Design And Error Analysis Of Randomly Encoded Hybrid Grating For Wavefront Sensing

In the Inertial Confinement Fusion (ICF), the high-power laser provides high intensity energy uniformly on the capsule which is filled with deuterium-tritium, which provides abundant and safe energy for human, and the density uniformity of D/T layer is one of the important parameters for successful ignition. In order to obtain the three-dimensional density field information of the target, the interference method is used for detecting the wavefront which carries the density uniformity information of the target, and to obtain the refractive index distribution inside the microspheres, which provides the basis for detecting the three-dimensional density field information of the target.This paper proposes using the quadriwave lateral shearing interferometer to detect the wavefront which carries the density uniformity information of the target, and a new type of randomly encoded hybrid grating is designed innovatively where only four diffraction orders exist in its diffraction field. The randomly encoded hybrid grating which includes an amplitude encoded grating and a phase chessboard is obtained according to the Fraunhofer diffraction principle and the randomly coding method based on the quantization of radiant flux. Compared with the modified Hartmann mask (MHM) and the phase grating, only four diffraction orders exist in its diffraction field, which reflects the obvious advantages of the randomly encoded hybrid grating in quadriwave lateral shearing interference.System parameters of the quadriwave lateral shearing interferometer are analyzed and determined. The shearing ratio of the quadriwave lateral shearing interferometer system is analyzed according to the grating equation, the results shows that the shearing ratio increases nonlinearly with the testing beam diameter increasing, whereas decreases nonlinearly with the grating pitch increasing, and tend to a constant value finally. At the same time, the distance from the grating to the imaging plane increases linearly with the grating pitch, and the testing beam diameter is larger, the greater the linear slope.The inherent system error of randomly encoded hybrid grating, the etching depth error and sidewall angle error in the processing and the alignment error in the experiment are analysed, and the tolerance range is then given to meet the current fabrication technique. Results shows that when the etching depth error is controlled in ±34.66 nm, the sidewall angle 6> 55°, and the alignment offset of the amplitude encoded grating and the phase grating is controlled in 3 ?m, the other diffraction orders in the Fraunhofer diffraction field of the randomly encoded hybrid grating can be neglected compared with the four main orders (±1,±1).Finally, the randomly encoded hybrid grating whose machining error is strictly controlled within the tolerance rang is placed in the quadriwave lateral shearing interference system, the far-field intensity distribution of the diffraction spots and the quadriwave lateral shearing interferograms are obtained. The experimental results shows that only four spots with equal intensity exist in the far-field diffraction of the randomly encoded hybrid grating, and the shearing interferogram is so stable that there is no Talbot phenomenon exist as the distance changes. It is capable of the detection of arbitrary wavefront aberrations.