Active optical transponder system

An active optical communication system uses an electromagnetic beam (preferably infra-red) from a mobile interrogation station to set up a communications path to a wanted one of a number of other terminals. The beam is aimed at the wanted terminal and modulated by a coder (1) with that terminal's code. At the wanted station the beam is picked up by infra-red diodes (A,B), and the code decoded and checked by a decoder (3). If it is correct for that station, a drive motor (4) directs the receiving diodes (A and B) towards the interrogator for maximum response. In addition a mirror (5) which directs the return beam from the terminal's transmitter (6) to the interrogation terminal is positioned with the receiving diodes. Thus the communication path is set up.