Study On Key Technologies Of Holographic Imaging Based On LCoS

Three dimensional display (3D) is an inevitable demand with the development of information display technology, since it can provide immersive visual perception. Holographic display is the most attractive 3D display technology for its capability of recording and reconstructing real-existing 3D images. With computer-generated-hologram (CGH) exerted on the spatial light modulator (SLM), computer holography can reconstruct the amplitude and phase of wavefront of the object.Liquid crystal on silicon (LCoS) is widely used for its advantages, like high-level integration, high pixel fill factor, and easy to achieve higher resolution and shorter response time over other types of spatial light modulators.The development of computer holography based on SLM is limited by three key scientific issues: 1. Huge computing loads of 3D scences.2. Limitation of device space-bandwidth product.3. Noises deteriorate image quality. The key technologies of holographic imaging based on LCoS provide solutions to the above three problems. The research on these key technologies is composed of modulation characteristics of LCoS and imaging characteristics of holographic imaging system. The main achievements are summarized as follows:1. For computer holography, pixel pitch limits the field of view, while the bottleneck of the micro devices is the fringing field effect. The simulation research on the fringing field effect was done. Novel device configurations of one dimensional phase grating and two dimensional phase array were proposed respectively to optimize the fringing field effect.Firstly, simulation models of LCoS based on the finite element method (FEM) was built. Secondly, the influence factors of the fringing field effect were studied. Then, the sinusoidal phase grating was simulated. Owing to the fringing field effect, the phase modulation depth of the conventional configuration doesn't increase with the increase of the driving voltage and the phase profile of the grating electrodes expand to neighboring electrodes. Correspondingly, the diffraction efficiency doesn't increase with the increasing of the voltages and the main energy of the diffraction pattern focuses on the zero order which cannot be used. In this thesis, a liquid crystal grating with equivalent capacitance configuration is proposed, where two layers of periodical ground electrodes are interlaced and aligned with the addressing electrodes. The generated electric fields between the stripe ground electrodes and grating electrodes resist the fringing field. Meanwhile the equivalent capacitance configuration is constructed with three layers of ground electrodes. By proper structure design and selection of dielectric material, distribution of the equivalent voltage exerting on the liquid crystal layer can be adjusted to further reduce the fringing field effect. The simulation results show that phase modulation depth of 2? is achieved. And phase modulation depth of 3.68 rad is obtained when the voltage is 3.7V, which means that the theoretical upper limit diffraction efficiency of the first-order diffraction 33.86% is achieved. The energy of the first order in the diffraction pattern is lower than 0.1, which indicates the efficiency is greatly enhanced. The simulation of two dimensional phase grating used chessboard pattern for the crosstalk of neighboring pixels is quite obvious. With regard to the cause of the fringing field effect:the inherent viscosity of LC molecules and expansion of the pixel electric field, a novel structure using dielectric separation to confine the LC molecules and slit-step electrodes to enhance local electric field where disclination lies is proposed. The simulation results show that the image quality is greatly improved.2. The dynamic nonlinear phase response of LCoS, namely the nonlinear relationship between phase modulation depth and driving voltage (grayscale), causes deviation between actual and ideal phase retardation, which reduces the image quality. Accuate phase modution characterization methods are proposed based on which gamma curve calibration is used to optimize the dynamic phase aberration.In this thesis, global phase measurement by double slits interference and local phase measurement by digital holographic microscopy were done. Based on the accurate characterization, gamma curve calibration was used for phase aberration, namely to find the best fitting polynomial equation between phase shift and LUT (look-up table) value from measurement results, then transform it to the relationship between gray level and LUT value which is the calibrated gamma curve. Calibrations for the nonlinear electro-optics effect and the nonlinear correlation effect were made respectively. Good results of calibrations were obtained.3.Holographic display has to use complex models to describe 3D objects and also has problems of massive computation, high requirements of device resolution and space-band width. Fractional Fourier transformation is used for CGH of LCoS. Time-sequence multi-plane method and multi-plane iteration algorithm were proposed to realize three dimensional holographic display.In true 3D techniques, volume display has complex system; holographic display has to use complex models to describe 3D objects and also has problems of massive computation, high requirements of device resolution and space-band width. Two multi-plane imaging methods were proposed to solve these problems. By using fractional Fourier transformation, different images are projected at different depth in the same system. By using synchronization information sent by the spatial light modulator, the circuits control the switch of the polymer doped liquid crystal screens.3D display by holographic display imaging on multiple focal planes is realized as a result of visual staying phenomenon. For multi-plane iteration algorithm, 3D light distribution is realized by simultaneously modulating coherent light in several parallel 2D planes. The system is simple and doesn't have extra requirements for power or display devices. By using a phase-only spatial light modulator on which the computer generated holograms load, images at different depth planes are sharply projected onto the closely spaced screens, which mimic the accommodation of natural vision.