The laser carries its momentum in a similar fashion to the Earth orbiting around the Sun

Twisted laser beams could boost fibre communications

A new type of laser that is able to output twisted beams could be used to improve optical communication, laser machining and medicine technologies.

The outputs and superposition of the new type of laser form a set of beams, called vector vortex beams.

"We are all familiar with angular momentum in our everyday lives: the spinning Earth carries spin angular momentum while the orbiting Earth carries orbital angular momentum (OAM). Light can also carry angular momentum: through its polarisation (spin), and through its pattern and phase OAM," said Professor Andrew Forbes with the University of the Witwatersrand who conceived of the idea.

Although producing light with a controlled spin in a laser has been known for decades, producing OAM beams inside a laser is not so simple.

Light carrying OAM is created by twisting the phase of light into a helical shape, forming a spiral. Because the twisting of the pattern gets tighter and tighter as you move towards the centre of the beam, the light disappears and such beams are often called doughnut beams or vortex beams.

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.

Moreover, combining spin and orbital components to produce general beams from a single laser that are mixtures of the two, have not been demonstrated before.

"Our novelty was to realise that by using custom-geometric phase optics to map polarisation to OAM, the laser could be designed to tell the difference between the clockwise and anticlockwise light," Forbes said.

"We like to call this a spiral laser because both the polarisation and OAM of the beam give rise to light that spins or twists in complicated ways."

This control is achieved by simply rotating a single optical element inside the laser, without any need for realignment. Such beams have been used in optical communication, optical trapping of microparticles and metrology but could not be produced from a single laser beam before this project.

The laser can be used for a wide number of applications, such as microscopy, imaging, laser machining, and communication in free space and in fibres.

Last year, Japanese researchers demonstrated an atomic laser capable of operating at shorter wavelengths than before which could pave the way for improvements in imaging of microscopic objects.

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