• S. Wesch, C. Behrens, B. Schmidt
  DESY, Hamburg
• P. Schmüser
  Uni HH, Hamburg

The observation of coherent optical transition radiation (COTR) has recently attracted a lot of attention because of its detrimental influence on OTR based diagnostic techniques, and also as evidence for a microbunching instability in magnetic bunch compressors. At FLASH, we have observed coherent visible and infrared radiation from bunches having passed the magnetic bunch compressor chicanes. The spectral distribution was measured from 400 nm to 1600 nm with high resolution for various settings of the magnet currents in the chicanes. Remarkably, the coherent visible radiation was found to be stronger for uncompressed bunches than for the compressed bunches needed for FEL operation. Additionally, images of the bunches using narrow band filters from 950 nm to 1650 nm have been recorded.

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

FLASH (The Free-Electron Laser At Hamburg) is a High-Gain SASE-FEL providing ultrashort pulses with a central wavelength of 6 to 40 nm. Measuring and controling the longitudinal shape of the electron bunches can dramatically improve the stability of the lasing process. Non-destructive electro-optical bunch profile diagnostics have proved to work with resolutions down to 100 fs. The electro-optical (EO) setup at FLASH relies on a standard Ti:sapphire laser delivering 80 fs pulses with 4 nJ pulse energy. For practical and physical reasons (i.e., space, costs, maintenance, performance) a new, ytterbium fiber laser system has been developed. This laser system supports pulse energies of 4.5 nJ and a bandwidth of 100 nm at a center wavelength of 1030 nm. Active repetition rate control allows to lock the laser to the RF based synchronisation system. A better EO signal-to-noise ratio is expected due to the improved group velocity matching in the EO crystal. First results from the prototype Yb laser system and comparison with the Ti:Sa based data will be presented. Furthermore, a structurally engineered version, promising enhanced stability and reliability will be introduced.