FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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group velocity. The velocity of propagation of the envelope produced when an electromagnetic wave is modulated by, i.e., mixed with, another wave of a different frequency. The group velocity is the velocity of information propagation and, loosely, of energy propagation. It is the velocity of transmission of energy associated with a progressing wave consisting of a group of sinusoidal components; i.e., the velocity of a certain feature of the wave envelope, e.g., the crest. The group velocity differs from the phase velocity. Only the latter varies with frequency. In general, the group velocity, representated as: is less than the phase velocity, represented as; ‘P “ IJJ/i3 where B is the angular velocity, ( u= 2n’f) and u is the propagation constant. guide. See waveguide. H heterodyne detection. In fiberoptic, detection based on the use or mixing of two or more frequencies. For example, if a lightwave is fed to a Bragg cell, it produces an output lightwave frequency that is the sum of the input lightwave frequency and the baseband frequency; an interferometer can be used to recover the baseband frequency. heterodyning. The mixing of an electromagnetic wave of one frequency with a wave of another frequency to produce one or more additional frequencies. Usually the sum and difference frequencies will be produced when waves of two different frequencies are combined in a nonlinear device, such as a nonlinear amplifier. heterojunction. In a laser diode, a boundary surface at which a sudden transition occurs in material composition across the boundary. For example, in a semiconductor, a change in the refractive index as well as a change from a positively-doped (p) region to a negatively-doped (n) region; or a positivelydoped region with a rapid change in doping level, i.e., a high concentration gradient of dopant versus distance. In most heterojunctions, change in geo - metric cross-section occurs across which a voltage or voltage barrier exists. Heterojunctions provide a controlled level and direction of radiation confinement. There usually is a step in the refractive index level at each heterojunction. Contrast with homojunction. See single heterojunction. hole. In physical electronics and solid-state devices, a semiconducting material containing a dopant that has one less electron for each atom than that required to complete the intrinsic bonding structure, a site at which an electron is missing to complete the bonding structure. Initially, the hole is created by the impurity atoms, but if an electron from a neighboring atom moves in to “fill” the hole, the neighboring atom will have a hole; thus, the hole can be considered to have migrated. Also see acceptor; donor; electron. homodyne detection. In fiberoptic, detection based on the use of only one frequency. For example, a detector that makes use of a varying-length fiber to achieve modulation of the phase of the lightwave at the output end of the fiber; the input baseband signal modulates the length of the fiber and thus there is no change in frequency of the lightwave. homogeneous medium. In optical systems, a transmission medium whose light-transmission parameters, such as the parameters of the constitutive relations, are spatially constant and not a function of space coordinates, although they may vary as a function of time, temperature, pressure, humidity, or other parameter uniformly throughout the medium. homojunction. In a laser diode, a single junction; i.e., a single region of shift in doping from positive to negative majority carrier regions, or vice versa, and a change in refractive index, all at one boundary. Hence one energy-level shift, one barrier, and one refractive-index shift constitute a aingle homojunction. Constrast with heterojunction. I incident ray. A ray of light that falls upon, or strikes, the surface of any object. The ray is said to be incident at the surface. incoherent light. Light of which not all parameters are predictable and correlated at any point in time or space, such as scattered or diffused light. Contrast with coherent light. index. See refractive index. index profile. See radial refractive index profile. See refractive-index-pro- index-profile mismatch loss. file mismatch loss. index profile parameter. parameter. See refractive index profile index-matching material. Alight-conducting material used in intimate contact with optical components such as optical fibers and lenses, to reduce optical power losses by using materials with refractive indices at interfaces that will reduce reflection, increase transmission, avoid scattering, and reduce dispersion. insertion loss. 1. In a communication system, the decrease in signal level that results from the insertion of a series component in a transmission circuit. It is expressed as a percent, in decibels, or as a per unit coefficient or fraction. The loss may be positive or negative, that is, greater or less than unity when expressed as a ratio (per unit fraction or coefficient). If the insertion loss is negative it is considered as a gain. When it is expressed as a fraction, ratio, or per unit, it is the decrease in signal level caused by the insertion divided by the signal level before insertion. 2. In an optical fiber, the optical power loss due to all causes, usually expressed as decibels/kilometer. Causes of loss may be absorption, scattering, diffusion, leaky modes, dispersion, microbending, or other causes or A-9
methods of coupling power outside the fiber. For example, in an optical component such as a 3-dB coupler, the insertion loss is considered as that in excess of the 3-dB associated with splitting the light between two fibers. 3. In lightwave transmission systems, the power lost at the entrance to a wavegufde due to any and all causes, such as Fresnel reflection, packing fraction, limited numerical aperture, axial misalignment, lateral displacement, initial scattering, or diffusion. insulator. A substance with a molecular structure in which all electrons remain in the valence band, rather than in the conduction band, even under the influence of high electric field gradients, and therefore a material that is used to prevent the flow of electric current when electric fields exist. tiso see dielectric. Contrast with conductor. integrated optical circuit (IOC). A circuit, or group of interconnected circuits, consisting of miniature solid state optical components. Examples of such components include light-emitting diodes, optical filters, photodetectors (active and passive), and thin-film optical waveguides on semiconductor or dielectric substrates. Components onan IOC chip might include semiconductor injection lasers, modulators, filters, lightguides, switches, couplers, logic gates, pulse shapers, differential amplifiers, and optical memories. Synonymous with optical integrated circuit. integrated optics. The design, development, and operation of circuits that apply the technology of integrated electronic circuits produced by planar masking, etching, evaporation, and crystal film growth techniques to microoptical circuits on a single planar dielectric substrate. Thus, a combination of electronic circuitry and optical waveguides are produced for performing various communication, switching, and logic functions, including amplification, gating, modulating, light generation, photodetecting, filtering, multiplexing, signal processing, coupling, and storing. intensity. See luminous intensity. intensity sensor. In fiberoptic, a fiberoptic sensor in which the optical intensity of a light ray (beam) is varied in accordance with a baseband signal by varying the light propagation properties of an optical fiber. For example, a microbend sensor. interference. See electromagnetic interference. interferometer. An instrument in which the interference effects of lightwaves are used for purposes of measurement, such as the measurement of the accuracy of optical surfaces by means of Newton’s rings, the measurement of optical paths, linear and angular displacements, phase changes due to pressure, rotation, and temperature effects on the sensing arm as compared to the reference arm. See Fabry-Perot interferometer; Mach-Zehnder interferometer; Michelson interferometer; Sagnac interferometer; _n- Green interferometer. interferometric sensor. In fiberoptic, a fiberoptic sensor that employs the principles of interferornetry to performa sensing function. For example, aFabry- Perot interferometer used as a fiberoptic sensor. Also see interferometer. interferometry. The scientific discipline devoted to the study and useful application of interference among electromagnetic waves. intermodal disperson. Dispersion (pulse broadening) that results from propagation time differences among the various modes in an electromagnetic pulse. Intermodal dispersion can be reduced by appropriate refractive index profile shaping. internal reflection. In an optical element in which an electromagnetic wave is propagating, a reflection at an outside surface from the inside such that a wave that is incident upon the surface is reflected wholly or partially back into the element itself. Optical fibers depend on internal reflection for the successful transmission of lightwaves in order that the waves do not leave the fiber; namely, the wave energy is confinded to or bound to the fiber. Also see total internal reflection. internal reflection sensor. See near total internal reflection sensor. interstitial defect. In the somewhat ordered array of atoms and molecules in optical fiber material, a site at which an extra atom or molecule is inserted in the space between the normal array. The defect can serve as a scattering center, causing diffusion, heating, absorption, and resultant attenuation. Also see vacancy defect. intrinsic absorption. In lightwave transmission media, the absorption of light energy from a traveling or standing wave by the medium itself, causing attenuation as a function of distance, material properties, mode, frequency, and other factors. Intrinsic absorption is primarily due to charge transfer bands in the ultraviolet region and vibration or multiphonon bands in the near infrared, particularly if they extend into the region of wavelengths used in optical fiber, namely, 0.7 to 1.2 microns. intramodal dispersion. The dispersion (pulse broadening) that occurs within one of the modea in an electromagnetic pulse. Intramodal dispersion in an optical fiber is a function of the spectral bandwidth of the light source and the material dispersion caused by the fiber. It is usually the only type of dispersion preaent in a monomode fiber. intrinsic fiber loss. Optical power loss in an optical fiber or optical fiber splice, connector, or coupling, caused in the manufacturing process, such as refractive index profile mismatch, diameter differences, scattering, absorption, and other causes not subject to the control of the user. intrinsic region. In a semiconductor junction, a region that lies between a positively-doped region and a negatively-doped region and that does not contain any dopant. Synonymous with i-region. inversion. See population inversion. Ioc. See integrated optical circuit. i-region. Synonym for intrinsic region. A-10
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FIBEROPTIC SENSOR TECHNOLOGY HANDBO
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FOREWORD THE FIBEROPTIC SENSOR TECH
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4.2 Phase 4.2.1 4.2.2 4.2.3 4.2.4 4
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CHAPTER 1 INTRODUCTION 1.1 BACKGROU
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CHAPTER 2 FIBEROPTIC SENSOR COMPONE
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fleeted each time it is incident on
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6=0 REFERENCE PLANE that propagate
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in the right center of Fig. 2.13. T
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The vertical height of the various
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60 40 20 10.C ~8 x z 6 n u 4 % c 62
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: 1.5 z o GeOz 0 ~ =1.49 u w > ~1.4
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tension it collapses to eliminate t
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MAXIMUM TENSILE STRENGTH (GN/m2 = l
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mately a 0.5 electron-volt increase
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I in one portion of the recombinati
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crated electron-hole paira that are
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sink fiber. Likewise angular misali
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fused together. The ends are bent a
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A laboratory beam splitter set-up u
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aged multichannel unit. High-streng
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CHAPTER 4 LIGHTWAVES IN FIBEROPTIC
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tor addition indefinitely, as indic
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modes. Since at point (a) the two e
Page 47 and 48: as a number of wavelengths. An expr
Page 49 and 50: 100 meters of fiber, length changes
Page 51 and 52: the aingle-sideband phaae noiae int
Page 53 and 54: sets up periodic spatial fluctuatio
Page 55 and 56: ient configuration in the lower rig
Page 57 and 58: sors or a pressure gradient sensor.
Page 59 and 60: nickel H. = 3.6 oersteds and for a
Page 61 and 62: optical source of constant intensit
Page 63 and 64: in a straight section of the fiber
Page 65 and 66: A simplified design of the cladding
Page 67 and 68: Referring once more to Fig. 5.35, i
Page 69 and 70: The path length AL traveled by ligh
Page 71 and 72: 5.4.9 Fiberoptic Rotation-Rate Sens
Page 73 and 74: a modulation scheme, the output of
Page 75 and 76: Fig. 5.60 lists several sources of
Page 77 and 78: CHAPTER 6 FIBEROPTIC SENSOR ARRAYS
Page 79 and 80: method of sensor energization will
Page 81 and 82: 1 The preceding discussion applied
Page 83 and 84: SIMPLEX FIBEROPTIC TRANSMITTER FIBE
Page 85 and 86: MAXIMUM ALLOWABLE RISETIME = 0.7/RN
Page 87 and 88: PROCESSING STATION TRANSMISSION wAV
Page 89 and 90: fiberoptic cable (optical cable), a
Page 91 and 92: angstrom. A unit of length equal to
Page 93 and 94: core. The central primary light-con
Page 95 and 96: diffraction. 1. The procesa by whic
Page 97: fiberoptic. Pertaining to optical f
Page 101 and 102: M Mach-Zehnder interferometer. An i
Page 103 and 104: N nanometer. One thousandth of a mi
Page 105 and 106: velocity is also the velocity at wh
Page 107 and 108: where nl and n2 are the reciprocals
Page 109 and 110: slab dielectric optical waveguide.
Page 111 and 112: v vacancy defect. In the somewhat o
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