THERMALLY-INDUCED SELF-LOCKING OF AN OPTICAL CAVITY BY OVERTONE ABSORPTION IN ACETYLENE GAS

Strong self-locking phenomena are observed when laser power is convert ed into heat by a weakly absorbing medium within a high-finesse cavity . Deposited heat leads to increased temperature and, for the case of w eakly absorbing intracavity gases studied here, to an associated reduc tion of density and refractive index. This thermal change in refractiv e index provides self-acting cavity tuning near resonant conditions. I n the experiments reported here a Fabry-Perot cavity of finesse 274 wa s filled with acetylene gas and illuminated with a titanium:sapphire l aser tuned to the P(11) line of the nu(l) + 3 nu(3) overtone band near 790 nm. The dependencies of maximum frequency-locking range on gas pr essure, laser power, and laser frequency sweep rate and direction were measured and could be well unified by analysis based on the thermal m odel. In the domain of strong self-tuning an interesting self-sustaine d oscillation was observed, with its several sharp frequencies directl y and quantitatively linked to the acoustic boundary conditions in our cylindrical cell geometry. The differences between the behavior of ac etylene near 790 nm and molecular oxygen with electronic transition ne ar 763 nm are instructive; whereas the absorbed powers were similar, t hey differed strongly in their rates for internal to translational ene rgy conversion by collisional relaxation. (C) 1996 Optical Society of America.