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