Studies On Large-Field-of-View Beam Steering Based On Liquid Crystal Phased Array

Beam steering technique is widely used in fields of laser communication, optical storage, target-tracing and so on. The conventional beam steering technique depends on mechanical systems to change the direction of the optical axis in order to control the propagating direction of the laser beam, resulting in complicated structures, substantial masses, expensive cost and high energy consuming. As the micro-devices become increasingly available, the nonmechanical beam steering techniques, a representative of which is liquid crystal optical phased array (LCOPA), have turned to be hot buttons for all related researchers. We studied the large-field-of-view beam steering technique based on liquid crystal optical phased array and volume Bragg gratings. The main work and results of this dissertation are listed as follows:1. The worldwide research status of beam steering based on liquid crystal devices is introduced. And then basic principles and performance indexes, in company with the research progress and bottlenecks, of these beam steering systems are explained detailedly. In particular, milestones of these techniques over recent years are reviewed. Comparisons between different liquid crystal based beam steering systems are given and the research trend of them is previewed.2. This dissertation made a systematic study on beam steering technique based on LCOPA. Based on wave optics theory, we established the physical model of LCOPA, and studied the performance of LCOPA (i.e. beam steering efficiency, beam-control approach, beam steering accuracy etc.) from theoretical analysis and numerical simulation'point of view. The results show that increasing the fill factor helps to enhance the beam steering efficiency and raising the phase quantized levels contributes to the steering accuracy. Particular polarization direction and wavelength are required by LCOPA for normal performance. Based on the theory of multiple beam interference, a novel approach for wavefront modulation is proposed, with which the problem of conventional beam steering techniques that the achievable steering angles are uncontinuous is solved. And then beam steering experiments based on LCOPA was finished and a beam scanning with a maximum steering angle 2.03°was achieved.3. This dissertation made a study on the characteristics of volume holographic gratings set as angular magnifier. Based on coupled wave theory, we established the physical model of multiplexed angular magnifiers (MAM), and summarized the designing principle and physical laws of MAM from uniform angular distribution and optimum diffraction efficiency's point of view. In addition, the effects of beam divergence and fabricating errors on performance of MAM were studied. Then we made an experimental study on MAM, whose characteristic curve of angular selecting was gained. Results indicate that single angular magnifier can realize angular amplifying by 103 times, with proper designed parameters, that means MAM can extend the scanning range. In addition, particular input/output angular distribution can be realized by controlling tilting angles and periods of gratings while optimized steering efficiency can be achieved by designing proper refractive index and thickness of gratings. Beam divergence causes a performance decline of the angular selection. Specifically, if the beam divergence is greater than half of the angular half-width, the optimal diffraction efficiency falls down to lower than 50%, resulting in disappearance of local minimum of the angular-selecting curve, and then considerable crosstalks of MAM appear.4. This dissertation made a study on large-field-of-view beam steering technique based on LCOPA and MAM. Compared with the current conventional beam steering technique based on LCOPA, the system designed in this paper raised the scanning range by about ten times. We measured the divergence of the input laser beam as well as the effect of tilted incident beam on the steering performance of LCOPA. Then we designed a large-field-of-view beam steering system based on LCOPA and MAM. Quasi-continual and programmable-control beam steering with a maximum steering angle of 11.82°was achieved. The angular resolution of this system is less than 0.02°while the energy transferring efficiency is not higher than 2% because of the energy loss brought by beam splitters.