Abstract

In today’s ultra-long haul WDM transmission systems, the reach of the system is increased by the use of forward error correction (FEC) coding.^1,2 A typical first generation FEC coding uses Reed-Solomon codes with an overhead of about 7%. This amount of overhead can at best give a coding gain of about 6 dB, which translates to both reach and system margins. FEC becomes most effective when the signal quality has degraded with the addition of white gaussian noise such that the optical signal to noise ratio (OSNR) under this situation causes a bit error rate error floor at around 2 × 10^–4. Gaussian noise typically originates in optical amplifiers used in between fiber spans. However, in a communication network, which has both amplified spans as well as optical add-drop multiplexers (OADM), there is one additional source of noise, namely, in-band coherent crosstalk. This originates at the OADM, when a dropped wavelength channel is not dropped in its entirety, and the remnant signal interferes with the added channel at the same wavelength, giving rise to in-band crosstalk at the point of detection. The statistics of this type of crosstalk is neither Gaussian nor purely Lorentz-ian.³ To-date there has been no investigation of the performance of FEC coding when interferometric noise is added to gaussian noise.

© 2002 Optical Society of America