The effects of scattering and mirror reflectivity on the performance of a ruby laser

Abstract

The output energy expected from a conventional ruby laser generator with plane parallel mirrors is calculated for a range of excitation energies, pulse lengths, mirror reflectivities and absorption, scattering and reflection losses. A linear dependence of output energy on excitation energy is expected only for a vanishingly small pulse length. The effects of localized losses such as those from reflections at the ends of the crystal are similar to scattering losses distributed through the crystal. The output mirror reflectivity giving maximum output energy falls as the excitation energy and scattering increase but is typically 50-60%. The reduction in output caused by scattering is less for lower reflectivities. The angular distribution of light scattered from the ruby when lasing is measured by varying the resonator length to assess the scattering. The predictions of output energy are in good agreement with experiment for all the excitation energies, introduced scattering losses, and mirror reflectivities tri d, except that above 70% reflectivity the prediction is up to 60% low. Possible reasons for this discrepancy are discussed. It was also found experimentally that the resonances between the ends of the ruby are suppressed when lasing, probably due to the optical inhomogeneities produced by the refractive index changes associated with the population changes during a laser spike. The output energy is thus reduced when the ends are not aligned in the resonator. These dynamic inhomogeneities are thought to override any static inhomogeneities already present in the crystal and are a major source of resonator loss.