Distortion Caused by the Interaction of Residual Chirp in Mach-Zehnder External Modulators with Optical Cavities, Fiber Dispersion, and EDFA Gain-Slope in Analog CATV Lightwave Systems

Author： Willems F.W. Muys W.

Publisher： Academic Press

ISSN： 1068-5200

Source： Optical Fiber Technology, Vol.2, Iss.1, 1996-01, pp. : 63-68

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Abstract

An imbalance in the modulation voltages for the two branches of a Mach-Zehnder (MZ) modulator causes frequency chirp. In combination with optical cavities, fiber dispersion, or EDFA gain-slope, this chirp introduces distortion in analog lightwave CATV systems. We have derived expressions for the frequency chirp for sinusoidal modulation that can be used to study the distortion in the same manner as previously done for systems employing a transmitter with a directly modulated laser diode. The effective cavity reflection coefficients must stay below -25 dB for the 42-channel NTSC frequency plan to keep the composite second order distortion (CSO) below -70 dB, which means that a CATV system using a MZ modulator can be use fiber sections with ordinary FC connectors. Reflection levels in optical amplifiers with gains around 20 dB must use components with reflection levels below -45 dB, which is usually the case. The dispersion-induced CSO is proportional to the fourth power of the distortion frequency. The residual chirp may limit the transmission distance to 50 km. The EDFA gain-slope must be several dB/nm to cause distortion problems.