Laser Microphone

Posted on Feb 7, 2014

This was a final project for an optics class EE 134 at Stanford. It was an open-ended project to make a fully functioning laser microphone the class was geared to demonstrate optics and photonics in the lab, not necessarily electrical schematics. There are real commercial devices that use lasers to sense sound acoustics from glass surfaces. The

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hardest aspects of this sort of project were getting all the optics to line up and work properly. The final write-up is included here. A laser microphone was constructed and demonstrated. The laser was able to successfully detect audio vibrations and recover the signal in a useful form for processing. Electrical circuits provided amplification and filtering which improved signal quality. After recovery, the audio signal was output to a speaker. The system had a signal-to-noise ratio of 3. 7 dB and recovered about 25% of the laser power at variable distances from 15 to 80 cm. Laser microphones are capable of detecting sound at distances and through mediums that traditional equipment cannot. Law enforcement or other surveillance experts employ many techniques and equipment in order to gather information. A traditional G ‚¬ bugG ‚¬ is an electronic device that is physically placed near or inside enclosures which transmits audio information via radio or other short-ranged frequencies. With the use of a laser, entry into a building in order to plant a bug is not necessary. Instead, only a direct line of sight to a window or other vibrating surface will suffice in order to sample and receive audio information. Techniques demonstrated in this paper could also be employed to perform high sensitivity studies on any vibrating surface and need not be limited to audio frequencies. Laser vibrometry is a technique to measure the velocity...

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