Fiber grating mode selection characteristic testing device and method

Abstract

The application provides a fiber grating mode selection characteristic testing device and method, and the output characteristic of testing light, the output characteristic of reflected light of a to-be-tested fiber grating and the output characteristic of transmitted light of the to-be-tested fiber grating are obtained by using the fiber grating mode selection characteristic testing device, the output characteristic of testing light, the output characteristic of reflected light of a to-be-tested fiber grating and the output characteristic of transmitted light of the to-be-tested fiber grating are compared and analyzed, and the mode selection characteristic of the to-be-tested fiber grating is obtained. The application quantitatively analyzes the characteristic of the fiber grating used in the fiber laser system based on a simple spatial light path, so as to provide technical support for the design of the fiber laser system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fiber lasers, and particularly relates to a fiber grating mode selection characteristic testing device and method. BACKGROUND

[0002] Fiber lasers have good beam quality, compact structure, stability and easy use, and are widely used in industries, national defense and the like. A fiber oscillator is used for generating a stable optical signal, and one of necessary components of the fiber oscillator is a fiber grating. The period and refractive index variation of the fiber grating can be designed and manufactured according to the required application, and can be used for selectively coupling or reflecting a specific mode of an optical signal in the fiber. By adjusting the characteristics (such as reflectivity, bandwidth, etc.) of the fiber grating, the working frequency and other performance parameters of the oscillator can be controlled.

[0003] The modes of laser light are mainly divided into transverse modes and longitudinal modes. The transverse mode is a spatial distribution mode on the cross-sectional plane of the laser beam. It describes the intensity distribution and wavefront shape of the laser beam in the transverse plane. The transverse mode is usually described by the Gaussian beam model and classified by the TEM label, such as TEM00, TEM01, TEM10, etc. Among them, the TEM00 mode is the most basic Gaussian mode, which has the smallest spot size and the lowest beam waist factor, and is therefore the most ideal in many applications. The longitudinal mode is the mode of laser light in the optical axis direction. It describes the frequency and wavelength characteristics of the laser beam, that is, it represents the resonant cavity structure and long-range coherence of the laser. The longitudinal mode is usually determined by the resonant cavity length and surrounding conditions of the laser. Different resonant modes correspond to different wavelengths of laser output. For example, in a continuous wave laser, the resonant cavity can support multiple longitudinal modes, thereby generating multiple stable output wavelengths.

[0004] The transverse mode and the longitudinal mode often exist simultaneously in the output of the laser. By properly designing and controlling the resonant cavity structure and working parameters of the laser, a specific transverse mode and longitudinal mode configuration can be achieved to meet the needs of specific applications. The stability and selectivity of the transverse mode and the longitudinal mode are crucial for the performance and application of the laser. The fiber grating is a key component for realizing the selection of laser transverse mode and longitudinal mode characteristics in the fiber resonant cavity, and convenient and rapid testing of the mode selection characteristics of the fiber grating is of great significance for subsequent laser system building and optimization. At present, some technologies have been used to test the temperature rise, polarization characteristics, diffraction efficiency, power and spectral characteristics of the grating, but none of them has systematically tested and analyzed the mode selection characteristics of the fiber grating. SUMMARY

[0005] In view of the problems in the prior art, the application provides a fiber grating mode selection characteristic testing device and method. The mode (lateral mode and longitudinal mode) selection characteristics of the fiber grating can be obtained by comparing and analyzing the reflection light characteristics (power, spectrum, and light spot) and transmission light characteristics (power, spectrum, and light spot) of the fiber grating with the output characteristics (power, spectrum, and light spot) of the light source.

[0006] The application aims to quantitatively analyze the characteristics of the fiber grating used in the fiber laser system based on a simple spatial light path, thereby providing technical support for the design of the fiber laser system. The main feature of the application is that the power and spectrum characteristics of the fiber grating can be quantitatively analyzed, and the spot characteristics of the fiber grating can be quantitatively analyzed. In combination with the mode decomposition technology, the lateral mode selection characteristics of the fiber grating can be optimized, thereby guiding the design of the fiber grating with the required specific mode output characteristics.

[0007] To achieve the above technical purpose, the application adopts the following technical scheme:

[0008] The fiber grating mode selection characteristic testing device comprises a testing light source, a fiber collimator, an aperture diaphragm, a broadband beam splitter, a fiber grating to be tested, and a plurality of testing units.

[0009] The testing light source is used for outputting testing light, and the output wavelength range of the testing light source covers the center reflection wavelength of the fiber grating to be tested.

[0010] The testing light output by the testing light source is sequentially incident on the broadband beam splitter after passing through the fiber collimator and the aperture diaphragm. The testing light is split into two beams by the broadband beam splitter. One of the two beams is reflected by the broadband beam splitter and then incident on the first testing unit through the first lens. The output characteristics of the testing light are monitored by the first testing unit. The other beam is transmitted by the broadband beam splitter and then coupled into the fiber grating to be tested through the second lens. The transmission light transmitted by the fiber grating to be tested is incident on the second testing unit through the third lens after passing through the third lens. The output characteristics of the transmission light of the fiber grating to be tested are monitored by the second testing unit. The reflection light reflected by the fiber grating to be tested is reflected by the broadband beam splitter to the fourth lens, and then incident on the third testing unit through the fourth lens. The output characteristics of the reflection light of the fiber grating to be tested are monitored by the third testing unit.

[0011] In another aspect, a fiber grating mode selection characteristic testing method is provided, which comprises the following steps:

[0012] Using the aforementioned fiber optic grating mode selection characteristic testing device, the output characteristics of the test light, the reflected light output characteristics of the fiber optic grating under test, and the reflected light output characteristics of the fiber optic grating under test are obtained. Each testing unit in the fiber optic grating mode selection characteristic testing device includes a power meter, a spectrometer, and a CCD, used to monitor power, spectrum, and spot characteristics, respectively. The output characteristics of the test light include power, spectrum, and spot characteristics; the transmitted light output characteristics of the fiber optic grating under test include power, spectrum, and spot characteristics; and the reflected light output characteristics of the fiber optic grating under test include power, spectrum, and spot characteristics.

[0013] The output characteristics of the test light, the reflected light output characteristics of the fiber grating under test, and the reflected light output characteristics of the fiber grating under test are compared and analyzed to obtain the mode selection characteristics of the fiber grating under test, wherein the modes of the fiber grating under test include transverse mode and longitudinal mode.

[0014] On the other hand, a method for evaluating fiber Bragg grating writing quality based on fiber Bragg grating mode selection characteristics is proposed, including:

[0015] Using the aforementioned fiber optic grating mode selection characteristic testing device, the output characteristics of the test light, the reflected light output characteristics of the fiber optic grating under test, and the reflected light output characteristics of the fiber optic grating under test are obtained. Each testing unit in the fiber optic grating mode selection characteristic testing device includes a power meter, a spectrometer, and a CCD, used to monitor power, spectrum, and spot characteristics, respectively. The output characteristics of the test light include power, spectrum, and spot characteristics; the transmitted light output characteristics of the fiber optic grating under test include power, spectrum, and spot characteristics; and the reflected light output characteristics of the fiber optic grating under test include power, spectrum, and spot characteristics.

[0016] The test light spot in the output characteristics of the test light, the reflected light spot in the output characteristics of the reflected light of the fiber optic grating under test, and the transmitted light spot in the output characteristics of the transmitted light of the fiber optic grating under test are decomposed using fiber mode decomposition technology to obtain the corresponding mode decomposition results.

[0017] The mode decomposition results of the test light spot and the reflected light spot are compared, and the fiber Bragg grating writing quality is evaluated based on the comparison results.

[0018] Compared with the prior art, the beneficial technical effects that this invention can produce are:

[0019] The purpose of the present application is to quantitatively analyze the characteristics of the fiber grating used in the fiber laser system based on the simple spatial light path, so as to provide technical support for the design of the fiber laser system. The main feature of the present application is that in addition to the quantitative analysis of the spectrum and power characteristics of the fiber grating, the spot characteristics of the fiber grating can also be quantitatively analyzed, and the transverse mode selection characteristics of the fiber grating can be optimized by combining the mode decomposition technology, thereby guiding the design of the fiber grating with the required specific mode output characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained according to the structures shown in the drawings without creative labor for those skilled in the art.

[0021] Figure 1 The structure diagram of the fiber grating mode selection characteristic testing device in the embodiment 1 of the present application;

[0022] Figure 2 The structure diagram of the fiber grating mode selection characteristic testing device in the embodiment 2 of the present application;

[0023] Figure 3 The reflection light spot diagram of the test light source and the fiber grating in an embodiment, wherein Figure 3 (a) is the reflection light spot diagram of the test light source measured by the CCD at the third test unit 12, Figure 3 (b) is the reflection light spot diagram of the fiber grating measured by the CCD at the first test unit 10;

[0024] Figure 4 The three-dimensional reflection light spot diagram of the test light source and the fiber grating in an embodiment, wherein Figure 4 (a) is the three-dimensional reflection light spot diagram of the test light source measured by the CCD at the third test unit 12, Figure 4 (b) is the three-dimensional reflection light spot diagram of the fiber grating measured by the CCD at the first test unit 10;

[0025] Figure 5 The spectrum diagram of the test light source, the transmission light of the fiber grating and the reflection light of the fiber grating in an embodiment, wherein Figure 5 (a) is the spectrum diagram of the test light source measured by the spectrometer at the third test unit 12; Figure 5 (b) is the transmission spectrum diagram of the fiber grating measured by the spectrometer at the second test unit 11; Figure 5 (c) is the reflection spectrum diagram of the fiber grating measured by the spectrometer at the first test unit 10;

[0026] Figure 6 Fig. 1 is a structural schematic diagram of a fiber grating to be tested in an embodiment of the present application.

[0027] The implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the following will clearly explain the spirit of the present application with the accompanying drawings and detailed description. Any person skilled in the art can make changes and modifications to the technology taught by the present application content after understanding the embodiments of the present application content, which does not deviate from the spirit and scope of the present application content. The illustrative embodiments of the present application and their descriptions are used to explain the present application, but not as a limitation of the present application.

[0029] In an embodiment, referring to Figure 1 , a fiber grating mode selection characteristic testing device is provided, which comprises a test light source 1, a fiber collimator 2, a pinhole diaphragm 3, a broadband beam splitter 4, a fiber grating to be tested 6 and a plurality of test units;

[0030] The test light source 1 is used to output test light, and the output wavelength range of the test light source 1 covers the center reflection wavelength of the fiber grating to be tested;

[0031] The test light output by the test light source 1 is sequentially incident to the broadband beam splitter 4 after passing through the fiber collimator 2 and the pinhole diaphragm 3, and is split into two beams by the broadband beam splitter 5, one of which is reflected by the broadband beam splitter 5 and then incident to the first test unit 10 through the first lens 9, and the output characteristics of the test light are monitored by the first test unit 10;

[0032] The other beam is transmitted through the broadband beam splitter 4 and then coupled into the fiber grating to be tested 6 through the second lens 5, and the transmitted light transmitted by the fiber grating to be tested 6 is incident to the second test unit 11 through the third lens 7, and the output characteristics of the transmitted light of the fiber grating to be tested are monitored by the second test unit 11;

[0033] The reflected light reflected by the fiber grating to be tested 6 is reflected by the broadband beam splitter 4 to the fourth lens 8 through the second lens 5, and then incident to the third test unit 12 through the fourth lens 8, and the output characteristics of the reflected light of the fiber grating to be tested are monitored by the third test unit 12.

[0034] The test units each include a power meter, a spectrometer and a CCD for monitoring power, spectrum and spot characteristics, respectively, the output characteristics of the test light include power, spectrum and spot characteristics, the transmission light output characteristics of the fiber grating to be tested include power, spectrum and spot characteristics, and the reflection light output characteristics of the fiber grating to be tested include power, spectrum and spot characteristics.

[0035] Further, the test light source 1 can be a white light source, an ASE source or a laser light source with a center reflection wavelength.

[0036] Further, the output spectrum bandwidth of the test light source 1 is greater than or equal to the reflection bandwidth of the fiber grating to be tested.

[0037] Further, the test light source 1 can be a continuous light source or a pulsed light source.

[0038] Further, the broadband beam splitter 4 used has a wavelength range of 500-2000 nm and can be a polarization-maintaining broadband beam splitter or a non-polarization-maintaining broadband beam splitter.

[0039] Further, the lenses used in the first lens, the second lens, the third lens and the fourth lens can be double convex lenses, or plano-convex lenses, or a combination of double convex lenses and plano-convex lenses.

[0040] Further, the type of the fiber grating to be tested is not limited and can be a fiber grating based on a gradient refractive index fiber or a fiber grating based on a step refractive index fiber, and the fiber can be a single-clad fiber or a multi-clad fiber, a conventional uniform fiber or a tapered fiber or other shaped fiber grating, and can be a polarization-maintaining fiber or a non-polarization-maintaining fiber. Figure 6 As shown in FIG. 1, the fiber grating to be tested in an embodiment is shown in a structural schematic diagram.

[0041] Referring to Figure 2 An embodiment provides a fiber grating mode selection characteristic testing device, which includes Figure 1The test light source 1, the fiber collimator 2, the pinhole diaphragm 3, the broadband beam splitter 4, the to-be-tested fiber grating 6, the first test unit 10, the second test unit 11 and the third test unit 12 shown in the figure are further provided with a plurality of reserved fibers. Specifically, a first reserved fiber 13 is provided at the tail fiber of the fiber collimator 2, a second reserved fiber 14 is provided at the input end of the to-be-tested fiber grating 6, and a third reserved fiber 15 is provided at the output end of the to-be-tested fiber grating 6. One end of the reserved fiber is fixed in the spatial light path, and the other end is used for fusion splicing with the tail fiber of the corresponding connecting fiber. The purpose of providing the reserved fiber is that when different center reflection wavelength gratings are written using the same specification fiber, the fusion splicing method can be directly used to avoid system errors caused by repeated adjustment of the optical path, so that the data comparison is more accurate and the debugging is more convenient and fast.

[0042] Because the fiber grating has mode selection characteristics, both the transverse mode of light and the longitudinal mode of light can be selected. At the same time, the mode selectivity of gratings of different writing methods (such as ultraviolet writing and femtosecond writing) differs greatly, so how to quickly and conveniently test and analyze the mode selection characteristics is of great significance to the subsequent establishment of fiber laser systems. Based on the broadband beam splitter, the output characteristics of the test light source, the transmission light output characteristics of the fiber grating and the reflection light output characteristics of the fiber grating can be monitored at the same time. Combined with data analysis tools such as fiber mode decomposition technology, the mode selection characteristics of the to-be-tested fiber grating can be quickly tested, and at least the number of modes contained in the reflection light of the to-be-tested fiber grating and the number of modes contained in the test light source are obtained. Further, the fiber mode decomposition technology used is the existing fiber mode decomposition method in the prior art, and the person skilled in the art can freely select the commonly used and conventional fiber mode decomposition method, including but not limited to the fiber mode decomposition method based on deep learning and readable medium disclosed in the publication No. CN111103120B, which can directly analyze the mode of the light spot collected by the CCD.

[0043] In an embodiment, a fiber grating mode selection characteristic testing method is provided, comprising:

[0044] The fiber grating mode selection characteristic testing device provided by any of the above embodiments is used to obtain the output characteristics of the test light, the reflection light output characteristics of the to-be-tested fiber grating and the reflection light output characteristics of the to-be-tested fiber grating. Each test unit in the fiber grating mode selection characteristic testing device includes a power meter, a spectrometer and a CCD, which are respectively used for monitoring the power, the spectrum and the light spot characteristics. The output characteristics of the test light include the power, the spectrum and the light spot characteristics. The transmission light output characteristics of the to-be-tested fiber grating include the power, the spectrum and the light spot characteristics. The reflection light output characteristics of the to-be-tested fiber grating include the power, the spectrum and the light spot characteristics.

[0045] The output characteristic of the test light, the reflection light output characteristic of the to-be-tested fiber grating, and the reflection light output characteristic of the to-be-tested fiber grating are compared and analyzed, and the mode selection characteristic of the to-be-tested fiber grating is obtained, wherein the modes of the to-be-tested fiber grating include transverse modes and longitudinal modes. The mode selection characteristic of the to-be-tested fiber grating includes the number of modes contained in the reflection light of the fiber grating.

[0046] Further, the performance of the to-be-tested fiber grating is evaluated based on the obtained mode selection characteristic of the to-be-tested fiber grating, and the fiber grating with a specific mode output characteristic is designed accordingly.

[0047] A method for evaluating the writing quality of a fiber grating based on a mode selection characteristic of the fiber grating, comprising:

[0048] The output characteristic of the test light, the reflection light output characteristic of the to-be-tested fiber grating, and the reflection light output characteristic of the to-be-tested fiber grating are obtained by using the fiber grating mode selection characteristic testing device provided in any of the embodiments, wherein each testing unit in the fiber grating mode selection characteristic testing device includes a power meter, a spectrometer, and a CCD, which are respectively used for monitoring power, spectrum, and spot characteristics. The output characteristic of the test light includes power, spectrum, and spot characteristics. The transmission light output characteristic of the to-be-tested fiber grating includes power, spectrum, and spot characteristics. The reflection light output characteristic of the to-be-tested fiber grating includes power, spectrum, and spot characteristics.

[0049] The test light spot in the output characteristic of the test light, the reflection light spot in the reflection light output characteristic of the to-be-tested fiber grating, and the transmission light spot in the transmission light output characteristic of the to-be-tested fiber grating are respectively mode-decomposed by using a fiber mode decomposition technology, and corresponding mode-decomposition results are obtained.

[0050] The mode-decomposition result of the test light spot is compared with the mode-decomposition result of the reflection light spot, and the writing quality of the fiber grating is evaluated based on the comparison result.

[0051] The number of modes contained in the reflection light of the fiber grating is determined according to the reflection light output characteristic of the to-be-tested fiber grating. When the number of modes contained in the reflection light of the fiber grating is less than or equal to 3 modes of the lowest order, the writing quality of the fiber grating is excellent. When the number of modes contained in the reflection light of the fiber grating is greater than 3 low-order modes, the writing quality of the fiber grating is poor. When the number of modes contained in the reflection light of the fiber grating is the same as the number of modes contained in the test light source, the fiber grating does not have a mode selection characteristic.

[0052] A specific application example of the present application is given below. The test light source used in the application example is a fiber laser with an output wavelength of 1015-1025 nm, a central wavelength of 1020 nm, and a 5% low reflectivity fiber grating with a reflection bandwidth of 1 nm. The fiber grating is based on a 20 / 400 um step-index fiber and is written by femtosecond laser with a numerical aperture of 0.06. The fiber grating mode selection characteristic testing device provided by any of the above embodiments is used to test the fiber grating mode selection characteristic of the fiber grating to be tested. Figure 3 The reflection light spot diagram of the test light source and the fiber grating is given below, wherein Figure 3 (a) The reflection light spot diagram of the test light source measured by the CCD at the third test unit 12, and the mode number obtained by mode decomposition of the spot is greater than 3. Figure 3 (b) The reflection light spot diagram of the fiber grating measured by the CCD at the first test unit 10, and the mode number obtained by mode decomposition of the spot is less than or equal to 3.

[0053] Figure 4 The reflection light spot diagram of the test light source and the fiber grating is given below, wherein Figure 4 (a) The reflection light spot diagram of the test light source measured by the CCD at the third test unit 12, Figure 4 (b) The reflection light spot diagram of the fiber grating measured by the CCD at the first test unit 10, and the mode number obtained by mode decomposition of the spot is less than or equal to 3. Figure 4 (a) more intuitive and Figure 4 (b) The reflection light spot diagram of the fiber grating is compared with the reflection light three-dimensional spot diagram of the fiber grating, and the transverse mode selection characteristic of the fiber grating can be obtained by analyzing the spot.

[0054] Figure 5 The spectrum diagram of the test light source, the fiber grating transmission light, and the fiber grating reflection light is given below, wherein Figure 5 (a) The spectrum diagram of the test light source measured by the spectrometer at the third test unit 12; Figure 5 (b) The transmission spectrum diagram of the fiber grating measured by the spectrometer at the second test unit 11; Figure 5 (c) The reflection spectrum diagram of the fiber grating measured by the spectrometer at the first test unit 10. By analyzing the spectrum, the longitudinal mode selection characteristic of the fiber grating can be obtained.

[0055] In addition, by monitoring the power of the fiber grating transmission light and the power of the reflection light (the normal power ratio is 95:5), it can be verified whether the optical path of the fiber grating mode selection characteristic testing device is adjusted well, and the reflectivity of the fiber grating can also be verified.

[0056] In addition, by monitoring the power of the transmission light and the power of the reflection light of the fiber grating, it can be verified whether the optical path of the fiber grating mode selection characteristic testing device is coupled well. When the reflectivity of the fiber grating is known, the ratio of the transmission light power to the reflection light power is normally (1-fiber grating reflectivity): fiber grating reflectivity. The reflectivity of the fiber grating used in this embodiment is 5%, so the ratio of the transmission light power to the reflection light power is (1-5%): 5%=95:5. When the ratio of the power is not 95:5, the optical path of the fiber grating mode selection characteristic testing device needs to be adjusted again, and then the light spot and the spectrum are measured.

[0057] The details of the present application are known.

[0058] The technical features of the above embodiments can be combined arbitrarily. In order to make the description simple, all possible combinations of the technical features in the above embodiments are not described, but as long as the combinations of the technical features do not contradict, they should be considered as the scope of the present application.

[0059] The above embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the scope of the present application. Therefore, the scope of the patent of the present application should be subject to the appended claims.

Claims

1. A device for testing the mode select properties of a fiber grating, characterized by, The test light source, the fiber collimator, the pinhole diaphragm, the broadband beam splitter, the to-be-tested fiber grating, and the plurality of test units are included. The test light source is configured to output test light, and an output wavelength range of the test light source covers a center reflection wavelength of the to-be-tested fiber grating. The test light output by the test light source is sequentially incident on the broadband beam splitter after passing through the fiber collimator and the pinhole diaphragm, and is split into two beams by the broadband beam splitter. One of the two beams is reflected by the broadband beam splitter and is incident on the first test unit through the first lens, and the output characteristics of the test light are monitored by the first test unit. The other beam is transmitted by the broadband beam splitter and is coupled into the to-be-tested fiber grating through the second lens, and the transmission light output by the to-be-tested fiber grating is incident on the second test unit through the third lens, and the transmission light output characteristics of the to-be-tested fiber grating are monitored by the second test unit. The reflection light output by the to-be-tested fiber grating is reflected by the broadband beam splitter and is incident on the third test unit through the fourth lens, and the reflection light output characteristics of the to-be-tested fiber grating are monitored by the third test unit. Each test unit includes a power meter, a spectrometer, and a CCD, which are respectively configured to monitor the power, the spectrum, and the spot characteristics. The output characteristics of the test light include the power, the spectrum, and the spot characteristics. The transmission light output characteristics of the to-be-tested fiber grating include the power, the spectrum, and the spot characteristics. The reflection light output characteristics of the to-be-tested fiber grating include the power, the spectrum, and the spot characteristics. The test light spot in the output characteristics of the test light, the reflection light spot in the reflection light output characteristics of the to-be-tested fiber grating, and the transmission light spot in the transmission light output characteristics of the to-be-tested fiber grating are respectively mode-decomposed by using a fiber mode-decomposition technology, and corresponding mode-decomposition results are obtained, and then the mode selection characteristics of the to-be-tested fiber grating are obtained.

2. The fiber grating mode selection property test apparatus according to claim 1, wherein, The test light source is a white light source, or the test light source is an ASE source, or the test light source is a laser light source corresponding to the center reflection wavelength of the fiber grating.

3. The fiber grating mode selection property test apparatus according to claim 2, wherein, The output spectral bandwidth of the test light source is greater than or equal to the reflection bandwidth of the to-be-tested fiber grating.

4. The fiber grating mode selection property testing apparatus according to claim 1 or 2 or 3, characterized in that, The first lens, the second lens, the third lens, and the fourth lens are double-convex lenses or plano-convex lenses.

5. The fiber grating mode selection property test apparatus according to claim 4, wherein, The to-be-tested fiber grating is a fiber grating based on a gradient refractive index fiber or a fiber grating based on a step refractive index.

6. The fiber grating mode selection property test apparatus according to claim 4, wherein The device further includes a remaining fiber, and the remaining fiber is arranged at the tail fiber of the fiber collimator, at the input end of the to-be-tested fiber grating, and at the output end of the to-be-tested fiber grating.

7. The fiber grating mode selection property test apparatus according to claim 4, wherein The modes of the to-be-tested fiber grating include transverse modes and longitudinal modes.

8. A method for evaluating the quality of writing of a fiber grating based on the mode selection characteristics of the fiber grating, characterized in that, The device further includes: The device is used to obtain the output characteristics of the test light, the reflection light output characteristics of the to-be-tested fiber grating, and the transmission light output characteristics of the to-be-tested fiber grating. The test light spot in the output characteristics of the test light, the reflection light spot in the reflection light output characteristics of the to-be-tested fiber grating, and the transmission light spot in the transmission light output characteristics of the to-be-tested fiber grating are respectively mode-decomposed by using a fiber mode-decomposition technology, and corresponding mode-decomposition results are obtained. The mode decomposition result of the test light spot is compared with the mode decomposition result of the reflected light spot, and the fiber grating inscription quality is evaluated based on the comparison result.

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