Low Divergence laser

[newtolasers260]

Hey guys i'm new to the whole laser thing. I'm working on a project for work creating a vehicle alignment system and need a fairly low cost low divergence laser. I purchased Calpac CP-TIM-218-4D-650 laser and the dot is just too big. I am shining the laser at a measurement plate that is about 120" from the laser. Any suggestions/pointers? Thanks!

 

[ARG]

If you don't mind a big dot, you could use a beam expander. It increases the beam size, but decreases the divergence.

 

[newtolasers260]

Sorry I thought that low divergence meant a small dot? Basically all I need is the smallest dot possible at 120"

 

[InfinitusEquitas]

Try a decent 5 or even 1mW green laser pointer first.

Laserglow, or optotronics won't let you down.

Divergence is a measure of how fast the laser beam spreads out, so your understanding is basically correct.

The thing is, you don't really need super low divergence to have a small dot at 120 inches. Nor do you need power, since the dot may appear bigger simply by virtue of being brighter.

At 120 feet, divergence would be a factor though, in which case a red laser pointer would work well.

 

[newtolasers260]

Thanks for the input. The laser I have now makes about an 1/8" dot even at 120". I am looking for an extremely small dot since this is for a measurement device and I will be reading the measurement based off of a laser pointed at a scale. Forgot to include all this info in the original post, my mistake

 

[InfinitusEquitas]

1/8" is a bit big. Still I were you, I would experiment with a couple of cheap, but quality 5mW greens first.

If that's not enough, you may need to increase power slightly and look into getting a beam expander as ARGLaser suggested. The beam diameter at aperture would increase, but the dot would be much smaller.

 

[DrSid]

All beams looks more or less like this: File:GaussianBeamWaist.svg - Wikipedia, the free encyclopedia

(from Gaussian beam - Wikipedia, the free encyclopedia)

There are two main types of focus:
1) You can focus the laser to infinity (beam is parallel at aperture) .. the beam will then slowly expand, and the dot will be always bigger then aperture. In this case, the aperture is at waist of the beam. With this setup you get smallest divergence angle - and thus smallest dot at large distance.
2) You focus closer. The beam will CONVERGE at aperture .. there will be some distance, at which the dot is smallest (waist of the beam), then the dot will get bigger again. With this you get steeper divergence angle (thus big dot at large distance) .. but the dot in the focus point will be smaller.

In both cases, the limits are defined by wavelength (shorter is better) and aperture size (bigger is better).

You could use 405nm laser for better focus .. but it would not be even twice as good as red, and even less then green .. and it is much less visible .. so it is not really a good choice.

Then you must enlarge the aperture. Some pointers come with beam expanders, which do just that.
Or you can do one yourself. Generally you use concave lens to make the beam highly divergent, thus increase diameter fast, then you use large diameter concave lens to switch the beam back into convergent or collimated. To get rid of the spherical aberration, which would ruin the dot, the concave lens should be plano-concave and the the convex lens should be two plano-convex lens .. with planar side toward more divergent part of the beam. In other words .. it's pretty tricky stuff.
Diode lasers are a bit simpler, since they come with rather divergent beam from the start .. you don't need the concave element.
Also for the convex element, you could use existing lens system with similar task .. namely photographic lens. If you could land your hands on some old mirror telephoto lens with 4 inch aperture .. and feed it with red laser .. that should work just fine.
But any photographic lens would give you a lot larger aperture then common pointers.

 

[InfinitusEquitas]

You could use 405nm laser for better focus .. but it would not be even twice as good as red, and even less then green .. and it is much less visible .. so it is really a good choice.

This doesn't really make sense to me... to get some visibility at least ~20mW would be needed, at which point safety comes into play a bit. I can't see a 20mW 405nm being a better option over 1mW of 532nm.

 

[Sigurthr]

If you need a super small spot at 120 inches out just get a FOCUSABLE 5mW Green Laser. Don't bother with beam expenders or high power lasers or anything complicated.

You can manually focus the spot to be as big or as small as you need it to be at a SET distance as long as the laser is focusable.

Green has the best visibility and very good divergence (not an issue here) and are rather inexpensive at low powers.

 

[DrSid]

This doesn't really make sense to me... to get some visibility at least ~20mW would be needed, at which point safety comes into play a bit. I can't see a 20mW 405nm being a better option over 1mW of 532nm.

Of course, it should read 'it is not a good choice' .. I fixed it ..

 

[DrSid]

If you need a super small spot at 120 inches out just get a FOCUSABLE 5mW Green Laser. Don't bother with beam expenders or high power lasers or anything complicated.

You can manually focus the spot to be as big or as small as you need it to be at a SET distance as long as the laser is focusable.

Green has the best visibility and very good divergence (not an issue here) and are rather inexpensive at low powers.

This is simply not true. You can set the dot as big as you want, but not as small as you want. There are limits to that, because of diffraction.

 

[Sigurthr]

I suppose I should have said "within reason". Still, I can focus any one of my focusable lasers to a spot less than 1mm in diameter. For measurement purposes I don't know that you would want smaller than that - that was my initial point. Theoretical limits of diffraction shouldn't be much of an issue.

 

[DrSid]

I suppose I should have said "within reason". Still, I can focus any one of my focusable lasers to a spot less than 1mm in diameter. For measurement purposes I don't know that you would want smaller than that - that was my initial point. Theoretical limits of diffraction shouldn't be much of an issue.

Ok .. 1mm at 120" would be about the limit from my experience too. But that already is the 'theoretical limit of diffraction'. I actually have one laser with extra thin beam .. which is 1cm at even such short distance .. and cannot be focused better.
If 1mm is enough, then sure, mostly anything will work. With 10x beam expander you should get 0.1mm at the same distance though ..

 

[newtolasers260]

Thanks for all of the input you guys have really saved me a lot of time trying to figure out this stuff on my own. So I think I found a laser that will work well for my application: Extended Life Green Laser Pointer Switch | Green Laser Pointers, OnPoint Lasers

It looks to be the same laser as the Galileo from Laserglow. Having a latching on/off switch will be really helpful for what I need it for. Anyone have any experience with this laser?

I was also looking into the beam expanders as ARGlaser suggested and found an affordable option from dragonlasers. Looks like they are out of stock, but does anyone know of another supplier for this expander or a comparable alternative?

 

[InfinitusEquitas]

Of course, it should read 'it is not a good choice' .. I fixed it ..

Just checking, no need to get all uppity:na: