One interesting way that fibers are not immune to RF noise is in the presence of lightning strikes [1]. The strong electromagnetic fields can cause a polarization rotation inside the fiber, which the DSP at the receiving end of coherent links has to track.
Spectral efficiency is also a nifty metric because, given some bandwidth and signal-to-noise ratio, there is an upper bound to which you can compare a result [0].
Frequency spacing makes sense as namibj points out. Most long-distance telecom links operate in the optical C-band, which is roughly 5 THz wide. (A wavelength of 1525nm has an optical frequency of 196.5 THz, and a wavelength of 1565nm has an optical frequency of 191.5 THz). You can select optical frequencies to modulate within this optical bandwidth. Given a certain modulation rate (>>GHz), separating the channels in units of 1 GHz is reasonable.
Figure 3 in this open-access paper [0] provides historical scaling trends in optical fiber communication (transport) compared to generation and processing. Depending on the time period under study, bandwidth increases at between 20% and 100% per year.
Though the improvement in transistor economics has definitely benefited transport, the large bulk of improvement over time is due to breakthroughs in manufacturing, materials science, semiconductor optics, and signal processing.
Howdy, I design optical integrated circuits for coherent communication. mmmBacon is correct. See here[0] or [1] for examples of the types of forward error correction used in coherent links. Pre-FEC bit error rate thresholds of 1e-3 or 2e-2 for <1e-15 post-FEC errors are fairly standard.
About using light: I don't believe light will replace the transistors performing logic except in a few niche applications (e.g. [1]). Light is physically constrained by it's wavelength. It's difficult to build interacting structures smaller than a few hundred nm and it's difficult to build light-generating elements with even shorter wavelengths--you're approaching the Deep UV and X-rays. Maybe you can get to the tens to low hundreds of nm with plasmonics, but this is still far from the realm in which it makes sense to replace an electronic transistor with an optical transistor. Furthermore, it's also difficult to achieve strong nonlinearities in optical systems, especially silicon. You need some sort of nonlinear element for switching.
Light-based communication probably will replace certain I/O blocks on chip. These tend to be quite large in terms of area after considering power and ESD constraints.
It's not a goal I guess to shrink these "photon CPU-s" to 5nm at start.
> but this is still far from the realm in which it makes sense to replace an electronic transistor with an optical transistor
The electromagnetic spectrum even at mid-near infrared wavelengths frequencies could help chips operate on the THz scale! You might list a mountain of reasons it can't work, but it's just fun to imagine that it might be possible to turn a cycle of light to an operation.
You can build interesting things at that scale, in this research they also refer to communication as you mentioned [1]
I think it's great that a data center operator is willing to relax their requirements. For too long we've been designing against telecom specs and operating environments.
I think in order to bring more OEM vendors in we need to see the other big players to also accept the relaxed specs.
Hopefully, we don't end up with another dozen different 100G or 200G MSAs that work from 15-55C.
I'm also curious what the pricing difference is for a CWDM4 transceiver and the OCP version.
I would guess the NRE to develop either is similar and that the design for either is almost the same. Perhaps Facebook is just trying to get the optics cost down by negotiating discounts on the non-yielding MSA parts that would have otherwise had to get thrown out?
Put it the other way: what would be the benefit of publicly disclosing their secret sauce? Perhaps to help fuel their recruiting pipeline, but I can't think of any other good reasons to do so.
They don't have a product out, their secret sauce might change, Google/MSR might be working on something similar and just need a few hints in the right direction to get there, they get to keep a big first-mover advantage in whatever area their tech enables them to move into, and there's no opportunity for possible early adopters like yourself to get in on the action yet.
I'm guessing their tech isn't so much "critically secret" as there just isn't a benefit to their business case for publicizing what they are doing
I believe that LIDAR will be to silicon photonics what the accelerometer was to MEMS.
The real challenge that the article only touched upon is to get lasers into the same package and keep the costs down. This is still an active area of pursuit in both research and industry--though, there are several promising methods emerging. The $10 cost used in the article is likely closer to the cost of the bare silicon die. Packaging is always the expensive part of optics (doubly so if the lasers are not monolithically or heterogeneously integrated onto the same die). That being said, even with today's technology, integrating a laser chip and a silicon photonics chip into a package is easily south of $1k, which is what they quoted competing technologies costing.
I look forward to seeing these sensors integrated into my self-driving car in 5-10 years.
From this part of the article, the team seems fairly confident in their ability to integrate the laser onto the same die as the waveguides. This seems very plausible to me, given how standard this integration is in communications applications (eg fiber optic transceivers)
> Our device is a 0.5 mm x 6 mm silicon photonic chip with steerable transmitting and receiving phased arrays and on-chip germanium photodetectors. The laser itself is not part of these particular chips, but our group and others have demonstrated on-chip lasers that can be integrated in the future.
The Watts group at MIT has done some fantastic work into rare-earth doped silicon waveguides to produce lasers on a silicon platform [0]. However, I believe this work is still very much in the research stage. I'm not convinced their method is scalable to production for this $10 & million-unit-per-year LIDAR application since Erbium-doped waveguide lasers still require an off-chip pump laser source.
Even at 2 meter range with centimeter resolution, these devices would be a much better solution to the "local obstacle" problem than ultrasonics today. Mobile platforms moving around in spaces with a lot of miscellaneous obstacles have to either be compliant (or padded) enough to just push through them or slow enough to detect them and move around them.
[1] PDF Warning: https://www.ofcconference.org/getattachment/d0ec1565-ce81-48...