A fiber-optic angular displacement measurement device and method based on orbital angular momentum beam interferometry.

Abstract

This application discloses a fiber-optic angular displacement measurement device and method based on orbital angular momentum beam interference, relating to the field of angular displacement measurement. During operation, a laser emitting component emits two beams of light, which are transmitted to a helical long-period fiber grating and a micro / nano polarization-maintaining fiber, respectively. The helical long-period fiber grating converts the received light from its fundamental mode into a first-order orbital angular momentum mode, which is then transmitted to a combiner via a first mirror. The micro / nano polarization-maintaining fiber rotates with a rotating displacement stage to generate angular displacement, simultaneously outputting the phase-changed light through a second mirror to a reflector and then reflecting it to a convex lens. The convex lens converts the light into a spherical wave, which is then transmitted to the combiner. The spherical wave received by the combiner interferes with the first-order orbital angular momentum mode light, and a detector detects the corresponding helical interference pattern, which is used to calculate the angular displacement. This application enables precise angular displacement measurement with both high accuracy and a wide range of measurement capabilities.

Description

Technical Field

[0001] This application relates to the field of angular displacement measurement, and in particular to a fiber-optic angular displacement measurement device and method based on orbital angular momentum beam interference. Background Technology

[0002] Optical angular displacement sensors, generally based on principles such as total internal reflection, interference, and birefringence, are less susceptible to external interference. However, their measurement systems typically involve volume gratings, prisms, and plane mirrors, resulting in large dimensions that hinder integration. Furthermore, the complex fabrication process of these components leads to low sensing accuracy, limiting their commercial application. Existing angular displacement sensors include grating-type, laser triangulation-type, and fiber Bragg grating-type angular displacement sensors, but most face the following problems:

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

[0004] (2) When image recognition 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 graph will affect the measurement accuracy of 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 high-precision and wide-range precision angular displacement measurement simultaneously.

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

[0007] In one aspect, this application provides a fiber-optic angular displacement measuring device based on orbital angular momentum beam interference, including a laser emitting component, a spiral long-period fiber grating, a micro-nano polarization-maintaining fiber, a first lens, a rotating displacement stage, a second lens, a reflector, a convex lens, a beam combiner, and a detector.

[0008] The laser emitting component is connected to the helical long-period fiber grating and the micro / nano polarization-maintaining fiber, respectively; the helical long-period fiber grating serves as a reference arm fiber and is connected to the first lens; the micro / nano polarization-maintaining fiber serves as a sensing arm fiber, is placed on the rotating displacement stage, and is connected to the second lens.

[0009] During operation, the laser emitting component emits two beams of light, which are transmitted to the helical long-period fiber grating and the micro / nano polarization-maintaining fiber, respectively. The helical long-period fiber grating converts the received light from its fundamental mode into a first-order orbital angular momentum mode, and then transmits it to the combiner via the first mirror. The micro / nano polarization-maintaining fiber rotates with the rotating displacement stage to generate angular displacement, and simultaneously outputs the phase-changed light to the reflector via the second mirror. The reflector then reflects the light to the convex lens, where it is converted into a spherical wave and transmitted to the combiner. The spherical wave received by the combiner interferes with the light in the first-order orbital angular momentum mode, and the detector detects the corresponding helical interference pattern. The helical interference pattern is used to calculate the angular displacement.

[0010] Secondly, this application provides a fiber-optic angular displacement measurement method based on orbital angular momentum beam interferometry, including:

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

[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 fiber follows the rotating displacement stage to rotate by a first angle; the second spiral interference image is obtained when the micro / nano polarization-maintaining fiber follows the rotating displacement stage to rotate by a second angle; the values ​​of the first angle and the second angle are different.

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

[0014] The median filtering algorithm is used to denoise the spiral interference image. Then, based on the center of the circle and the radius of the inner ring in the annular light field, the pixel values ​​in the denoised spiral interference image are read along the circular trajectory to fit the cosine function.

[0015] Cross-correlation calculations are performed on the cosine functions corresponding to the first and second spiral interference images to obtain the image rotation angle;

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

[0017] According to the specific embodiments provided in this application, this application has the following technical effects: This application provides a fiber-optic angular displacement measurement device and method based on orbital angular momentum beam interferometry. Micro-nano polarization-maintaining fiber is used as an angular displacement sensor in the measurement device, improving the anti-interference capability of the interferometric sensing system. A helical long-period fiber grating is applied, using the fiber grating to generate an orbital angular momentum beam, simplifying the optical path structure. In the interferometric system, a helical interference image is generated by the interference of light in a first-order orbital angular momentum mode with a spherical wave. When a directional angular displacement occurs, the interference image rotates directionally. Based on this, the angular displacement is calculated from the helical interference image detected by the detector, achieving a large-scale, high-sensitivity, and precise angular displacement measurement based on orbital angular momentum beam interferometry. In short, this application constructs an optical path structure based on orbital angular momentum beam interferometry, integrating a method for directly generating orbital angular momentum beams using fiber gratings, micro-nano polarization-maintaining fiber for large-scale, high-sensitivity angular displacement measurement, and an algorithm for interferometric image demodulation into the angular displacement interferometric system, thus achieving both high precision and large-scale precise angular displacement measurement. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a fiber-optic angular displacement measuring device based on orbital angular momentum beam interference, provided in an embodiment of this application.

[0020] Figure 2 This is a schematic diagram related to the fabrication of a helical long-period fiber grating provided in an embodiment of this application; wherein, Figure 2 (a) is a schematic diagram of the grating of an HLPFG magnified 20x by a microscope; Figure 2 (b) is a schematic diagram of a micro / nano polarization-maintaining grating magnified 20x by a microscope; Figure 2 (c) in the figure is a schematic diagram of the broadband transmission spectrum of HLPG.

[0021] Figure 3 This application provides a schematic diagram of fiber-optic angular displacement measurement based on orbital angular momentum beam interferometry, according to an embodiment of this application; wherein, Figure 3 (a) in the diagram is a schematic diagram of the image demodulation process; Figure 3 (b) in the figure is a schematic diagram of the extracted pixel intensity data and the fitted curve; Figure 3 (c) in the figure is the fitting curve obtained after data processing of two different interference images; Figure 3(d) in the figure is the cross-correlation result of the two curves in (c).

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

[0023] Figure reference numerals: 1-Laser, 2-Fiber optic attenuator, 3-Fiber optic coupler, 4-Helical long-period fiber grating, 5-First lens, 6-Micro-nano polarization-maintaining fiber, 7-Rotating displacement stage, 8-Second lens, 9-Reflector, 10-Convex lens, 11-Band combiner, 12-Linear polarizer, 13-Band expander, 14-Detector. Detailed Implementation

[0024] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

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

[0026] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Research has revealed that the proposed principle of using orbital angular momentum beam interferometry for angular displacement measurement can be mathematically explained. Typically, for an OAM (orbital angular momentum) beam propagating along the z-direction with order l, the electric field E1 is expressed as:

[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 beam; k = 2π / λ is the wave number; and λ is the wavelength of the light wave. Under ideal conditions, the electric field E² of a 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 intensities of the two beams are the same on the detection surface, i.e. The intensity of the superimposed interference time is:

[0034]

[0035] When an object undergoes angular displacement, the intensity and phase of the interference light change. At this point, the expression for the interference light intensity I becomes:

[0036]

[0037] As can be seen from the formula, when the order l = 1, the rotation angle of the light intensity center is consistent with the rotation angle of the spiral phase of the interference fringes. Therefore, 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, such as Figure 1 As shown, a fiber-optic angular displacement measurement device based on orbital angular momentum beam interference is provided, including a laser emitting component, a helical long-period fiber grating (HLPFG) 4, a micro-nano polarization-maintaining fiber (Micro-nano PMF) 6, a first mirror 5, a rotating platform 7, a second mirror 8, a reflector 9, a convex lens 10, a beam combiner 11, a linear polarizer 12, a beam expander 13, and a detector 14. The linear polarizer 12 and the beam expander 13 are arranged sequentially, and are positioned between the beam combiner 11 and the detector 14.

[0039] In a specific application example, the laser emitting assembly includes a laser 1 and an optical fiber coupling assembly arranged sequentially. The optical fiber coupling assembly is connected to the laser 1, the helical long-period fiber grating 4, and the micro / nano polarization-maintaining fiber 6, respectively. The optical fiber coupling assembly includes optical fiber attenuators 2 and optical fiber couplers 3 arranged sequentially. The optical fiber attenuators 2 are used to regulate the power of the light entering the optical fiber couplers 3. The optical fiber couplers 3 are used to split the received light beam into two beams, which are then transmitted to the helical long-period fiber grating 4 and the micro / nano polarization-maintaining fiber 6, respectively.

[0040] The laser emitting component is connected to the helical long-period fiber grating and the micro / nano polarization-maintaining fiber, respectively. The helical long-period fiber grating serves as a reference arm fiber and is connected to the first mirror. The micro / nano polarization-maintaining fiber serves as a sensing arm fiber, is placed on the rotating displacement stage, and is connected to the second mirror. Specifically, during operation, the laser 1 outputs light to the fiber coupling component, which splits the received beam into two beams, which are then transmitted to the helical 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 objectives, and the detector 14 is a photodetector CCD.

[0042] In another application example, the helical long-period fiber grating 4 is used to realize the conversion of the fiber's fundamental mode into an orbital angular momentum mode. The experiment utilizes a CO2 laser processing system to fabricate an HLPG to generate an orbital angular momentum beam. The fiber used in this application is a single-mode fiber with core and cladding diameters of 9 μm and 125 μm, respectively. To fabricate an HLPG with high coupling efficiency and high quality, a suitable focusing spot with appropriate energy must be selected, and the fabrication process must be monitored using a camera. The fabrication process of the helical long-period fiber grating 4 includes: using a CO2 laser processing system to inscribe helical lines on the single-mode fiber based on thermal effects to obtain the helical long-period fiber grating.

[0043] In the experiment, the fabrication process of the helical long-period fiber grating (HLPFG) 4 specifically included: fixing a section of single-mode fiber on a rotating fixture of a three-dimensional displacement platform, with the fiber axis of the single-mode fiber parallel to the X-axis. Then, based on the period parameters of the fiber grating, the rotational speed of the rotating fixture and the moving speed of the three-dimensional displacement platform were set to determine the helical shape of the grating. Next, the energy of the output laser from the CO2 laser processing system and the laser focusing position were adjusted to ensure that the laser was focused on the core of the single-mode fiber; appropriate energy can fabricate a high-conversion-efficiency fiber grating. Simultaneously, a broadband light source and an optical spectrometer were used to monitor the transmission spectrum of the HLPFG. Figure 2 Image (a) shows the grating region image of the HLPFG under a 20x microscope. Figure 2 (b) in the image is a micro / nano polarization-maintaining grating magnified 20x by a microscope. Figure 2 (c) in the figure represents the broadband transmission spectrum of the HLPG. When the grating period of the HLPG is 450 μm, the grating length is 9.25 mm, the resonant wavelength is 1581.8 nm, and the dip loss is 24.819 dB. When the input light wavelength is the resonant wavelength of the HLPG (1581.8 nm), the fundamental mode of the fiber, i.e., LP... 01 The mode was converted to OAM1 mode by HLFPG.

[0044] In another application example, the fabrication process of the micro / nano polarization-maintaining fiber 6 includes: heating the polarization-maintaining fiber with an oxyhydrogen flame and processing it using a fused taper method to prepare the micro / nano polarization-maintaining fiber. The core and cladding diameters of the polarization-maintaining fiber used are 10 μm and 125 μm, respectively. In a specific experiment, a section of polarization-maintaining fiber is fixed at both ends to fiber clamps on a three-dimensional displacement platform, with the fiber axis parallel to the X-axis. The middle of the polarization-maintaining fiber is heated with an oxyhydrogen flame, while the left displacement stage moves at 1.2 mm / s and the right displacement stage moves at 1.5 mm / s, repeated five times to achieve the fabrication of the micro / nano polarization-maintaining fiber. Compared to untreated polarization-maintaining fiber, which can only bend at small curvatures, the micro / nano polarization-maintaining fiber in this application, after taper processing, has a taper diameter of 22 μm and can bend at a larger angle, achieving a wide range of angular displacement strain, thus enabling angular displacement measurement.

[0045] During operation, 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. Specifically, the tunable laser outputs a beam with a wavelength of 1581.8 nm. The input optical power is controlled by the light attenuator 2, and the beam is uniformly split into two beams by the fiber coupler 3. The spiral long-period fiber grating 4 converts the received light from the fundamental mode into a first-order orbital angular momentum mode, and then transmits it to the beam splitter / combiner 11 via the first lens 5. The micro / nano polarization-maintaining fiber 6 rotates with the rotating displacement stage to generate angular displacement, and simultaneously outputs the phase-changed light to the reflector 9 via the second lens 8. Specifically, Gaussian light is output at the end face of the micro / nano polarization-maintaining fiber 6, reflected by the reflector 9 to the convex lens 10, and converted into a spherical wave by the convex lens 10 before being transmitted to the beam combiner 11. The spherical wave received by the beam combiner 11 interferes with the light in the first-order orbital angular momentum mode.

[0046] To improve interference efficiency, a linear polarizer 12 is added to the optical path. When the two received beams (i.e., a spherical wave and a first-order orbital angular momentum mode beam) interfere in the same polarization state, the beam expander 13 amplifies the light field by a factor of four, and the detector 14 ultimately detects the corresponding spiral interference image. The beam expander amplifies the light field of the interference image; a larger interference field allows the detector to obtain more pixel signals, and the resulting spiral interference image enables more accurate subsequent image calculations. Compared to the interference fringes between spherical waves, the interference fringes between OAM light and Gaussian light are spiral-arm shaped, which is more obvious and has higher precision in image observation and processing. Therefore, the spiral interference image is used to calculate angular displacement.

[0047] Based on the same inventive concept, this application also provides a fiber-optic angular displacement measurement method based on orbital angular momentum beam interference, including:

[0048] Step 100: Construct the fiber-optic angular displacement measurement device based on orbital angular momentum beam interference as described above. Before the angular displacement experiment, input light with a wavelength of 1581.8 nm, where the LP within the fiber grating... 01 After being converted to OAM1 and output from the optical fiber, a ring-shaped optical field can be obtained on the CCD.

[0049] Step 200: 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 fiber follows the rotating displacement stage to rotate by a first angle; the second spiral interference image is obtained when the micro / nano polarization-maintaining fiber follows the rotating displacement stage to rotate by a second angle; the values ​​of the first angle and the second angle are different. Generally, 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 a spiral interference image using image algorithms is as follows: identify the outer and inner rings of the annular light field and calculate the center of the concentric circles; use the image median filtering algorithm to denoise the spiral interference image; take 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 pixel values ​​with a sine function; perform cross-correlation calculation on the cosine fitting functions of the two interference images to obtain the maximum correlation point of the two sets of functions, and obtain the displacement value of the function, that is, the rotation angle value of the image. Figure 3 (b) shows the data obtained in the above steps. The cosine function is obtained by interpolating and fitting the data. Figure 3 (c) in the figure represents the fitting function curves after image processing of two spiral interference images at different angles. Specifically, the figure shows the interference patterns corresponding to the two angles of angular displacement of 62° and 32°. Figure 3 In the diagram, (d) represents the cross-correlation result of the two function curves in (c). The maximum value of the function is the point with the highest degree of cross-correlation and the highest similarity. In this case, the horizontal axis represents the angle of rotation of the interference image. The specific implementation process of the above procedure is shown below.

[0051] Step 300: For any helical interferometric image, identify the annular light field in the emitted light field image of the helical long-period fiber grating and calculate the center of the circle. In one application example, this specifically includes: using the Hough circle transform algorithm to perform edge fitting on the helical interferometric image to determine the inner and outer annulus in the helical interferometric image, and calculating the common center of the inner and outer annulus. The Hough circle transform algorithm mainly identifies and fits edges in the image by calculating the pixel gradient, thereby identifying the image edge contour. In this application, it is used to identify circles in the image and calculate the center and radius.

[0052] Step 400: The median filtering algorithm is used to denoise the spiral interference image. Then, based on the center of the circle and the radius of the inner ring in the annular light field, the pixel values ​​in the denoised spiral interference image are read along the circular trajectory to fit the cosine function.

[0053] Because the spiral interferometric images acquired by CCDs contain noise and fringe noise introduced by the linear polarizer, which degrades image quality and interferes with the interferometric image data, a median filtering algorithm is used to process the spiral interferometric images. The median filtering algorithm is a non-linear image processing method that can preserve image edge information as much as possible while removing noise. The principle of the median filtering algorithm is to select a window containing an odd number of points to scan the image, sort the pixel intensities within the window by size, and select the median value of the sorted pixels as the filtered pixel value of the central pixel of the window. After image processing, fringe noise caused by linear polarization can be effectively filtered out.

[0054] In one application example, 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. This includes: taking a circular trajectory on the denoised spiral interference image with the center of the circle and the radius of the inner ring in the annular light field; starting from the leftmost pixel of the circular trajectory, reading in a counterclockwise order, and using the radian corresponding to each pixel in the circular trajectory as the abscissa and the corresponding pixel value as the ordinate for data acquisition and plotting, and then obtaining the cosine function through interpolation fitting.

[0055] To calculate the image rotation before and after the angular displacement, the above processing is performed on the two corresponding frames before and after the angular displacement, respectively, to obtain two sets of cosine fitting functions.

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

[0057]

[0058] The maximum value of the cross-correlation function represents the angular offset. θ is the cross-correlation value; θ is the radian value corresponding to the pixel on the circular trajectory, ranging from 0 to 2π; x(θ) and y(θ) are two sets of sine function curves, which are the cosine functions corresponding to the first spiral interference image and the second spiral interference image, respectively; This represents a phase change.

[0059] Step 600: Based on the linear relationship, calculate the angular displacement according to the rotation angle of the image. Specifically, based on the rotation angle of the helical interferometer image before and after the angular displacement, and in conjunction with the readings of the rotation stage, the relationship between the angular displacement and the rotation of the interferometer image can be deduced.

[0060] In practical applications, a Python program based on spiral interference image processing is used to complete three sets of repeated experiments and process the results, as shown below. Figure 4 The test results of the angular displacement sensing optical measurement system are shown. The change in angular displacement and the rotation of the interferometric image maintain a good linear relationship during bending and restoration of the micro / nano polarization-maintaining fiber. Within 0°-2°, a step of 1° yields a sensitivity of 3524.16° (i.e., a 1° change in angular displacement corresponds to a 3524.16° rotation of the interferometric image); within 2°-0°, a step of 1° yields a sensitivity of 3233.22°; within 2°-152°, a step of 30° yields a sensitivity of 33.93°; and within 152°-2°, a step of 30° yields a sensitivity of 53.85°.

[0061] In summary, this application has the following advantages compared to the prior art:

[0062] (1) This application utilizes an oxyhydrogen flame heated fiber taper system to fabricate micro / nano polarization-maintaining fibers, thereby enabling the fabrication of large-angle angular displacement sensors. The oxyhydrogen flame heated fiber taper system utilizes the high temperature of an oxyhydrogen flame to heat the fiber, and the speed difference between the left and right displacement stages is generated by adjusting the speed. The diameter of the micro / nano fiber can be controlled by controlling the processing time. This fabrication method is highly efficient and has good repeatability. Due to its small fiber diameter, the micro / nano polarization-maintaining fiber can be directly bent at large curvatures and angles. Its simple structure and minimal external interference on polarization result in strong anti-interference capabilities in angular displacement measurement and enable high-sensitivity angular displacement sensing.

[0063] (2) This application utilizes a CO2 laser processing system to fabricate HLPGs on single-mode optical fibers, generating orbital angular momentum beams. The CO2 laser processing system uses thermal effects to inscribe helical lines on the optical fiber, fabricating high-conversion-efficiency HLPGs, thus realizing the direct generation of orbital angular momentum beams through fiber gratings. Compared to generating orbital angular momentum beams using space devices, the method using fiber gratings is smaller in size, can replace complex optical path systems, is simpler, more efficient, and lower in cost, and is more flexible in subsequent optical path systems.

[0064] (3) This application constructs an orbital angular momentum beam interferometry system to detect image data of interference fringe changes caused by large-scale angular displacement. Optical interferometry utilizes the changes in interference fringes to reflect changes in light phase, and has advantages such as high resolution and high sensitivity. This application uses an orbital angular momentum beam and spherical wave interferometry system to obtain a spiral interference image. Under the same light phase change conditions, a larger image change can be obtained than that of vertical interference fringes. Within the range of 0°-152°, an angular displacement change is applied to the micro / nano polarization-maintaining fiber by a rotating displacement 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 magnified by a beam expander, making fuller use of the CCD imaging area to obtain a larger interference field image. More data points are analyzed in image demodulation, enabling higher precision measurement.

[0065] (4) This application develops 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 accuracy and precision are greatly improved.

[0066] This application proposes an interference structure that utilizes an optical fiber to generate orbital angular momentum light. Micro-nano polarization-maintaining optical fibers are used as sensor devices. By demodulating the interference of orbital angular momentum light with spherical waves to obtain a spiral interference image, it is applied to precision angular displacement measurement, achieving a large range and high sensitivity. This precision measurement device is expected to meet the requirements of high precision, large range, and real-time performance in precision measurements such as micro-vibrations and micro-displacements, and has broad application prospects in precision measurement engineering.

[0067] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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 document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A fiber-optic angular displacement measuring device based on orbital angular momentum beam interference, characterized in that, The fiber-optic angular displacement measurement device based on orbital angular momentum beam interference includes a laser emitting component, a spiral long-period fiber grating, a micro / nano polarization-maintaining fiber, a first lens, a rotating displacement stage, a second lens, a reflector, a convex lens, a beam combiner, and a detector. The laser emitting component is connected to the helical long-period fiber grating and the micro / nano polarization-maintaining fiber, respectively; the helical long-period fiber grating serves as a reference arm fiber and is connected to the first lens; the micro / nano polarization-maintaining fiber serves as a sensing arm fiber, is placed on the rotating displacement stage, and is connected to the second lens. During operation, the laser emitting component emits two beams of light, which are transmitted to the helical long-period fiber grating and the micro / nano polarization-maintaining fiber, respectively. The helical long-period fiber grating converts the received light from its fundamental mode into a first-order orbital angular momentum mode, and then transmits it to the combiner via the first mirror. The micro / nano polarization-maintaining fiber rotates with the rotating displacement stage to generate angular displacement, and simultaneously outputs the phase-changed light to the reflector via the second mirror. The reflector then reflects the light to the convex lens, where it is converted into a spherical wave and transmitted to the combiner. The spherical wave received by the combiner interferes with the light in the first-order orbital angular momentum mode, and the detector detects the corresponding helical interference pattern. The helical interference pattern is used to calculate the angular displacement.

2. The fiber-optic angular displacement measuring device based on orbital angular momentum beam interference according to claim 1, characterized in that, The fiber-optic angular displacement measuring 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 disposed between the beam combiner and the detector; the linear polarizer is used to make the two received 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 can obtain more pixel signals.

3. A fiber-optic angular displacement measurement method based on orbital angular momentum beam interference, characterized in that, The fiber-optic angular displacement measurement method based on orbital angular momentum beam interferometry includes: Construct a fiber-optic angular displacement measuring device based on orbital angular momentum beam interference as described in any one of claims 1-2; 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 fiber follows the rotating displacement stage to rotate by a first angle; the second spiral interference image is obtained when the micro / nano polarization-maintaining fiber follows the rotating displacement stage to rotate by a second angle; the values ​​of the first angle and the second angle are different. For any helical interference image, identify the annular light field in the emitted light field image of the helical long-period fiber grating and calculate the center of the circle; The median filtering algorithm is used to denoise the spiral interference image. Then, based on the center of the circle and the radius of the inner ring in the annular light field, the pixel values ​​in the denoised spiral interference image are read along the circular trajectory to fit the cosine function. Cross-correlation calculations are performed on the cosine functions corresponding to the first and second spiral interference images to obtain the image rotation angle; Based on the linear relationship, the angular displacement is calculated according to the rotation angle of the image.

4. The fiber-optic angular displacement measurement method based on orbital angular momentum beam interference according to claim 3, characterized in that, Identify the annular light field in the emitted light field image of a helical 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 the inner and outer rings in the spiral interference image, and to calculate the common center of the inner and outer rings.

5. The fiber-optic 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 the 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 of the circular trajectory, the data is read in a counterclockwise order, and the radian corresponding to each pixel in the circular trajectory is used as the horizontal coordinate and the corresponding pixel value is used as the vertical coordinate for data picking and drawing. Then, the cosine function is obtained by interpolation fitting.

6. The fiber-optic angular displacement measurement method based on orbital angular momentum beam interference according to claim 5, characterized in that, The 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 as follows: in, θ represents the cross-correlation value; θ represents the radian value corresponding to the pixel on the circular trajectory; x(θ) and y(θ) are two sets of sine function curves, which are the cosine functions corresponding to the first spiral interference image and the second spiral interference image, respectively; This represents a phase change.

Images