Terabit-per-Second Fiber Optical Communication Becomes Practical

Humans used optical signals intuitively for the purpose of communication in ancient times. Modern day optical communication systems are instead based on the fundamental understanding of information theory and technological advances in optical devices and components. The Optical Society (OSA) played a vital role in making fiber-optic communication practical for the information age. It is well known that the capacity of a communication channel is constrained by the Shannon limit, W log2(1 + S/N), where W is the spectral bandwidth and S/N is the signal-to-noise ratio (SNR). The bandwidth of a communication channel is proportional to the carrier frequency, which is on the order of 200 THz for visible or near-infrared light. Therefore, a small fractional bandwidth around the optical carrier can provide a capacity much larger than the limited capacity supported by the spectrum of radio-frequency (RF) waves or microwaves [1]. The SNR of a communication channel is proportional to the received power and inversely proportional to the noise and distortion. The invention of the laser, which can produce high-power coherent optical radiation at the transmitter, fueled the migration from RF/microwave communication to optical communication. In fact, the first patent on lasers (more precisely masers) by Nobel Laureates Charles Townes and Arthur Schawlow, both OSA Honorary Members, was entitled “Maser and maser communication systems.” Tomake optical communication practical, however, the received optical power (not only the transmitted power) must be much stronger than the noise. This requires a low loss optical transmission channel. The loss in free-space transmission is determined by diffraction, which is much larger than that of RF/microwave in appropriate cables. Fortunately, light can also be guided by total internal reflection, a phenomenon known since the mid-nineteenth century. An optical fiber with a high-index core surrounded by a lower-index cladding can support guided “modes” inside the dielectric cylindrical waveguide that propagate without experiencing radiative loss [2]. As a consequence, the loss of the optical fiber is dominated by material loss. Glass fibers were initially deemed impractical for communication systems, as the measured attenuation was >1000 dB/km. In 1966, Kao and Hockham showed that the measured losses were due to impurities rather than fundamental loss mechanisms and, without impurities, glass fibers could achieve losses below 5 dB/km. They also identified that fused silica fiber could have the lowest losses. OSA Fellow Dr. Charles Kao was awarded the 2009 Nobel Prize in Physics for his “groundbreaking achievements concerning the transmission of light in fibers for optical communication,” which has fundamentally transformed the way we live our daily lives. It is the invention of the silica optical fiber and the semiconductor laser with significantly long life that ushered in the era of modern optical communication. (These inventions are described in separate essays in this section of this book.) The first-generation fiber-optic communication system in the 1980s used multimode fibers and 0.8-μm multimode Fabry–Perot semiconductor diode lasers, supporting a data rate 1975–1990