• M. Bousonville
  GSI, Darmstadt
• J. Rausch
  TUD, Darmstadt

In this paper, a method for measuring the absolute signal delays of active optical transmission lines will be presented. This measurement method is an essential part of the timing system for FAIR (Facility for Antiproton and Ion Research). To prevent interference of the timing signals whose delays are to be measured with the measurement signal sequence, the latter is transmitted on a separate optical carrier in the same fibre. By using a wavelength selective mirror at the end of the transmission line, the optical measurement signals are reflected and lead back to the measurement unit. The measurement sequence consists of a number of sinusoidal signals with different frequencies that are modulated one by one on the optical carrier. For each frequency a phase comparison of the outgoing and returning signal is performed. In the last step, the absolute delay is calculated from the obtained phase values by using an algorithm. It will be shown that this method enables cost efficient delay measurements with an accuracy of better than 100 fs.