The generation of a variable diameter annular flat-top laser beam in the far field based on an adaptive weight FFT-based iterative algorithm (AWFFT-IA) and a phase only liquid crystal spatial light modulators (poLCSLM) is demonstrated. The iterative algorithm is used to design the needed phase distribution written on the poLCSLM for the target diameter annular flat-top laser beam shaping. The experimental results show that the method proposed can concentrates about 71% of the incident laser energy into the desired region and the root mean square error (RMSE) of the tailored flat-top intensity profile is more or less 12%.
Multi-beam technology is one of the key technologies in optical phased array systems for multi-object treatment and
multi-task operation. A multi-beam forming and steering method was proposed. This method uses isosceles triangle
multilevel phase grating (ITMPG) to form multiple beams simultaneously. Phase profile of the grating is a quantized
isosceles triangle with stairs. By changing the phase difference corresponding to the triangle height, multiple beams can
be steered symmetrically. It took 34 ms to calculate a set of parameters for one ITMPG, namely one steering. A liquid
crystal spatial light modulator was used for the experiment, which formed 6 gratings. The distortion of which had been
compensated with the accuracy of 0.0408 λ. Each grating included 16 phase elements with the same period. Steering
angle corresponded to the triangle height, which is the phase difference. Relative diffraction efficiency for multiple
beams was greater than 81%, intensity nonuniformity was less than 0.134, and the deflection resolution was 2.263 mrad.
Experimental results demonstrate that the proposed method can be used to form and steer symmetrical multiple beams
simultaneously with the same intensity and high diffraction efficiency in the far field, the deflection resolution is related
to the reciprocal of grating period.
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.
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.
Wave front precision distortion correction and transformation using a Liquid Crystal Spatial Light Modulator (LC
SLM) is discussed. After passing through non perfect elements, a coherent beam becomes a distortional beam. The
distortion is expressed in either optical paths difference or phase difference way. The difference is measured
quantitatively. SLM modulates the phase of the beam in a pixel resolution. The beam is recovered to a plane wave
front again after the correction. The accuracy of the modulation is 0.0165λ.