A high-resolution wavefront detection method combined with oblique illumination

Abstract

The present invention discloses a high-resolution wavefront detection method combined with oblique illumination. A wavefront detection device is adopted, and the wavefront detection device includes an LED board, a collimating lens, a flat plate to be measured, a converging lens, and an image sensor arranged along the light path emission direction. The image sensor is fixed on a precision guide rail, and the image sensor is located at the defocus position of the converging lens. The present invention uses oblique illumination of the sample to be measured at different angles, sequentially collects diffraction spots, uses a phase retrieval algorithm to realize the splicing of a high-resolution and high-precision optical spectrum plane, and further uses a phase retrieval algorithm with negative feedback adjustment to realize high-resolution wavefront reconstruction. The method proposed by the present invention can achieve precise reconstruction of high-resolution wavefronts with high precision and high robustness.

Description

Technical Field

[0001] The present invention relates to the technical field of optical measurement, and in particular to a high-resolution wavefront detection method combined with oblique illumination. Background Art

[0002] With the wide application of modern computer-controlled small tools, fine polishing processes for various optical components have emerged. Periodic structures caused by small tools with specified traces are inevitable on the manufactured surface profiles. These periodic fluctuation errors with a certain amplitude (i.e., intermediate-frequency wavefront errors) will scatter the light source and are very harmful to the imaging performance in an optical system.

[0003] In a high-power laser optical system, due to the extremely high output power, the intermediate-frequency wavefront errors of optical components will, after superposition and non-linear growth, cause high-intensity focal spot side lobes in the focal plane. The focal spot side lobes hitting the edge of the target hole will form plasma plugging of the hole. At the same time, during high-throughput operation, the intermediate-frequency wavefront errors will also cause self-focusing and damage the optical components. Therefore, how to accurately measure the intermediate-frequency wavefront errors to provide feedback for the manufacturing process and thus improve the optical quality is very important.

[0004] In the prior art, there are already many phase retrieval methods. For example, the Chinese patent document with the publication number CN112629678A discloses a fast phase retrieval method for general-shaped non-diffracting iterative calculation. This method decomposes the wavefront to be measured using a numerical orthogonal polynomial mode, then calculates the diffraction basis function for each term of the numerical orthogonal polynomial based on the fast Fourier transform, and then iteratively solves the coefficient gradient using matrix operations between diffraction planes, realizing wavefront detection with high speed and general shape.

[0005] The Chinese patent document with the publication number CN110470245A discloses a phase retrieval method based on the fusion of Fresnel zone plate diffraction information. This method uses the diffraction light intensity distribution modulated by the Fresnel zone plate for recovery and can realize the reconstruction of a wider frequency band of the wavefront to be measured.

[0006] However, due to the particularity of the spatial scale of the intermediate-frequency error, effective intermediate-frequency error information often cannot be obtained through traditional phase retrieval methods. Summary of the Invention

[0007] Aiming at the deficiencies of the prior art, the present invention provides a high-resolution wavefront detection method combined with oblique illumination, which can solve the problem that effective intermediate-frequency error information cannot be obtained through traditional phase retrieval methods due to sampling constraints.

[0008] A high-resolution wavefront detection method combined with oblique illumination uses a wavefront detection device. The wavefront detection device includes an LED board, a collimating lens, a flat plate to be measured, a converging lens, and an image sensor arranged along the light path exit direction. The image sensor is fixed on a precision guide rail and is located at the defocus position of the converging lens. The method includes the following steps:

[0009] S1: Turn on the LED lights on the LED board in sequence, and collect n defocus diffraction spots containing the wavefront error of the flat plate to be measured at a certain defocus distance;

[0010] S2: Change the defocus position and repeat step S1;

[0011] S3: For the defocus diffraction spots collected in steps S1 and S2, perform diffraction spot synthesis according to the intensity position and intensity exposure time to form M×M defocus diffraction spot matrices I1 and I2;

[0012] S4: Respectively set the focal length s, aperture D of the converging lens, and the defocus position z of each defocus diffraction spot j , T is the total number of iterations, and set the initial complex amplitude g(x,y) of the measurement plane as a matrix of all ones;

[0013] S5: Diffract the initial complex amplitude g(x,y) of the measurement plane to the first defocus plane to obtain the diffraction light field G1(u,v) of the first defocus plane;

[0014] S6: Replace the amplitude of the calculated diffraction light field G1(u,v) with the first defocus diffraction spot matrix I1 and diffract it back to the measurement plane to obtain the updated complex amplitude g(x,y) of the measurement plane;

[0015] S7: Diffract the updated complex amplitude g(x,y) of the measurement plane to the second defocus plane to obtain the diffraction light field G2(u,v) of the second defocus plane;

[0016] S8: Replace the amplitude of the calculated diffraction light field G2(u,v) with the second defocus diffraction spot matrix I2 and diffract it back to the measurement plane to obtain the further updated complex amplitude g(x,y) of the measurement plane;

[0017] S9: Increment the iteration count t by 1. If t≤T, repeat steps S5~S8. Otherwise, output the phase θ(x,y) = arg(g(x,y)).

[0018] Furthermore, in step S3, the method of diffraction spot synthesis is: divide each collected diffraction spot by the exposure time and then superimpose and combine them into defocus diffraction spots.

[0019] In step S5, the calculation formula for the diffraction light field G1(u,v) is:

[0020] G1(u, v) = P[g(x, y)]

[0021] Wherein, P is a diffraction calculation operator.

[0022] The calculation formula in step S6 is as follows:

[0023]

[0024] Wherein, P -1 is an inverse diffraction operator.

[0025] In step S7, the calculation formula of the diffracted light field G2(u, v) is:

[0026] G2(u, v) = P[g(x, y)]

[0027] The calculation formula in step S8 is as follows:

[0028]

[0029] In steps S5 and S7, the diffraction operator is Fresnel diffraction.

[0030] In steps S6 and S8, the inverse diffraction operator is inverse Fresnel diffraction.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The present invention realizes high-resolution wavefront detection by combining oblique illumination. First, different-angle illumination is used to collect high-frequency information by illuminating the same sample with LEDs at different positions; second, high-signal-to-noise-ratio high-frequency information is collected by increasing the camera exposure time when collecting high-frequency information; third, high-resolution image reconstruction far exceeding the traditional method is realized through spectral plane intensity synthesis. Description of the Drawings

[0033] Figure 1 is a schematic structural diagram of the wavefront detection device adopted by the present invention;

[0034] Figure 2 is a flowchart of a high-resolution wavefront detection method combining oblique illumination according to the present invention;

[0035] Figure 3 is a recovery result diagram of applying the method of the present invention and the traditional method in the embodiment. Detailed Embodiments

[0036] The present invention will be further described in detail below in conjunction with the drawings and embodiments. It should be noted that the following embodiments are only for facilitating the understanding of the present invention and do not limit it in any way.

[0037] The wavefront detection device adopted by the present invention is asFigure 1 As shown in the figure, the wavefront detection device includes an LED board 1, a collimating lens 2, a flat plate to be measured 3, a converging lens 4, and an image sensor 5 arranged along the light path emission direction, and the collimating lens 2, the flat plate to be measured 3, the converging lens 4, and the image sensor 5 are coaxial. The image sensor 5 is fixed on a precision guide rail 6, and the image sensor 5 is located at the defocus position of the converging lens 4.

[0038] As Figure 2 shown, a high-resolution wavefront detection method combining oblique illumination uses the Figure 1 wavefront detection device shown, and includes the following steps:

[0039] S1: Turn on the LED lights on the LED board in sequence, and collect n defocus diffraction spots containing the wavefront error of the flat plate to be measured at a certain defocus distance.

[0040] S2: Change the defocus position and repeat step S1.

[0041] S3: For the diffraction spots collected in S1 and S2, perform diffraction spot synthesis according to the intensity position and the intensity exposure time to form an M×M defocus diffraction spot matrix I1 and I2.

[0042] S4: Respectively set the focal length s, aperture D of the converging lens, and the defocus position z of each defocus diffraction spot j , T is the total number of iterations, and set the complex amplitude g(x,y) of the measurement surface as a matrix of all ones.

[0043] S5: Diffract the initial solution of the measurement surface to the first defocus surface to obtain the diffraction light field G1(u,v) of the first defocus surface. The formula is as follows:

[0044] G1(u,v) = P[g(x,y)].

[0045] S6: Replace the amplitude of the calculated light field with the first defocus diffraction spot matrix I1 and diffract it back to the measurement surface to obtain the updated complex amplitude g(x,y) of the measurement surface. The formula is as follows:

[0046]

[0047] S7: Diffract the updated complex amplitude g(x,y) of the measurement surface to the second defocus surface to obtain the diffraction light field G2(u,v) of the second defocus surface. The formula is as follows:

[0048] G2(u,v) = P[g(x,y)].

[0049] S8: Replace the amplitude of the calculated diffraction light field G2(u, v) with the second defocus diffraction spot matrix I2, and inverse diffract it back to the measurement plane to obtain the updated complex amplitude g(x, y) of the measurement plane. The formula is as follows:

[0050]

[0051] S9: t = t + 1. If t ≤ T, repeat steps S5 to S8. Otherwise, output the phase θ(x, y) = arg(g(x, y)).

[0052] Considering energy conservation, in step S3, the diffraction spot synthesis method is: divide each collected diffraction spot by the exposure time, and then stack and combine them into defocus diffraction spots.

[0053] Considering the accuracy and efficiency of diffraction calculation, in S7, the diffraction operator is Fresnel diffraction, and in S8, the inverse diffraction operator is inverse Fresnel diffraction.

[0054] The traditional phase retrieval wavefront detection system is limited by the size of the target surface of the image detector and cannot collect all the high- and low-frequency diffraction information simultaneously. The present invention uses tilted illumination of the sample to be measured at different angles, sequentially collects diffraction spots, uses the phase retrieval algorithm to achieve the stitching of the optical spectrum plane with high resolution and high precision, and further uses the phase retrieval algorithm with negative feedback adjustment to achieve the high-resolution wavefront reconstruction. The phase retrieval technology proposed by the present invention can achieve the accurate reconstruction of the wavefront with high resolution, high precision and high robustness.

[0055] The following gives a specific embodiment of the method of the present invention to illustrate the technical effects of the method

[0056] Here, the focal length is selected as s = 500 mm, z1, z2 = [-5, -10] mm, the aperture D = 20 mm, the total number of iterations N for wavefront detection is 100, the effective spot sampling number is 512 × 512, and the number of LEDs is 25.

[0057] In this embodiment, 25 * 2 defocus diffraction images are collected for wavefront phase reconstruction. The interval between LEDs is 4 mm, and the selected diffraction calculation model is the Fresnel diffraction model, as Figure 3 shown, which shows the recovery result diagrams of the method proposed by the present invention and the traditional method. Among them, (a) is the recovered phase of the method proposed by the present invention, (b) is the recovered phase of the traditional method, and (c) is the phase of the real image. It can be seen from the figure that the method proposed in this paper has higher resolution than the traditional method.

[0058] The above-described embodiments have elaborated in detail 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 replacements made within the scope of the principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high-resolution wavefront detection method combined with oblique illumination, characterized in that, A wavefront detection device is adopted. The wavefront detection device includes an LED board, a collimating lens, a flat plate to be measured, a converging lens, and an image sensor arranged along the outgoing direction of the optical path. The image sensor is fixed on a precision guide rail, and the image sensor is located at the defocus position of the converging lens. The method includes the following steps: S1: Turn on the LED lights on the LED board in sequence, and collect n defocus diffraction spots containing the wavefront error of the flat plate to be measured at a certain defocus distance; S2: Change the defocus position and repeat step S1; S3: For the defocus diffraction spots collected in steps S1 and S2, perform diffraction spot synthesis according to the intensity position and intensity exposure time to form M×M defocus diffraction spot matrices I1 and I2, where M represents the length of the collected defocus diffraction spot matrix; S4: Set the focal length s, aperture D of the converging lens, and the defocus position z of each defocus diffraction spot respectively j , where T is the total number of iterations, and set the initial complex amplitude g(x, y) of the measurement plane as a matrix of all ones; S5: Diffract the initial complex amplitude g(x,y) of the measurement surface to the first defocus surface to obtain the diffraction light field G1(u,v) of the first defocus surface; S6: Replace the amplitude of the calculated diffraction light field G1(u,v) with the first defocus diffraction spot matrix I1, and diffract it back to the measurement surface to obtain the updated complex amplitude g(x,y) of the measurement surface; S7: Diffract the updated complex amplitude g(x,y) of the measurement surface to the second defocus surface to obtain the diffraction light field G2(u,v) of the second defocus surface; S8: Replace the amplitude of the calculated diffraction light field G2(u,v) with the second defocus diffraction spot matrix I2, and diffract it back to the measurement surface to obtain the complex amplitude g(x,y) of the measurement surface updated again; S9: Increment the iteration count t by 1. If t≤T, repeat steps S5~S8. Otherwise, output the phase θ(x,y)=arg(g(x,y)).

2. The high-resolution wavefront detection method combined with oblique illumination according to claim 1, characterized in that In step S3, the method of diffraction spot synthesis is: divide each collected diffraction spot by the exposure time, and then superimpose and combine them into defocus diffraction spots.

3. The high-resolution wavefront detection method combined with oblique illumination according to claim 1, wherein, In step S5, the calculation formula for the diffraction light field G1(u,v) is: G1(u,v)=P[g(x,y)] where P is a diffraction calculation operator.

4. The high-resolution wavefront detection method combined with oblique illumination according to claim 3, characterized in that The calculation formula in step S6 is: where P -1 is the inverse diffraction operator and i is the imaginary unit.

5. The high-resolution wavefront detection method combined with oblique illumination according to claim 4, characterized in that In step S7, the calculation formula for the diffraction light field G2(u,v) is: G2(u,v)=P[g(x,y)].

6. The high-resolution wavefront detection method combined with oblique illumination according to claim 5, characterized in that, The calculation formula in step S8 is: where i is the imaginary unit.

7. The high-resolution wavefront detection method combined with oblique illumination according to claim 1, wherein In steps S5 and S7, the diffraction operator is Fresnel diffraction.

8. The high-resolution wavefront detection method combined with oblique illumination according to claim 1, characterized in that In steps S6 and S8, the inverse diffraction operator is inverse Fresnel diffraction.

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