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Digital Optical Frequency Modulation

Discussion in 'Electronic Basics' started by Radium, Sep 16, 2006.

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  1. Radium

    Radium Guest

  2. Check out the first diagram on the page. It show an FM RF carrier being
    generated, which then goes to a comparator, which turns the light
    source on and off at the variable RF carrier frequency. There's no
    change in the light frequency.
  3. Radium

    Radium Guest

    So the "frequency" of the optical signal is the rate at which the light
    turns on and off?

    I thought the "frequency" of the optical signal is determined by the
    wavelength of the light.

    Light of a shorter wavelength has a higher frequency than light of a
    longer wavelength.
  4. That depends what you mean by "the optical signal". If you mean the
    signal carried by the optical beam, then yes.
    The frequency of the light is another thing. What you have is a double
    modulation scheme- the 70 MHz carrier is frequency modulated by the
    signal, and then the light source is pulse modulated by the FM carrier.
    True, but that's not the frequency they're talking about. Look at the
    diagram. Doesn't it indicate what I said? The only change of frequency
    of the light itself would be the (relatively) tiny spectral spreading
    implied by turning the light on and off at 70 MHz.
  5. Grandpa

    Grandpa Guest

    You may read Jesse Zheng´s "Optical Frequency Modulated Continuous
    Wave (FMCW) Interferometry" Springer 2004 , page 233.
  6. Guest

    Well, technically there is a change in the frequency of the light, but
    it's very small compared to the optical frequency (probably about 200
    THz). Any RF modulation will give you sidebands on the 200THZ at the
    modulation frequencies, though these modulation frequencies (100MHz)
    are probably well within the linewidth of the laser. If you had a 5MHz
    bandwidth to your 70MHz modulation, you'd see +/-70MHz sidebands, 5MHz
    wide, on your 200THz.
  7. Of course, pure optical frequency modulation is possible and used.

    Semiconductor (diode) lasers change their freuqency as a function of drive
    current. So, they are vert easy to modulate.

    It's also very easy to implement (at least in principle) for other types
    of lasers. One technique is to put a phase mouldator inside the cavity
    of the laser. Applying a signal to the phase modulator will result in
    frequency modulation of the optical output over up to the free spectral
    range (c/2L) of the laser.

    We have built microchip solid state lasers that can be frequency modulated
    at 10s of GHz. One implementation of these lasers is about the same size
    and looks similar to a composite crystal green laser pointer laser with
    LiNbO3 instead of KTP.

    --- sam | Sci.Electronics.Repair FAQ:
    Repair | Main Table of Contents:
    +Lasers | Sam's Laser FAQ:
    | Mirror Sites:

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
  8. That would be true for simple amplitude modulation in a steady state,
    but they are doing pulse modulation using a frequency modulated FM, so
    the picture would be a little more complicated, involving continuous
    spectra from the pulses. In any case, as I mentioned, it would amount
    to a teensy bit of vibrato on the laser's voice.
  9. Bob May

    Bob May Guest

    So you've assumed that there is only one frequency with this transmission
    system? How niave you are!
  10. Don Bowey

    Don Bowey Guest

    Notice how I quoted your post so mine would not be irrelevant and useless?

    Learn Bobby, learn.

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