AMPLIFIER INDUCED CROSSTALK IN MULTICHANNEL OPTICAL NETWORKS

We study the effect of crosstalk introduced due to gain saturation in an optical amplifier when it is used for amplifying multiple channels in a wavelength division multiplexed network employing ON-OFF keying with direct detection. The system power penalty is quantified as a function of the amplifier input power, the number of channels, and the extinction ratio. We consider two cases: the low-data-rate case, where the product of the data rate and the gain response time of the amplifier is small, typical of semiconductor-laser amplifiers, and the high-data-rate case, where the product is large, typical of doped-fiber amplifiers. We find that the system penalty due to the presence of other interfering channels can be decomposed into two components: a term arising from the steady-state reduction in the amplifier gain due to the larger average input power, and a crosstalk component because of the variation in the gain due to the randomness of the total input power around the mean. Results indicate that when the number of channels is small, the crosstalk component dominates, but when the number of channels is large the component due to the steady-state gain reduction is more significant. For the high-data-rate case, the crosstalk penalty is no longer present, and only the component due to the steady-state gain reduction, is retained. We study the performance of frequency-shift keying as an alternative to ON-OFF keying and suggest techniques for reducing the crosstalk penalty. Finally, we show that using amplifiers in a passive star network allows a significantly larger number of stations to be supported.