A free electron laser, or FEL, is a laser that generates tunable, coherent, high power radiation, currently ranging in wavelength from microwaves to the visible spectrum. While an FEL laser beam shares the same optical properties as conventional lasers such as coherent radiation, the operation of an FEL is quite different. Unlike gas or diode lasers which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free-electron.