Flatness and diffractive wavefront measurement of liquid crystal computer-generated hologram based on photoalignment technology

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Interferometry with computer-generated hologram (CGH) has been widely used in the field of precision optical testing and metrology. CGH can easily generate reference wavefronts of any desired shape by controlling the phase of the diffracted light. Traditional CGH is made by etching a specific pattern on a substrate, whose cost is extremely high and the phase of the diffraction wavefront is sensitive to the changes in etching depth. Based on photoalignment technology, liquid crystal (LC) can be fabricated into LC-CGH. The LC phase modulation elements made by this technology have the advantage of low cost and high accuracy. The existing phase modulation elements based on photoalignment technology are mostly qualitative phase controller, such as LC-grating and LC-wave plate. They are rarely used in high precision applications. In the field of optical testing, the high precision of diffractive wavefront of LC-CGH is critical. The wavefront changes due to phase retardation in adjacent areas of LC-CGH is affected by the flatness of the LC film. Therefore, it is essential to keep the surface of the LC-CGH flat. In this paper, we measure the surface flatness and the diffraction wavefront of the LC-CGH film to verify the feasibility of LC-CGH in optical testing. First, we introduce photoalignment technology and analyze the principle of LC-CGH. Second, we measure the surface flatness of LC-CGH. In the end, we evaluate the transmitted diffraction wavefront. The results can provide guidance for the LC-CGH process improvement.