A shear rate calibration device and method for a transverse shearing interferometer

Abstract

The present invention discloses a shear rate calibration device and method for a transverse shearing interferometer. The device allows the calibration plate to be tilted at any angle in the direction perpendicular to the optical axis and is simple to operate. The device is also applicable when the incident light is a spherical wave with a certain known divergence or convergence angle. The method can make adjustments when there is a rotation error in the shear direction, and reduces the steps of image processing, thereby improving the accuracy and computational efficiency of shear rate measurement. The device includes a calibration platform, a plug-in fixture, a double-slit calibration plate, a grating, a CCD detector, and a computer. First, the calibration component, the double slit, is placed between the light source and the detection surface of the CCD. The double slit is fixed with a fixture and can move freely in the axial direction of the system. The position of the shear rate required by the grating measured using the double slit is recorded, and the axial movement of the fixture is restricted. The double slit is removed from the plug-in fixture and the grating is inserted to complete the calibration.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a shear rate calibration device and method for a lateral shearing interferometer. Background Art

[0002] The grating-based four-wave transverse shearing interferometer is a common optical path interferometer technology proposed by French researcher Primot J in 2000. It has the characteristics of high precision and strong anti-interference ability. He proposed a modified Hartman Mask (MHM) transverse shearing interferometer, the structure of which is as follows: Figure 2 The MHM, consisting of a two-dimensional amplitude grating with a duty cycle of 2 / 3 and a checkerboard phase grating modulated to 0 and π with a period twice that of the amplitude grating, serves as the beam splitter. This design suppresses even-order and ±3-order diffracted light in the diffraction field. High-precision wavefront measurement is achieved by utilizing the interference patterns of the remaining four primary first-order diffracted beams at integer multiples of the 1 / 6 Talbot distance.

[0003] Among them, the MHM-based lateral shearing interferometer has high requirements for the placement of the grating. The MHM must be placed at a position that is an integer multiple of the 1 / 6 Talbot distance so that the contrast of the interference pattern formed can be high. Such constraints make it impossible to change the shear rate of the interferometer at will, which limits the measurement. In 2015, Zhejiang University proposed a random encoded grating (Random Encoded Grating, REG) based on flux constraints, which makes the shear rate continuously adjustable. As a high-precision optical measurement method, the four-wave lateral shearing interferometer method requires knowing the exact shear rate when reconstructing the wavefront. The REG-based lateral shearing interferometer loses the characteristic of high contrast of the interference pattern at 1 / 6 Talbot distance, and the shear rate cannot be directly obtained, resulting in the problem of shear rate calibration.

[0004] Zhejiang University proposed a calibration device based on a square phase plate. Figure 3, it is necessary to make a standard phase plate with a square pattern and place it between the grating and the light source, and then use a method based on differential wavefront feature extraction to obtain the shear rate in the shear direction. This method solves the problem of large errors introduced by the traditional geometric formula method when calculating the shear amount due to inaccurate placement of various components. However, in this method, the placement of the phase plate is required to be precisely along the shear direction, otherwise rotation errors will be introduced, and the device cannot measure the wavefront of spherical waves with a certain known divergence or convergence angle. In response to the problem of phase plate placement requirements, the Shanghai Institute of Precision Machinery proposed a calibration device based on a circular phase plate. The standard phase plate with a circular pattern is also placed between the grating and the light source. However, due to the symmetry of the circular pattern, the phase plate can be placed at any angle, but this will increase the difficulty of pattern feature extraction, and there are still problems when measuring the wavefront of spherical waves with a certain known divergence or convergence angle.

[0005] Existing shear rate calibration schemes have strict requirements on the placement of the calibration plate during the calibration process; the processing of the calibration image is relatively complex, and if the calibration plate is tilted at an angle perpendicular to the optical axis, accurate calibration results cannot be obtained. Summary of the Invention

[0006] In view of this, the present invention provides a shear rate calibration device and method for a lateral shearing interferometer. The device allows the calibration plate to be tilted at any angle in the direction perpendicular to the optical axis and is simple to operate. Moreover, the device is still applicable when the incident light is a spherical wave with a certain known divergence or convergence angle. The method can make adjustments when there is a rotation error in the shear direction, and reduces the image processing steps, thereby improving the accuracy and computational efficiency of the shear rate measurement.

[0007] To achieve the above object, the technical solution of the present invention is: a shear rate calibration device for a lateral shearing interferometer, comprising a calibration platform, a plug-in fixture, a double-slit calibration plate, a grating, a CCD detector and a computer.

[0008] The calibration platform is placed horizontally, and the insert-type fixture and CCD detector are both installed on the calibration platform; the central horizontal line direction of the calibration platform is the optical axis direction, and the detection surface of the CCD detector is perpendicular to the optical axis direction.

[0009] The insert type fixture has an insert area for clamping a fixed grating or a double-slit calibration plate.

[0010] The double-slit calibration plate is an opaque plate with two slits. The width of the two slits is limited to 1 / 2 or less of the distance between the two slits. When the double-slit calibration plate is in use, it is inserted into the insert area on the insert-type fixture, and the double-slit calibration plate is perpendicular to the optical axis.

[0011] When the grating is in use, it is inserted into the inserting area on the inserting fixture, and the grating plane is perpendicular to the optical axis.

[0012] The detection surface of the CCD detector sends the read grating imaging and interference fringe imaging to the computer.

[0013] The computer determines the grating period P based on the grating imaging and interference fringe imaging. c , the double-slit spacing d and the fringe spacing Δy in the interference fringe imaging, the shear rate σ is calculated according to the following formula;

[0014]

[0015] Where n is the grating period P c Half the ratio of θ to the double-slit spacing d; k is the incident light angle coefficient of the converging beam.

[0016] Preferably, the value of k is set in the computer as:

[0017]

[0018] Where NA is the numerical aperture, NA = sin(i); λ is the wavelength of the incident light; and i is the angle of the incident light.

[0019] Another embodiment of the present invention further provides a method for calibrating the shear rate of a lateral shearing interferometer, using the above-mentioned shear rate calibration device for the lateral shearing interferometer, wherein the computer is configured to execute the following process:

[0020] Step (1): When the double-slit calibration plate is inserted into the insert area on the insert fixture, the computer obtains the interference fringe imaging from the CCD detector and performs filtering and noise reduction.

[0021] Step (2): Use the correlation method to determine the symmetry axis of the interference fringe imaging and record the horizontal coordinate as C.

[0022] Step (3): Use the threshold method to determine the width of the interference fringe, and take the center of the fringe as the horizontal coordinate N of the fringe in the image. -m ,N -m+1 ,...,N -1 ,N0,...,N m , where the subscript is determined by the relationship between the specific value of the fringe and the center C of the interference pattern; 2m+1 is the total number of fringes.

[0023] Step (4): Calculate the current stripe spacing Repeated measurements are taken on a part of the image and the average is taken to reduce random errors, and the fringe spacing Δy in the interference fringe imaging is obtained.

[0024] Step (5): Utilize Calculate the shear rate σ; where n is the grating period P c Half the ratio of θ to the double-slit spacing d; k is the incident light angle coefficient of the converging beam.

[0025] Beneficial effects:

[0026] 1: The present invention provides a shear rate calibration device for a transverse shearing interferometer. The calibration device requires only a pair of double slits, which is simple to manufacture. When the double slits are placed, the double slits are added to the system using an insert method, which is simple to operate. When the double slits are placed, the position of the grating can be replaced from any direction, eliminating the error introduced by the original phase plate deviating from the shear direction during placement. When the incident light has a spherical wave front with a certain known divergence or convergence angle, the device can still achieve accurate measurement of the interferometer shear rate.

[0027] 2. The present invention provides a shear rate calibration method for a transverse shearing interferometer. This method not only adjusts for rotational errors in the shear direction but also reduces image processing steps, thereby improving the accuracy of shear rate measurements. Furthermore, the present invention has a wide range of applications, being applicable not only when the incident light is parallel but also when the incident light has a known divergence or convergence angle. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of grating diffraction; Figure 1 (a) in Figure 1 (a) shows the diffraction relationship diagram of parallel broadcast projected onto the CCD detection surface through the grating; Figure 1 (b) shows the geometric relationship between the grating plane used for shearing interferometry and the CCD detection surface;

[0029] Figure 2 Schematic diagram of the MHM lateral shearing interferometer device;

[0030] Figure 3 Schematic diagram of the calibration device based on square phase plate;

[0031] Figure 4 Schematic diagram of a calibration device using a double slit as a calibration device;

[0032] Figure 5 A schematic diagram of a calibration device using a double slit as a calibration device;

[0033] Figure 6The simulation diagrams are of double slits with a spacing of 120 μm, a slit length of 5 mm, and a slit width of 20 μm under 512*512 pixel sampling and with a placement angle, where (a) is a double slit with a rotation angle of 0.1 radians, and (b) is the interference pattern corresponding to (a); (c) is a double slit with a rotation angle of 0.05 radians, and (d) is the interference pattern corresponding to (c); (e) is a double slit with a rotation angle of 0.01 radians, and (f) is the interference pattern corresponding to (e). DETAILED DESCRIPTION

[0034] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0035] Example 1:

[0036] The present invention provides a shear rate calibration device for a transverse shearing interferometer, the structure of which is as follows: Figure 4 The device is a four-wave lateral shearing interference wavefront sensor shear rate calibration device, which includes a calibration platform, a plug-in fixture with adjustable optical axis direction fixed on the calibration platform, a double-slit calibration plate and a CCD detector fixed on the calibration platform.

[0037] The calibration platform is placed horizontally, and the insert-type fixture and CCD detector are both installed on the calibration platform; the central horizontal line direction of the calibration platform is the optical axis direction, and the detection surface of the CCD detector is perpendicular to the optical axis direction.

[0038] The insert type fixture has an insert area for clamping a fixed grating or a double-slit calibration plate.

[0039] The double-slit calibration plate is an opaque plate. For example, a plate made of fused quartz can be chrome-plated on the surface to achieve an opaque effect, or a chrome-plated surface of ordinary glass can be used as the double-slit calibration plate. Double slits are provided on the double-slit calibration plate, and the slit width of the double slits is limited to 1 / 2 or less of the double-slit spacing. When the double-slit calibration plate is in use, it is inserted into the insert area on the insert-type fixture, and the double-slit calibration plate is perpendicular to the optical axis. In the present invention, the thickness of the double slit is 1 mm, and the slit length needs to ensure that the interference pattern can be observed. The slit spacing of the double slits is designed according to different needs. During calibration, the double slits required for calibration replace the position of the grating, and the required shear rate is obtained after calculation by measuring the interference fringe spacing on the CCD.

[0040] When the grating is in use, it is inserted into the inserting area on the inserting fixture, and the grating plane is perpendicular to the optical axis.

[0041] The detection surface of the CCD detector sends the read grating imaging and interference fringe imaging to the computer.

[0042] According to the structural setting of the above device, the principle of determining the shear rate is as follows:

[0043] With the optical axis direction as the Z axis and the center of the CCD detection surface as the origin coordinate, the xy plane is constructed, and the grating plane ηξ plane is as follows: Figure 1 (a) shows the diffraction relationship diagram of parallel broadcast projected onto the CCD detection surface through the grating; assuming that the period of the grating is P c , the four diffracted lights diffracted from the center point of the grating plane to the xy plane include (+1, +1) order diffracted light, (+1, -1) order diffracted light, (-1, +1) order diffracted light, (-1, -1) order diffracted light, as shown in Figure 1 Assume that the period of the grating is P c , the angle between the projection of the (+1, +1) order diffracted light on the ηZ plane and the Z axis is θ1. Assuming that the incident angle is i, when the incident light is parallel light, according to Figure 1 (b) shows the geometric relationship between the grating plane and the CCD detection surface used for shearing interferometry. The shear amount s can be obtained as: λ is the wavelength, z is the distance from the grating plane to the CCD detection surface, according to the fringe spacing formula of double-slit interference: Where Δy represents the fringe spacing, d represents the double slit spacing, and if the double slit fringe spacing is to be equal to the shear amount or equal to n times the shear amount, then In parallel light mode, when the angle θ1 is very small, sinθ1≈tanθ1, so:

[0044] The formula means: when the distance between the double slits is equal to the grating period When the double slits are placed at the grating, the spacing of the interference fringes generated on the CCD is equal to n times the shearing amount of the global random encoding grating, that is,

[0045] Through relevant derivation, we can get: when the double slit spacing , where n can be any real number greater than 0, that is, d can be any real number greater than 0, and the shear rate σ is: Where px is the image sampling size.

[0046] Since double-slit interference has no special requirements for the parallelism of the light source, it is also applicable to the optical path of spherical waves with a certain known divergence or convergence angle.

[0047] Also assume that the incident angle of the incident light is i, and the exit angles are θ1 and θ2, as Figure 2 .

[0048] According to the grating equation, we can get: For marginal rays, sini = NA; NA is the numerical aperture;

[0049] Again, s = Z·(tanθ1-tanθ2)

[0050] s shear amount;

[0051] make in

[0052] That is, when using the same double slit to measure the shearing amount of a converging light beam, it is only necessary to multiply it by a coefficient k. When the accuracy requirement is not high, the coefficient k can also use an approximate value.

[0053] According to the above principle, the shear rate can be deduced as: That is, when using the same double slit to measure the shearing amount of a converging light beam, it is only necessary to multiply it by a coefficient k, where k is a variable related to the incident angle i.

[0054] Therefore, the computer determines the grating period P according to the grating imaging and interference fringe imaging. c , the double-slit spacing d and the fringe spacing Δy in the interference fringe imaging, the shear rate σ is calculated according to the following formula;

[0055]

[0056] Where n is the grating period P c Half the ratio of θ to the double-slit spacing d; k is the incident light angle coefficient of the converging beam.

[0057] The value of k set in the computer is:

[0058]

[0059] Where NA is the numerical aperture, NA = sin(i); λ is the wavelength of the incident light; i is the angle of the incident light, and K is the accuracy coefficient for non-zero incident light angles. When k is set to K, the calculated shear rate is more accurate. When k is within the range [K - 3% K + 3%], the shear rate deviation is within 3%.

[0060] Example 2:

[0061] An embodiment of the present invention provides a shear rate calibration method for a lateral shearing interferometer, and the method steps are as follows:

[0062] Step (1): When the double-slit calibration plate is inserted into the insert area on the insert fixture, the computer obtains the interference fringe imaging through the CCD detector and performs filtering and noise reduction;

[0063] Step (2): Use the correlation method to determine the symmetry axis of the interference fringe imaging, and record the horizontal coordinate as C;

[0064] Step (3): Use the threshold method to determine the width of the interference fringe, and take the center of the fringe as the horizontal coordinate N of the fringe in the image. -m ,N-m+1 ,...,N -1 ,N0,...,N m , where the subscript is determined by the relationship between the specific value of the fringe and the center C of the interference pattern; 2m+1 is the total number of fringes;

[0065] Step (4): Calculate the current stripe spacing Repeated measurements are taken on a portion of the image to obtain the average value to reduce random errors and obtain the fringe spacing Δy in the interference fringe imaging;

[0066] Step (5): Utilize Calculate the shear rate σ; where n is the grating period P c Half the ratio of θ to the double-slit spacing d; k is the incident light angle coefficient of the converging beam.

[0067] When using the device to calibrate the four-wave transverse shearing interferometer, such as Figure 5 First, place the calibration component double slit between the light source and the detection surface of the CCD; fix the double slit with a fixture and allow it to move freely in the system axis; record the position of the required shear rate of the grating measured using the double slit and limit the axial movement of the fixture; remove the double slit from the insert-type fixture and insert the grating to complete the calibration.

[0068] Based on this, simulations were conducted for a 120μm double-slit spacing, corresponding to a grating period of 240μm, a slit length of 5mm, a slit width of 20μm, and a 512x512 pixel sampling angle. The simulated measurement result was 0.0451 at a shear rate of 0.045, with a relative error of only 0.23%. Figure 6 These are simulation images of a double slit with a 120μm spacing, a 5mm slit length, and a 20μm slit width, with a 512x512 pixel sampling angle. (a) shows the double slits rotated at a 0.1 radian angle, and (b) shows the corresponding interference pattern. (c) shows the double slits rotated at a 0.05 radian angle, and (d) shows the corresponding interference pattern. (e) shows the double slits rotated at a 0.01 radian angle, and (f) shows the corresponding interference pattern. The simulation results show that when the shear rate is set to 0.05, and ignoring other system errors, the PV value of the reconstructed error is better than λ / 100 when the shear rate deviation is within 3%. When the measured interference fringes are error-free, the deviation of k is within 3%, and the reconstruction error remains small.

[0069] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A shear rate calibration device for a lateral shearing interferometer, characterized in that: Including calibration platform, insert type fixture, double slit calibration plate, grating, CCD detector and computer; The calibration platform is placed horizontally, and the insert-type fixture and the CCD detector are both installed on the calibration platform; the central horizontal line direction of the calibration platform is taken as the optical axis direction, and the detection surface of the CCD detector is perpendicular to the optical axis direction; The insert-type fixture is provided with an insert area for clamping and fixing the grating or the double-slit calibration plate; The double-slit calibration plate is an opaque plate with two slits formed thereon, the width of which is limited to 1 / 2 or less of the distance between the two slits. When in use, the double-slit calibration plate is inserted into the inserting area of ​​the inserting fixture, with the double-slit calibration plate perpendicular to the optical axis. When the grating is in use, it is inserted into the inserting area of ​​the inserting fixture, and the grating plane is perpendicular to the optical axis direction; The detection surface of the CCD detector sends the read grating imaging and interference fringe imaging to the computer; The computer determines the grating period P according to the grating imaging and the interference fringe imaging. c , the double-slit spacing d and the fringe spacing Δy in the interference fringe imaging, the shear rate σ is calculated according to the following formula; Where n is the grating period P c is half the ratio of the double-slit spacing d; k is the incident light angle coefficient of the converging beam, and px is the image sampling size.

2. A shear rate calibration device for a lateral shearing interferometer according to claim 1, characterized in that: The value of k set in the computer is: Where NA is the numerical aperture, NA=sin(i); λ is the wavelength of the incident light; i is the angle of the incident light, and K is the accuracy coefficient when the angle of the incident light is not 0.

3. A shear rate calibration method for a lateral shearing interferometer, characterized in that: Using the shear rate calibration device of a lateral shearing interferometer as claimed in claim 1 or 2, the computer is used to execute the following process: Step (1): When the double-slit calibration plate is inserted into the inserting area on the inserting fixture, the computer obtains interference fringe imaging through the CCD detector and performs filtering and noise reduction; Step (2): Use the correlation method to determine the symmetry axis of the interference fringe imaging, and record the horizontal coordinate as C; Step (3): Use the threshold method to determine the width of the interference fringe, and take the center of the fringe as the horizontal coordinate N of the fringe in the image. -m ,N -m+1 ,...,N -1 ,N0,...,N m , where the subscript is determined by the relationship between the specific value of the fringe and the center C of the interference pattern; 2m+1 is the total number of fringes; Step (4): Calculate the current stripe spacing Repeated measurements are taken on a portion of the image to obtain the average value to reduce random errors and obtain the fringe spacing Δy in the interference fringe imaging; Step (5): Utilize Calculate the shear rate σ; where n is the grating period P c Half the ratio of θ to the double-slit spacing d; k is the incident light angular coefficient of the converging beam.

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