Increased power injection in multimode optical-fiber buses through mode-selective coupling

The number of stations that can be put on an optical-fiber data bus is usually limited by the total allowable attenuation between transmitter and receiver. A fundamental assumption in local-area network designs has been that a passive bus coupler must remove from the bus the same fraction of optical power that it injects from the local transmitter. This mandates very inefficient power injection to the bus and limits the total number of stations that can be put on the bus. By considering mode-selective passive optical couplers, I show that this limitation can be overcome in principle, and practically all the local power can be injected. Calculations indicate that this can lead to an increase in injected power of around 7 dB, and an almost doubling of the number of possible stations on the bus.