Theory and implementation of a Raman active fiber delay line

In an optical delay line made of a single-mode reentrant fiber loop, Raman amplification is used to compensate for recirculating signal losses. Concurrent Stokes noise amplification limits the system performances. A theoretical model shows that the signal-to-noise ratio decays as the reciprocal of the number of signal recirculations. Experimental results obtained with a 760-m-long fiber loop operated atlambda = 1.12 mum are presented. A new pump modulation technique resulting in improved output signal stability is reported whereupon optical delays up to 3 ms were achieved.