Fiber Optics - Literature review Example

Fiber Optics A fiber optic cable is one of the mediums of transmitting telecommunication signals over long distances, particularly between continents. It is the best medium, especially for applications that require high bandwidth and immunity from electric fields. It can also link longer distances compared to copper. The cable works by transmitting signals in the form of modulated light pulses, which are fed at one end and relayed via thin glass connected to a receiver in the opposite end. The receiver changes the light signals back to electric signals. An optical fiber consists of a light-carrying core made of highly purified silica and germanium. The core is surrounded by an optic cladding that consists of pure silica. The cladding is covered with a buffer material, which protects the delicate inner components from damage. Because of the low tensile strength of the inner components, the fiber is reinforced with strength member, a layer typically made of aramid. The entire optical cable is then covered with an outer jacket, which provides protection against abrasion or other environmental damages (Ticker 77-78). Since it is made of glass, a fiber optic cable does not withstand considerable bending and longitudinal stress. Thus, it needs extreme care while handling. Manufacturers usually use special techniques to improve flexibility. Several fibers are placed together and encircled with a metal sheath. A strong central strength member also improves flexibility (Ticker 78). An optical fiber utilizes the principle of total internal reflection. Total internal reflection takes place when light travels from a medium with high optical density to another medium with a lower optical density (Brown 5). It only occurs at specific angles of incidence of the light. If the angle of incidence exceeds the critical angle of the medium, total internal reflection occurs at the interface of the two media (Halliday, Resnick, & Walker 823). The cladding and core of an optical fiber are made of different materials with different indexes of refraction. The core itself, which comprises highly purified silica and germanium, has a high index of refraction compared to the cladding (Fidanboylu & Efendiogu 2). Thus, a ray traveling from the core towards the cladding can be refracted or reflected internally depending on its angle of incidence. The angles of incidence of the rays entering the core are set in such a way that they exceed the critical angle of the material of the core. Therefore, a ray from one end of a fiber undergoes multiple reflections before reaching the opposite end. Although most of the rays undergo total internal reflection, some of them do not reach the destination. They are refracted out of the core or along its surface. The situation occurs if the angle of incidence is less or equal to the critical angle of the material of the core. If it is less than the critical angle, the ray is refracted out of the core, leading to signal loss. If the angle of incidence is equal to the critical angle, the ray is refracted along the surface of the optical fiber (Gleinair 2). Optical fibers are classified as single-mode or multimode. A single-mode fiber consists of a thin core, which can allow the propagation of one mode of light at a time. On the other hand, a multimode fiber comprises a relatively thick core that can allow the passage of many modes of light simultaneously. If the number of modes is high, the bandwidth reduces. Thus, a single-mode fiber, which allows one mode of light at a time, has a wider bandwidth than a multimode fiber. The bandwidth size primarily depends on the dispersion of light passing through the core. If dispersion in the core is low, the bandwidth is high. Multimode cables are used for general data applications such as connecting a desktop to other networks, segmenting existing networks, and operating alarm systems. On the other hand, a single-mode cable is used in high bandwidth applications. A single-mode fiber is preferred over a multimode fiber for long-distance transmission because it has a minimal dispersion of light (Ticker 77). Dispersion occurs when light propagates through an optical fiber. Its magnitude depends on the thickness of the core and the wavelength of incident light. If the core is large, the number of paths that light can traverse increases. However, if the core is small, the possible paths of light significantly reduce. Dispersion can cause variations in the time that light passing through the core reaches its destination. For example, if the core is large, the rays following a more direct path through the middle arrive at the destination earlier than those bouncing against the sides of the core (Downing 73-74). Dispersion of this nature is rectified by making the core small, forcing the light to pass through a more direct path. Apart from the thickness of the core, the wavelength of incident light can also cause dispersion. Different colors of light have different wavelengths. They also have different magnitudes of dispersion. If light consisting of various colors passes through the optical fiber, the colors will be dispersed at different magnitudes, distorting the nature of the light. Thus, a significant part of the light would not reach the destination the way it came from the source. Dispersion of this nature is rectified by limiting the number of wavelengths of light passing through the core (Brown 9). Attenuation occurs in the cable because of the absorption of light. Though the core consists of highly purified glass, the little impurities absorb some of the light (Dutton 26). Poor connections at junctions can also lead to considerable signal losses, especially if the surfaces are not polished properly. Bending of the cable also causes attenuation; it alters the angles of incidence of the light, causing some of them to refract out of the core (Udd & Spillman 25-26). How a Fiber-Optic Channel Works A fiber-optic communication channel consists of three major components. They include the transmitter, information channel, and receiver. The transmitter generates and converts messages into forms that are suitable for transmission (Torlak 3). The information channel relays messages from the transmitter to the receiver. The receiver extracts messages from the information channel and converts them into forms that are usable by the receiving device or individual (Sanger 18). The transmitter of a fiber-optic communication cable consists of a light source and detector. The source of light can be a semiconductor laser or light emitting diode (LED). Laser sources of light are of two types. They include single-frequency and tunable lasers. The single-frequency laser produces only one color of light. On the other hand, the tunable laser produces many colors, but it can only radiate one at a time (Sanger 18-19). The output light from the LED or laser is fed into a wavelength locker. The purpose of the locker is to stabilize the intensity and frequency of the output light signal from the source. If the intensity or frequency changes with time, it alters the transmitted signal, producing distortions in the receiver. Therefore, it is important that the intensity of light from the source remains constant in order to minimize distortions. The wavelength locker detects and rectifies any changes occurring in the frequency or intensity of the input light (Sanger 19). Because each locker can only stabilize one laser source, the output signals from various lockers could differ from one another in terms of intensity or frequency. Therefore, they are passed through an electronic variable optical attenuator (EVOP), which regulates the intensities of light from different channels. An EVOA ensures that light signals from all the sources have the same intensity (Sanger 20). After the intensity and the frequency stabilization process, the light signals are conveyed to a modulator, which impresses data on them. The process utilizes external electro-optical modulators to insert the data into the light signals. Although it is possible to modify the light without the external modulators, the process is not preferred because it alters the modulator’s optical gain, accelerating the rate of interference. After the modulation stage, the signals are directed to a multiplexer. At this stage, different color signals from various sources are directed to the optical fiber for transmission. There are several multiplexing technologies, but the commonly used technique is narrow-band pass thin film (Sanger 20). The information channel is the second major component of the fiber-optic channel. It relays signals from the transmitter to the receiver. In an ideal situation, the information channel comprises several optical fibers linking signal sources to their destinations. Such situation can only be true if the transmitter and receiver are at relatively short distances away from one another. However, if the distance from the signal source to the destination is large, attenuation and dispersion of the light rays occur. As a result, the light travels at different speeds, distorting the encoded information. Therefore, in a practical situation, repeater stations set are at specific intervals between the transmitter and receiver (Sanger 20). A repeater station typically regenerates and amplifies the signals. It starts by cleaning the degradations in the incoming signals before amplifying them. The degradations are mainly caused by chromatic dispersion. Chromatic dispersion is the spreading of light according to its colors (Halliday, Resnick, & Walker 820). It occurs when the light rays in the core travel for long distances. If chromatic dispersion continues for a sufficiently long distance, the light rays in different channels will spread further, eventually interfering with other channels. Chromatic dispersion also occurs within a single channel, causing the distortion of the light signals. A repeater station has a dispersion compensator, a device that balances the path difference of various colors of light (Sanger 20). The amplification stage utilizes a device known as erbium-doped fiber amplifier (EDFA). EDFA provides the light signals with sufficient energy capable of reaching the next repeater station with minimal distortions. EDFA is typically a laser fiber core doped with high concentrations of erbium. It amplifies every channel into high levels that can be transmitted along the fiber cable. However, it does not amplify the signals into uniform intensities. Therefore, a flattening filter should be placed in the output signals of EDFA in order to stabilize their intensities. The rectified and amplified signal is then redirected into routers, switches, and optical add-up multiplexers for transmission (Sanger 20). The receiver is the third major component in an optic-fiber channel. It consists of a demultiplexer, EVOA, and light detector. The demultiplexer separates the signals from the fiber optic cable before channeling them to the EVOA. Since the intensity of the incoming signals is high, it can potentially saturate the light detector. Thus, the EVOA is necessary in attenuating the signals in order to prevent them from saturating the light detector. The detector then converts the light rays into electrical signals, which can be used by a computer or telephone (Sanger 20). In summary, a fiber-optic communication channel consists of three major components. They include the transmitter, information channel, and receiver. The generator converts electrical signals into light pulses. The information channel is made of thin silica, which transmits signals in the form of modulated light pulses. The receiver converts the light pulses into electrical signals, which can be used by computers or other devices. Because of attenuation in the communication channel, repeater stations are placed between the signal source and destination. The repeater stations rectify and amplify distorted signals before they reach their destination. Since a fiber optic with a narrow core has a high bandwidth, it is preferred for transmitting signals over long distance. Short distance signal transmission is accomplished with a fiber with a thick core. Works Cited Brown, G. Tom. “Optical Fibers and Optical Communications. The Institute of Optics. n.d. Web. 24 November 2014. . Downing, N. James. Fiber-optic Communications. New York: Cengage Learning, 2004. Print Dutton, J.R. Harry. “Understanding Digital Communications.” IBM. n.d. Web. 24 November 2014. . Fidanboylu, K. and Efendiogu, H.S. “Fiber Optic Sensors and their Applications.” N.p., n.d. Web. 24 November 2014. . Halliday, David, Resnick, Robert, and Walker, Jearl. Fundamentals of Physics. Hoboken: Wiley, 2003. Print. “Introduction to Fiber Optic Interconnect Technology and Packaging.” Glenair. 2012. Web. 24 November 2014. . Sanger, Greg. “How Fiber Optics Works.” The Industrial Physicist. 8.1 (2002): 18-21. Print. Ticker, Ray. Optoelectronics and Fiber Optic Technology. London: Newnes, 2002. Print. Torlak, Murat. “Fiber Optic Communications.” N.p., n.d. Web. 24 November 2014. . Udd, Eric and Spillman, William. Fiber Optic Sensors: An Introduction for Engineers and Scientists. Hoboken: Wiley, 2011. Print. Read More