Sub-10 fs large scale timing distribution and synchronization

Next generation light sources or free electron laser, such as FLASH at DESY, LCLS at Stanford or FERMI@ELETTRA in Trieste require essentially drift-free distribution of a timing reference signal, which is usually provided by a stable RF oscillator, throughout the facility. The conventional method of distribution, via RF-cabling, allows synchronization within approximately 100 fs (rms) long-term. In collaboration with Prof. F.X. Kaertner at MIT, we have developed and engineered two key-solutions for long-term stable, pulsed optical timing distribution and synchronization in the sub-10-fs (rms) regime: A long-term stable RF-to-optical as well as a highly sensitive optical-to-optical phase detector.

A low-phase noise pulsed laser is locked to the RF timing reference using a balanced optical-microwave phase detection scheme, (BOM-PD) based on a Sagnac interferometer (1). The BOM-PD is to first order insensitive to temperature induced phase drifts between the reference and the signal, overcoming therefore a widely know problem when using traditional rf-components like rf-phase detectors or mixers. With this scheme it is possible to achieve long-term stable synchronization between the two oscillators. With the current implementation we have achieved a residual short-term jitter of < 7 fs (rms) from 10 Hz to 10 MHz, and a long-term drift of < 10 fs (rms) over eight hours. These measurements were done at the newly built Free-electron laser at FERMI@ELETTRA in Trieste, Italy.

The optical signal, synchronized to the RF master oscillator, is then distributed throughout the facility using fiber-optical links, sometimes hundreds of meters in length. The link length needs to be stabilized, since temperature changes, as well as other environmental factors affect the link length, and thus the propagation time of the pulses through the link. We achieve optical-to-optical synchronization with a single-crystal balanced cross-correlator (2). With a sensitivity of ~20 mV/fs the jitter introduced by any length variations of the link can be detected with atto-second resolution and compensated for using a feedback loop.

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BOM-PD short-term jitter from 10 Hz to 10 MHz.

Long-term drift between laser and RF oscillator.