Liquid Crystal Adaptive Optics Speed Up
Daniel C. McCarthy
ALBUQUERQUE, N.M. -- Conventional piezo technology, on which most adaptive optic arrays rely, tends to produce bulky, expensive and power-hungry devices. An alternative is liquid crystal arrays, which, with help from the video display industry, have evolved into two-dimensional devices equipped with multiple elements and lower weight, size, cost and power consumption.
One drawback is these materials' poor response times, which, for single-frequency liquid crystals, can last hundreds of milliseconds. The problem is not the speed at which the materials reach maximum phase shift, but rather how quickly they relax to the zero state.
Reaction times of single-frequency closed-loop devices translate as limited bandwidth. For example, last year a group of researchers from Applied Technology Associates, Optical Sciences Co. in Anaheim, Calif., and the US Air Force's Directed Energy Directorate in Kirtland reported a single-frequency liquid crystal device with closed-loop 3-dB bandwidth of 5 Hz.
In the May 20 issue of Applied Optics, however, they returned to report on an instrument that produced closed-loop tilt rejection at a 3-dB bandwidth of 40 Hz. Both the researchers and Meadowlark Optics in Frederick, Colo., which produced the device for them, owe part of their success to dual-frequency liquid crystal material from Niopik Organic Intermediates and Dyes Institute in Moscow.
Unlike single-frequency materials, Niopik's liquid crystals rapidly modulate phase retardance in two directions based on a low 7-kHz crossover frequency. This is the frequency of applied voltage at which the liquid crystal response changes direction. A frequency below the crossover point will drive phase retardance in one direction; a frequency above crossover will drive it in the opposite.
"The main point regarding crossover frequency is that a dual-frequency nematic crystal can achieve one wave of retardance at a much faster rate, up to 10 times a traditional nematic material," said Sergio Restaino, a project collaborator based at the Directed Energy Directorate.
The group compared experimental measurements and theoretical calculations of the device's dynamic response to further develop a pulse-amplitude control algorithm. The next step, Restaino said, is to integrate the technology into a large-aperture telescope and to study its performance in atmospheric compensation.
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