Beam profiling, best tools and diy ideas

[klick]

I'm new to lasers, only have a few thus far, but i'm trying to figure out the best way to determine the beam profile, or really just the primary diameter. Power isn't my primary concern, but of course using my techniques changing the power appears to change the size, although i'm not sure how much is just diffusement into the surface or what not.

Let me show you what i've done, and show you some pictures to illustrate, hopefully I can attach pictures here. All beams are 10 inches away, focused by hand, the usb microscope is pointing straight down and the laser is coming in at about 30 degree angle. Using a Mastech digital desktop power supply in constant amp mode, just modifying the amps. The laser I have is from DTR: "12X S06J Bluray Diode In Copper Module W/Microboost & 405-G-2 Glass Lens" So 405nm

I'll describe the pictures under the picture.


This is the first beam, 100mah, 2.1volts. I'm not sure how to upload will scale, but 37 pixels is 1mm, and on this picture, the beam was basically about 1mm, maybe a little less, like 0.85mm. That was all the focused I could get it.


Same exact beam and settings and I just moved the welders mask away. Oh yea, I was holding the welders mask close to the laser, that part of the beam is much larger as it's converging down before the focus point. It's about 2.5mm in the pic, but this is what confused me, it was the same focus, same distance, just the welding filter removed, so are we seeing lots of laser diffuse at the impact point (i don't know what terms to use haha) or is it really that size? I have no idea.


This is at 0.25amp, volts still read 2.1v, this is through the welders filter, and the beam appears to be 1.4mm


This is 0.25amp, with no welding filter, it seems like it's about 2.3mm, which is less then the other lower power one, but I'm sure I moved and re-focused a bit, so this is all just an estimation.

The only thing I can tell you that seems much smaller is the burn marks in the paper, some of the burn points and burn lines are 0.1mm, but that doesn't mean the beam only touched that spot

I looked into beam profilers, they all seem to be extremly expensive, like $2500 or more. edmunds optics has one for $2500, but that seems a bit much, was hoping there was some more reasonable solutions. I might consider one at $1000, but I dunno, perhaps i'm putting to much into the focusing of the beam. I also read about a technique using Logitech webcams in some special bayer mode, which might be kind of how the professional tools do it, but I couldn't find anybody who's actually done it, only info on potentially how to do it.

I'm working on a 3d printer using SLA, stereolithography with the laser and a galvo scanner, or some control technique similar to a galvo scanner. The focal distance will be about 10 inches away, and vary about half an inch, so like 10 to 10.5 inch, in that span, I want to have a small laser beam of course. Actually 1mm might not be to bad, and it might actually be what I want, but the point is I don't know if that's the case or not with my techniques. 1mm laser beam into UV curable resin is kinda like having a CNC machine with a 1mm end mill, which of course gives outstandingly accurate results. I've tested the beam in to the UV resin and it seems to work quite nicely, even at lower settings, but doing it by hand it hard to determine

One technique I juse thought of, I should focus the beam at the 10 inch point to the best I can by hand. Then shoot the beam 10 feet and measure the diamter, and then 15 feet and measure it, I can determine the diverging angle, and trace back to where the focus point should be... well actually that doesn't neccisarily tell me the size at the focus point, since the laser and optics aren't perfect, so it won't be perfect at the focus point, but it might tell me that I do have it focused at the right point... dunno.

So, basically, how do you guys measure beam size/profile? Just looking for some help. And if my technique is totally, wrong please tell me.

Thanks,
Ross

 

[Meatball]

Hi Ross,

Thanks for posting your results.

So, a nice beam profiler will be able to mark out relative intensity at any point along some axis that cuts through the beam. This can be done with a high density row of photodiodes, but in order to get the entire beam at once (IE, a sum of all horinzontal and vertical axis that cut through the profile) a two dimensional array of photo sensors can be used. So yeah, its like taking a picture of the beam head on.

This is usually done with a CCD array, or a CMOS array. But, if those were so easy to work with, they wouldn't be so expensive to purchase in a system...

So using a cheap webcam can yield similar results to a CCD profiler, but you have to be aware of the wavelength response of all optics in front of the camera, and the response of the camera sensors by themselves in order to get an accurate "relative intensity" profile. You'll also need to use a black and white mode so you can better relate intensity.

But, for your purposes I'm thinking you don't really need such accuracy. Beam diameter is your primary concern, not the profile so much - and for a diode laser the beam diameter is relatively easy to measure a opposed to a perfect gaussian profile.

Do you have laser safety glasses for use with this 405 nm laser? If you do, you'll notice how beam artifacts such as scattering through a piece of paper gets "cleaned up" when looking through the glasses. Consider that stuff that gets filtered out as something like "low power noise". Filter out that junk, and measuring the diameter of the beam itself is cake.

Try finding a yellow or orange theatre floodlight filter. Just those gel/plastic filters that go over a spot of flood light. You can buy them anywhere online. Try placing a small gel filter in between the laser dot and the microscope objectives, and you should be able to view a cleaned up version of the laser dot (don't use paper alone as a surface, try filling a good 1"x1" square on the paper surface with a #2 graphite pencil, and look at the dot incident on the graphite surface) through the scope. Stick a nice caliper or ruler under there, and you should be able to calibrate your laser spot to within tenths of a mm. Just sticking a thick plastic medium (like laser glasses) between the spot and the objectives can mess up the shape of your image, so stick with a flat, thin filter, like the theatre gels.

Just remember that a short focal length in the lens allows for a much smaller but harder to reach focal spot diameter. A longer focal length is more forgiving of tolerances, but does not allow for as small of a spot.

With the S0J-6, you'll most likely be alright either way. Its a single mode diode with a very nice oval shaped profile. Try using a 405-G2 lens from DTR if you aren't already using one. It will make a very noticable difference in intensity.

The multi-element coated lenses will give you a cleaner beam with less power however.

Good look with your lithography! Post your results as you go!

Tyler

 

[klick]

Thanks for your reply.

I'll try some of the gel to reduce the beam, the welders mask lens that I used was flat, which I thought would be alright since it shouldn't affect to focusing to much. It did cause some other anomalies so it's not the most ideal really.

Thanks for mentioning the multi-element will give a cleaner beam, because it does and I wondered if my G-2 lens was bad, or if the multi-element was better. I think the power reduction doesn't affect me that much, a clean beam is much more important, so the multi-element is the way to go for me. I'm going to order another multi-element for my original laser. I received the Osram PLTB450, after my initial post, with a multi-element and it focuses very nicely.

Yes I have safetly glasses, and yes the beam looks nicer through them. You mentioned putting the filter between the microscope and beam, I wonder if I could use the safety glasses there (I have more then one pair so i'm not exposing my eyes, don't worry) but it's curved, I dunno if that will screw up the focus. I'll try to get the filters you mentioned, but in the meantime I might try the safety glasses.

I think the multi-element focuses up probably good enough for my purposes. I need to start fabricating my 3d printer to start testing out the accuracy of the dot and what it can do. Trial and error wth that might be easier then focusing so much on the beam profile, I'm not sure.

Thanks again for your advice, I'll try to get some of those filters to help my beam profiling

Ross

 

[wee40811]

I have an idea of using a CCD camera with a OD3 neutral density filter. it should reduce the beam power into manageable range. Those filters are cheap form ThorLab about $40 for a glass one with 700w/cm^2 damage threshold.

 

[klick]

So are you thinking of item: NENIR40B-C from thorlabs.com?
desc: Ø25 mm Unmounted AR-Coated Absorptive Neutral Density Filter, 1050-1620 nm, OD: 4.0

OD 4.0 describes optical density, it says transmission is only 0.01%, the others are less restrictive.

I did do some other further testing. After moving the filter from between the laser and the focal point, I moved it to between the camera and the focal point, this made a nice difference in optical quality of the laser beam. I did manager to use this to get a pretty decent pinpoint, still it didn't filter as much as I had hoped, I had to turn the laser power down to the lowest point, then got a decent image.

I also learned how to change all the parameters of a webcam in linux, I turned off auto focus, and played with modifying exposure settings (this had the most impact, it's basically shutter speed, I think). Anyway, there are a number of webcam parameters to play with, using those allowed for some improvement to the beam picture as well.

I did try shooting the beam directly into the webcam CCD, this never resulted in a good picture, even locking in focus, and using the filter, the beam was just way to overpowering. I tried it with about 4 different webcams, 2 1080p ones, and some other older ones, the images basically looked the same through all of them. Perhaps with the filter you described above it might be better. For my purposes I think with all the webcam settings and the welder's filter (projecting it on the desk or some object like I initially did) produced good enough results to tell the point when the beam is focused the most.

I did order some standard DSLR camera filter sets of amazon, and an optical meter (General Tool UV513AB Digital UVAB Meter for Ultraviolet Light Measurement) Which isn't really for testing the beam size, but in part I will use it to test the filters I acquire. I needed the UVAB meter to test how well my UV filtering plexiglass will work in accordance to safety with the 3d printer enclosure, but I'm hoping it might be useful in other areas as well.

I've been looking to order some stuff from thorlabs, so I might pickup that filter as well if you think it'll help.

Thanks.

 

[wee40811]

Sorry not thorlab, the one I found is on Newport.
Neutral Density Filter, 2.0 Optical Density, 1.0 in. Diameter
You put 2 together and you got OD4 filter. And they work better with visible spectrum. You can adjust the OD by stacking several filter together. The one you indicated is for infrared. The OD drops significantly one you go into the visible light. These ones are more spectrally flat. they are pretty linear all the way from 400 to 1500.