Computer-generated hologram (CGH) method is a high-precision aspherical surface detection method. CGH produces wavefronts of any shape with extremely high precision and is adopted in null test. Liquid crystal CGH (LC-CGH) is a new type of CGH with short production cycle and low cost. It is a promising alternative to traditional CGH. In this paper, the overall process flow of LC grating preparation is presented. The influences of three process parameters, such as LC solution concentration, spin coating speed and time on the three physical quantities of LC grating diffraction efficiency, LC polymer film thickness and phase delay are studied. Based on the analysis, improvement measures are proposed for the preparation process. The research carried out in this paper has guiding significance for the processing and manufacturing of LC-CGH.
We propose a liquid crystal (LC) hologram fabricated with photoalignment technology, for the measurement of a cylindrical surface. A standard cylindrical surface reflects the incident planar wavefront and generates an interferogram with a planar reference wavefront. Photoalignment azo-dye material is then exposed by the interferogram and aligned with desired orientations, following with depositing LC monomer to generate the designed phase information. After ultra-violet curing, the fabricated LC hologram plate can generate a standard cylindrical wavefront when illuminated with a planar wavefront. The minimal line-width due to the limitation of LC molecules is sub-microns, which is smaller than the limitation in traditional CGH. The systematic design of the measurement is proposed, followed by a demonstration simulation.
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.