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| Fig. 1: The points show the capacity of optical communication systems demonstrated in research and commercially, the line indicates the world's total traffic use. (after [3]) |
Optical communication provides the high-capacity networking infrastructure that supports today's Internet. Consequently, it has enabled our global communication revolution, brought the world together at increasing speeds, and improved the quality of life for people everywhere. As the name suggests, optical communication consists of any technology that uses light as its transmission medium, typically transmitted from light-emitting diodes or laser-emitting diodes across fiber links. Modern optical networks feature transmission links that can reach high data-capacity on a single fiber by combining many wavelengths, operating at rates as high as 100 Gigabytes per second onto a single fiber. [1] Verizon Communications, one of the developers and users of this technology, commercially operates some of the world's most advanced fiber-optic networks that sustain the company's converged communications, information, and entertainment services. [1]
Energy is a topic of increasing importance for optical communication systems. This report aims to introduce a technical and historical overview of optical networking. Then it will consider some challenges of energy consumption in optical transmission systems.
Communicating across fiber-optic channels involves: creating the optical signal with the use of a transmitter, relaying the signal along the fiber, ensuring the signal does not become distorted or weak, receiving the optical, and then converting or encoding back into an electrical signal. Optical communications historically has specialized in the transmission of large amounts of data over long distances. [2] Since its introduction in 1970 and development in the following two decades, the transmission capacity of optical communication has steadily increased. [2] The gradual increase of bit rate capacity in the single optical channel supported on these early systems was influential, but not yet revolutionary. Then in the mid-1990s, a new technique was developed in optical communication networks that allowed transmission of tens of optical signals over a single optical fiber. [3] This technique called wavelength division multiplexing (WDM) ushered in the next generation of fiber-optic technology. [3]
Fig. 1 illustrates this rapid development of wavelength division multiplexing (WDM) and its predecessor, single-channel time division multiplexing (TDM). The figure juxtaposes optical communication development with the world's total traffic by year and extrapolates into the year 2020.
As shown, history suggests that data-network traffic follows the course of fiber- optic technology and will do so for some time. This could imply that rising network demand caused the rapid development of fiber-optic capacities but also possibly implies that the limit of possible future traffic will correlate with the advancement of optical communication.
This continued network growth will inevitably have significant consequences on the design and development of future optical communication networks. A key challenge for the industry would be to scale networks to the demands for increasing capacity, while minimizing the growth in network energy consumption.
An example of a current research challenge would be to expand the transmission band of each fiber, yielding more bandwidth thus more data-capacity, while not correspondingly scaling power. If a three-centimeter-diameter link can house one thousand fibers, continued progress in the development of multicore fibers and multiple multiplexing across each fiber could provide as much as five orders of magnitude of intrinsic capacity growth across the link. [4] The challenge undertaken by many research teams nowadays though is how to exploit this exponentially emergent bandwidth without causing a five-orders-of magnitude increase in energy. [4] Another field of optical communication research with the focus being on energy consumption is the issue of energy required for signal regeneration along long distance fiber propagation paths. If signal regeneration is needed along the path, then additional transceivers are required and the system's associated power for each regeneration quickly escalates total energy consumption. Since modulation format and channel coding determine the the reach of a transmission system, there is considerable opportunity for design tradeoffs in the transceiver system; exploiting them with regards to energy and cost will achieve the highest overall system efficiency for a given requirement. [4]
The evolution of optical networking solutions over the past years has enabled the internet revolution. Optical fiber networks provide a high capacity infrastructure for serving the world's escalating data traffic demand. Thus, optical networks have an increasing role to play in our society, quality of life, and other advancing technology. Optical data networks have matured over the past decade to a point in which they are starting to encounter challenging physical constraints associated with energy and capacity. There is a lot of research that can be done and likely will be done because people will always search for new ways to fulfill humans' increasing demand for data.
© Jomar Sevilla. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. Wang, "Requisite of High-Capacity Transmission Technology for Future Optical Internet," Proc. 7th Intl. Conf. Optical Internet (COIN 2008), 14 Oct 08.
[2] H. Kogelnik, "High-Capacity Optical Communications: Personal Recollections," IEEE J. Selected Topic Quanum Electronics 6, 1279 (2000).
[3] R. Tkach, "Scaling Optical Communications for the Next Decade and Beyond," Bell Labs Technical J. 14, No. 4 (February 2010), p. 3.
[4] D. Kilper et al., "Energy Challenges in Current and Future Optical Transmission Networks," Proc. IEEE 100, 1168 (2012).