of a different output mode, and the characteristic frequency of the multimode interference field is positively correlated with the total transmission delay accumulated when the corresponding output mode is transmitted in the fiber laser transmission link.

[0015] Furthermore, there is no restriction on the type, structure, and optical device composition of the fiber laser transmission link. Without loss of generality, the fiber laser transmission link can be various types of fiber laser systems. Fiber laser systems generally involve a combination of fiber optic devices, gain fibers, passive fibers, and other fiber optic devices. The various fiber optic devices in the fiber laser system are interconnected to form a complete laser transmission link.

[0016] Furthermore, methods for constructing a periodically varying multimode interference field by performing multi-wavelength injection at the input end of a fiber laser transmission link include but are not limited to the following methods:

[0017] A single-mode broadband light source with a 3dB bandwidth of no less than 10nm is injected into the input end of the fiber laser transmission link to construct a periodically varying multimode interference field.

[0018] Alternatively, a single-mode tunable light source is injected into the input end of the fiber laser transmission link, with a wavelength tuning range of not less than 10 nm and a minimum wavelength tuning interval of not more than 0.1 nm, to construct a periodically varying multi-mode interference field.

[0019] Compared with the prior art, the present invention can produce the following beneficial technical effects:

[0020] The present invention provides a method for detecting mode degradation in a fiber laser transmission link, which can simultaneously quantitatively evaluate the output light field mode components and determine the cause of mode degradation. It can be used to measure mode characteristics, diagnose the cause of mode degradation, and evaluate the effect of mode control in a fiber laser transmission link. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. 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 any creative work.

[0022] Figure 1 Schematic diagram of mode coupling characteristics when mode degradation occurs inside a dual-section optical fiber transmission link in one embodiment;

[0023] Figure 2 Schematic diagram of an optical fiber transmission link and mode degradation detection system constructed in one embodiment;

[0024] Figure 3 For Figure 2 The fiber transmission link and the preliminary mode degradation detection system show the mode degradation detection and beam quality M 2 Factor measurement results, where (a) is the beam quality M 2 Factor measurement results, (b) is the frequency domain analysis result obtained by mode degradation detection. DETAILED DESCRIPTION

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clearly understood, the following drawings and detailed descriptions clearly illustrate the spirit of the present invention. After understanding the embodiments of the present invention, any person skilled in the art will be able to make changes and modifications based on the techniques taught by the present invention without departing from the spirit and scope of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to illustrate the present invention and are not intended to limit the present invention.

[0026] The propagation of eigenmodes in optical fibers involves both discrete and coupled modes. Ideally, when no mode coupling is present within a fiber laser transmission link, eigenmodes propagate losslessly and uncoupled along their respective paths, allowing the theoretical propagation delay of each eigenmode to be determined based on the structure of the fiber laser transmission link. When mode coupling occurs within a fiber laser transmission link, energy redistribution occurs between eigenmodes within the link, exciting new component modes. The propagation characteristics of these newly excited component modes are influenced by both the characteristics of the eigenmodes that preceded their generation and their own propagation characteristics. At the output of the fiber laser transmission link, these modes manifest as the combined effects of multiple propagation characteristics. Therefore, each mode component in the superposition field output by a fiber laser transmission link has its own distinct "generation-coupling-output" path, and the sources and propagation characteristics of each mode component are not identical. Therefore, the propagation characteristics of each mode component in the superposition field output by a fiber laser transmission link can be used to separate and trace their origins, allowing for specific analysis of each mode component. One simple and easy method is to analyze mode transmission characteristics and trace mode degradation based on the intermodal dispersion effect. Due to differences in propagation constants, the intrinsic modes of few-mode or multimode optical fibers have different mode group delays during transmission, resulting in different time delays when different-order modes arrive at the output of the optical fiber link. If time-domain signal detection is performed at the output of the fiber laser transmission link, separated signals with different delays can be measured. In theory, each signal corresponds to a mode component. During transmission, each mode component produces a multimode interference effect, which also appears as a coherent superposition of multiple modes at the output of the fiber laser transmission link. When interference occurs between any two modes, the characteristic frequency of the interference field is positively correlated with the "cumulative group delay difference" between the two modes. Since different mode components have different cumulative group delays, the interference field between any two mode components corresponds to different characteristic frequencies. Therefore, it is possible to separate signals of different mode components through time-domain signal detection, and to analyze mode characteristics through frequency-domain analysis. By building a fiber laser transmission link, detecting the periodically changing multimode interference field at the output end of the fiber laser transmission link, and demodulating the characteristic frequency of the multimode interference field in the frequency domain, each high-frequency component corresponds to a mode component other than the fundamental mode. The mode component at each characteristic frequency corresponds to a mode "generation-transmission-coupling" path. Combined with the actual structure of the fiber laser transmission link, the main source of each mode component can be obtained, and the mode component can be specifically analyzed and mode controlled.

[0027] Based on the above analysis, in one embodiment, a method for detecting mode degradation in a fiber laser transmission link is provided, comprising:

[0028] The present invention provides a method for detecting mode degradation in a fiber laser transmission link, comprising:

[0029] Determine the fiber laser transmission link to be tested;

[0030] According to the actual structure of the fiber laser transmission link, the theoretical transmission delay of each eigenmode in the fiber laser transmission link is determined, and the theoretical transmission delay reference values ​​under all possible mode transmission and coupling paths are obtained;

[0031] By implementing multi-wavelength injection at the input end of the fiber laser transmission link to construct a periodically changing multi-mode interference field, the multi-mode interference field is detected and collected frame by frame at the output end of the fiber laser transmission link. The characteristic frequencies of the multi-mode interference field are obtained in the frequency domain. Each characteristic frequency strictly corresponds to a mode transmission and coupling path and has a corresponding transmission delay. Based on each characteristic frequency, the mode coupling events of different output modes in the fiber laser transmission link that cause beam quality degradation are determined.

[0032] The characteristic frequency corresponding to each mode coupling event is compared with the theoretical transmission delay of each eigenmode, and the transmission and coupling paths of the mode coupling events of different output modes are determined one by one, and the location where the mode degradation occurs is traced.

[0033] Furthermore, each characteristic frequency of the multimode interference field corresponds to a mode coupling event of a different output mode, and the characteristic frequency of the multimode interference field is positively correlated with the total transmission delay accumulated when the corresponding output mode is transmitted in the fiber laser transmission link.

[0034] By using the method for detecting mode degradation in a fiber laser transmission link provided by the present invention, a corresponding traceability analysis process is performed on the characteristic frequency of each mode coupling event to determine the transmission and coupling paths of all output modes. This allows the locations where all mode degradation occurs in the fiber laser transmission link to be traced.

[0035] There is no restriction on the type, structure, and optical device composition of the fiber laser transmission link. Without loss of generality, the fiber laser transmission link can be various types of fiber laser systems. The fiber laser system usually involves a combination of fiber optic devices, gain fibers, passive fibers, and other fiber optic optical devices. The various fiber optic devices in the fiber laser system are interconnected to form a complete laser transmission link.

[0036] The present invention makes full use of the transmission characteristics of the optical fiber mode of the optical fiber laser transmission link. When each intrinsic mode is transmitted in the optical fiber laser transmission link, according to its mode characteristics, it has a certain mode group delay in each section of the optical fiber laser transmission link. When mode coupling occurs at a certain node position of the optical fiber laser transmission link, mode degeneration occurs, which stimulates the generation of the next section mode, and is transmitted in the next section with the transmission characteristics of the newly generated mode, inheriting the transmission delay of the previous section and accumulating new transmission delays. By analogy, the total transmission delay at the output end of the optical fiber laser transmission link is manifested as the accumulation of the transmission delay of each section.

[0037] The present invention is based on the transmission characteristics of the light beam in the fiber laser transmission link. If mode degeneration occurs in the fiber laser transmission link and the new high-order mode is generated, resulting in beam quality degradation, a transmission component with a "new mode group delay" characteristic will appear at the output end of the fiber laser transmission link. This "new mode group delay" characteristic allows this mode to be distinguished from other non-degraded modes.

[0038] In one embodiment, a method for detecting mode degradation in a fiber laser transmission link is used to detect and trace the mode degradation in a dual-section fiber transmission link.

[0039] In a two-section optical fiber transmission link formed by connecting two optical fibers, it is assumed that the lengths of the first and second optical fibers are L1 and L2 respectively. If at any position (L 1-2 ) mode degeneration occurs, then the non-degenerate mode component corresponds to the multimode interference field I(x,y,ω) (1) , the transmission component with the new mode group delay characteristic corresponds to the multimode interference field I(x,y,ω) (2) ,I(x,y,ω) (1) and I(x,y,ω) (2) As shown in the following formula:

[0040]

[0041] Among them, (x, y) is the spatial coordinate, ω is the frequency of the light wave, A j (x, y, ω) is the complex amplitude of the j-th mode related to spatial and optical frequency, j = 1, 2, ..., N; Δτ 1,j For LP 01 The frequency-dependent differential inter-mode group delay between the mode and the j-th higher-order mode (the new mode generated by degeneration); For LP 01 The initial phase difference between the mode and the j-th higher-order mode (the new mode generated by degeneration); L 1～2 is the distance from the mode degeneration position to the output end of the double-section optical fiber transmission link; Δω is the unit frequency shift.

[0042] When there is no mode degeneration inside the dual-section optical fiber transmission link and no transmission component with new mode group delay characteristics is generated, the output interference field only contains I(x, y, ω) (1) , excluding I(x,y,ω) (2) According to I(x,y,ω) (1) The expression of the output interference field is Δτ. 1,j (L1+L2): There is no other position with mode degeneration in the transmission path of the entire dual-section optical fiber transmission link.

[0043] When mode degeneration occurs inside a dual-section optical fiber transmission link, a mode component with a new mode group delay characteristic is generated, and the output interference field also contains I(x, y, ω) (1) and I(x,y,ω) (2) At this time, the characteristic frequency components of the output interference field increase, except for the characteristic frequency Δτ 1,j In addition to (L1+L2), a characteristic frequency Δτ is also generated 1,j ·L 1～2 , the characteristic frequency is given by L 1～2 Determined by the length L of the mode degeneration position from the output end of the double-section optical fiber transmission link 1～2 By determining the position where the mode degradation occurs in the dual-section optical fiber transmission link,

[0044] Furthermore, in a multi-section optical fiber transmission link formed by connecting multiple sections of optical fibers, it is assumed that the lengths of the first section of optical fiber to the Nth section of optical fiber are L1, L2, L3, ..., L N , if at any position in a multi-section optical fiber transmission link (L (N-1)～N ) mode degeneration occurs, then the non-degenerate mode component corresponds to the multimode interference field I(x,y,ω) (1) ; The transmission component with the new mode group delay characteristic corresponds to the multimode interference field I(x,y,ω) (2) 、I(x,y,ω) (3) ......I(x,y,ω) (N) ,as follows:

[0045]

[0046] Among them, (x, y) is the spatial coordinate; ω is the frequency of the light wave; A j (x, y, ω) is the complex amplitude of the j-th mode related to spatial and optical frequencies; Δτ 1,j For LP 01 The frequency-dependent differential inter-mode group delay between the mode and the j-th higher-order mode (the new mode generated by degeneration); For LP 01The initial phase difference between the mode and the j-th higher-order mode (the new mode generated by degeneration); L (N-1)～N is the distance from the mode degeneration position to the output end of the double-section optical fiber transmission link; Δω is the unit frequency shift.

[0047] When there is no mode degeneration within the multi-section optical fiber transmission link, no component with new mode group delay characteristics is generated, and the output interference field only contains I(x, y, ω) (1) , excluding I(x,y,ω) (2) 、I(x,y,ω) (3) ......I(x,y,ω) (N) , according to I(x,y,ω) (1) The expression of the output interference field is Δτ. 1,j (L1+L2), there is no other position with mode degeneration in the transmission path of the entire multi-section optical fiber transmission link.

[0048] When mode degeneration occurs within a multi-section optical fiber transmission link, a mode component with a new mode group delay characteristic is generated, and the output interference field also contains I(x, y, ω) (1) 、I(x,y,ω) (2) 、I(x,y,ω) (3) ......I(x,y,ω) (N) , at this time the characteristic frequency components of the output interference field increase, except for the characteristic frequency Δτ 1,j In addition to (L1+L2), a characteristic frequency Δτ is also generated 1,j ·L 1～2 , Δτ 1,j ·L 2～3 ,…,Δτ 1,j ·L (N-1)～N and other components, and the corresponding characteristic frequencies are respectively represented by L 1～2 , L 2～3 ,…,L (N-1)～N The mode degradation position is determined by the distance between each mode degradation position and the output end of the multi-section optical fiber transmission link. In this way, the position where each mode degradation occurs inside the multi-section optical fiber transmission link can be determined one by one.

[0049] There is no limitation on the type, structure, and optical device composition of the fiber laser transmission link described in the present invention. Without loss of generality, the fiber laser transmission link can be various types of fiber laser systems. The fiber laser system usually involves a combination of fiber optic devices, gain fibers, passive fibers, and other fiber optic optical devices. The various fiber optic devices in the fiber laser system are interconnected to form a complete laser transmission link.

[0050] When establishing a fiber laser transmission link, methods for generating a periodically varying multimode interference field in the fiber laser transmission link, based on the wavelength of the signal laser to be transmitted in the fiber laser transmission link, include: using a single-mode broadband light source to inject into the fiber laser transmission link, with the 3dB bandwidth of the broadband light source being not less than 10nm; or using a single-mode tunable light source to inject into the fiber laser transmission link, with a wavelength tuning range of not less than 10nm and a minimum wavelength tuning interval of not more than 0.1nm.

[0051] In a fiber laser transmission link, the multimode interference phenomenon is used to detect the mode coupling source in the fiber laser transmission link.

[0052] like Figure 1 FIG. 1 is a schematic diagram of mode coupling characteristics when mode degeneration occurs within a dual-section optical fiber transmission link according to an embodiment. In the dual-section optical fiber transmission link formed by connecting two optical fibers, it is assumed that the lengths of the first and second optical fibers are L1 and L2, respectively, and the two optical fibers have the same parameters and length.

[0053] First, LP is excited simultaneously at the input end of the first fiber section #1. 01 Mode and LP 11 mode. Due to intermodal dispersion, the two modes are transmitted at different group velocities in the first section of optical fiber #1, and thus have different transmission delays. The cumulative delays of the two modes reaching the length L1 of the first section of optical fiber #1 are and At this time, the expression of the multimode interference field formed by the interference of the two modes at the length L1 of the first section of optical fiber #1 is:

[0054]

[0055] Where I is the light field intensity; (x, y) is the spatial coordinate; z is the transmission distance; ω is the light wave frequency; Δω is the unit frequency shift; ψ is the field distribution of each mode; A is the amplitude of each mode; is the initial inter-mode phase difference; τ 11 #1 For LP 11 The total transmission delay accumulated when the mode is transmitted in the first section of optical fiber; τ 01 #1 For LP 01 The total transmission delay accumulated when the mode is transmitted in the first section of optical fiber. From the above formula, we can see that the modulation frequency of the intermodal interference is determined by the difference in the accumulated delay between the two That is to say, the LP at L1 can be obtained by demodulating the frequency of the intermodal interference. 11 Modular relative LP 01 By performing Fourier transform on the multimode interference field, the intermodal interference at the length L1 of the first section of optical fiber #1 is calculated, and the characteristic frequency peak that appears is LP11 The modulus component, whose amplitude represents the relative LP 01 The content of mold.

[0056] The first section of optical fiber #1 is connected to the second section of optical fiber #2 at the length L1 to form a double-section optical fiber transmission link. At the splicing point of the first section of optical fiber #1 and the second section of optical fiber #2, due to the external disturbance of non-ideal splicing, mode coupling occurs. At this time, the LP in the first section of optical fiber #1 01 Model and LP 11 The modes are coupled to the corresponding modes of the second section of fiber #2 in a specific ratio. Mode coupling realizes the mode excitation in the second section of fiber #2, generating a new LP 01 Model and LP 11 The newly generated modes continue to propagate towards the output end at their respective group velocities. When they reach the output end of the second fiber section #2, the total delays transmitted in the two fiber sections are inconsistent due to the different sources of the various mode components. This means that although only LP can be observed from the perspective of mode components, 01 and LP 11 Two modes, but their respective components are different. Intuitively speaking, LP 01 The sources of the mode include the excitation component at the starting end and the excitation component at the splicing point, LP 11 The same applies to the other modes. In this way, the modulation frequency of inter-mode interference becomes more complicated.

[0057] Section 1: LP excited at the starting end of fiber #1 11 Model and LP 01 The interference field expression of the mode is:

[0058]

[0059] Where I is the light field intensity; (x, y) is the spatial coordinate; z is the transmission distance; ω is the light wave frequency; Δω is the unit frequency shift; ψ is the field distribution of each mode; A is the amplitude of each mode; is the initial inter-mode phase difference; τ 11 #1 For LP 11 The total transmission delay accumulated when the mode is transmitted in the first section of optical fiber; τ 01 #1 For LP 01 The total transmission delay accumulated by the mode when it propagates in the first section of optical fiber. τ 11 #2 For LP 11 The total transmission delay accumulated when the mode is transmitted in the second section of optical fiber; τ 01 #2 For LP 01The total transmission delay accumulated when the mode is transmitted in the second section of optical fiber. From the above formula, we can see that the modulation frequency of the intermodal interference is determined by the difference in the accumulated delay between the two. By Fourier transforming the interference field, the corresponding LP 11 The characteristic frequency peak of the mode corresponds to a larger differential delay value, and its amplitude represents the relative LP 01 The content of mold.

[0060] LP excited at the splicing point of the first section of fiber #1 and the second section of fiber #2 11 Model and LP 01 The interference field expression of the mode is:

[0061]

[0062] Where, I is the light field intensity; (x, y) is the spatial coordinate; z is the transmission distance; ω is the light wave frequency; Δω is the unit frequency shift; ψ is the field distribution of each mode; A is the amplitude of each mode; is the initial inter-mode phase difference; τ 11 #2 For LP 11 The total transmission delay accumulated when the mode is transmitted in the second section of optical fiber; τ 01 #2 For LP 01 The total transmission delay accumulated when the mode is transmitted in the second section of optical fiber. From the above formula, we can see that the modulation frequency of the intermodal interference is determined by the difference in the accumulated delay between the two. By Fourier transforming the interference field, the corresponding LP 11 The characteristic frequency peak of the mode corresponds to a smaller differential delay value, and its amplitude represents the relative LP 01 The content of mold.

[0063] In another embodiment, the Figure 2 The optical fiber transmission link and its mode degradation detection system shown in the figure include a signal laser source 1, a detection light source 2, a wavelength division multiplexer 3, a mode field adapter 4, and an energy transmission fiber 5. The mode degradation detection system includes a short-focus lens 6, a first beam splitter 7, a second high-reflection mirror 8, a third high-reflection mirror 9, a beam quality analyzer 10, a second beam splitter 11, a power meter 12, a long-focus lens 13, a fourth high-reflection mirror 14, a long-wave pass filter 15, a fifth high-reflection mirror 16, and a camera 17. The mode degradation detection system is used to perform mode degradation detection on the optical fiber transmission link to determine whether mode degradation has occurred in the optical fiber transmission link.

[0064] The signal laser (1080 nm) output by signal laser source 1 enters the optical fiber transmission link via the first input port of wavelength division multiplexer 3. The probe light (1120 nm-1130 nm) output by probe light source 2 enters the optical fiber transmission link via the second input port of wavelength division multiplexer 3. The signal laser and probe light are injected into the input port of mode field adapter 4 via the output port of wavelength division multiplexer 3. After passing through the output port of mode field adapter 4, they are simultaneously injected into power transmission fiber 5. At the output port of power transmission fiber 5, the signal laser and probe light are simultaneously emitted, collimated by short-focus lens 6 (focal length f1 = 50 mm), and then enter first beam splitter 7. The transmitted light from first beam splitter 7 is reflected by second high-reflection mirror 8 and third high-reflection mirror 9 in sequence before entering beam quality analyzer 10 for beam quality analysis. The reflected light from first beam splitter 7 is then reflected by second beam splitter 11. The transmitted light from second beam splitter 11 enters power meter 12 for power measurement. The reflected light from the first beam splitter 7 is used to characterize the real-time evolution of the transverse mode in the optical fiber transmission link. The reflected light from the first beam splitter 7 is focused by a telephoto lens 13 (focal length f2 = 500 mm), then sequentially reflected by a fourth high-reflection mirror 14 and a fifth high-reflection mirror 16 before entering the target surface of a camera 17. A long-wave filter 15 is placed between the fourth and fifth high-reflection mirrors 14, 16 to filter out the residual signal laser component in the beam, further preventing crosstalk between the signal laser mode and the detection light mode. The fiber optic dimensions of the input and output ends of the mode field adapter 4 are 10 / 125 μm and 20 / 400 μm, respectively. The power transmission fiber 5 is initially selected to be a passive fiber with dimensions of 20 / 400 μm, approximately 8 meters in length, and a nominal numerical aperture (NA) of 0.062.

[0065] It should be noted that Figure 2 The optical fiber transmission link and its mode degradation detection system shown in the figure are "lossless" spliced. The "lossless" splicing means that the optical fiber transmission link is aligned and spliced ​​according to the established splicing pattern without artificial deviation, that is, no mode generation is deliberately stimulated.

[0066] For the sake of Figure 2 The optical fiber transmission link and its mode degradation detection system have a preliminary understanding of the mode characteristics. First, a preliminary mode degradation detection and beam quality M are performed. 2 Factor measurement, the measurement results are as follows Figure 3 As shown. Figure 3 (a) The beam quality measurement results show that the output beam quality factor of the 20 / 400μm power transmission fiber is about 1.35. 2This value is relatively small, but there is still considerable room for improvement for 20 / 400μm fibers, indicating that a certain amount of higher-order modes are excited during laser transmission. To analyze the extent of higher-order mode excitation in the system, probe light ranging from 1110nm to 1130nm was injected into the system for mode degradation testing. Figure 3 The detection results of (b) show that there is indeed obvious high-order mode excitation in the entire link of the transmission system, and there are two characteristic positions of high-order mode excitation, corresponding to high-order mode characteristic peak ① and high-order mode characteristic peak ②, and the contents of the two high-order mode components are -13.9dB and -19.9dB respectively. Figure 3 The illustration in (b) is the corresponding reconstruction result of the high-order mode phase and light intensity distribution. It can be clearly interpreted as the LP11 mode, and the intensity distributions of the two high-order modes tend to be orthogonal.

[0067] The detection method of mode degradation in the fiber laser transmission link proposed by the present invention can simultaneously detect the composition and position information of the high-order modes excited in the fiber laser transmission link in the system, and realize the accurate measurement of the high-order mode content and the precise positioning of the mode degradation position. For 20 / 400μm dual-mode optical fiber, when LP 11 When the mode is excited at the incident end face of the optical fiber, due to the LP 01 and LP 11 The mode has a certain differential group delay, and the output mode interference light field will produce an inherent interference frequency at the output end of the optical fiber. Figure 3 There are two high-order mode characteristic peaks in the detection results of (b), namely two inherent interference frequencies, indicating that the LP 11 There are two sources of excitation for the mode.

[0068] Based on this, we can preliminarily judge that the reasons for the induced mode degradation in the system include: (1) The first high-order mode excitation source, that is, the process of taper inside the mode field adapter (MFA), which excites the high-order mode during adiabatic diameter change due to process defects; (2) The second high-order mode excitation source, that is, the output pigtail of the mode field adapter (MFA) and the 20 / 400μm power transmission fiber excite the high-order mode at the fusion point. Since the generation of the high-order mode characteristic peak is due to the inherent differential group delay existing in the transmission of the fundamental mode and the high-order mode in the optical fiber, the interference frequency (that is, the corresponding horizontal coordinate of the high-order mode characteristic peak) is the accumulation of the differential group delay of each group of modes (fundamental mode and a specific high-order mode) along the transmission length of the optical fiber. Therefore, the longer the transmission length, the greater the interference frequency, which means that the LP excited farther away from the output end of the optical fiber is 11 The mold has a greater delay, corresponding to Figure 3 (b) shows a larger horizontal axis. Based on this, combined with the actual fiber laser transmission link situation, it is preliminarily judged that Figure 3(b) The characteristic peak ① of the mid- to high-order mode corresponds to the source of the second high-order mode excitation, and the characteristic peak ② of the high-order mode corresponds to the source of the first high-order mode excitation. Based on these test results, we can provide guidance for beam quality optimization strategies in fiber laser transmission systems, achieving targeted results.

[0069] Matters not covered by the present invention are known technologies.

[0070] The technical features of the above embodiments can 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.

[0071] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and such modifications and improvements are intended to fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be determined by the appended claims.

[0072] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for detecting mode degradation in a fiber laser transmission link, characterized in that: include: Determine the fiber laser transmission link to be tested; Based on the actual optical path structure of the fiber laser transmission link, the theoretical transmission delay of each eigenmode in the fiber laser transmission link is determined, and the theoretical transmission delay reference values ​​under all possible mode transmission and coupling paths in the fiber laser transmission link are obtained; By implementing multi-wavelength injection at the input end of the fiber laser transmission link to construct a periodically changing multi-mode interference field, the multi-mode interference field is detected and collected frame by frame at the output end of the fiber laser transmission link. The characteristic frequencies of the multi-mode interference field are obtained in the frequency domain. Each characteristic frequency strictly corresponds to a mode transmission and coupling path and has a corresponding transmission delay. Based on each characteristic frequency, the mode coupling events of different output modes in the fiber laser transmission link that cause beam quality degradation are determined. Compare the characteristic frequencies corresponding to each mode coupling event with the theoretical transmission delay of each eigenmode, determine the transmission and coupling paths of the mode coupling events for each different output mode one by one, and trace the location where the mode degradation occurs; The optical fiber laser transmission link is a double-section optical fiber transmission link formed by connecting two sections of optical fiber or a multi-section optical fiber transmission link formed by connecting three or more sections of optical fiber; In a two-section optical fiber transmission link, let the lengths of the first section and the second section be L1 and L2 respectively; if mode degradation occurs at any position in the two-section optical fiber transmission link, then the non-degraded mode component corresponds to the multimode interference field , the transmission component with the new mode group delay characteristic corresponds to the multimode interference field , and As shown in the following formula: ； in, is the spatial coordinate, ω is the light wave frequency, is related to space and light frequency. j The complex amplitude of the first-order mode, j =1,2,..., N ; For LP 01 Mode and j Frequency-dependent differential inter-mode group delay between high-order modes; For LP 01 Mode and j The initial phase difference between the first and second order modes; L 1~2 is the distance from the mode degeneration position to the output end of the double-section optical fiber transmission link; is the unit frequency shift; When there is no mode degeneration inside the double-section optical fiber transmission link and no transmission component with new mode group delay characteristics is generated, the output interference field only contains , excluding ;according to The expression of , at this time the characteristic frequency of the output interference field is , there is no other position with mode degeneration in the transmission path of the entire dual-section optical fiber transmission link; When mode degeneration occurs inside a dual-section optical fiber transmission link, a mode component with a new mode group delay characteristic is generated, and the output interference field also contains and , at this time the characteristic frequency components of the output interference field increase, except for the characteristic frequency In addition, the characteristic frequency The characteristic frequency is determined by the length L of the mode degeneration position from the output end of the optical fiber transmission link. 1~2 By determining the position where the mode degradation occurs in the dual-section optical fiber transmission link, 2. The method for detecting mode degradation in a fiber laser transmission link according to claim 1, characterized in that: Each characteristic frequency of the multimode interference field corresponds to a mode coupling event of a different output mode, and the characteristic frequency of the multimode interference field is positively correlated with the total transmission delay accumulated when the corresponding output mode is transmitted in the fiber laser transmission link.

3. The method for detecting mode degradation in a fiber laser transmission link according to claim 1, characterized in that: Methods for constructing a periodically varying multimode interference field by performing multi-wavelength injection at the input end of a fiber laser transmission link include: A single-mode broadband light source with a 3 dB bandwidth of no less than 10 nm is injected into the input end of the fiber laser transmission link to construct a periodically varying multimode interference field. Alternatively, a single-mode tunable light source is injected into the input end of the fiber laser transmission link, with a wavelength tuning range of not less than 10 nm and a minimum wavelength tuning interval of not more than 0.1 nm, to construct a periodically varying multi-mode interference field.

4. The method for detecting mode degradation in a fiber laser transmission link according to claim 1, wherein: In a multi-section optical fiber transmission link formed by connecting 3 or more sections of optical fiber, it is assumed that the lengths of the first section to the Nth section of optical fiber are L1, L2, L3, ..., L N , if at any position L in a multi-section optical fiber transmission link (N-1)~N If mode degeneration occurs, then the non-degenerate mode components correspond to the multimode interference field ; The transmission component with new mode group delay characteristics corresponds to the multimode interference field 、 ...... ,as follows: ； in, is the spatial coordinate; is the frequency of the light wave; is related to space and light frequency. j The complex amplitude of the first-order mode; For LP 01 Mode and j Frequency-dependent differential inter-mode group delay between high-order modes, j The higher-order modes are new modes generated by degeneration; For LP 01 Mode and j The initial phase difference between the first and second order modes; L (N-1)~N is the distance from the mode degeneration position to the output end of the double-section optical fiber transmission link; is the unit frequency shift; When there is no mode degeneration inside the multi-section optical fiber transmission link, no component with new mode group delay characteristics is generated, and the output interference field only contains , excluding 、 ...... ,according to The expression of , at this time the characteristic frequency of the output interference field is , there is no other position with mode degeneration in the transmission path of the entire multi-section optical fiber transmission link; When mode degeneration occurs within a multi-section optical fiber transmission link, a mode component with a new mode group delay characteristic is generated, and the output interference field also contains 、 、 ...... , at this time the characteristic frequency components of the output interference field increase, except for the characteristic frequency In addition, the characteristic frequency 、 、…、 , and the corresponding characteristic frequencies are respectively given by 、 ,…,L (N-1)~N The mode degradation position is determined by the distance between each mode degradation position and the output end of the multi-section optical fiber transmission link. In this way, the position where each mode degradation occurs inside the multi-section optical fiber transmission link can be determined one by one.

5. The method for detecting mode degradation in a fiber laser transmission link according to claim 1, wherein: The fiber laser transmission link includes a fiber laser system, in which various fiber optic devices are interconnected to form a complete laser transmission link.

6. The method for detecting mode degradation in a fiber laser transmission link according to claim 5, characterized in that: The optical fiber transmission link includes a signal laser source, a detection light source, a wavelength division multiplexer, a mode field adapter, and an energy transmission optical fiber; The signal laser output by the signal laser source enters the optical fiber transmission link through the first input port of the wavelength division multiplexer, and the detection light output by the detection light source enters the optical fiber transmission link through the second input port of the wavelength division multiplexer. The signal laser and the detection light are injected into the input end of the mode field adapter through the output port of the wavelength division multiplexer, and then synchronously injected into the energy transmission optical fiber after passing through the output end of the mode field adapter and output through the energy transmission optical fiber.

7. The method for detecting mode degradation in a fiber laser transmission link according to claim 6, characterized in that: Performing mode degradation detection on the optical fiber transmission link using a mode degradation detection system to determine whether mode degradation occurs in the optical fiber transmission link; At the output end of the energy transmission optical fiber, the signal laser and the detection light are emitted synchronously, and enter the first beam splitter after being collimated by the focal length of the short-focus lens. The transmitted light transmitted by the first beam splitter is reflected by the second high-reflection mirror and the third high-reflection mirror in sequence and then enters the beam quality analyzer for beam quality analysis; the reflected light reflected by the first beam splitter is reflected to the second beam splitter, and the transmitted light transmitted by the second beam splitter enters the power meter for power measurement; the reflected light reflected by the first beam splitter is used for real-time characterization of the transverse mode evolution characteristics in the optical fiber transmission link; the reflected light reflected by the first beam splitter is focused by the telephoto lens, and then reflected by the fourth high-reflection mirror and the fifth high-reflection mirror in sequence and then enters the target surface of the camera, and a long-wave pass filter is placed between the fourth high-reflection mirror and the fifth high-reflection mirror to filter out the residual signal laser components in the light beam.

8. The method for detecting mode degradation in a fiber laser transmission link according to claim 7, characterized in that: The optical fiber sizes of the input and output ends of the mode field adapter are 10 / 125 μm and 20 / 400 μm respectively. The energy transmission optical fiber is a passive optical fiber with a size of 20 / 400 μm, a length of 8 meters, and a nominal numerical aperture of 0.062.

Images