• L. Fröhlich, O. Grimm
  DESY, Hamburg

The longitudinal charge distribution of short electron bunches can be characterized by a measurement of their coherent far-infrared radiation spectrum. This paper will present the results obtained at the DESY VUV-FEL linear accelerator by observation of synchrotron radiation with a Martin-Puplett interferometer. The reconstructed bunch shapes are strongly asymmetric with a full width at half maximum of about 1 ps.

• H. Delsim-Hashemi, O. Grimm, J. Rossbach, H. Schlarb, B. Schmidt, P. Schmuser
  DESY, Hamburg
• A.F.G. van der Meer
  FOM Rijnhuizen, Nieuwegein

Funding: DESY

FEL facilities are pushing to achieve higher peak currents mainly by means of compressing bunches longitudinally. This process defines a machine parameter that has to be fine-tuned empirically. Among the operational types of diagnostic tools for longitudinal phase-space are those based on IR spectroscopy. The most commonly used IR spectrometers at the FEL facilities are operating in the scanning mode and are not fast enough to be applicable for monitoring bunch compression. On the other hand, any non-scanning spectrometer may suffer from the low intensity that is available from coherent IR radiation in short time intervals in different wavelengths. The proposed "Single Shot Spectrometer" is based on using gratings as dispersive elements. Pioneering tests with a transmission grating have shown the feasibility of the concept. In a second step, a version with "Reflective Blazed Grating" will be tested and should allow getting the maximum available signal for the whole spectrum and improved resolution. Parallel to the study of optical parts, an array of pyroelectric detectors with integrated multi-channel readout is under development.

• D. Suetterlin, V. Schlott, H. Sigg
  PSI, Villigen
• D. Erni, H. Jäckel
  ETH, Zurich
• A. Murk
  University of Berne, Institute of Applied Physics, Berne

The polarization dependent intensity distribution of coherent transition radiation (CTR) emission has been studied theoretically and experimentally at an optical beam port downstream the 100 MeV SLS pre-injector LINAC. Based on these analyses, a spatial interferometer using the vertically polarized lobes of CTR has been designed and installed at this location. While a proof of principle of this bunch length monitor was achieved by step-scan measurements with a Golay cell detector, the single shot capability has been demonstrated by electro-optical correlation of the spatial CTR interference pattern with the fairly long Nd:YAG laser pulses in a ZnTe crystal. In single-shot operation variations of the bunch length due to different settings of the LINACs bunching cavities have been observed.

• J.W.  Staples, R.B. Wilcox
  LBNL, Berkeley, California

Funding: This work supported by the US Department of Energy under contract No. DE-AC03-76SF00098

Stabilization of the transit time through a glass fiber using an optical heterodyne technique promises to provide jitter reduction down to the few femtosecond level using inexpensive commodity hardware. An acousto-optical frequency shifter provides the optical frequency offset that is used to downconvert phase shifts at optical frequency to equivalent phase shifts at radio frequency which are used to close a phase-lock loop driving a piezoelectric phase shifter. Using the stabilized fiber transmission medium, two spectral lines of a mode locked laser lock two low-power CW lasers which are transmitted to a receiver which phase locks the same spectral lines of a second mode-locked laser to the first. The optical transmission system operates at low power and is linear, providing excellent signal-to-noise ratio and allows many signals to be transmitted without mutual interference. Experimental results will be presented.