Wavefront Measurement And Performance Analysis Of Liquid-Crystal Grating Phased Array

Liquid-crystal grating, as a kind of optical phased array, has been used in laserradar system, satellite communication system and other photoelectric systems to realizeagile beam steering. In order to control directional beam quality, the wavefront of liquidcrystal grating phased array needs to be tested and the wavefront distortion needs to beanalysised.To properly test the special dramaticly-changed wavefront contour of the liquidcrystal grating, the impact of the wavefront distribution on device performance, or onthe grating diffraction field, needs to be understood. This paper analyzes the errorsources of liquid crystal grating wavefront contour and divides the wavefront errors ofliquid crystal grating phased array into two types: periodic and non-periodic. Using theFourier series decomposition and angular spectrum diffraction theory, the influence ofdifferent types of wavefront errors on grating diffraction field has been calculated, andthe simulation analysis has been done.According to the different types of wavefront errors, different wavefront measuringmethods have been proposed. And theoretical analysis and computer simulation aboutthese methods have been done. For periodic wavefront error, optical field distribution ofthe back surface of liquid crystal grating (also known as the exit end of liquid crystalgrating) can be restructured approximately by testing the near-field intensities of all thediffraction orders, and thus the wavefront distribution is obtained from it. Fornon-periodic wavefront error, two kinds of test methods have been proposed. one isgetting wavefront distribution through directly measuring optical field distribution ofthe exit end of liquid crystal grating. The other is gaining wavefront error distributionthrough measuring optical field distribution of the blazed order and the inversediffraction propagation calculation.Measuring the wavefront of liquid crystal grating phased array by means of thenovel quadrilateral radial shearing interference light path has been proposed. And threekinds of measuring methods suitable for different frequency range wavefronts havebeen given: the own carrier modulation, fixed carrier modulation and a single diffraction order inversion. A10mm×10mm aperture radial shearing interferometer hasbeen developed. Using this interferometer and a commercial WYKO interferometertesting the same plane optical element, the difference of test results is less than λ/20forpeak to valley (P-V) value and less than/200for root mean square (RMS) value. Staticphase distribution of liquid crystal grating, voltage-phase shift characteristics and thewavefront of deflection beam have been measured utilizing this shearing interferometer,which provide a basis for performance analysis and voltage load scheme of liquidcrystal grating device.Mach-Zehnder conjugate phase shifting interferometry based on4f system hasbeen advanced to measure the wavefront of the exit end of liquid crystal grating. Thewavefronts of different periodic liquid crystal gratings have been tested with a10mm x10mm aperture Mach-Zehnder conjugate phase shifting interferometer. The far fieldintensity distributions of diffraction fields of liquid crystal gratings calculated from theresults are the same as the far field distributions measurement, which proves theaccuracy of test results.