Twisted photons get another spin

Inside fibre, this time

The principle that led to the controversy over “twisted radio waves” in 2012, known as orbital angular momentum, has also been applied to light for some time, but most demonstrations have been in free space.

A new experiment published in Science has successfully “twisted” a photonic beam in a fibre, using it to transport data at 1.6 Tbps over 1.1 Km using ten wavelengths and two orbital angular momentum (OAM) modes.

The speed's not going to suddenly depreciate the shares in optics outfits to zero, since the most advanced research labs routinely work at speeds well into petabits per second.kilometre, but as the abstract of the Science article notes: “OAM could provide an additional degree of freedom for data multiplexing in future fiber networks”.

The work was led by professor Siddharth Ramachandran of the University of Southern California, with collaborators from the University of Tel Aviv, Danish company OFS-Fitel, and Alan Willner, also of USC.

If that last name doesn't ring a bell, here's a refresher: last year, Willner demonstrated the OAM technique in free-space optics, achieving 2.5 Tbps transmission.

In fibre, that's more difficult, as professor Ramachandran explains: “For several decades since optical fibers were deployed, the conventional assumption has been that OAM-carrying beams are inherently unstable in fibres.”

The researchers concentrated on constructing fibres that could support the four modes required by OAM, instead of the more typical single or two modes in fibres. They created germanium-fluorine doped fibres to achieve the refractive index required for their experiment. For each mode, the researchers were able to use ten wavelengths to achieve the 1.6 Tbps transmission. ®

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