Pinhole diffraction conundrum

Question

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:

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:

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.

Answer 1

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).

Answer 2

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:

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:

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

A closer view of the outputs:

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

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.