The Evolution of Optical Communications Networks since 1990

Optical communication networks have played a critical role in the information/communication revolution and in turn have fundamentally changed the world and daily life for billions around the globe. Without cost-effective, high-capacity optical networks that span continents and connect them via undersea routes, the worldwide Internet would not be possible. Optical access systems, both fiber/cable and fiber-to-the home, are also essential to bring broadband access to that global Internet to homes and businesses. Increasingly important, ubiquitous broadband optical networks provide the high-bandwidth backhaul essential for wireless access networks that enable today’s smartphone users. These networks also provide the always available broadband access that will make cost-effective and energy-efficient cloud services available to all in the future. All this has been made possible because, as capacity demand has grown exponentially following the advent of the Internet, optical technology has made possible a dramatic reduction in the cost per bit carried over an optical fiber, allowing cost-effective capacity scaling. On average, transmission capacity over a single fiber has increased at a rate of ∼100-fold every ten years over the last thirty years. As a result, as traffic has grown and is aggregated at the ingress and disaggregated at egress nodes, new higher-capacity generations of long-haul and metro optical systems have been deployed at a total cost that has grown sub-linearly relative to capacity. Of course, the advantage of the optical frequencies for communication is the inherent ability to serve as a carrier for very-high-bandwidth information. Fiber provides an extremely attractive transmission medium that offers both ultra-low loss and low chromatic dispersion. The latter results in minimal pulse spreading, resulting in low inter-pulse (bit) interference after transmission over large distances. At its most basic implementation, an optical transmission system requires an optical source whose generated dc optical signal can be modulated with information at the information bandwidth of interest, a fiber, and an optical detector and supporting electronics. Figure 1 captures the progress of the “hero” research transmission experiments [1]. Shown is the maximum information capacity carried on a single fiber versus the year the research results were achieved. For this review, it is convenient to describe the research progress in fiber optic transmission capacity in three waves or eras. In what follows, we use those generations, each enabled by a set of critically important optical component technology innovations, to provide an overview of the advances in optical communications since 1990. At the start of the 1990s, commercially deployed systems provided per fiber capacities of about 1 Gb/s. They were used primarily in long-haul intercity applications to carry highly aggregated voice service. At that time, increase in capacity demand was still driven mostly by population growth as well as some increase in new services such as fax. The wavelength window utilized was the minimum chromatic dispersion 1.3-μm window. To increase time division multiplexed bit rates (TDM) for fixed distance between electrical regenerations, both signal strength relative to noise and quality of the detected signal with respect to pulse-to-pulse 1991-PRESENT