Hanbury Brown and Twiss interferometry at a free-electron laser

TL;DR

This study demonstrates that free-electron laser radiation exhibits high transverse coherence and Gaussian statistics, with measurements of intensity correlations revealing a pulse duration of about 50 femtoseconds.

Contribution

First measurement of higher-order intensity correlations at a free-electron laser, confirming its chaotic, laser-like coherence properties.

Findings

High transverse coherence (~80%) of FEL beam

FEL radiation obeys Gaussian statistics

Estimated pulse duration of 50 fs

Abstract

We present measurements of second- and higher-order intensity correlation functions (so-called Hanbury Brown and Twiss experiment) performed at the free-electron laser (FEL) FLASH in the non-linear regime of its operation. We demonstrate the high transverse coherence properties of the FEL beam with a degree of transverse coherence of about 80% and degeneracy parameter of the order 10^9 that makes it similar to laser sources. Intensity correlation measurements in spatial and frequency domain gave an estimate of the FEL average pulse duration of 50 fs. Our measurements of the higher-order correlation functions indicate that FEL radiation obeys Gaussian statistics, which is characteristic to chaotic sources.