Comparison of Stray-Light and Diffraction-Caused Crosstalk in Free-Space Optical Interconnects

In this paper we investigate for the first time the effect of the crosstalk introduced due to laser beam imaging in a freespace optical interconnect (FSOI) system. Due to the overfill of the transmitter microlens array by the vertical cavity surface emitting laser (VCSEL) beam, one part of the signal is imaged by the adjacent microlens to another channel, possibly far from the intended one. Even though this causes increase in interchannel and intersymbol interference, to our knowledge this issue has been neglected so far. The numerical simulation has been performed using a combination of exact ray tracing and the beam propagation methods. The results show that some characteristics of stray-light crosstalk are similar to that of diffraction-caused crosstalk, where it is strongly dependent on the fill factor of the microlens, array pitch, and the channel density of the system. Despite the similarities, the stray-light crosstalk does not affect by an increase in the interconnection distance. As simulation models for optical crosstalk are numerically intensive, we propose here a crosstalk behavioural model as a useful tool for optimisation and design of FSOIs. We show that this simple model compares favourably with the numerical simulation models.