Abstract
Optical networks are undergoing significant changes, fueled by the exponential growth of traffic due to multimedia services and by the increased uncertainty in predicting the sources of this traffic due to the ever changing models of content providers over the Internet. The change has already begun: simple on-off modulation of signals, which was adequate for bit rates up to 10 Gb/s, has given way to much more sophisticated modulation schemes for 100 Gb/s and beyond. The next bottleneck is the 10-year-old division of the optical spectrum into a fixed “wavelength grid,” which will no longer work for 400 Gb/s and above, heralding the need for a more flexible grid. Once both transceivers and switches become flexible, a whole new elastic optical networking paradigm is born. This new networking technology provides an opportunity to maximize spectral efficiency for each of many arbitrary bandwidth channels generated using one of many possible modulation formats. However, optical signals with high spectral efficiency are highly sensitive to physical layer impairments (PLIs), which accumulate differently depending on network configurations. In particular, bit error rates can rise due to changes in environmental conditions or degradations in optical signal-to-noise ratios (OSNR) of the affected optical links. The flexible bandwidth networking technique together with an effective optical performance monitoring method and an adaptive control plane can offer adaptive optimization of the network based on impairments (impairment-aware networking). By adopting a practical and real-time performance monitoring method that spans a broad and elastic spectral width, the network can dynamically and adaptively adjust the modulation format to maximize spectral efficiency while maintaining the required quality of service (QoS) and bit error rate (BER) performance even for signals experience time-varying PLIs.
In this thesis we first simulate and then experimentally demonstrate an impairment monitoring technique based on a low-frequency and low-modulation-index intensity modulation applied to the high-bandwidth data signals (10Gb/s and above). Using premeasured correlation data between the Bit Error Rate of the monitor signal and the data signal, the proposed monitoring technique allows to detect the quality of the link and to determine if the data signal is affected from linear impairments (i.e.. optical noise accumulation, in-band crosstalk) and nonlinear impairments such as Four-Wave-Mixing.
First, we carried out an extensive simulation study to verify the effectiveness of the proposed technique and determine the best design parameters such as line rate of the monitor signal and overmodulation index. This study was carried out for different modulation formats, i.e. OOK, QPSK, and 16QAM, considering both linear and non- linear impairments. Second, we verified, through an experimental study, some of the results obtained through simulation for the OOK case, demonstrating (a) the need of line coding techniques such as 8B/10B and/or 9B/10B to properly detect the monitor signal when the line signal is intensity modulated; (b) the correlation between the BER of an OOK line signal at 10-Gb/s and the BER of the overmodulating signal at 20Mb/s, in the case where the system is affected by a linear impairment such as optical noise accumulation.
