B Why Use a Photodetector in Interferometry?

[Silviu]

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!

 

[BvU]

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.

 

[Silviu]

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.

Sorry, I just edited, it is a photodetector, not an oscilloscope.

<Moderator's note: title changed>

 

[BvU]

Well, a photodetector gives off some signal that has to be converted into a reading. That's your brightness measure

 

[Andy Resnick]

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!

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.

 

[Silviu]

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.

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.

 

[Andy Resnick]

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.

I don't understand why you have a (single element) photodetector connect to a display screen.

 

[Silviu]

I don't understand why you have a (single element) photodetector connect to a display screen.

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

 

[BvU]

How big is the sensitive area of the photodetector ?

 

[Silviu]

How big is the sensitive area of the photodetector ?

It is around 5 times bigger than the area of the light dot produced by the laser.

 

[BvU]

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 ...

 

[Silviu]

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 ...

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?

 

[Andy Resnick]

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

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?

 

[Vanadium 50]

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?