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laser FM detector

Discussion in 'Electronic Design' started by [email protected], Feb 23, 2008.

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  1. Guest

    Greetings! I'm a fresh college student currently learning electronic
    communications. I have been planning on a project that makes use of
    lasers to transmit signals through the laser beam, shifting a beam
    using a mirror mounted on a vibrating speaker cone.

    For the receiving end, I'll be making a simple slope detector + audio
    amp circuit that would hopefully demodulate the signal. But instead of
    an antenna (which is particularly useless in my project), I will be
    using a simple photodiode that would change the frequency of a signal
    (from an AC source, most likely a signal gen) according to the angle
    shifting of the beam, even though there's a little difference.

    I'm currently stumped in how I would configure the photodiode + AC
    source part before the signal would go to the detector. Any help on
    this would be appreciated.

    (PS. I know, AM would've been better and easier, but still, I chose
    FM.)
     
  2. MooseFET

    MooseFET Guest

     
  3. MooseFET

    MooseFET Guest


    A moving speaker cone won't be able to make a very good signal for you
    to work with.

    If you take a sample of the unmodulated laser with a beam splitter you
    can send that light to the photodetector along with the modulated
    light. Interferance at the photodetector could then turn a phase
    modulation into an AM modulation at the photodetector.
     
  4. John Larkin

    John Larkin Guest

    How can a photodiode change the frequency of a signal from a signal
    generator?

    John
     
  5. Guest

    Is this supposed to be FMing the lasr freq? All it's going to do, is
    change the angle of reflection of the beam, changing to strength of
    the beam hitting the detector, no?
    A simple photodetector responses to the strength of the signal, not
    the freq.
     
  6. If you are doing FM by doppler, look what deviation you can get. If
    you have a speaker with 1 mm peak-peak excursion, and it is working at
    1 kHz, you will have 1 m/sec peak velocity. The relative change in
    frequency will be 1 m/sec divided by the speed of light, or about one
    part in 300 million times the frequency of the light. Since
    photodiodes don't respond at light frequencies, the slope detection
    has to take place before the photodiode, which would require an
    extremely selective optical filter. The only chance you would have of
    doing it would be to detect the signal before the photodiode with the
    unmodulated laser beam, as MooseFet suggested.
     
  7. Actually, 1 mm peak excursion --> 1 m/sec peak velocity.
     
  8. Phil Hobbs

    Phil Hobbs Guest

    A simple way of doing optical communication is to connect a small audio
    amplifier across the battery in a laser pointer, using a capacitor in
    series with one of the leads. This produces mostly AM, which you detect
    with a photodiode.

    It's quite possible to do the measurement you're talking about, but
    you'll need either an interferometer, which isn't too hard to make but
    will require some fairly significant care in alignment.

    Put a little spot of retroreflecting tape on the speaker cone where the
    laser beam hits--it'll increase your signal by at least 40 dB
    electrical, which is a big help.

    Another possibility is using an optical lever--mount a small mirror on
    the edge of the speaker cone so that it tilts when the cone moves, and
    put the photodetector at a distance, positioned so that half the beam
    hits it. Then speaker vibrations will cause the beam to move partly on
    and off the detector. You can improve this very much by using two
    photodiodes next to each other and subtracting the photocurrents (i.e.
    wire them in series between + and - bias supplies--with the right
    polarity!--and take the output from the middle).

    Cheers,

    Phil Hobbs
     
  9. MooseFET

    MooseFET Guest


    You use a thin one and work at too long of wavelengths for the
    detector. If you apply a forward current to the PIN junction, the
    carrier density in the diode changes its index of refraction and speed
    of light. By modulating the density, you can phase modulate the
    signal passing through. If you integrate the modulation signal, you
    have frequency modulation.

    I hope this helps :)
     
  10. Guest

    I think I haven't made myself clear on that. I know you got the gist
    on what I'm trying to do. But the thing here is I'm not trying to do a
    "true" frequency modulation (since it's utterly difficult in my
    configuration), but a sort of "emulation" of it. The frequency change
    happens through the change of distance of the laser beam. After that
    beam passes through a prism, the beam changes the angle.

    Probably if the laser beam distance is short, then we wouldn't see
    much difference in angle, but what if it's about a hundred meters?
    (Sometimes, I wish I can draw diagrams, so you guys might know what
    I'm trying to do. Heh.)
    Yes, but the photodetector (in my case a diode) is there to
    [supposedly] change the frequency of a pre-made signal using the
    strength of the beam signal it gets. Beam strength = change in
    frequency (not the frequency itself). That's what I *wanted* to happen.
     
  11. Guest

    But what happens after the beam hits the photodiode? You still don't
    have a signal after that, right? They're just photocurrents.
     
  12. Guest

    Using a photodiode to modulate a laser is news to me. If I follow you
    correctly, you are making the laser appear to move back and front,
    i.e. appear to change the distance to the receiver. How great is this
    modulation in terms of distance?

    When I first read this post, I had envisioned a laser beam moving from
    side to side, falling on a photodector with a tapered mask painted on
    it. As the beam moves, the position on the mask changes, and thus the
    output voltage due to the attenuation of the mask. Think of the mask
    as a V.

    How do laser printers move their spot around?
     
  13. Guest

    I quite don't get what you're saying here, but if there's much
    distance from the reflected laser (from a prism of some sorts) to the
    photodetector, the laser would appear to move back and forth even
    wider. And the photodiode doesn't modulate the laser. The photodiode
    [supposedly] modulates a pre-made signal according to the position of
    the laser to it.
    That's what I was planning to do with the photodiode: put a "V" mask
    on it.
    I have absolutely no idea. I don't own one, and I haven't seen how it
    operates.
     
  14. Guest

    I'll just put clearer, straight-forward question, since I feel that
    we're getting off-topic here:

    Is there a way to make an FM signal using an AC output and a
    photodiode? (AC output for the carrier, photodiode for changing the
    carrier frequency.)
     
  15. JosephKK

    JosephKK Guest

    Generally, but not always, they use a rotating mirror and the
    progression of the drum used to transfer the image to the paper. There
    are other techniques.
     
  16. default

    default Guest


    You had what sounded half way viable with your first paragraph - FM
    modulation of a light beam via a Doppler shift caused by moving a
    mirror - then you just need a way to demodulate it and recover the
    intelligence. And someone gave you a way to do that . . .

    Photo diodes are for converting light to electrical signals, that's
    all they do.

    Now you seem to be off on a different tangent combining some radio
    terms and trying to apply them to a/the laser transmission scheme.

    You could amplitude modulate the laser with a sub carrier then
    modulate the sub carrier with an FM signal to carry the intelligence -
    that would work and use the usual radio techniques for signal
    handling.

    Can you use an AC output to make and FM signal - yes. Can you use a
    photodiode to do it - no (unless perhaps you mean a "laser diode" when
    you say photodiode) Then there are ways to modulate it with FM but
    not directly since the laser diode is tuned to one wavelength.

    --
     
  17. John Fields

    John Fields Guest

    ---
    Yes.

    Use a VCO to generate the carrier, and get the deviation you need by
    amplifying the photodiode output as required.

    http://www.national.com/an/AN/AN-20.pdf#page=1

    http://www.national.com/an/AN/AN-74.pdf#page=1

    http://focus.ti.com/lit/ds/symlink/cd74hc4046a.pdf
     
  18. default

    default Guest

    That's the purpose of the interferometer
    --
     
  19. Phil Hobbs

    Phil Hobbs Guest

    No, you have a detected signal, because the speaker vibration moves part
    of the beam from the + photodiode to the - photodiode. It isn't an FM
    detector, but it does detect speaker displacement optically.

    Cheers,

    Phil Hobbs
     
  20. Guest

    Oh now I see what Moosefet meant. That was baffling. I though he
    meant the photodiode could reflect the beam and modulate it.

    This is what makes thing confusing. I thought the goal was to send a
    signal on the laser beam, i.e point to point communications. Now if
    the goal is to send information via a laser beam, then we have a
    different situation. Bear with me here.

    Lets label the areas point A and point B. At point A, there is a
    speaker and a mirror. At point B, there is a laser system. Now I see
    how to make the laser system. Modulate the laser intensity at a high
    frequency (a few MHz), i.e. put a carrier on the laser. Split the
    beam. Sense the direct laser energy with one photodiode, and the
    reflected laser energy with another photodiode. The wiggling of the
    speaker will phase modulate the reflected signal.
     
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