FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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AIR ACOUSTICS REFERENCE LEVEL ––– I UNDERWATER ACOUSTICS REFERENCE LEVEL . . . . . . - 1 - Fig. 5.30 TT71 NEWTONIMETER 2 ~0 ~B 1 PASCAL (Pa) 1 DYNE/CENTlMETER2 DYNEICENTIMETER2 20 MICRONEWTON/METER2 4’0.0C02 20 MICROPASCAL 26 dB 1 MlCRONEWTONfMETER2 1 MKROPASCAL (#Pa) Pressure levels and units for comparison of underwater acoustic pressure reference levels. average minimum detectable sound pressure for humans at 1000 Hz. Another is the currently accepted 1 m.lcronewton/m2 or 1 micropascal (1 ~Pa) reference pressure for underwater acoustics. To compare these on a decibel scale, recall first that for a pressure ratio, P1/P2, the number of decibels, N, in dB, is defined by: N = 20 loglo P1/P2 (5.18) Since 0.0002 dynes/cm 2 is equl to 26 upa, the air acoustic reference pressure is 26 dB above the underwater reference pressure. Similarl as indicated in Fig. 5.30, a pressure of 1 dyne/cm J’ is at a level of 74 dB re 0.0002 dyne/cm2 and at a level of 100 dB re 1 pPa. Finally, a pressure of 1 Pa corresponds to 120 dB re 1 pPa and 20 dBre 1 dyne/cm2. The table presented in Fig. 5.30 is an aid in interpreting hydrophore characteristics and in performing comparisons presented here and in later sections. Returning to the consideration of the Hughes- NRL microbend hydrophore, an experimental evaluation of the acoustic characteristic of their initial prototype was conducted at NRL. As indicated in Fig. 5.31, the hydrophone was placed in an acoustic test tank and measurements were made of its sensitivity and frequency response over a frequency range of 200 Hz to 2000 Hz. The H ~ MODE STRIPPER u \ results are shown in graphical form in the lower por– tion of the figure. The minimum detectable pressure, in a 1 Hz band and at a unity signal-to-noise ratio, was approximately 100 dB re 1 ma. The 10 dB fluctuations about this value were attributed to resonances in the outer case and deformer mount. It should be possible to eliminate these resonances without much difficulty. In addition, a significant increase in sensitivity was achieved in later designs by employing speciallydesigned graded-index fibers. This was to be expected since the fiber employed in the initial prototype was a readily available standard communications step-index fiber. Such optical fiber has been designed to have low microbend sensitivity to reduce losses due to bending introduced in cabling and other field use distortions. By employing graded-index fibers with enhanced microbend effects, sensitivity increases of more than 40 dB have been achieved. Thus, these improved microbend transducers are comparable to many of the more conventional hydrophores currently in use. It should be emphasized that with the microbend transducers discussed above, attempts are made to detect a very small change in the intensity of a relatively intense optical beam. This type of sensor is referred to as a brightfield microbend transducer. A second type of microbend device, a so called darkfield transducer, was proposed initially by a group at Catholic University and currently is under investigation by several different groups. The sensor is shown in Fig. 5.32. Beginning at the left, it is quite similar in OPTICAL SOURCE Fig. 5.32 FIBER DEFORMER MODE STRIPPER The microbend darkfield intensity-type fiberoptic sensor system. arrangement to the brightfield transducer up to and including the deformer. As with the brightfield transducer described above, light, possibly from a broadband incoherent source, is introduced into a multimode fiber. Care ia taken to remove cladding light prior to the deformer. The darkfield transducer differs from the brightfield transducer in that the light ejected from the core into the cladding is used to generate the output signal. Fig. 5.31 Test arrangement and sensitivity level of the Hughes Research Laboratories and U.S. Naval Research Laboratory microbend intensity-type fiberoptic senaor hydrophore. After Fields and Cole, APP1. Opt. ~, 3265 (1980)0 5.1 1 As indicated in Fig. 5.32, the Catholic University group used a more elaborate mode-stripper on the output section of the fiber (see Ref. 6 in Subsection 5.2.6). The fiber was stripped of ita outer coating and passed through a small chamber filled with a refractive-index matching fluid. A number of photodetectors mounted in the walls of the chamber responded to changes in the intensity of the core-to-cladding ejected light. In contrast with brightfield, the darkfield case has a relatively low level of background light that is modulated by changes in displacement of the deformer. The degree” of modulation - may be quite large and thus a very high sensitivity may be employed without overdriving it. detectable signals should, in principle, in the brightfield case. This has been recent studies. photodetector The minimum be lower than confirmed in
A simplified design of the cladding light monitor is indicated in Fig. 5.33. The pick-up fiber STRIPPER I o- SOURCE FIBER DEFORMER transducer outputs could also be accomplished prior to photodetection. The optical outputs may be combined with appropriate time delays determined by the transducer spacings and coupler fiber lengths. On the other hand, by employing a pulse modulated optical source, it is possible to employ a single return bus fiber into which the optical output pulses from the various transducers could be fed. Time domain multiplexing schemes could then be used to identify and process the signal from the individual sensors. These and other various types of fiberoptic sensor arrays and the telemetering of their outputs are discussed in detail in Chapter 6. Fig. 5.33 FIBER-FIBER COUPLER DETECTOR FIBER kAI A darkfield microbend intensity-type fiberoptic sensor with a fiber-fiber coupler following the deformer (microbender) developed by the Catholic University. is directly coupled to the outer surface of the cladding of the through-put fiber. Recent studies of this technique have ha~ p~omising results. darkfield The design and operation of a linear array of microbend sensors is shown in Fig. 5.34. Each This brief description of various intensity type sensors is not exhaustive. The discussion is aimed at introducing some basic concepts regarding their overall design and behavior. Many other fiberoptic intensity transducers are currently under investigation such as sensors employing fibers with temperature sensitive absorptive dopants, and several displacement and pressure transducers employing strain-induced birefringence as an intensity transduction mechanism. All of these have the advantage of being much simpler in design and operation, but they are less sensitive than interferometric fiberoptic sensors. Further improvements, including increases in sensitivity, are expected for the intensity-type fiberoptic sensors. 5.2.6 1. 2. 3. s. R. w. References Sheem and J. Cole, Opt. Lett. ~, 322 (1979). Phillips, Opt. Lett. ~, 318 (1980). Spillman, Appl. Opt. ~, 465 (1981). STR!-R DEFORMER SOURCE FIBER STRIPPER ,~fi \.- . ---- \ --- - DEFORMER K /’ FIBER-FIBER COUPLERS %T ) Fig. 5.34 A series of deformers used to control the intensity of light tapped from an optical fiber bus. sensor is an assembly consisting of a cladding-mode stripper, a microbend deformer (transducer), and a fiber-to-fiber coupler. Many of these assemblies may be mounted in series on a single optical fiber bus. The cladding-mode stripper removes any residual light in the cladding just prior to the microbend deformer. The deformer causes light from the core to enter the cladding according to the baseband (information-bearing force, pressure, or sound) signal. Following the deformer, the coupler removes the light (baseband signal) from the claddlng and dispatches it to a photodetector. A number of different multiplexing schemes could be used in the detection portion of the array. The moat direct, would be to feed each cladding light pick-up fiber to a separate photodetector. Temporal and spatial averaging of a number of closely spaced 5-12 4. 5. 6. 5.3 J. J. J. N. R. Fields, C. Asawa, O. Ramer, and M. Barnoski, Acoust. SOC. Am. Z, 816 (1980). Fields and J. Cole, Appl. Opt. ~, 3265 (1980). Lagakos, T. Litovitz, P. Macedo, R. Mohr, and Meister, Appl. Opt. ~, 167 (1981). <strong>FIBEROPTIC</strong> LINRAR ACCELEROMETERS The operation of interferometric fiberoptic sensors described in Section 5.1 depends primarily on phase changes associated with force-field-induced mechanical strains. Similarly the two-fiber optical accelerometer described in this section makes use of the change in fiber length due to a force resulting from the acceleration of a mass suspended between two fibers. The effect is to increase the tensile stress in the fiber in one arm of an interferometer and decrease the tensile stresa in the fiber in the other arm. The device for accomplishing this is shown in Fig. 5.35. A section of the fiber in one arm of the interferometer is attached both to the upper end of the case and to the mass. A similar section of fiber in the other arm of the interferometer is attached to the mass and to the lower end of the case. Thus, the mass, m, is suspended between the two fiber sections which effectively serve as springs. If the accelerometer case is given an acceleration, a, vertically upward, the upper fiber elongates by AL and the lower fiber shortens by the same amount in providing the force F required to accelerate the mass. This may be written as F=2AAT=ma (5.19)
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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
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.
Page 59 and 60: nickel H. = 3.6 oersteds and for a
Page 61 and 62: optical source of constant intensit
Page 63: in a straight section of the fiber
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 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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