I. Introduction
The Optical coherent transceiver system is the common solution for optical communications with very high data rate [1]. A single carrier coherent optical transmitter is shown in Fig. 1. It employs four digital-to-analog converters (DACs) to generate two complex signals on both - and -polarizations, respectively. The output of the DACs are linearly amplified to drive the optical modulators. The bandwidth and the resolution of the DACs are the keys to achieve higher data transmission rate and increase the utilization of the available spectrum.
Polarization-multiplexed square QAM optical transmission system.