A method for measuring mode purity of a few-mode ring-core fiber based on principal component prior information

Abstract

The application discloses a mode purity measurement method of a few-mode ring core fiber based on principal component prior information. First, all spatial modes supported by the few-mode ring core fiber to be measured are calculated to determine the highest order; second, a nonlinear equation set connecting the fiber output intensity mode spot and the complex amplitude of each spatial mode is listed, and an algorithm for solving the equation set by using the principal component order prior information is designed, so that the recovery of the complex amplitude of each mode and the measurement of the principal component mode purity are realized. Then, an optical device for detecting the fiber exit mode field is built. For the fiber near-field intensity mode spot at any time, the prior information of the principal component order is obtained through an interference pattern or the shape of the intensity mode spot, and the measurement of the mode purity can be realized by applying the above-mentioned algorithm. The application can accurately recover the mode purity of the few-mode ring core fiber from the fiber output near-field intensity mode spot only, and has strong applicability.

Description

Technical Field

[0001] This invention relates to a method for measuring the mode purity of a few-mode ring-core optical fiber based on principal component prior information, belonging to the fields of space-division multiplexing optical fiber communication systems and integrated optics. Background Technology

[0002] Orbital angular momentum (OAM) beams, characterized by helical wavefronts, hold great promise for applications in high-capacity communications, optical metrology, optical tweezers, and data storage. As cylindrical waveguides, few-mode fiber (FMF) is a natural container for OAM modes. In an ideal OAM fiber, each OAM mode is orthogonal and can carry independent information, significantly increasing communication channel capacity or the degrees of freedom in multi-parameter sensing. However, the unpredictability of inter-mode coupling in practical fibers hinders the reliability of using each OAM mode as a controllable unit and limits the application of FMF in various fields. Compared to traditional FMF, ring core fiber (RCF) is more suitable for the stable propagation of OAM modes due to its ring-shaped mode field distribution. RCF has also been shown to achieve weak coupling between higher-order modes and reduce the occurrence of radial higher-order modes. These advantages make RCF an important candidate transmission fiber for high-capacity OAM mode-division multiplexing communication systems.

[0003] In RCF-based communications and other applications, designing FMFs with lower crosstalk and devices with more efficient mode conversion and multiplexing capabilities is crucial. This has spurred the need for performance characterization of these FMFs and devices, with one key issue being the measurement of the purity of spatial modes actually transmitted in few-mode fibers. For few-mode fibers, high-precision purity measurement methods mean accurately characterizing inter-mode crosstalk, which can reflect the feasibility of fiber design methods and may reveal more details about the physical mechanisms of inter-mode coupling, thus pointing to further paths to suppress inter-mode coupling. For OAM spatial and all-fiber devices, experimental measurements of optical field purity can guide device development and evaluate device performance in practical applications, as pure mode sources are crucial for reducing system complexity. In addition to characterizing the performance of fibers and fiber devices, simple purity measurement methods can also provide new ideas for compensating for inter-mode coupling to address the challenges of FMFs in various fields, especially when combined with the measurement and inversion of fiber transmission matrices.

[0004] In principle, measuring mode purity requires not only determining the principal mode components in the emitted light field of an OAM fiber or device, but also the content of all possible other modes. Traditionally, researchers determine the principal components of an unknown light field by observing intensity mode spots or interferograms. In intensity mode spots, modes with different angular orders have different lobe numbers. In interferograms, they appear as different spiral or forked patterns. However, if the modes are not pure, the intensity mode spots and interferograms are complex, and determining the content of each mode to calculate the purity of the principal components remains challenging. This problem is often referred to as measurement of the OAM spectrum or mode decomposition. To address this issue, various methods based on diffraction, coordinate transformation, and interferometry have been reported, but these methods all struggle to achieve accurate measurements of RCF mode purity.

[0005] Methods based on diffraction and coordinate transformation principles typically require expensive spatial light modulators and complex experimental setups, and cannot be easily adapted to the purity measurement of higher-order modes in loop-core fibers. This is because the fundamental mode in a loop-core fiber is donut-shaped under near-field conditions and possesses modal characteristics very different from the intrinsic fundamental mode in free space, further increasing the complexity of practical setups. Since most mode converters use the fundamental mode as the source mode, it is usually a non-negligible impurity that should be measured simply and accurately.

[0006] Methods based on the principle of interferometry can be divided into three categories. The first category utilizes the propagation constants of different modes, such as spatial spectral resolution imaging (SPII). 2 The first category of methods includes sweep frequency interferometry (SQFID), vector network analyzer (VNA) measurements, and others. These methods are commonly used to characterize fibers with unknown structures, but their application is limited by the need for expensive tunable lasers or VNA analyzers. Furthermore, these methods require long fibers to generate sufficient group delay to distinguish different modes and cannot distinguish degenerate OAM modes with similar propagation constants. The second category of methods utilizes the principle that different mode combinations generate different intensity modes. These methods do not require complex equipment and can directly measure mode components using numerical algorithms, making them highly applicable. However, the number of modes that these methods can resolve is limited by their sensitivity to noise and initial iteration values, or the need for long-term neural network training. Therefore, using these methods to perform purity measurements in RCFs with a large number of modes is challenging. In experimental reports of this type of method, the highest angular order of the supported modes in few-mode fibers is only 3. The third category of methods requires a reference beam to obtain the phase distribution of the output optical field and uses the mode orthogonality property to calculate all mode components. These methods require precise optical path alignment and interferometric stability for in-situ measurements. However, achieving phase stability is sometimes difficult in practical applications of FMFs. Summary of the Invention

[0007] The purpose of this invention is to address the inaccuracies and poor applicability of existing OAM spectrum measurement or mode decomposition methods when applying them to the purity measurement of few-mode ring-core fibers. This invention proposes a novel method for measuring the mode purity of few-mode ring-core fibers based on prior information from principal components. This novel method utilizes only prior information about the order of the principal components of the optical field to obtain the amplitude of each degenerate mode through a single measurement of the fiber's output intensity mode spot, thereby calculating the purity of the principal components. The proposed method has the advantages of simple equipment, ease of implementation, high accuracy, and noise resistance. This technique is an important candidate method for accurately characterizing and evaluating the mode performance of few-mode ring-core fibers and their devices.

[0008] The technical solution adopted in this invention is:

[0009] A method for measuring mode purity in few-mode ring-core optical fibers based on principal component prior information, the method comprising:

[0010] Step 1: Based on the structural parameters of the few-mode ring-core fiber under test, calculate all spatial modes supported by the few-mode ring-core fiber under test using numerical calculation or analytical solution methods, and determine the highest order; the specific process is as follows:

[0011] For the few-mode ring-core fiber under test, its geometric parameters (including the shape and size of the few-mode ring-core fiber) and optical characteristic parameters (including refractive index distribution and other physical parameters that affect its optical properties) are first determined. For simple fibers for which analytical solutions can be obtained, the analytical expression for the mode field distribution of the spatial modes is derived using the analytical solution; for complex fibers for which analytical solutions are difficult to obtain, numerical calculation methods, such as the finite element method or the finite difference method, are used to calculate the numerical solution for the mode field distribution of the spatial eigenmodes and the corresponding mode order.

[0012] Step 2: Represent the spatial mode field in the few-mode ring-core fiber under test as the coherent superposition of all spatial modes that the fiber can support with arbitrary complex amplitudes. List the nonlinear equations between the intensity mode spot of the few-mode ring-core fiber under test and the complex amplitude of each spatial mode. Design a solution algorithm using the principal component prior information to obtain the complex amplitude of each spatial mode.

[0013] Step 3: An experimental setup was built to detect the emitted mode field of an optical fiber, and a space image detector was used to record the light field intensity mode spots and interference patterns at any given time.

[0014] Step 4: Determine the order of the principal component from the light field intensity mode or interferogram, and substitute the order of the principal component with the intensity mode information required by the nonlinear equation system in Step 2 into the equation system to solve the equation system and recover the amplitude of each mode to calculate the purity of the principal component.

[0015] The few-mode ring-core fiber refers to any few-mode fiber with axisymmetric refractive index characteristics, no geometric feature changes in the transmission direction, capable of conducting electromagnetic waves, and with a high refractive index ring structure introduced in the refractive index profile. It does not restrict the position of the ring structure or the specific refractive index, nor does it restrict the introduction of any other special design in the refractive index profile.

[0016] The term "spatial model" should refer to a series of spatial models that are orthogonal to each other and have undergone power normalization, obtained by numerical calculation, analytical solution, or actual measurement.

[0017] The nonlinear equation set refers to a set of equations that uses information from the intensity mode as known quantities, the complex amplitude of the spatial mode as unknown quantities, and is expressed in any form.

[0018] The algorithms for solving nonlinear equations include both analytical and numerical solutions; they include common nonlinear least squares algorithms (such as Levenberg-Marquardt and trust region methods), stochastic parallel gradient descent algorithms, and machine learning algorithms, as well as any numerical solution algorithm that can solve nonlinear equation systems; they include numerical computation methods implemented using computer programming languages, as well as numerical computation methods used in commercial software.

[0019] The space image detector refers to various detectors capable of recording the spatial mode field output by optical fiber.

[0020] The interferogram is any interferogram that can detect the phase information of the optical field output by the optical fiber to obtain the principal components of the optical field. It includes both the spiral interferogram in coaxial interference and the crosshair interferogram in off-axis interference.

[0021] Advantages and positive effects of the present invention:

[0022] The mode purity measurement method proposed in this invention overcomes the problem of poor applicability of existing OAM spectrum measurement or mode decomposition methods in the purity measurement of few-mode ring-core fibers. It has the advantages of simple equipment, easy implementation, high precision and noise resistance. It has excellent practical application value in characterizing the performance of few-mode ring-core fibers and mode division multiplexing devices based on few-mode ring-core fibers, and can promote the development of mode division multiplexing technology and its applications. Attached Figure Description

[0023] Figure 1 This is a simplified flowchart of the method for measuring the mode purity of few-mode ring-core optical fibers based on principal component prior information proposed in this invention.

[0024] Figure 2The refractive index profile of the ring-core fiber in the specific embodiment, the orbital angular momentum (OAM) modes supported by the ring-core fiber, and the radial field functions of these OAM modes are shown. (a) Relative refractive index difference between the core and cladding of the ring-core fiber ( (b) Intensity and phase distribution of each OAM mode, image size is [image size missing]. Pixel. (c) Radial field function of OAM modes of different angular orders.

[0025] Figure 3 This is the experimental setup used in the specific implementation. OC, optical coupler; SMF, single-mode fiber; PC, polarization controller; RCF, ring-core fiber; Pol., polarizing mirror; NPBS, non-polarizing beam splitter; CCD, charge-coupled device (camera).

[0026] Figure 4 The following is a flowchart illustrating the complete purity measurement algorithm in a specific implementation. (a) Flowchart of the angular Fourier transform algorithm. (b) Schematic diagram of determining the optical axis. (c) Cropped image; the dashed circles represent the one-dimensional intensity sequence sampled angularly. (d) Intensity values ​​of the one-dimensional intensity sequence, and the real and imaginary parts of each Fourier coefficient after performing a Fourier transform. (e) The recovered amplitude and phase spectra of each spatial mode. (f) Intensity mode reconstructed from the amplitude and phase spectra. Detailed Implementation

[0027] The present invention will be further illustrated below with reference to the accompanying drawings, using the mode purity measurement of a few-mode ring-core fiber that supports four modules as an example. The accompanying drawings are for illustrative purposes only and do not limit the scope of application of the present invention.

[0028] A method for measuring mode purity in few-mode ring-core optical fibers based on principal component prior information. (See the simplified flowchart below.) Figure 1 .

[0029] (I) Starting from the upper branch of the flowchart. Based on the given geometric parameters (including the shape and size of the few-mode ring-core fiber) and optical characteristic parameters (including refractive index distribution and other physical parameters that affect its optical properties), the number of modes that the few-mode ring-core fiber can support is calculated, and the mode field distribution and corresponding order of these spatial modes are obtained. For simple fibers for which analytical solutions can be obtained, the analytical expression of the mode field distribution of the spatial eigenmodes is derived using the analytical solution; for complex fibers for which analytical solutions are difficult to obtain, numerical simulation methods, such as the finite element method or the finite difference method, are used for calculation. The spatial modes in the fiber can be represented by different mode substrates, one of which is the orbital angular momentum mode (OAM) substrate, under which the characterization of the situation described in this invention is relatively convenient.

[0030] In this example, the refractive index profile of the few-mode ring-core fiber is cylindrically symmetrical, and the core-cladding relative refractive index difference near the core center is ( )like Figure 2 As shown in (a), the fiber cladding diameter is 125 µm, and the inner and outer radii of the ring core are 3.75 µm and 8.25 µm, respectively. The refractive index profile... The maximum value is 0.008. To reduce inter-module coupling caused by micro-perturbations, two notches were introduced in the refractive index profile. The first notch ranges from 3.75 µm to 4.6 µm. The value is 0.0065; the second notch ranges from 5.4 µm to 6.8 µm. The value is 0.0053. The intensity and phase distribution of each OAM mode supported by the fiber at 1550 nm were calculated using the finite element method with COMSOL, such as... Figure 2 As shown in (b). The angular order of each OAM mode can be determined by the number of times the phase distribution alternates between bright and dark in the angular direction. Therefore, this type of fiber supports four angular order modules, namely the 0th order (…). ), 1st order ( ), 2nd order ( ), 3rd order ( ). Figure 2 Each fiber mode shown in (b) also has two orthogonal polarization states (x-polarization state and y-polarization state). Each mode in the same module has the same intensity distribution and the same radial field function. Figure 2 (c) The normalized radial field functions for different order fiber modes are given. .

[0031] In a few-mode ring-core fiber, any optical field is a linear superposition of all spatial modes it can support. Therefore, describing the optical field in a fiber is equivalent to describing the optical field of each spatial mode superimposed with arbitrary complex amplitudes. Once such an optical field is captured by a space image detector at the output end, the intensity mode pattern can be obtained. Through a certain degree of mathematical analysis, a set of nonlinear equations relating the intensity mode pattern of the few-mode ring-core fiber under test to the complex amplitudes of each spatial mode can be established. The following section will theoretically analyze the mathematical form of the nonlinear equations connecting any intensity mode pattern to the complex amplitudes of each spatial mode, without restricting the angular order of the fiber modes.

[0032] We will limit our focus to one of the two polarization states, such as the x-polarization state, because applying the same analysis to two orthogonal polarization components can be extended to the entire vector field. Therefore, the electric field distribution of the optical field in the fiber can be expressed as:

[0033]

[0034] in , and Radial and angular coordinates, The electric field distribution in OAM mode. Angular order ( (The highest angular order), To represent the complex amplitude of the corresponding mode's amplitude and phase, Indicates the amplitude of the corresponding mode. Indicates the phase of the corresponding mode. This is the radial field function for the corresponding mode.

[0035] For any optical field to be measured, the mathematical expression for its near-field intensity mode is:

[0036]

[0037] in Let be the absolute value of the pairwise differences between the angular orders of all different modes. If a specific radius is selected... , It will be simplified to a one-dimensional sequence of azimuth sampling.

[0038]

[0039] Based on a simple correspondence with the standard expansion of Fourier series, the former Fourier coefficients ( The real and imaginary parts of a function can be represented as a system of equations.

[0040]

[0041] Equation (4) is the set of nonlinear equations connecting any intensity mode and the complex amplitudes of each spatial mode. If an angular sampling sequence is extracted from the intensity mode... By applying the Fast Fourier Transform (FFT) algorithm to obtain the corresponding Fourier coefficients, a set of equations described by equation (4) can be established.

[0042] If the system of equations is solved correctly and the radial field function for each mode is known, then it can be achieved through... By recovering the amplitude of each mode, the power of each mode can be measured, and then the purity of the mode of interest can be calculated. For few-mode ring-core fiber, the amplitude of each mode... They are very close, therefore each pattern can be approximated as being in... The radial field functions are equal at all points, which only introduces a small error when recovering the amplitude. The similarity of different radial field functions in a ring-core fiber is essentially that the intensity contributed by each mode at a single radius is similar, so the power of the entire mode can be evaluated by directly sampling a certain radius.

[0043] The above is a general expression for the nonlinear equation set between the connection strength mode spot and the complex amplitude of each spatial mode without limiting the angular order of the fiber mode. For the few-mode ring-core fiber described in this example, the highest angular order is 3. In this case, the equation set in equation (4) contains 14 equations and 14 unknowns to be measured (the amplitude and phase of the 7 modes, respectively). If the fiber mode is extended to more than 3 orders, then it is only necessary to determine the highest order L (which can be determined in the first step of this method for any fiber under test) to determine how many equations need to be solved in the equation set in equation (4).

[0044] The purity measurement problem has been transformed into solving a system of equations determined by equation (4). Equation (4) is a typical nonlinear system of equations. This system of equations is solvable, but naturally has multiple solutions. However, for an approximately pure light field to be measured, if the angular orders of the principal components of the light field are known, we can design a solution algorithm that utilizes the prior information of the principal components to solve this system of equations, thereby obtaining the complex amplitudes of each spatial mode.

[0045] Taking least squares algorithms (such as Levenberg-Marquardt and trust region methods) as examples, if the pure mode corresponding to the principal component of the optical field is set as the initial value for iteration, the correct solution can be obtained in one go. Information about the order of the principal component can be obtained from the intensity mode itself or the interferogram, depending on whether the principal component is a single OAM mode or a superposition of two OAM modes with opposite topological charges. For a single OAM mode, a Mach-Zehnder interferometer is often introduced to obtain a spiral or crosshair interferogram of the optical field under test; the number of lobes in the spiral or the difference in the interference fringes represents the angular order of the principal component. For a superposition of two OAM modes (such as the LP mode), the principal component can be determined by the number of lobes in the intensity mode itself. If the intensity mode has 2L petals, then the angular order is L, because modes of different angular orders have different numbers of angular nodes. Once the order of the principal components is determined from the intensity mode or interferogram, the system of equations represented by equation (4) can be solved correctly using a well-designed solution algorithm (such as the least squares algorithm), thereby realizing the measurement of the amplitude of each mode and the recovery of the purity of the principal components.

[0046] (ii) Next is the lower half of the flowchart, namely the experimental level. In this part, we built a simple mode purity measurement device to characterize the performance of different types of few-mode ring-core fibers or devices.

[0047] The experimental setup is as follows Figure 3 As shown, a tunable laser can be used to characterize the performance of the fiber and / or device under test at each single wavelength. At the heart of the device is a simple imaging system where the light field emitted from the ring-core fiber is imaged onto a CCD through a first lens (lens 1). The fundamental mode light from the tunable laser is split into two branches by a 5:5 optical coupler (OC).

[0048] The fundamental mode light of the first branch is converted into a higher-order mode in the loop fiber by the fiber or device under test. The light field emitted from the few-mode loop fiber or device under test enters the non-polarizing beam splitter (NBPS) through the first lens.

[0049] The second branch is used in an optional Mach-Zehnder interferometer system to probe the phase distribution of the optical field. In this branch, the fundamental mode light is collimated into a Gaussian beam by passing sequentially through a polarization controller (PC) and a second lens (lens 2), and its polarization can be adjusted by the polarization controller (PC). A non-polarized beam splitter (NBPS) is used to combine the two branches. A polarizer (Pol.) is also optional and is used to characterize the optical field at a specific polarization. For the user, whether to introduce a polarizer and a Mach-Zehnder interferometer system depends on the actual polarization of the output beam and how they determine the principal components. If the beam is linearly polarized and its principal components can be determined from the shape of the intensity mode spot itself, then these two parts are unnecessary.

[0050] (III) Below, we will introduce the detailed process of the purity measurement method using an actual model image with a principal component of order three taken in the experiment. The flowchart is as follows: Figure 4 As shown in (a). First, we should determine the optical axis position on the camera and crop the image. Because the near-field intensity mode spot of the ring-core fiber naturally approximates a donut shape, and the intensity distribution when different modes are superimposed will not exceed this ring range, although it may be uneven in the angular direction. We can select one of the images in a set of test images that is closer to a circle in the angular direction, perform image segmentation, and draw a circle to determine the optical axis, such as... Figure 4 As shown in (b). A group of images sharing the same optical axis (i.e., all intensity modalities whose light field purity needs to be determined when the light path remains unchanged) only requires determining the optical axis once. After determining the optical axis, the images can be cropped to a suitable size, such as... Figure 4 As shown in (c).

[0051] The second step is to select a sampling radius with high intensity and high signal-to-noise ratio, but without overexposure, and then perform angular sampling. If the intensity is too low or the image is overexposed, it will affect the accurate extraction of the Fourier coefficients. Figure 4 The dashed circles in (c) represent the one-dimensional intensity sequence at our chosen sampling radius. This intensity sequence is then subjected to a Fast Fourier Transform (FFT) to obtain the Fourier coefficients. Figure 4 (d) shows the intensity (dashed line) of the one-dimensional intensity sequence and the real and imaginary parts of each Fourier coefficient. Substituting these Fourier coefficients and the order of the principal components into the previously designed equation solving algorithm, the amplitude and phase spectra of each OAM mode can be recovered, as shown below. Figure 4 As shown in (e). Figure 4 (f) is the reconstructed intensity pattern, which has a correlation coefficient of 0.97958 with the actual intensity pattern, indicating that the algorithm can perform well in real-world scenarios.

[0052] This example is merely one illustration of the application of the present invention and does not limit the type of few-mode ring-core fiber, refractive index distribution, fabrication materials, operating wavelength, etc., nor is it limited to methods for determining the order information of principal components or methods for solving nonlinear equations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for measuring mode purity of a few-mode ring-core fiber based on principal component prior information, characterized in that It comprises the following steps: Step 1: According to the structural parameters of the measured few-mode ring-core fiber, the numerical calculation or analytical solution method is used to calculate all the spatial modes supported by the measured few-mode ring-core fiber, and the highest order is determined; Step 2: The spatial mode field in the measured few-mode ring-core fiber is represented as the coherent superposition of all spatial modes supported by the fiber with arbitrary complex amplitudes, the Fourier coefficients of the angular sampling sequence are extracted from the intensity mode spot of the measured few-mode ring-core fiber, a nonlinear equation set between the Fourier coefficients and the complex amplitudes of each spatial mode is listed, a solving algorithm using the principal component order prior information is designed, and the complex amplitudes of each spatial mode are obtained; Step 3: An amplification imaging optical device for detecting the output mode field of the fiber is experimentally set up, and a spatial image detector is used to record the light field intensity mode spot and the interference pattern at any time; Step 4: The order of the principal component is determined from the light field intensity mode spot or the interference pattern, and the order of the principal component is substituted into the nonlinear equation set together with the Fourier coefficients of the angular sampling sequence required by the nonlinear equation set to solve the equation set and recover the amplitude of each mode to calculate the purity of the principal component.

2. The method for measuring mode purity of a few-mode ring core fiber based on principal component prior information according to claim 1, characterized in that, The few-mode ring-core fiber refers to any few-mode fiber with an axisymmetric refractive index characteristic, no geometric characteristic change in the transmission direction, capable of conducting electromagnetic waves, and introducing a high refractive index ring structure on the refractive index profile, without limiting the position and specific refractive index of the ring structure, and without limiting the introduction of any other special design on the refractive index profile.

3. The method for measuring mode purity of a few-mode ring core fiber based on principal component prior information according to claim 1, characterized in that, In Step 1, for the measured few-mode ring-core fiber, the geometric parameters and optical characteristic parameters are first determined, for simple fibers that can obtain analytical solutions, the spatial mode field distribution analytical expression is obtained by analytical solution, and for complex fibers that are difficult to obtain analytical solutions, the spatial eigenmode mode field distribution numerical solution and corresponding mode order are obtained by numerical calculation method; The spatial mode is a series of spatial modes obtained by numerical calculation, analytical solution or actual measurement, which are mutually orthogonal and power normalized.

4. The method for measuring mode purity of a few-mode ring core fiber based on principal component prior information according to claim 1, characterized in that, The nonlinear equation set in Step 2 refers to an equation set represented in any form with information from the intensity mode spot as the known quantity, the complex amplitude of the spatial mode as the unknown quantity; the solving algorithm of the nonlinear equation set includes analytical solution solving method and numerical solving algorithm; it includes nonlinear least squares algorithm, stochastic parallel gradient descent algorithm, machine learning algorithm, and any numerical solving algorithm for solving nonlinear equation set; it includes numerical calculation method realized by computer program language and numerical calculation method used in commercial software.

5. The method for measuring mode purity of a few-mode ring core fiber based on principal component prior information according to claim 1, characterized in that, In Step 3, the spatial image detector refers to various detectors that can record the output spatial mode field of the fiber; the interference pattern is any interference pattern that can detect the phase information of the fiber output light field to obtain the principal component, including the spiral interference pattern in coaxial interference and the fork interference pattern in off-axis interference.

6. A device for realizing the method for measuring the mode purity of a few-mode ring core fiber based on principal component prior information according to any one of claims 1-5, characterized in that, The device is sequentially composed of a tunable laser, an optical coupler OC, a single-mode optical fiber SMF, a polarization controller PC, a first lens, a second lens, a fiber or device to be measured, a few-mode ring core fiber RCF, a non-polarization beam splitter NPBS, a polarizer Pol., and a camera CCD, wherein: The tunable laser is used to characterize the performance of the fiber or device to be measured at each single wavelength; the core of the device is a simple imaging system, the fundamental mode light from the tunable laser is divided into two branches through a 5:5 optical coupler OC, in the first branch, the fundamental mode light in the single-mode optical fiber is converted into high-order modes in the few-mode ring core fiber through the fiber or device to be measured, and is collimated into a high-order beam in free space after passing through the first lens, that is, the output light field of the few-mode ring core fiber; in the second branch, the fundamental mode light from the single-mode optical fiber sequentially passes through the polarization controller and the second lens and is collimated into a fundamental mode Gaussian beam in free space, which is used to detect the phase distribution of the output light field of the few-mode ring core fiber, and an interference pattern is obtained; the free space beams of the two branches are combined through the non-polarization beam splitter and selectively imaged onto the CCD through the polarizer.

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