Kvant makes me jizz in my pants

[White-Light]

Hi all

I am in KVANT right now and for next 3 days, so...like in heaven.

There are 24x1W RGBs and 1x25W RGB.

Made in Fiesta2 using Matrix interface.

Have a nice day.

Hey Martin,

Any chance of videoing a show for us?

That's too many lasers not too ask.

[White-Light]

A better example is a yellow HeNe. A yellow HeNe has mirrors that reflect light at 594 nm and pass other colors. Thus you only have gain in the cavity at 594 nm. And the output light is yellow at 594 nm.

Adam

I'm still not 100% convinced Adam because having read Springers on HeNe, it says the excitement of the Ne produces photons at different wavelengths and that the different lines form in the cavity containing that mixture of these photons pre the cavity that actually amplifies the light. As you say the line to be amplified is selected using mirrors pre amplification from this cavity. Unfortunately it doesn't explain how the different lines result - whether or not photons are produced natively at all the available wavelengths or whether there's any kind of collision or joining of photons going on to produce new wavelengths as is seen with yellow solid state lasers. However, as you know far more about this subject than me, I'll take your word for it alebit with an open mind to the possibility that the yellow is still formed from some kind of mixing of photons pre excitement in the same way that yellow solid state is mixed per amplification by the crystal but un seperable because amplification of the mixture causes that lasing crystal to lase natively at the same frequency as the mix. I have to still wonder if maybe that isn't what is happening here, the Ne photons are mixing and the amplification of the, yellow line that results causes the He to lase natively at the yellow frequency. I'd really like to see a text book definitive answer to put my mind at rest once and for all as the idea of non prime colours forming out of thin is air is a little hard to grasp if entirely possible.

[allthatwhichis]

I can say, I have a yellow HeNE; when I shoot it through a diffraction grating, it splits into ONLY one "colored" beam, yellow, at ONLY one wavelength, 594nm. To me, that is what Adam is saying. If it were a mixture of red and green; there would be the two colors/wavelegths after the grating.

[White-Light]

I can say, I have a yellow HeNE; when I shoot it through a diffraction grating, it splits into ONLY one "colored" beam, yellow, at ONLY one wavelength, 594nm. To me, that is what Adam is saying. If it were a mixture of red and green; there would be the two colors/wavelegths after the grating.

I understand that but so does solid state yellow but its actually made of green and red. When you look at the pump frequencies they're twice 532 and 671. Its only when you pump the crystal with both frequencies ie IR equivalents of green and red that the crystal gives out the IR equivalent of yellow which when the frequency is halved leaves you with native yellow which can't be seperated by diffraction. So on solid state yellow, yes the crystal is emitting native yellow but only after its pumped by both red and green equivolents which is what makes me wonder if this isn't what effectively also happens with HeNe and others.

[buffo]

the idea of non prime colours forming out of thin is air is a little hard to grasp if entirely possible.

You need to throw out your idea that "prime colors" are special. They're not. At least, not when it comes to the physical process of generating light in the first place. They are primary only in the sense that they *roughly* correspond to our own color vision setup. That is, the three "bands" of our color receptors, which are basically blue, green, and red. However, even this basic model is hopelessly flawed, both because the actual peaks for the three types of cones are actually closer to blue, green, and YELLOW, and also because there is significant overlap between the receptors, allowing the "long" wavelength cones to also detect red (albeit indirectly).

For more detail about human color vision, see the Wiki... But here is a relevant paragraph:

The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities: short (S), medium (M), and long (L) cone types, also sometimes referred to as blue, green, and red cones. While the L cones are often referred to as the red receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green, although they are often depicted as such. It is important to note that the RGB color model is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye.

So you see, RGB is nothing more than a model that makes it easy to make predictions about how the human eye will perceive various mixtures of light. It doesn't even correspond very well to the actual peaks in our vision, but more importantly, it has *NOTHING* to do with the physics of how that light is generated. For that, we need to talk about electrons... Specifically, electrons that have been excited to high energy levels and then allowed to fall back to lower levels (or even all the way to the ground state).

When electrons fall from high energy levels to lower ones, they emit electromagnetic radiation. The larger the change in energy level for a given electron transition, the more energy the resulting photon will have, and thus the shorter wavelength it will have. (eg. higher frequency) So, if you have a transisition of around 2.1 electron-volts, you'll get yellow. (594 nm works out to 2.08 eV) But if you have a slightly less energetic transistion - say, around 1.95 eV - then you'll get red (at 635 nm).

Allow me to run through a quick primer on laser operation, particularly as it relates to electron energy... Lasers exploit a couple unique features of these electron energy transisions in order to create the beams we all love. First, it's important to remember that electon energy levels are quantized. That means that they have specific, allowable values. They can't be at any arbitrary level; rather, they must be at one of a (relatively) small sub-set of energy levels. The next thing you need to remember is that electrons are always trying to get back to the ground state. They don't like being "excited", which means they don't like being at a high energy level.

When an electron is excited to a higher energy level (that is, a level above the ground state), it will spontaneously fall to a lower level. Sometimes it falls all the way to the ground state, but more often it will fall to a lower energy level that is still above the ground state. It make take two or three transisions for the electron to finally reach the ground state. But each time it falls to a lower energy level, it will emit a photon of electromagnetic radiation, and the frequency of that photon will be proportional to the magnitude of the change in energy of the electron. (The quick and dirty conversion is eV= 1240/wavelength in nm by the way.)

Some materials have "metastable" energy levels for their electrons. These are specific energy levels where, once the electron reaches this energy state, they will remain at that level for an unusually long period of time. In most cases, this time period is still very short (measured in milliseconds), but even so, this is a long time for an electron to stay excited. These metastable energy levels are the ones exploited by most lasers, since one of the goals is to obtain a population inversion, where the bulk of the electrons in the lasing medium are excited, instead of at the ground state. And obviously, it's easier to obtain a population inversion if you have some time to get all the electrons excited before they start falling back to the ground state. The metastable energy level buys us this time we need to finish the pumping.

The final key to laser action is what's known as stimulated emission. This is the process where an excited electron is struck by a passing photon of the exact same energy level. Such an interaction "stimulates" the excited electron to also fall to the same lower energy state, emitting a photon of the same wavelength. This is what allows lasers to create light of a single color. By trapping the photons in a resonating cavity consisting of a pair of mirrors, it is relatively easy to build up a strong beam through many, *many* iterations of stimulated emission. This is why you need a population inversion... You want to be sure that when the growing stream of photons strikes an electron, it's one that is excited. That way it will be stimulated to fall to a lower energy level, adding yet another photon to the beam.

Now, if the electrons in a given lasing medium have several different lower energy levels that they can fall into, then the resulting photon stream will have several different wavelengths - each corresponding to one of the energy transitions. Assuming that your mirrors will reflect all those different wavelengths, you'll get multiple colors out of the laser all at the same time. This is what happens in an Argon or Krypton laser (or even a Kr/Ar mixed gas unit).

As it turns out, in a Helium-Neon laser, the Neon (which is the only gas that lases) also has multiple energy levels that the excited electrons can drop into. Thus, it's also possible to get several different wavelengths. However, because the tube doesn't have nearly as much gain as an Argon or Krypton laser, the optics for a HeNe need to be selected such that only a single line is allowed to resonate (that is, only one color will be reflected and allowed to build up inside the cavity). Photons of any other wavelength simply pass through the mirrors and are lost, so they never get to make multiple trips through the tube to pick up strength.

If you install mirrors that will reflect 632.8 nm light on a HeNe, you get the familiar orange-red that we've all seen. However, if you change those mirrors for ones that only reflect 594 nm, you will get yellow. In this case, the electron transitions that yield a red photon are essentially wasted. But the ones that create yellow photons are saved in the cavity, so they can build up.

The problem here is that the electrons don't really want to fall to the specific energy level that makes yellow light. They'd rather fall to the level that emits red... This is why the same length HeNe that might make 15 mw as a red laser will only make 1 or 2 mw as a yellow laser. The gain for yellow is lousy. But so long as you can maintain the population inversion, eventually the yellow photons will build up enough to give you a beam.

A HeNe has a few other lines that it can lase at - given the proper optics. These include 543 nm (green), 604 nm (orange), 612 nm (darker orange), an even a few IR wavelengths as well.

So you see, "primary colors" have nothing whatsoever to do with the physics behind how light is generated. They are merly part of a convenient model for color theory that helps us make predictions about how our eyes will react.

Adam

PS: For those of you who are physics geeks, yes, I know I ignored quantum effects on the output wavelength, and no, I didn't talk about mode-locking or frequency drift or the difference between 2 state, 3 state, and 4 or more state lasers. I also conveniently omitted semiconducor lasers, mostly because the electron energy transitions are much harder to follow. This post was long enough without them, don't you think! Besides, none of it has any bearing on the core issue, which is that the generation of a particular frequency depends on energy level change, and not any restrictions implied by the color vision model in general, or the term "primary colors" in particular.

[allthatwhichis]

Ewwww... He just all over my screen...

[platinum]

<Educational Lecture>

Ya know, if you ever want to focus all of this non-money-earning energy into something that barely turns a profit, you'd make a great science teacher.

That was fun to read. Thanks!

-Jonathan

[Doc]

That Adam; was one of the easiest to visualise analogies about photon production i've ever read.

Apple for the teacher

[White-Light]

Yeah great explanation Adam and thank you. As you can probably tell, I never studied physics to any appreciable level but that all makes perfect sense.

[mccarrot]

To be honest, the amount of (RGB) lasers is of course wonderful and special, but the show is CRAP. Most of the effects can also be done using normal moving lights. Just give me a rig with the same amount of moving lights and a descent lighting controller with SMPTE and I'll win the race. Every lighting operator will win that race

The show is also DMX contolled like moving heads using the laser matrix.
http://www.showtacle.com/LaserMatrix.php

So no "complicated" showtime programming