Sidebands are observed at 72 GHz off an optical carrier at 633 nm. The laser beam is phase modulated in a LiNbO3 crystal which is exposed to a mm-wave field inside a Fabry-Perot resonator. Under optimum phase-matching conditions we obtain a modulation index of 5% derived from 200 mW microwave power. Phase matching is obtained by guiding the light beam on a zig-zag path under internal total reflections. For this unconventional type of phase matching, the optical wave fronts travel at twice the speed of the modulating wave along the resonator axis.