Cooking with Gas

[mixedgas]

[QUOTE=SoulFeast;294140]I hate to be blunt ... but there's no way that either of those results are the same as the laser output displayed earlier. For instance, there is no sequence of 5 lines that resemble C - G in your listings of results. There is no sequence of 3 lines resembling 587, 604, and 612. I'm not even bothering with accuracy of the wavelengths to anything beyond the decimal. I am simply taking note of simple observations.

QUOTES IN BLACK, MY REPLY IN VIOLET.

Be as blunt as you like, this spectrum set was taken with tubes C AND D, Tube A is too precious for transport for initial tests. Tube B just lases on a few lines due to Age. Until the methodology works, there is no reason to haul the good one around. I have four of these.

I do believe something very interesting is happening here.

Yes, your seeing four wave mixing clear as day.
Were those output spectra pictures taken immediately upon power on? After long warmup? Notice lines coming in and out besides J?

Lines come and go as four wave mixing lines have to occur where both the spectral line and the output line have N integer multiples of wavelengths fitting in the cavity. No line on this tube is that pressure sensitive as to die after warmup. They all appear at power up if the cavity length is correct and if the current is in the correct range. As the cavity length changes, what combination of M +N /2 integers that can lase varies in a cyclical pattern, thus four wave mixing and Raman lines come and go in a cycle. They are however current sensitive, and increasing the PSU current causes the balance of the upper state distribution to change. So the tube emits less lines at higher current. This is counter-intuitive to a person who has not studied the physics, but I get more lines at low current.

Active Cavity Length control with a Piezo is on the way. Tests with a cheap third mirror and a bench supply with a Steiner and Martins Piezo driving the third mirror show 0-2V from a bench supply take the tube through several full M+N/2 ranges. The Mixing and Raman lines clearly change in presence and intensity with cavity length. Fortune shines on us in that there is a fairly large range in each cycle where lasing is at peak intensity. The issue is I need a specialized supply with a ten turn potentiometer, it is too difficult to obtain stability on the weaker lines with a standard 270 degree potentiometer.

If the tube was tested upon immediate power up, this could be an issue. My experience with these tubes shows that even after a good run, one day later, or even hours later, causes a loss of lines upon immediate power on. It can take several minutes, to hours (pending time left unpowered) of runtime before all the lines begin to stably lase. Just food-for-thought.

That is explained above. A few lines may be pressure sensitive, but the four wave mixing lines, as is well documented, are simply length sensitive. By scanning the piezo I can watch them wink in and out. By slightly heating/cooling your end mirror assembly a fraction of a degree or pressing on it ever so gently with a piezo, you may see the line re-appear. Tiny in this case may mean simply placing a block of styrofoam insulation near the tube, the change in length required is in the range of a few micrometers.

Lastly, according to all of my research, 609.6nm is not dependent on Helium. It is an interaction between Argon and Neon. At least, excess Argon brings it out via interaction with Neon. Not 100% sure if that has any merit, but I'm just putting it out there.

Sam has tried to explain this to you, and I will as well. If the 609 line was a Argon or Argon related line, you'd see the Argon neutral spectral lines in the tube sidelight. There is a certain minimum amount of excited atoms needed for sustained lasing to occur. At any given time only a small percentage of atoms are in Population Inversion, the precursor to lasing. The amount of atoms lasing must exceed the threshold that overcomes the mirror losses and adsorption losses in the tube. At any given time only a small percentage of the large atoms are colliding with the Helium and the Walls of the Tube (Yes, you need wall collisions for HENE to lase), and are thus properly excited. To have that minimum population of metastable atoms present, means the tube would HAVE to contain enough Argon to be detected with a grating BY EYE from the sidelight spectrum.

Thus the "Money Shot" to start to prove your theory is a sideline spectrum taken with a cheap plastic diffraction grating in front of a cheap digital camera showing neutral argon spectral lines. I CHECKED the sideline spectrum of C and D with the spectrometer, there are no neutral Argon lines present. Just good old Helium and Neon.

My bet is that your line is easily current and length sensitive, and that reducing current and slightly altering length will cause it to be more stable. It may be right on the edge of conditions you can obtain with what you have. The pressure in the tube will rise as the tube heats up, but the percentage of pressure change is very low, and can easily be calculated from PV = NRT, the Ideal Gas Law. V is constant in the sealed tube, so P tracks T, and the rise in T is not that great..

I just verified that 730 is a known HENE lasing line, with no mixing required. So there is one more line from the spectrum verified. Ed only did the calibration at a few points, so there is going to be error in those numbers. One tries to work with what one has to work with.

Keep in mind that nearly every other HENE (632.8 only) on the planet is designed to prevent these lines from lasing, and has a low Q cavity to allow power to be released for use in the external beam. This laser has a cavity designed for exceptionally high Quality Factor (Q). The Q is so high across such a broad spectrum that we see lines that cannot normally lase in any other tube. The possibility presence of unknown lines is thus exceptionally high, and it need not be from a trace gas by any means.

Steve

[SoulFeast]

The four-wave mixing is now what I was referring to as "interesting". I was referencing the lack of similarity between the two spectrum sets posted, and the pictures above.

As far as four-wave-mixing not being dependent on pressure, I will not argue this. However, you and I both know that runtime affects the temperature of the tube, thus expansion of materials, thus cavity length increases. This is why my PMS lases 1-3 lines at start (629, 632, and 635) then begins lasing the other 4 lines after that. 612, 640, 650, and 653 (652.5 4-line-mixing line). So these other lines come in later. The 652.5 comes and goes, and eventually stabilizes. As do the other 6 lines. It gets to a point where all 7 are lasing in harmony.

This is why I questioned whether the results were taken upon powerup, or warmup. Though neither would explain the difference in similarity. I just was not aware that there were 4 tubes in question here. Which explains why the two spectrum sets do not match the pictures.

As far as 609.6 is concerned. Sam and I are ont he other side of the fence. We are both saying 609.06, however, I just wanted to add that part about 609.6 being a byproduct of Ne and Ar.

I feel as though we are all on the same page, but we just don't realize it.

A vast majority of our math (if I recall correctly) pointed at 609.06.

[142laser]

The high Q lasers are amazing but for sure the only gasses that can be at work are He and Ne...direct lasing, Raman or four wave mixing. Lots of lines. With IR mirrors you would find many, many lines but see none of them.