Laser range-gated underwater imaging including polarization discrimination

Abstract

A range-gated underwater imaging system is described that utilizes a frequency doubled, Q- switched Nd:YAG 30 Hz laser source for illumination and a fast-gated microchannel plate intensified CCD array camera for detection. Laser pulse widths of 7 ns are timed relative to comparable camera gates with subnanosecond jitter. Results for a test bed system obtained in a 4 X 4 X 40 ft water tank with various targets are presented. Water quality was varied with the addition of progressive concentrations of Maalox as scattering agent, and monitored with a home-built laser transmissometer. The 1/4 in., high contrast target lines were able to be resolved at ranges exceeding four attenuation lengths. A simple analytical model for image signal-to-noise ratios is presented and a straightforward polarization discrimination scheme suggested for contrast enhancement. Polarization optics were incorporated into the range-gated test bed system and results obtained for targets of varying characteristic depolarization. In all but a few cases where target and background depolarizations were similar, the signal to noise is enhanced in spite of rejection of orthogonal polarization signal. Issues regarding the evolution of the test bed system to field operation are discussed and significant progress in the development of appropriate miniaturized and ruggedized components is presented