In a free-electron laser the laser emission comes from the synchrotron radiation of a beam of accelerated electrons. Its principle is therefore very different from other types of laser. The beam is generated by a particle accelerator (like the ones used by physicists for studying particles). The electrons are injected into the middle of a succession of permanent magnets - the undulator - creating a periodic magnetic field. In the undulator two parallel mirrors reflect the synchrotron radiation emitted by the electron beam which becomes coherent (it therefore has the properties of laser radiation).
Costly to produce, free electron lasers can operate over a very wide range of frequencies from infrared to X-rays by modifying the speed of the electrons. The power can be adjusted by changing their rate.