Society has an insatiable desire for data. In fact, it is rather astonishing to think that average Internet traffic is several hundred terabits per second and consumes about eight percent of our electricity production. All of that for instant cat videos—and our desire for new cat videos is apparently insatiable, driving the need for more capacity and even more energy.
It would, however, be nice to scale capacity without energy requirements continuing to grow at the same rate. In a step toward achieving this goal, researchers have managed to encode an insane amount of information into the light of a single laser.
The problem with scaling bandwidth and power comes down to lasers and their inefficiency. A good laser is about 30-percent efficient. A typical telecommunications laser might emit 20mW, so that's at least 70mW for each laser (the amplifiers consume even more energy). To pack more data into a single optical fiber, the data is divided across different colors of light, called wavelength division multiplexing. Unfortunately, each color requires its own laser, meaning the energy cost increases with bandwidth.
Lasers with lots of colors
The researchers start in the same place that all optical communications systems begin: with a laser. But, instead of a very pure color, this laser emits pulses of light. These pulses are created by adding many very pure colors together, with the colors separated by evenly sized gaps in frequency.
By itself, the laser doesnt generate many of these colors, so the researchers use a trick to get more colors. The light is passed through a very fine wire (about 300 nanometers in diameter). The diameter is so small that the light is compressed and becomes very bright. The high intensity causes the material that makes up the wire to respond by generating new colors. The trick is that these new colors follow the spacing set by the laser pulse. So, out of the wire comes pulses of light that consist of thousands upon thousands of highly pure colors.
That means that a single laser generates all 80 colors required for the entire system, which is pretty cool. However, the researchers were not done yet.
Packing in the data
The emitted laser light is divided into two polarizations—polarization is the orientation of the electric field as it oscillates—so each color contributes two channels. Then, because the laser is pulsed, the information can be placed in four different time slots, called time division multiplexing. So, each color has a raw data rate of about 320Gbps. But, with 80 colors, that is 25Tbps.
The researchers were still not done.
The fiber that transported the signal consists of 30 light-guiding cores, surrounded by a single cladding. That means that each core is capable of transporting data at a rate of 25Tbps, bringing us to a grand total of 768Tbps. That, however, is the raw data rate. Data is always transmitted with some redundancy to allow for errors to be corrected, called forward error correction. Once redundancy is accounted for, the net data transfer rate is 661Tbps.
And that is an incredible amount of data by anyones standard.
As for power savings, Im not really sure how significant they will be. Each data stream still needs to have independent modulation to encode the data, so there is no saving in power there. However, the laser itself emits less than 90mW, which is about five percent of the optical power that it would use if each color were emitted independently. Assuming that the lasers have the same efficiency—and that is a big assumption—then those savings will translate into electrical energy savings as well.
I can imagine this being a great energy saver within data centers and such. But, for long haul connections, I suspect the major energy cost is in the amplifiers. That said, no matter how you look at it, this is an excellent bit of engineering.
Nature Photonics, 2018, DOI: 10.1038/s41566-018-0205-5