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# Collimation, What do people want to know ?

#### steve001

##### Well-known member
Reading the thread Laser Optics someone said many people would like to know more about it other than
Quote:
I think more than a few on the forum would like to better understand how a collimator works besides just "make the beam bigger and it doesn't expand as much."

I can answer some non mathmatical questions on the subject

#### laserrod

##### New member
See: Lasers / Companies & Reviews / (not so simple) Simple Beam Collimator reviewed
on: Jun 6th, 2007, 10:15pm

:

#### a_pyro_is

##### New member
My limited understanding is that a true point source could be collimated perfectly. But in this pesky 3D world we perceive (Please no Superstring Theory) there can be no true point source, so any LASER beam must diverge (Even if we cheat a few feet out of it and make it converge first, as converging is just diverging toward the focal point)
A LASER diode is pretty close to a point source, so if we put a lens in front of it at the focal point of the lens (focused at infinity) we get a pretty beam. But it's a beam with a divergence of some real number. Now if you expand the beam and then re-collimate it will the divergence be reduced by the same factor as the beam diameter was increased?

Example:
If I had a LASER with a diameter at the aperture of 0.05 Inches or about 2 mm, and a measured divergence of 0.7 mRad. Then at 1000 Feet it should have a spot size of about 8.45 inches according to pseudonomen137's calculator. http://www.pseudonomen.com/lasers/calculators/diameterCalculator.html
Now if I sent the .05 Inch beam through a 10X beam expander/collimator, would I have a 0.5 Inch beam with a 0.07 mRad divergence? Giving me at 1000 Feet a spot size of only 1.34 Inches?

Am I understanding this right?

**EDIT**
I guess I should refresh before posting.

**Another Edit**
I looked, but there wasn't a lot of info. Still wondering if a 10X expander makes for Divergence/10.

#### laserrod

##### New member
I just saw a link in a thread here to a Comp. called Meshlaser or something and now I can't find it.

They had a \$50 collimator.

Can anyone digg that post back UP?

It might be a interesting thread to re-review [smiley=laugh.gif]

Pyro,
I think that a 10x Coll. would reduce the initial laser Diverg. mRad spec. by 10x :-/
I think Psuedmondium said you can focus to spot. (Neg diverg.)

Cheers [smiley=beer.gif]

I missed the fight on this. Who got thrown out of the ring?

#### a_pyro_is

##### New member
Laserrod,
I looked a bit but couldn't find a \$50 collimator post. Maybe someone else will remember.

As to the "other" thread, it wasn't much of a fight from where I stood. I think SenKat took the brunt of it. Although I was hoping someone would use a "[crowbar] [/crowbar]" comment...I guess it wasn't needed, but would have been funny.

#### pseudonomen137

##### New member
I think some of you are confusing a collimator and an upcollimator. A collimation lens is used to take an uncollimated/high divergent beam, and collimate it. An upcollimator is multiple lenses that takes an already collimated beam, and increases the size of it, thus decreasing the divergence.

#### a_pyro_is

##### New member
OK, but if you replace "collimator" with "upcollimator" everywhere I used it in my post would it be correct? :-? I guess I should make my question a little shorter.

If you increase the beam diameter by one factor, will the divergence decrease by the same factor?

#### pseudonomen137

##### New member
a_pyro_is said:
Example:
If I had a LASER with a diameter at the aperture of 0.05 Inches or about 2 mm, and a measured divergence of 0.7 mRad. Then at 1000 Feet it should have a spot size of about 8.45 inches according to pseudonomen137's calculator. http://www.pseudonomen.com/lasers/calculators/diameterCalculator.html
Now if I sent the .05 Inch beam through a 10X beam expander/collimator, would I have a 0.5 Inch beam with a 0.07 mRad divergence? Giving me at 1000 Feet a spot size of only 1.34 Inches?

Am I understanding this right?
Yes, that's roughly the idea. To be technically correct, its not the beam diameter at aperture that matters, but the minimum beam diameter anywhere along the beam. However, yes, a 10x beam expander, aligned properly, could take your divergence down 10x.

Here is th eimportant relationship to remember:
Divergence is proportional to the wavelength, and inversely proportional to the minimum beam diameter.

Therefore, if you want to lower divergence, you either need to increase the beam diameter, or decrease the wavelength (the first being the more practical of the two for our purposes).

Also, keep in mind that there is a theorhetical perfect beam. The M^2 attribute is a measure of how many times worse than a perfect beam (at that wavelength) the laser is. For green DPSS lab lasers, you'll find the M^2 beam spec is usually <1.2, and the perfect beam would be exactly 1 (M^2 can never be less than 1 because of diffraction). With multi-mode red diodes, your M^2 is often closer to 20 (hence why I complain about them so much).

The M^2 is basically a factor based on the divergence, minimum beam diameter, and wavelength. So for instance, if you're considering the full-angle divergence, and 1/e^2 beam diameter (don't worry about what that means, I'm just trying to be correct so no smart-ass complains ) on a 532nm laser, the best possible beam would have a (divergence) x (minimum diameter) of roughly 0.678 mm*mRads

#### pseudonomen137

##### New member
a_pyro_is said:
OK, but if you replace "collimator" with "upcollimator" everywhere I used it in my post would it be correct?  :-? I guess I should make my question a little shorter.

If you increase the beam diameter by one factor, will the divergence decrease by the same factor?
Sorry! I was working on a response to that right after I made the first post. Check it out above ^

#### a_pyro_is

##### New member
Can you do the same thing with a single larger diameter, longer focal length lens placed further from the already highly divergent LD, as with one diverging and one collimating lens?

#### pseudonomen137

##### New member
a_pyro_is said:
Can you do the same thing with a single larger diameter, longer focal length lens placed further from the already highly divergent LD, as with one diverging and one collimating lens?
Sure. The difference is that the divergent/convergent lens combo would make a beam expander, which you would use on an already collimated beam. The weaker collimator lens would be for a diverging source.

#### a_pyro_is

##### New member
OK, thanks! I'll keep pretending that I fully understood all of that. Just so long as I can understand it in practice I think I'll be good. I guess I'll have to start collecting optics and just play with them.
Thanks again!

L

#### likewhat

##### Guest

it is at this link, you need to add the www part

www.imagehosting.com/show.php/1170171_beamresizing.bmp.html

The pictures show a beam that is being made smaller, but light is reversible so it would also work backwards to make a small beam bigger.

**edit** fixed link {SenKat}

#### rog8811

##### New member
I suspect I will still spend my time holding different lenses in the beam to see what happens...

@likewhat, are there some words to go with the picture? I am showing my ignorance here, I would like to know how the variables, f1/f2 etc, are calculated?
(I am sure someone will fix your link for you soon by the way )

Regards rog8811

#### pseudonomen137

##### New member
Well, the picture just shows the two basic configurations for a beam expander (can be used in reverse too). The f numbers are the focal lengths of the lens. Those are just properties of the lens you buy, not so much something you'd want to calculate yourself.

#### a_pyro_is

##### New member
I don't remember much about optics from Physics class but I DO remember NOT having much fun that day calculating focal lengths of lenses and mirrors.