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Pulse Transmission Mode (PTM) Nd:YAG Laser For Airborne Hydrography

Pulsed green lasers offer a very promising technique for conducting airborne coastal hydrographic surveys. When used as the transmitter of an airborne optical radar system, such lasers can chart miles of coastline quickly and accurately. The frequency-doubled Nd:YAG laser, operated in pulse transmission mode (PTM), can provide the required performance characteristics (wavelength, pulse duration, and repetition frequency) for this application. This paper describes a PTM Nd:YAG laser operating reliably at pulse repetition frequencies (PRF) of up to 400 Hz. Technical problems encountered include thermal effects in the laser rod, design of suitable Q-switch driving circuitry, and the problem of doubler lifetime. A stable resonator designed to correct thermal lensing and birefringence is described. A differential Q-switch driver for high PRF operation is described. A power output of 2.4 watts at 1.06 micrometers was obtained in PTM operation; at 400 Hz. Frequency doubling in DCDA at 400 Hz produced 0.176 watts at 532 nm. Thermally induced birefringence in the index-matched Q-switch imposed a limit on the average power of the PTM laser.