Orbital angular momentum multiplexing
optical multiplexing technique
Orbital angular momentum multiplexing is a physical layer method for multiplexing signals carried on electromagnetic waves using the orbital angular momentum (OAM) of the electromagnetic waves to distinguish between the different orthogonal signals.
OAM is one of two forms of angular momentum of light; it is distinct from, and should not be confused with, light spin angular momentum. The latter 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 constraint of real-world optics. The constraint has been clarified in terms of independent scattering channels or the degrees of freedom of scattered fields through angular-spectral analysis, in conjunction with a rigorous Green's function method.
The degrees of freedom limit is universal for arbitrary spatial-mode multiplexing, which is launched by a planar electromagnetic device, such as antenna, metasurface, etc., with a predefined physical aperture.
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, with numerous studies suggesting it can be modelled as a purely classical phenomenon by regarding it as a particular form of tightly modulated MIMO multiplexing strategy, obeying classical information theoretic bounds.
As of 2020, new evidence from radio telescope observations suggests that radio-frequency orbital angular momentum may have been observed in natural phenomena on astronomical scales, a phenomenon which is still under investigation.
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