Fundamentals of Fiber Optics Quiz

Hint

Fiber optics is the branch of optical technology concerned with the transmission of radiant power (light energy) through fibers.

The difference between conventional electronic systems and fiber optic systems is how the data is sent. Fiber optics transmits (photons) light through glass fibers. Electronic systems send electrons through wire. Radio-frequency and microwave communication (including satellite links) rely on radio waves and microwaves traveling through open space or air.

Hint

A fiber optic data link consists of four parts—transmitter, optical fiber, connectors/splices, and receiver. Figure 1-1 is an illustration of a fiber optic data-link connection. The transmitter, optical fiber, and receiver perform the basic functions of the fiber optic data link. Each part of the data link is responsible for the successful transfer of the data signal. A fiber optic data link needs a transmitter that can effectively convert an electrical input signal to an optical signal and launch the data-containing light down the optical fiber. A fiber optic data link also needs a receiver that can effectively transform this optical signal back into its original form. This means that the electrical signal provided as data output should exactly match the electrical signal provided as data input.

Hint

Light Emitting Diodes (LED)

LEDs are relatively restricted in their range of possible applications because of their relatively low data rate and power levels. LEDs are utilized in Local Area Networks (LANS) where transmissions of less than two kilometers are required with data rates usually no more than 680Mbps/km. They are also used for control signals such as opening and closing valves and vent dampers using programmable logic controllers. Their expected operating life usually exceeds 100,000 hours or about ten years. They are simple in design, require only a few components to power, drive and monitor the device and because of their low bias voltage no cooling circuits are needed. The output power of the typical LED ranges from -15dBm to -20dBm. They operate at wavelengths of 850nm and 1300nm.

Hint

Vertical Cavity Surface Emitting Laser (VCSEL)

The VCSEL is a short range high data rate transmitter for fiber optic data links. A VCSEL because of the increased bandwidth and mode field diameter requires a 50 micron multimode laser optimized fiber as its transmission medium. The most common emission wavelengths of VCSELs are in the range of 750–980nm (often around 850nm). Data rates with VCSELs of 10Gbps can be reached over a distance of a few hundred meters.

Hint

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.

Hint

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.

Hint

Fiber optics deals with the transmission of light energy through transparent fibers. How an optical fiber guides light depends on the nature of the light and the structure of the optical fiber. A light wave is a form of energy that is moved by wave motion. Wave motion can be defined as a recurring disturbance advancing through space with or without the use of a physical medium. In fiber optics, wave motion is the movement of light energy through an optical fiber. Before we introduce the subject of light transmission through optical fibers, you must first understand the nature of light and the properties of light waves.

Hint

If the surface of the medium contacted by the incident wave is smooth and polished, each reflected wave will be reflected back at the same angle as the incident wave. The path of the wave reflected from the surface forms an angle equal to the one formed by its path in reaching the medium. This conforms to the law of reflection which states: The angle of incidence is equal to the angle of reflection.

Hint

The electromagnetic energy of light is a form of electromagnetic radiation. Light and similar forms of radiation are made up of moving electric and magnetic forces. A simple example of motion similar to these radiation waves can be made by dropping a pebble into a pool of water, see figure 2-1. In this example, the water is not actually being moved by the outward motion of the wave, but rather by the up-and-down motion of the water. The up-and-down motion is transverse, or at right angles, to the outward motion of the waves. This type of wave motion is called transverse-wave motion. The transverse waves spread out in expanding circles until they reach the edge of the pool, in much the same manner as the transverse waves of light spread from the sun. However, the waves in the pool are very slow and clumsy in comparison with light, which travels approximately 186,000 miles per second.

Hint

The transmission of light along optical fibers depends not only on the nature of light, but also on the structure of the optical fiber. Two methods are used to describe how light is transmitted along the optical fiber. The first method, ray theory, uses the concepts of light reflection and refraction. The second method, mode theory, treats light as electromagnetic waves. You must first understand the basic optical properties of the materials used to make optical fibers. These properties affect how light is transmitted through the fiber.

Hint

Hint

Bending the fiber also causes attenuation. Bending loss is classified according to the bend radius of curvature: microbend loss or macrobend loss. Microbends are small microscopic bends of the fiber axis that occur mainly when a fiber is cabled. Macrobends are bends having a large radius of curvature relative to the fiber diameter. Microbend and macrobend losses are very important loss mechanisms. Fiber loss caused by microbending can still occur even if the fiber is cabled correctly. During installation, if fibers are bent too sharply, macrobend losses will occur.

Hint

Chromatic Dispersion (CD) is the term given to the phenomenon by which different spectral components of a light pulse travel at different speeds. CD arises for two reasons. The first reason is that the refractive index of silica is frequency dependent. Thus different frequency components travel at different speeds in silica. This component of CD is called material Dispersion. The second reason is that although material dispersion is the principle component of chromatic dispersion for most fibers, there is a second component called Waveguide Dispersion.

Hint

The acceptance angle (-a) is the maximum angle to the axis of the fiber that light entering the fiber is propagated. The value of the angle of acceptance (-a) depends on fiber properties and transmission conditions.

Hint

The mode theory, along with the ray theory, is used to describe the propagation of light along an optical fiber. The mode theory is used to describe the properties of light that ray theory is unable to explain. The mode theory uses electromagnetic wave behavior to describe the propagation of light along a fiber. A set of guided electromagnetic waves is called the modes of the fiber.