Performance Enhancement of WDM Optical System with Coherent Detection

Abstract

Wavelength division multiplexed (WDM) optical communication is rapidly growing with deployment of multilevel modulation formats. Efficient utilization of optical fiber bandwidth with high spectral efficiency is the key advantage of this technology for long transmission reach. However, fiber linear and nonlinear impairments are still the challenges which prevent the growth of WDM optical communication at high data rates. Coherent detection technique with digital signal processing (DSP) at the receiver end has shown the capability to overcome these challenges. Further, multi-carrier generation at transmitter and the use of polarization division multiplexed (PDM) multilevel modulation format are the solutions to enhance the spectral efficiency of the system with narrow spacing between channels. In this thesis, multi-carrier generation concept is applied at transmitter side employing coherent detection technique with DSP at receiver end to enhance the performance of WDM optical system. Polarization division multiplexed quadrature phase shift keying (PDM QPSK) and polarization division multiplexed 16- quadrature amplitude modulation (PDM 16-QAM) are used to realize the WDM optical system for long reach at high transmission rates. Hybrid transmission approach has also been considered in designing the system to represent the system performance in the dual modulation environment. Q-factor, Log of Estimated symbol error (Log (ESE)) and error vector magnitude (EVM) are considered as a performance metrics. Simulation results are obtained using optisystem software. The performance of 12 x 160 Gb/s (1.92 Tb/s) wavelength division multiplexed WDM optical system employing dual carrier and coherent detection with DSP is investigated for three different cases: gray coding, with differential coding and without any coding. Results are compared for 100 km transmission using polarization division multiplexed quadrature phase shift keying (PDM-QPSK) format. The results exhibit that system with differential coding performs better in comparison with gray coding and without any coding technique. Improvement of 2 to 5 dB in Q factor is found for system using differential coding. Moreover, performance evaluation for long transmission distance up to 8000 km is carried out for WDM optical system with differential coding technique with maintaining the Q factor beyond the FEC limit (BER value 3.8 x 10-3). An effort is made to investigate the performance of 4 x 200 Gbps (800 Gb/s) coherent WDM Optical systems employing dual carrier concept for polarization division multiplexed 16-quadrature amplitude modulation (PDM 16-QAM) modulation format. Results are compared for the system with nonlinearity compensation and without nonlinearity compensation at the receiver end. Improvement in Q factor is observed almost 2.43 dB to 1.95 dB for system with nonlinearity compensation than system without nonlinearity compensation at 6 dBm power per channel for 200 km to 1000 km transmission reach. Spectral efficiency of 4 b/s/Hz is achieved by keeping 50 GHz channel spacing between generated sub-carriers. Another approach is made to evaluate 10 x 100 Gbps coherent WDM optical system using hybrid modulation. WDM system is designed using PDM QPSK and PDM 16-QAM modulation formats in combination with dual carrier concept. Results are reported with ULAF fiber and SSMF fiber for WDM optical system. System with ULAF fiber shows better results due to less influence of fiber nonlinearity at 2500 km transmission reach. A Comparative analysis is carried out of of 23 x 100 Gbps (2.3 Tb/s) WDM Optical systems employing multicarrier generation for three different fibers: ultra large area fiber (ULAF), standard single mode fiber (SSMF) and large effective area fiber (LEAF). Enhancement in Q-factor is observed almost 3.21 dB and 4.63 dB for system with ULAF fiber than system with SSMF and LEAF fibers respectively for 5000 km transmission reach. Performance comparison is also carried out using ultra large area fiber (ULAF) and large effective area fiber (LEAF) for 39 x 100 Gbps (3.9 Tb/s) hybrid transmission WDM optical system employing multicarrier generation. Among 39 carriers, 20 sub-carriers utilize PDM-QPSK modulation format and 19 sub-carriers use PDM 16-QAM format. It is found that deployment of ULAF fiber outperforms than LEAF fiber in the system at long transmission reach.