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how to... reduce the mRAD of a laser?

IsaacT

IsaacT

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Instead of paying 80 bucks, go to optotronics dot com and go to their lenses section....get a convex and a concave, try to make the focal length very different. The farther the focal lengths are apart, the bigger Magnification Power, or MP, or the beam expander and as a result, a lower divergence. The space required between the lenses is the sum of the focal lengths. Watch out for negative focal lengths.

For example, they have a

-500mm focal length and a 85mm focal length, so if my calculations are correct, there should be 415mm between them. You are going to want the concave lens as the objective lens(front/closest to laser) and the concave as the image(back/farthest from laser), if you do it the otherway, there will be a focal point in between them and the point will heat the air, causing issues. Having the concave first means no focal point, and no heat.
 





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steve001

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wannaburnstuff said:
Instead of paying 80 bucks, go to optotronics dot com and go to their lenses section....get a convex and a concave, try to make the focal length very different. The farther the focal lengths are apart, the bigger Magnification Power, or MP, or the beam expander and as a result, a lower divergence. The space required between the lenses is the sum of the focal lengths. Watch out for negative focal lengths.

For example, they have a

-500mm focal length and a 85mm focal length, so if my calculations are correct, there should be 415mm between them. You are going to want the concave lens as the objective lens(front/closest to laser) and the concave as the image(back/farthest from laser), if you do it the otherway, there will be a focal point in between them and the point will heat the air, causing issues. Having the concave first means no focal point, and no heat.
Click to expand...


A Keplerian type expander use the sum of the focal lengths in spacing lenses. Galilean expanders are less than the sum of the focal lengths in lens spacing.

A simple Keplarian type expander use use two positive lenses. A simple Galilean type one negative lens and one positive focal length lens.

If your meant the lens focal lengths to be 85mm EFL and 500mm EFL then the correct spacing is 585 mm. This would giving a mrd of 0.2mrd if the beam is 1mm dia. and the initial beam divergence is 1.2mrd. It also gives a Rayleigh Range of 28835mm [ 94.6 feet] and a beam diameter of 5.9mm.

If the above lens becomes a negative focal length of -85 then the lens spacing is 417.1 mm. All the rest remains the same. See http://www.lightmachinery.com/gausbeam.php . Use Internet Explorer to see graphical presentation.
 
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IsaacT

IsaacT

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Correct me if I am wrong but it would still be sum, since a negative focal length + a positive focal length is less than the positive focal length.
This:
FL.gif


was in an article I read on Beam Expanders and the forumlas behind them. It would seem that the focal length of the objective lens is aligned at the focal point of the image lens, so

Focal Length(obj) 85mm
______

Focal length(img) -500mm
________________________________

aligned so that the sum of the focal lengths which would be -415, but you cannot have a negative length so you space them 415mm apart? Maybe it would be easier to say the difference of the focal lengths(absolute value?)
 
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steve001

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wannaburnstuff said:
Correct me if I am wrong but it would still be sum, since a negative focal length + a positive focal length is less than the positive focal length.
This:
FL.gif


was in an article I read on Beam Expanders and the forumlas behind them. It would seem that the focal length of the objective lens is aligned at the focal point of the image lens, so

Focal Length(obj) 85mm
______

Focal length(img) -500mm
________________________________

aligned so that the sum of the focal lengths which would be -415, but you cannot have a negative length so you space them 415mm apart? Maybe it would be easier to say the difference of the focal lengths(absolute value?)
Click to expand...

Notice the closely matched figures of 417.1mm spacing and yours of 415mm. From those numbers it seems we are saying the same thing. I'm just saying it in simpler language.
There's no getting around this. Keplarian expanders necessitate a longer working distance for their construction over Galilean expanders.
Punch in these numbers using the applet above. First go around keep both numbers positive 85mm elf and 500 elf. Spacing for the first lens [85] is 0mm because the initial beam is assumed to be optimally collimated and well within the Rayleigh Range. Then change the 85mm elf to -85mm elf [ include the minus sign]. Keep the beam diameter at 1mm and divergence at 1.2 mrd. only for the reason that most handheld lasers operate at those specs. After this is done you'll see the difference in actual spacing of these lenses for both types of expanders.
 
IsaacT

IsaacT

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woops, I missed the applet :yabbem: +1 for that btw! An applet will be a huge help when I make my own beam expander.
 
Bluefan

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Wannaburnstuff, the error you make is that the negative lens has the shortest absolute focal length, so you'd need a positive lens with f= 500mm, and a negative lens with f= -85mm. Adding them works for geometrical optics, which gives 415mm, but for gaussian optics the divergence slightly changes this.
 
IsaacT

IsaacT

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Oh! I guess I have some more reading to do! If I wanted to make a beam expander though, a 500mm and a -85mm would be good?
 
LazyBeam

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So could someone point me to some good small lenses for a homemade 3X or 5X beam expander? I'm poor when it comes to choosing optics for designs. I'm looking for short lens separation and lenses that are 20mm or less in diameter (preferrably smaller).

I would like to make a removable beam expander for my kryton groove I have ordered. 445nm diodes already have ~1.5mrad divergence on the fast axis. Even with a small 3X expander I'd get ~.5mrad and with 445nm power levels... talk about long range power density :evil:
 
IsaacT

IsaacT

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optotronics carries lenses but they require a 20 dollar purchase
 
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steve001

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wannaburnstuff said:
Oh! I guess I have some more reading to do! If I wanted to make a beam expander though, a 500mm and a -85mm would be good?
Click to expand...

Shoot for a 6mm dia. x -6mm EFL PCX lens. *That diameter and focal length appears to be generally the shortest focal length you can get. -6mm and 500. That would give you a Rayleigh Range of 3.595 miles. In other words you'd have a beam diameter at the collimating lens of 3.28 inches and at 3.595 miles the diameter is 4.95 inches. Divergence would be approximately 0.01mrd for this setup. Beam Diameter at Surface on the applet means how large the beam is at the collimating lens.

* There are smaller diameter pcx diameter lenses with shorter focal lengths, but they are hard to handle and there are a few pcx lenses that are around 6mm dia. with -5 EFL but you'll have to shop around. I like Edmund Optics because they have a large off the shelf lens selection in various glasses, diameters and are AR coated and uncoated and you can be certain you get what you order. For mere experimentation in the mean time look at their subsidiary Anchor Optics
 
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dpl

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Dear Gents,

and what about wider beam? I have laser diode module with focusable lens (0.1 rad - 10rad). I need to get cca 80-100 mRad divergence. Does anybody have idea how to increase divergence? I assume using some concave precision lens but I am not sure about focal lenght. I found nice lens shop Concave Lenses - AR Coated

Could anybody give me hint how design optical lens to get 10m spot at 100 meters?THX
 
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steve001

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dpl said:
Dear Gents,

and what about wider beam? I have laser diode module with focusable lens (0.1 rad - 10rad). I need to get cca 80-100 mRad divergence. Does anybody have idea how to increase divergence? I assume using some concave precision lens but I am not sure about focal lenght. I found nice lens shop Concave Lenses - AR Coated

Could anybody give me hint how design optical lens to get 10m spot at 100 meters?THX
Click to expand...

look at this sites app gausian beam rev 2.0.xls
what you need to do is create another beam expander.
If you need help using the app you may pm me or just reply.

To view the graphic use Internet Explorer or make sure you have updated Macromedia Flash installed and working with your particular browser
 
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Cyparagon

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Any lens will make the dot larger at a distance. Adjust the focus to fine-tune the size.
 
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dpl

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I would not say any lens. I am convinced just about concave(-planar?) lens type.
And what about colimator before divergence lens? I think to get good results source beam exiting of LD should be colimated first. My spot should be circular.
 
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dpl

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To Cyparagon>

I need uniform intensity of beam spot. That is most crucial requirement in my application.Do not you think that colimating lens just can help before getting beam diverged?

To all:
Q1 I wonder if some LPF member familiar with optical design would give me valuable general advice which kind of laser diode could be the best choice for uniform diverged >100mRAD beam? I assume I should use single mode LD.Another important aspect is gaussian exiting beam pattern and X/Y FWHM ratio near to 1. I would like to have number of lens in optical system as little as possible (no Anamorphic Prism Pairs for conversion to circle beam etc). I found decent LD and concave lens at Thorlabs.com - Laser Diodes

Q2 Could anybody advise me what particular -f of concave lens to use for 100mRad or 300mRad divergence? Assumed that LD will have in-bulid collimating lens
 
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