Distance Measurement System Based on Mode-Locked Laser

Measuring distance in air accurately is of a great importance for several applications such as surveying and metrology. The technique proposed here is based on measuring more than 50 m in air by measuring the change in repetition rate in a passively mode-locked laser when the measured distance be part of its cavity. The measurement uncertainty reached
$21\ \mu \mathrm{m}$
for the maximum measured distance.