Optical interconnects to silicon CMOS: Integrated optoelectronic modulators and short pulse systems

The performance of silicon CMOS integrated circuits has increased dramatically over the past three decades due to the steady reduction in transistor feature sizes. For these advances to continue, a new hurdle must be overcome: the stagnant performance of the electrical wiring used between and within computers. Optical interconnects promise to solve many of the challenges imposed by electrical interconnects because of the favorable physics governing optical signaling. This dissertation describes investigations of optoelectronic devices for use in optical interconnects, as well as several systems-level experiments.; The approaches taken in existing optical networks will need to be radically altered for use in future optical interconnects. Factors such as cost, bandwidth density, and power dissipation will necessitate the dense integration of two-dimensional optoelectronic device arrays with conventionally-processed silicon CMOS chips. Hence, arrays of surface-normal GaAs-based electroabsorption modulators and photodiodes were fabricated and flip-chip bonded directly to CMOS microelectronics. The process steps developed for fabricating the quantum-well modulators will be highlighted in this dissertation, and the resulting device performance will be discussed. Resonant-cavity enhancement was used to increase the low-voltage performance of the modulators, making them compatible with future generations of CMOS. A Fabry-Perot cavity-tuning technique will be described, as well as a novel modulator design that employs a first-order cavity to maintain a broad spectral bandwidth at very low voltages.; Using these integrated optoelectronic components, a free-space chip-to-chip optical interconnect demonstrator was constructed. The work described herein shows that ultrafast techniques can provide several benefits in such an optically-interconnected system. Bit error rate measurements were used to quantify the improvement in receiver sensitivity that can be achieved by employing “short pulse signaling” instead of the conventional non-return-to-zero (NRZ) data format. The short pulse duration and low jitter output of a modelocked laser enables the removal of transmitter skew and jitter, which was demonstrated using the link. The results of a short pulse pump-probe experiment will also be presented, in which precise time-domain measurements of circuit delay are used to determine the latency of interconnect transmitters and receivers. Finally, several additional advantages of using short pulses for optical interconnection will be described.