Wavelength-division multiplexing (WDM) has historically enabled the increase in the capacity of optical systems by progressively populating the existing optical bandwidth of erbium-doped fiber amplifiers (EDFAs) in the$C$-band. Nowadays, the number of channels—needed in optical systems—is approaching the maximum capacity of standard$C$-band EDFAs. As a result, the industry worked on novel approaches, such as the use of multicore fibers, the extension of the available spectrum of the$C$-band EDFAs, and the development of transmission systems covering$C$- and$L$-bands and beyond. In the context of continuous traffic growth, ultrawideband (UWB) WDM transmission systems appear as a promising technology to leverage the bandwidth of already deployed optical fiber infrastructure and sustain the traffic demand for the years to come. Since the pioneering demonstrations of UWB transmission a few years ago, long strides have been taken toward UWB technologies. In this review article, we discuss how the most recent advances in the design and fabrication of enabling devices, such as lasers, amplifiers, optical switches, and modulators, have improved the performance of UWB systems, paving the way to turn research demonstrations into future products. In addition, we also report on the advances in UWB optical fibers, such as the recently introduced nested antiresonant nodeless fibers (NANFs), whose future implementations could potentially provide up to 300-nm-wide bandwidth at less than 0.2 dB/km loss.

We investigate the potential of 100 Gb/s polarization division multiplexed (PDM-) quadrature phase shift keying (QPSK) paired with coherent detection to enable the future capacity upgrade of current networks designed for 10 Gb/s nonreturn to zero (NRZ) on-off keying data. We experimentally assess the tolerance to nonlinear effects of such a solution over optical systems based on low chromatic dispersion fibres. We particularly study the performance penalties brought by copropagating channels (either PDM-QPSK or 10 Gb/s NRZ) onto 100 Gb/s PDM-QPSK data, in which both polarisation tributaries are over-modulated with return-to-zero and temporally interleaved by half a symbol period. Besides, we investigate one option to overcome these limitations: introducing band-gaps in the multiplex.