Double-ring conjugate interference micro-displacement measurement method based on vortex light

Abstract

The invention discloses a vortex light-based double-ring conjugate interference micro-displacement measurement method, which adopts a Michelson interferometer structure, replaces a reflecting plane mirror of a reference arm with a hollow roof prism reflector, and replaces a reflecting plane mirror of a measurement arm with a cube-corner prism placed on a displacement platform. A spatial light modulator is used for generating double-ring vortex light, the angle of an instrument is adjusted to generate interference, a double-ring petal-shaped interference pattern is generated, and when displacement is introduced, an inner ring and an outer ring can rotate in opposite directions; an interference pattern collected by the CCD camera is processed by using an SIFT method to obtain a rotation angle, and then the displacement is calculated; due to the fact that the rotation angles of the inner ring and the outer ring are opposite, the rotation angle of an interferogram of the system is larger under the same displacement, the resolution of the system to infinitesimal displacement is improved, and the micrometric displacement measuring method and system based on conjugate interference are convenient to operate and higher in precision.

Description

Technical Field

[0001] The present application belongs to the field of optical measurement technology, and specifically relates to a double-ring conjugate interference micro-displacement measurement method based on vortex light. Background Art

[0002] Lasers are often used for high-precision measurements, including speed, distance, and angle, due to their monochromaticity, high brightness, and excellent directionality, along with good coherence. Laser ranging is widely used due to its non-contact and convenient operation. Laser ranging employs a variety of methods to meet diverse demands for micro-displacement, large-scale measurement, and high-precision measurement. In recent years, with the rapid development of precision machining, the demand for high-precision measurement of micro-displacements has become increasingly urgent, making research on precise micro-displacement measurement crucial.

[0003] Interferometers typically calculate displacement by measuring intensity changes at fixed points within the interference fringes or by counting the number of interference fringes. Both methods are susceptible to fluctuations in laser power and require high laser power stability. Furthermore, counting interference fringes is prone to errors.

[0004] The change in the interference pattern of the conjugate vortex light with displacement is no longer reflected in intensity, but rather in the rotation angle of the petal pattern. Therefore, the conjugate vortex light interferometer can directly calculate tiny displacement changes by measuring the angle of the pattern, greatly reducing the requirements for the power stability of the laser light source and the detection system. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a double-ring conjugate interferometry micro-displacement measurement method based on vortex light, which can reduce the requirements for laser power stability and improve the resolution of micro-displacement.

[0006] A micro-displacement measurement method adopts a Michelson interferometer structure, replaces the reflective plane mirror of the reference arm with a hollow roof prism reflector, and replaces the reflective plane mirror of the measuring arm with a corner cube placed on a displacement platform. The measurement method includes:

[0007] Step 1: In the superposition function area of ​​the vortex light function module of the spatial light modulator control software, set the superposition of two Laguerre-Gaussian lights with P = 0, L = +2 and P = 0, L = -15. The software automatically calculates and generates the corresponding phase hologram and displays it on the screen of the spatial light modulator;

[0008] Step 2: Adjust the placement angles of the corner cube prism and the hollow roof prism reflector respectively so that the reference light and the measurement light overlap coaxially after passing through the beam splitter and interfere with each other, thus generating an inner and outer double-ring interference pattern.

[0009] Step 3: Use CCD to take 20 interference patterns as reference images;

[0010] Step 4: Set the displacement Δz on the displacement platform control software so that the displacement of the corner cube in the light propagation direction is Δz, and the interference pattern rotates;

[0011] Step 5: Use the CCD to take 20 interference patterns as measurement patterns, and determine the displacement by the angle difference between the two captured images. Specifically:

[0012] Take the first 20 images for SIFT operation, save the coordinates of each spot center point and the rotation center point of the entire interference pattern; calculate the vector coordinates of the spot center feature point and the rotation center feature point; take the last 20 images for SIFT operation, save the coordinates of each spot center point and the rotation center point of the entire interference pattern; calculate the vector coordinates of the spot center point and the rotation center point; in the order of 1 to 20, calculate the angle Δθ between the vectors of the reference image and the measurement image N and Δθ W Further calculate the sum of the angles Δθ between the closest spots in the inner and outer rings H =Δθ N +Δθ W ; Calculate the average value of the angle Δθ H ; According to Δz=0.5kΔθ H , calculate the size of Δz;

[0013] Preferably, the light source used is a frequency-stabilized laser.

[0014] Preferably, the double-ring vortex light is generated by a spatial light modulator.

[0015] Preferably, the interference optical path is encapsulated in a sealed vacuum constant temperature cavity to reduce interference from the external environment.

[0016] Preferably, the captured interference spot pattern is pre-cropped to remove unnecessary dark areas to reduce SIFT calculation time.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. The present invention discloses a double-ring conjugate interference micro-displacement measurement method based on vortex light. The method adopts a Michelson interference structure, replaces the reflective plane mirror of the reference arm with a hollow roof prism reflector, and replaces the reflective plane mirror of the measuring arm with a corner cube prism placed on a displacement platform. The incident angle of the light beam is adjusted to cause interference, and the rotation angle of the interference pattern can reflect the displacement size. Since the superimposed topological charge vortex light forms inner and outer rings when it undergoes conjugate interference, and the rotation angles are opposite, the rotation angle of the interference pattern of the same displacement in the system will be larger, which can improve the system's resolution of displacement, making the micro-displacement measurement method and system based on conjugate interference easy to operate and more accurate.

[0019] 2. Using a double-ring vortex beam as the incident light source, when the CCD pixel is constant, the angle change caused by the same displacement is greater. Theoretically, the displacement that can be measured is smaller.

[0020] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the following embodiments are given in conjunction with the accompanying drawings for detailed description as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0022] Figure 1 This is a flow chart of a double-ring conjugate interferometry micro-displacement measurement method based on vortex light according to the present invention;

[0023] Figure 2 This is a schematic diagram of the principle of a double-ring conjugate interferometry micro-displacement measurement system based on vortex light in the present invention;

[0024] Figure 3 This is an interference image produced using vortex light;

[0025] Figure 4 The present invention uses the interference pattern of double-ring vortex light.

[0026] In the figure, 1-laser source, 2-beam expander, 3-reflecting plane mirror, 4-reflecting plane mirror, 5-non-polarizing beam splitter cube, 6-spatial light modulator, 7-non-polarizing beam splitter cube, 8-hollow roof prism reflector, 9-corner cone prism and 10-CCD camera. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work fall within the scope of protection of the present application.

[0028] It should be noted that similar numbers and letters represent similar items in the following figures, so once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. The terms "comprises", "comprising" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprising a..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the element. The terms "first", "second", etc. are used only to distinguish descriptions and are not to be understood as indicating or implying relative importance.

[0029] In view of this, the embodiment of the present application provides a dual-ring conjugate interferometry micro-displacement measurement system based on vortex light, such as Figure 2 As shown, it includes a laser source 1, a beam expander 2, a reflecting plane mirror 3, a reflecting plane mirror 4, a non-polarizing beam splitter cube 5, a spatial light modulator 6, a non-polarizing beam splitter cube 7, a hollow roof prism reflector 8, a corner cube prism 9 and a CCD camera 10;

[0030] Among them, the laser source 1, the non-polarizing beam splitter cube 7, the hollow roof prism reflector 8, and the corner cube prism 9 constitute a Michelson interferometer: the laser emitted by the light source 1 enters the non-polarizing beam splitter cube 7 and is split into two beams. The reflected beam serves as the reference light, is deflected by the hollow roof prism reflector 8, and then returns; the transmitted beam is reflected three times by the corner cube prism 9 and then returns. The two return beams overlap on the CCD to obtain an interference pattern.

[0031] Based on the traditional Michelson interferometer, the present invention replaces the reflecting plane mirror of the reference arm with a hollow roof prism reflector 8, and replaces the reflecting plane mirror of the measuring arm with a corner cube prism 9; the light of the reference arm is reflected a total of 3 times, and the light of the measuring arm is reflected a total of 4 times, which can ensure that the topological charges of the vortex light in the two arms are always of opposite signs, so as to produce a correct petal-shaped interference pattern.

[0032] like Figure 1 As shown, the method process of the present invention is specifically as follows:

[0033] Step 1. In the superposition function area of ​​the vortex light function module of the spatial light modulator supporting control software, set the superposition of two Laguerre-Gaussian lights with P = 0, L = +2 and P = 0, L = -15. The software automatically calculates and generates the corresponding phase-type hologram and displays it on the screen of the spatial light modulator, so that the light reflected by the spatial light modulator is a double-ring vortex light.

[0034] Step 2: Adjust the placement angles of the hollow roof prism reflector 8 and the corner cube prism 9 respectively so that the reference light and the signal light overlap coaxially in front of the CCD, interfere with each other, and generate a double-ring petal-shaped interference pattern.

[0035] Step 3: Use the CCD camera 10 to continuously capture interference images.

[0036] Step 4: Control the displacement platform to cause the corner cube prism 9 to move.

[0037] Step 5: Use the CCD camera 10 to continuously capture the interference image after rotation, and calculate the displacement size according to the rotation angle of the interference image, specifically:

[0038] Perform SIFT calculation on all the images taken, save the coordinates of the bright spot center and the rotation center of each image, and calculate the coordinates of each vector with the bright spot center as the starting point and the rotation center as the end point respectively; take the image taken before the displacement as the reference image, and group it with the image taken after the displacement in a one-to-one correspondence. Use the vectors corresponding to the bright spots before and after the displacement to calculate the rotation angle, and then average the rotation angles calculated for all the bright spots to get the rotation angle of the group; finally, average the rotation angles of all the groups to get the rotation angle Δθ of this displacement, and substitute it into the calculation formula Calculate the magnitude of the displacement Δz.

[0039] In the present invention, the laser source 1 is a Cobolt 08-DPL 532nm 100mW laser, and the spatial light modulator is a FSLM-2K39-P reflective, pure-phase type from Microstar. It has a pixel count of 1920×1080, an effective panel size of 8.64mm×4.86mm, and a wavelength range of 420nm-650nm. A computer-generated 256-level grayscale image (i.e., kinoform) is used to control the voltage applied to each pixel (liquid crystal) to control the phase change of light passing through it. The kinoform of a superimposed double-ring vortex light is loaded onto the liquid crystal surface by the computer, modulating the wavefront and converting plane light into double-ring vortex light. The generated double-ring vortex light is reflected by a non-polarizing beam splitter cube 5 and then perpendicularly incident on a non-polarizing beam splitter cube 7.

[0040] like Figure 3 As shown in , the interference image generated by vortex light is a single-ring petal-shaped central symmetric pattern; Figure 4 As shown in the figure, the interference image using double-ring vortex light is a double-ring petal-shaped centrally symmetrical pattern. When the measuring arm is displaced, the inner and outer rings rotate in opposite directions. Therefore, when the CCD pixel is constant, the angle change caused by the same displacement is greater. In theory, it can improve the system's resolution of small displacements.

[0041] In summary, the double-ring conjugate interferometry micro-displacement measurement method based on vortex light provided in this application reduces the requirements for laser power stability and improves the resolution of micro-displacement.

[0042] In this document, relational terms such as first and second, etc. are used merely to distinguish one entity or operation from another entity or operation, but do not necessarily require or imply any actual relationship or order between these entities or operations.

[0043] The above description is merely an embodiment of the present application and is not intended to limit the scope of protection of the present application. For those skilled in the art, various modifications and variations of the present application are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A micro-displacement measurement method, characterized in that The device comprises a laser source, a beam expander, a reflecting plane mirror, a non-polarizing beam splitter cube, a spatial light modulator, a hollow roof prism reflector, a corner cube prism, a CCD camera, and a displacement platform. The measuring method comprises: Step 1: In the SLM control software, set the superposition of two Laguerre-Gaussian beams with P = 0, L = +2 and P = 0, L = -15. The software automatically calculates and generates the corresponding phase hologram and displays it on the SLM screen. Step 2: Adjust the placement angles of the corner cube prism and the hollow roof prism reflector respectively so that the reference light and the measurement light overlap coaxially after passing through the beam splitter and interfere with each other, thus generating an inner and outer double-ring interference pattern. Step 3: Use CCD to take 20 interference patterns as reference images; Step 4: Set the displacement Δz on the displacement platform control software so that the displacement of the corner cube in the light propagation direction is Δz, and the interference pattern rotates; Step 5: Use the CCD to take 20 interference patterns as measurement patterns, and determine the displacement by the angle difference between the two captured images. Specifically: Take the first 20 images for SIFT operation, save the coordinates of the center point of each spot in the inner and outer rings and the rotation center point of the entire interference pattern; calculate the vector coordinates between the center point of the spot and the rotation center point; take the last 20 images for SIFT operation, save the coordinates of the center point of each spot in the inner and outer rings and the rotation center point of the entire interference pattern; calculate the vector coordinates between the center point of the spot and the rotation center point; in the order of 1 to 20, calculate the angle Δθ between the vectors of the reference image and the measurement image N and Δθ W Further calculate the sum of the angles Δθ between the closest spots in the inner and outer rings H =Δθ N +Δθ W ; Calculate the average value of the angle Δθ H ; According to Δz=0.5kΔθ H , calculate the size of Δz.

2. A micro-displacement measurement method according to claim 1, characterized in that: The measurement method replaces the reflecting plane mirror in the reference light path of the Michelson interferometer with a hollow roof prism reflecting mirror, replaces the reflecting plane mirror of the measuring arm with a corner cube prism, uses a spatial light modulator to generate double-ring vortex light, forms a double-ring petal-shaped interference pattern at the CCD camera, introduces displacement into the displacement platform of the measuring arm, rotates the interference pattern by a certain angle, and uses SIFT image processing to obtain the rotation angle to calculate the displacement.

3. A micro-displacement measurement method according to claim 1, characterized in that: Double-ring vortex light interference is used to form a double-ring petal-shaped interference pattern, and the inner and outer rings rotate in opposite directions. Finally, the displacement is calculated by the sum of the rotation angles of the light spots closest to the inner and outer rings.

4. A micro-displacement measurement method according to claim 1, characterized in that: After SIFT calculation of the interference pattern, the center coordinates of each light spot and the overall rotation center coordinates are saved. The vectors of the center coordinates of the light spot before and after displacement and the overall rotation center coordinates are calculated. Then the angle between the corresponding vectors is calculated and averaged to obtain the rotation angle of a ring.

5. A micro-displacement measurement method according to claim 1, characterized in that: Δz=0.5kΔθ H The parameter k is calculated by the displacement of a single loop and Δθ H The results obtained by linear fitting are as follows: Set up the spatial light modulator to generate a single-ring vortex light, perform steps 2 to 4 above, and then calculate the angle Δθ similar to step 5, using Calculate the displacement, so far it is a cycle, execute multiple cycles to get a series of displacement and angle data, and then the displacement data is used as the dependent variable, Δθ H As the independent variable, a linear fit was performed to obtain a coefficient of 0.5k.

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