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Fiber-optic Transmission and Networking: The Previous 20 and the Next 20 Years

Re-printed from Optics Express, a publication of OSA Publishing

Celebrating the 20th anniversary of Optics Express, this paper reviews the evolution of optical fiber communication systems, and through a look at the previous 20 years attempts to extrapolate fiber-optic technology needs and potential solution paths over the coming 20 years. Well aware that 20-year extrapolations are inherently associated with great uncertainties, we still hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us. Focusing on the optical transport and switching layer, we cover aspects of large-scale spatial multiplexing, massive opto-electronic arrays and holistic optics-electronics-DSP integration, as well as optical node architectures for switching and multiplexing of spatial and spectral superchannels.

Introduction: taking a 20-year horizon

Over the previous 20 years, since Optics Express was created in 1997, Internet Protocol (IP) traffic in North America has grown by a factor of 10,000; the capacity of IP router blades, which make sure that these packets reach their destination host, has grown by a factor of 1,000; the capacity of wavelength-division multiplexed (WDM) fiber-optic communication systems transporting the IP traffic across the globe has grown by the same factor of 1,000; and per-wavelength transponder interface rates have grown by a factor of between 10 and 100. These numbers reflect enormous growth in the demand for data traffic and its supply through information and communications technologies, on well understood long-term exponential scaling trends, as we shall discuss in this paper. Fitting for the occasion of the 20-year anniversary of Optics Express, we note that the number of pages published in this on-line only journal has grown by an equally impressive factor of 37, with ~900 pages published per year in its beginnings, and ~34,000 pages published annually today.

Adopting a 20-year view, we will review the evolution of optical fiber communication systems in this paper, and through a look at the previous 20 years attempt to extrapolate fiber-optic technology needs and potential solution paths for the coming 20 years. Well aware that 20-year technology extrapolations are inherently noisy, we hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us.

Similar to our companion paper [1], which “only” looks a decade into the future, and which may serve as both a complementary and an introductory source to the material discussed here, we base our 20-year extrapolations on well-established long-term historic traffic and technology growth trends, which we assume will continue going forward. Obviously, all exponential growth will eventually saturate, but many examples related to information and communications technologies have shown that exponential growth (as well as exponential energy reduction) can be maintained for decades [1], and even for a century [2]; examples of sustained exponential growth over multiple centuries can also be found, e.g., in economics [3].

While human perception is quick to accept long-term exponential scaling as a historic fact, there is often significant hesitation associated with accepting continued exponential scaling as a likely evolution path into the future. These conceptual difficulties likely arise because people tend to focus on the scaling of a given technology, while exponential scaling should rather be associated with functional scaling, whereby the scaling of a certain technology used to implement a certain function may saturate and be replaced by a new technology to continue the scaling of the considered function. Examples are the functional scaling of microprocessors, which first involved clock speed scaling and then scaling through multi-core architectures; the functional scaling of storage, which involved various generations of different magnetic technologies before transitioning to semiconductor technologies; and the functional scaling of long-haul transport, which first resorted to per-span regenerated time-division multiplexing using a succession of optical wavelength ranges over multi-mode, then single-mode waveguides, then incorporated optically amplified WDM, and is now shifting to parallel spatial paths.

Individual applications driving traffic growth also may exhibit saturation, only to be replaced by new applications that continue the scaling of traffic. Should established long-term scaling trends experience a pronounced change that prevents their underlying functional scaling at the exponential pace that our heavily information and communications centric society has gotten used to, the reduced growth is likely to have significant social and/or economic repercussions. These, we hope, will be avoided through the inventive spirit of the scientific and engineering communities addressed in this paper. In fact, while we expect there to be challenges in scaling networks over the next 20 years, we do not foresee any fundamental roadblocks, as we shall discuss in this paper.