Is a Krypton Ion laser monochromatic? Confused

[Asherz]

Hi,

I was just wandering if someone could give me a hand with a little query I have.

Basically in my physics classes we are taught that laser light is coherent and monochromatic, which is fair enough. Then I remembered that the Krypton ion laser gives off white light, so does that means it contradicts the "laws" of a laser.


I later understood a little more about the theory behind gas lasers, when we did some work on fluorescent lamp and electron energy levels, and how the high voltage from the cathode and anode generate heat which in turn causes the small blob of mercury to change into a liquid gas form which (memory gets a little blurry here) collides with atoms to bump the electrons upto a higher energy, as these return to normal emit photons of light in the UV range, this in turn then hits the phosphorous lining to emit white light (phosphorous electrons I assume release different bands of light to create white light when excited to a high energy plane.)

Now after that I understood more how gas lasers work, although the clue in krypton lasers is in the name, ion's so I assume that it works slightly differently if someone wishes to explain?

Basically I just want to know how a krypton laser is a classed a laser when it's light emitted is not monochromatic, although when ran through a diffraction it does give off the different beams of monochromatic light which make the white light, although in my eyes this is changing the actual laser... so the white beam it's self is still classed as monochromatic?

Hopefully you guys get at what I'm saying and can elaborate slightly for me

Cheers.

 

[ElektroFreak]

A krypton laser is not always monochromatic, no. Argon, Krypton, and Cu Vapor lasers have several "lines" that they lase at, lines being wavelengths. Each line is monochromatic, but it's quite possible with the proper mirrors to get the laser to output several lines simultaneously (multi-line). A laser in this configuration would not be monochromatic, but it is still very much a laser since the individual lines are monochromatic. It's also very possible, with different mirrors, to force the laser to produce only a single line, which would then make the laser monochromatic. Hope that helps!

 

[Asherz]

Thanks for the reply, basically my physics teacher is wrong and lasers don't have to be monochromatic, it is possible to have a single beam of chromatic coherent light via gas lasers. But then again I guess you could argue it's only white because it consists of all the different individual monochromatic beams, and would be technically the same as combing a RGB laser through a RGB cube or something, just a lot more accurate and stays true white without diffusing into different colours over a distance.

But then again again, the light from the actual laser emitted is white and coherent, so as far as I'm concerned lasers can be chromatic

 

[ElektroFreak]

You're exactly right, but I wouldn't necessarily say your teacher is wrong.. each line is monochromatic, which follows the definition of a laser perfectly.. he/she just didn't go into the more complex aspects of this idea like how to make a laser emit multiple monochromatic lines at once..

 

[Laser_Ben]

Thanks for the reply, basically my physics teacher is wrong and lasers don't have to be monochromatic, it is possible to have a single beam of chromatic coherent light via gas lasers. But then again I guess you could argue it's only white because it consists of all the different individual monochromatic beams, and would be technically the same as combing a RGB laser through a RGB cube or something, just a lot more accurate and stays true white without diffusing into different colours over a distance.

But then again again, the light from the actual laser emitted is white and coherent, so as far as I'm concerned lasers can be chromatic

Generally speaking, typical lasers are monochromatic, that is, emit a narrow range of wavelengths. There are certain exceptions to this, your krypton laser is one example of this. It emits light at several different wavelengths, each of which is specific to an energy transition in the laser.

At the same time, many lasers are barely coherent. Many diode lasers can have a coherence length of a few microns, sometimes less. Your Krypton laser is not coherent when you look at the entire output, but is coherent when you look at each individual transition.

 

[aryntha]

Asherz, best way to think about it is that a multiline (or white) laser is "several lasing actions in one device" -- simply put, "several lasers in one".

Each color in such a laser -is- monchromatic as ElectroFreak said, which is why it is called a "line" - it appears as a thin monochromatic line on a spectrograph - a single frequency. On a graphical plot, each line would appear as a severe thin spike.

What your teacher is probably trying to contrast with is nonmonochromatic light, as from an LED or tungsten, which is more of a "smear" or "spread" on a spectrograph, and a "hump" or "curve" on a graphical plot.

So your teacher is correct, its just that lasers can output more than one band of monochromatic light; i.e., 458nm and 488nm and 515nm.

You'll not see a laser outputting, however, say, "620-710nm" all at once -- as in, all-inclusive in that range (at various output levels) which an LED will do.

 

[Asherz]

Thanks for the info guys it's all really helpful.

Aryntha, that last explanation I think summed it up pretty well, the idea that lasers can emit different wavelengths but normally only in lines of a specific wavelength, where as an LED is over a range of wavelengths.

Kinda of like saying lasers are discrete (data) in their wavelengths, where as LED's have continuous (data) wavelengths? So Lasers have a set wavelength they can be, where as LED's can be anything in a certain range

I think I get it now.

 

[Pontiacg5]

So Lasers have a set wavelength they can be, where as LED's can be anything in a certain range

A laser can also be in a certain range (mostly diodes), but it will only be one wavelength at a time. For example a 635nm diode can shift to 640nm, but if it does shift that far it will not have any 635nm, 636nm, 637nm, etc.. light, only 640nm.

A LED will usually fill the range IIRC. If the range is 620-710nm, for example, you could find each wavelength of light in the output.

Aryntha explained a multiline perfectly, I think of it as a bunch of lasers stacked in the same tube.

 

[Bluefan]

And there's always the exception. Dye lasers can be tunable with a narrow linewith, but they can be broadband too. Ti:Sapphire lasers can be nonochromatic, but also can be quite broadband. In fact, for modelocked ultrashort pulses you simply need a large bandwidth, that is a wide spectrum. And it's still a laser. There's no hard border between coherent and incoherent.

 

[Asherz]

And there's always the exception. Dye lasers can be tunable with a narrow linewith, but they can be broadband too. Ti:Sapphire lasers can be nonochromatic, but also can be quite broadband. In fact, for modelocked ultrashort pulses you simply need a large bandwidth, that is a wide spectrum. And it's still a laser. There's no hard border between coherent and incoherent.

Oh that's annoying, there's always something to upset the rule lol.

 

[chipdouglas]

in simpler terms a laser is defined as coherent light right? (its a statement question)


michael

 

[Bluefan]

No, lasers are just usually coherent. Take a white light source and a good enough spectral filter, and it'll be coherent enough for some applications. Coherence is a property of the light and is related to the spectral width. Laser just happen to usually have a very narrow spectral width because of their working principle.

 

[chipdouglas]

i thought coherency defined lasers as all the protons are lined up and go in one direction?

 

[ElektroFreak]

What makes a laser source unique compared to other light sources gets rewritten each time a new threshold is crossed in their development. Coherence is a big one that never really changes much, since ALL lasers emit photons that are in phase with each other. It's probably the biggest defining point which separates laser light from regular light.

 

[Bluefan]

it's not that you have coherence or you don't, that's why a coherence length (or time if you want) is defined. Because normal light sources sometimes can be quite coherent, and some lasers can be quite incoherent.

 

[ElektroFreak]

While you're right, some lasers can be "quite" incoherent, they are NEVER fully incoherent. This is why coherence length is defined, not because lasers range from completely coherent to completely incoherent. They don't. With lasers there is ALWAYS some coherence and in most cases quite a lot of coherence when compared to normal light sources. Normal light sources produce light that is very often almost completely incoherent and NEVER anywhere near fully coherent. Therein lies the difference..

Rather than just state why you believe this info is inaccurate, it would be a good idea to add some facts about why for clarity and to add to the discussion..