Method for measuring photoinduced darkening effect of double-clad gain fiber

Abstract

The invention provides a method for measuring the photoinduced darkening effect of a double-clad gain fiber, and relates to the field of special fibers. Power loss data of double-clad gain fibers with different lengths under specific conditions are acquired, a photoinduced darkening effect model is built, power change of each fiber is processed, normalization processing is performed on data after length influence is eliminated, an objective function is set, model parameter optimization is performed, and the optical fiber power loss data of the double-clad gain fibers with different lengths is obtained. Obtaining an optimal data reference table of different optical fiber lengths and corresponding powers, calculating the change percentage of the power of the optical fiber to be measured before and after photoinduced darkening in a preset wave band by using the model, comparing a measurement result with reference data, and if a difference value is smaller than an error threshold value, regarding the measurement result as an effective measurement result of the optical fiber to be measured; according to the invention, systematized steps are carried out, and the accuracy of data processing is improved, so that the accuracy and reliability of measuring the photoinduced darkening effect of the double-clad gain fiber are improved.

Description

Technical Field

[0001] The present invention relates to the field of special optical fibers, and in particular to a method for measuring the photo-darkening effect of a double-clad gain optical fiber. Background Art

[0002] The core of the double-clad gain fiber is doped with rare earth elements. When the pump light is reflected and transmitted in the inner cladding, it will pass through the core many times, so that the rare earth ions in the fiber core can fully absorb the pump light, thereby increasing the absorption rate of the pump light. Photo-darkening refers to the situation where the laser power of the rare-earth-doped fiber decreases over time. This phenomenon is common in gain fibers. Photo-darkening can cause permanent damage to the gain fiber, which will shorten the service life of the fiber laser that uses the gain fiber as the gain medium. The development of high-quality double-clad gain fiber materials has made high-power fiber lasers possible.

[0003] Double-clad gain fiber is an indispensable part of fiber lasers or amplifiers. It is one of the core components of fiber lasers and can achieve a series of laser outputs in the visible light, near-infrared to mid-infrared bands. The United States has listed fiber products as technology export controls to China, which has seriously affected the development of fiber lasers in my country. With Fenghuo Ruiguang Technology Co., Ltd., Changfei Optical Fiber and Cable Co., Ltd., Wuhan Ruixin Special Optical Fiber Co., Ltd., Wuhan Changjin Laser Technology Co., Ltd., and National University of Defense Technology, etc., they have invested a lot of research and development work, and related work has achieved certain breakthroughs. With the rapid development of double-clad gain fiber, how to accurately, simply and quickly measure its photo-darkening effect is of great significance to the manufacturing process of optical fiber and the service life of fiber lasers.

[0004] JJ Koponen and others from the Finnish company Liekki conducted a series of experiments to test and characterize the photo-darkening effect more effectively. They used a 633nm wavelength light source to test the additional loss of photo-darkening of ytterbium-doped optical fibers and found that the additional loss at 633nm was 71 times the signal wavelength. According to the research of many researchers at home and abroad, the repeatability of the fiber core pump test is better, which is very suitable as a measurement standard for photo-darkening. Cladding pumping is more suitable for photo-darkening research at different levels of population inversion. Summary of the invention

[0005] The main purpose of the present invention is to provide a method for testing the photo-darkening effect of double-clad gain fiber with simple optical path construction, simple operation steps, high test optical path stability, short test cycle, high test efficiency and accurate test results.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for measuring the photodarkening effect of a double-clad gain fiber, comprising the following steps: S1: Acquire and utilize a dataset of different fiber lengths and corresponding power losses; S2: Calculate the power change of each optical fiber after eliminating the effect of length, normalize the data after eliminating the effect of length, set up the objective function, and build a photo-darkening effect model. The formula is: ; In the formula, and are the initial power and the final power, is the attenuation coefficient, is the optical fiber length; S3: Use the gradient descent algorithm to minimize the objective function and obtain the optimal data reference table of different optical fiber lengths and corresponding powers, that is, the table of changes before and after attenuation; S4: according to S3, the optimal length of the optical fiber to be tested is obtained, which is recorded as L1; the optical fiber to be tested with a length of L1 is connected to the photo-darkening effect model to obtain the power result to be tested, and after passing the pump light for a long time, the power test result is obtained again; then the power change percentage of the preset band of the test optical fiber before and after the photo-darkening is calculated; S5: Find the corresponding data in the data reference table in S3 and subtract it from the measurement result in S4. If the difference is less than the error threshold, it is the effective measurement result of the optical fiber to be tested. Then, the effective measurement result is analyzed and sorted according to S4 to obtain the power attenuation change diagram of the optical fiber to be tested over time; In the preferred solution, in step S2, the relationship between the power loss and the length of the optical fiber is considered, and the relationship between the power loss and the length of the optical fiber is fitted using linear regression, thereby eliminating the effect of the length of each optical fiber.

[0007] In the preferred embodiment, in step S4, the power change percentage of the preset wavelength band of the test optical fiber before and after photo-darkening is calculated, and the formula is: ; Where A is the red light power before photodarkening, and B is the red light power obtained after photodarkening.

[0008] In a preferred embodiment, obtaining and utilizing a data set of different optical fiber lengths and corresponding power losses comprises the following steps: Connect multiple optical fibers to be tested to the test optical path in sequence, that is, fuse the two ends with the high-reflection grating and the low-reflection grating respectively, couple the light in the light source into the optical fiber to be tested, use the first pump light source to perform scanning test, record the value of the power meter after stabilization and turn off the first pump light source, save the data, and record the data as A; The second pump light source is turned on and lasts for a preset period of time. The power meter records the power change curve in real time and processes the data into time series data. The data is also processed into a curve of the change of the additional loss of optical fiber photo-darkening at the second wavelength with the pump time. The second wavelength is the wavelength band length of the second pump light source. Turn off and on the second pump light source, turn on the first pump light source, record the data after the power meter is stable, turn off the first pump light source, save the data, and record the data as B; Among them, the test of the second optical fiber to be tested can be carried out only after the test of one optical fiber to be tested is completed.

[0009] In the preferred embodiment, the data processing is a curve showing the variation of the additional loss of the second wavelength optical fiber photo-darkening with the pumping time, and also includes: sorting out the collected time series data and the corresponding power values, and comparing them with the standard time power curve for consistency. If they are within the preset error range, they are valid data.

[0010] In the preferred solution, the stability of the recording power meter is determined by drawing a power curve according to the power collected in real time and dividing it by using the 3δ principle.

[0011] In the preferred solution, the test optical fiber is fusion-spliced ​​by a 100P+ Fujikura fusion splicer.

[0012] A system for measuring the photodarkening effect of a double-clad gain fiber, comprising: A data acquisition module, for acquiring and utilizing data sets of different optical fiber lengths and corresponding power losses; A model module was built to calculate the power change of each optical fiber after eliminating the effect of length. After normalizing the data after eliminating the effect of length, an objective function was established to build a photo-darkening effect model. The formula is: ; In the formula, and are the initial power and the final power, is the attenuation coefficient, is the optical fiber length; A data reference table module is used to minimize the objective function using a gradient descent algorithm to obtain an optimal data reference table of different optical fiber lengths and corresponding powers; The optical fiber test module is used to connect the optical fiber to be tested with a length of L1 to the photo-darkening effect model to obtain the test result, that is, to calculate the power change percentage of the preset wavelength band of the test optical fiber before and after photo-darkening; The test result module is used to find the corresponding data in the data reference table in S3 and subtract it from the measurement result of S4. If the difference is less than the error threshold, it is the valid measurement result of the optical fiber to be tested. Then, the valid measurement result is analyzed and sorted according to S4 to obtain the power attenuation change diagram of the optical fiber to be tested over time.

[0013] The present invention provides a method and system for measuring the photo-darkening effect of a double-clad gain fiber, obtaining power loss data of double-clad gain fibers of different lengths under specific conditions (such as the same pump power, working environment, etc.), and building a mathematical model capable of describing the photo-darkening effect, and then processing the power change of each optical fiber to eliminate the direct influence of the optical fiber length on the power loss, normalizing the data after eliminating the influence of the length, and setting up an objective function, and then optimizing and adjusting the model parameters to reduce the value of the objective function, so as to obtain an optimal data reference table of different optical fiber lengths and corresponding powers, and using the model to calculate the percentage change of the power of the optical fiber to be tested in a preset band (such as a specific wavelength range) before and after photo-darkening, and comparing the measurement result with the reference data. If the difference is less than an error threshold, it is regarded as a valid measurement result of the optical fiber to be tested. This embodiment performs systematic steps, improves the accuracy of data processing, and thus improves the accuracy and reliability of measuring the photo-darkening effect of the double-clad gain fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be further described below in conjunction with the accompanying drawings and embodiments: Figure 1 It is a test light path diagram of the photo-darkening effect of the present invention; Figure 2 It is a flow chart of the photodarkening effect method of the present invention.

[0015] In the figure: light source 1; light source output fiber 4; WDM input fiber 3; WDM4; 916nm pump 5; combiner 6; high reflection grating 7; fiber to be tested 8; low reflection grating 9; CPS9; first filter 11; second filter 12; power meter 13. DETAILED DESCRIPTION

[0016] Example 1 like Figure 1-2 As shown, a method for measuring the photo-darkening effect of a double-clad gain fiber is proposed, comprising the following steps: S1: Obtain and utilize a dataset of different fiber lengths and corresponding power losses.

[0017] S2: Calculate the power change of each optical fiber after eliminating the effect of length, normalize the data after eliminating the effect of length, set up the objective function, and build a photo-darkening effect model. The formula is: ; In the formula, and are the initial power and the final power, is the attenuation coefficient, is the fiber length.

[0018] S3: Use the gradient descent algorithm to minimize the objective function and obtain the optimal data reference table of different optical fiber lengths and corresponding powers.

[0019] S4: Connecting the optical fiber to be tested with a length of L1 to the photo-darkening effect model to obtain the test result, that is, calculating the power change percentage of the preset wavelength band of the test optical fiber before and after photo-darkening.

[0020] S5: Look up the corresponding data in the data reference table in S3 and subtract it from the measurement result in S4. If the difference is less than the error threshold, it is the effective measurement result of the optical fiber to be tested. Then, the effective measurement result is analyzed and sorted according to S4 to obtain a power attenuation change diagram of the optical fiber to be tested over time.

[0021] Through the above steps, the present embodiment obtains the power loss data of double-clad gain fibers of different lengths under specific conditions (such as the same pump power, working environment, etc.), and builds a mathematical model that can describe the photo-darkening effect. Then, the power change of each optical fiber is processed to eliminate the direct influence of the optical fiber length on the power loss. The data after eliminating the length influence is normalized, and an objective function is established. Then, the model parameters are optimized and adjusted to reduce the value of the objective function to obtain the optimal data reference table of different optical fiber lengths and corresponding powers. The model is used to calculate the percentage change of the power of the optical fiber to be tested in a preset band (such as a specific wavelength range) before and after photo-darkening. The measurement result is compared with the reference data. If the difference is less than the error threshold, it is regarded as a valid measurement result of the optical fiber to be tested. The present embodiment performs systematic steps to improve the accuracy of data processing, thereby improving the accuracy and reliability of measuring the photo-darkening effect of the double-clad gain fiber.

[0022] In this embodiment, in the test method of the photo-darkening effect of the double-clad gain fiber, the initial length L1 of the gain fiber to be tested is 5 m, and the error threshold can be set to no more than 0.03*reference data according to the actual situation, that is: .

[0023] In this embodiment, the first pump light source is a 976 nm pump light source, the second pump light source is a 633 nm wavelength pump light source, and the preset time period is 7200 seconds.

[0024] like Figure 1As shown, the 633nm light source 1 used uses THORLABS KLS635, WDM, (1+1)*1 combiner 6, high-reflection grating 7, low-reflection grating 9 and CPS9, which are all customized devices; compared with the aging copy machine test method, the test cycle is long and the efficiency is low; and compared with the spatial optical path test method, the optical path construction is cumbersome and complicated, and the platform stability requirements are high. The constructed optical path is simple and easy to operate, with a short test cycle and high efficiency. In the present invention, the optical signal generated by the light source 1 is transmitted to the WDM4 through the light source output optical fiber 4, and then enters the optical fiber 8 to be tested through the combiner 6. The high-reflection grating 7 and the low-reflection grating 9 are used to reflect and transmit the optical signal respectively. The filters 11 and 12 are used to select light of a specific wavelength from the output signal for measurement. Finally, the power meter 13 measures the power of the output optical signal to evaluate the performance of the optical fiber 8 to be tested; the test methods for the photo-darkening effect of the double-clad gain fiber described are all all-fiber structures, and the test optical path can be completed by using optical fiber fusion.

[0025] In the preferred solution, in step S2, the relationship between the power loss and the length of the optical fiber is considered, and the relationship between the power loss and the length of the optical fiber is fitted using linear regression, thereby eliminating the effect of the length of each optical fiber.

[0026] In the optical fiber communication system, as the length of the optical fiber increases, the attenuation of the optical signal during transmission will also increase, resulting in an increase in power loss. In this embodiment, a linear regression model is used. For each optical fiber, according to its length, the predicted power loss value is calculated using the linear regression model to confirm the accuracy of the S4 measurement result, thereby indirectly estimating the power loss of the optical fiber caused by the photo-darkening effect, providing a theoretical reference for the long-term life and reliability of the optical fiber.

[0027] In a preferred embodiment, obtaining and utilizing a data set of different optical fiber lengths and corresponding power losses comprises the following steps: Connect multiple optical fibers to be tested to the test optical path in turn, that is, fuse the two ends with the high-reflection grating and the low-reflection grating respectively, couple the light in the light source into the optical fiber to be tested, use the first pump light source to perform a scanning test, record the value of the power meter after it stabilizes, turn off the first pump light source, and save the data, which is recorded as A.

[0028] The second pump light source is turned on and continues for a preset period of time. The power meter records the power change curve in real time and processes the data into time series data. At the same time, the data is processed into a curve of the change of the additional loss of optical fiber photo-darkening of the second wavelength with the pumping time. The second wavelength is the band length of the second pump light source.

[0029] Turn off and on the second pump light source, turn on the first pump light source, record the data after the power meter is stable, turn off the first pump light source, save the data, and record the data as B.

[0030] Among them, the test of the second optical fiber to be tested can be carried out only after the test of one optical fiber to be tested is completed.

[0031] Specifically: 1) Connect only one fiber at a time. Connect the fiber to be tested with a length of L1. Figure 1 In the optical path shown, the two ends of the optical fiber to be tested are fused with the high-reflection grating and the low-reflection grating respectively; first, the 976nm pump light source is turned on (the first pump light source), and the value after the power meter is stable is recorded, and the data is recorded as A; after the data recording is completed, the 976nm pump light source is turned off.

[0032] 2) Turn on the 633nm light source (the second pump light source), and use the power meter to record the power change curve in real time until 7200S, then turn off the 633nm light source; process the data into the percentage of photo-darkening power decrease over time; and at the same time, process the data into the curve of the additional loss of 633nm wavelength optical fiber photo-darkening versus pumping time.

[0033] 3) After turning off the 633nm band light source, turn on the 976nm band light source and record the data after the power meter stabilizes. The data is recorded as B. After the data recording is completed, turn off the 976nm pump light source.

[0034] 4) Calculate the percentage change in the power of the 976nm band of the test fiber before and after photo-darkening.

[0035] In the preferred embodiment, the power change percentage of the preset wavelength band of the test optical fiber before and after photo-darkening is calculated using the formula: ; Where A is the power before photodarkening, and B is the power obtained after photodarkening.

[0036] In the preferred embodiment, the data processing is a curve showing the variation of the additional loss of the second wavelength optical fiber photo-darkening with the pumping time, and also includes: sorting out the collected time series data and the corresponding power values, and comparing them with the standard time power curve for consistency. If they are within the preset error range, they are valid data.

[0037] In this embodiment, the sorted time series data and the corresponding power values ​​are compared with the standard time power curve for consistency. The standard time power curve can be derived based on previous experiments or theoretical models to describe the power variation of the optical fiber over time under specific conditions (such as pump power, working environment, etc.).

[0038] The preset error range is used to evaluate the consistency between the experimental data and the standard curve. The error range can be set according to factors such as experimental requirements and fiber characteristics.

[0039] In the preferred solution, the stability of the recording power meter is determined by drawing a power curve according to the power collected in real time and dividing it by using the 3δ principle.

[0040] In this embodiment, the power data collected in real time is associated with time, and a curve of power variation over time is drawn to intuitively display the power variation trend of the optical fiber under the action of pumping, such as the increase, stability or decrease of power. The collected power data is statistically analyzed to calculate the mean (μ) and standard deviation (σ) of the power. According to the 3δ principle, the power data is divided into normal data and abnormal data: μ-3σ≤power≤μ+3σ. The data within this range is considered normal, reflecting the normal power variation of the optical fiber under the action of pumping. The normal data is analyzed to extract useful information, such as the rate of power variation, stable power value, etc., so as to more effectively evaluate the photo-darkening effect of the double-clad gain fiber.

[0041] In the preferred solution, the test optical fiber is fusion-spliced ​​by a 100P+ Fujikura fusion splicer.

[0042] In order to describe the technical solution of the present invention more clearly, a specific optical path device is provided: (1) Use a 633nm red light source, 976nm pump source, WDM, (1+1)*1 beam combiner, high-reflection grating, low-reflection grating, CPS, 100P+ welding machine, cutting knife and power meter; (2) The implementation case is used to measure the photodarkening of Yb-doped double-clad optical fiber. The test fiber is a double-clad optical fiber produced by Nufern, USA. The core diameter is 20um, the cladding diameter is 400um, the cladding shape is octagonal, the core numerical aperture is 0.065±0.005, the cladding numerical aperture is ≥0.46, and the test fiber length is 5m.

[0043] A specific implementation method is provided, comprising the following steps: 1. Optical path construction: (1) Connecting the 633nm light source to the WDM: First, remove the protective layer of the jumper at the output end of the 633nm light source to expose the bare optical fiber, and then cut the bare optical fiber with a Fujikura cutter. Use the Fujikura cutter to cut the WDM input end, and then fuse and solidify the two cut optical fibers. After the optical fiber at the other input end of the WDM is cut, the optical fiber end face is aligned with the blackbody absorption cavity. This can be simply understood as fusing the optical fiber in Figure 1 with the optical fiber in Figure 4, and placing the optical fiber in Figure 3 into the blackbody absorption cavity after cutting.

[0044] (2) WDM is connected to the (1+1)*1 combiner: the output fiber 4 in the figure is fused with the input fiber 6 in the figure; the pump source output fiber 5 in the figure is fused with the input pump fiber 6 in the figure.

[0045] (3) The (1+1)*1 beam combiner is connected to the high-reflection grating: The output end 6 in the figure is fused with the output end 7 in the figure. It is necessary to pay attention to the directionality of the high-reflection grating.

[0046] (4) Connection between low-reflection grating and CPS: 9 and 10 in the figure are welded together, and attention should be paid to the directionality of the low-reflection grating.

[0047] (5) After the other end of the CPS is cut, it is aligned with the filter. At the same time, the power meter is placed at the rear end of the filter. The filter and the CPS fiber end face form a 45° angle. The optical path is completed.

[0048] (6) The above welding was performed using a Fujikura welding machine and a cutting knife; all the above devices were placed on an optical water-cooling platform, and the water temperature was set to 23°C.

[0049] 2. Testing steps for the photo-darkening effect of double-clad gain fiber: (1) First, cut the fiber to be tested to a length of L1, and then connect the fiber to 8 in the figure, that is, fuse the two ends of the fiber to be tested with the high-reflection grating and the low-reflection grating respectively; first, turn on the 976nm pump light source, and record the value after the power meter stabilizes, and the data is recorded as A; after the data recording is completed, turn off the 976nm pump light source.

[0050] (2) Turn on the 633nm light source, and use the power meter to record the power change curve in real time until 7200s, then turn off the 633nm light source; process the data into the percentage of photo-darkening power decrease over time; and at the same time, process the data into the curve of the additional loss of 633nm wavelength optical fiber photo-darkening versus pumping time.

[0051] (3) After turning off the 633nm band light source, turn on the 976nm band light source and record the data after the power meter stabilizes. The data is recorded as B. After the data recording is completed, turn off the 976nm pump light source.

[0052] (4) Calculate the percentage change in the power of the 976 nm band of the test fiber before and after photodarkening.

[0053] When in use, first, except for the optical fiber to be tested, the remaining devices in the optical path are fusion-connected with the optical fiber, except for the filter; then the optical fiber to be tested is connected to the test optical path, and the optical fiber to be tested is tested according to the operating steps. After the test is completed, a red light attenuation curve and a power loss percentage at 976nm are obtained.

[0054] The optical path used in this embodiment is easy to build and takes less time than direct laser copying. It can conveniently test the photo-darkening of double-clad gain fibers and more accurately perform long-term reliability testing and life evaluation of double-clad gain fibers.

[0055] Example 2 In combination with Example 1, a system for measuring the photo-darkening effect of a double-clad gain fiber is provided, comprising: The data acquisition module is used to acquire and utilize data sets of different optical fiber lengths and corresponding power losses.

[0056] A model module was built to calculate the power change of each optical fiber after eliminating the effect of length. After normalizing the data after eliminating the effect of length, an objective function was established to build a photo-darkening effect model. The formula is: ; In the formula, and are the initial power and the final power, is the attenuation coefficient, is the fiber length.

[0057] The data reference table module is used to minimize the objective function by using a gradient descent algorithm to obtain an optimal data reference table of different optical fiber lengths and corresponding powers.

[0058] The optical fiber test module is used to connect the optical fiber to be tested with a length of L1 to the photo-darkening effect model to obtain the test result, that is, to calculate the power change percentage of the preset band of the test optical fiber before and after photo-darkening.

[0059] The test result module is used to find the corresponding data in the data reference table in S3 and subtract it from the measurement result of S4. If the difference is less than the error threshold, it is the valid measurement result of the optical fiber to be tested. Then, the valid measurement result is analyzed and sorted according to S4 to obtain the power attenuation change diagram of the optical fiber to be tested over time.

[0060] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for measuring the photodarkening effect of a double-clad gain fiber, characterized in that: The following steps are involved: S1: Acquire and utilize a dataset of different fiber lengths and corresponding power losses; S2: Calculate the power change of each optical fiber after eliminating the effect of length, normalize the data after eliminating the effect of length, set up the objective function, and build a photo-darkening effect model. The formula is: ； In the formula, and are the initial power and the final power, is the attenuation coefficient, is the optical fiber length; S3: Use the gradient descent algorithm to minimize the objective function and obtain the optimal data reference table of different optical fiber lengths and corresponding powers; S4: Connecting the optical fiber to be tested with a length of L1 to the photo-darkening effect model to obtain the test result, that is, calculating the power change percentage of the preset wavelength band of the test optical fiber before and after photo-darkening; S5: Look up the corresponding data in the data reference table in S3 and subtract it from the measurement result in S4. If the difference is less than the error threshold, it is the effective measurement result of the optical fiber to be tested. Then, the effective measurement result is analyzed and sorted according to S4 to obtain a power attenuation change diagram of the optical fiber to be tested over time.

2. The method for measuring the photodarkening effect of a double-clad gain fiber according to claim 1, characterized in that: In step S2, the relationship between the power loss and the length of the optical fiber is considered, and the relationship between the power loss and the length of the optical fiber is fitted by linear regression, so as to eliminate the influence of the length of each optical fiber.

3. The method for measuring the photodarkening effect of a double-clad gain fiber according to claim 1, characterized in that: In step S4, the power change percentage of the preset wavelength band of the test optical fiber before and after photodarkening is calculated, and the formula is: ； Where A is the red light power before photodarkening, and B is the red light power obtained after photodarkening.

4. The method for measuring the photodarkening effect of a double-clad gain fiber according to claim 1, characterized in that: Obtaining and utilizing a data set of different fiber lengths and corresponding power losses includes the following steps: Connect multiple optical fibers to be tested to the test optical path in sequence, that is, fuse the two ends with the high-reflection grating and the low-reflection grating respectively, couple the light in the light source into the optical fiber to be tested, use the first pump light source to perform scanning test, record the value of the power meter after stabilization and turn off the first pump light source, save the data, and record the data as A; The second pump light source is turned on and lasts for a preset period of time. The power meter records the power change curve in real time and processes the data into time series data. The data is also processed into a curve of the change of the additional loss of optical fiber photo-darkening at the second wavelength with the pump time. The second wavelength is the wavelength band length of the second pump light source. Turn off and on the second pump light source, turn on the first pump light source, record the data after the power meter is stable, turn off the first pump light source, save the data, and record the data as B; Among them, the test of the second optical fiber to be tested can be carried out only after the test of one optical fiber to be tested is completed.

5. The method for measuring the photodarkening effect of a double-clad gain fiber according to claim 4, characterized in that: The data processing is a curve of the change of the additional loss of the second wavelength optical fiber photo-darkening with the pumping time, and also includes: sorting the collected time series data and the corresponding power values, and comparing them with the standard time power curve for consistency. If it is within the preset error range, it is valid data.

6. The method for measuring the photodarkening effect of a double-clad gain fiber according to claim 4, characterized in that: Determine whether the recording power meter is stable, specifically: draw a power curve based on the power collected in real time, and divide it using the 3δ principle.

7. The method for measuring the photodarkening effect of a double-clad gain fiber according to claim 1, characterized in that: The test optical fiber is spliced ​​using a 100P+ Fujikura splicer.

8. A system for measuring the photodarkening effect of a double-clad gain fiber, characterized in that: include: A data acquisition module, for acquiring and utilizing data sets of different optical fiber lengths and corresponding power losses; A model module was built to calculate the power change of each optical fiber after eliminating the effect of length. After normalizing the data after eliminating the effect of length, an objective function was established to build a photo-darkening effect model. The formula is: ； In the formula, and are the initial power and the final power, is the attenuation coefficient, is the optical fiber length; A data reference table module is used to minimize the objective function using a gradient descent algorithm to obtain an optimal data reference table of different optical fiber lengths and corresponding powers; The optical fiber test module is used to connect the optical fiber to be tested with a length of L1 to the photo-darkening effect model to obtain the test result, that is, to calculate the power change percentage of the preset wavelength band of the test optical fiber before and after photo-darkening; The test result module is used to find the corresponding data in the data reference table in S3 and subtract it from the measurement result of S4. If the difference is less than the error threshold, it is the valid measurement result of the optical fiber to be tested. Then, the valid measurement result is analyzed and sorted according to S4 to obtain the power attenuation change diagram of the optical fiber to be tested over time.

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