Method and apparatus for testing mode coupling of fiber optic devices

Abstract

The application provides a kind of optical fiber device mode coupling test method and device, including the input signal fiber of the detection laser injection into the optical fiber device to be tested;After the transmission of detection laser from input signal fiber through optical fiber device to be tested, the core transmission mode of detection laser is changed by the influence of internal mode coupling of optical fiber device to be tested, and the transmission of modulated detection laser to the output signal pigtail of optical fiber device to be tested is in new mode state;Collect the light field information output by output signal pigtail and analyze mode field characteristics, obtain the mode number, high-order mode type and relative content induced by mode coupling when detection laser is transmitted in optical fiber device to be tested.The application can finely measure the internal mode coupling characteristics of the prepared optical fiber device, and assist the direct selection or yield screening of optical fiber device.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber device testing, and particularly relates to a mode coupling testing method and device for optical fiber devices. BACKGROUND

[0002] High-power fiber lasers based on all-fiber structures have made great progress in power improvement and defense industry applications in recent years, which is largely due to the wide application of all-fiber structures. In the all-fiber structure laser system, the combination of few-mode gain fibers, energy transmission fibers and fiber devices makes the overall structure of the system compact and strong in anti-interference. Among them, the most widely used fiber devices include pump combiner (signal-pump combiner), power combiner, mode field adapter, cladding light filter, fiber end cap, etc.

[0003] Because the fiber device usually involves integration between different fibers or mechanical / optical processing of the fiber structure, with the change of the waveguide structure, the transmission of laser in it is prone to mode coupling, which leads to serious mode degradation, which is particularly prominent at medium and high power output levels. For example, a fiber combiner is usually composed of at least two types of few (multi) mode fibers, the mode coupling between the fibers is complex, and the mode matching between the input fiber and the output fiber is affected by multiple factors such as the mode composition of the front-stage laser and the preprocessing technology. At present, there is still a lack of effective mode composition measurement and online process optimization means. In the existing method, the device manufacturing quality is usually evaluated subjectively based on output power measurement or output spot pattern direct acquisition, which is difficult to establish a "point-to-point" quantitative relationship between the key preparation process of the device and the mode preservation characteristics. In addition, the data obtained by direct power monitoring has no direct correspondence with the mode-related characteristics of the device, and is insufficient to indirectly represent the beam quality preservation characteristics of the device. Although the output spot pattern implies mode content information, the mode composition and the spot intensity distribution have a many-to-one relationship, which is easy to cause false judgment of the mode degradation of the fiber device. The method of directly measuring the beam quality to evaluate the mode preservation effect of the fiber device has low testing process efficiency, and the beam quality M 2 and the internal mode evolution of the fiber device have no direct correspondence. Therefore, in order to truly clarify the main causes of the internal mode degradation of the fiber device, quantify the mode content change rule, and explore the corresponding relationship between the process parameters and the internal mode state, a new internal mode coupling measurement tool for the fiber device needs to be developed.

[0004] For example, the fiber end cap is usually made by fusing the energy transmission fiber and the fused quartz block to reduce the output power density of the laser, and the quality of the fusion has a significant impact on the output beam quality. It is necessary to carry out online mode coupling analysis to quantify and optimize the fusion process and realize the mode characteristic preservation of high beam quality laser. For the preparation of combiner, mode field adapter and other devices containing the tapering process, although the process objectively meets the adiabatic tapering condition, the key process parameters still significantly affect the mode transmission characteristics, resulting in the degradation of the output beam quality. On the one hand, for the prepared fiber device, in addition to the need for testing the overall transmission efficiency of the device, the internal mode coupling characteristics of the beam quality preservation characteristics of the device also need to be tested. SUMMARY

[0005] In view of the problems in the prior art, the present application provides a fiber device mode coupling test method and device, which can quantitatively measure the mode coupling state and output mode composition of the fiber device, and assist in direct selection or yield screening of the fiber device.

[0006] To achieve the above technical purpose, the technical scheme adopted by the present application is:

[0007] On the one hand, a fiber device mode coupling test method is provided, comprising:

[0008] Injecting a probe laser into the input signal fiber of the fiber device to be tested;

[0009] After the probe laser is transmitted through the fiber device to be tested from the input signal fiber, the core transmission mode of the probe laser is changed due to the influence of the internal mode coupling of the fiber device to be tested, and the modulated probe laser is transmitted to the output signal tail fiber of the fiber device to be tested in a new mode state.

[0010] Collecting the light field information output by the output signal tail fiber and analyzing the mode field characteristics to obtain the mode number, high-order mode type and relative content induced by the mode coupling during the transmission of the probe laser in the fiber device to be tested.

[0011] Further, in the present application: the probe laser is a swept frequency laser; as the wavelength of the injected swept frequency laser changes, periodic interference light fields are generated at the output end of the output signal tail fiber of the fiber device to be tested for each core mode, and the periodic interference light fields contain a plurality of characteristic frequencies, and the frequency size is the differential mode group delay between different modes.

[0012] Intensity information of periodic interference light field at different wavelengths of the tail fiber output of the output signal is collected, intensity data at all positions in two-dimensional space at different wavelengths is extracted, Fourier transform is performed on intensity data at all positions in two-dimensional space, and mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be detected are obtained based on the Fourier transform result.

[0013] Further, in the application: if all output signal fibers of the fiber device to be detected are two-mode fibers, the peak values in the Fourier transform result are in turn base mode LP 01 characteristic peak, first high-order mode LP 11 characteristic peak;

[0014] If all output signal fibers of the fiber device to be detected are fibers supporting more modes, the peak values in the Fourier transform result are in turn base mode LP 01 characteristic peak, first high-order mode LP 11 characteristic peak, second high-order mode LP 21 characteristic peak, and so on, the order of the high-order mode special peak is determined by the relative difference delay of the current mode and the base mode of the fiber, and the smaller the difference delay is, the earlier the order is.

[0015] Under the premise that the total output power of the fiber device to be detected is constant, the greater the peak intensity in the Fourier transform result is, the higher the content of the corresponding mode is, so that the mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be detected are obtained.

[0016] On the other hand, the application provides a method for evaluating mode maintaining characteristics of a fiber device, comprising:

[0017] The mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be detected are obtained by using any one of the above fiber device mode coupling test methods.

[0018] According to the high-order mode type and relative content, the advantages and disadvantages of the fiber device to be detected in mode maintaining and beam quality are determined, wherein the fewer the high-order mode quantity is and the lower the relative content is, the better the characteristics of the fiber device to be detected in mode maintaining and beam quality are.

[0019] On the other hand, the application provides a fiber device mode coupling test device, comprising:

[0020] A probe laser injection unit is configured to generate a probe laser and inject the probe laser into an input signal fiber of a fiber device to be detected.

[0021] The to-be-tested fiber device at least includes two ports of input signal fiber and output signal pigtail; after the probe laser is transmitted from the input signal fiber through the to-be-tested fiber device, the fiber core transmission mode of the probe laser is changed due to the influence of the internal mode coupling of the to-be-tested fiber device, and the modulated probe laser is transmitted to the output signal pigtail of the to-be-tested fiber device in a new mode state.

[0022] The image acquisition and mode component analysis unit is used for acquiring the light field information output by the output signal pigtail and analyzing the mode field characteristics, so as to obtain the mode number, high-order mode type and relative content induced by the mode coupling during the transmission of the probe laser in the to-be-tested fiber device.

[0023] Further, in the present application: the probe laser injection unit includes a tunable laser and a single-mode jumper, the probe laser is a sweep frequency laser, and the sweep frequency laser is output through the tunable laser and the single-mode jumper and then injected into the input signal fiber of the to-be-tested fiber device.

[0024] Further, in the present application: the tunable laser has three parameters of wavelength tuning range, wavelength tuning resolution and wavelength tuning speed controllable, wherein the minimum wavelength tuning range of the tunable laser is not less than 5 nm, the minimum wavelength tuning interval is not greater than 0.5 nm, and the fastest wavelength tuning speed is not less than 0.1 nm / s.

[0025] Further, in the present application: the cutoff wavelength of the single-mode jumper is not greater than the working wavelength of the device application, the numerical aperture NA at the working wavelength is not greater than 0.1, and in the selectable range of the cutoff wavelength and the numerical aperture NA, the smaller the values of the cutoff wavelength and the numerical aperture NA of the single-mode jumper are, the better.

[0026] Further, in the present application: the to-be-tested fiber device is any fiber device with laser transmission purpose, the to-be-tested fiber device has an external processing technology area, and there are external processing factors inducing the coupling of the fiber core mode state or mechanical, chemical and optical processing processes inducing the coupling of the fiber core mode state in the internal area of the external processing technology area, so that the fiber device has non-ideal mode degradation.

[0027] Further, in the present application: the image acquisition and mode component analysis unit includes an imaging optical assembly, a high-frame-rate camera and a mode interference light field processing assembly.

[0028] The imaging optical assembly transmits the mode interference light field output by the output signal pigtail to the high-frame-rate camera.

[0029] The high-frame-rate camera records the intensity data of the mode interference light field.

[0030] The mode interference optical field processing assembly performs Fourier transform on intensity data of the mode interference optical field, and obtains mode quantity, high-order mode type and relative content induced by mode coupling during transmission of the probe laser in the optical fiber device to be detected based on the Fourier transform result.

[0031] Compared with the prior art, the present application can produce the beneficial technical effects that:

[0032] The present application can quantitatively measure the mode coupling state and output mode composition of the prepared optical fiber device. Specifically, for the prepared optical fiber device to be detected, the probe laser is injected into the input signal optical fiber of the optical fiber device to be detected. The probe laser starts from the input signal optical fiber, passes through the waveguide structure change region of the optical fiber device, and is affected by the external process parameters in the manufacturing process of the optical fiber device. The internal waveguide structure of the optical fiber device deviates from the ideal state, and there is mode mismatch during the fusion of the optical fiber. The core transmission mode of the probe laser changes in the corresponding region, and the modulated probe laser is transmitted to the output signal optical fiber of the optical fiber device in a new mode state. The output optical field information of the optical fiber device is collected and the mode field characteristics are analyzed, the mode quantity and relative content when the internal transmission optical field of the current device reaches the output end are obtained, and the advantages and disadvantages of the mode retention characteristics of the optical fiber device are analyzed according to the mode composition.

[0033] The present application can evaluate the mode retention performance of the prepared optical fiber device by accurately measuring the mode coupling state in the optical fiber device. On the other hand, the internal mode composition of the optical fiber device under different process parameters is quantified, which is beneficial to the targeted optimization of the device manufacturing process, and thus the mode retention performance of the optical fiber device is effectively improved, and the output beam quality is optimized. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0035] Figure 1 Flowchart of an embodiment;

[0036] Figure 2 Schematic diagram of mode coupling in an optical fiber device;

[0037] Figure 3 Structural schematic diagram of an embodiment;

[0038] Figure 4 Test optical fiber link diagram of an embodiment;

[0039] Figure 5 Fig. 1 is a diagram of internal mode coupling test results of two first-type mode field adapters (6 / 125 μm-20 / 400 μm mode field adapters) in an embodiment, wherein (a) is a diagram of internal mode coupling test results of a first first-type mode field adapter (6 / 125 μm-20 / 400 μm mode field adapter), and (b) is a diagram of internal mode coupling test results of a second first-type mode field adapter (6 / 125 μm-20 / 400 μm mode field adapter);

[0040] Figure 6 Fig. 2 is a diagram of internal mode coupling test results of two second-type mode field adapters (6 / 125 μm-25 / 250 μm mode field adapters) in an embodiment, wherein (a) is a diagram of internal mode coupling test results of a first second-type mode field adapter (6 / 125 μm-25 / 250 μm mode field adapter), and (b) is a diagram of internal mode coupling test results of a second second-type mode field adapter (6 / 125 μm-25 / 250 μm mode field adapter);

[0041] The purposes, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. Specific implementation method

[0042] To make the purposes, technical solutions and advantages of the embodiments of the present application more clear, the drawings and detailed descriptions will be used to clearly explain the spirit of the present application. Any person skilled in the art can make changes and modifications to the technology taught by the present application without departing from the spirit and scope of the present application. The schematic embodiments of the present application and the descriptions thereof are used to explain the present application, but not to limit the present application.

[0043] Reference Figure 1 Fig. 3 is a flowchart of an embodiment, which provides a mode coupling test method for an optical fiber device, comprising:

[0044] (1) injecting a probe laser into an input signal optical fiber of the optical fiber device to be tested;

[0045] (2) transmitting the probe laser from the input signal optical fiber through the optical fiber device to be tested;

[0046] During the transmission of the probe laser through the optical fiber device to be tested, the core transmission mode of the probe laser is changed due to the influence of the internal mode coupling of the optical fiber device to be tested, and the modulated probe laser is transmitted to an output signal pigtail of the optical fiber device to be tested in a new mode state;

[0047] (3) collecting the light field information output by the output signal pigtail of the optical fiber device to be tested and analyzing the mode field characteristics;

[0048] (4) Obtain the mode number, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the optical fiber device to be tested.

[0049] According to the mode coupling test result, the mode maintaining characteristics of the optical fiber device are quantitatively evaluated, and the test result can be used for direct selection or yield screening of the optical fiber device.

[0050] In an embodiment, a mode coupling test method of an optical fiber device is provided, comprising:

[0051] The probe laser is injected into the input signal fiber of the optical fiber device to be tested, and the probe laser is a swept laser with controllable wavelength tuning range, wavelength tuning resolution and wavelength tuning speed.

[0052] The output signal tail fiber of the optical fiber device to be tested generates a periodic interference light field with the change of the wavelength of the injected swept laser, and the periodic interference light field contains a plurality of characteristic frequencies, and the frequency size is the differential mode group delay between different modes.

[0053] The intensity information of the periodic interference light field at different wavelengths output by the output signal tail fiber is collected, the intensity data at all positions in the two-dimensional space at different wavelengths is extracted, the Fourier transform is performed on the intensity data at all positions in the two-dimensional space, and the mode number, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the optical fiber device to be tested are obtained based on the Fourier transform result.

[0054] The optical fiber device to be tested comprises at least two ports of input signal fiber and output signal fiber. The optical fiber device to be tested comprises not less than two types of optical fibers, or the optical fiber device to be tested is a non-single-mode optical fiber device processed by mechanical, optical or hybrid process, or the optical fiber device to be tested is any optical fiber device for laser transmission purpose such as a mode field adapter, a fiber combiner, a cladding optical filter, a fiber end cap, etc.

[0055] The optical fiber device to be tested has an external process treatment area, and the external process treatment area has an induced processing factor that induces the coupling of the core mode state of the optical fiber, or has a mechanical, chemical or optical processing process that induces the coupling of the core mode state.

[0056] The mode coupling types causing mode degeneration inside the fiber device mainly include: (1) high-order mode excitation caused by fiber waveguide structure change, (2) inter-mode energy exchange in the process of fiber-to-fiber waveguide coupling, (3) fiber waveguide structure defects caused by external processing disturbance. Ideally, the transmission of laser inside the fiber device should not generate new mode components, and the beam quality of the output beam should not be degraded compared with the input beam. In actual fiber devices, due to the process limitation, the mode coupling will inevitably exist in the disturbance area of the laser inside the device. The single transverse mode beam (high beam quality beam) injected into the fiber device will couple energy to the adjacent high-order mode beam in the mode disturbance area, and the newly generated high-order mode component will further induce the generation of higher-order high-order mode, so that the single transverse mode beam (high beam quality beam) is degraded to a multi-mode beam (low beam quality beam) at the output end of the fiber device.

[0057] Reference Figure 2 , a schematic diagram of mode coupling in a fiber device, wherein (a) is high-order mode excitation caused by fiber waveguide structure change; (b) is inter-mode energy exchange in the process of fiber-to-fiber waveguide coupling; (c) is fiber waveguide structure defects caused by external processing disturbance. The mode coupling inside the fiber device is characterized by energy transfer from low-order mode to high-order mode, which has a deteriorating effect. Once higher-order modes are generated, they will tend to couple to higher-order modes in the subsequent transmission process, especially in the area where the waveguide structure changes. High-order modes have larger transmission divergence angle and loss, so mode degeneration not only affects the beam quality of the device output beam, but also may cause local energy leakage to intensify, abnormal temperature rise of the device, and affect the safe working state. After the generation of new high-order mode components, the low-order mode light field and the high-order mode light field will coexist in the device waveguide and interfere with each other, resulting in optical beats with characteristic frequencies. Under the premise of dominant transmission of the fundamental mode (low-order mode), the higher the content of the high-order mode, the larger the amplitude value of the optical beat at the corresponding characteristic frequency.

[0058] Reference Figure 3 , an embodiment provides a fiber device mode coupling test device, comprising:

[0059] The probe laser injection unit 100 is used to generate probe laser and inject the probe laser into the input signal fiber of the fiber device to be tested;

[0060] The fiber device to be tested 200 at least includes two ports of input signal fiber and output signal tail fiber; the fiber core transmission mode of the probe laser is changed by the mode coupling inside the fiber device to be tested after the probe laser is transmitted from the input signal fiber to the output signal tail fiber of the fiber device to be tested; the probe laser modulated by the new mode state is transmitted to the output signal tail fiber of the fiber device to be tested;

[0061] The image acquisition and mode component analysis unit 300 is used to acquire the light field information output by the output signal tail fiber and analyze the mode field characteristics, so as to obtain the mode number, high-order mode type and relative content induced by mode coupling during the transmission of the probe laser in the optical fiber device to be detected.

[0062] The probe laser injection system 100 comprises a tunable laser and a single-mode jumper. After the swept laser is output by the tunable laser and the single-mode jumper, the swept laser is injected into the next stage system. The tunable laser has three controllable parameters of wavelength tuning range, wavelength tuning resolution and wavelength tuning speed. The minimum wavelength tuning range of the tunable laser is not less than 5 nm, the minimum wavelength tuning interval is not greater than 0.5 nm, and the fastest wavelength tuning speed is not less than 0.1 nm / s. The cutoff wavelength of the single-mode jumper is not greater than the working wavelength of the device application, and the numerical aperture NA is not greater than 0.1 at the working wavelength. In the optional range, the smaller the cutoff wavelength and the numerical aperture NA, the better.

[0063] The single-mode jumper can be a polarization maintaining single-mode fiber, which generates a single polarization swept laser and injects it into the next stage system for measuring the polarization maintaining and stress characteristics of the next stage system.

[0064] After the swept laser is output by the tunable laser and the single-mode jumper, the swept laser can be directly fused with the next stage system, that is, the single-mode jumper is directly fused with the input signal fiber of the optical fiber device to be detected 200, and the swept laser is input into the optical fiber device to be detected 200 from the input signal fiber.

[0065] As an optional solution, after the swept laser is output by the tunable laser and the single-mode jumper, the swept laser can also be injected into the next stage system through a spatial optical system, that is, the input signal fiber of the optical fiber device to be detected 200, and the swept laser is input into the optical fiber device to be detected 200 from the input signal fiber, wherein the spatial optical system can comprise a power attenuation element and a polarization optical element.

[0066] The optical fiber device to be detected 200 comprises at least two ports of input signal fiber and output signal fiber. The optical fiber device to be detected 200 comprises not less than two types of optical fibers, or the optical fiber device to be detected 200 is a non-single-mode optical fiber device processed by mechanical, optical or hybrid process, or the optical fiber device to be detected 200 is any optical fiber device for laser transmission purposes, such as a mode field adapter, a fiber combiner, a cladding optical filter, a fiber end cap, etc.

[0067] The optical fiber device to be detected 200 has an external process treatment area, and there are external treatment factors inducing the coupling of the core mode state of the optical fiber in the external process treatment area, or there are mechanical, chemical and optical processing techniques leading to the coupling of the core mode state, so that the optical fiber device has non-ideal mode degradation.

[0068] The output signal fiber of the to-be-tested fiber device 200 can be connected with the same type of fiber or other types of fiber as the output relay fiber.

[0069] The image acquisition and mode component analysis unit 300 comprises an imaging optical assembly, a high-frame-rate camera, and a mode interference optical field processing assembly; the imaging optical assembly transmits the mode interference optical field output by the output signal tail fiber to the high-frame-rate camera; the high-frame-rate camera records the intensity data of the mode interference optical field; and the mode interference optical field processing assembly performs Fourier transform on the intensity data of the mode interference optical field, and obtains the mode number, high-order mode type, and relative content induced by mode coupling during the transmission of the probe laser inside the to-be-tested fiber device based on the Fourier transform result.

[0070] As an optional solution, the imaging optical assembly can perform near-field imaging on the previous stage system or perform far-field imaging on the previous stage system.

[0071] As a preferred solution, the acquisition frame rate of the high-frame-rate camera can be greater than 500 fps, so as to realize real-time mode interference optical field data acquisition and processing.

[0072] For the measurement of the internal mode characteristics of the prepared to-be-tested fiber device, during the mode coupling test, the probe laser injection system 100 generates a swept-frequency laser, which is transmitted through the to-be-tested fiber device 200, and the mode components are affected by the internal mode coupling of the device, and the proportion of each core mode changes.

[0073] With the change of the injection swept-frequency laser wavelength during the injection of the swept-frequency laser into the to-be-tested fiber device 200, a periodic interference optical field is generated at the output end of the output signal tail fiber of the to-be-tested fiber device 200, and the periodic interference optical field contains multiple characteristic frequencies, and the frequency size is the differential mode group delay between different modes.

[0074] The image acquisition and mode component analysis system 300 acquires the intensity information of the periodic interference optical field under different wavelengths, extracts the intensity data at all positions in the two-dimensional space under different wavelengths, performs Fourier transform on the intensity data at all positions in the two-dimensional space, and obtains the mode number, high-order mode type, and relative content induced by mode coupling during the transmission of the probe laser inside the to-be-tested fiber device based on the Fourier transform result.

[0075] According to the type of the output signal fiber of the to-be-tested fiber device, if the output signal fiber of the to-be-tested fiber device is a two-mode fiber, the peak values in the Fourier transform result are the base mode (LP 01 ) characteristic peak, the first high-order mode (LP 11 ) characteristic peak in turn; if the output signal fiber of the to-be-tested fiber device is a fiber supporting more modes, the peak values in the Fourier transform result are the base mode (LP 01) characteristic peak, first high-order mode (LP 11 ) characteristic peak, second high-order mode (LP 21 ) characteristic peak, and so on. The order of the high-order mode characteristic peak is determined by the relative differential delay size of the current mode and the fiber fundamental mode. The smaller the differential delay, the higher the order. Under the premise of a certain total power of the output of the fiber device to be tested, the greater the peak intensity in the Fourier transform result, the higher the content of the corresponding mode. In this way, the mode number, high-order mode type, and relative content induced by mode coupling during the transmission of the probe laser in the fiber device to be tested are obtained.

[0076] Through the above test process, the high-order mode type and content induced by mode coupling during the transmission of the single-mode laser in the fiber device to be tested are analyzed, so that the device mode degradation behavior is accurately quantified, and the advantages and disadvantages of the current fiber device in terms of mode retention and beam quality are determined. Therefore, in an embodiment, a method for evaluating the mode retention characteristics of a fiber device is provided, which comprises:

[0077] The mode coupling test method provided in any of the above embodiments is used to obtain the mode number, high-order mode type, and relative content induced by mode coupling during the transmission of the probe laser in the fiber device to be tested.

[0078] According to the high-order mode type and relative content, the advantages and disadvantages of the fiber device to be tested in terms of mode retention and beam quality are determined. The fewer the number of high-order modes, and the lower the relative content, the better the mode retention and beam quality of the fiber device to be tested.

[0079] To verify the effectiveness of the fiber device mode coupling test method provided by the present application, a specific application example is provided below to test and analyze the internal mode coupling characteristics of a prepared mode field adapter. Through the fiber device mode coupling test, the test and analysis results obtained can be used for yield screening of the prepared fiber device, providing a "dissecting sparrow" single device in-depth analysis perspective for the subsequent construction of a fiber laser system based on the tested fiber device, which helps to analyze the system mode degradation causes in a fixed point and location.

[0080] In this embodiment, two types of mode field adapters, 6 / 125μm-20 / 400μm and 6 / 125μm-25 / 250μm, are tested. Each type of mode field adapter has two devices for direct comparison.

[0081] A setup as shown in FIG. 1 is built. Figure 4The shown test fiber link, single transverse mode swept fiber laser transmitted by single mode fiber SMF is injected into the fiber device to be tested, i.e. mode field adapter (MFA), through 6 / 125 μm single mode fiber. After the single transverse mode swept fiber laser is transmitted inside the mode field adapter, mode coupling behavior is generated, and the degraded laser mode is output through the 20 / 400 μm output signal fiber of the mode field adapter. Since the output signal fiber is short, a fiber with the same specification parameter, delay fiber, is spliced as a relay fiber or delay fiber. It can be understood that, Figure 4 The shown test fiber link is a test fiber link for the first type of mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter). For the second type of mode field adapter (6 / 125 μm-25 / 250 μm specification type mode field adapter), only the 6 / 125 μm-20 / 400 μm specification type mode field adapter in Figure 4 is replaced by a 6 / 125 μm-25 / 250 μm specification type mode field adapter.

[0082] Referring to Figure 5 The internal mode coupling test results of two first type mode field adapters (6 / 125 μm-20 / 400 μm specification type mode field adapters), wherein (a) is the internal mode coupling test result diagram of the first first type mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter), and (b) is the internal mode coupling test result diagram of the second first type mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter). From the test results, it can be seen intuitively that the ② type LP 11 mode content of the second first type mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter) is lower, and the rest of the high-order mode components are comparable to those of the first first type mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter), so it is preliminarily inferred that the second first type mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter) is better. In the actual construction process of the fiber laser system, from the perspective of system optimization of system mode control, the second first type mode field adapter (6 / 125 μm-20 / 400 μm specification type mode field adapter) can be preferably selected as a system component.

[0083] Referring to Figure 6The internal mode coupling test results of two second type of mode field adapters (6 / 125 μm-25 / 250 μm mode field adapter) are shown in (a) and (b). From the test results, it can be seen that the LP 11 The mode contents are equivalent. However, the ③ and ④ type of LP modes of the first second type of mode field adapter (6 / 125 μm-25 / 250 μm mode field adapter) are obviously lower than those of the second second type of mode field adapter (6 / 125 μm-25 / 250 μm mode field adapter). 02 The mode contents are equivalent. However, the ③ and ④ type of LP modes of the first second type of mode field adapter (6 / 125 μm-25 / 250 μm mode field adapter) are obviously lower than those of the second second type of mode field adapter (6 / 125 μm-25 / 250 μm mode field adapter).

[0084] The details of the present application are known.

[0085] The technical features of the above embodiments can be combined in any manner. In order to make the description simple, all possible combinations of the technical features in the above embodiments are not described, however, as long as the combinations of the technical features do not contradict, they should be considered as the scope of the present application.

[0086] The above embodiments only express several implementation manners of the present application, and the description is specific and detailed, however, it should not be understood as the limitation of the patent scope of the present application. It should be pointed out that, for the ordinary skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the patent protection scope of the present application should be subject to the appended claims.

Claims

1. A method of testing mode coupling in an optical fiber device, characterized by, The method comprises the following steps: a probe laser is injected into an input signal fiber of a fiber device to be tested, the probe laser being a swept laser; after the probe laser is transmitted through the fiber device to be tested from the input signal fiber, the core transmission mode of the probe laser is changed due to the influence of internal mode coupling of the fiber device to be tested, and the modulated probe laser is transmitted to an output signal fiber of the fiber device to be tested in a new mode state, and as the wavelength of the injected swept laser changes, periodic interference light fields are generated at the output end of the output signal fiber of the fiber device to be tested for each core mode, and the periodic interference light fields contain a plurality of characteristic frequencies, the frequency size being a differential mode group delay between different modes; light field information output by the output signal fiber is collected and mode field characteristics are analyzed to obtain the mode number, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be tested, including: collecting intensity information of the periodic interference light fields at different wavelengths output by the output signal fiber, extracting intensity data at all positions in two-dimensional space at different wavelengths, performing Fourier transform on the intensity data at all positions in two-dimensional space, and obtaining the mode number, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be tested based on the Fourier transform result. If all the output signal fibers of the fiber device under test are two-mode fibers, the peak values in the Fourier transform result are the fundamental mode LP 01 characteristic peak, first high-order mode LP 11 characteristic peak; If all the output signal fibers of the to-be-tested fiber device are multi-mode supporting fibers, the peak values in the Fourier transform result are the base mode LP 01 Characteristic peak, first high-order mode LP 11 Characteristic peak, second high-order mode LP 21 Characteristic peak, and so on. The order of the high-order mode characteristic peak is determined by the relative differential delay of the current mode and the base mode of the fiber. The smaller the differential delay, the higher the order. Under the premise that the total output power of the fiber device to be tested is constant, the greater the peak intensity in the Fourier transform result, the higher the content of the corresponding mode, so as to obtain the mode number, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be tested.

2. A method for evaluating a mode retention characteristic of an optical fiber device, characterized by, The method comprises the following steps: the mode coupling test method for the fiber device is used to obtain the mode number, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the fiber device to be tested; based on the mode number, high-order mode type and relative content, the advantages and disadvantages of the fiber device to be tested in terms of mode retention and beam quality are determined, wherein the fewer the number of high-order modes in the mode number and the lower the relative content of the high-order modes, the better the characteristics of the fiber device to be tested in terms of mode retention and beam quality.

3. An apparatus for testing mode coupling of fiber optic devices, characterized by, The method comprises the following steps: a probe laser injection unit is configured to generate a probe laser and inject the probe laser into an input signal fiber of a fiber device to be tested, the probe laser being a swept laser; the fiber device to be tested comprises at least two ports of an input signal fiber and an output signal fiber; after the probe laser is transmitted through the fiber device to be tested from the input signal fiber, the core transmission mode of the probe laser is changed due to the influence of internal mode coupling of the fiber device to be tested, and the modulated probe laser is transmitted to the output signal fiber of the fiber device to be tested in a new mode state, and as the wavelength of the injected swept laser changes, periodic interference light fields are generated at the output end of the output signal fiber of the fiber device to be tested for each core mode, and the periodic interference light fields contain a plurality of characteristic frequencies, the frequency size being a differential mode group delay between different modes. An image acquisition and mode component analysis unit is configured to acquire light field information output by the output signal fiber and analyze mode field characteristics, so as to obtain mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the optical fiber device to be detected, and comprises: acquiring intensity information of periodic interference light field output by the output signal fiber at different wavelengths, extracting intensity data at all positions in two-dimensional space at different wavelengths, performing Fourier transform on the intensity data at all positions in two-dimensional space, and obtaining mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the optical fiber device to be detected based on the Fourier transform result. If all the output signal fibers of the fiber device under test are two-mode fibers, the peak values in the Fourier transform result are the fundamental mode LP 01 characteristic peak, first high-order mode LP 11 characteristic peak; If all the output signal fibers of the to-be-tested fiber device are multi-mode supporting fibers, the peak values in the Fourier transform result are the base mode LP 01 Characteristic peak, first high-order mode LP 11 Characteristic peak, second high-order mode LP 21 Characteristic peak, and so on. The order of the high-order mode characteristic peak is determined by the relative differential delay of the current mode and the base mode of the fiber. The smaller the differential delay, the higher the order. Under the premise that the total output power of the optical fiber device to be detected is constant, the greater the peak intensity in the Fourier transform result is, the higher the content of the corresponding mode is, so as to obtain mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the optical fiber device to be detected.

4. The fiber device mode coupling test apparatus of claim 3, wherein, The probe laser injection unit comprises a tunable laser and a single-mode jumper, and the probe laser is output by the tunable laser and the single-mode jumper and then injected into the input signal fiber of the optical fiber device to be detected.

5. The fiber device mode coupling test apparatus of claim 4, wherein, The tunable laser has three controllable parameters of wavelength tuning range, wavelength tuning resolution and wavelength tuning speed.

6. The fiber device mode coupling test apparatus of claim 5, wherein, The minimum wavelength tuning range of the tunable laser is not less than 5 nm, the minimum wavelength tuning interval is not greater than 0.5 nm, and the fastest wavelength tuning speed is not less than 0.1 nm / s.

7. The fiber device mode coupling test apparatus of claim 4, wherein, The optical fiber device to be detected is any optical fiber device having a laser transmission purpose, and the optical fiber device to be detected has an external processing technology area, and the internal external processing technology area has an induced processing factor inducing coupling of the mode state of the optical fiber core or has a mechanical, chemical or optical processing technology leading to a coupling induced factor of the mode state of the optical fiber core, so that the optical fiber device has a non-ideal mode degradation.

8. The fiber device mode coupling test apparatus of claim 4, wherein, The image acquisition and mode component analysis unit comprises an imaging optical assembly, a high-frame-rate camera and a mode interference light field processing assembly. The imaging optical assembly transmits the mode interference light field output by the output signal fiber to the high-frame-rate camera. The high-frame-rate camera records intensity data of the mode interference light field. The mode interference light field processing assembly performs Fourier transform on the intensity data of the mode interference light field, and obtains mode quantity, high-order mode type and relative content induced by mode coupling when the probe laser is transmitted in the optical fiber device to be detected based on the Fourier transform result.

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