A team of physicists based in the UK, Germany, New Zealand and Canada have described how new research into 'optical angular momentum' (OAM) could overcome current difficulties with using twisted light across open spaces. This is an important step towards using 'twisted' light as a form of wireless, high-capacity data transmission which could make fibre-optics obsolete.
Scientists can 'twist' photons - individual particles of light - by passing them through a special type of hologram, similar to that on a credit card, giving the photons a twist known as optical angular momentum.
While conventional digital communications use photons as ones and zeroes to carry information, the number of intertwined twists in the photons allows them to carry additional data - something akin to adding letters alongside the ones and zeroes. The ability of twisted photons to carry additional information means that optical angular momentum has the potential to create much higher-bandwidth communications technology.
While optical angular momentum techniques have already been used to transmit data across cables, transmitting twisted light across open spaces has been significantly more challenging for scientists to date. Even simple changes in atmospheric pressures across open spaces can scatter light beams and cause the spin information to be lost.
The researchers examined the effects on both the phase and intensity of OAM carrying light over a real link in an urban environment to assess the viability of these modes of quantum information transfer.
Their free space link, in Erlangen, Germany, was 1.6 km in length and passed over fields and streets and close to high-rise buildings to accurately simulate an urban environment and atmospheric turbulence that can disrupt information transfer in space - a thorough approach that will be instrumental in moving OAM research forward.
Conducting this field tests in a real urban environment, has revealed exciting new challenges that will that must be overcome before systems can be made commercially available. Previous studies had indicated the potential feasibly of OAM communication systems, but had not fully characterised the effects of turbulent air on the phase of the structured light propagating over links of this length.
A complete, working optical angular momentum communications system capable of transmitting data wirelessly across free space has the potential to transform online access for developing countries, defence systems and cities around the world. Free space optics is a solution that can potentially give us the bandwidth of fibre, but without the requirement for physical cabling. This study takes vital steps forward in the journey towards high dimensional free space optics that can be a cheaper, more accessible alternative to buried fibre optics connections.
The turbulent atmosphere used in this experiment highlighted the fragility of shaped phase fronts, particularly for those that would be integral to high-bandwidth data transfers. This study indicated the challenges future adaptive optical systems will be required to resolve.
These findings allow researchers to address challenges - not previously observed - in developing adaptive optics for quantum information transfer to move closer towards a new age of free space optics that will eventually replace fibre optics as a functional mode of communication in urban environments and remote sensing systems.
The paper, titled "Free-space propagation of high dimensional structured optical fields in an urban environment," is published in Science Advances.
