Successful start of electro-optic sampling at the TTF-FEL

The free electron lasers (FEL) in the ultraviolet and X-ray range are designed to work with an extremely high time resolution of far less than a picosecond (one millionth of one millionth of a second). The ultra-short light pulses are produced by equally short electron bunches. In order to be able to measure them, the TESLA Test Facility (TTF) was equipped with an "electro-optic sampling" setup. The principle is as follows: the transition radiation pulses, emitted by the ultrashort electron bunches, induce a birefringence in an optically active crystal, and this birefringence is sampled with polarized ultrashort laser pulses. The main challenge was to establish the time overlap between the laser pulses and the electron bunches which have a repetition frequency of only 1 Hertz.

On March 17 it was finally possible to observe clear coincidence signals of less than 2 picoseconds width in two such scans. This is an important step towards an apparatus which permits time measurements with a resolution far below one picosecond.