Looking for information on how to build own lens assembly.

[Marco Polo]

Hey all,

A while back I posted a thread about some green Laser 301's that had damaged collimating lenses. I never did manage to find replacements, and the lasers have been sitting idle for the past year or so.

It turns out that the front part of an AixiZ module can be screwed into the open end of the Laser 301's DPSS module. Both use the same thread. The 5.6mm hole where a laser diode would go allows the DPSS output to pass. The laser can then be collimated/focused via the AixiZ mechanism and an appropriate lens. The original 301 focusing mechanism and its problems are totally eliminated.

However, I will need to build custom lens modules to make it work. The DPSS output is much less divergent than direct-diode output, so it needs a lens with a long focal length. Right now the idea is to use a 6mm diameter lens with 12mm, or perhaps 18mm, focal length. NA can be low due to the low DPSS divergence. Anchor Optics has sensible prices so I'll probably look into them first.

Does the lens need to be aspheric, or is spherical fine due to the low divergence of the DPSS output?

I'm interested in finding a tutorial, or some instructions, on how to assemble a 9x0.5 lens module, but forum search isn't turning up too much. The basics make sense - using spacers to position the lens(es) in the module, the module being held together by the retaining ring, etc. I'm less clear on what the right tools are, where to get spacers (or how to make them), problems to look out for, etc.

If anyone can link me to something or advise in any way, I'd appreciate it. I'll continue digging around in the meantime while waiting for parts to come in the mail. Thanks!

 

[IsaacT]

I have built a few custom DPSS lasers that required the use of long focal length lenses. The lenses I used were from Thorlabs.

Lens used:

LA1213-A N-BK7 Plano-Convex Lens, Ø1/2", f = 50.0 mm, ARC: 350-700 nm

This lens worked exceedingly well for my purposes with my 473nm build. My 532nm build already had a lens as I was merely transferring its corpse to a new body a la frankenstein or something.

For the 473nm, Ehgemus crafted a lens holder from acrylic and orings that slid up and down the inner tube of the host. It was very basic, but it did the job.

Better than that however, the 532nm he made with a threaded inner tube that the lens screwed into. Then you just stop it at a good focal length.

Crafting your own 9x0.5 lens holders sounds terrible. If the above options dont work you might try a few things:

1. Contact DTR about his "Long Focal Length G-2 Lenses". These might be long enough for your purpose and would come in a 9x0.5 lens barrel.

2. Look for a lens on Thorlabs that is both long focal length and the right diameter to fit in DTR's barrels. My guess is there won't really be anything though.

The inside of the barrel should be roughly 1/4 inch or about 6.4mm in diameter.

I hope something in there helps you on your journey. :beer:

 

[Marco Polo]

I looked on Thorlabs and lots of other places. There are a lot of good choices for this kind of project.

It might be even easier than all that. This thread started by LarryDFW proved extremely helpful. The focal lengths of the individual lens elements are mentioned... 0.5", 0.75". That's exactly the range I'm looking for.

I opened up one of my 3-element lenses and removed the first lens element. This is the element with the AR coating on it. I removed the other parts and replaced only the first lens element in the M9 mount. I then passed the laser through this single-element lens (holding the lens mount in my fingers since there's nowhere to screw it in) and the collimation was very good. I ordered a 10-pack of modules from AixiZ, so when they arrive this laser is as good as fixed.

The other lens element looks like two lenses glued together. It works also, although the focal length is different.

I just need to buy a few more 3-element lenses and then convert them to single-element, and I need to figure out something to use as a spacer since the single lens element is loose inside the lens mount and won't stay in position.

 

[The Lightning Stalker]

Some sleds have a long FL lens (coated, too!) and you don't have to sacrifice an expensive lens to get
one. I just measured FL on one here and it came in at 24mm.

 

[Marco Polo]

I've been taking measurements and generally doing tests on what will make a good setup and I think I'm going to use one of the lens elements out of a DTR 3-element.

There are actually only two lens elements in the "three-element" lens. All of the diagrams I've seen count multi-part lenses (e.g. two-part achromatics) as a single element.

The first lens is a relatively thin positive meniscus with the 405 AR coating on it. It seems to have a focal length of about 15mm.

The second lens looks like a plano-convex at first glance but is actually biconvex. There is no AR coating. This lens consists of a thinner biconvex bonded to a negative meniscus, leading me to believe this is an achromatic lens intended to keep the focal length of the system more or less constant and independent of wavelength. It has a shorter focal length, closer to 11 or 12mm, and is fairly thick.

Both will work to collimate the beam, apparently better than the original lens. In my circumstance, the fat lens works better because it's easier to position properly. The biconvex nature also makes it easy to clean compared to the meniscus. The best part is that the lenses are dirt cheap, under 9 dollars for the complete 3-element lens. That and the long focal lengths make them ideal for retrofitting Laser 301's.

Just playing the waiting game now. The worst part of DIY is waiting on the damn snail mail.

 

[KrowBar]

It's just semantics, but for lens assemblies the word "group" is sometimes used to refer to multiple elements which are cemented together like an achro doublet. So this is a 2 group, 3 element lens. Also, IIRC there have been multiple versions of the particular lens you are using, and some were found to have only 2 elements (although it could just be that 2 elements were formed into a doublet group on some of them - making it appear like only 2 elements like the ones you have)