Longitudinal mode width in excimer lasers

The time-averaged width of individual longitudinal modes of a multimode XeCl discharge laser oscillator was measured using time-delayed interferometry, a technique related to Fourier-transform spectroscopy. The beam from the laser was divided by the beam splitter of a Michelson interferometer. The arms of the interferometer were adjusted so that their lengths differed by an integral multiple of the laser cavity length. Interferograms were recorded using a CID camera and a videocassette recorder. Measurements of the interference fringe visibility, made at path length differences of up to three times the cavity length, were used to calculate the width of the longitudinal modes, averaged over the laser pulse. The width inferred in this way exceeds the transform-limited width by a factor of 7. This discrepancy is attributed to the time-varying refractive index of the gain medium, which causes the resonant frequencies of the optical cavity to change during the laser pulse. This phenomenon limits the temporal coherence of such lasers, unless a compensating change is made in the cavity length. The time-dependent populations of the atomic and molecular levels predicted by a computer model of the discharge plasma were used to calculate the refractive index. The principal cause of the refractive- index change was found to be the dissociation of HCl. A simple model of the effect of the time-varying refractive index on the laser frequency predicts fringe visibilities in good agreement with measured values.