FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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ground or lower state and at the expense of the resonant energy source (i.e. , the pump). This is not an equilibrium condition. It is forced and must be maintained, i.e, stimulated. The generation of population inversion is caused by pumping. When electrons revert to their lower levels, photons are emitted obtaining laser action. In a stimulated material, such as a semiconductor, the upper energy level of two possible electronic energy levels in a given atom, distribution of atoms, molecule, or distribution of molecules, has a higher probability (usually only slightly higher) of being occupied by an electron. When population inversion occurs, the probability of downward energy transition giving rise to radiation, is greater than the probability of upward energy transitions, giving rise to photon absorption, resulting in a net radiation level, thus obtaining stimulated emission, i.e., laser action. =“ See transmitted power. power budget. The allocation of available power in a system to the various functions that need to be performed. For example, in a fiberoptic data link, the distribution of available optical power among all the elements of the link, such as couplers, splices, fibers, and pigtails; or in a satellite communication system, the distribution of the available power in a satellite for maintaining orientation, maintaining orbit control, and for the reception and retransmission of signals. See optical power budget. power density. See optical power density. power margin. An extra amount of power that may be specified by a designer because of uncertainties in the empirical design method, loss, characteristics, material variability, and variation in equipment performance parameters and characteristics. For example, in an optical power budget, the optical power, PM, that remains after subtracting the sum of all the optical losses, PL, and the power input required by a photodetector, PR, from the available optical power output by a source, P5, i.e., profile. pM = ps - [pL+pR]. See radial refractive index profile. profile mlsmstch loss. mismatch loss. profile parameter. parameter. *“ See refractive-index-profile See refractive index profile See electromagnetic pulse (EMP). punch-through voltage. In a semiconductor junction, a voltage equal to that required to just overcome the potential barrier of the junction to permit the flow of electrons that are in the conduction band. R radial refractive-index profile. In an optical fiber with a circular cross section, the refractive index described as a function of the radial distance from the center. For example, the function: n=n~ f(r) where n is the refractive index at a radial distance r from the center, nl is the refractive index at the center, and f(r) is the function of r that expresses the index at the distance r from the center , usually independent of radial direction. Also see refractive index profile parameter. radiation. See light amplification by stimulated emission of radiation (laser). ~. See light ray; meridional ray; reflected ray; skew ray. Rayleigh scattering. The scattering of lightwaves propagating in material media due to the atomic or molecular structure of the material and variations in the structure as a function of distance. The scattering losses vary as the reciprocal of the fourth power of the wavelength. The distances between scattering centers must be small compared to the wavelength. Rayleigh scattering sets a theoretical lower limit to the attenuation of a propagating lightwave as a function of wavelength, ranging from 10 dB/km at 0.50 micron to 1 dB/km at 0.95 micron. Material scattering is caused primarily by Rayleigh scattering. Rayleigh scattering is also due to the variation in molecular density of intrinsic material; for example, the familiar light green color of pop bottles is due to Rayleigh scattering from distributed iron atoms. This represents the limiting absorption. Also since it decreases with A, this has resulted in using higher wavelengths to achieve low-loss fiber. Minimum attenuation is due to decreases in absorption due to Rayleigh scattering and increases due to OH- vibration. Also see scattering. ray trapping. Preventing a lightwave from leaking from a waveguide. For example, total internal reflection causes ray trapping. recombination. See electron-hole recombination. reflected ray. A ray of electromagnetic radiation, usually light leaving a reflecting surface. The ray Indicates the path after reflection. reflection. When electromagnetic waves, such as light rays, strike a smooth, polished surface, their return or bending back into the medium from whence they came. Specular or regular reflection from a polished surface, such as a mirror, will return a major portion of the light in a definite direction lying in the plane of the incident ray and the normal. After specular reflection, light can be made to form a sharp image of the original source. Diffuse reflection occurs when the surface is rough and the reflected light is scattered from each point in the surface. These diffuse rays cannot be made to form an image of the original source, but only of the diffusely reflecting surface itself. Also see Snell’s law; internal reflection; total internal reflection. reflection coefficient. 1. The ratio of the reflected field strength to the incident field strength when an electromagnetic wave is incident upon an interface surface between dielectric media of different refractive indices. If, at oblique incidence, the magnetic field component of the incident wave is parallel to the interface, the reflection coefficient is given by: A-17 R = (nlcosA - n2cosB)/(nlcosA + n2cosB)
where nl and n2 are the reciprocals of the refractive indices of the incident and transmitted mediums, respectively, and A and B are the angles of incidence and refraction (with respect to normal), respectively. If, at oblique incidence, the electric field component of the incident wave is parallel to the interface, the reflection coefficient is given by: R = (n2cosA - nlcosB)/(n2cosA + n~cosB) These equations are known as the Fresnel equations for such cases. For large smooth surfaces, the reflection coefficient may be near unity, such as for highly polished mirrors. At near grazing incidence angles, that is nearly 90° from the normal to the surface, even rough surfaces may reflect relatively well, i.e., total reflection occurs. 2. At any specified point in a transmission line between a power source and a power sink (absorber), the vector ratio of the electric field associated with the reflected wave to that associated with the incident wave. The reflection coefficient, RC, is given by RC = (Z2-Zl)/(Z2+Zl) = (SWR-1)/(SWR+l), where Z1 iS the impedance looking toward the source, Z2 is the impedance looking toward the load, and SWR ia the standing wave ratio. Also see transmission coefficient. reflection sensor. sensor. See near total internal reflection reflective star-coupler. An optical-fiber coupling device that enables signals in one or more fibers to be transmitted to one or more other fibers by entering the signals into one side of an optical cylinder, fiber, or other piece of material, with a reflecting back-surface in order to reflect the diffused signals back to the output ports on the same aide of the material, for conduction away in one or more fibers. refraction. The bending of oblique (non-normal) incident electromagnetic waves or raya as they cross the interface between a transmission medium of one refractive index into a medium of a different refractive index. The velocityof propagationof theelectromagnetic waves change when passing from a medium to one with a different refractive index. Also see Snell’s law refraction angle. When an electromagnetic wave strikes an interface surface between media with different refractive indices and is wholly or partially transmitted into the new medium, the acute angle between the normal to the surface at the point of incidence and the refracted ray. refractive index. 1. The ratio of the velocity of light in a vacuum to the velocity of a given frequency of light in the transmission medium whose refractive index is desired; e.g., n = 2.6 for certain kinds of glass. 2. The ratio of the sines of the incidence angle and the refraction angle when light passes from one medium to another. The index between two media ia the relative index, while the index when the first medium is a vacuum is the absolute index of the second medium. The refractive index expressed in tables is the abaolute index, i.e., vacuum-to-substance at a certain temperature, with light of a certain frequency. Examples: vacuum, 1.000; air, 1.000292; water, 1.33; ordinary crown glass, 1.516. Since the index of air is very cloae to that of vacuum, the two are often used interchangeably. The refractive index of a substance is given as: where p is the magnetic permeability of the substance, E ia the electric permittivity, P. is the magnetic permeability of a vacuum, and E. is the electric permittivity of a vacuum, although nearly the same relative to air. refractive index profile. profile. See radial refractive index refractive-index-profile mismatch loss. A 10SS Of Sig - nal power that occurs when two optical fibers are butt-coupled and their refractive index profiles are not the same. refractive index profile parameters. The exponent i in the relation that expresaes the refractive index as a function of the radial diatance from the central axis of an circular optical fiber, i.e., in the expression: nfnl = [1-(r/a)i]l/2 where n is the refractive index at the radial distance r and a is the radiua of the fiber core. For a step-index fiber i= co for ra; for a parabolic graded-index fiber, i=2. A value of i= 2.25 will minimize or eliminate intermodal dispersion and maximum the length-bandwidth product. rejection ratio. See common-mode rejection ratio (CMRR). ~. A bounded region in a material medium, such as a free rectangular space in a laser crystal, a length of metal hollow tubing closed on both ends, a short length of optical fiber with mirrors on both ends, or the reflection could be due to a 3-dB coupler or a splice associated with the insertion loss, or a region of such geometrical dimension, such as two parallel walls that are a multiple or submultiple of wavelengths apart, that a standing wave (electromagnetic, acoustic, or elastic) can be austained and raised to high intensity by application of stimulation (applied energy of appropriate frequency) from outside or inside the cavity. Resonant cavities are used in some lasers in which they form part of the laser head. ribbon. See optical fiber ribbon. risetime. In pulse circuits, the time required for a pulse to reach a specific magnitude from a given level. For example, the time required for a voltage or a light pulse to change from 0.1 to 0.9 of its maximum value. risetime budget. In fiberoptic transmission systems, an accounting of all the riaetimea (time required for a lightpulse to reach a apecified level) in a aequence of optical and electrical components. Because the distribution of photon energies in lightwaves are Gausaian, the risetimes of a aequence of components are combined as the aquare root of the sum of the squarea of the individual ristimes. A-18
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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
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as a number of wavelengths. An expr
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100 meters of fiber, length changes
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the aingle-sideband phaae noiae int
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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 and 98: fiberoptic. Pertaining to optical f
Page 99 and 100: methods of coupling power outside t
Page 101 and 102: M Mach-Zehnder interferometer. An i
Page 103 and 104: N nanometer. One thousandth of a mi
Page 105: velocity is also the velocity at wh
Page 109 and 110: slab dielectric optical waveguide.
Page 111 and 112: v vacancy defect. In the somewhat o
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FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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