In the evolution of optical networks, spectral efficiency (SE) enhancement has been the most cost-efficient and thus the main driver for capacity increase for decades. As a result, the development of optical transport systems has been focused on the$C$- and$L$-bands, where silica optical fiber exhibits the lowest attenuation, and erbium-doped fiber amplifiers provide an efficient solution to compensate for the optical loss. With a gradual maturity in the SE growth, however, the extension of the optical bandwidth beyond the$C + L$-band is expected to play a significant role in future capacity upgrades of optical networks and, thus, attract increasing research interests. In this article, we discuss the merits and challenges of ultrawideband optical transport systems and networks beyond conventional bands.

We derive a closed form formula of SNR penalty induced by polarization dependent loss (PDL) in optically amplified optical fiber dual-polarization systems by employing perturbation analysis based on the reversed channel model. The SNR penalty in dB of each polarization tributary is Gaussian distributed where the mean and standard deviation are determined by the longitudinal distribution profiles of PDL and noise as well as the accumulated PDL value. The simulation verifies the proposed theory under various transmission link configurations.