30 to 50 ns liquid-crystal optical switches.

The optical switching time of twisted-nematic liquid-crystal cells using the liquid crystals, 5CB (C(5)H(11)-Ph-Ph-CN), 5OCB(C(5)H(11)-O-Ph-Ph-CN) and PCH5 (C(5)H(11)-Cy-Ph-CN) have been characterized as a function of temperature, bias voltage and switching voltage, V. The transition time from 90% to 10% transmission scales as V(-1.9) and is limited to 30 to 50 ns by the liquid-crystal breakdown electric field, ~100 V µm(-1). The time from the initial switching voltage step to 90% transmission, delay time, decreases with increasing bias and switching voltage. For 5CB and 5OCB the delay time approaches a constant value at higher electric fields, >10 V µm(-1). Both the transition and delay times decrease with increasing temperature. The minimum transition time at temperatures a few degrees below the nematic-isotropic temperature are 32, 32, and 44 ns and delay times are 44, 25 and 8 ns for 5CB, 5OCB, and PCH5 respectively.

Processing of medical images using real-time optical Fourier processing.

Optical image processing techniques are inherently fast in view of parallel processing. A self-adaptive optical Fourier processing system using photoinduced dichroism in a bacteriorhodopsin film was experimentally demonstrated for medical image processing. Application of this powerful analog all-optical interactive technique for cancer diagnostics is illustrated with two mammograms and a Pap smear. Microcalcification clusters buried in surrounding tissue showed up clearly in the processed image. By playing with one knob, which rotates the analyzer in the optical system, either the microcalcification clusters or the surrounding dense tissue can be selectively displayed. Bacteriorhodopsin films are stable up to 140 degrees C and environmentally friendly. As no interference is involved in the experiments, vibration isolation and even a coherent light source are not required. It may be possible to develop a low-cost rugged battery operated portable signal-enhancing magnifier.

Transient optical modulation with a disperse-red-1-doped polymer film.

Transient optical modulation properties of a disperse-red-1-doped polymer film are investigated by use of four-wave mixing with two-color light. Interesting behavior is observed for the transient modulation on variation of the two-color light intensities because of competition between the two mechanisms of one-photon (reorientation) and biphoton (isomerization) gratings. Positive and negative modulation as well as double-frequency modulation relative to blue-light input is observed in the experiment. The results may have applications in optical information processing.

Spatial light modulation with an azobenzene-doped polymer by use of biphotonic holography.

Spatial light modulation by use of a biphotonic holographic grating has been demonstrated in an azobenzene-doped polymer film. The mechanism of incoherent-to-coherent optical conversion originates from the photoinduced trans <--> cis isomerization of azobenzene molecules by two color lights. It is also found that when the intensity of the blue beam is varied the output replica of the input information changes sign.