Fiber Optics

Optical fiber is a very thin strand of glass or plastic capable of transmitting light from one point to another. Optical fiber can also be called an optical waveguide, since it is a device that guides light.

Optical fibers consist of a light-carrying core and a cladding surrounding the core. There are generally three types of construction: glass core/cladding, glass core with plastic cladding, or all-plastic fiber. Optical fibers typically have an additional outside coating which surrounds and protects the fiber (see Figure 1).

Commonly available glass fiber diameters range from 8 micron core/125 micron cladding to 100 micron core/140 microns cladding, whereas plastic fibers range from 240 micron core/250 micron cladding to 980 micron core/1,000 micron cladding. The human hair, by comparison, is roughly 100 microns in diameter.

Fiber optic communications

Why is the propagation of pulses of light through optical fibers important? Voice, video, and data signals can be encoded into light pulses and sent across an optical fiber. Each time someone makes a phone call, a stream of pulses passes through an optical fiber, carrying the information to the person on the other end of the phone line.

A fiber optic communication system generally consists of five elements: the encoder or modulator, the transmitter, the fiber, the detector, and the demodulator (see Figure 8).

Electrical input is first coded into a signal by the modulator, using signal processing techniques. The transmitter converts this electrical signal to an optical signal and launches it into the fiber. The signal experiences attenuation as it travels through the fiber, but it is amplified periodically by repeaters. At the destination, the detector receives the signal, converting it back to an Figure 6. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 7. Illustration by Hans & Cassidy. Courtesy of Gale Group.
electrical signal. It is sent to the demodulator, which decodes it to obtain the original signal. Finally, the output is sent to the computer or to the handset of your telephone, where electrical signals cause the speaker to vibrate, sending audio waves to your ear.

Advantages of fiber optic cable

Communication via optical fiber has a number of advantages over copper wire. Wires carrying electrical current are prone to crosstalk, or signal mixing between adjacent wires. In addition, copper wiring can generate sparks, or can overload and grow hot, causing a fire hazard. Because of the electromagnetic properties of current carrying wires, signals being carried by the wire can be decoded undetectably, compromising communications security. Optical fiber carries light, no electricity, and so is not subject to any of these problems.

The biggest single advantage that optical fiber offers over copper wire is that of capacity, or bandwidth. With the rising popularity of the Internet, the demand for bandwidth has grown exponentially. Using a technique called wavelength division multiplexing (WDM), optical networks can carry thousands of times as much data as copper-based networks.

Most copper networks incorporate a technique known as time division multiplexing (TDM), in which the system interleaves multiple conversations, sending bits of each down the line serially. For example, the system transmits a few milliseconds of one conversation, then a few milliseconds of the next, a few milliseconds of a the next, then returns to transmit more of the first conversation, and so on. For many years, network designers increased carrying capacity by developing electronics to transmit shorter, more closely spaced data pulses.

Electronics can operate so quickly, however, and eventually copper wire hit a maximum carrying capacity. To increase bandwidth, network operators had to either lay more copper cable in already packed underground conduits, or seek another method. Enter fiber optics.

Figure 8. Illustration by Hans & Cassidy. Courtesy of Gale Group.

The electrons in copper wire can only carry one stream of time-division multiplexed data at a time. Optical fiber, on the other hand, can transmit light at many wavelengths simultaneously, without interference between the different optical signals. Fiber optic networks can thus carry multiple data streams over the same strand of optical fiber, in a technique known as wavelength division multiplexing. A good analogy is a that of a ten-lane expressway compared to a one-lane county road.

Wavelength division multiplexing is an incredibly powerful technique for increasing network capacity. Transmitting data over two wavelengths of light instantly doubles the capacity of the network without any additional optical fiber being added. Transmitting over sixteen wavelengths of light increases the capacity by sixteen times. Commercially deployed WDM systems feature 64 wavelengths, or channels, spaced less than 1 nanometer (nm) apart spectrally. Researchers have built WDM networks that operate over hundreds of channels, sending the equivalent of the amount of data in the Library of Congress across the network in a single second.

Attenuation, dispersion, and optimal communications wavelengths

As mentioned previously, signals carried by optical fiber eventually lose strength, though the loss of attenuation is nowhere near as high as that for copper wire. Singlemode fiber does not incur as much attenuation as multimode fiber. Indeed, signals in high quality fiber can be sent for more than 18.6 mi (30 km) before losing strength. This loss of signal strength is compensated for by installing periodic repeaters on the fiber that receive, amplify, and retransmit the signal. Attenuation is minimized at 1,550 nm, the primary operating wavelength for telecommunications.

Signals in optical fiber also undergo dispersion. One mechanism for this is the modal dispersion already discussed. A second type of dispersion is material dispersion, where different wavelengths of light travel through the fiber at slightly different speeds. Sources used for fiber optics are centered about a primary wavelength, but even with lasers, there is some small amount of variation. At wavelengths around 800 nm, the longer wavelengths travel down the fiber more quickly than the shorter ones. At wavelengths around 1,500 nm, the shorter wavelengths are faster. The zero crossing occurs around 1,310 nm: shorter wavelengths travel at about the same speed as the longer ones, resulting in zero material dispersion. A pulse at 1,310 nm sent through an optical fiber would arrive at its destination looking very much like it did initially. Thus, 1,310 nm is an important wavelength for communications.

A third kind of dispersion is wavelength dispersion, occurring primarily in single-mode fiber. A significant amount of the light launched into the fiber is leaked into the cladding. This amount is wavelength dependent and also influences the speed of propagation. High volume communications lines have carefully timed spacings between individual signals. Signal speed variation could wreak havoc with data transmission. Imagine your telephone call mixing with someone else's! Fortunately, wavelength dispersion can be minimized by careful design of refractive index.

Based on dispersion and attenuation considerations, then, the optimal wavelengths for fiber-optic communications are 1,300 and 1,550 nm. Despite the dispersion advantages of operating at 1,310 nm, most modern fiber optic networks operate around 1,550 nm. This wavelength band is particularly important to the WDM networks that dominate the major cross-country fiber optic links because the erbium-doped fiber amplifiers (EDFAs) incorporated in the repeaters provide signal amplification only across a range of wavelengths around 1,550 nm. Thus, most modern fiber optic networks operate around the so-called EDFA window. These signals are in the infrared region of the spectrum, that is, these wavelengths are not visible. Diode lasers are excellent sources at these wavelengths.

Telecommunications companies have developed singlemode optical fiber that addresses the problem of dispersion. Dispersion-shifted fiber is designed so that the region of maximum dispersion falls outside of the socalled telecommunications window. Although dispersion-shifted fiber is sufficient for basic transmission, in the case of WDM systems with tightly spaced channels, the fiber triggers nonlinear effects between channels that degrades signal integrity. In response, fiber manufacturers have developed non-zero dispersion-shifted fiber that eliminates this problem.