1.Key Laboratory of High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2.National Laboratory on High Power Laser and Physics, Chinese Academy of Sciences and China Academy of Engineering Physics, Shanghai 201800, China 3.School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
Abstract:As one of the coherent diffractive imaging (CDI) techniques, coherent modulation imaging (CMI) is a lensless phase imaging technology with diffraction limited resolution in theory. Unlike multiple measurement phase retrieval algorithms, the CMI can achieve fast convergence speed with single-shot measurement by introducing a pre-characterized random phase modulator. Besides, it has simple structure without reference wave based on iterative engine. Despite the fact that the matured phase imaging can be used to implement the on-line wave diagnostics of laser pulse, in this work we accurately measure the face-type of optical component with peak-to-valley value below 0.5λ (λ = 632.8 nm) by using the CMI for the first time. In order to verify its measurement capability, 10 quartz windows with a diameter of 80 mm and PV value between 0.1λ and 0.5λ are repeatedly measured. Compared with the results of commercial interferometer, the root mean square error (Root MSE) of the peak-to-valley (PV) ratio of the results of the CMI is 0.0305λ, and the Root MSE of the root mean square (RMS) is 0.00522λ. The measurement accuracy of PV ratio and RMS can reach 0.1λ and 0.01λ respectively. In addition, the parallel flat with PV ratio = λ/20 is measured and analyzed with CMI, and its noise level is also analyzed. Considering that the potential improvement of CMI is available in the future, the CMI is expected to become a new technique for optical metrology with high precision, which is different from interferometry. Keywords:phase retrieval/ iterative engine/ optical metrology/ coherent diffractive imaging
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2.基本原理利用CMI实现光学元件检测的基本光路如图1(a)所示, 相干光源经过准直后通过会聚透镜会聚, 经过位于焦点附近的相位板调制后, 其衍射光斑由电荷耦合器件传感器(charge-coupled device sensor, CCD)记录, 为消除会聚透镜和准直光本身像差带来的影响, 待测元件紧贴会聚透镜位于准直光一侧, 当存在和不存在待测元件时, 假定会聚透镜的出射光波前分布分别为${\varphi _{\rm{P}}}$和${\varphi _{\rm{n}}}$, 则两者相位差$\delta = {\rm{angle}}\{ {{{\varphi _{\rm{P}}}} / {{\varphi _{\rm{n}}}}}\} $即为待测元件本身带来的相位变化, 其中${\rm{angle}}\{ \} $表示取相位. 图 1 (a) CMI测量光学元件的基本光路; (b) 实验装置照片; (c), (d) 由ePIE算法标定的随机相位板振幅和相位分布, (c)中标尺长度为0.198 mm Figure1. (a) Basic scheme for the measurement of optical components using CMI; (b) photo of the experimental setup; (c) amplitude and (d) phase of the center part of the random phase plate reconstructed by ePIE. The scale bar of (c) is 0.198 mm.
为了验证CMI应用于光学元件测量的可靠性, 定制了10片口径为80 mm, 相位峰谷值PV从0.1λ到0.45λ (λ = 632.8 nm)的石英窗口, 其中两片如图3(a)所示, 当存在待测窗口时, 其中一幅8 bit衍射光斑如图3(b)所示, 利用上述迭代算法计算得到的窗口对应相位分布图如图3(c)所示, 其中黑色虚线直径为79.1 mm, 略小于窗口口径, 通过计算可得到该区域内的PV值和均方根RMS值, 并作为最终测量结果, 同时每个石英窗口随机旋转一定角度后重复测量了10次, 每次重建时初始猜测为不同的随机数. 用于迭代的矩阵像素数是3072 × 3072, 为提升计算速度, 采用一块NVIDIA Tesla计算卡进行加速计算, 单次迭代约0.2 s, 共迭代500次. 图 3 (a) 作为被测物的石英窗口; (b) CCD记录的衍射光斑; (c) 通过相位相减得到的石英窗口相位图, 其中由黑色虚线标记的区域的直径为79.1 mm Figure3. (a) Photo of the plate glasses used in experiments; (b) diffraction pattern recorded by CCD; (c) phase map of plate glass obtained directly by phase subtraction. The section marked by the black dashed circle with a diameter of 79.1 mm is used for the analysis of PV and RMS. The constant phase slope is not removed for these calculations.
表1CMI和干涉仪的测量结果(λ) Table1.CMI and interferometer results (λ).
图 4 CMI和Zygo干涉仪的测量结果下, PV (a)和RMS (b)的最小二乘线性回归曲线 Figure4. Least-squares linear regressions of PV (a) and RMS (b) comparing the measurements from the CMI and Zygo interferometer.
图 5 分别由CMI和干涉仪测量的10个不同石英窗口的相位图 Figure5. Phase maps of ten different plate glasses measured by CMI and inteferometer.