Optical behaviorReading assignmentSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Refractive index nSlide 10Slide 11Slide 12Slide 13Slide 14DispersionSlide 16Slide 17Reflectance (reflectivity) RSlide 19Slide 20Slide 21When 1 > c, there is no refracted ray and all the incident ray is reflected.Slide 23Slide 24Applications of optical fiberOptical fiberSlide 27Slide 28Slide 29Core materialAttenuation of lightSlide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44An optical fiber may have different diameters of the core.Slide 46Slide 47Slide 483 types of optical fiberSlide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60Slide 613 types of optical fiber sensorTransmission-gap sensorEvanescent-wave sensorInternal-sensing sensorSlide 66Slide 67Slide 68Slide 69PhotoresponseLuminescenceSlide 72Slide 73Slide 74Slide 75Slide 76Slide 77Slide 78Slide 79Slide 80Slide 81Slide 82Slide 83Slide 84Slide 85ElectroluminescenceLight-emitting diode (LED)Slide 88Slide 89Slide 90Slide 91Slide 92Slide 93Slide 94Slide 95Photonic bandgap materialsSolar cellSlide 98LaserCharacteristics of a laser beamNearly monochromaticCoherenceCondition for coherenceSlide 104Slide 105Slide 106Slide 107Slide 108Slide 109Solid-state lasersSlide 111Slide 112Slide 113Slide 114Slide 115Thermal emissionSlide 117Slide 118Slide 119Optical behaviorTopic 10Reading assignment•Askeland and Phule, The Science and Engineering of Materials, 4th Ed. ,Ch. 20.•Shackelford, Materials Science for Engineers, 6th Ed., Ch. 16.•Chung, Composite Materials, Ch. 8.•Light is energy, or radiation, in the form of waves or particles called photons that can be emitted from a material. •The important characteristics of the photons—their energy E, wavelength λ, and frequency ν—are related by the equationThe Electromagnetic SpectrumFigure 20.1 The electromagnetic spectrum of radiation; the bandgaps and cutoff frequencies for some optical materials are also shown. (Source: From Optoelectronics: An Introduction to Materials and Devices, by J. Singh. Copyright © 1996 The McGraw-Hill Companies. Reprinted by permission of The McGraw-Hill Companies.)Refraction of light as it passes from vacuum (or air) into a transparent material.High indexLow indexRefractive index n n = Speed of light in vacuum (essentially the same as that in air), divided by the speed of light in a transparent material.n1 sin 1 = n2 sin 2 Snell’s LawIf n1 > n2, then 2 > 1 Since a larger refractive index means lower speed, n1 > n2 means v2 > v1. Thus, the medium with the larger speed is associated with a larger angle between the ray in it and the normal.DispersionFrequency dependence of the index of refractionReflection of light at the surface of an opaque metal occurs without refraction.Reflectance (reflectivity) R•R = Fraction of light reflected•Fresnel’s formula R = [(n-1)/(n+1)]2 Strictly valid for θi = 0 (normal incidence) High n results in high R (i.e., R approaches 1)Reflection of light at the surface of a transparent material occurs along with refraction.•When 2 = 90, the refracted ray is along the interface .nnsin 121 The value of 1 corresponding to 2 = 90 is called c (the critical angle).When 1 > c, there is no refracted ray and all the incident ray is reflected. Total internal reflection when θ1 exceeds θcCable with 144 glass fibers (right)Copper-wire cable (left)Copper-wire cable (left)Applications of optical fiber•Communication•Digital processing•Sensing (extrinsic smartness)Optical fiber An optical fiber guides the light in it so that the light stays inside even when the fiber is bent.Optical fiber•This is because the fiber has a cladding of refractive index n2 and a core of refractive index n1, such that n1 > n2 and total internal reflection takes place when 1 > c. •This means that the incident ray should have an angle of incidence more than c in order to have the light not leak out of the core. Hence, incoming rays that are at too large an angle (exceeding NA) from the axis of the fiber leak.The coaxial design of commercial optical fibersCore diameter: 5-100 micronsCore materialHigh-purity silica glassAttenuation of light Power loss through a 16-kilometer (19-mile) thickness of optical fiber glass is equivalent to the power loss through a 25-millimeter (1-inch) thickness of ordinary window glass.Light scattering is the result of local refraction at interfaces of second-phase particles or pores. The case for scattering by a pore is illustrated here.Specular reflection occurs relative to the “average” surface, and diffuse reflection occurs relative to locally nonparallel surface elements.©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.Figure 20.3 (a) When a ray of light enters from material 1 into material 2, if the refractive index of material 1 (n1) is greater than that of material 2 (n2), then the ray bends away from the normal and toward the boundary surface. [1, 9] (b) Diagram a light beam in glass fiber for Example 20.1.•The acceptance angle of the fiber is defined as twice NA. Rays within the acceptance angle do not leak.•The numerical aperture (NA) of the fiber is defined as n1 sin NA. Since NA = 90 - c, n1 sin NA = n1 sin (90 - c) = n1 cos c.nnsin 12c212221212cnnnnn1cos nnn 12221nnnn 122211Numerical aperture nn 2221=•An optical fiber (or optical wave guide) has a low-index glass cladding and a normal-index glass core. •The refractive index may decrease sharply or gradually from core to cladding, depending on how the fiber is made. • A sharp decrease in index is obtained in a composite glass fiber; a gradual decrease is obtained in a glass fiber that is doped at the surface to lower the index. •A gradual decrease is akin to having a diffuse interface between core and cladding. As a consequence, a ray does not change direction sharply as it is reflected by the interface•A sharp decrease in index corresponds to a sharp interface and a ray changes direction sharply upon reflection by the interface.©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.Step-index fiberGraded-index fiberPath of rays entering