Many roads lead to dynamic gain equalization in optical networks

摘要

Current methods of dynamic gain equalization are typically based on attenuating the stronger channels, but the requirements of next-generation dynamic optical networks may demand a different approach. An ideal optical amplifier, like an ideal electronic amplifier, would have uniform gain across a wide range of frequencies or wavelengths. Likewise, an ideal transmission system also should have uniform response across its operating range. Real systems, however, fall short of those ideals, creating a need to equalize gain across the spectrum. The problem with optical amplifiers is evident in the gain spectrum of erbium atoms. Erbium exhibits gain from 1525 to about 1610 nm, but this gain peaks sharply near 1535 nm, and declines at longer wavelengths (see Fig. 1). Such wide variation presented little problem in early systems that transmitted only a single optical channel through the erbium-doped fiber amplifier (EDFA). Serious issues arose, however, with the introduction of wavelength-division multiplexing (WDM). The variation of gain with wavelength meant that the channels with the least gain would fade away as they passed through a series of EDFAs. To control this imbalance, engineers inserted static optical filters to selectively attenuate the wavelengths at which gain was the highest in order to offset the excess gain, thus balancing gain across the spectrum. This static gain equalization proved its worth in the first generation of WDM systems, but its application is limited. Minor dynamic fluctuations accumulate over long chains of amplifiers, particularly when the gain per amplifier is high, as in terrestrial systems. Furthermore, static gain flattening cannot cope with dynamic optical networks in which the channel loading changes over time when channels are added or dropped. Overcoming these limitations of static gain equalization requires an active system that automatically provides dynamic gain equalization. Dynamic gain equalization can balance the strengths of optical channels in optical networks that switch optical channels, changing the loads on optical amplifiers. They also can balance minor fluctuations in the transmission system as well as in the optical amplifier, which can accumulate over long chains of amplifiers.