Capacitively-Coupled CMOS VCSEL Driver Circuits

A capacitively-coupled laser diode driver (LDD) for common-cathode vertical-cavity surface-emitting lasers (VCSELs) designed in 65 nm CMOS intended for short-reach parallel optical interconnects is presented. The LDD consists of a capacitively-coupled voltage-mode high-frequency path and a direct-coupled path that provides low-frequency signal components. By increasing the low-frequency path bandwidth an order of magnitude compared to previous capacitively-coupled drivers, the on-chip coupling capacitor is reduced to 1.05 pF, occupying 3 times less area than prior art. The driver incorporates capacitively-coupled two-tap pre-emphasis to equalize the VCSEL's optical response. The VCSEL was modulated with an optical modulation amplitude (OMA) of up to +5.1 dBm and an extinction ratio of 9 dB at 15 Gb/s, the highest extinction ratio achieved in high-speed anode-driving CMOS LDDs. The driver is programmable and is also demonstrated with +2.3 dBm OMA at a power consumption of only 30 mW, corresponding to an energy efficiency of 2 pJ/bit.