Optical Class of Service Segregation using the State of Polarization in Circuit and Packet Switched Networks

Abstract

The topic of this thesis is service class segregation using the state of polarization in fiber-optical networks. A particular network concept is studied: a hybrid circuit and packet switched network which can support a wide variety of services and service quality classes. Although polarization multiplexing is a way of achieving spectral transmission efficiency and is an enabling technology for future 100 Gb/s optical core networks, here focus is on polarization multiplexing as a bit-rate independent way of achieving service class segregation without additional header processing. One key technology in this context is automatic polarization control (APC). Research on APC has so far focused exclusively on continuous transmission systems. However, a major point of interest in this thesis has been to investigate APC with non-continuous data transmission, thus enable APC in future circuit and packet switched systems.

The work can be divided into three parts: a study of physical impairments in polarization multiplexing schemes for class of service (CoS) segregation,experimental demonstration of the hybrid network concept,and investigation of automatic polarization control with packet transmission. For class of service segregation we compare the traditional polarization multiplexing (PolMUX) method to a recently proposed polarization and time-division multiplexed (PolTDM) scheme. The robustness of the polarization multiplexing scheme is the main priority in order to transmit both circuit and packet switched data in the same wavelength channel and in order to support data traffic with high signal quality requirements. Our simulation results, which investigate the impact of polarization dependent loss (PDL) and polarization-mode dispersion (PMD) on CoS segregation, show that PolTDM is more robust than PolMUX towards these impairments. PolTDM is thus more suitable for traffic which requires low bit-error rates. Due to a PMD compensating effect, the performance of PolTDM will in some circumstances surpass that of a conventional system without polarization multiplexing. With respect to PDL, the penalty of using the polarization state in PolTDM is negligible compared to the conventional system. Although PolTDM is demonstrated in the OpMiGua (Optical Migration capable packet switched network with service Guarantees) hybrid network concept, the results are applicable to systems in general using polarization multiplexing for CoS segregation.

The PolTDM scheme was implemented in an optical test-bed. The purpose of this test-bed was to demonstrate experimentally both physical layer and network layer principles of OpMiGua: absolute service guarantee and priority of the circuit switched class; high lightpath utilization by inserting packet switched traffic in-between circuit switched traffic; adding and dropping of packets using polarization multiplexing; and automatic polarization control with packet transmission. This test-bed demonstrated the flexibility, bandwidth-efficiency and cost-efficiency of the hybrid circuit/packet switched concepts.

Automatic polarization control is essential in, e.g., optical PMD compensation methods as well as for de-multiplexing of service classes in OpMiGua. We have studied the effect of packet transmission with non-continuous light, and hence non-continuous feedback signal, on polarization control. Ideally, APC performance should be independent of the transmitted traffic pattern and of traffic load. One method has been demonstrated experimentally in the OpMiGua test-bed and was shown to be traffic pattern/load independent. This was the first reported demonstration of APC with packet transmission. A general method for use in optical packet and burst switched systems was investigated by numerical simulations, providing guidelines for system design when combining packet transmission and polarization dependent network functionality. Efficient control was possible when the sampling-time interval was on the order of or longer than the packet length.