Word Length Dependent Sensitivity Penalty in High Speed VCSEL-Based Optical Interconnects

The use of long pseudo-random bit sequences to emulate random data in optical transmission links often gives rise to degradation of the receiver sensitivity for a given target bit error rate resulting in a power budget penalty. This is a well know fact and conventionally attributed to the non-ideal response of the system at very low frequencies. Here, we investigate this in the context of directly-modulated vertical-cavity surface emitting laser (VCSEL) based optical interconnects. We observe, both in simulations and in experiments, a substantial increase in the penalty with long sequences especially at symbol rates exceeding the VCSEL bandwidth. Our results show that this is related to the high frequency dynamic behavior of the VCSEL itself and we identify a few certain short bit sequences that cause the large penalty observed. We also show that the penalty can be significantly reduced by a simple modification of the sequences.