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UBC Theses and Dissertations

A differential push-pull voltage mode driver for vertical-cavity surface emitting laser Sivadhasan Ramani, Ajith


The unabated demand for data communication has led to a rapid growth in warehouse-sized datacenters where high-end servers transfer terabytes of data per second between the racks using optical data links. Vertical cavity surface-emitting laser (VCSEL) based optical links are widely popular in such datacenters for short-reach (< 300 m) interconnects due to their compact footprint, low cost, ease of integration with multimode fiber and flexibility in implementing arrays to achieve high aggregate data rate. Improving power-conversion efficiency (PCE), defined as the ratio of desired output optical power to the total electrical power of VCSEL driver, is paramount to improve the overall energy efficiency of the entire optical link. VCSEL diodes are normally driven single-ended with pseudo-differential current-mode drivers to maintain signal integrity. However, such conventional drivers consume significant power and are often unable to compensate for supply switching noise due to package parasitics at high data-rates. We propose a differential push-pull voltage-mode VCSEL driver to mitigate bondwire parasitics, reduce power consumption and leverage complementary meral-oxide semiconductor (CMOS) process scaling to its maximum advantage. A proof-of-concept prototype in a 65nm CMOS process achieves the highest reported PCE to-date of 18.7% for VCSEL drivers when normalized to VCSEL slope efficiency. It uses an asymmetric 3-tap rise and fall based pre-emphasis to achieve a total energy efficiency of 1.52 pJ/b at 16 Gb/s with an average optical power output of 1.34 dBm, optical modulation amplitude (OMA) of 2.1 dBm and extinction ratio of 5.92 dB.

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