Fundamental switching contrast and extinction ratio degradations in ultrafast all-optical switching using nonlinear optical loop mirrors

Nonlinear optical loop mirrors (NOLM) and nonlinear amplifying loop mirrors (NALM) are two good high-speed time-division multi-/demultiplexers. Two key parameters in determining their performance are the switching contrast and the extinction ratio. A poor switching contrast degrades the extinction ratio of the switched signal, which in turn degrades the bit-error-rate (BER) performance. Although complete switching can be achieved in NALM theoretically, only incomplete switching was shown in most of the experimental results reported previously. In this paper, we explain the degradation both theoretically and numerically. We have shown that in ultrafast all-optical switching using NOLM/NALM, the extinction/contrast ratio is degraded due to the pedestal effect and directional gain difference. Possible solutions to solve this problem are to use the walk-off effect or to use an additional NOLM to improve the extinction ratio.