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Old 01-22-2017, 12:57 AM #1
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Default focusing lens for sls 3d printer

I am very new to optics and lasers. I understand some of the concepts involved with focusing a laser diode, but it is very general and a very small amount. I am currently building a 3d sls printer and I want to have my laser source about 8 inches (200 mm) away from the powder that is going to be sintered using the laser.

I plan to couple the laser diode to a fiber optic and I understand I will need a lens to focus the laser into the fiber optic and a lens after the fiber optic to focus the beam again. I currently do not know what kind of laser diode I will use but I need a focused laser spot of 0.1 mm.

I am currently looking at using a collimating lens and a focus lens to get the laser spot correct because from the information i gathered the ratio of the two lens multiplied by the diameter of the fiber optic will give me my laser spot diameter.

Is it possible to have my laser source 8 inches (200 mm) away and still get a laser spot of 0.1 mm?

I have looked at lens focal lengths and it seems hard to find them in the correct ratio to get that small of a laser spot at that far of a distance. I also do not want to have a big focusing head because it will be moving around a lot.

Also, I have seen focusing lens with a 200 mm focal length could I just use that after the fiber optic and get the same laser spot?

If there are any tips or suggestions or any info you could provide that could lead me in the right direction I would greatly appreciate it.

Thank you.


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Old 01-23-2017, 02:29 PM #2
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Default Re: focusing lens for sls 3d printer

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Originally Posted by Team3D View Post
If there are any tips or suggestions or any info you could provide that could lead me in the right direction I would greatly appreciate it.
Look through some thread I posted my answers. They had very similar questions about focusing.

Keep in mind your power budget - coupling a laser diode radiation into a fiber, and keeping everything small and short comes at a price when dealing with a multimode high power laser diodes.
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Old 01-23-2017, 03:50 PM #3
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Default Re: focusing lens for sls 3d printer

Yes that size spot is possible, even smaller at 200mm. Depends on the diode though. May not be possible for some of the red diodes but I know for sure an NDB7875 can focus to a pinpoint at that distance. An M140 can do the same. Goes with most blue diodes. Violet can be focused to the smalllest dot of them all.
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Old 01-24-2017, 02:21 AM #4
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Default Re: focusing lens for sls 3d printer

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Originally Posted by lazeristasUVISIR View Post
Look through some thread I posted my answers. They had very similar questions about focusing.

Keep in mind your power budget - coupling a laser diode radiation into a fiber, and keeping everything small and short comes at a price when dealing with a multimode high power laser diodes.

lazeristasUVISIR thanks for the response I really appreciate it. I looked at your older post and I saw in one of the threads you said:


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).

So, what you are saying is i need to use a beam expander before my focusing lens? I have seen some post state this in other threads, but it is hard to find concrete information confirming this.

I assumed it has to do with changing the divergence of the beam. If so does this mean that when I focus the beam after the expander the amount the beam will converge increases, which gives me a smaller spot size at the focal point?

How would I do the calculations for figuring out the specific numbers?

Last edited by Team3D; 01-24-2017 at 02:34 AM.
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Old 01-24-2017, 02:31 AM #5
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Default Re: focusing lens for sls 3d printer

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Originally Posted by Crazlaser View Post
Yes that size spot is possible, even smaller at 200mm. Depends on the diode though. May not be possible for some of the red diodes but I know for sure an NDB7875 can focus to a pinpoint at that distance. An M140 can do the same. Goes with most blue diodes. Violet can be focused to the smalllest dot of them all.

Thanks for the response Crazlaser

I have read that smaller wavelengths will create smaller spots. Much of the info I found online states that smaller distances are better for creating smaller spots and as you increase distance the spot size increases.

So, if I uses a single focal lens with a focal length of 200mm and one of the diodes you stated above. Does that mean the lens will focus it back to a spot with a small diameter as close to its originating source? To change spot size would that depend on distance between diode and lens?
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Old 01-24-2017, 05:09 AM #6
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Default Re: focusing lens for sls 3d printer

After taking a look at beam expanders I found these equations from this website http://www.parallax-tech.com/faq.htm

for calculating spot size and depth of field

spot diameter (in mm) = 1.27 x focal length x wavelength x M^2 / Diameter

DOF = 2.5 x wavelength x ( f /D )^2

so by having a larger diameter beam at the focus lens should make the laser spot diameter smaller

can anyone confirm if this is the correct approach?

if so my calculations show that I would need to expand my beam diameter to be about 1000 mm (39 inches) and that seems way too big

Last edited by Team3D; 01-24-2017 at 05:29 AM.
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Old 01-24-2017, 05:39 AM #7
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Default Re: focusing lens for sls 3d printer

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Originally Posted by Team3D View Post

so by having a larger diameter beam at the focus lens should make the laser spot diameter smaller

That is correct, a large input beam, shorter wavelength or shorter focal length will result in a smaller spot.

I think you're complicating this too much - why not buy a laser diode that's already fiber coupled? You'd only need one lens to focus the output of that to a point.

Also, I'm curious, what material are you planning on using? Further, do you have experience with high power lasers at all?
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Last edited by diachi; 01-24-2017 at 05:40 AM.
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Old 01-24-2017, 10:54 PM #8
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Default Re: focusing lens for sls 3d printer

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Originally Posted by diachi View Post
That is correct, a large input beam, shorter wavelength or shorter focal length will result in a smaller spot.

I think you're complicating this too much - why not buy a laser diode that's already fiber coupled? You'd only need one lens to focus the output of that to a point.

Also, I'm curious, what material are you planning on using? Further, do you have experience with high power lasers at all?

I have seen fiber focusers and diode couplers, which may be the option we choose.

Fiber Focuser

This website is selling fiber focuser with the possible specifications I require

Clean Gaussian Beam
Few microns up to 1mm spot sizes
High power handling to over 20 Watts
Large working distance 5 - 400 mm
Wide operating wavelength 350 - 1800 nm
Compact size for remote operation

What I don't understand is if this is such an easy method how come many of the people online asking the same question I am are trying to use multiple lenses instead of a single focuser. I am only using a laser diode in the 1-3 W range if possible.

The printer was originally meant to be a metal printer but because of the high cost of a high powered laser and the lack of experience with lasers we are only trying to use a low powered diode to sinter plastic powder. Maybe possibly nylon.

Of course with any laser their is a need for laser safety and we are going to take the best precautions to make sure no one gets hurt in the process.
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Old 01-29-2017, 12:37 AM #9
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Default Re: focusing lens for sls 3d printer

It seems we may be looking more into using an infrared laser diode in the 800-900 nm range. Will this change my optics? Can I still use a focusing lens and a collimator to get the job done for a 100 micron spot at between 100-200mm working distance?

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Old 02-08-2017, 04:33 PM #10
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Default Re: focusing lens for sls 3d printer

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Originally Posted by Team3D View Post
After taking a look at beam expanders I found these equations from this website Frequently Asked Questions about CO2 laser lenses for cutting

for calculating spot size and depth of field

spot diameter (in mm) = 1.27 x focal length x wavelength x M^2 / Diameter

DOF = 2.5 x wavelength x ( f /D )^2

so by having a larger diameter beam at the focus lens should make the laser spot diameter smaller

can anyone confirm if this is the correct approach?

if so my calculations show that I would need to expand my beam diameter to be about 1000 mm (39 inches) and that seems way too big

Opps I feel embarrassed now. I did the calculation incorrectly. I converted 455nm to mm incorrectly. The correct calculations should be

0.1 mm = 1.27 x 200 mm x 0.000455 mm x 1/D

D = 1.15 mm

So, If I focus my beam when its diameter is 1.15 mm with a 200 mm focal lens I should get the spot size of 0.1 mm.
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Old 02-08-2017, 04:36 PM #11
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Default Re: focusing lens for sls 3d printer

Quote:
Originally Posted by Team3D View Post
Opps I feel embarrassed now. I did the calculation incorrectly. I converted 455nm to mm incorrectly. The correct calculations should be

0.1 mm = 1.27 x 200 mm x 0.000455 mm x 1/D

D = 1.15 mm

So, If I focus my beam when its diameter is 1.15 mm with a 200 mm focal lens I should get the spot size of 0.1 mm.

The equation assumes a Gaussian beam, which you won't get from an LD at high powers, so it won't quite be 0.1mm...
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Old 02-08-2017, 06:16 PM #12
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Default Re: focusing lens for sls 3d printer

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The equation assumes a Gaussian beam, which you won't get from an LD at high powers, so it won't quite be 0.1mm...
haha thanks for the response diachi. I understand their will be a lot of other things that come into play that will affect spot size.

but if this were tried with a lower powered diode like the m140 at around 1W would the diameter be close to 0.1 mm?

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Old 02-08-2017, 06:36 PM #13
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Default Re: focusing lens for sls 3d printer

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Originally Posted by Team3D View Post
haha thanks for the response diachi. I understand their will be a lot of other things that come into play that will affect spot size.

but if this were tried with a lower powered diode like the m140 at around 1W would the diameter be close to 0.1 mm?

The M140 is one of the higher powered diodes I was referring too The beam is nowhere near Gaussian, it's more rectangular/stripe shaped. You'll get close to 0.1mm, but how close I can't say.
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Old 02-09-2017, 12:07 AM #14
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Default Re: focusing lens for sls 3d printer

Wow, I was thinking only over 10 watts would be considered a high powered laser. I guess no matter what I'll be working with a high powered laser.

Well, I guess this is where the collimating lens comes into play. To help round out the beam. I know the beam still diverges slightly and isn't perfectly round, but it is a lot better than a rectangular shape.

Then using a focusing lens after the collimator to get the beam spot at the focal length I want should give me much better results.

I know it is still not a Gaussian beam, but because it is a lot closer to a rounder beam does that mean the equation I used previously can be applied with only a small degree of error?

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Old 02-09-2017, 12:16 AM #15
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Default Re: focusing lens for sls 3d printer

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Originally Posted by Team3D View Post
Wow, I was thinking only over 10 watts would be considered a high powered laser. I guess no matter what I'll be working with a high powered laser.

Well, I guess this is where the collimating lens comes into play. To help round out the beam. I know the beam still diverges slightly and isn't perfectly round, but it is a lot better than a rectangular shape.

Then using a focusing lens after the collimator to get the beam spot at the focal length I want should give me much better results.

I know it is still not a Gaussian beam, but because it is a lot closer to a rounder beam does that mean the equation I used previously can be applied with only a small degree of error?

It'll be close enough to what you want, you can always get a shorter FL focusing lens if it doesn't quite cut it. Say drop down to 150mm.

FYI, the M140's beam isn't even close to Gaussian or round, it isn't as bad as some other high power blue diodes, but it's still far from Gaussian.

Here's an example of the M140:



And here's a pretty much perfect Gaussian beam:




Try it with the 200mm lens, see if it works, it likely will.
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Last edited by diachi; 02-09-2017 at 12:17 AM.
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Old 02-09-2017, 12:23 AM #16
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Default Re: focusing lens for sls 3d printer

Oh wow haha the m140 was a lot more square and rectangular than I thought it would be. Ok, Diachi thanks a lot for the info. Its been a great help.
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