FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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detectors or display devices. The sensor may be connected to optical fibers and cables by various types of connectors, splices, couplers, mixers, and cables, the connector and splice being used primarily to join fibers and cables, the couplers being used to connect one source to many fibers (divergence), the mixer being used to couple many fibers to one photodetector (convergence). The operation of fiberoptic sensors cannot be well understood without an understanding of the varioua actions and interactions that can take place by and among lightwaves. Many of these interactions are the basis for sensing physical parameters. The lightwave characteristics discussed in Chapter 4 include interferometrics, polarization, and intensity and phase modulation in relation to homodyne and heterodyne detection. Chapter 4 ends with lightwave control technique used in integrated optical circuits. Chapter 5 turns primary attention from general principles and techniques used in the operation of fiberoptic aensora to a description of the sensors themselves and their components. Intensity modulation sensors; phase modulation devices, such aa those used in hydrophores and magnetometera; rotation sensors, accelerometers, and geophones are discuased as examplea of fiberoptic sensor applications. The grouping of fiberoptic aensors into senaor arrays and the telemetering of their outputs to other locations are covered in Chapter 6. Design options; basic configuration; riaetime, power, and coat budgeta; and specific configurations of fiberoptic arrays and telemetry aystema are diacuased in depth, along with light sourcea, transmission lines and endterminals as they relate to fiberoptic senaors. Bibliographies pertinent to the subject are included at the end of each chapter. The appendix contains an authoritative glossary of terms and definitiona in the field of fiberoptic with emphasia on the terms used to describe the design, fabrication, and operation of fiberoptic aenaors. Particular attention in the gloasary is devoted to the terms used in this manual. Many topics and concepta related to fiberoptic senaors, their operating principles, and supporting theory are included in the glosaary ao aa not to overburden the reader with too many detaila while the main topics are being discussed. For example, concepta concerning diaperaion, Msxwell’a equations , modulation, polarization, reflection and transmission coefficient, and various tvpes of fiberoptic sensors and interferometers are described in the glosaary. 1-2
CHAPTER 2 <strong>FIBEROPTIC</strong> <strong>SENSOR</strong> COMPONENTS 2.1 OPTICAL FIBER PROPERTIES In this section the basic properties of optical fibers are discussed in some detail with emphasis placed on concepts that are important in optical fiber sensor technology. The structure of optical fibers is quite simple, as shown in Fig. 2.1. Basically they consist of layered cylinders of glass or plastic with small diameters. There is a central cylinder called the core, which is made up of one type of glass or plastic. Surrounding the core is a cylindrical shell called the cladding that is made of a slightly different type of glass or plastic. The difference between the core and cladding materials will be discussed later. Finally, this layered cylinder is usually surrounded by a Protective jacket. The light-guiding capability of the fiber is dependent upon the properties of the core and cladding while the mechanical strength of the fiber is maintained by the jacket that is usually made of plastic. CL’ ‘A’!” A INPUT Fig. 2.2 MODULATING SIGNAL LIGHT PULSES OPTICAL DET&CT- . A basic optical fiber link. OUTPUT Four major optical fiber design objectives will be discussed. The first major objective is the desirability of maximizing the amount of available light that is transferred (coupled) into the core of the fiber. It is only the light in the core that is propagated along the length of the fiber with relatively low optical power loss. In order to maximize the amount of light transferred (coupled) into the core, it is necessary to maximize the numerical aperture (NA) of the fiber. This is one of four important fiber parameters that strongly affect the behavior of the simple system shown in Fig. 2.2. After introducing the other three important parameters, each will be discussed in some detail. The second fiber design objective is the de- 1/ L sizability of minimizing the light lost from a beam as it travels through the core from the input end to the output end of the fiber. This light loss is described as the attenuation (power leas) rate, usually expressed in dB (decibel) per kilometer of fiber. CORE Fig. 2.1 The basic structure of an optical fiber. 2.1.1 Design Objectives Some of the design objectives considered in the development of a good optical fiber are illustrated by the simple system shown in Fig. 2.2. The system consists of a pulse-modulated optical source. The input signal at the left represents the intelligence (information) that is impressed on (modulates) the light beam that, after emerging from the source, is focused with a lens into one end of an optical fiber. The light travels through the fiber and emerges from the opposite end, where it is directed into an optical detector (photodetector), possibly focused again with a second lens. The third optical fiber design objective is the desirability of maximizing the information carrying capacity of the fiber. The input to the fiber may be a light beam of continuously-varying intensity or a group of well defined pulses of light as shown in Fig. 2.2. As the light pulses propagate through the fiber, their amplitude will decrease due to attenuation. In addition, due to a number of other effects to be discussed, the individual pulses also may broaden (spread). If they become too broad they will overlap or coincide with one another in both time and space. If this OCcurs, the intelligence (information) originally impressed on the light beam would be lost. Pulse broadening that occurs in the fiber is called dispersion. This parameter places a limit on a fiber’s information carrying capacity (signaling rate). The fourth design objective is the desirability of maximizing the strength of fibers when they are Initially drawn and maintaining this strength when the fibers are formed into cables or are used in sensors and other applications. Before considering these and other fiber design objectives in some detail, the basic theory of light propagation in optical fibers will be discussed, beginning with light-ray propagation in layered media. 2-1
Page 1 and 2: FIBEROPTIC SENSOR TECHNOLOGY HANDBO
Page 3 and 4: FOREWORD THE FIBEROPTIC SENSOR TECH
Page 5 and 6: 4.2 Phase 4.2.1 4.2.2 4.2.3 4.2.4 4
Page 7: CHAPTER 1 INTRODUCTION 1.1 BACKGROU
Page 11 and 12: fleeted each time it is incident on
Page 13 and 14: 6=0 REFERENCE PLANE that propagate
Page 15 and 16: in the right center of Fig. 2.13. T
Page 17 and 18: The vertical height of the various
Page 19 and 20: 60 40 20 10.C ~8 x z 6 n u 4 % c 62
Page 21 and 22: : 1.5 z o GeOz 0 ~ =1.49 u w > ~1.4
Page 23 and 24: tension it collapses to eliminate t
Page 25 and 26: MAXIMUM TENSILE STRENGTH (GN/m2 = l
Page 27 and 28: mately a 0.5 electron-volt increase
Page 29 and 30: I in one portion of the recombinati
Page 31 and 32: crated electron-hole paira that are
Page 33 and 34: sink fiber. Likewise angular misali
Page 35 and 36: fused together. The ends are bent a
Page 37 and 38: A laboratory beam splitter set-up u
Page 39 and 40: aged multichannel unit. High-streng
Page 41 and 42: CHAPTER 4 LIGHTWAVES IN FIBEROPTIC
Page 43 and 44: tor addition indefinitely, as indic
Page 45 and 46: 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.
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nickel H. = 3.6 oersteds and for a
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optical source of constant intensit
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in a straight section of the fiber
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A simplified design of the cladding
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Referring once more to Fig. 5.35, i
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The path length AL traveled by ligh
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5.4.9 Fiberoptic Rotation-Rate Sens
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a modulation scheme, the output of
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Fig. 5.60 lists several sources of
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CHAPTER 6 FIBEROPTIC SENSOR ARRAYS
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method of sensor energization will
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1 The preceding discussion applied
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SIMPLEX FIBEROPTIC TRANSMITTER FIBE
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MAXIMUM ALLOWABLE RISETIME = 0.7/RN
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PROCESSING STATION TRANSMISSION wAV
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fiberoptic cable (optical cable), a
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angstrom. A unit of length equal to
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core. The central primary light-con
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diffraction. 1. The procesa by whic
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fiberoptic. Pertaining to optical f
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methods of coupling power outside t
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M Mach-Zehnder interferometer. An i
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N nanometer. One thousandth of a mi
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velocity is also the velocity at wh
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where nl and n2 are the reciprocals
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slab dielectric optical waveguide.
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v vacancy defect. In the somewhat o
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