Phase Noise and Polarization Effects in Fiber-Optic Communication Systems: Modeling, Compensation, Capacity, and Sensing

Doktorsavhandling, 2024

High data rate optical communications are susceptible to phase noise and state of polarization (SOP) perturbations. The dynamic nature of phase noise and SOP fluctuations requires a comprehensive investigation of their effects on the channel capacity and the development of robust and efficient communication technologies. This thesis unravels phase and polarization challenges in optical communication systems by characterizing polarization drift channels, introducing polarization tracking algorithms, utilizing polarization data for fiber sensing, and investigating capacity implications. 

The SOP drifts at a much slower rate than typical transmission rates in buried or underwater fibers. Thus, we first characterize the capacity of the block-constant polarization drift channel under an average power constraint and imperfect channel knowledge. An achievable information rate is derived, showing strong dependence on the channel estimation technique. A novel data-aided channel estimator is proposed, enforcing the unitary constraint, and its superior performance is validated through Monte Carlo simulations. However, in aerial fibers, the SOP drift does not follow the block-constant assumption and can drift quickly over time. Hence, the next contribution involves investigating the robustness of polarization tracking algorithms in the presence of fast SOP drift and polarization-dependent loss. Novel tracking algorithms are proposed, showing a higher tolerance to SOP drift compared to the gradient descent-based algorithms without the need for parameter tuning. Thereafter, we explore the application of polarization for fiber sensing by proposing a physics-based learning approach. The proposed approach shows lower sensitivity to additive noise compared to previous inverse scattering methods.

Next, we turn our attention to phase noise and investigate the capacity of a discrete-time multiple-input-multiple-output channel with correlated phase noises originating from electro-optic frequency combs (EO-comb). We derive capacity bounds and show that the multiplexing gain is M - 1 where M is the number of channels. Moreover, a constant gap between the bounds is observed in the high signal-to-noise ratio regime, which vanishes for the special case of M =2. Finally, we study optimal pilot placement for channels impaired by phase noise from EO-combs. Contrary to regular multichannel systems, it is demonstrated that allocating the first and last channels for pilots is optimal under a fixed pilot overhead.