FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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2.2.3 Fiber Strength The operational and shelf-life of fiberoptic sensors will depend to a large extent on the mechanical strength of the glass fibers used in them. In a certain way, glass fiber is much atronger than steel. Short, pristine silica fibers, immediately after drawing, have elastic limits, and ultimate and breaking tensile strengths, greatly exceeding that of steel wires. Stress-strain curves for priatine silica fiber and steel wire are shown in Fig. 2.35. The elaatic 1 I I 1 ! , ( ! , , , 99 - *# .: 80 - 60 - ./” ./””” ~ 40 - / u m 3 ~ 20 GAGE LENGTH= 60cm if 8 - 4 10’0 - 109 “E ~ $ 108 w u G 107 106 WIRE FIBER 10–4 10–3 10–2 10–’ 10° STRAIN Fig. 2.35 Stresa versus strain curves for steel wire and pristine silica (Si02) fiber under ideal conditions. limit of steel typically uaed in wire is about 0.2 x 10-9 Newtons/m2 at a atress of about 0.1 percent. Steel wires tend to break at strains of the order 0.5 percent and a stress of about 1.5x109 N/m2. On the other hand, unscratched fibers remain elastic to strains in excess of 10 percent, corresponding to stresses of about 5 x 109 Newtona per square meter. However, unlike steel, which may be made malleable and capable of flow-healing small surface cracks, glass is brittle. Thus, very fine cracks in glass fibers tend to become stress concentration centers that propagate transversely across the fiber. They lead to exceasive strain and ultimately to complete rupture. A length of fiber is only as strong as its weakest section. Under constant tension a length of fiber will tend to break at its weakest point. The break will most likely occur where there is a scratch on the outer surface of the fiber. A long length of fiber ia more likely to have a weakest point that is weaker than the weakest point of a short length of fiber. Thus, fiber strength determination is a procesa of collecting statistics of failure. This is illustrated by the reaults of a seriea of tests performed on a group of test samples, each 60 cm long, cut from sections distributed uniformly along a 1 km length of fiber. Each sample was streased to rupture in a tension test machine. The percentages of the total number of specimens that failed below a given stress level are shown as a function of the breaking stress in Fig. 2.36. Fig. 1 , I I , [ 0.5 0.8 10 2.0 3.0 4.0 TENSILE STRENGTH (GN/m2) 2.36 The percentage of optical fiber specimena that failed as a function of breaking tenaile strength. From the graph it may be seen that the first apecimen broke at approximately 0.5 x 109 N/m2, approximate 10 percent of the specimens broke at 0.8 x 109 N/m J or leas another 10% broke at atresses between 0.8 and 1.0 x 10 4N/m2, and so forth. A few of the s eclmens, however, withstood stresses of 4.0 x 109 N/m % before breaking. From a practical, application-oriented viewpoint, it is the weakest point in a length of fiber that determines ita overall strength. Thus, in specifying fibers for a given application, the maximum streas or strain to be encountered ahould be determined and the entire length of fiber to be employed should be pre-tested at some acceptable safety margin above this level. Such testing is usually done by reeling the fiber from one spool to another at a fixed rate, while maintaining the interreel section under the fixed specified stress (tension). There is evidence that preform preparation and treatment, as well aa the manner in which fibers are handled after they are drawn, contribute substantially to their overall atrength. Data on the breaking or tensile strength of a particular type of fiber drawn from identically produced preforms is shown in the bargraph in Fig. 2.37. In the upper portion of Fig. 2.37, the number of specimens versus breaking strength ia plotted for 40 specimens taken from a length of fiber drawn from an ordinarily prepared preform that was heated as usual in an RF induction furnace. As indicated, the breaking strengths ranged from approximately 0.50 x 109 N/m2 to 5.5 N/m2 with a maximumof 12 specimena that broke at 3.0 x 10 4N/m2. Results are presented in the center aection of Fig. 2.37 on 46 specimens from a length of fiber drawn, using the RF induction furnace, from a preform that had been fire-polished prior to drawing in an effort to eliminate any fine cracks (microcracks) and other imperfections that might have existed in its outer surface. In this caae, the first specimen to break withstood stresses up to 1.8 x 109 N/m2 and 36 of the specimens broke at a stress exceeding 3.8 x 102 N/m2. In the third case, as ahown in the lower portion of Fig. 2.37, 42 specimens were tested from a fiber that waa drawn using an infrared laser to heat the preform, that also had been fire polished prior to drawing. All of the specimens withstood tensile atresses up to 4.0 x 109 N/m2 before breaking. However, instead of being distributed over a very wide range of breaking tensile strengths, the breaking points were all in the range from 4.0 to 5.25 x 109 N/m2. 2-16
MAXIMUM TENSILE STRENGTH (GN/m2 = lo g N/m2) INSULATOR CONDUCTOR NO FIRE POLISH N = 40 FURNACE FIRE POLISH ~ N = 46 ; : 10 5 ; ~z”NDucT’oN’’NE~ K Lu BAND-GAP EG — 61~ VALENCE BAND — — b m LASER FIRE POLISH : N = 42 ~ z :L_.—dJu o 200 400 600 800 1,000 P-TYPE SEMICONDUCTOR N-TYPE SEMICONDUCTOR I 1 I 1 I 1 DONOR LEVEL I a. a. * a. I ACCEPTOR LEVEL MAXIMUM TENSILE STRESS (KPSI) Fig. 2.37 The variation of maximum tension stress of a number of optical fibers for unpolished, furnace polished, and laser polished fibers showing the number of fibers that failed at the various stress levels in total tested for each polishing condition. These results clearly indicate how important it ia to carefully prepare the preforms and to accurately control the drawing process in order to maintain the strength of the final drawn fibers and thus get as cloae as possible to the ideal strength of silica glass. 2.3 SOLID STATE <strong>FIBEROPTIC</strong> LIGHT SOURCES Solid state optical sources and detectors utilized in compact fiberoptic sensors will be discussed in this section. This information will serve as a background for understanding later discusaiona of sensor noise and packaging. In order to understand the trade-offs required, a knowledge of light production mechanisms and fabrication processes is helpful. Finally, such information is important for estimating what is likely to be available in the future. 2.3.1 Energy Levels In Semiconductors Electrons in free atoms are normally tightly bound in discrete energy levels. When the atoms are located in a crystalline structure these discrete energy levels are replaced by energy bands. Some of the electrons remain tightly bound to the atom while other, more energetic electrons, have energies corresponding to the valence or conduction bands. Those in the valence band are atill localized at individual atoms but have the highest energy of such bound electrons, while electrons in the conduction band are free to move throughout the crystal. Materials can be divided into a number of classes depending on the energy gap (separation between the top energy level of the valence and the bottom energy level of the conduction band) and upon the number of electrons, if any, in the conduction band and lack of electrons in the valence band as shown in Fig. 2.38. Electrons cannot possess energies that lie in the gap. In an insulator the valence and conduction energy bands are separated by a wide energy gap. If the gaps in Fig. 2.38 were drawn to scale, the gap between the valence and conduction bands of the insulator would be much wider than that of the other materials. The conduction band in insulators is normally devoid of electrons while the valence band is filled. Therefore, when an electric field is applied across Fig . 2.38 Energy band diagrams in which the crosshatching symbolizes that there are many electrons in the various energy bands for various types of materials. the insulator, no current flows. If sufficiently high temperatures are applied (thousands of degrees) it is possible to excite some of the electrons with valence band energies up to the energy level of the conduction band. At such an elevated temperature, insulators become conductors with conduct ivities that increase with temperature. Electrical conductors, such as metals, consist of materials in which electrons fill the valence band and about half the conduction band. In this case when an electric field is applied the electrona move through the crystal easily and the material is referred to as a conductor. In metals an increaae in temperature increases lattice vibrations and electron scattering, therefore the conductivity decreases with increasing temperature. Materials with properties between insulators and conductors are known as semiconductors. Semiconductors are similar to insulators in that the valence band is filled and the conduction band is empty. However, the energy gap separating the conduction and valence bands is much smaller than that of insulators. For such semiconductors, thermal energy can excite a few electrons from the valence to the conduction band. Such materials are known as intrinaic semiconductors. Their conductivity increases with increasing temperature. By doping these materials with certain impurities, it is possible to greatly increase the number of carriers and increase the conductivity. If the dopant has carriers with an energy level that lies in the band gap just slightly below the conduction band, then thermal motions can readily excite electrona from these impurities (or dopants) into the conduction band where they are free to move through the crystal causing the material to become more conductive. Such dopants are known as donors and the resultant materials are known as negative or n-type semiconductors due to the fact that the carriers are electrons. Galium arsenide (GaAs) crystalline materials are important as roomtemperature light-emitting diodes (LED’s) and diode (or injection) lasers. In these materials, tin and tellurium serve as dopants that contribute (or donate) electrons to the conduction band while germanium (an acceptor impurity) introduces trapping sites with energy levels slightly above the valence band in the band gap itself. In the case of an acceptor, thermal motions will provide sufficient energy to permit electrons from the valence band to be trapped by an acceptor impurity atom. The holes left behind in the valence band act as positive conductors. p-type semiconductors. An important These materials are known as semiconductor energy state is 2-17
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 and 8: CHAPTER 1 INTRODUCTION 1.1 BACKGROU
Page 9 and 10: CHAPTER 2 FIBEROPTIC SENSOR COMPONE
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: tension it collapses to eliminate t
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.
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
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1 The preceding discussion applied
Page 83 and 84:
SIMPLEX FIBEROPTIC TRANSMITTER FIBE
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MAXIMUM ALLOWABLE RISETIME = 0.7/RN
Page 87 and 88:
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
Page 99 and 100:
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
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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