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As global IP traffic demands are exponentially growing, the increase in sub-system and system level requirements for higher speeds and longer transmission distances from multi-mode fiber (MMF) based vertical-cavity surface-emitting laser (VCSEL) optical links are fundamentally inhibited by bandwidth limits due to inherent fiber dispersions as well as transmitter noise impairments. Noise variances such as spectral linewidth (Δ𝜆rms), relative intensity noise (RIN), mode noise (MN), mode partition noise (MPN), chromatic dispersion (CD), frequency chirp and reflection feedback etc., all have serious ramifications on signal-to-noise ratio (SNR) in degrading receiver sensitivity. In order to drive the systems to higher bandwidth levels, it is important to identify the source of noise impairments, and try to minimize them with optimum component designs that facilitate solutions to achieve high SNR at the receivers. In this contest the author propose a multipurpose effective refractive index (Δneff) model of VCSEL cavity to push data transmission of non-return zero (NRZ) based 100G SR4 to beyond eSR4 limits that can potentially offer stable industrial temperature narrow Δ𝜆rms and low RIN at chip level, low MPN in optical fiber, and high SNR at receiver, respectively. This simple and effective concept with further bandwidth, power and thermal budget improvements likely to have high potential for 850nm VCSEL use in PAM-4 based top of the rack (TOR)-leaf and leaf-spine 400G (SR8, SR4.2)/800G (SR16) Datacenter optical interconnects (DCI). The Δneff occurred from multilayer distributed Bragg reflector (DBR) stacks and quantum well active regions due to oxide layer is critical in fixing Δ𝜆rms of VCSELs as the data transmission distance is inversely proportional to Δ𝜆rms. The author discusses the impact of Δneff in creating narrow Δ𝜆rms and its benefit on low MPN in basic light transmitter units of NRZ 25.78125Gb/s and 28.05Gb/s transmissions of VCSELs in 100G eSR4 optical links with performance margin up to 400m in OM4 MMF at 85C with inexpensive power penalties at the receiver without equalization, pre-emphasis and error correction techniques.
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