High Speed Vertical Cavity Surface Emitting Lasers for Short Reach Communication

Doctoral thesis, 2011

The vertical cavity surface emitting laser (VCSEL) is a low cost light source with attractive performance characteristics such as low power consumption, high speed capabilities at low currents, and a circular output beam. These features have made the VCSEL an established component in digital communication networks, particularly in short reach optical data transmission applications where links consisting of GaAs-based 850nm VCSELs and multimode fiber have become the standard solution. However, VCSELs available on the market today are limited to bit-rates of ~10 Gbit/s and as current trends point towards single channel bit-rates being standardized and implemented far beyond this point, the development of higher speed VCSELs is crucial to meet the market’s need for higher speed digital communication services in the near future. The objective of this work has been to extend the maximum possible data transmission rate for 850nm VCSELs beyond the bit-rate limit of today’s commercially available devices. The work includes detailed studies of intrinsic and extrinsic device characteristics of importance for high speed performance and illuminates design trade-offs necessary for optimized performance. By tailoring the epitaxial and component design we demonstrate that 10 Gbit/s is far from the end point for VCSEL technology. Using strained InGaAs quantum wells (QWs) for high differential gain, a thin graded separate confinement heterostructure (SCH) for short carrier capture time, a binary compound in the bottom mirror for improved thermal conductivity, and a component design optimized for low electrical parasitics we are able to reach a modulation bandwidth of 20 GHz and demonstrate error-free data transmission at bit-rates up to 32 Gbit/s in the first generation devices. After optimization of the photon lifetime and a further reduction of electrical parasitics, we ultimately reach a record high modulation bandwidth of 23 GHz and demonstrate 40 Gbit/s error-free data transmission for the second generation 850nm VCSELs.