DEVELOPMENT OF AN ALTERNATIVE, PHOTODIODE-BASED, FEMTOSECOND STABLE DETECTION PRINCIPLE FOR THE LINK STABILIZATION IN THE OPTICAL SYNCHRONIZATION SYSTEMS AT FLASH AND XFEL

Abstract The fs-stable timing information in the optical synchronization system at FLASH and the upcoming European XFEL is based on the distribution of laser pulses in optical bers. The optical length of the bers is continuously monitored and drifts in signal propagation time are actively compensated in order to provide a phase stable pulse train at the end of the ber link. At present, optical cross-correlation is used to measure the optical length changes. To overcome some of the disadvantages of the current scheme, a different approach for the detection of the optical ber link length variation was developed. This new scheme uses 10 GHz photodiodes to measure the amplitude modulation of harmonics created by overlapping two pulse trains. The long-term stability of the prototype of this detector over 33 h was demonstrated to be below 5 fs (peakto-peak) with a rms jitter of about 0.86 fs. The detection principle itself is practically insensitive to environmental in uences and needs only about 10 % of the optical power, compared to the optical cross-correlator