Quasi-Single Mode VCSELs for Longer-Reach Optical Interconnects
The vertical-cavity surface-emitting laser (VCSEL) is the standard light source in optical interconnects connecting racks in supercomputers and datacenters hosting services such as online data sharing, storage and processing. VCSELs have numerous advantages such as low power consumption, fast direct modulation at low currents, circular output beam for efficient fiber coupling and low-cost fabrication. However, today's commercial optical interconnects, operating at around 10 Gbit/s over up to 300 m of multimode fiber, have insufficient speed and reach for future datacenters. The goal of this work has been to extend the maximum reach of GaAs-based 850 nm VCSEL-based optical interconnects. Recently developed high-speed VCSELs featuring strained InGaAs quantum wells and multiple oxide layers are highly transverse multimode with large root-mean-square (RMS) spectral widths around 1 nm. This leads to problems with chromatic and modal fiber dispersion, limiting the maximum reach to around 100 m at 25 Gbit/s. By using quasi-single mode VCSELs with a side-mode suppression ratio of ~15-20 dB and RMS spectral widths around 0.3 nm, the reduced dispersive effects enable extended reach at high data rates. Two different techniques to reduce the spectral width are investigated. By using a small oxide aperture, the number of modes guided by the VCSEL waveguide is significantly reduced. A 3 um oxide aperture VCSEL was used to transmit 22 Gbit/s over 1100 m of OM4 fiber. However, small aperture devices have high resistance and relatively low output power. The spectral width of larger aperture VCSELs can be reduced by etching a shallow surface relief that induces a mode selective loss to suppress higher order modes. In effect, the etched feature acts as an integrated mode filter. A 6 um oxide aperture surface relief VCSEL enabled transmission at 20 Gbit/s over 2000 m of fiber, setting a new bit-rate-distance product record for directly modulated 850 nm VCSEL links. Benefits and drawbacks of both methods are discussed and compared.
vertical-cavity surface-emitting laser