DESY, Hamburg
Uni HH, Hamburg
At FLASH, UV and soft X-Ray pulses with durations in the order of 10 fs are generated. To fully exploit the opportunities provided by these short laser pulses, an optical synchronization system provides the possibility to synchronize external lasers and stabilize the electron bunch arrival time with 10 fs precision. A seeded free-electron-laser (FEL) section, called sFLASH, is installed upstream of the existing SASE undulators. After higher-harmonic-generation, the femtosecond seed laser pulse needs to be temporarily and spatially overlapped with the electron bunch. Furthermore, for time-resolved pump-probe experiments, using an experimental laser and the FEL pulse, either of sFLASH or of the ordinary SASE process, the synchronization between pump and probe laser pulses is crucial. While the best performance for synchronizing these lasers within 10 fs will be achieved by using an optical cross-correlator, in this paper we present a precursor that relies on an RF-based locking mechanism. The setup includes a coarse and a fine phase measurement between the laser pulses of the reference and the synchronized system after their conversion to an RF signal.
Uni HH, Hamburg
DESY, Hamburg
The optical synchronization system of the free-electron laser in Hamburg (FLASH) is based on the stabilized pulse-train distribution of a passively mode-locked laser. This master laser oscillator is based on erbium-doped fiber technology and is built in a σ-configuration, enabling passive mode-locking through nonlinear polarization evolution. Recently, a commercial laser system has been installed in addition to the existing laser. Besides maintenance-free operation, this SESAM-based laser shows an even lower timing jitter, enabling a tighter synchronization to the accelerator's RF reference. In this paper we report on the commissioning, the characterization and the long-term stabilty of the new laser system, as well as on the performance of the laser with the existing pulse-train distribution scheme and optical front-ends of the synchronization system in comparison to the old one.