The Laser Microphone

2009-05-16 00:14 by Ian

Here is yet another unintentionally wonderful product of the Cold War. This device is designed to detect slight disturbances in the reflection of a laser beam and convert them into sound. Very nifty to use!
I wrote the following reflections during my 9:15 economics class…

Performance > Expectation:
I was happily surprised with the unexpected characteristics of the laser. After I burnt out the laser diode I was using for testing I tried to remove it painlessly from the collimator-heatsink housing. This failed, forcing me to destroy the housing and scavenge its lens. An elaborate act of improvisation resulted in a new collimator-sink housing comprised of the following:

I expected degraded performance, and indeed received it. But the fact that the beam now diverges slightly is useful for the purpose of illuminating the target at long distance. Furthermore the divergence causes concentric interference patterns in the axis of the beam to be readily visible past about six feet.
The mic is very sensitive to changes in amplitude. I used a small 32ohm speaker with a plastic membrane as my target. It was driven with an adjustable output function-generator. I was amazed by what I found. When properly aligned, the mic was able to pickup tones from 15 feet away that were barely audible from two inches away.

In the pictures below you can see the aluminum enclosures that house the receiver and transmitter, The lens-assembly with mounted laser, the rear of the lens-assembly which gives a detailed view of the LEDs that form the optical-squelch, and finally the target that I used for testing.