Orbital angular momentum multiplexing
|This article needs attention from an expert in Physics. (July 2012)|
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 multiplexing can (at least in theory) access a potentially unbounded set of OAM quantum states, and thus offer a much larger number of channels, subject only to the constraints of real-world optics.
As of 2013[update], 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.
OAM multiplexing was demonstrated using light beams in free space as early as 2004. Since then, research into OAM has proceeded in two areas: radio frequency and optical transmission.
An experiment in 2011 demonstrated OAM multiplexing of two incoherent radio signals over a distance of 442m. 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 eight millimetre wave channels multiplexed using a combination of OAM and polarization mode multiplexing to achieve an aggregate bandwidth of 32 Gbits/s over a distance of 2.5 metres.
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 eight 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 the fiber, making direct-detection OAM multiplexing still not being realized in long-haul communications. As of 2012, it was possible to transmit OAM states with 97% purity after 20 meters over specialty fibers. Making OAM multiplexing work over future fibre optic transmission systems, possibly using similar techniques to those used to compensate mode rotation in optical polarization multiplexing, is a subject of ongoing research.
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 1.1km test path. The test system was capable of using up to four 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.