I. Introduction
Relentless growth of internet-based services is rendering the constant challenge to upgrade the capacity of optical links. Coherent optical modulation in conjunction with polarization division multiplexing (PDM) has proved its capability to increase the single wavelength-channel bitrate by utilizing the optical amplitude and phase of dual polarization states [1]–[4] . A recent experiment employing 64-quadrature amplitude modulation (64-QAM) with PDM has demonstrated data transmission with single-carrier bitrate exceeding 1 Tb/s [5]. With the advent of semiconductor photonic integrated circuit (PIC) technologies, polarization-multiplexed coherent transceivers operating beyond 100 Gb/s per channel have been realized on InP [6] –[21] and silicon [22]– [28] platforms.