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B.Tech sem I Engineering Physics U-I Chapter 1-Optical fiber
Optical fibers carry light along their length and work on the principle of total internal reflection. Light is kept in the core of the fiber due to the difference in refractive index between the core and cladding materials. Total internal reflection occurs when light traveling through the higher refractive index core strikes the core-cladding boundary at an angle greater than the critical angle, causing it to reflect back into the core rather than refracting out of the fiber. Optical fibers come in different types depending on their construction, propagation mode, refractive index profile, and applications in telecommunications and local area networks.
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B.Tech sem I Engineering Physics U-I Chapter 1-Optical fiber
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- 2. Basic principle TotalInternal Reflection in Fiber An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Light is kept in the "core" of the optical fiber by total internal reflection.
- 3. 3 What Makes TheLight Stay in Fiber Refraction The light waves spread out along its beam. Speed of light depend on the material used called refractive index. Speed of light in the material = speed of light in the free space/refractive index Lower refractive index higher speed
- 4. 4 The Light isRefracted This end travels further than the other hand Lower Refractive index Region Higher Refractive index Region
- 5. 5 Refraction When alight ray encounters a boundary separating two different media, part of the ray is reflected back into the first medium and the remainder is bent (or refracted) as it enters the second material. (Light entering an optical fiber bends in towards the center of the fiber – refraction) Refraction LED or LASER Source
- 6. 6 Reflection Light insidean optical fiber bounces off the cladding - reflection Reflection LED or LASER Source
- 8. 8 Critical Angle Iflight inside an optical fiber strikes the cladding too steeply, the light refracts into the cladding - determined by the critical angle. (There will come a time when, eventually, the angle of refraction reaches 90o and the light is refracted along the boundary between the two materials. The angle of incidence which results in this effect is called the critical angle). Critical Angle n1Sin X=n2Sin90o
- 9. 9 Angle of Incidence Also incident angle Measured from perpendicular Exercise: Mark two more incident angles Incident Angles
- 10. 10 Angle of Reflection Also reflection angle Measured from perpendicular Exercise: Mark the other reflection angle Reflection Angle
- 11. 11 Reflection Thus light isperfectly reflected at an interface between two materials of different refractive index if: The light is incident on the interface from the side of higher refractive index. The angle θ is greater than a specific value called the “critical angle”.
- 12. 12 Angle of Refraction Also refraction angle Measured from perpendicular Exercise: Mark the other refraction angle Refraction Angle
- 13. 13 Angle Summary Refraction Angle Three important angles The reflection angle always equals the incident angle Reflection Angle Incident Angles
- 14. 14 Refractive Index n= c/v c = velocity of light in a vacuum v = velocity of light in a specific medium light bends as it passes from one medium to another with a different index of refraction air, n is about 1 glass, n is about 1.4 Light bends in towards normal - lower n to higher n Light bends away from normal - higher n to lower n
- 15. 15 Snell’s Law Theamount light is bent by refraction is given by Snell’s Law: n1sinθ1 = n2sinθ2 Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection) Light can either bounce off the cladding (TIR) or refract into the cladding
- 16. 16 Snell’s Law Normal Incidence Angle(Φ1) Refraction Angle(Φ2) Lower Refractiveindex(n2) Higher Refractive index(n1)Ray of light
- 17. 17 Critical Angle Calculation The angle of incidence that produces an angle of refraction of 90° is the critical angle n1sin(qc) = n2sin(90°) n1sin(qc) = n2 qc = sin-1 (n2/n1) Light at incident angles greater than the critical angle will reflect back into the core Critical Angle, θc n1 = Refractive index of the core n2 = Refractive index of the cladding
- 18. OPTICAL FIBER CONSTRUCTION Core– thin glass center of the fiber where light travels. Cladding – outer optical material surrounding the core Buffer Coating – plastic coating that protect the fiber.
- 19. OPTICAL FIBER Thecore, and the lower-refractive-index cladding, are typically made of high-quality silica glass, though they can both be made of plastic as well.
- 20. 20 NA & ACCEPTANCEANGLE DERIVATION In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.” optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, θmax.
- 21. 21 When a lightray is incident from a medium of refractive index n to the core of index n1 , Snell's law at medium-core interface gives
- 22. Substituting forsin θr in Snell's law we get: By squaring both sides Thus, 22
- 23. from wherethe formula given above follows. NUMERICAL APERATURE IS ACCEPTANCE ANGLE θmax = 23
- 24. Definition:- Acceptance angle:- Acceptance angle is defined as the maximum angle of incidence at the interface of air medium and core medium for which the light ray enters into the core and travels along the interface of core and cladding. Acceptance Cone:- There is an imaginary cone of acceptance with an angle .The light that enters the fiber at angles within the acceptance cone are guided down the fiber core Numerical aperture:- Numerical aperture is defined as the light gathering capacity of an optical fiber and it is directly proportional to the acceptance angle. 24
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- 26. 26 Three commontype of fiber in terms of the material used: • Glass core with glass cladding –all glass or silica fiber • Glass core with plastic cladding –plastic cladded/coated silica (PCS) • Plastic core with plastic cladding – all plastic or polymer fiber
- 27. Plastic and SilicaFibers
- 28. BASED ON MODEOF PROPAGATION Two main categories of optical fiber used in fiber optic communications are multi-mode optical fiber single-mode optical fiber. 28
- 29. Single-mode fiber Carrieslight pulses along single path Multimode fiber Many pulses of light generated by LED travel at different angles 29
- 30. Based on theindex profile 30 The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded- index fiber
- 31. 31 Step Index Fibers A step-index fiber has a central core with a uniform refractive index. An outside cladding that also has a uniform refractive index surrounds the core; however, the refractive index of the cladding is less than that of the central core. The refractive index profile may be defined as n(r) = n1 r < a (core) n2 r ≥ a (cladding)
- 32. GRADED-INDEX In graded-indexfiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. 32
- 33.
- 34. 34 multimode step-indexfiber the reflective walls of the fiber move the light pulses to the receiver multimode graded-index fiber acts to refract the light toward the center of the fiber by variations in the density single mode fiber the light is guided down the center of an extremely narrow core
- 35. Figure 2.10 Twotypes of fiber: (Top) step index fiber; (Bottom) Graded index fiber
- 36. Attenuation Definition: aloss of signal strength in a lightwave, electrical or radio signal usually related to the distance the signal must travel. Attenuation is caused by: Absorption Scattering Radiative loss 36
- 37. Losses Losses inoptical fiber result from attenuation in the material itself and from scattering, which causes some light to strike the cladding at less than the critical angle Bending the optical fiber too sharply can also cause losses by causing some of the light to meet the cladding at less than the critical angle Losses vary greatly depending upon the type of fiber Plastic fiber may have losses of several hundred dB per kilometer Graded-index multimode glass fiber has a loss of about 2–4 dB per kilometer Single-mode fiber has a loss of 0.4 dB/km or less 37
- 38. Macrobending Loss: Thecurvature of the bend is much larger than fiber diameter. Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region. As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius. For any radius a bit smaller than this point, the losses suddenly becomes extremely large. Higher order modes radiate away faster than lower order modes. 38
- 39. Microbending Loss MicrobendingLoss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the microbended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber. 39
- 40. Dispersion The phenomenonin an optical fibre whereby light photons arrive at a distant point in different phase than they entered the fibre. Dispersion causes receive signal distortion that ultimately limits the bandwidth and usable length of the fiBer cable The two main causes of dispersion are: Material (Chromatic) dispersion Waveguide dispersion Intermodal delay (in multimode fibres) 40
- 41. Dispersion infiber optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others Single-mode fibers are relatively free from dispersion except for intramodal dispersion Graded-index fibers reduce dispersion by taking advantage of higher-order modes One form of intramodal dispersion is called material dispersion because it depends upon the material of the core Another form of dispersion is called waveguide dispersion Dispersion increases with the bandwidth of the light source 41
- 42. Advantages of OpticalFibre Thinner Less Expensive Higher Carrying Capacity Less Signal Degradation& Digital Signals Light Signals Non-Flammable Light Weight
- 43. Advantages of fiberoptics Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber Immunity to Noise - Immune to electromagnetic interference (EMI). Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline. High Security - Impossible to “tap into.”
- 44. Advantages of fiberoptics Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation) Reliability - More resilient than copper in extreme environmental conditions. Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is physically very flexible.
- 45. Fiber Optic Advantagesgreater capacity (bandwidth up to 2 Gbps, or more) smaller size and lighter weight lower attenuation immunity to environmental interference highly secure due to tap difficulty and lack of signal radiation
- 46. Disadvantages include thecost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult. Disadvantages of fiber optics
- 47. Areas of Application Telecommunications Local Area Networks Cable TV CCTV Optical Fiber Sensors
- 48. 48 Formula Summary Indexof Refraction Snell’s Law Critical Angle Acceptance Angle Numerical Aperture v c n = 2211 sinsin θθ nn = = − 1 21 sin n n cθ ( )2 2 2 1 1 sin nn −= − α 2 2 2 1sin nnNA −== α
- 49. STUDENTS YOU CANALSO REFER IT…… 49 http://hank.uoregon.edu/experiments/Dispersion-in- Optical-Fiber/Unit_1.6%20(2).pdf http://www1.ceit.es/asignaturas/comuopticas/pdf/chapter4. pdf http://course.ee.ust.hk/elec342/notes/Lecture %206_attenuation%20and%20dispersion.pdf 1 Engineering Physics by H Aruldhas, PHI India 2 Engineering Physics by B K Pandey , S. Chaturvedi, Cengage Learning 3Resnick, Halliday and Krane, Physics part I and II, 5th Edition John Wiely 4Engineering Physics by S.CHAND 5Engineering Physics by G VIJIYAKUMARI