Fibre optic cables experience losses from attenuation and dispersion. Attenuation includes material absorption from intrinsic effects like electronic and molecular vibrations, and extrinsic effects like impurities. Scattering losses occur from irregularities causing Rayleigh, Brillouin, Raman, waveguide and Mie scattering. Nonlinear losses include micro/macro bending, leaky modes and mode coupling. Proper fibre design and manufacturing can minimize these losses to support high bandwidth signal transmission.
Unit 3- OPTICAL SOURCES AND DETECTORS tamil arasan
This document discusses optical sources and detectors used in fiber optic communications. It describes light emitting diodes (LEDs) and laser diodes as the main optical sources. LEDs use a double heterostructure to provide carrier and optical confinement for high efficiency. They emit incoherent light without an optical cavity. Laser diodes function as coherent sources using a Fabry-Perot cavity formed by cleaved facets to provide optical feedback, producing highly directional and monochromatic output. Factors such as modulation capability and fiber characteristics must be considered when choosing an optical source.
Optical fibers experience various intrinsic and extrinsic losses that limit signal strength over long distances. Intrinsic losses include material absorption and scattering due to fiber imperfections. Absorption is caused by molecular vibrations and impurities, while scattering results from refractive index fluctuations. Extrinsic losses include bending, launching, and connector losses. Bending losses occur from macroscopic or microscopic bends, launching losses are from imperfect coupling into the fiber, and connector losses are due to core misalignments between joined fibers. Together these losses contribute to the overall attenuation of signals transmitted through optical fibers.
This document discusses optical waveguides and fiber optic modes. It begins by describing the mode patterns seen in the end faces of small diameter fibers. It then discusses multimode propagation and explains that many modes are excited, resulting in complex field and intensity patterns. Finally, it summarizes the key parameters and solutions used to determine the modes in cylindrical optical fibers.
Optical fiber communication Part 2 Sources and DetectorsMadhumita Tamhane
For optical fiber communication, major light sources are hetero-junction-structured semiconductor laser diode and light emitting diodes. Heterojunction consists of two adjoining semiconductor materials with different bandgap energies. They have adequate power for wide range of applications. Detectors used are PiN diode and Avalanche Photodiode. Being very small in size and feeding to small core optical fiber, it is very important to study emission characteristics of sources and their coupling to fiber. As it can operate for low power over a long distance, received power is very small, hence study of noise characteristics of detectors is very essential...
There are two main types of optical fiber signal loss: scattering and absorption. Scattering losses include Rayleigh scattering caused by molecular irregularities and Mie scattering caused by larger defects. Absorption losses are caused by intrinsic material properties like ultraviolet and infrared absorption in silica glass, as well as extrinsic impurities introduced during manufacturing. Proper fiber design and high material purity can minimize these signal losses to enable effective optical fiber communication.
This narrated power point presentation attempts to explain the various dispersion mechanisms that are observed in optical fibers. Some fundamental terms and concepts are also discussed. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
Graded index (GRIN) optical fibers have a refractive index that decreases continuously from the core center to the cladding. This results in curved ray paths inside the core rather than straight lines, reducing intermodal dispersion. The optimal refractive index profile for minimizing dispersion is parabolic. Attenuation in optical fibers is due to various factors including material absorption, scattering, and bending losses. Rayleigh scattering increases at shorter wavelengths, while absorption peaks exist for hydroxyl and metal impurities.
The document discusses optical detectors used in fiber optic communications systems. It describes the functioning of PIN photodetectors and avalanche photodetectors (APDs). PIN photodetectors convert received light photons into an electric current through the photoelectric effect. Their performance is characterized by quantum efficiency and responsivity. APDs have higher gain than PIN photodiodes through impact ionization, but also higher noise. Both device types aim to optimize sensitivity while minimizing noise.
Unit 3- OPTICAL SOURCES AND DETECTORS tamil arasan
This document discusses optical sources and detectors used in fiber optic communications. It describes light emitting diodes (LEDs) and laser diodes as the main optical sources. LEDs use a double heterostructure to provide carrier and optical confinement for high efficiency. They emit incoherent light without an optical cavity. Laser diodes function as coherent sources using a Fabry-Perot cavity formed by cleaved facets to provide optical feedback, producing highly directional and monochromatic output. Factors such as modulation capability and fiber characteristics must be considered when choosing an optical source.
Optical fibers experience various intrinsic and extrinsic losses that limit signal strength over long distances. Intrinsic losses include material absorption and scattering due to fiber imperfections. Absorption is caused by molecular vibrations and impurities, while scattering results from refractive index fluctuations. Extrinsic losses include bending, launching, and connector losses. Bending losses occur from macroscopic or microscopic bends, launching losses are from imperfect coupling into the fiber, and connector losses are due to core misalignments between joined fibers. Together these losses contribute to the overall attenuation of signals transmitted through optical fibers.
This document discusses optical waveguides and fiber optic modes. It begins by describing the mode patterns seen in the end faces of small diameter fibers. It then discusses multimode propagation and explains that many modes are excited, resulting in complex field and intensity patterns. Finally, it summarizes the key parameters and solutions used to determine the modes in cylindrical optical fibers.
Optical fiber communication Part 2 Sources and DetectorsMadhumita Tamhane
For optical fiber communication, major light sources are hetero-junction-structured semiconductor laser diode and light emitting diodes. Heterojunction consists of two adjoining semiconductor materials with different bandgap energies. They have adequate power for wide range of applications. Detectors used are PiN diode and Avalanche Photodiode. Being very small in size and feeding to small core optical fiber, it is very important to study emission characteristics of sources and their coupling to fiber. As it can operate for low power over a long distance, received power is very small, hence study of noise characteristics of detectors is very essential...
There are two main types of optical fiber signal loss: scattering and absorption. Scattering losses include Rayleigh scattering caused by molecular irregularities and Mie scattering caused by larger defects. Absorption losses are caused by intrinsic material properties like ultraviolet and infrared absorption in silica glass, as well as extrinsic impurities introduced during manufacturing. Proper fiber design and high material purity can minimize these signal losses to enable effective optical fiber communication.
This narrated power point presentation attempts to explain the various dispersion mechanisms that are observed in optical fibers. Some fundamental terms and concepts are also discussed. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
Graded index (GRIN) optical fibers have a refractive index that decreases continuously from the core center to the cladding. This results in curved ray paths inside the core rather than straight lines, reducing intermodal dispersion. The optimal refractive index profile for minimizing dispersion is parabolic. Attenuation in optical fibers is due to various factors including material absorption, scattering, and bending losses. Rayleigh scattering increases at shorter wavelengths, while absorption peaks exist for hydroxyl and metal impurities.
The document discusses optical detectors used in fiber optic communications systems. It describes the functioning of PIN photodetectors and avalanche photodetectors (APDs). PIN photodetectors convert received light photons into an electric current through the photoelectric effect. Their performance is characterized by quantum efficiency and responsivity. APDs have higher gain than PIN photodiodes through impact ionization, but also higher noise. Both device types aim to optimize sensitivity while minimizing noise.
Optical multiplexers allow multiple signals to be transmitted simultaneously over a single optical fiber link. There are different optical multiplexing techniques, including wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM). WDM assigns each signal a unique wavelength, while OTDM separates signals in the time domain. Optical multiplexers and demultiplexers use passive optical filters to combine and separate the wavelength signals. This increases bandwidth utilization and reduces transmission costs.
An optical fiber coupler is a device that splits light from one fiber into multiple fibers. There are different types of couplers classified by their shape, including Y, T, X, star, and tree couplers. Couplers work by transferring power between fibers through their cores or surfaces. Examples show how to calculate excess loss, insertion loss, crosstalk, and splitting ratios using the measured input and output powers. Optical couplers have applications in splitting and combining optical signals in fiber networks and communication systems.
This document discusses semiconductor optical amplifiers (SOAs). It explains that SOAs use stimulated emission to amplify optical signals, like lasers, but have anti-reflection coatings on the facets so light passes through only once. The main types are traveling-wave amplifiers, which are widely used because they amplify signals with a single pass and have a large bandwidth. SOAs have a core made of InGaAsP for gain and InP cladding layers. External pumping by current injection provides carriers that undergo stimulated emission to amplify optical signals. Amplifier gain increases with length and current but saturates with increasing optical power due to depletion of excited carriers.
Passive optical components play a key role in optical networks by coupling, splitting, and multiplexing signals. Common passive components include fused fiber couplers and splitters, arrayed waveguide gratings (AWGs), and isolators/circulators. Couplers combine or split signals depending on fiber spacing, while AWGs multiplex and demultiplex wavelengths. Isolators and circulators use non-reciprocal polarization rotation to allow transmission in only one direction.
Optical fiber communication Part 1 Optical Fiber FundamentalsMadhumita Tamhane
Optical fiber systems grew from combination of semiconductor technology, which provided necessary light sources and photodetectors and optical waveguide technology. It has significant inherent advantages over conventional copper systems- low transmission loss, wide BW, light weight and size, immunity to interferences, signal security to name a few. One principle characteristic of optical fiber is its attenuation as a function of wavelength. Hence it is operated in two major low attenuation wavelength windows 800-900nm and 1100-1600nm . Light travels inside optical fiber waveguide on principle of total internal reflection. Fiber is available as single mode and multiple mode, step index and graded index depending on applications and expenditures. Principle of fiber can be understood by ray theory or mode theory. ...
The document discusses different types of linear-beam microwave tubes, specifically focusing on klystron tubes. It provides details on the operation of two-cavity klystrons and reflex klystrons. Two-cavity klystrons work by velocity modulating electrons in the first cavity which become current modulated before interacting with the second cavity to produce microwave power. Reflex klystrons use a single cavity and repeller field to reflect electrons, allowing them to interact twice with the cavity field and function as an oscillator. Quantitative analyses of velocity modulation, power output, and efficiency are also presented.
The document summarizes key components and concepts in basic microwave engineering. It discusses waveguides and their operating frequencies based on dimensions. It also describes electric and magnetic fields in rectangular waveguides. Additional components summarized include coaxial to waveguide transitions, choke joints, coupling loops, phase shifters, junctions, tuners, mixers, isolators, circulators, directional couplers, and cavity resonators. Isolators, circulators, and directional couplers are multi-port devices that control the direction of signal propagation with differing levels of attenuation.
This document discusses different types of dispersion in optical fibers, including modal dispersion, material dispersion, waveguide dispersion, and polarization mode dispersion. It defines important terms related to dispersion like group velocity and group delay. It also examines how dispersion causes pulse broadening over distance as different wavelengths within a pulse propagate at different speeds through the fiber. Finally, it compares the dispersion characteristics of different fiber types like dispersion shifted and flattened fibers which are designed to reduce dispersion effects.
1. The document discusses optical fibers, specifically step index fibers. It describes step index fibers as having a core with a constant refractive index n1 surrounded by a cladding with a slightly lower refractive index n2.
2. It discusses several factors that determine the number of propagating modes in a step index fiber, including the V-number which is a function of the core radius, wavelengths, and refractive index differences. Fibers with V<2.405 support only one mode.
3. Dispersion effects in step index fibers include intermodal dispersion from different propagation speeds of fiber modes, and material dispersion from the wavelength dependence of the core refractive index.
Optical fibers transmit light and operate based on the principles of total internal reflection. They consist of a core and cladding material, with the core having a higher refractive index. This allows light to be guided along the fiber due to total internal reflection at the core-cladding boundary. There are two main types of optical fibers - single-mode fibers which only allow one mode of light to propagate, and multi-mode fibers which allow multiple light modes. Dispersion and attenuation are two factors that limit the performance of optical fibers by causing light pulses to broaden as they travel along the fiber.
Optical Fiber Cables :- An Introduction Pradeep Singh
This document discusses fiber optic cables and their components. It begins by classifying optical fibers into single-mode fibers, which carry light along a single path, and multi-mode fibers, which carry multiple light paths. It then describes the core, cladding and coating layers that make up an optical fiber. Total internal reflection is discussed as the mechanism that keeps light confined in the fiber. Common fiber optic components like connectors, couplers and circulators are also outlined.
Microwave attenuators are electronic devices that reduce the power of signals without distorting their waveforms. They are the opposite of amplifiers in that they reflect and absorb energy through dissipative elements. There are fixed and variable types of attenuators. Fixed attenuators provide a set amount of power reduction and are used for impedance matching and where a fixed power level is required. Variable attenuators allow step-wise or continuous adjustment of attenuation through mechanisms like rotary wheels, flaps, or vanes made of lossy dielectric materials inserted into the signal path. Both types have characteristics like impedance, power handling, frequency response, and temperature dependence that are important to their performance.
This document discusses optical amplifiers and their future uses. It introduces different types of optical amplifiers including erbium doped fiber amplifiers and semiconductor optical amplifiers. Erbium doped fiber amplifiers were first demonstrated in the 1980s and have wide bandwidth, high gain, and are fiber compatible. Semiconductor optical amplifiers can provide exponential gain increases with length and operate from 1250-1650 nm. The document also discusses amplifier comparisons, gain dynamics, noise in optical amplifiers, and applications of optical amplifiers including 5G networks and long haul communications.
Dispersion Compensation Techniques for Optical Fiber CommunicationAmit Raikar
This document discusses dispersion in optical fiber communication systems and various techniques to compensate for it, including dispersion compensating fibers, fiber Bragg gratings, electronic dispersion compensation, digital filters, and optical phase conjugation. Dispersion increases pulse spreading and affects signal quality. These techniques help reduce dispersion to improve transmission over long distances. The document compares the advantages and disadvantages of each technique.
The attached narrated power point presentation attempts to explain the working principle, types, classifications, merits, demerits, applications,safety and deployment issues related to Raman Amplifiers. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
The document discusses various optical phenomena including reflection, refraction, and total internal reflection. It explains that optical fibers use total internal reflection to guide light along the fiber. Optical fibers have a core with a higher refractive index than the cladding. This allows total internal reflection to contain light within the core. The document also discusses the historical development of optical fiber communications, describing the progression from early generations with lower data rates and shorter distances to current generations with multi-terabit capacities over extremely long ranges. Overall, the document provides an overview of fundamental optical concepts and the evolution of optical fiber communication technology.
This document provides an overview of optical amplifiers, including their necessity, basic concepts, types, and applications. Optical amplifiers are needed to compensate for attenuation losses over long transmission distances. The main types discussed are semiconductor optical amplifiers, erbium-doped fiber amplifiers (EDFAs), and Raman amplifiers. EDFAs use stimulated emission in erbium-doped fiber to amplify signals, while Raman amplifiers rely on stimulated Raman scattering in fiber. Both can provide wavelength-independent amplification but have different noise and gain characteristics. Optical amplifiers play a critical role in modern long-haul optical networks by enabling transmission over thousands of kilometers.
This document discusses various topics related to transmission characteristics of optical fibers, including:
- The main types of losses in optical fibers are attenuation due to absorption and scattering. Absorption includes material absorption from defects, ions, and molecular vibrations. Scattering includes Rayleigh and Mie scattering.
- Other losses include bending losses from micro- and macro-bends, core-cladding losses, and polarization mode dispersion.
- Signal dispersion spreads optical pulses as they propagate and can cause intersymbol interference. The main types are material dispersion, waveguide dispersion, modal dispersion, and polarization mode dispersion.
- Design of single mode fibers aims to optimize parameters like cutoff wavelength, dispersion, mode field diameter
Opto electronics by er. sanyam s. saini me (reg) 2012-14Sanyam Singh
This document discusses materials used for optical fibers and losses in optical fibers. It describes how silica is commonly used due to its good optical transmission, mechanical strength, and chemical inertness. Fluoride glasses are also mentioned but are difficult to manufacture without crystallization. The main losses discussed are absorption, scattering, and bending losses. Absorption can be intrinsic to the material or due to impurities. Scattering transfers power between modes. Bending losses include macro bending from sharp curves and micro bending from microscopic bends in cables. The wavelength of minimum attenuation is around 1550 nm.
Optical multiplexers allow multiple signals to be transmitted simultaneously over a single optical fiber link. There are different optical multiplexing techniques, including wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM). WDM assigns each signal a unique wavelength, while OTDM separates signals in the time domain. Optical multiplexers and demultiplexers use passive optical filters to combine and separate the wavelength signals. This increases bandwidth utilization and reduces transmission costs.
An optical fiber coupler is a device that splits light from one fiber into multiple fibers. There are different types of couplers classified by their shape, including Y, T, X, star, and tree couplers. Couplers work by transferring power between fibers through their cores or surfaces. Examples show how to calculate excess loss, insertion loss, crosstalk, and splitting ratios using the measured input and output powers. Optical couplers have applications in splitting and combining optical signals in fiber networks and communication systems.
This document discusses semiconductor optical amplifiers (SOAs). It explains that SOAs use stimulated emission to amplify optical signals, like lasers, but have anti-reflection coatings on the facets so light passes through only once. The main types are traveling-wave amplifiers, which are widely used because they amplify signals with a single pass and have a large bandwidth. SOAs have a core made of InGaAsP for gain and InP cladding layers. External pumping by current injection provides carriers that undergo stimulated emission to amplify optical signals. Amplifier gain increases with length and current but saturates with increasing optical power due to depletion of excited carriers.
Passive optical components play a key role in optical networks by coupling, splitting, and multiplexing signals. Common passive components include fused fiber couplers and splitters, arrayed waveguide gratings (AWGs), and isolators/circulators. Couplers combine or split signals depending on fiber spacing, while AWGs multiplex and demultiplex wavelengths. Isolators and circulators use non-reciprocal polarization rotation to allow transmission in only one direction.
Optical fiber communication Part 1 Optical Fiber FundamentalsMadhumita Tamhane
Optical fiber systems grew from combination of semiconductor technology, which provided necessary light sources and photodetectors and optical waveguide technology. It has significant inherent advantages over conventional copper systems- low transmission loss, wide BW, light weight and size, immunity to interferences, signal security to name a few. One principle characteristic of optical fiber is its attenuation as a function of wavelength. Hence it is operated in two major low attenuation wavelength windows 800-900nm and 1100-1600nm . Light travels inside optical fiber waveguide on principle of total internal reflection. Fiber is available as single mode and multiple mode, step index and graded index depending on applications and expenditures. Principle of fiber can be understood by ray theory or mode theory. ...
The document discusses different types of linear-beam microwave tubes, specifically focusing on klystron tubes. It provides details on the operation of two-cavity klystrons and reflex klystrons. Two-cavity klystrons work by velocity modulating electrons in the first cavity which become current modulated before interacting with the second cavity to produce microwave power. Reflex klystrons use a single cavity and repeller field to reflect electrons, allowing them to interact twice with the cavity field and function as an oscillator. Quantitative analyses of velocity modulation, power output, and efficiency are also presented.
The document summarizes key components and concepts in basic microwave engineering. It discusses waveguides and their operating frequencies based on dimensions. It also describes electric and magnetic fields in rectangular waveguides. Additional components summarized include coaxial to waveguide transitions, choke joints, coupling loops, phase shifters, junctions, tuners, mixers, isolators, circulators, directional couplers, and cavity resonators. Isolators, circulators, and directional couplers are multi-port devices that control the direction of signal propagation with differing levels of attenuation.
This document discusses different types of dispersion in optical fibers, including modal dispersion, material dispersion, waveguide dispersion, and polarization mode dispersion. It defines important terms related to dispersion like group velocity and group delay. It also examines how dispersion causes pulse broadening over distance as different wavelengths within a pulse propagate at different speeds through the fiber. Finally, it compares the dispersion characteristics of different fiber types like dispersion shifted and flattened fibers which are designed to reduce dispersion effects.
1. The document discusses optical fibers, specifically step index fibers. It describes step index fibers as having a core with a constant refractive index n1 surrounded by a cladding with a slightly lower refractive index n2.
2. It discusses several factors that determine the number of propagating modes in a step index fiber, including the V-number which is a function of the core radius, wavelengths, and refractive index differences. Fibers with V<2.405 support only one mode.
3. Dispersion effects in step index fibers include intermodal dispersion from different propagation speeds of fiber modes, and material dispersion from the wavelength dependence of the core refractive index.
Optical fibers transmit light and operate based on the principles of total internal reflection. They consist of a core and cladding material, with the core having a higher refractive index. This allows light to be guided along the fiber due to total internal reflection at the core-cladding boundary. There are two main types of optical fibers - single-mode fibers which only allow one mode of light to propagate, and multi-mode fibers which allow multiple light modes. Dispersion and attenuation are two factors that limit the performance of optical fibers by causing light pulses to broaden as they travel along the fiber.
Optical Fiber Cables :- An Introduction Pradeep Singh
This document discusses fiber optic cables and their components. It begins by classifying optical fibers into single-mode fibers, which carry light along a single path, and multi-mode fibers, which carry multiple light paths. It then describes the core, cladding and coating layers that make up an optical fiber. Total internal reflection is discussed as the mechanism that keeps light confined in the fiber. Common fiber optic components like connectors, couplers and circulators are also outlined.
Microwave attenuators are electronic devices that reduce the power of signals without distorting their waveforms. They are the opposite of amplifiers in that they reflect and absorb energy through dissipative elements. There are fixed and variable types of attenuators. Fixed attenuators provide a set amount of power reduction and are used for impedance matching and where a fixed power level is required. Variable attenuators allow step-wise or continuous adjustment of attenuation through mechanisms like rotary wheels, flaps, or vanes made of lossy dielectric materials inserted into the signal path. Both types have characteristics like impedance, power handling, frequency response, and temperature dependence that are important to their performance.
This document discusses optical amplifiers and their future uses. It introduces different types of optical amplifiers including erbium doped fiber amplifiers and semiconductor optical amplifiers. Erbium doped fiber amplifiers were first demonstrated in the 1980s and have wide bandwidth, high gain, and are fiber compatible. Semiconductor optical amplifiers can provide exponential gain increases with length and operate from 1250-1650 nm. The document also discusses amplifier comparisons, gain dynamics, noise in optical amplifiers, and applications of optical amplifiers including 5G networks and long haul communications.
Dispersion Compensation Techniques for Optical Fiber CommunicationAmit Raikar
This document discusses dispersion in optical fiber communication systems and various techniques to compensate for it, including dispersion compensating fibers, fiber Bragg gratings, electronic dispersion compensation, digital filters, and optical phase conjugation. Dispersion increases pulse spreading and affects signal quality. These techniques help reduce dispersion to improve transmission over long distances. The document compares the advantages and disadvantages of each technique.
The attached narrated power point presentation attempts to explain the working principle, types, classifications, merits, demerits, applications,safety and deployment issues related to Raman Amplifiers. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
The document discusses various optical phenomena including reflection, refraction, and total internal reflection. It explains that optical fibers use total internal reflection to guide light along the fiber. Optical fibers have a core with a higher refractive index than the cladding. This allows total internal reflection to contain light within the core. The document also discusses the historical development of optical fiber communications, describing the progression from early generations with lower data rates and shorter distances to current generations with multi-terabit capacities over extremely long ranges. Overall, the document provides an overview of fundamental optical concepts and the evolution of optical fiber communication technology.
This document provides an overview of optical amplifiers, including their necessity, basic concepts, types, and applications. Optical amplifiers are needed to compensate for attenuation losses over long transmission distances. The main types discussed are semiconductor optical amplifiers, erbium-doped fiber amplifiers (EDFAs), and Raman amplifiers. EDFAs use stimulated emission in erbium-doped fiber to amplify signals, while Raman amplifiers rely on stimulated Raman scattering in fiber. Both can provide wavelength-independent amplification but have different noise and gain characteristics. Optical amplifiers play a critical role in modern long-haul optical networks by enabling transmission over thousands of kilometers.
This document discusses various topics related to transmission characteristics of optical fibers, including:
- The main types of losses in optical fibers are attenuation due to absorption and scattering. Absorption includes material absorption from defects, ions, and molecular vibrations. Scattering includes Rayleigh and Mie scattering.
- Other losses include bending losses from micro- and macro-bends, core-cladding losses, and polarization mode dispersion.
- Signal dispersion spreads optical pulses as they propagate and can cause intersymbol interference. The main types are material dispersion, waveguide dispersion, modal dispersion, and polarization mode dispersion.
- Design of single mode fibers aims to optimize parameters like cutoff wavelength, dispersion, mode field diameter
Opto electronics by er. sanyam s. saini me (reg) 2012-14Sanyam Singh
This document discusses materials used for optical fibers and losses in optical fibers. It describes how silica is commonly used due to its good optical transmission, mechanical strength, and chemical inertness. Fluoride glasses are also mentioned but are difficult to manufacture without crystallization. The main losses discussed are absorption, scattering, and bending losses. Absorption can be intrinsic to the material or due to impurities. Scattering transfers power between modes. Bending losses include macro bending from sharp curves and micro bending from microscopic bends in cables. The wavelength of minimum attenuation is around 1550 nm.
The document discusses signal degradation in optical fibers. It notes that attenuation and distortion limit the distance and capacity of fiber optic communication. Attenuation is the decay of signal strength as light pulses propagate through fiber. It is caused by absorption and scattering from fiber material imperfections. Nearly 90% of attenuation is from Rayleigh scattering, which is dependent on wavelength. Absorption results from defects, impurities in the glass composition, and intrinsic absorption by glass constituents. Radiation exposure can also increase attenuation by damaging the fiber structure.
Transmission characteristics of optical fibersaibad ahmed
This document discusses the transmission characteristics of optical fibers, specifically focusing on signal attenuation. It explains that as optical signals transmit through fibers, they experience power losses known as attenuation. The main factors that cause attenuation are absorption, scattering, macro bending, and microbending. Absorption can be intrinsic due to electron/atom resonance or extrinsic from impurities. Scattering changes the signal direction and includes Rayleigh and Mie scattering. Macro and microbending cause losses from large and small scale fiber bending, respectively. Examples are provided to illustrate how to calculate attenuation levels from given transmission parameters.
Signal Degradation In Optical Fiber
Losses in an optical fibre:-
The types of losses in a optical fibre are
Attenuation loss
Absorption
Scattering
Bending loss
Dispersion loss
Coupling loss
Losses in optical fibers include attenuation from absorption and scattering, as well as dispersion effects. Attenuation is caused by absorption of light energy through heating of impurities in the fiber, resulting in a loss of optical power over length. Dispersion causes pulse broadening and occurs from intermodal and intramodal effects such as material and waveguide dispersion. An optical time domain reflectometer (OTDR) can be used to detect faults, splices, and bends in fibers by emitting light pulses and measuring backscattered light over time to map reflections in the fiber.
Losses in optical fibers include attenuation from absorption and scattering, as well as dispersion from material and waveguide effects. Attenuation is caused by absorption of light energy through intrinsic effects like interactions with glass components and extrinsic effects from impurities. Dispersion spreads optical pulses during transmission and has intermodal and intramodal components. An optical time domain reflectometer (OTDR) detects backscattered light to locate faults, splices, and bends in fibers by measuring return time and intensity.
Losses in optical fibers include attenuation from absorption and scattering, as well as dispersion effects. Attenuation is caused by absorption of light energy through heating of impurities in the fiber, resulting in a loss of optical power over length. Dispersion causes pulse broadening and occurs from intermodal and intramodal effects such as material and waveguide dispersion. An optical time domain reflectometer (OTDR) can be used to detect faults, splices, and bends in fibers by emitting light pulses and measuring backscattered light over time.
This document discusses key characteristics of optical fibers that affect their performance as a transmission medium. It describes how wavelength, frequency, reflection, refraction, polarization, and attenuation properties influence fiber optic communication. Specific bands used in optical fibers, including O, C, E, S and L bands, are defined. The document also examines intrinsic and extrinsic factors contributing to fiber attenuation, as well as dispersion which limits bandwidth by spreading out light pulses over time as they travel through the fiber.
This document discusses various sources of attenuation in optical fibers:
(1) Attenuation is mainly caused by absorption and scattering. Absorption includes intrinsic absorption which is a natural property of glass, and extrinsic absorption due to impurities in the glass.
(2) Scattering includes Rayleigh scattering due to refractive index fluctuations and Mie scattering from fiber imperfections. Both result in loss of power.
(3) Other losses come from bending of fibers, connections, and nonlinear effects like stimulated Brillouin and Raman scattering at high powers. Careful fiber design and manufacturing can reduce losses from many of these sources.
The document discusses several disadvantages of fiber optic communication systems:
1. Fiber optic systems have high upfront costs due to the expense of installing optical fiber cables compared to copper wiring, though the raw materials are inexpensive.
2. Signal scattering from linear and non-linear effects causes light to transfer between fiber modes, reducing the strength of the transmitted signal over long distances.
3. Optical fibers are susceptible to additional signal loss if bent below their minimum bend radius, which can vary significantly depending on the fiber type.
Presentation on Optical Fiber for UG Physics students by Dr. P D Shirbhate assistant Professor, Department of Physics G S Gawande college, Umarkhed Dist Yavatmal.
B.Tech ECE IV Year I Sem, MWOC UNIT 5 Optical CommunicationsUNIT 5 MWOC.pptxjanakiravi
Optical detectors convert received light signals into electrical signals. PIN photodiodes are commonly used and have an intrinsic layer between the p-region and n-region to widen the depletion zone. Avalanche photodiodes provide internal gain through collisions that generate more electrons. Optical detectors have advantages like high sensitivity, wide bandwidth, low noise and reliability but also have disadvantages like limited dynamic range and sensitivity to temperature changes.
Em and optics project 3 (1st) convertedDurgeshJoshi6
This document is a lab report submitted by Ashok Kumar Sahoo for the course Electromagnetism & Optics at the Indian Institute of Technology Kharagpur. The report discusses experiments and measurements performed with optical fibers and optoelectronic devices. In the first part, experiments are described to analyze the working of single mode and multimode optical fibers by calculating properties like numerical aperture, bending loss, and splice loss. The second part analyzes the characteristics of various optoelectronic devices including solar cells, light dependent resistors, LEDs, phototransistors, photodiodes, and optocouplers. Basic theories of total internal reflection, optical fibers, and these components are also outlined.
(1) Optical signals propagating through optical fiber experience attenuation and dispersion, which degrade the signal.
(2) Attenuation reduces the signal power as it travels along the fiber and limits the maximum transmission distance. The primary attenuation mechanisms are absorption and scattering.
(3) Dispersion causes different frequency components of the signal to travel at different velocities, resulting in signal broadening and limiting transmission capacity. The primary dispersion mechanisms are material dispersion, waveguide dispersion, and polarization mode dispersion.
1) Optical communication systems use optical fibers to transmit messages as light signals. Optical fibers consist of a core and cladding material that guides light through total internal reflection.
2) Information is encoded onto light signals using transmitters like LEDs or lasers, which are then sent through the fiber. Receivers like photodiodes detect the light signals and reproduce the original message.
3) Attenuation and dispersion are the main factors limiting signal quality in optical fibers. Attenuation is caused by absorption and scattering within the fiber material. Dispersion causes pulse spreading and is a result of differences in propagation speeds between light modes and wavelengths.
Losses in optical fibers can occur due to bending, material absorption, scattering, and dispersion. The main types of bending losses are microbending from small bends and macrobending from larger radius bends. Material absorption losses include intrinsic losses from the fiber material and extrinsic losses from impurities. Proper fiber design and coating can help minimize bending and material absorption losses to improve signal transmission.
This narrated power point presentation attempts to analyse the reasons for attenuation in optical fibers due to linear effects such as absorption, scattering and fiber bend. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
The document discusses the optical properties of materials and their applications. It begins by defining optical materials as substances that manipulate light flow and describes their interactions with electromagnetic radiation. It then covers classification of materials as transparent, translucent, or opaque based on light transmission. Specific optical properties like reflection, refraction, absorption, and transmission are defined. Applications such as luminescence, lasers, photoconductivity, and optical fibers are also summarized.
Similar to Optical Instrumentation 12. Optical Fibre Losses (20)
This article speaks about the different energy domains, sensors, actuation techniques, transduction techniques, fabrication materials, physical strength requirements, substrate materials and De Vries formula used in MEMS technology.
This article discusses MEMS, i.e. Micro-Electro Mechanical Systems.
It gives a rudimentry idea of MEMS technology, its block diagram, applications, advantages and disadvantages. It also gives a brief idea on the working principle of MEMS devices.
Eqautions_1_Industrial Instrumentation - Flow Measurement Important Equations...Burdwan University
This document summarizes important equations for flow measurement. It includes equations for:
1. Newton's law of viscosity relating shear stress and velocity gradient.
2. Hagen-Poiseuille equation relating pressure drop, flow rate, viscosity, pipe diameter and length for laminar flow through a pipe.
3. Reynolds number, a dimensionless number used to determine if flow is steady or turbulent based on velocity, diameter, density and viscosity.
The document provides these equations along with definitions of the variables and parameters in the equations. It is a technical summary of key equations for analyzing and calculating fluid flow.
Industrial instrumentation flow measurement important equationsBurdwan University
This document summarizes important equations for flow measurement, including:
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Discharge is calculated using factors for the throat area, velocity of flow, and square root of the differential pressure.
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Introduction- e - waste – definition - sources of e-waste– hazardous substances in e-waste - effects of e-waste on environment and human health- need for e-waste management– e-waste handling rules - waste minimization techniques for managing e-waste – recycling of e-waste - disposal treatment methods of e- waste – mechanism of extraction of precious metal from leaching solution-global Scenario of E-waste – E-waste in India- case studies.
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3. Fibre Losses:
Fibre Losses: Optical fibre cables suffer few losses. They are classified as Attenuation
and Dispersion. These two are further classified into several other losses.
Attenuation Coefficient: Signal attenuation or transmission loss is defined as the ratio of
the input transmission optical power 𝑃𝑖𝑛 into a fibre to the output (received) optical
power 𝑃𝑜𝑢𝑡 from the fibre. This ratio is a function of the operating wavelength.
The symbol α 𝑑𝐵 is commonly used to express the attenuation in decibels (dB) per
kilometre (L).
𝜶 𝒅𝑩 =
𝟏
𝑳
[𝟏𝟎 𝐥𝐨𝐠 𝟏𝟎
𝑷𝒊𝒏
𝑷 𝒐𝒖𝒕
]
3
7. Nonlinear/ Radiative Loses:
Nonlinear/ Radiative Loses
1. Bending Loss
A. Micro Bending
B. Macro Bending (Constant Radius Bending)
C. Leaky Mode (Skew Ray)
D. Mode Coupling Loss
7
9. Attenuation – Material Absorption:
Material absorption is a loss mechanism related to both the material composition and the
fabrication process for the fibre.
The optical power is lost as heat in the fibre.
The light absorption can be intrinsic (due to the material components of the glass) or
extrinsic (due to impurities introduced into the glass during fabrication).
Pure silica-based glass has two major intrinsic absorption mechanisms at optical
wavelengths:
1. Fundamental UV absorption
2. Fundamental IR or Far-IR absorption
9
10. Material Absorption: Fundamental UV Absorption
In fundamental UV absorption edge, the peaks are centred in the UV region.
Fused silica valence electrons absorb light and can be ionized to conduction electrons.
This gives rise to an energy loss in the light field contributing to transmission loss.
The absorption loss increases with the decrease of wavelength.
The UV edge of electron absorption band in both crystalline and amorphous materials
follows Urbach’s Rule
𝜆 𝑢𝑣 = 𝐶𝑒
𝐸
𝐸0
Here C and E0 are empirical constants. E is the photon energy.
𝜆 𝑢𝑣 = attenuation constant in the UV region
10
11. Material Absorption – Fundamental IR Absorption:
Fundamental IR and Far-IR absorption edge is due to the molecular vibrations (Si-O).
The tail od these absorption peaks may extend into the longer wavelengths.
IR absorption occurs because the photons are absorbed by atoms within the glass
molecules and converted to random mechanical vibrations typical of heating.
11
12. Material Absorption – Intrinsic:
Electronic Absorption: The bandgap of fused silica is about 8.9 eV (~140 nm). This
causes strong absorption of light in the UV spectral region due to electronic transitions
across the band gap.
An amorphous material like fused silica generally has very long bandtails. These
bandtails lead to an absorption tail extending into the visible and infrared regions.
Empirically, the absorption tail at photon energies below the bandgap falls off
exponentially with photon energy.
Molecular Absorption: In the infrared region, the absorption of photons is
accompanied by transitions between different vibrational modes of silica molecules. The
fundamental vibrational transition of fused silica causes a very strong absorption peak at
about 9 μm wavelength.
Nonlinear effects contribute to important harmonics and combination frequencies
corresponding to minor absorption peaks at 4.4, 3.8 and 3.2 μm wavelengths.
A long absorption tail extending into the near infrared, causing a sharp rise in absorption
at optical wavelengths longer than 1.6 μm.
12
13. Material Absorption – Extrinsic:
Ion – Resonance Absorption: Major extrinsic loss mechanism is caused by absorption
due to water (as the hydroxyl or OH- ions) introduced in the glass fiber during fiber
pulling by means of oxyhydrogen flame. This leads to Ion – Resonance Absorption.
The lowest attenuation for typical silica-based fibers occur at wavelength 1.55 μm at
about 0.2 dB/km, approaching the minimum possible attenuation at this wavelength.
Impurity Absorption: Most impurity ions such as OH-, Fe2+ and Cu2+ form
absorption bands in the near infrared region where both electronic and molecular
absorption losses of the host silica glass are very low.
Near the peaks of the impurity absorption bands, an impurity concentration as low as
one part per billion can contribute to an absorption loss as high as 1 dB km-1.
Today, impurities in fibers have been reduced to levels where losses associated with
their absorption are negligible, with the exception of the OH- radical.
13
14. Scattering Loss:
Scattering results in attenuation (in the form of radiation) as the scattered light may not
continue to satisfy the total internal reflection in the fiber core.
The scattered ray can escape by refraction according to Snell’s Law.
Scattering is due to irregularity of materials.
When a beam of light interacts with a material, part of it is transmitted, part it is
reflected, and part of it is scattered.
Mainly there are five such losses viz.
1. Rayleigh
2. Brillouin
3. Raman
4. Wave Guide
5. Mie
14
15. Scattering Loss – Rayleigh:
Rayleigh scattering results from random inhomogeneities that are small in size
compared with the wavelength. It takes place due to the variations in the refractive
index in glass. The glass used is amorphous one, prepared by allowing glass to cool
from molten state at high temperature until it freezes.
During this transition two defects may arise.
1. Glass being amorphous is composed to randomly connected network of molecules.
Ans therefore it may contain regions in which the molecular density is higher or lower
than the average density in the glass.
2. Since the glass is made up of several oxides, such as SiO2, GeO2 and P2O5,
compositional fluctuations may occur.
For a single component glass, the Rayleigh scattering coefficient is given by
𝜏 𝑅 =
8𝜋3
3𝜆4 𝑛1
8
𝑝2
𝐵 𝐶 𝐾 𝐵 𝑇 𝐹
15
16. Scattering Loss – Rayleigh:
For a single component glass, the Rayleigh scattering coefficient is given by
𝜏 𝑅 =
8𝜋3
3𝜆4 𝑛1
8
𝑝2
𝐵 𝐶 𝐾 𝐵 𝑇 𝐹
Where 𝜏 𝑅 = Rayleigh scattering coefficient, λ = wave length of optical radiation
n1 = refractive index of the medium, p = average photo elastic coefficient,
BC = isothermal compressibility at fictive temperature TF and KB = Boltzmann constant.
The fictive temperature of glass is defined as the temperature at which glass can reach a state
of thermal equilibrium and closely related to the anneal temperature.
Sub microscopic variations in the glass density and doping impurities are frozen into glass
during manufacture and they act as the reflecting and refracting facets to scatter a small
portion of light through the glass.
These defects may be in the form of trapped bubbles, unreacted starting materials and
crystallized regions in the glass.
16
17. Scattering Loss – Brillouin:
It may be regarded as the modulation of light through thermal molecular vibration
within the fibre.
The incident photons of light undergo nonlinear interaction to produce vibrational
energy or phonons in the glass as well as the scattered light or photons.
The scattered light is found to be frequency modulated by the thermal energy and both
upward and downward frequency shifts are observed.
The amount of frequency shift and the strength of scattering vary as the function of the
scattering angle maximum occurring at the backward direction and the minimum or zero
being observed in the forward direction.
Thus Brillouin scattering mainly occurs in the backward direction which directs the
power to the source and the power of the receiver is reduced.
17
18. Scattering Loss – Brillouin:
The optical power level at which Brillouin scattering becomes significant in a single
mode fibre is given by an empirical formula. The threshol2d power level PB is given by
𝑃 𝐵 = 10−3
𝑑2
𝜆2
𝛼 𝑑𝑏∆𝜏
Where d and λ are the core diameter and the operating wavelength respectively, αdb is the
fibre attenuation in dB/km. ατ is the source bandwidth in GHz.
18
19. Scattering Loss – Raman:
The non-linear interaction in Raman scattering produces a high frequency phonon and a
scattered photon, where as low frequency phonons are produced in Brillouin scattering.
In Raman scattering, light is predominantly in the forward direction and thus the power
is not reduced in the receiver.
The threshold power level for the significant Raman scattering to occur is given by
𝑃 𝑅 = 5.9 ∗ 10−2
𝑑2
𝜆𝛼 𝑑𝐵
Where d is the diameter of the fibre in μm, λ is the wavelength emitted by the source in
μm, 𝛼 𝑑𝐵 is the fibre loss in dB/km and PR is the threshold optical power.
19
20. Scattering Loss – Wave Guide:
Imperfections in the waveguide structure of a fiber, such as non-uniformity in the size
and shape of the core, perturbations in the core-cladding boundary, and defects in the
core or cladding, can be generated in the manufacturing process.
Environmentally induced effects, such as stress and temperature variations, also cause
imperfections.
The imperfections in a fiber waveguide result in additional scattering losses.
They can also induce coupling between different guided modes.
20
21. Scattering Loss – Mie:
Linear scattering may occur at inhomogeneities which are comparable in size with the
guided wavelength.
When the size of scattering inhomogeneity is greater than λ/10, the scattering intensity
has an angular dependence and can be quite large.
The scattering occurring due to such inhomogeneity is mainly in the forward direction
and is known as Mie Scattering.
Depending on the fibre material, design and manufacture, Mie scattering can cause
considerable power loss. The inhomogeneity can be minimized by
1. Reducing imperfection during glass manufacturing process
2. Careful controlled extrusion and coating of the fibre
3. Increasing the fibre guidance by increasing the relative refractive index between
core and cladding.
21
22. Non linear/Radiative Losses:
As light is confined over long distances in an optical fiber, nonlinear optical effects can
become important even at a relatively moderate optical power.
Nonlinear optical processes such as stimulated Brillouin scattering and stimulated
Raman scattering can cause significant attenuation in the power of an optical signal.
Other nonlinear processes can induce mode mixing or frequency shift, all contributing
to the loss of a particular guided mode at a particular frequency.
Nonlinear effects are intensity dependent, and thus they can become very important at
high optical powers.
Radiative losses occur whenever an optical fibre undergoes a bend of finite radius of
curvature.
Fibres can be subject to two types of bends viz. Micro bending and Macro bending or
Constant Radius Bending.
22
23. Non linear/Radiative Losses - Micro Bending:
It is a microscopic bending with repetitive changes in the axis of the core and it takes
place due to the slightly different contraction rate between the core and the cladding
materials.
It occurs due to non uniform lateral pressure created during cabling.
Losses in the micro bending take place because the small bends act as the scattering
facets and these facets cause mode coupling to occur.
Energy from the guided modes is cross coupled to the leaky mode and is lost through
the cladding.
Micro bending are randomly distributed over the length of the fibre.
23
24. Non linear/Radiative Losses - Micro Bending:
Careful precaution in manufacturing and handling of fibres will reduce the loss.
One method to minimize is done by extruding a compressible jacket over the fibre
which will be able to take on external tension without deforming the core.
Potential micro bending losses may be minimized by
1. Designing fibres with large relative refractive index differences between the core
and the cladding.
2. Operating at the shortest possible wavelength.
24
25. Non linear/Radiative Losses - Macro Bending:
It is also called Constant Radius Bending.
Bends are introduced while installing cable ducts to join corners.
Sometimes these bends are quite sharp.
These large radius bends introduce losses in the fibre.
The bending may provide incidence angles less than the critical angle thereby allowing
a part of the light energy to escape from the fibre through the cladding.
It is therefore necessary to ensure that no sharp bends are introduced in the path of the
fibre.
25
26. Critical radius of Bend:
Critical radius of Bend: The relationship between the radius of curvature of the bend
and radiation attenuation coefficient 𝜆 𝑟 is given by
𝜆 𝑟 = 𝐶1exp(−𝐶2 𝑅)
R = radius of curvature; C1 and C2 are constants independent of R.
Large bending losses tend to occur in multi mode fibre at a critical radius of curvature
R 𝐶 given by 𝑅 𝐶 =
3𝑛1
2 𝜆
4𝜋(𝑛1
2−𝑛2
2)
3
2
26
27. Non linear/Radiative Losses – Skew Rays
At each reflection of a ray propagating in helical type of path (skew mode) the angle of
incidence 𝜃1 at the core – cladding surface is more than the critical angle 𝜃 𝐶 and the
mode will propagate through the fibre.
But at some higher order modes, the 𝜃 𝐶 may be less than 𝜃 𝐶 and a part of the
propagation will escape the core by refraction.
Successive such refractions will make the intensity weak and eventually will be lost.
It is desirable to remove this leaky mode from the core and cladding as rapidly as
possible to reduce signal dispersion.
This is accomplished by surrounding thin cladding by a third party layer of pure silica
having refractive index more than cladding but less than core.
This provides mechanical strength to the fibre and acts to remove the partially refracted
ray from the leaky mode and possesses rays from the cut off modes by total refraction.
27
28. Non linear/Radiative Losses: Mode – Coupling
Power may be launched successfully into a propagating mode but it may be coupled into
a leaky or radiating mode at some point further down the fibre.
This type of improper coupling can occur for several reasons.
Small imperfections in the core glass or in the core – cladding interface may occur due
to small variations in the core diameter, cross sectional shape or bubbles in the glass and
these are responsible for causing the energy to be coupled into one of the leaky modes.
Losses from these sources will be uniform along the length of the fibre.
Another source of mode coupling is the imperfectly formed splices or imperfectly
aligned connectors.
These are discrete losses which can be reduced by decreasing the number of splices or
connectors in a given fibre.
28
29. DISPERSION:
Dispersion is the primary cause of limitation on the optical signal transmission
bandwidth through an optical fiber.
Dispersion is referred to widening the pulse as the light travels through the fiber optics.
When a short pulse of light travels through an optical fiber its power is “dispersed” in
time so that the pulse spreads into a wider time interval.
There are four sources of dispersion in optical fibers: modal dispersion, material
dispersion, wave guide dispersion and nonlinear dispersion.
Both material dispersion and waveguide dispersion are examples of chromatic
dispersion because both are frequency dependent.
Waveguide dispersion is caused by frequency dependence of the propagation constant β
of a specific mode due to the wave guiding effect.
The combined effect of material and waveguide dispersions for a particular mode alone
is called intra mode dispersion.
29
30. DISPERSION: Modal
Modal dispersion occurs in multimode fibers as a result of the differences in the group
velocities of the modes.
A single impulse of light entering an M-mode fiber at z = 0 spreads into M pulses with the
differential delay increasing as a function of z.
For a fiber of length L, the time delays encountered by the different modes are
𝜏 𝑞 = 𝐿/𝑣 𝑞
q = 1, ....., M, where vq is the group velocity of mode q.
If vmin and vmax are the smallest and largest group velocities, the received pulse spreads over
a time interval are
𝐿
𝑣 𝑚𝑖𝑛
−
𝐿
𝑣 𝑚𝑎𝑥
.
Since the modes are generally not excited equally, the overall shape of the received pulse is a
smooth profile.
An estimate of the overall rms pulse width is
𝜎 𝜏 =
1
2
( 𝐿
𝑣 𝑚𝑖𝑛
− 𝐿
𝑣 𝑚𝑎𝑥
)
This width represents the response time of the fiber.
30
31. DISPERSION: Modal
In a step-index fiber with a large number of modes,
𝑣 𝑚𝑖𝑛 ≈ 𝑐1 1 − ∆ 𝑎𝑛𝑑 𝑣 𝑚𝑎𝑥 ≈ 𝑐1
Since (1 − ∆)−1
≈ 1 + ∆, the response time is
𝜎 𝜏 ≈
𝐿
𝑐1
∆
2
(response time for multi mode step – index fibre)
i.e., it is a fraction ∆/2 of the delay time 𝐿 𝑐1.
Modal dispersion is much smaller in graded-index fibers than in step-index fibers since the
group velocities are equalized and the differences between the delay times
𝜏 𝑞 = 𝐿/𝑣 𝑞
of the modes are reduced.
In a graded-index fiber with a large number of modes and with an optimal index profile,
𝑣 𝑚𝑖𝑛 ≈ 𝑐1(1 − ∆2
/2) 𝑎𝑛𝑑 𝑣 𝑚𝑎𝑥 ≈ 𝑐1
The response time is therefore 𝜎 𝜏 ≈
𝐿
𝑐1
∆2
4
(𝑟𝑒𝑠𝑝𝑜𝑛𝑠𝑒 𝑡𝑖𝑚𝑒 𝑓𝑜𝑟 𝑔𝑟𝑎𝑑𝑒𝑑 𝑖𝑛𝑑𝑒𝑥 𝑓𝑖𝑏𝑟𝑒)
which is a factor of ∆/2 smaller than that in a step index fibre.
31
32. DISPERSION: Modal
The pulse broadening arising from modal dispersion is proportional to the fiber length L
in both step-index and graded-index fibers.
This dependence, however, does not necessarily hold when the fibers are longer than a
certain critical length because of mode coupling.
Coupling occurs between modes of approximately the same propagation constants as a
result of small imperfections in the fiber (random irregularities of the fiber surface, or
inhomogeneities of the refractive index) which permit the optical power to be
exchanged between the modes.
Under certain conditions, the response time στ of mode-coupled fibers is proportional to
L for small L and to L1/2 when a critical length is exceeded, so that pulses are broadened
at a slower rate.
32
33. DISPERSION: Material
Glass is a dispersive medium; i.e. its refractive index is a function of wavelength.
An optical pulse travels in a dispersive medium of refractive index n with a group
velocity 𝑣 = 𝑐0
𝑁 where N = n – λ0, dn/d λ0.
Since the pulse is a wavepacket, composed of a spectrum of components of different
wavelengths each travelling at a different group velocity, its width spreads.
The temporal width of an optical impulse of spectral width σλ (nm), after travelling a
distance L, is
𝜎 𝜏 = 𝑑
𝑑𝜆0
𝐿/𝑣 𝜎 𝜆 = 𝑑
𝑑𝜆0
(𝐿𝑁/𝑐0) 𝜎 𝜆
from which
𝝈 𝝉 = 𝑫 𝝀 𝝈 𝝀 𝑳 (response time for material dispersion)
Where
𝐷 𝜆 =
𝜆0
𝑐0
𝑑2
𝑛
𝑑𝜆2
0
is the material dispersion coefficient.
33
34. DISPERSION: Material
The response time increases linearly with the distance L.
Usually L is measured in km, 𝜎 𝜏 in ps and 𝜎 𝜆 in nm, so that 𝐷 𝜆 has units of ps/km-nm.
This type of dispersion is called material dispersion (as opposed to modal dispersion).
34
35. DISPERSION: Wave guide
The group velocity of the modes depends on the wavelength even if material dispersion
is negligible.
This dependence, known as waveguide dispersion, results from the dependence of the
field distribution in the fiber on the ratio between the core radius and the wavelength
(a/λ0).
If this ratio is altered, by altering λ0, the relative portions of optical power in the core
and cladding are modified.
Since the phase velocities in the core and cladding are different, the group velocity of
the mode is altered.
Waveguide dispersion is particularly important in single-mode fibers, where modal
dispersion is not exhibited, and at wavelengths for which material dispersion is small
(near λ0 = 1.3 μm in silica glass).
35
36. DISPERSION: Wave guide
The group velocity 𝑣 = (𝑑𝛽/𝑑𝜔)−1
and the propagation constant β are determined
from the characteristic equation which is governed by the fibre V parameter
𝑉 = 2𝜋
𝑎
𝜆0
𝑁𝐴 = (𝑎. 𝑁𝐴/𝑐0)𝜔
In the absence of material dispersion (i.e. when NA is independent of ω), V is directly
proportional to ω, so that
1
𝑣
=
𝑑𝛽
𝑑𝜔
=
𝑑𝛽
𝑑𝑉
𝑑𝑉
𝑑𝜔
=
𝑎. 𝑁𝐴
𝑐0
𝑑𝛽
𝑑𝑉
The pulse broadening associated with a source of spectral width 𝜎 𝜆 is related to the time
delay 𝐿/𝑣 by
𝜎 𝜏 = 𝑑/𝑑𝜆0 (𝑙/𝑣) 𝜎 𝜆
Thus
𝜎 𝜏 = 𝐷 𝑤 𝜎 𝜆 𝐿
𝐷 𝑤 =
𝑑
𝑑𝜆0
1
𝑣
= −
𝜔
𝜆0
𝑑
𝑑𝜔
1
𝑣
36
37. DISPERSION: Wave guide
𝐷 𝑤 is the waveguide dispersion coefficient.
Thus we obtain
𝐷 𝑤 = (
1
2𝜋𝑐0
)𝑉2 𝑑2
𝛽
𝑑𝑉2
Thus the group velocity is inversely proportional to
𝑑𝛽
𝑑𝑉
and the dispersion coefficient is
proportional to 𝑉2 𝑑2
𝛽
𝑑𝑉2.
Since β varies nonlinearly with V, the waveguide dispersion coefficient 𝐷 𝑤 is itself a
function of V and is therefore also a function of the wavelength.
The dependence of 𝐷 𝑤 on λ0 may be controlled by altering the radius of the core or the
index grading profile for graded-index fibres.
37
38. DISPERSION: Combined Material & Wave guide
The combined effect of material dispersion and waveguide dispersion is also known as
“Chromatic Dispersion”.
Chromatic Dispersion may be determined by including the wavelength dependence of
the refractive indices n1 and n2 and therefore NA, when determining
𝑑𝛽
𝑑𝑉
from the
characteristic equation.
Although generally smaller than material dispersion, wavelength dispersion does shift
the wavelength at which the total chromatic dispersion is the minimum.
Since chromatic dispersion limits the performance of single – mode fibres, more
advanced fibre designs aim at reducing this effect by using graded – index cores with
refractive – index profiles selected such that the wavelength at which waveguide
dispersion compensates material dispersion is shifted to the wavelength at which the
fibre is to be used.
38
39. DISPERSION: Combined Material & Wave guide
Dispersion – shifted fibres have been successfully made by using a linearly tapered
core refractive index and a reduced core radius.
This technique can be used to shift the zero – chromatic – dispersion wavelength from
1.3 μm to1.55 μm where the fibre has its lowest attenuation.
However the process of index grading itself introduces losses since dopants are used.
Other grading profiles have been developed for which the chromatic dispersion vanishes
at two wavelengths and is reduced for wavelengths between.
These fibres, called dispersion – flattened, have been implemented by using a
quadruple – clad layered grading.
39
40. DISPERSION: Combined Material and Modal
The effect of material dispersion on pulse broadening in multimode fibres may be
determined by returning to the original equations for the propagation constants βq of the
modes and determining the group velocities
𝑣 𝑞 = (𝑑𝛽 𝑞/𝑑𝜔)−1
with n1 and n2 being functions of ω.
Although n1 and n2 are dependent on ω, it is reasonable to assume that the ratio
∆= (𝑛1 − 𝑛2)/𝑛1
is approximately independent of ω.
40
41. DISPERSION: Combined Material and Modal
Using this approximation and evaluating
𝑣 𝑞 = (𝑑𝛽 𝑞/𝑑𝜔)−1
we obtain
𝑣 𝑞 =
𝑐0
𝑁1
1 −
𝑝 − 2
𝑝 + 2
(
𝑞
𝑀
)
𝑝
(𝑝+2)
∆
Where 𝑁1 = 𝑑
𝑑𝜔 𝜔𝑛1 = 𝑛1 − 𝜆 𝑜(𝑑𝑛1/𝑑𝜆0) is the group index of the core material.
p is the profile parameter and q is the mode of fibre.
For a step – index fibre (p = ∞), the group velocities of the modes vary from 𝑐0 𝑁1 to
𝑐0 𝑁1 (1 − ∆), so that the response time is
𝝈 𝝉 ≈
𝑳
𝒄 𝟎 𝑵 𝟏
∆
𝟐
(response time for multi mode step index fibre with material dispersion).
41
42. DISPERSION: Non-linear
Another dispersion effect occurs when the intensity of light in the core is sufficiently
high, since the refractive indices then become intensity dependent and the material
exhibits nonlinear behaviour.
The high-intensity parts of an optical pulse undergo phase shifts different from the low-
intensity parts, so that the frequency is shifted by different amounts.
As because of material dispersion, the group velocities are modified, and consequently
the pulse shape is altered.
Under certain conditions, nonlinear dispersion can compensate material dispersion, so
that the pulse travels without altering its temporal profile.
The guided wave is then known as a solitary wave, or a soliton.
42