Focusing on mirrors

[mjk]

Hi, all! I've been reading the forum for some time now. Finally decided to register and ask a question! Thanks in advance for the help.

I have a few M140s that I'm experimenting with. I've attached one directly to the moving head on my CNC machine and it's been neat. I also have a cheap Chinese 40w laser cutter that uses a mirrored system that I'm curious about.

If I have my M140 point directly at fixed mirror (like this one, since I'm assuming a regular old mirror doesn't work) and I focus the beam on the fixed mirror...and the fixed mirror points to a mirror that moves on direct line closer and further away from the fixed mirror (between 1-5 inches away) would my point remain focused and strong enough to etch/cut/burn?

Example attached.

 

[Chrisbee]

Yeah, regular mirrors don't work. You need a first surface mirror. And you'll need to focus the laser to the total distance. So how ever far the actual working surface is at. A variable distance type mirror setup can change the final focus point, but depending on the initial distance, and how far the mirror actually moves will determine how much it'll change the focus, if any (i.e. if the working surface is far enough out, that the lens is already set to infinite, but with a cutter/burner it probably won't be).

 

[mjk]

if the working surface is far enough out, that the lens is already set to infinite, but with a cutter/burner it probably won't be

I wish I had paid better attention in science class. I'm assuming my 40W laser can accomplish variable distance without noticeable loss in focus due to brute strength...?

I wonder if the M140 can accomplish variable mirror distances even on a fractional level...say between 6-12in without re-focus...

Also, is it correct to say there are other forces at work when bouncing through multiple mirrors? i.e. a laser shot 12in in one direction will have different properties/output than the same laser shot 12in through 2 front first mirrors?

 

[HydroSean]

I wish I had paid better attention in science class. I'm assuming my 40W laser can accomplish variable distance without noticeable loss in focus due to brute strength...?

I wonder if the M140 can accomplish variable mirror distances even on a fractional level...say between 6-12in without re-focus...

Also, is it correct to say there are other forces at work when bouncing through multiple mirrors? i.e. a laser shot 12in in one direction will have different properties/output than the same laser shot 12in through 2 front first mirrors?

It can't hurt to do trial and error can it? If there is no expense involved then I say just try a bunch of things out and see what gives you the best result! SCIENTIFIC METHOD YEEEHAW! Just remember to record your procedure, variables, observations, and results. Draw a conclusion and then develop a better procedure with some theory behind it. Be sure to show and tell us what your results are!

 

[mjk]

Love that spirit! I definitely will! I'm going to set up a pretty simple layout like my diagram above tomorrow (pending some mirrors from eBay) and let you guys know how it goes. What's the worst that could happen... :lasergun:

 

[HydroSean]

The worst would be burning your eyes or your house down. Since you're dealing with mirrors I would highly recommend glasses and to do in a flame retardant area. Sounds like you have a fun project ahead of you

 

[mjk]

The worst would be burning your eyes or your house down. Since you're dealing with mirrors I would highly recommend glasses and to do in a flame retardant area. Sounds like you have a fun project ahead of you

Agreed! :thanks: I wear my safety glasses even when I know my variable PSU is at 0 amps / 0 volts. I have this insane fear I forgot to turn the knobs last time I used it. Probably a good habit.

Stay tuned for updates!

 

[diachi]

The focus point will remain the same distance from the laser, so if you're moving the mirror the focus point will change relative to that and relative to whatever you're trying to cut/etch. Best to collimate the laser and then have a focusing lens after the mirror that's moving.

 

[mjk]

Best to collimate the laser and then have a focusing lens after the mirror that's moving.

Hi, diachi! Thanks for this.

Excuse my ignorance but I'm not exactly sure I understand the process/properties of collimating the beam. Am I basically focusing the beam twice? Once when it leaves my diode, and again when I know the remaining distance to the object to be cut?

 

[diachi]

Hi, diachi! Thanks for this.

Excuse my ignorance but I'm not exactly sure I understand the process/properties of collimating the beam. Am I basically focusing the beam twice? Once when it leaves my diode, and again when I know the remaining distance to the object to be cut?

If you're collimating a beam you're essentially focusing it to infinity. I.e. it'll never reach a focused point.

Practically that's not possible, so the focal point will either be very far away (The beam will converge very slowly and then diverge after a certain point) or entirely non-existant (It'll just diverge from the get go).

You want to have the laser "focused to infinity" until after it hits the moving mirror, at which point you'll have another lens - attached to the same fixture as the mirror, so that it moves with the mirror - that focuses it to a point.

Let me know if I'm not being clear enough...

 

[mjk]

Let me know if I'm not being clear enough...

I'm 100% certain your clarity is spot on and my knowledge is lacking.

No, this makes a ton of sense and I greatly appreciate it. Final question is in regards to the actual application:

I'm using a G-2 glass lens directly attached to the diode and (fingers crossed) can use a second G-2 lens to be the "final" focus.

It's easy to visibly see when a beam is focused 6 inches away... how do I know exactly how to focus to infinity? And if it's focused to infinity, I'm slightly confused about how it can be recaptured in a tiny 9mm lens but I'm willing to learn through application if that's the answer...

 

[HydroSean]

The focus point will remain the same distance from the laser, so if you're moving the mirror the focus point will change relative to that and relative to whatever you're trying to cut/etch. Best to collimate the laser and then have a focusing lens after the mirror that's moving.

That is a truly great contribution of knowledge diachi. Did you look that up or come up with it? I am asking because its ideas like that which make billions of dollars on products like CNC laser cutting instruments.

I'm 100% certain your clarity is spot on and my knowledge is lacking.

No, this makes a ton of sense and I greatly appreciate it. Final question is in regards to the actual application:

I'm using a G-2 glass lens directly attached to the diode and (fingers crossed) can use a second G-2 lens to be the "final" focus.

It's easy to visibly see when a beam is focused 6 inches away... how do I know exactly how to focus to infinity? And if it's focused to infinity, I'm slightly confused about how it can be recaptured in a tiny 9mm lens but I'm willing to learn through application if that's the answer...

Here's the quick way to collimate your laser. Point it at a flat surface as far away as possible (>100 meters) and focus it to the smallest dot you can make, then lock it in place. Essentially you just collimated it.

 

[mjk]

I am asking because its ideas like that which make billions of dollars on products like CNC laser cutting instruments.

Agreed! I love this community.

Here's the quick way to collimate your laser. Point it at a flat surface as far away as possible (>100 meters) and focus it to the smallest dot you can make then lock it in place, essentially you just collimated it.

Genius, thank you!

 

[diachi]

That is a truly great contribution of knowledge diachi. Did you look that up or come up with it? I am asking because its ideas like that which make billions of dollars on products like CNC laser cutting instruments.

That's just how a lot of CNC laser cutters do it and makes good sense in many scenarios(really it's the only practical way in some scenarios).

You've got a few choices, fiber couple and run the fiber out to the cutting head, mount the laser to the X/Y "gantry" and have the whole laser move, move the entire work piece (keeping the laser fixed) or use bounce mirrors to reach the final output lens.

With CO2 lasers (And some others I'm sure) fiber doesn't work and mounting the tube to the "gantry" (for lack of a better word...) isn't exactly practical due to the size of your typical CO2 laser. The solution then is to use mirrors and bounce the beam over to the cutting head which contains the final turn mirror, focusing lens and gas assist nozzle (see below for a picture!).

 

[mjk]

Here's the quick way to collimate your laser. Point it at a flat surface as far away as possible (>100 meters) and focus it to the smallest dot you can make, then lock it in place. Essentially you just collimated it.

Hmm, doing something wrong here. I've got my M140 w/ G-2 lens pointed down the hallway (50+ ft) and focused as much as possible...dot the size of a fingernail. Collimated?

When I grab a second G-2 lens and place it in the beams path (a foot away from the collimated diode) I get this spilled "flashlight" effect and no defined point.

I'm not sure if the problem is I'm placing the second lens by hand which is giving me this unstable, unaligned, placement or if the second lens should be something other than a G-2 lens.

Science is fun.

Perhaps this has something to do with it... invert the lens'?