Optically linearized modulators: chirp control for low-distortion analog transmission

We show that an increase of composite-second-order (CSO) distortion with increasing fiber length can occur in analog transmission systems using optically linearized Mach-Zehnder modulators and that the degradation is due to generation of chirp in the modulator. We extend a first-principles model to calculate the modulator chirp parameter and show that system CSO calculations using this model are in excellent agreement with measured results. Finally we propose and demonstrate a novel modulator design where chirp is greatly reduced while preserving excellent linearity. For an 80-channel North American frequency plan an optimized low-chirp modulator is predicted to extend the CSO-limited range of the system over standard single-mode fiber from approximately 50 km to greater than 300 km.