The phase-only liquid crystal on silicon (LCOS) spatial light modulator has been used as a wavefront generator and reconstruction device for display applications due to its phase-only modulating property of incident light. In order to achieve the best reconstructed phase modulated wavefront, the properties of LCOS have to be fully known. The intensity of reflected light from LCOS decreases under normal incidence by going through a beam splitter. In order to improve the intensity of the reflected light beam, the oblique incidence is preferred during many applications. The oblique incidence characterization of a parallel aligned LCOS is investigated at a working wavelength of 532 nm on the basis of double-hole interferometric method. Through experiments, the phase modulation characterization of LCOS under the oblique incidence is obtained. In addition, an image postprocessing method is proposed to overcome the effect of flicker and coherent noise by simplifying the computation and increasing the 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 LCOS, where the phase modulation decreases with the increasing angle of incidence.
The digital holographic method is used to characterize the phase modulation depth of phase-only LCOS. Compared with the conventional ways, the digital holographic method could obtain the information around the whole field of view. Besides, the digital holographic method is a non-contact, lossless, high-fidelity way to achieve the phase distribution. In this paper, the lensless Fourier transform digital holography is employed, due to its simple setup and reconstruction process. In LCOS the phase modulation is controlled by displaying the gray level images on its active area. Usually for the phase modulation characterization, the total of all 255 gray level images are displayed in a step change of 10, for each recording. That is why it takes time for the complete calibration. In this method a mask with the entire range of gray-level i.e. from 0-255 is displayed on the LCOS active area and the hologram is recorded, which on reconstruction gives the depth of phase modulation of LCOS for the entire range of gray level. 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 masks. Also, the sorting by reliability, following a non-continuous path (SNRCP) phase unwrapping algorithm is used for unwrapping the final result. The main advantages of this method are the less number of required recording holograms, the easy and real time calibration. Results are then compared with the conventional method that is young double slit method, which is widely proposed to obtain the phase modulation depth of the LCOS and they are in good agreement with each other. The efficiency of our method is verified by comparison.
Phase-only spatial light modulator (SLM) based on liquid crystal on silicon (LCOS) is a kind of device based on
electrically controlled birefringence effect to realize phase modulation. Due to its low cost, programmable, high
resolution, fast response time, LCOS has been widely used in multi-channel imaging, adaptive optics, diffraction optical
elements (DOEs), dynamic holographic, optical tweezers and other fields. It is necessary to numerically evaluate the
modulation characterization of LCOS before application. Firstly, the phase modulation characterization of the LCOS
(PLUTO HED6010XXX by Holoeye Company) was measured based on the Twyman-Green interferometer, and the
curves of both phase shift and normalized intensity as grey level functions were obtained. Experimental results indicated
that phase modulation of the LCOS could be achieved to 3.99π, and the root-mean-square value (RMS) of normalized
intensity was less than 0.01, which demonstrated that LCOS could be regarded as a phase-only modulation device. This
method is also suitable for the evaluation of modulation characterization of other LCOS devices. Secondly, a phase
pattern of thin lens written onto LCOS was demonstrated. Because of the pixel structure of LCOS, the theory of
discretization of lenses was studied. Both simulation and experimental results were obtained. The experimental results
proved that the convergence character of the lens written onto LCOS was similar to optical lenses. In the experiment, the
measured focal length was in a good agreement to the theoretical deduction, and the relative error (RE) of which was
below 1%. Both simulation and experimental results showed that LCOS could be used as lens to converge the plane
wave, and replace the optical lens successfully.