Long-term Stable, Large-scale, Optical Timing Distribution Systems with Sub-femtosecond Timing Stability*

Sub-fs X-ray pulse generation in kilometre-scale FEL facilities will require sub-fs long-term timing stability between optical sources over kilometer distances. We present here key developments towards a completely fiber-coupled, pulsed optical timing distribution system capable of delivering such stability. First, we developed a novel 1.2-km dispersion-compensated, polarizationmaintaining fiber link to eliminate drifts previously induced by polarization mode dispersion. Link stabilization for 16 days showed 0.6 fs RMS timing drift and during a 3-day interval only 0.13 fs drift. Second, we verified that ultralow-noise optical master oscillators for sub-fs timing distribution are available today; the measured jitter for two commercial femtosecond lasers is less than 70 as for frequencies above 1 kHz. Lastly, we fabricated a hybrid-integrated, balanced optical crosscorrelator using PPKTP waveguides to eliminate alignment drifts and for future reduction of the link operation power by a factor of 10-100.