Adaptive Optics Systems Compared

Anne Fischer Lent

Increasing demands in military observation and intelligence are accelerating research in the field of adaptive optics. Researchers from Kirtland Air Force Base in New Mexico have compared the performance of adaptive optics systems employing an electrostatic membrane mirror and a dual-frequency nematic liquid crystal. The compact, low-cost systems could enable the construction of networks of fixed or mobile stations for military, commercial or civilian use.

The researchers tested the systems by imaging solar-illuminated satellites in low Earth orbit through atmospheric turbulence, using the Air Force Directed Energy Directorate's 3.67-m telescope on Maui, Hawaii. The telescope's aperture was stopped down to 1.12 m to match the number of actuators and the dynamic range of the devices.

A typical microelectromechanical systems (MEMS) setup consists of a silicon chip mounted over a printed circuit board holder, with an array of electrodes etched onto the board, usually in a hexagonal pattern. The system in the demonstration, however, arranged Shack-Hartmann lenslets in a square array because the geometry matched that of the pupil-plane detector array.

The array consisted of 600-µm-wide lenslets with 72-mm focal lengths. A 64 x 64, 12-bit wavefront sensor camera from Lincoln Laboratory in Lexington, Mass., ran at up to 1300 fps with an exposure time of 1.0 ms, which allowed for an open-loop bandwidth of up to 1 kHz for the wavefront control system. A 256 x 256, 12-bit camera from Dalsa recorded open- and closed-loop focal-plane data, and it ran 10- to 25-ms exposures. Frame grabbers supplied by Imagine Technology of Lincoln, Neb., were used with both cameras. Two 20-channel digital-to-analog converters drove the high-voltage amplifiers that controlled the mirror's electrostatic actuators.

Using theoretical influence function and matrix-inversion techniques, the researchers developed a multi-input/output law to control the electrostatic membrane mirror using measurements from the wavefront sensor. A 500-MHz Pentium computer measured a 3-dB closed-loop bandwidth of 80 Hz.

The advantage that this system has over conventional setups is that the solid-state materials lend themselves to precision mass production. These devices can be made in semiconductor fabrication shops and, because they will likely find application in the mass market, the cost should be relatively low.

Liquid crystal vs. MEMS

The scientists compared the performance of this system with that of one that employed a multisegment dual-frequency liquid-crystal phase retarder. They discovered that the liquid-crystal setup was more difficult to control but that it offers the potential of large numbers of elements. The MEMS setup was easier to control, but it is limited in the number of actuators that can be implemented.

Investigations into these approaches will continue. The results may be useful for military applications, laser communications, information processing, and laser beam and cavity control.