In-line predistorter for linearization of electronic and optical signals

Abstract

An electronic circuit provides a linear output from a nonlinear transmission device such as a laser. Second and higher order distortion of the nonlinear device is compensated by applying a predistorted signal equal in magnitude and opposite in sign to the real and imaginary components of distortion produced by the nonlinear device. The input signal for the nonlinear device is applied to an in-line electrical path coupled to the nonlinear device. The in-line path contains at least one component for generating primarily real components of distortion. In some applications, at least one component for generating imaginary components of distortion is located on the in-line path. Filter stages are used to provide frequency dependent predistortion. In a preferred embodiment, an attenuator, a MMIC amplifier, a CATV hybrid amplifier, and a varactor in line with a semiconductor laser, provide the predistorted signal. In another embodiment, the real component of predistortion is generated by a FET configured as a voltage controlled resistor. In still another embodiment, the real component of predistortion is generated by the parallel combination of a diode and a resistor connected in series with the RF signal path. Also provided is a separate circuit including antiparallel diodes and reactive elements for generating frequency dependent third-order predistortion.