The on-off contrast in an all optical switch based on stimulated Raman scattering in optical fibers

We investigate optical switching based on stimulated Raman scattering. The circuit 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. Second stage is fed by 1-mW power at signal wavelength. 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. The on-off contrast is deteriorated by the pulse shape because the pump saturation is observed in the central part of pulses, by fiber GVD, etc. These effects were not considered before. We used 2-ns pulses at 1528 nm as the pump and a 1620-nm cw as the signal. We used in the first stage both fibers with normal and anomalous dispersion. In fibers with anomalous dispersion pump saturation was affected by modulation instability. We found that the contrast 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 6-W pump peak power.