Raman circuit for all-optical switch

We experimentally investigate optical switching based on stimulated Raman scattering in optical fibers. The experimental setup consists of two fiber stages connected in series with a spectral filter rejecting a signal inserted between them. When both pump and signal are launched to the input, the pump is saturated because of the signal amplification in the first stage; the amplified signal is rejected by the filter, so that only the low-power pump enters the second stage and no signal pulses appear at the output. When pump only enters at the input, it passes through the first stage without saturation, enters the second stage and amplifies the signal entering this stage; strong signal pulses appear at the output. We used 2-ns pulses at 1528 nm as the pump and a cw 1620-nm diode as the signal source for the first as well as for the second stages. In fibers with anomalous dispersion pump saturation was affected by modulation instability. We found that the contrast (the ratio of energies) may be improved using fibers with normal and anomalous dispersion connected in series in the first stage provided that the ratio between the lengths of the fibers with normal and anomalous dispersion is appropriately selected. The best achieved contrast was 15 dB at peak pump power of 6 W.