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When will optical CPUs become a reality? Instead of pushing electrons with a voltage difference, to signify 0 or 1, a light wave of red or blue, can be used in

When will optical CPUs become a reality? Instead of pushing electrons with a voltage difference, to signify 0 or 1, a light wave of red or blue, can be used instead. Anyone here a photonics expert?

rwmj 7y ago
There was a RISC-V processor with optical components manufactured (https://riscv.org/wp-content/uploads/2016/01/Wed1015-riscv-sunchen-1-6-2016.pdf https://riscv.org/wp-content/uploads/2016/01/Wed1015-riscv-s... https://escholarship.org/content/qt4dh1v4px/qt4dh1v4px.pdf https://escholarship.org/content/qt4dh1v4px/qt4dh1v4px.pdf). This isn't necessarily an "optical CPU" because the cores themselves are still regular CMOS, but it's a step towards that. Interestingly one of the authors is the chief architect at SiFive.
Symmetry 7y ago
There's lots of optical data transport, even plausible research for on-chip communication, but nobody really has a plausibly commercial optical transistor except at a very conceptual stage.
beisner 7y ago
https://www.luminouscomputing.com/ https://www.luminouscomputing.com/ There's at least some cool work going on in the GPU/TPU area.
- jlokier 7y ago
  Thanks for that link, I hadn't heard of Luminous.
icapybara 7y ago
Yeah I work in photonics (phd student in a lab that does optical microresonators). The problem is that photonic structures are massive (10s-100s of um) and waste a lot of space. Photonics will never beat electronics in terms of density on the chip because you're limited by the wavelength of the light you're using. They are also quite lossy. Every time the waveguide turns, you have some radiation loss, for example. You can't contain an optical signal on a chip as well as you can contain an electrical signal.
jlokier 7y ago
I'm not a photonics expert, but I have done some development on a device using optical parametric amplification (OPA) for high performance bulk compute. The project goal was to outperform ASICs on a very well-defined, highly regular problem, and achieve it without the capital cost of typical ASICs, so that specialised circuits could realistically be built and used for different problems. So, not a general purpose CPU, but something that can compute. OPA is potentially an extremely high bandwidth signal processing process, which doesn't involve converting to electrical signals, through transistors and then back to optical the way that some photonic designs do. It is more like the way optical communications amplifiers work, directly amplifying the modulated light that is passing through. We are talking >1THz bit rates per logic element, and it's also quite an energy efficient process (despite limited OPA conversion efficiency, because you can recycle some of the light that hasn't converted), so it was worth exploring. In the process, no insurmountable technical obstacles were found during the time of the project, but we ran out of time and money. But it was surprising to find that, despite the superficial promise of photonics, it wasn't obviously a lot faster, or faster per Watt, than the best silicon electronics after all. This is because silicon transistors are pretty fast and efficient these days, and because you can fit a huge number of them in an area much smaller than the wavelength of visible light. You can confine light too, and there has been some published progress at nanoscale OPA elements, but it's a much more complicated structure and process (plasmons etc) than OPA in bulk materials, and nanoscale OPA may be just as difficult to manufacture as nanoscale transistors. Also, quantum: Just due to light being quantized as photons, there comes a point where to carry enough information at high data rates, the power density needed is an issue. Sorry, I didn't answer your question :-) My guess: Actually making a photonic CPU is economically and motivationally constrained rather than science constrained at this point, even though there's plenty of R&D still needed to do it. The motivation isn't that strong because the benefits aren't that obvious, and I think if they were obvious, the big commercial labs would have shipped a working prototype already.
jariel 7y ago
Though optical parts are still 'big' and nobody seems to know how to make small IC's - 20 years ago I worked in networking and they were trying to make optical switches with mirrors! At the long haul level, they use a transport layer tech called SONET. Rather than demuxing the optical signal into bits, then back into optical and out another pipe, they wanted to switch using fancy mirrors. Frankly, I'm not sure of the advantage. Maybe some performance? Security? I'm doubtful though.

When will optical CPUs become a reality? Instead of pushing electrons with a voltage difference, to signify 0 or 1, a light wave of red or blue, can be used instead. Anyone here a photonics expert?

rwmj 7y ago

There was a RISC-V processor with optical components manufactured (https://riscv.org/wp-content/uploads/2016/01/Wed1015-riscv-sunchen-1-6-2016.pdf https://riscv.org/wp-content/uploads/2016/01/Wed1015-riscv-s... https://escholarship.org/content/qt4dh1v4px/qt4dh1v4px.pdf https://escholarship.org/content/qt4dh1v4px/qt4dh1v4px.pdf). This isn't necessarily an "optical CPU" because the cores themselves are still regular CMOS, but it's a step towards that. Interestingly one of the authors is the chief architect at SiFive.

Symmetry 7y ago

There's lots of optical data transport, even plausible research for on-chip communication, but nobody really has a plausibly commercial optical transistor except at a very conceptual stage.

beisner 7y ago

https://www.luminouscomputing.com/ https://www.luminouscomputing.com/ There's at least some cool work going on in the GPU/TPU area.

jlokier 7y ago
Thanks for that link, I hadn't heard of Luminous.

icapybara 7y ago

Yeah I work in photonics (phd student in a lab that does optical microresonators). The problem is that photonic structures are massive (10s-100s of um) and waste a lot of space. Photonics will never beat electronics in terms of density on the chip because you're limited by the wavelength of the light you're using. They are also quite lossy. Every time the waveguide turns, you have some radiation loss, for example. You can't contain an optical signal on a chip as well as you can contain an electrical signal.

jlokier 7y ago

I'm not a photonics expert, but I have done some development on a device using optical parametric amplification (OPA) for high performance bulk compute. The project goal was to outperform ASICs on a very well-defined, highly regular problem, and achieve it without the capital cost of typical ASICs, so that specialised circuits could realistically be built and used for different problems. So, not a general purpose CPU, but something that can compute. OPA is potentially an extremely high bandwidth signal processing process, which doesn't involve converting to electrical signals, through transistors and then back to optical the way that some photonic designs do. It is more like the way optical communications amplifiers work, directly amplifying the modulated light that is passing through. We are talking >1THz bit rates per logic element, and it's also quite an energy efficient process (despite limited OPA conversion efficiency, because you can recycle some of the light that hasn't converted), so it was worth exploring. In the process, no insurmountable technical obstacles were found during the time of the project, but we ran out of time and money. But it was surprising to find that, despite the superficial promise of photonics, it wasn't obviously a lot faster, or faster per Watt, than the best silicon electronics after all. This is because silicon transistors are pretty fast and efficient these days, and because you can fit a huge number of them in an area much smaller than the wavelength of visible light. You can confine light too, and there has been some published progress at nanoscale OPA elements, but it's a much more complicated structure and process (plasmons etc) than OPA in bulk materials, and nanoscale OPA may be just as difficult to manufacture as nanoscale transistors. Also, quantum: Just due to light being quantized as photons, there comes a point where to carry enough information at high data rates, the power density needed is an issue. Sorry, I didn't answer your question :-) My guess: Actually making a photonic CPU is economically and motivationally constrained rather than science constrained at this point, even though there's plenty of R&D still needed to do it. The motivation isn't that strong because the benefits aren't that obvious, and I think if they were obvious, the big commercial labs would have shipped a working prototype already.

jariel 7y ago

Though optical parts are still 'big' and nobody seems to know how to make small IC's - 20 years ago I worked in networking and they were trying to make optical switches with mirrors! At the long haul level, they use a transport layer tech called SONET. Rather than demuxing the optical signal into bits, then back into optical and out another pipe, they wanted to switch using fancy mirrors. Frankly, I'm not sure of the advantage. Maybe some performance? Security? I'm doubtful though.