  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 6s2
1S0 -> 5d6s 3D1 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.
