Investigation of different optical modulation schemes

Abstract

The optical communication system has become a major part of global infrastructure in the past years because of its several benefits. The main objective of an optical communication system is to utilize minimum bandwidth over long distances with minimum possible errors during the transmission of signals. Today, the core of the telecom network consists of optical Wavelength Division Multiplexed (WDM) systems as it provides an advantage of high spectral efficiency (defined as the overall transmitted capacity per unit bandwidth). The commercial WDM systems provided a spectral efficiency of 0.2-0.8 b/s/Hz but in order to cope up with the exponential increase in demand of capacity due to fast growth in telecommunication industry, there is a need to develop communication systems with higher spectral efficiencies. Traditionally, optical communication systems employed on-off keying (OOK) as its modulation format but it is no more a suitable choice for next generation optical transmission systems. So choosing an appropriate modulation scheme for the optical transmission systems that can provide higher capacities at high data rates is a critical issue. While selection of an optical modulation scheme, there is a trade-off between various requirements, namely Optical Signal to Noise Ratio (OSNR) requirement, optical filter tolerance, spectral efficiency, Chromatic Dispersion (CD) tolerance, Polarization Mode Dispersion (PMD) tolerance, non-linear tolerance and transponder complexity. Each of these requirements has a different importance which depends on the design of the optical transmission system. This necessitates the designer to have an optimal choice of optical modulation scheme. The objective of this dissertation is to investigate the performance of different optical modulation schemes by means of numerical simulations. In order to achieve this goal, first of all, different optical modulation formats, namely, OOK, Differential Phase Shift Keying (DPSK), non-return-to-zero Differential Quadrature Phase Shift Keying (NRZ-DQPSK) and return-to-zero (RZ) DQPSK have been compared with each other at different data rates. The results showed that RZ-DQPSK gives the best performance at high data rates. Secondly, the performance of dual carrier polarization multiplexed (POLMUX) DQPSK at 112 Gb/s with respect to changes in state of polarization (SOP) and the differences between two formats (RZ and NRZ) of POLMUX DQPSK signal have been analysed. The POLMUX RZ-DQPSK systems gave an improvement of 1.25 dB in OSNR for BER=10−3 viii than POLMUX NRZ-DQPSK system. Thirdly, the impact of cross-phase modulation (XPM) and cross-polarization modulation (XpolM) imposed by OOK or DPSK channels was investigated on a 112 Gb/s POLMUX QPSK signal along with increase in number of channels and increase in bit rate. A hybrid WDM system consisting of alternate channels of OOK and DPSK signals with 50 GHz spacing between them was also proposed to minimize the impact of XPM by OOK channels.