Modeling transverse coupled cavity VCSELs for ultra-high-speed data communications

Messaging applications and social media platforms have taken center stage as the primary means of human interaction. Entertainment experiences are becoming increasingly smarter, encompassing a wide range of activities such as online gaming and audio/video streaming services. In this context, optical communications have emerged as a crucial catalyst driving this transformative evolution. Consequently, the focus of research and development for ICT engineers has shifted from long-distance communications to short-range optical links. Optical transceivers designed for intradatacenter communications are currently pushing the boundaries to achieve data rates of up to 200 Gbps. To accomplish this feat, they employ advanced modulation techniques such as pulse amplitude modulation (PAM). Cutting-edge optical transceivers harness the power of PAM, allowing for increased data throughput within datacenters. In this context, vertical-cavity surface-emitting lasers (VCSELs) have emerged as the ideal choice for high-speed low-power data communications light sources, thanks to their compact active region, sub-mA threshold currents, and optimal packaging and coupling with optical fibers due to their circular symmetry. Yet, reaching 200 Gbps is going to require redefining the concept of high-speed VCSELs. In this view, one of the hottest research topics is that of VCSELs featuring transverse coupled cavities (TCC-VCSELs). These devices are based on introducing an interaction between the main VCSEL cavity and one or more additional cavities coupled to it, so that if one optical mode closely matches the resonant frequency of another mode, various nonlinear optical effects occur, such as a huge broadening of the intensity modulation bandwidth. The scope of this thesis is to develop competencies in TCC-VCSELs. The thesis starts with a critical appraisal of the literature, considering different TCC-VCSEL implementations and models, ranging from bowtie devices, modeled in terms of a Lang-Kobayashi feedback model, to the very recent daisy-like VCSELs. It is discussed how these ideas share a common denominator, which is: IM bandwidth enhancement can be achieved by implementing technologically a VCSEL featuring two or more phase-shifted fields, in partial analogy with the idea of photon-photon resonance, consolidated in the context of edge-emitting lasers. The models are implemented and solved with two different approaches. The first is the model stability analysis based on the determination of bias point and small-signal IM response. Being this approach semi-analytic, it enables extensive, yet very quick parametric investigation campaigns of the number, extension, and coupling strength of the transverse cavities, allowing to understand and engineer the physical mechanism causing bandwidth enhancement. The second implemented solution is a fully-numerical time-domain analysis based on numerical integration techniques optimized for the delay differential equations arising from our feedback-like models, which allows, in addition to cross-validation of the small-signal solution, the investigation of large-signal modulations such as NRZ or 4-level PAM.