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B Interferometry on photodetector

  1. Sep 13, 2016 #1
    Hello! I am trying an interferometry experiment using the Michelson Morley experiment. I managed to obtain the interference pattern on a screen, but I want to use an photodetector and I am not sure what should I obtain. As I know, the oscilloscope shows the intensity of the light, so if I modify the interference pattern, the intensity should change, but I don't understand why should I see the bright and dark lines on the photodetector. And if I don't see this, what is the point of using an photodetector in this case? Thank you!
     
    Last edited: Sep 13, 2016
  2. jcsd
  3. Sep 13, 2016 #2

    BvU

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    Hi,
    An oscilloscope is useful for looking at rapid variations in voltage.
    If you scope shows light intensity, that means that intensity (at teh central spot of the interference pattern?) has been converted into a voltage some way or other.
    If your interference patttern moves in and out you see dark as low voltage and bright as high voltage. Useful is one of the mirrors moves periodically with a suitable frequency.
     
  4. Sep 13, 2016 #3
    Sorry, I just edited, it is a photodetector, not an oscilloscope.

    <Moderator's note: title changed>
     
    Last edited by a moderator: Sep 13, 2016
  5. Sep 13, 2016 #4

    BvU

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    Well, a photodetector gives off some signal that has to be converted into a reading. That's your brightness measure
     
  6. Sep 13, 2016 #5

    Andy Resnick

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    Are you asking if the interference pattern is small enough/fine enough fringes to 'fit' onto the active area of a photodetector? Usually photodetectors are used under conditions that the intensity is (nearly) constant over the detector area.
     
  7. Sep 13, 2016 #6
    Hello! I just want to know if the interference pattern should appear in any way on the screen connected to the photodetector or i just get a straight line.
     
  8. Sep 13, 2016 #7

    Andy Resnick

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    I don't understand why you have a (single element) photodetector connect to a display screen.
     
  9. Sep 13, 2016 #8
    So I have this screen connected to an oscilloscope that shows me the intensity of the light on the screen as a function of time
     
  10. Sep 13, 2016 #9

    BvU

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    How big is the sensitive area of the photodetector ?
     
  11. Sep 13, 2016 #10
    It is around 5 times bigger than the area of the light dot produced by the laser.
     
  12. Sep 13, 2016 #11

    BvU

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    If your interferometer produces a ring pattern then that pattern will move inward/outward if one of the optical paths changes. Your photodetector should only see the central area of the pattern, otherwise the amount of light that it picks up doesn't vary that much ....
     
  13. Sep 13, 2016 #12
    So how can I prove the interference pattern this way? Like what response of the detector should prove it is interference and not just a single source of light?
     
  14. Sep 13, 2016 #13

    Andy Resnick

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    Ok- so if I understand you, the detector is measuring the intensity (essentially) at a point, and the intensity value is shown on the screen/scope. So, if you vary the path length difference by a small amount (fractions of a wavelength), the intensity reading should oscillate as sin^2(path length difference), but should not vary with time (mechanical vibrations excepted). Precisely varying the path length difference by this amount can be difficult, but there are methods- a QHQ retarder is one:

    http://bnonlinear.com/pub/spectralF...r_wide_spectral_tuning_of_an_optical_null.pdf (Figure 2)

    Rotating the half-wave plate an angle θ introduces a phase change 2θ.

    Without seeing your setup, it's hard to come up with suggestions, but here's two more:

    If you are using fiber optics, there are devices that bend/stretch the fiber by small amounts to introduce a phase shift.
    If you have expanded the beam, you can introduce a slight tilt or defocus to one arm (wavefront shearing interferometer) and the signal at the photodiode will oscillate.

    A simple thing to try is to use the heat of your hand to introduce a path length difference in the air- it's not controlled or precise, but if you place your hand below one of the (open air) arms, you should be able to see the signal.

    Does this help?
     
  15. Sep 14, 2016 #14

    Vanadium 50

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    The one thing you haven't done is described the equipment you are using and how it's hooked up. Without that, people are guessing. Do you think guessing is helpful?
     
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