1. The Evolution of Optical Transport Networks

This introductory chapter describes the role of optics in networks, their capabilities, and their scaling limitations (different multiplexing techniques, i. e., (time-division multiplexing), (code-division multiplexing), (frequency-division multiplexing), (space-division multiplexing)). Types of optical networks installed around the globe are summarized, as well as their impact on society, market structure, and future perspectives.

Optical fiber transmission links were first deployed in the mid 1970s to provide \({\mathrm{45}}\,{\mathrm{Mb/s}}\) capacity in metropolitan networks at a time when most traffic was wireline telephone communication. Few would have imagined then, when bandwidth demand on the telephone network was growing only as rapidly as population growth, that this new technology would radically alter business and everyday life by enabling the worldwide Internet. As the Internet grew in popularity and extended to all parts of the world, new devices and transmission technologies were invented and developed to cost-effectively achieve the required higher capacity transmission, and fiber was laid across continents and under the oceans to cover the globe. Thus began a virtuous cycle of higher capacity optical transmission systems and, ultimately, reconfigurable optical networks enabled by new technologies to meet increased demand, which resulted in new applications and services, such as video, which drove ever greater capacity and flexibility demand, which was, again, achieved via new technology innovation at significantly lower costs/capacity.

First-era systems grew the capacity of optical links by increasing the bit rate of information carried on the fiber. The second era, which was achieved cost-effectively by the optical fiber amplifier, increased capacity by multiplexing many wavelengths, each carrying independent information, onto a single fiber. The result was an increase in single fiber transmission equal to the number of wavelength channels and long spans over which all the wavelength channel signals could be periodically boosted with a single optically-powered optical fiber amplifier. The next era took the giant step of moving optics from transmission links only to fully reconfigurable, wavelength-channel-based networks to achieve higher efficiency (and, therefore, capacity), flexibility, and restorability by allotting and managing network capacity at the optical layer level.

This step required cost-effective optical switching components to provide the reconfigurable wavelength add/drop and cross connect functions. Today’s optical networks provide superhighways of information bandwidth of the order 100 wavelength-defined lanes each providing hundreds of \(\mathrm{Gb/s}\) information capacity. These networks provide network optimization to address changing capacity needs under software control via configurable wavelength on-and-off ramps and route switching centers. Without these optical networks, the global internet, cloud computing, and high bandwidth mobile services, including video, would not be possible. This chapter provides a view of the evolution of these optical networks—the market drivers, network architectures, transmission system innovations and enabling devices, and module technologies—which was a result of the efforts of a global community of researchers, developers, and, ultimately, manufacturers.