An optical ranging system which can see through opaque material has been demonstrated. It uses picosecond pulses, generated in a mode-locked dye laser for viewing and, as a very fast gate, a second frequency offset pulse train with a variable delay at the detector. The system could detect a reflection from a mirror inside an opaque mixture with 64-dB round-trip loss and with 1.7-mm range resolution. Transverse resolution of 0.1 mm was also observed. With certain obvious changes in instrumentation this could be converted into a scanning device. The present depth of penetration (round-trip attenuation) would correspond to 0.67 cm in human tissue.
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Park et al. (1981) studied this question.
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