The dynamical response of a relativistic bunch of electrons injected in a p
lanar magnetic undulator and interacting with a counterpropagating electrom
agnetic wave is studied. We demonstrate a resonance condition for which the
free-electron-laser (FEL) dynamics is strongly influenced by the presence
of the external field. It opens up the possibility of control of short wave
length FEL emission characteristics by changing the parameters of the micro
wave field without requiring change in the undulator's geometry or configur
ation. Numerical examples, assuming realistic parameter values analogous to
those of the TTF-FEL, currently under development at DESY, are given for p
ossible control of the amplitude or polarization of the emitted radiation.