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optical heterodyne detection for spectrometer

Discussion in 'Electronic Design' started by Jamie M, Jan 5, 2013.

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  1. Jamie M

    Jamie M Guest

    Hi,

    I came across optical heterodyne detection on wikipedia:

    http://en.wikipedia.org/wiki/Optical_heterodyne_detection

    Basically hitting a photodiode with two closely matched
    light sources, and then the electrical signal from the
    photodiode will output the frequency difference between
    the two light sources. Could something like this be
    rigged up to function as a spectrometer? If one signal
    was a reference light, a light from a supercontinuum
    laser that is swept over the desired light frequency range,
    then it might be similar to a spectrometer with a rotating
    grating and a photodiode. Would there be a way to operate
    in continuous mode so no frequency sweep is required?

    cheers,
    Jamie
     
  2. Sure, that's exactly how I measure the linewidth of our diode lasers.

    (LW about 1 MHz on the second time scale.... Mostly jitter, I think of
    the optics in the signal path, but it could be something else.
    On shorter time scales the LW looks to be
    ~1-200kHz, but that's close to the 'scope resolution.)
    And then of course I'm measureing the LW of two lasers, and assuming
    it's shared equally.

    George H.
     
  3. Bret Cannon

    Bret Cannon Guest

    "Jamie M" wrote in message
    Hi,

    I came across optical heterodyne detection on wikipedia:

    http://en.wikipedia.org/wiki/Optical_heterodyne_detection

    Basically hitting a photodiode with two closely matched
    light sources, and then the electrical signal from the
    photodiode will output the frequency difference between
    the two light sources. Could something like this be
    rigged up to function as a spectrometer? If one signal
    was a reference light, a light from a supercontinuum
    laser that is swept over the desired light frequency range,
    then it might be similar to a spectrometer with a rotating
    grating and a photodiode. Would there be a way to operate
    in continuous mode so no frequency sweep is required?

    cheers,
    Jamie

    Yes, it has been done by Jun Ye at JILA in Boulder, CO. He and his
    colleagues combined a pair of femtosecond frequency combs with slightly
    different spacings on a photodiode and used an rf frequency analyzer to
    measure the optical spectrum change due to molecular absorption of one of
    the beams. A femtosecond comb is a phase locked set of optical waves where
    each "tooth" in the comb has a width below 1 Hz, the teeth are separated by
    say 100 MHz, and the frequencies range from say 3E14 Hz to 6E14 Hz (1000 nm
    to 500 nm in wavelength). The second femtosecond comb has the "teeth"
    separated by 100 MHz + 50 Hz. A pair of "teeth", one from each comb, are
    phase locked to be 50 MHz apart, then the beat from the next pair of teeth
    is 50 MHz + 10 Hz, etc. So the beats from the (6E14-3E14)Hz/100 MHz = 3E6
    pairs of teeth in the two frequency combs are mapped into the range between
    50 MHz and 80 MHz.

    Bret Cannon
     
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