Timing the Laser and the FEL

Electron bunch time pattern of FLASH

Electron bunch time pattern of FLASH with 10 Hz repetition rate and up to 500 bunches in an 500-μs-long bunch train. The separation of electron bunches within a train is 1 μs. To a certain extent, the bunch distance in a train can be varied; for instance to 2, 5, or 10 μs and some other distances. The duration of the electron bunches is 20 to 300 fs. The nonlinear FEL process reduces the duration of the photon pulses down to 10 - 200 fs.

optical laser pulse and the synchrotron radiation pulse

Image of the optical laser pulse and the synchrotron radiation pulse as a reference for the FEL arrival time on the streak camera. The shown time frame is about 140 ps.

Since the FEL and the optical laser are independent sources of fs-pulses (few 10 femtoseconds pulse duration for the FEL and 60 fs for the optical laser), the synchronization between both is of vital importance to perform well defined pump-probe experiments.

In order to use the optical pulses in combination with the XUV pulses it is mandatory that the relative jitter between the two pulses is less than the pulse duration (or at least known to this precision). Tremendous effort is made to enable stable synchronization and precise measurement of the remaining temporal jitter and drift.
The synchronization of the optical laser to the FEL relies on a low phase noise optical master clock which is synchronized to the accelerators 1.3GHz RF master clock to prevent drifts of the optical master. The signal from the optical master clock is optically distributed via length stabilized links to the pump probe laser hutch. Together with optimized photo diode based phase detectors and a digital lock controller, the short term jitter of the optical laser vs the optical master oscillator could be reduced to ~30fs RMS.First tests showed that with an optical cross-correlator as phase detector even a jitter well below 10fs can be reached.

S. Schulz, et al., Femtosecond all-optical synchronization of an X-ray free-electron laser,
Nature Communications 6, 5938 (2015);

In addition an optical cross correlator monitors and corrects for the temporal drift induced by the amplifiers. The optical cross correlator measures the relative arrival time of the seed and amplified laser beam inside the laser hutch. Due to changes of the refractive index inside the laser hutch or thermal expansion of the optical breadboards the path length and hence the arrival time of the laser could change. Without correction this drift caused by the laser can be as big as 3ps/day. The remaining drift between the optical laser and the FEL pulse is monitored using a streak camera. Part of the optical laser pulse is reflected to a streak¬ camera, which records its arrival time . For the timing reference of the FEL pulse the optical portion of a synchrotron radiation pulse is used. This synchrotron radiation pulse is produced when the electrons are deflected by a dipole magnet into the dump. This synchrotron radiation pulse is inherently synchronized to the FEL pulse and guided to the streak camera that also records its arrival time. The temporal separation between FEL pulse and pulse from the optical laser on the Streak-Camera is set to an arbitrary value by a delay line. This temporal separation is then measured and recorded permanently. In that way drifts happening in the time frame of a few seconds can be monitored with 0.3 ps resolution.