A group of researchers from Tsinghua University (China) has developed a technique for listening to conversations in a room where an optical cable runs, for example, used to connect to the Internet. Sound vibrations create air pressure drops, due to which microvibrations occur in the optical cable, modulated with a light wave transmitted through the cable. The resulting distortions can be analyzed at a sufficiently large distance using a laser Mach-Zehnder interferometer.
During the experiment, it was possible to fully recognize the sounding speech in the presence of a three-meter open piece of optical cable (FTTH) in front of the modem. The measurement was carried out at a distance of 1.1 km from the end of the cable located in the eavesdropped room. The listening range and the ability to filter interference correlate with the length of the cable in the room, i.e. as the length of the cable in the room decreases, the maximum distance from which listening is possible also decreases.
It is shown that the detection and restoration of an audio signal in optical communication networks can be implemented covertly, imperceptibly for the listening object and without violating the used communication functions. For inconspicuous wedging into the communication channel, the researchers used a wavelength division multiplexer (WDM, Wavelength Division Multiplexer). An additional reduction in the background noise level is achieved by balancing the arms of the interferometer.
Measures to counter eavesdropping include reducing the length of the optical cable in the room and placing the cable in rigid cable channels. You can also use angled APC (Angled Physical Connect) optical connectors instead of flat end connectors (PC) to reduce listening efficiency. For fiber optic cable manufacturers, it is recommended to use materials with a high modulus of elasticity, such as metal and glass, as fiber cladding.
Source: opennet.ru