Abstract: An electro-optic modulator for modulating a beam of electro-magnetic radiation having a beam path and a beam axis is of a transverse type and comprises: a modulator element that is positioned in the beam path, a housing for receiving the modulator element, which housing forms a resonant cavity, and a coupling device for inputting modulating energy into the housing such that a resonant standing wave is generated within the housing. The housing comprises at least one sidewall that extends substantially parallel to the beam axis over a length, wherein the at least one sidewall comprises a deformed portion such that in the resonant cavity a distance between the sidewall and the beam axis in a direction perpendicular to the beam axis varies over at least a portion of the length of the sidewall.