In this Paper we investigate the optimization of constellations for polarization-division multiplexed transmission
in standard single-mode fiber systems and for space-division multiplexing in a multi-mode fiber system.
On the following pages we address some actual topics of analog and digital electronic equalisation for 10 and 40 Gbit/s transmission. Analog circuits currently overcome the bandwidth limits for 40Gb/s operation. With first 5 tap feed forward equaliser (FFE) SiGe chips CD and PMD tolerance enhancement can be demonstrated. According to numerical simulations, the combination of the FFE with a one-stage decision feedback equaliser (DFE) should be a good choice to mitigate signal degradation caused by various effects. At 10 Gb/s more complex signal processing based on digitalsignal-processing (DSP) is already implemented in first products as maximum likelihood sequence estimator (MLSE, also referred to as Viterbi equaliser VE) or in a lab prototype of soft error correction (soft FEC). Detailed numerical
studies on the performance of the VE reveal that opposite to the analog FFE+DFE, low electrical bandwidth cannot be handled by the standard MLSE scheme and an extension of the processing algorithm is needed. Though the combination of an MLSE based equaliser and soft-FEC has the potential to lead to further improvement, numerical analysis of turbo equalisers combining distortion mitigation and error correction in an iterative way indicate that with a FEC overhead in the range of 7% only moderate equalisation improvement seems to be possible.
Nonlinear optical fiber loop mirrors (NOLM) are attractive devices for multiplexing optical signals with bitrates of up to 100 Gbit/s and beyond. So far, less attention has been paid to the relation between design of the demultiplexer and the resulting system performance characterized by parameters like input power penalty and wavelength tolerance. Based on our own measurements of a polarization-independent 40 Gbit/s demultiplexer and in conjunction with a new theoretical approach modeling NOLM transmission and worst-case input power penalty of the demultiplexer, a device capable of 100 Gbit/s operation is analyzed. The survey illustrates the potential and trade-offs of the NOLM concept assuming a limited cw power of the switching wave. E.g., the calculations show that a 14.5 dBm booster EDFA together with a loop fiber length between 1 and 2 km allow low penalty drop rates (< 6 dB worst-case penalty) of the demultiplexed channel between 1.3 and 8.8 Gbit/s. Then 100 Gbit/s operation should be possible within a wavelength range of at least 5 nm.