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Acceptance of optical interconnects into spaceborne and ground-based military systems will be limited by the risk and maturity of the technology. Multimode fibers are currently the interconnect medium of choice for military application due to the availability of qualified parts. Multimode optical interconnects also offer lower assembly cost than singlemode interconnects, making them attractive for use in commercial computers. In both applications, modal noise may limit the attainable bit error rate in a digital system. We report on an investigation of the validity of simple expressions for determining modal noise in multimode systems, and consider in particular waveguides supporting fewer modes than typical multimode fibers and waveguides with incomplete modal excitation. We conclude that the simple expressions are valid for losses greater than approximately 0.5 dB per interface, but that the actual signal-to-noise ratio is significantly poorer than that predicted by theory for large displacements of waveguides in which a small fraction of the total modal volume is excited. We describe two simulation techniques for determining the statistics of the transmission associated with a waveguide bend, and find that the simple formula widely used for straight waveguide intersections do not describe curved waveguide interconnects accurately. We also describe a practical demonstration of two optical interconnect systems in which multimode interconnect media incorporating several imperfect interfaces and excited with coherent sources can achieve useful bit error rates in digital systems. In the first interconnect, a fiber optica data bus for satellite use employs six multimode connectors and multimode fiber to transmit data at 1.2 Gbps over distances from 1 meter to 100 meters. In the second interconnect, passive multimode polymide waveguides compatible with intracabinet optical interconnects are used to implement a multichip module (MCM)-to-MCM interconnect in which optoelectronic die are incorporated in a 'chip first' multichip module technology.
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