For future advances in accelerator physics in general and seeding of FELs in particular, precise synchronization between seed radiation, low-level RF-systems and photo-injector laser is required. Typical synchronization methods based on direct photodetection are limited by the detector nonlinearities, which lead to amplitude-to-phase conversion and introduce timing jitter. A new synchronization scheme for extraction of low jitter RF-signals from optical pulse trains distributed by mode-locked lasers is proposed. It is robust against photodetector nonlinearities. The scheme is based on a transfer of timing information into an intensity imbalance between the two output beams from a Sagnac-loop interferometer. As a first experimental demonstration, sub-100 fs timing jitter between the extracted 2 GHz RF-signal and the 100 MHz optical pulse train from a mode-locked Ti:sapphire laser is demonstrated. Numerical simulations show that scaling to sub-femtosecond precision is possible. Together with mode-locked fiber lasers and timing stabilized fiber-link, this scheme can be applied for the large-scale precise timing distribution and synchronization of free-electron laser facilities.

A new technique, combining electro-optic detection of the Coulomb field of an electron bunch with single-shot cross-correlation of optical pulses is used to enable single-shot measurements of the electric field profile of sub-picosecond electron bunches. As in our previous "spectral decoding" technique (I. Wilke et al., Phys. Rev. Lett. 88(12) 2002), the electric field of the electron bunch is encoded electro-optically on an optical pulse. However, the new "temporal decoding" method offers a much better time resolution since it overcomes a fundamental time-resolution limit of the spectral decoding method, which arises from the inseparability of time and frequency properties of the probing optical pulse. The temporal decoding technique has been applied to the measurement of 50 MeV electron bunches in the FELIX free electron laser, showing the longitudinal profile of single bunches of around 650 fs FWHM. The method is non-destructive and real-time, and therefore ideal for online monitoring of the longitudinal shape of single electron bunches. At FELIX we have used it for real-time optimization of sub-picosecond electron bunches.