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I've been playing around with laser diffraction quite a bit lately.

Yesterday I tried to have a shot at diffraction of a laser beam with a pinhole.

The pinhole was created by slowly penetrating a piece of heavy duty kitchen tin foil ($\text{Al}$), which is easier to manipulate than the thin, cheap stuff, with a thin sewing pin. The pinhole is about $\approx 0.2\,\mathrm{mm}$ ($200\,\mathrm{\mu m}$)

The assembly is simple:

pinhole 2

The lasers are typical pointer/presentation low power pen-type lasers (although the green $532\,\mathrm{nm}$, $500\,\mathrm{mW}$ is quite powerful)

With a red laser I got almost nothing although a little diffraction could be observed, without any of the expected rings.

With the green laser the result was more interesting:

pinhole 1

Clearly, going by the mosaic of maxima and minima, there's lots of diffraction going on. The 'cloud' is about $8\,\mathrm{cm}$ in radius.

But onle again, no rings at all...

I suspect the following may be the cause:

  • shape of the laser spot (although I can't a priori see much wrong with it),
  • shape of the pinhole, especially in the beam's direction.

Does anyone here have any ideas/experience with this?

Thanks for reading.

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  • $\begingroup$ I found your post yesterday. Unfortunately there's linkrot. Thanks nonetheless. $\endgroup$
    – Gert
    Feb 13, 2022 at 13:56
  • $\begingroup$ There is no reason in the world for a 500 mW laser pointer to exist. Be very careful. Reflections can be blinding. Better yet: destroy it. $\endgroup$
    – garyp
    Feb 13, 2022 at 14:17
  • $\begingroup$ @garyp. Yep, beause there isn't enough mindless scaremongering in the world! I've been using this laser for years now and am not about about to 'destroy it'. Thankfully perhaps this site require us to be polite or I possibly wouldn't be on this occasion. $\endgroup$
    – Gert
    Feb 13, 2022 at 14:40
  • $\begingroup$ Have you examined your pinhole under a microscope? Tearing a hole through metal foil with the relatively blunt point of a sewing pin* might not yield a clean, circular hole. [*Sewing pins are meant to find a way between the fibers of a piece of cloth. A truly sharp point could tear the fibers.] $\endgroup$ Feb 13, 2022 at 15:54
  • $\begingroup$ @SolomonSlow Good point of course but my USB microscope is out of order, right now. I'm also very careful when making the hole, very slowly... $\endgroup$
    – Gert
    Feb 13, 2022 at 16:43

2 Answers 2

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The radius of the first maximum is approximately $$\frac{\lambda l}{d}\sim\frac{0.5\cdot 10^{-6}m\cdot0.3 m}{0.2\cdot 10^{-3}m}\sim 1mm.$$ As "The 'cloud' is about 8cm in radius", you are probably looking for the rings in wrong places. It is possible that even minimums of the "ring" diffraction picture look very bright (except for narrow areas).

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  • $\begingroup$ I'm going to accept that answer based on the fact that it's correct and that I feel now very stupid for not having checked the basic diffraction formula. It also explains the result with the red laser, of course. So the quest will be on for smaller pinholes in the range of $20 - 50\,\mathrm{\mu m}$. Thank you! $\endgroup$
    – Gert
    Feb 13, 2022 at 13:55
  • $\begingroup$ @Gert : or try a longer distance. I am not sure about the effect of the foil thickness though. $\endgroup$
    – akhmeteli
    Feb 13, 2022 at 14:13
  • $\begingroup$ Yes, I'm trying that as I write. But I need to await evening. Also, I can reduce the pinhole's $\phi$ to probably about $0.1\,\mathrm{mm}$, using a finer sewing pin. But I'm still stumped as to why I get that interesting diffraction pattern with the green laser? $\endgroup$
    – Gert
    Feb 13, 2022 at 14:32
  • $\begingroup$ @Gert I am not sure, but laserpointerforums.com/threads/… may be relevant. $\endgroup$
    – akhmeteli
    Feb 13, 2022 at 17:32
  • $\begingroup$ I'll have a look at that, thanks. $\endgroup$
    – Gert
    Feb 13, 2022 at 17:37
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Even though there is already a good answer that has been upvoted (and I will upvote right after this is posted) and accepted, this looked like a fun one to try, given that I have all the necessary components. So here is the setup I put together an hour ago:

Setup photo 1

The green (532 nm) laser, removed from its laser pointer housing and powered by an external adjustable DC power supply, is at left. The green beam is split at the non-polarizing beamsplitter cube. The beam that goes straight through strikes a 100 micron pinhole. The reflected beam from the beamsplitter cube is reflected from a front surface mirror and then strikes the adjustable slits (from a 1950s 3.4 m J-A spectrograph).

Another view:

Setup photo 2

The resulting outputs of the 100 micron pinhole and the adjustable slits:

Setup and outputs

A closer view of the outputs:

Outputs of 100 micron pinhole and slits

Replacing the 100 micron pinhole with a Thorlabs 30 micron pinhole yields the following:

Outputs of 30 micron pinhole and slits

So the slits work fine, but the pinhole results are really poor. I assume this is due to the relatively bad beam quality of my inexpensive green laser pointer. Presumably spacial filtering the laser would help a great deal. I might try it later.

Sorry about the stray light: I should add iris apertures, but this was just a quick setup.

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    $\begingroup$ Thank you very much!. Very interesting (+1). Your pinpoint results are very much like mine, indicating it isn't the quality of the whole that's at fault here (I thought perhaps poorly defined edges). What do you use for a 'non-polarizing beamsplitter cube', please? $\endgroup$
    – Gert
    Apr 9, 2022 at 16:16
  • $\begingroup$ It is just one of the beamsplitter cubes I had in a optics “junk” drawer. I got the beamsplitter cubes long ago, from a former (now deceased) colleague, and I have no idea where he got them originally. I posted this just because you mentioned maybe using a smaller pinhole and I happened to have the Thorlabs 30 micron pinhole I purchased several months ago. I figured you might like to see what happens with it. Frankly, I was surprised by just how bad the results were. $\endgroup$
    – Ed V
    Apr 9, 2022 at 16:44
  • $\begingroup$ I'm not sure whatb we're to understand by 'poor beam quality': the slit diffraction pattern is perfect and identical to what I achieved with my 532 nm pen laser. $\endgroup$
    – Gert
    Apr 9, 2022 at 17:27
  • $\begingroup$ I just meant the beam is not nice, like a Gaussian beam from a He-Ne laser. But I think it can be cleaned up and that would give better results. This shows a good quality green laser profile: en.m.wikipedia.org/wiki/Gaussian_beam. $\endgroup$
    – Ed V
    Apr 9, 2022 at 17:31

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