FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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arated by a small gap in which is placed a pair of gratings, consisting of a cyclic grid of totally transmissive and totally reflecting (opaque) parallel line elements of equal width. When the gratings are moved relative to one another there is a change In the transmitted intensity. This type of transducer has been employed in a hydrophore, as sketched in Fig. 5.23. Here one DIAPHRAGM n \ n L r HYDROPHORE /HOUSING / be seen that the sensitivity will be greatest when the quiescent or bias point is set at a relative displacement of 2.5 pm, 7.5 pm, 12.5 pm, etc. In addition, decreasing the width of the grating elements will increase the sensitivity but decrease the dynamic range. 5.2.5 Microbend Fiberoptic Sensor The intensity-type sensors to be considered last are based on microbend-induced ejection of light from the core of a fiber into the cladding (See Ref. 4 in Subsection 5.2.6). Referring to Fig. 5.25, the transduction element in this type of sensor consists of a deforming device such as a pair of toothed or serated plates that introduce small bends in a fiber. As FIBER \ I F I DEFORMER F I I , I OPPOSED GRATING Fig. 5.23 A moving grating intensity-type fiberoptic sensor used in a hydrophore. Adapted from W. Spillman, Appl. Opt. ~, 465 (1981). of the gratings is shown mounted on the rigid base plate of the housing while the other is attached to a flexible diaphragm. The diverging lightbeam from the input fiber on the left is collimated using a short graded-index self-focusing (Selfocc) lens and then partially transmitted through the gratings as a parallel (collimated) beam that is focused into the output fiber using a second self-focuaing (Selfocc) lens. Assuming the two gratings each consist of 5 pm-wide grating elements that are spaced 5 Bm apart, the transmitted light intensity will vary cyclically as sketched in Fig. 5.24, passing through successive maxima each time the grating displacement changes by 10 ~m. From this graph it may 1.0 MODE1: E1-sin(LJt-~lz) MODE2: E2-sin(Ut–~2z) COUPLING CONDITION: L=— 13;:132 Fig. 5.25 A microbend intensity-type ser. fiberoptic senahowo in the figure, the distance L between adjacent teeth defines the apatial frequency of the deformer. By increasing the force, F, applied to the plates the amplitude of the deformations can be increased. In the earlier brief discussion of attenuation mechanisms in fibera, it was shown that random bends in fibers can cause light to be ejected from the core into the cladding. Thia process is illustrated in the enlargement of the deformer shown in Fig. 5.26. Rays propagating ~
in a straight section of the fiber at an angle less than the critical angle may have their angle of incidence on the core-cladding interface increased by the bends and thus be partially transmitted into the cladding. A more detailed wave theory analysis of periodic bend-induced coupling indicates that the level of coupling between modes, including non-attenuating core modes as well as attenuating core and cladding modes, is strongest when the difference between the effective propagation constants of a pair of modes, (Bi - Bj), is equal to 2 /1.. range 0.5N < F > 1.5 N. In the latter caae, 9“ corresponded closely to the critical incidence angle, thus the light waa injected mainly into the highest order propagating modes and therefore was more eaaily ejected from the core into the cladding. loofiT_..- ,_L 1::::-,- 1 I 1 I I I I I I I I I I T – - * — - 0 - - 1 A number of studies of this phenomenon have been conducted. For example, the system shown in Fig. 5.27 was used at the Hughes Research Laboratories. As EO ~ — L I FIBER DETECTOR MODE DEFORMER STRIPPER Fig. 5.27 A microbend intensity-type sor system developed by the Laboratories. fiberoptic sen- Hughes Research indicated in the figure, light was injected into a multimode step-index fiber which was passed through a deformer element. In this case the intensity of the core light reaching the end of the fiber was monitored. By using a helium-neon laser with a well-collimated beam it was possible to vary the incidence angle of light into the fiber and thus inject light into a fairly well defined set of propagating core modes. On sections of the cladding, just before and just after the deformer, mode strippers were employed. These elements, which in their simplest form might consist of black paint applied to a few centimeters of the outer surface of the cladding, absorb almost all of the light that my be propagating in the cladding of the fiber. The use of cladding mode strippers first insured that only core light reached the section of fiber in the deformer and then that any core light ejected into the cladding by the deformer was absorbed so that it did not reach the photodetector. Uaing the system outlined in Fig. 5.27, the Hughes’ inveatigatora measured the transmitted optical intensity as a function of the force applied to the transducer (microbend deformer). This was done for several different angles of incidence of the input light and the reaulting data la preaented in Fig. 5.28. Aa shown in that figure, when the input incidence angle was set at 0°, i.e., for light injected along the axis of the fiber, the output intensity decreased by about twenty percent as the force applied to the deformer increaaed from O to 2 newtons. On the other hand, for light incident at 9°, the transmitted intensity decreased to approximately 40 percent of the input when the applied force waa again increased to 2 N. In addition, the slope of the transmission intensity, I, veraus applied force, F, curve was nearly conatant over the 1 r r ——— O.ODEG . 20 ----- 7.ODEG 0 -“---”- 8.ODEG ❑ 1 } ------- 9.0 DEG ● 4 oo~ 0.5 1.0 1.5 FORCE N Fig. 5.28 The percent transmission of core input light obtained at the output as a function of applied force in a microbend intensitytype fiberoptic sensor. After J. Fields, et al., J. Acouat. Soc. Am. ~, 816 (1980). Using the results of this and aimilar experiments, the Hughes investigatora, in cooperation with the Physical Acouatica Branch of the U.S. Naval Reaearch Labora~ory, deaigned and teated a hydrophore employing such a microbend deformer as the transducer element. Their first prototype unit ia aketched in Fig. 5.29 DIAPH /-- FIBERLEADS / Fig. 5.29 A microbend intenaity-type fiberoptic sensor hydrophore developed by the Hughes Research Laboratories and the U.S. Naval Research Laboratory. After Fields and Cole, Appl. Opt. ~, 3265 (1980), (See Ref. 5 in Subsection 5.2.6). One deformer plate was rigidly mounted to the cylindrical ahell of the hydrophore while the other was attached to a thin diapragm. In addition to the through-put fiber, a second inactive fiber was included to insure that the deformer plates remain parallel during operation. At this point it would be uaeful to review some of the basic acoustical levels and unita of meaaure. Referring to Fig. 5.30, one frequently encountered acoustic reference pressure level encountered in air acouatics is 0.0002 dynea/cm2. This is the accepted 5-1o
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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 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: optical source of constant intensit
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 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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