Amplified Spontaneous Emission'

We review the coherence properties of the self-amplified spontaneous emission (SASE). Temporally, SASE is similar to the spontaneous undulator radiation except that the spectral bandwidth is about ten times narrower compared with typical undulator radiation. The situation is quite different in the transverse dimension, where SASE is fully coherent. INTRODUCTION Several laboratories are pursuing the R and D of the linac-based light source based on the SASE principle [l], [2] for its promise of extreme high brightness and time resolution [3]. It therefore is important to clearly understand the coherence properties of SASE, in particular, compared to the current generation of synchrotron radiation sources. This paper is a review of this topic. In the temporal (longitudinal) dimension, the first order coherence of SASE sources is improved about ten times compared with typical undulator sources in current generation synchrotron radiation facilities. The second order coherence has to do with the intensity fluctuation, and SASE in this regard is very similar to undulator radiation, consisting of a random superposition of wave packets. The fluctuation property of such light, known as thermal or chaotic light, is well-known [4]: the fluctuation of intensity at a given time or frequency is 100%. However, the fluctuation for an integrated intensity is smaller by a factor 6, where m, is the number of modes in the integration interval. The probability distribution of the integrated intensity is given by I) This work was supported by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Division of High Energy Physics, of the U.S. Department of Energy under Contract No. DEAC 03-76SF00098. 1 @ 1997 American Institute of Physics