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]
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 2012[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 not yet been used in practical commercial telecommunications systems.
History
OAM multiplexing had been demonstrated using light beams as early as 2004.[2]
An experiment in 2011 demonstrated OAM multiplexing of two incoherent radio signals over a distance of 442m.[3] However, OAM seems to be of little use for 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] However, utilizing the angular-momentum properties of OAM by going from multimode (MIMO) to multistate (quantum) methods, has been proposed to have the potential of increasing the radio channel spectral capacity by a very large factor[citation needed].
OAM multiplexing looks much more promising 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.[1][5] Work is ongoing on applying OAM techniques to long-range practical free-space optical communication links.[6]
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,[7] making direct-detection OAM multiplexing still not being realized in long-haul communications. In some specialty fibers, OAM states were transmitted with 97% purity after 20 meters.[8] 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.[citation needed]
Alternative to direct-detection OAM multiplexing is a computationaly complex coherent-detection with (MIMO) digital signal processing (DSP) approach, that can be used to achieve long-haul communication,[9] where strong mode coupling is suggested to be beneficial for coherent-detection based systems.[10]
Criticism
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.[11][12][13]
References
- ^ a b Sebastian Anthony (2012-06-25). "Infinite-capacity wireless vortex beams carry 2.5 terabits per second". Extremetech. http://www.extremetech.com/extreme/13 1640-infinite-capacity-wireless-vorte x-beams-carry-2-5-terabits-per-second. Retrieved 2012-06-25.
- ^ Gibson, G.; Courtial, J.; Padgett, M. J.; Vasnetsov, M.; Pas'Ko, V.; Barnett, S. M.; Franke-Arnold, S. (2004). "Free-space information transfer using light beams carrying orbital angular momentum". Optics Express 12 (22): 5448–5456. doi:10.1364/OPEX.12.005448. PMID 19484105. edit
- ^ Tamburini, F.; Mari, E.; Sponselli, A.; Thidé, B.; Bianchini, A.; Romanato, F. (2012). "Encoding many channels on the same frequency through radio vorticity: First experimental test". New Journal of Physics 14 (3): 033001. doi:10.1088/1367-2630/14/3/033001. edit
- ^ Edfors, O.; Johansson, A. J. (2012). "Is Orbital Angular Momentum (OAM) Based Radio Communication an Unexploited Area?". IEEE Transactions on Antennas and Propagation 60 (2): 1126. doi:10.1109/TAP.2011.2173142. edit
- ^ "'Twisted light' carries 2.5 terabits of data per second". BBC News. 2012-06-25. http://www.bbc.co.uk/news/science-env ironment-18551284. Retrieved 2012-06-25.
- ^ Djordjevic, I. B.; Arabaci, M. (2010). "LDPC-coded orbital angular momentum (OAM) modulation for free-space optical communication". Optics Express 18 (24): 24722–24728. doi:10.1364/OE.18.024722. PMID 21164819. edit
- ^ McGloin, D.; Simpson, N. B.; Padgett, M. J. (1998). "Transfer of orbital angular momentum from a stressed fiber-optic waveguide to a light beam". Applied optics 37 (3): 469–472. doi:10.1364/AO.37.000469. PMID 18268608. edit
- ^ Bozinovic, Nenad; Steven Golowich, Poul Kristensen, and Siddharth Ramachandran (July 2012). "Control of orbital angular momentum of light with optical fibers". Optics Letters 37 (13): 2451–2453. http://dx.doi.org/10.1364/OL.37.00245 1.
- ^ Ryf, Roland; Randel, S. Gnauck, A.H. Bolle, C. Sierra, A. Mumtaz, S. Esmaeelpour, M. Burrows, E.C. Essiambre, R. Winzer, P.J. Peckham, D.W. McCurdy, A.H. Lingle, R. (February 2012). "Mode-Division Multiplexing Over 96 km of Few-Mode Fiber Using Coherent 6 x 6 MIMO Processing". Journal of Lightwave Technology 30 (4): 521–531. http://ieeexplore.ieee.org/xpls/abs_a ll.jsp?arnumber=6074912.
- ^ Kahn, J.M.; K.-P. Ho and M. B. Shemirani (March 2012). "Mode Coupling Effects in Multi-Mode Fibers". Proc. of Optical Fiber Commun. Conf.. http://ee.stanford.edu/~jmk/pubs/mode .coupling.ofc.12.pdf.
- ^ Jason Palmer (8 November 2012). "'Twisted light' data-boosting idea sparks heated debate". BBC News. http://www.bbc.co.uk/news/science-env ironment-20217938. Retrieved 8 November 2012.
- ^ Tamagnone, M.; Craeye, C.; Perruisseau-Carrier, J. (2012). "Comment on 'Encoding many channels on the same frequency through radio vorticity: First experimental test'". New Journal of Physics 14 (11): 118001. doi:10.1088/1367-2630/14/11/118001. edit
- ^ Tamburini, F.; Thidé, B.; Mari, E.; Sponselli, A.; Bianchini, A.; Romanato, F. (2012). "Reply to Comment on 'Encoding many channels on the same frequency through radio vorticity: First experimental test'". New Journal of Physics 14 (11): 118002. doi:10.1088/1367-2630/14/11/118002. edit
See also
