History of Fiber Optics Technology
The earliest attempts to communicate via light undoubtedly go back thousands of years. Early long distance communication techniques, such as “smoke signals”, developed by native North Americans and the Chinese were, in fact, optical communication links. A larger scale version of this optical communication technique was the “optical telegraph” developed by Claude Chappe and deployed in France in the late 18th century. However, the development of fiber optic communication awaited the discovery of TIR (Total Internal Reflection) and a host of additional electronic and optical innovations.
In 1854, John Tyndall, using a jet of water that flowed from one container to another and a beam of light, demonstrated that light used internal reflection to follow a specific path. As water poured out through the spout of the first container, Tyndall directed a beam of sunlight at the path of the water. The light, as seen by the audience, followed a zigzag path inside the curved path of the water. This simple experiment, illustrated in Figure 1-7, marked the first research into the guided transmission of light.
People have used light to transmit information for hundreds of years. However, it was not until the 1960s, with the invention of the laser that widespread interest in optical (light) systems for data communications began. The invention of the laser prompted researchers to study the potential of fiber optics for data communications, sensing, and other applications. Laser systems could send a much larger amount of data than telephone, microwave, and other electrical systems. The first experiment with the laser involved letting the laser beam transmit freely through the air. Researchers also conducted experiments letting the laser beam transmit through different types of waveguides. Glass fibers, gas-filled pipes, and tubes with focusing lenses are examples of optical waveguides.
Charles Kao and Charles Hockham, working at the Standard Telecommunication Laboratory in England in 1966, published a landmark paper proposing that optical fiber might be a suitable transmission medium if its attenuation could be kept under 20 decibels per kilometer (dB/km). At the time of this proposal, optical fibers exhibited losses of 1,000 dB/ km or more. Even at a loss of only 20 dB/km, 99% of the light would still be lost over only 3,300 feet. In other words, only 1/100th of the optical power that was transmitted reached the receiver. But, even with this loss, the power was enough to drive the receiver.
A decibel is a ratio of output power compared to the input power or mathematically, dB = 10 log (output/input). The decibel is the unit of measurement used in optics to describe loss or attenuation. Loss is the difference in power between the transmitter and the receiver measured in dB.
The problem was developing a process in glass manufacturing to achieve the 20 dB threshold. Intuitively, researchers postulated that the current, higher optical losses were the result of impurities in the glass and not the glass itself. An optical loss of 20 dB/km was within the capability of the electronics and optoelectronic components of the day.
Intrigued by Drs. Kao and Hockham’s proposal, glass researchers began to work on the problem of purifying glass. In 1970, Drs. Robert Maurer, Donald Keck, and Peter Schultz of Corning Glass Works succeeded in developing a glass fiber that exhibited attenuation at less than 20 dB/km, the threshold for making fiber optics a viable technology. It was the purest glass ever made.
There are two basic types of optical fibers, multimode fibers and single mode fibers. Chapter 2 discusses the differences between the fiber types. In 1972, Corning made a high silica-core multimode optical fiber with 4dB/km minimum loss. Currently, multimode fibers can have losses as low as 0.5 dB/km at wavelengths around 1300 nm. Single mode fibers are available with losses lower than 0.25 dB/km at wavelengths around 1500 nm.
The early work on fiber optic light sources and detectors was slow and often had to borrow technology developed for other reasons. For example, the first fiber optic light sources were derived from visible indicator LED’s. As demand grew, light sources were developed for fiber optics that offered higher switching speed, more appropriate wavelengths, and higher output power.
Fiber optics developed over the years in a series of generations that can be closely tied to wavelength. Figure 1-8 shows three curves. The top, dashed, curve corresponds to early 1980’s fiber, the middle, dotted, curve corresponds to late 1980’s fiber, and the bottom, solid, curve corresponds to modern optical fiber. The earliest fiber optic systems were developed at an operating wavelength of about 850 nm. This wavelength corresponds to the so-called “first window” in a silica-based optical fiber. This window refers to a wavelength region that offers low optical loss. It sits between several large absorption peaks caused primarily by moisture in the fiber and Rayleigh scattering.
The 850 nm region was initially attractive because the technology for light emitters at this wavelength had already been perfected in visible indicator and infrared (IR) LED’s. Low-cost silicon detectors could also be used at the 850 nm wavelength. As the technology progressed, the first window became less attractive because of its relatively high 3 dB/km loss limit.
Most companies jumped to the “second window” at 1310 nm with lower attenuation of about 0.5 dB/km. In late 1977, Nippon Telegraph and Telephone (NTT) developed the “third window” at 1550 nm. It offered the theoretical minimum optical loss for silica-based fibers, about 0.2 dB/km.
Today, 850nm, 1310nm, and 1550nm systems are all manufactured and deployed along with very low-end, short distance, systems using visible wavelengths near 660nm. Each wavelength has its advantage. Longer wavelengths offer higher performance, but always come with higher cost. The shortest link lengths can be handled with wavelengths of 660nm or 850nm. The longest link lengths require 1625nm wavelength systems. This fourth window was developed in 2007.
