Laser Listening Devices
Published: 22/5/09


Laser listening devices—sometimes termed laser-bounce listening devices—are remote-eavesdropping systems that do not require the placement of a microphone or bug in the space to be monitored. Instead, they measure changes in light reflected from some surface (usually a window) that is made to vibrate by sound waves in the monitored space. As the window-glass is made to move to and fro by the alternating pressure of incident sound waves, some component of its motion will be toward and away from an observer viewing the glass at any angle except 90° to the direction of vibration. Laser light reflected from the glass toward the operator of the laser listening device will, therefore, be Doppler shifted by a continuously changing amount. By detecting this Doppler shift, the vibrations of the reflecting surface, and thus, of the adjacent air, can be detected. Doppler shift occurs when light (or any other moving wave) is reflected from a moving surface or radiated by a moving source. Laser light reflected when the window is vibrating toward the laser listening device is shifted upward in frequency, while light reflected when the window is vibrating away is shifted downward. Laser listening devices have the drawback that they require line-of-sight access to an appropriate reflector; they have the advantage that they can record conversations from a considerable distance and without access to the monitored space itself ever being needed.

Infrared laser light is used in this application both because (a) infrared laser light is invisible, making naked-eye detection of the eavesdropping device unlikely, and(b) because a laser's light is all emitted at one frequency, measurement of Doppler shift of the reflected beam is straightforward.

Effective laser listening devices are available from commercial suppliers; more sophisticated versions have long been used by various national security agencies. Embassies and other locations wishing to be secure against laser listening devices can deploy such countermeasures as multi-paned windows, exterior meshes to break up laser light, infrared laser detectors, and noise generators that add random vibrations to those caused by conversation.

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