Germanium Diodes Mean Progress Toward Silicon-Chip Lasers 66
David Orenstein writes "Teams at Stanford and MIT have each reported getting
strong light signals from germanium-based diodes on silicon at room temperature. Engineers have long sought to do this because, with further refinement into lasers, such diodes would allow for optical interconnects on chips. Optical interconnects could operate much faster and with less power than electrical (metal) ones that are becoming bottlenecks on current chips."
Re:I thought they already existed (Score:5, Informative)
I know for sure that I used Germanium diodes before and I'm pretty sure Germanium-based LED's have been developed before. Dunno what the news is.
They seem to have improved on Germanium LEDs by doping them differently to the point where the can look into using photons to transmit information around a silicon chip in place of electrons. I imagine they will look into building light pipes out of silicon, ie, little optical fibres.
OT: somebody should teach ascribe how to use the title tag.
Re:I thought they already existed (Score:5, Informative)
Inter-chip communication? (Score:3, Informative)
I can see this technology being able to be used to help with inter-chip communication, perhaps to help with running more tasks in parallel, or locking/unlocking memory segments shared by the CPUs.
The only thing I see that would be a limit is having to mux/demux a lot of signals before they get put on the fiber optic cable. However fiber optic cables have a lot of bandwidth, so this may not be a big issue.
It would be nice if silicon chip lasers could replace most signal circuits on a PC board. Mainly because it would allow positioning of components to allow for better cooling and heat dissipation. Ultimately, if several fiber optic connections can replace the hundreds (going on thousands) of pins needed on a CPU to the motherboard, it would be a great advance in reliability.
Fiber optics on chips isn't new though. I remember talk about the PowerPC 603 having the ability to have this for better SMP communication.
Re:The 40-year old promise (Score:3, Informative)
Yes, Germanium has an indirect band-gap, but SiC, I thought, had a direct one. The problem with SiC was (is?) to grow a crystal of a determined orientation. As it is now, the crystal structure of SiC is pretty much random. That said, growing a thin layer of SiC (by simple CVD) on Si is promising.
Re:Optics Express and Optics Letters (Score:3, Informative)
The Stanford team's abstract is at
http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-12-10019 [opticsinfobase.org]
and the full paper is downloadable there
The MIT abstract is at
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-34-11-1738 [opticsinfobase.org]
but you have to pay to read the paper
Re:The 40-year old promise (Score:3, Informative)
That "some other mechanism" is phonons-- lattice vibrations. The lattice vibrations temporarily turn it into a direct band gap semiconductor).
Even after all these years of research, it's still largely inefficient to have to create the phonons (heat) so that you can create the photons (the laser).
Re:I thought they already existed (Score:3, Informative)
Also notice how we've all but abandoned parallel buses in PCs. This is because it is almost impossible with copper to ensure the multiple signals of a parallel bus will arrive at their destination at the same time - creating what is called a race condition. At higher frequencies, you'll start seeing data bits from the same message arriving before and after the clock signal which results in an unreliable interconnect. We have reached this point years ago, and that's why everything is using serial today - SATA, PCIe, USB, etc - serial buses eliminate the possibility for a race condition at the cost of requiring much higher clock speeds for the same throughput. Optical interconnects on the other hand could be used to much, much higher frequencies before we start seeing race conditions thus allowing for a very high speed parallel bus.
I don't think adding fiber to a PCB would really increase the cost of a PCB all that much, at least not when it becomes a common practice. I don't think the fabrication process would be all that much different from copper in the end; I don't see it being much more than routing a transparent plastic instead of copper. Granted, you'd be more limited in your routing paths, but that's nothing an extra layer wouldn't fix. So yeah, I guess it would increase costs a bit, at least at first, but it would probably reach the commodity level pricing we are at now pretty quickly. (The real cost increase will probably be the connectors and not the PCB itself.)
The point is we have just about maxed out copper and there is no other long term option but to move to optical interfaces.