Research On Power Control Technologies Of Multiple Beams By Using Phase-Only Liquid Crystal Spatial Light Modulator

Multi-beam control technology can be used to synchronously control the number, pointing and energy distribution of multiple laser beams. It has such advantages as high parallel capability, large throughput, high integration and so on, and has many applications in field of optics, physics, microelectronics, laser communications, laser radar, biological engineering and medicine etc. Thus it has already gradually become an important branch of laser technology.In recent years, phase-only liquid crystal spatial light modulator (LCSLM) has been able to simulate phase-only diffractive optical element to generate controllable multiple beams. Because of such advantages as high spatial resolution, high phase resolution, no moving parts, low driving voltage, high integration, small size, light weight, low cost, LCSLM can promote multi-beam control technology toward high integration, miniaturization, multi-function.Power control is one of the most important parts for multi-beam control technology. Incresing its speed and accuracy are conducive to enhancing the function of multi-beam control technology, and are conducive to developing new mode to expand applications. However, current power control technologies of multiple beams based on phase-only LCSLM have still suffered from phase errors of LCSLM and conflicting relation between control accuracy and computing time, which both make power control technologies hard to meet applications.The purpose of this thesis"Research on power control technologies of multiple beams by using phase-only liquid crystal spatial light modulator"is to propose a feasible method to inhibit phase errors of LCSLM, and make the method of controlling multi-beam power have shorter response time with a common accuracy. Two key parameters of power control, i.e. the ratio of intensities and the diffraction efficiency, are mainly researched in theory and experiments, the main contents and achievements of which are describled as following,(1) As a typical multilevel 1×4 beam splitter, Isosceles Triangle Multilevel Phase Grating (ITMPG) is studied in order to inhibit the bad influence to intensity uniformity from phase errors of LCSLM. After researching the beam-splitter theory of ITMPG, which can generate equal-intensity beams with controllable pointing, a method of adjusting grating structure parameter, namely adjusting phaseφh for ITMPG, is proposed to control its intensity uniformity. Experiment results demonstrated that ITMPG can generate 4 beams symmetrical to diffractive zeroth order which can be steered symmetrically. Compared with the linear compensation method, the method of adjusting grating structure parameter can increase the intensity uniformity by 5%, and further increase the intensity uniformity after using the linear compensation method. Therefore, the method of adjusting grating structure parameter has stronger capability of inhibiting phase errors, which would provide theoretical and experimental basises for controlling total power of multiple beams with high accuracy.(2) When conventional Weighted Gerchberg-Saxton (GSW) algorithm is used to control total power of multiple 3D spots with a common accuracy, there is a problem of how to shorten the computing time. In order to solve this problem, a control method is proposed by combining a GSW algorithm with a tunable binary phase grating. An initial phase hologram can be generated very fast by the GSW algorithm; however, it usually generates multiple spots without desired total power. By superimposing the initial phase hologram with a tunable binary phase grating, the total power of multiple spots follows a simple cosine function of the grating structure parameter (phase depth of binary phase gratingφ), thus a desired phase depth of the binary phase grating can be obtained in an order ofμs, namely the total spot power can be controlled fast and accurately by the proposed method. In theory, performed by a LCSLM with 1600 linear phase levels, the proposed method can linearly control total spot power to 500 equidistant normalized power, and its computering time is 2 orders less than the conventional GSW algorithm with the same accuracy, therefore the computing time can be shorten obviously.(3) In order to demonstrate accuracy and ramdon feature of the proposed method, a LCSLM from USA BNS Inc. is used to generate multiple beams. Firstly, the validity of the proposed method is demonstrated by the comparison with GSW algorithm. Secondly, the proposed method is preliminarily demonstrated by the experiments of controlling power of a single beam, including in experiments of controlling power linearly and of controlling power randomly. At last, its performance of controlling total power is demonstrated by the 2×2 three dimension spots experiments. In these experiments, the method of adjusting grating structure parameter made the control accuracy of normalized total power to be in good agreement with the simulated value. Experimental results indicate that the proposed method can control the total power to a random power value with repeatability standard deviation better than 0.0007.According to the research above, the accuracy influences from phase errors of LCSLM can be inhibited, and the conflicting relation between the control accuracy and the computing time can be catabatic, therefore this paper provides theoretical basis and key techniques for controlling multi-beam power fast and accurately.