Chapter 5 - Dynamic and Static Gain Changes of Optical Amplifiers at ROADM Nodes

Abstract

This chapter focuses on two of the major amplifier-based sources of interactions between channels, erbium-doped fiber amplifier (EDFA) gain dynamics, and spectral hole burning (SHB) resulting from inhomogeneous broadening. It reviews the current understanding of the underlying physics and the complex spectral structure of hole burning. Gain dynamics are important because when channel loading in the EDFAs changes, the co-propagating surviving channels are impacted. This could happen because of reconfiguration of the channels through the network or because of a fault, such as a fiber break or connector pull. In either case, the surviving channels that were co-propagating through EDFAs with the affected channels will experience changes in their gains if the amplifier is not controlled. And the longer the chain of concatenated amplifiers is and the harder these amplifiers are driven, the faster the gain dynamics can be. This chapter also presents methods of fast gain control that can be incorporated in amplifier designs to manage these dynamic effects. As a result of SHB, the gain spectrum of the EDFAs depends on the wavelengths and powers of the individual channels that are populated, even if the amplifier is maintained at the constant inversion. A numerical model of SHB is developed, and its use to predict the gain spectra for different allocations of provisioned channels is demonstrated. As the phenomenon is intrinsic to the physics of gain in EDFAs, it must be managed using the capability of the reconfigurable optical add/drop multiplexers (ROADMs) to adjust the powers of individual channels to compensate for SHB.