Study of the Mechanism of Dye Laser Intracavity Absorption.

The process of Intracavity Absorption (ICA) in a cw multimode dye laser is studied in an attempt to look for a possible mechanism of the process. An atomic beam of barium is used as an absorber because it very closely represents a two level atom, simplifying the interpretation of the data. The line shape of the signals observed by intracavity absorption is complex consisting of both absorption and enhancement features. The detailed line shape depends upon the absorber density in the path of the laser and the total optical power in the laser cavity. In the high absorber density and low cavity power regime, absorption feature is predominant where as at low absorber density and high cavity power enhancement is observed. The absorber density and the cavity power dependence of the enhancement and the absorption features is experimentally determined. The line shape also depends upon the laser scan rate across the adsorption profile. All these results disagree with the existing steady state rate equation models. The newly developed Super-Regen model seems successful in qualitatively predicting the observed line shapes and the time dependent behavior. When the laser is not wavelength scanned, spectral condensation and locking of the laser to the absorption line occurs. This is termed as the "Enhancement Effect". The enhancement signal is found to increase linearly with the absorber density at low densities, rises exponentially and then saturates at the highest densities used. Also the laser beam waist in the long leg of the cavity is found to be larger for frequencies locked to the absorption line compared to the ones away from it. No satisfactory explanation of the phenomenon is available at the present time.