PCX Lens FL Question

[Alaskan]

removed

 

[steve001]

Guys,

Trying to get an understanding of the relationship of FL to infinity focus collimation of a laser diode. Is infinity focus found near the specified FL of the lens, or something a bit different, longer or shorter?

Here's the project: I have a 2"/50.8 mm dia. PCX lens with a FL of 2.64 inches I want to use to collimate a 638 nm 1.2 watt CW HL63283HD laser diode with 35 and 10 degree divergences for the slow and fast axis output. My assumption is only one axis will be close to, or at, infinity focus, since they are different. Is that assumption correct?

More info:

https://www.ebay.com/itm/USHIO-HL63...ode-Single-Beam-High-Power-Diode/262887287538

https://www.ebay.com/itm/CVI-MELLES...2-64-FL-AR-coated-for-633nm-HeNe/122793535161

To make sure the lens has a large enough diameter at its FL for this diode, an online calculator (I am lazy) is giving me a beam diameter of just over 1.6 inches for the 35 degree diverging side of the diode, so the lens I have found on ebay (with a superb AR coating) should work fine. Of course I will need to find a way to adjust the distance the lens is from the diode to have error wiggle room in the device to be able to achieve near perfect infinity focus (for one axis).

I learned the answer to your first question via an Edmund Optics video on YouTube a while back. Using a bare diode and a single lens place the lens at a distance equal to the focal length. Video. https://youtu.be/ac-vVNbQt2I

 

[Lifetime17]

Hi Alaskan
I just checked out the vid will watch more after work today

Rich

 

[lazeristasUVISIR]

Don't expect a great spot with a PCX lens.

 

[RedCowboy]

If the diode was putting out a round beam like the HeNe it talks about in the description, even if it does diverge but stayed round, otherwise you may want to correct the diodes output with a C lens pair or a prism pair to correct for uneven divergence before your lens.

 

[steve001]

lazeristasUVISIR, if I could find a quality 2 inch aspheric at close to this FL with AR coating for red I would rather use that, but I can't find one cheap. PCX is a middle choice for a collimating lens, better than bi-convex.

RCB, yea, it won't be round. I've just come to accept for most of my pointers I will have a rectangle output without correction, but when expanded, a much lower divergence rectangle

Doing some quick reading and visualizing it appears the reason long focal length plano-aspheres are not found is because as the curvature of the lens is reduced the plano-asphere becomes a plano-convex lens. There are other reasons why these lenses are aspherical.
I wonder how a long focal length positive meniscus lens would perform? Cheap enough to experiment with. https://www.edmundoptics.com/optics...xperimental-quality-positive-meniscus-lenses/

 

[steve001]

I don't know, never heard of a meniscus lens being used for something like this, but I will study up on it. There was an optics guru over at PL feeding the crowd some time back, his advice was that the order of desirability for laser collimation lenses was Plano-Aspherical first, Plano-Convex second and BCX or Bi-Convex lenses last. I've found plano-aspherical lenses up to 2 inches diameter on ebay, but few and far between and no larger unless they were junk made for LED's with cheap glass.

Now that I'm familiar with the lens in question it's seems plano-aspheres would typically be used were a long Rayleigh length is not the objective. That's why I think you can't find them with long focal length.

Going from memory now. Excluding cemented or closely spaced three elements Gaussian or Keperian expanders two elements expanders use either plano-convex or double convex lens as the exit lens.

 

[steve001]

I was looking for large diameter plano-aspheres because I had the impression they were the best lens to use to collimate laser diodes. I wanted the best lens I could find to produce a fat beam to lower the divergence, otherwise I didn't care so much what the focal length was, as long as the beam was fat to get that reduced divergence. Although you are right, you won't get a fat beam with a short focal length lens, not without using a concave lens to expand the beam quickly.

Focal length is what matters. Diameter too to capture all of the beam. You can collimate a beam with just a single lens.

 

[lazeristasUVISIR]

Why best? I suppose due to less spherical aberration, I don't know what else it would be.

That's a solid reason that they are used to collimate laser diodes.

If the length is not a problem, a short focal length lens with a telescope will make you a wide well-collimated beam.

 

[RedCowboy]

Lets look at the shape of a G2 type aspherical lens.

I think we need this or the 3 element because of the very aggressive divergence?

I know the real G2 has a rear FL of 2.39mm and this copy is close at @ 2.357mm working distance, but it says 4.02mm is the effective FL so there is apparently rear FL that's the distance and effective FL that I assume is to the center of the lens?

From wiki >>> https://en.wikipedia.org/wiki/Astigmatism_(optical_systems)

The asphere's more complex surface profile can reduce or eliminate spherical aberration and also reduce other optical aberrations such as astigmatism, compared to a simple lens. A single aspheric lens can often replace a much more complex multi-lens system. The resulting device is smaller and lighter, and sometimes cheaper than the multi-lens design.[1] Aspheric elements are used in the design of multi-element wide-angle and fast normal lenses to reduce aberrations. They are also used in combination with reflective elements (catadioptric systems) such as the aspherical Schmidt corrector plate used in the Schmidt cameras and the Schmidt-Cassegrain telescopes. Small molded aspheres are often used for collimating diode lasers.

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So it appears the multi mode diodes do better with an aspheric, also consider that the vertical and horizontal changes proportion because of the uneven divergence, but you can see that in the beam of our MM laser beams by rotating them a quarter turn.

The reason an aspheric works better is likely because of the multi mode bands, if you look at the raw diode output on a wall and slowly turn up the power you can see the side bands grow around the dominant center wavelength, so this would cause an astigmatism, I have a slight astigmatism in both of my eyes lenses that I did not have just 10 years ago, now a bright dot has some fuzzy crap around it and it's different with each eye, I only see it with a laser dot or my red dot rifle sights that are not of the prism type.

 

[RedCowboy]

The lenses I have taken out of old projection TV sets I think are aspherical and they are 2.5-3.0 inches wide.
I remember our old host maker built one a while back, can't remember his name but you have a lot of his work.

I have a bunch of these that might work for you, just have to try and see, I could send you some but I don't know what shipping would be to where you are.

The outer 2 are acrylic but the center aspheric lens is glass.

 

[RedCowboy]

Nice clear glass, no coating that I can see, they are aspheric, that is convex on both sides but at a different grind and right about 3 inches wide plus or minus a hair on this one, I have a couple different ones, just PM me where to send them, it's no problem. Maybe they work, maybe not, all you can do is try and see.

P.s. The back side has a double grind it seems, like not flat in the center 1.5 inches but maybe less of a curve.

 

[RedCowboy]

You can also buy some on flebay if you like but I am glad to send you a couple different sizes, the inner round black plastic bodies all have a glass center lens with an acrylic inner and outer that protects the glass lens pretty well, I have taken apart many and they are all built that way, just different sizes.

https://www.ebay.com/sch/i.html?_fr...tv+lens.TRS0&_nkw=projection+tv+lens&_sacat=0

 

[RedCowboy]

Part of the joy of getting older my friend, just let me know if I can help later on. :beer:

 

[RedCowboy]

https://www.ebay.com/sch/i.html?_fr...tv+lens.TRS0&_nkw=projection+tv+lens&_sacat=0

Unscrew the wing nut and peel out the center plastic carrier, then pop the little side tabs and open it.

 

[RedCowboy]

That's odd, I have opened up a dozen and they all have a nice glass center with lens with plastic/acrylic front and back lens, here's the label on some I have.