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Old 03-08-2015, 08:10 PM #1
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Default Laser diode chip aperture, lens aperture & wavelength relationships to divergence?

From what I've been able to pull together over the last year of reading posts, and searching on google etc., is that the divergence of a laser is the product of:

1. Laser diode chip aperture (and if single mode?), the larger the laser chips aperture the lower the divergence?

2. Wavelength, i.e. 532nm, 405nm etc., I have searched for hours to find a graph which shows the relationship between wavelength and divergence but haven't been able to find one yet.

3. Clear aperture of the lens used to collimate the laser diode as well as wavelength being collimated.

Can someone please explain the relationships of the above in simple language from a big picture or top down understanding? I found some calculators and math to determine each, but haven't been able to pull it all together yet. If you look at my signature, you can find links to info as well as calculators about each of the above, but I don't have a full grasp of how the puzzle goes together yet to make the lowest divergence laser pointer possible from the available parts.

But here are my assumptions:

1. Single mode large aperture laser diode chip.
2. Shortest wavelength laser diode.
3. Largest diameter lens.

Perhaps using a large lens to reduce the divergence is simply the product of having an expanded beam? Please see this online lens divergence calculator: US-Lasers - Beam Divergence Calculator

Maybe I do understand all of the relationships well enough to buy what I need for a ultra low divergence laser pointer, if someone can confirm my above assumptions, I'd sure appreciate the help. Thanks!


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Old 03-08-2015, 08:34 PM #2
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Default Re: Laser diode aperature vs lens aperture vs wavelength... ???

Quote:
Originally Posted by Alaskan View Post
Perhaps using a large lens to reduce the divergence is simply the product of having an expanded beam?
You've got it

That's what beam expanders do, they reduce divergence and increase beam size.

More on beam expanders:
http://assets.newport.com/webdocumen...expander_5.pdf

The Wikipedia article explains this simply:
https://en.wikipedia.org/wiki/Beam_divergence

Last edited by ARG; 03-08-2015 at 08:37 PM.
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Old 03-08-2015, 08:38 PM #3
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Default Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

Thanks ARG!

One more question: Last night when looking at some 5um aperture laser diodes vs 100um aperture diodes I was surprised by the math that a larger aperture diode is better for lower divergence. I was so sure a smaller point source meant a lower divergence, now I'm confused.

OK I understand the beam-expander, but that link I found with the calculator where you input the wavelength, i.e. 532nm, and then the lens clear aperture had me scratching my head, because they didn't mention "expansion".
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Last edited by Alaskan; 03-09-2015 at 02:46 AM. Reason: added remark about expander
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Old 03-08-2015, 08:52 PM #4
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Default Re: Laser diode aperature vs lens aperture vs wavelength... ???

I think this article explains that/
https://en.wikipedia.org/wiki/Gaussi...eam_divergence

"Because the divergence is inversely proportional to the spot size, a Gaussian beam that is focused to a small spot spreads out rapidly as it propagates away from that spot. To keep a laser beam very well collimated, it must have a large diameter. This relationship between beam width and divergence is due to diffraction."

The larger the starting diameter of the beam, the better the divergence.

Last edited by ARG; 03-08-2015 at 08:52 PM.
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Old 03-08-2015, 09:11 PM #5
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Default Re: Laser diode aperature vs lens aperture vs wavelength... ???

That makes sense, it appears I have had a bad assumption, that single mode laser diodes have a small aperture. Somewhere along the line I misunderstood that the smaller the dye, the lower the divergence when I am now seeing the situation is reversed.

Last night I tried to find the aperture of some single mode diodes to see if they all have small apertures or not, but none of the documents I could find for their specs gave the aperture. Perhaps the size of the aperture is not relative to the mode, whether single or multi, and the reason a single mode diode has a lower divergence is due to something else.

OK, now that I know my concepts were off, can you help align them for me? Is there such a thing as a 5um aperture multimode? I know there are 5um single mode diodes, I see them on ebay. I'm trying to understand the relationship to the aperture and possible mode, now that I know a larger aperture produces reduced divergence in single mode diodes.
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Sincerely investigate any of these three short quotes as new concepts and you've taken your first step into a larger world:

"I regard consciousness as fundamental. I regard matter as derivative from consciousness..." - Max Planck. "Everything we call real is made of things that cannot be regarded as real" - Neils Bohr. "What we call physical things and events do not exist independently of subjective experience..." - Deepak Chopra.

Each of these three rabbit holes go deep, ending up in the same place.

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Last edited by Alaskan; 03-08-2015 at 10:06 PM. Reason: Reduced the amont of text to be more concise regarding what I need to figure out.
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Old 03-08-2015, 10:19 PM #6
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Default Re: Laser diode aperature vs lens aperture vs wavelength... ???

Quote:
Originally Posted by Alaskan View Post
I was so sure a smaller point source meant a lower divergence, now I'm confused.
Before, or after collimation?

A larger aperture will lower divergence at first glance, but the side-effect is that other transverse modes usually appear with anything much above a few microns. Of course, multi-mode beams diverge more rapidly than the same sized single-mode beam. A smaller emitter area is better for overall divergence (after collimation).
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Old 03-08-2015, 10:24 PM #7
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Default Re: Laser diode chip aperture, lens aperture & wavelength relationships to divergence

edit:

After collimation.

I received an answer regarding a question I had about the aperture of a single mode diode from an ebay seller, his response was:

Quote:
The spec sheet doesn't list the emitter width, but we can assume it's on the order of 5um (likely smaller). Raw divergence in the fast axis is 29 degrees or about 500mrad. I don't know the focal length of the G2 lenses, but the collimating lenses I use have an effective focal length of 8mm. By my trig (which you should double check and / or redo for the focal length of your G2 lens), that gives a beam size around 4mm. 4um -> 4mm is 1000X expansion so the divergence is 1/1000 the original or about 0.5mrad. That's about what I'd expect just from experience. SM diodes make really nice beams.
With the advice of your last post, and the above, this tells me that I should buy as small an aperture single mode diode as I can find if I want to have a small amount of divergence after collimation, due to the expansion between the diode output and the FL of the lens. Looks like I'm on my way to pulling the trigger on a candidate SM diode soon, at 5um aperture.
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To shorten my signature I have moved most of my laser related web links to this forum page, the second post in that thread shows most of my builds... Alaskan's Laser Links: http://laserpointerforums.com/f44/al...ml#post1449395


Sincerely investigate any of these three short quotes as new concepts and you've taken your first step into a larger world:

"I regard consciousness as fundamental. I regard matter as derivative from consciousness..." - Max Planck. "Everything we call real is made of things that cannot be regarded as real" - Neils Bohr. "What we call physical things and events do not exist independently of subjective experience..." - Deepak Chopra.

Each of these three rabbit holes go deep, ending up in the same place.

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Last edited by Alaskan; 03-09-2015 at 11:43 PM.
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