A free-electron laser ( FEL ) , is a type of laser that uses very-high-speed electrons that move freely through a magnetic structure , hence the term free electron as the lasing medium .
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 .