[HN Gopher] TSMC bets on unorthodox optical tech
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TSMC bets on unorthodox optical tech
Author : Rohitcss
Score : 94 points
Date : 2025-05-26 17:15 UTC (5 hours ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| Liftyee wrote:
| As I understand it (from designing high-speed electronics), the
| major limitations to data/clock rates in copper are signal
| integrity issues. Unwanted electromagnetic interactions all
| degrade your signal. Optics is definitely a way around this, but
| I wonder if/when it will ever hit similar limits.
| moelf wrote:
| luckily photons are boson (if we ever pushes things to this
| level of extreme)
| Taek wrote:
| This comment appears insightful but I have no idea what it
| means. Can someone elaborate?
| scheme271 wrote:
| Electrons are fermions which means that two electrons can't
| occupy the same quantum state (Pauli exclusion principle).
| Bosons don't have the limit so I believe that implies that
| you can have stronger signals at the low end since you can
| have multiple photons conveying or storing the same
| information.
| cycomanic wrote:
| What the previous poster is implying is that electrons
| interact much more strongly than photons. Hence electrons
| are very good for processing (e.g. building a transistor),
| while photons are very good for information transfer. This
| is also a reason why much of the traditional "optical
| computer" research was fundamentally flawed, just from
| first principles one could estimate that power requirements
| are prohibitive.
| xeonmc wrote:
| Fermions can "hit each other" while bosons "passes right
| through each other".
|
| (Strong emphasis on the looseness of the scare quotes.)
| EA-3167 wrote:
| The energy densities required for photon-photon interactions
| are so far beyond anything we need to worry about that it's a
| non-issue. Photons also aren't going to just ignore local
| potential barriers and tunnel at the energy levels and scales
| involved in foreseeable chip designs either.
| notepad0x90 wrote:
| isn't attenuation also an issue with copper? maybe with small
| electronics it is negligible given the right amps? in other
| words, with with no interference, electrons will face impedance
| and start losing information.
| to11mtm wrote:
| Attenuation is going to be an issue for any signal, but in my
| experience Fiber can go for many miles without a repeater
| whereas something like Coax you're going one to two orders of
| magnitude less. [0]
|
| [0] - Mind you, some of that for Coax is due to other issues
| around CTB and/or the challenge that in Coax, you've got many
| frequencies running through alongside each frequency having
| different attenuation per 100 foot...
| bgnn wrote:
| This is the main mechanism of interference anyhow, called
| inter-symbol-interference.
| lo0dot0 wrote:
| Optics also have signal integrity issues. In practice OSNR and
| SNR limit optics. Cutting the fiber still breaks it. Small
| vibrations also affect the signal's phase.
| cycomanic wrote:
| Phase variations will not introduce any issues here, they
| most certainly are talking about intensity modulation. You
| can't really (easily) do coherent modulation using incoherent
| light sources like leds.
|
| SNR is obviously an issue for any communication system,
| however fiber attenuation is orders of magnitude lower than
| coax.
|
| The bigger issues in this case would be mode-dispersion,
| considering that they are going through "imaging" fibres,
| i.e. different spatial components of the light walking off to
| each other causing temporal spread of the pulses until they
| overlap and you can't distinguish 1's and 0's.
| m3kw9 wrote:
| If each cable is 10gb/s and uses 1 pixel to convert into
| electrical signals, would that mean they need a 10 giga frame per
| second sensor?
| cpldcpu wrote:
| I think that's just a simplifying example. They would most
| likely not use an image sensor, but a photodetector with a
| broadband amplifier.
| lo0dot0 wrote:
| No, not necessarily. If you can distinguish different amplitude
| levels you can do better. For example four amplitude modulation
| (4AM) carries two bits per symbol. There is also the option to
| use coherent optics, which can detect phase, and carry
| additional information in the phase.
| amelius wrote:
| > The transmitter acts like a miniature display screen and the
| detector like a camera.
|
| So if I'm streaming a movie, it could be that the video is
| actually literally visible inside the datacenter?
| tehjoker wrote:
| maybe a low res bit packed binary motion picture thats
| uncompressed
| tails4e wrote:
| No, this is just an a analogy. The reality is the data is
| heavily modulated, and also the video is encoded so at no point
| would something that visually looks like an image be visible in
| the fibre.
| lo0dot0 wrote:
| Obviously this is not how video compression and packets work
| but for the sake of the argument consider the following. The
| article speaks of a 300 fiber cable. A one bit per pixel square
| image with approx. 300 pixels is 17x17 in size. Not your
| typical video resolution.
| fecal_henge wrote:
| Not your typical frame rate either.
| speedbird wrote:
| Headline seems misleading. They're buildings detectors for
| someone, not 'betting' on it.
| ls612 wrote:
| Forgive the noob question but what stops us from making optical
| transistors?
| qwezxcrty wrote:
| I think the most fundamental reason is that there is no
| efficient enough nonlinearity at optical frequencies. So two
| beams(or frequencies in some implementation) tends not to
| affect each other in common materials, unless you have a very
| strong source (>1 W) so the current demonstrations for all-
| optical switching are mostly using pulsed sources.
| ls612 wrote:
| I wonder if considerably more engineering and research effort
| will be applied here when we reach the limit of what silicon
| and electrons can do.
| cycomanic wrote:
| No this is not an engineering issue, it's a problem of
| fundamental physics. Photons don't interact easily. That
| doesn't mean there are not specialised applications where
| optical processing can make sense, e.g. a matrix
| multiplication is really just a more complex lens so it's
| become very popular to make ML accelerators based on this.
| elorant wrote:
| Photons are way more difficult to control than electrons. They
| don't interact with each other.
| qwezxcrty wrote:
| Not an expert in communications. Would the SerDes be the new
| bottleneck in the approach? I imagine there is a reason for
| serial interfaces dominating over the parallel ones, maybe timing
| skew between lanes, how can this be addressed in this massive
| parallel optical parallel interface?
| to11mtm wrote:
| > timing skew between lanes
|
| That's a big part of it. I remember in the Early Pentium 4
| days, starting to see a lot more visible 'squiggles' on PCB
| traces on motherboards; the squiggles essentially being a case
| of 'these lines need more length to be about as long as the
| other lines and not skew timing'
|
| In the case of what the article is describing, I'm imagining a
| sort of 'harness cable' that has a connector on each end for
| all the fibers, and the fibers in the cable itself are all the
| same length, there wouldn't be a skew timing issue. (Instead,
| you worry about bend radius limitations.)
|
| > Would the SerDes be the new bottleneck in the approach
|
| I'd think yes, but at the same time in my head I can't really
| decide whether it's a harder problem than normal mux/demux.
| fecal_henge wrote:
| SerDes is already frequently parallelised. The difference is
| you never expect the edges or even the entire bits to arrive at
| the same time. You design your systems to recover timing per
| link so the skew doesnt become the constraint on the line rate.
| bgnn wrote:
| one can implement SerDes at any point of the electro-optical
| boundary. For example, if we have 1 Tbps incoming NRZ data
| from the fiber, and the CMOS technology at hand only allows
| 10 GHz clock speed for the slicers, one can have 100x
| receivers (photodiode, TIA, slicer), or 1x photodiode, 100x
| TIA + slicer, or 1x photodiode + TIA and 100x slicers. The
| most common id the last one, and it spits out 100x parallel
| data.
|
| Things get interesting if the losses are high and there needs
| to be a DFE. This limits speed a lot, but then copper
| solutions moved to sending multi-bit symbols (PAM 3,
| 4,5,6,8,16.. ) which can also be done in optical domain. One
| can even send multiple wavelengths in optical domain, so
| there are ways to boost the baud rate without requiring high
| clock frequencies.
| waterheater wrote:
| >serial interfaces dominating over the parallel ones
|
| Semi-accurate. For example, PCIe remains dominant in computing.
| PCIe is technically a serial protocol, as new versions of PCIe
| (7.0 is releasing soon) increase the serial transmission rate.
| However, PCIe is also parallel-wise scalable based on
| performance needs through "lanes", where one lane is a total of
| four wires, arranged as two differential pairs, with one pair
| for receiving (RX) and one for transmitting (TX).
|
| PCIe scales up to 16 lanes, so a PCIe x16 interface will have
| 64 wires forming 32 differential pairs. When routing PCIe
| traces, the length of all differential pairs must be within
| <100 mils of each other (I believe; it's been about 10 years
| since I last read the spec). That's to address the "timing skew
| between lanes" you mention, and DRCs in the PCB design software
| will ensure the trace length skew requirement is respected.
|
| >how can this be addressed in this massive parallel optical
| parallel interface?
|
| From a hardware perspective, reserve a few "pixels" of the
| story's MicroLED transmitter array for link control, not for
| data transfer. Examples might be a clock or a data frame
| synchronization signal. From the software side, design a
| communication protocol which negotiates a stable connection
| between the endpoints and incorporates checksums.
|
| Abstractly, the serial vs. parallel dynamic shifts as
| technology advances. Raising clock rates to shove more data
| down the line faster (serial improvement) works to a point, but
| you'll eventually hit the limits of your current technology.
| Still need more bandwidth? Just add more lines to meet your
| needs (parallel improvement). Eventually the technology
| improves, and the dynamic continues. A perfect example of that
| is PCIe.
| cycomanic wrote:
| They are doing 10 Gb/s over each fibre, to get to 10 Gb/s you
| have already undergone a parallel -> serial conversion in
| electronics (clock rates of your asics/fpgas are much lower),
| to increase the serial rate is in fact the bottleneck. Where
| the actual optimum serial rate is highly depends on the cost of
| each transceiver, e.g. long haul optical links operate at up to
| 1 Tb/s serial rates while datacenter interconnects are 10-25G
| serial AFAIK.
| cycomanic wrote:
| That article is really low on details and mixes up a lot of
| things. It compares microleds to traditional WDM fiber
| transmission systems with edge emitting DFB lasers and ECLs, but
| in datacentre interconnects there's plenty of optical links
| already and they use VCSELs (vertical cavity surface emitting
| lasers), which are much cheaper to manufacture. People also have
| been putting these into arrays and coupling to multi-core fiber.
| The difficulty here is almost always packaging, i.e. coupling the
| laser. I'm not sure why microleds would be better.
|
| Also transmitting 10 Gb/s with a led seems challenging. The
| bandwidth of an incoherent led is large, so are they doing
| significant DSP (which costs money and energy and introduces
| latency) or are they restricting themselves to very short (10s of
| m) links?
| tehjoker wrote:
| short links it's in the article
| cycomanic wrote:
| Ah I missed the 10m reference there. I'm not sure it makes
| more sense though. Typical intra-datacenter connections are
| 10s-100s of meters and use VCSELs, so introducing microleds
| just for the very short links instead of just parallelising
| the VCSEL connections (which is being done already)? If they
| could actually replace the VCSEL I would sort of see the
| point.
| qwezxcrty wrote:
| I guess they are doing direct modulated IMDD for each link so
| the DSP burden is not related to the coherence of diodes? Also
| indeed very short reach in the article.
| cycomanic wrote:
| The problem with both leds and imaging fibres is that modal
| dispersion is massive and completely destroys your signal
| after only a few meters of propagation. So unless you do MMSE
| (which I assume would be cost prohibitive), you really can
| only go a few meters. IMDD doesn't really make a difference
| here.
| albertzeyer wrote:
| There is also optical neuromorphic computing, as an alternative
| to electronic neuromorphic computing like memristors. It's an
| fascinating field, where you use optical signals to perform
| analog computing. For example:
|
| https://www.nature.com/articles/s41566-020-00754-y
|
| https://www.nature.com/articles/s44172-022-00024-5
|
| As far as I understood, you can only compute quite small neural
| networks until the noise signal gets too large, and also only a
| very limited set of computations works well in photonics.
| cycomanic wrote:
| The issue with optical neuromorphic computing is that the field
| has been doing the easy part, i.e. the matrix multiplication.
| We have known for decades that imaging/interference networks
| can do matrix operations in a massively parallel fashion. The
| problem is the nonlinear activation function between your
| layers. People have largely been ignoring this, or just
| converted back to electrical (now you are limited again by the
| cost/bandwidth of the electronics).
| seventytwo wrote:
| Seems hard to imagine there's not some non-linear optical
| property they could take advantage of
| cycomanic wrote:
| The problem is intensity/power, as discussed previously
| photon-photon interactions are weak, so you need very high
| intensities to get a reasonable nonlinear response. The
| issue is, that optical matrix operations work by spreading
| out the light over many parallel paths, i.e. reducing the
| intensity in each path. There might be some clever ways to
| overcome this, but so far everyone has avoided that
| problem. They said we did "optical deep learning" what they
| really did was an optical matrix multiplication, but saying
| that would not have resulted in a Nature publication.
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