| 摘要 |
An optical communications system is provided comprising first and second signalling devices 3, 7 in which a free space duplex communications link 11 between the devices is established. The first device 3 includes a modulating retro-reflector and the second device 7 includes at least one light source for directing light towards the retro-reflector. A full duplex link is described which uses small signal modulation techniques to differentiate the transmitted uplink modulation data from the reflected downlink modulation data. The strength of the reflected signal is monitored and used as part of a control loop to control the power level of the transmitted light. In one embodiment the modulation depth (the difference in modulated intensity levels p1, p2 in fig. 5) of the transmitted uplink signal is varied in accordance with the attenuation characteristics of the channel. A drive circuit for the light source is disclosed in detail (see figure 11). Use of lower signal frequencies, such as microwave, is also envisaged. |
