Diffracted light spectroscopy of the 3.39-micron CH4 system

A differential analysis of the line shapes of the intracavity saturated absorption peak of methane, observed simultaneously on both the output beam of the laser and on the diffracted beam, points out frequency shifts on the order of several tens of kHz, and also asymmetries of opposite signs. The value and sign of the shift on the principal output beam depends essentially on the cavity geometry. A 'critical geometry', analogous to that found for the classical Lamb dip, is proposed for elimination of these important shifts related to the Gaussian character of the beams and for obtaining symmetric line profiles.