John Crisp

What makes the light stay in the fiber

This chapter provides the practical knowledge related to the functioning of light waves in the fiber. Light waves spread out along its beam. At a long distance from the light source, the wavefront remains virtually straight. In a short interval of time, each wavefront moves forward a set distance. There is a widely held view that light always travels at the same speed. This fact is not true. The speed of light depends upon the material through which it is moving. In free space, light travels at its maximum possible speed, close to 300 million meters (almost eight times round the world) in a second. When it passes through a clear material, it slows down by an amount dependent upon a property of the material called its refractive index. Refractive index is a ratio of the speed of light in a material to the speed of light in free space and does not have any units.

LANs and topology

This chapter provides information on the topology and characteristics of local area networks (LANs). As computers have became more compatible, it seems like a good idea to exchange information by a cable connecting two or more systems. This makes it possible to have shared facilities, be it massive storage areas, printers or software. It also allows staff to work on the same project and share the corporate facilities. Sharing data is called networking and is categorized by the physical area that is interconnected. The smallest subdivision is called LAN. A LAN can be small or it could interconnect a whole building or a collection of buildings, or a large manufacturing site or a university with several thousand connections. A LAN uses a privately owned communication system rather than the normal telephone system. A whole city can be interconnected or possibly several LANs can be joined to provide a larger system and, in this case, the network is referred to as a MAN, “Metropolitan Area Network”. Any larger network is called a “Wide Area Network” or WAN.

The transmission of signals

This chapter provides an overview on the transmission of signals along with the consideration of the design and operation of fiber optic systems for the transmission of light. The chapter discusses the analog transmission that is the simple method and used for short range work. This analog transmission is illustrated by a figure in the chapter. The incoming information signal, speech, music, video, etc. is used to control the power output from the light-emitting diodes (LED) or the laser. The light output is, as near as possible, a true copy of the electrical variations at the input, At the far end of the fiber, the receiver converts the light back into an electrical signal that is same as the original electrical signal. Any non-linearity of the characteristics of the transmitter or receiver reduces the accuracy of the electrical/optical (E/O) and optical/electrical (O/E) conversions and give rise to distortion in the output signal. It also outlines the digital transmission of a system in which the information signal is represented by a sequence of on/off levels.

Testing a system

This chapter presents several tests carried out on an optic fiber system to determine the efficiency of a system. One of the tests is visible light continuity test commonly used to test short connecting cables called patchcords, patch cables, or jumper cables. These are short lengths of easily replaceable cable, typically 5–20 meters in length, used to connect between enclosures and instruments. Light source and power meter is used to measure the actual power loss of the fiber system. Mode stripper or mode filter makes the light escape much faster than in a normal fiber so that all the unwanted modes in about 300 mm (12 in) of fiber are lost. The optical time domain reflectometer (OTDR) is connected to one end of any fiber optic system up to 250 km in length. It enables measuring the overall loss, or the loss of any part of a system, the overall length of the fiber, and the distance between any points of interest, within few seconds. Fault locator devices are used to locate faults quickly and easily rather than providing a detailed analysis of a system, and are therefore more likely to be met in a repair environment than at a new installation.

Losses in optic fibers

This chapter highlights the light discontinuity in optic fiber. There are two ways of losing light: either the fiber is not clear enough or the light is being diverted in the wrong direction. Impurities that remain in the fiber after manufacture block some of the light energy. The worst culprits are hydroxyl ions and traces of metals. Hydroxyl ions are actually the form of water that causes the large losses at 1380 nm. In a similar way, metallic traces can cause absorption of energy at their own particular wavelengths. These small absorption peaks are also visible. In both cases, the answer is to ensure that the glass is not contaminated at the time of manufacture and the impurities are reduced as far as possible. The chapter also talks about Rayleigh scatter, which is the scattering of light due to small localized changes in the refractive index of the core and the cladding material. The amount of scatter depends on the size of the discontinuity compared with the wavelength of the light; therefore, the shortest wavelength, or highest frequency, suffers most scattering. A sharp bend in a fiber can cause significant losses as well as the possibility of mechanical failure. It is easy to bend a short length of optic fiber to produce higher losses than a whole kilometer of fiber in normal use.

Connecting optic fibers—the problems

This chapter focuses on the problems involved in connecting optic fibers. A curious feature of these compatibility problems is that they result in the degree of loss being dependent on the direction of travel of the light along the fiber. Multimode fibers come in a wide variety of core sizes between 7 μm and 3 mm, of which the most usual are 50 μm, 62.5 μm, 100 μm, and 200 μm. The industry standard for data communications is now 50/125 and 62.5/125 μm multimode, using silica fiber with a slow but steady growth of single mode. Telecommunications uses exclusively single mode. Similarly, the all-plastic fibers range from 0.25 mm to 3 mm, of which 1 mm is the most common. On connecting a multimode fiber with a large core to one with a smaller core (as shown by a figure in the chapter), only some of the light emitted by the larger core enters the smaller core because of the reduced area of overlap, and a power loss also occurs. If the light travels from the smaller core to the larger, the entire active core is overlapped and no losses occur.

Propagation of light along the fiber

This chapter focuses on the light propagation along the ultra thin optic fiber. A diagram in the chapter suggests that a useful transmission system can be built from a simple length of clear glass. The original glass, called the core, now has a new layer, the cladding, added around the outside during manufacture. The chapter also clears the difference between the optic fiber bending in a circle and the sheet of window glass, is the presence of minute surface scratches. To protect the optic fiber from surface scratches, a layer of soft plastic must be added to the outside of the cladding. This extra layer is called the primary buffer (sometimes called the primary coating or buffer ) and is presented only to provide mechanical protection and has nothing to do with light transmission. The choice of material for the outer layer, called the jacket, depends upon the use to which the cable is to be put.

The choice of frequency

This chapter discusses the frequency parameter of electromagnetic waves for encouraging transmission speed. It gives an account of radio and light waves that belong to the category of electromagnetic waves. The rate at which they alternate in polarity is called their frequency (f) and is measured in Hertz (Hz), where 1 Hz denotes 1 cycle per second. The speed of the electromagnetic wave (v) in free space is approximately 3 × 10 8 ms -1 . The term ms -1 means meters per second and the distance traveled during each cycle, called the wavelength (λ). In the early days of radio transmission when the information transmitted was restricted to the Morse code and speech, low frequencies (long waves) were used. The range of frequencies to be transmitted, called the bandwidth, was very low. As time went by, the need for a wider bandwidth emerged to send more complex information and to improve the speed of transmission. To do this, the high frequency of radio signal is explored. In addition, the fiber optics promises for increasing transmission rates. Infrared light covers a wide range of wavelengths and is generally used for all fiber optic communications. Visible light is used for very short range transmission using a plastic fiber or perhaps for fault testing where leaking visible light can be a diagnostic aid.

Dispersion and our attempts to prevent it

This chapter discuses the spreading effect called dispersion of optic fibers. Dispersion causes the pulses to spread out and they blend together and the information can lost. This degree of dispersion can be made acceptable by decreasing the transmission frequency, thus allowing larger gaps between the pulses. This type of dispersion is called intermodal dispersion. The chapter also approaches the problem of intermodal dispersion in two ways: one could redesign the fiber to encourage the modes to travel at the same speed along the fiber or eliminate all the modes except one. The first strategy is called graded index optic fiber. Chromatic dispersion is the combined effect of two other dispersions: material dispersion and waveguide dispersion. Both result in a change in transmission speed, the first is due to the atomic structure of the material and the second is due to the propagation characteristics of the fiber.

Optic fiber and light—a brilliant combination

This chapter discuses the combination between optic fiber and light. It is a well-known fact that as light travels in straight lines, it is impossible to make it follow a curved path to shine around corners. In Boston, Mass., U.S.A., 1870, an Irish physicist by the name of John Tyndall gave a public demonstration of an experiment that not only disproved this belief but gave birth to a revolution in communications technology. After conducting an experiment, Tyndall found a way to guide light and observed that light stayed inside the water column and followed the curved path. Light can be guided around any complex path as is described in the chapter. A single light source can be used to power many optic fibers. This technique is used in traffic signs to indicate speed limits, lane closures, etc. There are many applications of light guiding and more are being devised every day, few of which are also described in the chapter.