Talk:Laser/Archive 5

This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archive 1

←

Archive 3

Extra spectra pics

I don't think that we need three different spectra pictures in the categories by type section.

First of all, having all three of them (sized thumbs in opposition to the MOS) screws up where the edit links end up for the subsections
Second, two of the pictures are output from someone's spectrometer and as such represent original research
Third, one of those two is a spectra of an LED which is not even a laser.
fourth, the navy pic covers a broad range and is a good pic

-- Patrick Berry 19:35, 29 January 2007 (UTC)

"output from someone's spectrometer" does not constitute original research any more than "output from someone's digital camera" does. There is no "research" involved first of all (just because someone is using a scientific device doesn't make something research) and second there is not a thing original about it, as information on the spectral bandwidth of lasers is absurdly widely available. I don't know why the led spectra was added, I don't think that was relevant ehough to be there but I will re-add the spectrum of the HeNe laser as it demonstrates a property of lasers not shown by any other image on the page, specifically, the high monochromaticity of lasers. --Deglr6328 14:50, 30 January 2007 (UTC)

I am fine with that. You should cite a source with the pic then confirming that it accurately represents the linewidth of that HeNe. In addition, not every laser is that highly monochromatic, so the pic only represents certain types of lasers. --Patrick Berry 15:17, 30 January 2007 (UTC)

The linewidth of a HeNe is exceptionally fine and as is explained in the image info page the spectrometer can not resolve the actual linewidth. The image serves instead to simply contrast the "ultra"-monochromatic nature of (most) lasers with that of other light sources (like leds) conventionally described as such. I have placed a link to a site showing the linewidth of a brillouin scattered HeNe on the image info page for reference.--Deglr6328 18:02, 30 January 2007 (UTC)

"Original research (OR) is a term used in Wikipedia to refer to material that has not been published by a reliable source. It includes unpublished facts, arguments, concepts, statements, or theories, or any unpublished analysis or synthesis of published material that appears to advance a position" -- in your own image page you state: "The emission spectrum of the HeNe laser is even more monochromatic than seen here and the broadening of the peak in this spectrum is actually a result of the imperfect optics and scattering of light inside the spectrometer which results in some light being detected on the parts of the (linear CCD) sensor which surround the central peak." <-- Are these your conclusions about why the peak is broader or do you have a citation for this?

Also, you add "Spectrum of a very high resolution Brillouin scattered HeNe laser can be seen here [1] for reference." This is citing a webpage which cites a journal article about sideband generation, not linewidth broadening, which makes it not really germane. I am not trying to be a total jerk about this, I just don't think that an individual's spectra is the same as posting a picture and should not be used as an image unless it is in a properly cited and peer reviewed article. I will look for a replacement from a citable source --Patrick Berry 18:30, 30 January 2007 (UTC)

Oh come on now, let's don't go crazy here. The plot I linked to there clearly shows the linewidth of the nonscattered main beam to be ~2GHz fwhm, this is in perfect agreement with our own page on modelocking that states it to be ~1.5GHz (and from many other sources [2], this stuff is widely published) this means of course that the actual linewidth of a HeNe beam is somewhere around an absurdly fine 2 PICOmeters. That's like a thousandth of the resolution the spectrometer is capable of achieving, not to mention the fact that the linewith is in the few ppm level for a plot spanning 500nm. The red line itself in the image is hundreds of times larger than the natural HeNe width! So yes, it is my conclusion for the reason that the line in the plot I uploaded is broadened is probably because of imperfect optics or really..... who knows what. And no this is not original research because that fact is not at all going into the article and the reason for such broadening is totally irrelevant to the purpose that the image is being used for here, which is merely to illustrate and compare the spectral purity of a common laser with that of other sources. If I uploaded a photo of an everyday object that had a slight artefact for some reason I would note that just the same, as othes have done [3]. --Deglr6328 23:38, 30 January 2007 (UTC)

Vandalism, or Fact?

Someone added the name "Jeremy J. Wittig" to the list of those who produced the first maser. Is this correct, or is it vandalism? I would suspect someone would like to add his own name to the list, so it might be a malicious edit. · AndonicO ^{Talk · Sign Here} 21:00, 14 February 2007 (UTC)

I removed it. I can't find it in any quick resource, so if the anon IP wants it in, they should cite it. -- Patrick Berry 02:45, 15 February 2007 (UTC)

Intro section

I recently rewrote the intro section of this article into:

The simplest type of laser consists of a gain medium surrounded by two mirrors of which one is partially transparent and with a means to supply energy to the gain medium . The gain medium is a material (gas, liquid, or solid) chosen to have appropriate optical properties. Light of the right wavelength that passes through the gain medium is amplified (increases in intensity); the surrounding mirrors ensure that most of the light makes many passes through the gain medium. Part of the light that is between the mirrors (i.e., is in the cavity) passes through the partially transparent mirror and appears as a beam of light. The energy required for the amplification is typically supplied as an electrical current or as light at a different wavelength from a flash lamp or other laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

It is now changed into

A laser generally consists of a gain medium inside of an optical cavity along with a means to supply energy to the gain medium . Light of the right wavelength that passes through the gain medium is amplified while the optical cavity ensures that most of the light makes many passes through the gain medium. The optical cavity is typically designed so that a portion of the light in the cavity escapes at each round trip, producing the laser output. The energy required for the amplification is typically supplied through either electrical or optical pumping. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

Although I agree that my version could be improved, I'm don't think this is the kind of improvement I was thinking of. The reason I rewrote the intro section is that it was illegible for someone who doesn't already know what a laser is and has a background in physics. In the current version, the terms gain medium, optical cavity, and pumping are introduced without any explanation. The idea of the lead section is that all specialist terminology is explained on the spot. Although I sinned against this guideline myself in specialistic articles such as Pulse shaping, I think this article is likely to draw readers with no background in physics. Han-Kwang 20:27, 9 April 2007 (UTC)

You have a valid point about the lack of explanation for the lay-person. I was writing under the idea that the wiki-links allowed someone who did not know what the words meant to find further information on them. I will however attempt to expand the paragraph somewhat to offer more insight. --Chuck Sirloin 20:49, 9 April 2007 (UTC)

Wikilinks are nice for the person who wants to know more about a particular thing, but annoying when you have to click on lots of them just to understand a passage, especially for a high visibility topic like this. A slight diversion like, an optical cavity, that is, a special arrangement of partially reflecting mirrors, or some such can cost little while saving a lot of clicking. — Laura Scudder ☎ 17:09, 10 April 2007 (UTC)

I have another point which is more subtle about this phrase: it is coherent, monochromatic and collimated.. (1) The vast majority of lasers are in cd and dvd players and they are not collimated. The semiconductor laser itself produces a very divergent beam and rather than being recollimated, it is focused again onto the disc. (2) coherence and monochoromaticity are equivalent properties (apart from the fact that they are unexplained terminology); the degree of coherence is simply the inverse of the linewidth. And lasers are in general not monochromatic. For example, all lasers that I work with have a bandwidth of tens of nanometers. Also in telecom, lasers have a fairly large bandwidth because they are pulsed lasers. Han-Kwang 08:11, 10 April 2007 (UTC)

I understand what you are saying and I agree to some extent. However, the counter examples you give are more advanced topics and run counter to your argument that the intro be for the masses. In general, lasers are described this way in most text books because these three properties are typically defining of a laser. Also, coherence and monochromaticity are not equivalent properties; one has to do with bandwidth and the other with photons being in phase with each other. Light can easily be monochromatic without being coherent (however I do not know that the converse is true). With respect to collimated light, any laser, when you get into the fine details, is not really collimated (i.e. its ROC is infinity at all points) but is still much more collimated than other light sources. --Chuck Sirloin 14:20, 10 April 2007 (UTC)

Well, I don't think the counterexamples should be mentioned explicitly, but the wording should not exclude those counterexamples. My version was "well-defined color", which is much less strict than monochromatic. Regarding coherence, I actually meant coherence time, i.e. what is relevant if you try to do interference. Light from a fairly monochromatic source (e.g. sodium emission line) does not have spatial coherence, but use a pinhole and collimate the light that passes through and it will have coherence properties that are very similar to that of a laser. Regarding "photons being in phase with each other": this is an incorrect notion of what photons are. Quantum-mechanically, a photon is an amount of energy that is added to or taken from a radiation field. Coherence is a property of the radiation field as a whole, not of individual photons. A true single photon has a completely undefined phase; the coherent radiation field in a cavity actually has an undefined number of photons (phase and number are mutually exclusive in QM). Han-Kwang 19:54, 10 April 2007 (UTC)

I always worked with pulsed lasers, so monochromatic is a special problem for me. In general, I would say that the main commonality of lasers is coherence, being rather key for the whole process. Non-monochromatic pulsed lasers also exhibit spatial and temporal coherence, otherwise no pulse. And collimation is merely a by-product of a particular kind of spatial coherence. I'll also resort to the appeal to authority and point out that Verdeyen's Laser Electronics says that the main thing distinguishing lasers from lamps is coherence (page 23).— Laura Scudder ☎ 17:09, 10 April 2007 (UTC)

Works for me. I'll revert. --Chuck Sirloin 17:33, 10 April 2007 (UTC)

Gas Lasers

Carbon monoxide is not a hazard to health or equipment when dealing with a CO2 laser. Adding a little bit of water vapour in the laser tube causes the CO to recombine back into CO2. I removed the information stating so from the article. I'm going to add something about the efficiency. Photonicsguy 00:49, 16 April 2007 (UTC)

I added something about the efficiency of the CO2 laser, and compared it to the He-Ne laser. I think mention should be made of the efficiencies of the various types of lasers. I'll write something when I have time, or if anyone would like to be involved. Photonicsguy 00:53, 16 April 2007 (UTC)

Maybe the CO hazard was actually supposed to be mentioned at the CO laser. I didn't know that these exist, though. Han-Kwang 10:30, 16 April 2007 (UTC)

How powerful?

I'm just a little bit confused over the laser classes... Everything is rated by eye damage. Does anyone know a different way to judge how powerful something is? Could someone explain this please? I just want to hear an example, like which class of laser would be able to scorch paper, or pop a balloon, or something everyone can relate to something other than a laser. Thanks a bunch! :-) Ilikefood 21:26, 26 April 2007 (UTC)

They're usually rated by eye damage because that's a major concern. Any other measure would depend on too many things, for instance, laser wavelength and the color of the paper or balloon. Dark paper obviously absorbs more light than white paper, although some white papers have flourescent dyes that will cause them to burn at lower energies.

I can tell you that a 50 fs infrared pulse with 540 GW/cm² average burned paper and singed the skin surface, creating an itchy kind of sensation. And blue light at I think maybe 10 W/cm² took a long while to become warm on the skin but set a navy shirt on fire rather quickly. — Laura Scudder ☎ 22:55, 26 April 2007 (UTC)

You touched half a terrawatt/cm^2!? I've felt the heat of a 20 watt 1064nm cw beam expanded to ~10cm, it was definitely warm after a few seconds. The unexpanded ~2mm diameter beam would smoke paper easily but only when shone on the black toner covered areas. --Deglr6328 08:16, 28 April 2007 (UTC)

She said average, but she actually meant peak power (or average over 50 fs). She didn't mention the repetition frequency of the pulses, which is probably on the order of 100 Hz, i.e. 2.5 W average. The main difference with 2.5 W continuous and ultrashort-pulsed regarding burning things is multi-photon processes: an 800 nm pulsed beam can also be absorbed by materials that absorb at 400 nm. Han-Kwang 10:12, 28 April 2007 (UTC)

Thanks. :-) Ilikefood 16:26, 29 April 2007 (UTC)

Yes, sorry, I meant average over 50 fs, not peak power or total average power. — Laura Scudder ☎ 18:40, 16 July 2007 (UTC)

Why are lasers used for cutting?

A lot of energy is pumped in to a laser to produce an industrial cutter. What makes a laser so special for cutting? Why aren't normal light sources used to cut metal? Is a laser more efficient at converting electrical energy to light than say a carbon arc? Why not collimate the light of normal light sources?

A related question, is the coherence of laser light important in cutting applications? Would a collimated light source of the same power and frequency but incoherent be any better or worse for cutting?

Chgenly 15:47, 6 May 2007 (UTC)

For cutting, a laser is focused to a small cross-section. This increases the energy density at the point of operation. Because a laser provides essentially monochromatic light, a lens can be very effective in producing a well controlled, small cross-sectional area. Normal, or polychromatic, light sources emit light in a wide range of wavelengths. If you use a lens to focus that light, even after you have columnated it, there will be a larger variance because the focal length of the lens varies with wave legnth of the light being focused, so there is some "spreading" of lens-focused light around the nominal point of interest. As a result, light from a laser is more readilly controlled with greater precision. (This is intended to be a common language explanation and will undoubtedly grate with imprecision for the more technically oriented. My apologies.) Pzavon 02:35, 4 May 2007 (UTC)

I don't agree. You could use an achromatic lens, but even then I believe that for cutting purposes the focus is rather weak - the depth of the focus (not the focal length) has to be bigger than the thickness of the material you want to cut. Chromatic abberations would not be very problematic in that case. The deeper issue is that you need (spatial) coherence in order to be able to create a collimated beam in the first place. As I mentioned earlier, the only way to collimate an incoherent source is by sending the light through a minuscule pinhole, followed by a lens. This will actually create spatial coherence, but at the cost of a huge loss of all the light that didn't go through the pinhole. Han-Kwang 19:22, 6 May 2007 (UTC)

Could it be that the infrared light (heat) generated by a cutting laser is just a lot more efficient at cutting than visible light? --Rick Sidwell 04:03, 7 May 2007 (UTC)

If anything, rather the opposite, since most metals are pretty good reflectors for IR light. The main reason is that a CO2 laser is relatively cheap to build with kilowatts of output power. Han-Kwang 06:56, 7 May 2007 (UTC)

Max power of CO2 lasers?

The current revision of Carbon dioxide laser says "The CO2 laser can be constructed to have powers between milliwatts (mW) and gigawatts (GW)."; this contradicts the current revision of Laser, which says "Carbon dioxide lasers emit up to 100 kW". Neither article gives references for these figures, so can we please have some? -- Whitepaw 19:11, 21 May 2007 (UTC)

I clarified this and added a reference --Chuck Sirloin 20:45, 21 May 2007 (UTC)

actually the laser was invented

I don't know how to actually post this but i read an article that says the laser was invented by Edison here's the source http://www.soundandvisionmag.com/columns/1363/laser-driven-dlp-projectors.html check it out. otherwise good job to all. 05:07, 29 May 2007 User:Virgilisleading

no, it says that the light bulb was invented by Edison, which is mostly correct. Han-Kwang 06:52, 29 May 2007 (UTC)

Gaussian shaped beam

This edit by LambOfDog changed

The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are often Gaussian beams. If the beam is not a pure Gaussian shape, ...

to read

The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are, at best, low order Gaussian beams. However this is rarely the case with powerful lasers. If the beam is not a low-order Gaussian shape, ...

Which is probably a very sensible thing to say, unless it is similar to the nonsense from this user that seems to be a subtle attempt to compromise the integrity of Wikipedia. In reaction I said to LambOfDog "at first glance [this] appears entirely reasonable. However, I think this highlights the problem in the original article as the underlying concept of a photonic probability density function has not first been explained or refered to in order to give the concept of beam intensity and shape a theoretical context in terms of wave particle duality. I am now wondering about your qualifications to write so eruditely upon the subject and are concerned that this knowledge that you have disclosed might be an official secret." Having said that, more or less as a joke, I now wonder if this portion of the article really does need some clarification, because it is not clear to me what is being talked about, or why it appears in that point in the article. Also the physics of what causes the beam to be Gaussian shaped or what we are really talking about, is it intensity, power, photon density, energy density, electric or magnetic field strength in the time domain, or frequency domain, in two or 3 dimensions or more. To me, there is something missing. -- Cameron Dewe 07:36, 15 July 2007 (UTC)

The interesting thing about the Gaussian function is that (1) the fourier transform of it is another gaussian function and (2) the function squared is another gaussian function. Therefore if the electric and magnetic fields of a beam have a (TEM 00) gaussian cross sectional beam profile in the time domain, they will also have a gaussian profile in intensity (and therefore in photon density). This is a rather nifty thing and is why we just say that the beam is gaussian without specifying how. Some pulsed lasers are gaussian in the time domain, and therefore also in the frequency domain, but that's never what's meant by "gaussian beam".

It is true that most lasers more complicated than a He-Ne have some sort of higher order beam modes present, although it's so helpful usually to have the same sort of beam shape after a lens (due to the fourier transform property of gaussian functions) that you usually try to maximize the lower order as much as possible. — Laura Scudder ☎ 13:47, 15 July 2007 (UTC)

I think that a couple of really important fundamental facts have been overlooked by all the experts editing this article. That is electromagnetic radiation has both an electric field and a magnetic field and what we are talking about is the field strength of those fields over both space and time. Thus the article should talk about the field strength of the beam rather than the beam. Also, the observation that the Fourier Transform of a Gaussian Function is another Gaussian Function is interesting and may be worth a mention. But it does not explain the, presumably random, process that gives rise to the existence of a field strength that possesses those Gaussian Function properties in the first place. My question is Why Gaussian? and not any other shape, or does nobody know. -- Cameron Dewe 10:11, 16 July 2007 (UTC)

The simple answer is because the Gaussian field distribtution 'fits' the mirror surfaces. See J. T. Verdeyen, Laser Electronics, Third ed., Prentice Hall series in solid state physical electronics, Chapter 3. (Prentice Hall, Upper Saddle River, 2000). The Gaussian mode comes straight from the free space divergence equation. --Chuck Sirloin 16:40, 16 July 2007 (UTC)

This article does skip a lot of the physics, but that might be because the physics of it easily fills a graduate class.

So far as why gaussian, the answer is that you set up your cavity to make sure that mode is stable and has the highest gain-to-loss ratio. A method for this is to use the complex beam parameter to find how a gaussian beam would propagate in a certain cavity design and then appropriately choose the distances and focal lengths so that the beam is the same on the n + 1 th trip as on the nth trip.

The other nice thing about gaussian beams, by the way, is that they have the smallest divergence and highest spatial coherence. — Laura Scudder ☎ 18:30, 16 July 2007 (UTC)

So what you are saying is that by careful design of the lasing cavity one obtains a gaussian beam. Indeed, a gaussian beam is the likely outcome of an optimal design for lasing stability and efficiency. -- Cameron Dewe 11:27, 17 July 2007 (UTC)

Yeah, the norm is for the gaussian mode to be the most efficient. Picture a cavity with a focus within your lasing medium. Because the TEM-00 focuses to the smallest beam waist the power in the focus will be higher, meaning more stimulated emission meaning higher gain-to-loss. In such a situation, as long as TEM-00 is stable, then it will be the most efficient. There are weird cavities designed for higher order modes, but in your typical laser, the predominate mode is TEM-00 unless something is really wrong (like someone badly maladjusted the optics or there's an irregularity on an optic). — Laura Scudder ☎ 16:00, 18 July 2007 (UTC)

If a gaussian mode is efficiently focused in a gain medium then significant gain saturation will result. The level of gain for this this mode will be significantly reduced. However the regions of the gain medium surrounding this focal spot will have little laser flux passing through them. Hence spontaneous emission will be significant leading to the generation of higher order spatial modes.

A common type of laser cavity is the positive branch confocal unstable cavity. This cavity is useful for producing relatively high power and low divergence laser beams. A result of the unstable cavity is a hole in the centre of the output beam. In this case, the Gaussian mode makes no sense.

In many gain media the gain is not evenly distributed across the gain medium. In many cases the gain is higher on the outside of the gain medium than in the centre. This can be especially pronounced in flashlamp-pumped lasers. In this situation a gaussian beam is unlikely to be formed. Azimuthal Laguerre modes are more likely. --LambOfDog.

I think most of what you are talking about takes place in flashlamp (and maybe electrically) pumped lasers. In most optically pumped lasers, higher order gaussian modes are not caused by spontaneous emission, but by improper mode matching with the pump beam. I also don't know that I agree with the statement that confocal, unstable cavity lasers are a common type, but even so, there are higher order Gaussian modes with holes in the middle. That being said, Gaussian modes are by no means the only self-consistent field distribution available, just the most common (now that optically pumped lasers are the norm).--Chuck Sirloin 19:53, 25 July 2007 (UTC)

Mirrorless lasers

It came up on the Science RD that coherent light diodes are lasers. I disagreed - stating that they can be laser-like, but not lasers. I am alone in this opinion and obviously wrong. Therefore, it seems that the laser article should be rewritten. Apparently, coherent light is all that is required to form a "laser". No mirrors or cavity is required. It should state that it is possible to create a laser with a mirrored cavity, but it is not common since coherent light diodes are far more common than conventional lasers. -- Kainaw^(what?) 17:03, 2 August 2007 (UTC)

You are wrong, because you misunderstand. Laser diodes have an optical cavity designed into the semiconducting substrate, and light bounces back and forth within the material. No additional mirrors are required because that function is already part of the design. Dragons flight 17:39, 2 August 2007 (UTC)

I stated that a gallium arsenide diode shining through a pinhole is not a true laser. Then, I was told that it is because it is the basis of a laser diode. Perhaps I simply misunderstood the replies. -- Kainaw^(what?) 17:42, 2 August 2007 (UTC)

A laser diode is definitely much more than a normal LED shining through a pinhole. Dragons flight 17:44, 2 August 2007 (UTC)

Laser Application article

Something weird is going on in the article: Laser Application. Seems to have been created for spam, can someone please have a look and maybe delete it. I havn't a clue howto do this - Roidroid 03:38, 6 August 2007 (UTC)

It possibly could've been speedily deleted, but I put it up for WP:AFD since there is some sort of claim of notability made. You can also mark articles for deletion by placing {{prod}} on them. — Laura Scudder ☎ 04:26, 6 August 2007 (UTC)

Solid-state lasers and ruby

Dears, ruby and sapphire are varieties of the mineral Corundum. It's wrong, an incoherence, to say ruby is the chromium doped sapphire.

Sapphire and Ruby are populars names. According to IMA (International Mineralogical Association) the mineral name is Corundum.

It is correct to say Ti-sapphire because there are Fe-sapphire and perhaps Mn-sapphire but is wrong to say Cr-ruby because it is only that exists.

Zimbres 21:53, 11 August 2007 (UTC)

Gamma Ray Annihilation Laser

BBC just reported a story which mentioned these "Gamma Ray Annihilatoin Lasers" in the passing, http://news.bbc.co.uk/2/hi/science/nature/6991030.stm but apprently (doing a search within the page) there isn't anything about "gamma ray" lasers on this page..(other than under "gaser" at the end but that's a different phrase than BBC uses..could someone who is knows more about it, update the page to include mention of this type of laser people are trying to create?

--Harel 03:26, 13 September 2007 (UTC)

If so, http://news.bbc.co.uk/2/hi/science/nature/6991030.stm has been edited since. As at 05:48, 13 September 2007 (UTC) it does not include the word "gaser", but it says that positronium annihilation may be used to drive a gamma-ray laser. Anthony Appleyard 05:48, 13 September 2007 (UTC)

SVG graph needs equations fixed

I have started a tracing of the history graph at commons:Image:History of laser intensity.svg, but I gave up trying to get Greek characters in inkscape on Windows. If I get around to fixing it on my Linux box at home, I'll upload it, otherwise maybe one of you can beat me to it? --Slashme 11:23, 17 September 2007 (UTC)

Note: I see Inkscape didn't put in the arrowheads I asked for. Another thing to fix. --Slashme 11:25, 17 September 2007 (UTC)

OK, I got the Greek letters into the equations, but Inkscape's spacing is still messing me around, because the text size differs between implementations of the SVG standard (probably due to font availability). And the arrowheads are still missing. Note that my svg graph is now on commons. --Slashme 14:25, 17 September 2007 (UTC)

Well, User:Rugby471 sorted the graph out, so I'm putting it on the page. The jpeg was not clear enough for me to be sure about some of the exponents, so please check it for accuracy.--Slashme 07:38, 18 September 2007 (UTC)