Boffins build laser that can twist its own light

South African university spins up OAM-capable laser

The world has known that information can be encoded on “twisted light” for some time, but only with complex equipment. Now, boffins from South Africa have demonstrated a laser that can add the twist at its output.

The Register will resist the temptation to call this some kind of game-changer, but it's a useful development, since it means that orbital angular momentum (OAM) can be generated and controlled in an integrated device, rather than with more complex lab setups.

Professor Andrew Forbes, of the University of the Witwatersrand (Wits), worked with collaborators from South Africa's Council for Scientific and Industrial Research (CSIR) and Italy's University of Naples on the project, published in Nature Photonics.

The university's press release explains the challenge in applying OAM at the laser source: “The problem is that usually lasers cannot tell the difference between light that is twisted clockwise and light that is twisted anti-clockwise, and so the laser simply gives a combination of both in an uncontrolled manner”.

As the Arxiv pre-print version of the paper explains, most of the laser is standard and easy-to-implement.

“We couple SAM (spin angular momentum) to OAM inside the laser cavity by means of a wave-plate and a nonhomogeneous polarisation optic (q-plate) so that polarisation control maps to OAM mode control”, the paper says.

This means, Forbes says in the press release, that the laser can tell the difference between a clockwise and anti-clockwise OAM, “by simply rotating a single optical element inside the laser, without any need for realignment.”

Even generating and detecting two OAM states relatively cheaply in the laboratory would be a quick kill for optical communications if it can be manufactured at scale, since it would double the bandwidth of an optical link without needing higher-speed conventional modulation or tighter wavelength multiplexing.

There are also applications in laser machining, microscopy, and imaging applications, the university says. ®


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