VCSEL-based Single-Sideband Transmission for Sustainable Metro-Access Links

The increasing demand for traffic is piling up the throughput on the optical metro-acces network. Despite the well-known ensured high capacity and reach performances, coherent detection is not a feasible choice due to sustainability (cost and power consumption) requirements. Instead, the combination of intensity modulation (IM) and direct detection (DD) in IM-DD systems is a sustainable solution and represents the most used paradigm in the mentioned context. At the transmitter side, sustainability can be achieved using innovative commercially available light sources, such as vertical cavity surface emitting lasers (VCSELs), now also emitting in the C band, featuring a 17-GHz bandwidth, a few μms footprint, low-cost thanks to mass production, low-power consumption (the bias current is a few mAs) and a light quality comparable to the more used distributed feedback (DFB) lasers. To enhance the system performance, discrete multitone (DMT) modulation can be applied to optimize the transmission thanks to the bit and power loading operations, returning a throughput 2-4 times the signal bandwidth. In case of dual sideband (DSB) signals, IM-DD systems suffer from the power fading issue, deriving from the combination of the fiber chromatic dispersion (CD) and the DD square-law. Such phenomenon causes dips in the spectrum, inducing a loss of information and the reach and effective bandwidth reduction. Single sideband (SSB) signals represent a solution, removing one signal sideband, keeping the transmitted information unchanged with respect to DSB signals. In this way, the spectrum dips due to the bands interference at the detector are removed. In this paper, by means of simulations of an IM-DD system, we compare the capacity performance as a function of the distance of two methods to generate SSB DMT signals. On one hand, optical filtering (OF) is considered: a directly modulated VCSEL generates a DSB optical signal, then filtered by a super-gaussian OF (VCSEL-OF in Fig.1) to suppress one of the sidebands [1]. The other option is the use of an electro-optical (E/O) conversion of a signal and its Hilbert transform using a novel so-called dual modulator (DM) scheme [2], a sustainable solution consisting of a double light modulation, first by a directly modulated laser (acting as phase modulator owing to its chirp) and then by an IM, driven to produce SSB optical signals. The DM implementiation is based on the cascade of a VCSEL and a Mach-Zehnder modulator (MZM) [3], named VCSEL-MZM DM in Fig.1. The performance of DSB signals obtained with a MZM external modulation of a VCSEL is also considered (VCSEL+MZM DSB in Fig.1). The results, shown in Fig.1, highlight the potential of the proposed VCSEL-MZM DM scheme to assure more than 50 Gb/s after 50 km with a very sustainalbe approach.