Back in March, a group of Italian and Swedish radio researchers demonstrated that a characteristic of radio waves called orbital angular momentum (OAM) can be used as a multiplexing technique, vastly increasing the theoretical capacity of wireless transmissions.
Now, according to Nature Photonics, the same technique has been applied to optical waves.
The original breakthrough was specific to radio waves. It showed that the “spin” (an imprecise term but OAM is difficult to describe) characteristic of the radio wave can be used alongside more familiar amplitude, frequency and phase modulation. Using different OAM characteristics, different transmissions can be recovered at the same frequency over the same path, without interfering with each other.
The new research, conducted by American scientists from the University of Southern California in Los Angeles, NASA’s Jet Propulsion Laboratory, the University of Tel Aviv and China’s Huazhong University of Science and Technology, demonstrated a 2.5 Tbps free-space optical transmission using the same technique.
The peak speed was achieved by applying OAM values to eight polarization-multiplexed beams. The demonstrations also showed that OAM can be used with different conventional modulation techniques – both QAM and phase shift keying were used in the experiments.
In the terabit range, the demonstration doesn’t come even close to challenging current lab work for optical fibre transmissions (Alcatel showed off a 100 Petabit/second.kilometer system in 2009), but the Nature Photonics article is focused on free-space optics communication.
El Reg notes that while OAM is hailed as offering “infinite” bandwidth, it’s limited by how finely “spin” can be manipulated and detected. It is, however, a very young technology – eight beams is only the beginning.
USC researcher Alan Willner told the BBC that atmospheric turbulence would degrade the transmission as distances increase. However, it could be used for satellite-to-satellite communications, where atmosphere doesn’t get in the way.
Bootnote: Orbital angular momentum in waves is difficult to explain, because frankly, any non-mathematical explanation is at best an imperfect metaphor.
The mathematics of OAM in light or radio waves describes a helical wavefront, where the direction (clockwise or anti-clockwise) of the helix is shown by whether it’s given a positive or negative value, and how “tightly” the helix is wound (another imperfect metaphor), given by the integer value.
Hence: if two signals with OAM +1 and -1 are sent across the same path, they can be detected without causing interference to each other. To increase the density of OAM-based multiplexing, you need only be able to apply different values to different beams – and recover those values at the receiver. ®