Retrodirective tracking of a moving target using phase conjugate light generated in a Fabry–Pérot Nd:YAG laser

Abstract

This report describes the retrodirective tracking of a target moving at a constant speed using a Fabry–Pérot laser resonator-type self-starting phase conjugator. Three-dimensional pointing ability was confirmed with this system: the generated phase conjugate light can focus at a target position. To track the moving targets, target-reflected light is Doppler-shifted, which causes wash-out of the gain grating. To address this problem, numerical analysis and experiments showed that shortening the pulse width is an efficient means of eliminating the gain grating wash-out effect. For the measured target speeds of 0–80 mm/s, the phase conjugate light output is unaffected when the pulse width of 18 ns was used, in contrast to the case of 400 μs pulse width, where the phase conjugate light output rapidly decreased as the target speed approached 10 mm/s. Numerical simulation revealed that phase conjugate light can even be generated for fast-moving targets such as space debris in low-earth-orbit with pulses of several tens of picoseconds. Unlike conventional mechanical gimbal steering methods, this system is applicable for wireless energy transfer to rapidly moving objects.