Twisted laser beams: 1.6Tbps over a fibre optic network

The team of researchers that produced infinite-capacity wireless vortex beams has deployed the same technology in optical fibres, using twisted laser beams to transfer data at 1.6 terabits per second over 1.1 kilometres (0.68 miles).

In both cases, the principle at play is called orbital angular momentum. In essence, electromagnetic radiation – whether it’s a wireless signal or a laser pulse – can have two kinds of twist: spin angular momentum (SAM) and orbital angular momentum (OAM). If you picture the Earth, SAM is equivalent to the planet spinning on its axis (producing the day/night cycle), and OAM is equivalent to us rotating around the Sun (producing seasons). In every commercial network topology, we only modify the SAM.

What Bo Thide of the Swedish Institute of Space Physics proved in 2012, however, was that we could modify both the SAM and the OAM. In effect, this creates a three dimensional wave that’s modulated in one plane (SAM), but also twisted through another (OAM), creating a vortex or corkscrew.

As you can probably imagine, by shifting from a 2D wave to 3D, you gain a huge amount of spectral efficiency. In fact, if you have enough control over the OAM, you can effectively transmit infinite amounts of data on the same carrier frequency. This is what led a group of researchers, headed by Andy Willner at the University of Southern California, to create a wireless network link capable of 2.5 terabits per second – or about 40GB (one Blu-ray disc) per second.

Now, Willner has teamed up with Siddharth Ramachandran, a fibre expert at Boston University, to implement orbital angular momentum in a fibre optic network. This is slightly more difficult, as optical fibres tend to be single-mode – i.e. they can only transport a single beam of light.

OAM requires that you transmit multiple beams of light (or radio waves), twisted like a corkscrew to prevent interference between the beams. To get around this, the researchers used a special kind of fibre that has had special chemicals added to it (doped) to create different spatially separated pathways. Using a single colour of light with four twists, the team obtained a data rate of 400Gbps; using 10 colours, each with two twists, 1.6Tbps was achieved.

Moving forward, with orthogonal multiplexing reaching its limits, and global Internet traffic showing no signs of growth, spatial multiplexing could be the solution. The problem is that almost every fibre that criss-crosses the Earth, including those in the oceans, are single-mode fibres that can’t carry OAM-modulated signals. These fibres could be upgraded, but it would be a very slow and prohibitively expensive process. This isn’t to say that OAM mode division multiplexing won’t ever be used, though – but for the foreseeable future, the only real applications are in new installations, or short runs between servers in a data centre.

Research paper: DOI: 10.1126/science.1237861 – "Terabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers"