Gabriella Bosco

Accurate estimation of Non-Linear Interference (NLI) is critical for optimizing ultra-wideband optical networks. This paper investigates the performance of Longitudinal Power Monitoring (LPM)-based NLI estimation, focusing on heterogeneous links. While LPM offers robust, link-agnostic estimation of Self-Channel Interference (SCI), it requires correction for Cross-Channel Interference (XCI) to fully assess NLI. Two correction approaches are compared: a simplified Gaussian Noise (GN)-model-based approximation and the more accurate Polynomial Closed-Form GN/EGN model (PCFM). Experimental validation is conducted over both homogeneous and heterogeneous links, including scenarios with unequal channel power, on two different modulation formats. Results show that while estimation accuracy decreases in complex scenarios, LPM-based methods still provide sufficiently reliable performance for practical optimization tasks, such as determining optimal launch power. These findings support the use of LPM-based NLI estimation as a promising tool for real-time network monitoring and digital twin modeling.

Distributed acoustic sensing (DAS), implemented through phase-sensitive optical time-domain reflectometry ($\phi$OTDR), has gained attention in geophysical and telecommunications environments. By utilizing the existing telecom infrastructure, DAS facilitates self-monitoring of networks and external events observation, from human activities to large-scale geophysical phenomena. However, integrating it with data signals on the same fiber is challenging, as DAS employs higher power signals that can interact with data channels. Although increasing DAS performance with higher bandwidths is possible, practical applications have mostly been limited to 1 GHz due to cost and complexity constraints. This paper investigates the coexistence conditions of DAS, specifically direct detection (DD) chirped-pulse-$\phi$OTDR (CP-$\phi$OTDR), with intensity-modulated and DD (IM-DD) telecommunication systems, both operating at comparable complexities (5 GHz chirp and 10 Gbit/s), particularly interesting for metro/access contexts. We varied the frequency spacing of data and DAS channels, the DAS pulse peak power, and the receiver optical filter bandwidth, identifying the conditions for feasible coexistence. Our analysis shows that nonlinear Kerr-induced modulation instability (MI) is the main impairment affecting IM-DD channels co-propagating with DAS. In this scenario, MI cannot be approximated as originating from a single frequency source, and the observed nonlinear impairments vary along the pulse with its sweeping central frequency. Numerical simulations validate the experimental results, showing close alignment.

This letter presents an experimental demonstration of the Linear-Least Squares Longitudinal Power Monitoring (LLS-LPM) technique over a mixed fiber link. The link is made by five spans, including four spans of G.652 Single-Mode Fiber (SMF) and one span of G.655 Non-Zero Dispersion-Shifted Fiber (NZDSF) with negative dispersion in the C-band. The study highlights the challenges posed by the negative dispersion of G.655 fiber and its impact on the convergence of the LLS-LPM algorithm. The experimental results demonstrate that the LLS-LPM algorithm can accurately estimate the power profile, only in the link portion where there is a one-to-one relation between the cumulated chromatic dispersion and the distance. This work provides valuable insights into the application of LLS-LPM in real-world fiber networks with mixed fiber types.

Given the ubiquity of optical fiber networks in both terrestrial and submarine environments, leveraging these facilities for sensing anomalous conditions alongside telecommunications can provide significant added value. In this context, distributed acoustic sensing (DAS) systems have been widely employed and discussed due to their sensitivity and ability to locate events. However, integrating them within existing networks is complex and expensive. On the other hand, the received state of polarization (SOP) is also sensitive to external factors, and it can be used for sensing: in this case, no extra hardware would be required since the SOP is already estimated in coherent receivers for data demodulation. The sensing information is provided “for free” by the already installed hardware, potentially requiring only a software upgrade. In this work, we analyze the feasibility of using polarization-based sensing to detect anomalous conditions in metropolitan environments. A polarimeter was used to evaluate SOP noise induced by urban factors, while a commercial coherent transceiver was employed to assess SOP estimation noise. We propose two algorithms for processing polarization data: a time-based method called SOP angular speed (SOPAS) and an adaptive, frequency-based approach named SOP-power spectral density gap (SOP-PSDG). These algorithms were compared by processing Stokes vector samples from the polarimeter when different sinusoidal vibrations are applied to the fiber through a mechanical shaker. Results demonstrate that a sampling rate of just a few tens of Hz is sufficient to effectively identify various hazardous conditions, with SOP-PSDG consistently outperforming SOPAS. Additionally, preliminary findings on the performances of these algorithms using SOP samples from a commercial coherent receiver are discussed.

A novel method for the estimation of nonlinear interference (NLI) penalties in optical networks is presented. Its theoretical derivation is grounded in linear least squares (LLS) longitudinal power monitoring (LPM), an algorithm capable of estimating the absolute power evolution of a signal along the link. We demonstrate that closed-form NLI estimation is inherently integrated into LPM. However, this method does not account for the cross-channel interference (XCI) contribution to NLI, since the receiver typically lacks knowledge of other WDM channels. To address this limitation, simple analytical expressions to compensate for the bias originating from neglecting XCI are proposed for the scenario of homogenous WDM combs. These formulas are derived from closed-form NLI models (e.g., the GN-model) and require only limited information about the signal configuration. The effectiveness of the proposed approach is validated through a wide range of realistic numerical simulations and experiments, including C-band transmissions over 300-km and 1100-km EDFA-amplified optical links. The obtained results confirm the reliability of the method, yielding nonlinear SNR estimates with a maximum absolute error consistently below 0.97 dB when compared to well-established analytical NLI models or measured values. The mean absolute errors in simulation and experiments are 0.3 dB and 0.5 dB, respectively. Additionally, a practical demonstration of transmit power optimization is provided, leveraging the values obtained through the proposed method.

As i write this final editorial as Editor-in-Chief (EiC) of the Journal of Lightwave Technology (JLT), I am filled with mixed emotions: pride in our accomplishments, sadness in parting from close teammates, and relief at completing this fulfilling journey. Serving as EiC of JLT over the past six years has been both an honor and a privilege. When I began this role, I had only a vague sense of what to expect, but it has proven to be a rewarding journey of continuous learning.

This paper focuses on a closed-form ultra-wide-band solution of the GN and EGN model integrals (CFM, for closed form model), collaboratively developed by CISCO and Politecnico di Torino. The version presented here is capable of handling the frequency-dependence of all main system and fiber parameters and accounts accurately for Interchannel Stimulated Raman Scattering (ISRS). In the paper, we first describe the CFM in detail and then we present a validation experiment carried out over a 5-span 128-channel C+L transmission line over SMF. The experiment was designed to explore transmission scenarios ranging from quasi-linear to deeply non-linear, with significant ISRS. We describe the thorough characterization of the link and the careful instrumentation of the set-up, necessary to minimize all errors that might compromise the significance of the measurements. The results are extensively discussed and show a good match between the CFM predictions and the measured performance. The CFM provides a closed-form solution of the GN and EGN model integrals, but other steps of system performance assessment are not closed-form. However, since GN/EGN integrals are by far the most computationally heavy element of the calculation, the overall speed-up granted by using the CFM is quite significant, about four orders of magnitude. The real-time system performance assessment capability thus achieved makes it possible to perform physical-layer-aware numerical optimization of system and link parameters, a critical aspect in multi-band and ultra-wide-band optical networks. Overall, we believe that both the speed and the reliability of the CFM make it a valuable tool with significant potential for practical application in the field.

The efficiency of optical networks employing flexible wavelength division multiplexing (WDM) can be increased by maximizing the throughput of each individual channel, provided that the position of its neighboring channel is known with sufficient accuracy in order to avoid inter-channel interference. In this paper, we propose a digital signal processing (DSP) algorithm, leveraging the use of an artificial neural network (ANN), to estimate the neighboring channels’ distance by processing raw digital samples from a standard coherent receiver. We present an efficient dataset design approach, based on Latin hypercube sampling (LHS), in order to effectively optimize and validate the algorithm under different assumptions on the optical WDM channels. We investigate the accuracy of the ANN-based DSP scheme through simulation analysis, highlighting its potential in relation to the characteristics of the optical network. Finally, we validate our approach in an experimental setup using standard commercial coherent transceivers. The experimental results show that the distance from the neighboring WDM channels can be estimated with a root mean square error of less than 1.5 GHz for a channel under test with a symbol rate of 52 GBaud.

We investigate the use of NANF in various system scenarios and specifically we investigate the prospective capability of future NANF to increase span length. NANF appears to have the potential for carving various niches for itself and perhaps achieve widespread adoption in certain contexts.