# Laser Microphones

1. Mar 30, 2014

### Ghassan99

I have been trying to write the mathematical simulation for this device but unfortunately I found no clue to start with .
the simulation should be starting with the Gaussian beam equation of the laser source and then the vibrations in the glass pane cause the window to flex, changing the center of curvature of the window, thereby causing the "focal length(=measure of how strongly the system(glass) converges or diverges light) of the window to change, albeit very slightly. This creates a varying divergence and converges in the reflected laser beam which then can be directed to a photo diode causes the voltage across the detector to fluctuate.
I'm really desperate for a mathematical simulation for this process.

Last edited: Mar 30, 2014
2. Mar 30, 2014

### Staff: Mentor

That's not how a laser microphone works. What makes you think it is a modulation in a focal length?

3. Mar 30, 2014

### Ghassan99

that's what I have studied , do you have any other idea i assume?

4. Mar 30, 2014

### davenn

the usual way is that the laser is reflected off the window back to a receiver
The flexing glass FM modulates the laser signal which is then demodulated and the audio recovered

Dave

5. Mar 30, 2014

### meBigGuy

I guess it is possible to build a system based on an optical lensing receiver system with an extremely shallow depth of field such that microscopic defocusing would cause the field to increase enough that the amount of light on a critically sized sensor would vary enough to be detected.

But, I think the more common approach is an interferometer based approach where the reflected signal's phase varies due to the path length changes and is summed with a reference from the transmitter. This produces an AM light signal.

6. Mar 30, 2014

### davenn

hi mebigguy

it may well be AM ... Berkeman PM'ed me with same thoughts
my reasoning for it being FM was that there was a Doppler effect in action which would produce an FM'ing of the laser beam

I don't understand how the laser is AM modulated by a sheet of glass vibrating back and forward ?

cheers
Dave

7. Mar 31, 2014

### meBigGuy

It is an interferometer. The distance to the reflector and back changes as the reflector vibrates. This causes a phase change which is detected by summing with a portion of the transmitted signal.

http://www.williamson-labs.com/laser-mic.htm explains different approaches, including one that is not an interferometer.

8. Mar 31, 2014

### Ghassan99

9. Mar 31, 2014

### davenn

interesting link, saved that one :)

fig 3 is the way I was thinking, but that isnt listed as the interferometer as the others are
So is that one PM, AM or FM or a mixture ?

Dave

10. Mar 31, 2014

### meBigGuy

Figure three is just detecting angular deflection of the window the same as figure 2.

It has to work such that the laser light boundary falls through the middle of the detector, and the deflections change the area exposed to the laser. (half the detector is lit with no deflection, all or none with full deflection)

So in principle there is no "modulation" of the light in that application. It is just that the beam is moved off the detector.

BTW, doppler shift of light is not detectible by a photodetector. FM modulation is constant amplitude.
The interferometer actually AM modulates the light intensity. The detector tracks the intensity.