# Multipath Interference Impact Due to Fiber Mode Coupling in C+L+S Multiband Transmission Reach
## 1 Introduction  
The announced optical network capacity crunch due to the continuous demand for high bandwidth services, such as cloud computing, video streaming, and 5G/6G services, has ignited the research for solutions that can potentially bring more capacity to optical transport networks [1, 2, 3]. The two major promising solutions that are being studied for mitigating this problem are the exploitation of the spatial division multiplexing (SDM) technique in the optical domain and the utilization of the unused spectrum in single-mode fibers (SMFs). The first solution consists of either installing additional SMFs, creating a set of parallel fibers in each link, or installing new fibers, like the multicore fibers or the multimode fibers [1, 2]. In the second solution, called multiband (MB), the installation of new fibers is precluded, and the unused spectrum available in SMFs working on the C-band, i.e., the O, E, S, L, and U bands, is exploited [3]. Both solutions have pros and cons. For example, the SDM solution has a large capital expenditure associated, despite having the advantage of providing much more capacity than the MB solution, so it has been regarded by network operators and equipment vendors as a medium- to long-termsolution. In contrast, MB is seen as a near to midterm solution, since there is no need to install new fibers. Several commercial systems are already using the C+L-bands in SMFs, with an aggregate bandwidth of around 10 THz [4]. Further research with other bands, namely the exploitation of the C+L+S- bands scenario, which allows for a capacity increase of more than three times in comparison with the common C-band scenario, is being analyzed [5]. Also, recently, an experimental work has used 37.6 THz of the SMF spectrum to transmit a record capacity of 402 Tbps over 50 km, occupying the O+E+S+C+L+U-bands, which confirms the desire to extend the life of already-deployed optical fibers [6]. Nevertheless, the MB solution has several technical shortcomings, unlike those found in common C-band systems that have been addressed over the last few years [3]. For example, the optical amplified technologies for bands other than the C- and L-bands must be improved, such as the thulium-doped fiber amplifier (TDFA) technology used in the S-band, which has a high noise factor and consumes much more power than the common erbium-doped fiber amplifier (EDFA) [7]. Also, the MB optical node architecture is more complex and has a superior cost than common C-band nodes [8]. Lastly, several physical layer impairments (PLIs) are enhanced when bands other than the C-band are used for transmission, like the inter-channel stimulated Raman scattering (ISRS), a wideband phenomenon that is responsible for power transfer from high to low frequencies [5; 9]. In 2016, the G.654E SMF was standardized by ITU-T G.654.E for terrestrial long-haul links [10]. The main attributes of this SMF are its low loss, large effective area, and shifted-cable cut-off wavelength, in comparison with the standard G.652 SMF [11], which allows for reducing the effect of non-linear transmission effects, hence improving the transmission performance for high-data-rate and high-spectral-efficiency signals in C-band transmission scenarios, as has been demonstrated over the last years in several field trials [12; 13]. However, when the G.654E SMF is used in MB transmission systems, power mode coupling arises when bands with wavelengths below the cut-off wavelength are used, which is the case for the S, E, and O bands, inducing multipath interference (MPI) [14]. MPI is a PLI commonly encountered in optical communications systems operated in the C-band [15], which occurs when multiple replicas of the transmitted signal propagate over different optical paths. When the differential delay between the signal and its replicas is much smaller than the laser source coherence time, it is called coherent MPI; otherwise, it is called incoherent MPI [16].