Predistortion of Mach–Zehnder Modulator Using Symmetric Imbalance

Impairments of Mach-Zehnder modulators (MZM) set a sever performance limit on transmitting high-order modulation signals such as M-QAM, optical OFDM or M-PAM in high-speed fiber optics communication links. Various predistortion methods were proposed for compensating MZM effects; however, they do not compensate for all types of impairments and they require significant computation resources for each additional impairment needed to be corrected. In order to overcome these difficulties, a new predistortion approach is proposed. The method is based on adding an imbalance symmetrically to the information applied to the MZM arms. The imbalance predistorts the signal in a way that the overall effect of the symmetric imbalance and MZM impairments is minimized by means of mean square error. A closed form as well as an optimization method that compensates for all MZM impairments with a single and low complexity infrastructure is presented. Analysis includes floating point as well as fixed point, considering digital to analog converter effects. It is shown that the overall method performance is limited only by the number of bits N of the digital-to-analog converter so that the MZM output error vector magnitude is bounded by 10-6N [dB].