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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.
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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.
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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.
These are — 1. Subscriber Channel SC connector. Straight Tip ST connector. MT-RJ connector. SC connectors are general purpose connections. It has push-pull type locking system. ST connectors are most suited for networking devices. It is more reliable than SC connector. ST connector has bayonet type locking system. Low coupling loss. Inter-changeability — No variation is loss whenever a connector is applied to a fiber. Ease of assembly.
Low environmental sensitivity. Low cost — The connector should be in expensive also the tooling required for fitting. Reliable operation. Ease of connection. Repeatability — Connection and reconnection many times without an increase in loss.
Connector Types Connectors use variety of techniques for coupling such as screw on, bayonet-mount, puch-pull configurations, butt joint and expanded beam fiber connectors. Butt Joint Connectors. Fiber is epoxied into precision hole and ferrules are used for each fiber. The fibers are secured in a precision alignment sleeve. But joints are used for single mode as well as for multimode fiber systems.
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Two commonly used butt-joint alignment designs are: In straight sleeve mechanism, the length of the sleeve and guided ferrules determines the end separation of two fibers.
In tapered sleeve or biconical connector mechanism, a tapered sleeve is used to accommodate tapered ferrules. The fiber end separations are determined by sleeve length and guide rings.
Due to this unwanted feedback the optical frequency response may degrade, also it generates internal noise within the source affecting overall system performance. The return loss for the index-matched gap region is given by, Where, D is the separation between fiber ends.
R is reflectivity constant. State the considerations for power coupling and power launching in a fiber optic system. Derive the expression for power coupling to a step index fiber by a surface emitting LED. Derive the expression for power coupling to a graded index fiber by a surface emitting LED. Explain various types of misalignments in fiber cables.
Derive the expression for power received by fiber for axial misalignment. Give the expressions for various fiber-related losses. State the steps involved in cleaving process 8. Explain controlled fracture technique of cleaving. Define finer splicing. Explain different types of splicing.
State the factors on which the power launching capability of source is dependent. What is lensing schemes? With simple sketch show different lensing scheme. State the drawback of lensing schemes also.
Explain equilibrium numerical aperture. Write note on laser diode to fiber coupling. State the principles of good connector design. List the steps involved in process of installing fiber optic connectors. Noise generated in receiver must be controlled precisely as it decides the lowest signal level that can be detected and processed.
Hence noise consideration is an important factor in receiver design. Another important performance criteria of optical receiver is average error probability. Receiver Configuration Configuration of typical optical receiver is shown in Fig.
All noise sources are Gaussian in statistics. All nose sources are flat in spectrum. All noise sources are uncorrelated statistically independent.
Binary digital pulse train incident on photodector is fiven by — Where, P t is received optical power.
Tb is bit period. Preamplifier Types The bandwidth, BER, noise and sensitivity of optical receiver are determined by preamplifier stage. Preamplifier circuit must be designed with the aim of optimizing these characteristics. Commonly used preamplifier in optical communication receiver are — 1.
Low — impedance preamplifier LZ 2. High — impedance preamplifier HZ 3. Transimpedance preamplifier TZ 1. Low — impedance preamplifier LZ In low-impedance preamplifier, the photodiode is configured in low — impedance amplifier. The bias resister Rb is used to match the amplifier impedance. Rb along with the input capacitance of amplifier decides the bandwidth of amplifier. Low — impedance preamplifier can operate over a wide bandwidth but they have poor receiver sensitivity.
Therefore the low — impedance amplifier are used where sensitivity is of not prime concern. High — impedance preamplifier HZ In high — impedance preamplifier the objective is to minimize the noise from all sources.
This can be achieved by — - Reducing input capacitance by selecting proper devices. As the high impedance circuit has large RC time constant, the bandwidth is reduced. A differentiating, equalizing or compensation network at the receiver output corrects for this integration. Transimpedance preamplifier TZ The drawbacks of ghigh input impedance are eliminated in transimpedance preamplifier. A negative feedback is introduced by a feedback resistor Rf to increase the bandwidth of open loop preamplifier with an equivalent thermal nose current if t shunting the input.
An equivalent circuit of transimpedance preamplifier is shown in Fig. Basic noise sources in the circuit are — - Thermal noise associated with FET channel. Thermal noise characteristic equation is a very useful figure of merit for a receiver as it measures the noiseness of amplifier. For this the detector output signal is integrated amplifier input resistance. It is to be compensated by differentiation in the equalizer. The integration — differentiation known as high input impedance preamplifier design technique.
However, the integration of receive signal at the front end restricts the dynamic range of receiver. It may disrupt the biasing levels and receiver may fail. To correct it the line coded data or AGC may be employed such receivers can have dynamic ranges in excess of 20 dB. Of course, FET with high gm is selected. Spectral density of input noise current source because shot noise of base current is — Spectral height of noise voltage source is given as — Where, gm is transconductance.
The performance of receiver is expressed by thermal noise characteristic equation W Substituting Rin, SI and SE in characteristic equation. A transimpednace amplifier is a high-gain high-impedance amplifier with feedback resistor Rf Fig. Shunt feedback transimpedance receiver. Bandwidth BW To find BW, the transfer function of non-feedback amplifier and feedback amplifier is compared. The transfer function of non-feedback amplifier is Where, A is frequency independent gain of amplifier.
Now the transfer function of feedback transimpedance amplifier is — This yields the BW of transimpedance amplifier. BW of transimpedance amplifier is A times that of high-impedance amplifier. Because of this equalization becomes easy. Characteristic equation The thermal noise characteristic equation W is reduced to — Where, WHZ is noise characteristic of high-impedance amplifier non-feedback amplifier.
Thus thermal nose of transimpedance amplifier is sum of ooutput noise of non-feedback amplifier and noise associated with R f. Benefits of transimpedance amplifier 1. Wide dynamic range: As the BW of transimpedance preamplifier is high enough so that no integration takes place and dynamic range can be set by maximum voltage swing at preamplifier output.
No equalization required: Since combination of Rin and Rf is very small hence the time constant of detector is small. Less susceptibe to external noise: Easy control: Transimpedance amplifiers have easy control over its operation and is stable.
Compensating network not required: Since integration of detected signal does not occur, compensating network is not required. High Speed Circuit Now fiber optic technology is widely employed for long-distance communication, LAN and in telephone networks also because of improvement in overall performance, reliable operation and cost effectiveness.
Because of advancement in technology minimized transmitters and receivers and available in integrated circuits package. Noise is an unwanted electric signal in signal processing. The noise sources can be internal or external to the system.
Only the internal sources of noise are considered here. The nose is generated by spontaneous fluctuations of current and voltage e. When photons incident on the photo detector are random in nature, quantum noise shot noise is generated. Other sources of photo detector noise are from dark current and leakage current.
These noise can be reduced considerably by choosing proper components. Thermal noise is generated from detector load resistances. Intersymbol interference ISI also contributes to error which is causing from pulse spreading. Because of pulse spreading energy of a pulse spreads into neighbouring time slots, results in an interfering signal.
Noise Mechanisms Where, vN t is noise voltage. The noise voltage can be expressed as — Where, Vshot t is quantum or shot noise. The mean square noise voltage is expressed as — i Thermal noise of load resistor Rb: A is amplifier gain.
Bbae is noise equivalent BW. Where, SE is the spectral density of amplifier noise voltage source. Be is noise equivalent BW of amplifier. Where, All constituents of mean square noise voltage are summarized here. Substituting these values and solving equation 5. W is thermal noise characteristic of receiver.
X is photodiode factor. I2 is normalized BW. The incident optical power is nW - 35 dBm an the receiver bandwidth is 20 MHz. Find the various nosie terms of the receiver. The current generated at optical receiver by analog optical signal is given as — For a photodiode detector mean noise current is sum of i Mean square quantum noise current.
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