A free-electron laser (FEL) is a fourth generation light source producing extremely brilliant and short pulses of radiation. An FEL functions much as a laser but employs relativistic electrons as a gain medium instead of using stimulated emission from atomic or molecular excitations. In an FEL, a bunch of electrons passes through a magnetic structure called an undulator or wiggler to generate radiation, which re-interacts with the electrons to make them emit coherently, exponentially increasing its intensity.
The free-electron laser FELIX Radboud University, Netherlands.
The undulator of FELIX.
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.
CW dye laser based on Rhodamine 6G. The dye laser is considered to be the first broadly tunable laser.
A typical laser diode. When mounted with external optics these lasers can be tuned mainly in the red and near infrared.