Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers

Buy Site Supporter Role (remove some ads) | LPF Donations

Links below open in new window

FrozenGate by Avery

Smallest Gas Laser

R

Roam

0
Joined
Nov 21, 2011
Messages
106
Points
18
What is the minimum tube length we need that would still support lasing, if we only know that the maximum gain coefficient of the gas medium is, say 0.5 m^-1?

I've never seen a commercially available gas laser smaller than ~4-5" in length. But I did some calculation that turns out to be such an unrealistic tiny number...

I equated the gain coefficient to the distributed loss coefficient (amount lost in one trip across the etalon) since this is the minimum gain to start oscillation:

γ (ν) = α_r

γ (ν) = 1/2L * ln(1/R1*R2)

^ we can then evaluate L if the end mirrors for optical feedback have reflectivities of say 99% and 90% for instance. Anyway we'll get an extremely tiny length. Is something wrong with the calculation or am I missing something? :confused:
 





Depends on the optics. But frankly anything smaller than a few inches is not going to really see a useful amount of gain outside of theory. Typical amount of gas in one of those small Tubes is less than a square cm at STP. That and error comes into play.
 
Last edited:
I seem to remember reading about a 2" or 2.5" HeNe designed for early laser disc players. Also, there are some ~5W CO2 lasers that are pretty tiny, not sure on dimensions though.
 
Sounds about right. Smallest hene I saw was about 2 inches if memory serves. It was used as a laser gun sight.
 
Hmmm... interesting, ~2" is quite small. I knew the very early video disk recorder invented by Phillips used a compact model of HeNe laser but didn't how small they were.

But how come the equation evaluates to ~4"? I calculated this based on the threshold gain condition γ (ν) > α_r. How do they lower the gain coefficient of the gas in order to shorten the tube?
 
Sigurthr said:
I seem to remember reading about a 2" or 2.5" HeNe designed for early laser disc players. Also, there are some ~5W CO2 lasers that are pretty tiny, not sure on dimensions though.
Click to expand...

There are some HV discharge sealed co2 lasers , but they are 30mm in diameter and 250cm long HR mirror to OC mirror .
 
That actually makes sense. Not positive how big it was, saw it a long time ago, I'll see if I can find out more. 4 sounds actually a bit more reasonable.
 
I don't know much about the equations here.

However, I can say that you can throw reflectivity out the window if you're talking about gas lasers of any kind. The OC on a green HeNe averages about 99.92 - 99.95%. The OC on a N2 laser or CuBr/CuCl/CVL is <5%.

The mirrors will be different wavelength to wavelength, maker to maker, laser to laser.

If you want to know of the smallest possible, that's a big stretch. The smallest HeNe, N2, CO2, or ion? Now you're in the fairly "simple" ballpark.

Tubes with a smaller RoC, with a higher reflectivity, will allow for smaller lasers to be built. Though there comes a point where you're reflecting more light, to gain more light, than you will see. So there will be a point where the reflectivity you will need for significant gain, will actually surpass the transmission necessary for visibility.

Sure, you could possibly squeeze out a few uW from a 1" HeNe with short RoC and a pair of decent HR mirrors. Perhaps one mirror would be a gold 99.9999% HR. Though is it really worth it? A laser that small would almost NEED to be a wide bore multimode tube.

As far as CO2, well, they can be very small indeed. I'm building one now with an active discharge length of about 30cm, before the mirrors. After the extension to mirrors, it'll be about 40cm. That's from an HV tube! There are RF and waveguide tubes much shorter than that.
 
Last edited:
Took me a while but I dug up the old image I remember seeing:

mghenes1.jpg
 
The smallest gas laser I own is a Uniphase 1007. 6" from mirror to mirror, 4" tube.
 
the one in the sight was a melles griot. I just asked my friend. he actually still has it. no model number, but it looks like the one on the far left on the bottom. it's also a 6" cavity I think by eyeballing it in the pic.
 
Of the group of three bare tubes on the bottom right of the image, the top one looks to be <5" long, mirror to mirror.
 
Sigurthr said:
Of the group of three bare tubes on the bottom right of the image, the top one looks to be <5" long, mirror to mirror.
Click to expand...

Did you mean bottom left? See image.

5e6k.jpg


(credit goes to Sam for the image, I just modified it)
 
Last edited:
Yeap that one, my phone likes to autocorrect "lrft" to "right" when I mistype "left" haha. Didn't catch that one.
 
bloompyle said:
I don't know much about the equations here.

However, I can say that you can throw reflectivity out the window if you're talking about gas lasers of any kind. The OC on a green HeNe averages about 99.92 - 99.95%. The OC on a N2 laser or CuBr/CuCl/CVL is <5%.

The mirrors will be different wavelength to wavelength, maker to maker, laser to laser.

If you want to know of the smallest possible, that's a big stretch. The smallest HeNe, N2, CO2, or ion? Now you're in the fairly "simple" ballpark.
Click to expand...

To be specific I guess my question concerns the HeNe. I wanted to know the smallest tubes ever made commercially available, as well as the smallest "working" HeNe theoretically possible.

Around ~99% reflectivity I get ~4". However, I assumed a lossless medium because I didn't know an experimental value for the attenuation coefficient.

Sure, you could possibly squeeze out a few uW from a 1" HeNe with short RoC and a pair of decent HR mirrors. Perhaps one mirror would be a gold 99.9999% HR. Though is it really worth it? A laser that small would almost NEED to be a wide bore multimode tube.
Click to expand...

Why does it need to be a wide bore? I thought atoms relax back to the gnd state due to collisions with the wall of the tube, therefore the gain should increase with decreasing tube diameter, no?

You could be right though, I have to look up some textbooks as I'm not sure how the bore width factors into the equation.
 
They made smaller tubes do higher output, by widening the bore. Getting 10mW from a 12" tube won't work unless the tube has a wider bore, thus multimode. There is more light population, and more artifacts, but more gain.

It's the accumulation of many factors, laws, and concepts, that determins the gain of a laser.

Wavelength
Resonator length
Mirror quality
Mirror H/T percentages
Mirror ROC
Vacuum pressure
Gas purity
Gas ratio
Voltage and current provided
Coating quality on the mirrors, Bore width
Bore quality
How well the laser was outgassed.

All these things come into play not only from kaer and maker, wavelength to wavelength, but from each individual laser to laser. You could compare the exact same moduel number, with the serial number 98MG0334001 to 98MG0334002, and find at least 3 or 4 differences in some of the above factors alone.

Gas lasers aren't so much a science as it is an art. The perfect "recipe" or masterpiece comes with generations of trial and error.

To come up with the perfect recipe for the smallest tube would take years and decades of factoring changes in one of 2 or 3 dozen factors, one at a time. Something as small as a 0.025torr difference can determine output in some lasers.

I don't want to discourage you, I just want you to know it's a bit more difficult that crucnhing numbers for mirror ROR and HR%, and spitting out a number. There's a TON more to it than that.
 
You must log in or register to reply here.


Back
Top