Target ranging and identification using timing modulation of a mode-locked laser

The U.S. Air Force has an ongoing interest in improved techniques for ranging and high-resolution depth profiling of ground and air targets. We have investigated the feasibility of a target ID system based on a mode-locked laser and a high-speed detection system. Mode-locked lasers easily produce multi-Watt output with pulsewidths in the tens of ps for excellent resolution, and repetition rates in the tens of MHz allowing signal averaging over multiple pulse trains to improve sensitivity. While peak powers may be high, average powers are well below the damage thresholds of typical optics. One drawback inherent in this approach is aliasing caused by the high repetition rate. This is easily remedied by modulation of the laser output with a pseudo-random pattern.