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Old 02-02-2014, 03:24 AM #1
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Default Smallest Gas Laser

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?


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Old 02-02-2014, 09:44 PM #2
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Default Re: Smallest Gas Laser

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.
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Last edited by ultimatekaiser; 02-02-2014 at 09:47 PM.
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Old 02-02-2014, 11:35 PM #3
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Default Re: Smallest Gas Laser

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.
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Old 02-03-2014, 01:52 AM #4
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Default Re: Smallest Gas Laser

Sounds about right. Smallest hene I saw was about 2 inches if memory serves. It was used as a laser gun sight.
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Old 02-03-2014, 09:48 AM #5
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Default Re: Smallest Gas Laser

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?
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Old 02-03-2014, 01:25 PM #6
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Default Re: Smallest Gas Laser

Quote:
Originally Posted by Sigurthr View Post
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.
There are some HV discharge sealed co2 lasers , but they are 30mm in diameter and 250cm long HR mirror to OC mirror .
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Old 02-03-2014, 05:46 PM #7
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Default Re: Smallest Gas Laser

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.
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476.5nm: Argon Ion Line

483nm Omni-643RYB Whitelight Line
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496.5nm: Argon Ion Line
501.7: Argon Ion Line
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520nm: Trustfire A8 Custom

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561nm: Melles Griot 85-YCA-015, Coherent Sapphire 561-20
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577nm: Coherent Genesis MX Taipan OPSL
583.8nm: DPSS line
586.2nm: DPSS line
588.6nm: CNI PGL-M-80

591nm: DPSS line
593.5nm CNI MGL-H-50 Lab Laser
594.1nm: Coherent 31-2230-000 (4.5mW)

604.6nm: LOR-150 HeNe line
609nm: Raman HeNe line?
611.8nm: Melles Griot LOR-150 HeNe (5mW), LOR-006 HeNe (0.5mW)

632.8nm: Spectra Physics Model-127 (>35mW), Model-124B, Model-120, Model-117, others
Melles Griot LHB-580, LHB-190, WHR-570

641nm: Melles Griot RCS
647nm: Omni-643RYB Whitelight Line
659.6nm: Laser Quantum IGNIS DPSS laser 0.5W

671nm: CNI PGL-III-C-100
676nm: Omni-643RYB Whitelight Line
690nm: custom diode laser
694nm: Portable Q-switched Ruby Laser

1152.6nm: SP-120 HeNe alt. mirrors
1523.5nm: Melles Griot LIR-151 HeNe
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Old 02-03-2014, 11:25 PM #8
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Default Re: Smallest Gas Laser

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.
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Old 02-04-2014, 12:46 AM #9
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Default Re: Smallest Gas Laser

Took me a while but I dug up the old image I remember seeing:

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Old 02-04-2014, 01:19 AM #10
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Default Re: Smallest Gas Laser

The smallest gas laser I own is a Uniphase 1007. 6" from mirror to mirror, 4" tube.
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Old 02-04-2014, 09:07 AM #11
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Default Re: Smallest Gas Laser

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.
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457.9nm: Argon Ion Line

472nm Omni-643RYB Whitelight Line
473nm: CNI MGL-473-100, CNI GLP-473
476.5nm: Argon Ion Line

483nm Omni-643RYB Whitelight Line
488nm: Argon Ion Line, Spectra Physics >20mW Cyan DECSL Laser, Coherent Sapphire 488-20 OPSL

496.5nm: Argon Ion Line
501.7: Argon Ion Line
514.5nm: Argon Ion Line
520nm: Trustfire A8 Custom

532nm: CNI MGL-F-3W, Coherent Genesis MX Taipan OPSL, LG Hercules-325
543.5nm: 05-LGR-193 Melles Griot HeNe 3.8mW
Melles Griot LGB-563 HeNe

561nm: Melles Griot 85-YCA-015, Coherent Sapphire 561-20
568nm: Omni-643RYB Whitelight Line

577nm: Coherent Genesis MX Taipan OPSL
583.8nm: DPSS line
586.2nm: DPSS line
588.6nm: CNI PGL-M-80

591nm: DPSS line
593.5nm CNI MGL-H-50 Lab Laser
594.1nm: Coherent 31-2230-000 (4.5mW)

604.6nm: LOR-150 HeNe line
609nm: Raman HeNe line?
611.8nm: Melles Griot LOR-150 HeNe (5mW), LOR-006 HeNe (0.5mW)

632.8nm: Spectra Physics Model-127 (>35mW), Model-124B, Model-120, Model-117, others
Melles Griot LHB-580, LHB-190, WHR-570

641nm: Melles Griot RCS
647nm: Omni-643RYB Whitelight Line
659.6nm: Laser Quantum IGNIS DPSS laser 0.5W

671nm: CNI PGL-III-C-100
676nm: Omni-643RYB Whitelight Line
690nm: custom diode laser
694nm: Portable Q-switched Ruby Laser

1152.6nm: SP-120 HeNe alt. mirrors
1523.5nm: Melles Griot LIR-151 HeNe
3391nm: SP-124 alt. mirrors
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Old 02-04-2014, 04:33 PM #12
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Default Re: Smallest Gas Laser

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.
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Old 02-04-2014, 06:13 PM #13
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Default Re: Smallest Gas Laser

Quote:
Originally Posted by Sigurthr View Post
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.
Did you mean bottom left? See image.



(credit goes to Sam for the image, I just modified it)
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457nm Tunable OMNI 532
465nm Tunable OMNI 532
467nm 1.8W NDB7675
472nm Tunable Omni 532
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476nm Tunable OMNI 532
488nm Single line 35mW
496nm Tunable OMNI 532
502nm Tunable OMNI 532
514nm Tunable OMNI 532
520nm 40mW MSV2
532nm Various pointers
543nm GreeNe 2mW
568nm ~20mW ArKr whitelight line
589nm 126mW PGL-III-C
594nm Rigel-6; 4.75mW + <1mW
604nm 2x <1mW multiline
609nm Ne Penning mixture Ar line from REO 612nm
612nm HeNe 1mW ML 4 line; Raman 6 line
629nm <1mW Raman 6 line
633nm 1-4mw red HeNe
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HeNe/gas reference thread

Last edited by bloompyle; 02-04-2014 at 06:14 PM.
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Old 02-04-2014, 06:47 PM #14
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Default Re: Smallest Gas Laser

Yeap that one, my phone likes to autocorrect "lrft" to "right" when I mistype "left" haha. Didn't catch that one.
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Old 02-05-2014, 01:44 AM #15
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Default Re: Smallest Gas Laser

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Originally Posted by bloompyle View Post
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.
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.

Quote:
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.
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.
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Old 02-05-2014, 09:22 PM #16
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Default Re: Smallest Gas Laser

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.
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405nm 500mW
442nm 15mW Melles Griot HeCd
445nm 1W
455nm 100mW LaserBTB
457nm Tunable OMNI 532
465nm Tunable OMNI 532
467nm 1.8W NDB7675
472nm Tunable Omni 532
473nm 2x 100mW Melles Griot lab heads w/ PSUs
476nm Tunable OMNI 532
488nm Single line 35mW
496nm Tunable OMNI 532
502nm Tunable OMNI 532
514nm Tunable OMNI 532
520nm 40mW MSV2
532nm Various pointers
543nm GreeNe 2mW
568nm ~20mW ArKr whitelight line
589nm 126mW PGL-III-C
594nm Rigel-6; 4.75mW + <1mW
604nm 2x <1mW multiline
609nm Ne Penning mixture Ar line from REO 612nm
612nm HeNe 1mW ML 4 line; Raman 6 line
629nm <1mW Raman 6 line
633nm 1-4mw red HeNe
635nm Raman 6 line
638nm 20mW single mode
640nm Raman 6 line
647nm ~10mW ArKr whitelight Kr line
650nm Raman 6 line
658nm LPC-826
685nm 20mW
808nm Modded NewWish pen


HeNe/gas reference thread
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