Contents I. Introduction
The advent of optical repeaters based on erbium-doped fiber amplifiers (EDFAs) has opened the new era of optical transmission technologies, allowing us to use wavelength-division-multiplexing (WDM) technologies with simple, compact, and economical approaches [1]. In fact, the demonstrated capacity for long-haul optical transmission has been growing remarkably, and more than a thousandfold increase in capacity has been achieved over the past ten years. Schematic diagram of dispersion map of first-generation optically amplified transoceanic system. The price we have to pay for such success is the combat with the accumulated impact of fiber nonlinearity, interplaying with the chromatic dispersion of the transmission fiber, which grows with transmission distance and, therefore, becomes significant for ultra-long-haul systems. Dispersion-management technologies have been invented to overcome such inherent problems in optically amplified transmission systems [2]. The basic idea is to use two kinds of fiber with the opposite sign of dispersion to configure a “sawtooth” pattern of the dispersion map along the system, as shown in Fig. 1. For example, the first-generation dispersion map used for 5-Gb/s single-channel transoceanic systems consisted of −0.2-ps/nm/km dispersion-shifted fiber (DSF) as a main transmission fiber and some length of single-mode fiber (SMF) placed at a certain period, something like several hundreds of kilometers. The main mission of DSF was to suppress four-wave mixing (FWM) between the signal and the amplified spontaneous emission (ASE) noise coming from the repeaters, and the role of SMF was to offset the accumulated dispersion in the main transmission fiber.
