The free-electron laser (FEL) is a particular type of laser in which the optical element, that is, the medium that is excited to produce the photons, consists of a beam of very fast (relativistic) electrons.

• When electrons are forced to move along a curved trajectory they emit electromagnetic energy whose frequency depends both on the speed of the electrons and on the curvature of their trajectory.
• In the FEL the trajectory of the electrons is curved by a device made of a sequence of magnets with alternating north and south poles, called an undulator.
• The radiation produced in this way is captured by the mirrors placed at the ends of a resonant cavity.
• The oscillatory motion of the electrons between the magnets of the undulator stays in phase with the radiation, and therefore causes the amplification of the light, exactly as happens in an ordinary laser.
• By adjusting the speed of the electrons and the magnetic field of the undulator, photons can be emitted at different frequencies, from microwaves up to X-rays. Unlike an ordinary laser, therefore, the radiation emitted by the FEL is widely tunable.