405nm Laser with <100µm Focus

[Matyro]

Hey,
im new and have some questions to the professionals here

Im building a Laser Exposer for PCB's, i use a old laser printer polygon mirror to control my beam. The laser diode is a "BDR S06J 12x" at ~350mA.

My main problem is now to focus this beam to a size smaller then 100µm (0.1mm). I dont know if the "G2" lense is good enough.

The focus point should be 25cm away from my lense/laser and im ready to pay 100$ if the result is good.

Someone know which optic elements should i use? Should i use a collimator?

Thanks
Matyro

 

[SKeeZ]

I dont think you'll be able to get it that small at 25cm because of beam divergence.

 

[Matyro]

This is a problem...

The distance difference between the (pcb) border to the mirror and (pcb) center to the mirror are to big, when the space between pcb amd mirror is to small.
(Sry hope you understand what i tried to explain)

Edit:
I recalculate a bit: maybe 15cm beam length is enough.

 

[Wolfman29]

You may be able to do it. But you'd probably need a telescopic lens to expand the beam and then another lens closer to focus it again.

 

[Cyparagon]

That's three lenses and lots of alignment. You can do the same thing by using a single large collimating lens with a longer focal point.

There is no law of physics that mandates a collimating lens needs to fit in an aixiz module. For example:

 

[Matyro]

The alignment is no problem. I have a big Al Block to mount all things on.
Lense size are ok if they are smaller than ~50mm.

 

[Meatball]

Well. We can consider if each of your PCBs will be the same thickness or not. I'm assuming your design illuminates the board from up top so its the PCBs top surface that needs to be a very constant distance away from the diode to maintain such a tight tolerance.

Next, you can consider that taking the FL out to say 15 cm will allow for better distance errors that might occur along the beam waist, but a long FL also limits your beam diameter to a minimum size.

A short FL would enable sub 100um beam waists, but then tiny changes in PCB position along that waist makes a setup that is very sensitive and perhaps unreliable.

You will need to find a balance between these two. With a good clean lens, 100um is very achievable and certainly measurable, but long long should the waist remain at a 100um size? Its up to you, and limited by the maths.

See the following: optics - Physics of Focusing a Laser - Physics Stack Exchange

Focusing and Collimating

 

[Matyro]

Thx for the links, i will try to understand and calculate the lenses i need. But its not so easy for a beginner in Gauß Optics

 

[Matyro]

Hey,
I find the so called "f-theta" lense today. This seems to be the sollution for my problem.
But more then 1000€ (edmund) for a lense is extreme over my budget.

Small Picture of my Build:

The focus lense alias "Magic Lense" is my problem. The distance between the mirror and pcb can i adjust to fit the lense data.

 

[lazeristasUVISIR]

Try to make all distance as short as possible. Rearrange angles.
You have many limiting parameters like a scanner mirror size/height; the laser diode emitter size.
You can not enlarge your beam (w/ longer focal length lenses) too much as it will overfill your scanner.
You can not go too far as the magnification ratio become large and the spot size increases. This is only partially true as a f-theta lens overcomes limitation, though the lens is over your budget :/

Enjoy your project.

 

[Matyro]

Maybe i found a much cheaper way for a "flat field lens".

I try to simulate (with WinLense) a Lense Array out of three lenses. I think it can work and give the precission i need for less than 200$.