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Old 06-22-2010, 03:32 AM #1
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Default Building a HeNe...

Hio

Backround: I got hired for a summer internship with the DOD in....a LASER testing facility!

Pretty sweet right? I get to play with anywhere from 10W-100W lasers everyday. Most of its IR though, so not as much fun.

Anyway to the point. My boss says I can build a HeNe laser, but he want's me to figure out what I need and buy it. He's already got some stuff, like a power supply. The main thing he was saying I needed to look at is mirrors. He says I need like a 100% reflective one, and then a 98% reflective one, with exactly the right concavity.

So anybody know anything about mirrors, where to get them, and what kind I need?

So, I'd appreciate any help in this regard.

Coolios! Thanks,
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Last edited by ShaOwned; 06-22-2010 at 03:36 AM.
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Old 06-22-2010, 03:42 AM #2
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Default Re: Building a HeNe...

I would just buy a premade tube, or are you on a government grant? If it's a grant, I'd get a larger HeNe from Newport or Edmund Optics, they're expensive but worth it, they're brand new tubes and power supplies. Another option, on eBay there are 23mW polarized JDSU tube setups for a few hundred $$$. I don't think you need mirrors unless you've got a huge HeNe tube with external mirrors. Otherwise you'd need access to a glass shop and vacuum pumps and nearly 100% pure Helium/Neon mixtures. Hope it helps.
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Old 06-22-2010, 04:02 AM #3
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Default Re: Building a HeNe...

Mirrors in the US come from Melles Griot or REO optics in Denver. Its not a easy mirror to make.

Do you really mean build a tube or just buy a single or dual brewster tube and build a external mirror cavity?

632.8 or the IR lines in a soft sealed or indium sealed tube is doable by a amateur, but the other wavelengths or hard sealed optics are a art form.

What are you really trying to do?

We have links and plans in Sam's Laser FAQ, in the gas laser section, and I have tons of mirrors, and the cavity design software, but I need to know more to help you.

I know guys who have done the hene from scratch, its a good lesson for a intern, if the lab your in has vacuum to at least 10^-5 torr. The glass to metal seals and indium based mirror mounts are easy enough if you have access to a lathe, a real full blown glass blower is not needed, unless your making a really long tube. Co2 lasers are much easier though!

Costs, based on a indium based seal and getting lucky on the mirrors.

I've seen mirrors for red as low as 60$ each for a one off purchase, or you can pull them from dead gas analysis machines on ebay.

Mirror, HR, someplace between free and 50-250$
Mirror, OC, someplace between free and 50-250$
Bore capillary tubing, about 12-24$ for a short piece.
1/4" diameter glass to metal seal, kovar to pyrex, 3 needed, around 48$ total if you get the right supplier.
flanges, 1.3", vacuum, 16$ each, 4 needed,
Gas, helium neon blend, disposable tank, non isotopic, 7:1 mix, about 150$.
Some scrap steel, some brazing material, a bit of stainless steel, and a AR coated 1/4th inch diameter window for one end of the tube, and aluminum for the resonator. About 2" of fine indium wire.
Conflat gaskets, 10 for 15$

The flanges and indium go away if you make a soft seal tube. In that case you need a box of hysol 1c for the epoxy at around 10$ a box, and your tube lifetime will be measured in days or weeks.

A one Brewster hene tube sells for around 700$ new, and 200$ used, add up the above, your call.

I sent you a PM.


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Old 06-22-2010, 04:37 AM #4
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Default Re: Building a HeNe...

I do know its going to be red, but other than that I'm not yet sure of specifics.
I'm going to find everything out at work tomorrow and then I'll update you guys.

I do think its an external mirror cavity though. I think he's already got a tube. I'm probably not qualified enough to build that =P

Thanks for the posts!
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Old 06-23-2010, 01:32 AM #5
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Default Re: Building a HeNe...

So, I've got the details.

632.8, we are buying a tube, 2 brewster, and 2 external mirrors.

He wants me to find Silver 1-2cm diameter mirrors, AR coated. One 100% reflective(or close) and the other ~98% reflective.

I've checked all the sites I've been given so far without much luck. Any ideas?
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Old 06-23-2010, 02:11 AM #6
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Default Re: Building a HeNe...

[QUOTE=ShaOwned;723138]So, I've got the details.

632.8, we are buying a tube, 2 brewster, and 2 external mirrors.


This is unclear...

OK, Am I correct in assuming your buying the hene resonator mirrors with the tube and need external mirrors for something else? Because silvered mirrors will not lase a hene tube. Only dielectric mirrors will achieve the red line. Are you buying factory dielectric coated mirrors with a radius for the laser cavity and then 2 more mirrors for your experiment?

Not enough details. I know where to send you, but I need a bit of a better picture.

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Old 06-23-2010, 02:22 AM #7
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Default Re: Building a HeNe...

Hmm, I'm not quite sure.

I know it's external mirrors, and he wants silvered ones.
I think he said he wanted the tube to have two "windows"

Does that help? If you haven't noticed, I'm not too knowledgeable about this stuff yet >.>

EDIT: From Sam's FAQ "High performance HeNe lasers may have Brewster angle windows on the tube for use with external mirrors."
I don't know if thats what we're doing, but that sounds like it to me.

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Old 06-23-2010, 03:41 PM #8
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Default Re: Building a HeNe...

Well if you've got an external mirror HeNe tube, then the tube will already have two brewster windows.
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Old 06-23-2010, 06:42 PM #9
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Default Re: Building a HeNe...

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Originally Posted by MarioMaster View Post
Well if you've got an external mirror HeNe tube, then the tube will already have two brewster windows.
Yes, that sounds correct.

So are we saying that Silver coated mirrors will NOT work for this use and I need dielectric ones?
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Old 06-24-2010, 01:30 AM #10
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Default Re: Building a HeNe...

Quote:
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Yes, that sounds correct.

So are we saying that Silver coated mirrors will NOT work for this use and I need dielectric ones?
HENE will only lase with dielectric cavity mirrors at 632.8

Also 2% is way too high a transmission.

What is the active length of the tube, I can run some calculations.

SEE page 261 of this, which has complete calculations for designing a typical hene. The 2nd edition is better.

http://books.google.com/books?id=x3V...page&q&f=false

Oh, and if you did use a silvered mirror, the IR lines share a upper level and will deplete if not kill the red lasing.

SEE ALSO: http://www.repairfaq.org/sam/laserhen.htm

Quoting Sam, and my measurements are similar:

Here are some (approximate) typical OC reflectances for red (632.8 nm) HeNe lasers determined by measuring the actual transmission (R = 100 - T) of a red HeNe laser beam through the optic with a simple photodiode based laser power meter:

* OC from 0.5 mW, 12.5 cm Melles Griot model 05-LHR-002-246 internal mirror HeNe tube: 99.3 percent.
* OC from 2.25 mW, 26 cm Spectra-Physics model 084-1 internal mirror HeNe tube: 99 percent.
* OC from 20 mW, 75 cm Aerotech model unknown internal mirror HeNe tube: 97.7 percent.
* OC from 50 mW, 177 cm Spectra-Physics model 125 large frame external mirror HeNe laser: 99.4 percent

Steve

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Old 06-24-2010, 02:02 AM #11
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Default Re: Building a HeNe...

hard sealed mirrors will look like this, they can be used for brewster tubes, but may be a little harder to align.

Building a HeNe...-untitled.jpg
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Old 06-24-2010, 03:21 AM #12
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Default Re: Building a HeNe...

Quote:
Originally Posted by LSRFAQ View Post
HENE will only lase with dielectric cavity mirrors at 632.8

Also 2% is way too high a transmission.

What is the active length of the tube, I can run some calculations.

SEE page 261 of this, which has complete calculations for designing a typical hene. The 2nd edition is better.

Laser fundamentals - Google Books

Oh, and if you did use a silvered mirror, the IR lines share a upper level and will deplete if not kill the red lasing.

SEE ALSO: Sam's Laser FAQ - Helium-Neon Lasers

Quoting Sam, and my measurements are similar:

Here are some (approximate) typical OC reflectances for red (632.8 nm) HeNe lasers determined by measuring the actual transmission (R = 100 - T) of a red HeNe laser beam through the optic with a simple photodiode based laser power meter:

* OC from 0.5 mW, 12.5 cm Melles Griot model 05-LHR-002-246 internal mirror HeNe tube: 99.3 percent.
* OC from 2.25 mW, 26 cm Spectra-Physics model 084-1 internal mirror HeNe tube: 99 percent.
* OC from 20 mW, 75 cm Aerotech model unknown internal mirror HeNe tube: 97.7 percent.
* OC from 50 mW, 177 cm Spectra-Physics model 125 large frame external mirror HeNe laser: 99.4 percent

Steve
I haven't yet ordered a tube. I'm having a bit of difficulty finding one. Apparantly Melles Griot doesn't have them listed on their site, but I ordered a calalog. I think we're planning around 9-12 inches. I did see one on a seemly more questionable site that was about 13.7in I believe.

I'll take a look at those links and pass them on to my boss, thanks.
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Old 06-24-2010, 04:17 AM #13
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Default Re: Building a HeNe...

[QUOTE=LSRFAQ;723198]
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dielectric coated mirrors
While we're here, is there a difference between dielectric coated mirrors, dielectric mirrors, dichroic mirrors, and dichroic filters?
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Old 06-24-2010, 02:15 PM #14
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Default Re: Building a HeNe...

[QUOTE=Cyparagon;724070]
Quote:
Originally Posted by LSRFAQ View Post

While we're here, is there a difference between dielectric coated mirrors, dielectric mirrors, dichroic mirrors, and dichroic filters?
At first, I was tempted just to post, "What is the sound of one photon bouncing off a tree in the woods, if not one was there to see it."

Or, "In the beginning there was man...."

And then run like hell.

OK, can of worms time, this could be a whole masters dissertation:

A bunch of terms, all meaning in reality the same thing. There is NO difference between a dielectric mirror and a dichroic filter, in concept , for example.

A conventional mirror is a film of a bright metal, on a polished substrate.
The outer electrons in the metal do a pretty good job, for the most part, of redirecting photons that hit them, and for bare silver or bare aluminum, hit between 70-80% reflection, over a broad range of wavelengths. Some do better at other wavelengths outside the visible, such as gold or silicon for the far IR.

Sometime in the 1920s or 1930s, as math and instruments got better, people noticed that some glass lenses exposed to chemicals or the elements, for many years, were much more efficient at transmitting light, and no longer showed the 4% loss from Fresnel Reflection on the exposed side. Snell's law and the index of refraction (How much a given material bends light by slowing its speed) Were already well known, but the technology of glassmaking was improving by leaps and bounds. Investigation of the weathering showed that minimal reflection loss happened when the coating was 1/4 wavelength thick, at the targeted color. It also showed that the coating had a lower index of refraction then the lens.

At first done chemically, (Zeiss GMBH, Germany) and then by vacuum deposition, the simple antireflection "V" coat was a revolution in optics, allowing more then 2 or 3 lenses to be used in a telescope with low loss and a dramatic reduction in ghost images.

The next step was to try depositing more then one layer. A modern reflective coating, in its simplest form, consists of alternating one fourth wave thick stacks of high and low index materials. Every time the light passes through the interface of a 90' Low to High stack, a good percentage of it has its phase shifted 180' and is thus reflected. More stacks results in more reflection, thus a 15 layer stack of coatings can approach 99% reflection for one wavelength.

Now this is where the fun starts. If you put down, just for example, 10 red stacks, and 10 green stacks, and 5 blue stacks, you can start to approach a white mirror. By changing the thickness and type of stacks, you can go the other way and make a bandpass filter. Ie put down a reflect red and a reflect green stack, blue can still pass through. By moving away from just 1/4 wave thicknesses, the math gets much more complex, but you can then really start to tune things and make it efficient.


Ok so far we're dealing with 0 degree incidence, where the light hits the mirror at exactly 90 degrees (A "normal", in geometry) At normal incidence, its easy to make a 1/4 wave thickness. Now here is the rub, if the simple dielectric coating, made for normal incidence, are tilted, the reflectance falls off as the cosine of the angle. This is why light show folks end up "angle tuning" their combining dichros. So if you know what your target angle is, you can make the coatings a little thicker or thinner, as needed to correct for the thickness error of the light hitting at a angle. Hence the ability to make a dichro for combining laser lines.

So all dielectric coatings use the same physics, its just the designers change the thickness and number of layers to tune for a given set of properties. It gets even more fun if you add thin metal layers , between the layers of high and low index materials. metal films less then 1/4 wavelength thick are just spacers, they don't do much, allowing for some even crazier interactions.

So a dichro, is a dielectric coating, which means "insulator" and can be from one to say 100 layers, tuned to do a specific function, either bandpass, band stop, lowpass or highpass, or antireflect, or a combination of those functions. Dichroic simply means, two colors. A dichroic mirror is a subset of dielectric optics, if you want to draw one of those Gawd Aweful "Venn Diagrams" they make you learn in Algebra.

Thus something like a Newport BD-1 coating, which is reflect 99% of S (vertical) polarization from 0 to 45 degree incidence, from 400 to 700 nanometers wavelength, is about a 80 layer stack.

Since dichroic/dielectric coatings are not that expensive, and are getting cheaper all the time, professionals wonder why laser hobbyists still buy aluminum (75%) or enhanced aluminum (90-93%) coatings and loose large amounts of expensive laser power. As the laser power goes up, in the visible, that loss turns to heat, and its not uncommon for a 1-2 watt visible laser to start to burn a cheap metal film coating if a speck of dust lands on it or it gets focused. Dielectric coatings can handle huge amounts of power.

Modern "hard" dielectric coatings are also scratch resistant, meaning they will last much, much , longer then a aluminum or silver metal mirror.

For a low gain laser amplifier, such as helium neon, where the per pass gain is perhaps one tenth of a per cent, it becomes essential to use dielectric mirrors for the laser cavity. The losses from even a overcoated aluminum mirror, would prevent lasing.

Sometimes you see the terms "enhanced aluminum" or "Overcoated Aluminum", this is a compromise, it is a metal film mirror with two or three dielectric layers coated over the aluminum to bring the reflectivity up while keeping costs down. These are used in things like photocopiers or video projectors where 95% reflection is good enough.

Let me give one example. A simple AL mirror, 70% reflection at 532 nm. Launch power is 1 watt. At after the first mirror, you have 700 mW, after the second mirror, you have .70 x 700 = 490 mW. So if you used simple AL mirrors in a laser show projector, your down to 1/2 power just after the galvos alone. (OUCH!, and I've seen projectors with simple AL mirrors!)




A good source for low cost Dielectric mirror is One Stop Laser Shop.

Steve

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