Proc. SPIE. 7658, 5th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Optoelectronic Materials and Devices for Detector, Imager, Display, and Energy Conversion Technology
In practical application of Liquid crystal optical phased array (LCOPA) device, diffraction efficiency and deflection
range are two important performance parameters, which should be improved further more. Both diffraction efficiency
and deflection range are directly related to the width of flyback region. Therefore, the flyback region problem was
analyzed quantitatively in this paper. According to elastic continuum theory of liquid crystal and crystal optics theory,
we calculated LC director and its phase delay profile. Then we analyzed quantitatively the influence of device parameters
including electrode size, alignment layer thickness and LC cell thickness on the width of flyback region. Results show
that there exists a critical value of electrode size. When electrode size is larger than this critical value, the width of
flyback region increases nearly linear to the increase of electrode size. The width of flyback region is not sensitive to
alignment layer thickness and increases monotonously with the increase of LC cell thickness.
The discreteness of driving electrodes in liquid crystal optical phased array (LCOPA) device causes phase valley in the
region between two adjacent electrodes. When a one-dimensional transmission-type LCOPA device was used, there was
a pair of diffraction sidelobes with considerable intensity on either side of the deflected beam, which decreased the
diffraction efficiency of deflected beam greatly. An analytical numerical model of phase valley was proposed for the
quantitative analytic analysis on diffraction efficiency. Results showed that phase valley is the cause for diffraction
sidelobes. The diffraction efficiency of deflected beam decreases steeply as the phase valley depth increases. When
valley depth decreases close to zero, the main factor having affect on diffraction efficiency turns to be flyback region
size. The influence of electrode space on diffraction efficiency was quantitatively analyzed as well. An effective way to
reduce or even eliminate the phase valley is to reduce the space between electrodes.
The structure of a storage capacitor in nematic liquid crystal optical phased array causes the data refresh rate to be higher
than the liquid crystal switching frequency, which demands a high data bandwidth. A novel driving method was
therefore proposed to reduce the data refresh rate. Without using a storage capacitor, the proposed method uses digital
scanning instead of conventional analog scanning. Furthermore, digital scanning is able to drive all pixels in parallel and
transmit driver data only once for one frame. 1-D nematic liquid crystal optical phased array was used to test the novel
driving method. During the experiment two frames of driver data were alternately transmitted at a refresh rate of 10 Hz,
and diffraction patterns were acquired. Experimental results demonstrate that the proposed method can be used to keep
the data refresh rate lower than the liquid crystal switching frequency.
A practical numerical model for liquid crystal cell is set up based on the geometry structure of liquid crystal optical phased arrays (LCOPA). Model parameters include width and space of electrodes, thickness of liquid crystal layer, alignment layers, electrodes and glass substrates, pre-tilted angles, dielectric constants, elastic constants, the refractive indexes and so on. Especially, the thickness of alignment layer is first considered to the best of our knowledge. According to electrostatic field theory and Frank-Oseen elastic continuum theory, two dimension (2D) electric potential distribution and 2D director distribution are calculated by means of the finite difference method on non-uniform grids. And the phase delay of LCOPA is derived from crystal optics. The influence of cell sizes on phase delay distribution is analyzed. The evaluation function of fringing field effect is provided. And the methods to decrease fringing field effect between electrodes are also discussed.