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I'm using a Raspberry Pi camera v2.1 (lens removed) to make some observations on a laser. I have the laser coupled into a single-mode fiber which outputs a smooth gaussian spread onto the camera. From the end of the fiber, the camera has an angular size of about .02 radians. When the laser is turned on, the camera sees an airy disk interference pattern, much like what you might see in a Fabry-Perot interferometer, though it's not perfectly centered. I've been busting my brain trying to figure out what's causing that pattern and I keep coming up blank. Here's what I've considered so far:

At first I thought it might be thin-film interference in the glass covering the photoarray, but the fringes are too small for any reasonable thickness of glass. I did the calculations, and it would need to be like 8cm thick.

Then I thought maybe it was just single-slit diffraction from the fiber, but again, the fringes are too small for the aperture and distance.

Finally, realized that, if it was interference in the glass, it's too thick for thin-film, so I thought it might be a sort of thick-film interference, where the incident ray at each angle will have an infinite series of rays at smaller angles, which each reflect a certain number of times before all ending at the same spot. I did those calculations and still came up short.

The effect seems very similar to the airy disk interference in a Fabry-Perot etalon, and it seems like that type of effect should happen, since the glass has two parallel surfaces, but I can't figure it out for the life of me.

UPDATE: I've looked at the beam without the camera, and I've looked at the camera with the laser off, and there are no rings in either case. Also, the pattern phase shifts slightly when I tune the laser to slightly different frequencies, and significantly if the camera is moved at all.

I'm not at the lab now, but I can get a picture to post tomorrow.

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    $\begingroup$ A photo would help, but here are some thoughts: Any aperture causes diffraction, so if you laser light is passing through any hard-edged round aperture you'll see circular fringes. This might be at the laser, or it might be in your camera. $\endgroup$ – S. McGrew Dec 19 '18 at 0:40
  • $\begingroup$ Ah- it won't be at your laser since you're going through a fiber. Something to try: illuminate a piece of white paper using the fiber, and take a photo of the illumination patch. That way, the camera can't cause the fringes. If you see no fringes in that case, I'd be suspicious of the fiber. $\endgroup$ – S. McGrew Dec 19 '18 at 0:44
  • $\begingroup$ Sure sounds like an etalon effect. There may be a very thin cover glass over the image sensor in your camera. You didn't mention what kind of laser you're using, or how far the fiber tip is from the camera. Is the laser single-frequency? And, what happens if you keep the fiber tip in precisely the same place but tilt it so the Gaussian beam is offset at a slight angle to the system axis? If the fringes pretty much stay put but behave like the rings in a bull's eye target illuminated by a flashlight that's directed off center, it's almost certainly an etalon effect at the camera sensor. $\endgroup$ – S. McGrew Dec 19 '18 at 1:36
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I think the fiber itself is the aperture. Also if the fiber has mechanical couplers they are also apertures. Even the laser cavity itself (the size of a small chip for laser diodes) exhibits extreme diffraction (but not circular). The laser diode is often in a can package with a collimating lens, these are apertures. If you are using a lens to focus the beam into the fiber that is also an aperture but bigger than the other apertures.

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  • $\begingroup$ from OP: UPDATE: I've looked at the beam without the camera, and I've looked at the camera with the laser off, and there are no rings in either case $\endgroup$ – Manu de Hanoi Dec 19 '18 at 23:29
  • $\begingroup$ Also the pixel array can act as a diffraction grating. $\endgroup$ – PhysicsDave Dec 20 '18 at 1:12
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The effect seems very similar to the airy disk interference in a Fabry-Perot etalon, and it seems like that type of effect should happen, since the glass has two parallel surfaces, but I can't figure it out for the life of me.

Perhaps interference isnt from internal reflection within the glass cover but rather reflection between the glass cover(that is probably an IR filter) and the end of the fiber (you mentionned a 0.2 rad slight angle just like a Fabry Perot right ?).

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