The study on measurement methods of phase modulation characteristics for universal liquid crystal spatial light modulator

The universal liquid crystal spatial light modulator (LC-SLM) was widely used in many aspects of optical study. The working principles and application of LC-SLM were introduced briefly. The traditional Twyman-Green interference method which was used to measure the phase modulation characteristics of universal liquid spatial light modulator had some obvious disadvantages in the practical use, such as high environmental requirement and difficult interference fringes acquisition. The disadvantages of traditional Twyman-Green interference method gained the difficulty of carrying out corresponding optical measurement experiments. To avoid this, the traditional Twyman-Green interference method was improved in the paper. The experimental light path was designed anew. Distinct and stable interference fringes could be acquired by controlling the optical path difference (OPD) dynamically. To verify the validity of the newly proposed measurement method, the phase modulation characteristics of P512-1064 LC-SLM produced by Meadowlark Company were measured by utilizing the improved Twyman-Green interference method at the wavelength of 632.8 nm which was beyond the working wavelengths of the LC-SLM. A series of gray images covering the gray degree from 1 to 256 which were generated by computer were used in the experiment. An extra lens was added in front of a reflector in the optical path to control the OPD dynamically. 256 interference images were acquired after loading the gray image into the LC-SLM in order. After that, the acquired interference images should be pre-processed by several digital image processing methods for easier measurement later. Specifically, the method of gray filtering and morphological processing were adopted to make the interference fringes clearer and thinner in the corresponding processing. Then, the phase modulation curve of the LC-SLM was acquired through numerical computation of the cycles of the interference fringes. In general, the phase modulation curve we acquired was not so accurate in the practical use, as the LC-SLM was required to work from zero to 2pi in the linear interval. So, the nonlinear interval among the phase modulation curve should be compensated and corrected. Here, the method of inverse interpolation which was regarded as one of the most common phase correcting methods was utilized. The corrected phase modulation curve was acquired after numerical computation. The results shows that the improved Twyman-Green interference method could realize the dynamic control of the interference fringes. As a result, the method's requirement for external experiment environment was reduced and its feasibility was improved. Also, the Root Mean Squared Error (RMSE) between the calibrated phase modulation curve and the ideal phase modulation curve was reduced.