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Cylindrical lens setup in combination with beam expander

gozert

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Recently I have been thinking of doing a NDG7475 520nm build, but this time I want to optically correct it.

My idea was to simply expand the beam with a cylindrical concave lens first and then place a cylindrical convex lens after that. Now here's what I'm wondering. Will it be worth optically correcting the beam when there will be a beam expander at the end anyways?

Also, would anybody mind explaining me how optically correcting a beam with these two lenses exactly works? I've found many drawings of the convex lens working similar to a magnifying glass. If that would be the case, then it would be very hard to achieve a thin, low divergent beam right? On every example I found it showed that the light diverged immensely after reaching the focal point.

Here a picture of what I mean:

 
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Alaskan

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If you dont optically correct prior to expansion your output will be more of a ribbon and slightly different infinity focal points... But for myself, I'm fine with those minor issues.
 

gozert

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What exactly do you mean by the beam being "more of a ribbon"?
 

Alaskan

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Rectangle due different fast and slow aixis divergences.
 

steve001

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As Alaska said, you need to correct for both axes. But you need to use this type of lens to expand the beam at a certain rate because each axes expands at a different rate. The slow axis should be expanded to match the rate of expansion of the fast axis
See this http://www.newport.com/Beam-Shaping-with-Cylindrical-Lenses/144888/1033/content.aspx

By the way. That drawing does not illustrate a cylindrical lens. That is a bi-convex lens also known as a double convex lens. This is what a cylindrical lens is. This is one example.

Such a lens focuses the light into a line. This form of cylindrical lens would be easiest to use.


http://www.edmundoptics.com/technical-resources-center/optics/using-cylinder-lenses/

http://www.edmundoptics.eu/optics/optical-lenses/cylinder-lenses/?site=WE&countryid=155_ga=1.52934056.165698216.1441915846
 
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gozert

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Alright. I only saw examples like the one I showed in my first post, even when searching for cylindrical lenses. I think I can figure it out now, thanks.
 

Alaskan

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If the beam is wide enough, you can use a line lens like that first one to help square it up some as I have done for my NDG7475 which helped, but alone as just a PCX cylinder it can only help a little. To make it more of a round shape a pair of cylinder lenses in front of the beam are needed, one a concave cylinder the next a convex cylinder.

Depending on how much correction is needed, or which diode you are correcting for, the magnification factor will be higher or lower. The NUBM44 appears to need a 6X magnification cylinder pair which are not available at a reasonable hobby price right now in the sizes we need for our pointers but a GB is underway for them if we have enough interest. However here's an example of a pair which can be used to help "square" a NDG7475 520nm laser diode output:

520nm Cylindrical Lens Pair Brass Mount Nichia Laser Cylinder Anamorphic | eBay

Scroll down to near the bottom of the above ebay link to see what this lens pair will do to your beam.
 
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gozert

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Thanks a bunch Alan. That's exactly what I was looking for. My first idea was to do a similar thing with round lenses from Thorlabs. Only they charge nearly 180$ for the two lenses shipped, and then I'd still have to mount them myself.

Going to order this asap.
 

Alaskan

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Except for mechanical, they can be round or square etc.

Edit: except what is shown in the photo of two round PCX cylinder lenses will not work, one needs to be concave which expands the slow aixis.

chris
 
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gozert

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I've thought of a concept that I think would work, but I could use some opinions on it.



This is what I thought of. The only thing that I'm not sure will work is the PBS cube, because the beams get altered by the lenses that I've put in front of them.

The beam expander would come somewhere after the PBS cube.
 

Alaskan

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That's how I believe CDBEAM777 recommends correcting for two into a PBS.

Edit: I have an unbreakable rule, no post can be made without thinking of more to add, it seems:

Only thing is, you will need to turn one set of optics on its side relative to the other set so you can have the polarities into the PBS cube correct, or 90 degrees apart.
 
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RedCowboy

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I've thought of a concept that I think would work, but I could use some opinions on it.



This is what I thought of. The only thing that I'm not sure will work is the PBS cube, because the beams get altered by the lenses that I've put in front of them.

The beam expander would come somewhere after the PBS cube.
You do know you will also need a S1/G2/G9 right in front of your diode.
And you turn one module or it's diode 90 so its polarity is not the same.
 
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gozert

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You do know you will also need a S1/G2/G9 right in front of your diode.
And you turn one module or it's diode 90 so its polarity is not the same.
Yeah, that I know. The only thing I'm worried about is the polarity after the corrective lenses. I know it widens the beam, but does it change the polarity too? Would it be needed to use a 1/2 wave plate before the beam goes into the pbs cube on one side?

Same here Alan. I rarely don't edit my posts to add additional info.
 
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Alaskan

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The polarity will remain the same, just that the slow aixis will be wider, that's all. You can just turn one of the diode holders so the beam is 90 degrees different from the other diode to match that side of the PBS cube. So, in sum, the changes are the two diodes are ninety degrees different polarity as well as their associated cylinder lenses.

Once corrected, the output of a laser diode will become more of a square when you widen the slow axis to match the fast axis. Regardless, the polarity of the wave itself will be retained.
 
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CDBEAM777

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The polarity will remain the same, just that the slow aixis will be wider, that's all. You can just turn one of the diode holders so the beam is 90 degrees different from the other diode to match that side of the PBS cube. So, in sum, the changes are the two diodes are ninety degrees different polarity as well as their associated cylinder lenses.

Once corrected, the output of a laser diode will become more of a square when you widen the slow axis to match the fast axis. Regardless, the polarity of the wave itself will be retained.
Yes....the Polarity will not be changed by going thru the corrective Cylindrical lenses.

Disclaimer !! I have NOT experimented with Beam Expanders....but they expand the entire beam....and greatly lessen divergence.

Unlike Cylindrical lenses....which expand only one axis of the beam....and leave the other axis alone....a Beam expander expands both axis at the same time.

I speculate that for the best possible beam obtainable .... with the more divergent multi mode high power diodes...a set of Cylindrical lenses should be placed after the collimation lens.....then....that beam should enter the Beam Expander.

SOooo....correct it first....to achieve the best possible aspect ratio ( Best aspect ratio is.... Width geometry as close to height geometry....as in most like a square....not a rectangle/bar)

Then...dump the beam into a Beam Expander !!!....and the Beam Expander will do what they do....greatly reduce the divergence....and increase the overall beam diameter !!

I am working on just such a project ...right now....with another member.....and If he wants to share the details....then...OK !!!

IMNSHO...This concept will work with our high power multi- mode diodes.....!!

Laser Diode==> Collimation lens ( Like a G2)==> Cylindrical lens set==>
Beam Expander=====>

Again...Disclaimer....These optical manipulation will not give a perfect round beam !!! and they will slightly reduce to power of the beam....that is just the way it is !!! BUT....they go a long way to improve the beam quality !!

NOW.....working with a PBS cube to combine beams....is a whole other process....

I will do a search....for I am sure....someone has written a great explaination...much better than I can write !!......and I will post....somewhere !! :na::na:
 

RedCowboy

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Exactly right CDBEAM, your exit beam diameter will be wider out of the expander, but your impact point of focus will be tighter.
The wider the exit beam diameter is, the longer the range and tighter your point of focus can be.
 




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