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How to estimate how many wavelengths wide a laser is?

  1. Mar 27, 2012 #1
    1. The problem statement, all variables and given/known data

    I've conducted two laser experiments. One with single slit, and one with a double slit.

    I've produced two graphs which look something like these;


    http://www.google.co.uk/imgres?q=la...&w=604&h=437&ei=8vJxT9qaC6ap0AWJ8eX4Dw&zoom=1


    http://www.chegg.com/homework-help/...ed-small-distance-slits-3-meters-awa-q1840625


    Would anyone know how to go about roughly estimating how many wavelengths wide the laser beam used is?

    Possibly from the graphs? (or maybe from other data)

    Thank you.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 28, 2012 #2

    rude man

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    Homework Helper
    Gold Member

    You can't use your graphs since the resolution is totally inadequate. You're looking for slight flattening of the peaks and troughs on the interferogram & you won't see that.

    I think you need a "pure" monochromatic source to mix with your laser. If the laser spectrum is extremely small ( < 10 GHz or so) you could theoretically mix the two sources onto a photodiode which woud then output an electrical frequency spectrum you could measure: {sin(wt) + sin(w + Δw)t}2 gives a term 2sin(wt)*sin(w + Δw)t which of course produces the difference-frequency term in Δw.

    For a broader laser spectrum I can ony think of a $100,000 spectrum analyzer. These use precision diffraction gratings, Fourier transforms & whatnot. I don't know enough about how they work. Agilent and Anritsu make them, you might go to their Websites.

    For a somewhat lower monetary expenditure you might get hold of the classic introductory text: https://www.amazon.com/Optics-4th-Edition-Eugene-Hecht/dp/0805385665
     
    Last edited by a moderator: May 5, 2017
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