Characterization Of Reflective Liquid Crystal On Silicon(lcos) Spatial Light Modulator And Its Applications In Digital Holography

The versatility of liquid crystal on silicon(LCOS) spatial light modulator made it a real-time recon?gurable device capable of modifying the amplitude(intensity), phase, or polarization of an optical wave front as a function of position across the wave front. In order to use LCOS for any application, it is important to know its phase modulating properties as function of the gray level because different LCOS of the same trademark and models show different optical behaviors from one another. Traditional optical components are difficult to design due to their higher production cost, size and being static but the versatility of LCOS provides the ?exibility to design real time diffractive optical elements.The thesis mainly deal with the characterization of phase modulation property of parallel aligned LCOS spatial light modulator(PA-LCOS) under two different conditions of illumination that is normal angle of incidence and oblique angle of incidence, and its applications as diffractive optical element in digital holography. The details of the topic covered by the thesis are as follows.1. Study of the Phase modulation Characterization of PA-LCOS. Different interferometric methods such as Young-double hole, Michelson, Mach-Zehnder and digital holographic method(lensless Fourier transform method) are used for the characterization of LCOS by displaying a range of gray levels(0-255) on the LCOS with the step change of 10 gray level and their results are compared with each other. All the above stated methods need a number of recordings and measurements for the characterization which are very time consuming. For the real time analysis of phase modulation of PA-LCOS, a mask with the entire range of gray-level i.e. from 0-255 is displayed on its active area and the hologram is recorded, which on reconstruction gives the depth of phase modulation for the entire range of gray levels. In order to avoid the aberration a double exposure method is used in which two holograms are recorded; one with the 0-255 and other with the zero gray level mask. Main advantages of this method are the less number of required recordings, the easy and real time measurements.2. Study of the phase modulation dependence of PA-LCOS on the angle of illumination. The oblique incidence characterization is investigated at a working wavelength of 532 nm, on the basis of double hole interferometric method. Through experiments, the phase modulation characterization of PA-LCOS under the obliqueincidence is obtained. In addition, image post processing method is proposed to overcome the effect of ?icker and coherent noise, by simplifying the computation and enhancing their measurement accuracy. The comparison of experimental results for different incident angles indicates that it plays an important role in the performance of phase modulation of PA-LCOS, where the phase modulation decreases with the increasing angle of incidence.3. Study of PA-LCOS as diffractive optical element. Thereafter, the LCOS as a “lens”is simulated and tested experimentally to verify the feasibility of LCOS being used as diffractive optical element. We used LCOS to solve the twin image problem in dual plane in-line digital holography by displaying a computer-generated chirp like complex re?ectance pattern on it, to adapt the object beam at two planes for recording of two holograms which are processed to eliminate the DC term and twin image. The proposed approach not only improves the speed, accuracy and stability of the experiment but also time saving as it requires no manual operation. Computer simulation and experiments for both amplitude and phase objects are carried out to validate the proposed method. Furthermore, LCOS is used for the suppression of speckle in off-axis lensless Fourier transform digital holography as a complex phase pattern for the decorrelation of recorded holograms by longitudinal displacement of object beam. The simulation has been carried out successfully.