Liquid Crystal Arrays Produce Variable-Focus Lenses
Liquid crystal arrays may be used to modulate the intensity of light transmitted through a specific region. Whether in a reflective configuration, such as in the first liquid crystal display (LCD) watch introduced by Seiko in 1973, or in transmissive systems, such as the displays in the latest portable DVD players, voltage is applied to a liquid crystal pixel to modify the amount of light delivered through it.
Different voltages on liquid crystals introduce different phase delays. By tailoring the phase profile of pixels in a commercial liquid crystal array, researchers created variable-focus reflective lenses.
LCDs also modify the phase of transmitted light, but when used in conjunction with polarizers, they modulate intensity. A group from the University of Fudan in Shanghai, China, has used the phase-modifying property of LCDs to produce a tunable reflective lens.
The researchers considered a thin lens as a phase-modifying device. By introducing phase changes that vary as a function of position across the aperture, a convex thin lens brings light to a focus. The new device, rather than using a varying thickness of glass to introduce relative phase delays, uses LCD pixels with various applied voltages to introduce comparable phase delays.
The liquid-crystal-on-silicon lens was demonstrated on a commercially available ASI 1200 LCD array from Aurora Systems of San Jose, Calif. To apply the proper voltages to introduce phase delays for a given focal length, the researchers drove the 1024 × 768-pixel array using the red channel of a standard display driver to produce a 6 × 6 array of lenses with 100-pixel diameters. With the maximum voltage -- corresponding to a red value of 255 -- the lenses had a focal length of 1 m.
To verify the ability to change the focal length, they illuminated the lens array with the expanded beam from a CW solid-state laser and sent the 5-mW beam through a polarizer aligned parallel with the input director of the LCD array. The beam was incident on the array at an angle of 5°. The 532-nm light propagated through the LCD pixels, reflected off an integral backside mirror and propagated back through the pixels to an analyzer parallel to the initial polarizer. Finally, it was detected at a CCD camera.
With a uniform red value applied to the liquid-crystal-on-silicon device, it functioned as a flat mirror. With a red pattern, variable phase delays were applied across the device. When varying red values defined a lens array, the focal length of individual lenses was adjustable down to 1 m. At that focal length, the individual 0.98-mm-radius lenses produced a spot smaller than 0.12 mm, with a peak intensity 25 times higher than when the mirror was flat. According to Xin Wang, one of the university researchers, the focal length near 1 m is adjustable in 1.3-cm steps, limited by the granularity of the pixels.
With the flexibility and adjustability verified, the investigators are looking at large-angle reflection. Wang noted that the approach should be useful for the handling of off-axis light, such as an ellipsoidal reflector, and is excited about the possibilities opened up by the freedom of phase modulation in the liquid-crystal-on-silicon lens.
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