Gerd keiser optical fiber communications 3rd edition pdf


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Gerd Keiser Optical Fiber Communications 3rd Edition Pdf

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Low losses Fiber optic cables offers very less signal attenuation over long distances. This enables longer distance between repeaters. Immune to cross talk Fiber optic cables have very high immunity to electrical and magnetic field. Since fiber optic cables are non-conductors of electricity hence they do not produce magnetic field.

Thus fiber optic cables are immune to cross talk between cables caused by magnetic induction. Interference immune Fiber optic cable immune to conductive and radiative interferences caused by electrical noise sources such as lighting, electric motors, fluorescent lights.

Light weight As fiber cables are made of silica glass or plastic which is much lighter than copper or aluminum cables. Light weight fiber cables are cheaper to transport. Small size 7. The diameter of fiber is much smaller compared to other cables, therefore fiber cable is small in size, requires less storage space.

More strength Fiber cables are stronger and rugged hence can support more weight. Security Fiber cables are more secure than other cables. It is almost impossible to tap into a fiber cable as they do not radiate signals. No ground loops exist between optical fibers hence they are more secure. Long distance transmission Because of less attenuation transmission at a longer distance is possible. Environment immune Fiber cables are more immune to environmental extremes.

They can operate over large temperature variations. Also they are not affected by corrosive liquids and gases. Sage and easy installation Fiber cables are safer and easier to install and maintain.

They are non-conductors hence there is no shock hazards as no current or voltage is associated with them.

Their small size and light weight feature makes installation easier. Less cost Cost of fiber optic system is less compared to any other system. High initial cost The initial cost of installation or setting up cost is very high compared to all other system. Maintenance and repairing cost The maintenance and repairing of fiber optic systems is not only difficult but expensive also.

Jointing and test procedures Since optical fibers are of very small size. The fiber joining process is very costly and requires skilled manpower. Tensile stress Optical fibers are more susceptible to buckling, bending and tensile stress than copper cables. This leads to restricted practice to use optical fiber technology to premises and floor backbones with a few interfaces to the copper cables. Short links Even though optical fiber cables are inexpensive, it is still not cost effective to replace every small conventional connector e.

Fiber losses The amount of optical fiber available to the photo detector at the end of fiber length depends on various fiber losses such as scattering, dispersion, attenuation and reflection. Applications of Optical Fiber Communications Applications of optical fiber communications include telecommunications, data communications, video control and protection switching, sensors and power applications.

Telephone networks Optical waveguide has low attenuation, high transmission bandwidth compared to copper lines; therefore numbers of long haul co-axial trunks links between telephone exchanges are being replaced by optical fiber links. Urban broadband service networks Optical waveguide provides much larger bandwidth than co-axial cable, also the number of repeaters required is reduced considerably.

All these can be supplied over a single fiber optic link. Optical Fiber Waveguides In free space light ravels as its maximum possible speed i. When light travels through a material it exhibits certain behavior explained by laws of reflection, refraction. Electromagnetic Spectrum The radio waves and light are electromagnetic waves.

The rate at which they alternate in polarity is called their frequency f measured in hertz Hz. Infrared light covers a fairly wide range of wavelengths and is generally used for all fiber optic communications. Visible light is normally used for very short range transmission using a plastic fiber. Ray Transmission Theory Before studying how the light actually propagates through the fiber, laws governing the nature of light m ust be studied.

These was called as laws of optics Ray theory. There is conception that light always travels at the same speed. This fact is simply not true. The speed of light depends upon the material or medium through which it is moving. In free space light travels at its maximum possible speed i. Reflection The law of reflection states that, when a light ray is incident upon a reflective surface at some incident angle 1 from imaginary perpendicular normal, the ray will be reflected from the surface at some angle 2 from normal which is equal to the angle of incidence.

Refraction Refraction occurs when light ray passes from one medium to another i. Refraction occurs whenever density of medium changes. The refraction can also observed at air and glass interface. When wave passes through less dense medium to denser medium, the wave is refracted bent towards the normal.

The refraction bending takes place because light travels at different speed in different mediums. The speed of light in free space is higher than in water or glass. Refractive Index The amount of refraction or bending that occurs at the interface of two materials of different densities is usually expressed as refractive index of two materials.

Refractive index is also known as index of refraction and is denoted by n. Based on material density, the refractive index is expressed as the ratio of the velocity of light in free space to the velocity of light of the dielectric material substance.

The refractive index for vacuum and air os 1. Equation can be written as, This equation shows that the ratio of refractive index of two mediums is inversely proportional to the refractive and incident angles. As refractive index substituting these values in equation f.

Critical Angle When the angle of incidence 1 is progressively increased, there will be progressive increase of refractive angle 2. At some condition 1 the refractive angle 2 becomes 90o to the normal. The angle of incidence 1 at the point at which the refractive angle 1 becomes 90 degree is called the critical angle. It is denoted by c. The critical angle is defined as the minimum angle of incidence 1 at which the ray strikes the interface of two media and causes an angle of refraction 2 equal to 90o.

Fig 1. Total Internal Reflection TIR When the incident angle is increase beyond the critical angle, the light ray does not pass through the interface into the other medium. This gives the effect of mirror exist at the interface with no possibility of light escaping outside the medium.

In this condition angle of reflection 2 is equal to angle of incidence 1. TIR can be observed only in materials in which the velocity of light is less than in air. The two conditions necessary for TIR to occur are: The refractive index of first medium must be greater than the refractive index of second one.

The angle of incidence must be greater than or equal to the critical angle. Then above equation reduces to, The angle 0 is called as acceptance angle and omax defines the maximum angle in which the light ray may incident on fiber to propagate down the fiber. The Cone of acceptance is the angle within which the light is accepted into the core and is able to travel along the fiber.

The launching of light wave becomes easier for large acceptance cone. The angle is measured from the axis of the positive cone so the total angle of convergence is actually twice the stated value.

Numerical Aperture NA The numerical aperture NA of a fiber is a figure of merit which represents its light gathering capability. Larger the numerical aperture, the greater the amount of light accepted by fiber.

The acceptance angle also determines how much light is able to be enter the fiber and hence there is relation between the numerical aperture and the cone of acceptance.

NA is not a function of fiber dimension. Example 1. A light ray is incident from medium-1 to medium If the refractive indices of medium-1 and medium-2 are 1. Optical Fiver as Waveguide An optical fiber is a cylindrical dielectric waveguide capable of conveying electromagnetic waves at optical frequencies.

The electromagnetic energy is in the form of the light and propagates along the axis of the fiber. The structural of the fiver determines the transmission characteristics.

The propagation of light along the waveguide is decided by the modes of the waveguides, here mode means path. Each mode has distict pattern of electric and magnetic field distributions along the fiber length. Only few modes can satisfy the homogeneous wave equation in the fiver also the boundary condition a waveguide surfaces. When there is only one path for light to follow then it is called as single mode propagation. When there is more than one path then it is called as multimode propagation.

Single fiber structure A single fiber structure is shown in Fig. This cylinder is called as core of fiber. The core is surrounded by dielectric, called cladding. The index of refraction of core glass fiber is slightly greater than the index of refraction of cladding. Modes of Fiber Fiber cables cal also be classified as per their mode.

Light rays propagate as an electromagnetic wave along the fiber. The two components, the electric field and the magnetic field form patterns across the fiber. These patterns are called modes of transmission.

The mode of a fiber refers to the number of paths for the light rays within the cable. According to modes optic fibers can be classified into two types. Multimode fiber. Multimode fiber was the first fiber type to be manufactured and commercialized. The term multimode simply refers to the fact that numerous modes light rays are carried simultaneously through the waveguide. Multimode fiber has a much larger diameter, compared to single mode fiber, this allows large number of modes.

Single mode fiber allows propagation to light ray by only one path. Single mode fibers are best at retaining the fidelity of each light pulse over longer distance also they do not exhibit dispersion caused by multiple modes.

Thus more information can be transmitted per unit of time. This gives single mode fiber higher bandwidth compared to multimode fiber. Some disadvantages of single mode fiber are smaller core diameter makes coupling light into the core more difficult.

Precision required for single mode connectors and splices are more demanding. Fiber Profiles A fiber is characterized by its profile and by its core and cladding diameters. One way of classifying the fiber cables is according to the index profile at fiber. The index profile is a graphical representation of value of refractive index across the core diameter. There are two basic types of index profiles.

Step index fiber. Graded index fiber. Step Index SI Fiber The step index SI fiber is a cylindrical waveguide core with central or inner core has a uniform refractive index of n1 and the core is surrounded by outer cladding with uniform refractive index of n2.

The cladding refractive index n2 is less than the core refractive index n1. But there is an abrupt change in the refractive index at the core cladding interface. Refractive index profile of step indexed optical fiber is shown in Fig. The refractive index is plotted on horizontal axis and radial distance from the core is plotted on vertical axis.

In the graded index GRIN fiber the refractive index is not uniform within the core, it is highest at the center and decreases smoothly and continuously with distance towards the cladding. The refractive index profile across the core takes the parabolic nature.

Optical fiber communications keiser pdf

In graded index fiber the light waves are bent by refraction towards the core axis and they follow the curved path down the fiber length. This results because of change in refractive index as moved away from the center of the core.

A graded index fiber has lower coupling efficiency and higher bandwidth than the step index fiber. Comparison of Step Index and Graded Index Fiber Optic Fiber Configurations Depending on the refractive index profile of fiber and modes of fiber there exist three types of optical fiber configurations. These optic-fiber configurations are - Single mode step index fiber.

Multimode graded index fiber. Single mode Step index Fiber In single mode step index fiber has a central core that is sufficiently small so that there is essentially only one path for light ray through the cable. The light ray is propagated in the fiber through reflection.

Single mode fiber is also known as fundamental or mono mode fiber. Single mode fiber will permit only one mode to propagate and does not suffer from mode delay differences.

These are primarily developed for the nm window but they can be also be used effectively with time division multiple TDM and wavelength division multiplex WDM systems operating in nm wavelength region. The core fiber of a single mode fiber is very narrow compared to the wavelength of light being used. Therefore, only a single path exists through the cable core through.

Usually, 20 percent of the light in a single mode cable actually travels down the cladding and the effective diameter of the cable is a blend of single mode core and degree to which the cladding carries light. The disadvantage of this type of cable is that because of extremely small size interconnection of cables and interfacing with source is difficult.

Another disadvantage of single mode fibers is that as the refractive index of glass decreases with optical wavelength, the light velocity will also be wavelength dependent. Thus the light from an optical transmitter will have definite spectral width. It is easy to manufacture. The light rays are propagated down the core in zig-zag manner. There are many paths that a light ray may follow during the propagation.

The light ray is propagated using the principle of total internal reflection TIR. Since the core index of refraction is higher than the cladding index of refraction, the light enters at less than critical angle is guided along the fiber. Light rays passing through the fiber are continuously reflected off the glass cladding towards the centre of the core at different angles and lengths, limiting overall bandwidth. The disadvantage of multimode step index fibers is that the different optical lengths caused by various angles at which light is propagated relative to the core, causes the transmission bandwidth to be fairly small.

Because of these limitations, multimode step index fiber is typically only used in applications requiring distances of less than 1 km. The light ray is propagated through the refraction. The light ray enters the fiber at many different angles. As the light propagates across the core toward the center it is intersecting a less dense to more dense medium. Therefore the light rays are being constantly being refracted and ray is bending continuously. This cable is mostly used for long distance communication.

The modes travelling in a straight line are in a higher refractive index so they travel slower than the serpentine modes. This reduces the arrival time disparity because all modes arrive at about the same time. It is seen that light rays running close to the fiber axis with shorter path length, will have a lower velocity because they pass through a region with a high refractive index.

Rays on core edges offers reduced refractive index, hence travel more faster than axial rays and cause the light components to take same amount of time to travel the length of fiber, thus minimizing dispersion losses.

Typical attenuation coefficients of graded index fibers at nm are 2. The main advantages of graded index fiber are: Reduced refractive index at the centre of core. Comparatively cheap to produce. Mode Theory for Cylindrical Waveguide To analyze the optical fiber propagation mechanism within a fiber, Maxwell equations are to solve subject to the cylindrical boundary conditions at core-cladding interface.

Hence the analysis of optical waveguide is more complex than metallic hollow waveguide analysis. The two lowest order does are HE11 and TE Overview of Modes The order states the number of field zeros across the guide. The electric fields are not completely confined within the core i. The low order mode confines the electric field near the axis of the fiber core and there is less penetration into the cladding. While the high order mode distribute the field towards the edge of the core fiber and penetrations into the cladding.

Therefore cladding modes also appear resulting in power loss. In leaky modes the fields are confined partially in the fiber core attenuated as they propagate along the fiber length due to radiation and tunnel effect.

Cladding also improves the mechanical strength of fiber core and reduces surface contamination. Plastic cladding is commonly used. Materials used for fabrication of optical fibers are silicon dioxide SiO2 , boric oxide-silica. If the core refractive index is 1. A step index multimode fiber with a numerical aperture of a 0.

Mode Field Diameter and Spot Size The mode filed diameter is fundamental parameter of a single mode fiber. This parameter is determined from mode field distributions of fundamental LP01 mode. In step index and graded single mode fibers, the field amplitude distribution is approximated by Gaussian distribution. In single mode fiber for fundamental mode, on field amplitude distribution the mode filed diameter is shown in fig.

The material must be transparent for efficient transmission of light. It must be possible to draw long thin fibers from the material. Fiber material must be compatible with the cladding material. Glass and plastics fulfills these requirements. Most fiber consists of silica SiO2 or silicate.

Various types of high loss and low loss glass fibers are available to suit the requirements. Plastic fibers are not popular because of high attenuation they have better mechanical strength. Glass Fibers Glass is made by fusing mixtures of metal oxides having refractive index of 1. The principal raw material for silica is sand and glass. The fiber composed of pure silica is called as silica glass. The desirable properties of silica glass are: Resistance to breakage from thermal shocks low thermal expansion.

Better transparency. Other types of glass fibers are Halide glass fibers. Active glass fibers Chalgenide glass fibers Plastic optical fibers Fiber Fabrication Methods The vapor-phase oxidation process is popularly used for fabricating optical fibers. In this process vapors of metal halides such as SiCl4 and Gecl4 reactive with oxygen and forms powder of SiO2 particles.

The SiO2 particles are collected on surface of bulk glass and then sintered to form a glass rod called Preform. The preforms are typically mm diameter and cm long from which fibers are drawn. A simple schematic of fiber drawing equipment The preform is feed to drawing furnace by precision feed mechanism.

The preform is heated up in drawing furnace so that it becomes soft and fiber can be drawn easily. The fiber thickness monitoring decides the speed of take up spool.

The fiber is then coated with elastic material to protect it from dust and water vapor. Fig, 1. During the SiO2 deposition O2 and metal halide vapors can be controlled so the desired core-cladding diameters can be incorporated. The mandrel is removed when deposition process is completed; this preform is used for drawing thin filament of fibers in fiber drawing equipment.

The rod is continuously rotated and moved upward to maintain symmetry of particle deposition. The advantages of VAD process are - Both step and graded index fibers are possible to fabricate in multimode and single mode. The performs does not have the central hole.

The performs can be fabricated in continuous length. Clean environment can be maintained. A hollow silica tube is heated to about oC and a mixture of oxygen and metal halide gases is passed through it.

The soot that develops from this deposition is consolidated by heating. The tube is rotated while the heater is moved to and along the tube and the soot forms a thin layer of silica glass.

The rotation and heater movement ensures that the layer is of constant thickness. Graded index fiber is produced by careful continuous control of the constituents.

The temperature is now increased to about oC and the tube is collapsed to form a solid rod called a preform. The preform is about 25 mm in diameter and 1 meter in length. This will produce 25 km of fiber. The preform is placed at a height called a pulling tower and its temperature is increased to about oC.

To prevent contamination, the atmosphere is kept dry and clean. Laser gauges continually monitor the thickness of the fiber and automatically adjust the pilling rate to maintain required thickness. After sufficient cooling the primary buffer is applied and the fiber is drummed. It reduces mechanical stress on glass films. There is no soot formation and hence sintering is not required.

Non-isothermal microwave plasma at low pressure initiates the chemical reaction. Double-Crucible Method Double-crucible method is a direct melt process.

In double-crucible method two different glass rods for core and Cladding are used as feedstock for two concentric crucibles. The inner crucible is for core and outer crucible is for cladding. The fibers can be drawn from the orifices in the crucible.

Major advantage of double crucible method is that it is a continuous production process. Fiber Optic Cables The fiber optic cable is to be used under variety of situations such as underground, Outdoor poles or submerged under water. The structure of cable depends on the situation where it is to be used, but the basic cable design principles remain same. Maximum allowable axial load on cable decides the length of the cable be reliably installed.

Also the fiber cables must be able to absorb energy from impact loads.

The outer sheath must be designed to protect glass fibers from impact loads and from corrosive environmental elements. Fiber Arrangements Several arrangements of fiber cables are done to use it for different applications. The most basic form is two fiber cable designs. It is also known as basic building block of fiber cable. For providing strength to the core several coatings of different materials are applied as shown in fig 1. Multiple fiber cable can be combined together using similar techniques.

The basic fiber building blocks are used to form large cable. These units are bound on a buffer material which acts as strength element along with insulated copper conductor. The fiber building blocks are surrounded by paper tape, PVC jacket, yarn and outer sheath. To ease identification, individual fibers are colour coded Table 1. Plastic Fiber Optic Cables Fibers can also be manufactured from transparent plastic which offers advantages of larger diameter 1 mm , increased flexibility, can be cut using a hot razor blade, ease of termination.

But because of high intrinsic loss use of plastic fibers is normally restricted to only few metres. Plastic optic fiber POF offers noise immunity and low cable weight and volume and is competitive with shielded copper wire making it suitable for industrial applications.

Also, silica fiber can tolerate higher temperature than plastic fiber. On the other hand, POF is more flexible, less prove to breakage, easier to fabricate and cost is low than glass fibers. These advantages and disadvantages are summarized in Table 1. Recommended Questions 1. State and explain the advantages and disadvantages of fiber optic communication systems? State and explain in brief the principle of light propagation?

Explain the important conditions for TIR to exit in fiber.? Derive an expression for maximum acceptance angle of a fiber? Explain the acceptance come of a fiber? Define numerical aperture and state its significance also? Explain the different types of rays in fiber optic? Explain the following —?

A Step index fiber B Graded index fiber What is mean by mode of a fiber? Write short notes on following — A Single mode step index fiber B Multimode step index fiber C Multimode graded index fiber.

Explain the fiber materials used in fabrication requirements? In case of glass fibers how the refractive index can be varied? Briefly explain following techniques of fabrication? Introduction One of the important properties of optical fiber is signal attenuation. It is also known as fiber loss or signal loss. The signal attenuation of fiber determines the maximum distance between transmitter and receiver.

The attenuation also determines the number of repeaters required, maintaining repeater is a costly affair. Another important property of optical fiber is distortion mechanism. As the signal pulse travels along the fiber length it becomes broader.

After sufficient length the broad pulses starts overlapping with adjacent pulses. This creates error in the receiver. Hence the distortion limits the information carrying capacity of fiber. Attenuation Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses propagate through the length of the fiber.

In optical fibers the attenuation is mainly caused by two physical factors absorption and scattering losses. Absorption is because of fiber material and scattering due to structural imperfection within the fiber. Micro bending of optical fiber also contributes to the attenuation of signal.

The rate at which light is absorbed is dependent on the wavelength of the light and the characteristics of particular glass.

Glass is a silicon compound; by adding different additional chemicals to the basic silicon dioxide the optical properties of the glass can be changed. The Rayleigh scattering is wavelength dependent and reduces rapidly as the wavelength of the incident radiation increases. The attenuation of fiber is governed by the materials from which it is fabricated, the manufacturing process and the refractive index profile chosen. Let the couples optical power is p 0 i. This parameter is known as fiber loss or fiber attenuation.

Attenuation is also a function of wavelength. Optical fiber wavelength as a function of Wavelength is shown in Fig. Example 2. Determine — 1 Overall signal attenuation in dB. Each splice introducing attenuation of 1 dB. A continuous 12 km long optical fiber link has a loss of 1. Given data: Absorption loss results in dissipation of some optical power as hear in the fiber cable. Although glass fibers are extremely pure, some impurities still remain as residue after purification. The amount of absorption by these impurities depends on their concentration and light wavelength.

Absorption is caused by three different mechanisms. Absorption by Atomic Defects Atomic defects are imperfections in the atomic structure of the fiber materials such as missing molecules, high density clusters of atom groups.

These absorption losses are negligible compared with intrinsic and extrinsic losses. The radiation dames the internal structure of fiber. The damages are proportional to the intensity of ionizing particles. This results in increasing attenuation due to atomic defects and absorbing optical energy. The total dose a material receives is expressed in rad Si , this is the unit for measuring radiation absorbed in bulk silicon.

Extrinsic Absorption Extrinsic absorption occurs due to electronic transitions between the energy level and because of charge transitions from one ion to another. A major source of attenuation is from transition of metal impurity ions such as iron, chromium, cobalt and copper.

The effect of metallic impurities can be reduced by glass refining techniques. Another major extrinsic loss is caused by absorption due to OH Hydroxil ions impurities dissolved in glass. Vibrations occur at wavelengths between 2. The absorption peaks occurs at , and nm. These are first, second and third overtones respectively.

Between these absorption peaks there are regions of low attenuation. Thus intrinsic absorption sets the fundamental lower limit on absorption for any particular material. Intrinsic absorption results from electronic absorption bands in UV region and from atomic vibration bands in the near infrared region. The electronic absorption bands are associated with the band gaps of amorphous glass materials.

Absorption occurs when a photon interacts with an electron in the valene band and excites it to a higher energy level. In the IR infrared region above 1. The inherent IR absorption is due to interaction between the vibrating band and the electromagnetic field of optical signal this results in transfer of energy from field to the band, thereby giving rise to absorption, this absorption is strong because of many bonds present in the fiber.

Attenuation spectra for the intrinsic loss mechanism in pure Ge is shown in Fig. The loss in infrared IR region above 1.

The expression is derived for GeO2-SiO2 glass fiber. Rayleigh Scattering Losses Scattering losses exists in optical fibers because of microscopic variations in the material density and composition. As glass is composed by randomly connected network of molecules and several oxides e. These two effects results to variation in refractive index and Rayleigh type scattering of light.

Rayleigh scattering of light is due to small localized changes in the refractive index of the core and cladding material. There are two causes during the manufacturing of fiber. The first is due to slight fluctuation in mixing of ingredients.

The random changes because of this are impossible to eliminate completely. The other cause is slight change in density as the silica cools and solidifies. When light ray strikes such zones it gets scattered in all directions. The overall losses in this fiber are more as compared to single mode fibers.

Mie Scattering: Careful control of manufacturing process can reduce mie scattering to insignificant levels. This is shown in Fig. As the core bends the normal will follow it and the ray will now find itself on the wrong side of critical angle and will escape. The sharp bends are therefore avoided. The radiation loss from a bent fiber depends on — Field strength of certain critical distance xc from fiber axis where power is lost through radiation.

The radius of curvature R. The higher order modes are less tightly bound to the fiber core, the higher order modes radiate out of fiber firstly. For multimode fiber, the effective number of modes that can be guided by curved fiber is given expression: Microbending Microbending is a loss due to small bending or distortions.

This small micro bending is not visible. The losses due to this are temperature related, tensile related or crush related. The effects of microbending on multimode fiber can result in increasing attenuation depending on wavelength to a series of periodic peaks and troughs on the spectral attenuation curve.

These effects can be minimized during installation and testing. Macrobending The change in spectral attenuation caused by macrobending is different to micro bending. Usually there are no peaks and troughs because in a macrobending no light is coupled back into the core from the cladding as can happen in the case of microbends. The macrobending losses are cause by large scale bending of fiber. The losses are eliminated when the bends are straightened. The losses can be minimized by not exceeding the long term bend radii.

For step index fiber, the loss for a mode order v, m is given by, For low-order modes, the expression reduced to For graded index fiber, loss at radial distance is expressed as, The loss for a given mode is expressed by, Where, P r is power density of that model at radial distance r. Signal Distortion in Optical Waveguide The pulse gets distorted as it travels along the fiber lengths.

Pulse spreading in fiber is referred as dispersion. Dispersion is caused by difference in the propagation times of light rays that takes different paths during the propagation.

The light pulses travelling down the fiber encounter dispersion effect because of this the pulse spreads out in time domain.

The distortion effects can be analyzed by studying the group velocities in guided modes. Information Capacity Determination Dispersion and attenuation of pulse travelling along the fiber is shown in Fig. At certain distance the pulses are not even distinguishable and error will occur at receiver. Therefore the information capacity is specified by bandwidth distance product MHz. For step index bandwidth distance product is 20 MHz.

Group Delay Consider a fiber cable carrying optical signal equally with various modes and each mode contains all the spectral components in the wavelength band. All the spectral components travel independently and they observe different time delay and group delay in the direction of propagation. The velocity at which the energy in a pulse travels along the fiber is known as group velocity.

Group velocity is given by, Thus different frequency components in a signal will travel at different group velocities and so will arrive at their destination at different times, for digital modulation of carrier, this results in dispersion of pulse, which affects the maximum rate of modulation. Material dispersion exists due to change in index of refraction for different wavelengths. This results in time dispersion of pulse at the receiving end of fiber.

A plot of material dispersion and wavelength is shown in Fig. An LED operating at nm has a spectral width of 45 nm. What is the pulse spreading when a laser diode having a 2 nm spectral width is used? Find the the material-dispersion-induced pulse spreading at nm for an LED with a 75 nm spectral width?

Waveguide dispersion is significant only in fibers carrying fewer than modes. Since multimode optical fibers carry hundreds of modes, they will not have observable waveguide dispersion. As frequency is a function of wavelength, the group velocity of the energy varies with frequency. The produces additional losses waveguide dispersion. The propagation constant b varies with wavelength, the causes of which are independent of material dispersion. Chromatic Dispersion The combination of material dispersion and waveguide dispersion is called chromatic dispersion.

These losses primarily concern the spectral width of transmitter and choice of correct wavelength. A graph of effective refractive index against wavelength illustrates the effects of material, chromatic and waveguide dispersion. Material dispersion and waveguide dispersion effects vary in vary in opposite senses as the wavelength increased, but at an optimum wavelength around nm, two effects almost cancel each other and chromatic dispersion is at minimum. Attenuation is therefore also at minimum and makes nm a highly attractive operating wavelength.

The net effect is spreading of pulse, this form of dispersion is called modal dispersion. Modal dispersion takes place in multimode fibers.

It is moderately present in graded Index fibers and almost eliminated in single mode step index fibers. This results in pulse broadening is known as polarization mode dispersion PMD. PMD is the limiting factor for optical communication system at high data rates. The effects of PMD must be compensated. Pulse Broadening in GI Fibers The core refractive index varies radially in case of graded index fibers, hence it supports multimode propagation with a low intermodal delay distortion and high data rate over long distance is possible.

The higher order modes travelling in outer regions of the core, will travel faster than the lower order modes travelling in high refractive index region. If the index profile is carefully controlled, then the transit times of the individual modes will be identical, so eliminating modal dispersion.

The r. From this the expression for intermodal pulse broadening is given as: The intramodal pulse broadening is given as: Solving the expression gives: Briefly explain material dispersion with suitable sketch?

Give expression of pulse broadening in graded index fiber?. Elaborate dispersion mechanism in optical fibers? Differentiate between intrinsic and extrinsic absorption? Derive an expression for the pulse spread due to material dispersion using group delay concept?

Explain the significance of measure of information capacity? Describe the material dispersion and waveguide Dispersion? Discuss Bending Loss? Explain absorption losses? Describe attenuation mechanism?

Optical Sources Optical transmitter coverts electrical input signal into corresponding optical signal. The optical signal is then launched into the fiber. Optical source is the major component in an optical transmitter. Characteristics of Light Source of Communication To be useful in an optical link, a light source needs the following characteristics: As the carriers are not confined to the immediate vicinity of junction, hence high current densities can not be realized.

The middle layer may or may not be doped. The carrier confinement occurs due to band gap discontinuity of the junction. Such a junction is call heterojunction and the device is called double heterostructure. LEDs are best suitable optical source. LED Structures Heterojunction A heterojunction is an interface between two adjoining single crystal semiconductors with different band gap. Heterojunction are of two types, Isotype n-n or p-p or Antistype p-n.

Double Heterojunction DH In order to achieve efficient confinement of emitted radiation double heterojunction are used in LED structure. A heterojunction is a junction formed by dissimilar semiconductors. Double heterojunction DH is formed by two different semiconductors on each side of active region.

The crosshatched regions represent the energy levels of free charge. Recombination occurs only in active InGaAsP layer. The two materials have different band gap energies and different refractive indices. The changes in band gap energies create potential barrier for both holes and electrons.

The free charges can recombine only in narrow, well defined active layer side. A double heterojunction DH structure will confine both hole and electrons to a narrow active layer.

Under forward bias, there will be a large number of carriers injected into active region where they are efficiently confined.

Antoer advantage DH structure is that the active region has a higher refractive index than the materials on either side, hence light emission occurs in an optical waveguide, which serves to narrow the output beam. Surface emitting LED. Edge emitting LED. Both devices used a DH structure to constrain the carriers and the light to an active layer.

A DH diode is grown on an N-type substrate at the top of the diode as shown in Fig. A circular well is etched through the substrate of the device.

A fiber is then connected to accept the emitted light. The current flows through the p-type material and forms the small circular active region resulting in the intense beam of light.

Optical Fiber Communications by Gerd Keiser

The isotropic emission pattern from surface emitting LED is of Lambartian pattern. The beam intensity is maximum along the normal. The radiation pattern decides the coupling efficiency of LED. It consists of an active junction region which is the source of incoherent light and two guiding layers. The refractive index of guiding layers is lower than active region but higher than outer surrounding material.

Thus a waveguide channel is form and optical radiation is directed into the fiber. The beam is Lambartian in the plane parallel to the junction but diverges more slowly in the plane perpendicular to the junction. In this plane, the beam divergence is limited. In the parallel plane, there is no beam confinement and the radiation is Lambartian.

To maximize the useful output power, a reflector may be placed at the end of the diode opposite the emitting edge. Features of ELED: Linear relationship between optical output and current. Modulation bandwidth is much large. Not affected by catastrophic gradation mechanisms hence are more reliable. ELEDs have better coupling efficiency than surface emitter. ELEDs are temperature sensitive.

LEDs are suited for short range narrow and medium bandwidth links. Long distance analog links. Light Source Materials The spontaneous emission due to carrier recombination is called electro luminescence.

To encourage electroluminescence it is necessary to select as appropriate semiconductor material. The semiconductors depending on energy band gap can be categorized into, 1. Direct band gap semiconductors. Indirect band gap semiconductors. Some commonly used band gap semiconductors are shown in following table 3. Hence direct recombination is possible.

The recombination occurs within to sec. In indirect band gap semiconductors, the maximum and minimum energies occur at Different values of crystal momentum. The recombination in these semiconductors is quite slow i. The active layer semiconductor material must have a direct band gap.

In direct band gap semiconductor, electrons and holes can recombine directly without need of third particle to conserve momentum. In these materials the optical radiation is sufficiently high. Some tertiary alloys Ga Al As are also used. The peak output power is obtained at nm.

The width of emission spectrum at half power 0. Different materials and alloys have different bandgap energies. The bandgap energy Eg can be controlled by two compositional parameters x and y, within direct bandgap region. Where, Rr is radiative recombination rate. Rnr is non-radiative recombination rate. It is also known as bulk recombination life time. The external quantum efficiency is used to calculate the emitted power.

The external quantum efficiency is defined as the ratio of photons emitted from LED to the number of photons generated internally. The radiative and non radiative recombination life times of minority carriers in the active region of a double heterojunction LED are 60 nsec and 90 nsec respectively.

Determine the total carrier recombination life time and optical power generated internally if the peak emission wavelength si nm and the drive currect is 40 mA. Simple design. Ease of manufacture. Simple system integration.

Low cost. High reliability. Disadvantages of LED 1. The average life time of a radiative recombination is only a few nanoseconds, therefore nodulation BW is limited to only few hundred megahertz. Low coupling efficiency. Large chromatic dispersion. The operation of the device may be described by the formation of an electromagnetic standing wave within a cavity optical resonator which provides an output of monochromatic highly coherent radiation. Material absorption light than emitting. Three different fundamental process occurs between the two energy states of an atom.

Laser action is the result of three process absorption of energy packets photons spontaneous emission, and stimulated emission. These processes are represented by the simple two-energy-level diagrams.

Where, E1 is the lower state energy level. E2 is the higher state energy level. Quantum theory states that any atom exists only in certain discrete energy state, absorption or emission of light causes them to make a transition from one state to another. The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the two states.

If E1 is lower state energy level. An atom is initially in the lower energy state, when the photon with energy E2 — E1 is incident on the atom it will be excited into the higher energy state E2 through the absorption of the photon. The emission process can occur in two ways. A By spontaneous emission in which the atom returns to the lower energy state in random manner.

B By stimulated emission when a photon having equal energy to the difference between the two states E2 — E1 interacts with the atom causing it to the lower state with the creation of the second photon.

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Spontaneous emission gives incoherent radiation while stimulated emission gives coherent radiation. Hence the light associated with emitted photon is of same frequency of incident photon, and in same phase with same polarization.

It means that when an atom is stimulated to emit light energy by an incident wave, the liberated energy can add to the wave in constructive manner. The emitted light is bounced back and forth internally between two reflecting surface. The bouncing back and forth of light wave cause their intensity to reinforce and build-up. The section titles in the TOS are for convenience only and have no legal or contractual effect.

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