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.
[2] 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.
[10] These results agree well with predictions about severely limited distances made by Edfors et al.[6] 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[11] showing that there is no real gain beyond traditional spatial multiplexing in terms of capacity and overall antenna occupation.
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.
[citation needed] 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,[17] where strong mode coupling is suggested to be beneficial for coherent-detection-based systems.
[18] In the beginning, people achieve OAM multiplexing by employing several phase plates or spatial light modulators.
[21] 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.
[23] 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.