A group of researchers from the University of Qinghua (China) designed the technique of listening to conversations in the room in which the optical cable takes place, 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 distortions can be analyzed at a fairly large distance using a laser interferometer MAHA CENDER .
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During the experiment, it was possible to completely recognize the sounding speech if there is a three -meter open piece of optical cable (FTTH) in the room 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 eavesdated room. The listening range and the ability to filter interference correlates with the length of the cable in the room, i.e. With a decrease in the length of the cable in the room, the maximum distance from which is possible listening is reduced.
It is shown that the definition and restoration of the sound signal in the networks of optical communication can be implemented secretly, imperceptibly for the list of listening and without violating the communication functions used. For inconspicuous wedding into the communication channel by researchers, a multiplexer with a division in wavelength (WDM, Wavelength Division Multiplexer) was involved. An additional decrease in background noise is achieved through the balancing of the interferometer shoulders.
As measures for countering listening, the reduction in the length of the optical cable in the room and the placement of the cable in hard cable channels are called. To reduce listening efficiency, you can also use when connecting inclined optical connectors APC (Angled Physical Connect) instead of connectors with a flat end (PC). For manufacturers of optical cables, it is recommended to use materials with a high elastic module such as metal and glass as a fiber coating.