Unidirectional device developed for optical vortices
Scientists at the Max Planck Institute for the Science of Light have created a unidirectional device that increases the quality of a special class of transmitted signals in optical communications: optical vortices. By transmitting selective optical vortex modes exclusively unidirectionally, the developed device reduces detrimental backscattering to a minimum. The discovery has great practical utility in many optical systems, with applications ranging from mode division multiplexed communications, optical tweezers and vortex lasers to quantum manipulation systems.
Optical communication can be improved by increasing the amount of optical information that is transmitted. This can be achieved by using multiplexed channels such as using many optical wavelengths, different polarisation states or multiple time slots. In the last decade, optical spatial modes, which are the eigenfields in the waveguides, have been widely exploited to improve the communication capacity due to the little crosstalk between orthogonal spatial modes. In classical and quantum communication, the use of vortex modes in multiplexing methods has proven to be advantageous. This special mode set possesses a helical optical phase distribution and allows an additional degree of freedom for multiplexing optical signals. Devices like vortex generators, lasers and signal amplifiers were demonstrated and are in great demand.
A limiting effect on the applicability is that there has not yet been a device that permits transmission of certain vortex modes in one direction but not the opposite one. However, this kind of device — an optical vortex insulator — is of crucial importance for the improvement of signal quality and purity. The difficulty in developing such a device is a fundamental principle of optics: reciprocity. It requires a symmetrical response of a transmission channel when the source and observation points are interchanged.
Now, a team of researchers have used sound waves that propagate only in one direction to break the light transmission reciprocity for chosen vortex modes. The effect of so-called topology-selective Brillouin-Mandelstam scattering in chiral photonic crystal fibre allows for a unidirectional interaction of vortex-carrying light waves with travelling sound waves. A specific optical vortex can be suppressed or amplified with a well-designed control light. The experimental results, published in Science Advances, show a significant vortex isolation rate, preventing random backscattering and signal degradation in the system.
Xinglin Zeng, the first author on the paper, said that this is the first non-reciprocal system for vortex modes, which could open up a new perspective in non-reciprocal optics. “The same physical effect can happen not only on the fundamental modes but also on higher-order modes,” Zeng said.
“The light-driven optical vortex isolator will have great impact on the applications such as optical communications, quantum information processing, optical tweezers and fibre lasers. I find the possibility of selective manipulation of vortex modes solely by light and sound waves a very fascinating concept,” said Birgit Stiller, the leader of the Quantum Optoacoustics Research Group.
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