Optical `feedthrough´ transmission systems for the local loop

In order to obtain bidirectional working, additional splitters or WDM multiplexers need to be installed on the fibre, either by splicing or by the use of optical connectors. The authors describe a novel transceiver design in which the separation of send and receive signals is performed after the incoming signal has passed through the `send´ laser. This technique, `optical feedthrough´, enables bidirectional links to be configured without the need for line side components such as couplers, thereby reducing costs and network complexity. Two systems are described, one operating in a WDM mode transmitting digital data at 2 and 140 Mbit/s, and an alternative technique employing similar wavelengths for both directions but using analogue subcarrier modulation to provide simultaneous telephony channels