  Electro-optical detection has proven to be a valuable technique to study
temporal profiles of THz pulses with pulse durations down to femtoseconds. As
the Coulomb field around a relativistic electron bunch resembles the current
profile, electro-optical detection can be exploited for non-invasive bunch
length measurements at accelerators. We have developed a very compact and
robust electro-optical detection system based on spectral decoding for bunch
length monitoring at the European XFEL with single-shot resolution better than
200~fs. Apart from the GaP crystal and the corresponding laser optics at the
electron beamline, all components are housed in 19\" chassis for rack mount and
remote operation inside the accelerator tunnel. An advanced laser
synchronization scheme based on radio-frequency down-conversion has been
developed for locking a custom-made Yb-fiber laser to the radio-frequency of
the European XFEL accelerator. In order to cope with the high bunch repetition
rate of the superconducting accelerator, a novel linear array detector
(KALYPSO) has been employed for spectral measurements of the Yb-fiber laser
pulses at frame rates of up to 2.26~MHz. In this paper, we describe all
sub-systems of the electro-optical detection system as well as the measurement
procedure in detail, and discuss first measurement results of longitudinal
bunch profiles of around 400~fs (rms) with an arrival-time jitter of 35~fs
(rms).
