Optical performance monitoring for next generation coherent optical communication systems

Abstract

Optical performance monitoring (OPM) is a technique used to assess the performance and limiting factors of optical networks, especially the physical channel impairments. In conventional intensity modulation/direct detection (IM/DD) systems, only linear channel impairments including optical signal-to-noise ratio (OSNR), chromatic dispersion (CD) and polarization mode dispersion (PMD) are concerned by network management. The monitoring data from OPM enables the network management system to implement impairment-aware route-wavelength-assignment (IA-RWA) algorithms for better quality of transmission (QoT) by taking into account real-time signal distortions in vanous links between the source and destination during route-wavelength assignment. Moreover, physical states of a particular channel provided by OPM can help automatic negotiation of optimal configuration of network components between two transmission ends to achieve more flexible, scalable reliable and efficient network operation. Fault localization using conventional fault diagnose techniques is limited to identifying serious signal degradation and complete connection lost. Since OPM directly provide the state of the physical channel, it can not only identify subtle faults such as drift in operating parameters of system components but can also differentiate the origin/cause of the faults. It is believed that OPM will be a fundamental function instead of a subroutine in future optical communication networks. Coherent detection with electronic digital signal processing (DSP) receivers is considered as the most promising solution for the next generation optical networks transmitting at 100 Gbps and beyond. Besides the merits of OPM discussed above, information of critical channel parameters monitored by OPM is necessary for improving the performance of channel equalization function such as backpropagation (BP) at the coherent receiver. However, high order modulation formats used in coherent communication systems such as QPSK and 16 QAM also bring new challenges to OPM. In addition to the aforementioned linear channel impairments, nonlinear impairments and other sources of phase noise, including self-phase modulation (SPM), cross phase modulation (XPM), laser phase noise and laser frequency offset between the transmitter and receiver can no longer be simply ignored. High quality of service (QoS) in the next generation coherent networks, therefore, will rely on OPM to provide accurate information of these impairments for system configuration, packet switching and channel impairment compensation. As the control plane of the management system requires a complete map of real-time channel conditions across various links in a network to coordinate routing for maximum network efficiency, OPM is desired to be deployed at intermediate network nodes such as optical cross-connect (OXC) unit as well as end terminals. Thus, the cost of OPM becomes another major concern. Moreover, modem OPM is also expected to be transparent to various modulation formats and bit rates that may coexist in coherent optical systems. In this thesis, we first studied the application of asynchronous amplitude histogram (AAH) in OSNR monitoring for higher order modulation formats. Since no timing/clock recovery circuitries are required and the sampling speed could be much lower than the symbol rate, AAH can significantly reduce the cost of monitoring units. Simulation results indicate that the proposed technique can be applied to M order quadrature amplitude modulation (QAM) formats with high accuracy. The influences of CD and PMD are also evaluated. As independent and simultaneous monitoring of OSNR, CD and PMD are almost impossible using AAH alone, we further proposed the use of artificial neural network (ANN) trained with AAH for simultaneous monitoring of multiple channel impairments. Simulation results verified that the combination of ANN and AAH can independently monitor OSNR, CD and PMD simultaneously with high accuracy with a wide monitoring range. For monitoring of nonlinear parameters of the channel, we proposed a DSP-based monitoring technique using the statistics of the received pilot symbols. It is demonstrated that the proposed nonlinear parameter monitoring technique can simultaneously monitor phase noise introduced by laser linewidth, laser frequency offset between the transmitter and local oscillator and ASE noise. The number of spans between the two transmission ends can be obtained as well in case such information is not available from upper level network protocols. Finally, faults can also be localized by evaluating the relation between the variance of received signal power and phase.