How does a fiber optic cable work?

The development of information technologies has continued since the existence of humanity, facilitated the transfer of information, and enabled the fastest, most efficient, and most accurate way of information exchange between people. In the first experiments in the 1800s, it was not predicted that fiber optic technology would become as common as it is today. In the past, fiber optic was not a welcomed solution due to high costs and difficulties in supplying materials. Today, these problems are not experienced and fiber optic technology is developing rapidly. Today, “fiber optic cables” and “optical systems” are used to transmit light. Optical fibers show us that large amounts of information can be transmitted along with a single fiber at the speed of light. When you learn the working principle of fiber optic cable, you will better understand how it transmits.

Fiber optic cable construction

Fiber-optic cables are composed of three sections: the core, the cladding, and the sheath.

The core is composed of either glass or plastic. Glass has a higher bit rate of transmission or bandwidth than plastic, and it has less line loss than plastic. Glass fibers are also able to withstand higher temperatures and are less affected by corrosive atmospheres and environments. Plastic core fibers are more flexible and can be bent to a tighter radius than glass. Plastic fibers are stronger and can be cut, spliced, and terminated with less difficulty than glass.

The cladding or clad surrounds the core and is made of glass or plastic also. The clad serves two basic functions. It protects the core from the surrounding environment, and it increases the size and strength of the cable itself. Increasing the size of the cable makes it easier to handle. The core and cladding are considered fiber optic.

The sheath is a polyurethane jacket that surrounds the cable. The sheath protects the fiber optics from the environment. Fiber-optic cables may be packaged as a single fiber, fiber pairs, or several thousand fibers.

How does a fiber optic cable work?

Light travels in a straight line. Fiber-optic cable, however, makes it possible to bend light around corners and conduct it to any desired location.

The reason that light can travel through an optical fiber is because of refraction. Imagine that you are standing on the shore of a clear mountain lake on a calm, windless day. If you looked out over the surface of the lake, you would probably see the sun, clouds, and trees reflected on the surface of the water. If you looked directly at the water at your feet, you would no longer see reflections of the clouds or trees, but you would see down into the water. This is an example of refraction instead of reflection. The angle at which you stopped seeing the reflection of clouds and trees and started seeing down into the water is called the critical angle or acceptance angle. The critical angle occurs because air and water have different optical properties. The optical property is the speed at which light can travel through a material. Optical property is generally expressed as a term called index of refraction (IR), or n. The index of refraction is the speed at which light travels through a material. It is determined by comparing the speed of light traveling through the vacuum to the speed of light traveling through a particular material.

In glass optical fibers, the index of refraction is approximately 1.46 to 1.51.

When an optical fiber cable is connected to a light source, light strikes the cable at many different angles.

Some photons strike at an angle that cannot be refracted and are lost through the cladding and absorbed by the sheath. Photons that can be refracted bounce down the core to the receiving device. This bouncing action of the photons causes a condition known as modal dispersion. Because photons enter the cable at different angles, some bounce more times than others before they reach the end of the cable, causing them to arrive later than photons that bounce fewer times. This causes a variance in the phase of the light reaching the source. Modal dispersion can be greatly reduced by using fiber cables called single-mode cables. Single-mode cables have a diameter of 1 to 2 micrometers. The cladding also affects modal dispersion. If the cladding thickness is kept to within three times the wavelength of the light, modal dispersion is eliminated.

Another type of cable that is much larger than single-mode cable is multimode cable. Multimode cable ranges in thickness from about 5 to 1000 micrometers. Multimode cable can cause severe modal dispersion in long lengths of several thousand feet. For short runs, however, it is generally preferred because it is larger in size and easier to work with than a single-mode cable. Multimode cable is also less expensive than single-mode cable, and for short runs the modal dispersion generally is negligible.

Another type of multimode cable called graded cable has a core made of concentric rings. The rings on the outside have a lower density than that of the rings beneath. This produces a sharper angle of refraction for the outer rings. This arrangement helps to eliminate modal dispersion.

Fiber optic cable losses

Fiber-optic cables do suffer some losses or attenuation. No fiber-optic cable is perfect, and some amount of light does escape through the cladding and is absorbed by the sheath. The greatest losses generally occur when the cable is terminated or spliced. The ends of the fiber-optic cable must be clean and free of scratches, nicks, or uneven strands. It is generally recommended that the ends of fiber-optic cables be polished when they are terminated. A special hot knife cutting tool is available for cutting fiber-optic cable.

Transmitters

Several factors should be considered in selecting a transmitter or light source for a fiber-optic system. One is the wavelength of the light source. Many fiber-optic cables specify a range of wavelengths for best performance. The wavelength can be measured by the color of the emitted light.

Another consideration is the spectral width. The spectral width is a measure of the range of colors that are emitted by the light source. The spectral width affects the color distortion that occurs in the optic fiber.

The numerical aperture (NA) is a measure of the angle at which light is emitted from the source. If the NA of the source is too wide, it can overfill the NA of the optical fiber. If the NA of the source is too small, it will underfill the fiber. A low-NA light source helps reduce losses in both the optical fiber and at connection points. Transmitter light sources are generally light-emitting diode (LED) or laser.

LEDs are relatively inexpensive, operate with low power, and have a wide spectral width. They are generally used for short distances of about 7 kilometers or 4.3 miles. LEDs have relatively low bandwidths of about 200 MHz or less. They can be used for data bit transmission rates of about 200 MBPS or less. LEDs have wavelengths that range from about 850 to 1300 nanometers.

Laser diodes are expensive, require a large amount of operating power, and have a narrow spectral width. They can be used for extremely long-distance transmission and can handle very high rates of data transmission. Laser diodes are generally used for telephone and cable television applications. Laser diodes operate at a wavelength of about 1300 nanometers.

Receivers

Receivers convert the light input signal into an electrical signal that can be used by the programmable controller or other devices. Receiver units generally consist of a photodiode.

Photodiodes are preferred over other types of photodetection devices because of their speed of operation.

Transceivers

Transceivers house both a transmitter and receiver in the same package. Transceivers are often used as photodetection devices. Assume that half of the fiber-optic fibers in a cable are connected to the transmitter, and the other half are connected to the receiver. If a shiny object, such as a can on an assembly line, should pass in front of the cable, the light supplied by the transmitter would be reflected off the can back to the receiver.

The output of the receiver could be connected to the input of a programmable controller that causes a counter to step each time a can is detected. Another device that contains both a transmitter and receiver is called a repeater. A repeater is used to boost the signal when the fiber-optic cable is run long distances. Repeaters not only amplify the signal but also can reshape digital signals back to their original form. This ability of the repeater to reshape a digital signal back to its original form is one of the great advantages of digital-type signals over analog. The repeater “knows” what the original digital signal looked like, but it does not “know” what an original analog signal looked like. A great disadvantage of analog-type signals is that any distortion of the original signal or noise is amplified.