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Quantum-Cascade Polychromatic Lasers
eznihm writes: "This article describes a new laser, developed by Lucent and others, that emits a band of light and operates at room temperature. "The result: a beam of high intensity at every wavelength from 6 to 8 micrometers, in the so-called midinfrared range.""
Well, really, a LASER is anything that operates by lasing. You remember, the light amplification by stimulation of... bla bla bla.
The truth is, lasers (even the standard HeNe) don't have to emit a strait beam or a single wavelength of light to be lasers.
- monochromatic
- coherent
- low-divergence/parallel
- high intensity
The devices build in this article don't have the first two properties, at least. Indeed the device sounds more akin to a "white" light LED (white in the sense of broad spectrum, I know they are infrared).Of course, these are just properties, and might not actually be a working definition of a laser - maybe if you satisfy the acronym, you're a LASER (light amplification by stimulated emission of radiation, for those that don't know).
I could reach behind me to look at Svelto's "Laser Physics" book, but I'm too lazy.
XML causes global warming.
LASER = Light Amplification by Stimulated Emission of Radiation.
Essentially, some material (usually a crystal) is excited, which causes it to emit photons. Usually, because of properties unique to the material, it will only emit photons of a certain wavelength, but this is not a requirement. Lucent's LASER is simply built using a material that emits photons of many wavelenths.
The laser does not produce an infinite amount of wavelengths. Each layer produces one dominant wavelength, and one to several weaker wavelengths.
As for consumer applications, don't hold your breath. Unless these are cheaper to produce than your supermarket price-checker, they'll stay in the realm of science for now. Multi-spectrum lasers are useful simply because they're light that all goes in one direction, which makes them useful for observing molecules.
Bell Labs has a page up on a Quantum Cascade Laser at http://www.bell-labs.com/org/physicalsciences/psr/ qc/ with info about its design, applications, and other related info from a few years ago.
The key part of the laser acronym is "Stimulated". What happens is electrons are temporarily pumped to higher energy levels. What would normally happen is that the electron would spontaneously drop to a lower energy level resulting in an emitted photon (this happens in all kinds of material and light emitting devices). In a laser however, an existing photon passing the atom with the excited electron Stimulates the emission of the photon, and in doing so, the two photons will be in phase. This is how you get coherent light out of a laser. Of course, it takes special selection of materials, controlled electron pumping, and good optics for this process to build up to appreciable levels
Furthermore, this:
"some material (usually a crystal)"
is wrong: While the first lasers were cryptals (ruby), and some still are (Nd:Yag and others), I think you'll find that lasers these days are usually semiconductors (as measured by shear number - think CD/DVD players) or some variant of gas / chemical reaction (as measured by total power - think Chemical Oxygen Iodine Lasers (COIL) or HF/DF lasers)
XML causes global warming.
Technically speaking this isn't quite true - it depends on what your lasing medium consists of. While each colour line emitted will be monochromatic, a single laser is capable of producing multiple lines.
In the case of a Krypton or Helium Neon ion gas laser you will get a single line out (usually, but not necessarily, red for either of these).
However, if I look at an Argon laser with apropriate optics you get primarily Blue and Green (514nm "Green" and 488nm "Blue") lines (with combinations in between). If I put a prism to the output of my little American Laser 60x I can see 7 individual lines - 5 are of such lower power as to be virtually useless, but the primary Green and Blue are strong.
Then if you look at a Copper Vapor laser which works by evaporating copper you get two lines: an emerald green and *gold* (this type of laser was made famous during the Pink Floyd Division Bell tour).
Newer solid state are very much single line. If you ever see a very harsh green beam you are probably looking at a Nd:YAG laser. The new solid state stuff is really looking promising... much more reliable with a much longer lifespan. Now, if they could just get the Blue solid states more powerful reliable we would be laughing. A low to mid powered white-light lasershow that could fit in a briefcase! On the down side though, typically much lower power output than their ion cousins (and the YAG green is, in my opinion, really nasty).
Could almost make me miss lugging 909's around... :)
"They do not preach that their god will rouse them, a little before the Nuts work loose." Kipling, 'The Sons of Martha'
Typically, you're correct. Traditional lasers emit almost all their energy at a single wavelength, with very small deviations of energy (determined by the time it takes an atom to emit a photon, thanks to the good ol' uncertainty principle, dE*dt>h/(2pi)). What Lucent did here is to create a whole mess of lasers in one package, which all emit slightly different wavelengths. The wavelength uncertainties overlap enough that you get a fairly smooth distribution of energy, rather than a single, well-defined peak at one wavelength.
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ation:)
In answer to all those going, "huh? i thought the whole point of lasers was that they're coherent/all the same wavelength"
Any device which excites one or more substances electrons to jump up an energy level and then fall back generating a specific wavelength (per substance) photon. these are usually bounced back and forth in a chamber and released at one end.
This article is about a quantum cascade laser, which is a bit more complicated than my simplified (even for normal lasers) explaination.
The point is, that while coherent lasers are the norm and coherence has many uses, this is still a laser and the technology may have many different uses itself.
Monocromacity is not an inherent property of lasers.
It's a limitation we could not overcome until now.
Lasers are coherent.
Lucent has created a multichromatic coherent laser.
Simply put, multiple quantum wells laze at different frequencies. Stacks of these multiple quantum wells create multiple lasers in one cavity, if I understand it correctly.
Each frequency is indeed coherent. You get multiple frequencies, however, in one resonant cavity. I'm guessing here, but the reason why you don't see each frequency shooting emitting from the cavity at different times is because it's either a continuous laser, or because the energy spread between the different colors is much smaller than the energy of activation to escape the cavity.
In either case, an analogy would be to place multiple crystals stacked together into one laser, and stimulating all of them. If you assume that there are no diffraction problems, and that they all emit at roughly the same period, you have a very crude multi-chromatic laser.
GPL Deconstructed
If they can make them powerful enough, I can imagine this being used in laser target designators to make them more immune to changes in the absorption properties of the atmosphere. Also, a lot of FSU tanks have optional laser warning receivers, which might not pick up this "spread spectrum" laser.
Comments, anyone?
Score:-1, Wrong
For better or worse (to it's employee's)
I can't see it being simple to modulate the laser to do that, but generally modulating the laser isn't much good for high bitrates.
The trouble is the very power modulation can cause shifts in frequency (due to ohmic heating), leading to unwanted losses and cross talk.
Better to have an external modulator which can change its loss at various frequencies, and use this as the source.
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Hmrr, parent post is a valid question and deserves a proper answer.
Lasing phenomena occurs in some media if you have so called "inverse population" of metastabile states of molecules. "Metastabile" means that those (excited) states cannot de-excite in to the lower energy level by spontaneous photon emission (e.g. momentum conservation forbids singlet-triplet transitions). However, if such metastabile molecule is hit by the photon with the energy that corresponds to the difference between upper and lower energy level of the molecule, a stimulated emission occurs. Emitted photon has the same wavelength, phase and direction as the incident one. In the conditions of inverse population (lots of metastbiles and sparsely populated lower levels), something similar to chain reaction happens. The initial photon gets multiplied in the geometric progression as it propagates trough the medium. This accounts for "Amplification" in the acronym LASER. In many cases upper and lower energy levels are well defined i.e. discrete, but they can be energy bands or even continuum. In the later case the wavelength of the "triggering" photon can lie in the range of values. This is actually the answer to the parent of this thread.
The lasing medium is usually confined in so called resonant cavity consisting of parallel mirrors. The reason for this is to effectively enhance the length of propagation in one preferential direction. The bunch of photons are bouncing back and forth between the mirrors many times and each time they traverse medium their number is increased. One of the mirrors is somewhat transparent and the portion of the beam exits the cavity.