High-Capacity Short-Range Optical Communication Links

Over the last decade, we have observed a tremendous spread of end-user mobile devices. The user base of a mobile application can grow or shrink by millions per day. This situation creates a pressing need for highly scalable server infrastructure; a need nowadays satisfied through cloud computing offered by data centers. As the popularity of cloud computing soars, the demand for high-speed, short-range data center links grows. Vertical cavity surface emitting lasers (VCSEL) and multimode fibers (MMF) prove especially well-suited for such scenarios. VCSELs have high modulation bandwidths, are energy efficient, reliable, and cheap to fabricate. MMFs are highly tolerant to coupling misalignment and bending. However, because of the large spectral width of VCSELs and, consequently, chromatic and modal dispersion effects in the fiber, the VCSEL-MMF links have a limited bandwidth{distance product: their achievable distance is limited to 100 m at 25 Gbps for non-return-to-zero (NRZ) signaling. This thesis introduces several methods to tackle this limitation and increase the capacity of a VCSEL-MMF link based on intensity modulation (IM)/direct detection (DD). First, we apply the MultiCAP modulation format to increase the transmission net rate to 65.7 Gbps over 100 m MMF using a single VCSEL and equalization at the receiver. Second, we demonstrate that using a novel block-based 8-dimentional/8-level (BB8) advanced modulation format improves the receiver sensitivity compared to to the equally spectrally ecient PAM-4 modulation format. Single VCSEL transmission over 100 m is demonstrated at 54.5 Gbps. Third, we explore the potential of extending the transmission reach by using a lower chromatic dispersion region with a pre-emphasized 1060 nm VCSEL. Fourth, we discuss short-range wavelength division multiplexing where the total capacity is the product of the single wavelength's capacity and the number of wavelengths. The presented simulations and experiments validate the capacity improvement it introduces. Finally, we apply selective modal launch to the multimode VCSEL-multimode fiber scenario. This way, we achieve 10 Gbps over 400 m and then conrm the approach in an optimized system at 25 Gbps over 300 m. The techniques described in this thesis leverage additional degrees of freedom to better utilize the available resources of short-range links. The proposed schemes enable higher speeds and longer distances of the IM/DD optical interconnects.