On the saturation effects and start jump of Gaussian modes in oscillators

The present paper deals with the interaction of a Gaussian mode with a homogeneous and an inhomogeneous laser transition. The interaction of the beam with the medium in a laser is treated by taking into account the spatial distribution of both radiation and gain. The intensity characteristics are very different from those obtained for a one-dimensional interaction of a plane wave with a saturating medium. In the presence of a small-signal gain profile the threshold condition requires much higher inversion densities along the optic axis than by ignoring this profile. For gas lasers, for instance, having a small-signal gain profile that is approximately described by a zero-order Bessel function, the threshold inversion density can be about 50 percent higher. For high-power systems the saturation of the medium by the Gaussian intensity distribution results in a considerable amount of radial radiation transport. For homogeneous transitions this amount is about equal to the stimulated emission and for inhomogeneous transitions it is about half of it, independent of the beamwidth. Further, it is found that if one slowly passes the threshold condition for laser action, the intensity jumps from zero to a certain value and vice versa. This effect has also been verified experimentally.