Engineering vertical-cavity surface-emitting lasers for high-speed operation

As the performance of microprocessors constantly improves with scaling and integration, copper-based electrical interconnects are slowly becoming the bottleneck for the system performance. Optics is a viable solution to this interconnect problem because it can have less signal distortion, higher throughput and density, and potentially requires less power. Among all the technologies pursued for optical interconnects, vertical-cavity surface-emitting lasers (VCSELs) are very attractive due to their capability of direct modulation, small footprint, ease of fabrication in arrays, support of on-wafer testing, and low power consumption.;Most of the high-speed VCSELs are relatively large from 5 to 8mum in diameters and thus require currents that are more than necessary to achieve high bandwidth. In this work, small-dimension, high-efficiency, high-speed VCSELs operating at 980 nm wavelength were developed. To improve the high-speed performance without increasing the power dissipation, the mode volume has to be reduced, which increases the optical scattering loss if not designed properly. By using tapered oxide apertures and carefully choosing the taper length, we showed that the mode volume of VCSELs can be greatly reduced without sacrificing their static performance. To reduce the parasitic capacitance in the mesa, we proposed to use the deep oxidation layers, which are simple, effective, and can be easily incorporated into our devices without complicating the fabrication process. Furthermore, we optimized the p-mirror to achieve low loss and low resistance. The pad capacitance was reduced by using BCB, removing the n-contact layer, and shrinking the pad dimension.;With all these improvements, our 3mum demonstrated a very low threshold current of 0.14 mA and a high differential quantum efficiency of 54%. This indicates that the static performance was not noticeably degraded by these modifications for high-speed operation. On the other hand, > 20 GHz bandwidth, the highest for 980 nm VCSELs, was achieved at just 2 mA. A data rate of 35 Gb/s was demonstrated with only 10 mW power dissipation, corresponding to the highest data-rate/power-dissipation ratio of 3.5 Gbps/mW for any laser sources.