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‘Twisted wave’ trick works with light
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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. ®
COMMENTS
Re: I have to ask
What they've demonstrated isn't multiplexing as we used to it as that's about sharing multiple data lines or signals over a single carrier. What they've done with OAM is to use the same carrier multiple times concurrently.
They've demontrated that they can fill a single carrier (radio or optical) with 8 times the capacity we know today by putting 8 signals over it at the same time using the 'spins' without changing the carrier frequency and without those signals interfering with each other as they would normally.
As for infinite - RTFA. "...OAM is hailed as offering “infinite” bandwidth, it’s limited by how finely “spin” can be manipulated and detected."
Re: Infinite bandwidth... pfffft.
First time I have seen the name Shannon mentioned anywhere near this research! Yay!
Infinite bandwidth... pfffft.
Sure, I could give you infinite bandwith if I had a zero noise channel and an infinitely powerful DSP at either end.
As a bear of little brain, this appears to be another mode of orthogonality.
Shannon still applies.
How noisy is and OAM modulated channel? If it is related to the relative polarisation of the carrier, the channel is going to be good and noisy particularly in a multipath environment.
Not naysaying, quite the contrary: WTG you folks, nice one! But tone down the rhetoric for pity's sake.
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