Wdm Optical Networks, Optical Performance Monitoring Techniques Based On The Subcarrier

During the past decade, optical transport has enabled the rapid growth of data traffic in the network backbone. Further increases in capacity are gained by moving to dense wavelength division multiplexing (DWDM) with large channel counts. Optical performance monitoring (OPM) is essential for managing such high capacity optical transmission and switching systems, Ultralong haul and optically switched networks, with new optical layer functionality such as dynamic reconfiguration and link level restoration, promise improved operations, reduced footprint and cost. However, these benefits come with the added complications required by managing transparent networks and have stimulated interest in OPM for enhanced fault management applications. All of these issues bring focus to OPM as an enabling technology for next generation optical networks.There have developed many techniques for OPM, such as optical spectrum monitoring, histogram monitoring, Q-factor monitoring, sub-carrier multiplexing (SCM) monitoring and so on. Compared with other monitoring techniques, SCM has many advantages: it is simple, cost-effective, and can be used for monitoring various optical parameters in a WDM networks. Moreover, it is well suited for use in a dynamic WDM networks with OADMs and OXCs. In this paper, we reviewed the SCM technique and its typical applications, and estimated the scalability of this technique by analyzing its potential limitations.The former researches are mainly based on single sub-carrier multiplexing monitoring techniques, and there are few reports on how to use multiple sub-carriers multiplexing for the performance monitoring. The multi-SCM can be used to monitor several parameters together and improve the flexibility and capability of monitoring. But the usage of multiple sub-carriers will definitely bring more penalties to the optical signal's quality and have more chance for some deteriorative factors such as fiber nonlinearity to affect the monitoring functions than the single SCM case. The objective of this paper is to study these problems when using multi-SCM, which willbe a helpful guidance for the research in multi-SCM techniques.We first study the deteriorative effects of multi-SCM to the Q-fector or BER of the optical receiver. Based on the conventional optical receiver model, we put forward a new approach to analyze the degradation of the optical transmission link affected by the low-frequency sub-carrier multiplexing. We use a sinusoidal distributed random variable as a substitute for the multiplexed sub-carrier and employ its standard deviation into the formula derivation. Resultantly, the Q factor and the optimal decision threshold level can be described as an analytic form respectively. Then we extend the results into multiple sub-carriers case and analyze the BER and Q penalty of the optical receiver versus modulation index, input OSNR and the number of sub-carriers. Our method coincides with the proved theory with numerical analysis and is more convenient in calculating for the mapping between Q factor and other parameters is one-to-one.When the SCM monitoring technique is used in an amplified WDM network, its performance could be deteriorated by ghost-tones caused by cross-gain modulation (XGM) of an erbium-doped fiber amplifier (EDFA) and/or stimulated Raman scattering (SRS). These ghost tones could not only cause measurement errors of the monitoring technique, but also mislead the network operators (to interpret the dropped channels as still existing). This problem will become more serious in the multi-SCM case. The effect of XGM could be mitigated simply by using high-frequency sub-carriers and/or automatic gain-controlled EDFAs. However, the SRS effect could not be suppressed only by increasing the sub-carrier frequency. Taking the dual sub-carriers case for tracking the optical path as an example, we make an elementary research about the effect of SRS on the multi-SCM monitoring techniques. We proposed a math model of the optical power dither due to dual sub-carriers multiplexing in the time domain. Using FFT to this model, we analyzed the spectrum of the dual sub-carriers, the relationship between the sub-carrier power and the transmission distance, and the scalability of the dual-SCM monitoring technique due to the effect of SRS. The numerical simulation shows that although the maximum network size supported by the dual-SCM technique is smaller than the single SCMtechnique, the dual-SCM monitoring technique is still well suited for use in the metropolitan network. Moreover, the usage of dual-SCM for the identification of optical path and channel could save numerous sub-carrier frequencies, and make the channel management more effective when compared with single SCM case.