Laser Microphone: Understanding Physics Behind Its Working

[DBB]

Hi,

I am creating a laser microphone in labs at the moment, and was wondering if anyone could help explain how/ why it works.

We are shining a laser a a reflective surface and using a photo-diode and some basic op amp electronics to detect the reflected beam and produce a voltage output.

Everything works and we are picking up an audible sound signal. However I'm trying to work out the physics behind it. Is it because the vibrations of the reflective surface are causing the reflected laser beam to travel by different amounts, therefore causing interference, causing the intensity to vary? Or is it something else all together.


Thanks

 

[sophiecentaur]

Yes, that's the basic principle.
However, the variation of intensity of the light entering the receiving sensor will not be very linear as a function of the sound pressure on the reflective surface so you can expect a pretty distorted version of the sound unless you are lucky in your choice of reflector.

Ideally, you would get a variation of intensity from zero to a max level over the maximum range of displacement of the reflector. But you may get several max-zero-max-zero excursions over one cycle of the sound. This could result in a Fuzz Buzz effect. OK for intelligibility of speech but lousy music quality.

 

[Bob S]

It is perported that a laser reflection off of the U.S. Embassy in Moscow was used as a spy technique in the 1960's:

"...Perhaps the most advanced - and talked about - technique of all is bouncing a laser beam off a window. The window pane vibrates slightly from the sound pressure generated by the conversation inside. The returning laser beam is modulated by these vibrations, and the original voice signals are recovered.

One of the textbooks designed to train security personal contains a section on such devices. It says one can be built using a General Electric H1A1 laser, which radiates about 35 watts of power in the infrared band. It is pulsed with a simple transistor circuit at 10 kilohertz. The receiver is an astronomical reflecting telescope bought from Edmund Scientific Company. A photomultiplier tube, which turns the pulsed infrared signal into a series of electrical pulses, is mounted in place of the eyepiece. The output of the photomultiplier is then fed to an amplifier to recover the voice signal from within the room."

From http://www.bugsweeps.com/info/bugsweepers.html