WO2016183874A1 - 数字相移点衍射干涉仪及光学系统波像差测量方法 - Google Patents

数字相移点衍射干涉仪及光学系统波像差测量方法 Download PDF

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WO2016183874A1
WO2016183874A1 PCT/CN2015/080420 CN2015080420W WO2016183874A1 WO 2016183874 A1 WO2016183874 A1 WO 2016183874A1 CN 2015080420 W CN2015080420 W CN 2015080420W WO 2016183874 A1 WO2016183874 A1 WO 2016183874A1
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spatial light
light modulator
optical system
phase shift
computer
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French (fr)
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戴凤钊
王向朝
唐锋
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中国科学院上海光学精密机械研究所
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
    • G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G01M11/02—Testing optical properties

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  • the invention belongs to the field of optical detection, and in particular relates to a digital phase shift point diffraction interferometer and a method for measuring wave aberration of an optical system.
  • Wave aberration measurement of optical imaging systems is very important for imaging quality control.
  • Interferometry is an important method for measuring the wave aberration of optical imaging systems.
  • the traditional interferometry uses a spherical reference mirror as a reference surface to generate a spherical wave as a reference light wave, which interferes with the light wave of the object to be measured to form interference fringes.
  • the measurement accuracy of this type of method is limited by the surface accuracy of the reference surface. Due to the fabrication technique and cost limitation of the high-precision reference surface shape, such methods cannot meet the high-precision wave aberration measurement requirements.
  • Smartt proposes a point diffraction interferometry technique (prior art [1]: RN Smartt and WHSteel, Jpn. J. Appl. Phys.
  • Point diffraction produces a reference spherical wave instead of the traditional method of generating a reference spherical wave using a reference surface, eliminating the need for a high-precision reference surface.
  • the measurement accuracy is no longer limited by the manufacturing accuracy of the reference surface, and high measurement accuracy can be achieved.
  • the extreme ultraviolet lithography projection objective wavefront aberration detecting interferometer developed by the American Berkeley National Laboratory and the Japanese EUVA and other organizations based on the technology has a detection repeatability of 0.1 nm RMS (root mean square value).
  • the point diffraction interferometer is a common optical path structure, and the reference light and the object light path are almost the same, making it insensitive to environmental disturbances and mechanical vibrations, but also making the interference fringe density very small, which is not suitable for the Fourier transform method.
  • Phase extraction is extracted from the interferogram, so phase-shifting phase extraction techniques are the best choice. Since the object light and the reference light are almost in common, the introduction of the phase shift is very difficult.
  • H. Medecki proposes a phase shift point diffraction interferometer (prior art [2]: H. Medecki, E.
  • phase shift by PZT mechanically moving grating introduces phase shift by PZT mechanically moving grating.
  • the phase shift accuracy is affected by PZT nonlinear effects (such as hysteresis effect, thermal drift, etc.), and in order to achieve accurate phase shift, accurate voltage calibration is required.
  • V. Akondi proposes a phase shift point diffraction interferometer (prior art [3]: Vyas Akondi, A.R.Jewel, And Brian Vohnsen, "Digital phase-shifting point diffraction interferometer," Opt. Lett.
  • This method uses a digital method to introduce a phase shift, which avoids the problem of phase shift inaccuracy using PZT mechanical movement, but the distance between the pinhole and the window is limited, and the center of the window is at the focus of the optical system.
  • the pinhole must be placed near the focus to ensure that sufficient light intensity is diffracted through the aperture, so that the reference light diffracted through the aperture matches the intensity of the object light passing through the window to achieve higher interference fringes. Contrast. If the distance between the pinhole and the window is too large, the light intensity transmitted through the pinhole will be weak, resulting in poor contrast of the interference fringes. Due to the limitation of the distance between the pinhole and the window, the interference fringe density is also relatively small, and a large interference fringe density cannot be achieved.
  • the object of the present invention is to overcome the above-mentioned deficiencies of the prior art, and to provide a digital phase shift point diffraction interferometer and an optical system wave aberration measuring method, wherein the interferometer structure respectively places a spatial light in front of the optical system to be tested and on the image plane.
  • a modulator that sets a spatial light modulator in front of the optical system as a grating by a computer, and digitally shifts the grating by moving the grating digitally in a direction perpendicular to the grating line, and sets the spatial light modulator of the image plane as a pinhole window mask
  • the distance between the image mask pinhole and the window can be adjusted, the grating period is small, the distance between the pinhole windows is large, and the grating period is large, and the pinhole is The distance between the windows is small, thereby avoiding the problem of poor contrast of the interference fringes and also making the density of the interference fringes adjustable.
  • a digital phase shift point diffraction interferometer characterized in that the interferometer comprises a light source, and the output beam direction along the light source is a small aperture mask, a first spatial light modulator, a second spatial light modulator and two dimensions.
  • a photodetector the output end of the two-dimensional photodetector being connected to an input end of the computer;
  • the first spatial light modulator and the second spatial light modulator being respectively placed in the first XYZ three-dimensional stage and the second XYZ three-dimensional a shifting stage; the input ends of the first spatial light modulator and the second spatial light modulator are respectively connected to an output end of the computer;
  • the aperture mask is a square visor having a light-transmissive aperture in the center of the visor, the diameter of the aperture being smaller than the object resolution of the optical system to be tested;
  • the first spatial light modulator is a transmissive spatial light modulator, and is controlled by a computer to generate a digital grating.
  • the wavefront to be measured is diffracted into multi-level diffracted light, and the grating line along the first spatial light modulator
  • the x direction or the y direction includes a light transmitting portion and an opaque portion;
  • the second spatial light modulator is a transmissive spatial light modulator, and is controlled by a computer to generate a digital pinhole
  • the window mask is used as a filter element to filter diffracted light other than 0 and +1 or -1.
  • the 0th order light is diffracted through the aperture to generate a spherical wave as the reference light, and the +1 or -1 order light passes.
  • the window serves as the object light, and the mask comprises a light-transmissive aperture and a light-transmissive window, and other portions of the mask are set to be opaque;
  • the two-dimensional photosensor is a CCD, CMOS or two-dimensional photodetector array.
  • the grating period p is set, and the grating period p is an integer of the pixel width of the first spatial light modulator.
  • the optical system to be tested is placed between the first spatial light modulator and the second spatial light modulator, and the optical system to be tested is adjusted so that the small aperture mask is in the object surface of the optical system to be tested, so that the aperture mask is transparent.
  • the center of the small hole is located on the optical axis of the digital phase shifting point diffraction interferometer, and the displacement stage of the second spatial light modulator is adjusted so that the surface of the second spatial light modulator is perpendicular to the optical axis and is in the optical system to be tested
  • the surface of the first spatial light modulator is adjusted such that the surface of the first spatial light modulator is perpendicular to the optical axis and the distance from the object aperture mask is said d;
  • the computer sets the first spatial light modulator as a grating, the raster line direction is along the y direction, and the grating period is set to a set value p; the computer sets the second spatial light modulator in the image plane as a pinhole window mask, so that the computer
  • the center position of the pinhole is located on the optical axis, the width of the pinhole is smaller than the image resolution of the optical system to be tested, and the position of the center of the window in the image plane two-dimensional coordinate system is (x 1 , 0
  • the amplitude transmittance of the light-transmitting portion of the pinhole window mask is set to 1, and the amplitude transmittance of the other portion is set to 0; the light source is turned on, and the interference light intensity signal I x1 is detected by the photodetector, the I x1 Incoming computer save;
  • the computer controls the first spatial light modulator grating to be translated in the +x or -x direction, the translation amount is the ⁇ p, the interference light intensity signal I x2 is detected by the photodetector, and the I x2 is transmitted to the computer for storage; Repeat n times, n is the phase shift step of the phase shift algorithm, and finally the computer saves n interference light intensity signals I x1 , I x2 , ..., I xn ;
  • the n interferograms stored by the computer are used to calculate the phase and perform the unwrapping (see See TRJudge and PJ Bryanston-Cross, "A review of phase unwrapping techniques in fringe analysis,” Opt.Lasers Eng. 21, 199-239 (1994)), to eliminate geometrical optical path differences (see: KAGoldberg, "Extreme ultraviolet Interferometry, "Ph. D. thesis (University of California, Berkeley, Berkeley, Calif., 1997).), obtains the wave aberration of the optical system to be tested.
  • the principle of the invention is that two spatial light modulators are used, and the two are respectively set as a digital grating and a digital pinhole window filter by a computer, and the two can cooperate to make the distance between the pinhole windows adjustable, thereby A higher interference fringe contrast is obtained.
  • the interference fringe density is adjustable accordingly.
  • the grating period is set larger, the distance between the pinhole windows on the filter mask is smaller, the interference fringe density is correspondingly smaller, and the grating period is set smaller, and the pinhole on the filter mask is filtered.
  • the distance between the windows is large, and the interference fringe density is also relatively large.
  • a phase shift can be introduced between the object light (i.e., the 0th order diffracted light) and the reference light (i.e., the 1st order or the -1st order diffracted light) by the digital moving grating.
  • the present invention has the following advantages:
  • the present invention can introduce a phase shift between the object light and the reference light as compared with the prior art [1];
  • the invention adopts the digital phase shift, avoids the problem that the phase shift is inaccurate and the voltage needs to be accurately calibrated by using the PZT phase shift, and the phase shift is more precise and convenient;
  • the reference light and object light of the present invention are taken from the 0th order and 1st order diffracted light of the grating, so that the distance between the object light and the reference light image collecting point is adjustable, thereby making the interference fringe Density and contrast are adjustable.
  • FIG. 1 is a schematic structural view of a digital phase shift point diffraction interferometer of the present invention
  • FIG. 2 is a schematic diagram of a surface aperture mask in a digital phase shifting point diffraction interferometer of the present invention
  • 3 is a first space in the optical system for performing wave aberration measurement using the digital phase shift point diffraction interferometer of the present invention
  • Figure 4 is a second spatial light modulator for performing wave aberration measurement using the digital phase shift point diffraction interferometer of the present invention.
  • the digital phase shifting point diffraction interferometer of the present invention is characterized in that the interferometer includes a light source 1, along the The output beam direction of the light source 1 is, in order, an aperture mask 2, a first spatial light modulator 3, a second spatial light modulator 6, and a two-dimensional photodetector 8, the output of the two-dimensional photodetector 8 and the computer 9 Inputs connected; said A spatial light modulator 3 and a second spatial light modulator 6 are respectively disposed on the first XYZ three-dimensional stage 4 and the second XYZ three-dimensional stage 7; the first spatial light modulator 3 and the second spatial light modulation The input ends of the device 6 are respectively connected to the output end of the computer 9;
  • the aperture mask 2 is a square visor having a light-transmissive aperture 21 in the center of the visor.
  • the diameter of the aperture 21 is smaller than that of the optical system 5 to be tested.
  • the first spatial light modulator 3 is a transmissive spatial light modulator controlled by a computer 9 to generate a digital grating.
  • the wavefront to be measured is diffracted into multi-level diffracted light, and the first space is
  • the grating line of the light modulator 3 includes a light transmitting portion 31 and an opaque portion 32 in the x direction or the y direction;
  • the second spatial light modulator 6 is a transmissive spatial light modulator controlled by a computer 9 to generate a digital pinhole window mask as a filter component, which will be divided into 0 levels and +1 or -1 stages.
  • the external diffracted light is filtered out, the 0th order light is diffracted through the aperture to generate a spherical wave as the reference light, and the +1 or -1 order light passes through the window as the object light, and the mask includes the light transmitting aperture 61 and the light transmitting window 62.
  • the other part of the mask is set to be opaque;
  • the two-dimensional photosensor 8 is a CCD, CMOS or two-dimensional photodetector array.
  • the method for performing optical aberration measurement of an optical system using the digital phase shift point diffraction interferometer includes the following steps:
  • the number of interference fringes N is set, generally between 20 and 100, and the grating period p is set according to the pixel size of the first spatial light modulator 3 used, and the grating period p is the first spatial light modulator 3 pixel width.
  • the optical system 5 to be tested is placed between the first spatial light modulator 3 and the second spatial light modulator 6, and the optical system 5 to be tested is adjusted so that the aperture mask 2 is in the object surface of the optical system 5 to be tested, so that the small
  • the center of the light-transmissive aperture 21 of the aperture mask 2 is located on the optical axis of the digital phase shifting point diffraction interferometer, and the displacement stage 7 of the second spatial light modulator 6 is adjusted so that the surface of the second spatial light modulator 6 is perpendicular to
  • the optical axis is in the object plane of the optical system 5 to be tested, and the displacement stage of the first spatial light modulator 3 is adjusted such that the surface of the first spatial light modulator 3 is perpendicular to the optical axis and is opposite to the object aperture mask 2
  • the distance between them is the stated d;
  • the computer 9 sets the first spatial light modulator 3 as a grating with the raster line direction in the y direction and the grating period set to the set value p; the computer 9 sets the second spatial light modulator 6 in the image plane as a pinhole window mask.
  • the center of the pinhole 61 is located on the optical axis, the width of the pinhole 61 is smaller than the image resolution of the optical system 5 to be tested, and the center of the window 62 is in the image plane two-dimensional coordinate system.
  • the position is (x 1 , 0), the transmission transmittance of the transparent portion of the pinhole window mask is set to 1, and the amplitude transmission of other portions is set to 0; the light source 1 is turned on, and is detected by the photodetector 8. Interfering with the light intensity signal I x1 , the I x1 is transmitted to the computer 9 for storage;
  • the computer 9 controls the first spatial light modulator 3 grating to be translated in the +x or -x direction, the translation amount is the ⁇ p, the interferometric light intensity signal I x2 is detected by the photodetector 8, and I x2 is transmitted
  • the computer 9 saves; repeats n times, n is the phase shift step of the phase shift algorithm, and finally the computer 9 stores n pieces of interference light intensity signals I x1 , I x2 , ..., I xn ;
  • the n-step phase shift algorithm is used to calculate the phase from the n interferograms stored by the computer 9, and the process of unwrapping and eliminating the geometric optical path difference is performed to obtain the wave aberration of the optical system 5 to be tested.
  • the light source adopts He-Ne laser, the wavelength ⁇ is 632.8 nm, and the number of interference fringes N is 20.
  • the first and second spatial light modulators adopt the amplitude-type transmissive spatial light modulator developed by the Chinese Academy of Sciences Xi'an Opto-mechanical Co., Ltd., and the pixel width is 8.5 ⁇ m, the grating period of the first spatial light modulator is set to 4 pixels width, that is, 34 ⁇ m, the optical system image to be tested has an image numerical aperture of 0.01, the magnification is 1/5, and the image resolution is 31.6 ⁇ m (0.5 ⁇ / NA I ), the object side resolution is 158 ⁇ m, and the distance d between the first spatial light modulator 3 and the aperture mask 2 is calculated to be 17 cm, and the diameter of the light transmission aperture on the object aperture mask is set to 10 ⁇ m.
  • the substrate has a thickness of 100 nm and the material is chrome.
  • the diameter of the small hole on the square aperture window mask is set to one pixel, that is, 8.5 ⁇ m, and the window width is set to 50 pixels or 425 ⁇ m, and the distance between the center of the small hole and the center of the window is set to 632.8 ⁇ m, about 75 pixels, the experiment uses a three-step phase shift algorithm, the phase shift amount per step is ⁇ /2, and the calculation formula is For the phase distribution to be measured, I 1 , I 2 , and I 3 are the intensity distributions of the interferogram obtained by three phase shifts.
  • the moving distance of each step of the grating is p/4, that is, 8.5 ⁇ m, and the number of pixels of the two-dimensional photodetector is 1024 ⁇ 768. CCD.
  • the present invention can generate a phase shift by means of a digital moving grating, avoiding the problem that the PZT phase shift needs to accurately calibrate the voltage and the nonlinear effect, and the object light and the reference light are between the image surface convergence points.
  • the distance is adjustable to achieve a large interference fringe density without reducing the interference fringe contrast.
  • the main limiting factor of the present invention is that the spatial size of the spatial light modulator is currently a few micrometers, so that the period of the grating and the diameter of the pinhole can only be on the order of a few micrometers, and the numerical aperture that can be measured is very small, or at a large value.
  • the measurement accuracy under the aperture is not high and the application range is limited.
  • the pixel size will become smaller and smaller, and the application range of the present invention will become wider and wider, and the measurement accuracy will be higher and higher.

Abstract

一种数字相移点衍射干涉仪，该干涉仪由光源（1）、小孔掩模（2）、第一空间光调制器（3）、第二空间光调制器（6）、二维光电探测器（8）及计算机（9）组成，通过计算机（9）将第一空间光调制器（3）设置为光栅，作为分光器件，将第二空间光调制器（6）设置为针孔窗口掩模，作为滤波器件，滤除除0级与+1或-1级以外的衍射级次，使0级光通过针孔（61）发生衍射产生准理想球面波作为参考光波，+1或-1级光通过窗口（62）作为物光波，两者发生干涉，获取干涉图，从干涉图中提取待测光学系统波像差。还提供了一种利用数字相移点衍射干涉仪对光学系统进行波像差测量的方法。通过物光和参考光在像面汇聚点之间的距离可调，在不降低干涉条纹对比度的情况下实现较大的干涉条纹密度。

Description

数字相移点衍射干涉仪及光学系统波像差测量方法 技术领域
本发明属于光学检测领域，具体涉及一种数字相移点衍射干涉仪及光学系统波像差测量方法。
背景技术
光学成像系统的波像差测量对于成像质量控制非常重要。干涉测量法是光学成像系统波像差测量的一种重要方法，传统的干涉法以球面参考反射镜作为参考面产生球面波作为参考光波，与待测的物光波产生干涉，形成干涉条纹。这类方法的测量精度受限于参考面的面形精度，由于高精度参考面面形的制作技术和成本的限制，使得这类方法不能满足高精度的波像差测量需求。Smartt提出了一种点衍射干涉测量技术(在先技术[1]:R.N.Smartt and W.H.Steel,Jpn.J.Appl.Phys.14,Suppl.14-1,351(1975))，这种方法使用针孔点衍射产生参考球面波，代替传统的使用参考面产生参考球面波的方法，消除了对高精度参考面的需求，测量精度不再受限于参考面的制造精度，可以实现很高的测量精度，例如美国伯克利国家实验室、日本EUVA等组织基于该技术研发的极紫外光刻投影物镜波像差检测干涉仪，检测重复性达到了0.1nm RMS(均方根值)。这种点衍射干涉仪是共光路结构，参考光和物光光程几乎相同，使其对环境扰动和机械振动不敏感，但是也使得干涉条纹密度非常小，不适合采用傅里叶变换法从干涉图中提取相位，因此相移法相位提取技术成为最佳选择。由于物光和参考光几乎共光路，使得相移的引入非常困难。H.Medecki提出一种相移点衍射干涉仪(在先技术[2]：H.Medecki,E.Tejnil,K.A.Goldberg,et al.,Phase-shifting point diffraction interferometer,Optics Letters,21(19),1526-1528(1996))，采用光栅做为分光器件，将待测波前分为多个衍射级次，在像面使用针孔窗口掩模作为滤波器，使+1级(或-1级)衍射光通过窗口作为物光，零级衍射光通过小孔衍射为理想球面波作为参考光，其他级次的衍射光被针孔窗口掩模滤除，通过在垂直于光轴和光栅线条的平面内移动光栅，在零级光和+1级(或-1级)光之间引入相移。这种方法通过PZT机械移动光栅引入相移，相移精度受PZT非线性效应(如迟滞效应、热漂移等)的影响，且为实现精确的相移，需要对电压进行精确校准。V.Akondi提出一种相移点衍射干涉仪(在先技术[3]：Vyas Akondi,A.R.Jewel, and Brian Vohnsen,"Digital phase-shifting point diffraction interferometer,"Opt.Lett.39,1641-1644(2014))，在待测光学系统的像面放置一个空间光调制器，将空间光调制器设置为一个针孔窗口掩模，待测波前一部分通过窗口作为物光波，一部分通过针孔衍射产生球面波，作为参考光波。通过计算机控制改变空间光调制器针孔窗口掩模上的针孔的相位在物光和参考光之间引入相移。这种方法使用数字的方法引入相移，避免了使用PZT机械移动存在的相移不准等问题，但是这种方法针孔和窗口之间的距离受限，窗口的中心在光学系统的焦点，针孔必须设置在焦点的附近，以保证有足够的光强通过小孔发生衍射，从而使通过小孔发生衍射的参考光和通过窗口的物光的光强相匹配，实现较高的干涉条纹对比度。如果针孔和窗口之间的距离过大，透过针孔的光强会很弱，从而使得干涉条纹对比度很差。由于针孔和窗口之间距离的限制，也使得干涉条纹密度比较小，不能实现较大的干涉条纹密度。
发明内容
本发明的目的在于克服上述在先技术的不足，提供一种数字相移点衍射干涉仪与光学系统波像差测量方法，该干涉仪结构在待测光学系统前和像面分别放置一个空间光调制器，通过计算机将光学系统前的空间光调制器设置为光栅，通过在垂直于光栅线条的方向数字移动光栅可以实现数字相移，将像面的空间光调制器设置为针孔窗口掩模，通过两个空间光调制器的配合使用，可以使像面掩模针孔和窗口之间的距离可调，光栅周期小，则针孔窗口之间的距离大，光栅周期大，则针孔窗口之间的距离小，从而避免干涉条纹对比度差的问题，同时也可使干涉条纹密度可调。
本发明的技术解决方案如下：
1、一种数字相移点衍射干涉仪，其特征在于该干涉仪包括光源，沿该光源输出光束方向依次是小孔掩模、第一空间光调制器、第二空间光调制器和二维光电探测器，该二维光电探测器的输出端与计算机的输入端相连；所述的第一空间光调制器和第二空间光调制器分别置于第一XYZ三维位移台和第二XYZ三维位移台上；所述的第一空间光调制器和第二空间光调制器的输入端分别与计算机的输出端相连；
所述的小孔掩模是一块方形的遮光板，在该遮光板的中心有一个透光的小孔，该透光小孔的直径小于待测光学系统的物方分辨率；
所述的第一空间光调制器为透射式空间光调制器，受计算机控制产生数字光栅，作为分光器件，将待测波前衍射为多级衍射光，第一空间光调制器的光栅线条沿x方向或者y方向，包含透光部分和不透光部分；
所述的第二空间光调制器为透射式空间光调制器，受计算机控制产生数字针孔 窗口掩模，作为滤波器件，将除0级与+1或-1级外的衍射光滤除，0级光透过小孔发生衍射产生球面波作为参考光，+1或-1级光通过窗口作为物光，掩模包含透光小孔和透光窗口，掩模其他部分设置为不透光；
所述的二维光电传感器是CCD、CMOS或二维光电探测器阵列。
2、利用权利要求1所述的数字相移点衍射干涉仪对光学系统进行波像差测量的方法，其特征在于该方法包含以下步骤：
1)计算干涉仪系统的参数：
设定干涉条纹数N，一般在20～100之间取值，根据采用的第一空间光调制器的像素大小，设定光栅周期p，光栅周期p为第一空间光调制器像素宽度的整数倍，在设定的干涉条纹数N及光栅周期p的条件下，由所述的光源的波长λ及待测光学系统的物方数值孔径NAO，根据式d＝N×p/(2NAO)计算第一空间光调制器与小孔掩模之间的距离d；由设定的干涉条纹数N和待测光学系统的像方数值孔径NAI由下式x1＝N×λ/2NAI计算1级衍射光在像面汇聚点的位置x1；根据拟采用的相移算法及相移步数n及每一步的相移量
Figure PCTCN2015080420-appb-000001
计算每一步所述的第一空间光调制器的数字光栅需要移动的距离Δp，计算公式为
Figure PCTCN2015080420-appb-000002
为相移量；
2)搭建干涉仪的测量系统：
将待测光学系统置于第一空间光调制器和第二空间光调制器之间，调整待测光学系统使小孔掩模处于待测光学系统的物面，使小孔掩模的透光小孔的中心位于数字相移点衍射干涉仪的光轴上，调整第二空间光调制器的位移台，使第二空间光调制器的表面垂直于光轴，并处于待测光学系统的物面，调整第一空间光调制器的位移台，使第一空间光调制器的表面垂直于光轴且与物面小孔掩模之间的距离为所述的d；
3)进行光学系统波像差测量：
计算机将第一空间光调制器设置为光栅，光栅线条方向沿y方向，光栅周期设置为设定值p；计算机将处于像面的第二空间光调制器设置为针孔窗口掩模，使所述的针孔的中心位置位于光轴上，该针孔的宽度小于待测光学系统的像方分辨率，所述的窗口的中心在像面二维坐标系中的位置为(x1,0)，所述的针孔窗口掩模的透光部分振幅透过率设置为1，其他部分振幅透过率设置为0；开启光源，由光电探测器探测干涉光强信号Ix1，该Ix1传入计算机保存；
计算机控制第一空间光调制器光栅，使其沿+x或-x方向平移，平移量为所述的Δp，由光电探测器探测干涉光强信号Ix2，并将Ix2传入计算机保存；共重复n次，n为相移算法的相移步数，最终计算机保存n幅干涉光强信号Ix1、Ix2、…、Ixn；
采用n步相移算法，由计算机存储的n幅干涉图计算出相位，并进行解包裹(参 见T.R.Judge and P.J.Bryanston-Cross,“A review of phase unwrapping techniques in fringe analysis,”Opt.Lasers Eng.21,199～239(1994))、消除几何光程差的处理(参见：K.A.Goldberg,“Extreme ultraviolet interferometry,”Ph.D.dissertation(University of California,Berkeley,Berkeley,Calif.,1997).)，获得待测光学系统的波像差。
本发明的原理为：采用两个空间光调制器，通过计算机分别将二者设置为数字光栅和数字针孔窗口滤波器，两者配合，可以使针孔窗口之间的距离可调，从而可以获得较高的干涉条纹对比度。干涉条纹密度相应可调，光栅周期设置较大则滤波掩模上的针孔窗口之间的距离较小，干涉条纹密度也相应较小，光栅周期设置较小，则滤波掩模上的针孔窗口之间的距离较大，干涉条纹密度也相应较大。通过数字移动光栅可以在物光(即0级衍射光)与参考光(即1级或-1级衍射光)之间引入相移。
本发明与在先技术相比，具有以下优点：
1、与在先技术[1]相比，本发明可以在物光和参考光之间引入相移；
2、与在先技术[2]相比，本发明采用可以实现数字相移，避免了使用PZT相移带来的相移不准、电压需要精确校准等问题，相移更精确、方便；
3、与在先技术[3]相比，本发明参考光和物光取自光栅的0级和1级衍射光，使得物光和参考光像面汇聚点的距离可调，从而使得干涉条纹密度和对比度可调。
附图说明
图1为本发明数字相移点衍射干涉仪结构示意图；
图2为本发明数字相移点衍射干涉仪中的物面小孔掩模示意图；
图3为用本发明数字相移点衍射干涉仪进行光学系统波像差测量时在第一空间
光调制器上设置的光栅示意图；
图4为用本发明数字相移点衍射干涉仪进行波像差测量时在第二空间光调制器
上设置的针孔窗口掩模示意图；
具体实施方式
下面结合实施例和附图对本发明作进一步说明，但不应以此限制本发明的保护范围。
先请参阅图1，图1为本发明数字相移点衍射干涉仪的光路结构图，由图1可见，本发明数字相移点衍射干涉仪，其特征在于该干涉仪包括光源1，沿该光源1输出光束方向依次是小孔掩模2、第一空间光调制器3、第二空间光调制器6和二维光电探测器8，该二维光电探测器8的输出端与计算机9的输入端相连；所述的第 一空间光调制器3和第二空间光调制器6分别置于第一XYZ三维位移台4和第二XYZ三维位移台7上；所述的第一空间光调制器3和第二空间光调制器6的输入端分别与计算机9的输出端相连；
请参阅图2，所述的小孔掩模2是一块方形的遮光板，在该遮光板的中心有一个透光的小孔21，该透光小孔21的直径小于待测光学系统5的物方分辨率；
请参阅图3，所述的第一空间光调制器3为透射式空间光调制器，受计算机9控制产生数字光栅，作为分光器件，将待测波前衍射为多级衍射光，第一空间光调制器3的光栅线条沿x方向或者y方向，包含透光部分31和不透光部分32；
请参阅图4，所述的第二空间光调制器6为透射式空间光调制器，受计算机9控制产生数字针孔窗口掩模，作为滤波器件，将除0级与+1或-1级外的衍射光滤除，0级光透过小孔发生衍射产生球面波作为参考光，+1或-1级光通过窗口作为物光，掩模包含透光小孔61和透光窗口62，掩模其他部分设置为不透光；
所述的二维光电传感器8是CCD、CMOS或二维光电探测器阵列。
使用所述的数字相移点衍射干涉仪进行光学系统进行波像差测量的方法包含以下步骤：
1)计算干涉仪系统的参数：
设定干涉条纹数N，一般在20～100之间取值，根据采用的第一空间光调制器3的像素大小，设定光栅周期p，光栅周期p为第一空间光调制器3像素宽度的整数倍，在设定的干涉条纹数N及光栅周期p的条件下，由所述的光源1的波长λ及待测光学系统5的物方数值孔径NAO，根据式d＝N×p/(2NAO)计算第一空间光调制器3与小孔掩模2之间的距离d；由设定的干涉条纹数N和待测光学系统5的像方数值孔径NAI由下式x1＝N×λ/2NAI计算1级衍射光在像面汇聚点的位置x1；根据拟采用的相移算法及相移步数n及每一步的相移量
Figure PCTCN2015080420-appb-000003
计算每一步所述的第一空间光调制器3的数字光栅需要移动的距离Δp，计算公式为
Figure PCTCN2015080420-appb-000004
为相移量；
2)搭建干涉仪的测量系统：
将待测光学系统5置于第一空间光调制器3和第二空间光调制器6之间，调整待测光学系统5使小孔掩模2处于待测光学系统5的物面，使小孔掩模2的透光小孔21的中心位于数字相移点衍射干涉仪的光轴上，调整第二空间光调制器6的位移台7，使第二空间光调制器6的表面垂直于光轴，并处于待测光学系统5的物面，调整第一空间光调制器3的位移台，使第一空间光调制器3的表面垂直于光轴且与物面小孔掩模2之间的距离为所述的d；
3)进行光学系统波像差测量：
计算机9将第一空间光调制器3设置为光栅，光栅线条方向沿y方向，光栅周 期设置为设定值p；计算机9将处于像面的第二空间光调制器6设置为针孔窗口掩模，使所述的针孔61的中心位置位于光轴上，该针孔61的宽度小于待测光学系统5的像方分辨率，所述的窗口62的中心在像面二维坐标系中的位置为(x1,0)，所述的针孔窗口掩模的透光部分振幅透过率设置为1，其他部分振幅透过率设置为0；开启光源1，由光电探测器8探测干涉光强信号Ix1，该Ix1传入计算机9保存；
计算机9控制第一空间光调制器3光栅，使其沿+x或-x方向平移，平移量为所述的Δp，由光电探测器8探测干涉光强信号Ix2，并将Ix2传入计算机9保存；共重复n次，n为相移算法的相移步数，最终计算机9保存n幅干涉光强信号Ix1、Ix2、…、Ixn；
采用n步相移算法，由计算机9存储的n幅干涉图计算出相位，并进行解包裹、消除几何光程差的处理，获得待测光学系统5的波像差。
下面给一个实施例的具体参数：
光源采用He-Ne激光器，波长λ为632.8nm，干涉条纹数N取20，第一和第二空间光调制器均采用中科院西安光机所研发的振幅型透射式空间光调制器，像素宽度为8.5μm，第一空间光调制器的光栅周期设置为4个像素宽度，即34μm，待测光学系统像方数值孔径0.01，放大倍率为1/5，像方分辨率为31.6μm(0.5λ/NAI)，物方分辨率为158μm，计算第一空间光调制器3与小孔掩模2之间的距离d为17cm，物方小孔掩模上的透光小孔的直径设置为10μm，基板厚度为100nm，材料为铬，像方小孔窗口掩模上的小孔直径设置为一个像素，即8.5μm，窗口宽度设置为50像素即425μm，小孔中心和窗口中心的距离设置为632.8μm，约75个像素，实验采用三步相移算法，每步相移量π/2，计算公式为
Figure PCTCN2015080420-appb-000005
为待测相位分布，I1、I2、I3为三次相移得到的干涉图强度分布，光栅每一步移动距离p/4，即8.5μm，二维光电探测器为像素数为1024×768的CCD。
与在先技术相比，本发明可以通过数字移动光栅的方式产生相移，避免了PZT相移需要精确校准电压及非线性效应的问题，而且物光和参考光在像面汇聚点之间的距离可调，可以在不降低干涉条纹对比度的情况下实现较大的干涉条纹密度。
本发明的主要限制因素在于目前空间光调制器的像素尺寸最小为数微米，使得光栅的周期和针孔的直径都只能做到数微米量级，能够测量的数值孔径非常小，或者在大数值孔径下的测量精度不高，应用范围受到限制。但是随着空间光调制器技术的发展，像素尺寸会越来越小，本发明的应用范围也将越来越广，测量精度也会越来越高。
Figure PCTCN2015080420-appb-000006

Claims (1)

  1. 将待测光学系统(5)置于第一空间光调制器(3)和第二空间光调制器(6)之间，调整待测光学系统(5)使小孔掩模(2)处于待测光学系统(5)的物面，使小孔掩模(2)的透光小孔(21)的中心位于数字相移点衍射干涉仪的光轴上，调整第二空间光调制器(6)的位移台(7)，使第二空间光调制器(6)的表面垂直于光轴，并处于待测光学系统(5)的物面，调整第一空间光调制器(3)的位移台，使第一空间光调制器(3)的表面垂直于光轴且与物面小孔掩模(2)之间的距离为所述的d；
    3)进行光学系统波像差测量：
    计算机(9)将第一空间光调制器(3)设置为光栅，光栅线条方向沿y方向，光栅周期设置为设定值p；计算机(9)将处于像面的第二空间光调制器(6)设置为针孔窗口掩模，使所述的针孔(61)的中心位置位于光轴上，该针孔(61)的宽度小于待测光学系统(5)的像方分辨率，所述的窗口(62)的中心在像面二维坐标系中的位置为(x1,0)，所述的针孔窗口掩模的透光部分振幅透过率设置为1，其他部分振幅透过率设置为0；开启光源(1)，由光电探测器(8)探测干涉光强信号Ix1，该Ix1传入计算机(9)保存；
    计算机(9)控制第一空间光调制器(3)光栅，使其沿+x或-x方向平移，平移量为所述的Δp，由光电探测器(8)探测干涉光强信号Ix2，并将Ix2传入计算机(9)保存；共重复n次，n为相移算法的相移步数，最终计算机(9)保存n幅干涉光强信号Ix1、Ix2、…、Ixn；
    采用n步相移算法，由计算机(9)存储的n幅干涉图计算出相位，并进行解包裹、消除几何光程差的处理，获得待测光学系统(5)的波像差。
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