Optical fiber type angular displacement measuring device and method based on orbital angular momentum light beam interference

Abstract

The invention discloses an optical fiber type angular displacement measuring device and method based on orbital angular momentum light beam interference, and relates to the field of angular displacement measurement. When the device works, a laser emitting assembly emits two beams of light, and the two beams of light are transmitted to a spiral long-period fiber grating and a micro-nano polarization maintaining optical fiber respectively; the spiral long-period fiber grating converts the received light from a fundamental mode into a first-order orbital angular momentum mode, and then the light is transmitted to the beam combiner through the first lens; the micro-nano polarization-maintaining optical fiber rotates along with the rotary displacement table to generate angular displacement, outputs the light after phase change to the reflector through the second lens, reflects the light to the convex lens, changes the light into spherical waves through the convex lens, and transmits the spherical waves to the beam combiner; interference is formed between the spherical wave received by the beam combiner and the light in the first-order orbital angular momentum mode, and a corresponding spiral interference image is obtained through detection of the detector and used for calculating the angular displacement amount. According to the invention, high-precision and large-range precise angular displacement measurement can be realized at the same time.

Description

Technical Field

[0001] The present application relates to the field of angular displacement measurement, and in particular to an optical fiber type angular displacement measurement device and method based on orbital angular momentum beam interference. Background Art

[0002] Angular displacement sensors based on optical principles are generally based on total internal reflection effect, interference effect, birefringence effect and other principles, and are less affected by external interference. However, their measurement systems generally involve volume gratings, prisms and plane reflectors, so they have large volume sizes, which are not conducive to integration. In addition, the processing process of their system components is complex, and the sensing accuracy is low, which is not conducive to commercial applications. Existing angular displacement sensors include grating angular displacement sensors, laser triangulation angular displacement sensors, and fiber grating angular displacement sensors, but most of them face the following problems:

[0003] (1) Large range and high precision are generally a contradiction in sensors. The existing optical methods for measuring angular displacement are limited by the characteristics of optical devices and measurement methods, and the application scenarios of optical angular displacement measurement are limited.

[0004] (2) When image recognition and demodulation technology is used to improve measurement accuracy, the image calibration and recognition technology will directly affect the final angular displacement sensing accuracy, while the direct analysis of the experimental graphics will affect the measurement accuracy of the angular displacement. Summary of the invention

[0005] The purpose of this application is to provide a fiber-optic angular displacement measurement device and method based on orbital angular momentum beam interference, which can achieve precise angular displacement measurement with high accuracy and large range at the same time.

[0006] To achieve the above objectives, this application provides the following solutions:

[0007] In a first aspect, the present application provides a fiber-type angular displacement measurement device based on orbital angular momentum beam interference, including a laser emission component, a spiral long-period fiber grating, a micro-nano polarization-maintaining fiber, a first lens, a rotational translation stage, a second lens, a reflector, a convex lens, a beam combiner, and a detector;

[0008] The laser emission assembly is connected to the spiral long period fiber grating and the micro-nano polarization-maintaining fiber respectively; the spiral long period fiber grating is used as a reference arm fiber and is connected to the first lens; the micro-nano polarization-maintaining fiber is used as a sensing arm fiber and is placed on the rotation platform and connected to the second lens;

[0009] When working, the laser emitting component emits two beams of light, which are transmitted to the spiral long-period fiber grating and the micro-nano polarization-maintaining fiber respectively; the spiral long-period fiber grating converts the received light from the fundamental mode to the first-order orbital angular momentum mode, and then transmits it to the combiner via the first lens; the micro-nano polarization-maintaining fiber rotates with the rotation stage to generate angular displacement, and at the same time outputs the light after phase change to the reflector via the second lens, which is reflected by the reflector to the convex lens, converted into a spherical wave through the convex lens and transmitted to the combiner; the spherical wave received by the combiner interferes with the light of the first-order orbital angular momentum mode, and the corresponding spiral interference image is detected by the detector; the spiral interference image is used to calculate the angular displacement.

[0010] In a second aspect, the present application provides a fiber-optic angular displacement measurement method based on orbital angular momentum beam interference, comprising:

[0011] Build a fiber-optic angular displacement measurement device based on orbital angular momentum beam interferometry;

[0012] Acquire a first spiral interference image and a second spiral interference image detected by the detector; wherein the first spiral interference image is obtained when the micro-nano polarization-maintaining optical fiber follows the rotation stage to rotate at a first angle; the second spiral interference image is obtained when the micro-nano polarization-maintaining optical fiber follows the rotation stage to rotate at a second angle; and the value of the first angle is different from the value of the second angle;

[0013] For any spiral interference image, identify the annular light field in the spiral long-period fiber grating output light field image and calculate the center of the circle;

[0014] The spiral interference image is denoised by using a median filter algorithm, and then pixel values ​​in the denoised spiral interference image are read along a circular trajectory based on the center of the circle and the radius of the inner ring in the annular light field to obtain a cosine function by fitting;

[0015] Performing cross-correlation calculation on the cosine function corresponding to the first spiral interference image and the cosine function corresponding to the second spiral interference image to obtain an image rotation angle;

[0016] Based on the linear relationship, the angular displacement is calculated according to the image rotation angle.

[0017] According to the specific embodiments provided by the present application, the present application has the following technical effects: the present application provides a fiber-type angular displacement measurement device and method based on orbital angular momentum beam interference, and applies micro-nano polarization-maintaining optical fiber as an angular displacement sensor device in the measurement device, so that the anti-interference ability of the interference sensing system is improved; the spiral long-period fiber grating is applied, and the optical path structure is simplified by using the fiber grating to generate the orbital angular momentum beam; in the interference system, the light of the first-order orbital angular momentum mode and the spherical wave interfere to generate a spiral interference image, and when the angular displacement is generated in a directional manner, the interference image rotates in a directional manner. Based on this, the angular displacement is calculated according to the spiral interference image detected by the detector, and the large-scale high-sensitivity precision angular displacement measurement based on the orbital angular momentum beam interference is realized. In short, the present application builds an optical path structure based on orbital angular momentum beam interference, integrates the method of directly generating orbital angular momentum beams with fiber gratings, the micro-nano polarization-maintaining optical fiber that can be used for large-scale high-sensitivity angular displacement measurement, and the algorithm for interferometric image demodulation into the angular displacement interference system, that is, the precision angular displacement measurement with high precision and large range is realized at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 A schematic structural diagram of an optical fiber-based angular displacement measurement device based on orbital angular momentum beam interference provided in one embodiment of the present application.

[0020] Figure 2 A schematic diagram of the preparation of a spiral long-period fiber grating provided in one embodiment of the present application; wherein: Figure 2 (a) is a schematic diagram of the grating of HLPFG under a microscope with a magnification of 20 times; Figure 2 (b) is a schematic diagram of a micro-nano polarization-maintaining grating magnified 20 times by a microscope; Figure 2 (c) is a schematic diagram of the broadband transmission spectrum of HLPFG.

[0021] Figure 3 A schematic diagram of an optical fiber angular displacement measurement based on orbital angular momentum beam interferometry provided in one embodiment of the present application; wherein: Figure 3 (a) is a schematic diagram of the image demodulation process; Figure 3 (b) is a schematic diagram of the intensity data of the extracted pixels and the fitting curve; Figure 3 (c) is the fitting curve diagram obtained after data processing of two different interference images; Figure 3(d) in the figure is the cross-correlation result diagram of the two curves in (c).

[0022] Figure 4 The experimental results and fitting schematic diagram provided for an embodiment of the present application; wherein, Figure 4 (a) and (b) are schematic diagrams of experimental results and linear fitting results for angular displacements of 0°-2° and 2°-0°, respectively; Figure 4 (c) and (d) are schematic diagrams of the experimental results and linear fitting results for angular displacements of 2°-152° and 152°-2°, respectively.

[0023] Figure numerals: 1-laser, 2-fiber attenuator, 3-fiber coupler, 4-helical long-period fiber grating, 5-first lens, 6-micro-nano polarization-maintaining fiber, 7-rotational translation stage, 8-second lens, 9-reflector, 10-convex lens, 11-beam combiner, 12-linear polarizer, 13-beam expander, 14-detector. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0025] This application uses micro-nano polarization-maintaining optical fiber as a large-range angular displacement measurement device, combined with an optical interference system, and utilizes its advantages of high precision and high sensitivity to achieve a precision angular displacement measurement system that simultaneously achieves high precision and large range; at the same time, relying on the characteristics of interference image detection, it achieves real-time detection with fast response; in addition, this application can achieve stable interference pattern acquisition, which is the key to image recognition and demodulation technology.

[0026] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0027] Through research, it is found that the proposed principle of orbital angular momentum beam interference for angular displacement measurement can be explained mathematically. Usually, for an OAM (orbital angular momentum) beam propagating along the z direction, the order is l, and the electric field E1 expression is:

[0028]

[0029] Where r is the radial component in cylindrical coordinates; ω0 is the waist radius of the Gaussian beam; θ represents the azimuth angle of the vortex light; k = 2π / λ is the wave number; λ is the wavelength of the light wave. Under ideal conditions, the electric field E2 of the spherical wave is expressed as:

[0030]

[0031] Where A represents the amplitude of the spherical wave. The complex amplitude of the coherent superposition of two beams is:

[0032]

[0033] To simplify the calculation, it is assumed that the light intensity of the two beams on the detection surface is the same, that is, Then the light intensity during interference superposition is:

[0034]

[0035] When the object produces angular displacement, the phase of the interference light intensity changes At this time, the expression of interference light intensity I becomes:

[0036]

[0037] It can be seen from the formula that when the order l=1, the rotation angle of the light intensity center is consistent with the rotation angle of the interference fringe spiral phase, so the phase change can be determined by measuring the rotation angle of the interference spiral fringes.

[0038] Based on the above research results, in an exemplary embodiment, Figure 1 As shown, a fiber-type angular displacement measurement device based on orbital angular momentum beam interference is provided, including a laser emission component, a spiral long period fiber grating (HLPFG) 4, a micro-nano polarization-maintaining fiber (Micro-nano PMF) 6, a first lens 5, a rotating platform (RotatingPlatform) 7, a second lens 8, a reflector 9, a convex lens 10, a beam combiner 11, a linear polarizer 12, a beam expander 13 and a detector 14. Among them, the linear polarizer 12 and the beam expander 13 are arranged in sequence, and the linear polarizer 12 and the beam expander 13 are arranged between the beam combiner 11 and the detector 14.

[0039] In a specific application example, the laser emission component includes a laser 1 and a fiber coupling component which are arranged in sequence; the fiber coupling component is connected to the laser 1, the spiral long-period fiber grating 4 and the micro-nano polarization-maintaining fiber 6 respectively; the fiber coupling component includes a fiber attenuator (FiberAttenuators) 2 and a fiber coupler 3 (Fiber Couplers) which are arranged in sequence. The fiber attenuator 2 is used to adjust the power of the light entering the fiber coupler 3; the fiber coupler 3 is used to divide a received beam of light into two beams of light, and transmit them to the spiral long-period fiber grating 4 and the micro-nano polarization-maintaining fiber 6 respectively.

[0040] The laser emission component is connected to the spiral long-period fiber grating and the micro-nano polarization-maintaining fiber respectively; the spiral long-period fiber grating is used as a reference arm fiber and is connected to the first lens; the micro-nano polarization-maintaining fiber is used as a sensing arm fiber, which is placed on the rotation platform and connected to the second lens. Specifically, when working, the laser 1 outputs light to the fiber coupling component, and the fiber coupling component divides the received light into two beams, and transmits them to the spiral long-period fiber grating 4 and the micro-nano polarization-maintaining fiber 6 respectively.

[0041] In another specific application example, the laser 1 is a tunable laser, the first lens 5 and the second lens 8 are both 40x objective lenses, and the detector 14 is a photodetector CCD.

[0042] In another application example, the spiral long-period fiber grating 4 is used to realize the conversion of the fundamental mode of the optical fiber into the orbital angular momentum mode, and the CO2 laser processing system is used to prepare the HLPFG to generate an orbital angular momentum beam. The optical fiber used in this application is a single-mode optical fiber, and its core and cladding diameters are 9μm and 125μm, respectively. In order to prepare an HLPFG with high coupling efficiency and high quality, it is necessary to select a focused light spot with appropriate energy, and monitor the production process of the HLPFG by a camera. The production process of the spiral long-period fiber grating 4 includes: using a CO2 laser processing system to write a spiral line on a single-mode optical fiber based on a thermal effect to prepare a spiral long-period fiber grating.

[0043] In the experiment, the production process of the spiral long-period fiber grating 4 specifically includes: fixing a section of single-mode optical fiber on the rotating fixture of the three-dimensional displacement platform, and the fiber axis of the single-mode optical fiber is parallel to the X-axis. Then, according to the periodic parameters of the fiber grating, the rotation speed of the rotating fixture and the movement speed of the three-dimensional displacement platform are set to formulate the spiral shape of the grating. After that, adjust the energy of the laser output by the CO2 laser processing system and the laser focusing position to ensure that the laser is focused on the core of the single-mode optical fiber. The appropriate energy can prepare a fiber grating with high conversion efficiency. At the same time, a broadband light source and an optical spectrum analyzer are used to monitor the transmission spectrum of the HLPFG. Figure 2 (a) shows the grating area imaging of HLPFG under a 20x microscope. Figure 2 (b) is a 20-fold magnification of the micro-nano polarization-maintaining grating. Figure 2 (c) is the broadband transmission spectrum of HLPFG. When the grating period of HLPFG is 450μm, the grating length is 9.25mm, the resonant wavelength is 1581.8nm, and the dip loss is 24.819dB. When the input light wavelength is the resonant wavelength of HLPFG, 1581.8nm, the fundamental mode of the optical fiber is LP. 01 mode is converted to OAM1 mode by HLFPG.

[0044] In another application example, the production process of the micro-nano polarization-maintaining optical fiber 6 includes: heating the polarization-maintaining optical fiber with an oxyhydrogen flame, and processing the polarization-maintaining optical fiber by a fusion taper method to prepare a micro-nano polarization-maintaining optical fiber. The core and cladding diameters of the polarization-maintaining optical fiber used are 10 μm and 125 μm, respectively. In a specific experiment, the two ends of a section of polarization-maintaining optical fiber are respectively fixed on the optical fiber clamp of the three-dimensional displacement platform, the optical fiber axis is parallel to the X-axis, the middle part of the polarization-maintaining optical fiber is heated with an oxyhydrogen flame, and the left displacement stage is controlled to move at 1.2 mm / s, and the right displacement stage is controlled to move at 1.5 mm / s, and the process is pulled back and forth 5 times to achieve the preparation of micro-nano polarization-maintaining optical fiber. Compared with the untreated polarization-maintaining optical fiber that can only be bent with a small curvature, the cone area diameter of the micro-nano polarization-maintaining optical fiber after the taper treatment in this application is 22 μm, and it can be bent at a larger angle to achieve a large range of angular displacement strain, thereby enabling angular displacement measurement.

[0045] When working, the laser emission assembly emits two beams of light, which are transmitted to the spiral long-period fiber grating and the micro-nano polarization-maintaining fiber respectively; specifically, the tunable laser outputs a beam with a wavelength of 1581.8nm, and the input light power is regulated by the optical attenuator 2, and is evenly divided into two light paths through the optical fiber coupler 3. The spiral long-period fiber grating 4 converts the received light from the fundamental mode to the first-order orbital angular momentum mode, and then transmits it to the beam splitter and combiner 11 through the first lens 5. The micro-nano polarization-maintaining fiber 6 rotates with the rotation stage to generate angular displacement, and outputs the light after phase change to the reflector 9 through the second lens 8. Specifically, Gaussian light is output at the end face of the micro-nano polarization-maintaining fiber 6, reflected to the convex lens 10 through the reflector 9, and converted into a spherical wave through the convex lens 10 and transmitted to the beam combiner 11; the spherical wave received by the beam combiner 11 interferes with the light of the first-order orbital angular momentum mode.

[0046] In order to improve the interference efficiency, a linear polarizer 12 is added to the optical path, so that when the two received light beams (i.e., the spherical wave and the light of the first-order orbital angular momentum mode) interfere in the same polarization state, the light field is amplified 4 times by the beam expander 13, and finally the corresponding spiral interference image is detected by the detector 14. Among them, the beam expander can amplify the light field of the interference image, and the larger interference light field can enable the detector to obtain more pixel point signals, and the formed spiral interference image can make the subsequent image calculation more accurate. Compared with the interference fringes between spherical waves, the interference fringes between OAM light and Gaussian light are spiral arm-shaped, which is more obvious and more precise in image observation and processing, so the spiral interference image is used to calculate the angular displacement.

[0047] Based on the same inventive concept, the embodiment of the present application also provides a fiber-type angular displacement measurement method based on orbital angular momentum beam interference, comprising:

[0048] Step 100: construct the above-mentioned fiber-type angular displacement measurement device based on orbital angular momentum beam interference. Before the angular displacement experiment, input light with a wavelength of 1581.8 nm, wherein the LP 01 After being converted into OAM1 and output from the optical fiber, a ring-shaped light field can be obtained on the CCD.

[0049] Step 200, obtaining a first spiral interference image and a second spiral interference image detected by the detector; wherein the first spiral interference image is obtained when the micro-nano polarization-maintaining optical fiber follows the rotation stage to rotate by a first angle; the second spiral interference image is obtained when the micro-nano polarization-maintaining optical fiber follows the rotation stage to rotate by a second angle; the value of the first angle is different from the value of the second angle. In general, the first angle is the angle before rotation, and the second angle is the angle after rotation.

[0050] Combination Figure 3 In (a), the general process of demodulating the spiral interference image with the help of image algorithm demodulation is as follows: identify the outer ring and the inner ring of the annular light field, and calculate the center of the concentric circles; use the image median filter algorithm to denoise the spiral interference image; use the center coordinates and the radius of the inner ring as the circular trajectory, read the pixel values ​​of the interference image on the circular trajectory, and fit the data group composed of the pixel values ​​to the sine function; perform cross-correlation calculation on the cosine fitting functions of the two interference images, and obtain the maximum correlation value point of the two groups of functions, and obtain the displacement value of the function, that is, the rotation angle value of the image. Figure 3 (b) in the figure shows the data obtained in the above steps, and the cosine function is obtained by interpolation fitting of the data. Figure 3 (c) in the figure is a fitting function curve after image processing of two spiral interference images at different angles, as shown in the figure, which specifically shows the interference patterns corresponding to the two angles of angular displacement of 62° and 32°. Figure 3 (d) in (c) is the cross-correlation result of the two function curves. The maximum value of the function is the point with the largest cross-correlation degree and the highest similarity. At this time, the horizontal axis represents the rotation angle of the interference image. The specific implementation process of the above process is as follows.

[0051] Step 300, for any spiral interference image, identify the annular light field in the spiral long-period fiber grating output light field image, and calculate the center of the circle. In an application example, it specifically includes: using the Hough circle transform algorithm to perform edge fitting on the spiral interference image to determine the inner circle and the outer circle in the spiral interference image, and calculate the common center of the inner circle and the outer circle. Among them, the Hough circle transform algorithm mainly calculates the image pixel gradient, identifies the edge in the image for fitting, and then identifies the image edge contour. In this application, it is used to identify the circle in the image and calculate the center and radius.

[0052] Step 400, denoising the spiral interference image using a median filtering algorithm, and then reading pixel values ​​in the denoised spiral interference image along a circular trajectory based on the center of the circle and the radius of the inner ring in the annular light field to fit a cosine function.

[0053] Since the spiral interference image acquired by CCD has noise points and stripe noise caused by the linear polarizer, the image quality is reduced, resulting in interference of the interference image data. Therefore, the median filter algorithm is used to process the spiral interference image. The median filter algorithm is a nonlinear image processing method that can retain the image edge information as much as possible while removing noise. The principle of the median filter algorithm is to select a window containing an odd number of points to scan the image, sort the intensity of the pixels in the window by size, and select the median value in the sorting as the pixel value after filtering of the central pixel of the window. After image processing, the stripe noise on the image caused by linear polarization can be effectively filtered out.

[0054] In an application example, based on the center of the circle and the radius of the inner circle in the annular light field, the pixel values ​​in the denoised spiral interference image are read along a circular trajectory to fit a cosine function, including: on the denoised spiral interference image, a circular trajectory is taken with the center of the circle and the radius of the inner circle in the annular light field; starting from the leftmost pixel point of the circular trajectory, reading in a counterclockwise order, and picking up and drawing data with the radian corresponding to each pixel point in the circular trajectory as the horizontal coordinate and the corresponding pixel value as the vertical coordinate, and then obtaining the cosine function through interpolation fitting.

[0055] In order to calculate the image rotation amount before and after the angular displacement, the two frames of images corresponding to the two frames before and after the angular displacement are processed as above, and two sets of cosine fitting functions can be obtained.

[0056] Step 500, performing cross-correlation calculation on the cosine function corresponding to the first spiral interference image and the cosine function corresponding to the second spiral interference image to obtain the image rotation angle; wherein the cross-correlation calculation is used to calculate the similarity between two signals, and the fitting function in the present application can be approximated as a continuous function, and the function formula of the cross-correlation calculation is:

[0057]

[0058] Among them, the maximum value of the cross-correlation function is the angle offset, is the cross-correlation value; θ is the arc value corresponding to the pixel point on the circular trajectory, ranging from 0 to 2π; x(θ) and y(θ) are two sets of sinusoidal function curves, which are the cosine function corresponding to the first spiral interference image and the cosine function corresponding to the second spiral interference image respectively; is the phase change.

[0059] Step 600, based on the linear relationship, calculate the angular displacement according to the image rotation angle. Specifically, according to the rotation angle of the spiral interference image before and after the angular displacement, in conjunction with the reading of the rotation stage, the relationship between the angular displacement and the interference image rotation can be calculated.

[0060] In practical applications, Python is used to write a program based on spiral interference image processing, and then three sets of repeated experiments can be completed and the experimental results can be obtained. Figure 4 The test results of the angular displacement sensing optical measurement system are shown. The change in angular displacement of the micro-nano polarization-maintaining optical fiber during bending and recovery maintains a good linear relationship with the rotation of the interference image. Within 0°-2°, the step is 1°, and the sensitivity is 3524.16° (that is, the angular displacement changes by 1°, corresponding to the interference image rotation of 3524.16°); within 2°-0°, the step is 1°, and the sensitivity is 3233.22°; within 2°-152°, the step is 30°, and the sensitivity is 33.93°; within 152°-2°, the step is 30°, and the sensitivity is 53.85°.

[0061] In summary, compared with the prior art, the present application has the following advantages:

[0062] (1) The present application uses an oxyhydrogen flame heating fiber taper system to prepare micro-nano polarization-maintaining optical fiber and realize the preparation of a large-angle angular displacement sensor. The oxyhydrogen flame heating fiber taper system uses an oxyhydrogen flame to heat the optical fiber at high temperature, adjusts the speed of the left and right translation stages to produce a speed difference, and controls the processing time to control the diameter of the micro-nano optical fiber. The preparation method has high preparation efficiency and good repeatability. Due to its tiny fiber diameter, the micro-nano polarization-maintaining optical fiber can be directly bent at a large curvature and a large angle, has a simple structure, and has little external interference on polarization, which makes it have a strong anti-interference ability in angular displacement measurement and realizes high-sensitivity angular displacement sensing.

[0063] (2) This application uses a CO2 laser processing system to prepare HLPFG on a single-mode optical fiber to generate an orbital angular momentum beam. The CO2 laser processing system uses thermal effects to write helical lines on the optical fiber to prepare a high-conversion-efficiency HLPFG, thereby directly generating an orbital angular momentum beam through a fiber grating. Compared with using space devices to generate an orbital angular momentum beam, the use of a fiber grating is small in size, can replace complex optical path systems, is simpler, more efficient, and low-cost, and is more flexible to use in subsequent optical path systems.

[0064] (3) The present application constructs an orbital angular momentum beam interference system to detect image data of interference fringe changes caused by large-scale angular displacement changes. The optical interferometry method uses the interference fringe changes to reflect the light phase changes, and has the advantages of high resolution and high sensitivity. The present application uses the orbital angular momentum light and spherical wave interference system to obtain a spiral interference image. Under the same conditions of light phase change, a larger image change than the vertical interference fringe can be obtained. Within the range of 0°-152°, an angular displacement change is applied to the micro-nano polarization-maintaining optical fiber through a rotating translation stage. As the angular displacement increases or decreases, the spiral interference image rotates clockwise or counterclockwise. At the same time, the interference light field is amplified with the help of a beam expander, making more full use of the imaging area of ​​the CCD, obtaining a larger interference field image, and having more data points to analyze in image demodulation, which can achieve higher-precision measurements.

[0065] (4) The present application prepares an image data demodulation program to calculate the rotation angle of the spiral interference image. Compared with the commonly used centroid method or tangent method, this method is simpler in terms of image data processing requirements, but its precision and accuracy are greatly improved.

[0066] The present application proposes an interference structure that uses an optical fiber to generate orbital angular momentum light, uses a micro-nano polarization-maintaining optical fiber as a sensor, and uses the spiral interference image obtained by demodulating the interference between the orbital angular momentum light and the spherical wave to apply to precision angular displacement measurement, achieving a wide range and high sensitivity. This precision measurement device is expected to meet the requirements of high precision, large range, and real-time performance in the precision measurement needs of micro-vibration, micro-displacement, etc., and has broad application prospects in precision measurement engineering.

[0067] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0068] This article uses specific examples to illustrate the principles and implementation methods of this application. The description of the above embodiments is only used to help understand the method and core ideas of this application. At the same time, for those skilled in the art, according to the ideas of this application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. An optical fiber angular displacement measuring device based on orbital angular momentum beam interference, characterized in that: The optical fiber angular displacement measuring device based on orbital angular momentum beam interference comprises a laser emission component, a spiral long-period fiber grating, a micro-nano polarization-maintaining optical fiber, a first lens, a rotation platform, a second lens, a reflector, a convex lens, a beam combiner and a detector; The laser emission assembly is connected to the spiral long period fiber grating and the micro-nano polarization-maintaining fiber respectively; the spiral long period fiber grating is used as a reference arm fiber and is connected to the first lens; the micro-nano polarization-maintaining fiber is used as a sensing arm fiber and is placed on the rotation platform and connected to the second lens; When working, the laser emitting component emits two beams of light, which are transmitted to the spiral long-period fiber grating and the micro-nano polarization-maintaining fiber respectively; the spiral long-period fiber grating converts the received light from the fundamental mode to the first-order orbital angular momentum mode, and then transmits it to the combiner via the first lens; the micro-nano polarization-maintaining fiber rotates with the rotation stage to generate angular displacement, and at the same time outputs the light after phase change to the reflector via the second lens, which is reflected by the reflector to the convex lens, converted into a spherical wave through the convex lens and transmitted to the combiner; the spherical wave received by the combiner interferes with the light of the first-order orbital angular momentum mode, and the corresponding spiral interference image is detected by the detector; the spiral interference image is used to calculate the angular displacement.

2. The optical fiber angular displacement measuring device based on orbital angular momentum beam interference according to claim 1, characterized in that: The optical fiber type angular displacement measurement device based on orbital angular momentum beam interference also includes a linear polarizer and a beam expander arranged in sequence; The linear polarizer and the beam expander are arranged between the beam combiner and the detector; the linear polarizer is used to make the two received light beams interfere in the same polarization state; the beam expander is used to amplify the light field of the interference image so that the detector obtains more pixel point signals.

3. A fiber-optic angular displacement measurement method based on orbital angular momentum beam interference, characterized in that: The optical fiber type angular displacement measurement method based on orbital angular momentum beam interference comprises: Constructing a fiber-optic angular displacement measurement device based on orbital angular momentum beam interference as described in any one of claims 1 to 2; Acquire a first spiral interference image and a second spiral interference image detected by the detector; wherein the first spiral interference image is obtained when the micro-nano polarization-maintaining optical fiber follows the rotation stage to rotate at a first angle; the second spiral interference image is obtained when the micro-nano polarization-maintaining optical fiber follows the rotation stage to rotate at a second angle; and the value of the first angle is different from the value of the second angle; For any spiral interference image, identify the annular light field in the spiral long-period fiber grating output light field image and calculate the center of the circle; The spiral interference image is denoised by using a median filter algorithm, and then pixel values ​​in the denoised spiral interference image are read along a circular trajectory based on the center of the circle and the radius of the inner ring in the annular light field to obtain a cosine function by fitting; Performing cross-correlation calculation on the cosine function corresponding to the first spiral interference image and the cosine function corresponding to the second spiral interference image to obtain an image rotation angle; Based on the linear relationship, the angular displacement is calculated according to the image rotation angle.

4. The optical fiber angular displacement measurement method based on orbital angular momentum beam interferometry according to claim 3, characterized in that: Identify the annular light field in the output light field image of the spiral long-period fiber grating and calculate the center of the circle, including: The Hough circle transform algorithm is used to perform edge fitting on the spiral interference image to determine an inner ring and an outer ring in the spiral interference image, and a common center of the inner ring and the outer ring is calculated.

5. The optical fiber type angular displacement measurement method based on orbital angular momentum beam interference according to claim 3, characterized in that: Based on the center of the circle and the radius of the inner ring in the annular light field, pixel values ​​in the denoised spiral interference image are read along a circular trajectory to fit a cosine function, including: On the denoised spiral interference image, a circular trajectory is taken with the center of the circle and the radius of the inner ring in the annular light field; starting from the leftmost pixel point of the circular trajectory, reading is done in a counterclockwise order, and data is picked up and drawn with the arc corresponding to each pixel point in the circular trajectory as the horizontal coordinate and the corresponding pixel value as the vertical coordinate, and then the cosine function is obtained by interpolation fitting.

6. The optical fiber type angular displacement measurement method based on orbital angular momentum beam interference according to claim 5, characterized in that: The calculation formula for cross-correlation calculation of the cosine function corresponding to the first spiral interference image and the cosine function corresponding to the second spiral interference image is: in, is the cross-correlation value; θ is the arc value corresponding to the pixel point on the circular trajectory; x(θ) and y(θ) are two sets of sinusoidal function curves, which are the cosine function corresponding to the first spiral interference image and the cosine function corresponding to the second spiral interference image respectively; is the phase change.

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