VCSEL driver

Team Members: Mayank Raj and Manuel Monge

Increasing bandwidth requirements have pushed the traditionally electrical wireline interconnects within computing systems and data centers to their limits. As data-rates scale, the shortcomings of electrical channels are becoming more severe. Technology scaling favors I/O circuit performance, but the bandwidth of electrical channels does not scale with the same trend. Several receiver and transmitter equalization techniques have been proposed to overcome this bandwidth limitation. However, these compensation techniques consume considerable power and die area, and, as a result, current high-speed I/O link designs are increasingly becoming power and channel-limited.

The lab has worked on an ultralow-power VCSEL transmitter in 32 nm SOI CMOS. To increase its power efficiency, the VCSEL is driven at a low bias current. Driving the VCSEL in this condition increases its inherent nonlinearity. Conventional pre-emphasis techniques cannot compensate for this effect because they have a linear response. To overcome this limitation, a nonlinear equalization scheme is proposed. A dynamic VCSEL modeling technique is used to generate the time-domain optical responses for “one” and “zero” bits. Based on the asymmetry of the two responses, the rising and falling edges are equalized separately. Additionally, instead of using fixed bit delays, the equalization delay is selected based on the bias current of the VCSEL. The efficiency of the proposed modeling and equalization technique is evaluated through simulations and measurements. The transmitter achieves energy efficiency of 0.77 pJ/b at 20 Gb/s and occupies 100 µm × 60 µm active silicon area.

Related Publication

Mayank Raj, Manuel Monge, Azita Emami “A Modelling and Nonlinear Equalization Technique for a 20 Gb/s 0.77 pJ/b VCSEL Transmitter in 32 nm SOI CMOS,” IEEE Journal of Solid-State Circuits, vol.51, no.8, pp.1734 – 1743, July. 2016, DOI: 10.1109/JSSC.2016.2553040

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