Abstract

The two output pulse trains from a passively mode-locked ring dye laser are combined spatially and temporally to produce a beat signal. The beat note frequency is a sensitive measurement of the difference of the optical path length experienced by the two counterpropagating pulses inside the laser cavity. Tested elements, such as a Pockel cell, are inserted into the cavity away from the crossing points of the pulses to avoid beat frequency lock-in. The two pulses reach the tested element with a relative time delay determined by the distance between the element and a pulse crossing point, and they probe a different phase of index change induced in the element by a synchronized external driving force (e.g., an electrical or optical pulse train) or by the previous pulses of each other. An optical path length difference of 10^-6 of a wavelength has been resolved simply on a digital scope. These intracavity experiments combine the high time resolution of ultrashort laser pulses (about 100 fs) and the high sensitivity of intracavity interferometry (better than 10^-13).

© 1992 Optical Society of America