| Beam steering, used to target the laser beam to certain direction, is critical technology in laser radar. Liquid crystal based beam steering, one of the most promising non-mechanical beam steering technologies, is attracting people's attention for its potential capability of steering a large aperture within a wide angle range with high efficiency and high rate. In this dissertation, the physics underlying the limits of a particular type of liquid crystal beam steering systems are explored. Two major types of beam steering devices that are based on the use of liquid crystal electro-optical elements considered here are (1) Conventional Liquid Crystal Optical Phased Array (LC-OPA); (2) Liquid Crystal De-centered Lens Array (LC-DLA). The efficiency of the first type device is limited by the reset that are set to be a multiple of the wavelength of light that is being steered. The second type of device is free of resets but consists of multiple layers, which makes it a thick diffractive element with diffraction loss associated with wave optics. To study the efficiency issues, a theoretical model is established by studying a standard blazed grating. It's found that three factors affect the diffraction efficiency: two geometrical factors and one wave-optics factors. These factors are studied for both of these devices. It shows that the efficiency of first type device will drop rapidly at large steering angle, which limits it good only for small angle (less than 1 degree, for example) beam steering. The efficiency of second type of device can be kept above 90% within its steering angle range under certain conditions. Therefore it's capable of relatively larger angle beam steering (7 degree, for example). Besides efficiency issue, another concern is the broadband beam steering with these devices. Both of these devices suffer from grating dispersion that strongly depends on wavelength. Regarding the broadband beam steering, the factors determining the dispersion is studied. The approaches for implementing broadband beam steering, high order grating and multiple-channel approach, are explored. |
