To highlight research being carried out to ensure that the capacity of fiber optic communication systems can handle this rapidly increasing demand, and thus to address the Internet capacity crunch, the editors of the Optical Society's (OSA) open-access journalOptics Expresshave published a special Focus Issue on Space Multiplexed Optical Transmission. The issue is organized and edited by Guifang Li of the University of Central Florida and Xiang Liu of Bell Labs, Alcatel-Lucent.
The Focus Issue describes different approaches to increase the information carrying capacity of optical fibers. One of these, Space-Division Multiplexing (SDM), uses fibers with seven cores, rather than a single one. By using all seven cores, seven independent signals can be carried, and the fiber capacity can thus, in principle, be increased seven-fold. In other approaches, such as Mode-Division Multiplexing (MDM) a single core carries multiple information channels, each of which is associated with a different physical shape of the light field, also increasing the fiber capacity.
According to Xiang Liu, "Our goal in producing this focus issue was to provide a comprehensive survey of the state-of-the-art research activities in multiplexed transmission, which holds potential for addressing the challenges of capacity growth in optical communications."
"We hope this collection will stimulate future research on this subject to address the remaining fundamental and practical challenges, potentially enabling dramatic capacity growth in future optical communication systems", stated Guifang Li.
It is well known that the capacity of a communication channel is constrained by the Shannon limit. In optical fiber communication, fiber nonlinearity imposes an additional limit in the high power or high
signal-to-noise ratio (SNR) regime. Digital coherent detection increases optical signal tolerance to linear noise such as amplified spontaneous emission noise, and sometimes non-linear phase noise. However, the single-channel capacity increase scales logarithmically with the increase in SNR. This logarithmical
single-channel capacity scaling ultimately will not be able to support exponential traffic growth.
While it might be impossible to provide exponential single-channel capacity growth in optical communication, multiplicative growth in optical communication capacity has satisfied traffic demand in the past, especially when dense wavelength-division multiplexing (DWDM) with a multiplicative factor on the order of 100 was invented. As today's DWDM coherent optical communication technology has already taken advantage of all degrees of freedom of a light wave in a single-mode fiber, namely frequency, polarization, amplitude, and phase, further multiplicative growth has to explore new degrees of freedom. MDM using few-mode fibers (FMF) and SDM using multi-core fibers (MCF) have emerged as promising candidates for the next multiplicative capacity growth for optical communication. ThisOptics ExpressFocus Issue features the state-of-the-art research activities in MDM and SDM aimed at order-of-magnitude capacity growth in future optical communication systems.
The following papers are some of the highlights of theOptics ExpressFocus Issue on Space Multiplexed Optical Transmission. All are included in Volume 19, issue 17 and can be accessed on-line at http://www.opticsinfobase.org/oe
A paper from B. Zhu et al. of OFS Labs and Bell Labs reports a transmission capacity of 112 Tb/s over a distance of 76.8-km using SDM in a 7-core fiber using SDM and DWDM in the C and L bands. Each of the 7 cores carried 160 polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) channels at 107-Gb/s on a 50-GHz grid, resulting in an aggregate spectral efficiency of 14 b/s/Hz. The impact of the inter-core crosstalk was experimentally investigated and the system implications of core-to-core crosstalk on long-haul transmission were discussed. pp. 16665-16671 ( http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-19-17-16665 )
T. Hayashi et al. from Sumitomo Electric Industries report an ultra-low-crosstalk MCF that is suitable for ultra-long-haul transmission. A remarkably low inter-core crosstalk level of below -62 dB was achieved in a 17.4-km 7-core fiber, by using a trench-assisted index profile. A transmission capacity of 109 Tb/s was recently demonstrated by using SDM and DWDM with this ultra-low-crosstalk MCF. pp. 16576-16592 ( http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-19-17-16576 )
S. Randel et al. from Bell Labs and OFS labs present the transmission of three 112-Gb/s PDM-QPSK signals over the fundamental mode (LP01) and two orthogonal LP11 modes of a 33-km FMF. 6x6 MIMO processing was used to achieve the separation of the three spatial modes and the two polarizations of the transmitted signals. These experiments show the feasibility of scaling capacity using MDM in FMF in combination with MIMO signal processing. pp. 16697-16707 (<A href="http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-19-17-16697">http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-19-17-16697</a>)
K. S. Abedin et al. from OFS Labs have demonstrated a multicore Erbium-doped fiber (MC-EDF) amplifier for simultaneous amplification in its 7 cores. The pump and signal were coupled to individual cores of the MC-EDF using two tapered fiber bundled (TFB) couplers. The average gain achieved in the MC-EDF amplifier was 30 dB, and noise figure was less than 4 dB. pp. 16715-16721 ( http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-19-17-16715 )