Predistortion Techniques for Linearization of External Modulators

Advances in externally-modulated transmitters and erbium-doped fiber amplifiers (EDFAs) have lead to an explosion in the number of systems utilizing 1550-nm sources to extend supertrunks and to enable extensive optical splitting. External intensity modulators, such as Mach-Zehnder (MZ) and Electroabsorption (EA) modulators, have the low chirp required for distortionless transmission through standard fiber at 1550 nm but exhibit nonlinear light-vs-voltage transfer functions. Electronic predistortion has been the dominant linearization technique commercially because it adds nothing to the complexity of the optics. To data, all commercial 1550-nm CATV transmitters utilize MZ modulators. However, integrated EA modulated lasers (EMLs) may soon be used in WDM systems carrying QAM channels. This paper reviews techniques commonly used to predistort both MZ modulators and EMLs and discusses both fundamental and practical performance limitations.