Diffraction wavefront optimization of liquid crystal polarization gratings via geometric phase differential compensation.

Liquid crystal polarization gratings (LCPGs) are considered ideal candidates for optical antennas in free-space optical communications and LiDAR, yet the diffracted wavefront aberrations degrade the signal-to-noise ratio in coherent detection. In this work, the LCPG diffracted wavefront is derived to equal the wavefront difference between the two arms of the polarization interferometer. Accordingly, an in-situ differential compensation approach is proposed using a liquid crystal geometric phase differential compensation plate (LC-GPDC). Unlike conventional thickness-modulated phase plates, the LC-GPDC is fabricated by zero-angle polarization interference, where the relative wavefront aberration is recorded directly into the liquid crystal director distribution through the spatial polarization azimuth. The LC-GPDC is then placed back into one interference arm to equalize the wavefronts prior to LCPG exposure. The root-mean-square (RMS) value of the relative wavefront aberration between the two interference arms was reduced from 0.346λ to 0.079λ (@457 nm) after introducing the LC-GPDC. A 7-μm-period LCPG was fabricated with a diffracted wavefront of 0.238λ RMS (@632.8 nm) over a 50 mm aperture, showing a 39% reduction compared to that without compensation. This work demonstrates a practical geometric-phase compensation scheme for optimizing LCPG diffraction wavefronts, advancing their application in high-performance coherent detection systems.