Specific lenslike effects and resonant diffraction losses in two-isotope gas lasers

The possible frequency-dependent diffraction loss mechanisms and the associated asymmetries of the output power versus frequency profiles are theoretically and experimentally investigated in equally mixed two-isotope gas lasers. It is shown that the evolution of resonant diffraction losses due to population lenslike effects is unchanged with respect to the single-isotope case. However, the resonant diffraction losses due to saturation lenslike effects in two-isotope systems are shown to exhibit three different possible behaviors, depending on the relative values of the homogeneous width and the isotope shift of the transition and the free spectral range of the laser cavity. In particular, it is shown that in the case where the homogeneous width of the transition is smaller than the half isotope shift, as long as the free spectral range of the cavity is larger than the isotope shift, the output power profile exhibits two reversed asymmetries, for the center and the wings of the profile. The case of usual ring-laser gyroscopes is also investigated and shows that in this case the output power is more important on the high-frequency side than on the low-frequency side, independently of the nature of the predominant lenslike effect. In every case, a good agreement is obtained between the experimental profiles and the theoretical results, computed either from a third-order perturbative calculation or a high-intensity exact theory. Such a high-intensity calculation is shown to be necessary to make quantitative comparisons between experiments and theory in ring-laser gyroscopes.