Physics of Electronic Devices SPH1102 Unit -I FIBER OPTICS

Transcription

1 Physics of Electronic Devices SPH1102 Unit -I FIBER OPTICS

2 Introduction/ Definition An optical fiber is a flexible, transparent fiber made of high quality extruded glass or plastic, slightly thicker than a human hair. It can function as a waveguide, or light pipe to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths than other forms of communication.

3 This is used in telephone and cable TV cables to carry the signals. Light as an information carrier is much faster and more efficient than electrons in an electric current. Also, since light rays don t interact with each other (whereas electrons interact via their electric charge), it is possible to pack a large number of different light signals into the same fibre optics cable without distortion. Principle of Optical fiber communication An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection.

4 Total internal reflection Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflectance occurs.

5 The critical angle is given by Snell s law, Principle of Total Internal Reflection

6 Acceptance cone and Numerical Aperture Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber s core and cladding. The sine of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA.

7 Expression for Numerical Aperture The ray is refracted along OB at an angle in the core as shown in Fig. At this angle the ray just moves along BC. Hence the angle of incidence ( ( 90 ) at the interface of core and cladding will be more than the critical angle. Hence the ray is botally internally reflected ray. have

8 Where ncore is the refractive index along the central axis of the fiber. Note that when this definition is used, the connection between the NA and the acceptance angle of the fiber becomes only an approximation. In particular, manufacturers often quote NA for single-mode fiber based acceptance angle for single-mode fiber is determined from the indices of refraction alone. on this formula, even though the quite different and cannot be

9 Two types of rays can propagate along an optical fiber. Meridional rays Skew rays The first type is called meridional rays. Meridional rays are rays that pass through the axis of the optical fiber. Meridional rays are used to illustrate the basic transmission properties of optical fibers. Meridional rays can be classified as bound or unbound rays. Bound rays remain in the core and propagate along the axis of the fiber. Bound rays propagate through the fiber by total internal reflection. Unbound rays are refracted out of the fiber core. Fig shows a possible path taken by bound and unbound rays in a step-index. The second type is called skew rays. Skew rays are rays that travel through an optical fiber without passing through its axis. Skew rays are not confined to a single plane, but instead tend to follow a helical-type path along the fiber as shown in Fig. These rays are more difficult to track as they travel along the fiber, since they do not lie in a single plane.

10 Classification of fibers based on materials based on number of modes based on refractive index Classification of fibers basis on materials Glass fiber Glass optical fibers are almost always made from silica, Silica and fluoride glasses usually have refractive indices of about 1.5, but some materials such as the chalcogenides can have indices as high as 3. Typically the index difference between core and cladding is less than one percent. Example : SiO2 - P2O3; SiO2 Plastic Fiber Plastic fiber optic differs from single-mode and multi-mode fiber optic because the cables are made of plastic instead of glass. Therefore, plastic fiber optic is generally not used to transmit data because it does not have the capability to offer reliable data transmission. Instead plastic fiber optic is used to create impressive light displays and other ornamental purposes. Technology specialists have experimented with plastic fiber optic for data transmission because it is easier to install and work with, however the results so far have been inconclusive as to whether it can be a possible alternative to traditional fiber optic cable connections. Example : PMMA - Co-Polymer, Polystyrene - PMMA

11 Classification of fibers based on number of modes Single-Mode Fiber Optic: A single-mode fiber optic connection is used for longer cable runs due to the fact that it only allows for a single stream of data flow. The single stream of data is transmitted through a single light pulse which carries a significant amount of bandwidth providing for a faster transmission of data. Single-mode fiber optic is used for Ethernet connections that require long distances for data transmission.

12 Multimode fibre In a multimode fibre, the core diameter is much bigger than the wavelength of the transmitted light. A number of modes can be simultaneously transmitted. Fibre modes are related to the possible ways the light travels inside the fibre. The primary mode travels parallel to the axis of the fibre and therefore takes the minimum time to reach the end of the fibre. When the incoming beam enters with an angle respect to the fibre axis, the light will follow a longer path and therefore will take longer to reach the end. The number of modes that can be transmitted along the fibre increases with the core diameter. Multimode fibres may be divided in step and gradient index. In step index fibres the refraction index of the core is constant and the light travels in straight paths. In gradient index fibres the refraction index decreases gradually (parabolic) from the core out through the cladding and therefore the light travels along smooth curves.

13 Classification of fibers based on refractive index profile Step index fibre For an optical fiber, a step-index profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding interface so that the cladding is of a lower refractive index. The step-index profile corresponds to a power-law index profile with the profile parameter approaching infinity. The step-index profile is used in most single-mode fibers and some multimode fibers. Step index fibres are the most used fibres in fields other than telecommunications. They are relatively cheap and they have the widest range of core diameters: basically from 50 ìm up to 2mm.The material may be plastic, liquid or glass. Plastic fibres are not wide used nowadays; their optical transmission is poor and the core relatively big (0.5 to 2 mm). The most efficient fibres are made in acrylic and they are mainly used for short length telecommunication networks. In spite of their limited performances, new developments in plastic fibres might open applications in the field of high speed home networks (Gigabit/s). New polymers are being proposed with attenuations approaching the silica fibres. Most common step index fibres are made in silica glass (core and cladding) because of its high optical transmission in a very broad spectral range.

14 Graded index fiber In fiber optics, a graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber. Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fiber. The most common refractive index profile for a graded-index fiber is very nearly parabolic. The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion. Multi-mode optical fiber can be built with either graded index or step index. The advantage of the graded index compared to step index is the considerable decrease in modal dispersion. In the simplest optical fiber, the relatively large core has uniform optical properties. Termed a step index multimode fiber, this fiber supports thousands of modes and offers the highest dispersion and hence the lowest bandwidth.

15 By varying the optical properties of the core, the graded-index multimode fiber reduces dispersion and increases bandwidth. Grading makes light following longer paths travel slightly faster than light following a shorter path. Put another way, light traveling straight down the core without reflecting travels slowest. The net result is that the light does not spread out nearly as much. Nearly all multimode fibers used in networking and data communications have a graded index.

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17 Fiber Properties Numerical aperture Numerical aperture (NA) of the fiber defines which light will be propagated and which will not. NA defines the light-gathering ability of the fiber. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated. A high NA gathers more light, but lowers the bandwidth. A lower NA increases bandwidth. NA has an important consequence. A large NA makes it easier to inject more light into a fiber, while a small NA tends to give the fiber a higher bandwidth. A large NA allows greater modal dispersion by allowing more modes in which light can travel. A smaller NA reduces dispersion by limiting the number of modes.

18 Bandwidth Fiber bandwidth is given in MHz-km. A product of frequency and distance, bandwidth scales with distance: if you half the distance, you double the frequency. If you double the distance, you half the frequency. What does this mean in premises cabling? For a 100-meter run (as allowed for twisted pair cable), the bandwidth for 62.5/125-micrometer fiber is 1600 MHz at 850 nm and 5000 MHz at 1300 nm. For the 2-km spans allowed for most fiber networks, bandwidth is 80 MHz at 850 nm and 250 MHz at 1300 nm. With single mode fibers, the bandwidth for a 100-meter run is about 888 GHz. Attenuation is loss of power. During transit, light pulses lose some of their energy. Attenuation for a fiber is specified in decibels per kilometer (db/km). For commercially available fibers, attenuation ranges from approximately 0.5 db/km for single mode fibers to 1000 db/km for large-core plastic fibers.

19 Fabrication of Optical Fiber by Double crucible method A double crucible is used of which at least the inner crucible has a double-walled construction. The double wall accommodates a metal wire coil. During the fabrication of the optical fiber the core glass is heated by means of a high-frequency electric field and the crucible wall is cooled with a cooling liquid. The double crucible method is a known technique for the continuous production of optical fibers of the graded index type as well as of the stepped index (monomode and multimode) type. The double crucible method utilizes an arrangement of at least two, usually concentrically arranged melting vessels, each having an opening in its bottom. These openings are arranged on one line. In most cases, the contents of the vessels are heated to the operational temperature by means of an oven in which the vessels are placed. The core glass is melted in the innermost vessel while the cladding glass or glasses are melted in the vessel or vessels surrounding the central vessel. At the fiber drawing temperature, the glasses in the vessels usually flow together from the apparatus (at least in part) by the gravitational force of the earth.

20 The glasses are then drawn directly into a fiber. Depending on (i) the composition of the glasses (e.g. the presence of exchangeable cations), (ii) the operational temperature, (iii) the time of contact, an exchange of cations between the core and the cladding glass may occur or diffusion of a cation from the core glass to the cladding glass or from the cladding glass to the core glass may occur. It is however also possible to manufacture stepped index fibers by the double crucible technique. The melting vessels used in the double crucible method usually consist of platinum or quartz. At temperatures above 900 of platinum vessels is not practicable. Above 900 becomes dispersed into the glass, predominantly via the gas phase. Moreover, platinum particles may become detached from the vessel wall and be carried along by the outflowing glass. In addition, at these temperatures impurities such as, for example, chromium, diffuse from the platinum into the glass melts.

21 In the apparatus which is schematically shown in Fig. the inner crucible 1 has a cylindrical shape. The outer crucible is provided with a longer outflow channel 20, which is also heated by the oven 14. By locating the metal wire winding 3 deeper or shallower in the space enclosed by the double wall of crucible 1 (using a device such as the rack 22 and the pinions 23) the thicknesses of the layers of solid glasses 8 and 9, particularly at the bottom, can be controlled, which determine the diameter of the outflow opening 2 and consequently the diameter ratio between the core glass and the overall diameter of the fiber 21.

22 Chemical Vapour Deposition method (CVD)

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24 Splicing Techniques Splices create a permanent joint between two fibers, so its use is limited to places where cables are not expected to be available for servicing in the future. The most common application for splicing is concatenating (joining) cables in long outside plant cable runs where the length of the run requires more than one cable. Splicing can be used to mix a number of different types of cables such as connecting a 48 fiber cable to six 8 fiber cables going to various locations. Splicing is generally used to terminate single mode fibers by splicing preterminated pigtails onto each fiber. And of course, splicing is used for OSP restoration. Splicing is more common in outside plant (OSP) applications than premises cabling, where most cables are pulled in one piece and directly terminated. Splicing is only needed if the cable runs are too long for one straight pull or you need to mix a number of different types of cables (like bringing a 48 fiber cable in and splicing it to six 8 fiber cables.) And of course, we often use splices for OSP restoration, after the number one problem of outside plant cables, a dig-up and cut of a buried able, usually referred to as backhoe fade for obvious reasons. There are two types of splices, fusion and mechanical. Fusion splicing is most widely used as it provides for the lowest loss and least reflectance, as well as providing the strongest and most reliable joint. Fusion splicing machines are available in two types that splice a single fiber or a ribbon of 12 fibers at one time. Virtually all single mode splices are fusion. Mechanical splicing is mostly used for temporary restoration and for multimode splicing

25 Fusion splicing Fusion splicing requires special (and expensive) equipment containing an electric welder and a precision mechanism for aligning fibers. They often include a video camera or microscope so the operator can observe fibers during the alignment process and instruments to measure optical power before and after splicing. Many of them are automated to assist the operator. Fusion splicing two fibers involves several steps. First, the fiber ends are exposed by stripping back any protective buffer or jacket. Then the ends are cleaved to provide clean perpendicular faces and aligned so they butt together accurately. First-generation fusion splicing equipment relied on technicians to manually align the fiber ends. In the latest generation of equipment this process is computer-automated with motorized positioning equipment. The computer aligns the fiber ends until the best optical transmission is achieved before welding. Finally, an electric arc is established to weld or fuse the two ends together with heat. The resulting joint is re-measured optically to ensure minimal light loss, then protected mechanically and environmentally with either a coating or enclosure. The cost of a fusion splice is low compared to that of a typical connector because less mechanical hardware is required.

26 Mechanical splicing A mechanical splice is a junction of two or more optical fibers that are aligned and held in place by a self-contained assembly (usually the size of a large carpenter s nail). The fibers aren t permanently joined, just precisely held together so that light can pass from one to another. Mechanical splices are used to create permanent joints between two fibers by holding the fibers in an alignment fixture and reducing loss and reflectance with a transparent gel or optical adhesive between the fibers that matches the optical properties of the glass. Mechanical splices generally have higher loss and greater reflectance than fusion splices, and because the fibers are crimped to hold them in place, do not have as good fiber retention or pullout strength. The splice component itself, which includes a precision alignment mechanism, is more expensive than the simple protection sleeve needed by a fusion splice. The advantage of mechanical splices is they do not need an expensive machine to make the splices.

27 Alignment Mechanisms The biggest difference between mechanical splices is the way the fibers are aligned. Here are some typical methods. Capillary Tube method The simplest method of making a mechanical splice is to align two fibers in a small glass tube with a hole just slightly larger than the outside diameter of the fibers. This type of splice works well with UV-cured adhesive as well as index-matching gel between the fibers. The Ultra splice is a capillary splice. V-Groove Method V-Groove splices are quite simple and work well. They work for single fibers or even for fiber ribbons as shown here. The Grooved alignment plates can be made of many types of materials and are quite inexpensive.

28 Elastomeric Tube splicing An elastomeric splice contains two elastomeric (rubber like) inserts inside a glass sleeve as shown in Fig. A V-Groove is molded into one insert, while the other has a flat surface. The triangular shaped space formed where the two insert halves mate is slightly smaller in dimension than the diameter of the fibers being joined. When the fiber ends are pushed into the inserts the elastomer compresses equally on each side in contact with the fiber. As a result, the fibers are aligned on their center axes. Even fibers with different diameters are centered along their respective axes, maximizing the overlap of their end faces. The fibers are usually held in place using an adhesive cured with ultraviolet (UV) light. As in the capillary splice an index matching gel is often applied to minimize Fresnel losses. Many manufacturers include the gel within the splice body, which reduces this assembly step for the technician.

29 Connectors

30 Plastic materials. The only difference in the tapered sleeve connector is the ferrules and the alignment sleeve are tapered. The fiber is send into the drilled hole of the ferrule and are aligned properly with the help of alignment sleeve. The distance between the fiber is minimized by adjusting the alignment sleeve and the guide ring, and is used to match the ends of the fibers. Once the matching was done, the light from one fiber can be easily coupled to the other fiber with minimum losses. Expanded beam connectors Fig. shows the expanded beam connector arrangement. It consists collimating lens at the ends of transmitting fiber and focusing lens at entrance of the receiving fiber. The fiber to lens distance is equal to the focal length of the lens. Light coming out from the transmitting fiber is made to fall over the collimating lens. The collimating lens make the beam parallel and is focused into the focusing lens. After passing through the focusing lens, the light is coupled into the receiving fiber without any loss. Thus the loss is minimized.

31 LOSS IN OPTICAL FIBERS The following are the factors which affect the transmission of light waves through optical fibers: Absorption Scattering Fiber bending Dispersion Fiber attenuation, which necessitates the use of amplification systems, is caused by a combination of material absorption, Rayleigh scattering, Mie scattering, and connection losses. Although material absorption for pure silica is only around 0.03 db/km impurities in the original optical fibers caused attenuation of about 1000 db/km. Other forms of attenuation are caused by physical stresses to the fiber, microscopic fluctuations in density, and imperfect splicing techniques. The attenuation of an optical fiber measures the amount of light lost between input and output. Total attenuation is the sum of all losses. Optical losses of a fiber are usually expressed in decibels per kilometer (db/km). The expression is called the fiber s attenuation coefficient and the expression is where P(z) is the optical power at a position z from the origin, P(0) is the power at the origin.

32 Optical Fiber Loss Mechanisms Absorption Absorption is uniform. The same amount of the same material always absorbs the same fraction of light at the same wavelength. If you have three blocks of the same type of glass, each 1-centimeter thick, all three will absorb the same fraction of the light passing through them. Absorption also is cumulative, so it depends on the total amount of material the light passes through. If the absorption is 1% per centimeter, it absorbs 1% of the light in the first centimeter, and 1% of the remaining light the next centimeter, and so on. (i) Intrinsic Material Absorption Intrinsic absorption is caused by interaction of the propagating light wave with one more major components of glass that constitute the fiber s material composition. These looses represent a fundamental minimum to the attainable loss and can be overcome only by changing the fiber material. (ii) Extrinsic Impurity Ions Absorption Extrinsic impurity ions absorption is caused by the presence of minute quantity of metallic ions (such as Fe2+, Cu2+, Cr3+) and the OH- ion from water dissolved in glass.

33 Scattering Scattering losses occur when a wave interacts with a particle in a way that removes energy in the directional propagating wave and transfers it to other directions. The light isn t absorbed, just sent in another direction. However, the distinction between scattering and absorption doesn t matter much because the light is lost from the fiber in either case. There are two main types of scattering: linear scattering and nonlinear scattering. For linear scattering, the amount of light power that is transferred from a wave is proportional to the power in the wave. It is characterized by having no change in frequency in the scattered wave. On the other hand, nonlinear scattering is accompanied by a frequency shift of the scattered light. Nonlinear scattering is caused by high values of electric field within the fiber (modest to high amount of optical power). Nonlinear scattering causes significant power to be scattered in the forward, backward, or sideways directions.

34 Rayleigh scattering (Linear Scattering) Rayleigh scattering (named after the British physicist Lord Rayleigh) is the main type of linear scattering. It is caused by small-scale (compared with the wavelength of the lightwave) inhomogeneities that are produced in the fiber fabrication process. Examples of inhomogeneities are glass composition fluctuations (which results in minute refractive index change) and density fluctuations (fundamental and not improvable). Rayleigh scattering accounts for about 96% of attenuation in optical fiber.

35 Macro bending Loss Macrobending happens when the fiber is bent into a large radius of curvature relative to the fiber diameter (large bends). These bends become a great source of power loss when the radius of curvature is less than several centimeters. Macrobend may be found in a splice tray or a fiber cable that has been bent. Macrobend won t cause significant radiation loss if it has large enough radius. However, when fibers are bent below a certain radius, radiation causes big light power loss as shown in the figure below. Micro bending Loss Microbendings are the small-scale bends in the core-cladding interface. These are localized bends can develop during deployment of the fiber, or can be due to local mechanical stresses placed on the fiber, such as stresses induced by cabling the fiber or wrapping the fiber on a spool or bobbin. Microbending can also happen in the fiber manufacturing process. It is sharp but microscopic curvatures that create local axial displacement of a few microns and spatial wavelength displacement of a few millimeters. Micro bends can cause 1 to 2 db/km losses in fiber cabling process.

36 Dispersion in Fibers Dispersion is the broadening of light pulses as it propagates through the fiber. It increases with length of the fiber. Excessive dispersion causes over-lapping of adjacent pulses or inter symbol interference. So dispersion has a negative effect on the bandwidth of a fiber. The higher the dispersion, lie lower will he bandwidth of the system. Dispersion also decreases the peak optical power of the pulse and therefore increases the effective attenuation of a fiber. Dispersion may be classified into two categories depending on the cause. These are (i) Modal and (ii) Material dispersion. (i) Modal dispersion: These are dominant in multimode fibers where the optical rays propagate in different modes. Thus the path lengths are different for different modes and consequently propagation time is different for different rays, This results in broadening or dispersion of light pulses. In order to solve this problem, graded index fibers have been discovered where the refractive index varies accross he cross-section of the fiber. Therefore the speed of optical rays arc different for different modes. The refractive index profile is so designed that the propagation lime is almost the same for different modes. Another solution to tie problem of modal dispersion is to reduce the core diameter to such an extent that only a single mode of propagation is possible. Such a fiber is called the monomode fiber. With the introduction of this type of fiber it has been possible to virtually eliminate modal dispersion. The fiber bandwidth, therefore, is much greater now a days.

37 (ii) Material dispersion: The refraction index of a optical material varies with the wavelength of operation. The variation pattern is shown below: It can be seen from the above description,that the refractive index decreases with the increasing lengths. Practical optical sources have a non-zero spectral width. So, there is a difference in refractive index and, consequently, speeds of light rays are different for this non-zero spectral width. This results in broadening or dispersion of optical signals. The LED source has a much larger spectral line width and thereby material dispersion is much higher. Later on LASER diode sources have been developed with much less spectral line width and higher optical output power. Thus it has been possible to reduce the material dispersion to a great extent. The unit of material dispersion is ps/nm/km.

38 Function of Fiber optical communication system The typical optical communications system essentially consists of a transmitter with a diode laser, a receiver with a photo-diode and an optical fibre between the two serving as the transmitting medium. Modern fiber-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fiber, a cable containing bundles of multiple optical fibers that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers and telephone systems. Principle of optical fiber communication The principle of optical fiber communication is to transfer the information (such as voice) into electrical signals firstly, then send to laser modulation of the laser beam, strengthen the light signal with the amplitude (frequency) change, and send out through the optical fiber; at the receive end, after receiving light signalÿ the detector transform it into electrical signals, recover to original information after demodulation. An optical fiber communication system uses a digital communication scheme.

39 Transmitters The most commonly used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs) and laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient, and reliable, while operating in an optimal wavelength range, and directly modulated at high frequencies. Optical sources The fundamental function of optical source in optical fiber communications is to convert electrical energy in the form of current into optical energy. The main optical sources currently used in optical fiber communications are lasers and light emitting diodes (LEDs).

40 Light emitting diode (LED) An LED is essentially a semiconductor p-n junction under forward bias. In this condition, electrons cross the pn junction from the n-type material and recombine with holes in the p-type material. When recombine takes place, the recombining electrons release energy in the form of heat and light. A large exposed surface area on one layer of the semiconductive material permits the photon to be emitted as visible light. This process is called electroluminescence. Various impurities are added during the doping process to establish the wavelength of the emitted light. The wavelength determines the color of the light and if it is visible or invisible (infrared). LEDs are made of gallium arsennide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP). Silicon and germanium are not used because they are essentially heat-producing materials and are very poor at producing light. GaAs LEDs emit infrared (IR) radiation, which is nonvisible, GaAsP produces either red or yellow visible light, and GaP emits red or green visible light. LEDs that emit blue light are also available. Red is the most common. In LED, the dominant photon generation is spontaneous emission in which the electron drops to the lower energy level in an entirely random way. The output spectrum of an LED is relatively wide. Receivers The main component of an optical receiver is a photo detector, which converts light into electricity using the photoelectric effect. The primary photo detectors for telecommunications are made

41 from Indium gallium arsenide The photo detector is typically a semiconductorbased photodiode. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photo detectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers. Amplifiers The transmission distance of a fiber-optic communication system has traditionally been limited by fiber attenuation and by fiber distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than it was before. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high.

42 Working Like all other communication system, the primary objective of optical fiber communication system also is to transfer the signal containing information (voice, data, video) from the source to the destination. The general block diagram of optical fiber communication system is shown in the figure. The source provides information in the form of electrical signal to the transmitter. The electrical stage of the transmitter drives an optical source to produce modulated light wave carrier. Semiconductor LASERs or LEDs are usually used as optical source here. The information carrying light wave then passes through the transmission medium i.e. optical fiber cables in this system. Now it reaches to the receiver stage where the optical detector demodulates the optical carrier and gives an electrical output signal to the electrical stage. The common types of optical detectors used are photodiodes (p-i-n, avalanche), phototransistors, photoconductors etc. Finally the electrical stage gets the real information back and gives it to the concerned destination. It is notable that the optical carrier may be modulated by either analog or digital information signal. In digital optical fiber communication system the information is suitably encoded prior to the drive circuit stage of optical source. Similarly at the receiver end a decoder is used after amplifier and equalizer stage.

43 Advantageous Of Optical Fibers Communication: 1. Information bandwidth is more. 2. Optical fibers are small in size and light weighted. 3. Optical fibers are more immune to ambient electrical noise, electromagnetic interference. 4. Cross talk and internal noise are eliminated in optical fibers. 5. There is no risk of short circuit in optical fibers. 6. Optical fibers can be used for wide range of temperature. 7. A single fiber can be used to send many signals of different wavelengths using Wavelengths Division Multiplexing (WDM). 8. Optical fibers are generally glass which is made up of sand and hence they are cheaper than copper cables. 9. Optical fibers are having less transmission loss and hence less number of repeaters are used. 10. Optical fibers are more reliable and easy to maintain. Disadvantageous of Optical Fibers Communication: 1. Attenuation offered by the optical fibers depends upon the material by which it is made. 2. Complex electronic circuitry is required at transmitter and receiver. 3. The coupling of optical fibers is difficult. 4. Skilled labors are required to maintain the optical fiber communication. 5. Separated power supply is required for electronic repeaters at different stages.

44 Engineering applications of optical fiber Telecommunication Telecommunication applications are widespread, ranging from global networks to desktop computers. These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs. Network Carriers use optical fiber to carry plain old telephone service (POTS) across their nationwide networks. Local exchange carriers (LECs) use fiber to carry this same service between central office switches at local levels, and sometimes as far as the neighborhood or individual home. Transmission Optical fiber is also used extensively for transmission of data. Multinational firms need secure, reliable systems to transfer data and financial information between buildings to the desktop terminals or computers and to transfer data around the world. Cable television companies also use fiber for delivery of digital video and data services. The high bandwidth provided by fiber makes it the perfect choice for transmitting broadband signals, such as high-definition television (HDTV) telecasts. Intelligent transportation systems, such as smart highways with intelligent traffic lights, automated tollbooths, and changeable message signs, also use fiber-opticbased telemetry systems.

45 1.22 Other applications of optical fiber Another important application for optical fiber is the biomedical industry. Fiber-optic systems are used in most modern telemedicine devices for transmission of digital diagnostic images. Other applications for optical fiber include space, military, automotive, and the industrial sector..xxxxxxxxxxxxxxxxxxxxxx

46 Part - A Questions 1. What is the basic priniciple of Fiber optics? 2. Define Acceptance angle and Acceptance core? 3. Define total internal reflection.? 4. What are conditions to be satisfied for total internal reflection.? 5. What are the clssification of optical fiber? 6. Define Numerical Aperture.? 7. What is meant by Attenuation? 8. What is intrirsic Absorption? 9. What is dispersion? 10. What are the different applications of optical fiber? 11. What is splicing? 12. What is connector? 13. Explain micro Bending and macro Bending? 14. What are the advantages of OFC? 15. What is meat by skew rays and meridioral rays? 16. Explain material dispersion? 17. Define micro Bending lossses and macro Bending losses? 18. Define chromatic dispersion? 19. What is meant by V-Groove connector? 20. Explain the Elastic Tube Splicing?

47 Part - B Questions 1. Write short notes on the following, a) Acceptance Angle, b) Numerical Aperture, c) Step and Grade index Fiber. 2. Discuss in detail the Basic principle structure and advantages of optical fiber. 3. Classify the optical fiber on the basis of materials, modes of propagation and refractive index. 4. Discuss How optical fiber is used in communication field. What is its advantages over the conventional methods. 5. Explain with a neat Block diagram the instrumentation system adopted to explain the communication system. 6. Write a note on how the light propagates through step index, Graded Index, Single mode and multimode Fiber. Draw a suitable diagram. 7. Explain the Engineering Applications of optical Fiber. What are the disadvantages of optical fiber?