Converging a beam to a small point?

[Athiril]

I feel like I've asked this before.

Using a general lens, or a microscope objective, can we focus a laser to a much smaller point, say 5 microns or less? Converging the beam, which then diverges rapidly after the smallest point is fine, I'm not talking about keeping it collimated.

I'd like to take something like a low power green laser, and pass it through a transparency with a sensor on the other side to read in the optical density essentially (how much light the sensor picks up) as it sweeps across, I'd like a small point, for a more detailed reading of course.

So the other restriction is, it shouldn't be powerful enough to burn anything or wreck what I'm reading. Of course I could use ND filters.

 

[Bionic-Badger]

Yes, you just need a lens, preferably a more precision lens. You can see what Surplus Shed has (look for plano-convex), or see what Newport, Thorlabs, or Edmund's Optics has (more expensive). The real challenge is keeping both the lens, laser, and subject the perfect distance apart. For that you might need some kinematic mounts.

 

[Athiril]

Thanks, so a single element converging lens could produce such small spots?

I was wondering if the size of the spot from a beam or imaged point would be limited by the picture taking resolving power of the lens, of if a beam/point dead on center to the center of the lens at a flat angle wouldn't, since there'd be no overlap.

 

[Bionic-Badger]

Technically yes, but you'll also have to choose the right lenses that will give you the needed spot sizes, etc. That's beyond my expertise, but you can read up on it on sites like this.

 

[lazeristasUVISIR]

I don't see how far from the lens you want to have a small beam.

Your cheap options could be CD or Blu-ray sledges.

http://www.cd-info.com/images/blu-ray/blu-ray-illustration.jpg

 

[Athiril]

Ideally under 30cm but above 1-2cm.

I'll have a look into that.

 

[Cyparagon]

The focusing lens from any optical drive will be adequate for this, as they're designed for spots as small as 0.58 microns. Focal length is <1cm though, if you can live with that. Greens are difficult to find in very low powers, so you'll want to at least consider a standard red.