VCSELs for High-Speed, Long-Reach, and Wavelength-Multiplexed Optical Interconnects

Doctoral thesis, 2015

The vertical-cavity surface-emitting laser (VCSEL) is the standard light source in short-reach fiber-optic links in datacenters and supercomputers. These systems use an enormous number of links, making cost and energy efficiency pressing issues. GaAs-based 850 nm VCSELs are therefore attractive due to low-cost fabrication, a small device footprint enabling compact integration into 2-D arrays, and above all, the capability of high-speed direct modulation at low power consumption. However, present commercial VCSELs, operating at around 25 Gbit/s over up to 100 m of multimode fiber, have insufficient speed, energy-efficiency, and reach for future links. Many of the attractive VCSEL properties stem from their small modal and active region volumes. The first part of this thesis explores the limits of optical and electrical confinement in high-speed VCSELs by using the shortest possible cavity length, and positioning the current-confining oxide aperture close to the active region. This enabled small-oxide-aperture VCSELs with record-high modulation bandwidth of 30 GHz, capable of energy-efficient data transmission at 25-50 Gbit/s with record-low dissipated energy per bit in the VCSEL of <100 fJ/bit. High-speed VCSELs are usually transverse multimode with large spectral widths. This leads to penalties from chromatic and modal fiber dispersion, limiting the feasible transmission distance to around 100 m at 25 Gbit/s, which is too short for large datacenters. The second part of this thesis demonstrates that VCSELs with narrow spectral widths, realized using either a small oxide aperture or an integrated mode filter, can transmit data at high bit rates over much longer distances. VCSELs with mode filters enabled transmission at 20 Gbit/s over 2000 m, setting a bit-rate-distance product record for directly modulated 850 nm VCSEL links. To enable higher link capacity, wavelength division multiplexing may be used, where several channels at different wavelengths are transmitted in the same fiber. The final part of the thesis presents the design, fabrication, and experimental results for monolithically integrated 980 nm multi-wavelength VCSEL arrays. By using high-contrast gratings with different parameters as top mirrors, the VCSEL resonance wavelength may be set in a post-growth process. Lasing over a wavelength span of 15 nm was realized.