We show that the cavity round-trip Gouy phase leads to pulse-to-pulse varia
tion of the absolute phase and temporal profile of circulating few- or sing
le-cycle pulses in empty resonators. This pulse-to-pulse variation can be e
liminated by the proper insertion of a lens into the cavity. An application
to terahertz resonators with phase-locked feedback is discussed, (C) 2000
Optical Society of America [S0740-3232(00)01211-4].