Self-amplified stimulated (or spontaneous) emission (SASE) is a process within a free-electron laser (FEL) by which a laser beam is created by the high-energy electron beam. The lasing starts up from the random microbunching (i.e., shot noise

Shot noise

Shot noise is a type of electronic noise that may be dominant when the finite number of particles that carry energy is sufficiently small so that uncertainties due to the Poisson distribution, which describes the occurrence of independent random events, are of significance...

) on the electron beam instead of being coherently produced by an input seed laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 source. This source is fully transversely coherent at saturation, but, because the radiation starts from random noise at many radiation wavelengths, the longitudinal coherence of the radiation is less than that of the amplifier case but better than that of spontaneous radiation.

This concept has been demonstrated at the SPring-8

SPring-8

SPring-8 is a synchrotron radiation facility located in Hyōgo Prefecture, Japan and run by the Japan Synchrotron Radiation Research Institute. The machine consists of a storage ring containing an 8 GeV electron beam...

 FEL in Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, the Free electron LASer (FLASH) in Hamburg and the Linac Coherent Light Source (LCLS) at SLAC.