Beam expander to focus laser beam

[steve001]

Re: Beam expander to focus laser bearm

Steve001, Alaskan, Singlemode, and others.
Thank you for commenting in this thread.

I tried to summarize what I can understand from the discussion and from the source out there.
Please evaluate attached files. Whether theoretically I am right or completely misleading.

I think focusing laser at a distance using galilean expander is possible. Not only collimating, but also focusing at a point at where the beam spot is the smallest.
This can be done depend on the fineness and size of the lenses.

One of my reference is here:
https://lightmachinery.com/optical-...tical-design-tools/gaussian-beam-propagation/

Give me your comments. Thanks.

Use the Gaussian app.

Try this: http://www.holoor.co.il/Diffractive_Optics_Products/Calculators.php?PN=DL

With a perfect TEMoo beam focusing down to a few mm spot size at a mile out should be doable with reasonably sized optics (neglecting for the moment any distortion of the beam by air currents). Unfortunately (or maybe it's a good thing?) our high power multimode diodes don't have nice clean Gaussian beams even when corrective optics are used.

It's a pretty cool website with some handy optical calculators.

That calculator isn't useful in this instance because it's purpose is to focus the beam at close distances from what I see.

 

[WizardG]

Re: Beam expander to focus laser bearm

Use the Gaussian app.



That calculator isn't useful in this instance because it's purpose is to focus the beam at close distances from what I see.

Look closer. The screen shot I attached is the calculation to focus a 450 nm laser with a perfect beam at a distance of one mile. It shows you'll need a beam expander that will produce a 400 mm diameter beam in order to get a spot size of <3 mm. The calculator assumes perfect optics

 

[steve001]

Re: Beam expander to focus laser bearm

Look closer. The screen shot I attached is the calculation to focus a 450 nm laser with a perfect beam at a distance of one mile. It shows you'll need a beam expander that will produce a 400 mm diameter beam in order to get a spot size of <3 mm. The calculator assumes perfect optics

First thing let's keep this practically doable. In the "EFL" box you inputed a lens with an efl of 1.6 kilometers. Finding a lens like that I'd would be difficult to impossible. Second the important number to look at too is the Rayleigh Length. That figure in your calculation is much to short to apply to Far Field performance. What this app is intended for is spot size a close distance. The Light Machinery Gaussian app is the one to use.

 

[WizardG]

Re: Beam expander to focus laser bearm

First thing let's keep this practically doable. In the "EFL" box you inputed a lens with an efl of 1.6 kilometers. Finding a lens like that I'd would be difficult to impossible. Second the important number to look at too is the Rayleigh Length. That figure in your calculation is much to short to apply to Far Field performance. What this app is intended for is spot size a close distance. The Light Machinery Gaussian app is the one to use.

Thanks, after a another look at my old physics textbook I saw my mistake. That calculator for diffraction limited spot size has some limits. Putting in an EFL that's far longer than the Rayleigh length gives....optimistic results. Rep on ya if the system will let me.

 

[steve001]

Re: Beam expander to focus laser bearm

Thanks, after a another look at my old physics textbook I saw my mistake. That calculator for diffraction limited spot size has some limits. Putting in an EFL that's far longer than the Rayleigh length gives....optimistic results. Rep on ya if the system will let me.

Good. Here are two plano-cinvex lenses that would be interesting to use. EFL's of 2600mm and 4000mm
Search Result

 

[RedCowboy]

So for a divergence corrected 450nm MM beam 7mm wide what input lens would be a good match for this exit lens and how long would a sliding BE need to be to focus from 10 feet to 500 feet > https://www.ebay.com/itm/NEWPORT-OR...135895?hash=item4b3c3c5c17:g:IZgAAOSwHcpapGEP

 

[Singlemode Laser]

The solution is very easy, if one solves the problem backwards.

First decide what spot size 2w you want to have at a distance d. Than calculate the far field divergence for this waist (basic formula from wikepeia).
With the divergence calculate the resulting beam diameter b at the distance d.

You are done! You need a collimated beam with a diameter b and a lense with a focal length d to create your spot.

Singlemode

 

[RedCowboy]

Ok, the lens I linked has a 750mm FL that's some 29 inches and I don't want to make the sliding expander that long, so I will be striking this lens with a large print and really want to be able to focus to a tight point between 15 - 150 feet.

So I may want to divide the input lens 25% to keep the device shorter, around 7 - 8 inches long or so.

Also I am not finding the formula at Wikipedia but I will look again.

---edit---

If I use a Keplarian what is the formula for, well it's a corrected MM beam, not a Gaussian beam, but just to get in the ballpark what formula do I use for a Keplarian BE ?

It says I have to add both FL together for the device length, this is why the Galilean works better for keeping the BE shorter, with the trade off of adjustment being faster, that is threaded lens holders rather than sliding PVC pipe.

Here is basic info: http://assets.newport.com/webDocuments-EN/images/How_to_Build_a_Beam_Expander_5.pdf


I am thinking about a build with a 3.0 inch wide round aluminum heat sink that my PVC exit lens holder can slide over, possibly tapped for a polymer thumbscrew to hold the position.

 

[steve001]

Ok, the lens I linked has a 750mm FL that's some 29 inches and I don't want to make the sliding expander that long, so I will be striking this lens with a large print and really want to be able to focus to a tight point between 15 - 150 feet.

So I may want to divide the input lens 25% to keep the device shorter, around 7 - 8 inches long or so.

Also I am not finding the formula at Wikipedia but I will look again.

---edit---

If I use a Keplarian what is the formula for, well it's a corrected MM beam, not a Gaussian beam, but just to get in the ballpark what formula do I use for a Keplarian BE ?

It says I have to add both FL together for the device length, this is why the Galilean works better for keeping the BE shorter, with the trade off of adjustment being faster, that is threaded lens holders rather than sliding PVC pipe.

Here is basic info: http://assets.newport.com/webDocuments-EN/images/How_to_Build_a_Beam_Expander_5.pdf


I am thinking about a build with a 3.0 inch wide round aluminum heat sink that my PVC exit lens holder can slide over, possibly tapped for a polymer thumbscrew to hold the position.

There are somethings that can't be worked around. Correct Keplerian beam expander lens separation is the sum of both lenses focal lengths. For a Galilean expander it will be less. It won't be 7-8 inches. Use the Light Machinery "Gaussian Beam Propagation" app. There's a link a few posts back as I recall. A single lens can be used, but that would require the lens to be placed a distance away from the diode equal to the focal length.

 

[Rintec]

Re: Beam expander to focus laser bearm

Use the Gaussian app.



That calculator isn't useful in this instance because it's purpose is to focus the beam at close distances from what I see.

Thanks a lot.
I think I have figured out what I have to do.
Now I am waiting for my laser source and lenses to make experiment.

Regards

 

[steve001]

Re: Beam expander to focus laser bearm

Thanks a lot.
I think I have figured out what I have to do.
Now I am waiting for my laser source and lenses to make experiment.

Regards

Post your results.

 

[Bacon]

This has got my interest now too. I've got one of those TV lens units too, and some 3x expanders. From what I've seen on google this Delta digital 250 unit has 2 plastic lenses of some type, and a biconvex glass lens. Ill have to take it apart some time, but I fear doing so will make me want to buy a 3D printer. :san: $$

 

[Alaskan]

That's what I found inside, a glass one.

 

[Hemlock_Mike]

A friend of mine gave me a projector lens like that too. It is from a unit
which had three lenses and a big screen on the front of the cabinet.
One lens is a bit weird because I think it made the image square on the
flat screen. Also, I shined my green pointer at it and the reflection was BAD news.
HM

 

[Bacon]

A friend of mine gave me a projector lens like that too. It is from a unit
which had three lenses and a big screen on the front of the cabinet.
One lens is a bit weird because I think it made the image square on the
flat screen. Also, I shined my green pointer at it and the reflection was BAD news.
HM

Ya, the reflections are terrible with this thing. It might just end up as a toy for flashlights.

 

[paul1598419]

It has been a long time since I saw a projection TV lens. There was a time when I saw them all the time and could have had as many as I wanted for the taking. I don't know how good they would be for a BE. There are pretty large lenses.