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Coherence length of monochromatic laser

  1. May 14, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi.I need an experiment to study and present it.the subject is how to measure coherence length of a monochromatic laser ? lm also search on google.However,is there any advice from you ?
    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 14, 2015 #2


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    You can use a Michelson interferometer and see how long is the path difference when the interference is not blurred yet.
    But lasers can have very long coherence lengths. Choose a not too good laser.
  4. May 14, 2015 #3
    But the teacher ask for also long coherent length.I learn that I can use a spectrometry to find bandwith and then calculate the coherent length.But I couldnt find how can I get bandwith measurement? And I wonder up to what coherent length I can use michelson interferometer
    thank you so much
  5. May 14, 2015 #4


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    To measure the spectral bandwidth of the laser radiation you need very high resolution of the spectrometer. The resolution must be much better than the bandwidth of the laser.

    The coherence length L is c/Δf, where Δf is the bandwidth, difference of the frequencies where the power is half the maximum.

    The coherence length of He-Ne lasers are in the range of several cm - several dm. But high-quality lasers can have even 100 m coherence length.

    Some things to read:

  6. May 14, 2015 #5
    I think I read all of them before asking that question :) But I need to describe how can use a spectrometer to measure bandwith ? what is the procedure...etc.For michelson I know what I need to do.Also do you know which spectrometer should I use ?
  7. May 15, 2015 #6


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    Sorry, I can not help as it would need a very-high resolution spectrometer. In principle, you have to scan the source spectrum around the nominal wavelength and find the frequency, where the intensity falls to half of the maximum.
    You can find the linewidth for given coherence length at the page http://www.rp-photonics.com/coherence_length.html. The resolution must be much better then the linewidth.
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