Orbital angular momentum multiplexing
Orbital angular momentum (OAM) multiplexing is a physical layer method for multiplexing signals carried on electromagnetic waves using the orbital angular momentum of the electromagnetic waves to distinguish between the different orthogonal signals.
Orbital angular momentum is one of two forms of angular momentum of light. OAM is distinct from, and should not be confused with, light spin angular momentum. The spin angular momentum of light offers only two orthogonal quantum states corresponding to the two states of circular polarization, and can be demonstrated to be equivalent to a combination of polarization multiplexing and phase shifting. OAM on the other hand relies on an extended beam of light, and the higher quantum degrees of freedom which come with the extension. OAM multiplexing can thus access a potentially unbounded set of states, and as such offer a much larger number of channels, subject only to the constraints of real-world optics.
As of 2013, although OAM multiplexing promises very significant improvements in bandwidth when used in concert with other existing modulation and multiplexing schemes, it is still an experimental technique, and has so far only been demonstrated in the laboratory. Following the early claim that OAM exploits a new quantum mode of information propagation, the technique has become controversial; however nowadays it can be understood to be a particular form of tightly modulated MIMO multiplexing strategy, obeying classical information theoretic bounds.
An experiment in 2011 demonstrated OAM multiplexing of two incoherent radio signals over a distance of 442 m. 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.
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.
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. These results agree well with predictions about severely limited distances made by Edfors et al.
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 showing that there is no real gain beyond traditional spatial multiplexing in terms of capacity and overall antenna occupation.
OAM multiplexing is used in the optical domain. In 2012, researchers demonstrated OAM-multiplexed optical transmission speeds of up to 2.5 Tbits/s using 8 distinct OAM channels in a single beam of light, but only over a very short free-space path of roughly one metre. Work is ongoing on applying OAM techniques to long-range practical free-space optical communication links.
OAM multiplexing can not be implemented in the existing long-haul optical fiber systems, since these systems are based on single-mode fibers, which inherently do not support OAM states of light. Instead, few-mode or multi-mode fibers need to be used. Additional problem for OAM multiplexing implementation is caused by the mode coupling that is present in conventional fibers, which cause changes in the spin angular momentum of modes under normal conditions and changes in orbital angular momentum when fibers are bent or stressed. Because of this mode instability, direct-detection OAM multiplexing has not yet been realized in long-haul communications. In 2012, transmission of OAM states with 97% purity after 20 meters over special fibers was demonstrated by researchers at Boston University. Later experiments have shown stable propagation of these modes over distances of 50 meters, and further improvements of this distance are the subject of ongoing work. Other ongoing research on making OAM multiplexing work over future fibre-optic transmission systems includes the possibility of using similar techniques to those used to compensate mode rotation in optical polarization multiplexing.
Alternative to direct-detection OAM multiplexing is a computationally complex coherent-detection with (MIMO) digital signal processing (DSP) approach, that can be used to achieve long-haul communication, where strong mode coupling is suggested to be beneficial for coherent-detection-based systems.
Practical demonstration in optical-fiber system
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. 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.
Practical demonstration in conventional optical-fiber systems
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, 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 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.
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