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Best lens setup for smallest most perfect focus dot possible at around 300mm distance

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educa

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Jun 25, 2011
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Hi,

I am busy designing a few prototypes of 3d printers based on laser stereolithography. I have experience with setting up a 445nm 0.7W laser for cnc engraving, but this time the purpose is to be able to focus a 405nm (445nm is multipoint so unusable) to as small as possible dot as possible.

The distance between the laser diode and the actual place where the focus spot should be is around 300mm. I can not go too much smaller because the light also has to pass a ganvanometer positioning system.

Now I did allready read quite some info and I'm also in the lucky position to be able to tell that I don't need full burning power. In other words I actually don't care that much if I loose some optical power because of the lenses. The UV resin which is used does even cure with very low 20-50mW laser and I'll be using a 500+mW diode at less power for long life. The diode will be housed in active cooled alu housing and driven by a TTL controllable constant current source.

But now..... the important question. What lens could I use to get the smallest possible focal spot with also the best possible round spot? without Flares or lightrings around the spot.

If there is light around the spot, then this will cure resin too, so that should be avoided.

From what I have read until now, I need a 3-element AR coated glass lens and not a G1/G2 single element lens. Is that right ? Did I understand that OK? As far as I read a 3 element lens would give me a much better spot, but I would looks power where a single element would give me more power but at the cost of a lower quality dot?

Any help would be very much appreciated here, because optics isn't that easy. I just read a paper where they state that the 405nm diode in a bluray player is focussed using a 0.85 aperture lens and therefore can focus to a dot of 480nm. That sounds extremely small and I know from previous tests with a aixis 445nm that it was hard to get a spot of 0.1mm

I would like to test how small I could go and what lens would be needed.



Also, 2nd question is about the same but then for a simpler setup. We are also making a prototype where the laserdiode is moved on a XY motion platform and pointing down onto a resin bath. The distance between the resin and the laserlens will be only 5mm there, so I know for sure that I'll get smaller focus point there than when I would focus at 300m distance. Would that setup need the same 3-element lens or is there better stuff available ?



Kind regards,

Bart
 





well good choice w/ the 405=IMHO it has the smallest dot and is the quickest burner-AixiZ Laser (.com) had and may still have a great 405 lens -its referred to as a 'half thread' glass lens ( avoid any plastic lenses)-- for 445 AixiZ has a single element G lens for blue and 405 ($8usd iirc)...while not the very best 'spot shape; it does give us the highest power---there will be posts here soon from members with much more expertise than me..Quite a few use laser CNC--hope this is helpful welcome to LPF -

looking forward to reading your 'intro' in the WELCOME section..hak
 
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On Ebay, you can get 3-element coated lens for 405..465nm LASER at prices starting from $3.50 (to around $15). There are also uncoated acrylic lens, but I'm not sure how they perform at P>0.5W.
Which lens would make smaller spot at 30cm (300mm=30cm). It might be the most expensive, but it might be the cheapest - no guarantee here. Talking about smallest spot, the best known are the 3-element glass lens. But the brands and quality of the lens are highly mysterious.
Also, I've found 3 kinds of lens with specified something called "F" (focus distance?):
F=8.3, F=9.8 and F=11, they are being sold at ~ $7.8 on ebay.
I guess F=11 stands for F=11 mm.

As for the Sanwu's lens, they re here: Sanwu shop - lens.
The Gxxx ones are single-element and more expensive, $15 at the time of writing vs $8 for his 3-element lens. Their pros are they reportedly allow for up to 10-20% more energy to pass. Their cons - higher price and slightly inferior focus than 3-element.

I asked staff from techhood about the difference between their $13 UV lens and another, unknown $3USD glass coated 3-element UV lens, sold on taobao. What they know is that they are from different factory in China and different coating. So the information is next to nothing :can:.
I have bought the $3.5 ones from another ebay supplier (eama/nanma) but they are in transit for the last 20 days...
I can compare the spot only with acrylic uncoated (when they arrive), but I don't know how to shoot it - on white paper it has lot of glare, probably on black paper or best - direct expose the 1/2.4 inch CMOS matrix (of web camera).

There is some additional stuff like beam expanders, which may help you to maintain low divergence at distances at the cost of expanded beam, but then you'll need additional focusing lens near the subject to focus the low divergence expanded beam in a tiny all-vaporizing spot :yh:
 
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educa said:
Hi,

I am busy designing a few prototypes of 3d printers based on laser stereolithography. I have experience with setting up a 445nm 0.7W laser for cnc engraving, but this time the purpose is to be able to focus a 405nm (445nm is multipoint so unusable) to as small as possible dot as possible.

The distance between the laser diode and the actual place where the focus spot should be is around 300mm. I can not go too much smaller because the light also has to pass a ganvanometer positioning system.

Now I did allready read quite some info and I'm also in the lucky position to be able to tell that I don't need full burning power. In other words I actually don't care that much if I loose some optical power because of the lenses. The UV resin which is used does even cure with very low 20-50mW laser and I'll be using a 500+mW diode at less power for long life. The diode will be housed in active cooled alu housing and driven by a TTL controllable constant current source.

But now..... the important question. What lens could I use to get the smallest possible focal spot with also the best possible round spot? without Flares or lightrings around the spot.

If there is light around the spot, then this will cure resin too, so that should be avoided.

From what I have read until now, I need a 3-element AR coated glass lens and not a G1/G2 single element lens. Is that right ? Did I understand that OK? As far as I read a 3 element lens would give me a much better spot, but I would looks power where a single element would give me more power but at the cost of a lower quality dot?

Any help would be very much appreciated here, because optics isn't that easy. I just read a paper where they state that the 405nm diode in a bluray player is focussed using a 0.85 aperture lens and therefore can focus to a dot of 480nm. That sounds extremely small and I know from previous tests with a aixis 445nm that it was hard to get a spot of 0.1mm

I would like to test how small I could go and what lens would be needed.



Also, 2nd question is about the same but then for a simpler setup. We are also making a prototype where the laserdiode is moved on a XY motion platform and pointing down onto a resin bath. The distance between the resin and the laserlens will be only 5mm there, so I know for sure that I'll get smaller focus point there than when I would focus at 300m distance. Would that setup need the same 3-element lens or is there better stuff available ?



Kind regards,

Bart
Click to expand...
As you noted it's quite complicated more so than what people are suggesting you do so far. There are apps available that may help
light machinery.com > https://lightmachinery.com/optical-design-center/

For beam shaping beam shaping optics are required.

One thing I do know with certainty is, if you expand the beam then focus the beam very high power densities can be achieved.
 
dden4012 said:
I purchased both the 7mm and the 9mm G7 lens. The short one got stuck in a 12mm module. I don't know why it won't thread on right.
Click to expand...

It seems that some of the threads are a little messy on their lens barrels. Try cleaning it a bit and see how that goes.
 
dden4012 said:
I hear ya. You would think that since their hosts can accept other lenses that there wouldn't be an issue. They thread just fine into Sanwu hosts but in our 12mm modules they get jammed a few turn in. Ill have another go at it with an empty module just to be sure but there really isn't any benefit to be had.
Click to expand...

I've had a few that are messy and I ran a carpenters knife along the grooves, which seemed to help some. The tools they use to thread their modules and such might be every so slightly different then what we use, so as to make screwing in lenses a bit more difficult.
 
Correct me if I'm wrong, but I think that he talks about the most suitable lens for focusing of 405 nm single mode LASER diode at ~30 cm distance in the smallest possible spot in an industrial host, not in a Jedi - style torch.
I think he doesn't want to invent new lens or custom-order them for a 4-digit sum. IMHO I happened to have similar problem, which I'm trying to solve with the 3-element coated glass UV lens. The last time I managed to focus the beam very well was with cheating - I used an additional acrylic lens after the mirror.
 
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All else being equal, a shorter focal length will make a smaller spot. All else equal, a larger initial beam diameter will make a smaller spot. Since op is insisting on getting rid of option 1, option 2 is the one left. This means a long focal length and very large collimating lens. If you limit yourself to the standard 12mm lenses available, you're going to get shit results.
 
We're doomed :( - said C-3PO ... e.t. tied to the "shit results" because of the 12 mm hosts.
Thanks, Cyparagon for explaining this. But I did not understand this: "Since op is insisting on getting rid of option 1".
If shorter focal length makes smaller spot, then among the unknown 3-element glass lens @ Ebay, the one with F=8.3 will make the smallest spot, right ?
For the G2 lens, Bionic-Badger (member here) has said that "G2" is just the name for a coated 6.3mm lens (this is not the FL!!) with a short focal length that allows it to capture a lot of light. I have dug the thread: LFL - long focal length G2 Lens (they search the opposite effect, but there's some NFO).
 
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AngelG said:
We're doomed :( - said C-3PO ... e.t. tied to the "shit results".
Thanks, Cyparagon for explaining this. But I did not understand this: "Since op is insisting on getting rid of option 1".
If shorter focal length makes smaller spot, then among the unknown 3-element glass lens @ Ebay, the one with F=8.3 will make the smallest spot, right ?
For the G2 lens, Bionic-Badger (member here) has said that "G2" is just the name for a coated 6.3mm lens (this is not the FL!!) with a short focal length that allows it to capture a lot of light. I have dug the thread: LFL - long focal length G2 Lens
Click to expand...


The focal length of the G2 is shorter than the 3 element. The numbers I've seen put it at ~4mm EFL while the shortest 3 element is 8.3mm EFL.
 
The pdf at laser66 says this for G2: 6.33mm dia., 4.00mm EFL, 2.40mm BFL, NA= 0.60, so according the theory above, they should create even smaller spot than the Chinese @ ebay. But then they should also have inferior divergence than the loger FL lens, am I right ?
And the members here buy lot of G2 lens to create lasers, designed for low divergence beams (not for smallest spots at 30 cm). This makes me ... confused.:thinking:
Is it possible so many people to do it wrong ?
 
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AngelG said:
The pdf at laser66 says this for G2: 6.33mm dia., 4.00mm EFL, 2.40mm BFL, NA= 0.60, so according the theory above, they should create even smaller spot than the Chinese @ ebay. But then they should also have inferior divergence than the loger FL lens, am I right ?
And the members here buy lot of G2 lens to create lasers, designed for low divergence beams (not for smallest spots at 30 cm). This makes me ... confused.:thinking:
Is it possible so many people to do it wrong ?
Click to expand...

Here's what you need to do. Expand the beam using a low negative focal length lens. Then input that beam into a long focal length lens. Just be sure the diameter of the *long focal length lens is wide enough to accommodate the beam. Now focus the beam. What in essence you are constructing is a refractor telescope that's what a Galilean type beam expander is. See if Surplus Shed has any suitable lenses.

You may be confused or others may be so, I do not know, but a G2 lens at 4mm FL will not make for a low diverging beam.

* look for a lens with a focal length of 300mm.
 
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There can be at least two approaches:

ONE LENS:
A lens collected power from the laser and focuses to a spot at a certain distance. The longer focal length lens is better to get a small spot. However, there are some constrains like $$$, scanning mirror sizes, etc. Thus, it would be necessary to optimize lens parameters - focal length, diameter (or NA).

TWO LENSES:
One collects and collimates, other focuses. The second lens focal length is equal of the distance from it to a focusing location. The first lens should make the beam as large as possible (again limiting factors will be mirror sizes).

Keep in mind that "lens" really means a lens system.

That's the starting point of the design! The next step is optimizing the shape of the focused spot. I'll stop here as it enters into the NDA area.
 
lazeristasUVISIR said:
........................
ONE LENS:
A lens collected power from the laser and focuses to a spot at a certain distance. The longer focal length lens is better to get a small spot.
.......... .....................
The next step is optimizing the shape of the focused spot. I'll stop here as it enters into the NDA area.
Click to expand...
Now I'm really confused, because what lazeristasUVISIR said about the longer focal length is the opposite of what Cyparagon said:
Cyparagon said:
All else being equal, a shorter focal length will make a smaller spot .....................
Click to expand...
I begun to think that the question is not stated clearly enough.
We're talking about Diode-->Lens-->Mirror->Surface kind of system where the mirror does not distort the beam. The diode's beam is wider at higher distance, because of it's natural divergence.
lazeristasUVISIR is right that we talk about lens systems: the 3-layer glass lens are such.

I'm grateful for all users who try to help, including those who thought we're trying to get low divergence while we want the smallest spot at a distance (30cm).

As for the NDA thing - I haven't signed such, since my job isn't related to optics, so I'll post 2 brute-force approaches that come to my mind:
1. Use special lens who distort the shape in the opposite direction to correct it, e.t. close to what the cylindrical lens do;
2. Sacrifice power and simply cut out part of the beam to get better projection.
These boys also elaborated on the "NDA" stuff:
Beam shaping with cylindrical lenses


To return on the lens problem - longer or shorter focus distance lens will be the best solution for the given system - if we voted, up to now most of the votes would be for shorter EFL.
 
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