Scientists at the Max Planck Institute for the Science of Light have succeeded for the first time in creating a unidirectional device that dramatically increases the quality of a special class of signals transmitted in optical communications: optical vortices. By transmitting selective optical vortex modes exclusively unidirectionally, the developed device largely reduces harmful backscatter to a minimum. The scientists point to the great practical utility of their discovery in many optical systems, with applications ranging from mode division multiplexed communications, optical tweezers, vortex lasers to quantum manipulation systems.
Optical communication can be improved by increasing the amount of optical information transmitted. This can be achieved using multiplexed channels such as using many optical wavelengths, different polarization states, or multiple time slots. In the past decade, optical spatial modes, which are the eigenfields in waveguides, have been widely exploited to further improve the communication capability due to low crosstalk between orthogonal spatial modes.
In classical communication as in quantum communication, the use of vortex modes in multiplexing methods has proven to be advantageous. This special mode set has a helical optical phase distribution and allows an additional degree of freedom for multiplexing optical signals. Devices such as vortex generators, lasers and signal amplifiers have been demonstrated and are in high demand.
A limiting effect on the applicability is that there is not yet a device allowing the transmission of certain vortex modes in one direction but not in the opposite direction. However, this type of device – a so-called optical vortex isolator – is of crucial importance for improving signal quality and purity. The particular 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 the observation points are inverted.
Researchers succeed in building an optical vortex isolator
Now a team from the Max Planck Institute for Light Science led by Xinglin Zeng, Philip Russell and Birgit Stiller have achieved a breakthrough that makes this possible: they have used sound waves that travel in a single direction to breaking the reciprocity of light transmission for chosen vortex modes. The effect of so-called selective topological Brillouin-Mandelstam scattering in the chiral photonic crystal fiber allows unidirectional interaction of vortex-carrying light waves with traveling sound waves. A specific optical vortex can be strongly suppressed or amplified with a well-designed control light. The experimental results published in Scientists progress show a significant vortex isolation rate, preventing random backscatter and signal degradation in the system.
“This is the first non-reciprocal system for vortex modes, which opens a new perspective in non-reciprocal optics — the same physical effect can occur not only on fundamental modes but also on order modes superior,” says Xinglin Zeng, the first author of this paper. “The light-driven optical vortex isolator will have a great impact on applications such as optical communications, quantum information processing, optical tweezers, and fiber lasers. I find the possibility of mode-selective manipulation vortexing only by light and sound waves is a very fascinating concept,” says Birgit Stiller, head of the quantum optoacoustics research group.
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