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South African, Italian research collaboration yields all-in-one laser breakthrough

1st April 2016

By: Samantha Herbst

Creamer Media Deputy Editor

  

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A South African-Italian research team has conceptualised an all-in-one laser that produces several types of beams to serve a variety of industrial applications, including microscopy, imaging, laser machining and communication in free space and fibres, with one solution.

Spearheaded by Professor Andrew Forbes of the University of the Witwatersrand (Wits), the research team comprised scientists from Wits, the Council for Scientific and Industrial Research (CSIR) and the University of Naples.

While Forbes led the collaboration, all the key experiments were performed by the CSIR’s Dr Darryl Naidoo, who was also lead author of the paper and performed the experiments as part of his doctoral studies. The other team members included Professor Stef Roux of Wits and the CSIR; Dr Angela Dudley, of Wits and the CSIR; Dr Igor Litvin, of the CSIR; and professors Lorenzo Marrucci and Bruno Piccirillo, of the University of Naples.

Together, the team compiled a paper, which was this week published by peer-reviewed scientific journal Nature Photonics.

Forbes’ initial idea was for a new type of laser that was able to produce laser beams “with a twist”, using geometic-phase inside lasers for the first time. The output of this laser would pave the way for the future of laser applications in optical communication, laser machining and medicine.

The outputs and superpositions of the team’s laser formed a set of beams, called vector vortex beams. The custom geometric-phase optics, without which the realisation of the idea would not have been possible, were produced by the Italian team.

“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,” explained Forbes.

He noted that the ability to produce light with a controlled spin in a laser had been known for decades, but that producing OAM beams inside a laser was not so simple.

The team’s study noted that light-carrying OAM was created by twisting the phase of light into a helical shape, forming a spiral. The twisting of the pattern became tighter and tighter as one moved towards the centre of the beam, making the light disappear. Such beams were often called donut beams or vortex beams.

However, Forbes explained that the problem was that lasers could not tell the difference between light that was twisted clockwise and light that was twisted anticlockwise, therefore the team’s laser simply produced a combination of both in an uncontrolled manner. He added that combining spin and orbital components to produce general beams from a single laser that were mixtures of the two momenta, had 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,” said Forbes.

The control was achieved through the rotation of a single optical element inside the laser, without any need for realignment. Such beams had been used in optical communication, optical trapping of microparticles and metrology – and now a single laser could create them on demand.

While the geometric phase of light was an abstract concept, first appearing in quantum theory, the South African-Italian research team used the theory to create particular types of twisted light.

The custom optic, called a q-plate, changed the handedness of the OAM twist according to the handedness of the polarisation twist, mapping one to the other. For example, if clockwise polarised light with no twisted phase was passed through the optic, the output was anticlockwise polarised light with a clockwise twist in phase. By placing this element inside the laser, the twist in polarisation (spin) controlled the handedness of the twist in OAM, so the output could be controlled in either.

“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,” noted Forbes.

He added that the same laser could produce any combination of these OAM beams and various polarisations of light.

For example, radially polarised light used a field that pointed away from the centre of the circle, which was useful for cutting and drilling metals. Such beams were called vector beams as the polarisation changed across the beam. However, when the polarisation pattern stayed constant across the beam, it was called a scalar beam. In the reported work, the researchers showed that either could be created from their laser.

“You have to understand that vector vortex laser beams have proven immensely useful in machining metals and other materials with lasers, for example, in the automotive industry. But until now, we have not been able to produce all of them in one laser,” concluded Naidoo.

Edited by Martin Zhuwakinyu
Creamer Media Senior Deputy Editor

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