SPATIAL LIGHT MODULATORS: Liquid-crystal phased array steers beams efficiently

Phased arrays have been used successfully for beam steering at wavelengths of 1.06 and 10.6 µm by researchers at Raytheon Corp. (Lexington, MA). The devices, which are based on commercially available liquid-crystal material, can deflect beams by up to 5 with a diffraction efficiency of 35%-95%. The researchers hope that, when the technology is fully developed, liquid-crystal spatial light modulators will form the basis of completely nonmechanical systems for two-dimensional (2-D) beam steering. Applications might include laser radar, communications switching, and adaptive optics.

Conventional phased arrays are used with microwave sources and are similar to phase holograms, which manipulate the phase of coherent light to produce a new wavefront. With microwaves, manipulation is accomplished by several phase-synchronized microwave sources operating with a time delay to change the phase of individual beams. This approach, though, is not possible with visible or near-visible light because phased-array sources should be less than one-half wavelength apart. Coherent diode-laser arrays do exist, but they are not small enough or individually controllable enough to make a source-array system work. Hence, a single coherent source with a spatial light modulator to impart phase changes on individual sections of the beam provides a viable alternative.

Although the idea is not new, its implementation in a near-practical device is; problems such as achieving sufficient resolution and obtaining phase changes of up to 2—without needing high voltages or losing too much light have been difficult to overcome. Early experiments with solid crystals such as lithium tantalate and lithium niobate have been useful for proof-of-principle work but have had impractical results: the solid-crystal approach requires that 600 V be applied across the phase-shifting elements.

Promising alternative

Liquid crystals are a promising alternative given that they are much more birefringent than other electro-optic materials. Whereas a 5-µm-thick liquid-crystal layer could implement a 2 phase change for 1.06-µm light, a 7-mm-thick lithium tantalate crystal would be required to accomplish the same change. Experiments using a liquid-crystal TV for beam steering—with the output polarizer removed so the TV worked as a phase modulator—showed that phase control was poor and the phase modulation and pixel fill factor were both too low. Researchers designed a custom device in which the electrode pitch, phase delay, voltage control, and active area all meet exacting specifications.

The Raytheon device uses stripe electrodes (instead of classic pixel structures) to control the liquid-crystal material. Although these only steer a beam in one dimension, 2-D steering is made possible by putting crossed devices in series with a half-wave plate in the middle. The stripe structure is also easier to fabricate, reduces pinout problems, and allows voltage control circuitry—which holds the voltage steady as data are refreshed—to exist on the same chip as the electrodes. The on-device circuitry requires an electronic backplane and, therefore, beam-steering in reflection. Hence, because the light passes through the liquid-crystal twice—once on the way in and once on the way out—a given phase delay can be implemented with half the liquid-crystal thickness.

Design considerations

Three configurations of the device were built: coarse and fine steerers for use with 1.06-µm light and a coarse steerer for 10.6-µm light.^1,2 Both of the coarse devices had similar diffraction efficiencies of up to 95% for small steering angles and about 35% at an angle of 5. Insertion losses were very different, however, as were the engineering considerations. The input window of the 10.6-µm device, for example, is made of gallium arsenide instead of glass. Using nonoptimal liquid crystal (British Drug House, E7), 28% of the light was lost through liquid-crystal absorption and a further 7% from other factors. The 1.06-µm device, though, had almost no losses from absorption.

Electrode periods of the devices were 2 and 4 µm for the 1.06- and 10.6-µm coarse steerers, respectively, and 80 µm for fine control. Although the pitch size is similar for both wavelengths, the effect is different. At the longer wavelength, the grating period is significantly smaller than one-half wavelength, so the grating profile created will appear relatively smooth to the beam. For the shorter wavelength, though, grating jumps consist of more than two wavelengths, thus reducing the diffraction efficiency from the high theoretical maximum.

Unfortunately, a further reduction of electrode pitch does not help. If the liquid-crystal layer is thick, the electric fields will "smear out" from one electrode into the adjacent cell, thereby removing the phase variation.