Experimental investigation of the laser phase noise effect on next generation high order MQAM optical transmission

Abstract

Next generation 400Gbps and 1Tbps per wavelength optical transmission system will require the use of high spectral efficiency optical modulation schemes. Rather than wireless communication system, laser phase noise (PN) has a major impact on the performance of optical systems due to high speed of communication and high order modulations recently proposed/demonstrated in these communication systems. In this paper, we prototype a reference optical transmitter, that digitally emulates the effect of the laser phase noise. This experiment enables us to generate laser linewidths ranging from a kilohertz up to hundreds of megahertz. Moreover, this enables us to measure the performance of the generated high order MQAM modulated signals in case inexpensive low quality or alternatively expensive high quality laser is exploited. In our experimental performance investigation, we have addressed state-of-the-art symbol rates of 14, 16, and 25Gbaud and modulation orders ranging from QPSK up to 16QAM. In order to mitigate the phase noise effect a Kalman filter (KF) phase tracker is employed at the receiver side. Our simulation and experiment show that our transmitter makes successful emulation of the PN and our tracking algorithm makes appropriate tracking of 16QAM for a laser linewidth of 1MHz. However, when inexpensive lower quality 10 MHz laser is used, the recovery capacity of the algorithm becomes limited to QPSK.