Beam expander to focus laser beam

[steve001]

Yes a large PCX is a good choice for use with high divergence MM beams, the lens in those projection TV housings are aspheric and make huge wings when I have tested them.

They could be made to work, but I don't think there the optimum shape......I have seen beam expanders with a double output lens arrangement, but the more uncoated glass the more parasitic loss.

Yes, that's why I am thinking a long FL very " thin in the middle " PCX because it's the more aggressive curve near the edges that throws the wings, or so it seems, granted I have not worked with all the lenses I have, but I have seen what you are describing.....maybe a thick edged PCX such as a larger lens with the outer circumference reduced so the edge is thick........there's experimenting yet to be done. :beer:


This 350mm FL would need 14 inches from your G2 that would be focused so that it's beam crossed over and printed to cover 80% of this 3 inch wide lens around 14 inches away, but that's positive/positive. Really correcting the MM beam 1st is the best way, then expanding with a concave to print on a PCX. That's what seems to be the best way from what I have read.

https://www.ebay.com/itm/NEWPORT-OR...m=222873847295&_trksid=p2047675.c100005.m1851

I think I wrote that wrong, I meant, I am not sure what that thick lens will produce with a mere 75 mm of focal length, that is fairly short compared to the results I was able to get with a large 300 mm FL PCX lens with the relatively shallow curve, I just can't see at that camera angle if that thick lens has a relatively shallow curve, it may not need as much curve though, it makes sense, with a given refractive index, a thicker lens will require less curve.

A few comments. The Light Machinery Gaussian Beam Propagation app assumes thin lenses. I infer that's because thick lenses introduce aberrations such as the wings we complain about. Inferring some more if laser companies would start using wider lenses with longer focal length lenses those wings might he eliminated. I remember measuring the focal lengths of early green dpss laser pointer and the focal lengths were about 15mm, whereas G lenses have focal lengths of about 4mm.

 

[Alaskan]

Looking forward to sorting this out. The problem with wings I've experienced were with the NUBM44 450 nm laser diode, using a huge photographic condenser lens from a enlarger pair has worked for me to eliminate the wings, but only if using a long focal length large diameter lens, and using 50% or less of the lens diameter. The lenses I was using were close to 300 mm focal length.

 

[Singlemode Laser]

A few comments. The Light Machinery Gaussian Beam Propagation app assumes thin lenses. I infer that's because thick lenses introduce aberrations such as the wings we complain about..

That is not true. This app uses thin lenses because a thin lens has only one parameter, namely its focal lenght. To account for a thick lens one would need to enter the curvature of the front and rear side along with the thickness of the lens.

Thicker lenses in general introduce less aberrations. Especially achromats (a lens constructed from two lenses who are glued together) introduce the smallest amount of spherical aberrations (1 order aberrations) from every single lens.

The best (less aberrations) fixed camera lenses consist of more than 10 single lenses all of them thick.

Inferring some more if laser companies would start using wider lenses with longer focal length lenses those wings might he eliminated. I remember measuring the focal lengths of early green dpss laser pointer and the focal lengths were about 15mm, whereas G lenses have focal lengths of about 4mm.

The key is here: wider. To avoid beam clipping the 15mm lens needs to be ~4x larger compared to the 4mm lens! Impractical for almost every application especially if the beam size is adjusted as needed with a telescope anyway.

Singlemode

 

[steve001]

That is not true. This app uses thin lenses because a thin lens has only one parameter, namely its focal lenght. To account for a thick lens one would need to enter the curvature of the front and rear side along with the thickness of the lens.

Thicker lenses in general introduce less aberrations. Especially achromats (a lens constructed from two lenses who are glued together) introduce the smallest amount of spherical aberrations (1 order aberrations) from every single lens.

The best (less aberrations) fixed camera lenses consist of more than 10 single lenses all of them thick.

Ok. Maybe I'm mistaken.

The key is here: wider. To avoid beam clipping the 15mm lens needs to be ~4x larger compared to the 4mm lens! Impractical for almost every application especially if the beam size is adjusted as needed with a telescope anyway.

Singlemode

How did you determine 4x wider? What do telescopes have to do with a beam expander?

 

[Singlemode Laser]

Ok. Maybe I'm mistaken.



How did you determine 4x wider? What do telescopes have to do with a beam expander?

I was talking about collimating the diode with a single lens, refering to your statement about companys using higher f lenses.

The bare diode emits light with a fixed divergence. If you use a lens with a 4x larger focal length for beam collimation you have to put the lens in a distance 4x higher away from the diode. At this distance the beam expanded to a size 4x times larger.

If wings are present in the collimated beam or not depends on type of the lens (spherical or aspherical) and its f number size/focal lenght.

Singlemode

PS: telescopes and beam expander are the same thing.

 

[steve001]

I was talking about collimating the diode with a single lens, refering to your statement about companys using higher f lenses.

The bare diode emits light with a fixed divergence. If you use a lens with a 4x larger focal length for beam collimation you have to put the lens in a distance 4x higher away from the diode. At this distance the beam expanded to a size 4x times larger.

If wings are present in the collimated beam or not depends on type of the lens (spherical or aspherical) and its f number size/focal lenght.

Singlemode

PS: telescopes and beam expander are the same thing.

The key is here: wider. To avoid beam clipping the 15mm lens needs to be ~4x larger compared to the 4mm lens! Impractical for almost every application especially if the beam size is adjusted as needed with a telescope anyway.

Your wording is awkward here. Especially so because a single lens is not a telescope. Well on my part it is assumed a longer focal length lens necessarily needs to be wider because it will be further from the diode. But you still have not stated why it has to be 4x larger. I think the determining variable is the beam diameter at 15mm or any other distance. That will determine how much wider any focal length lens needs to be to avoid beam truncation.

I think you're trying to say this in your most recent reply to me . > To collimate with a single lens the lens needs to be place a distance from the diode equal to the lens focal length. Laser collimation video.

https://youtu.be/ac-vVNbQt2I

 

[Singlemode Laser]

Your wording is awkward here. Especially so because a single lens is not a telescope. Well on my part it is assumed a longer focal length lens necessarily needs to be wider because it will be further from the diode. But you still have not stated why it has to be 4x larger. I think the determining variable is the beam diameter at 15mm or any other distance. That will determine how much wider any focal length lens needs to be to avoid beam truncation.

I think you're trying to say this in your most recent reply to me . > To collimate with a single lens the lens needs to be place a distance from the diode equal to the lens focal length. Laser collimation video.

https://youtu.be/ac-vVNbQt2I

I read our conversation now 3 times in a row and think all my answers were very clear. If not please formulate your question more precisely so I can understand what you really want to know so I can help you.

Thank you also for sharing this video. I hope you watched it yourself since all your answers are covered there.

Singlemode

 

[RedCowboy]

This video helps to demonstrate why our MM diodes produce that far field shadow when collimated through a single lens, adjacent to the dominant center wavelength are the weaker side bars, you can see them fill in on your wall when increasing current and viewing the raw diodes output, also the uneven divergence is no doubt a factor as you can see how your collimated beams wider axis at the aperture becomes the thinner axis in the far field.....rather the thinner axis at your aperture diverges faster and becomes the wider axis in the far field, this can be seen even in the early 1w 445nm diodes collimated beams by rotating your device.

Correcting the divergence of the aggressive axis first is the best way to go for producing the tightest spot in the far field when employing a beam expander, it will also help reduce the spherical aberration " wings " because we are not working with a TEM00 DPSS Gaussian beam, that is our MM beam is neither monochromatic nor TEM00, this is why the focused bar looks sharper as seen through attenuation safety glasses in the daylight and is surrounded by overspray at night, the sun drowns out the less energetic side bars " wings " or chromatic aberration as well as spherical aberration, I have noticed this before at night when adjusting for the hottest burning bar, it's going to burn better when you seem to go past the larger spot and get a tight center bar surrounded by spray or wings.

@ Alaskan: The thicker edged lens is essentially a wider lens with the center cut out, so as you get less spherical aberration by using the central portion of your objective/output lens you can do the same with a thicker edge lens in a more manageable size with less unused area.

Notice how a thick edged PCX looks a lot like the combined aberration correction lenses in the video.

https://en.wikipedia.org/wiki/Gaussian_beam

 

[Alaskan]

Agreed, that's why it works for me using 50% of the lens diameter and smaller, thanks for sharing the info.

 

[RedCowboy]

Glad to share, I enjoy learning the correct terminology even if I don't get my concepts 100% right, at least the things I understand in my mind are easier to explain.

Do you still have that 4W 532nm unit ? If so I wonder how well you could miniaturize it into a HH unit, that would be a great beam to run through a proper negative - positive BE.

I have seen your pic where you held up a large lens but were you using the output from the crystal or the output from the crystal with the factory concave/GRIN lens in place ?

 

[Alaskan]

I still have the 4 watt 532 nm, wanted to fire it up before coming to this new job, but didn't. For that photo, I put a double concave lens in the front of the output and then recollimated with a 12 inch diameter PCX lens

I wouldn't dare do anything to that lab unit, but I did get some nice KTP and LBO as well as some massive YVO4 crystals I can try to make a HH unit with, but I am not sure I will ever get to it with all of my projects waiting.

 

[Hemlock_Mike]

Chris -- I've got a couple DCV (-15 and -19) and a +87 PCX coming from
Surplus Shed for that 808 FAP.
My computer won't access their site tonight because of "certificate
errors" but the lenses are on the way. I hope the double CV lenses work OK.
HM
PS: Are you at your "new" home now?

 

[Alaskan]

Yep, in Qatar now, the big city of Doha working on a military base, but living in town, on the economy, as they say on base. Those negative lens ought to spread the light out to a wide expanded circle, do you have a camera to pick up the IR to see it?

 

[RedCowboy]

Hemlock_Mike: For working with IR I am going to attach this cam to these goggles, cam may need battery but goggles have a 4 pin video input so no need for a video transmitter, should plug and play as cam has it's own board and 4 pin I am thinking should power the cam as it says that's what it's for, if not a batt and transmitter board, but I think it will plug and play.

Would make a nice hands free way to see the IR beam/spot and protect eyes, of course safety first and use at your own risk, yada yada yada....

https://www.ebay.com/itm/Visionplus...984836&hash=item1ece42a3e8:g:x98AAOSwH-9Zo48n

https://www.ebay.com/itm/Night-Visi...304178&hash=item25e69944e8:g:MiMAAOSwjZJZ-C9-

I bought one of these too just for fun, connect battery and set it outside to watch the birds.

https://www.ebay.com/itm/GOQOTOMO-S...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649

 

[steve001]

Chris -- I've got a couple DCV (-15 and -19) and a +87 PCX coming from
Surplus Shed for that 808 FAP.
My computer won't access their site tonight because of "certificate
errors" but the lenses are on the way. I hope the double CV lenses work OK.
HM
PS: Are you at your "new" home now?

That combination will give an expansion of 5.8 and 4.6 mrd.