Follow Slashdot stories on Twitter

 


Forgot your password?
Close
typodupeerror
×
Science

Quantum-Cascade Polychromatic Lasers 158

Posted by michael from the tan-don't-burn dept.
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.""
This discussion has been archived. No new comments can be posted.

Quantum-Cascade Polychromatic Lasers More

Quantum-Cascade Polychromatic Lasers

Comments Filter:

  • Uh oh, someone find Gordon (Score:2, Funny)

    by Anonymous Coward
    With this type of technology, pretty soon Lucent (Aka Black Mesa) is going to attempt some experiment that will cause a resonance cascade, then a cascade failure.

    Egads, someone find Gordon, and find him quick! I don't want to be a slave to some bigheaded multidimensonal being! (Bill Gates is bad enough)

  • Pardon my ignorance (it's been a very long time since my last physics class) but aren't lasers *SUPPOSED* to be monochromatic? I thought one thing that characterizes a laser is that all the photons are in the same phase. It must be a limitation of my imagination but how can this happen if you have multiple wavelengths?

    Can someone who know more tell me what's going on here?
    • 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.
    • 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.

      • Even if the term LASER refers to the process of generation of the laser, i think there is one common property of devices that we like to call a "laser":

        A laser is capable of emitting a lot of bosons that are in the same state (with some tolerance).

        If you get yourself to accept that a single photon can be polychromatic, you will also be able to imagine coherent polychromatic light. (However I am not sure if it applies to the laser at hand.)

        • Can a single photon be polychromatic?

          • Every real photon is polychromatic since the norm of the free monochromatic photon \psi(x,t) = e^{i(kx-\omega t)} would be divergent. A completely monochromatic photon cannot be localized. On the other hand if you have a localized wave-packet, it is not sharp in the momentum space. Remember Heisenberg: \Delta x * \Delta p >= \hbar/2. And for photons, momentum p is reciproc colour. (p = \frac{\hbar}{\lambda})

            Less mathematically speaking: don't imagine a photon having a colour. The colour is decided at the measurement. Imagine a photon with a function of wavelengths, some wavelengths will be measured more probably, some will be measured less probably. A conventional LASER would have one peak at a certain wavelength and a fall of on the sides. But with the LASER at hand it will look differently.

      • Re:Isn't this a contradiction? (Score:4, Interesting)

        by renoX ( 11677 ) on Saturday February 23, 2002 @06:43PM (#3058642)
        For me the device that they build is a way to have multiple lasers in the same packaging, each with a slightly different wavelength.

        Each of the individual emmiting layer is monochromatic and coherent, but the resulting beam isn't, you're right.

        What would be interesting is to see a diagram which would give the intensity of the light for each wavelength.

        I suspect that it isn't totally flat, but more something like this:
        /\/\/\/\

        • looking at the Nature article, the "above threshold" spectrum goes from 6 to 8 um and looks almost rectangular, with some broad irregualr modulations and a maximum at 7.6 um. At higher spectral resolution one can see the individual Fabry-Perot modes (i.e. the individual modes of the resonator(s)).
    • 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.
      • This is a pretty poor definition of a laser, and would include such things as glow in the dark toys, light bulbs, and just about any thing that emits light.

        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)

        • FYI, semiconductors are crystals.
          • Semiconductor lasers aren't crystal lasers.
            • The only thing that is different between semiconductor lasers and crystal-based lasers is the method of pumping (and required electronics etc.) and the materials.

              But in both cases, crystals are the source and the amplification medium.

              Feel free to pick up a Solid State Physics book, you will see that semiconductors are crystals.

              • "Principles of Lasers, 3rd Edition" - Orazio Svelto, 1989.

                Chapter 6: Types of Lasers

                Section 6.2 Solid-State Lasers

                Section 6.3 Gas Lasers

                Section 6.4 Liquid Lasers (Dye Lasers)

                Section 6.5 Chemical Lasers

                Section 6.6 Semiconductor Lasers

                ....

                Should I go on? Of course, most solid-state physics books, including the one I could pick up here, are useless in the context of Lasers Types.

                • Yes you should, because what you have written doesn't prove anything.

                  I never said that solid-state/crystal lasers are exactly the same as semiconductor lasers. I only said that crystal lasers and semiconductor lasers both use crystals.

                  You should also learn that having books on your shelf doesn't make you knowledgable. You try reading them.
                  • I've read them. Great, semiconductor lasers and crystal lasers both use crystals. You've really demostrated your great understanding of the subject. Guess what? They both use atoms too. They're both atom lasers. Woo hoo. To insist on repeating the fact that semiconductors are crystals in the context of a discussion of lasers is both pedantic and indiciative of a lack of knowledge of the subject. Thanks for breaking the ice in this thread with the first round of insults.
                    • Fair enough, the last comment was unnecessary. I thought we, including everyone else in this thread, were trying to gain more understanding of the topic. My original reply to the parent was to point a slight discrenpancy in someone else's post. This often occurs to me, so I assumed it was reasonable.

                      I was never trying to appear smarter than you, but I'm not sure what kind of response you were expecting to your previous post. Well whatever, think what you will.

                    • That's hardly insulting. Don't be so sensitive. BTW, an attempt at a definition was made, it was just implied. When someone says:

                      DEVICE
                      HOW DEVICE WORKS

                      I'd say that is a definition, albeit an implied one.

                      YMMV

                    • Truce. ;) Anyway, you're probably right that the distinction isn't all that important for the level of this forum. Opening that book reminded me why I like my new job as coder better than my previous life doing physics.

                      Hung over and ashamed, Scotch

                    • Sounds good. I'm an EE, about to graduate. Looking forward to the real world. Why did you leave physics?
                    • I liked physics while I was getting my degree, and I got to work on some interesting things in the field after I graduated, but I found that I wasn't getting to do the king of problem solving that I liked in school. I kind of got stuck doing systems engineering / project management of projects that involved cool physics, I just wasn't doing the physics. I always liked to code, and it gave the same problem solving feeling, so a few years ago I made the career switch. Haven't regretted it.

                      Good luck

                    • I liked it better when you two guys were arguing!
        • I'd say the key part of the laser acronym is "amplification". That eliminates glow in the dark toys, light bulbs, etc from the definition.
        • My definition was very general, but there was nothing incorrect about it. I was just explaining how the new Lucent laser can emit photons of various wavelengths and still fit the definition of a laser.

          By the way, most lasers ARE crystals. As you know, a semiconductor is a crystal.

    • Re:Isn't this a contradiction? (Score:5, Informative)

      by caffeinated_bunsen ( 179721 ) on Saturday February 23, 2002 @04:42PM (#3058259)
      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.
    • If you look at the Bell Labs page which someone linked in a lower comment, then you'll see that's it's not really emitting from a broad range of light.

      There are lots of layers of slightly different sizes, and the size of the layer largely determines the color (wavelength) of the light emitted. In effect it's like there are lots of lasers on different frequencies located in nearly the same space. Since they are all firing at the same time the net effect is a broad distributed band of coherent light, but you can still make out individual peaks [bell-labs.com].

    • Laser Physics Digest (Score:2, Informative)

      by pdp11e ( 555723 )
      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.

  • I always thought that the whole point of a laser was that it was generating a perfect coherent waveform, so is this new laser generating light at a large number of wavelengths in the infrared, or is it an infinite number of wavelengths in the range specified?

    Also, it seems like it would take a lot more development to extend this wideband laser work for something more consumer-oriented, like room lighting, as the light would need to be relatively omni-directional, which it doesn't seem like lasers are very good at producing now.
    • 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.
    • The whole point of a laser is to get an intense and highly directional beam of light. The reason for this would be for coupling to an optical fiber or other such application. Also, since the energy density of the spot would be auite large compared to an LED, you would never want to use it as a light source. The naked eye would be damaged.

    • Lasers are very good at producing omnidirectional light.

      Pop the columnating lens out of a laser pointer to see just how omnidirectional it is (and please don't look right at it). It's just like any other light source in that respect.. the only difference is that the light it produces is of one wavelength, and in-phase. Both reasons why it's not an ideal room light. But direction has nothing to do with it.

      Lasers are focused using a columnating lens.. just like any other light.

      As the article said... this produces laser lightevenly distributed across the mid-infrared range.

      • ... why is a laser pointer more useful for directional things than a simple flashlight? After all, an incandescent flashlight is simply an omnidirectional light source with a broad frequency range, roughly focused with a (usually cheap) parabolic reflector. Why could a flashlight not produce as potent and well-focused beam of light with the same colimating lens? After all, you can build a very cheap arc lamp that puts out thousands of watts of light. If you could focus THAT on a small spot, it would be as hot as the arc that produced it in the first place. So what's the problem with doing that?

        (This is a serious question. I'd love to hear the answer from a qualified physics type. I'm even willing to burn a karma point to show this at a 2 rating, so I stand a chance of getting a real answer.)

        I'm just guessing, but it's probably because the monochromatic nature of most laser pointers means that ALL the light focuses the same thru the collimating lens. This is of course because different frequencies of light have different refraction angles thru a lens. Therefore, with a broad-spectrum source like a light bulb, you cannot sharply focus the beam, because each frequency focuses at a different place. As I write this, it occurs to me that's why a flashlight uses a parabolic reflector - because reflection is not affected by wavelenght like refraction is. But with a parabolic lens, the fully parallel beam can never be smaller than the diameter of the lens.
        • I believe you just answered your own question perfectly.

        • You are right about the chromatic abberations and how to avoid them using reflective optics. Another limit for collimating (or focusing) beams is the size of your light source: the larger the source the harder to achieve a good collimation/focus (in classical geometrical optics). Typically, flashlights have a quite large source (tungsten filament), so only part if it will be in the focus point of the parabolic reflector.
          OTOH, if your source is too small or you have small apertures (on the order of the wavelength) your beam divergence will be limited by diffraction.

          Another issue with flashlights is that the light that directly goes towards the exit and does not hit the reflector will not be collimated and thus contribute to the divergence. Of course, flashlights are optimized for brightness, so you _have_ to use a reflector in order to gather the light that is emitted in the "wrong" direction. A lens would only collimate light that already goes towards the exit of the lamp.
    • Also, it seems like it would take a lot more development to extend this wideband laser work for something more consumer-oriented

      The point of this is not for a general consumer device any time soon. This device will be used in spectroscopy where there are very few line source lights that operate in the mid-infrared range.
  • The tight physical confinement of many of those stacked layers makes them act as so-called quantum wells, in which electrons can only have certain amounts of energy, Gmachl explains. Those specific energy levels are determined by the laws of quantum mechanics.


    Sounds like the 1 electron processor from Signal to Noise. I wonder if anyone has a simple explanation of how it exactly works? It seems that the electrons build up so much energy which pentrates diffrent strata of semiconductors and that creates the diffrent wavelenghes. Is that anywhere close to right?
  • I wonder if this would have applications in optical communications?

    It's not clear if the different wavelengths have to appear together, or can be manipulated individually.

    If they can be manipulated individually think of the coolness.

    So many questions.

    • Re:Multiplex? (Score:3, Informative)

      by Yarn ( 75 )
      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.
    • Since the lasing lines overlap, I think it would be very difficult to separate them to modulate individually. In addition, multi-line operation decreases the lasing efficiency of any particular line is probably pretty bad. Both these considerations would make it very difficult to integrate such a device into a wavelength-multiplexed communication system. The only way I can think of using this device in optical communication is to mode-lock the output and use it in a very fast single-channel link.

    • Why am I smacking my forehead. I should be smacking yours. Basically, now they have a laser that can replace the 10 or so lasers that would normally feed an optical fiber. Now they can make things smaller and fit more stuff in those central offices at the phone company and stuff.


      Before I mouth off, I should research the terms I'm looking for. Also I should get some sleep.

  • Sounds like Zippy (Score:3, Funny)

    by strredwolf ( 532 ) on Saturday February 23, 2002 @04:18PM (#3058186) Homepage Journal
    Quantum-Cascade Polychromatic Lasers!
    Quantum-Cascade Polychromatic Lasers!
    Quantum-Cascade Polychromatic Lasers!
    Quantum-Cascade Polychromatic Lasers!

    (I took one look at that and thought Zippy. Oh boy. Excuse me while I fix a peanut butter and olive sandwich...)
  • In principle, the same technique might someday lead to a white-light device for room lighting, computer displays, or video projection ...

    Of course, they don't mention the weapons potential

    or laser headlights for *really* bright high beams on the car.

    • Lasers Make Poor Weapons! (Score:5, Interesting)

      by Myriad ( 89793 ) <myriad@the[ ]d.com ['bso' in gap]> on Saturday February 23, 2002 @05:06PM (#3058339) Homepage
      Of course, they don't mention the weapons potential.

      Argh. It seems there is one of those unwritten Slashdot rules that dictates any story about lasers must be accompanied by posts about laser weapons... usually in the Death Ray variety. For the most part lasers make poor offensive weapons, at least in the Burning a Hole Through X variety.
      (Thank you Alien54 for not suggesting such a thing)

      I've posted this before [slashdot.org] but here it is again:
      ***************

      It seems inevitable that whenever there is a story regarding lasers we get to see all sorts of silly posts about blasting people with laser.

      Even antimissle lasers have a long way to go. Between power requirements, beam handling, divergence, and atmospheric interference, lasers do not make great destructive weapons.

      However, they would be damned good for some nasty tricks like blinding the enemy army (or, unfortunately, civilians).

      Take this scenario: a bomber/cargo style aircraft has been outfitted with a large infrared laser (similar things have been done). Fly said aircraft over the people you wish to 'zap'. Release some fireworks or other attention getting devices and when the crowd looks up turn on and start scanning the laser.

      Since the laser is infrared nobody would know they are being exposed to blinding levels of light, nor would the blink/aversion reaction take place. By the time you noticed anything the permanent damage has been done. Scary huh?

      Another scenario under serious consideration by police (at least here in Canada, I've participated in meetings on the subject) is the use of lasers against commercial aircraft. The idea isn't to shoot down the aircraft, but to scan at temporarily blind the pilot during final night approaches. The effect is like someone flashing a camera flash in your face when your in a dark room.

      As the few moments prior to landing are the most critical, distracting and flash blinding the pilot could easily lead to the plane crashing.

      Worse, new solid state lasers are available in the 3watt (plenty of power to cause permanent blindness) range and can be powered off a car with an inverter. Simply park at the end of a convenient runway at night, plug 'er in and away you go. Ok, so it's not quite that easy, but the concept is...

      Doesn't that all just scare you a bit more than some silly death ray?

      Note: after saying all that I want to point out that I do not support the insane regulations placed against the use of lasers in the United States by the CDRH. It's totally ridiculous and overzealous.

      • Another scenario under serious consideration by police ... to .. temporarily blind the pilot during final night approaches.

        Pardon my ignorance of Canadian culture but why on earth would the police want to do that?
      • As the few moments prior to landing are the most critical, distracting and flash blinding the pilot could easily lead to the plane crashing.

        Back when the (iirc) MGM Grand in Las Vegas was new, they had a laser show they ran off their roof.

        They shut it down when it scanned the eyes of a pilot during takeoff from McCarron.

        Here's a link to google's cached page for it (the original is behind a login dialog):

        Laser blinds flight crew. [google.com]

        Now, if blinding pilots for 5-10 seconds while they're climbing under full power at 500 feet isn't considered "life-threatening risk" to everyone on board and lots of people in the city below, then I must be some sort of chicken...

        --Blair
      • You're forgetting that most posts are based on "what we want to be true" and not necessarily "what is true".

        I sure as hell want a massive, Death Star, lasing-zapper-blaster raygun. (If only to blast the god-damned pigeons that nest in my gutters -- I'm just waiting for the GPS-phone-tracking version so I blast telemarketers).

        So, I can guarantee that anytime there is a "laser" post, you'll see some "death ray" posts because most of us want a Death Star floating over the house so we can unleash death and destruction on anything that bothers us.

  • Finally!! (Score:3, Funny)

    by woodix ( 167920 ) on Saturday February 23, 2002 @04:26PM (#3058216) Journal
    I don't know about you guys, but I've been looking for a geek-friendly method of removing hair from my body and that IGEA thingy that does it one at a time just wasn't cutting it. Now I can just burn it all right off with a big nasty room temp laser. They should probably package it with a warning. Something like, "avoid using this device near mirrors." Can you imagine how stupid you'd feel if you ricocheted your hair burning beam off the bathroom mirror and vaporized your dog in the next room?


    And when you weren't burning hair off your body, you could use it to super heat your evil Science Teacher's house so that popcorn exploded out of it everywhere...and you wouldn't even need to hijack the computers on a nearby B-1 bomber!!


    Finally, my life can emulate Real Genius!! (thanks Cliff!)

  • bell lab's laser (Score:3, Informative)

    by I Want GNU! ( 556631 ) on Saturday February 23, 2002 @04:37PM (#3058244) Homepage
    Bell Labs has a page up on a Quantum Cascade Laser at http://www.bell-labs.com/org/physicalsciences/psr/ qc/ [bell-labs.com] with info about its design, applications, and other related info from a few years ago.

  • Not quite right... (Score:5, Informative)

    by Myriad ( 89793 ) <myriad@the[ ]d.com ['bso' in gap]> on Saturday February 23, 2002 @04:41PM (#3058253) Homepage
    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 [ufl.edu] 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 [laserfx.com] 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... :)

    • Nd:YAG is fairly narrow linewidth at 1064nm. The green is frequency doubled(532nm), in a nonlinear crystal.

      To get blue frequency tripling is often used, but each nonlinear element reduces the intensity greatly.
      • Actually, when you triple it, don't you get ultraviolet (~355 nm)? Of course, the 400nm-700nm model of human color vision is messed up, so it's possible that some people could see that as a really deep purple...
    • Jedis use Argon in their lightsabers, and the Sith use Krypton or Helium Neon ion gas in theirs.
      • lightsabers of either side do not make use of lasers. Every try to tune coherent light to de-cohere at a point a couple feet away, giving the appearance of a sword? not easy, and somewhere i believe it is mentioned that the sabers are comprised of plasma or something equally non-laser.
    • Lasers operate on the priciple that an atom 'detects' a wavelength of energy going by it that it happens to be holding, and releases it in the same phase as the other. Repeat X1000^2. Voila, laser light.
      The beam gets repeated by two parallel mirrors, one is opaque, the other is part-way(leaky), which lets the light out, lasing maerial between.
      Replace the opaque mirror with a prism, which spreads the spectrum out, pick a color and place a mirror perpendicular to the beam, and it will reenter the lasing material. The stimulation will reinforce the one color, and the others will degrade(filters would reduce the frequency you want due to imperfections), works with multiple frequncies.
      What should be really fun would be a EL-display type laser device, which might be tunable with the voltage across.
      (As far as rude colors, ever see a neon *plasma* pink?)

  • Light Amplification by Stimulated Emission of Radi (Score:3, Informative)

    by slithytove ( 73811 ) on Saturday February 23, 2002 @04:46PM (#3058268) Homepage
    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! (Score:4, Informative)

    by 2nd Post! ( 213333 ) <gundbear@pacbe l l .net> on Saturday February 23, 2002 @04:47PM (#3058272) Homepage
    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.
    • The light you see coming out the laser is a steadt state condition. Although there is a small transient time when the device is first "turned on", it would be very short. Therefore, you would never see that difference.

      The timing of the light being emitted by the material does not matter since the resonance cavity, through multiple reflections, and the materials, through gain stimulated emission, will cause the light of a single wavelength to be in phase.

      • Let me clarify what I meant. Light of differing wavelength would not be in phase and see some spread. If this were a continuous laser, then only the wavefront and tail of the beam would show this spread, but for the average case the spread would not be of consequence. If this were not a continuous laser (perhaps all semiconductor lasers are continuous lasers, I don't know) but fired in pulses, if the pulse width were sufficiently short, you might see each frequency occupy a different part of the pulse.

        Am I thinking about a non-issue?
        • I'm not sure but I think you might be confusing pulse spreading (in time) with homogeneous broadening (in wavelength/frequency). The former occurs since shorter wavelength light travels faster than longer wavelength light in a medium other than air. The latter results from the finite transition width (in wavelength) of the source. this is probably more confusing. I think I'm not understanding you though, oh well.

    • What i did not get (Score:2, Interesting)

      by hokanomono ( 530164 )

      Do they really have a polychromatic laser or is it just a laser that allows the user to choose an arbitrary wavelength in a given range? The article explains how they get spontaneous emission with a wide range of wavelengths, however it does not explain how to build a cavity that keeps a polychromatic wavepacket (wide in the momentum space) in a stationary state.

      I had the chance to play with a 200mW Argon laser last year. It can laze at many different wavelengths, but i can set the cavity only to a discrete set of wavelengths. This was crucial for deciding between the 351.1nm and the 351.4nm (iirc) line. *g*

      • The cavity is very large compared to the lasing wavelength(s). Therefore, it will have resonances at several wavelengths/frequencies. Each emitting layer has gain at a different wavelength, which can couple to the corresponding resonance of the cavity.

        In the case of the Argon laser, you were using the cavity to force the source to emit light at a slightly different wavelength. You wouldn't be able to tune too much since the source has limited bandwidth, unlike the quantum cascade laser which uses multiple sources.

        • I should do some calculating, before i post.
          If L is the length of the cavity and l is a wavelength, then the distance of the next wavelength that has a resonance in the cavity is l^2/L and this is VERY small under usual circumstances. I think i should read something about laser basics.

          You are right, thank you!
      • I think the point is that you can mess with the material so you can shift the cross section to different points. I didn't manage to get the abstract, but when Nature arrives I may post my understanding of the paper.

        From the ScienceNews Site it doesn't seem that revolutionary to be honest.
      • 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.
      Yeah, but isn't the point to be able to control each of these frequencies seperately?
      • Actually, I don't think it is. From the article:
        One such device, already in the works, would operate at the shorter infrared wavelengths now widely used for optical-fiber telecommunications, Gmachl says. In principle, the same technique might someday lead to a white-light device for room lighting, computer displays, or video projection, she notes.
        It sounds like it's more or less a glorified micro-optoelectronic spotlight that you might be able to mode-lock to use for fiber-optic communication.

  • High bandwidth lasers (Score:5, Interesting)

    by Yarn ( 75 ) on Saturday February 23, 2002 @04:57PM (#3058310) Homepage
    These aren't exactly new, but this is a new(ish) way of getting larger bandwidths. Before you have been more limited by the physical properties of your material, which meant you had to use interesting carcinogenic dyes, or expensive crystals to get broad stimulated emission cross sections.

    I'd publish my course notes on the issue, but I'm not certain my prof would be too happy. I'll check with him when I see him Monday.
  • "In any quantum-cascade laser, a high voltage coerces an electric current to penetrate layer after layer in the stack. The tight physical confinement of many of those stacked layers makes them act as so-called quantum wells, in which electrons can only have certain amounts of energy,..." Anyway it was good for me :-) and now I have to go and smoke something. The future's so bright I have to ware shades.
  • "Here, let me try this new gizmo out by beaming these MP3s over to ya. Just line up your comp. right next to mine."

    [Star Trek phaser sound]

    "Wow! that was the fastest transfer I've ever seen!!! Sorry about the big hole in your computer though.."

  • Military uses (Score:4, Informative)

    by europrobe ( 167359 ) <daniel@@@perup...net> on Saturday February 23, 2002 @05:21PM (#3058382) Homepage
    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?
  • My TV already came with a remote control.

  • Applications (Score:2, Interesting)

    by JumboMessiah ( 316083 )
    I'm going out on a ledge, but I'd believe one of the major applications of this is in DWDM down single mode fiber. It simplifies manufacturing costs to have a laser that can output on different wavelengths vs. multiple lasers all specifically designed for a single wavelength. I was reading a telecom book today at Barnes and Noble that pointed to that specifically as one reason for the current high costs of DWDM. Of course I have to take the book for a grain of salt considering it was yellow and black :).
  • MIDIfrared? Let me know when we get to OGGfrared, then I'll be more interested.

Slashdot Top Deals

I've noticed several design suggestions in your code.

Close