Method of determining the signal level at the input of the optical receiver of surveillance equipment

The paper considers typical designs of surveillance devices for acousto-optic leakage channels. To assess the quality of the channel, it is proposed to use the criterion of the minimum permissible power of the information signal at the input.To analyze the processes that occur while the probe signal propagates to the pick-up point and back to the surveillance device, the authors propose a detailed model of the information leakage channel with the indication of the main transformations of the intercepted signal. The paper presents the main criteria for the correct reception of an optical signal and for the extraction of acoustic information from it. A number of assumptions are formulated regarding the practical implementation of the optoelectric converter of the receiving device, and a simplified equivalent functional model of the optical information transmission channel is developed. The authors also propose a method for calculating the voltage at the load of the photodiode depending on the distance to the vibrating surface, power and the coefficient of directional action of the transmitter emitter. The numerical dependences of the voltage at the load of the photodiode on the distance are obtained for specific numerical values of the load resistance, supply voltage, directional coefficient of the emitter and receiver, as well as the geometric dimensions of the information interception zone. A simplified technique is developed that allows determining the signal level at the input of the sensitive element of a photodetector under idealized conditions, if the minimum permissible optical signal power condition is met and there is no background illumination of the photodetector of the surveillance device.