Self-amplified spontaneous emission (SASE) is a process within a

free-electron laser 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 active laser medium, gain medium instead of using ...

(FEL) by which a laser beam is created from a high-energy

electron beam Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy. Mechani ...

. The SASE process starts with an electron bunch being injected into an

undulator An undulator is an insertion device from high-energy physics and usually part of a larger installation, a synchrotron storage ring, or it may be a component of a free electron laser. It consists of a periodic structure of dipole magnets. These ca ...

, with a velocity close to the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

and a uniform density distribution within the bunch. In the undulator the electrons are wiggled and emit light characteristic of the undulator strength but within a certain energy bandwidth. The emitted photons travel slightly faster than the electrons and interact with them each undulator period. Depending on the relative phase between electrons and photons, electrons gain or lose energy (velocity), i.e. faster electrons catch up with slower ones. Thereby the electron bunch density is periodically modulated by the radiation which is called microbunching. The structured electron beam amplifies only certain photon energies at the cost of kinetic energy until the system goes into saturation. SASE energy spectra show a noise-like distribution of intense spikes on top of a lower-amplitude background. The micro-bunch structuring reduces the

phase space The phase space of a physical system is the set of all possible physical states of the system when described by a given parameterization. Each possible state corresponds uniquely to a point in the phase space. For mechanical systems, the p ...

available to the photons, thus they are also more likely to have a similar phase and the emitted beam is quasi-coherent. This concept has been demonstrated at the SPring-8 FEL SACLA in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, the Free electron LASer in Hamburg (

FLASH Flash, flashes, or FLASH may refer to: Arts, entertainment, and media Fictional aliases * The Flash, several DC Comics superheroes with super speed: ** Flash (Jay Garrick) ** Barry Allen ** Wally West, the first Kid Flash and third adult Flash ...

) and the Linac Coherent Light Source (LCLS) at SLAC.

References

Laser science Synchrotron radiation {{Physics-stub

See also

References