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.""

Comments

Re:Isn't this a contradiction?

[scotch]

The way I learned it in my laser physics class (a long time ago) was that lasers in general had these properties:

• 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.

bell lab's laser

[I Want GNU!]

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.

Not quite right...

[Myriad]

An ordinary laser emits only a single color because it's built with a light-emitting substance that naturally generates one wavelength of light when energized.

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... :)

Re:Isn't this a contradiction?

[caffeinated_bunsen]

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.

Light Amplification by Stimulated Emission of Radi

[slithytove]

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.

Wow, I thought everyone knew how lasers worked!

[2nd Post!]

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.

Military uses

[europrobe]

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?

Re:Multiplex?

[Yarn]

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.