Optimum Damping Level for High-Speed Large Signal VCSEL modulation

Poster (konferens), 2014

The commercial optical interconnects used in datacenters and high performance computers have recently been pushed to data rates of 25-28 Gbps. Near future standards will require even higher data rates and further work to increase the speed is therefore necessary. Typically the 850 nm vertical-cavity surface-emitting laser (VCSEL) is employed as light source in such optical interconnects, due to its low-cost fabrication and excellent high-speed properties at low drive currents. Previously, we have shown that a reduction of the damping of the VCSEL small signal modulation response can increase the modulation bandwidth (BW) and that there exists an optimum damping for maximum BW. This allowed us to demonstrate a record-high small signal modulation BW of 28 GHz and error-free [bit error rate (BER) <10^-12] transmission up to 57 Gbps. However, the optimum damping for maximum BW does not necessarily equate to the damping that produces the highest quality eye and best receiver sensitivity under large signal modulation. Recently we showed that the optimum damping for large signal modulation is dependent on the data rate. The damping (quantified by the K-factor) should be tuned to yield sufficiently fast rise time, high BW, and low timing jitter to produce a clear eye opening. At low data rates, the BW requirement is low and one can afford to use a low bias and a large damping which results in a low timing jitter, whereas an increase of the bias and a reduction of the damping is necessary at higher data rates to achieve sufficiently fast rise time and large BW to keep the eye open. This induces more timing jitter. The dependence of optimum damping on data rate was illustrated by BER measurements, which showed that the most damped VCSELs achieved the best receiver sensitivity at 10 Gbps, whereas a slightly less damped VCSEL achieved the best receiver sensitivity at 40 Gbps. Using a too low damping will induce too much timing jitter yielding an unusable high BER.