Propagation of Electromagnetic Waves in Optical Fibers PDF

Module -6 What is Optical Fiber? Propagation of Electromagnetic Waves in Optical Fibers Introduction to optical fiber Architecture of optical fibre communication system, com...

Module -6 What is Optical Fiber? Propagation of Electromagnetic Waves in Optical Fibers Introduction to optical fiber Architecture of optical fibre communication system, components involved in communication system - light the system, basics of optical fiber propagation through fibers Acceptance angle - Numerical Total internal reflection, Snell's law, Acceptance cone, Numerical aperture aperture V-parameter - Types of fibers – Types of fibers based on materials, modes and refractive index, different losses Attenuation happening during transmission Dispersion-intermodal and Types of dispersion, relation how much intermodal dispersion occurs when the intramodal. light propagates through the optical fiber Application of fiber in medicine - The role of optical fibre in the field of medicine, eg. Endoscopy Endoscopy. Class 32: 05/11/2024 3 Class 32: 05/11/2024 6 Module -6 What is Optical Fiber? Propagation of Electromagnetic Waves in Optical Fibers An optical fiber is a thin fiber of glass or plastic or Introduction to optical fiber communication system fiber that can carry light from one end to the other Light propagation through fibers Thin strands of pure glass Acceptance angle Numerical Aperture Carry data over long distances Types of fibers At very high speeds Step index and Graded index Single mode and multimode Fiber can be bent or twisted Attenuation Optical fibers transmission of information in form of ‘light’ Dispersion Intermodal and intramodal Jean-Daniel Colladon; Physicist in 18th Century, core principles of modern-day optical fiber Class 32: 05/11/2024 2 Class 32: 05/11/2024 5 Electromagnetic Waves Module -6 Propagation of Electromagnetic Waves in Optical Fibers Light is an electromagnetic radiation Light can also be a photons, massless packets of energy, each travelling with wavelike Engineering Physics (BPHY101L) properties Fall -Semester 2024-25 Electromagnetic waves are nothing but electric and magnetic fields travelling through free space with the speed of light c. If the frequency of oscillation of the charged particle is f, then it produces an Dr. A. Joseph Nathanael, MSc, MPhil, PhD, PGDCA, MRSC. electromagnetic wave with frequency f. The wavelength λ of this wave is given by λ = c/f. Associate Professor Senior Vellore Institute of Technology (VIT), Vellore - 632 014. Electromagnetic waves transfer energy through space. Class 32: 05/11/2024 Class 32: 05/11/2024 4  Applications of Optical Fiber Applications of Optical Fiber Class 32: 05/11/2024 9 Class 32: 05/11/2024 12 Applications of Optical Fiber Applications of Optical Fiber Class 32: 05/11/2024 8 Class 32: 05/11/2024 11 Applications of Optical Fiber Applications of Optical Fiber Class 32: 05/11/2024 7 Class 32: 05/11/2024 10  History of Optical Fiber Refractive index and Snell’s law Refractive index What is Snell's law? The amount of bending when light travels from one For given pair of media the ratio of the medium to another medium can be measured by a sine of angle of incidence to the sine physical quantity names as Refractive Index (n) or (µ) of angle of refraction is a constant 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 medium 1 𝑠𝑖𝑛 𝑖 = Refractive index = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 2 𝑠𝑖𝑛 𝑟 Constant is equal to refractive index of medium 2 w.r.t medium 1 A medium in which speed of light is more is known as optically rarer medium and a medium in which speed of light is less is said to be optically denser medium. For example, in air and water, air is rarer, and water is a denser medium. Class 32: 05/11/2024 15 Class 32: 05/11/2024 18 History of Optical Fiber Cross-section of Optical Fiber Cross-section of Optical Fiber :Core, cladding and coating Class 32: 05/11/2024 14 Class 32: 05/11/2024 17 History of Optical Fiber Structure of Optical Fiber Optical fibers are long, thin strands about the Core—Central thin part of the fiber with diameter of a human hair. higher refractive index where the light They are arranged in bundles called optical travels cables. Cladding—Outer optical material (less refractive index) surrounding the core that reflects the light back into the core Buffer coating—Plastic coating that protects the fiber from damage and moisture. Jacket—Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable’s outer covering, called a jacket. Class 32: 05/11/2024 13 Class 32: 05/11/2024 16  Relation between Refractive Index and Critical Angle Light rays Propagating Through a Bent Fibre Let a ray of light travels from denser medium of refractive index n1 towards rarer medium of refractive index with an angle of incidence and angle of refraction 90° n2 ic Even for a bent fibre light guidance can take place Then by Snell’s law, n1 sin i = n2 sin r; through multiple total internal reflections. substitute i = θc and r = 90° 𝑛2 𝒏𝟐 Note that the angle of incidence (at the core-cladding sin θc = θc = 𝐬𝐢𝐧−𝟏 ( ) Critical angle interface) remains constant in a straight fibre, whereas it 𝑛1 𝒏𝟏 changes in a bent fibre. Thus, a ray may eventually hit the core-cladding interface To confine the optical signal in the core, the at an angle less than the critical angle and be refracted refractive index of the core must be greater away. than that of the cladding to support total Representation of critical angle internal reflection. Class 32: 05/11/2024 21 Class 32: 05/11/2024 24 Total Internal Reflection in Optical Fiber Light rays propagating through Straight Optical Fiber If the angle of incidence (at the core-cladding interface) is grater than the critical angle The light ray travel from core of refractive index n1 to 𝒏𝟐 θc or ic = sin−𝟏 ( ) 𝒏𝟏 cladding of refractive index n2 Then the ray will undergo total internal reflection at the interface. Because of cylindrical symmetry in the fibre structure, this ray will suffer total internal reflection at the lower interface also a) When i< θc, it is refracted into rarer medium and therefore will get guided through the core by repeated b) When i = θc , it traverses along the interface so that angle of refraction is 90° total internal reflection. c) When i > θc it is totally reflected into the denser medium itself. Refractive index distribution of a cladded optical fibre that consists of a cylindrical glass structure surrounded by a material of slightly lower refractive index. Class 32: 05/11/2024 20 Class 32: 05/11/2024 23 Principle of Optical Fiber Conditions for Total Internal Reflection The optical fiber works on the principle of Total Internal Reflection (TIR) 1. The ray of light must travel from denser medium towards rarer medium. What is total internal reflection ? n2 Rarer medium Clad This phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle. n1 Denser medium Core 2. The angle of incidence in the denser medium must be greater than the critical angle for the pair of the media in contact. Class 32: 05/11/2024 19 Class 32: 05/11/2024 22  Derivation for Angle of Acceptance of a Fiber Numerical Aperture In triangle ABC, r =90-θ Fractional Refractive Index change (Δ): From Snell’s law It is the ratio of refractive index difference in core and cladding to the refractive index of the core. At air and core interface (point A) 𝑛1 n0 sin i = n1 sin r sin i = sin r 𝑛0 𝑛 Substitute r = 90 - θ sin i = 1 sin (90-θ) 𝑛0 Let the light ray enter at an angle i to the axis of the fiber 𝑛 sin i = 1 cos θ The ray refracts at an angle r. 𝑛0 The ray strikes the core – cladding interface at an angle θ. If θ is less than the critical angle θc, the ray will be lost If θ is greater than the critical angle θc, the ray undergoes by refraction. Therefore, limiting value for containing total internal reflection at the interface. the beam inside the core by total internal reflection is Let us now find out up to what maximum value of i at A θc. Let imax be the maximum possible angle of total internal reflection at B is possible. incidence at the fiber end face A for which θ = θc. Class 32: 05/11/2024 27 Class 32: 05/11/2024 30 Acceptance Angle and Cone Numerical Aperture The angle of acceptance is the maximum angle from the fiber axis at which light may enter When light traveling in a dense medium hits a boundary at a steep angle (larger than the the fiber so that it will propagate in the core of the fiber without any refraction to the “critical angle” for the boundary), the light will be completely reflected. This effect is cladding. used in optical fibers to confine light in the core. Light travels along the fiber bouncing The sine of the angle of acceptance is called the Numerical Aperture (NA) of the fiber. back and forth off of the boundary. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain NA = sin im n0 = 1 for air range of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of 𝑛1 2 − 𝑛2 2 NA = NA = 𝑛12 − 𝑛22 the refractive index difference between the fiber’s core and cladding. The half-angle of n0 this acceptance cone is called the acceptance angle. Numerical aperture determines the light gathering ability of the fiber. It is a measure of amount of light that can be accepted by a fiber. NA depends only on the refractive indices of the core and cladding materials. A large NA implies that a fiber will accept large amount of light from the source. Class 32: 05/11/2024 26 Class 32: 05/11/2024 29 𝑛2 2 Acceptance Angle or Angle of Acceptance cos θc = (1 − ) 𝑛2 1 Acceptance angle : Now, i = imax and θ= θc Acceptance angle is defined as the maximum angle that a light ray can have relative to the axis of the fiber 𝑛1 𝑛1 2 − 𝑛2 2 𝑛1 sin imax = and propagate down the fiber. Or the maximum angle at or below which the light can suffer Total Internal sin imax = cos θc 𝑛1 2 𝑛0 𝑛0 reflection is called acceptance angle. At the interface of core (n1) and cladding (n2); at point B 𝑛1 2 − 𝑛2 2 Acceptance cone: sin imax = 𝑛2 n0 An optical fiber accepts only those rays which are incident within a cone having a semi angle im or imax sin θc = 𝑛1 𝑛1 2 − 𝑛2 2 The light rays contained within the cone having a full Using trigonometrical relationship; -1 angle 2im are accepted and transmitted along the fiber. imax = sin n0 Therefore, the cone is called the acceptance cone. sin2 θ + cos2 θ = 1 Light incident at an angle beyond im refracts through the cladding and the corresponding optical energy is lost. It cos2 θ = 1- sin2 θ imax = sin-1 𝑛1 2 − 𝑛2 2 n0 = 1 for air is obvious that the larger the diameter of the core, the larger the acceptance angle. Cos θc = (1 − 𝑠𝑖𝑛2θc) This angle imax is called the acceptance angle of the fiber. Class 32: 05/11/2024 25 Class 32: 05/11/2024 28  Module -6 Types of Optical Fibers Propagation of Electromagnetic Waves in Optical Fibers Step Index Fibre Based on the type of the In step index fiber, the refractive index profile makes a step Single mode step index fiber Introduction to optical fiber communication system material used, they are change at the core cladding interface. RI of core is uniform Multimode step index fiber classified into two types throughout the core medium, and its value is greater than Light propagation through fibers the cladding refractive index. 1. Glass fiber: Acceptance angle Example: In step index fibers, if core has refractive index n1 and cladding has refractive index n2. Numerical aperture Core: SiO2 Cladding: SiO2 Then n1>n2 and this is responsible for total internal Core: GeO2- SiO2 Cladding: SiO reflectance in optical fibers. V-parameter Graded Index Fibre 2. Plastic fiber: Types of fibers Example: In graded fibers, refractive index will not be constant in the Core: polymethyl core, but RI of the core decreases with increasing radial Attenuation methacrylate distance from the core. Cladding: Co- Polymer Dispersion-intermodal and intramodal Core: Polystyrene The light rays travel inside the core by the phenomena of total internal reflectance. Since the core has higher RI than Application of fiber in medicine - Endoscopy. Cladding: Methyl that of the cladding, when the light falls on the core cladding methacrylate Multimode graded index fiber interface (moves from denser to rarer medium) it returns back into the core. Class 33: 06/11/2024 2 Class 33: 06/11/2024 5 Module -6 Types of Optical Fibers Propagation of Electromagnetic Waves in Optical Fibers Optical fibers are classified into three major categories Engineering Physics (BPHY101L) Fall -Semester 2024-25 Dr. A. Joseph Nathanael, MSc, MPhil, PhD, PGDCA, MRSC. Associate Professor Senior Vellore Institute of Technology (VIT), Vellore - 632 014. Class 33: 06/11/2024 Class 33: 06/11/2024 4 Numerical Module -6 Propagation of Electromagnetic Waves in Optical Fibers An optical fibre core and its cladding have refractive indexes of 1.46 and 1.45 respectively. Calculate the critical angle, acceptance angle, and numerical aperture? Introduction to optical fiber Architecture of optical fibre communication system, components involved communication system - light in the system, basics of optical fiber propagation through fibers 𝒏𝟐 Critical angle, ic = sin−𝟏 ( ) Acceptance angle - Numerical Total internal reflection, Snell's law, Acceptance cone, Numerical aperture 𝒏𝟏 aperture imax = sin-1 𝑛1 2 − 𝑛2 2 V-parameter - Types of fibers Types of fibers based on materials, modes and refractive index, different – Attenuation losses happening during transmission Dispersion-intermodal and Types of dispersion, relation how much intermodal dispersion occurs when NA = 𝑛12 − 𝑛22 intramodal. the light propagates through the optical fiber Application of fiber in The role of optical fibre in the field of medicine, eg. Endoscopy medicine - Endoscopy. Class 32: 05/11/2024 31 Class 33: 06/11/2024 3  Graded Index Fiber Difference between Step Index fiber and Graded Index fiber The index variation represented as : STEP INDEX FIBER GRADED INDEX FIBER The refractive index of the core is uniform throughout and The refractive index of the core is made to vary gradually n1 (1-2Δ (r/a)α)1/2 r < a undergoes on abrupt change at the core cladding such that it is maximum at the center of the core. Δ = n12 – n22/2n12 boundary n(r) = n1 (1-2Δ)1/2 = n2 r ≥ a The diameter of the core is about 50-200 μm in the case of The diameter of the core is about 50 μm in the case of multimode fiber and 10 μm in the case of single mode multimode fiber where Δ is the relative refractive index difference and fiber α is the profile parameter which gives the characteristic The path of light propagation is zig- zag in manner The path of light is helical in manner refractive index profile of the fiber core, The refractive index profile and ray transmission in a a – core radius; r – distance from the fiber axis multimode graded index fiber Attenuation is more for multimode step index fiber but for Attenuation is less. single mode it is very less For graded index fibers the situation is more complicated since the numerical aperture is a function of the radial distance from the fiber axis. This fiber has lower bandwidth This fiber has higher bandwidth The light ray propagation is in the form of meridional rays, The light propagation is in the form of skew rays, and it will Graded index fibers, therefore, accept less light than corresponding step index fibers with the same relative and it passes through the fiber axis. not cross fiber axis. refractive index difference. No of modes of Propagation: No of modes of Propagation: For refractive indices that do not differ by more than 1%, the refractive-index contrast is given by the relation Δ ≈ (n1 − n2)/n1 where Δ is the refractive-index contrast, n1 is the refractive index of the denser material and n2 is the refractive index of the less dense medium. V – number determines how many modes a fiber can support Class 33: 06/11/2024 8 Class 33: 06/11/2024 11 Step Index Fiber Graded Index Multi Mode Optical Fiber The two major types of step index fiber. Characteristics The refractive index profile may be defined as : Low attenuation Step index fiber: n1 r < a (core) intermediate band width Refractive index of the core remains constant throughout the dimension of the n(r) = Small N.A core and also RI of the cladding remains n2 r ≥ a (cladding) LED/laser can be used constant throughout the dimension of the a – core radius; r – distance from the fiber axis cladding. Advantages Multimode (a) shows a multimode step index fiber with a core Dimension of the core is large compared to diameter of around 50 μm or greater, which is large Intermodal dispersion can be reduced enough to allow the propagation of many modes cladding d1>d2 within the fiber core. Many different possible ray Disadvantages single mode paths through the fiber. (b) shows a single-mode or monomode step index fiber which allows More expensive Dimension of the core is less compared to the propagation of only one transverse electromagnetic mode and cladding d1 n2> n3> n4 > n5 > n6 Step index fiber Because of decrease in refractive index the ray gets gradually curved towards the upward direction and at ✓ The optical fiber with a core of constant refractive index n1 and a cladding of a slightly lower one place , where in it satisfies the condition for total internal reflection, it is totally internally reflected. refractive index n2 is known as step index fiber. ✓ This is because the refractive index profile for this type of fiber makes a step change at the core- n1 >n2 cladding interface. Graded index fiber or inhomogeneous core fibers n2 >n3 ✓ Graded index fibers do not have a constant refractive index in the core but a decreasing core index n3 >n4 n(r) with radial distance from a maximum value of n1 at the axis to a constant value n2 beyond the core radius a in the cladding. An expanded ray diagram showing refraction at the various high to low index interfaces within a graded index fiber. Class 33: 06/11/2024 6 Class 33: 06/11/2024 9  Based on Modes Propagation in Optical Fiber Characteristics of Multimode Step Index Fiber Optical fibers based on modes of propagation of optical signal through the fiber Single mode fiber Characteristics: Advantage Multimode fiber High attenuation Easy for manufacturing In simple terms, modes can be visualized as the possible number of paths of light in an optical fiber. Ideal for long short distance Less expensive The number of modes that a fiber will support depends upon the ratio of d/λ communication Disadvantages where d is the diameter of the core and λ is the wavelength of the wave being transmitted Low band width Suffer from intermodal dispersion High N.A Applications LED can be used Data links which require low band width Single-mode and multimode fibers Class 33: 06/11/2024 14 Class 33: 06/11/2024 17 V - Number / Parameter Characteristics of Single Mode Step Index Fiber Characteristics: Advantage The Normalized Frequency Parameter of a fiber, also called the V number, is a Low attenuation 80% of the world is using this fiber for communications useful specification. Ideal for long distance communication Very high information carrying capacity Many fiber parameters can be expressed in terms of V, such as: Very high band width Disadvantages Low N.A Highly expensive the number of modes at a given wavelength, mode cut off conditions, and Very small core diameter propagation constants. More difficult for manufacture Need laser LED cannot be used For example, the number of guided modes in a step index multimode fiber is given by V2/2, Applications Undersea cables A step index fiber becomes single-mode for a given wavelength when V < 2.405. Submarine cable systems Mathematically, V=2 π·NA·a/λ where “a” is the fiber core radius. Class 33: 06/11/2024 13 Class 33: 06/11/2024 16 Difference between Step Index fiber and Graded Index fiber Single-mode and Multi-mode Fibers The international standard for outer cladding diameter of most single-mode optical fibers is 125 μm for the glass and 245 μm for the coating. This standard is important because it ensures compatibility among connectors, splices, and tools used throughout the industry. Standard single-mode fibers are manufactured with a small core size, approximately 8 to 10 μm in diameter. Multimode fibers have core sizes of 50 to 62.5 μm in diameter Dimensions of the single-mode and multi-mode fibers Fiber optic splicing involves joining two fiber optic cables together Class 33: 06/11/2024 12 Class 33: 06/11/2024 15  Optical Fiber Types and Light Propagation Mechanisms Module -6 Propagation of Electromagnetic Waves in Optical Fibers Introduction to optical fiber communication system Light propagation through fibers Acceptance angle Numerical aperture V-parameter Types of fibers Attenuation Dispersion-intermodal and intramodal Application of fiber in medicine - Endoscopy. Class 33: 06/11/2024 20 Class 34: 08/11/2024 2 Differences Between Single and Multimode Fibre Module -6 Single mode fibre Multimode fibre Propagation of Electromagnetic Waves in Optical Fibers In single mode fiber only one mode can propagate In multimode it allows a large number of paths or through the fiber modes for the light rays travelling through it. It has smaller core diameter and the difference between It has larger core diameter and refractive index the refractive index of the core and cladding is very difference is larger than the single mode fiber. small. Engineering Physics (BPHY101L) Advantages: Disadvantages: Fall -Semester 2024-25 No dispersion (i.e. there is no degradation of signal Dispersion is more due to degradation of signal owing to during propagation) multimode. Since the information transmission capacity is inversely Information can be carried to shorter distances only. proportional to dispersion, the fiber can carry information to longer distances Disadvantages: Advantages: Dr. A. Joseph Nathanael, MSc, MPhil, PhD, PGDCA, MRSC. Launching of light and connecting of two fibers difficult. Launching of light and also connecting of two fibers is Associate Professor Senior Installation (fabrication) is difficult as it is more costly easy. Vellore Institute of Technology (VIT), Vellore - 632 014. Fabrication is easy and the installation cost is low. Class 33: 06/11/2024 19 Class 34: 08/11/2024 Optical Fiber Types and Light Propagation Mechanisms Step Index fiber Graded Index fiber Single mode fiber Class 33: 06/11/2024 18 Class 33: 06/11/2024 21  Optical Fiber Types and Light Propagation Mechanisms Light Signal at Input and Output of the Optical Fiber What happened in this two cases? 1 0 1 Fiber optic communication system suffers from the following three major impediments 1 0 1 Attenuation Dispersion Nonlinear effects Class 34: 08/11/2024 5 Class 34: 08/11/2024 8 Differences Between Single and Multimode Fibre What are the Factors Causing Distortion in Optical Fibers? Single mode fibre Multimode fibre In single mode fiber only one mode can propagate In multimode it allows a large number of paths or through the fiber modes for the light rays travelling through it. Attenuation: Transmission loss; It has smaller core diameter and the difference between It has larger core diameter and refractive index Reduction in intensity of the refractive index of the core and cladding is very difference is larger than the single mode fiber. the light beam (or signal) small. with respect to distance Advantages: Disadvantages: traveled through a No dispersion (i.e. there is no degradation of signal Dispersion is more due to degradation of signal owing to transmission medium during propagation) multimode. Since the information transmission capacity is inversely Information can be carried to shorter distances only. proportional to dispersion, the fiber can carry Dispersion: information to longer distances Spreading of light pulse Disadvantages: Advantages: as it travels down the Launching of light and connecting of two fibers difficult. Launching of light and also connecting of two fibers is length of an optical fiber Installation (fabrication) is difficult as it is more costly easy. Fabrication is easy and the installation cost is low. Class 34: 08/11/2024 4 Class 34: 08/11/2024 7 Module -6 Optical Fiber Types and Light Propagation Mechanisms Propagation of Electromagnetic Waves in Optical Fibers Introduction to optical fiber Architecture of optical fibre communication system, components involved communication system - light in the system, basics of optical fiber propagation through fibers Step Index fiber Acceptance angle - Numerical Total internal reflection, Snell's law, Acceptance cone, Numerical aperture aperture V-parameter - Types of fibers – Types of fibers based on materials, modes and refractive index, different Graded Index fiber Attenuation losses happening during transmission Dispersion-intermodal and Types of dispersion, relation how much intermodal dispersion occurs intramodal. when the light propagates through the optical fiber Single mode fiber Application of fiber in The role of optical fibre in the field of medicine, eg. Endoscopy medicine - Endoscopy. Class 34: 08/11/2024 3 Class 34: 08/11/2024 6  Mechanisms Responsible for Signal Attenuation in Fibers Geometric Effect-Induced Optical Loss or Radiative Loss Intrinsic Material property Geometric effect-induced optical loss is caused when bending a fiber. Such bending loss is categorized to into two types: Absorption Extrinsic Impurities in glass Fe2+, Cu2+, Cr2+ and OH- Macroscopic bending loss Atomic defects Missing mole, O2 defects Microscopic bending loss Macroscopic bending loss is produced whenever apply curvatures to the fiber. Material composition Material component density The preparation and purification technique The practical examples are wrapping in a spool and fiber installation at the corner The waveguide structure Compositional fluctuations Scattering Macro bending loss can be explained from the viewpoint of changing of Randomly distributed material defects the angle when traveling light interacting with interface of the core Geometric effects and cladding. Inhomogeneity Such variation of angle tends to become smaller than the critical angle, where the total reflection does not occur anymore, and portion of the Macroscopic bends, diameter of fiber >> diameter of core light launches into the fiber cladding. Radiative Loss In particular, the light with traveling angle close to critical angle is more Microscopic bends, diameter of core > diameter of fiber sensitive to the macro bending loss. This loss is negligible for small bends. Class 34: 08/11/2024 11 Class 34: 08/11/2024 14 How the Signal Attenuation is Measured/Calculated? Scattering Losses In metallic conductors the attenuation is usually expressed in the logarithmic unit of the decibel. Scattering losses in glass arise : The decibel, which is used for comparing two power levels, may be defined for a particular Microscopic variations of fiber material component density optical wavelength as the ratio of the input (transmitted) optical power Pi into a fiber to the output (received) optical power Po from the fiber as: Compositional fluctuations Randomly distributed material defects Inhomogeneous material structure In optical fiber communications the attenuation is usually expressed in decibels per unit Rayleigh scattering is mainly responsible for the scattering loss in glass optical fiber. length (i.e. dB km−1) following: The Rayleigh scattering is the process of energy scattering when the traveling light interacts with the small objects. The metric of Rayleigh scattering is proportional to 1/λ4, therefore the significance of the Where αdB is the signal attenuation per unit length in decibels which is also referred to as the fiber loss parameter and L is Rayleigh scattering reduced with the increase of wavelength. the fiber length. Class 34: 08/11/2024 10 Class 34: 08/11/2024 13 What is Attenuation and Why it is Important ? Material Absorption Losses Material absorption is a loss mechanism related to the material composition and the fabrication process for the Attenuation in optical fibers, also known as transmission loss fiber, which results in the dissipation of some of the transmitted optical power as heat in the waveguide. Reduction in intensity of the light beam (or signal) with respect to distance traveled The absorption of the light may be intrinsic (caused by the interaction with one or more of the major through a transmission medium components of the glass) or extrinsic (caused by impurities within the glass). The attenuation or transmission loss of optical fibers has proved to be one of the most important factors in bringing about their wide acceptance in telecommunications. Intrinsic Attenuation is an important factor limiting the transmission of a digital signal in the Extrinsic Absorption by Absorption is caused by three absorption by absorption by atomic defects in form of light rays across large distances. basic constituent impurity atoms the composition different mechanism: atoms of fibre in silica material of glass material Usually absorption of light occurs due to imperfections of the atomic structure such as missing molecules, hydroxyl ions, high density cluster of atoms etc., which absorbs light. Class 34: 08/11/2024 9 Class 34: 08/11/2024 12  Dispersion Intramodal and Intermodal Dispersion Spreading of light pulse as it travels down the length of an optical fiber Dispersion Intramodal dispersion Intermodal dispersion (Modal or ‘Mode’ (chromatic dispersion) dispersion) Present in all type of Present in multimode step index and graded fibers index fibers Dispersion phenomenon related to the ‘Intra’ means within ‘Inter’ means between i.e. between different Dispersion causes temporal pulse spreading variation in velocity of different i.e. within mode modes. Hence, it will not be presented in Pulse overlap results in indistinguishable data frequencies (wavelengths) or different This dispersion is due single mode fiber to finite bandwidth of This dispersion is due to change in velocity Inter symbol interference (ISI) modes the signal from one mode to other. Inter symbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. Class 34: 08/11/2024 17 Class 34: 08/11/2024 20 Factors Responsible for Losses in Optical Fiber Pulse Broadening due to Dispersion Class 34: 08/11/2024 16 Class 34: 08/11/2024 19 Microscopic Bending Loss Pulse at Input and Output of Optical Fiber Micro-bends losses are caused due to non-uniformities or micro bends inside the fiber. Input signal Output signal This micro bends in fiber appears due to non uniform pressures created during the cabling of the fiber or even during the manufacturing itself. The loss of light by leakage through the fiber. Optical fiber Microscopic bending loss is much weaker. It is caused by the strain or stresses distributed along the fiber, which is subjected to temperature variation along the length or setup in the cabling process. The amount of microscopic bending loss depends on the how well the fiber is coated or packed. How to minimize the loss ? Micro-bend losses can be minimized by extruding (squeezing out) a compressible jacket over the fiber. A series of pulses, each of width t1 (at the input end of the fiber), after transmission through the fiber emerges a series of pulses of width t2 (> t1). In such cases, even when the external forces are applied, the jacket will be deformed but the fiber will If the broadening of the pulse is large, then adjacent pulses will overlap at the output end and may not be tend to stay relatively straight and safe, without resolvable. Thus, pulse boarding determines the minimum separation between adjacent pulses, which in causing more loss. turn determines the maximum information carrying capacity of the optical fiber Class 34: 08/11/2024 15 Class 34: 08/11/2024 18  Intramodal Dispersion or Chromatic Dispersion Module -6 Different wavelengths travel at different speeds through the fiber Propagation of Electromagnetic Waves in Optical Fibers This spreads a pulse in an effect named chromatic dispersion Chromatic dispersion occurs in both single mode and multimode fiber The chromatic dispersion is the combination of the material and waveguide dispersion effects. Engineering Physics (BPHY101L) Winter -Semester 2024-25 Dr. A. Joseph Nathanael, MSc, MPhil, PhD, PGDCA, MRSC. Associate Professor Senior Intramodal dispersion play a major role in limiting the bandwidth of a single mode optical fiber. Vellore Institute of Technology (VIT), Vellore - 632 014. Class 34: 08/11/2024 23 Class 35: 12/11/2024 Intermodal Dispersion Questions The more modes the grater the modal dispersion Class 34: 08/11/2024 22 Class 34: 08/11/2024 25 Intermodal Dispersion Intramodal Dispersion or Chromatic Dispersion When an optical pulse is launched into the fiber, the optical pulse is distributed over all modes of fiber Here we consider the propagation of light within the fiber in terms of guided electromagnetic waves called “modes”. Different modes will travel with different propagation angles, hence these modes takes different routes, travel with the different velocity, but at the end of fiber they come at different timings. It causes pulse widening, This is called intermodal dispersion or modal dispersion This dispersion is due to change in velocity from one mode to other. Intermodal dispersion present in only multimode optical fiber. Class 34: 08/11/2024 21 Class 34: 08/11/2024 24  Two Types of Intramodal Dispersion Intermodal Dispersion in Multimode Step Index Fiber Dispersion The delay difference Δts between the extreme meridional ray and the axial ray may be obtained by subtracting 𝒏𝟏𝟐𝑳 𝒏𝟏𝑳 Intra-modal dispersion Inter modal dispersion 𝐓 = Δ𝒕 = 𝒕𝒎𝒂𝒙 − 𝒕𝒎𝒊𝒏 = − 𝑾𝒆 𝒌𝒏𝒐𝒘 𝚫 = 𝒏𝟏−𝒏𝟐 𝑪 𝒏𝟐 𝑪 𝒏𝟐 𝒏𝟏 𝑳 𝒏𝟏 𝒏𝟏 𝑳 T = −𝟏 𝐓 = 𝚫 𝚫 Relative refractive index difference 𝒄 𝒏𝟐 𝒄 Material dispersion Waveguide dispersion NA = n1 (2Δ)1/2 (NA)2 = n12 (2Δ) The Intra-modal dispersion consists of two parts: Material dispersion: This is due to intrinsic properties of the material, glass, dispersive medium. 𝒏𝟏 𝑳 𝐓 = 𝚫 Different colors (wavelengths) have different velocity in glass 𝒄 Waveguide dispersion: Due to the difference in the refractive index between the core and cladding The approximate expressions for the delay difference in below are usually employed to estimate the maximum pulse the light will travel with different velocities which results in spreading of light. The rays which broadening in time due to intermodal dispersion in multimode step index fibers. travel through cladding will reach early than that of the rays which travel through core. Class 35: 12/11/2024 4 Class 35: 12/11/2024 7 Module -6 Intermodal Dispersion in Multimode Step Index Fiber Propagation of Electromagnetic Waves in Optical Fibers If the ray moves with the φ = φc (critical angle) φ =90-θ sinφ = sin(90-θ) =cosθ The time taken is maximum Introduction to optical fiber Architecture of optical fibre communication system, components involved 𝒏𝟏𝑳 communication system - light 𝐭𝒎𝒂𝒙 = 𝒏𝟏𝑳 in the system, basics of optical fiber 𝑪 (𝒔𝒊𝒏φ𝒄) 𝒏𝟏𝑳 propagation through fibers 𝐭= 𝐭= 𝑪 (𝒔𝒊𝒏φ) 𝑪 𝒄𝒐𝒔θ Acceptance angle - Numerical From the Senll’s law, at the core-clad interface Total internal reflection, Snell's law, Acceptance cone, Numerical aperture aperture If the ray moves along the axis it takes minimum time; θ = 0 V-parameter - Types of fibers – Types of fibers based on materials, modes and refractive index, different 𝒏𝟐 𝒔𝒊𝒏φ𝒄 = 𝒏𝟏𝑳 Attenuation losses happening during transmission 𝒏𝟏 𝒏𝟏𝑳 𝒏𝟏𝑳 𝐭𝒎𝒊𝒏 = 𝐭𝒎𝒊𝒏 = 𝐭𝒎𝒊𝒏 = 𝑪 𝒄𝒐𝒔θ 𝑪 𝒄𝒐𝒔(𝟎) 𝑪 Dispersion-intermodal and Types of dispersion, relation how much intermodal dispersion occurs 𝑨𝑪 intramodal. when the light propagates through the optical fiber In another way writing, tmin is : 𝐭𝐦𝐢𝐧 = 𝑪 𝒏𝟏 Application of fiber in The role of optical fibre in the field of medicine, eg. Endoscopy 𝒏𝟏𝑳 medicine - Endoscopy. 𝐭𝒎𝒂𝒙 = 𝒏𝟏𝟐𝑳 𝒏𝟏𝑳