Historical Perspective of Optical Communication

Light has always been with us. Communications using light occurred early in our development, when human beings first communicated by using hand signals. This is obviously a form of optic communications. It does not work in darkness. During the day, the sun is the source of light for this system. The information is carried from the sender to the receiver on the sun's reflected radiation. Hand motion modifies, or modulates, the light. The eye is the message-detecting device, and the brain processes this message. Information transfer by such a system is slow, the transmission distance is limited, and the chances for error are great. While primitive by today's standards, this early form of visual communication laid the groundwork for understanding how light could carry information—an idea that would eventually lead to the development of the fiber optic cable.

The First Optical Communicators

Early humans relied on visual cues and hand signals to convey messages across short distances. This rudimentary form of optical communication, while limited, demonstrated a fundamental principle that remains critical in modern systems: light can be modulated to carry information. These early methods, though simple, established the conceptual framework that would one day evolve into sophisticated technologies like the fiber optic cable.

Smoke Signals: Early Encoded Communication

A later optic system, useful for longer transmission paths, was the smoke signal. The message was sent by varying the pattern of smoke rising from a fire. This pattern was again carried to the receiving party by reflected sunlight. This system required that a coding method be developed and learned by the communicator and receiver of the message. This is comparable to modern digital systems that use pulse codes. The development of coding systems represented a significant advancement, as it introduced structured information transfer—much like how modern fiber optic cable systems use complex encoding to transmit data efficiently over long distances.

Smoke signals enabled communication over much greater distances than hand signals, though they were still dependent on weather conditions and line of sight. The encoded nature of these signals demonstrated an important evolution: the separation of the message from the medium. In modern terms, this is analogous to how data is encoded independently of the fiber optic cable that carries it, allowing for standardized communication protocols across different physical implementations.

An artistic representation of smoke signals being used to transmit messages across distances

Alexander Graham Bell's Photophone

In 1880, Alexander Graham Bell invented a light communication system, the photophone. It used sunlight reflected from a thin voice-modulated mirror to carry conversation. At the receiver, the modulated sunlight fell on a photoconducting selenium cell, which converted the message to electrical current. A telephone receiver completed the system. The photophone never achieved commercial success, although it worked rather well. Bell himself considered the photophone his most important invention, and with good reason—it represented the first practical device for transmitting sound over a light beam, a principle that would eventually be perfected in the fiber optic cable.

The photophone's operation was remarkably similar to modern fiber optic communication in concept, if not in execution. It used a light source (sunlight), a modulator (voice-controlled mirror), a transmission medium (air), a detector (selenium cell), and a receiver (telephone). The primary difference was that instead of using a controlled environment like a fiber optic cable to guide the light, Bell's invention relied on open air—which made it vulnerable to atmospheric interference.

Despite its limitations, the photophone demonstrated that light could carry complex information like speech, paving the way for future innovations. The fundamental principles Bell established—modulating light to carry information and converting it back to an electrical signal at the receiver—remain the basis for all modern optical communication, including fiber optic cable systems.

Alexander Graham Bell demonstrating his photophone, a precursor to modern fiber optic communication

Lamp-based Optical Communication

The advent of lamps allowed the construction of simple optic-communications systems, such as blinker lights, for ship-to-ship and ship-to-shore links, automobile turn signals, and traffic lights. In fact, any type of indicator lamp is basically an optic-communications system. These systems expanded the practical applications of optical communication beyond what was possible with natural light sources, providing reliable signaling regardless of time of day or weather conditions—though they still lacked the bandwidth and distance capabilities that would later be achieved with the fiber optic cable.

Ship-to-ship communication using signal lamps became particularly important in maritime navigation, allowing vessels to exchange messages at night or in reduced visibility. These systems used coded patterns of light flashes, typically based on Morse code, to transmit simple messages. While limited in data capacity compared to modern standards, they represented an important step toward more reliable optical communication—foreshadowing the directed light transmission that would later be optimized in fiber optic cable technology.

Maritime Signal Lamps

Ship-to-ship communication using coded light signals was a critical advancement in optical communication, enabling reliable messaging at sea long before radio technology became widespread.

Early Traffic Signals

Traffic lights represented one of the first widespread applications of optical communication in urban environments, using standardized light patterns to convey instructions to many recipients simultaneously.

These lamp-based systems, while simple by today's standards, established important conventions in optical signaling—such as standardized patterns and meanings—that would influence later communication technologies. They also demonstrated the value of optical communication in situations where electrical or radio-based systems were impractical or unavailable, a principle that remains relevant in modern fiber optic cable deployments in remote or challenging environments.

The Laser: A Revolutionary Breakthrough

All these systems have low information capacities. A major breakthrough that led to high-capacity optic communications was the invention of the laser, in 1960. The laser provided a narrowband source of optic radiation suitable for use as a carrier of information. Lasers are comparable to the radio-frequency sources used for conventional electronics communications. This breakthrough was pivotal, as it provided a coherent, focused light source that could carry much more information than previous light sources—creating the technical foundation that would eventually make the fiber optic cable a practical communication medium.

The unique properties of laser light—monochromaticity (single wavelength), coherence (uniform wavefront), and directionality—made it ideal for communication. Unlike natural light or conventional artificial light sources, laser light could be modulated to carry complex information over much greater distances with minimal distortion. This capability was essential for developing high-capacity communication systems, eventually leading to the fiber optic cable's development as the optimal medium for guiding this specialized light.

Early laser research in the 1960s laid the groundwork for modern optical communication systems

Unguided optic communications systems (non-fiber) were developed shortly after the discovery of the laser. Communication over light beams traveling through the atmosphere was easily accomplished. The disadvantages of these systems include dependence on a clear atmosphere, the need for a line-of-sight path between transmitter and receiver, and the possibility of eye damage to persons who unknowingly look into the beam. These limitations highlighted the need for a controlled transmission medium—the role that would eventually be filled by the fiber optic cable.

Although somewhat limited in their use, these early laser applications aroused interest in optic systems that would guide the light beam and thus overcome those disadvantages. In addition, guided beams could bend around corners and could be buried in the ground. The early work on atmospheric laser systems provided much of the fundamental theory and many of the actual components required for communications over fibers. Ironically, it is now known that laser sources are not required for all fiber systems. In many cases the broader band light-emitting diode is suitable. (The choice of the proper light source is a matter we discuss in this book.) This flexibility in light sources, combined with the versatile transmission medium of the fiber optic cable, has contributed to the technology's widespread adoption.

The Development of Practical Fiber Optics

In the 1960s, the key element in a practical fiber system was missing—that is, an efficient fiber. Although light could be guided by glass fibers, those available attenuated light by far too large an amount. Glass produced by the ancient Egyptians was opaque. The artisans of Venice fabricated glass of much greater purity in the Middle Ages. Venetian glass was moderately transparent, but still orders of magnitude too lossy for modern long-distance communications. The quest for a more transparent glass would ultimately lead to the fiber optic cable as we know it today.

A modern fiber optic cable showing internal light transmission through the core

It was not until 1970 that the first truly low-loss fiber was developed and fiber optic communications became practical. This occurred just 100 years after John Tyndall, a British physicist, demonstrated to the Royal Society that light can be guided along a curved stream of water. Guiding of light by a glass fiber and by a stream of water are evidence of the same phenomenon: total internal reflection. This principle, which allows light to bounce along the inside of a transparent medium without escaping, is what makes the fiber optic cable such an efficient transmission medium.

The development of low-loss glass fibers—with attenuation levels below 20 decibels per kilometer—marked the critical turning point that made the fiber optic cable a viable alternative to copper wires and coaxial cables. This breakthrough suddenly made it possible to transmit light over long distances without significant signal loss, opening the door to the high-speed, high-capacity communication networks we rely on today.

The fiber optic cable revolutionized communication by offering several key advantages over traditional copper-based systems. It could carry much more information (higher bandwidth) over much greater distances without signal regeneration. It was immune to electromagnetic interference, making it ideal for industrial environments. It was also lighter and more compact than copper cables, and less susceptible to tapping, providing better security. These advantages quickly established the fiber optic cable as the preferred medium for long-distance communication, eventually forming the backbone of the global telecommunications network.

Since the 1970s, the fiber optic cable has continued to evolve, with ongoing improvements in bandwidth, transmission distance, and manufacturing efficiency. Today, fiber optic cable forms the backbone of the internet, connecting continents through undersea cables, linking cities, and increasingly reaching directly to homes and businesses. The journey from ancient hand signals to modern fiber optic communication represents one of humanity's most remarkable technological evolutions—a testament to our persistent quest to communicate more effectively across ever-greater distances.