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U of W  EE Dept., Project

Optics, 4th Ed


Fig. 1

Simplified Michaelson Interferometer
The Problem:
To detect sound within a room (having an outside window) at a distance. 

There are several methods that might accomplish this, with some fidelity; [1] however this page will describe the use of a LASER Microphone. 

The use of a LASER to transduce sound from a window pane/glass is commonly used in movies which leave the mistaken impression that any such task is easy and the LASER Microphone, itself, is highly portable and easily set up. My recent experience is just the opposite. 



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Fig. 2


Angular, or Grazing Detection 45°

         
There are several forms the LASER Mic can take; the several that follow are, by no means all: 

1)_  The one that is probably the most intuitive is a system that "grazes" a LASER beam at an angle to the plane of the glass (e.g., 45-deg.), with the photo detector at a complementary angle, and located at a near distance on the other side of the window, see fig. 2. The principle being that sound vibrations will cause the window glass to move sufficient to deflect the LASER beam across the receiving photo detector.



Fig, 3 

Direct Reflection
(0° Angular, Bounce-back Detection) 
2)_  The second method might be the same as above, with the difference being the LASER and detector are co-located, i.e., incident and reflected beam boresighted.


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Fig, 4

LASER Interferometer (Michaelson)
Interferometry:
By using Interferometry, one stands the best chance for success. However, the difficulty of use has increased somewhat. Also, this particular approach has several deficienses--the most glaring (sorry) one is the very large differences in the "leg" length. Ideally both legs of an interferometer should be near equal length. This is due to temporal or longitudinal coherence: where the phase coherence of a LASER beam changes over time. If the two jointly arriving beams are not phase synchronized, the constructive and destructive interference is degraded, or nonexistent, thus limiting the device's sensitivity.


Fig. 5

LASER Interferometer (Modified Michaelson)
       
3)_  A third approach might be to use an Interferometer. This takes the form of fig. 5, except one of the "arms" of the interferometer is the long path to and from the window. This, of course, suffers "coherence degradation," as explained above. 
   


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Fig. 6
4)_ The final approach is an interferometer similar to the one above, but having both legs of equal length--the so called "Dual Beam LASER Mic." 

The main principle is the differential measurement of glass movement (acoustic vibration) across a small section of the glass pane. 

This has the advantages of ~equal leg length for temporal coherence; common mode rejection of gross window movements, and some rejection of commom-mode path disterbances. 
 

Note: There is nothing magic about the LASER's Wave Length. The bound being between visible (e.g., < 670-nm), and Infrared (e.g., >1500-nm). The ideal wavelength (WL) is what is termed the "NIR," (Near-Infra-Red) band, e.g., ~ 790-nm. 

There are only a finite number of WLs available in this band, i.e., it's a Physics thing. Often the determining factor is cost and ease of use. Speaking of which, alignment can be a 'Bitch,' if you can't see the beam. So while experimenting, you might consider using a visible LASER!
 

Dual Beam LASER Interferometer
 


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 Some Suggested Optical Configurations
LASER Beam Collimation, Expansion, and Focusing
 


Circuit has Wide Dynamic Range 
 
 
 

PIN Diode detector can be substituted with less expensive devices.

PIN Photo Detector Circuit
 


Links:  U of W  EE Dept., Project
 
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Frequently-Asked-Questions
 Q:
I would like to make an optical microphone using your method , but there is a thing that i don't understand (my teacher too):  in the dual beam microphone, if both legs of the interferometer are on the window, they vibrate together, and there is no path difference?! (or just a little phase difference) and it would not be possible to get the frequency of the vibration.

Could you explain to me if i'm wrong ? it would REALLY help me out.

How could I realized one? Does it worked well?
 

A:
There is a differential measurement between the beams. This tends to cancel the common mode sound waves, especially in the lower frequencies; caused by wind, elevator noise, heating and air conditioning, etc. Of course the wider the spacing of the beams, the lower the frequency response.

There are several other things that complement this situation:
1) the audio bandwidth for optimum intelligibility in a noisy environment is in the area of 400 Hz to 2800 Hz (military), and the telephone industry uses 300 Hz to 3600 Hz (all measured at -3 dBm).

2) The speed of sound in air is ~ 1100 ft/sec, in glass the speed of sound is in the neighborhood of  ~20,000 ft/sec.

I have tested this method, it Does Work. 

While experimenting, aligning, and testing, you should Seriously Consider using a visible LASER!
 

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Misc Notes:

Finding the components for this and other projects: 

In the majority of cases, in today's consumer market, it is significantly cheaper to purchase either new or used devices and cannibalize them for the parts. 

Also, and more importantly, this assures you of not being the victim of promises  not kept by parts vendors. 
 

You can fail utterly if you have to WAIT for parts that either never get to you or when they finally do, are either the wrong part or defective. 

This is a lesson one hopes to learn sooner than later, especially if the fortunes of your business is riding on same.

In the case of the LASER microphone, finding an inexpensive red LASER pointer, of which there are many, would be a good starting point. 

As for photo detectors, capable of detecting reflected LASER light, with voice frequencies impressed on it, TV remote control detectors (that reside in the TV set) might be a good source. 

As I mention on this page, if possible use a RED visible LASER; as oppose to using an invisible Near Infrared (NIR) LASER. 

Alignment is a bitch if you can't see the LASER light!! 

Of course, once you have perfected the system, then you can substitute the invisible NIR LASER. 
 


 

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