A critical geometry for lasers with internal lenslike effects

We point out here an interesting property concerning Gaussian mode size evolution when a thin lens or induced lenslike effects are inserted in a laser cavity. When the distance d between mirrors of a half-symmetric Fabry-Perot with a lens at half-length is changed, the variations of the mode size at the curved mirror cancel for dc ≅ 4 R/5, and change sign when d ≧ dc (where R is the mirror curvature radius). This property explains simply opposite asymmetries observable on Lamb dips, saturated absorption peaks, and intracavity absorption spectroscopy resonances. The experimental verification is performed on the Lamb dip at 3.39 μm by diffracted light spectroscopy.