Enhanced transmission of nonlinear optical loop mirrors using SOAs with robust transparency

Nonlinear optical loop mirrors based on transparent SOAs (TSOA-NOLMs) exhibit fast switching with a recovery time of the order of a few picoseconds, cascadable switching functionalities, low crosstalk between adjacent channels and low noise figures. In a TSOA-NOLM, following the passage of the pump pulse, the SOA may be considered to be transparent to the probe pulse if the carrier density is immediately restored to the value existing prior to the arrival of the pump pulse. The transparency carrier density is, however, significantly dependent upon the pump pulse power. Furthermore, due to the small net optical gain in a TSOA-NOLM, a high pump pulse power is usually required to achieve a specific switching output energy. Therefore, both the strong pump power dependence of the transparency carrier density of SOAs and the requirement of high switching powers would seriously limit the utilisation of TSOA-NOLMs in practice. In this paper, calculations predict that long SOAs (>500 μm) can be employed in TSOA-NOLMs to not only considerably reduce the pump power dependence of the transparency carrier density of the SOAs but also to significantly increase the transmission of the switches