VCSEL-based multimode optical links [electronic resource]

High Performance Computing and Data Centers are driving the demand for high bandwidth interconnects. Future exascale supercomputers capable of more than a quintillion operations per second will demand higher performance networks characterized by lower power consumption, lower cost, and higher bit rate. Vertical cavity surface emitting laser (VCSEL) based multimode optical links are one key solution to enable this growth. The work presented here aims to demonstrate solutions that increase bit rate, decrease power consumption, decrease cost, and increase the usable distance of VCSEL-based multimode optical links. Equalization is known to be an effective solution for overcoming the bandwidth limitations in a copper interconnect; however, here it is used to achieve higher bit rate, lower power consumption, and increased margins for a multimode optical link. This dissertation presents a full link employing a 90 nm CMOS driver and 90 nm CMOS receiver amplifier fabricated with IBM's standard bulk 90 nm CMOS process. Using feed-forward equalization at the transmitter, the link achieves a highest serial bit rate of 30 Gb/s, a lowest power consumption of 2.95 pJ/bit at 20 Gb/s, an increased timing margin of 0.17 UI at 23.5 Gb/s, and improved receiver sensitivity by 4 dB at 23.5 Gb/s. Wavelength division multiplexing (WDM) is a technique to combine multiple parallel channels of different wavelengths into a single fiber to reduce system cost. This dissertation presents the design and characterization of a grating-based wavelength division multiplexer formed with injection molded plastic to reduce the number of fibers needed in parallel links. This design uses fewer components than previously demonstrated dielectric filter-based approaches in an effort to minimize the complexity of manufacturing to reduce cost. Care is taken to optimize the design for alignment tolerances associated with assembly, and to ensure the device works across the wide temperature ranges VCSEL-based links must tolerate. Simulations and experimental characterization show 4 channels at 850, 880, 910, and 980 nm multiplexed into a single multimode fiber. The coupling conditions between VCSELs and multimode fiber are studied in this dissertation to improve fiber bandwidth. A novel characterization system based on a spatial light modulator imaging system is used to characterize a VCSEL both spatially and spectrally. The system applies a blazed grating to the spatial light modulator to scan an image of the VCSEL across the core of a single mode fiber and measures the full optical spectrum at each point in the scan. A mode solver is used to develop an analytical model for light propagation down a multimode fiber, which is combined with the experimental VCSEL characterization to design a coupling solution to improve fiber bandwidth. A 6th order spiral phase mask is experimentally shown to modify coupling conditions to increase fiber bandwidth from 6.3 GHz to 17.5 GHz over 300 m of OM3 fiber under typical alignment.

Keywords: Electrical Engineering.
Advisor: Solgaard Olav
Harris J. S. (James Stewart)
Kahn Joseph M. (Joseph Mardell) advisor. Thesis advisor