• M. Felber, V.R. Arsov, M.K. Bock, P. Gessler, K.E. Hacker, F. Löhl, F. Ludwig, K.-H. Matthiesen, H. Schlarb, B. Schmidt, A. Winter
  DESY, Hamburg
• S. Schulz, L.-G. Wißmann, J. Zemella
  Uni HH, Hamburg

Next generation FEL light sources like the European XFEL require timing stability between different subsystems of 10^-20 fs. In optical synchronization systems, the timing information is distributed across the facilities via sub-ps laser pulses travelling on length stabilized optical fibers. Different methods are available for RF extraction from the pulse train. In this paper, we characterize the long-term phase stability of a 1.3 GHz signal gained from the direct conversion of a higher harmonic of the pulse repetition frequency, and from a voltage controlled oscillator locked with a PLL that uses a Sagnac-Loop as balanced optical-microwave phase detector.

• P. Gessler, K. Rehlich
  DESY, Hamburg
• C. Bohm, A. Hidvégi
  Stockholm University, Stockholm

For the operation of the European X-Ray Free Electron Laser (XFEL), a system wide synchronous low-jitter clock and precise, adjustable triggers must be generated and distributed throughout the approximately 3.5 km long facility. They are needed by numerous diagnostics, controls, and experiments. Fast ADCs require the jitter of the distributed 1.3GHz clock to be in the order of a few pico seconds (RMS) and that it is synchronized to the accelerating RF. The phase of the 1.3GHz clock must therefore be adjustable at every endpoint. Due to cable lengths, and the temperature dependence of the propagation speed, temperature drifts are a serious issue. Therefore a complex monitoring and compensation mechanism has been developed to minimize these effects. Triggers must also be distributed throughout the system to synchronize different control or measurement tasks. The triggers must be adjustable in time in order to compensate for different cable lengths and should have a resolution of one ns but with ps stability. A prototype of this clock and trigger system has been developed and first measurements have shown, that the strong requirements can be fulfilled.

• S. Schulz, L.-G. Wißmann, J. Zemella
  Uni HH, Hamburg
• V.R. Arsov
  PSI, Villigen
• M.K. Bock, M. Felber, P. Gessler, F. Ludwig, K.-H. Matthiesen, H. Schlarb, B. Schmidt
  DESY, Hamburg
• F. Löhl
  CLASSE, Ithaca, New York
• A. Winter
  ITER, St Paul lez Durance

The free-electron laser FLASH and the planned European XFEL generate X-ray light pulses on the femtosecond time-scale. The feasibility of time-resolved pump-probe experiments, special diagnostic measurements and future operation modes by means of laser seeding crucially depend on the long-term stability of the synchronization of various laser systems across the facility. For this purpose an optical synchronization system is being installed and tested at FLASH. In this paper, we report on the development and the performance of a background-free optical cross-correlation scheme to synchronize two individual mode-locked lasers of different center wavelengths and repetition rates with an accuracy of better than 10 fs. The scheme can be tested by linking a Ti:sapphire oscillator, used for electro-optical diagnostics at FLASH, to both a locally installed erbium-doped fiber laser and the end-point of an actively length-stabilized fiber link distributing the pulses from a master laser oscillator. After the commissioning of this fiber link, the diagnostics laser can be synchronized to the electron beam and first accelerator based measurements on the performance of the system will be carried out.