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Old 08-05-2016, 10:28 AM #17
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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Originally Posted by Alaskan View Post
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.


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Old 08-05-2016, 11:23 AM #18
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

I still haven't been able to wrap my mind around the reason why divergence is reduced when the beam is expanded.
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Old 08-05-2016, 01:23 PM #19
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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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Old 08-05-2016, 01:33 PM #20
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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

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.. This would mean if you have an extremely powerful multimode diode and let its beam expand to 6 inches diameter (or what would be, but cut off inside the tube), using a 3 inch diameter PCX lens to collimate that beam would allow the divergence to be just as low as a six inch expanded beam, although gained by wasting 75% the power due to so much of the beam being cut off. That might seem like a huge loss, but if the divergence approaches the amount you would get from a single mode diode, but at several times the power output, I call it good. Although a 3 inch lens is huge

I don't know yet, this could all be a wash when comparing what a single mode diode could do for power delivered at a distance using that same 3 inch diameter lens.
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Old 08-05-2016, 02:10 PM #21
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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Originally Posted by Alaskan View Post
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

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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Old 08-05-2016, 02:34 PM #22
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

Of course, do I really come off that niave? LOL. It is indeed the physics part of divergence I am trying to find the answer for. If I understood you correctly about the diameter of the beam prior to collimation, regardless of the lens being smaller, determining divergence after collimation.

The focal length plays a part, but a tiny lens can have a long focal length and much higher divergence than one larger but a much shorter focal length.
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"I regard consciousness as fundamental. I regard matter as derivative from consciousness..." - Max Planck. "Everything we call real is made of things that cannot be regarded as real" - Neils Bohr. "What we call physical things and events do not exist independently of subjective experience..." - Deepak Chopra.

Each of these three rabbit holes go deep, ending up in the same place.

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Old 08-05-2016, 04:16 PM #23
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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.
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Old 08-05-2016, 04:45 PM #24
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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.
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Sincerely investigate any of these three short quotes as new concepts and you've taken your first step into a larger world:

"I regard consciousness as fundamental. I regard matter as derivative from consciousness..." - Max Planck. "Everything we call real is made of things that cannot be regarded as real" - Neils Bohr. "What we call physical things and events do not exist independently of subjective experience..." - Deepak Chopra.

Each of these three rabbit holes go deep, ending up in the same place.

Useless troll fighting.

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Old 08-05-2016, 05:46 PM #25
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

Quote:
Originally Posted by Alaskan View Post
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.
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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.
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Old 08-06-2016, 12:55 AM #27
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

Now we are getting into chemistry with valances

So much to learn when it comes to laser diodes, the physics are cool but to really know this subject, takes a lot of time. You have chemistry, electronics, materials and optics all in one.
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Old 08-09-2016, 01:52 PM #28
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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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.

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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.


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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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Old 08-09-2016, 02:01 PM #29
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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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.
Would like to know how big is that lens to get a spot of 1 mm on the Moon (I'm lazy to calculate myself
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Old 08-09-2016, 05:26 PM #30
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

I've wondered how far I can converge a beam to a small point using a 12 inch diameter lens, but haven't dived deep enough into the formulas to find out. I was able to get through an intense electronics degree program using math approximations, but since then, haven't had to use much of it again.
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Old 08-09-2016, 06:04 PM #31
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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I've wondered how far I can converge a beam to a small point using a 12 inch diameter lens, but haven't dived deep enough into the formulas to find out.
You need to tell a focus length of this lens.
Do you want to use it as a single collimating lens?

Building a telescope should give you a better divergence.
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Old 08-09-2016, 10:54 PM #32
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Default Re: Question for the optics savvy in regard to beam expansion to reduce divergence.

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I've wondered how far I can converge a beam to a small point using a 12 inch diameter lens...
You'd be approaching a diffraction limited system. The absolute minimum diameter of the spot is equal to the wavelength of light itself, under perfect conditions. This is why a BRD holds more data than a DVD.
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