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UCLA Researchers Demo First Silicon Laser
An anonymous reader submits "Researchers at University of California, LA have demonstrated the first silicon laser. The lack of a silicon laser has been a major roadblock in the progress of silicon optoelectronics and photonics. This development shows that despite popular belief, a laser can indeed be made on a silicon chip. Modern electronic computers are getting closer to being optical in any case (gigahertz range). This discovery makes optical computers much closer to reality."
Cue the Raman effect jokes...
What about the laser diodes in laser pointers? Aren't they made of silicon? I know they're really crappy quality lasers, but they're still technically lasers...
From the UCLA "A key attribute of the new technology is that it can produce mid-infrared radiation without any cooling," Jalali said
Now this sounds really intersting, how come they dont need cooling?
The lunatic is in my head
Santa Claus, The Tooth Fairy, and The Easter Bunny, not to mention the sun rising and setting.
No, they aren't made of silicon. They're usually made of compound semiconductors, such as gallium arsenide, etc.
Ce n'est pas un vrai mouvement de robot!
In this case, the researchers are using a different mechanism to create photons (and more specifically population inversion and stimulated emission required for a laser) using something similar to what happens with erbium-doped fiber amplifiers.
Modern electronic computers are getting closer to being optical in any case (gigahertz range).
Could someone explain this to me? I don't get it.
But can it be attached to a sharks head?
Execute? [Y/N] _
This is nowhere near practical for integrated photonics. For one, the repetition rate is limited by the free carrier lifetime, which can range anywhere from about a nanosecond to several hundred nanoseconds, depending on how the waveguides are fabricated. So, the fastest the pulses can be repeated is about 1 GHz. Which is just slow even by todays standards. In this work they didn't even do that good, they repeated it at 25 MHZ. Slooow! And second, and the biggest problem, the Raman effect relies on an external laser to pump the Silicon laser! These can easily cost $25000 dollars! Using the raman effect there is NO way to do electric pumping. Another external laser will always be necessary, so I don't ever see this becoming practical.
And at this point there is no other easy way to make a silicon laser using anything other than the raman effect. Any other method would require the use of exotic materials (i.e. erbium, heterojunctions, etc.). It's been tried and the results aren't all that promising.
Regardless, this is some great progress and with some more research it can be improved on. For one, they are putting the silicon chip in a fiber ring to form the laser. The next obvious step is to make the laser work just on the chip. One easy way to do this would be to put mirrors on both sides of the chip. You can easily make mirrors with some fairly simple fabrication.
>Modern electronic computers are getting closer to >being optical in any case (gigahertz range).
Huh, last time I checked lasers were up there in the *TERA* hertz range.
So what is the punchline here?
If the poster means we need faster interconnects, then yeah verily, but that isn't the problem for CPU
design. Feeding the buggers is. Since the 386 we haven't had memory which could keep them without grumbly tummy syndrome. Intel tried to lie to everybody about this, but even they eventually caved in and started using caches of static ram.
Even back in 386 days you needed 35ns memory (or thereabouts). Nothing has changed since. Memory
technology *hasn't delivered*.
Modern "fine tuned" deep pipelined, superscalar up to the wazoo processors just don't scale anymore. Go watch *this* video reffed below for some insight. Suddenly you'll see why Intel has
backed off from their MHz == Performance kick...
Hint: It's 90 minutes and well worth the laughs
for the insights. I had to fight hard to see it
in a whole day (two Cretans in one office makes
for a really unproductive work environment...)
(I really need to borrow some hardware from ESR...)
Stanford is where the infamous and rather splendid
Don Knuth resides, and they have a 64 bit design I'd love to see (anyone want to give me one?) so I
can play with a real CPU.
http://www.theinquirer.net/?article=14310
(and thanks to whoever it was on slashdot for pointing me at it in an earlier post).
Two problems arise. Driving a signal from one end of the chip to the other end is very slow because the wires present a high RC load to the puny transistor. The other problem is simply routing the wires.
The laser solves the first problem because we can simply transmit a bit (0 or 1) by modulating the laser light. The bit will travel at the speed of light.
The laser also solves the second problem because there is no need to route the optical paths. The light from one laser can cross the path of light emanating from a second laser and can continue, unimpeded, to the detector intended for the first laser.
Modern electronic computers are getting closer to being optical in any case (gigahertz range).
Could someone explain this to me? I don't get it.
It refers to a couple of things.
One is that people have been talking about using optics to route signals on motherboards for a long time, as high-frequency electrical signals are a pain to keep clean over the relatively long distances invovled. I'm skeptical of this happening any time soon, for the same reason I'm skeptical of CMOS-on-silicon dying any time soon: the technologies are still scalable, a vast amount of study has produced a very good understanding of them, and we already have all of the tools we need to produce devices using them. An optical scheme would involve considerable retooling and a considerable learning curve. One of the important pieces of an optical communications scheme for electrical chips is good, fast, and cheap electro-optic transcievers. Cheap means implemented on silicon, on the same die as the logic elements. Receivers are no problem, as photodiodes work fine in an indirect bandgap material, but transmitters are very difficult (you want a direct bandgap if you're trying to produce an LED or a semiconductor laser). This paper shows a way of producing a semiconductor laser on something resembling a conventional silicon CMOS process (though I'd have to read the paper carefully to see if they did any tweaking). The catch is that I'm not sure if they're producing light in a band that can be picked up by a silicon photojunction (I _think_ they are, but would have to read the paper in detail).
The second thing the line could be referring to is that the distinction between an electrical signal on a bus and a microwave signal in a waveguide gets a bit blurry as frequency goes up. However, that's definitely overstating the case for the time being. If you were designing motherboards to use waveguide style signal transmission, they'd be set up quite differently. Right now, microwave emission is an unwanted side effect, and is for the most part minimal (it causes crosstalk and illegal RF/microwave interference).
Means the clock frequencies of modern processors are approaching the frequency range of visible light.
I don't suffer from insanity. I enjoy every minute of it.
And at this point there is no other easy way to make a silicon laser using anything other than the raman effect. Any other method would require the use of exotic materials (i.e. erbium, heterojunctions, etc.). It's been tried and the results aren't all that promising.
Actually, it turns out that if you can produce structures small enough, you can get silicon acting like something closer to a direct-bandgap material (you no longer have a crystal of near-infinite extent, so energy level analysis changes).
The best I've heard of is efficiencies in the single-digit percent range, though.
I think anyone actually building an optically-communicating chip would just use gallium arsenide (there's enough of an infrastructure for it to be practical).
Or I could be just talking out of my ass, the optical frequencies are actually almost into the petahertz range. Sorry 'bout that, the "(gigahertz range)" thingy fooled me. Should've checked the actual figures first.
I don't suffer from insanity. I enjoy every minute of it.
A laser on silicon essentially solves the wiring problem of traditional digital integrated circuits (ICs). Modern digital ICs consist mostly of wires; a small percentage of the silicon area is the transistors that perform the computation.
Not true last I checked, and I am presently doing IC design. Yes, you need plenty of space for your routing channels, but especially with the number of metal layers we have now, there's plenty of space _above_ the active circuitry even after you take out the layers used for routing within the gates.
Two problems arise. Driving a signal from one end of the chip to the other end is very slow because the wires present a high RC load to the puny transistor. The other problem is simply routing the wires.
The RC load problem is only _severely_ nasty if you lay out the bus as one long wire. If you segment it and put repeaters, time scales as length, not as length squared. You can also pack adjacent data stages more closely together as linewidth decreases, so chip shrinks continue speeding things up.
Routing is only a crippling problem if you're trying to do an anywhere-to-anywhere communications mesh. Even then, there are well-known topologies that minimize pain. This is part of why the movement to multi-core chips is happening - a massively superscalar chip _does_ have to route results from any part of the chip to functional units at any other part. Smaller cores in parallel are a win/win scenario (simpler design, easier communications).
See above re. routing. Routing is actually _easier_ now than it was a decade ago, because we have six or more metal layers to do it in rather than two or three. For a pipelined data flow, the routing complexity between stages doesn't really _increase_ with chip size. It's aggressively superscalar designs that do that, and those reached their peak complexity a few years back (with emphasis shifting to multi-core now).
The laser solves the first problem because we can simply transmit a bit (0 or 1) by modulating the laser light. The bit will travel at the speed of light.
Electrical signals travel somewhere between 10% and 50% C, usually. The real problem is capacitive loads, and an electro-optic system has to deal with those too, believe me. Electro-optic systems will actually have it _worse_, because you need large photosensitive structures to pick up the light (more capacitance), and have relatively small photocurrents to charge or discharge them with. You no longer have electric networks with huge fan-out (and large capacitance), but if you build a segmented hierarchical scheme, you don't with an electrical implementation either. And you get an _optical_ fan-out in return - you're only producing so much light, and it has to be split among N potential receivers.
Light is very far from being a magic bullet for intra-chip communication.
The laser also solves the second problem because there is no need to route the optical paths. The light from one laser can cross the path of light emanating from a second laser and can continue, unimpeded, to the detector intended for the first laser.
This turns out not to be the case. Your emitters are small enough that diffraction will spread out the beams quite a bit. So, you're either using a broadcast system (which reduces your communications bandwidth drastically - you only have one channel shared among all components), or you build waveguides. Waveguides have to be routed, and have the same kind of crossover problems wires have. They're actually a bit worse, as they have more constraints on geometry than a low-current signal wire does.
In summary, I don't think that optical communication works as well as you think it does, and the problems you note with electrical communication are largely solved.
The Silicon laser isn't working with population inversion, it is based on the Raman effect - a nonlinear process. Pump photons are converted to photons with a lower energy given a phonon (a lattice vibration at 15.6 THz in this case). The laser works because the fact that Silicon is NOT strongly absorptive at the lasing wavelength. The lasing wavelength is 1650nm if I recall correctly. The bandgap of Silicon is ~1100nm. So there is no way that Silicon can be used to detect this light. Bandgap engineering or a completely different material system is needed to detect this light.
Will this make laser tag more fun?