Optimal Modulation Index of the Mach-Zehnder Modulator in a Coherent Optical OFDM System Employing Digital Predistortion

We study the impact of nonlinear distortions caused by the external optical modulator in coherent optical OFDM systems and determine by simulation the optimal modulation index that minimizes the OSNR penalty introduced by nonlinear distortion and modulation excess loss. To reduce this penalty a digital predistortion device implemented by a look-up table is proposed and the achievable gain in receiver sensitivity is quantified. These results are compared for the word lengths 6 bit and 8 bit of the digital-to-analog converter and look-up table, respectively.