Adaptive null interferometric test using spatial light modulator for free-form surfaces

We report a method of using a liquid-crystal spatial light modulator (LC-SLM) as reconfigurable multi-level interferogram-type computer generated holograms (ICGHs) to perform dynamic null tests for aspheric and free-form surfaces. With the proposed multi-level ICGHs encoding method, amplitude and accuracy of the applicable aberration of LC-SLMs are both suitable for interferometric test. No other equipment is required to monitor the dynamic phase of LC-SLM for guaranteeing test accuracy,. Moreover, complicated phase response calibration of the LC-SLM is not required. Besides being used in collimated beams, the LC-SLM is demonstrated for the first time to be used in divergent beams; hence. concave surfaces with apertures larger than that of the LC-SLMs can be tested. For realizing practical tests. the calibration of inherit wavefront distortion of the LC-SLM, diffraction orders isolation, and alignment arc analyzed in detail. Two free-form surfaces with about 20 mu m departure from flat and spherical surfaces arc successfully measured in collimated beam and divergent beam, respectively. Cross tests arc provided to verify the test accuracy. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement