Single-shot determination of focused FEL wave fields using iterative phase retrieval.

Determining fluctuations in focus properties is essential for many experiments at Self-Amplified-Spontaneous-Emission (SASE) based Free-Electron-Lasers (FELs), in particular for imaging single non-crystalline biological particles. We report on a diffractive imaging technique to fully characterize highly focused, single-shot pulses using an iterative phase retrieval algorithm, and benchmark it against an existing Hartmann wavefront sensor. The results, both theoretical and experimental, demonstrate the effectiveness of this technique to provide a comprehensive and convenient shot-to-shot measurement of focused-pulse wave fields and source-point positional variations without the need for manipulative optics between the focus and the detector.