KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE, SINGAPUR - PowerPoint Presentation

1. KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE, SINGAPURClass : I B. Tech. II Semester Branch: CSEAcademic Year : 2020-21 Regulation : R18Subject : Applied PhysicsTopic : Fiber opticsName of the Staff : K. Rajashekhar

2. Fiber optics

3. Optical fiberThe information carrying capacity of a transmission system is directly proportional to the carrier frequency of the transmitted signals. A light wave acting as a carrier wave is capable of carrying more information than radio waves and microwaves. Optical fiber is a guiding medium .It is a fine transparent dielectric medium and designed to guide the light over long distances with a very little leakage through the walls.Optical fiber is works on the principle of “Total Internal Reflection”.

4. Principle of Optical fiberConsider a light ray traveling in a medium of refractive index n1 strikes the second medium of refractive index n2. If θ1 and θ2 are the angles of incidence and refraction, according to Snell’s law n1 sin θ1=n2 sin θ21) If n1 = n2 ; θ1= θ2 ---- no deviation is observed2) If n1 < n2 ; θ1> θ2 ---- the refracted ray bends towards the normal3) If n1 > n2 ; θ1< θ2 ---- the refracted ray bends away from the normal

5. Critical angleA ray traveling from an optically dense medium to a less dense medium, for a particular angle of incidence θ the refracted ray travels along the interface or the angle of refraction in the rare medium becomes maximum(90o), this angle of incidence in the denser medium is known as ‘critical angle of incidence (θc )’

6. Total Internal Reflection When a light ray traveling from an optically dense medium to a less dense medium, If the angle of incidence exceeds the critical angle, the ray gets reflected into the same medium, this phenomenon is known as “total internal reflection”.There is no loss of energy during the reflection, hence the optical fibers are able to sustain the light signal transmission over along distances.

7. Optical fiber constructionOptical fiber is a very thin and flexible medium having a cylindrical shape consisting of three major parts as follows.Core : It is an inner cylindrical material made up of glass or plastic dielectric materialCladding : It is an outer cylindrical shell surrounds the core and serves to reflect the light back into the core.Buffer coating: Coatings are usually multi-layers of plastics applied to preserve fiber strength, absorb shock and provide extra fiber protection.The refractive index of core n1 is greater than the refractive index of cladding n2

8. Acceptance angleLet the light is launched from the medium of refractive index n0 into the core of refractive index n1. Let the incident ray make an angle θi with the fiber axis and proceeds after refraction at an angle θr from the axis. It is then undergoes the total internal reflection at the core cladding interface at an angle ϕ

9. Let θa be the maximum possible angle of incidence the fiber end face for which ϕ is equal to ϕc. For a ray the angle of incidence exceeds θa then ϕ is less than ϕc and hence the ray will be refracted.

10. This maximum angle is called the acceptance angle or the acceptance half cone angle. Rotating the acceptance angle about the fiber axis describes the acceptance cone.

11. Numerical ApertureLight collecting capacity of the fiber is expressed in terms of acceptance angle using the terminology ‘Numerical aperture’.Sine of the maximum acceptance angle is called the numerical aperture (NA) of the fiber.

12. Types Of Optical FibersThere are two basic ways of classification of fibers. They are based on Refractive index profile of core & cladding and mode of propagation.Step index fiber Single mode step index fiber (One ray path) Multimode step index fiber (More than one path) Graded index fiberMultimode Graded index fiber

13. Step Index FiberIn step index fiber the refractive index of the core remains constant throughout the core and undergoes an abrupt change (or step) at the core cladding boundary.In practical step index fibers the core of radius ‘a’ has the refractive index n1 which is typically 1.48. This is surrounded by a cladding of slightly lower refractive index n2.The refractive index profiles may be defined as n(r) = n1 for r<a (core) n(r) = n2 = n1(1-Δ) for r ≥ a (cladding) The parameter Δ is called the core cladding refractive index difference The value of n2 are chosen such that Δ is nominally 0.01.

14. Single Mode Step Index FiberA step index fiber will become single mode by reducing core refractive index or by reducing Δ or by reducing the core diameterThe core size is about 8 to 12μm and cladding is about 60 to 125μm.Single mode fibers have high information carrying capacityIt is difficult to make and hence very expensive.Handling, splicing and making interconnections are also more difficult It requires the laser source.

15. Multi Mode Step Index FiberThe diameter of the core is 50 to 200μm and cladding is 125 to 400μmIt can allow to propagate a large number of modes.The large core diameter makes easier to launch optical power Handling, splicing and making interconnections are easy. The disadvantage of multimode fiber is large intermodal dispersionlight emitting diodes can be used as sources

16. Graded Index FiberIn graded index fiber the core refractive index is made to vary as the function of the radial distance from the center of the core. The core refractive index n(r) decreases from a maximum value n1 at the axis to a constant value n2 beyond the core radius ‘a’. Thus the index variation can be represented as: n(r) = n1 for r<a (core) n(r) = n1 n1⌊1−2∆⌋=n2 for r ≥ a (cladding)where Δ is the relative index difference, and is the profile parameter  

17. Multimode Graded Index FiberThe diameter of the core is about 50 -100μm and the cladding is about 125- 140μm. The core consists of a number of layers having gradual variation in refractive index.Light launched into the fiber core, undergoes refraction from one layer to the other layer and there is change in the path of the light ray in this way the light ray travels in curved path.The intermodal dispersion is reducedIt is easier to splice, interconnect and cheaperLED’s are used as sources.

18. V-Number It is used to characterize the optical fibersV= Where a is the radius of the core and λ is the wavelength, we know that NA= V= = The maximum number of modes M supported by a step index fiber is given by The wavelength corresponding to the value V=2.405 is known as cutoff wavelength λc of the fiber. = The total number of modes that are supported of graded index is given by.  

19. Single Mode1Light can propagate through the fiber in only one mode2The fiber core diameter is very small (8-12µm) and also the difference the refractive indices of the core and cladding is very small ( Δ=0.2 to 1%).3V number ≤2.4054Launching of light into the fiber and coupling process is not easy5Dispersion is minimum6High bandwidth, high information carrying capacity.7Optical light power using LEDs cannot be connected , It r quires laser sources8It used in haul (long distance) communication. 9Since, the fabrication is difficult, the production cost is also high.Multi Mode1Light can propagate through the fiber with large number of modes.2The fiber core diameter is large(50-200µm) and also the difference the refractive indices of the core and cladding is large( Δ=1 to 3%).3V number >2.4054Launching of light into the fiber and coupling process is easy5Dispersion is maximum6low bandwidth, information carrying capacity is less.7LEDs can be used as optical sources8Used in short distance communications, LAN (Local Area Net works)9Since, the fabrication is easy, the production cost is also low.

20. Step Index FiberGraded Index Fiber1Refractive index of the core remains constant in the core and there is an abrupt change in refractive index at core cladding interfaceRefractive index of the core vary as the function of the radial distance from the center of the core. It is maximum at the center of the core and decreases gradually with distance towards core cladding boundary.2The refractive index profile is n(r) = n1 for r<a (core) n(r) = n2 = n1(1-Δ) for r ≥ a (cladding)The refractive index profile is n(r) = n1 for r<a (core)n(r) = n1 n1⌊1−2∆⌋=n2 for r ≥ a (cladding)3Numerical aperture (NA) is very less for single mode step index fiber but is more for multimode step index fiberNumerical aperture (NA) is high4NA of step index fiber is constant. The NA for graded index fibers varies as a function of the radial distance (r) in core and is zero in cladding . 5Light rays are propagated in the core in a zigzag manner. Light rays are propagated in the core helical path6Number of modes propagation for a multimode step index fiber is given by Number of modes propagation for a multimode Graded index fiber is given by 7Intermodal dispersion is largeIntermodal dispersion is less8Bandwidth of single mode step index fiber is more Multimode graded index fiber has a higher band width 9It is used for long distance communicationsIt is used for short distance communicationsStep Index FiberGraded Index Fiber1Refractive index of the core remains constant in the core and there is an abrupt change in refractive index at core cladding interfaceRefractive index of the core vary as the function of the radial distance from the center of the core. It is maximum at the center of the core and decreases gradually with distance towards core cladding boundary.2The refractive index profile is n(r) = n1 for r<a (core) n(r) = n2 = n1(1-Δ) for r ≥ a (cladding)3Numerical aperture (NA) is very less for single mode step index fiber but is more for multimode step index fiberNumerical aperture (NA) is high4NA of step index fiber is constant. The NA for graded index fibers varies as a function of the radial distance (r) in core and is zero in cladding . 5Light rays are propagated in the core in a zigzag manner. Light rays are propagated in the core helical path67Intermodal dispersion is largeIntermodal dispersion is less8Bandwidth of single mode step index fiber is more Multimode graded index fiber has a higher band width 9It is used for long distance communicationsIt is used for short distance communications

21. Fiber Optic Communication SystemThe major components of an optical communication system are Transmitter Optical fiber Receiver

22. Transmitter It consists of Light source (laser diode or light emitting diode) and Drive circuitry The output of the light source can be modulated according to the input electrical signal to produce a optical signal which can be transmitted through the optical fiber Two types of light modulation are possible i) Analog: In this modulation the intensity of light from the laser is varied continuously ii) Digital: In this modulation, the intensity is changed in an on/off fashionOptical Fiber The modulated optical signal launched in to the fiber Optical fiber transmit the signal by the principle of total internal reflection Receiver At the receiver the optical signal is converted into electrical signal It consists of a photo detector (photodiode or avalanche photodiode) and amplifier

23. Advantages Of Optical Fiber CommunicationsWider Band width - High information carrying capacityElectrical isolationImmunity to interference and cross talkSignal securitySmall size and weightLow transmission lossLow costImmune to Adverse moisture and temperature conditionsSafely used in any environmentLow maintenance costLong life span

24. AttenuationAttenuation is the loss of optical power as light travels through the fiber. It results from Absorption Scattering Bending losses Dispersion spreads the optical pulses as it ravels along the fiber. Attenuation i.e. loss = The decibel loss of optical power = - 10 log Where Pout is the power coming out of the fiber, Pin is the power launched in to the fiber The loss in decibels per unit length ( i.e. dB/km) = - logwhere α is the signal attenuation ( attenuation coefficient) and L is the length of the fiber 

25. Absorption:Absorption is caused by three different mechanisms .Atomic defects in the glass composition Extrinsic absorption by impurity atoms in the glass material Intrinsic absorption by basic constituent atoms of the fiber material.  Atomic defects : Missing molecules, high density clusters of atom groups, or oxygen defects in glass structures causes the fiber loss. Absorption loss due to these defects are negligible compared with other loss mechanisms lossExtrinsic absorption: Metal ion impurities such as Fe, Cu, V, Co, Ni, Mn and Cr absorbs strongly in the region of interest (0.5 μm to 1.6 μm ) and must not exceed 1 to 10 ppb to cause the losses below 20 dB/kmAbsorption by OH ion impuritiesThe absorption peak by molecular vibration (ion resonance) of OH is around is 2.7 μm and its overtones occur at 1.38 μm, 1.25 μm, 0.95 μm and 0.725 μm . This absorption can be reduced by reducing the water content in the fiber below 1ppb.

26. Intrinsic absorption:It is associated with the basic fiber material (SiO2)It results from the electronic absorption band in the UV region and the atomic vibration bands in near infrared region.UV loss contribution decays exponentially with increasing the wavelengthThe tail of ultraviolet absorption presents at lower wavelengths near 0.8 μmAn interaction between the vibrating bond and the electromagnetic field of the optical signal results the transfer of energy from the field to the bond there by giving rise the absorption.In pure silica fibers, tail of infrared absorption Si-O occurs at higher wavelength around 1.4μm to 1.6 μm In pure silica fibers the transmission losses are reduced to a minimum value at 1.55 μm

27. Scattering:Scattering losses in glass arise from Microscopic variations in the material density, Compositional fluctuations andStructural inhomogeneities or defects If the dimensions of these fluctuations are of the order of λ/10 or less, each irregularity acts as a point source of scattering center. This type of scattering is known as Rayleigh scattering. It varies as 1/λ4 and it becomes important a lower wavelength region

28. Bending Losses:Bending ( radiative) losses occurs whenever an optical fiber undergoes a bend of finite radius of curvature. There are two types of bending lossesMacroscopic bending lossesMicroscopic bending losses

29. Macroscopic bending losses: These losses occur when a fiber cable turns at corners, if the radius of curvature of bend is greater than the fiber diameter. To maintain the pulse in the fiber at bend the part of propagation mode outside the bend has to travel faster, than that inside which is not possible.If the cable is installed with a bend radius is less than the minimum bending radius. The light ray incident the core-cladding boundary at an angle less than the critical angle and it will be lost into the cladding

30. Microscopic bending losses: These losses occur due to random microscopic bends on the fiber axis. This situation arises when the fibers are incorporated in to cables. Due to micro bending the light ray may incident the core-cladding boundary at an angle less than the critical angle and it will be refracted from the cladding. These losses can be minimized by providing a compressible jacket over the fiber.

31. Optical Fiber Applications1) Communication systems: Voice: Telephone trunk, Near power plants, Along the power lines, Field communication Video: Broadcast TV, Community area TV (CATV), Remote monitoring, Fiber missile, Fiber to the home Data: Computers : CPU to peripherals, CPU to CPU, Interoffice data links, Local area networks(LAN)2) Medical: Optical fibers are used in the design of endoscope, which is used to inspect internal organs of the human body for diagnosis purpose.3) Industrial: Fibers are used to design borescope which is used to inspect the machinery parts where the direct inspection with naked eye is not possible.4) Domestic: They can be used in decorator articles, display etc.5) Sensor applications: Optical fiber sensors two types namely active and passive. In active sensors, the quantity to be measured acts directly on the fiber itself to modify the radiation passing through the fiber. In passive sensors, the modulation takes place outside the fiber. The fiber merely acts as a convenient transmission channel for the radiation. Various kinds of fiber optic sensors including the temperature sensor, pressure sensor, acoustic sensor, current sensor, flow meter, chemical sensor, etc. These sensors have inherently high sensitivity, immunity to E.M interference, geometric flexibility and light weight.

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