System and method for monitoring return light of high-power multimode optical fiber unit on line

Abstract

The invention is applicable to the technical field of optical fiber measurement, and provides a system and a method for online monitoring return light of a high-power multi-mode optical fiber unit, the system comprises a high-power laser signal light source, a three-port optical fiber device working in a double-axis mode and a power meter, the three-port optical fiber device is a passive optical device applicable to non-polarization-maintaining and polarization-maintaining laser, and the power meter is used for measuring the return light of the high-power multi-mode optical fiber unit. The three-port optical fiber device is provided with three ports, the three-port optical fiber device comprises a Faraday rotating mirror, a half-wave plate, two polarization beam splitters, two reflecting prisms and three optical fiber collimators, and connecting tail fibers of the three optical fiber collimators serve as the three ports of the three-port optical fiber device. The structure of the three-port optical fiber device is ingeniously designed, an accurate method is provided for measuring the return light of the high-power multi-mode optical fiber unit, and an online monitoring mode is provided for manufacturing the high-performance high-power multi-mode optical fiber unit.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber measurement, and in particular relates to a system and method for online monitoring of return light of a high-power multimode optical fiber unit. Background Art

[0002] Currently, most laser passive fiber optic components are manufactured and tested using low-signal fundamental mode light of a few to tens of milliwatts for online adjustment or parameter measurement. However, most components, especially multimode fiber components, operate at high power levels, ranging from hundreds of milliwatts to tens of watts, or even tens of thousands of watts. Consequently, device parameters measured using low-power fundamental mode light often differ from actual performance. This is not only due to the difference in spot pattern between low-power fundamental mode light and high-power multimode light, but also primarily due to the differences in the interaction between the components within these multimode fiber components and the laser light at high power levels.

[0003] Therefore, developing a high-power light return test solution is extremely important. Traditionally, fiber couplers are used to test the return light of optical fiber devices or systems using low-power fundamental mode light. However, fiber couplers are difficult to manufacture for multimode and large-mode-area optical fibers, and coupler losses are also difficult to control. Summary of the Invention

[0004] In view of the above problems, the purpose of the present invention is to provide a system and method for online monitoring of the return light of a high-power multimode optical fiber unit, aiming to solve the technical problem that the existing return light testing scheme under high power is relatively difficult.

[0005] The present invention adopts the following technical solutions: The system for online monitoring of return light from a high-power multimode optical fiber unit includes a high-power laser signal light source, a biaxial three-port optical fiber device, and a power meter. The three-port optical fiber device is a passive optical device suitable for non-polarization-maintaining and polarization-maintaining lasers and has three ports. The three-port optical fiber device includes a Faraday rotator, a half-wave plate, two polarization beam splitters, two reflecting prisms, and three optical fiber collimators. The connecting pigtails of the three optical fiber collimators serve as the three ports of the three-port optical fiber device. The first optical fiber collimator is used to connect to the high-power laser signal light source, the second laser collimator is used to connect to the high-power multimode optical fiber unit to be measured or the first power meter, and the third laser collimator is used to connect to the second power meter. On the one hand, the laser input from the first fiber collimator is transmitted to the first polarization beam splitter and then split into two laser beams. One laser beam passes through the first reflecting prism, the Faraday rotator, and the half-wave plate, and the other laser beam passes through the Faraday rotator, the half-wave plate, and the second reflecting prism. The two laser beams are irradiated to the second polarization beam splitter and then transmitted to the second laser collimator for output; on the other hand, the return light from the second laser collimator is transmitted to the second polarization beam splitter and then split into two laser beams. One laser beam passes through the second reflecting prism, the half-wave plate, and the Faraday rotator, and the other laser beam passes through the half-wave plate, the Faraday rotator, and the first reflecting prism. The two laser beams are irradiated to the first polarization beam splitter and then transmitted to the third laser collimator for output.

[0006] Furthermore, the output wavelength of the high-power laser signal light source matches the operating wavelength of the high-power multimode optical fiber unit to be measured, and the power matches the actual use environment of the high-power multimode optical fiber unit to be measured.

[0007] Furthermore, the polarization beam splitter is used to split the input laser into two linearly polarized laser beams with mutually perpendicular polarization states, namely, p-light and s-light. If the input laser is non-linearly polarized light, the polarization beam splitter separates the input light into p-linearly polarized light and s-linearly polarized light according to the direction of its crystal axis, and propagates in two perpendicular directions. If the input laser is linearly polarized light, it will be transmitted in the p-light or s-light direction according to the direction of the crystal axis relative to the polarization beam splitter, and will not be split into two laser beams.

[0008] Furthermore, the reflecting prism is used to totally reflect the transmitted laser light at a certain angle; the Faraday rotator is a crystal used to rotate the polarization direction of the incident laser light by 45° in one direction; and the half-wave plate is used to rotate the polarization angle of the incident linearly polarized laser light by a certain angle.

[0009] Furthermore, the second power meter is used to monitor the laser power of the port where the third laser collimator is located, with a measurement accuracy at the nW level, and the first power meter is used to monitor the laser power of the port where the second laser collimator is located, with a measurement accuracy at the W level.

[0010] In addition, the method for online monitoring of return light of a high-power multimode optical fiber unit is implemented based on the system, wherein the three ports of the three-port optical fiber device correspond to three laser collimators, respectively recorded as port one to port three, and the method comprises the following steps: Step S1: After fusing the output fiber pigtail of a high-power laser signal light source to port one of a three-port optical fiber device operating in a dual-axis manner, the fiber pigtail of port two is processed so that it generates no or very little return light, a first power meter is connected to port two, and a second power meter is connected to port three; the high-power laser signal light source is turned on, and the power count value P1 outputted from port two is monitored and recorded, while the power count value P2 outputted from port three is monitored and recorded; Step S2: Turn off the high-power laser signal light source, remove the first power meter, fuse port 2 of the three-port optical fiber device with the input optical fiber of the high-power multimode optical fiber unit to be measured, turn on the high-power laser signal light source, and monitor and record the power count value P3 of port 3 again; Step S3: Calculate the return light ratio (P3-P2) / P1 of the high-power multimode optical fiber unit to be tested at the power count value P1.

[0011] Furthermore, in step S1, the specific optical path process of the high-power laser signal light source entering from port 1 and outputting from port 2 is as follows: The output laser of the high-power laser signal light source passes through the first fiber collimator and is then transmitted to the first polarization beam splitter. The p-vibration direction laser light contained in the output laser light is directly output from the first polarization beam splitter, and the s-vibration direction laser light is refracted 90° and output to the side. The p-polarization laser output from the first polarization beam splitter passes through the Faraday rotator and half-wave plate, where its polarization state is changed to s-polarized light. After being reflected by the second reflecting prism, its propagation direction is rotated 90° before entering the second polarization beam splitter. The propagation direction is then rotated 90° again and hits the second fiber collimator, thereby coupling into the optical fiber at port two. The s-polarization laser beam output from the first polarization beam splitter is reflected by the first reflecting prism, and its propagation direction is rotated 90°. After passing through the Faraday rotator and the half-wave plate, its polarization state is changed to p-polarized light, and it is incident on the second polarization beam splitter. Then, the propagation direction remains unchanged and it directly hits the second fiber collimator, thereby coupling into the optical fiber of port 2. Furthermore, in step S2, the specific optical path process of the laser reflected from the high-power multimode optical fiber unit to be tested entering from port 2 and outputting from port 3 is as follows: After passing through the second fiber collimator, the reflected light is transmitted to the second polarization beam splitter. The p-vibration direction laser light contained in the output laser is directly output from the second polarization beam splitter, and the s-vibration direction laser light is refracted 90° and output to the side. The laser beam output from the second polarization beam splitter in the p-direction remains polarized after passing through the half-wave plate and Faraday rotator. After being reflected by the first reflecting prism, the propagation direction is rotated 90° and enters the first polarization beam splitter. Then, the propagation direction remains unchanged and directly hits the third fiber collimator, thereby coupling into the optical fiber of port three. The laser output from the second polarization beam splitter with the s vibration direction is reflected by the second reflecting prism, and its propagation direction is rotated 90°. After passing through the half-wave plate and Faraday rotator, the polarization state remains unchanged. It is incident on the first polarization beam splitter, and its propagation direction is rotated 90° to hit the third fiber collimator, thereby coupling into the optical fiber of port three.

[0012] The beneficial effects of the present invention are as follows: the optical path design of the present invention is simple, and the optical path structure within the three-port optical fiber device operating in a dual-axis manner is cleverly designed, enabling low-loss, multi-directional laser transmission. Specifically, the input high-power multimode laser can be transmitted to the high-power multimode optical fiber unit under test with low loss and a guaranteed spot pattern; at the same time, the reflected light reflected from the high-power multimode optical fiber unit under test can be transmitted relatively completely to port three for monitoring. This not only enables accurate measurement of the reflected light power reflected from the high-power multimode optical fiber unit under test, but also enables real-time monitoring of the high-power multimode optical fiber unit under test, facilitating unit parameter optimization. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 1 is a schematic diagram of the principle structure of a system for online monitoring of return light from a high-power multimode optical fiber unit provided by an embodiment of the present invention; Figure 2 This is a schematic diagram of the transmission of a high-power laser signal light source laser in a three-port device; Figure 3 This is a schematic diagram of the transmission of return light from a high-power multimode fiber unit in a three-port device. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0015] In order to illustrate the technical solution of the present invention, specific embodiments are provided below.

[0016] like Figure 1 As shown, the system for online monitoring of return light from a high-power multimode fiber unit provided in this embodiment includes a high-power laser signal source 1, a three-port optical fiber device 12 operating in a dual-axis manner, and a power meter. A second power meter 13 is shown, while the first power meter is not. The three-port optical fiber device is a passive optical device suitable for both non-polarization-maintaining and polarization-maintaining lasers and has three ports, designated as ports 1 through 3 in the diagram. Both polarization-maintaining and non-polarization-maintaining lasers can be input from port 1 and output from port 2, and vice versa. The first power meter is used to detect the output laser power from port 2, while the second power meter is a precision instrument for monitoring the power of the three return-reflected lasers from the port. The second power meter has much higher measurement accuracy than the first power meter. For example, as a specific example, the second power meter has a measurement accuracy of nW, while the first power meter has a measurement accuracy of W.

[0017] The high-power laser signal source's output fiber pigtail matches a three-port dual-axis optical fiber device, and its output power is adjustable based on the specifications of the high-power multimode optical fiber unit under test. The high-power multimode optical fiber unit under test is a device or system with specialized internal functions that uses a special multimode optical fiber as its input. The output wavelength of the high-power laser signal source matches the operating wavelength of the high-power multimode optical fiber unit under test, and its power matches the actual operating environment of the high-power multimode optical fiber unit under test.

[0018] like Figure 1 The specific structure of the three-port optical fiber device 12 shown includes a Faraday rotator 6, a half-wave plate 7, two polarization beam splitters (shown as the first polarization beam splitter 2 and the second polarization beam splitter 9), two reflecting prisms (shown as the first reflecting prism 4 and the second reflecting prism 8), and three optical fiber collimators (shown as the first optical fiber collimator 2, the second optical fiber collimator 10 and the third optical fiber collimator 5). The connecting fiber pigtails of the three optical fiber collimators serve as the three ports of the three-port optical fiber device, wherein the first optical fiber collimator 2 is used to connect to the high-power laser signal light source 1, the second laser collimator 2 is used to connect to the high-power multimode optical fiber unit 11 to be measured or the first power meter, and the third laser collimator 5 is used to connect to the second power meter 13.

[0019] The three optical fiber collimators in the dual-axis three-port optical fiber device 12 have output pigtails that are consistent with the input pigtails of the high-power multimode optical fiber unit 11 to be tested. The internal parameters of the optical fiber collimators are related to the actual internal optical path length, operating band and operating power of the three-port optical fiber device.

[0020] The two polarization beam splitters in the dual-axis three-port fiber optic device 12 are used to split the input laser into two linearly polarized laser beams, namely p-light and s-light, with polarization states perpendicular to each other. Light with different polarization states propagates in different directions. If the input laser is non-linearly polarized, the polarization beam splitter separates the input light into p-polarized light and s-polarized light based on the direction of its crystal axis, and the two beams propagate in two perpendicular directions. If the input laser is linearly polarized, it propagates in either the p-light or s-light direction based on the direction of the crystal axis relative to the polarization beam splitter, and is not split into two laser beams.

[0021] The two reflecting prisms in the dual-axis three-port optical fiber device 12 are used to totally reflect the transmitted laser light at a certain angle.

[0022] The Faraday rotator 6 in the dual-axis three-port optical fiber device 12 is a crystal used to rotate the polarization direction of incident laser light (regardless of the direction from which the laser light enters) by 45° in one direction.

[0023] The half-wave plate 7 in the dual-axis three-port optical fiber device 12 is used to rotate the polarization angle of the incident linearly polarized laser light by a certain angle. In this embodiment, the rotation angle is set to 45 degrees, and the rotation direction is related to the incident direction of the laser light.

[0024] As can be seen from the above structure, on the one hand, the laser light input from the first fiber collimator 2 is transmitted to the first polarization beam splitter 3 and then split into two laser beams. One laser beam passes through the first reflecting prism 4, the Faraday rotator 6, and the half-wave plate 7, and the other laser beam passes through the Faraday rotator 6, the half-wave plate 7, and the second reflecting prism 8. The two laser beams are irradiated to the second polarization beam splitter 9 and then transmitted to the second laser collimator 10 for output. On the other hand, the return light from the second laser collimator 10 is transmitted to the second polarization beam splitter 9 and then split into two laser beams. One laser beam passes through the second reflecting prism 8, the half-wave plate 7, and the Faraday rotator 6, and the other laser beam passes through the half-wave plate 7, the Faraday rotator 6, and the first reflecting prism 7. The two laser beams are irradiated to the first polarization beam splitter 3 and then transmitted to the third laser collimator 5 for output.

[0025] Based on the above system, this embodiment also provides a method for online monitoring of return light from a high-power multimode optical fiber unit. The three ports of the three-port optical fiber device correspond to three laser collimators, which are respectively marked as port one to port three. The method includes the following steps: Step S1: After the output fiber pigtail of the high-power laser signal light source is fused to port one of the dual-axis three-port optical fiber device, the fiber pigtail of port two is processed so that it generates no or very little return light, a first power meter is connected to port two, and a second power meter is connected to port three; the high-power laser signal light source is turned on, and the power count value P1 output from port two is monitored and recorded, while the power count value P2 output from port three is monitored and recorded.

[0026] In this step, the high-power laser signal light source is incident from port 1 and output from port 2. Figure 2 As shown, the specific optical path process is as follows: S11, the output laser of the high-power laser signal light source passes through the first fiber collimator and is transmitted to the first polarization beam splitter. The p-vibration direction laser contained in the output laser is directly output from the first polarization beam splitter, and the s-vibration direction laser is refracted 90° and output to the side. S12. The p-polarization laser light output from the first polarization beam splitter is changed in polarization state after passing through the Faraday rotator and the half-wave plate, becoming s-polarized light. After being reflected by the second reflecting prism, the propagation direction is rotated 90° and then enters the second polarization beam splitter. The propagation direction is then rotated again by 90° (because the polarization state is exactly s-polarization relative to the crystal axis of the second polarization beam splitter, the propagation direction is rotated by 90°) and hits the second fiber collimator, thereby coupling into the optical fiber of port 2. S13. The s-vibration direction laser output from the first polarization beam splitter is reflected by the first reflecting prism, and its propagation direction is rotated 90°. After passing through the Faraday rotator and the half-wave plate, its polarization state is changed to p-polarized light, and it is incident on the second polarization beam splitter. Then, the propagation direction remains unchanged (because the polarization state is exactly p-polarization relative to the crystal axis of the second polarization beam splitter, the propagation direction will not change), and it directly hits the second fiber collimator, thereby coupling into the optical fiber of port two.

[0027] It should be noted that if the high-power laser signal light source is a linearly polarized light source with the same vibration direction as p or s, it will not be split into two beams after passing through the first polarization beam splitter, but will only be transmitted along one optical path of p or s.

[0028] Step S2: Turn off the high-power laser signal light source, remove the first power meter, fuse port 2 of the three-port optical fiber device with the input optical fiber of the high-power multimode optical fiber unit to be tested, turn on the high-power laser signal light source, and monitor and record the power count value P3 of port 3 again.

[0029] To ensure accurate measurement, the pigtails at all ports of the three-port optical device should be consistent with the pigtails at the input end of the multimode fiber unit to be tested. In this step, the reflected light from the high-power multimode fiber unit to be tested is incident from port two and output from port three. Figure 3 As shown, the specific optical path process is as follows: S21, after the reflected light passes through the second fiber collimator, it is transmitted to the second polarization beam splitter. The p-vibration direction laser light contained in the output laser light is directly output from the second polarization beam splitter, and the s-vibration direction laser light is refracted 90° and output to the side. S22, the laser light in the p-vibration direction output from the second polarization beam splitter passes through the half-wave plate and the Faraday rotator, and its polarization state remains unchanged (since the transmission direction is opposite to that in step S13, the polarization state will not be changed). After being reflected by the first reflecting prism, the propagation direction is rotated 90° and enters the first polarization beam splitter. Then, the propagation direction remains unchanged (since the polarization state is exactly p-polarization relative to the crystal axis of the first polarization beam splitter, the propagation direction remains unchanged) and directly hits the third fiber collimator, thereby being coupled into the optical fiber of port three. S23. The laser light with s vibration direction output from the second polarization beam splitter is reflected by the second reflecting prism, and its propagation direction is rotated 90°. After passing through the half-wave plate and the Faraday rotator, the polarization state remains unchanged and it is incident on the first polarization beam splitter. The propagation direction is rotated 90° (because the polarization state is exactly s polarization relative to the crystal axis of the first polarization beam splitter, the propagation direction changes by 90°) and hits the third fiber collimator, thereby being coupled into the optical fiber of port three.

[0030] It should be noted that if the reflected light from the high-power multimode optical fiber unit to be tested is a linearly polarized light source with the same vibration direction as p or s, it will not be split into two beams after passing through the second polarization beam splitter, but will only be transmitted along one optical path of p or s.

[0031] Step S3: Calculate the return light ratio (P3-P2) / P1 of the high-power multimode optical fiber unit to be tested at the power count value P1.

[0032] For example, in this online monitoring system, the output continuous optical power of the high-power laser signal source 1 is 5W, with a center wavelength of 1550nm. The output fiber pigtail is a 25 / 300 double-clad fiber. The three port pigtails of the three-port optical fiber device 12 all use 25 / 300 double-clad fiber. The operating center wavelength of each component is 1550nm. The insertion loss from port 1 to port 2 is <1dB, the insertion loss from port 1 to port 3 is >50dB, the insertion loss from port 2 to port 1 is >50dB, and the insertion loss from port 2 to port 3 is <1dB. The test port fiber of the high-power multimode optical fiber unit to be tested is also a 25 / 300 double-clad fiber. The second power meter 13 has a measurement range of 10nW-1mW and a resolution of nW. The first power meter has a measurement range of 100mW-20W and a resolution of W.

[0033] Using the above monitoring method, the power count value P1 recorded at port one in step S1 is 4.1W, and the power count value P2 monitored and recorded at port three is 46uW. The power count value P3 monitored at port three in step S2 is 55uW. By calculating (P3-P2) / P1 = 8uW / 4.1W, we can determine the proportion of return light generated by the high-power multimode fiber unit under test at power count value P1.

[0034] In summary, the present invention provides a system for online monitoring of the return light of a high-power multimode optical fiber unit, which not only provides an accurate method for measuring the return light of a high-power multimode optical fiber unit, but also provides an online monitoring method for the production of high-performance high-power multimode optical fiber units.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A system for online monitoring of return light from a high-power multimode optical fiber unit, characterized in that: The system includes a high-power laser signal light source, a biaxial three-port optical fiber device, and a power meter. The three-port optical fiber device is a passive optical device suitable for non-polarization-maintaining and polarization-maintaining lasers, and has three ports. The three-port optical fiber device includes a Faraday rotator, a half-wave plate, two polarization beam splitters, two reflecting prisms, and three optical fiber collimators. The connecting pigtails of the three optical fiber collimators serve as the three ports of the three-port optical fiber device, wherein the first optical fiber collimator is used to connect to the high-power laser signal light source, the second laser collimator is used to connect to the high-power multimode optical fiber unit to be measured or the first power meter, and the third laser collimator is used to connect to the second power meter. On the one hand, the laser input from the first fiber collimator is transmitted to the first polarization beam splitter and then split into two laser beams. One laser beam passes through the first reflecting prism, the Faraday rotator, and the half-wave plate, and the other laser beam passes through the Faraday rotator, the half-wave plate, and the second reflecting prism. The two laser beams are irradiated to the second polarization beam splitter and then transmitted to the second laser collimator for output; on the other hand, the return light from the second laser collimator is transmitted to the second polarization beam splitter and then split into two laser beams. One laser beam passes through the second reflecting prism, the half-wave plate, and the Faraday rotator, and the other laser beam passes through the half-wave plate, the Faraday rotator, and the first reflecting prism. The two laser beams are irradiated to the first polarization beam splitter and then transmitted to the third laser collimator for output.

2. The system for online monitoring of return light of a high-power multimode optical fiber unit according to claim 1, characterized in that: The output wavelength of the high-power laser signal light source matches the operating wavelength of the high-power multimode optical fiber unit to be measured, and the power matches the actual use environment of the high-power multimode optical fiber unit to be measured.

3. The system for online monitoring of return light from a high-power multimode optical fiber unit according to claim 2, wherein: The polarization beam splitter is used to split the input laser into two beams of linearly polarized laser light with mutually perpendicular polarization states, namely p-light and s-light. If the input laser light is non-linearly polarized light, the polarization beam splitter will separate the input light into two beams of p-linearly polarized light and s-linearly polarized light according to the direction of its crystal axis, and propagate them in two perpendicular directions. If the input laser light is linearly polarized light, it will be transmitted in the direction of p-light or s-light according to the direction of the crystal axis relative to the polarization beam splitter, and will not be split into two laser beams.

4. The system for online monitoring of return light from a high-power multimode optical fiber unit according to claim 3, wherein: The reflecting prism is used to totally reflect the transmitted laser light at a certain angle; the Faraday rotator is a crystal used to rotate the polarization direction of the incident laser light by 45° in one direction; the half-wave plate is used to rotate the polarization angle of the incident linearly polarized laser light by a certain angle.

5. The system for online monitoring of return light of a high-power multimode optical fiber unit according to claim 4, characterized in that: The second power meter is used to monitor the laser power of the port where the third laser collimator is located, and the measurement accuracy is at the nW level. The first power meter is used to monitor the laser power of the port where the second laser collimator is located, and the measurement accuracy is at the W level.

6. A method for online monitoring of return light from a high-power multimode fiber unit, the method being implemented based on the system of any one of claims 1 to 5, wherein the three ports of a three-port fiber device correspond to three laser collimators, respectively designated as port 1 to port 3, and the method comprising the following steps: Step S1: After fusing the output fiber pigtail of a high-power laser signal light source to port one of a three-port optical fiber device operating in a dual-axis manner, the fiber pigtail of port two is processed so that it generates no or very little return light, a first power meter is connected to port two, and a second power meter is connected to port three; the high-power laser signal light source is turned on, and the power count value P1 outputted from port two is monitored and recorded, while the power count value P2 outputted from port three is monitored and recorded; Step S2: Turn off the high-power laser signal light source, remove the first power meter, fuse port 2 of the three-port optical fiber device with the input optical fiber of the high-power multimode optical fiber unit to be measured, turn on the high-power laser signal light source, and monitor and record the power count value P3 of port 3 again; Step S3: Calculate the return light ratio (P3-P2) / P1 of the high-power multimode optical fiber unit to be tested at the power count value P1.

7. The method for online monitoring of return light of a high-power multimode optical fiber unit according to claim 6, characterized in that: In step S1, the specific optical path process of the high-power laser signal light source entering from port 1 and outputting from port 2 is as follows: The output laser of the high-power laser signal light source passes through the first fiber collimator and is then transmitted to the first polarization beam splitter. The p-vibration direction laser light contained in the output laser light is directly output from the first polarization beam splitter, and the s-vibration direction laser light is refracted 90° and output to the side. The p-polarization laser output from the first polarization beam splitter passes through the Faraday rotator and half-wave plate, where its polarization state is changed to s-polarized light. After being reflected by the second reflecting prism, its propagation direction is rotated 90° before entering the second polarization beam splitter. The propagation direction is then rotated 90° again and hits the second fiber collimator, thereby coupling into the optical fiber at port two. The s-vibration direction laser output from the first polarization beam splitter is reflected by the first reflecting prism, and its propagation direction is rotated 90°. After passing through the Faraday rotator and the half-wave plate, its polarization state is changed to p-polarized light and incident on the second polarization beam splitter. Then, the propagation direction remains unchanged and it directly hits the second fiber collimator, thereby coupling into the optical fiber of port two.

8. The method for online monitoring of return light of a high-power multimode optical fiber unit according to claim 6, wherein: In step S2, the specific optical path process of the laser reflected from the high-power multimode optical fiber unit to be tested is as follows: After passing through the second fiber collimator, the reflected light is transmitted to the second polarization beam splitter. The p-vibration direction laser light contained in the output laser is directly output from the second polarization beam splitter, and the s-vibration direction laser light is refracted 90° and output to the side. The laser beam output from the second polarization beam splitter in the p-direction remains polarized after passing through the half-wave plate and Faraday rotator. After being reflected by the first reflecting prism, the propagation direction is rotated 90° and enters the first polarization beam splitter. Then, the propagation direction remains unchanged and directly hits the third fiber collimator, thereby coupling into the optical fiber of port three. The laser output from the second polarization beam splitter with the s vibration direction is reflected by the second reflecting prism, and its propagation direction is rotated 90°. After passing through the half-wave plate and Faraday rotator, the polarization state remains unchanged. It is incident on the first polarization beam splitter, and its propagation direction is rotated 90° to hit the third fiber collimator, thereby coupling into the optical fiber of port three.

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