• J. Bödewadt, A. Azima, F. Curbis, H. Delsim-Hashemi, M. Drescher, Th. Maltezopoulos, V. Miltchev, M. Mittenzwey, J. Roßbach, S. Schulz, R. Tarkeshian, M. Wieland
  Uni HH, Hamburg
• S. Düsterer, J. Feldhaus, T. Laarmann, H. Schlarb
  DESY, Hamburg
• R. Ischebeck
  PSI, Villigen
• S. Khan
  DELTA, Dortmund
• A. Meseck
  BESSY GmbH, Berlin

Funding: This work is supported by the Bundesministerium für Bildung und Forschung under contract 05 ES7GU1

A seeded free-electron laser operating in the soft X-ray (XUV) spectral range will be added to the SASE FEL facility FLASH. For this purpose, a 40 m long section upstream of the existing SASE undulator will be rebuilt during the shutdown in fall 2009. This includes the injection of the seed beam into a new 10 m variable-gap undulator, the out-coupling of the seeded FEL radiation and all diagnostics for photon- and electron beams. The XUV seed pulse is generated by high harmonics (HHG) from a near-infrared laser, optically synchronized with FLASH. After amplification within the undulators the XUV light will be guided towards diagnostic stations. Besides a proof-of-principle demonstration for seeding at short wavelength the purpose of this development is to provide future pump-probe experiments with a more stable FEL source in terms of spectral properties and timing.

• 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.

• 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.