Hello there,
Recently, I have been uprgading all laser components for better performance. I came to the focus lens. I have 2" chienese one from cloudray, pretty the standard one. Than I looked up on the internet and realized that there are 1.5", 2.5", 3", american one, chienese one, all different materials and so on… I got confused and now I do not know which lens is the best performant? I am guessing the american one, and also is there any store with these type of lens for shipping to europe?
Of course there is a difference in quality, but also in price.
To find out what is economically justifiable, you need to find out how much more you can get out of your machine.
The best but also most expensive way is to measure the real output of your machine with a “laser watt meter”. If the output is close to what corresponds to your machine, that’s fine. If you lack power, the tube may not be quite perfect anymore or is defective.
With very optimistic glasses on, you can get 5-7% more out of the machine with high-quality optics compared to very cheap components. But you can get significantly more out of the laser by having the machine optimally aligned. This is also where most people start with priming, after they have cleaned the mirrors and lens.
Do you have reference tests of your machine so you can see what it can present under optimal conditions? and what are your expectations?
Your profile states that you have a 100 watt CO2 laser, it is a very powerful machine and should cut almost everything except metal. However, for fine engraving it is not the most optimal machine and this cannot be significantly improved with expensive optics.
I do not know at what power does my tube operates, but I am pretty sure it has lost some power because it is old. Currently I am using cloudray - China PVD, ZnSe Focusing Lens, focal length: 50.8mm, coating: AR/AR @ 10.6um. According to my research this type of lens are like base model.
Now I am wondering if for example II-VI meniscus focus lens 20mm, USA CVD, ZnSe Focusing lens, which according to internet is more robust and more durable than the one mentioned. Would this lens make a difference when discussing the lens’s lifespan, heat tolerance, and actual performance?
Usually I do not make detailed engravings, mostly my work is continuously cutting 5 - 8mm wood.
Price is not the problem, on one website the USA one is only like 20 euros more than the Chinese one.
It is also my standard lens and I am extremely happy with this cheap lens, this also applies to my “standard” mirrors (Cloudray). In addition to a little experimentation I have used 2 sets of the same cheap type for 5 years. I cut primed 3 to 6mm
BB plywood and Acrylic up to 8mm.(60Watt CO2)
Have you checked your tubes TEM mode?
The laser beam is not a perfect circle, it is a weird shape, I think that my laser is cutting well for its working hours and years. I am just trying to find something that can boost my performance by little because it would mean a lot on a long term. Also I did not find any problem with the cheap one, but if these “american” CVD focusing lens allegedly made out of more quality materials can be more durable and robust and also more precise, than there is no room for debate as the mentioned one does not cost much more.
You need to pick the lens based on what kind of work you are doing.
Just like a camera lens, it has the same properties as all other lenses. Quality of the lens can mater, but there is a lot of setup that goes along with a well operating machine and a simple optical path change.
The lens is what makes these actually able to do work.
You should consider depth of focus (dof) and spot size. There are on-line calculators to give you a good estimate of what you can do with a lens.
I got most of this data from the tubes manufacturer, but it can be had in other ways. I don’t know how you’d measure beam quality ( M²) without a rather expensive machine. These are three screenshots of the calculator I use. The difference on the input is only lens length. Note the output.
Generally when cutting thicker materials you want a lens that will maintain the spot size throughout the area you need to engrave/cut. Left is a compound lens, making the smallest spot size of 968 microns (0.968mm), however it’s dof is only 922 microns, or less than 1mm.
You can pick the highest possible dof or the smallest spot, but they go opposite ways. Can’t have a small spot with a long lens.
With a 2" (50.8mm) I get a spot size of about 129 microns but a dof of over 1.5mm. I use my 7" (101.6mm) for cutting thicker material, such as 5mm sub flooring, as it has a 6.5mm dof. You have to focus at the midpoint of the material to use the entire dof. If you measure to top of material 1/2 of your dof is lost or wasted.
Does any of this make any sense?
If you have a watt meter, then it might be worth upgrading to more expensive mirror/lens combination. I agree with @bernd.dk that it’s lower cost and makes more of a different if the machine is properly set up before messing with the optics directly. Without a watt meter you can’t really tell what’s what.
On my fiber, when doing rocks, the surface is rarely flat. Using my longest lens F420mm gives me more dof to make up for irregular suffices.
Any questions, sing out.
can you show a test shot on m1, it should be a nice round filled brown dot of approx. 6mm.
You can certainly buy high quality optics and get more for your money, but you can’t fix a bad tube with that.
As I said, I run a (moderate) production, primarily cutting jobs and spend very little money on lenses and mirrors but always keep an eye on alignment and cleanliness of the optical path.
Yes I understand what you want to say and I am aware that I can not fix my tube. The only thing here that I want to solve is that I find the best lens for my money and my needs.
This does make sense. Is it possible for example that 3" lens can cut 8mm wood faster than the my 2" lens. I know higher the focal length is the thicker you can cut - but can you faster?
Also, I do not clearly understand why is worth to upgrade optics only if I would know the power of my tube?
I ask all of this because as I already said according to internet sources the lens that I have China PVD ZnSe is worse than the Meniscus USA CVD ZnSe lens. Is there any better lens that I could buy or is this one mentioned enough for my needs?
The only way to find out is to try it. You asked if there is a difference, yes there is a difference. Can it be measured? - possible. Is it worth it? - it depends on who you ask. If you can increase the quality, power or focus by 5%, it does not mean that you can cut 6mm plywood 5% better or faster. But buy a set of mirrors and lenses and test it yourself. I personally cannot measure/see a difference that justifies doubling(?) the cost of a lens/mirror change from “normal good” to “extra good”.
The finest lens in the world cannot focus the beam from a tube that’s not resonating in TEM00 mode, because the incoming beam has the wrong energy distribution.
The focused spot thus has the wrong shape, so it will not cut or engrave correctly, regardless of the lens.
Your money would be better spent solving the actual problem.
I pretty much doubt it. There are two types of machining, additive or 3d type and subtractive where material is removed. Most of these machines work throughout a wide range of speed and power settings. However each has a, so called, sweet spot. This a much small range - where it works best, last longest, uses less power and delivers the best results. With milling machines for metal, you can look up the bit/flutes and type of steel and get a good speed/feed that’s pretty accurate. With lasers, there are too many variables, so you have to dial it in, to get the setting to work the best.
If you go from a 4" lens with a spot size of 400 microns and then use a shorter lens, like a 2" lens with a spot size of 200 microns, you have reduced the diameter by 1/2. When you do this, the same power over a smaller area, appears as 4 times more power to the user. Other than the fact that your source produces X amount of power and a lens isn’t going to change that, maybe you could consider it more powerful. 
The user should see an apparent increase in power of 4 times.
This really shows up with fiber lasers. As I have 4 lenses and commonly use a very long lens with stuff like rocks. An uneven surface can be handled by a long lens with a deep dof.
As far as these extra cost items, I usually buy the lowest cost items. I’m sure there must be some gain in it, but you’d have to have a watt meter to find out for sure if it’s worth it. These other people can put their 2 bits in about this. I’ve been able to do about anything I want with what I have, no extra cost mirrors. I do have a different head assembly, but other than that, it’s pretty much like it was originally… at least optically.
I cut 5mm sub flooring with a 2" lens using my 43W tube, 45mm/s@45% power. Even though it doesn’t have enough dof and you really can’t see much if anything of the spot size changing on this type of wood. I do slow down for longer lenses.
Lean from others, who have wandered here before, then whatever works for you is fine.
Good luck.
Here it is, the stop on the M1 straight from the tube, actually I did not expect to be good as it is.
We’ve definitely seen worse! 
Set the controller for a shorter manual pulse to reduce charring in the center part of the spot and fire it at white paper with less texture. If the spot has a uniformly dark center tapering off smoothly to the edges, then the tube is likely working fine.
If that’s the case, then spend as much as you’d like on fancy lenses & mirrors, keeping in mind they won’t make as much difference as you’d like.
