Even smaller spot size by using a mask?

Here is an idea:

Let's say I want to make really tiny holes in a thin plastic film (~0.2 mm thin plastic film).
My laser gives me a spot size of ~0.1 mm, but I want to make even smaller holes, say only 0.03 mm in diameter (30 microns!).

If I order a metal plate with a 30 micron small hole in it, and it on top of my plastic film as a "mask", and direct the laser beam at it... what do you think would happen? Has this any chance of producing 30 micron holes? – In other words, could I use a "mask" on top of my material to get a even smaller effective spot size? (Assuming that the laser is not powerful enough to damage the mask etc...)

Have this been done previously (I would guess so?)

While I have not tried the masking idea personally, the idea seems sound, although it may not be the best way to get small holes. In general, you can decrease the focused spot size by decreasing the distance to the target (use a short focal length lense), decreasing the wavelength and number of modes (use a single mode 405nm from a blu-ray burner) , or (maybe counter intuitively) start with an expanded beam. To find out more about your masking idea try looking up pinhole aperture or spatial filtering as it relates to laser optics.

So a few problems. First, if your mask is any distance away from your film, then you'll get diffraction and actually you will get a larger spot size, not a smaller one. But, assuming that your mask is touching your film, it should work. The only issue is that you will decrease the power at the spot accordingly such that it may not be able to burn through the film. If I were you, I would attempt to get a 405nm diode (for the shortest focal length) then get a short focal length lens. The spot should be close to diffraction limited, so you should get a sufficiently small spot.

I've never tried something like this, but I think it would stay the same or even create a larger dot.
Fiat of all, no matter what size the aperture is, the laser will still diverge at the same rate, whatever that may be.
Hence the same size spot.
Here's a thought: a beam expander reduces divergence by creating effectively a thicker beam. Would decreasing the beam diameter at aperture possibly increase the divergence as being almost the opposite of a beam expander?
That got kind of rambly, but at any rate the dot would be the same size (or maybe bigger if my theory is correct).

Interestingly the pinhole can both create and remove noise around a central bright spot. Passing a collimated beam through an aperture will result in an Airy disk pattern with alternating light and dark rings surrounding the central spot due to the sharp edges of the hole. But if a beam with this type of noise is focused using a short focal length lens and then passed through an appropriately sized pinhole at the waist, the noise can be blocked and a more Gaussian profile can be passed through. This new clean profile will of necessity, however, be diverging as it is past the waist, so your final imaging plane (the stuff you want to burn holes in) would need to be as close as possible to the pinhole.

Here's a thought: a beam expander reduces divergence by creating effectively a thicker beam. Would decreasing the beam diameter at aperture possibly increase the divergence as being almost the opposite of a beam expander?

Click to expand...

Yes, passing a beam through a hole will result in a narrower beam with higher divergence, but it will at least initially have a smaller diameter.

So to conclude the comments...

...it may work, provided that:

1) The mask is in close contact with the material (since the beam will diverge after the aperture)
2) The material is thin (since the beam will diverge after the aperture)
3) The beam has enough power (since we won't use the full original beam)

...but it might be easier to use a 405nm laser diode and short focal length lens.

I will start with a 405 nm diode and see if I can reach 30 micron holes. If it doesn't work I might try the "mask" idea. Any suggestions for a short focal length lens for 405 nm?

Milton, what lense are you using currently? Since the beam from the emitter is already diverging, you can adjust the distance to the collimating lens to adjust the distance to your beam waist. If you position a 6mm focal length lens such that your beam gets focused on a plane 10 cm from the lens, you might get a spot size of say 0.1 mm, but if you move the lens further from the diode such that the focal plane is only about 3.5 cm away, then you might be able to get a spot size of 0.03mm. I think the "standard" lenses sold and used by other forum members would be adequate to try this.

Now I did think of something else that might be important. Your beam does not really have a well defined "edge", but at best has a Gaussian profile (although you can buy optics to adjust to a top hat profile), which means it gets gradually dimmer away from the center. The effective dot size (either what your eyes/camera perceive, or what has enough power density to burn/melt your target) will be a function of total beam power. So you might have too much power no matter how well focused your beam gets. To get a smaller hole you might just need to reduce your power such that only the highest intensity peak at the center on the focused dot does the burning.