We live and die by data these days. Data rates and latencies are everything, with data centers and chips designed to maximize communication speeds.

The hero in the world of data is the optical fiber. Thanks to light's very high base frequency, it is possible to modulate it very quickly without using a huge amount of bandwidth. Optical fiber's ability to modulate light quickly allows network designers to choose a wavelength band, divide it up into slots, and use each slot to communicate its own data. So a typical fiber will carry several channels, each operating at multi-gigabit-per-second speeds. This approach, already many, many years old, has served us very well.

But all good things come to an end. Researchers are always looking for ways to carry more information, and one idea—that one, at the back of the class, ignored by all the other ideas—is to use special states of light to encode information. These orbital angular momentum (OAM) states have the potential to vastly increase bandwidth, but they are difficult to handle. Some recent research, however, suggests that we might well be using OAM states before too long.

The barriers to expanding bandwidth are multiple: the lasers and amplifiers that produce the light only operate over a limited range of colors, so you have to cram a lot of channels close together. When you modulate very fast, which is required for high data rates, the data fills and then over-fills its allotted spectrum, spilling into neighboring channels. This cross talk kills communication. What's more, the higher the data rate, the sharper the light pulses you need. Sharp light pulses can themselves introduce all sorts of nasty effects that result in coupling to adjacent channels and generating noise.

What we really need is a way to re-use the same channels over and over again. That requires a new degree of freedom, another way of encoding data into light. Enter orbital angular momentum: use the same wavelength, but ensure each wavelength channel carries multiple data streams, each encoded in different orbital angular momentum states.

What is orbital angular momentum anyway?

Be warned, the idea of orbital angular momentum in light is a bit bizarre. First, let's start with an ordinary light beam that is circularly polarized. In this case, if the light field is travelling towards us, we would see that the electric field always has the same amplitude, but its orientation rotates as the light travels. And, since there is a degree of freedom here, the electric field can rotate either clockwise or counterclockwise. So these light beams have spin angular momentum because of the rotation of the electric field.



Further Reading Polarization

Orbital angular momentum is a completely different animal. To understand OAM, I'm going to change topics entirely and talk about waves at the beach (a much more relaxing topic). Think of yourself as looking down at waves coming onto a beach. Now, freeze the picture for an instant. In this frozen instant, you can draw a line along the top of a single wave: this is a wavefront.

What we care about is how that wavefront moves. When we unfreeze the picture, the wave rushes forward. But the wave does not necessarily move evenly because the beach doesn't have the same slope everywhere. So the line that we drew becomes curved as parts of the wave slow down relative to others. If we look closer, we see that where the wavefront curves, the water wave isn't running directly up the beach—it travels at an angle.

The same behavior occurs in light. We can draw this same line along the peak of a light wave. It forms a smooth curve that becomes closer and closer to a straight line as a light wave moves away from its source. The local wavefront always points away from the source and in the direction that the local bit of light is traveling. In general, the wavefront never does anything very strange.

For orbital angular momentum, however, the situation is different. If a beam of light is travelling towards you, the local wavefronts are never pointed towards you. Instead, they always point a bit inwards. So, if you were to track a little patch of wavefront, it would rotate around the axis of travel. It makes a kind of corkscrew motion towards you.

As a result, the wave has to have both a maximum and a minimum at the center of the beam. So, a beam with orbital angular momentum has a dark spot right in the center. If you like to think in terms of photons, then you can't picture them moving in straight lines anymore. Instead, you have to think of them as corkscrewing along the direction of the beam. And, to be honest, this doesn't make much sense. It is probably more accurate to discuss the probability of finding a photon at a particular moment in time and space is periodically modulated due to OAM.

Stuffing a square peg in round hole

How does orbital angular momentum help us pack more information into a data stream? Well, unlike spin angular momentum, which is restricted to just two values, OAM can take on as many values as you like. For each unit of OAM, you also get an additional channel by separating the spin angular momentum states.

So, in addition to dividing your data among a fixed number of frequency channels, you can reuse each frequency many times by encoding data in different orbital angular momentum states to increase data capacity even more.

That sounds pretty cool. So why isn't it already here?

One big problem is the optical fibers required to support OAM states. Ordinary fibers have a core with a high refractive index surrounded by a cladding with a low refractive index. For orbital angular momentum, we need to reverse that: a low refractive index core, a high refractive index inner cladding. That's not all; you have to add a second, outer cladding that has a refractive index higher than the core, but lower than the inner cladding. To make matters more difficult, the difference in refractive index between the inner core and inner cladding has to be large (compared to that used in ordinary fibers). So the material of choice for the inner core is air.

Using air has a couple of consequences: first, during fabrication, you have an additional surface that is exposed to atmosphere, so water can leak into the glass. Modern fibers are as good as they are because the fabrication process keeps water out of the fiber (a Nobel Prize was awarded for that effort). With a hollow core, that becomes more challenging. The second problem is that light scatters on imperfections, especially on the inner walls. For a normal fiber, some imperfections can be tolerated because the refractive index difference between the core and the cladding are tiny, making the scattering weaker. Here, however, the differences in refractive index are high, so scattering is much stronger.

These two fabrication issues increase the amount of light lost as it travels down the fiber, and they increase the amount of light scattered from one orbital angular momentum state into another. In other words, data transfers may be prone to errors.

A recent publication in Optics Express highlights some of the progress in this area. The authors show that the optical loss in their fiber is now below one dB/km. To put that in perspective, a loss of three dB means you lose half the power, and modern hardware can typically cope with the loss of six or more dB. So this fiber would be capable of links on the order of six km. This is way too short for long haul, but is perfect for data centers and is within a factor of two for metro links. That is progress.

Cross talk is now the largest problem. The researchers showed that some orbital angular momentum states are quite resistant to cross talk, while others are very sensitive. For those sensitive modes, even unwinding and re-winding the fiber changed the amount of cross talk.

So, for the right modes, the researchers claim that links of just under six kilometers are possible. But possible isn't the same as practical; the links won't be useful until more modes can be used to transmit data without having them interfere with each other.

In the end, these are all solvable problems. And the number of orbital angular momentum states of light is infinite, making this a very attractive solution. It's just a question of being able to create them, combine them in the fiber, and separate them accurately at the destination (not to mention guiding them efficiently through a low loss optical fiber). Solutions for all these problems are coming, and then my nice fiber connection at home will be out of date.

Optics Express, 2016, DOI: 10.1364/OE.24.018938