Talk:Laser pumping

Flashlamp inaccuracies

"Quartz tubes can operate at 900°C and the cathode needs to be hot, so cooling with water seems to cool down the lamp too far[citation needed]." This line in the article doesn't seem to make much sense. In flashlamps, every attempt is made to keep the electrodes cool, as too much thermal expansion can crack the seal. Often, the glass is shrunken around the electrodes to allow the cooling water to affect electrode cooling as well. Quarts glass will maintain its strength at red-hot temperatures, and can even be plunged into water without cracking. Strobe and timing lights often run at red-hot temps. Higher average powered flashtubes need water cooling, and, with such, temperatures rarely get hot enough to crack the glass, except in long pulse durations where the inner wall temp may suddenly exceed the outerwall temp, (temperature gradient), and the glass may crack from this.

"Used lamps have deposited their cathode material on the glass and are therefore inefficient." Not entirely accurate, as sputter is usually only a problem at very long pulse durations or low pressures. More of a problem is wall ablation, vaporizing the glass. This can cloud up the glass and release oxygen, affecting the pressure.

"If the lamp is thick enough the light in the lamp is thermal equilibrium with the gas and optimal brightness is achieved." This doesn't make any sense at all. "Optimal brightness" is a relative term, as it is not clear which part of the spectrum this is referring to, (I assume visual). Brightness is is a product of power, (watts), which is merely the speed at which the energy is released. Brightness can be affected by bore diameter, as too large a bore will lead to plasma self-absorbtion, (in effect, the smaller the bore; the more compact the arc, the brighter it usually is). It is important to realize that the lamp's output can be centered anywhere from the near infrared to the far ultraviolet. Optimal brightness in the visual spectrum is achieved at a current density (balance of current and voltage) that produces "greybody radiation", centering the output in the visual. (The effect of greybody radiation is similar to what happens to sunlight when it passes through a cloud, changing from colored to white.) Ion spectum radiation, (excessively high voltage and low current) appears less bright because the output is centered in the near IR, whereas the same is true for blackbody radiation, (excessively high current and low voltage), which is centered in the UV.

"For pulsed lasers the lamp voltage may be turned off for up to 10 ms after each laser's output pulse (before too many ions recombine)." ? Ions recombine almost immediately, producing all of the emmitted light. The voltage is usually "turned off" when the lamp stops conducting. (Although voltage is similar to water pressure, it drops when the faucet is on and spikes when it is turned off.) It almost sound like this is trying to describe a "prepulse technique", like dye lasers often use.

"Generally cathodes suffer due to sputtering because of high voltage spikes; in this respect, arc lamps need to be distinguished from flash lamps." What? Sputter is caused by current, (in flashtubes, peak current), and not voltage, although pressure does have an effect on sputter. The biggest difference between a flashlamp and a DC arc lamp is the cathode shape. Arc lamps have a cathode with a sharp tip, to keep the arc centered. Flashtube have a flattened radius for the same reason, but avoid the sharp tip to eliminate any hot spots caused by the enormous peak currents, (often well over 1000 amps, compared to an average of 35 amps for an arc lamp). This information can be found at http://optoelectronics.perkinelmer.com/content/RelatedLinks/CAT_flash.pdf . Zaereth (talk) 01:23, 5 September 2009 (UTC)

That paragraph seems quite poorly written. The phrase "cooling with water seems to cool down the lamp too far" seems oddly worded, as if the author didn't quite understand what he was trying to say. I doubt the comment about "optimal brightness" relates to the visible spectrum. This is an article on laser pumping, not illumination. The main constraint I recall on lamp thickness from the flashlamp-pumped laser I built years ago, was that one wanted the lamp diameter to be about the same as the rod diameter for good imaging of the light into the rod. A lamp that is thicker than the rod doesn't couple light as efficiently with an elliptical reflector. A lamp that is narrower than the rod doesn't illuminate the rod uniformly. I presume that a "thick enough" lamp produces a blackbody spectrum, while a thinner lamp may produce a spectrum with atomic lines, but it's been so long since I have seen a flashlamp I can't recall. The spectrum of the lamp is important, though, for matching to the lasing medium's absorption lines or bands. I don't follow the comment about turning off the lamp for 10 ms either. It sounds like you know a lot about this. Why don't you make some changes in the article?

That is very interesting, as I had not heard that about the diameter needing to match the rod size, (although length is often critical, particularly in the case of ruby, where the entire rod needs to be pumped otherwise dark spots at the ends will absorb the lasing radiation). It sounds reasonable, but in most cases it's the current level that determines the optimum bore diameter. I'll provide this from A Comparison of Rare-Gas Flashlamps: "One should carefully note that for a particular input energy the optimum lamp diameter (and also gas pressure) is dependent upon the level of lamp current. Excessively large lamp bores give rise to insufficient excitation of the plasma; lamp bores that are too small result in high wall losses."

Your presumtion is partially correct, as too large a diameter will quickly lead to radiation being emanated from only a thin sheath around the surface of the plasma, but this tends to be ion radiation, as current density at the surface is usually very low. Too small a diameter, however, creates a denser arc and blackbody radiation can result from that, but the main problem is the wall losses. Pressure and bore diameter need to be optimzed together with current. Take a small camera flashlamp, for example. Bore diameter is often incredibly small, but is compensated by using extremely high pressures, often as high as 3000 Torr, (60 PSI).

I'm sure that I can fix the section, and provide some new information specifically related to lasers. I just got back from the holiday weekend, so give me a couple more days to compose something. Thanks for the response. Zaereth (talk) 17:00, 8 September 2009 (UTC)

Other questionable info

There seems to be a bit of info in this article which I find a little questionable. The first three references, which don't seem to provide much useful information, belong to the illustration, and should probably go in the picture's summary, along with the description of the color's meaning.

The entire remainder of the optical section is unsourced, save #4 (which is a mirror catalog). I think the section could be broken into subsections, such as "Flashlamp pumping," "Arc lamp pumping" "External laser pumping", or something along those lines. Much of the information seems a bit jumbled up. There are some things that seem dubious, and I would need to clarify before I can sort this out.

Paragraph 1

Flash lamps are the oldest energy source for lasers. They are used for lower energies in both solid-state and dye lasers. I'm not sure what is meant by the term "lower energies", as energy levels in these often exceed hundreds of joules. Also, the term "oldest" may be slightly misleading, as the helium/neon laser quickly followed with semiconductors not far behind. Perhaps "first" would be better, but I'm not one to quibble over synonyms.

Paragraph 2

Typically, the lamp is surrounded by a cylindrical jacket with a dielectric coating that reflects unsuitable wavelengths of light back into the lamp. I'm not sure if this is typical.

This light is absorbed and some of it is re-emitted at suitable wavelengths by means of fluorescence. I've never heard anything about electrical arc fluorescence. (Although I do have a cool photo of quartz glass phosphorescence here.)

Cylindrical laser rods support whispering gallery modes due to total internal reflection between the rod and the cooling water, which is not true for other rod cross-sections. This should be clarified, but I have no idea what it means.

Inexpensive rods have unpolished outer diameters, while expensive rods can have a cylindrical lens on one side to focus the pump light into the rod. An unpolished rod lowers the intensity at the center of the rod worsening the beam profile. A lamp jacket or rod without an antireflection coating also leads to losses. This, I've never heard of either. Every laser rod I've ever seen is roughened around the circumference. This helps prevent focal points (hot spots) from being created in the rod, and helps prevent stray internal reflections, (TEM modes, I believe is the correct term).

Paragraph 4

Another configuration uses a rod and a flashlamp in a cavity made of a diffuse reflecting material such as spectralon. I have no clue what this paragraph is talking about. Can somebody please clarify?

Other types

I believe there are several other exotic types of pumping, such as the supersonic carbonmonoxide laser. I'll do a little research into this, but first want to get the optical section sorted out. Any help would be greatly appreciated. Zaereth (talk) 19:52, 10 September 2009 (UTC) PS. I've got a few laser pumping cavities, flashlamps, rods, etc... I'll see if I can get some good photos for this article. Zaereth (talk) 00:42, 11 September 2009 (UTC)

I disagree about the figure. The description of the colors, and the references that support the figure, belong in the caption. It would be a good idea to copy them to the image description page as well, however.

Para 1

I agree that "first" would be better than "oldest", and the claim that they are "are used for lower energies" should be removed. If anything, flashlamps are used for higher energy systems, since diodes are (or have in the past been) more expensive for higher power pumping.

Para 2

I'm not sure how common it is to have a reflective filter coating on the flow tubes. It might well be common in commercial lasers. It is true that any light that returns to the plasma and is absorbed is re-emitted, with the same spectrum as the rest of the source's emission. If undesired wavelengths are reflected back to the source, some of the light will be re-emitted at the desired wavelength. I'm not sure if this is accurately described as "fluorescence" however; it is just that the reflected light contributes to the heating of the plasma, and the energy is eventually re-emitted.

The comment about whispering gallery modes is probably true. These are modes where light circulates around the circumference through multiple reflections at shallow angles.

I don't know about the polished, AR-coated rods. Like you, the rods I have used (years ago) had ground sides. It seems plausible, though, that some commercial lasers might use a polished rod with an AR coating to get more efficient pumping.

Para 4

I might have added the bit about spectralon, a long time ago. I've seen such pump arrangements in the literature (maybe in Koechner's book on solid state laser engineering?) The idea is that rather than trying to image the pump source into the laser rod with an elliptical reflector, you put the rod and lamp in a chamber with walls that reflect light diffusely, but with very little absorption. You get nice uniform illumination, like in an integrating sphere. It's been a long time, though, and I don't remember much about these pump cavities. There was formerly an article on spectralon, which was deleted as non-notable. I have an archived copy of it at User:Srleffler/Spectralon. I had meant to restore the article with references that establish notability, but I forgot.--Srleffler (talk) 05:12, 11 September 2009 (UTC)

Thanks for the clarification, Srleffler. This will help me very much. I'll gather my sources and begin working this this weekend. Zaereth (talk) 16:18, 11 September 2009 (UTC)

Actually, I'm in complete agreement with you about the picture caption, sorry I wasn't more clear.

I know that in dye lasers it's often common to have a filter tube around the lamp, usually just for water flow, to absorb the NIR before it causes thermal shock in the liquid. I'll look into this more.

Thanks for your explanation of spectralon, as that makes a great deal of sense. Yes, it would've been helpful if the article had not been deleted. It is often helpful to include a small parenthetical explanation of specialized words anyway. I can see where using a polished and AR coated rod would be beneficial in such a cavity, coupling more light than difussing it through the surface of the rod. I know from sad experience that keeping the light diffused either way is almost a necessity, to keep the rod from cracking. :-{ I believe I also read that on the Kentek website, and in one of the most accurate sources I've found on lasers, Principles of Lasers by Oratio Svelto, although the book is a little difficult to follow as 80% of it is written in math.

Anyhow, thanks again for all of your assistance :-D Zaereth (talk) 22:56, 11 September 2009 (UTC)

Electrical pumping

• There needs to be an extension of the sources that describes how lasers are electrically pumped. -Srodrig 10/25/2012 I'll try and get on it. Eventually, no promises though.

Any help you can provide would be appreciated. Personally, my knowledge of electrical pumping is very limited, so thanks for whatever assistance you can give. Zaereth (talk) 21:16, 7 November 2012 (UTC)

Proposed changes

I've been working out some changes and some new information for the "optical" section of this article. It's still in progress, and I have to insert the sources still, but any help or comments would be welcome. If anyone would like to review what I have so far, please check it out at User:Zaereth/Laser pumping. Thanks Zaereth (talk) 20:55, 24 September 2009 (UTC)

Ok, through my research I'm coming up with some more inconsistancies. From this book, Solid State Laser Engineering, the main purpose for diffusing the light, besides eliminating focal points (hot spots) in the rod, is to provide very even pumping throughout the rod, utilizing up to 80% of the cross section for gain. The most effective way to do this is by roughening (frosting) the barrel of the rod. This also helps diffuse any stray internal reflections which could sap energy from the beam, providing better overall gain at a slight cost in transfer efficiency.

The most effective cavities are "close coupled". That is, the smaller the cavity, the fewer reflections before light reaches the rod. The most efficient imaging of the light into the rod, from a single lamp, comes from an ellipse that is twice as wide as it is high, not "closer to circular" as the article states, (which always seemed a bit dubious to me). A double ellipse is extremely inefficent, which is why the ruby laser in the photo doesn't use them, but instead, pumps the light evenly from both sides. With three or more lamps, multiple ellipses become common.

Diffusing the light from the reflector, or even through a frosted flow tube, before diffusing it through the barrel of the rod is also common. (The ruby laser shown has spectralon inserts that effectively diffuse the light before it reaches the frosted rod.) This helps provide even better distribution of the light throughout the rod for a better gain at a slight cost in transfer.

The article mentions rods with a lens on one side. I have found nothing to support this, as it is usually desirable to pump the rod as evenly as possible from all sides. This may be confused with the "grooved barrel" configuration of rod, whereas cylindrical grooves along the length of the barrel act as concave lenses, spreading out the light without diffusing it. This is according to the Kentek website, rods are available with frosted, polished, and grooved barrels.

I'm still looking for more info myself. My Nd:YAG laser, (which can be seen in the article), uses a powdery coating in an elliptical reflector that is highly but diffusely reflective of near-IR, but easily passes visible light.

Before making any changes, I'd like to ask, does anyone have any sources that support the statements in the article? Zaereth (talk) 17:36, 28 September 2009 (UTC)

I don't, and Koechner's book is the one I would recommend for info on flashlamp pumping. It's been years since I have worked with flashlamp-pumped lasers, so my memory is shaky on the kind of details you are asking about. Feel free to remove uncited claims from the article, especially if you can replace them with something that is supported by a citation.--Srleffler (talk) 03:57, 29 September 2009 (UTC)

Thanks. I'll work out any changes in my user space before incorporating them into the article. It'll be a few more days, as I prefer accuracy over speed. Thanks for your help on this! Zaereth (talk) 16:19, 29 September 2009 (UTC)