Beam Expander Design Theory

[SomeBodyElse]

I have been trying to wrap my head around beam expanders to design my own. I think I understand a majority of how I want it to work but have a couple questions.

I plan to have the laser come out of the diode and pass through a PCX or G2 lens to be collimated.

After that I would pass it through 2 more off the shelf lenses that make a keplerian beam expander.

Heres where my question comes in, everything I read indicates I should expect a collimated final output, which I understand means that if I have a 1" beam leaving the expander, I would have ~1" spot + minimal divergence an arbitrary distance away.

I want to focus to a fine or as fine as possible a point at an arbitrary distance and have this focusing distance adjustable from minimum to infinity.

Which distances would I want to make adjustable in my design? Diode to G2? G2 to Start of expander? Internal distance of beam expander?

Also is the diagram I drew correct in concept?

 

[Alaskan]

I've never built a Keplerian beam expander, but that type of expander probably acts the same as any other beam expander if you vary the distance of the output or collimating lens on the end to produce a focused spot too.

For my own pointers, I use a very unconventional expander where instead of a concave lens in front of a collimated output to expand the beam to a larger size, I use a G2 or three element etc. lens to do the same thing by defocusing the lens to a broad output spot which is then collimated with a large PCX lens. With this setup, to focus my beam expander to a spot I just vary the distance the output lens is from everything else (G2 lens or what have you inside prior to it) and my focus will then have a range from a few feet out to infinity. For a close in focal point I believe being able to do so depends on how much mechanical throw your focus has, I've found 1 inch of adjustment isn't enough to have both a close in focal point as well as infinity focus. You might try having two or three inches of adjustment to be able to focus to a spot close in, or at a distance for that matter. I'd buy some inexpensive lenses on ebay and experiment!

Hope someone with direct experience with the Keplerian expander chimes in, this is all I have to offer. Check my signature for some links which you might find helpful

 

[SomeBodyElse]

I've never built a Keplerian beam expander, but it probably acts the same as any other beam expander in regard to focusing the beam to a point (although that is not what an "expander" is designed to do, expanders can be without adjustment and when built that way are set to only expand to infinity but with focal adjustment, I think that type of expander can do what you want).

I use a very unconventional expander on my pointers where instead of a concave lens in front of a collimated output to expand the beam to a larger size, I use a G2 or three element etc. lens to do the same thing by defocusing the lens to a broad output spot which is then collimated with a large PCX lens. With this setup, to focus my beam expander to a spot I just vary the distance the output lens is from everything else (G2 lens or what have you inside prior to it) and my focus will then have a range from a few feet out to infinity. Much depends on how much mechanical throw your focus has, I've found 1 inch of adjustment isn't enough to have both a close in focal point as well as infinity focus. You might try having two or three inches of adjustment to be able to focus to a spot close in, or at a distance for that matter.

Hope someone with direct experience with the Keplerian expander chimes in, this is all I have to offer.

Ok, yes this is what I was wondering, so by moving the exit lens of the beam expander while everything else stands still adjusts focus distance, and I will want to start with 3 inches of throw for experimenting.

Also what happens if the beam entering a beam expander is not collimated, but is either converging or diverging instead before it gets there?

 

[Alaskan]

If you have an adjustable beam expander and the input beam you are shooting into it is not fully collimated to infinity, from my experience with my own adjustable units, you can adjust it to infinity with the expander settings anyway. I believe this works with converging or expanding inputs, but of course, if converging too sharply, or expanding too sharply, I am not sure you can adjust for that. All of that said, if you move the focus of any lens in the string, it affects the output beam too when moving it one way or the other from infinity focus, but I've found moving the focus of the G2 lens has the least affect when using that as an expander lens/out of focus. However, moving the focus of the G2 lens DOES affect the focal point of the output lens which will need to be moved to a new distance away to compensate for the change, but not very much. The most pronounced change adjusting the G2 lens out of focus makes is the spot size which falls upon the face of the final collimating lens, but this is for a two lens expander using a G2 out of focus as a cheat, not a classic beam expander like you are building, for that, I believe you will want your G2 (or what ever lens) adjusted to infinity focused output.

 

[SomeBodyElse]

If you have an adjustable beam expander and the input beam you are shooting into it is not fully collimated to infinity, from my experience with my own adjustable units, you can adjust it to infinity with the expander settings anyway. I believe this works with converging or expanding inputs, but of course, if converging too sharply, or expanding too sharply, I am not sure you can adjust for that. All of that said, if you move the focus of any lens in the string, it affects the output beam too, moving it one way or the other from infinity focus, but I've found moving the focus of the G2 lens has the least affect when using that as an expander lens/out of focus. However, moving the focus of the G2 lens DOES affect the focal point of the output lens which will need to be moved to a new distance away to compensate for the change, but not very much. The most pronounced change adjusting the G2 lens out of focus makes is the spot size which falls upon the face of the final collimating lens, but this is for a two lens expander using a G2 out of focus as a cheat, not a classic beam expander like you are building, for that, I believe you will want your G2 (or what ever lens) adjusted to infinity focused output.

Great, thankyou for sharing your experience, I can't wait to get something going for testing.

 

[Alaskan]

Remember though, I have not tried any of this on a Kep. beam expander, maybe someone can confirm or deny my thoughts.

 

[Cyparagon]

The only disadvantage with Keplerian as far as I'm aware is the length of your optics assembly needs to be longer.

I prefer a simpler approach - use as few lenses as possible. Even one lens has adjustable focus - further out means the focus moves closer, and vice versa. The focal length needs to be quite short, and needs to be in proportion to your diode's raw divergence. Old photo of an experiment I did:

 

[Alaskan]

Thanks Cy.

My last 5 inch diameter beam expander only has one lens, no G2 etc, just a PCX lens spaced 8 inches away, its focal length. The raw output of the diode into that lens.

 

[SomeBodyElse]

The only disadvantage with Keplerian as far as I'm aware is the length of your optics assembly needs to be longer.

I prefer a simpler approach - use as few lenses as possible. Even one lens has adjustable focus - further out means the focus moves closer, and vice versa. The focal length needs to be quite short, and needs to be in proportion to your diode's raw divergence.

I was thinking that more lenses would create a cleaner beam with less divergence, and that the Keplerian style had an advantage in beam quality.

If that's not the case, then I should just go with the simpler beam expander designs.

 

[steve001]

I was thinking that more lenses would create a cleaner beam with less divergence, and that the Keplerian style had an advantage in beam quality.

If that's not the case, then I should just go with the simpler beam expander designs.

A Keplerian needs only two positive focal length lenses, as does the Galilean type, but with this type one lens must be a negative focal length. Both types are simple. A refractor telescope can be used as a Galilean expander. A cheap Tasco gun scope or monocular can be used as a Keplerian expander. Read this Using Laser Beam Expanders - Wavelength Technology Singapore
Old thread. http://laserpointerforums.com/f39/diy-beam-expander-41981.html

Lenses for experimentation. AnchorOptics.com - Supplies affordable, quality commercial grade and experimental grade optics

 

[Gabe]

Haha, just finished an experiment where I attached a tube from a pair of binoculars onto the end of my Pro and worked like a charm. Surprising thing is though, there are only two convex lenses, one at each end. So I thought it would use a keplerian system, but wouldn't that yield a rightside-left and upside-down image when looking through it? There aren't any prisms or mirrors to flip the image rightways. Anyways, it works well and gives me an eventually needle thin beam into the sky. I had a different binocular tube that expanded it much more, but there were some optics in there that really reduced output power, and didn't work cause the beam is misaligned.
Ugly as a wart, though:

 

[SomeBodyElse]

Steve, thanks for the info and link, I was only aware of Thor labs before, and thanks Gabe for the info.

Something else I was wondering, if I use a lower powered diode and expanded the beam to about 1" collimated, how do I calculate weather or not it's safe to look at without eye protection. I seem to have read that 5mw is the magic number, does surface area influence that number, or a different rule for wide beams? I'm thinking it should have something to do with power level per an area?

 

[steve001]

Steve, thanks for the info and link, I was only aware of Thor labs before, and thanks Gabe for the info.

Something else I was wondering, if I use a lower powered diode and expanded the beam to about 1" collimated, how do I calculate weather or not it's safe to look at without eye protection. I seem to have read that 5mw is the magic number, does surface area influence that number, or a different rule for wide beams? I'm thinking it should have something to do with power level per an area?

Click link. It's free. Laser Visuals - Scanguard laser show analysis software : Lite Version

 

[SomeBodyElse]

Click link. It's free. Laser Visuals - Scanguard laser show analysis software : Lite Version

That's awesome, I'm reading on it now, I'm thinking of mounting lasers on gimbal under a quad copter to simulate phaser fire like in star trek for fun

 

[steve001]

That's awesome, I'm reading on it now, I'm thinking of mounting lasers on gimbal under a quad copter to simulate phaser fire like in star trek for fun

Here's another useful program. https://lightmachinery.com/optical-design-center/lasers-and-laser-beams/gaussian-beam-propagation/

 

[Alaskan]

I guess the thing to do is to calculate the power density of a wider beam against the power density of a small 5mw pointer and see how different they are. One thing to keep in mind though, even a five milliwatt laser pointer can damange your eyes when focused on to the back of your retina and I'm not sure 5mw of a collimated 2-4mm diameter laser is really safe either.