Is a Krypton Ion laser monochromatic? Confused

[Bluefan]

I disagree. Unless your bandwidth is infinite, you can never reach something called fully incoherent. Not even normal light sources can be fully incoherent, just as lasers can't be fully incoherent. And it goes the other way too, normal light can't be perfectly coherent, nor can lasers even be fully coherent. Take a look at the fourier theorem.

Lasers don't emit special photons, it's just light like any other source. Lasers just can have special properties compared to the average light source.

Which facts do you want to know? I'd say start at the fourier theorem, but you may have some study ahead of you if fourier isn't known to you.

 

[ElektroFreak]

Also very true, but for the purposes of this thread here we'll just say that without getting into ridiculous amounts of physics (that not everyone even cares about) that the thing that separates a laser light source from all other light sources is that lasers tend to be more coherent, fair enough? Sometimes you just want a simple answer, ya know?

If that is blatantly incorrect, then please tell us what is it that separates a laser from all other light sources? Or are we all delusional and lasers are exactly the same as any other light source?

Here's a few quotes:

Quote from wikipedia:

"The laser's beam of coherent light differentiates it from light sources that emit incoherent light beams"

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Quote from: Quantum properties of light

"Coherence is one of the unique properties of laser light. It arises from the stimulated emission process which provides the amplification. Since a common stimulus triggers the emission events which provide the amplified light, the emitted photons are "in step" and have a definite phase relation to each other. This coherence is described in terms of temporal coherence and spatial coherence, both of which are important in producing the interference which is used to produce holograms. Ordinary light is not coherent because it comes from independent atoms which emit on time scales of about 10^-8 seconds. There is a degree of coherence in sources like the mercury green line and some other useful spectral sources, but their coherence does not approach that of a laser."

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And my personal favorite from wiki answers (because of it's simplicity):

http://wiki.answers.com/Q/How_is_a_laser_different_from_other_light

"A laser is different in three main ways:

1- The light emitted from a laser is monochromatic, that is, it only has one wavelength (color). Ordinary light on the other hand, is a combination of many different wavelengths (colors).

2- It is coherent, that is, the waves of light of the laser are in phase. Just like two sea waves when they are both at their maximum height, all waves of the light in a laser are exactly at the same amplitude, all time.

3- It is highly directional. Laser light is emitted as a narrow beam in a specific direction. Ordinary light coming from the sun, a light bulb or a candle is emitted in various directions from the source. Since laser rays are so concentrated they also have much more brilliance than rays from other sources."

although we've already discussed how not all lasers are strictly monochromatic (multi-line emissions, diodes and other exotic laser systems)

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And let's not forget there are more than one type of coherence:
Encyclopedia of Laser Physics and Technology - coherence, coherent, light, spatial and temporal coherence, monochromaticity

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And you can also just google "difference between laser light and ordinary light" and tell us what the most common answer is.


So you'll forgive me for not accepting that you know better that all these web resources and the people behind them..

 

[ElektroFreak]

Not to double post, but ^^^^^that's^^^^^ what I mean by providing facts to back your side of things up..

 

[Bluefan]

Also very true, but for the purposes of this thread here we'll just say that without getting into ridiculous amounts of physics (that not everyone even cares about) that the thing that separates a laser light source from all other light sources is that lasers tend to be more coherent, fair enough? Sometimes you just want a simple answer, ya know?

Usually, this is true, but it's not a fundamental law.

If that is blatantly incorrect, then please tell us what is it that separates a laser from all other light sources? Or are we all delusional and lasers are the same as any other light source?

The acronym says it, it's Light Amplification by Stimulated Emission of Radiation. Doesn't mention spectrum or coherence, but it is a property that sets them apart.

Here's a few quotes:

Quote from wikipedia:

"The laser's beam of coherent light differentiates it from light sources that emit incoherent light beams"

Again, true in general, but not a defining property of a laser. Quotes on the same page: "The emitted laser light is (usually) a spatially coherent, narrow low-divergence beam, ", notice the usually. Again: Laser light is generally a narrow-wavelength electromagnetic spectrum monochromatic light; yet, there are lasers that emit a broad spectrum of light, or emit different wavelengths of light simultaneously."
You did quote a bit selectively, but I think the wikipedia article sums it up quite nicely.

I think a lot of sites say normal light sources are all fully incoherent and lasers are all completely coherent because that keeps it simple. It's not that simple if you look close, but that brings more complicated optics. If something as an absolute coherent and incoherent would exist, why define an interferometric visibility?

(edit again)
bottom of the page, see also:
Degree of coherence

 

[ElektroFreak]

There are exceptions to every rule, that's what you're saying if I'm following you right I think. Certainly quite true in many, many cases.

We're not talking about what defines a laser, though, we're talking (I think) about what SEPARATES a laser from other light sources. Perhaps what I've said isn't specific enough. There are certain special properties of the light itself that SEPARATE laser light sources from other light sources. Also, would you care to offer any rebuttal to the rest of the quotes I posted? It's nice that you're skilled at rearranging words to suit your side of a discussion, but I still have issues believing that I should take your word over all these references.

When will I be able to add a link to Bluefan's Laser FAQ to my sig?

 

[Bluefan]

It's not a rule, but the "non-coherent" lasers are a (very) small minority.
Again, usually (!) the coherence is what separates a laser from a normal light source.

Fine, if you want me to say it: Some of the quotes are incorrect. They are too much simplified that they may be true in general, but not completely. Because the complete theory is more complicated which is not always needed for a basic understanding which these sites aim to give.
but from your link to Encyclopedia of Laser Physics and Technology - coherence, coherent, light, spatial and temporal coherence, monochromaticity:

For other applications, the coherence of the light used should be as low as possible. For example, very low temporal coherence (but combined with high spatial coherence) is required for coherence tomography, where images are created with a kind of interferometry, and a high spatial resolution requires low temporal coherence. Suitable light sources for such applications can be based on amplified spontaneous emission (ASE) from a laser amplifier (→ superluminescent sources) or on supercontinuum generation in nonlinear media. A low degree of temporal coherence can also be beneficial for laser projection displays, imaging and pointer applications, as it reduces the tendency for speckle and similar interference effects.

You started quoting selectively from wikipedia, and now I'm rearranging words? If I only highlight my statement, it's because you already know yours. Don't be offended (I'm not trying to), but it does get us both thinking of how we understand the physics.

But to get back: Do you understand fouriers theorem? Anything from fourier optics? I posted some links, your thoughs about them?

 

[ElektroFreak]

I don't have any thoughts on fourier optics whatsoever. You're stating obvious truths that span all forms of science. It's kind of like a fractal, there's always a deeper more complicated side of any aspect of science. Pursuing these deeper more complicated sides is what drives scientific innovation, but this is a rather entry-level laser forum not a meeting place for PhDs.. I don't mind discussing these things, but to say that all these quotes and people's responses are "wrong" just because they're simplified is not correct. We're merely trying to respond to the widest audience possible. I respect the fact that you are knowledgeable in physics, but I have no interest is playing a game of "who's the smartest" with you.

For anyone interested in Fourier's theorem, Fourier Series, the Fourier Transform, or Fourier optics, here are a few basic links. A good working understanding of fairly advanced mathematics (algebra, trigonometry primarily) is suggested prior to reading.

http://en.wikipedia.org/wiki/Fourier_series

http://en.wikipedia.org/wiki/Fourier_transform

http://press.princeton.edu/books/maor/chapter_15.pdf

I haven't been entirely through this one, but it seems to be fairly accurate and detailed from what I've seen:

http://www.erbion.com/258 PHYS-Optics.htm