[1][2] The SASE process starts with an electron bunch being injected into an undulator, with a velocity close to the speed of light and a uniform density distribution within the bunch.
The emitted photons travel slightly faster than the electrons and interact with them each undulator period.
[3] Thereby the electron bunch density is periodically modulated by the radiation which is called microbunching.
SASE energy spectra show a noise-like distribution of intense spikes on top of a lower-amplitude background.
This concept has been demonstrated at the SPring-8 FEL SACLA in Japan, the Free electron LASer in Hamburg (FLASH) and the Linac Coherent Light Source (LCLS) at SLAC.