Computer Backplane With Free Space Optical Links: Air Turbulence Effects

Over recent decades, there have been increasing demands for high data-rate backplanes for enhanced performance in computers. The present technology of electronic backplanes cannot meet the required data-rate demands for next-generation computers, indicating a need for a technology shift from the electronic to the optic domain. In this paper, we develop a mathematical model to describe the effect of air turbulence on a short range free space optical communication link, such as would be found in computer backplane communication, and conduct experiments to validate the model. The air turbulence under discussion resembles that which would result from the high temperature of chips on the board together with the ventilation of the air by the chip fan. In our experiment, the communication performance, expressed as bit error rate (BER), is presented as a function of the location of the turbulence source and the log amplitude variance. The log amplitude variance was evaluated by two independent methods. One of the main results of this study is the indication that the performance of the backplane deteriorates even for very small values of air turbulence strength. This is exacerbated by the fact that extremely low BER performance is required at the backplane (e.g., from 10
^-14
to 10
^{- 18}
). We also demonstrate that increasing the distance between the turbulence source and the optical link reduces the influence of the air turbulence in an exponential manner. This fact is important for the design of future-generation optical backplanes. It is noted that operation at wavelengths around 1550 nm yields communication performance that is less severely degraded by air turbulence effects than at 670 nm.