Method and apparatus for measuring relative refractive index difference of optical fiber core and cladding

Abstract

The application relates to a method and device for measuring the relative refractive index difference between a fiber core and a cladding. The method comprises: inputting single transverse mode laser beams of different wavelengths in a target application wave band into a fiber to be measured at equal wavelength intervals, obtaining a light spot pattern of the fiber to be measured at each wavelength, and then obtaining a two-dimensional measurement matrix with the row and column sizes being the number of pixel points of the light spot pattern and the number of sampling wavelengths; performing Fourier transform on each row of data of the two-dimensional measurement matrix and adding the results to obtain a differential mode group delay test value of a first high-order mode relative to a base mode; and searching a lookup table of the change of a differential mode group delay theoretical value corresponding to a high-order mode of the fiber to be measured with the core-cladding refractive index difference to find a differential mode group delay theoretical value closest to the differential group delay test value under the core diameter of the fiber to be measured, and obtaining the core-cladding refractive index difference of the fiber to be measured. The method can realize efficient, accurate and non-invasive measurement of the core-cladding refractive index difference of a long link fiber.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber testing, in particular to a method and device for measuring the relative refractive index difference between the core and cladding of an optical fiber. BACKGROUND

[0002] As a kind of circularly symmetric waveguide, the key indicators determining the mode characteristics and transmission performance of optical fiber mainly include fiber core diameter and core-cladding refractive index difference. If the above characteristic parameters are given, the normalized frequency, transverse mode number, guided mode characteristics and other information of the optical fiber are also determined. Among them, the fiber core diameter, as a geometric feature of the optical fiber, can be quickly measured by an optical microscope, and the core-cladding refractive index difference usually needs to be measured by a more complex testing device. At present, the mainstream methods include near-field estimation method, scanning end-face reflection method, refractive near-field method, digital holography method and focusing method, etc.

[0003] However, the traditional method can generally only test a piece of fiber sample with a length of several centimeters, which means that the tested optical fiber must be sampled, and the test results may have local particularity. In optical communication and optical fiber laser applications, the length of the optical fiber used is usually tens of meters, hundreds of meters or even thousands of meters. If the Δn (core-cladding refractive index difference) of the long-link optical fiber is to be measured integrally and non-invasively, new technical means need to be developed. Considering that the process of optical fiber production usually includes several steps such as preform preparation, fiber drawing and rewinding screening, even if the optical fibers are based on the same preform and the same batch, the Δn may still fluctuate. Therefore, how to segmentally and globally test the Δn of a large number of long-link optical fibers is a problem to be solved.

[0004] Further, in the application process of few-mode optical fiber, especially in the application of large-core, low-NA (Numerical Aperture, numerical aperture) few-mode optical fiber for optical fiber laser, the tens of meters of gain optical fiber are usually tightly coiled on a heat sink device. Bending will produce tensile stress and compressive stress on the outside and inside of the optical fiber, respectively, and then cause changes in the refractive index distribution. This refractive index modulation cannot be measured using conventional refractive index testing instruments. Although the refractive index change caused by stress can be estimated according to the photoelastic effect and stress optical model, the virtual refractive index distribution of the bent waveguide is simulated based on conformal transformation, and the refractive index distribution in the straight waveguide is corrected, but whether the mapping optical fiber can well correspond to the actual situation still needs further study.

[0005] In addition, for few-mode or multi-mode optical fibers, it is generally difficult to achieve pure mode excitation in the near-field output, and the high-order mode components will introduce another potential measurement error; the measurement accuracy of the scanning end-face reflection method is limited by the spatial sampling interval of the end-face reflected light intensity, and it is difficult to provide sufficient spatial resolution; the measurement time of the near-field refraction method is generally longer, the pre-processing process of the optical fiber sample is required to be higher, and the measurement environment cleanliness and vibration are sensitive, and it is difficult to achieve fast and efficient measurement; the digital holography method has high accuracy and wide application range, but the measurement equipment is relatively expensive, and a complex pre-adjustment is required before testing; the focusing method has high stability requirements for the light source, and the measurement accuracy is generally poor. Due to the limitations of measurement efficiency, measurement accuracy, spatial resolution, sample preparation and other factors, it is generally difficult to achieve high efficiency and high accuracy of the core-cladding refractive index difference measurement. SUMMARY

[0006] Therefore, it is necessary to provide a method and device for measuring the relative refractive index difference between the core and the cladding of an optical fiber.

[0007] A method for measuring the relative refractive index difference between the core and the cladding of an optical fiber, the method comprising:

[0008] A tunable laser source is used to inject single transverse mode laser beams at different wavelengths in the target application band into the input end of the optical fiber to be measured at equal wavelength intervals, and the spot patterns of the optical fiber to be measured at each wavelength are obtained at the output end of the optical fiber; the optical fiber to be measured is a long optical fiber; the optical fiber to be measured transmits at least two core modes of the fundamental mode and the first high-order mode, and the number of high-order modes can be further increased on this basis;

[0009] The intensity data of each pixel point of the spot pattern at each wavelength is extracted to obtain a two-dimensional measurement matrix with the number of rows and columns being the number of pixel points of the spot pattern and the number of sampling wavelengths, respectively. The Fourier transform results are obtained by adding each row of data of the two-dimensional measurement matrix after Fourier transform.

[0010] According to the characteristic peak of the first high-order mode in the Fourier transform result, a differential mode group delay test value of the first high-order mode relative to the fundamental mode is obtained.

[0011] According to the high-order mode transmitted by the optical fiber to be measured, a lookup table of the change of the theoretical value of the differential mode group delay corresponding to the high-order mode with the core-cladding refractive index difference is established, and the theoretical value of the differential group delay closest to the differential group delay test value under the core diameter of the optical fiber to be measured is found according to the lookup table, to obtain the core-cladding refractive index difference of the optical fiber to be measured.

[0012] In one of the embodiments, further comprising: performing Fourier transform on intensity data corresponding to each row vector of the two-dimensional measurement matrix to obtain F sets of one-dimensional Fourier transform results, where F is the number of pixel points of the spot pattern, and the intensity data is:

[0013]

[0014] where I is the field intensity distribution of the interference light field, I1 is the field intensity of the fundamental mode, I2 is the field intensity corresponding to the high-order mode used to establish the lookup table, is a constant phase difference, Δτ is a differential group delay, L is the length of the optical fiber to be measured, Δω is the difference frequency of the two frequency classifications of the interference light field, is the group refractive index of the fundamental mode, is the group refractive index of the high-order mode, c is the speed of light, Δn eff is the effective refractive index difference of the mode; and the F sets of one-dimensional Fourier transform results are added to obtain the Fourier transform result.

[0015] In one of the embodiments, further comprising: obtaining a wavelength scanning interval corresponding to a current measurement accuracy according to a correspondence relationship between a preset measurement accuracy threshold and the wavelength scanning interval; obtaining a scanning start wavelength and a scanning end wavelength of the tunable laser light source according to a target application wavelength band of the optical fiber to be measured and the wavelength scanning interval; the tunable laser light source is used to output a laser beam with a tuned wavelength; the laser beam is incident into an input end of the optical fiber to be measured at an equal wavelength interval by using the tunable laser light source, and a core mode of the optical fiber to be measured is excited; and an optical fiber output light field at each wavelength within the scanning start wavelength and the scanning end wavelength is frame captured at an optical fiber output end to obtain a spot pattern of the optical fiber to be measured at each wavelength.

[0016] In one of the embodiments, further comprising: when the measurement accuracy threshold is 0.001, the wavelength scanning interval is not less than 5 nm; when the measurement accuracy threshold is 0.0008, the wavelength scanning interval is not less than 18 nm; when the measurement accuracy threshold is 0.0005, the wavelength scanning interval is not less than 25 nm; and when the measurement accuracy threshold is 0.0001, the wavelength scanning interval is not less than 30 nm.

[0017] In one of the embodiments, further comprising: a capture frame rate of frame capturing the optical fiber output light field at each wavelength within the scanning start wavelength and the scanning end wavelength at the optical fiber output end is:

[0018]

[0019] where R c is the capture frame rate, R s is the wavelength tuning resolution of the tunable laser light source, and A is the wavelength scanning speed of the tunable laser light source.

[0020] In one embodiment, further comprising: the mode group delay accumulation of the first high-order mode relative to the fundamental mode in the transmission fiber of the current length is not less than 5 times the Fourier transform resolution; the Fourier transform resolution is:

[0021]

[0022] wherein, R f is the Fourier transform resolution, λ1 is the initial scanning wavelength of the tunable laser source, λ2 is the terminal scanning wavelength of the tunable laser source, and c is the vacuum light speed; the mode group delay accumulation of the first high-order mode relative to the fundamental mode in the transmission fiber of the current length is not more than the maximum measurable group delay; the maximum measurable group delay is:

[0023]

[0024] wherein, DMGD max is the maximum measurable group delay, R s is the wavelength tuning resolution of the tunable laser source.

[0025] In one embodiment, further comprising: the fiber under test includes a gain fiber, an energy transfer fiber, or a microstructure fiber.

[0026] In one embodiment, further comprising: the fiber under test is in a flat state or a coiled state.

[0027] In one embodiment, further comprising: the acquisition object of the each frame of spot pattern includes a near-field light intensity distribution or a far-field light intensity distribution.

[0028] A device for measuring the relative refractive index difference between a fiber core and a cladding, the device comprising:

[0029] a spot pattern acquisition module configured to use a tunable laser source to input single-mode laser beams of different wavelengths in a target application band into an input end of a fiber under test at equal wavelength intervals, and obtain spot patterns of the fiber under test at each wavelength at an output end of the fiber under test; the fiber under test is a long fiber; the fiber under test transmits at least a fundamental mode and a first high-order mode, and the number of high-order modes can be further increased;

[0030] an intensity data processing module configured to extract intensity data of each pixel point of the spot pattern at each wavelength, obtain a two-dimensional measurement matrix with the number of rows and columns being the number of pixel points of the spot pattern and the number of sampling wavelengths respectively, and add each row of data of the two-dimensional measurement matrix after Fourier transform to obtain a Fourier transform result;

[0031] a differential group delay test module configured to obtain a differential mode group delay test value of the first high-order mode relative to the fundamental mode according to a characteristic peak of the first high-order mode in the Fourier transform result;

[0032] a result output module configured to establish a lookup table of differential mode group delay theoretical values corresponding to the high-order modes and varying with a core-cladding refractive index difference, and find a differential mode group delay theoretical value closest to the differential group delay test value at a core diameter of the to-be-tested optical fiber according to the lookup table, to obtain the core-cladding refractive index difference of the to-be-tested optical fiber.

[0033] A computer device includes a memory and a processor, the memory stores a computer program, and the processor implements the following steps when executing the computer program:

[0034] Single transverse mode laser beams of different wavelengths in a target application waveband are incident on an input end of a to-be-tested optical fiber at equal wavelength intervals by using a tunable laser source, and spot patterns of the to-be-tested optical fiber at each wavelength are obtained at an output end of the optical fiber; the to-be-tested optical fiber is a long optical fiber; the to-be-tested optical fiber at least transmits two core modes of a fundamental mode and a first high-order mode, and the number of high-order modes can be further increased on this basis;

[0035] Intensity data of each pixel point of the spot patterns at each wavelength is extracted, to obtain a two-dimensional measurement matrix with a row size and a column size being a number of pixel points of the spot pattern and a number of sampling wavelengths, respectively, Fourier transform is performed on each row of data of the two-dimensional measurement matrix and then the data are added, to obtain a Fourier transform result;

[0036] A differential mode group delay test value of the first high-order mode relative to the fundamental mode is obtained according to a characteristic peak of the first high-order mode in the Fourier transform result;

[0037] A lookup table of differential mode group delay theoretical values corresponding to the high-order modes and varying with a core-cladding refractive index difference is established according to the high-order modes transmitted by the to-be-tested optical fiber, and a differential mode group delay theoretical value closest to the differential group delay test value at a core diameter of the to-be-tested optical fiber is found according to the lookup table, to obtain the core-cladding refractive index difference of the to-be-tested optical fiber.

[0038] A computer readable storage medium stores a computer program, and the computer program is executed by a processor to implement the following steps:

[0039] Single transverse mode laser beams of different wavelengths in a target application waveband are incident on an input end of a to-be-tested optical fiber at equal wavelength intervals by using a tunable laser source, and spot patterns of the to-be-tested optical fiber at each wavelength are obtained at an output end of the optical fiber; the to-be-tested optical fiber is a long optical fiber; the to-be-tested optical fiber at least transmits two core modes of a fundamental mode and a first high-order mode, and the number of high-order modes can be further increased on this basis;

[0040] extracting intensity data of each pixel point of the light spot pattern at each wavelength to obtain a two-dimensional measurement matrix with the row size and the column size being the number of pixel points of the light spot pattern and the number of sampling wavelengths, respectively, performing Fourier transform on each row data of the two-dimensional measurement matrix and adding the results to obtain a Fourier transform result;

[0041] obtaining a differential mode group delay test value of the first high-order mode relative to the base mode according to the characteristic peak of the first high-order mode in the Fourier transform result;

[0042] establishing a lookup table of the differential mode group delay theoretical value corresponding to the high-order mode varying with the core-cladding refractive index difference according to the high-order mode supported by the fiber to be measured, and finding the differential mode group delay theoretical value closest to the differential group delay test value at the core diameter of the fiber to be measured according to the lookup table to obtain the core-cladding refractive index difference of the fiber to be measured.

[0043] The method and device for measuring the relative refractive index difference between the fiber core and the cladding, by obtaining each frame of light spot pattern of the fiber to be measured at the target application wavelength range, obtaining the corresponding interference light field intensity information, then performing Fourier transform on the interference light field intensity information to obtain the differential group delay test value of the first high-order mode relative to the base mode in the fiber to be measured, and finally obtaining the core-cladding refractive index difference with high accuracy through the differential group delay test value with high precision and high accuracy. In the case where multiple related parameters are not determined, the core-cladding refractive index difference cannot be directly obtained from the differential group delay test value. The present application establishes the relationship between the core-cladding refractive index difference and the differential group delay theoretical value corresponding to the high-order mode, establishes the lookup table of the differential mode group delay theoretical value corresponding to the high-order mode varying with the core-cladding refractive index difference according to the high-order mode supported by the fiber to be measured, confirms the differential group delay theoretical value closest to the differential group delay test value in the lookup table, and outputs the core-cladding refractive index difference corresponding to the theoretical value. The embodiment of the present application establishes the relationship between the mode interference behavior and the important optical parameter of the fiber, and can realize efficient, accurate and non-invasive measurement of the core-cladding refractive index difference of the long link fiber, which is more convenient to test and has higher accuracy of the measurement result. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 It is a flowchart of the method for measuring the relative refractive index difference between the fiber core and the cladding in one embodiment;

[0045] Figure 2 It is a structural schematic diagram of the light spot pattern acquisition device for measuring the relative refractive index difference between the fiber core and the cladding of the few-mode fiber in one embodiment;

[0046] Figure 3 It is the light spot pattern of LP 01 and LP11 a schematic diagram of a change trend of effective refractive index difference of modes;

[0047] Figure 4 a schematic diagram of processing results of test data of a fiber to be measured in another embodiment where NA is 0.12 and different core diameters; 11 a schematic diagram of a change trend of differential group delay of modes;

[0048] Figure 5 a schematic diagram of processing results of test data of a fiber to be measured in an embodiment where NA is 0.115;

[0049] Figure 6 a schematic diagram of Fourier transform results of interference fields of a first high-order mode and a fundamental mode in a range of 1070-1080 nm in an embodiment where the core diameter is 15 μm and NA is 0.115;

[0050] Figure 7 a structural block diagram of a measuring device of a relative refractive index difference between a fiber core and a cladding in an embodiment;

[0051] Figure 8 an internal structural diagram of a computer device in an embodiment. DETAILED DESCRIPTION

[0052] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application is 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 application and do not limit the present application. The core-cladding refractive index difference in the present application refers to the relative refractive index difference between the core and the cladding, which is denoted by Δn.

[0053] In an embodiment, as shown in Figure 1 a measuring method of a relative refractive index difference between a fiber core and a cladding is provided, which comprises the following steps:

[0054] In step 102, a single transverse mode laser beam at different wavelengths in a target application wavelength band is incident on an input end of a fiber to be measured by using a tunable laser source at equal wavelength intervals, and a spot pattern of the fiber to be measured at each wavelength is obtained at an output end of the fiber.

[0055] The to-be-measured optical fiber is a long optical fiber rather than a centimeter-length optical fiber sample used in traditional refractive index measurement. By measuring the core-cladding refractive index difference of the long optical fiber, a more accurate reference can be provided for mode characteristic evaluation of the optical fiber system. The to-be-measured optical fiber can be any optical fiber, as long as a tunable laser source with a proper wavelength range is selected so that the to-be-measured optical fiber is in a non-single-mode transmission state in the corresponding injection wavelength range. In the test wavelength range, the to-be-measured optical fiber can work in a two-mode or higher mode form. The to-be-measured optical fiber at least transmits two core modes of a fundamental mode and a first high-order mode, and the to-be-measured optical fiber is not limited to supporting two modes. On this basis, the number of high-order modes can be further increased, and the measurement of the core-cladding refractive index difference can be realized by selecting a suitable injection wavelength. The first high-order mode can be any high-order mode such as LP 11 , LP 21 , LP 02 , etc.

[0056] In step 104, the intensity data of each pixel point of the light spot pattern at each wavelength is extracted to obtain a two-dimensional measurement matrix with the row size and the column size being the number of pixel points of the light spot pattern and the number of sampling wavelengths. The Fourier transform results are obtained by performing Fourier transform on each row of data of the two-dimensional measurement matrix and then adding the results.

[0057] The intensity data refers to the intensity of the interference light field. Each frame of image data is an m*n two-dimensional intensity matrix, where m is the number of rows of the matrix, and n is the number of columns of the matrix. The intensity data of each pixel point of each light spot pattern is extracted one by one to form a p*q two-dimensional measurement matrix, where p=F, indicating that each row vector contains the intensity data of a specific pixel point in the light spot pattern; q=m*n, indicating that the number of rows of the p*q two-dimensional measurement matrix is equal to the number of pixel points of each frame of image. The Fourier transform is performed on each row of data of the p*q two-dimensional measurement matrix to obtain q one-dimensional Fourier transform results. The q one-dimensional Fourier transform results are further added to obtain the final Fourier transform result.

[0058] In step 106, the differential mode group delay test value of the first high-order mode relative to the fundamental mode is obtained according to the characteristic peak of the first high-order mode in the Fourier transform result.

[0059] The corresponding position (zero-frequency position) of the first-order characteristic peak in the Fourier transform result is the mode group delay of the fundamental mode. The corresponding position of the second-order characteristic peak is the differential group delay of the first high-order mode relative to the fundamental mode. By measuring the differential group delay, the core-cladding refractive index difference can be further measured. Compared with the traditional method, the test is more convenient, and the measurement result is more accurate.

[0060] In step 108, a lookup table of the change of the differential mode group delay theoretical value corresponding to the high-order mode with the core-cladding refractive index difference is established according to the high-order mode to be tested, and the differential mode group delay theoretical value closest to the differential group delay test value is found at the core diameter of the fiber to be tested according to the lookup table, so as to obtain the core-cladding refractive index difference of the fiber to be tested.

[0061] In the establishment of the lookup table of the change of the differential mode group delay theoretical value corresponding to the high-order mode with the core-cladding refractive index difference, the differential group delay theoretical values of each mode of the few-mode fiber are calculated at different core diameters when the core-cladding refractive index difference of the fiber takes different values. In the measurement, it is not necessary to take the first high-order mode as the basis for the equivalent refractive index lookup. If the fiber to be tested supports multiple high-order modes, any high-order mode can be selected as the basis for the core-cladding refractive index difference measurement. The calculation object of the high-order mode can be LP 11 , LP 21 , LP 02 , etc. Further, according to the core diameter of the fiber to be tested and the test result, the differential group delay corresponding to the characteristic peak position of the corresponding high-order mode in the test result is matched with the theoretical value in the lookup table, so as to determine the core-cladding refractive index difference of the fiber.

[0062] In the above method for measuring the relative refractive index difference between the fiber core and the cladding, the interference light field intensity information corresponding to each frame of the light spot pattern of the fiber to be tested at the target application wavelength is obtained, then the interference light field intensity information is subjected to Fourier transform to obtain the differential group delay test value of the first high-order mode relative to the base mode in the fiber to be tested, and finally, the high-precision and high-accuracy differential group delay test value is used to obtain the core-cladding refractive index difference with high accuracy. In the case where multiple related parameters are not determined, the core-cladding refractive index difference of the fiber cannot be directly obtained from the differential group delay test value. According to the relationship between the core-cladding refractive index difference and the differential group delay theoretical value corresponding to the high-order mode, the lookup table of the change of the differential mode group delay theoretical value corresponding to the high-order mode with the core-cladding refractive index difference is established according to the high-order mode supported by the fiber to be tested. The differential group delay theoretical value closest to the differential group delay test value is found in the lookup table, and the core-cladding refractive index difference corresponding to the theoretical value is output. In the embodiment of the present application, the mode interference behavior is associated with the important optical parameters of the fiber, so that efficient, accurate and non-invasive core-cladding refractive index difference measurement of long-distance fiber can be realized, the test is more convenient, and the measurement result is more accurate.

[0063] In one embodiment, the step of adding the Fourier transform results after the Fourier transform of each row of data of the two-dimensional measurement matrix includes: performing Fourier transform on the intensity data corresponding to each row vector of the two-dimensional measurement matrix to obtain F one-dimensional Fourier transform results, where F is the number of pixel points of the light spot pattern; and the intensity data is:

[0064]

[0065] wherein I is the intensity distribution of the interference optical field, I1 is the intensity of the fundamental mode, I2 is the intensity of the high-order mode corresponding to the establishment of the lookup table, is the constant phase difference, Δτ is the differential group delay, L is the length of the optical fiber to be measured, Δω is the two-frequency classification difference frequency of the interference optical field, is the group refractive index of the fundamental mode, is the group refractive index of the high-order mode, c is the speed of light, Δn eff is the mode effective refractive index difference; the Fourier transform results of the F groups are added to obtain the Fourier transform result.

[0066] In one embodiment, as Figure 2 shown, a structure diagram of a spot pattern acquisition device for relative refractive index difference between a few-mode fiber core and a cladding is provided, and the steps of obtaining the spot pattern of the optical fiber to be measured at each wavelength at the output end of the optical fiber by using a tunable laser source to input single transverse mode laser beams at different wavelengths in the target application wavelength band into the input end of the optical fiber to be measured with equal wavelength intervals include: obtaining the wavelength scanning interval corresponding to the current measurement accuracy according to the corresponding relationship between the pre-set measurement accuracy threshold and the wavelength scanning interval; obtaining the scanning start wavelength and the scanning end wavelength of the tunable laser source according to the target application wavelength band of the optical fiber to be measured and the wavelength scanning interval; the tunable laser source is used to output the wavelength-tuned laser beam; the tunable laser source is used to input the laser beam into the input end of the optical fiber to be measured with equal wavelength intervals, and excite the core mode of the optical fiber to be measured; the output optical field of the optical fiber at each wavelength within the scanning start wavelength and the scanning end wavelength is frame collected at the output end of the optical fiber, and the spot pattern of the optical fiber to be measured at each wavelength is obtained.

[0067] Specifically, as Figure 2 shown, the tunable laser source 100, the mode excitation assembly 200, the optical fiber to be measured 300, the imaging system 400 and the high-frame-rate image acquisition system 500 are connected in sequence, the feedback control acquisition system 600 is connected with the tunable laser source 100 and the high-frame-rate image acquisition system 500 respectively, and in the measurement process, first, the optical fiber to be measured with a length of L is selected, and the optical fiber to be measured is loosely coiled with a bending diameter greater than 40 cm; the scanning start wavelength λ1 and the end wavelength λ2 are set at the tunable laser source 100 end according to the target application wavelength band of the optical fiber to be measured 300, the wavelength tuning resolution R s is set; the mode excitation assembly 200 excites the core mode of the optical fiber to be measured 300 with a certain injection offset, and the imaging system 400 frame collects the output optical field of the optical fiber at different wavelengths at the frame rate R c .

[0068] The tunable laser light source 100 has the output performance of fast, equal interval, stable wavelength, and repeatable wavelength scanning, the wavelength scanning interval range is adjustable, the wavelength tuning resolution is up to 0.001 nm, and the output power is continuously adjustable.

[0069] The mode excitation assembly 200 is used for coupling the output laser of the tunable laser light source to the next stage system to realize controllable excitation of the transmission mode in the next stage system. The mode excitation assembly 200 has the characteristic that the mode excitation state of the next stage system is adjustable, and can be set as a spatial optical system, a fiber jumper assembly or a spatial-jumper composite system. When the mode excitation assembly 200 is a spatial optical system, the assembly includes a double-lens 4f system, and an attenuation sheet or a polarization optical element is arranged between the double lenses to attenuate the output power and select the output field polarization state, respectively. When the mode excitation assembly 200 is a fiber jumper assembly, the jumper used must be a single-mode fiber, and the NA value of the fiber must be less than 0.1 in the output wavelength band of the tunable laser light source. In the selectable range, the smaller the NA value of the single-mode fiber is, the better. When the mode excitation assembly 200 is a spatial-jumper composite system, pure core mode coupling must be realized between the spatial optical element and the single-mode fiber jumper, and the NA value of the fiber must be less than 0.1 in the output wavelength band of the tunable laser light source. In the selectable range, the smaller the NA value of the single-mode fiber is, the better. When the output port of the mode excitation assembly 200 is a spatial optical output, controllable excitation of the transmission mode in the next stage system is realized through mechanical adjustment. When the output port of the mode excitation assembly 200 is a fiber output, controllable excitation of the transmission mode in the next stage system is realized through alignment fusion or eccentric fusion.

[0070] The to-be-measured fiber 300 is a fiber with controllable length in any working state.

[0071] The imaging system 400 is used for imaging the output light field of the to-be-measured fiber into the next stage system. The selectable configurations of the imaging system 400 include: (1) a double-lens 4f system, an attenuation sheet or a polarization optical element is arranged between the double lenses to attenuate the output power and select the output field polarization state, respectively; (2) a single-lens 2f system, an attenuation sheet or a polarization optical element is arranged between the single lens and the next stage system to attenuate the output power and select the output field polarization state, respectively, and the imaging system 400 should select an antireflection optical element to avoid end reflection of the optical element as much as possible.

[0072] The high-frame-rate image acquisition system 500 is used for acquiring the output light field of the previous stage system, and the acquisition frame rate R c The acquisition frame rate R should be not less than the wavelength scanning speed of the tunable laser light source. When the wavelength scanning speed is A nm / s, the acquisition frame rate R satisfies

[0073] The feedback control acquisition system 600 records and displays the current output wavelength of the tunable laser source in real time, and controls the high frame rate image acquisition system to record the light spot pattern. The optional data acquisition methods of the feedback control acquisition system 600 include: (1) parallel acquisition, where the tunable laser source reaches a certain output wavelength λ at a certain time t. t At this time, the laser is locked in a frequency-stabilized state, and the high frame rate image acquisition system acquires a frame of light spot pattern I1. After acquisition, the tunable laser source is tuned at tuning interval R. s (1) Scanning to the next characteristic wavelength, the high frame rate image acquisition system continues to acquire one frame of spot pattern, repeating this process until the acquisition of one frame of spot pattern corresponding to the termination wavelength is completed; (2) Serial acquisition, the wavelength tuning process of the tunable laser source and the spot acquisition process of the high frame rate image acquisition system are carried out synchronously. In the actual scanning and acquisition process, the acquisition frame rate R c The wavelength scanning speed A nm / s of the tunable laser source satisfies the following condition.

[0074] In one embodiment, the correspondence between the measurement accuracy threshold and the wavelength scanning interval includes: when the measurement accuracy threshold is 0.001, the wavelength scanning interval is not less than 5 nm; when the measurement accuracy threshold is 0.0008, the wavelength scanning interval is not less than 18 nm; when the measurement accuracy threshold is 0.0005, the wavelength scanning interval is not less than 25 nm; and when the measurement accuracy threshold is 0.0001, the wavelength scanning interval is not less than 30 nm. In this embodiment, the measurement accuracy of the core-cladding refractive index difference depends on the wavelength scanning interval of the tunable laser source, i.e., the difference between the scanning start wavelength and the scanning end wavelength. The larger the wavelength scanning interval, the higher the measurement accuracy.

[0075] In one embodiment, the frame rate for frame acquisition of the fiber output optical field at each wavelength within the scan start wavelength and scan end wavelength at the fiber output end is:

[0076]

[0077] Among them, R c For the frame rate, R s Let A be the wavelength tuning resolution of the tunable laser source, and let A be the wavelength scanning speed of the tunable laser source.

[0078] In one embodiment, the step of obtaining the length of the optical fiber to be tested includes: after transmission of the current length, the intermodal group delay accumulation of the first higher-order mode relative to the fundamental mode in the optical fiber is not less than 5 times the Fourier transform resolution; the Fourier transform resolution is:

[0079]

[0080] Among them, R fFor the Fourier transform resolution, λ1 is the starting scanning wavelength of the tunable laser light source, λ2 is the terminal scanning wavelength of the tunable laser light source, and c is the vacuum light speed; the mode group delay accumulation of the first high-order mode in the optical fiber after transmission through the current length relative to the base mode is not greater than the maximum measurable group delay; the maximum measurable group delay is:

[0081]

[0082] Wherein, DMGD max is the maximum measurable group delay, R s is the wavelength tuning resolution of the tunable laser light source. DMGD (Differential Mode Group Delay) is the differential mode group delay value.

[0083] In one embodiment, the optical fiber to be measured is in a flat state or a coiled state. In this embodiment, for a long optical fiber link, the bending and coiling state of the optical fiber will exert refractive index modulation, and different bending and coiling states have different core-cladding refractive index differences. Based on the non-invasive and global measurement of the long optical fiber link, the present application can measure the core-cladding refractive index difference of the optical fiber in any working state. Measuring in different working states can provide more valuable parameters for actual application, make the measurement results more accurate, and provide an accurate equivalent parameter set for the theoretical and experimental evaluation of the system, which is of great significance for the design and optimization of high-power fiber lasers.

[0084] In one embodiment, the optical fiber to be measured includes a gain optical fiber, an energy transmission optical fiber or a microstructure optical fiber. In this embodiment, the optical fiber to be measured can be any optical fiber, as long as an appropriate wavelength tunable laser light source is selected so that the optical fiber to be measured is in a non-single mode transmission state in the corresponding injection wavelength range, and the measurement of the relative refractive index difference can be completed.

[0085] In a specific embodiment, when the optical fiber to be measured 300 is a few-mode optical fiber supporting two core modes (LP 01 and LP 11 ), the output end interference light field intensity distribution can be simply represented as:

[0086]

[0087] Wherein, A1 and A2 are the field amplitudes of LP 01 and LP 11 modes, β1 and β2 are the propagation constants of LP 01 and LP 11 modes, and I1 and I2 are the field amplitudes of LP 01 and LP 11The field strength of the mode, I, is the total intensity distribution of the two-mode interference field; Taylor expanding the propagation constant β about the frequency ω, denoted as

[0088]

[0089] where the first term is the value of the propagation constant at the frequency ω0, the second term is related to the mode characteristics of the few-mode fiber, causing mode dispersion, denoted as the intermodal dispersion term, and the third term is related to the waveguide dispersion and material dispersion terms of the fiber, denoted as the chromatic dispersion term or the higher-order dispersion term; the intermodal dispersion term is denoted as the group delay time τ of the mode, i.e., the mode group delay τ, then

[0090]

[0091] where v g is the group velocity of the mode field; for a few-mode fiber with a length L, the intensity distribution of the output end interference field is further denoted as

[0092]

[0093] where is a constant phase difference, and Δτ is the mode group delay difference of the two propagation modes, referred to as the differential group delay. As can be seen, the intensity distribution at the output end of the fiber is a periodic interference field with a modulation frequency of ΔτxL;

[0094] Further, the differential group delay Δτ can be rewritten as an expression related to the group refractive index of the mode:

[0095]

[0096] where n g is the group refractive index of the transmission mode, n eff is the effective refractive index of the transmission mode, and λ0is the free space wavelength; for a few-mode fiber with a length L, the intensity distribution of the output end interference field is further denoted as

[0097]

[0098] Therefore, the characteristic frequency of the periodic interference field at the output end of the fiber is proportional to the effective refractive index difference between the fundamental mode and the higher-order mode. As shown in Figure 3 , a schematic diagram of the variation trend of the effective refractive index difference between the LP 01 and LP 11 modes with different core diameters is provided. Generally speaking, when the numerical aperture NA is given, the effective refractive index difference between the fundamental mode and the higher-order mode decreases with the increase of the core diameter.

[0099] Further, as shown in Figure 4As shown, a LP 11 The differential group delay variation trend diagram of the mode is shown. The value of the differential mode group delay represents the frequency of the interference optical field. As can be seen, when the numerical aperture NA is given, the larger the core diameter, the smaller the periodic frequency of the interference optical field of each mode at the output end of the optical fiber. Therefore, there is a linear correspondence between the numerical aperture NA of the optical fiber and the periodic frequency of the interference optical field, and the NA measurement can be realized by measuring the mode field interference frequency of the few-mode optical fiber. When the winding state of the optical fiber changes, the core-cladding refractive index difference of the optical fiber in different states can be further measured.

[0100] In one embodiment, the method of the present application can also be used for the measurement of the equivalent numerical aperture, and the equivalent numerical aperture measurement result of a typical fiber to be measured is given. During the test, the scanning wavelength range of the tunable laser source 100 is set to 1070-1080 nm, the wavelength tuning resolution is 0.02 nm, the tuning speed is 0.2 nm / s, and a total of 501 characteristic wavelengths are correspondingly obtained, and the total scanning time is about 50 s. The mode excitation assembly 200 adopts a single-mode jumper with a cutoff wavelength of 780 nm and a numerical aperture of about 0.1, which ensures that it is an absolutely single-mode optical fiber in the scanning wavelength range. The mode excitation assembly 200 adopts a core offset fusion method to excite the core mode of the fiber to be measured 300. During the test, the length of the fiber to be measured 300 is 20 m, the core diameter is 14.9 μm, the nominal numerical aperture is 0.115, and the fiber is loosely wound with a bending diameter of about 40 cm. The imaging system 400 is a double-lens 4f system for imaging the output near-field spot of the fiber to be measured 300. The high-frame-rate image acquisition system 500 acquires images at a frame rate of 10 fps, and the feedback control acquisition system 600 controls the camera to acquire images in series. The total acquisition time is about 50 s, and a total of 501 frames of spot patterns are acquired. The acquisition serial port is 128x128 pixels. After one wavelength scan and spot pattern acquisition, the intensity data of each pixel of each frame of spot pattern is extracted one by one to form a 16384x501 two-dimensional measurement matrix, wherein the number of rows of the matrix is 16384=128x128, indicating that each column vector contains the intensity data of each frame at a specific pixel in the spot pattern; the number of columns of the matrix is 501, indicating that the number of columns of the two-dimensional measurement matrix is equal to the number of acquisition frames. The Fourier transform is performed on each column of data of the 16384x501 two-dimensional measurement matrix to obtain 501 groups of one-dimensional Fourier transform results. The 501 groups of one-dimensional Fourier transform results are further added to obtain the final Fourier transform result. The corresponding position of the first-order characteristic peak in the Fourier transform result is the fundamental mode (LP 01 ) mode group delay, and the corresponding position of the second-order characteristic peak is the differential mode group delay of the high-order mode (LP 11 ) relative to the LP 01 ) mode. The experimental results are as follows Figure 5The NA is 0.115, and the test data processing result of the tested optical fiber is shown in the figure, the differential mode group delay value corresponding to the first high-order mode characteristic peak is 2.07ps / m, according to the established first high-order mode differential mode group delay lookup table under different fiber core diameters and different NAs, when the fiber core diameter is 15μm, the corresponding equivalent numerical aperture E-NA value is found at the position of 2.07ps / m, and the measurement value of the equivalent numerical aperture E-NA value is 0.115, which is completely consistent with the nominal data. Further, as shown in the figure, Figure 6 The theoretical Fourier transform result of the first high-order mode and the base mode interference field in the range of 1070-1080nm when the fiber core diameter is 15μm and the NA is 0.115 is shown in the figure, and the theoretical Fourier transform result of the first high-order mode and the base mode interference field in the range of 1070-1080nm when the fiber core diameter is 15μm and the numerical aperture is 0.115 is calculated. In summary, the method of the present application can measure the key parameters of long-distance few-mode optical fiber link, and the theoretical and experimental results have good consistency.

[0101] It should be understood that, although Figure 1 The steps in the flowchart of the figure are displayed in sequence according to the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified herein, the execution of these steps is not strictly limited in sequence, and these steps can be executed in other orders. Moreover, Figure 1 At least part of the steps in the figure can include multiple sub-steps or multiple stages, which are not necessarily executed at the same time, but can be executed at different times, and the execution order of these sub-steps or stages is not necessarily sequential, but can be alternately executed with other steps or at least part of the sub-steps or stages of other steps.

[0102] In one embodiment, as shown in the figure, Figure 7 A device for measuring the relative refractive index difference between the fiber core and the cladding is provided, comprising: a spot pattern acquisition module 702, an intensity data processing module 704, a differential group delay test module 706, and a result output module 708, wherein:

[0103] The spot pattern acquisition module 702 is used to use a tunable laser source to input single transverse mode laser beams of different wavelengths in a target application wavelength band into the input end of the tested optical fiber at equal wavelength intervals, and obtain the spot patterns of the tested optical fiber at each wavelength at the output end of the optical fiber; the tested optical fiber is a long optical fiber; the tested optical fiber transmits at least two core modes of the base mode and the first high-order mode, and the number of high-order modes can be further increased;

[0104] The intensity data processing module 704 is configured to extract intensity data of each pixel point of the spot pattern at each wavelength, obtain a two-dimensional measurement matrix with the number of rows and columns being the number of pixel points of the spot pattern and the number of sampling wavelengths, and add each row of data of the two-dimensional measurement matrix after Fourier transform to obtain a Fourier transform result.

[0105] The differential group delay test module 706 is configured to obtain a differential mode group delay test value of a first high-order mode relative to a base mode according to a characteristic peak of the first high-order mode in the Fourier transform result.

[0106] The result output module 708 is configured to establish a lookup table of the differential mode group delay theoretical value corresponding to the high-order mode changing with the core-cladding refractive index difference according to the high-order mode transmitted by the to-be-tested optical fiber, find the differential mode group delay theoretical value closest to the differential group delay test value at the core diameter of the to-be-tested optical fiber according to the lookup table, and obtain the core-cladding refractive index difference of the to-be-tested optical fiber.

[0107] In one of the embodiments, the intensity data processing module 704 is further configured to perform Fourier transform on intensity data corresponding to each row vector of the two-dimensional measurement matrix to obtain F groups of one-dimensional Fourier transform results, where F is the number of pixel points of the spot pattern, and the intensity data is as follows:

[0108]

[0109] where I is the field intensity distribution of the interference light field, I1 is the field intensity of the base mode, I2 is the field intensity corresponding to the high-order mode used to establish the lookup table, is a constant phase difference, Δτ is the differential group delay, L is the length of the to-be-tested optical fiber, and Δω is the two-frequency classification difference frequency of the interference light field, is the group refractive index of the base mode, is the group refractive index of the high-order mode, c is the speed of light, and Δn eff is the mode effective refractive index difference; and the F groups of one-dimensional Fourier transform results are added to obtain the Fourier transform result.

[0110] In one of the embodiments, the intensity data processing module 704 is further configured to obtain a wavelength scanning interval corresponding to a current measurement accuracy according to a preset correspondence relationship between measurement accuracy thresholds and wavelength scanning intervals, obtain a scanning start wavelength and a scanning end wavelength of the tunable laser light source according to a target application wavelength band of the to-be-tested optical fiber and the wavelength scanning interval, and use the tunable laser light source to output a wavelength-tuned laser light beam; and the tunable laser light source is configured to use the wavelength-tuned laser light beam to enter the to-be-tested optical fiber input end at an equal wavelength interval, excite the core mode of the to-be-tested optical fiber, and perform frame acquisition on the optical fiber output light field at each wavelength within the scanning start wavelength and the scanning end wavelength to obtain the spot pattern of the to-be-tested optical fiber at each wavelength.

[0111] In one of the embodiments, the wavelength scanning interval is not less than 5 nm when the measurement precision threshold is 0.001; the wavelength scanning interval is not less than 18 nm when the measurement precision threshold is 0.0008; the wavelength scanning interval is not less than 25 nm when the measurement precision threshold is 0.0005; and the wavelength scanning interval is not less than 30 nm when the measurement precision threshold is 0.0001.

[0112] In one of the embodiments, the acquisition frame rate of the frame acquisition of the fiber output light field at each wavelength within the scanning start wavelength and the scanning end wavelength at the fiber output end is:

[0113]

[0114] wherein R c is the acquisition frame rate, R s is the wavelength tuning resolution of the tunable laser source, and A is the wavelength scanning speed of the tunable laser source.

[0115] In one of the embodiments, the mode group delay accumulation of the first high-order mode relative to the base mode in the current length of transmission fiber is not less than 5 times the Fourier transform resolution; and the Fourier transform resolution is:

[0116]

[0117] wherein R f is the Fourier transform resolution, λ1 is the start scanning wavelength of the tunable laser source, λ2 is the end scanning wavelength of the tunable laser source, and c is the vacuum light speed; and the mode group delay accumulation of the first high-order mode relative to the base mode in the current length of transmission fiber is not more than the maximum measurable group delay; and the maximum measurable group delay is:

[0118]

[0119] wherein DMGD max is the maximum measurable group delay, R s is the wavelength tuning resolution of the tunable laser source.

[0120] In one of the embodiments, the fiber to be measured includes a gain fiber, an energy transmission fiber or a microstructure fiber.

[0121] In one of the embodiments, the fiber to be measured is in a flat state or a coiled state.

[0122] In one of the embodiments, the acquisition object of each frame of the light spot pattern includes a near-field light intensity distribution or a far-field light intensity distribution.

[0123] The specific definition of the measuring device of the relative refractive index difference between the fiber core and the cladding can refer to the definition of the measuring method of the relative refractive index difference between the fiber core and the cladding in the above, which will not be repeated here. Each module in the above measuring device of the relative refractive index difference between the fiber core and the cladding can be realized by software, hardware and combination thereof in whole or in part. The above modules can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory in the computer device in software form, so as to be called and executed by the processor to perform the operations corresponding to the above modules.

[0124] In one embodiment, a computer device is provided, which can be a terminal, and its internal structure diagram can be as shown in Figure 8 The computer device includes a processor, a memory, a network interface, a display screen and an input device connected through a system bus. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operating system and the computer program in the non-volatile storage medium to run. The network interface of the computer device is used to communicate with external terminals through network connection. The computer program is executed by the processor to implement a measuring method of the relative refractive index difference between the fiber core and the cladding. The display screen of the computer device can be a liquid crystal display screen or an electronic ink display screen. The input device of the computer device can be a touch layer overlaid on the display screen, or a key, trackball or touchpad arranged on the shell of the computer device, or an external keyboard, touchpad or mouse, etc.

[0125] Those skilled in the art can understand that Figure 8 The structure shown in the above is only a block diagram of part of the structure related to the scheme of the present application, and does not constitute a limitation on the computer device to which the scheme of the present application is applied. The specific computer device can include more or fewer components than those shown in the figure, or combine certain components, or have a different component arrangement.

[0126] In one embodiment, a computer device is provided, which includes a memory and a processor. The memory stores a computer program. The processor executes the computer program to implement the steps of the method in the above embodiments.

[0127] In one embodiment, a computer readable storage medium is provided, which stores a computer program. The computer program is executed by a processor to implement the steps of the method in the above embodiments.

[0128] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a non-volatile computer readable storage medium, and when the computer program is executed, the processes of the above-mentioned embodiments of the methods can be included. Any reference to memory, storage, databases, or other media in the embodiments provided by the present application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration but not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.

[0129] The technical features of the above embodiments can be combined in any way. In order to make the description simple, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not exist, they should be considered as the scope of the present application.

[0130] The above embodiments only express several implementation manners 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, some 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 method for measuring the relative refractive index difference between the fiber core and cladding, characterized in that, The method includes: A tunable laser source is used to incident single transverse mode laser beams of different wavelengths within the target application band onto the input end of the fiber under test at equal wavelength intervals, and the spot patterns of the fiber under test at various wavelengths are obtained at the output end of the fiber. The fiber under test is a long fiber. The fiber under test transmits at least two core modes: the fundamental mode and the first higher-order mode. The number of higher-order modes can be further increased based on this. The intensity data of each pixel of the spot pattern under each wavelength is extracted to obtain a two-dimensional measurement matrix with the number of pixels of the spot pattern and the number of sampling wavelengths, respectively. The Fourier transform of each row of the two-dimensional measurement matrix is ​​performed and then summed to obtain the Fourier transform result. Based on the characteristic peak of the first higher-order mode in the Fourier transform result, the differential mode group delay test value of the first higher-order mode relative to the fundamental mode is obtained. Based on the higher-order modes transmitted in the optical fiber under test, a lookup table is established to show the theoretical value of the differential mode group delay corresponding to the higher-order modes as a function of the core-cladding refractive index difference. The theoretical value of the differential group delay that is closest to the measured value of the differential group delay is found at the core diameter of the optical fiber under test, thus obtaining the core-cladding refractive index difference of the optical fiber under test.

2. The method according to claim 1, characterized in that, The steps of performing a Fourier transform on each row of data in the two-dimensional measurement matrix and then summing the results to obtain the Fourier transform result include: Perform a Fourier transform on the intensity data corresponding to each row vector of the two-dimensional measurement matrix to obtain... The results of the one-dimensional Fourier transform are as follows: The number of pixels in the light spot pattern; the intensity data is: in, For the field intensity distribution of the interference light field, For fundamental mode field strength, The field strength corresponding to the higher-order modes used to build the lookup table. For a constant phase difference, For differential group delay, The length of the optical fiber to be measured. To classify the difference frequency between the two frequencies of the interfering optical field, The refractive index of the fundamental mode group, For higher-order mode group refractive index, At the speed of light, The effective refractive index difference of the mode; Will The results of the one-dimensional Fourier transforms are added together to obtain the Fourier transform result.

3. The method according to claim 1, characterized in that, The step of using a tunable laser source to incident single transverse mode laser beams of different wavelengths within the target application band at equal wavelength intervals onto the input end of the fiber under test, and obtaining the spot patterns of the fiber under test at various wavelengths at the output end of the fiber, includes: Based on the pre-set correspondence between the measurement accuracy threshold and the wavelength scanning interval, the wavelength scanning interval corresponding to the current measurement accuracy is obtained; Based on the target application wavelength band of the optical fiber under test and the wavelength scanning interval, the scanning start wavelength and scanning end wavelength of the tunable laser source are obtained; the tunable laser source is used to output a wavelength-tuned laser beam. A tunable laser source is used to incident the laser beam at equal wavelength intervals onto the input end of the fiber under test and excite the fiber core mode of the fiber under test. At the output end of the fiber, the output optical field of the fiber under test at each wavelength within the scanning start wavelength and scanning end wavelength is frame-by-frame acquired to obtain the light spot pattern of the fiber under test at each wavelength.

4. The method according to claim 3, characterized in that, The correspondence between the measurement accuracy threshold and the wavelength scanning interval includes: When the measurement accuracy threshold is 0.001, the wavelength scanning interval is not less than 5 nm; When the measurement accuracy threshold is 0.0008, the wavelength scanning interval is not less than 18 nm; When the measurement accuracy threshold is 0.0005, the wavelength scanning interval shall not be less than 25 nm; When the measurement accuracy threshold is 0.0001, the wavelength scanning interval is not less than 30nm.

5. The method according to claim 3, characterized in that, The frame rate for frame acquisition of the fiber output optical field at each wavelength within the scan start wavelength and scan end wavelength at the fiber output end is: in, To collect frame rate, For the wavelength tuning resolution of tunable laser sources, This refers to the wavelength scanning speed of a tunable laser source.

6. The method according to claim 1, characterized in that, The step of obtaining the length of the optical fiber to be tested includes: After transmission over the current length, the intermodal group delay accumulation of the first higher-order mode relative to the fundamental mode in the optical fiber is not less than 5 times the Fourier transform resolution; the Fourier transform resolution is: in, For Fourier transform resolution, The starting scanning wavelength of the tunable laser source. The termination scanning wavelength of the tunable laser source. It is the speed of light in a vacuum; After transmission over the current length, the intermodal group delay accumulation of the first higher-order mode relative to the fundamental mode in the optical fiber is no greater than the maximum measurable group delay; the maximum measurable group delay is: in, For the maximum measurable group delay, This refers to the wavelength tuning resolution of a tunable laser source.

7. The method according to claim 1, characterized in that, The optical fiber under test is either straight or coiled.

8. The method according to claim 1, characterized in that, The optical fiber under test includes gain fiber, power transmission fiber, or microstructure fiber.

9. The method according to claim 1, characterized in that, The objects collected for each frame of light spot pattern include near-field light intensity distribution or far-field light intensity distribution.

10. A device for measuring the relative refractive index difference between the fiber core and cladding, characterized in that, The device includes: The spot pattern acquisition module is used to use a tunable laser source to incident single transverse mode laser beams of different wavelengths within the target application band onto the input end of the fiber under test at equal wavelength intervals, and obtain the spot pattern of the fiber under test at each wavelength at the output end of the fiber; the fiber under test is a long fiber; the fiber under test transmits at least two core modes, namely the fundamental mode and the first higher-order mode, and the number of higher-order modes can be further increased on this basis. The intensity data processing module is used to extract the intensity data of each pixel of the spot pattern under each wavelength, and obtain a two-dimensional measurement matrix with row and column sizes of the number of pixels of the spot pattern and the number of sampling wavelengths, respectively. The Fourier transform of each row of the two-dimensional measurement matrix is ​​performed and then the data are added together to obtain the Fourier transform result. The differential group delay test module is used to obtain the differential group delay test value of the first higher-order mode relative to the fundamental mode based on the characteristic peak of the first higher-order mode in the Fourier transform result. The result output module is used to establish a lookup table for the theoretical value of the differential group delay corresponding to the higher-order mode transmitted in the optical fiber under test, which varies with the core-cladding refractive index difference. Based on the lookup table, the module finds the theoretical value of the differential group delay that is closest to the measured value of the differential group delay under the core diameter of the optical fiber under test, and obtains the core-cladding refractive index difference of the optical fiber under test.

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