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Credit: by Zhensong Wan, Yijie Shen, Zhaoyang Wang, Zijian Shi, Qiang Liu, and Xing Fu
As the intensity, polarization, wavelength, and other physical dimensions of light have already been desperately exploited to scale the capacity of optical communications, it is far more difficult now to meet the ever-increasing demand of information bandwidth, especially considering the current revolution from 5G to 6G. Multiplexing of orbital angular momentum (OAM) beam, which can theoretically support infinite independent data channels, is a promising solution, but in practice the number of OAM modes is severely limited mostly under 20. This is because the beam diverges rapidly as the OAM order enlarges, giving rise to unacceptable power loss for a limited-size receiver aperture. To break the limit, the structured light carriers with high divergence degeneracy and the associated manipulation techniques are urgently required.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Xing Fu, and Professor Qiang Liu from Department of Precision Instrument, Tsinghua University, China, have introduced a new type of structured light as high-dimensional information carriers for free-space optical communication. It has novel degrees of freedom (DoFs) as sub-beam OAM, and coherent-state phase, in addition to the traditional one (central OAM). The orthogonality of all DoFs as information carriers are well verified for the first time. Importantly, researchers show the novel modal set is extremely densely packed in beam quality space and has a highly consistent propagation behavior that it can possess a divergence degeneracy as high as 20, and a divergence variation by merely 18% among 100 independent lowest order spatially multiplexed modes, in contrast to 900% for OAM counterpart. As a result, thousands of independently spatial channels in new modal basis can be supported in a free space optical communication system, two orders of magnitude larger than that in traditional OAM basis.
In addition to the novel concept, this work develops efficient method for sorting and demultiplexing superposed modes of new basis, well validating the concept and making the approach available and handy. Furthermore, the reported results indicate another distinct advantage of new basis in demultiplexing with much lower bit error rates caused by center offset and background noise, compared with traditional OAM basis. This work broadens the horizon of spatially multiplexing of structured light, and offers a promising modal basis set for next generation of large-scale dense data communication.
These scientists summarize the features and advantages of their novel modal basis set:
“At the heart of our work is the exploitation of modes in ray-wave duality state, whereby crafted spatial modes appear to be both wave-like and ray-like, which allows us access to higher divergence degeneracy and more consistent propagation behavior among all modes, dramatically increasing the addressable number of independent spatial channels.”
“We validate the high dimensional information carriers by proof-of-concept experiments of mode (de)multiplexing and free space data transmission. The mode multiplexing approach is compatible with and can combine with other techniques, such as wavelength and polarization division multiplexing, and may also work within fibers.” they added.
“Our technique could be extended to other types of structured light, to explore even more spatial DoFs and higher divergence degeneracy. The concept of ultra-DoF modal basis can also be applied to the quantum data channels.” the scientists forecast.