An experimental study of saturable absorbers for ruby lasers

This paper presents the results of a study of the behavior of some saturable dye filters used with ruby lasers. We review some apparently contradictory results obtained earlier and present modified forms of energy level models, which are consistent with the known photophysical properties of the molecules. We then present experimental evidence to elucidate the nature of the overall relaxation time, the source of the residual absorption, and the nature of the transient spectral hole burning, which gives rise to frequency-locking between different lasers. On the basis of the frequency-locking experiments, and our model, it is possible to calculate the rate at which the broad molecular absorption band couples into the narrow spectral width of the radiation field during pumping. This time is of the order of
5 \times 10^{-12}
second for chloro-aluminum phthalocyanine in pyridine. Taking into account considerations such as excited-state absorption, mode of bleaching, etc., we conclude that the phthalocyanine dyes are best suited as saturable absorbers at low power levels (<5 MW/cm
²
but are unsuited for mode-locking applications. Cryptocyanine has lower residual losses above 10 MW/cm
²
and can be used for mode locking. Both types of dye exhibit very narrow spectral hole burning, which can be used to frequency-lock separate lasers.