Twofold critical geometry for lasers.

Use of a twofold critical cavity is proposed to obtain a symmetrical saturated absorption peak on a symmetrical background. The experimental verification is performed on the CH(4)3.39-microm system. This geometry is expected to provide optical frequency standards with better reproducibility.

Spatial-asymmetry distribution of a saturated-absorption peak.

The amount of asymmetry of saturated-absorption peaks observed outside a cavity with two collinearly counter-propagating Gaussian beams is analyzed at different points in the cross section of the probe beam. Although the total beam exhibits a quasi-symmetric line shape, a spatial distribution of asymmetries with different amounts and signs is obtained versus the distance to the beam axis. A simple model taking account of pure focusing and defocusing effects induced by a saturated Gaussian beam leads to agreement between experiment and theory for the 5944-A neon line in the case of saturated-absorption spectroscopy.