Green Optical Communications—Part I: Energy Limitations in Transport

Abstract

The capacity and geographical coverage of the global communications network continue to expand. One consequence of this expansion is a steady growth in the overall energy consumption of the network. This is the first of two papers that explore the fundamental limits on energy consumption in optical communication systems and networks. The objective of these papers is to provide a framework for understanding how this growth in energy consumption can be managed. This paper (Part I) focuses on the energy consumption in optically amplified transport systems. The accompanying paper (Part II) focuses on energy consumption in networks.A key focus of both papers is an analysis of the lower bound on energy consumption. This lower bound gives an indication of the best possible energy efficiency that could ever be achieved. The lower bound on energy in transport systems is limited by the energy consumption in optical amplifiers, and in optical transmitters and receivers. The performance of an optical transport system is ultimately set by the Shannon bound on receiver sensitivity, and depends on factors such as the modulation format, fiber losses, system length, and the spontaneous noise in optical amplifiers. Collectively, these set a lower bound on the number of amplifiers required, and hence, the amplifier energy consumption. It is possible to minimize the total energy consumption of an optically amplified system by locating repeaters strategically. The lower bound on energy consumption in optical transmitters and receivers is limited by device and circuit factors. In commercial optical transport systems, the energy consumption is at least two orders of magnitude larger than the ideal lower bounds described here. The difference between the ideal lower bounds and the actual energy consumption in commercial systems is due to inefficiencies and energy overheads. A key strategy in reducing the energy consumption of optical transport systems will be to reduce these inefficiencies and overheads.

Minimising total energy requirements in amplified links by optimising amplifier spacing

“…The left hand (3a) plot, calculated for negligible input and output losses, corresponds to the case considered in [9]. However if coupling losses are neglected the lowest overall output power strategy suggests that distributed amplification represents the minimum power consumption, consistent to [9], although we find that an optimum amplifier spacing for a system operated at the nonlinear threshold (46.7 km) still remains. The first conclusion is clearly unphysical, as common sense suggests that there must be some energy penalty for the infinite number of pump couplers associated with infinitesimally short amplifier spacing.…”

“…There is an obvious similarity between Eqs. (7) and (9) in that both indicate a universal optimum span length only dependent on loss, and in consequence a universal amplifier gain. Ignoring the correction factor W(.)…”

“…This power is always below that of the blue dotted line and intercepts the x-axis at a spacing given by Eq. (9). A simple illustration will show how to use Fig.…”

“…Of course, optimization based on the functional power requirements as described here tells only part of the story, with overheads associated with control and management also contributing to the total power consumption [9], and hence optimization as described here. We note however that since their first introduction in the 1990s [11], a wide range of power overheads have been associated with optical amplifiers ranging from zero [11] to several Watts per amplifier.…”

“…The underlying assumption is that the electrical power needed to operate an amplified optical link is directly related to the total optical power employed by the sum of the amplifiers. Previous work on optimum power [9] was restricted to the linear regime and showed that for a lossless amplifier the minimum power per bit is obtained with distributed amplification i.e. zero gain amplifiers.…”