Small-dimension power-efficient high-speed vertical-cavity surface-emitting lasers

Introduction: Recently, VCSELs have received considerable interest for short-distance optical interconnects owing to their small footprint, natural occurrence in arrays and, most importantly, cost effectiveness. Two of the main challenges for the devices to be used in board-toboard and chip-to-chip interconnects are speed and power dissipation. For directly modulated lasers, higher speed can be achieved with higher bias current at the expense of higher power dissipation. High power dissipation can be fatal in these compact systems with limited power budgets and stringent thermal restrictions. Most of the highspeed VCSELs have diameters from 6 to 8 mm and have threshold currents of 0.3 mA or above [1–3]. For these devices, 15 GHz bandwidth, which should enable 20 Gbit=s operation, was achieved at a current of at least 1.5 mA, but usually a much higher current was required owing to their high threshold currents and large mode volumes. Smaller devices are potentially faster and require less power, and thus are more suitable for optical interconnects. One of the obstacles for small devices to have high speed is the parasitics. By implementing deep oxidation layers [4] and optimising the doping schemes, we were able to reduce the parasitics to enable high-speed operation for smaller devices. Our 3 mm diameter device shows 15 GHz bandwidth at a bias current of 0.9 mA, corresponding to only 1.2 mW power dissipation. A bandwidth=power-dissipation ratio of 12.5 GHz=mW is achieved for 15 GHz, the highest ever reported.