Divergence....Can it be adjusted with success?

[Mk2_GTi]

I purchased a Greenie from Arnold (40mw) and I would like to collimate it better than it is from the factory. Is this possible with the optics provided, or is adjusting the focus pretty much a lost cause? FYI, I have attempted to adjust the focus, but I cant get it better than a factory Atlas nova, it either gets bigger when you turn it one way, or it gets bigger when you turn it the other way. My guess is I have hit the sweet spot, which at a distance is pretty unimpressive. What is the best results people have received as far as divergence is concerned? Thanks Guys.

 

[comozo]

There is an optimum position with those optics, you can do no better. The easiest way to lower beam divergence would be to buy a longer focal length plano-convex lens (singlet), AR coated. You'd have to extend the length of the pointer case to accomodate the new lenses focal length. Or you could buy 2 lenses, a plano-concave and a plano-convex and attach them to the front.
Do what I did read up on beam expanders etc. You can also read postings by me on this subject, here. Experiment with this applet. First lens should have a negative focal length ex. -6mm, second lens is positve. If you construct your own collimator, lens orientation is thus plano-concave>plano-convex
[(------[) http://www.lightmachinery.com/gausbeam.html

 

[jkaiser3000]

One way of reducing the divergence would be by means of a pair of binoculars (or small telescope). In that case, the binoculars will be acting like a beam expander. If you use, say, a 7x50, it means your beam will be 7x bigger at the exit point, but the divergence will decrease 7x also. For short distances, the spot will be bigger with the expander, but on longer distances, it will actually be smaller than without it.

Granted, it's not really practical to do this for a pointer. :shrug:

It really depends on what you want it for, but if you want to keep the pointer form, then your coices are a bit limited.

 

[WildRice]

If the spot gets larger turning both ways then is it optimum for that distance. Basically, you are setting the focus at a set distance. Smallest spot at near field may not be smallest spot at far field. It really needs to be set at distance.

Jeff

 

[AJ_Dual]

IIRC, there are also fundamental limits to convergence based on the fundamental physical nature of light and the frequency itself.

Isn't that "Railegh length" (sp?) or some such?

Although, I imagine that it's lots better than what the standard pointer optics can achive.

 

[WildRice]

IIRC, divergence is mostly dependent on the interference of the photons in the beam path and the frequency they resonate at. From experience I can safely say that a YAG laser running at 1064nm has CRAPPY divergence. Spot size right out of the resonator went from 5mm to about 12" at about 5feet. CO2 (10600nm) is WAY tighter. I have asked why this is.. is it the frequency itself, or does it have something to do how narrow the frequency range is.. dunno that.

Jeff

 

[comozo]

Divergence in part is wavelength dependent, the shorter wavelengths will have lower divergence.
They want optimum divergence or minimum divergence for all distances this equals a beam that does not expand very much with the optics used. To do this the plano-convex (pcx) lens should be at a distance from the plano-concave lens somewhere near to the pcx lenses focal length but not more than the focal length. A beam whose sides converge or expand is not optimal. Do what WildRice suggests set it for distance, smallest spot size.

 

[cbfull]

Source size is a big limiter. Technically, a source that is 0.1µ by 0.1µ can theoretically produce a projection far away the same size. Many factors make this ideal impossible (no lens can ever be perfect, they are made of atoms!)

One of the problems with all these greenies, is that they design it so the beam is focused almost to it's minimum size when it hits the collimation lense. This TREMENDOUSLY limits the ability of the collimation lens to do a good job. To explain, if you were to look VERY closely at a lens at the place where the laser hits it, you will begin to see the imperfections on the surface of the lense, depending on how close you are looking. The smaller the area, the more the beam is effected by imperfections in the lens. In a sense, the effect the surface error has on a beam increases exponentially as the diameter of the beam gets smaller. While it is nice that they are attempting to build the laser to start thin and stay thin, there is a bit of a tradeoff.

So, to improve collimation, you would likely need to get new lenses. One that lets the beam be just a little bit wider when it hits the collimation lens, and then the focal length of the new collimation lens must be much longer, and if you have the beam too wide when it hits the coll., you will end up having to buy a custom lens (ouch$$$$). The only major drawback to this is that the beam will come out of your laser a little wider, but you can make a spot far away that should be significantly smaller than before.

I'ver never tried this but I have studied optics just a little.

 

[comozo]

A simple rule. Expand the beam reduce divergence.
Like this:

laser-------[(<<<<<<<<<])===============

 

[bg2vo]

Source size is a big limiter. Technically, a source that is 0.1µ by 0.1µ can theoretically produce a projection far away the same size. Many factors make this ideal impossible (no lens can ever be perfect, they are made of atoms!)
.

Actually, if source that is 1µm by 1µm can theoretically produce a projection K time bigger. Where K = D/f.
D is the distance, f is the focal length of the lens used.
For instance, if a 10cm lens is used and the distance is 100m, then K=100/0.1=1000, you will get a 1000µm = 1mm by 1mm.
Another thing, you cannot get a 0.1µm spot given that the light wave is >0.5µm.
The min. spot we can get for 532nm wave with a lens of NA=0.5 is 1064nm=1µm.

 

[cbfull]

Good point about the source size I chose, I was just tossing out an example. Perhaps a few microns would have been a better choice.

Another thing is that it is a relatively simple concept that a source cannot be focused with a simple lens or arrangement of lenses to produce a spot that is smaller than the source, *while preserving the source intensity* (that last part is the key, preserving the intensity usually means that "d" is as close to zero as possible) I think we are saying the same thing.

The importance of what I was trying to describe is that you cannot "concentrate" the image of a source (which would mean smaller AND brighter than the source). If it were possible to concentrate a source in this way, then you could technically focus diffuse energy to a spot of essentially infinitesimal size (limited by the wavelength, of course), and the energy density would be astronomical.

This does not apply to coherent light of course.

I hope I am making sense.