Effect of Plano-Concave lens in standard focusable dot laser

[Vandis2]

Question from a newbie.

Anyone have an educated guess what kind of beam divergence I might get by replacing a standard collimating lens (~.5mRad) in a common dot focusable laser module (808nm 500mW) with a 6mm plano-concave lens (-8.111mm focal length) with the module having an original beam diameter of ~2.5mm?

On the other hand, given the original collimating lens what might be the expected beam spread out to say 100m after it crosses over at point of focus, say 1 meter?

 

[steve001]

I don't quite understand you intention. Is it too collimate the beam? Or. Is it to only expand the beam?

 

[Vandis2]

I don't quite understand you intention. Is it too collimate the beam? Or. Is it to only expand the beam?

My goal is to expand the beam to illuminate objects in the night.
A "laser pointer" will not serve my needs at all.

I am inquiring as to what the beam will do with this plano-concave lens of -8.111 focal length.
As I have thought more about what the original lens will do to the beam after crossover, I realize that it will be much less than I need.
After crossing over I assume that the beam will continue diverging at the same rate (.5mRad), which would yield a beam diameter of 50mm or ~2" at 100 meters. I assume that .5mRad is the maximum divergence of that lens where the manufacturer states it is <.0.5

What I would like to achieve, if possible, is a beam diameter of 3000mm or 10' (or even greater) at 100 meters, which would require that the plano-concave have minimally the effect of diverging the beam at 30mRad (1mRad = 1mm/1m). What are the chances of that?

 

[steve001]

My goal is to expand the beam to illuminate objects in the night.
A "laser pointer" will not serve my needs at all.

ok

I am inquiring as to what the beam will do with this plano-concave lens of -8.111 focal length.

It will obviously expand the beam

As I have thought more about what the original lens will do to the beam after crossover, I realize that it will be much less than I need.

After crossing over I assume that the beam will continue diverging at the same rate (.5mRad), which would yield a beam diameter of 50mm or ~2" at 100 meters. I assume that .5mRad is the maximum divergence of that lens where the manufacturer states it is <.0.5

You are confused here. There is no [beam my emphasis] crossover because placing a negative focal length lens in the path of the beam, expands the beam only

What I would like to achieve, if possible, is a beam diameter of 3000mm or 10' (or even greater) at 100 meters, which would require that the plano-concave have minimally the effect of diverging the beam at 30mRad (1mRad = 1mm/1m). What are the chances of that?

Chances are excellent if you have the right lens. Do you? I don't know. I think you'll have to experiment with different plano-concave lenses to find out what works.

 

[Vandis2]

Steve,
Thanks for posting.
I need to clarify for you that I am considering two (2) options.

The first is leaving the original lens in (collimating lens), which I am now guessing will crossover at a meter or two and then began expanding at the original lens divergence of .5mRad. And this answers for myself one of the questions of my first post.

The second is to buy this 6mm -8.111 focal length lens (plano-concave) with hopes that it will produce a divergence of >300 mRad.
This is my key concern, whether it will do at least this.

IMHO there are those among us who are much better educated than I and cannot only apply a mathematical formula to determine this but can also accurately guess at other factors that would influence the outcome, which I cannot even begin to fathom...

 

[BShanahan14rulz]

Even if we found out the focal length of the current lens and told you how far to position it from the emitting face of the laser diode, how would you position it to that point exactly?

Can you just fiddle with it until it looks right? That's what most folks would do. You will want to move the lens closer to the diode to make the beam diverge only. You can move it farther from the diode to have the beam condense and then spread out again. Either way, you will end up with a diverging beam, the rate of divergence depends on how close or far the lens is from its focal length from the laser.

 

[steve001]

Can you just fiddle with it until it looks right? That's what most folks would do. You will want to move the lens closer to the diode to make the beam diverge only. You can move it farther from the diode to have the beam condense and then spread out again. Either way, you will end up with a diverging beam, the rate of divergence depends on how close or far the lens is from its focal length from the laser.

He has a concave lens. Moving it's position in or out will never make the beam converge and it won't change how fast the beam diverges either.

 

[steve001]

Steve,
Thanks for posting.
I need to clarify for you that I am considering two (2) options.

The first is leaving the original lens in (collimating lens), which I am now guessing will crossover at a meter or two and then began expanding at the original lens divergence of .5mRad. And this answers for myself one of the questions of my first post.

ok, I see now. But it depends whether or not the collimating lens has collimated the beam optimally for lowest possible divergence based upon the collimating lenses focal length.

The second is to buy this 6mm -8.111 focal length lens (plano-concave) with hopes that it will produce a divergence of >300 mRad.
This is my key concern, whether it will do at least this.

This will require some experimentation on your part.

 

[BShanahan14rulz]

I think steve001's suggestion to just experiment may be the best way of achieving what you are after.

This video was posted over at another forums, not mine, so not hotlinking it:
3W 808nm IR laserpointer - YouTube

It is a focusable 808nm laser pointer. The effect was done by changing the focus of the original collimating lens. Perhaps is this what you are trying to achieve? Many thanks to the creator, youtube user neophyte101.