Carrier frequency interference detection device and method based on polarization grating

Abstract

The invention discloses a carrier frequency interference detection device and method based on a polarization grating, the polarization grating is used to separate reference light and test light at a certain angle to realize carrier frequency, and the surface shape is demodulated through a least square iteration space carrier phase shift demodulation algorithm, so that the space phase shift is converted into time phase shift, and the detection precision is improved. The demodulation precision is improved, the polarization error between pixels introduced by a polarization camera is avoided, and high-precision carrier frequency interference detection is realized. According to the carrier frequency interference detection method based on the polarization grating, the surface type can be calculated only through a single carrier frequency interference pattern, the method is suitable for dynamic measurement, and the problem of polarization errors between pixels in a traditional carrier frequency interference detection method is effectively solved. According to the invention, the dynamic carrier frequency interference detection of the surface shape of the to-be-detected object can be effectively realized with high precision.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical precision detection, and in particular relates to a carrier frequency interference detection device and method based on polarization grating. Background Art

[0002] Carrier frequency interferometry has a very important application in interferometric detection. It can be used for high-precision surface measurement and can also be used for interferometer calibration and optimization. In addition, carrier frequency interferometry can achieve sub-micron displacement or distance measurement and is widely used in systems such as laser radar.

[0003] For example, the Chinese patent document with the publication number CN107560565A discloses a surface shape detection device and detection method based on dynamic time-sharing tilted carrier frequency interference. According to the surface shape of the detected element, the required spatial interference point source array position is calculated, and the time-sharing spatial interference point source array is generated by using the Tip / Tilt mirror. Each time the spatial posture of the Tip / Tilt mirror is adjusted, a spatial point source is arranged. Through the designed posture adjustment position, the scanning of the planned spatial point source is completed in sequence. The interference fringes are resolved to complete the surface shape reconstruction of the detected element, thereby realizing the surface shape measurement of the high-precision surface shape element of the present invention.

[0004] Compared with traditional interference detection methods, carrier frequency interferometry can directly extract effective phase information from the interference pattern acquired in a single time, thereby greatly improving the detection efficiency. At the same time, carrier frequency interferometry can avoid the influence of displacement error, alignment error and environmental disturbance introduced by multiple image acquisition on the measurement, and can provide more stable and reliable results in unstable environments.

[0005] Carrier frequency interferometry uses the principle of spatial carrier frequency phase shifting. The difference between spatial phase shifting and temporal phase shifting is that it converts the inter-frame phase shift in temporal phase shifting into the inter-pixel phase shift and controls the inter-pixel phase shift by controlling the carrier frequency. However, this also introduces polarization errors between pixels, which in turn affects the detection accuracy. Therefore, a carrier frequency modulation method that can stabilize the carrier frequency and has low polarization errors is urgently needed. Summary of the invention

[0006] The present invention provides a carrier frequency interference detection device and method based on polarization grating, which can realize high-precision surface measurement with low polarization error.

[0007] A carrier frequency interference detection device based on polarization grating comprises an interference detection system and a computer processing module.

[0008] In the interference detection system, the outgoing light beam of the tunable laser passes through the first linear polarizer and is incident on the light receiving surface of the collimating beam expansion system. The outgoing light beam after collimation and expansion by the collimating beam expansion system is incident on the polarization beam splitter prism for splitting; one path of vertical polarized light passes through the first quarter wave plate after reflection, and then passes through the reference mirror and returns to the original path as the reference light; the other path of horizontal polarized light passes through the second quarter wave plate, passes through the first lens of the beam expander and the second lens of the beam expander successively, and then passes through the to-be-measured mirror and reflects and returns to the original path as the to-be-measured light after beam expansion; the two beams of light are respectively passed through the first quarter wave plate and the second quarter wave plate twice, and the polarization states are converted, and they are combined into one beam after passing through the polarization beam splitter prism again, and then are incident on the surface of the beam splitter; a part of the light is transmitted and converged by the first imaging lens and incident on the alignment camera for adjusting the optical path, and a part of the light is reflected and converged by the second imaging lens and then passes through the third quarter wave plate first, and then passes through the polarization grating to generate the carrier frequency, and finally passes through the second linear polarizer to unify the polarization state and image the interference fringes on the CMOS camera to obtain the interference pattern;

[0009] The computer processing module includes a phase control module, an image acquisition module, and an interference pattern data analysis and processing module; wherein the phase control module is connected to the controller of the tunable laser, and the wavelength of the tunable laser is changed by adjusting the controller voltage of the tunable laser, thereby realizing phase control; the image acquisition module is connected to the CMOS camera, and after obtaining the carrier frequency interference pattern image of the interference imaging of the mirror to be measured and the reference mirror, the data is transmitted to the interference pattern data analysis and processing module for analysis and processing, so as to obtain the surface information of the mirror to be measured.

[0010] Furthermore, the interference detection system is installed on an air-floating vibration-isolating optical platform.

[0011] Furthermore, the polarization grating can convert left-handed circularly polarized light and right-handed circularly polarized light into -1-order right-handed circularly polarized light and +1-order left-handed circularly polarized light. Since the polarization grating has a compact structure, it can be placed in close proximity to the polarizer and CMOS camera, so the reference light and the light to be measured will not be separated, thereby achieving a carrier frequency effect.

[0012] Furthermore, the relationship between the tilt angle θ between the reference light and the light to be measured, the optical constant d of the polarization grating, and the wavelength λ of the light is:

[0013]

[0014] Furthermore, the contrast of interference fringes is adjusted by rotating the linear polarizer and the quarter wave plate.

[0015] A carrier frequency interference detection method based on polarization grating, using the above-mentioned carrier frequency interference detection device based on polarization grating, comprises the following steps:

[0016] Step 1, adjusting the clamping angles of the reference mirror and the standard mirror by an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast;

[0017] Step 2, calibrating the fixed carrier frequency of the polarization grating; performing aperture selection on the phase-shifting interference pattern, using the least squares iterative spatial carrier phase-shifting algorithm to demodulate the surface shape and calculate the phase shift amount, and inferring the fixed carrier frequency of the polarization grating;

[0018] Step 3, adjusting the clamping angles of the reference mirror and the mirror to be measured by an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast;

[0019] Step 4: demodulate the surface shape of the mirror to be tested according to the fixed carrier frequency of the polarization grating; select the aperture of the interference pattern, demodulate the surface shape using the fixed phase shift demodulation algorithm, unwrap, and de-tilt the aberration fitting to obtain the surface shape diagram.

[0020] In step 2, the least squares iterative spatial carrier phase shift algorithm is used to demodulate the surface shape and calculate the phase shift amount, specifically:

[0021] Using the least squares iterative spatial carrier phase shifting algorithm, a single spatial carrier frequency interferogram is converted into multiple random phase-shifted time domain interferograms. The light intensity distribution of a single spatial carrier frequency interferogram is expressed as:

[0022] I(x,y)=a(x,y)+b(x,y)cos[φ(x,y)+2π(f x x+f y y)]

[0023] In the formula, a(x,y) and b(x,y) represent the interference fringes and background modulation at point (x,y), respectively, and f x and f y Respectively represent the carrier amount along the x and y directions, is the phase distribution to be measured;

[0024] The interference pattern is then transformed into four time-domain interference patterns with uncertain phase shifts, and the phase is extracted from the four interference patterns through the iterative least squares method.

[0025] In step 2, GPU acceleration is used to accelerate the matrix operations in the least squares iterative spatial carrier phase shifting algorithm.

[0026] In step 4, during the process of performing aberration fitting to remove the tilt, Zernike polynomials are used to perform aberration fitting to remove the tilt for regular circular apertures, and Schmidt orthogonalization is used to perform aberration fitting to remove the tilt for irregular apertures.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The present invention uses a polarization grating to separate the reference light and the test light at a certain angle to realize the carrier frequency, and demodulates the surface shape through a least squares iterative spatial carrier phase shift demodulation algorithm, thereby converting the spatial phase shift into a temporal phase shift, improving the demodulation accuracy and avoiding the polarization error between pixels introduced by the polarization camera, thereby realizing high-precision carrier frequency interference detection.

[0029] 2. The carrier frequency interferometry detection method based on polarization grating of the present invention only needs a single carrier frequency interferogram to calculate the surface shape, which is suitable for dynamic measurement and effectively solves the polarization error problem between pixels in the traditional carrier frequency interferometry detection method. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The figure is a schematic structural diagram of a carrier frequency interference detection device based on polarization grating of the present invention.

[0031] Figure 2 The interference fringes are obtained by magnifying the interference pattern obtained on the CMOS camera in the embodiment of the present invention by 30 times.

[0032] Figure 3 Schematic diagram of interference pattern transformation in step 2 in an embodiment of the present invention.

[0033] Figure 4 This is a surface diagram obtained after being processed by the carrier frequency interference detection method in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0035] like Figure 1 As shown, a carrier frequency interference detection device based on polarization grating includes an interference detection system and a computer processing module.

[0036] The entire interference detection system is installed on an air-floating vibration-isolating optical platform, including a tunable laser 1, a first linear polarizer 2, a collimating beam expansion system 3, a polarization beam splitter prism 4, a first quarter-wave plate 5, a reference mirror 6, a second quarter-wave plate 7, a first lens of a beam expander 8, a second lens of a beam expander 9, a mirror to be measured 10, a beam splitter 11, a first imaging lens 12, an alignment camera 13, a second imaging lens 14, a third quarter-wave plate 15, a polarization grating 16, a second linear polarizer 17, and a CMOS camera 18.

[0037] The output light beam of the tunable laser 1 passes through the first linear polarizer 2 and is incident on the light receiving surface of the collimating and expanding system 3. The output light beam after collimation and expansion by the collimating and expanding system 3 is incident on the polarization beam splitting prism 4 for splitting. The vertically polarized light passes through the first quarter wave plate 5 after reflection, and then is reflected by the reference mirror 6 and then returns to the original path as the reference light wave. The other horizontally polarized light passes through the second quarter wave plate 7, passes through the first lens 8 and the second lens 9 of the beam expander successively, and after expansion, is reflected by the to-be-measured mirror 10 and then returns to the original path as the to-be-measured light wave. The two light waves are The light passes through the first quarter wave plate 5 and the second quarter wave plate 7 twice, and the polarization state is converted. The light passes through the polarization splitter prism again to combine into a light beam, and then is incident on the surface of the splitter 11. A part of the light is transmitted and converged by the first imaging lens 12 to be incident on the alignment camera 13 for adjusting the light path. A part of the light is reflected and converged by the second imaging lens 14, and then passes through the third quarter wave plate 15, and then passes through the polarization grating 16 to generate the carrier frequency. Finally, the interference fringes are imaged on the CMOS camera 18 after the second linear polarizer 17 unifies the polarization state to obtain an interference pattern.

[0038] The tunable laser 1 can be a tunable He-Ne laser with a wavelength of 632.8 nm, the polarization grating 16 can be a polarization grating with a period of d=5 μm and a line number of 200, and the focal length f of the first imaging lens and the second imaging lens can both be 250 mm.

[0039] The computer processing module includes a phase control module, an image acquisition module, and an interference pattern data analysis and processing module; the phase control module is connected to the controller of the tunable laser 1, and the wavelength of the tunable laser 1 is changed by adjusting the controller voltage of the tunable laser 1, thereby realizing phase control; the image acquisition module is connected to the CMOS camera 18, and after obtaining the carrier frequency interference pattern image of the interference imaging of the sample to be tested and the reference mirror, the data is transmitted to the interference pattern data analysis and processing module for analysis and processing to obtain the surface information of the sample to be tested.

[0040] Carrier frequency interference detection based on polarization grating is performed using the above device, including the following steps:

[0041] Step 1: Adjust the clamping angles of the reference mirror 6 and the standard mirror by means of an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast.

[0042] Step 2, calibrate the fixed carrier frequency of the polarization grating; select the aperture of the phase-shifting interferogram to include the complete interferogram as much as possible, use the least squares iteration-based spatial carrier phase-shifting demodulation surface type and calculate the carrier frequency. The algorithm converts a single carrier frequency interferogram into four time-domain phase-shifting interferograms, and then uses the least squares iteration method to obtain the phase shift between the interferograms, and use this to deduce the fixed carrier frequency of the polarization grating.

[0043] Among them, the transformation method of a single carrier frequency interference pattern is as follows: Figure 3 shown.

[0044] Step 3, adjusting the clamping angles of the reference mirror 6 and the mirror to be measured 10 by means of an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast.

[0045] Step 4, demodulate the surface shape of the mirror to be measured according to the fixed carrier frequency of the polarization grating; select the aperture of the phase-shifting interference pattern, including the complete interference pattern as much as possible, use the fixed phase-shifting demodulation algorithm to demodulate the surface shape, and unwrap the wrapped phase.

[0046] For regular circular apertures, Zernike polynomials are used to fit the aberrations and remove the tilt. For irregular apertures, Schmidt orthogonalization is used to fit the aberrations and remove the tilt.

[0047] Figure 4 The results of applying the present invention to the surface shape detection of planar components are shown. The PV value is 12.628nm and the RMS value is 2.8943nm.

[0048] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A carrier frequency interference detection device based on polarization grating, characterized in that: including an interference detection system and a computer processing module; In the interference detection system, the outgoing light beam of the tunable laser (1) passes through the first linear polarizer (2) and then enters the light receiving surface of the collimating and expanding system (3). After being collimated and expanded by the collimating and expanding system (3), the outgoing light beam enters the polarization beam splitting prism (4) for splitting. One path of the vertically polarized light is reflected and then passes through the first quarter-wave plate (5), then is reflected by the reference mirror (6) and then returns along the original path as the reference light. The other path of the horizontally polarized light is transmitted and then passes through the second quarter-wave plate (7), then passes through the first lens (8) and the second lens (9) of the beam expander, then is expanded and then is reflected by the to-be-measured mirror (10) and then returns along the original path as the to-be-measured light. The two beams The light passes through the first quarter wave plate (5) and the second quarter wave plate (7) twice, and the polarization state is converted. The light passes through the polarization beam splitter prism (4) again to be combined into a beam, and then is incident on the surface of the beam splitter (11); a part of the light is transmitted, converged by the first imaging lens (12) and incident on the alignment camera (13) for adjusting the optical path; a part of the light is reflected, converged by the second imaging lens (14), and then passes through the third quarter wave plate (15), and then passes through the polarization grating (16) to generate a carrier frequency, and finally passes through the second linear polarizer (17) to unify the polarization state, and then the interference fringes are imaged on the CMOS camera (18) to obtain an interference pattern; The computer processing module comprises a phase control module, an image acquisition module, and an interference pattern data analysis and processing module; wherein the phase control module is connected to a controller of a tunable laser (1), and the wavelength of the tunable laser (1) is changed by adjusting the controller voltage of the tunable laser (1), thereby realizing phase control; the image acquisition module is connected to a CMOS camera (18), and after obtaining a carrier frequency interference pattern image of interference imaging between a mirror to be measured (10) and a reference mirror (6), the data is transmitted to the interference pattern data analysis and processing module for analysis and processing, thereby obtaining surface shape information of the mirror to be measured (10).

2. The carrier frequency interference detection device based on polarization grating according to claim 1, characterized in that: The interference detection system is installed on an air-floating vibration-isolating optical platform.

3. The carrier frequency interference detection device based on polarization grating according to claim 1, characterized in that: The polarization grating (16) converts the left-handed circularly polarized light and the right-handed circularly polarized light into -1st order right-handed circularly polarized light and +1st order left-handed circularly polarized light.

4. The carrier frequency interference detection device based on polarization grating according to claim 1, characterized in that: The relationship between the tilt angle θ between the reference light and the light to be measured, the optical constant d of the polarization grating, and the wavelength λ of the light is:

5. The carrier frequency interference detection device based on polarization grating according to claim 1, characterized in that: The contrast of interference fringes can be adjusted by rotating the linear polarizer and the quarter-wave plate.

6. A carrier frequency interference detection method based on polarization grating, characterized in that: The carrier frequency interference detection device based on polarization grating according to any one of claims 1 to 5 comprises the following steps: Step 1, adjusting the clamping angle of the reference mirror (6) and the standard mirror by an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast; Step 2, calibrating the fixed carrier frequency of the polarization grating (16); performing aperture selection on the phase-shifting interference pattern, using a least squares iterative spatial carrier phase-shifting algorithm to demodulate the surface shape and calculate the phase shift amount, and inferring the fixed carrier frequency of the polarization grating; Step 3, adjusting the clamping angle of the reference mirror (6) and the mirror to be measured (10) by means of an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast; Step 4, demodulating the surface shape of the mirror to be measured according to the fixed carrier frequency of the polarization grating (16); selecting the aperture of the interference pattern, demodulating the surface shape using a fixed phase shift demodulation algorithm, unwrapping, and de-tilting the aberration fitting to obtain a surface shape diagram 7. The carrier frequency interference detection method based on polarization grating according to claim 6 is characterized in that: In step 2, the least squares iterative spatial carrier phase shift algorithm is used to demodulate the surface shape and calculate the phase shift amount, specifically: Using the least squares iterative spatial carrier phase shifting algorithm, a single spatial carrier frequency interferogram is converted into multiple random phase-shifted time domain interferograms. The light intensity distribution of a single spatial carrier frequency interferogram is expressed as: I(x,y)=a(x,y)+b(x,y)cos[φ(x,y)+2π(f x x+f y and)] In the formula, a(x,y) and b(x,y) represent the interference fringes and background modulation at point (x,y), respectively, and f x and f y Respectively represent the carrier amount along the x and y directions, is the phase distribution to be measured; The interference pattern is then transformed into four time-domain interference patterns with uncertain phase shifts, and the phase is extracted from the four interference patterns through the iterative least squares method.

8. The carrier frequency interference detection method based on polarization grating according to claim 7 is characterized in that: In step 2, GPU acceleration is used to accelerate the matrix operations in the least squares iterative spatial carrier phase shifting algorithm.

9. The carrier frequency interference detection method based on polarization grating according to claim 6, characterized in that: In step 4, during the process of performing aberration fitting to remove the tilt, Zernike polynomials are used to perform aberration fitting to remove the tilt for regular circular apertures, and Schmidt orthogonalization is used to perform aberration fitting to remove the tilt for irregular apertures.

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