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Laser Listening Device

  1. Jul 11, 2005 #1
    I wonder if it's possible to create the laser listening device as a project .
    i 've been fascinated by the idea ever since i saw it on SPIES
    It'd be a great help if anyone could could tell me how to get started first ,like
    how does the thing actually work n how to create a laser n stuff .
  2. jcsd
  3. Jul 11, 2005 #2


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    This is an educated guess:
    I do not believe lasers can listen in on just any old sounds. Laser listening devices are used to extract sounds from within enclosed rooms. They do so by treating the window panes as eardrums. Sounds within the room are transmitted as vibrations through the window panes themselves. Lasers pick up this extremely tiny vibration via Doppler Effect and turn it back into sound.

    I would think that this is precision engineering, way beyond amateur level, but that is a guess.
  4. Jul 11, 2005 #3
    Ha ha. One of the final year projects for the Physics Degree at Bristol Uni is to create a device that detects sounds using a laser, based on the principle that a laser is a pressure wave, therefore the refractive index of the air will change when the sound wave passes through, deflecting the laser. A friend of mine's doing this - should be interesting.
  5. Jul 11, 2005 #4
    A friend of mine told me they're used in the millitary right now. Then again, this friend also plays a lot of Splinter Cell.

    Here's a site I found by googling "Laser Mic": http://www.williamson-labs.com/laser-mic.htm
  6. Jul 11, 2005 #5

    Claude Bile

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    Pressure changes are easily detected by a laser (especially in a gas) since a change in pressure corresponds directly to a change in density and hence the optical path length of any laser beam that happens to be travelling through such a region (which is the basis of any interferometric device).

    The difficult part would be reconstructing the original acoustic signal, using the optical signal, which is just a set of (moving) fringes. Difficult, but far from impossible.

  7. Jul 12, 2005 #6
    I saw a demo lecture where the professor was showing us a laser listening device he'd built - he shone the laser at the clock at the back of the lecture hall and you could hear the mechanism ticking amplified through the device.

    I'm pretty sure he built it himself
  8. Jul 12, 2005 #7


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    When i saw this threads title, "Splinter cell!!" immediately came to mind haha. But yah, as some people have stated, they are real devices.
  9. Jul 12, 2005 #8
    Claude - the supervisor for the project is the head of the nanostructures group, so I believe they're looking at using techniques from the AFM (atomic force microscopy) work, where the minute deflections of a cantalever are read off with a laser.
  10. Jul 12, 2005 #9

    Claude Bile

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    Sounds interesting. What is the motivation? Are they trying to build a minaturised device?

  11. Jul 12, 2005 #10


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    Several years ago, The magazine Radio Electronics had a project where you built such a laser eavesdropping device. You could do it all yourself or you could order some of the specialty items form them (the lasers and the etched pc boards/).
    I remember that they included the disclaimer to the effect the the device was for educational purposes only and was not meant for actual eavesdropping. They also used to include like disclaimers for their cable de-scrambler. Yeah, right. Like some guy was going to spend all that time and money to make the cable de-scrambler and then say, " Well, that was educational", put it away on the shelf and never use it.
  12. Jul 14, 2005 #11
    hey thanks jelfish for the link ,well it did provide basic structures of the device but the biggest problem is the detector ,notin about that ,how do you convert the laser signals after they bounce back from the window pane into sound ?i guess you'd have to build some kinda MODEM for that

    saw the circuit diagram but c'mon not good enough for practical construction!

    i wonder if a standard cd plyayer could be used as a source for the laser ,
    can some one give me the wavelenghts of the laser used in diskmans and cd players and the optimal wavelength for building a listening device and what is the wavelenght of the visible red laser and which appliance is most likely to use a visible red laser ?


  13. Jul 14, 2005 #12


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    This was several years ago, late 80's or early 90's. The mag is out of print and the publisher went out of business in 2002. It probably wouldn't do you much good either as you would most likely not be able to get some the components anymore.
  14. Jul 16, 2005 #13
    So The World's Not A Freindly Place For A Geek After All .
    Thanks Nyway
  15. Jul 16, 2005 #14

    One of the hard parts to find is a fast sensitive optical pin diode for under a dollar. I need a response time of 10 nanoseconds.
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