By Dr. Ioannis Tomkos
A common issue that the OFC/NFOEC community has tried to address since the first OFC took place 38 years ago is how to increase the capacity of optical communications systems, which remains an important and challenging problem to this day. To address these capacity scaling needs, optical communications scientists try to explore all possible “dimensions” and properties of light, most recently the multiple spatial modes in multi-mode fibers.
Over the last few decades, optical networks have relied on exploiting different properties of light for achieving the required capacity upgrades dictated by significant traffic growth rates. The first systems deployed in the ’70s used single-channel transmission over multi-mode fibers (MMFs) and soon after (to avoid the severe limitations introduced by inter-modal dispersion) over single-mode fibers (SMFs). To multiply the capacity of the overall connection between different points in the network in the early days of optical fiber communications, the main solution was to use space-division-multiplexing by incorporating more fibers in parallel. However, the costs of the systems were increasing linearly, thus making this solution unsustainable. The big change occurred in the late ’80s/early ’90s, when the invention of the Erbium-doped fiber amplifier (EDFA) and new stable optical filtering technologies enabled the use of wavelength as an extra dimension, and the advent of dense wavelength division multiplexing (DWDM) allowed for the transmission of multiple signals within an SMF. Initially, the deployed WDM systems used the amplitude dimension (using simple “on/off keying” to start with) to encode the information of the wavelength channels, but in the early ’00s the use of amplitude, phase, and polarization (alone or all combined) enabled the required capacity scaling.
While modern communication networks have been based on single-mode fibers for more than three decades now, their capacity moves closer to exhaustion as the available bandwidth in the EDFA wavelength band is limited and the spectral efficiency of the transmission systems approaches the fundamental limits set by Shannon’s theorem. To overcome the fundamental capacity limitations of SMFs, one possible solution may be the exploitation of a new dimension that is once again in the space domain, but this time in the form of signal transmission over a set of different paths within the same optical fiber. The signal “paths” would be over either multiple-core fiber (MCF) or over different modes of a multi-mode fiber. Theoretically, MMFs exhibit a higher transport capacity limit than SMFs, provided that we can exploit the various modes as independent communication channels with individual scattering paths (as in the case of a wireless channel with multipath transmission features), using advanced digital signal processing techniques.
Despite the hype around this new research topic, we have to consider that MMFs as a transmission medium for long-haul communications have some glitches, and it will take a while until they achieve similar transmission distances at similar costs as SMFs. In addition, we have to consider that all deployed long-haul fiber infrastructure to date is SMF-based. It may take quite some time until network operators are convinced to make the decision to deploy new fiber cables with MCFs or MMFs. To support a complete MMF-based system, we also need something more than the multi-mode fiber itself to be deployed. We need specially-designed novel photonic components including multi-mode optical amplifiers, multi-mode transmitters and receivers and multi-mode mux/demux units, all making use of efficient optical adaptors between conventional single-mode fibers to the new multi-mode fibers. Needless to say, such technology is in its infancy and far from becoming a commercial reality. Therefore, even if the spatial multiplexing technology exists for significantly increasing the transmission capacity over such new single fibers, it will take a long while until it will become a commercial reality (at least for anything beyond niche green-field applications).
Though there is limited commercialization potential of such MMF-based technologies in the near/medium term, I have to admit that it is a fascinating research topic! OFC/NFOEC 2013 will address these and related issues through extensive workshops, tutorials, invited talks and sessions of contributed papers. Among the many such relevant talks and events, I would like to highlight the workshop titled “If the technology for SDM exists, do we want to use it?” by Chongjin Xie and Georgios Zervas, the tutorial “Mode-Division Multiplexing Systems: Propagation Effects, Performance, and Complexity” by Joseph Kahn and Keang-Po Ho, as well as the following invited talks:
“Nonlinear Performance of SDM Systems Designed with Multimode or Multicore Fibers” by Govind Agrawal,
“Space Division Multiplexed Transmission” by Sebastian Randel,
“Nonlinearities in Space-division Multiplexed Transmission” by Antonio Mecozzi,
“Recent Progress in Multi-core and Few-mode Fibers” by Shoichiro Matsuo,
“Weakly-coupled Few-mode Fibers for Single-mode and Mode-division-multiplexed Transmissions” by David Boivin,
“Devices and Components for Space-Division Multiplexing in Few-Mode Fibers” by Nicholas Fontaine,
“Optical Amplifiers for Space Division Multiplexed Systems” by David Richardson, and
“Recent Progress in Space-division Multiplexed Transmission Technologies” by Toshio Morioka.
I will certainly try to attend all of these!
In closing, I would like to mention that the OFC/NFOEC 2013 final technical program is now available online and you can check out the relevant contributed papers that were accepted from the open call. I’m interested to see what the readers of this blog consider novel important topics and achievements. Perhaps a new paper (I have not managed to look at them all so far…) on yet another dimension that can be explored for increasing the capacity
Ioannis Tomkos (B.Sc., M.Sc. Ph.D.), has been with AIT since September 2002 (serving as Professor, Research Group Head and Associate Dean). In the past he was Senior Scientist at Corning Inc., USA (1999 – 2002) and Research Fellow at University of Athens, Athens, Greece (1995 - 1999).
Posted: 24 January 2013 by
Dr. Ioannis Tomkos
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