This problem can be partially solved by utilizing a multiple-input, multiple-output (MIMO) approach. MIMO is a popular technique in radio-frequency (RF) systems and can untangle some of the crosstalk between modes using digital signal processing.52,53 However, it is unclear as to the upper limit of performance and number of modes that can be supported.
Orthogonal Frequency Division Multiplexing (OFDM). OFDM uses a single carrier wave and multiple orthogonal subcarrier waves; it has produced some of the most exciting results in the RF world. Each subcarrier that carries independent data can be densely packed in the spectrum, since orthogonality is maintained by advanced electronic processing in the transmitter/receiver.54 OFDM is spectrally efficient and is tolerant to many fiber-based impairments, but it does require a fair amount of high-speed electronic data signal processing, such as fast Fourier transforms and analog-digital converters.55
Device and Subsystem Achievements
These recent systems-level advances have been enabled by device and subsystem achievements, which include those described below.
High-Speed Local and Access Area Networks. The advances of local area networks (LANs) made it possible to deliver the high bandwidth from long-haul optical networks to desktops. Started with light-emitting diodes (LEDs) and Fabry-Perot lasers as transmitters, multi-mode optical fibers and fiber ribbon cables became major LAN standards in the mid-1990s. In the late 1990s, direct modulated gigabits
52 Kaminow et al. 1996. A wideband all-optical WDM network.
53 Ryf, R., A. Sierra, R. Essiambre, A. Gnauck, S. Randel, M. Esmaeelpour, S. Mumtaz, P.J. Winzer, R. Delbue, P. Pupalaikis, A. Sureka, T. Hayashi, T. Taru, and T. Sasaki. 2011. “Coherent 1200-km 6 × 6 MIMO Mode-Multiplexed Transmission over 3-CoreMicrostructured Fiber.” Conference paper. European Conference and Exposition on Optical Communications. Available at http://www.opticsinfobase.org/abstract.cfm?URI=ECOC-2011-Th.13.C.1. Accessed June 26, 2012.
54 Shieh, W., and I. Djordjevic. 2009. Orthogonal Frequency Division Multiplexing for Optical Communications. New York, N.Y.: Elsevier/Academic Press.
55 Liu, X., S. Chandrasekhar, B. Zhu, P.J. Winzer, A.H. Gnauck, and D.W. Peckham. 2011. 448-Gb/s reduced-guard-interval CO-OFDM transmission over 2000 km of ultra-large-area fiber and five 80-GHz-Grid ROADMs. IEEE Journal of Lightwave Technology 29:483-490.