A free-electron laser, or FEL, is a laser that produces a very bright beam of light. It is basically a super flashlight. It shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power. The FEL used operating principles to form the beam that are very different operating principles of a conventional laser. Unlike gas, liquid, or solid-state lasers such as diode lasers, in which electrons are excited while bound to atoms, FELs use a relativistic electron beam as the lasing medium which moves freely through a magnetic structure, hence the term free electron.^[1] The free-electron laser has the widest frequency range of any laser type, and can be widely tunable,^[2] currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to ultraviolet, to X-rays.^[3]

Free—electron lasers were invented by John Madey in 1976 at Stanford University. The work builds on research done by Hans Motz and his coworkers who made the first undulator at Stanford in 1953 using the wiggler magnetic configuration which is at the heart of a free-electron laser. Madey used a 24 MeV electron beam and 5 m long wiggler to amplify a signal. Soon afterward, other laboratories with accelerators started developing such lasers.

Free-electron lasers use a lot of electricity when they operate. To reduce the energy needed to keep them operating, scientist use an energy recovery linear accelerator to recycle the high-energy electron beam that activates the laser.