# Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs
## 1 Introduction  
In recent years, we have witnessed continuous growth of bandwidth demand in the access and metropolitan area networks (MANs), as traffic remains almost confined to where it was generated. MANs must therefore evolve toward more efficient and agile architectures which are able to address multi-Tb/s transmission and routing over variable distances and topologies, supporting any kind of services such as 5G, Mobile Edge Computing, ultra-high density TV, etc. [1]. Moreover, other challenges should be faced, enabling a \"pay-as-you-grow\" scheme according to traffic scaling, minimizing the operational expenditures (OpEx) by achieving low footprint and low power consumption. The current transmission devices for metro network are evolving from the traditional approach by proposingenhanced modulation schemes or, as an alternative, coherent transmission solutions, which today are used mainly for long haul transmission, and are characterized by high cost and high-power consumption. With these approaches, the expected performance enhancement in terms of throughput is limited to a factor \(\times 2\)-\(\times 4\), which seems to be inadequate to support 5G-and-beyond network requirements. Moreover, a migration towards more flexible, efficient, and agile paradigms is expected, owing to the aggregation of the metro/access networks, and for the support of heterogeneous networks. Alternative energy- and cost-efficient photonic solutions can be effectively used, starting from the adoption of direct modulation (DM) of the laser sources; in particular, vertical cavity surface emitting lasers (VCSELs) allow a reduction of transmitter cost, power consumption, and footprint. Recently, a sliceable bandwidth/bitrate variable transceiver (S-BVT) based on VCSEL has been proposed for MAN applications [2]. In order to obtain high transmission capacities, long-wavelength InP VCSELs emitting in the C band can be directly modulated by multicarrier formats, such as discrete multitone (DMT) format, as demonstrated for reaches of a few tens of kms with direct detection (DD) [3; 4], and for longer reach by DD and single sideband (SSB) modulation [2]. Thanks to bit and power loading at the digital signal processing (DSP) level, DMT is capable of addressing the efficient usage of the bandwidth resource as a function of the requested capacity and transmission distance, and allowing dynamic and flexible adaptation to traffic/channel conditions and spectrum fragmentation mitigation in MANs [5]. Multi-Tb/s transmitters can be obtained by aggregating multiple DM-VCSELs emitting in the C-band with dense wavelength division multiplexers in SOI chips [6; 7] providing a fine granularity, e.g., 25 GHz. When the VCSEL is DM to obtain high capacities, the generated frequency chirp impacts significantly on the propagation performance [8], playing an important role, especially if the multiplexer filter bandwidths are comparable with the electrical signal bandwidth, and potentially limiting the VCSEL employment in high-capacity MAN applications. In this paper, long-wavelength VCSEL sources are employed to realize multi-Tb/s transceivers for MAN applications; in particular, we focus the analysis on VCSEL sources emitting in the C-band, taking into account realistic optical sources and network filters. In the following sections, we analyze the impact of the frequency chirp induced by DM VCSELs when DMT is used to provide a rate higher than 50 Gb/s per wavelength. We take into account both dual side-band (DSB) and single side-band (SSB) DMT modulation while maintaining the modulated optical spectrum confined to 25-GHz spacing to guarantee very dense wavelength-division multiplexing (WDM) transmission. At the receiver side, in order to be able to bridge typical MAN distances, coherent detection (COH-D) is employed, which allows chromatic dispersion (CD) compensation by DSP post-processing. In Section 2 chirp experimental measurements performed on two different kinds of InP long-wavelength VCSELs are presented.