Random coded hybrid grating design for four-wave transverse shear interferometric wavefront detection

Transverse shear interference technology uses the wavefront itself to carry out dislocation interference, so as to achieve direct measurement of the phase of the wave front, because it adopts a common channel system, no reference beam, so the interference fringe is stable, strong anti-interference ability, simple instrument structure, can be used for short coherence length beam quality detection. Based on the above advantages, the transverse shear interferometry technique is commonly used in the inspection and measurement of optical materials and components, the detection of beam properties and parameters, the calibration, inspection and evaluation of optical systems.

The traditional transverse shear interferometer uses a plate or a prism as the wavefront splitter, and requires two optical systems to generate the absolutely orthogonal transverse shear interferogram along the x and y directions, and the system structure is relatively complex. The cross grating transverse shear interferometer uses a two-dimensional cross grating as a splitting element, which can diffract in the x direction and y direction at the same time, and produce diffracted light of different orders. Then, the order selection window selects the diffracted light, so that only the ±1 light in the x and y directions passes through, and other orders are blocked. Finally, shear interference occurs between the four beams of light through the order selection window. Although cross-grating transverse shear interferometer can directly obtain shear interferogram of two orthogonal directions to realize real-time detection of transient wavefront, the existence of hierarchical selection window leads to complex system adjustment structure. In the process of instrument adjustment, it is necessary to ensure that only level 1 light in x and y directions passes through the window, and other levels are completely blocked. Therefore, the precision of the instrument adjusting mechanism is high and the adjustment is difficult. Moreover, the size of the order selection window will affect the range of the wavefront distortion that can be measured. In addition, the position and size of the order selection window will also affect the transverse shear interference between the four beams, thus reducing the accuracy of the transient wavefront detection. Now the common four-wave transverse shear interferometer uses the modified Hartmann template (MHM) as the splitting element, and the phase information of the wave front to be detected can be obtained without the order selection window. The spectral element MHM consists of a checkerboard phase grating and an amplitude grating. The period of the phase grating is twice that of the amplitude grating, and the duty cycle of the amplitude grating is 2:3. The even-order light and ± 3-order diffractive light in the diffractive light field of MHM can be well eliminated. However, the diffraction light of ±5, ±7, ±11 and other high order still exists and affects the transverse shear interference between ±1 order, resulting in obvious difference in interferogram contrast at different observation positions. Therefore, wavefront detection can only be carried out at a limited Tabor distance and its integer multiple, which limits the selection of shear rate.

The author proposes a four-wave transverse shear interference wavefront detection system based on random coded mixed grating, and carries out in-depth research on random coded mixed grating, introduces the design principle and coding method of random coded mixed grating, and compares its Fraunhofer diffraction light field distribution with MHM and phase grating. It is found that only four orders exist in the diffraction field of randomly coded hybrid grating. Based on the grating equation and geometric relation, the system parameters such as incident beam aperture, grating distance and observation distance are analyzed and determined. The far-field spot distribution and four-wave transverse shear interferogram of the random coded mixed grating and MHM obtained by experiments are given respectively, which shows the obvious advantage of random coded mixed grating in four-wave transverse shear interferogram.