Physikalisch-Technische Bundesanstalt

Progress in Large-Scale Long-Term Stable Timing Distribution

Kolloquium der Abteilung 4

Next generation X-ray Free-Electron Lasers will need timing distribution systems enabling synchronization of all laser and rf-sources to sub-10-fs precision today and scalability to potentially 100 attosecond precision in the future. In this talk, a set of ultrafast optical techniques for long-term stable femtosecond synchronization of these large-scale X-ray FELs will be presented. Low-noise optical pulse trains generated from mode-locked lasers, and tightly synchronized to a microwave reference, are distributed via timing-stabilized fiber links to end-stations where precise timing and synchronization is required. The timing links are length stabilized using balanced optical cross-correlators. At these end-stations, optical and RF sub-systems are synchronized with the delivered timing signals. For recovery of low-jitter RF-signals a balanced optical-microwave phase detector has been developed. Using these techniques, we demonstrate that remotely located lasers and microwave sources can be synchronized with femtosecond accuracy over typical operating periods of FELs. Experimental results, that demonstrate synchronization of lasers over 300 m standard single-mode fiber links with sub-10 fs precision over one week, and of RF sources over 10 hours, are presented. Due to the robustness of the timing of short pulses to noise, femtosecond lasers may enable timing distribution well below 1 fs.  We demonstrate first results using polarization maintaining fiber links that show this potential. Long term stability can be further improved using integrated balanced optical cross correlators.

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