Orbital angular momentum multiplexing

An experiment in 2011 demonstrated OAM multiplexing of two incoherent radio signals over a distance of 442 m.[4] It has been claimed that OAM does not improve on what can achieved with conventional linear-momentum based RF systems which already use MIMO, since theoretical work suggests that, at radio frequencies, conventional MIMO techniques can be shown to duplicate many of the linear-momentum properties of OAM-carrying radio beam, leaving little or no extra performance gain.[5]

In November 2012, there were reports of disagreement about the basic theoretical concept of OAM multiplexing at radio frequencies between the research groups of Tamburini and Thide, and many different camps of communications engineers and physicists, with some declaring their belief that OAM multiplexing was just an implementation of MIMO, and others holding to their assertion that OAM multiplexing is a distinct, experimentally confirmed phenomenon.[6][7][8]

In 2014, a group of researchers described an implementation of a communication link over 8 millimetre-wave channels multiplexed using a combination of OAM and polarization-mode multiplexing to achieve an aggregate bandwidth of 32 Gbit/s over a distance of 2.5 metres.[9] These results agree well with predictions about severely limited distances made by Edfors et al.[5]

The industrial interest for long-distance microwave OAM multiplexing seems to have been diminishing since 2015, when some of the original promoters of OAM-based communication at radio frequencies (including Siae Microelettronica) have published a theoretical investigation[10] showing that there is no real gain beyond traditional spatial multiplexing in terms of capacity and overall antenna occupation.

In 2019, a letter published in the Monthly Notices of the Royal Astronomical Society presented evidence that OAM radio signals had been received from the vicinity of the M87* black hole, over 50 million lightyears distant, suggesting that optical angular momentum information can propagate over astronomical distances.[2]

A paper by Bozinovic et al. published in Science in 2013 claims the successful demonstration of an OAM-multiplexed fiber-optic transmission system over a 1.1 km test path.[21][22] The test system was capable of using up to 4 different OAM channels simultaneously, using a fiber with a "vortex" refractive-index profile. They also demonstrated combined OAM and WDM using the same apparatus, but using only two OAM modes.[22]

A paper by Kasper Ingerslev et al. published in Optics Express in 2018 demonstrates a MIMO-free transmission of 12 orbital angular momentum (OAM) modes over a 1.2 km air-core fiber.[23] WDM compatibility of the system is shown by using 60, 25 GHz spaced WDM channels with 10 GBaud QPSK signals.

In 2014, articles by G. Milione et al. and H. Huang et al. claimed the first successful demonstration of an OAM-multiplexed fiber-optic transmission system over a 5 km of conventional optical fiber,[24][25][26] i.e., an optical fiber having a circular core and a graded index profile. In contrast to the work of Bozinovic et al., which used a custom optical fiber that had a "vortex" refractive-index profile, the work by G. Milione et al. and H. Huang et al. showed that OAM multiplexing could be used in commercially available optical fibers by using digital MIMO post-processing to correct for mode mixing within the fiber. This method is sensitive to changes in the system that change the mixing of the modes during propagation, such as changes in the bending of the fiber, and requires substantial computation resources to scale up to larger numbers of independent modes, but shows great promise.

In 2018 Zengji Yue, Haoran Ren, Shibiao Wei, Jiao Lin & Min Gu[27] at Royal Melbourne Institute of Technology miniaturised this technology, shrinking it from the size of a large dinner table to a small chip which could be integrated into communications networks. This chip could, they predict, increase the capacity of fibre-optic cables by at least 100-fold and likely higher as the technology is further developed.