Date:16 March 2016
A new type of spiral laser beam discovered by a team of South African and Italian researchers could have far-reaching applications in optical communication, machining and medicine.
Nature Photonics has published research by the team, demonstrating a new type of laser able to produce laser beams with a twist as its output. The idea was conceived by team leader Professor Andrew Forbes of the University of the Witwatersrand. Also involved were personnel from the CSIR and the University of Naples, the latter providing the custom geometric phase optics used in the laser.
“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,” says Forbes.
Producing light with a controlled spin in a laser has been known for decades, but 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 donut beams or vortex beams. However, usually lasers can’t 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.
“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,” says Forbes. The 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 – and now a single laser can create them on demand.
In addition to the special cases of OAM beams, the same laser also produces radially and azimuthally polarised light, where the polarisation (direction of the electric field) changes in space. For example, radially polarised light has the field always pointing away from the centre of the circle, which is very useful for cutting and drilling metals.
Often it’s necessary to decide beforehand which beam is the most desirable and then design a laser for it. Now it is possible to have such beams available on demand from a single laser.
The paper can be downloaded at: Laser with a twist