The four-wave transverse shear interference wavefront detection system based on random coded mixed grating is mainly composed of random coded mixed grating and CCD, among which the random coded mixed grating includes amplitude coded grating and chess plate phase grating, as shown in Figure 1. The wave front to be detected is diffracted by random coded hybrid grating, and the two orthogonal directions respectively generate ±1 order diffracted light. Finally, the four diffracted light mispositions each other and interferes with each other during the propagation process, and the CCD is used to receive the shear interferogram.
FIG. 1 Four-wave transverse shear interference system based on randomly coded hybrid grating
The interferogram obtained on the CCD image plane is a transverse shear interferogram in two mutually orthogonal directions. The spectrum diagram is obtained by Fourier transform, and the +1 level spectrum in two orthogonal directions is filtered respectively. Then the inverse Fourier transform is applied to extract the shear differential interferogram in these two directions respectively. Finally, the wavefront to be detected can be reconstructed by differential Zernick fitting.
The ideal four-wave transverse shear interference requires that only four orders of equal light intensity exist in the diffraction field of the grating. Therefore, based on the desired ideal diffraction spectrum, a random coded hybrid grating composed of amplitude coded grating and phase grating can be obtained according to Fraunhofer's diffraction principle and the random coding method constrained by luminous flux. In addition, the aperture of the incident beam, the grating distance and the observation distance affect the shear rate and the wavefront measurement results, and are important parameters of the transverse shear interference system, which need to be considered in the research process.
A random coded hybrid grating is proposed for four-wave transverse shear interference wavefront detection. Only four equal intensity levels exist in the diffractive light field, which achieves the four-wave transverse shear interference well. The design principle of random coded hybrid grating and the method of determining system parameters such as grating spacing are introduced in detail. The Fraunhofer diffraction order distribution of random coded hybrid grating is obtained by computer simulation, and compared with MHM and phase grating, it shows the obvious advantage of random coded hybrid grating in four-wave transverse shear interference. The experiments verify that only four spots of equal intensity exist in the far-field diffraction field of the random coded hybrid grating, and the four-wave transverse shear interferogram based on the random coded hybrid grating is stable, and does not produce periodic Taber effect with the change of observation distance, and can achieve arbitrary distortion wavefront detection compared with the cross grating transverse shear interferometer.
There are only four levels in the Fraunhofer diffraction field, and the four-wave transverse shear interference is well realized. Therefore, the four-wave transverse shear interferometer based on the random coded hybrid grating is not only compact in structure, simple in instrument adjustment, stable in interference fringes, but also can detect arbitrary wavefront distortion. It can be used in wavefront sensing and wave aberration measurement of photographic projection objective.
