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Question for the optics savvy in regard to beam expansion to reduce divergence.

Alaskan

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I'm finding it difficult to find cheap ebay AR coated lenses of the correct focal length for use with beam expansion. What is the effect of using a PCX lens 50% too small for the beam diameter at its focal length? Just a waste of power otherwise ok? What about if the lens is 50% too large for the beam diameter? Any plusses for having the lens twice as large as it needs to be? I did notice astigmatism wings from a NUBM44 laser diode were eliminated when using a lens twice as large as the beam diameter, is this a commonly known spherical PCX lens property with the newer high power multimode laser diodes? Any other plusses for using PCX lenses which are have diameters which are twice as large or more than they need to be for the beam diameter?
 
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RedCowboy

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Sawnu's 3X expander works well, I use them with a G2/G7/3element lens.
I set the primary G2/G7/3ele to about 12-17 feet and the expander will focus to a clean bar at any distance, the further the larger of course but it's a clean focus.

The expander has a lens at the input and another at the exit so it is possible to calculate lens curvature to work with our MM diodes after the primary lens in a wide range of adjustment, I don't know the formula or even how to figure for the formula, but I see it can be done.

With a NUBM44 and a 3 element focused to 15 feet, then attach the 3X expander and it can zoom to set paper on fire 20 feet across the yard or set wood on fire just right in front of me and all in between, the math on the 3X lenses works.

If I use a 44 diode and a Sanwu G2 zeroed at 15 feet and the 6X cyl pair zeroed at 15 feet then the 3X I can zoom to burn cardboard at 75 feet and likely further, or burn wood 5 feet to 20 feet away, but in my backyard I only have 75 feet to work with.

 
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ElectricPlasma

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I've got to admit Alaskan, I would think with a title directed to the "optics savvy" that the thread would be by a newer member. I'd think of you as pretty optics savvy already :p Otherwise great to see the info from this thread, pretty helpful stuff.
 

RedCowboy

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I am learning lenses as I go.

The 3X from Sawnu proves it can be done.

When you order the expander get some adaptors, they are metal.

I drill out the hole in the adaptor a little wider but not enough to get into the threads, then screw my G2 or 3 ele into the adaptor tight.
Once I set my G2 or 3ele focus to about 15 feet I push the module into the heat sink and tighten the heat sink set screw, this holds the G2/3ele zero, then just screw on the expander.

It's so very worth it and you will see the lenses work well, so it can be done.

The laser in the pic with the spacers are to hold my zero distance setting of my 3 element, I could not slide my module back because I used mounting heat past that is rock solid.

This is just an easy fun way to do it, but what is really cool is how well the 3X lenses work over a wide range, so a bigger 7X or 15X could be built.



--------edit--------
Yes it is actually 3.3X and you are right about the smaller emitters being the one factor that's hard to get around, I'm just doing what I can on a budget and learning a bit as I go, I would love to truly understand lenses and be able to design expanders with a center relay lens so beam width and focal point could be adjusted.
 

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RedCowboy

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Yes the 3X is AR coated for VIS, it's not a heavy coating but if you tip it you can see the coating, so far they have been super tough expanders that make a real difference in the burning fun of the horribly divergent 44 diode. They work for red and green diodes as well.
I just ordered some more yesterday I like them so much, remember to get the adaptors, without the acrylic lens they are inexpensive and surprisingly ridged, just be sure to snug them down tight.



SANWU
 
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RedCowboy

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No the 3X lenses are GLASS, the adaptor comes with an acrylic lens screwed in that's useless for high power diodes, tell him you want just the adaptor and that makes them inexpensive.
 
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RedCowboy

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I just bought 2 more 3X expanders for 55.00 each plus 26.00 for speedy post shipping and the adaptors are just a few bucks, they are metal.\, tell Sanwu that you don't need the acrylic lens in the adaptor.
I set my G2/3 ele at 15 feet then back off a half turn and push the module with lens and adaptor into the heat sink so the adaptor sits against the heat sink, then tighten my set screw and run the adaptor back down that 1/2 turn and it will be nice and snug against your heat sink and zeroed at 15 feet, then screw the 3x in tight and its solid.
I find that a 15 foot zero is a good median point and the expander can make up for a lot, you can tweak it but 15 feet works well, if you are indoors you could go 10 or 12 feet but it's not a big deal, the 3X has a lot of range.
 
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steve001

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I'm finding it difficult to find cheap ebay AR coated lenses of the correct focal length for use with beam expansion. What is the effect of using a PCX lens 50% too small for the beam diameter at its focal length? Just a waste of power otherwise ok? What about if the lens is 50% too large for the beam diameter? Any plusses for having the lens twice as large as it needs to be? I did notice astigmatism wings from a NUBM44 laser diode were eliminated when using a lens twice as large as the beam diameter, is this a commonly known spherical PCX lens property with the newer high power multimode laser diodes? Any other plusses for using PCX lenses which are have diameters which are twice as large or more than they need to be for the beam diameter?
I learned something new.

You'll truncate the sides of the beam depending on how small the diameter is.

They're might be other pluses. If you find any please pass them on.

When retail expanders are constructed there's a warning of a sort emphasizing at the input, a maximum beam diameter. That means obviously that you'll want a lens with a diameter large enough to accommodate the input beam. It also means the output lens needs to be larger in diameter.

Something to try. One can collimate a laser beam with just one positive lens off a bare diode ( no optics in place). Just place the lens at a distance from the diode about equal to it's focal length.
 
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steve001

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[qute]hYou'll truncate the sides of the beam depending on how small the diameter is.
Steve, I'm not tracking, are you recommending a lens smaller than the beam diameter to cut off the wings? What I did was use a PCX lens twice the diameter of the uncollimated NUBM44 output beam and had no wings at all. i.e. a 2 inch lens with a focal length long enough to produce a 1 inch spot on it from the diode.[/QUOTE]

Read carefully what is written, it's clear. But no, just the opposite.
 
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steve001

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OK, I wouldn't do that to get rid of the wings, my NUBM44 works great by having a lens twice the diameter of the beam; look Ma, no wings!

You are right to respond with remark about the trunking or cut-off of the beam using a lens too small, I forgot I had also mentioned that in my first post because my main interest is in whether there is a draw-back from having the lens too large. But regarding lenses smaller than the beam size, even if the beam is cut off, doesn't the output maintain the same divergence after collimation, if the input beam is far wider than the lens? I mean the same divergence it would have had if the lens were large enough to pass the whole beam through?
Yes, it does.
 

RedCowboy

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When you expand the beam then focus it to a distant point the divergence is negative rather than positive because the now wide beam is getting smaller as it travels over distance, at least for a long way and to the eye indefinitely.

The trade off is a larger beam.

Have you ever taken apart a 532 module? There is a little expander after the crystal, what comes out of the crystal is needle thin until it passes through a little concave or gradient index lens that expands it until the final output lens catches it at an expanded width and focuses the beam to infinity.

So it trades a needle thin beam with a higher divergence ratio for a larger beam with a lower divergence ratio, sometimes actually a negative divergence until the beam crosses over and runs positive again, but positive at a lower ratio.

The divergence is a ratio of change against the beam diameter.

I hope I said that right, if not I am happy to be corrected.
 
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steve001

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I have taken apart a DPSS laser to see how the lenses worked and was surprised a expander lens was needed, seeing that the beam is fairly thin until going through that lens, at least, close to the crystal assembly. They could leave that expander completely out of the pointer and just let the beam diverge to expand to a larger size on its own, which it will, and then collimate, but then pointer would need to be much longer for a 6mm diameter lens :p

My gap in understanding is why a beam diverges, regardless of being a thin or extremely fat beam, or a beam expanded to 100 feet wide, it will still diverge, but why? Steve answered one of my questions regarding divergence which surprised me, if I understood correctly; if prior to the collimating lens, the beam is expanded to several times its diameter, the resulting divergence after collimation will have the same divergence the larger diameter beam would have had, if collimated. If true, the lens only collimates and has little to do with the property of divergence (if the right lens), regardless of being able to only pass a fraction of the original beam diameter through it which surprises me..
The focal length of the positive lens plays a big role in how fast a beam will or won't diverge.

Why light expands from a source is a physics question. Google will give an answer.

Do you understand what the word collimate means? It means to bring light into a column; to make more narrow than it was previously before collimating.
 
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RedCowboy

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A beam is always converging or diverging by some amount.
I understand your center concept and if you can get it down to 1 string of photons then you still have atmospheric conditions.
 

steve001

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You guys must be yanking my chain! Ok, I found an answer I can understand:

Light does not actually travel as a bundle of straight rays. This is an approximate model that does reasonably well if the wavelength of light involved is much smaller than any significant feature of the rest of the system. Light travels as a self-interfering, self-propagating, oscillating electromagnetic field. Every light beam with non-infinite beam width will diverge because of the way the field interferes with itself, even if is was at one point somewhat collimated. Some people call this diffraction and other call it interference. Many books make it sound like diffraction is caused by a light beam interacting with an obstacle (such as a screen with a slit), but in reality the diffraction is caused by the beam itself after being given a certain shape by an obstacle.

Edit: I found more, I asked the right question on Google and a bunch of information coins started flowing:

There are physical limitations that prevent us from having a beam of photons that continue in a perfectly parallel fashion. The closest we can get is a beam we call collimated--that is, not focusing or diverging, but staying roughly the same diameter as it travels--that's what we think of as a laser beam. But even collimated beams spread apart with some small divergence angle. The deep reality of this is the uncertainty principle. We know the laser's position to a certain accuracy because the photons had to originate within the laser material. So, since we know the position (perpendicular to the beam's direction of travel) of every photon to some accuracy, we know there is some spread in the momentum in that same direction. If we know with less certainty the position (that is, a larger laser material or larger beam), we know with more certainty the momentum. Momentum is just another name for speed, so it follows that if we know a certain photon started in a finite-size laser crystal, we don't know its speed perpendicular to the beam exactly. Thus we can't correct for it. We can use a lens to trade between position and momentum uncertainty--we can make the beam bigger so it doesn't diverge as much, or vice versa--but we can never make divergence zero without making an infinitely-wide beam.

If you want a 1 mm speck on the moon, it's certainly possible to use a lens to focus the laser--essentially making it non-collimated, so we increase the spread in angle to decrease the spread in position. The governing quantity for how small you can focus the beam with a lens is the f-number: the distance between the lens and the focus divided by the diameter of the laser beam when it enters the lens. So, you could place a small lens close to the moon so the beam is focused rapidly to the 1 mm spot, or you could place a huge lens on earth so it converges over its entire journey to the moon.

Focusing light from stars is a slightly different problem. Stars are so far away that the light we see from them doesn't look like it's spreading out much. A telescope can catch some of that light and focus it to a small point, just like we did with the lens on the moon focusing the laser beam from earth. The only difference is that the star emits everywhere, so we're only catching a small portion of its light. We build bigger telescopes to catch more of it and thus be able to focus the light to a smaller point. You'd be doing the exact same thing if the lens on the moon were smaller than the laser beam was when it got there.

Ching ching, more flowing:

The waves in laser light are not parallel. It is theoretically impossible to construct a beam with perfectly parallel rays unless you have an infinitely wide beam. As described in the textbook “Principles of Lasers” by Orazio Svelto, even a perfectly spatially coherent beam will spread out due to diffraction. Diffraction means that all waves – including sound, water, radio, and light – bend around corners. And it's not just the edge of the wave that bends around the corner. It is the entire wave. This means that a beam of light that is shone through a hole spreads out as it travels. A beam with perfectly parallel rays would never spread out. Every beam of light has a finite beam width and therefore can be thought of as emanating from a hole. Diffraction is a wave effect, so it applies to laser beams as well.

Now I have a pocket full of divergence understanding, finally. Don't know why I couldn't ask the right questions before.
Very good grasshopper.
There are two ways to describe a laser beam divergence, either as a conjugate beam or as an infinite conjugate beam.

Rayleigh Range aka Rayleigh Length is an interesting thing to look into. Beam expanders increase it.
 

RedCowboy

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Electrons in the molecules of the gain medium are driven into a higher orbit and when they fall back to their natural orbit that energy is released as a photon of light, the chain reaction effect is something I read was not fully understood why it happens, just that it does.
I have read that trash is introduced into the edges of the gain medium inside the p/n junction to help guide the photon flow and with fiber lasers they are learning tricks to get a high quality beam, it's interesting and some of the high end single mode lasers can have excellent divergence.

But out MM diodes with a dominant center wavelength and less energetic side bars does appear to have defined lines in the raw output, maybe blocking all but the dominant center wavelength could offer better beam quality on the cheap.

I notice while burning that getting the center bar tightest with the side bars as splash burns faster at a distance, the closest side bars do seem to pre heat the material if burning a slot the hard way, blocking the side bars at the start could aid in finding that tightest focus of the beam and eliminating those wings.

This is not exactly what you were talking about but a thought.
 

lazeristasUVISIR

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What is the effect of using a PCX lens 50% too small for the beam diameter at its focal length? Just a waste of power otherwise ok?
You loose power & you introduce diffraction patterns in your beam.

What about if the lens is 50% too large for the beam diameter?
You paying more $. I'd need to open books to confirm but from memory - 1.3x of the beam size is good enough for optics diameter.


Any other plusses for using PCX lenses which are have diameters which are twice as large or more than they need to be for the beam diameter?
the PCXs are not great lenses to collimate laser diodes with high NA.
 
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