Download Optical Fiber Communications: Principles and Practice By John M. Senior – Senior is an established core text in a field that is growing fast, and in. Page 1. Optical Fiber. Communications. - Gerd Keiser. Second Edition. TiiiTIT. MCGRAW-HILL INTERNATIONAL EDITIONS. Electrical & Electronic Engineering . Main Characteristics of Fiber Optics Communication System. - Light propagation in of Plastic Fibers. (Source: musicmarkup.info).
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Optical fiber communications: principles and practice / John M. Senior, assisted by musicmarkup.info, with permission from Fujikura Limited; Figure (a) from. Yoga is, indeed, an excellent form of exercise that carries with it many. own sequences of yoga poses, which were char Load more similar PDF files. Optical Fibre Communications musicmarkup.info - Free ebook download as PDF File .pdf), Text File .txt) or read book online for free. E-Book Optical Fibre.
During this period tremendous advances have been achieved with optical fibers and components as well as with the associated optoelectronics. As a result this new technology has now reached the threshold of large scale commercial exploitation.. Installation of optical fiber communication systems is progressing within both national telecommunication networks and more localized data communication and telemetry environments. Furthermore, optical fiber communication has become synonymous with the current worldwide revolution in information techno1ogy.. The relentless onslaught will undoubtedly continue over the next decade and the further predicted developments will ensure even wider application of optical fiber communication technology in this 'infermation age'.. The practical realization of wide-scale optical fiber communications requires suitable education and training for engineers and scientists within the technology. In this context the book has been developed from both teaching the subject to final year undergraduates and from a successful series of short courses on optical fiber communications conducted for professional engineers at Manchester Polytechnic.
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.
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.
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 result in a high brilliance, single frequency light beam providing amplification. Emission and Absorption Rates It N1 and N2 are the atomic densities in the ground and excited states.
Under equilibrium condition the atomic densities N1 and N2 are given by Boltzmann statistics. Where, KB is Boltzmann constant. T is absolute temperature. Under equilibrium the upward and downward transition rates are equal. Fabry — Perot Resonator Lasers are oscillators operating at frequency. The oscillator is formed by a resonant cavity providing a selective feedback. The cavity is normally a Fabry-Perot resonator i. The two heterojunctions provide carrier and optical confinement in a direction normal to the junction.
The current at which lasing starts is the threshold current. Above this current the output power increases sharply. Lasing light amplification occurs when gain of modes exceeds above optical loss during one round trip through the cavity i. Now the expression for lasing expressing is modified as, The condition of lasing threshold is given as — i For amplitude: Resonant Frequencies At threshold lasing m is an integer.
Gain in any laser is a function of frequency. For a Gaussian output the gain and frequency are related by expression — where, g 0 is maximum gain. The frequency spacing between the two successive modes is — The wavelength Spacing is given as — Optical Characteristics of LED and Laser The output of laser diode depends on the drive current passing through it.
At low drive current, the laser operates as an inefficient Led, When drive current crosses threshold value, lasing action beings. Advantages of Laser Diode 1. Simple economic design. High optical power.
Production of light can be precisely controlled. Can be used at high temperatures. Better modulation capability. High coupling efficiency. Low spectral width 3. Ability to transmit optical output powers between 5 and 10 mW. Ability to maintain the intrinsic layer characteristics over long periods. At the end of fiber, a speckle pattern appears as two coherent light beams add or subtract their electric field depending upon their relative phases.
Laser diode is extremely sensitive to overload currents and at high transmission rates, when laser is required to operate continuously the use of large drive current produces unfavorable thermal characteristics and necessitates the use of cooling and power stabilization. The main requirement of light detector or photo dector is its fast response. For fiber optic communication purpose most suited photo detectors are PIN p-type- Intrinsic-n-type diodes and APD Avalanche photodiodes The performance parameters of a photo detector are responsivity, quantum efficiency, response time and dark current.
The cut-off wavelength is determined by band gap energy Eg of material. Pin is average optical power incident on photo detector. Absorption coefficient of material determines the quantum efficiency. It is normally expressed in percentage. Responsivity is denoted by Responsivity gives transfer characteristics of detector i. Germanium pin photodiode at 1.
In GaAs pin photodiode at 1. As the intensity of optical signal at the receiver is very low, the detector has to meet high performance specifications. At present, these requirements are met by reverse biased p-n photodiodes. In these devices, the semiconductor material absorbs a photon of light, which excites an electron from the valence band to the conduction band opposite of photon emission. The increases the material conductivity so call photoconductivity resulting in anincrease in the diode current.
The diode equation is modified as — Where, Id is dark current i. Is is photo generated current due to incident optical signal. PIN Photodiode PIN diode consists of an intrinsic semiconductor sandwiched between two heavily doped p-type and n-type semiconductors as shown in Fig.
Sufficient reverse voltage is applied so as to keep intrinsic region free from carries, so its resistance is high, most of diode voltage appears across it, and the electrical forces are strong within it. The incident photons give up their energy and excite an electron from valance to conduction band.
Thus a free electron hole pair is generated, these are called as photocarriers. These carriers are collected across the reverse biased junction resulting in rise in current in external circuit called photocurrent. In the absence of light, PIN photodiodes behave electrically just like an ordinary rectifier diode.
If forward biased, they conduct large amount of current. PIN detectors can be operated in two modes: Photovoltaic and photoconductive.
In photovoltaic mode, no bias is applied to the detector. In this case the detector works very slow, and output isapproximately logarithmic to the input light level.
Real world fiber optic receivers never use the photovoltaic mode. In photoconductive mode, the detector is reverse biased. The output in this case is a current that is very linear with the input light power. The intrinsic region some what improves the sensitivity of the device.
It does not provide internal gain. Jdiff is diffusion current density due to carriers generated outside depletion region. The drift current density is expressed as — where, A is photodiode area.
Pn is hole concentration in n-type material. Pn0 is equilibrium hole density. The transit time is given by — The diffusion process is slow and diffusion times are less than carrier drift time. By considering the photodiode response time the effect of diffusion can be calculated. The detector behaves as a simple low pass RC filter having pass band of where, RT, is combination input resistance of load and amplifier. CT is sum of photodiode and amplifier capacitance. APDs uses the avalanche breakdown phenomena for its operation.
The APD has its internal gain which increases its responsivity. In this region, the E-field separates the carriers and the electrons drift into the avalanche region where carrier multiplication occurs.
If the APD is biased close to breakdown, it will result in reverse leakage current. Thus APDs are usually biased just below breakdown, with the bias voltage being tightly controlled.
List the characteristics of light sources required in optical communication. Describe the construction and working of LED. Explain the structure of surface emitting and edge emitting LEDs. Deduce the expression at internal quantum efficiency and internally generated optical power for LED. From this expression how external efficiency and power is calculated?
Explain the principle of laser action. Explain also the spontaneous and stimulated emission process. Give the necessary conditions for lasing threshold. Explain the structure of — i Fabry-Perot resonator. Derive expression for lasing condition and hence for optical gain. Explain the power current characteristics of laser diode. Give the expression for — i External quantum efficiency. State the significance of each parameter in the expression.
With a proper sketch briefly explain the structure of PIN diode. Explain the following term relating to PIN photodiode with proper expressions. Explain the structure and principle of working of APD. Deduce the expression for total current density for APD. How the response time of APD is estimated? Give expression for pass band of APD detector. The interconnection of fiber causes some loss of optical power. Different techniques are used to interconnect fibers. A permanent joint of cable is referred to as splice and a temporary joint can be done with the connector.
The fraction of energy coupled from one fiber to other proportional to common mode volume Mcommon. The fiber — to — fiber coupling efficiency is given as — where, ME is number of modes in fiber which launches power into next fiber.
If the radiation cone of emitting fiber does not match the acceptance cone of receiving fiber, radiation loss takes place. The magnitude of radiation loss depends on the degree of misalignment.
Different types of mechanical misalignments are shown in Fig. Angular misalignment Angular misalignment occurs when fiber axes and fiber end faces are no longer parallel. The axial or lateral misalignment is most common in practice causing considerable power loss. The axial offset reduces the common core area of two fiber end faces as shown in Fig. The common area is given by expression — Where, a is core radius of fiber. The coupling efficiency for step index fiber is the ratio of common core area to the end- face area.
For graded index fiber, the total received power for axial misalignment is given by — Where, P is the power in emitting fiber. These includes, - Variation in core diameter. Coupling loss when emitter fiber radius aE and receiving fiber radius aR is not same, is given as — where, aE is emitter fiber radius.
Coupling loss when numerical apertures of two fibers are not equal, to expressed as — Coupling loss when core refractive index of two fibers are not same, is expressed as Precaution If the stress distribution is not properly controlled, fiber can fork into several cracks, various types of defects can be introduced in the fiber, few of them are mentioned here. And the process of joining two fibers is called as splicing. Typically, a splice is used outside the buildings and connectors are used to join the cables within the buildings.
Splices offer lower attenuation and lower back reflection than connectors and are less expensive. Types of Splicing There are two main types of splicing i Fusion splicing.
Fusion splicing is normally done with a fusion splicer that controls the alignment of the two fibers to keep losses as low as 0. Fiber ends are first pre aligned and butted together under a microscope with micromanipulators. The butted joint is heated with electric arc or laser pulse to melt the fiber ends so can be bonded together.
Mechanical splices may have a slightly higher loss and back reflection. These can be reduced by inserting index matching gel. V groove mechanical splicing provides a temporary joint i. The fiber ends are butted together in a V — shaped groove as shown in Fig.
The splice loss depends on fiber size and eccentricity Source-to-Fiber Power Launching Optical output from a source is measured in radiance B. Radiance is defined as the optical power radiated into a solid angle per unit emitting surface area.
Radiance is important for defining source to fiber coupling efficiency. Source Output Pattern Spatial radiation pattern of source helps to determine the power accepting capability of fiber. The emission pattern of Lambartian output is shown in Fig.
Both radiations in parallel and normal to the emitting plane are approximated by expression — Where, T and L are transverse and lateral power distribution coefficients. Power Coupling Calculation To calculate power coupling into the fiber, consider an optical source launched into the fiber as shown in Fig. Numerical aperture for graded index fiber is given by, Is source radius rs is less than fiber core radius a i. The power couple is reduced by factor, where, n is the refractive index of medium.
R is the Fresnel reflection or reflectivity. Lensing Schemes for Coupling Improvement When the emitting area of the source is smaller than the core area of fiber, the power coupling efficiency becomes poor.
In order to improve the coupling efficiency miniature lens is placed between source and fiber. Micro lens magnifies the emitting area of source equal to core area. The power coupled increases by a factor equal to magnification factor of lens. Important types of lensing schemes are: Rounded — end fiber. Spherical — surfaced LED and Spherical-ended fiber. Taper ended fiber. Non imaging microsphere. Cylindrical lens, 6. Imaging sphere.
There are some drawbacks of using lens. Complexity increases. Fabrication and handling difficulty. Precise mechanical alignment is needed. Equilibrium Numerical Aperture The light source has a short fiber fly lead attached to it to facilitate coupling the source to a system fiber.
The low coupling loss, this fly lead should be connected to system fiber with identical NA and core diameter. At this junction certain amount of optical power approximately 0. Also excess power loss occurs due to non propagating modes scattering out of fiber. The excess power loss is to be analyzed carefully in designing optical fiber system.
This excess loss is shown in terms of fiber numerical aperture. If the optical powers of measured in long fiber lengths under equilibrium of modes, the effect of equilibrium numerical aperture NAeq is significant.
Optical power at this point is given by, Where, P50 is optical power in fiber at 50 m distance from launch NA. The degree of mode coupling is mainly decided by core — cladding index difference. Hence NAeq is important while calculating launched optical power in telecommunication systems. Connectors are mechanisms or techniques used to join an optical fiber to another fiber or to a fiber optic component. Different connectors with different characteristics, advantages and disadvantages and performance parameters are available.
Suitable connector is chosen as per the requirement and cost. However, selected proofs are developed in important areas throughout the text. It is assumed that the reader is conversant with differential and integral calcu1us and differential equations.
In addition, the reader will find it useful to have a grounding in optics as well as a reasonable familiarity with the fundamentals of , solid state physics. Chapter 1 gives a short introduction to optical fiber communications by considering the historical development, the general system and the major advantages provided by this new technology. Chapter 2 the concept of the optical fiber as a transmission medium is introduced using a simple ray theory approach.
This is followed by discuslion of electromagnetic wave theory applied to optical fibers prior to consideration of Ii,htwave transmission within the various fiber types, The major transmission cha r acteris tics of optical fi bers are th en di sCU ssed in so me detail in C h a pter 3. Chapters 4 and 5 deal with the more practical aspects of optical fiber communication. In Chapter 4 the x PREFACE manufacture and cabling of the various fiber types are described, together with fiber to fiber connection or jointing, Chapter 5 gives a genera] treatment of the major measurements which may be undertaken on optical fibers in both the laboratory and the field.
This chapter is intended to provide sufficient background for the reader to pursue u se f u l 1 a bora tory wo r k with opti cal fibers. The other important semiconductor optical source, namely the light emitting diode, is dealt with in Chapter 7. The next two chapters are devoted to the detection of the optical signal and the amplification of the electrical signal obtained, Chapter 8 discusses the basic principles of optical detection in semiconductors; this is followed by a description of the various types of photodetector currently utilized.
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