Fiber Bragg gratings and chromatic dispersion

Since the late 1990s, steady advances in wavelength division multiplexing (WDM) technology have provided better ways to increase the capacity of optical networks. As the industry makes the transition to advanced optical networks, three significant trends become evident in long-haul transmission system, namely the continual increase in the number of dense WDM channels, the increase in data rates from 2.5 Gb/s to today's 10 Gb/s, to tomorrow's 40 Gb/s, and finally longer distances between electrical regeneration sites. These trends towards an increased optical network capacity are now clashing with chromatic dispersion. The management of chromatic dispersion due to the optical fiber and optical components is one of the critical challenges for present and future telecommunication systems operating at data rates of 10 Gb/s and higher. Chromatic dispersion, while inherently troublesome, does however bring with it some advantages, in that it keeps undesirable non-linear effects (such as self-phase modulation and four-wave mixing) to acceptable levels. Hence, rather than eliminating completely chromatic dispersion with specialized fiber like dispersion-shifted fiber, it is better to compensate for it using additional devices. This paper will discuss the application of high-end FBGs to telecommunication systems, focusing on their performances with respect to chromatic dispersion. Two types of components will be discussed: low-dispersion FBG WDM filters and FBG dispersion compensators. High-quality ultra-low dispersion FBGs have been fabricated successfully and their key attributes will be discussed. Advanced applications of FBGs for chromatic dispersion compensation, such as broadband multi-channel dispersion and slope compensation, will be covered. In particular, FBG dispersion slope compensators can be used in conjunction with Dispersion Compensating Fiber (DCF) to fully manage the dispersion over a large number of WDM channels. The need for tunable dispersion compensation at 40 Gb/s transmission rates will be discussed. Experimental results will also be presented.