Broadband linearization of externally modulated fiber-optic links

Transmission of analog amplitude modulated signals puts stringent demands on the link linearity. Transmitter nonlinearities cause the modulated subcarriers to mix and generate intermodulation products, which usually fall into other channels. It is well known that externally modulated laser transmitters demonstrate higher dynamic range than direct modulated DFB laser transmitters. A prominent advantage of the external modulator is that it offers the quadrature biasing point where even-order distortions are nulled out. To deal with the remaining odd-order terms of distortion, electronic linearization techniques have been conceived. The linearizer has an arcsine transfer function that is the inverse of the raised cosine transfer function of the modulator. The purpose of the local feedback loop is to stabilize the transmitter against long term drifts. We present the design of a CMOS linearization circuit and its performance in an experimental externally modulated fiber optic link.