beam divergence questions

[darklandz]

what is the main factor in a laser that determines its divergence?
i know even a very high powered greenie would have a dot a few meters wide in diameter on the target, a mile away from the aperture.
what adjustments would needed to be made for the beam to diverge with much more delay (say a 50% delay) so the dot is about half the size compared to the 2 meter dot u get when shone from a mile away?
it was just on my head so thought id ask, im sorry if this question has been asked before, just not too familiar with the search feature.

 

[pseudonomen137]

Coherence, wavelength, and minimum beam diameter .

Based on your wavelength their is a "diffraction limited" beam parameter product. This basically is if you take the diameter at the thinnest part anywhere on the beam (if it converges first and then diverges), and multiply it by the divergence. That mm*mRad number will have an absolute minimum depending on the wavelength. For instance, if you are talking 532nm, 1/e^2 beam diameter and full angle divergence the minimum would be ~0.678mm*mRads.

Now what this implies is that beam diameter and divergence are inversely proportional - you can trade them off. Just like with torque and speed, if you make one worse, you can make the other better and vice versa. So if you have a beam with a 1mm minimum diameter and 1mRad, you could improve your divergence to just ~0.1mRad simply by expanding the beam by 10x and recollimating it.

The other factor is coherence. That determines how close the ACTUAL beam parameter product is to the theoretical limit. Better coherence, the closer you get to M^2 of 1.0 - wherein the beam is as perfect as you can get (Greenies are usually spec'd to M^2 of <1.2, IE very close to perfect).

So yeah, if you want a lesser diverging beam, you need to compromise. That compromise is expanding the beam to a larger diameter, and reaping the benefit of lower divergence in the process.

 

[darklandz]

cheers pseudo im still trying to absorb all the algebra lol.
so basically, if u just stuck a "beam expander" lens infront of the aperture and tried to point on a tower say 4 miles away (if u cud see it), would the help of a beam expander make it easier for the laser to project the dot at much further distances?
if that makes sense, i may look into buying 1 if theyr not ridiculously over-priced.

 

[Switch]

Yea theoretically if you want long distance pointing that is the way to go.
With a 1mm beam at aperture you will get an ugly 8 meter dot at 5 miles away(1mRad).
But if you make that beam an ugly 300 mm at aperture you could have a 0.003mRad angle of divergence and your dot at 5 miles away will be only 320 mm, almost the same as the dot at aperture.Way better than the 8 meter one ,huh?

Just a sorta related question: If the only thing stoping you to have a 0mRad laser is the diffraction limit, would it ever be possible to achieve a non diverging laser without any lenses, just by having some sort of cool diode or whatever that spits out photons with paralel paths? :-/

 

[darklandz]

haha i always wondered if that were possible but i doubt it.
it would be cool if u could point a target so many miles away, with the dot being a nice small size, so the laser beam could just carrying on travelling.

 

[Switch]

Well it should be possible.I made those calculations considering an M[sup]2[/sup] factor of 1.You can make a more realistic scenario with a 1.2 M[sup]2[/sup] factor.With a 50cm beam to begin with(you would need a big ass colimating lens though....) It will still be under a meter 10 miles away .And a 300mW greenie would still be plenty visible spreaded across 1m in the dark.

 

[pseudonomen137]

Switch, that isn't quite how M^2 works... but now I'm just being picky  

As for a non diverging laser - keep in mind that even a stream of single photons has a divergence, as wacky as that sounds.

Darklandz, a basic beam expander would be a diverging lens followed by a converging lens. Pretty simple in theory really. One just diverges the beam, and the other re-collimates it once its wider. If f1 is the focal length of the converging and -f2 is the focal of the diverging, your beam gets expanded by a factor of f1/f2, and your diverges decreases by that factor if you get everythign aligned properly.

You can buy beam expanders like Melles Griots 3x or 10x ones, but expect to pay in the 3 figures. If you need your divergence even lower, you need even larger optics and you end up paying even more  :-/

 

[steve001]

Initially it's the beam waist. The beam waist occurs within the crystal normally. Adding a negative focal length lens increases the beam waist followed by a positive focal length lens to collimate. Here's a handy graphical applet that will aid you. It's called Thin Lens Gaussian Beam Program. Use Internet Explorer to see the graphical part. You want to increase the Rayleigh range as much as possible. Any ?'s just ask. For $50 you can get a beam expander from Lasermate

 

[darklandz]

was bored so thought id shop for a bargain
i found a few items of interest on ebay but not sure if these are what im lookin for. need someone to confirm which 1 or any of these are the right ones and if they will work with 532nm.

http://cgi.ebay.co.uk/Laser-Collima...ageNameZWD2VQQcmdZViewItem?_trksid=p1638.m122

and

http://cgi.ebay.co.uk/Melles-Griot-...ageNameZWD2VQQcmdZViewItem?_trksid=p1638.m122