A polarization grating-based carrier frequency interference detection device and method

Abstract

The application discloses a kind of based on polarization grating carrier frequency interference detection device and method, utilize polarization grating to produce certain angle separation of reference light and test light to realize carrier frequency, and through least square iteration spatial carrier phase shift demodulation algorithm demodulation profile, to convert spatial phase shift into time phase shift, improve demodulation precision and avoid the polarization error between pixel and pixel introduced by polarization camera, realize high-precision carrier frequency interference detection.The carrier frequency interference detection method based on polarization grating only needs single carrier frequency interferogram to calculate surface type, suitable for dynamic measurement, and effectively solve the polarization error between pixel in traditional carrier frequency interference detection method.Problem.Utilizing the application, can effectively and high-precision realize the dynamic carrier frequency interference detection of the surface shape of object to be measured.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical precision detection, and particularly relates to a carrier frequency interference detection device and method based on a polarization grating. BACKGROUND

[0002] Carrier frequency interference has very important applications in interference detection, and can be used for high-precision surface shape measurement and can be used for calibration and optimization of an interferometer. In addition, carrier frequency interference can realize measurement of sub-micron level displacement or distance and is widely used in laser radar systems and the like.

[0003] A surface shape detection device and method based on dynamic time-sharing tilt carrier frequency interference are disclosed in Chinese Patent Document CN107560565A. According to the surface shape of a detected element, a required spatial interference point source array position is calculated, a Tip / Tilt mirror is used to generate a time-sharing spatial interference point source array, and the spatial posture of the Tip / Tilt mirror is adjusted once to realize arrangement of a spatial point source. Through a designed posture adjustment position, scanning of the planned spatial point source is sequentially completed. Interference fringes are solved, surface shape reconstruction of the detected element is completed, and thus high-precision surface shape measurement of the surface shape element is realized.

[0004] Compared with a traditional interference detection method, carrier frequency interference can directly extract effective phase information from a single collected interference image, thereby greatly improving detection efficiency. Meanwhile, carrier frequency interference can avoid influences of displacement errors, alignment errors and environmental disturbances and the like on measurement caused by multiple collected images, and can provide more stable and reliable results in an unstable environment.

[0005] Carrier frequency interference adopts a spatial carrier frequency phase shift principle. Spatial phase shift and time phase shift are different in that the spatial phase shift converts inter-frame phase shift in time phase shift into phase shift between pixels and controls the phase shift between pixels by controlling the amount of carrier frequency, but this introduces polarization errors between pixels and affects detection precision. Therefore, a carrier frequency modulation method capable of stabilizing carrier frequency and having low polarization errors is urgently needed. SUMMARY

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

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

[0008] The tunable laser beam is incident on the light receiving surface of the collimating and expanding system after passing through the first linear polarizer, and the collimated and expanded beam is incident on the polarization beam splitter prism; one of the beams is reflected and then passes through the first quarter-wave plate, and then passes through the reference mirror and returns to the original path as the reference light; the other beam is transmitted and then passes through the second quarter-wave plate, and then passes through the first lens and the second lens of the beam expander, is expanded, is reflected by the test mirror, and returns to the original path as the test light; the two beams are converted in polarization state due to passing through the first quarter-wave plate and the second quarter-wave plate twice, 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; part of the light is transmitted and is incident on the alignment camera after being condensed by the first imaging lens, and is used for adjusting the optical path; part of the light is reflected, is condensed after passing through the second imaging lens, passes through the third quarter-wave plate, generates a carrier frequency after passing through the polarization grating, and is uniformly polarized after passing through the second linear polarizer, and then the interference fringes are imaged on the CMOS camera, and an interference pattern is obtained.

[0009] The computer processing module comprises a phase control module, an image acquisition module, and an interference pattern data analysis processing module; the phase control module is connected with the controller of the tunable laser, the wavelength of the tunable laser is changed by adjusting the voltage of the controller, and thus the phase control is realized; the image acquisition module is connected with the CMOS camera, the carrier frequency interference pattern image obtained by interference imaging of the test mirror and the reference mirror is transmitted to the interference pattern data analysis processing module for analysis and processing, and the surface shape information of the test mirror is obtained.

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

[0011] Further, 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 is small and can be placed close to the polarizer and the CMOS camera, the reference light and the test light cannot be separated, and thus the carrier frequency effect can be realized.

[0012] Further, the relationship between the inclination angle θ between the reference light and the test light, the optical constant d of the polarization grating, and the wavelength λ of the light is as follows:

[0013]

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

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

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

[0017] Step 2, the fixed carrier frequency quantity of the polarization grating is calibrated; the aperture selection is performed on the phase-shifted interference pattern, the least square iterative spatial carrier phase shift algorithm is used to demodulate the surface shape and calculate the phase shift quantity, and the fixed carrier frequency quantity of the polarization grating is calculated;

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

[0019] Step 4, the surface shape of the to-be-measured mirror is demodulated according to the fixed carrier frequency quantity of the polarization grating; the aperture selection is performed on the interference pattern, the fixed phase shift quantity demodulation algorithm is used to demodulate the surface shape, the surface shape is unwrapped, and the surface shape is obtained by performing aberration fitting and removing the tilt

[0020] In step 2, the least square iterative spatial carrier phase shift algorithm is used to demodulate the surface shape and calculate the phase shift quantity, and the specific process is as follows:

[0021] The single spatial carrier frequency interference pattern is converted into multiple time-domain interference patterns with random phase shifts by using the least square iterative spatial carrier phase shift algorithm, and the light intensity distribution of the single spatial carrier frequency interference pattern is represented 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 the background modulation degree of the (x,y) point respectively, f x and f y represent the carrier quantities along the x and y directions respectively, is the to-be-measured phase distribution;

[0024] Then, the interference pattern is converted into four time-domain interference patterns with uncertain phase shift quantities, and the phase is extracted from the four interference patterns by using the least square method.

[0025] In step 2, the GPU is accelerated to accelerate the matrix operation in the least square iterative spatial carrier phase shift algorithm.

[0026] In step 4, in the process of aberration fitting and removing the tilt, the Zernike polynomial is used for aberration fitting and removing the tilt for a regular circular aperture, and the Schmidt orthogonalization is used for aberration fitting and removing the tilt for an irregular aperture.

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

[0028] 1. The present application utilizes 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 the least square iteration spatial carrier phase shift demodulation algorithm, so as to convert the spatial phase shift into the time phase shift, improve the demodulation precision, avoid the polarization error between pixels introduced by the polarization camera, and realize the high-precision carrier frequency interference detection.

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

[0030] Figure 1 It is a structure schematic view of the carrier frequency interference detection device based on the polarization grating.

[0031] Figure 2 It is an interference fringe after the interference pattern obtained on the CMOS camera in the embodiment of the present application is enlarged 30 times.

[0032] Figure 3 It is a conversion schematic view of the interference pattern in step 2 in the embodiment of the present application.

[0033] Figure 4 It is a surface shape diagram obtained after the carrier frequency interference detection method in the embodiment of the present application is processed. DETAILED DESCRIPTION

[0034] The present application will be further described in detail below in combination with the drawings and embodiments, and it should be pointed out that the following described embodiments are intended to facilitate the understanding of the present application and do not have any limiting effect on the present application.

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

[0036] The whole interference detection system is installed on an air-floating vibration isolation optical platform, and includes a tunable laser 1, a first linear polarizer 2, a collimating beam expander 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 beam expander first lens 8, a beam expander second lens 9, a to-be-measured mirror 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 light beam emitted by the tunable laser 1 is incident on the light receiving surface of the collimating and expanding system 3 after passing through the first linear polarizer 2, and the collimated and expanded light beam is incident on the polarization beam splitter prism 4 after passing through the collimating and expanding system 3; the vertically polarized light is reflected and then passes through the first quarter-wave plate 5, and then passes through the reference mirror 6 and returns to the original path as the reference light wave; the horizontally polarized light is transmitted and then passes through the second quarter-wave plate 7, and then passes through the first lens 8 and the second lens 9 of the expanding mirror, and then passes through the to-be-measured mirror 10 and returns to the original path as the to-be-measured light wave; the polarization states of the two light waves are converted because the two light waves pass through the first quarter-wave plate 5 and the second quarter-wave plate 7 twice, and then the two light waves are combined into one light beam after passing through the polarization beam splitter prism again, and then the light beam is incident on the surface of the beam splitter 11, a part of the light is transmitted and then converges on the alignment camera 13 after passing through the first imaging lens 12, and the light is used for adjusting the optical path, and another part of the light is reflected and then converges after passing through 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, and an interference pattern is obtained.

[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 d=5 μm and a line number of 200, and the focal lengths 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 processing module; the phase control module is connected with the controller of the tunable laser 1, the wavelength of the tunable laser 1 is changed by adjusting the voltage of the controller of the tunable laser 1, so that the phase control is realized; the image acquisition module is connected with the CMOS camera 18, after the carrier frequency interference pattern image of the to-be-measured sample and the reference mirror is obtained, the data is transmitted to the interference pattern data analysis processing module for analysis and processing, and the surface shape information of the to-be-measured sample is obtained.

[0040] The above device is used for carrier frequency interference detection based on a polarization grating, and includes the following steps:

[0041] Step 1: The clamping angle of the reference mirror 6 and the standard mirror is adjusted by the electric five-dimensional adjusting frame, and the interference pattern with clear focus and good fringe contrast is obtained.

[0042] Step 2: The fixed carrier frequency of the polarization grating is calibrated; the aperture selection is performed on the phase-shifted interference pattern, and the complete interference pattern is included as much as possible, the surface shape is demodulated based on the least square iteration space carrier phase shift, and the carrier frequency is calculated; the algorithm converts a single carrier frequency interference pattern into four time domain phase-shifted interference patterns, and then the least square iteration method is used to obtain the phase shift between the interference patterns, and the fixed carrier frequency of the polarization grating is calculated based on the phase shift.

[0043] The conversion of single carrier frequency interferogram is shown in Figure 3 .

[0044] Step 3: Adjust the clamping angle of the reference mirror 6 and the measured mirror 10 by the electric five-dimensional adjustment frame to obtain the interferogram with clear focus and good fringe contrast.

[0045] Step 4: According to the fixed carrier frequency of the polarization grating, the surface shape of the measured mirror is demodulated; aperture selection is performed on the phase shifting interferogram to include the complete interferogram as much as possible, the surface shape is demodulated by using the fixed phase shifting amount demodulation algorithm, and the wrapped phase is unwrapped.

[0046] For regular circular apertures, Zernike polynomials are used for aberration fitting to remove tilt, and for irregular apertures, Schmidt orthogonalization is used for aberration fitting to remove tilt, to obtain the surface shape

[0047] Figure 4 The results of the application of the present application to the surface shape detection of a planar element are shown, wherein the PV value of the surface shape is 12.628 nm, and the RMS value is 2.8943 nm.

[0048] The above-described embodiments have described the technical solutions and beneficial effects of the present application in detail, and it should be understood that the above-described only serves as specific embodiments of the present application and is not used to limit the present application. Any modification, supplement and equivalent replacement made within the principle range of the present application should be included in the protection range of the present application.

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 is incident on the light receiving surface of the collimating beam expansion system (3). After being collimated and expanded by the collimating beam expansion system (3), the outgoing light beam is incident on the polarization beam splitting prism (4) for splitting. One path of the vertically polarized light is reflected and passes through the first quarter-wave plate (5), and then is reflected by the reference mirror (6) and returns to the original path as the reference light. The other path of the horizontally polarized light is transmitted and passes through the second quarter-wave plate (7), passes through the first lens (8) and the second lens (9) of the beam expander, and after being expanded, is reflected by the to-be-measured mirror (10) and returns to 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 form a beam, and then is incident on the surface of the beam 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 optical 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 a carrier frequency, and finally passes through the second linear polarizer (17) to unify the polarization state and image the interference fringes 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 changes the wavelength of the tunable laser (1) by adjusting the voltage of the controller of the tunable laser (1), thereby realizing phase control; and 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), transmits the data 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 was adjusted by rotating the linear polarizer and 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-shift interferogram, using a least squares iterative spatial carrier phase-shift 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 an electric five-dimensional adjustment frame to obtain an interference pattern with clear focus and good fringe contrast; Step 4, demodulating the surface 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 using a fixed phase shift demodulation algorithm, unpacking, 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, 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)] Where 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 Represent the carrier amount along the x and y directions respectively, is the phase distribution to be measured; The interferogram is then converted into four time-domain interferograms with uncertain phase shifts, and the phase is extracted from the four interferograms through iterative least squares method.

8. The carrier frequency interference detection method based on polarization grating according to claim 7, 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 tilt, Zernike polynomials are used to perform aberration fitting to remove tilt for regular circular apertures, while Schmidt orthogonalization is used to perform aberration fitting to remove tilt for irregular apertures.

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