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The Basics of Fiber Optics, Part 2

Aug. 1, 2003
Certain characteristics affect the performance of typical optical fiber. Knowing what it is will help you make more knowledgeable design and purchasing

Certain characteristics affect the performance of typical optical fiber. Knowing what it is will help you make more knowledgeable design and purchasing decisions.

This second part of a three-part article continues our discussion of fiber optics. Last month, we discussed how we use light to send communication signals and why this method works so well. This month, we'll talk about total internal reflection, numerical aperture, chromatic and modal dispersion and the various types of optical fibers in use today.

Total internal reflection. Optical fiber functions well for signal transmission because of the principle of total internal reflection. Here's how this works. When light goes from one material to another of a different density (the index of refraction), the light's path will bend. This is why you can see the bottom of a clear pond at the edge you're standing at, but when you look across the pond, you can see only a reflection of the other side. At a certain angle, light will not pass through the surface, but bounces off.

Optical fiber uses this phenomenon to bend the light at its core/cladding boundary and trap the light in its core. By choosing the material differences between the core and cladding, you can select the angle of light at which total internal reflection occurs.

Numerical aperture. The selected angle defines a primary fiber specification: the numerical aperture (NA) of a fiber. NA designates the angle (called the angle of acceptance). This is the angle beyond which the light rays injected into an optical fiber are no longer guided. Instead, they will pass through the core/clad boundary and be lost.

Fibers with higher NAs will accept a wider range of light paths. (The technical term for paths is modes.) And because there are so many modes, the signal distorts. So, a fiber with a higher NA will have increased signal distortion and be able to carry less signal. We can then say high NA fibers have less bandwidth while low NA fibers have greater bandwidth.

Index of refraction (IR). The IR of a material is the ratio of the speed of light in vacuum to that in the material. In other words, the IR is a measure of how much light slows down after it enters the material. Since light has its highest speed in a vacuum (approximately 300,000 kilometers per second), and slows down whenever it enters any medium (water, plastic, glass, crystal, oil, etc.), the IR of any material is greater than one.

For example, the IR of a vacuum is 1.0 while the IR of glass and plastic optical fibers is about 1.5. Water has an IR of about 1.3. Light signals moving through an optical fiber travel at much less than the speed of light. The "speed of light" is its speed in a vacuum, not in all materials.

Pulse spreading. Signal distortion comes from two primary causes:

?The colors of light through the fiber (chromatic dispersion) and ?The path the light takes as it moves through the fiber (modal dispersion). Both of these reasons have the same final effect: distortion of the signal by pulse spreading. You can see this in Fig. 1 on this page.

Notice the digital signal input into the fiber is square. As the signal travels down the fiber, it distorts and begins to spread. Pulse spreading is not a loss of light. In fact, as much light is leaving the fiber as entering it. However, the light signals distort. If the pulses spread too much, they will be unintelligible to the receiver. Let's see why both color and path cause this pulse spreading.

Chromatic dispersion. This phenomenon (pulse spreading due to the colors of light sent through the fiber) occurs because of one fact: Different colors of light (which we also call different wavelengths) travel at different speeds in an optical fiber.

For example, if you send two different wavelengths (colors) of light into a long fiber at the same time, one will reach the far end before the other. Here, it's obvious the time difference between the wavelengths arriving at the end of a long fiber would spread a data pulse. This causes the pulse spreading shown in Fig. 1.

Because of chromatic dispersion, you should use light sources that put out only one color of light. Many of the newer lasers do this well. And even though these lasers are more expensive than LED light sources, they often cause far less chromatic dispersion. Most use LED-type sources only for shorter runs, where higher chromatic dispersion is more manageable.

Good laser sources have a narrow spectral bandwidth, putting out light within a 1-nanometer (nm) range. (A nanometer is .000000001 meter.) So, the light output from a 1550nm laser will be within a range of 1549.5nm and 1550.5nm.

LED sources have a broad spectral bandwidth. Many LEDs have a spectral bandwidth of 20nm. So, the light output from an 850 nm LED would be between 840nm and 860nm.

Modal dispersion. This phenomenon (pulse spreading due to the paths of light sent through the fiber), occurs because some paths through a fiber are more direct than others. You will see this in Fig. 2. Here, one of the light rays goes right down the middle of the fiber while others enter the fiber at angles and bounce from side to side.

You can see how the light ray going down the middle of the fiber has a significantly shorter path and will reach the far end considerably sooner than the other ray of light. This causes a data pulse to spread. Modal dispersion is a major factor in determining the design of optical networks-even in the design of fibers.

Optical fibers. Let's discuss the three main types of optical fibers the datacom industry uses today. Fig. 3 on this page shows all three types.

*Single-mode. This type of fiber allows only one ray to transmit down the core. The core is small, usually between eight and nine microns. Because of quantum mechanical effects, the light traveling in the narrow core stays together in packets, rather than bouncing around the core. Thus, single-mode fiber has an advantage: It can handle more signal over far greater distances.

*Multimode, graded-index. This type of fiber contains many layers of glass, each with a lower IR as you move outward from the fiber's center. Since light travels faster in the glass with lower indexes of refraction, the light waves refracted to the outside of the fiber speed up to match those traveling in the center. The result: This type of fiber allows for high-speed data transmission over a long distance.

Most use multimode fibers with LED light sources, because they are less expensive than the laser-light sources, which most use for single-mode fiber. Graded-index fibers come in core diameters of 50, 62.5, 85, and 100 microns. (One micron equals one millionth of a meter. For comparison purposes, a sheet of paper has a thickness of approximately 25 microns.)

*Multimode, step-index. Most people use this type of fiber less than others, because it has a lower capacity. It has a wide core (like the multimode, graded-index fiber). But since it's not graded, the light bounces wildly through the fiber. This results in high levels of modal dispersion (pulse spreading due to path losses).

Fiber sizing. The size of the optical fiber is the outer diameter of its core and cladding. For example, a size given as "62.5/125" indicates a fiber having a core of 62.5 microns and a cladding of 125 microns. We don't typically mention the coating in the size, because it has no effect on the light-carrying characteristics of the fiber.

The core is the part of the fiber that actually carries the light pulses for transferring data. It consists of plastic or glass. The core sizes of joined fibers must match. Larger cores have greater light-carrying capacity than smaller cores, but may cause greater signal distortion.

The cladding sets a boundary around the fiber, so light running into this boundary reflects back into the cable. This keeps the light moving down the cable, keeping it from escaping. Claddings can be glass or plastic, and they always have a different density than that of the core.

Coatings are multiple layers of ultraviolet curable acrylate plastic. This adds strength to the fiber, protects it, and absorbs shock. These coatings come between 25 microns and 100 microns in thickness. You can strip coatings from the fiber (which you must do for terminating) either mechanically or chemically, depending on the type of plastic.

Manufacturing fiber. Manufacturing optical fiber is a difficult and complex process. In general, the process entails three parts:

1. The manufacture of a preform. This is a cylinder of glass from which the optical fiber will be made. It's generally about 3 ft long and 1 in. wide. The preform has a physical makeup identical to the final fiber (including both core and cladding), except it's much wider and shorter. (See Fig. 4 on page 41.)

2. Pulling the fiber. The manufacturer heats the preform very precisely to pull a thin strand of glass off of one end. Variances in heating and pulling tension control the diameter of this strand. This is the optical fiber, which contains both the core and cladding.

3. Cooling, coating and winding. Once the fiber is pulled off of the preform, it's carefully and slowly cooled, covered with the final coating and wound onto reels.

After the fiber is pulled, the manufacturer applies a protective coating just after the formation of the hair-thin fiber. The coating provides protection and prevents the ingress of water into any fiber surface cracks. The coating consists of a soft inner and harder outer coating. The thickness of the coating varies between 62.5 micrometers and 187.5 micrometers.

You can strip these coatings by mechanical means. You must remove them before splicing fibers or fitting them with connectors.

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