The information age demands fat pipes. But making fat pipes is not always as easy as it sounds. Consider our current generation of fiber optic communications. Compared to microwave systems, where every symbol communicates something like one or two bytes of data, most current optical systems are limited to one to four bits per symbol.

This hasn’t mattered so much because many lasers, each with a different wavelength—called a channel—can be used on the same fiber, and the rate at which we send those bits is astonishingly high. Single channel capacities are way in excess of 40Gb/s range—40Gb/s was in testing the last time I taught a telecommunications course, and in 2012, various companies were testing 160Gb/s per channel. These incredible capacities, however, are achieved under very stringent conditions: the optical power must remain low, and the optical properties of the fiber must be carefully controlled (something called dispersion management).

The increase from 40Gb/s to 160Gb/s also represented the switch from encoding one bit per symbol to four bits per symbol. However, these encoding systems require that there is considerably more optical power per channel, and this causes problems with the stringent conditions mentioned above. This has made increases beyond four bits per symbol difficult. Funnily enough, everyone has kind-of-sorta known how to solve the problem, but no one was willing to simply bite the bullet and do it. At least until now.

Dimming the lights

Before we come to the latest and greatest, let us discuss how current fiber optic technology sucks. Let's start with something called dispersion. Every material has a refractive index. This can be thought of as the degree to which light is slowed by the material. For glass, the refractive index is about 1.5, so light travels about a third slower than its speed in a vacuum. This, by itself, is not a problem—the problem is that the refractive index is different for each color of light.

One way to communicate optically is to blink the light source. In a very simple encoding scheme, a bright flash might encode a one, while darkness encodes a zero. We set a clock speed for the maximum rate of blinking and then simply measure the light intensity in each time slot. To communicate faster, we make the light pulses shorter and shorter and make the corresponding time slots smaller and smaller. But to make a pulse of light requires a range of colors, rather than a single pure color. The shorter the pulse, the broader the range of colors.

The first problem you run into is that as the bandwidth required by a channel increases (due to the range of colors required to generate the short pulse), the spacing between the frequencies used for channels must increase. At some point, the total capacity no longer increases because the increased capacity of the individual channel comes at the cost of reducing the total number of channels.

But before you even get that far, the dispersion of the fiber destroys the link. Typically, red colors experience a slightly smaller refractive index and blue colors a slightly higher refractive index. The result is that the red colors move faster and “run away” from the blue colors, so the pulse of light spreads out. This causes two problems: light leaks into neighboring time slots, which potentially changes their bit value; and the peak light intensity drops, making it harder to detect the bright flashes that were originally in their time slots.

An accidental glow

So imagine we have a special optical fiber. This fiber has a refractive index that ensures light pulses will not spread out, even if they have a range of colors. Unfortunately, the refractive index is the linear response of a material to light, and materials don’t just respond linearly; they also have a nonlinear response.

You can think of it like this: light is an electromagnetic wave, where the electric field amplitude varies smoothly with time in a particular pattern, called a sinusoid. The electrons in the material vibrate in sympathy with this field—their movement follows the field, albeit with a small time delay. That is the linear response of the material. However, the electrons can't always exactly follow the field, causing tiny deviations. These tiny deviations are the nonlinear response of the medium, and they are, as intimated, individually tiny.

The importance of this only comes clear when you realize that electrons radiate light as they accelerate. So, when electrons follow the incoming light’s field exactly, they accelerate continuously and radiate light with exactly the same color as the incoming light. However, when electrons don’t follow the field exactly, the color of light that they radiate is not exactly the same as the incoming light.

If this happened in a purely random way—say a white glow that was evenly spread across all channels—then this would cause problems, but probably not serious ones. But the nonlinear behavior of the fiber is exactly like a bored kid: it does specifically the thing that you most want it not to do. Let's imagine we have ten channels that are separated from their neighbors by constant frequency gaps. The electrons don’t know about this, so they simply try to follow the total electric field, which is the sum of the fields from the light of all these different channels.

The result is that the nonlinear response of the medium will cause channel one to mix with adjacent channels and generate light that has exactly the frequency for another channel. So, for example, light from channels one, three, and four will combine to generate light in channel two. And, as the light propagates down the fiber, this process continues, reducing the light-contributing signal and increasing the noise in other channels.

Low-power solutions

This problem is solved by keeping the total optical power per channel as low as possible. And we ensure that the pulses are allowed to spread out (using dispersion) and then pull them back together by using a section of fiber with exactly the opposite dispersion. But, by keeping the power low, we intrinsically limit data speeds and ensure that the distance between repeaters is smaller, making the link more expensive.

What was known was that the physics of these processes are all deterministic. That is, since we know the dispersion of a fiber and the range of colors used in a link, network engineers use a short section of fiber with strong dispersion (but where the blue light travels faster than the red) to compensate for the dispersion. Likewise, if we know about the individual channels and the in-channel power, we should be able to predict the crosstalk between channels. And, if you know that, you can do the reverse: that is, compute a waveform that will result in a desired, clean set of pulses exiting the fiber at the other end.

In other words, before a symbol is encoded, we calculate how that symbol will be distorted in the fiber by all the other symbols that are being transmitted at the same time. Then we turn that solution around so that the waveform we input will be shaped by this distortion so as to exit as the undistorted, encoded symbol. This seems simple, but earlier attempts were not notably successful, because each channel was handled independently of the other.

There was seemingly no way to avoid this. If you have a ten-channel system, and each channel consists of a laser diode that has been tuned to the center frequency of the channel, over time, this frequency drifts back and forth over a small range around that center frequency. Each laser does this independently, so you cannot accurately predict the relative frequency spacing of the lasers at any given time. This small deviation is enough to severely degrade the accuracy of the calculation, making its usefulness limited.

There is, however, a way around this. A laser's wavelength can be locked to that of another. This by itself doesn't help because both lasers will have the same wavelength, even if that wavelength drifts. The researchers got around this by using something called a frequency comb. Under the right conditions, a laser will emit a series of wavelengths that are all separated by the same frequency (on a spectrum analyzer, it looks like a hair comb, hence the name). One of the frequencies in the comb can be locked to a master laser. Now, as the master laser drifts, the entire comb of frequencies drifts together.

At this point, we have a nice stable set of colors but cannot transmit information. So, some of the combs are used to lock the individual laser diodes. By doing this, the researchers could be certain that each channel laser had a set and fixed center frequency. They used these wavelengths in their calculations to figure out the waveform that they needed to send to compensate for the fiber's nonlinearities.

Under these conditions, however, the new work showed a reasonably dramatic improvement in link transmission. Their test link, which was the equivalent of 1020km long and transmitting at a rate of 64Gb/s (so not top of the line), was designed to emulate a typical long-haul fiber link. They showed that without locking the channel lasers, they could achieve 64Gb/s for channel powers of around 0.2mW. However, once the lasers were locked, their computational scheme allowed them to increase the channel power to 2mW.

This is a nice proof of principle, but, unfortunately, I don’t think we will see it outside the lab for a couple of years yet. In the researchers' system, the slaving between a master laser and the slaves was achieved via a third laser that generates something called a frequency comb. Although frequency combs are no longer things of mystery, maintained by graduate students through dark arts, they are still not telecom-cheap either. However, I also know of lots of quite successful efforts to produce frequency combs in a way that is suitable for telecommunications (e.g., cheap and reliable), so I can’t imagine that it will remain in the lab for too long.

Science, 2015, DOI:10.1126/science.aab1781