Salt Lenses , Technology from Laser Mirrors

[Mustafa Umut Sarac]

I am researching laser hardware making nearly 20 years and I think I found something interesting.
As you know , common laser technology contains a gas , liquid , solid rod and two half transparent mirror at the two ends. CO2 lasers are the most powerful lasers you can do it yourself and they create thousand degree heat. The salt mirrors can live with that heat and transparent to this beam.
I never saw a salt mirror , i do not know its transparency but i think it could be used to make lenses.
May be it can even be doped with rare earth metals and so.
Anyone know about this ?
Salt is very sensitive to moisture but it can protected with filters.

Best ,

Mustafa Umut Sarac

 

[AgX]

First: What is a salt mirror anyway?

A mirror where the reflective layer is layed upon a crystal instead of on an amorphous base as glass?


Well, there had been lenses made in history out of crystals.
And there are those modern fluorite lenses.

 

[Mustafa Umut Sarac]

Salt lens is simply build with salt not with a glass

 

[Roger Hicks]

Salt lens is simply build with salt not with a glass

Dear Mustafa,

Fluorite elements were used by Canon many years ago.

In response to another post, liquid-filled wide-angles are not new either -- I haven't time to do the research right now (going to buy groceries), but they're in most standard histories -- and you might want to take a look at oil-immersion objectives for microscopy for further information on non-glass lens components.

Cheers,

R.

 

[Loose Gravel]

CO2 lasers operate at 10.6 microns. This is the far infrared. CO2 lasers are very dangerous and not to be attempted unless you know what you are doing. Salt mirrors, as you called them, are windows that use an optical property called the Brewster angle. At this angle the window either reflects perfectly or transmits perfectly, depending on the polarization of the light. This is why they are half transparent. At the power levels of CO2, you don't want any absorbion or the mirror would get very hot very fast.

 

[Photo Engineer]

Salt is used in Infra Red spectrophotometers instead of glass, as the glass absorbs IR wavelengths. The salt windows were usually KBr IIRC, and they were very difficult to maintain as they rapidly frosted over and began falling apart even when protected in a vacuum dessicator when not in use. We had to change the lenses or windows, as the case may be, quite often due to the ease with which they frosted over or scratched. They often cracked as well.

PE

 

[AgX]

confusing

I never saw a salt mirror , i do not know its transparency but i think it could be used to make lenses.
Mustafa Umut Sarac

Mustafa,

This is getting confusing. If you mean such a mirror as Loose Gravel has described above (Brewster mirror?), how could it be used in making lenses?

If you deduct from the fact that salts are used in optics as mirrors, that one could also make lenses from salts, then the answer has been given twice in posts above.

 

[Photo Engineer]

Well, that was part of my point.

Since salt is transparent to IR and is used for spectrophotometry from about 2 - 15 microns, is it a good mirror?

PE

 

[Little_shiny_Light]

Sodium Chloride lenses

I am researching laser hardware making nearly 20 years and I think I found something interesting.
As you know , common laser technology contains a gas , liquid , solid rod and two half transparent mirror at the two ends. CO2 lasers are the most powerful lasers you can do it yourself and they create thousand degree heat. The salt mirrors can live with that heat and transparent to this beam.
I never saw a salt mirror , i do not know its transparency but i think it could be used to make lenses.
May be it can even be doped with rare earth metals and so.
Anyone know about this ?
Salt is very sensitive to moisture but it can protected with filters.

Best ,

Mustafa Umut Sarac

Even this topic is a bit old, the answer is yes. Salt lens are still used for infrarer lasers on professional environments.
Proper name is "window" rather than lens.
http://www.korth.de/eng/lagerverkauf/50372895570d30604/index.htm

 

[Mustafa Umut Sarac]

Thank you very much for your link.
What about the r index ?
May be we can create a extraordinary big lens and shoot a very big glass negative and throw the lens.
As photo engineer says , it can be cracked easily but it can be manufactured as easy as it is.
Is there anyone who knows something about their manufacturing process ?
I think I will look to the patents.

Best ,

Mustafa Umut Sarac

 

[Ole]

Salt is crystalline, soft, and extremely difficult to polish since it would rather crack up in a million little pieces at any attempt at mechanical shaping.

Salt doesn't make particularly good mirrors, it's just that at the deep IR wavelengths there are very few transparent substances to choose from. Simple metal halides (KBr, NaCl, and similar) are among the least bad. They are also optically isotropic, which is very important for optical use of crystalline substances.

 

[Little_shiny_Light]

Hi, happy it helps.
below is a link with salient properties of salt lenses.
http://www.crystran.co.uk/products.asp?productid=119
The most "popular" method is the so called "Kyropoulos method", large ingots can be produced reliably as soon as one takes them away from moisture .. ehm ..
The ingots are then machined in a suitable shape with standard technology ... so to speak ...
The cited method is well described on literature, it makes no point to refer it here due to the extreme difficulty also in skilled environments.
Just Google for it.
BRs
A Little Shiny Light

 

[Little_shiny_Light]

Thank you very much for your link.
What about the r index ?
May be we can create a extraordinary big lens and shoot a very big glass negative and throw the lens.
As photo engineer says , it can be cracked easily but it can be manufactured as easy as it is.
Is there anyone who knows something about their manufacturing process ?
I think I will look to the patents.

Best ,

Mustafa Umut Sarac

MMmmm no auto quote on re edit of msg .. sorry.

 

[Mustafa Umut Sarac]

I think , hard thing is to polish the raw salt. I had been thought that i can mold and use but final polishing and making the surfaces transparent is necessary

 

[Photo Engineer]

I have seen IR filters made of salt that looked just like glass - until you start using them and then they rapidly frost over due to moisture.

PE

 

[Mustafa Umut Sarac]

Why cant we coat the salt lens with something like anti reflection coating ?
I think mixing titanium nanoparticles in to plastic is faraway more sensible idea but i think someone will find some reson for making salt lenses , their r index is around with plastic.
And mixing same nanoparticle with liquid salt , interesting.

 

[keithwms]

I don't know how much I am repeating- I didn't quite understand some of the comments above, but the salt itself is not the mirroring element, of course. If you want a mirror for 10.6 microns then the best choices are metals such as Copper, Aluminum, Gold etc. to reflect the beam, and the underlying substrate is then chosen to transmit the wavelength of interest because no mirror coating is 100% reflective... and you don't want to fry your mirror surface. So you let the unreflected (transmitted) beam pass through and then you dump it, either by routing it to a power meter or a beam dump, if high powers are involved.

CaF2 is chosen because it transmits from about 100 nm to well beyond the CO2 transition that is normally used. CaF2 mirrors can have a very good level of flatness and they take a coating quite well. But CaF2 is rather hygroscopic and kinda fragile. There are hardier things that are reasonably transmissive at 10.6 microns, for example ZnSe. I used ZnSe optics with a 150W cw CO2 laser in grad school. Other popular choices are things like KCl. As I vaguely recall when I was working on a CO2 pyrolysis system, we went with ZnSe because it was the least expensive option and seemed less likely to scratch- our windows were under vacuum so that was important.

CO2 lasers are pretty interesting, you can fairly easily build an open air TEA version which requires no sealed gas cavity.

 

[Photo Engineer]

I have seen a small and large CO2 laser being tested. (also a gigantic one but that is another story).

The small laser was being used to cut razor blades in the standard demo and cut 1 in 1 second. It was said to be rated at 1 gilette. The large laser was used to cut a meat cleaver in half in 1 second and was said to be rated at 1000 gilettes as the cleaver had the mass and thickness of 1000 razor blades.

I thought it was a nice interesting demo and very wittily done.

PE

 

[TheFlyingCamera]

Ron- but how many gilettes is the giant one? or is it measured in Boeings, or NORADs?

 

[Mustafa Umut Sarac]

may be , you did not understand the nanoparticle part , as you know , if you want a plastic lens around 1.8 to 2.4 , you have to mix plastic with nanoparticles like titanium oxide.
i m saying , may be it could be possible to mix salt with nanoparticles...
i dont interest with mirrors but easy to shape transparent , hard materials like nacl.
if salt transparent to uv , it could be used to expose pt pd negative at site.
I wrote about video digital camera connected lcd filtered uv lamp ,camera to
hybridphoto.com / it is interesting

 

[Mustafa Umut Sarac]

If you want to hit ufos , you can read oxygen iodine chemical lasers patents at google patents

 

[keithwms]

What about something like PMMA, might it have the transmission spectrum you want? As I recall, it is totally transparent down to about 220 nm, I don't know how far out into the IR but that would be easy to check. And you can mix your particles into your monomer or coat them on top after baking. PMMA in solvent can be poured like a liquid and then baked to form sheets or lenses or whatever.

The obvious problem with mixing salts and nanoparticles is that the former is soluble in water, whereas the latter usually dissolve best in nonaqueous solvents and only after ligand attachment. There are some ligands that will allow water dissolution of some nanoparticles, but it's nontrivial chemistry.

So what do you want, to dissolve a host matrix and nanoparticles and make a n IR -transmissive sheet? I'm still not clear on what your final goal is, how about a sketch!

RE: CO2 lasers, my favourite demo to do was to wave a piece of paper in front of my beam, it would promptly explode into flames, no matter how quickly I waved it through. 150W cw is very, very dangerous, I taped up pieces of paper all over the lab so that I could immediately spot the burn marks if there were any light leaks.

 

[Photo Engineer]

Ron- but how many gilettes is the giant one? or is it measured in Boeings, or NORADs?

Actually, I think it was in Shermans.



PE

 

[Mustafa Umut Sarac]

PMMA , is it acrylic ? Do we need to bake acrylic for harden ? My friend made some pieces from acrylic and i did not hear about baking ?
Keith , can you explain this ? If we harden a liquid with heat
a - it rods
b - it streches
What about the precision of lens ? Do we need to calculate the deformation margin before we cnc the mold ?
I think , this method requires high pressure injection molding ?
I think I loved salt because it does not require lots of it ?

 

[keithwms]

PMMA is easy. You get it in a solvent, you can bake it at 150 degrees for a half hour or so, the solvent leaves and the PMMA hardens into a nice clear plastic with excellent transparency. Looks like plexiglas/perspex (in fact, maybe it's the exact same thing, I don't know offhand). There may be other ways to polymerize PMMA besides baking, but I routinely use it in lithography because it can be baked on a simple hotplate in layers as thin as ~20nm or as thick as whatever. And of course there are other acrylates that you could consider.

Don't ask me how they make lenses with it but I have heard that it can be done! I think if you have a mold then you can probably just pour it in and bake. You wouldn't want to polish/grind it.

Salt optical pieces are a pain in the arse, I avoid them at all cost. There are salt crystals and then there are salt crystals, and polishing a salt crystal, oh boy... there are very good reasons why salt optical pieces are priced so high.