It's a half-year project. Leading a 5 people team, I developed a eavesdrop device prototype by sensing the interference pattern of two beams laser light. The device was developed while I was a physics undergraduate student at University of Science and Technology of China (USTC) in 2011. The simplified 3D model is shown below for demonstration.

Demo Audio (Audio information collected by the device)

The audio above was collected by my device. The sound source is a iphone2G playing musice behand a window. The laser light beam from the device shot to the glass of the window. The sound was collected remotely by decoding the reflected light.

Sensing Vibration through Interference of Laser


Optical Path

Sound is the traveling vibration. Objects near the sound source will appear similar virbation.

The device devided the laser ray into two. One was shooted to a subject near the sound source. The reflect light interfered with the other ray. The interference pattern is decided by the lengths difference the two ray travelled.

The sound information is underneath the time series interference pattern.

Photo of Interference Pattern


Silicon Photocell

The light strongth sensor was placed in the middle of the concentric circles like interference pattern. I selected the silicon photocell as the light sensor because of its short response time and high light sensitivity. Before using the silicon photocell, I tried photoresistor. However, the slow response of photoresistor make the high frequency sound disappeared.

Amplifying Circuit

The voltage signal from silicon photocell is amplified by the amplifying circuit. Then the amplifyed signal go to the 3.5mm mic-in port of my laptop. A C++ program developed by me read the data from mic-in port and translate the data to the final audio signal.

Experment Result & Photos

Comparing with the old laser eavesdrop technque which does not use interference, the frequency data of sound from my device is more close to the sound source.

I won the first prise of the National Undergraduate Innovation Experment Competiton because this device.

Photos of my teammates and Mine