A convex grating detection system based on CGH and a design method thereof

Abstract

The application discloses a convex grating detection system and a design method based on CGH. Test light emitted from an interferometer of the system is incident on a first CGH compensator; after phase modulation of the first CGH compensator, first-order diffraction light of the first CGH compensator is reflected to a convex grating to be measured; after diffraction of the convex grating to be measured, working-order diffraction light of the convex grating to be measured is reflected to a second CGH compensator; after phase modulation of the second CGH compensator, a convergence point of first-order diffraction light of the second CGH compensator coincides with a curvature center of a standard spherical mirror; after the standard spherical mirror, the test light returns to the interferometer along an original path. The test light interferes with reference light, and the interferometer can measure wave aberration of the path according to interference fringes; the application has the advantages of simple structure, easy installation and adjustment, high light energy utilization rate and wide measurement object range.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of optical detection, and relates to a convex grating detection system and a design method. BACKGROUND

[0002] An imaging spectrometer can be used to acquire spatial information and spectral information of a target scene, has the characteristics of graph and atlas integration, and has been widely applied in the fields of earth remote sensing, machine vision and target recognition. An optical system of the imaging spectrometer is generally composed of a pre-system and a light splitting system, wherein the light splitting system is a core part of the imaging spectrometer and determines the spectral performance. In order to realize high imaging quality and compact volume, the light splitting system often adopts an Offner light splitting structure based on a convex grating. As shown in FIG. 1, the structure is a total reflection type structure, which is composed of a primary mirror, a convex grating and a third mirror. Figure 1

[0003] In the Offner type imaging spectrometer system, the performance of the convex grating will affect the final performance of the instrument, and the period, diffraction efficiency and wavefront quality of the convex grating need to be evaluated. However, due to the limitation of existing detection methods and devices, the conventional convex grating evaluation currently only includes the test of the period and the diffraction efficiency, and it is difficult to detect the wavefront quality. In “Design, fabrication, and testing of convex reflective diffraction gratings”, two wavefront detection methods of convex gratings are reported. The first method is to use 0-order light for detection, and the problems are: a) for blazed gratings, the 0-order diffraction light energy is low, b) for convex gratings, there is a large astigmatism in the 0-order light. The second method is to detect the wavefront based on the actual Offner light splitting structure, and the problem of this method is that the test results simultaneously contain the system adjustment error and the element processing error, and the test results cannot accurately evaluate the wavefront quality of the grating. In addition, for aspheric and freeform gratings, the wavefront detection is more difficult than the conventional spherical grating. SUMMARY

[0004] To solve the above technical problems, a convex grating detection system and a design method based on CGH (Computer Generated Hologram) are provided. The purpose is to improve the light energy utilization rate and the measurement object range.

[0005] A convex grating detection system based on CGH, comprising an interferometer, a first CGH compensator, a second CGH compensator and a standard spherical mirror.

[0006] The test light emitted from the interferometer is incident on the first CGH compensator;

[0007] After the phase modulation of the first CGH compensator, the first-order diffraction light is reflected to the convex grating to be measured.​

[0008] After the diffraction of the convex grating to be measured, the working order diffraction light of the convex grating to be measured is reflected to the second CGH compensator;

[0009] After the phase modulation of the second CGH compensator, the convergence point of the first order diffraction light coincides with the curvature center of the standard spherical mirror;

[0010] After the standard spherical mirror, the test light returns to the interferometer.

[0011] The light path of the test light returning to the interferometer is that: after the light passes through the standard spherical mirror, it is first reflected to the second CGH compensator, then the first order diffraction light of the second CGH compensator is reflected to the convex grating to be measured, after the diffraction of the convex grating to be measured, the working order diffraction light of the convex grating to be measured is reflected to the first CGH compensator, and finally the first order diffraction light of the first CGH compensator is reflected back to the interferometer.

[0012] After the standard spherical mirror, the test light returns to the interferometer, the test light and the reference light interfere, and then the wavefront information of the convex grating is obtained. The interferometer can measure the wave aberration of the light path according to the interference fringes obtained by the test light returning to the interferometer through the detection system. The standard spherical mirror can achieve high surface accuracy and reduce the influence on the detection accuracy.

[0013] Preferably, a first spatial filter is further included; the first spatial filter is arranged at the curvature center of the standard spherical mirror. The first spatial filter only allows the first order diffraction light from the first CGH compensator, the working order diffraction light of the convex grating to be measured, and the first order diffraction light of the second CGH compensator to pass through.

[0014] Preferably, a second spatial filter is further included; the second spatial filter is arranged at the focal point of the interferometer. The second spatial filter only allows the first order diffraction light from the first CGH compensator, the working order diffraction light of the convex grating to be measured, and the first order diffraction light of the second CGH compensator to pass through.

[0015] Preferably, the first CGH compensator and the second CGH compensator are arranged on the same substrate. Arranging them on the same substrate can reduce the number of elements and reduce the difficulty of assembling and adjusting the convex grating detection system. In addition, the first CGH compensator and the second CGH compensator can be prepared on the same substrate at the same time by using a photolithography process.

[0016] Preferably, the first CGH compensator and the second CGH compensator are both binary optical surfaces; taking the meridian direction as the Y axis, the sagittal direction as the X axis, and the optical axis direction as the Z axis, the binary optical surface changes the wavefront phase passing through the surface according to the following XY polynomial:

[0017] Φ=M(a1x 1 y 0 +a2x 0 y 1 +a3x 2 y 0 +a4x 1 y 1 +a5x 0 y 2 +a6x 3 y 0 +a7x 2 y 1 +a8x 1 y 2 +a9x 0 y 3 +…);

[0018] M is the diffraction order; a1 to a9 are coefficients of monomial, x is the coordinate on the X axis, and y is the coordinate on the Y axis.

[0019] Preferably, the whole system is symmetrical about the YOZ plane, for which the odd-order terms of x in the phase polynomial are set to 0, only even-order terms are used in the optimization process, the highest power term is the fourth order, and the optimized form is as follows:

[0020] Φ=M(a2x 0 y 1 +a3x 2 y 0 +a5x 0 y 2 +a7x 2 y 1 +a9x 0 y 3 +…)。

[0021] Preferably, the aperture of the first CGH compensator and the second CGH compensator is not more than 100 mm.

[0022] In addition, on the basis of the above-mentioned CGH-based convex grating detection system, the application further provides a design method of a CGH-based convex grating detection system, comprising:

[0023] Step 1, first design an Offner spectrometer system according to the convex grating to be measured, wherein the light emitted at the slit is incident to the primary mirror, reflected by the primary mirror, incident to the convex grating to be measured, reflected by the convex grating to be measured, and then incident to the third mirror, and then imaged to the image plane after being reflected by the third mirror;

[0024] Step 2, calculate the initial phase of the first CGH compensator so that the first-order diffracted light is reflected to the convex grating to be measured, and the light completely fills the aperture of the convex grating to be measured; calculate the initial phase of the second CGH compensator so that the first-order diffracted light is converged to the convergence point;

[0025] The initial distance d1 between the focal point of the interferometer and the first CGH compensator is equal to the distance between the slit and the primary mirror in the Offner spectrometer system;

[0026] The initial distance d2 between the first CGH compensator and the convex grating to be measured is equal to the distance between the convex grating to be measured and the primary mirror in the Offner spectrometer system;

[0027] The initial distance d3 between the convex grating to be measured and the second CGH compensator is equal to the distance between the convex grating to be measured and the third mirror in the Offner spectrometer system;

[0028] The initial distance d4 between the second CGH compensator and the convergence point is equal to the distance between the third mirror and the image plane in the Offner spectrometer system;

[0029] Step 2, calculate the initial phase of the first CGH compensator so that the first-order diffracted light is reflected to the convex grating to be measured, and the light completely fills the aperture of the convex grating to be measured; calculate the initial phase of the second CGH compensator so that the first-order diffracted light is converged to the convergence point;

[0030] Step 3, set the phase of the first CGH compensator and the second CGH compensator as the optimization variable;

[0031] Step 4, perform optimization, use point spread diagram and wavefront aberration as image quality evaluation, when the image quality meets the design requirements, output the optimized structure parameters of the CGH-based convex grating detection system;

[0032] Step 5, set the standard spherical mirror behind the first-order diffracted light convergence point of the second CGH compensator; the convergence point coincides with the center of curvature of the standard spherical mirror, so that the test light incident to the standard spherical mirror returns to the interferometer.

[0033] During the optimization process of step 4, attention should be paid to:

[0034] Avoid the convex grating to block the test light emitted by the interferometer during optimization;

[0035] Avoid the convex grating to block the first-order diffracted light of the second CGH compensator during optimization.

[0036] Preferred: in step 1, the numerical aperture of the object side of the CGH-based convex grating detection system is equal to the numerical aperture of the object side of the convex grating Offner spectrometer system.

[0037] Preferred: in step 3, d1, d2, d3, d4 are not used as optimization variables.

[0038] Preferred: in step 4, the distance d2 between the first CGH compensator and the convex grating to be measured is equal to the distance d3 between the convex grating to be measured and the second CGH compensator.

[0039] Advantages of the present application:

[0040] The present application provides a CGH-based convex grating detection system and a design method, which uses working order diffraction light when the test light of the detection system is reflected by the convex grating, and has high light energy utilization rate; the CGH compensator element has the advantages of simple alignment and low development cost; the measurement object range is wide, including spherical convex grating, aspherical convex grating, and free-form surface convex grating.

[0041] The above description is only a summary of the technical solutions of the present application, in order to more clearly understand the technical means of the present application, the content of the specification can be implemented, and in order to make the above-mentioned purposes, characteristics and advantages of the present application more obvious and easy to understand, the following preferred embodiments are described in detail below, and the accompanying drawings are described as follows. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is a structure schematic diagram of an Offner spectrometer system;

[0043] Figure 2 is a structure schematic diagram of a CGH-based convex grating detection system provided by an embodiment of the present application;

[0044] Figure 3 is a structure schematic diagram of a design example provided by an embodiment of the present application;

[0045] Figure 4 is a layout schematic diagram of a first CGH compensator and a second CGH compensator on the same substrate provided by an embodiment of the present application;

[0046] Figure 5 is a point diagram of the system provided by an embodiment of the present application;

[0047] Figure 6 is a wave aberration diagram of the system provided by an embodiment of the present application;

[0048] Label description: 1, interferometer; 2, first CGH compensator; 3, convex grating to be measured; 4, second CGH compensator; 5, standard spherical mirror. DETAILED DESCRIPTION

[0049] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention.

[0050] This application proposes a convex grating detection system based on CGH. In this system, test light emitted from an interferometer is incident on the first CGH compensator. After phase modulation by the first CGH compensator, its first-order diffracted light is reflected to the convex grating to be measured. After diffraction by the convex grating to be measured, the working-order diffracted light of the convex grating to be measured is reflected to the second CGH compensator. After phase modulation by the second CGH compensator, the convergence point of its first-order diffracted light coincides with the center of curvature of a standard spherical reflector. After passing through the standard spherical reflector, the test light returns to the interferometer along the original path.

[0051] Next, the convex grating detection system and method proposed in this application will be described with reference to the accompanying drawings.

[0052] like Figure 2 As shown, a convex grating detection system based on CGH in an embodiment of the present invention includes: an interferometer 1, a first CGH compensator 2, a second CGH compensator 4, and a standard spherical reflector 5.

[0053] The test light emitted from the interferometer 1 is incident on the first CGH compensator 2;

[0054] After phase modulation by the first CGH compensator 2, the first-order diffracted light is reflected to the convex grating to be measured;

[0055] After being diffracted by the convex grating 3 to be measured, the working order diffracted light of the convex grating 3 to be measured is reflected to the second CGH compensator 4;

[0056] After phase modulation by the second CGH compensator 4, the convergence point of its first-order diffracted light coincides with the center of curvature of the standard spherical reflector;

[0057] After passing through the standard spherical reflector 5 , the test light returns to the interferometer 1 along the original path.

[0058] The optical path of the test light returning to the interferometer 1 is specifically as follows: after passing through the standard spherical reflector 5, the light is first reflected to the second CGH compensator 4, and then the first-order diffraction light of the second CGH compensator 4 is reflected to the convex grating 3 to be measured. After being diffracted by the convex grating 3 to be measured, the working-order diffraction light of the convex grating 3 to be measured is reflected to the first CGH compensator 2; finally, the first-order diffraction light of the first CGH compensator 2 is reflected back to the interferometer 1, and interferes with the reference light in the interferometer 1.

[0059] An example of simulation is provided below. The system test wavelength λ of the design example is 632.8 nm, and the F number is 6.75; the measured object is a free curved surface convex grating used in an Offner spectrometer containing a free curved surface, with a vertex curvature radius of 403.94 mm, an aperture of 64 mm, a period of 45 lp / mm, and the free curved surface being characterized by a Fringe Zernike polynomial. The surface shape description parameters are shown in Table 1.

[0060] The Fringe Zernike polynomial expression is as follows:

[0061]

[0062] In the formula, z is the sag, c is the vertex curvature, k is the quadratic surface coefficient, r is the radial aperture, Z j is the jth polynomial, C j is the coefficient of Z j .

[0063] The optical path of the design example is shown in Figure 3 , and the optical parameters are shown in Table 2. The first CGH compensator and the second CGH compensator are arranged on the same CGH substrate, and the layout is shown in Figure 4 .

[0064] Both the first CGH compensator and the second CGH compensator are binary optical surfaces. The binary optical surface changes the wavefront phase passing through the surface according to the following XY polynomial:

[0065] Φ = M (a1x 1 y 0 +a2x 0 y 1 +a3x 2 y 0 +a4x 1 y 1 +a5x 0 y 2 +a6x 3 y 0 +a7x 2 y 1 +a8x 1 y 2 +a9x 0 y 3 +…)

[0066] The entire system is symmetric about the YOZ plane. Therefore, the odd-order terms of x in the phase polynomial are set to 0, and only even-order terms are used in the optimization process, with the highest power being 4th order. The optimization form is as follows:

[0067] Φ = M (a2x 0 y 1+a3x 2 y 0 +a5x 0 y 2 +a7x 2 y 1 +a9x 0 y 3 +…)

[0068] The parameters of the phase polynomial of the first CGH compensator and the second CGH compensator are shown in Table 3 and Table 4 respectively.

[0069] The spot diagram of the design example is shown in Figure 5 The wave aberration is shown in Figure 6 The root mean square radius is 0.174 μm, the wave aberration PV value is 0.018 λ, and the RMS value is 0.003 λ, and it can be seen that the imaging quality is good.

[0070] Table 1 is a free-form surface shape description parameter table, as shown below

[0071] ZF5 ZF8 ZF9 ZF11 0.0517 2.4433e-05 6.7150e-06 -4.5546e-06 ZF12 ZF15 ZF16 ZF17 2.9842e-05 3.2346e-06 1.5922e-06 2.6115e-05

[0072] Table 1

[0073] Table 2 is a system parameter table,

[0074]

[0075]

[0076] Table 2

[0077] Table 3 is a first CGH compensator phase polynomial parameter table

[0078] Diffraction order Constructive wavelength Y X 2 ]] Y 2 ]]> X 2 Y 1 632.8 0.0051 -0.0012 -0.0012 -1.5226e-007 Y 3 ]]> X 4 ]]> X 2 Y 2 ]]>

[00006] 4 ]] X 4 Y -2.0211e-007 5.4478e-010 3.8516e-010 1.9426e-010 -1.4753e-012

[0079] Table 3

[0080] Table 4 is a second CGH compensator phase polynomial parameter table

[0081] Diffraction order Constructive wavelength Y X 2 ]]>

[0007] Y 2 ]] X 2 Y 1 632.8 0.0091 -0.0013 -0.0013 2.5875e-008 Y 3 ]] X 4 ]] X 2 Y 2 ]]> Y 4 ]] X 4 Y 4.7671e-008 2.5950e-010 1.2654e-09 6.3360e-010 6.9272e-013

[0082] Table 4

[0083] The design method of the convex grating detection system based on CGH in the embodiment of the application, and the specific design method comprises:

[0084] Step 1, first, according to the Offner spectrometer system of the to-be-measured convex grating, a single light path of the convex grating detection system based on CGH is constructed;

[0085] The object side numerical aperture of the embodiment of the application is equal to the object side numerical aperture of the convex grating Offner spectrometer system, and is 0.074.

[0086] The initial distance d1 between the focus of the interferometer 1 and the first CGH compensator 2 is equal to the distance between the slit and the main mirror in the Offner spectrometer system, which is 799.78 mm;

[0087] The initial distance d2 between the first CGH compensator 2 and the convex grating 3 to be measured is equal to the distance between the convex grating to be measured and the main mirror in the Offner spectrometer system, which is 394.00 mm;

[0088] The initial distance d3 between the convex grating 3 to be measured and the second CGH compensator 4 is equal to the distance between the convex grating and the third mirror in the Offner spectrometer system, which is 394.00 mm;

[0089] The initial distance d4 between the second CGH compensator 4 and the image point is equal to the distance between the third mirror and the image plane in the Offner spectrometer system, which is 799.84 mm;

[0090] Step 2, the initial phase of the first CGH compensator 2 is calculated so that the first-order diffracted light is reflected to the convex grating, and the light completely fills the aperture of the convex grating; the initial phase of the second CGH compensator 4 is calculated so that the first-order diffracted light converges;

[0091] Step 3, the optimization variables are set, and the optimization variables include the phases of the first CGH compensator 2 and the second CGH compensator 4;

[0092] Step 4, the optimization is performed, and the point spread function and the wave aberration are used as the image quality evaluation, and when the image quality meets the requirements, the designed single optical path system is outputted;

[0093] Step 5, the standard spherical mirror 5 is arranged behind the converging point of the first-order diffracted light of the second CGH compensator 4, so that the test light incident to the standard spherical mirror 5 returns to the interferometer, forming a double optical path, and the convex grating detection system design based on CGH is completed.

[0094] The above embodiments are only preferred embodiments for fully illustrating the present application, and the protection scope of the present application is not limited thereto. The equivalent substitutions or transformations made by the person skilled in the art on the basis of the present application are within the protection scope of the present application. The protection scope of the present application is subject to the claims.

Claims

1. A convex grating detection system based on CGH, characterized in that, Comprise: an interferometer, a first CGH compensator, a second CGH compensator and a standard spherical mirror, the first CGH compensator and the second CGH compensator are arranged on the same substrate, The test light emitted in the interferometer is incident on the first CGH compensator; After phase modulation by the first CGH compensator, the first order diffraction light thereof is reflected to the convex grating to be measured; After diffraction by the convex grating to be measured, the working order diffraction light of the convex grating to be measured is reflected to the second CGH compensator; After phase modulation by the second CGH compensator, the convergence point of the first order diffraction light thereof coincides with the center of curvature of the standard spherical mirror; After the standard spherical mirror, the test light returns to the interferometer; further comprising: A first spatial filter is arranged at the center of curvature of the standard spherical mirror; A second spatial filter is arranged at the focal point of the interferometer.

2. The CGH-based convex grating detection system of claim 1, wherein: The first CGH compensator and the second CGH compensator are both binary optical surfaces; the phase expression of the binary optical surface is: Φ = M (a1x 1 y 0 +a2x 0 y 1 +a3x 2 y 0 +a4x 1 y 1 +a5x 0 y 2 +a6x 3 y 0 +a7x 2 y 1 +a8x 1 y 2 +a9x 0 y 3 +…), Wherein: M is the diffraction order; a1 to a9 are the coefficients of monomial, the meridian direction is Y axis, the sagittal direction is X axis, x is the coordinate on X axis, and y is the coordinate on Y axis.

3. The convex grating detection system based on CGH according to claim 1, wherein: The aperture of the first CGH compensator and the second CGH compensator is not more than 100mm.

4. A design method of a convex grating detection system based on CGH, characterized in that: The method comprises the following steps: Step 1: First, design an Offner spectrometer system according to the convex grating to be measured, wherein the light emitted at the slit of the Offner spectrometer system is incident on the primary mirror, reflected by the primary mirror, incident on the convex grating to be measured, reflected by the convex grating to be measured, and then incident on the third mirror, and then imaged on the image plane after being reflected by the third mirror; According to the designed Offner spectrometer system, determine the initial structure of the convex grating detection system based on CGH according to any one of claims 1 to 3; The initial distance d1 between the focal point of the interferometer and the first CGH compensator is equal to the distance between the slit and the primary mirror in the Offner spectrometer system; The initial distance d2 between the first CGH compensator and the convex grating to be measured is equal to the distance between the convex grating to be measured and the primary mirror in the Offner spectrometer system; The initial distance d3 between the convex grating to be measured and the second CGH compensator is equal to the distance between the convex grating to be measured and the third mirror in the Offner spectrometer system; The initial distance d4 between the second CGH compensator and the convergence point is equal to the distance between the third mirror and the image plane in the Offner spectrometer system; Step 2: Calculate the initial phase of the first CGH compensator so that the first order diffraction light thereof is reflected to the convex grating to be measured, and the light completely fills the aperture of the convex grating; calculate the initial phase of the second CGH compensator so that the first order diffraction light converges; Step 3: Set the phases of the first CGH compensator and the second CGH compensator as optimization variables. Step 4, performing optimization, using point spread function and wavefront aberration as image quality evaluation, when the image quality meets the design requirements, output the optimized structure parameters of the CGH-based convex grating detection system; Step 5, setting the standard spherical mirror behind the converging point of the first order diffraction light of the second CGH compensator; the converging point coincides with the curvature center of the standard spherical mirror, so that the test light incident to the standard spherical mirror returns to the interferometer.

5. The design method of the CGH-based convex grating detection system according to claim 4, characterized in that: In step 1, the numerical aperture of the object side of the CGH-based convex grating detection system is equal to the numerical aperture of the object side of the Offner spectrometer system.

6. The design method of the CGH-based convex grating detection system according to claim 4, characterized in that: In step 3, d1, d2, d3 and d4 are not used as optimization variables.

7. The design method of the CGH-based convex grating detection system according to claim 4, characterized in that: In step 4, the distance d2 between the first CGH compensator and the convex grating to be measured is equal to the distance d3 between the convex grating to be measured and the second CGH compensator.

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