Research On Instantaneous Phase Shift Interferometry Based On Point Diffraction

Point diffraction interferometry（PDI）has provided an effective way for ultrahigh accuracy measurement of optical surface.As a nearly ideal spherical wavefront was diffracted by a micro size round pinhole,this method can get rid of the shape error limitation of the real reference lens and reach a nano or even sub-nano measurement accuracy.The traditional point diffraction interferometers mostly use high-precision piezoelectric transducer（PZT）as phase-shifter to drive the tested optics moving several times,so as to collect multiple interference images for phase extraction.This kinds of time-domain multi-step phase-shift PDI has two shortcomings,on the one hand,it depends on expensive imported PZT,on the other hand,it is easily be affected by environmental disturbance during its long time multi interference images acquisition,which will result in low actual measurement accuracy,especially low repeatability accuracy.If a synchronous phase-shift technology can be introduced into traditional PDI,multiple interference images with different phase-shifts can be obtained through singal measurement.It can realize transient measurement of ultra surface and expected to provide the possibility to solve the above bottleneck problems in the development of PDI.In order to meet the requirements of transient detection,a new transient PDI method based on polarization synchronous phase shift is proposed in this paper.With the help of checkerboard phase grating,the test light and reference light in the PDI measurement system are divided,and four ±1-order diffracted light through 2×2 polarizer array is phase shifted,and then four interference images with different phase shifts are obtained on the CCD at one time.Finally,the measured surface shape information is obtained by preprocessing,phase extracting,phase unwrapping and wave surface fitting for the four instantaneous interference images in a single frame.In this paper,the specific optical path of instantaneous phase-shift interference based on small hole point diffraction is constructed,and the related key technologies and systems are studied from both theoretical and experimental aspects.Firstly,for the detection of concave spherical surface with high reflection,the point diffraction synchronous phase-shift interferometry optical path of reflective semi common optical path is proposed.Based on Jones matrix theory,the theoretical analysis model of system polarization state and interference light intensity analysis model are constructed,and the feasibility of the method is verified theoretically.Secondly,through studying the specific implementation technology of instantaneous phase shift,the combination of two-dimensional phase grating splitting and polarization array phase-shift is selected.And based on the simulation analysis of the influence of grating and polarization array parameters on the measurement,the two-dimensional phase checkerboard grating,small aperture and polarization array are designed and processed.Thirdly,the realization of the experimental system of small hole point diffraction instantaneous phase-shift interference detection is studied.And the system design scheme,the key parameters in the system,the calibration of the azimuth between the optical axis of the quarter wave plate and the light transmission axis of the polarization array,and the pixel level registration of the preprocessed image are discussed.Finally,based on 2.5μm diffraction aperture,a real PDI phase-shift interferometry system is successfully built,which can complete the surface shape transient detection of 50 mm small aperture spherical mirror,and obtain the test results that the peak valley error（PV）is 0.314 λ and the root mean square error（RMS）is 0.0422λ.In order to verify the feasibility and correctness of the method proposed in this paper,the same measured object is detected by Zygo interferometer.The measurement results are the same surface shape,in which the PV difference is 0.01λ,RMS difference is 0.004 λ.Through the comparison and analysis of several groups of experiments,the repeatability accuracy of the proposed point diffraction transient interferometry system’s PV and RMS are better than λ/200 and λ/1600,which can verify that the system is feasible and has good stability.The system proposed in this paper not only takes into account the advantages of high accuracy of point diffraction and strong anti-interference ability of synchronous phase shift,but also has simple principle which is easy to implement.The research results are expected to establish a certain theoretical and technical foundation for the development of practical PDI.