We have investigated the compression mechanism of picosecond optical pulses in a far infrared waveguide free electron laser driven by a radio frequency linear accelerator. With a pertinent waveguide gapband the required original phase velocity of the electron bunch, the longitudinal length of a picosecond optical pulse will be continuously shortened so that the pulse power increases quickly until saturation occurs. In order to continue the compression process, an optimal cavity detuning &Dgr;Lcis necessary. ©1995 American Institute of Physics.