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13 December 2001 Free-space optical backplanes based on beaconlike links or on a ring bus
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We studied the feasibility of two different topologies for board-to-board free-space optical interconnects: a unidirectional ring and a beacon-type star. In each node of the ring bus, the incident VCSEL beam is split both to the next node and to the detector. The star consist of beacon-like nodes based on cones; in the transmitter it reflects the VCSEL beam into all directions on the PCB plane and in the receivers it reflects the incoming beams towards the detector. 3-D models of the optical systems were optimized considering realistic mechanical requirements and tolerances of a typical PCB system. The power budgets and optomechanical tolerances were analyzed by ray-trace simulations. With four nodes and total length of 80 cm, the simulated path loss of the ring bus was 27 dB. At the corresponding 40-cm range, the beacon link had 38 dB signal attenuation. At the same bit-rate, the ring bus provides longer separation between nodes whereas the beacon system allows more nodes. The ring bus also enables parallel interconnects, but the alignment requirements would be very tight. The beacon link was demonstrated. The resulted attenuation was 10 dB higher than in the simulations, mostly due to losses in the receiver optics.
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Mikko Karppinen, Jukka-Tapani Maekinen, Antti Kemppainen, Marianne Borenius, and Pentti Karioja "Free-space optical backplanes based on beaconlike links or on a ring bus", Proc. SPIE 4455, Micro- and Nano-optics for Optical Interconnection and Information Processing, (13 December 2001);

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