Why do Fresnel rings appear in laser beam experiments?

[ppoonamk]

Hi,

I have a very simple test bench set up with a HeNe laser, followed by a spatial filter, then an Iris and then a couple of lenses. I can observe many fresnel rings on my image plane. Changing the size of the iris, and its distance changes the number of of rings. I would like to know why this happens. Is there any way I can get uniform illumination from my laser? Is it because the spatial filter pinhole diameter is too big? It has a 40X, 0.25NA objective.

 

[jfizzix]

When you say Fresnel rings, do you mean the Airy rings you'll see when you image the light going through a small circular aperture? If I had to guess, I would assume these rings get fewer and wider the smaller the iris is?

 

[ppoonamk]

Hi,

You are right! So I am quite sure these rings appear due to the iris I have in my setup. The rings get smaller as the diameter of the iris is reduced. Is there an alternative arrangement to the spatial filter + iris setup?

 

[jfizzix]

If you're looking to get a nice Gaussian laser beam profile, ring-free, I would try (if you have the equipment) to focus the light into a single mode optical fiber. You won't get more than like a third of the light through, depending on the quality of the beam coming in, The light coming out the other end will be ring-free, and already spatially filtered. After that, you can recollimate the beam by placing a lens a focal distance away from the output, and then you'll have a well-behaved Gaussian laser beam. (all this is easier said than done, of course).

If your beam is well-behaved and ring-free already after the spatial filter (but before the iris), you need only lenses to collimate and adjust the size of the beam.

This is all assuming that you're trying to adjust the size of a laser beam with irises. I would do it with lenses to collimate and magnify.

 

[ppoonamk]

If you're looking to get a nice Gaussian laser beam profile, ring-free, I would try (if you have the equipment) to focus the light into a single mode optical fiber. You won't get more than like a third of the light through, depending on the quality of the beam coming in, The light coming out the other end will be ring-free, and already spatially filtered. After that, you can recollimate the beam by placing a lens a focal distance away from the output, and then you'll have a well-behaved Gaussian laser beam. (all this is easier said than done, of course).

If your beam is well-behaved and ring-free already after the spatial filter (but before the iris), you need only lenses to collimate and adjust the size of the beam.

This is all assuming that you're trying to adjust the size of a laser beam with irises. I would do it with lenses to collimate and magnify.

HI,

Thank you for your suggestion. I don't have a single mode optical fiber. Seems like a very good way to get a Gaussian. Are there other ways to do it?

 

[jfizzix]

If you see rings after the iris, but not after the spatial filter, I would just forget the iris and collimate with lenses.

 

[ppoonamk]

Yes! I see rings only after the iris. But my beam after the spatial filter isn't a gaussian. There are other orders along with it. I clip the pattern to allow only the bright order through. The spatial filter setup alone does not clean the beam. I am using a 40X, .6 NA objective with a 10 um aperture iris.

 

[jfizzix]

At this point, I don't think I know enough about the light source coming into exactly solve your problem.

Maybe it's a matter of alignment; perhaps the distance between the spatial filter and your focusing lens isn't quite perfect, or the positioning is off slightly. With a 10 micron diameter, the translation in z where you're maximally focused is similarly small.