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Degree of coherence of light from a red semiconductor laser

  1. Jul 25, 2012 #1

    htg

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    I would like to have at least a vague idea how coherent is light from an inexpensive 1 mW red semiconductor laser which I have.
    I tried to find the manufacturer using Google, but I haven't found it.
    I would like to know the coherence length and the degree of spatial coherence.
    Is there anyone who may give me some information?
     
  2. jcsd
  3. Jul 25, 2012 #2

    Dale

    Staff: Mentor

    It is not possible to get that information anywhere other than either the manufacturer or direct measurements. Even for a single manufacturer with different versions of the same model of laser you can get coherence lengths ranging from 1 mm to 300 m.

    http://www.crystalaser.com/new/greenlaser.html
     
  4. Jul 25, 2012 #3

    htg

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    1mm already is a decent coherence length, because it is >>> λ.
    But what about spatial coherence?
     
  5. Jul 25, 2012 #4

    Dale

    Staff: Mentor

    You missed the point. My point was that even minor variations in the model can give you coherence lengths which vary over 5 orders of magnitude.

    There is simply no way that someone on an internet forum can answer your question. You either need to measure it yourself, or get the specifications from the manufacturer.
     
  6. Jul 25, 2012 #5

    htg

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    I think if they call it a laser diode, there should be decent lower limit of coherence length (which should be >> λ ) and there should be significant degree of spatial coherence.

    Do you have a suggestion how I can measure spatial coherence and coherence length?
     
    Last edited: Jul 25, 2012
  7. Jul 26, 2012 #6

    Andy Resnick

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    2016 Award

    Spatial coherence is usually measured by a Young interferometer, letting the slit spacing vary. Coherence length can be measured with a Michaelson interferometer, letting one arm change in length. The variation of fringe visibility is a measure of the coherence properties.

    One way to infer the coherence length is to measure the spectral bandwidth of the output, and IIRC you can infer the coherence area by measuring the size of a single 'speckle'. Finally, both can usually be calculated from manufacturer's specifications: the output spectral FWHM, the beam diameter at the output face, and the beam divergence.
     
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