1, add a layer of shutters or other objects that can block the laser on the glass window.
2. The glass window is changed into special-shaped glass, and the surface of the special-shaped glass is not smooth, which does not affect the light transmission and makes the scattered laser unacceptable.
3. Install the glass window at a certain angle to make the incident laser beam reflect to the nearby ground;
4, the window with enough thick glass, make * * * vibration is difficult to make a sound;
5. Stick the audio noise source of piezoelectric body or motor on the glass window or near the window to make the noise adhere to the reflected light beam;
6. When talking, the recording is played indoors (it is best to record noisy sounds in public places), and the conversation is submerged in noise. This measure can also be used to prevent eavesdropping by other means;
7. Use a laser detector to detect whether there is a laser in the room. If there is super laser intensity in the room, it will send out an alarm signal.
With the development of science and technology, people have made various eavesdropping instruments according to the characteristics of acoustic vibration. From the early "big-eared" bug, directional microphone bug and "shotgun" bug to the later micro microphone bug, radio bug, microwave bug, telephone bug and laser bug.
Laser eavesdropping, which we have only heard of in film and television dramas before, is usually used in political and military espionage.
The Prism Gate incident in the United States revealed that during the Gulf War, American intelligence personnel used laser eavesdropping technology in Iraq to capture the voice of senior Iraqi generals in the car from the mirror of the driving car, and mastered all the information of the conversation in the car through technical processing.
The origin of early laser eavesdropping technology
The earliest technical data is the patent of optical telephone invented by Alexander Graham Bell and Charles Sumner tainter, which is a kind of telephone that makes wireless audio calls by modulating light beam (narrow projection of electromagnetic wave). In that distant era, public utilities did not provide electricity and lasers. Even in science fiction, their inventions had no practical application, which was largely influenced by sunshine and fine weather.
Similar to free space optical communication, sound generator also needs a clear line of sight between transmitter and receiver. It will take decades for the principles of optical scientists to find their first practical application in military communications and later optical fiber communications.
What is the principle of laser eavesdropping?
The principle of laser eavesdropping is to use a laser generator to generate a very fine laser beam and emit it to the glass of the eavesdropped room. When someone is talking in a room, the glass will vibrate slightly due to the change of indoor sound, and the laser reflected by the glass contains the vibration information of indoor sound waves. People can use a special laser receiver to receive the sound signal and demodulate it in a certain position outdoors, so as to monitor the conversation of people in the room.
Three Technologies of Laser Eavesdropping at Present
The theoretical basis of laser eavesdropping technology has long been perfect. At present, there are no more than three technical schemes: the eavesdropping technology of positive reflection laser intensity detection for detecting glass vibration information, the eavesdropping technology based on Doppler interference laser frequency or phase change for detecting object vibration through glass, and the eavesdropping technology of image processing based on speckle detection.
In 2009, researchers developed a new method of remote sensing sound by laser beam, which can extract and separate remote sound sources. The principle is to observe the movement of the secondary speckle pattern generated at the top of the target when the laser beam is irradiated, and image the optical processing and extract the sound signal. Various sound sources image in different spatial pixels, thus realizing optical listening.
Just like children's wired telephone, except that the wire is replaced by laser, and the microphone on the other side is any solid object that can vibrate, and it is thin enough to monitor the changes caused by the vibration when the red laser bounces back, which is invisible to the human eye.
The biggest feature of laser eavesdropping is concealment and portability! Because laser eavesdropping equipment does not need to be placed in the listener's room, it is difficult to be found. Early laser bugs need great stability and are often implemented in relatively fixed places. Now the vehicle-mounted laser eavesdropping has been realized, and the laser eavesdropping is even more difficult to prevent.