Lineshape-asymmetry elimination in weak atomic transitions driven by an intense standing wave field

Owing to the ac-Stark effect, the lineshape of a weak optical transition in an atomic beam can become significantly distorted when driven by an intense standing wave field. We use an Yb atomic beam to study the lineshape of the 6s²S1₀→5d6sD3₁ transition, which is excited with light circulating in a Fabry-Perot resonator. We demonstrate two methods to avoid the distortion of the transition profile. Of these, one relies on the operation of the resonator in multiple longitudinal modes, and the other in multiple transverse modes.