Board-to-board optical free-space interconnect: models and validations

Massively parallel computers raise a major challenge to the interconnection designer by asking for more than the electrical connector and PCB backplane technology are able to perform. Many new configurations are being explored in optics such as the fiber and holographic interconnects. We describe a third option: optical interconnects between adjacent boards by means of free space beams. They present the significant advantage of being possibly packed (4 to 16 per mm2) much more heavily than electrical interconnects because they can be put at the output of the processor, therefore decreasing delay. They are working with short wavelength (around 0.85 micrometers ) LEDs to decrease the cost, the sensitivity to temperature shift, and increase the reliability. Crosstalk can be kept below 20 dB and still have reduced sensitivity to misalignment of the boards. LEDs and microlenses are solder bumped which provide mounting accuracy of around one micron.