5.1 INTRODUCTION

System performance can be maximized and total system cost savings can be realized by choosing an optical fiber design optimized for a particular system application. The cabled optical fiber that forms the backbone of the physical layer is one part of an optical transmission line that also comprises amplifiers and dispersion compensation modules (DCMs). The designs of the amplifier, DCM, and cabled transmission fiber are not mutually independent, and an integrated view of the transmission line design is necessary to optimize performance and drive cost out of the total system.

A digital transmission system relies on the ability of a receiver to discriminate whether a transmitted bit is a 1 or 0. The ability to do this can be degraded by multiple factors. First, the signal-to-noise ratio (SNR) can be degraded in either the optical or the electrical domain. Insufficient optical SNR (OSNR) can be due to noise in the transmitter, optical attenuation in the fiber span, the injection of noise by an amplifier, or insufficient receiver sensitivity. Second, intersymbol interference (ISI) occurs as adjacent bits (symbols) spread into one another and overlap due to dispersion in the fiber. ISI can cause a 0 to be corrupted by a neighboring 1. ISI arises from both chromatic and polarization mode dispersion. In a purely linear system, ISI can be fully compensated, at least theoretically, by a dispersion compensation device.

Nonlinearity in the optical transmission...