Phase and Polarization Modulations based on LCOS SLM
Liquid crystal on silicon (LCOS) technology has initially been developed for imaging and display applications. This technology combines the unique light-modulating properties of liquid crystal materials and the advantages of high-performance silicon complementary metal oxide semiconductor (CMOS) technology through dedicated LCOS assembly processes. Phase-only LCOS SLM are becoming an important tool for laser processing in a range of systems. A recent breakthrough in laser-induced self-assembled nanostructure in glass has made it possible to store data in fused silica. The technology of 5D optical data storage in transparent materials paves a promising way to almost unlimited lifetime data storage for future cloud. The phase-only SLM has already shown its potential for this application in tailoring ultrafast lasers writing beam for optical data storage. In data writing process, a light filed of target data pattern (multi-beam arrays) with target linear polarization state is required to encode information onto the nanograting structure created by laser pulses. In data writing beam generation, phase-only LCOS SLM can generate arbitrary data pattern by using diffractive holographic imaging. However, the polarization control of output image is still achieved by using an external polarization modulator. This leads to the complication, bulkiness, and large delay in current methods. Therefore, this research aims to develop a phase and polarization modulation method based on phase-only LCOS SLMs to simultaneously control both the holographic image and its polarization state. Phase-only modulation of light can tailor the output image by using computer-generated holograms onto LCOS SLMs. This is very useful for generate arbitrary multi-beam data writing light field. To fully control the polarization state of the data writing light field, the amplitude and relative phase of two orthogonal polarization components needs to be independently modulated. However, the phase-only modulation cannot directly affect the polarization of light. Therefore, three methods are proposed to achieve the phase and polarization control using LCOS SLMs. Also, two customized LCOS SLMs were designed and fabricated by using in-house-developed die-level assembly technique. They are used in all experiments of this research. First method is parallel coding and two-beam combining. Two orthogonal input beams are parallelly and independently encoded with the same target image information but with the different amplitude information by using two phase holograms on two LCOS SLMs. Then, two modulated beams are now considered as two polarization components and be spatially superposed to form target polarization state. The first-order diffraction efficiency change controlled by using phase modulation depth on hologram is used to design a technique to encode amplitude information onto the hologram. This technique is used in all three methods for amplitude modulation of polarization components. This method requires high precision alignment process in beam combining. In the second method, to avoid beam combining in polarization control, instead of modulating two beams, the object of modulation is changed to two polarization components of a single input beam. By using the characteristic of polarization sensitivity of phase-only LCOS SLM, two polarization components of single beam are sequentially and independently coded with information target image and amplitude using two LCOS SLMs. Because the spatial status to components of single beam remain unchanged in the modulation, the phase and polarization control is achieved automatically. The third method is a compact system using only single LCOS SLM device. The principle of this method is two polarization components coding like in the second method, but the fundamental difference is in a polarization components rotation technique in compact system. Using this polarization rotation technique, two light components can be independently coded by separately using two holograms on the two halves of LCOS SLM. The prototype of the compact system is developed and fabricated, and the effectiveness of the system has been experimentally verified. To sum up, the phase and polarization modulation methods based on LCOS SLM has been explored and developed. They can provide dynamical control of both polarization sate and image of light field using only LCOS. The proposed methods can provide a more promising way to largely increase the data writing speed in the 5D optical data storage technology.