Optical transmission links are generally composed of optical fibers, optical amplifiers, and optical filters. In this paper, we present a channel reconstruction method (CRM) that extracts physical characteristics of multiple link components such as longitudinal fiber losses, chromatic dispersion (CD), multiple amplifiers' gain spectra, and multiple filters' responses, only from receiverside (Rx) digital signal processing (DSP) of data-carrying signals. The concept is to reconstruct a virtual copy of an actual transmission channel in the digital domain, where optical fibers and amplifiers are modeled as the split-step Fourier method for the Manakov equation while optical filters are emulated as complex-valued finite impulse response filters. We estimate the model parameters such as losses, CD, gains, and filter responses from boundary conditions, i.e., transmitted and received signals. Experimental results show that, unlike traditional analog testing devices such as optical time-domain reflectometers and optical spectrum analyzers, CRM visualizes multi-span characteristics of fibers, amplifiers, and filters in Rx DSP, and thus localizes anomaly components among multiple ones without direct measurement.
We numerically and experimentally investigate the laser phase noise tolerance of probabilistically shaped (PS) and uniformly shaped (US) quadrature amplitude modulation (QAM) signals. In the simulations, we compare PS-64QAM to US-16QAM, PS-256QAM to US-64QAM, and PS-1024QAM to US-256QAM under the same information rate (IR). We confirm that a sufficient shaping gain is observed with narrow linewidth lasers, whereas degradation of the shaping gain is clearly observed when large phase noise and high order modulation formats are assumed. In our experiments, we compare polarization-division-multiplexed (PDM) 16-GBd PS-1024QAM and US-256QAM under the same IR using lasers with 0.1-kHz and 40-kHz linewidths. For carrier phase recovery (CPR), we employ a pilot-assisted digital phase locked loop. Results reveal that PS-1024QAM achieves high performance with the 0.1 kHz-laser or > 5% pilot ratio, whereas US-256QAM outperforms PS-1024QAM when lasers with 40-kHz linewidth and < 5% pilot ratio are used. We also evaluate the pilot ratio dependency of the required optical signal-to-noise ratio at the forward error correction limit and the achievable information rate. Additionally, we compare the performance of two types of CPR updating schemes: updating phase estimation at only the pilot symbol or at all symbols.
We experimentally demonstrate simultaneous localization of optical excess loss points and spans with different dispersion in multi-span fiber links using a neural-network based digital backpropagation.
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