Orbital angular momentum multiplexing

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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. [1]

In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer. This layer may be implemented by a PHY chip.

Multiplexing method by which multiple analog or digital signals are combined into one signal over a shared medium

In telecommunications and computer networks, multiplexing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in communications. In telephony, George Owen Squier is credited with the development of telephone carrier multiplexing in 1910.

Contents

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.

The angular momentum of light is a vector quantity that expresses the amount of dynamical rotation present in the electromagnetic field of the light. While traveling approximately in a straight line, a beam of light can also be rotating around its own axis. This rotation, while not visible to the naked eye, can be revealed by the interaction of the light beam with matter.

In quantum physics, quantum state refers to the state of an isolated quantum system. A quantum state provides a probability distribution for the value of each observable, i.e. for the outcome of each possible measurement on the system. Knowledge of the quantum state together with the rules for the system's evolution in time exhausts all that can be predicted about the system's behavior.

Circular polarization

In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electric field of the wave has a constant magnitude but its direction rotates with time at a steady rate in a plane perpendicular to the direction of the wave.

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.

History

OAM multiplexing was demonstrated using light beams in free space as early as 2004. [2] Since then, research into OAM has proceeded in two areas: radio frequency and optical transmission.

Radio frequency

An experiment in 2011 demonstrated OAM multiplexing of two incoherent radio signals over a distance of 442 m. [3] 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. [4]

MIMO Use of multiple antennas in radio

In radio, multiple-input and multiple-output, or MIMO, is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wireless communication standards including IEEE 802.11n (Wi-Fi), IEEE 802.11ac (Wi-Fi), HSPA+ (3G), WiMAX (4G), and Long Term Evolution. More recently, MIMO has been applied to power-line communication for 3-wire installations as part of ITU G.hn standard and HomePlug AV2 specification.

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. [5] [6] [7]

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. [8] These results agree well with predictions about severely limited distances made by Edfors et al. [4]

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 [9] showing that there is no real gain beyond traditional spatial multiplexing in terms of capacity and overall antenna occupation.

Optical

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. [1] [10] Work is ongoing on applying OAM techniques to long-range practical free-space optical communication links. [11]

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, [12] 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. [13] Later experiments have shown stable propagation of these modes over distances of 50 meters, [14] 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.[ 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, [15] where strong mode coupling is suggested to be beneficial for coherent-detection-based systems. [16]

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. [17] [18] 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. [18]

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, [19] [20] [21] 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 [22] 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.

See also

Related Research Articles

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Optical vortex

An optical vortex is a zero of an optical field; a point of zero intensity. The term is also used to describe a beam of light that has such a zero in it. The study of these phenomena is known as singular optics.

Optical fiber light-conducting fiber

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer excessively. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers.

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Double-clad fiber

Double-clad fiber (DCF) is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices.

A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.

Fiber-optic communication method of transmitting information from one place to another by sending pulses of light through an optical fiber

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Bo Y. Thidé is a Swedish physicist who studies radio waves and other electromagnetic radiation in space, particularly their interaction with matter and fields. He received his B.Sc. in 1972, his M.Sc. in 1973, and defended his Ph.D. thesis on semiclassical quantum theory at Uppsala University in 1979. His Ph.D. was obtained under the supervision of professor Per Olof Fröman at the Department of Theoretical Physics, Uppsala University. He has worked at the Swedish Institute of Space Physics in Uppsala since 1980, where he has been a professor since 2000.

Radial polarization

A beam of light has radial polarization if at every position in the beam the polarization vector points towards the centre of the beam. In practice, an array of waveplates may be used to provide an approximation to a radially polarized beam. In this case the beam is divided into segments, and the average polarization vector of each segment is directed towards the beam centre.

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The spin angular momentum of light (SAM) is the component of angular momentum of light that is associated with the quantum spin and the wave's circular or elliptical polarization.

The orbital angular momentum of light (OAM) is the component of angular momentum of a light beam that is dependent on the field spatial distribution, and not on the polarization. It can be further split into an internal and an external OAM. The internal OAM is an origin-independent angular momentum of a light beam that can be associated with a helical or twisted wavefront. The external OAM is the origin-dependent angular momentum that can be obtained as cross product of the light beam position and its total linear momentum.

Polarization-division multiplexing

Polarization-division multiplexing (PDM) is a physical layer method for multiplexing signals carried on electromagnetic waves, allowing two channels of information to be transmitted on the same carrier frequency by using waves of two orthogonal polarization states. It is used in microwave links such as satellite television downlinks to double the bandwidth by using two orthogonally polarized feed antennas in satellite dishes. It is also used in fiber optic communication by transmitting separate left and right circularly polarized light beams through the same optical fiber.

Miles J. Padgett Professor of Optics

Miles John Padgett, is Professor of Optics in the School of Physics and Astronomy at the University of Glasgow. He has held the Kelvin Chair of Natural Philosophy since 2011 and has been Vice Principal for Research at Glasgow since 2014.

Q-plate

A q-plate is an optical device which can generate light beams with orbital angular momentum of light (OAM) from a beam with well-defined Spin angular momentum of light (SAM). It is currently realized using liquid crystals, polymers or sub-wavelength gratings.

Siae Microelettronica

SIAE MICROELETTRONICA is an Italian multinational corporation and a global supplier of telecom network equipments. It provides wireless backhaul and fronthaul solutions that comprise microwave and millimeter wave radio systems, along with fiber optics transmission systems provided by its subsidiary SM Optics.

References

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