3 ms·

It is not particularly hard to do a "hero" experiment like this. Shannon limits to fiber transmission have pretty much been reached experimentally a long time a

It is not particularly hard to do a "hero" experiment like this. Shannon limits to fiber transmission have pretty much been reached experimentally a long time ago. Muxing several wavelengths together is also of course the backbone of fiber optics transmission since its very origin. The current buzz in the field is to use micro-combs like they did as opposed to an array of lasers to provide the multiple wavelengths - but it still comes with its particular set of challenges to make it practical. The micro-comb provides the other wavelengths from a single source through a nonlinear process. My understanding from just looking quickly at the paper is that they don't modulate all the wavelengths independently, meaning that they duplicate the info they send several times to reach that high terabit rate. The laser source is only one part of a transceiver, and once you have 400+ independent modulators/receivers, the laser source becomes a much smaller concern when it comes to make it practical. A conventional laser source can be made very compact too (in a semiconductor platform) and integrated with the rest of the transceiver on the same chip. This is still where the industry is putting its efforts. These micro-combs come with some disadvantages too, relating to stability, low SNR, and uneven power among the wavelengths (that then need to be equalized).

cycomanic 6y ago
I agree with you that microcombs come with their specific kind of challenges (and SNR is the big one), however making many semiconductor lasers so small as too fit 100 on a single chip poses lots of challenges (in particular thermal management and wavelength stability). That said, combs (not necessarily microcombs) over opportunities for additional functionality/optimizations. Because the comb lines are locked to each other (normally individual lasers have small wavelength fluctuations) let's you space channels even closer together, as well as process multiple channels at the same time. Regarding your comment about these hero experiments not being hard, I would argue it's actually the other way around, we are now so close to the limits that it is becoming incredibly hard to observe further gains. Also regarding not modulating lines independently, this is the common way how everyone (even the industry labs) demonstrates these systems, using 100 independent transceivers would be prohibitevely expensive, moreover research has shown that you actually receive a penalty from using this approach so the demonstration is a lower bound on what could be achieved with individual tx modules.
- dasudasu 6y ago
  Modulating individual lines is the only way for such a scheme to become practical in a real environment and achieve the claimed data rates. Making hundreds of modulators and receivers fit within a single chip is about as hard as making hundreds of laser fit a single chip, hence why it's not realistically being pursued by the industry. My point is that if you already require separate chips for the rest of the transceiver, integrating the laser itself becomes much less of an issue, and the benefits of a single laser source common to all much more muted.

cycomanic 6y ago

I agree with you that microcombs come with their specific kind of challenges (and SNR is the big one), however making many semiconductor lasers so small as too fit 100 on a single chip poses lots of challenges (in particular thermal management and wavelength stability). That said, combs (not necessarily microcombs) over opportunities for additional functionality/optimizations. Because the comb lines are locked to each other (normally individual lasers have small wavelength fluctuations) let's you space channels even closer together, as well as process multiple channels at the same time. Regarding your comment about these hero experiments not being hard, I would argue it's actually the other way around, we are now so close to the limits that it is becoming incredibly hard to observe further gains. Also regarding not modulating lines independently, this is the common way how everyone (even the industry labs) demonstrates these systems, using 100 independent transceivers would be prohibitevely expensive, moreover research has shown that you actually receive a penalty from using this approach so the demonstration is a lower bound on what could be achieved with individual tx modules.

dasudasu 6y ago
Modulating individual lines is the only way for such a scheme to become practical in a real environment and achieve the claimed data rates. Making hundreds of modulators and receivers fit within a single chip is about as hard as making hundreds of laser fit a single chip, hence why it's not realistically being pursued by the industry. My point is that if you already require separate chips for the rest of the transceiver, integrating the laser itself becomes much less of an issue, and the benefits of a single laser source common to all much more muted.