High-Resolution Mode-Locked Laser Rangefinder With Harmonic Downconversion

The development of new pulsed optical sources with high pulse stability allows the improvement of common contactless measurement techniques. A new two-way optical rangefinder, employing a femtosecond laser, is presented here from design to implementation. The system is based on the evaluation of the phase delay of the returning optical pulse train with respect to the outgoing one. The main idea is to expand the time interval corresponding to the phase delay by harmonic heterodyne downconversion of a high harmonic of the pulse repetition rate (up to 10 GHz), which is applied symmetrically to the two channels. The system has shown 10 repeatability with a resolution that improves proportionally to the number of the employed harmonic.