Transient Wavefront Measurement Based On Near Infrared Reflective Shearing Point Diffraction Interferometer

A point diffraction interferometer(PDI) can be realized by using the near-perfect spherical wave diffracted by a pinhole as reference wavefront, which can satisfy high precision measurement. PDI uses common optical path structure. So the reference beam and test beam is difficult to separate which makes it difficult to shift phase. The interferogram just can be processed by interactive way that can’t meet the requirement of the automation testing. In order to measure the dynamic near infrared laser wavefront, near infrared reflective shearing point diffraction interferometer is proposed which can satisfy automation testing.Firstly, in order to measure the dynamic near infrared laser wavefront, a structure of oblique incidence of near infrared reflective shearing point diffraction interferometer is proposed. The point diffraction interferometer is integrated in flat substrate plated with special films. The coherent beams are reflected at the front and rear surfaces of the substrate respectively. The shear of the two beams introduces linear spatial carrier frequency to the point diffraction interferogram. The single shot interferogram is processed by Fourier transform method to retrieve the near infrared wavefront under test automatically.Secondly, the algorithm of dealing with the interferogram collected by near infrared reflective shearing point diffraction interferometer is studied. The accuracy of the algorithm is simulated. The accuracy of RMS is better than λ/50 by processing the simulated linear spatial carrier frequency interferogram containing spherical aberration, astigmatism, coma. The feasibility of the algorithm is proved.Thirdly, the key technology of near infrared reflective shearing point diffraction interferometer is studied. The quality of reference wavefront is proved by the size and sharp of the pinhole on the point diffraction plate. In order to match the intensity of reference light and test light and eliminate the stray light and multiple reflection light, the films are coated on the point diffraction interferometer. In order to solve the problem of the pinhole alignment in near infrared band, the method of the image alignment is proposed. Quick alignment of pinhole is realized.Finally, the experiment of near infrared reflective shearing point diffraction interferometer is studied. In order to verify the correctness and feasibility of the interferometer, measurement of transmitted wavefront of double lens optical system is set up. The result is in agreement with that obtained by Hartmann wavefront sensor. The measurement of a lens is set up and the result is also agreement with that obtained by the point diffraction interferometer of lateral defocus. The precision of near infrared reflective shearing point diffraction interferometer is tested. The PV is better than λ/1Oand the RMS is better than λ/100. The fringe contrast, light source monochromaticity and systematical error of near infrared reflective shearing point diffraction interferometer are discussed.