Newb Optics Question

[CycloWizard]

I'm embarassed to have to ask this, but I'll suck it up I suppose. 😛

If I have a laser beam that is 3 mm in diameter and does not scatter, then pass it through a lens, is it possible that the focal point (waist?) of the beam will have a smaller diameter than the incident beam?

 

[RossGr]

Yes. the size of the resulting spot is a function of the lens and the diameter of the input beam.

 

[CycloWizard]

Originally posted by: RossGr
Yes. the size of the resulting spot is a function of the lens and the diameter of the input beam.

So, is there a lower limit on the size of the spot? Do you know of an equation that I can look at that will tell me this information? Thanks! :beer:

 

[f95toli]

I think the theoretical limit is roughly the wavelengh of the laser with some prefactor which is of the order of one.
In reality I don't think you can even come close with ordinary optics. The beam is never truly circular, the lens is not perfect etc.

Anyway, try googling "optical tweezers"; they use systems of lenses to focus lasers to very small spots so you should be able to find some usefull information about real world limitations.

 

[CycloWizard]

Originally posted by: f95toli
I think the theoretical limit is roughly the wavelengh of the laser with some prefactor which is of the order of one.
In reality I don't think you can even come close with ordinary optics. The beam is never truly circular, the lens is not perfect etc.

Anyway, try googling "optical tweezers"; they use systems of lenses to focus lasers to very small spots so you should be able to find some usefull information about real world limitations.

Actually, I've read all about optical tweezers... I'm not sure why I didn't think of that. Thanks. :beer:

 

[silverpig]

Originally posted by: f95toli
I think the theoretical limit is roughly the wavelengh of the laser with some prefactor which is of the order of one.
In reality I don't think you can even come close with ordinary optics. The beam is never truly circular, the lens is not perfect etc.

Anyway, try googling "optical tweezers"; they use systems of lenses to focus lasers to very small spots so you should be able to find some usefull information about real world limitations.

That sounds about right.

 

[RaynorWolfcastle]

Here's a slide from my course on photonics this semester that can probably help you out. Using a Gaussian beam approximation...
Text
z_r is the Rayleigh range: Pi*w_0^2/lambda

It's likely that the main problem you'll run into getting a small beam waste will be to get the optics close enough, since the beam waste is inversely related to the beam's divergence (tight waste = quick divergence). I hope this is the kind of answer you were looking for 🙂.