Beam Expander Question: Two or More Diverging Lenses Before Collimation?

[Alaskan]

Has anyone ever seen two concave lenses being used one right after the other to expand an already diverging beam further before collimation inside a beam expander? Would such reduce the divergence even more than a single lens?

 

[steve001]

Has anyone ever seen two concave lenses being used one right after the other to expand an already diverging beam further before collimation inside a beam expander? Would such reduce the divergence even more than a single lens?

Of course two would over one.

 

[Alaskan]

I've only seen expanders which use one diverging lens at the input, not one following another. Only reason I am considering doing this, due to the added loss, is that I have lots of lenses but not the right combination to do what I want without a double expansion on the input. I don't quite understand the physics regarding why expanding the lens reduces the divergence, after collimation, so was hoping someone could answer, thank you.

 

[KrowBar]

I don't quite understand the physics regarding why expanding the lens reduces the divergence, after collimation

I assume you mean expanding the beam. A beam with some given quality has a parameter called the beam parameter product (BPP) which is more or less constant. This BPP is the product of the beam waist (it's smallest diameter) and the divergence angle. If this product remains constant, then the only way to have a smaller divergence angle is to have a larger waist diameter. If the beam is collimated (focused to infinity) then the waist will be right next to the lens. Now, you can hurt the BPP by using a poor quality lens, or by having bad alignment, but you aren't going to make it any better without specifically designed (expensive) corrective optics.

Someone might use 2 diverging lenses either to reduce the magnitude of the effective focal length of the input (thereby allowing a higher expansion ratio for a given output lens focal length) or to correct some aberration produced by using just a single lens. (Using 2 lenses with spherical faces might be a cheaper way to get a result similar to that of using a single lens with engineered surfaces)

 

[Cyparagon]

Has anyone ever seen two concave lenses being used one right after the other to expand an already diverging beam further before collimation inside a beam expander? Would such reduce the divergence even more than a single lens?

Only assuming the collimator was large enough to contain the larger beam, and assuming you couldn't get a single lens with half the focal length, and assuming you have enough room mechanically to vary the distance between the expansion lens and collimating lenses. It's complicated.

 

[hakzaw1]

BE are something that a lot here have trouble wrapping their brains including me at first.

In the ,most simplest terms --this BE makes a fatter beam near the laser and extends the 'sweet' focal point farther away from the laser-- you can burn things from a longer distance for example.

good helping by both posters--+6 (when I can Cyp)

Here are some pics of the current BE from JETLASERS- made to use this adaptor for the JL Pro Pl-E series- note that it has threrads on both ends which IMO is by far the best way to attach-- the first ones JL offered all (except the Pro Pl-Ds) had set screws to hold the BE in place and that was a PITA -made some scratches etc took a lot of fiddling about - it was near impossible to make the big 'ring' go away.. but with threaded adaptor --no rings..

this is thier 10X BE...

see it at jetlasers.com

I can see that in this one there is at least two lenses inside.

hope this helps----hak

 

[Alaskan]

I have the lenses, just need the tube to make a very long beam expander. My thought is when using a beam expander, as long as the laser light at the input shooting through the concave lens produces a spot large enough to cover 75 percent or more of the PCX lens in front of it, it should work. However, I'm being told this isn't quite so... Maybe this only works when the concave lens is at the focal point of the PCX lens coupled with it.

I'm wondering what happens if the concave expander lens is further away than the focal point of a PCX lens, yet the spot does not overshoot the diameter of the collimation lens? I'm guessing the focus would not be at infinity and instead of exiting as parallel rays, either expanding out or perhaps bending in, depending upon which side of the focal point of the output lens you are on?

Wish I could find a animation showing this, so far not.

 

[hakzaw1]

Looking backwards thru my JL BE is like looking through the front of binoculars-

things get smaller.... mine has a very small lens close to the laser and bigger at the other end-- whne you look at a diagram/drawing of laser beams ity sometimes looks like there are two beams when it is just showing how large the beam is as it goes to the focul point where it starts to grow larger again- which I know you understand by your 'interesting fact' in your sig..

I think 'focused to infinity' should NOT be taken literally as 'its' ( the fp)never THAT far away.