The Basics of Fiber Optics — Part 1

May 1, 1999 12:00 PM, By Paul Rosenberg, Datacom Consulting Editor

Learning fiber optics is much easier than learning the laws of electricity. The only problem is it's relatively new to electricians and technicians entering the datacom industry. Here's an overview of how we use light to send communication signals, and why this method works so well.

You've probably heard the terms "light tube" and "conduit for light" in reference to an optical fiber. Although these terms might lead you to believe there's some type of hole in the middle of an optical fiber, this is not the case. To better explain its operation, let's look at a typical optical fiber cross section (Fig. 1, original article).

Here, you can see the three concentric layers in an optical fiber. Light pulses through the glass core of the fiber only. The cladding (which is a different type of glass) serves as a barrier to keep the light within the core; functioning much like a mirrored surface. The coating has nothing to do with light transmission and is only for mechanical strength and protection.

As you can see, light flows down the center of a fiber like water through a pipe. We could even say the fiber is a "virtual" tube. Light stays in the center of the fiber, not because of a physical opening, but because the cladding glass reflects any escaping light back to the core.

These "light tubes" are very thin strands of ultra-pure glass. The dimensions of typical fiber components are:

• Core: 8 microns to 62.5 microns (a micron is one millionth of a meter)
• Cladding: 125 microns
• Coating: 250 microns

The core is one density of glass, the cladding is a second grade of glass, and the coating is a plastic.

Cabling. To suitably protect our glass fibers, we package them in cabling. Many people believe if you drop a fiber cable on a hard surface, it will shatter. After all, it's made of glass! This is a misconception. The fiber itself (not the cable, but only the thin fiber) needs to be protected, although it's surprisingly flexible. (It's several times as strong as steel; but because it's so thin, it breaks easily.)

However, fiber cables are not at all fragile. In fact, they're often more durable than copper communication cables. Optical cables encase the glass fibers in several layers of protection (shown in Fig. 2, original article). The first protective layer is the coating we mentioned earlier. The next layer is a buffer layer, which extrudes over the coating to further increase the strength of the single fibers. This buffer can be of either a loose-tube or tight-tube design. Note: Most datacom cables have one of these two constructions; however, a third type, ribbon cable, is frequently used for telecommunications transmissions. Ribbon cable uses a modified type of tight buffering.

After the buffer layer, the cable contains a strength member. Most commonly, it's a Kevlar(r) fabric, the material found in bulletproof vests. The strength member not only protects the fiber, but it also carries the tensions of pulling the cable. (You should never pull fiber cable by the fibers themselves.)

After the strength member, you can see the outer jacket of the cable, which is typically some type of polyethylene or PVC. In many cases there will be additional stiffening members, which increase the cable's strength and durability.

Important field-installed components. As with copper cabling, you have field-installed components to make a working datacom system.

Connectors. You use fiber connectors to make nonpermanent connections at fiber ends. Since they have such a small diameter, you must hold optical fibers rigidly in place and accurately align them to mate with other fibers, light sources, or light detectors.

Advances in design and technology make connector installation easy today. However, this wasn't always so.

As the fiber-optic field began to develop, one of the biggest mechanical problems was how to permanently fix the fibers at their ends. The first fiber connectors were difficult to install. They used a variety of glues, ovens, and long, difficult polishing methods. However, since then, things have drastically changed.

While terminating a fiber is not yet as easy as installing a coaxial cable connector, it's much simpler than before. You can terminate a fiber in about half the previous time, and the process continues to get easier with time. In a few years, it should be quite simple.

Splices. You use splices to permanently join the ends of fibers. There are two primary ways to do this: by fusion (the melting of pieces of glass together) or mechanical means. When installing a splice, you must address two critical factors:

• The fiber joint must be able to pass light without loss, and
• The joint must be mechanically secure so it cannot easily break.

Testing. When installing a fiber system (we often call the whole system of optical fibers a cable plant), you must test it to verify performance. You are basically making sure light will pass through the system properly. There are three types of optical testing: continuity, power, and Optical Time Domain Reflectometer (OTDR) testing.

Continuity testing. This is a simple visible light test. Its purpose is to make sure the fibers in your cables are continuous (unbroken). You do this with a modified type of flashlight device and the naked eye. This takes only a few minutes.

Power testing. This accurately measures the quality of optical fiber links. A calibrated light source puts infrared light into one end of the fiber, and a calibrated meter measures the light arriving at the other end. You measure the loss of light in decibels.

OTDR testing. This testing uses a piece of equipment called an OTDR. This device uses light backscattering to analyze fibers. The OTDR takes a snapshot of the fiber's optical characteristics by sending a high-powered pulse into the fiber and measuring the light scattered back toward the instrument.

You can use the OTDR to locate fiber breaks, splices, and connectors as well as to measure loss. However, due to the different methods of measuring loss, this method may not give the same value for loss as a source and power meter. The OTDR gives a graphic display of the status of the fiber under test. Another advantage is it requires access to only one end of the fiber.

The OTDR is not always necessary and is also quite expensive, so many installers prefer to rent one.

Technical concepts. In addition to the things we've covered so far, you must understand several technical concepts.

Attenuation. This performance characteristic is the measure of weakening of an optical signal as it passes through a fiber. In other words, it's a measure of signal loss. Attenuation in an optical fiber is a result of two factors: absorption and scattering.

Absorption is just that: absorption of light and its conversion to heat by molecules in the glass. Primary absorbers include residual deposits of chemicals used in the manufacturing process to modify the characteristics of the glass. This absorption occurs at definite wavelengths. (Remember, the wavelength of light signifies its color and place in the electromagnetic spectrum.) It's determined by the elements in the glass and is most pronounced at wavelengths around 1000 nanometers (nm), 1400 nm, and above 1600 nm.

Scattering is the largest cause of attenuation. It occurs when light collides with individual atoms in the glass, which knocks it off its original course. Fiber-optic systems transmit in the "windows" created between the absorption bands at 850 nm, 1300 nm, and 1550 nm wavelengths, for which lasers and detectors can be easily made.

Networks. To communicate between several pieces of equipment (for example, between 20 different computers in an office), you must connect them. To do this, you must:

• Develop a logical method of connection. (Should they all be tied to a central point or connected in a loop?)
• Provide a definite protocol for communicating. (If the machines don't "talk" to each other in some type of order, the whole system will collapse in a jumble of signals they can neither separate nor interpret.

There are many types of networks, each with their own strengths and weaknesses. In fact, you've probably heard of these: Ethernet, 10base T, FDDI, ATM, and Token Ring. These are simply different methods of connecting computers.

Bandwidth. This is the range of signal frequencies or bit rate at which a fiber system can operate. It's a measure of the amount of signal able to be put through a fiber. Higher bandwidth means more data per second; lower bandwidth means less signal.

Dispersion. Two potentially confusing terms you'll come across in your readings are chromatic dispersion and modal dispersion. In both of these terms, "dispersion" refers to the spreading of light pulses until they overlap. This distorts and causes the loss of the data signal.

Chromatic refers to color. Modal refers to the light's path. Thus, we can state in simple terms that chromatic dispersion is signal distortion due to color, while modal dispersion is signal distortion due to path.

Note: Dispersion is not a loss of light; it's a distortion of the signal. Thus, dispersion and attenuation are two very different and unrelated problems: Attenuation is a loss of light; dispersion is a distortion of the light signals.

Sidebar: Why Use Optical Fiber

We use optical fiber to transmit all types of data and communications signals over all distances. Since the amount of data and communications signals used for business and personal use has grown exponentially in recent years, sending high amounts of signal has become necessary for modern life.

For example, in 1987 an average home used about 3000 Hz worth of bandwidth (about one telephone line). Now, the same home might use two 56 KHz modems transferring data; 14,400 baud for fax transmissions; several hundred MHz of cable television; and 3000 Hz of voice; all at the same time. This situation is often more dramatic for businesses. As you can see, our need for more signal transmission capacity is urgent.

The copper wire communications networks simply cannot keep up. Yes, copper is reliable and served us well for decades, but it cannot operate well at high signal transfer rates. You need special systems copper wiring for data transfer rates of above 50 million bits per second (Mb/s). Above 150 Mb/s, even the best copper wiring is questionable.

On the other hand, optical fiber can handle transmission rates many times that high. At the time of this writing, systems operating at 40 billion bits per second (gigabits per second, or Gb/s) over a single fiber are common, and we are not sure how high our bit rates will be able to go. We are certainly nowhere near the limit yet.

Another advantage to optical fiber is its electrical immunity. Because optical fiber cable is nonmetallic, it cannot emit or pick up electromagnetic interference (EMI) or radio frequency interference (RFI); each of which is a problem with metallic conductors. Furthermore, cross talk between FO cables does not exist. Additionally, FO cabling has no grounding or shorting problems. This is also an important feature when installing communications wiring in hazardous environments. Finally, optical fiber cabling causes no sparking or excessive heat, even when broken.

The security of optical fiber is also far superior to that of copper wire. Electronic bugging depends on electromagnetic monitoring. Because optical fibers carry light rather than electricity, they are immune to bugging. To plant a bug on an optical fiber cable, you must physically tap the cables. However, this is easily detectable because the signal diminishes, and error rates increase.

Currently, the industry uses optical fiber for all long-distance telephone traffic and increasingly for local telephone circuits. Other communications systems, from cable television to computer networks, are also changing to optical fiber. Eventually, almost every communication signal will be sent over optical fiber.

Sidebar: What's the Nature of Light

Light is a series of electromagnetic waves, with a few properties of particles thrown in. (We don't have to worry about the particle properties of light in our applications; that's for physicists to ponder.)

The first characteristic of light is color. Some light is red, some blue, etc. After years of research, scientists discovered each color of light has its own wavelength; or in other words, its own frequency (measured in cycles per second, or Hertz). For instance, waves of red light have a lower frequency than blue light, and green is somewhere in between.

Our eyes perceive different frequencies of light waves like radios tune into different frequencies of radio waves. Rather than specifying light by its frequency, we signify the color by the wavelength of the light.

So, a certain wavelength of light means a certain color of light. We measure wavelengths in nanometers (nm), which are billionths of a meter. For example, a deep red light would have a wavelength of about 600 nm.

Wavelength and frequency really signify the same thing, and you can interchange them by using a mathematical equation. We specify wavelength rather than frequency, since the former is generally easier to use and found in most scientific literature.

There is also such a thing as invisible light; but this light is not actually invisible. It has a wavelength beyond a person's visual range. It's like a radio that can pick up the AM and FM bands but not the shortwave bands. The shortwave signals are out there waiting, but your receiver can't tune in to them.

A good example of invisible light is infrared light, which is the type of light our television remote control uses. Invisible light is what we use in optical fibers to transmit signals. It would be handy to use visible light, but it simply doesn't work well. Wavelengths of 850 nm, 1300 nm, and 1550 nm seem to be the best. These wavelengths are outside of our visual spectrum, in the infrared range.