VCSEL Cavity Engineering for High Speed Modulation and Silicon Photonics Integration

Abstract: The GaAs-based vertical-cavity surface-emitting laser (VCSEL) is the standard light source in today's optical interconnects, due to its energy efficiency, low cost, and high speed already at low drive currents. The latest commercial VCSELs operate at data rates of up to 28 Gb/s, but it is expected that higher speeds will be required in the near future. One important parameter for the speed is the damping of the relaxation oscillations. A higher damping is affordable at low data rates to reduce signal degradation due to overshoot and jitter, while lower damping is required to reach higher data rates. A VCSEL with the damping optimized for high data rates enabled error-free transmission at record-high data rates up to 57 Gb/s. For future interconnect links it is of interest with tighter integration between the optics and the silicon-based electronics. Techniques to heterogeneously integrate GaAs-based VCSELs on silicon could potentially enable integrated multi-wavelength VCSEL arrays, thus increasing the data rate through wavelength division multiplexing. Heterogeneous integration of GaAs-based VCSELs would also benefit applications that need short-wavelength light sources, such as photonic integrated circuits for life sciences and bio photonics. Silicon-integrated short-wavelength hybrid-cavity VCSELs with up to 2.3 mW optical output power and 12 GHz modulation bandwidth, which enables data transmission at up to 25 Gb/s, are demonstrated by employing ultra-thin adhesive bonding. Further, a vertical-cavity silicon-integrated laser (VCSIL) with in-plane waveguide emission is demonstrated by employing an intra-cavity waveguide with a weak diffraction grating that couples light from the standing wave in the vertical cavity into an in-plane waveguide.