FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

More documents

Recommendations

Info

$Course Material\Efficiently Coding Communication Protocols in C++ ...$

vacuum, and t= is the overall risetime to be calculated later in this chapter. This value of ta can be reduced to the extent that the pulse width can be reduced. If there are only a few sensors in the array the second term in bracketa in Eq. (6.6) is significant. As the number of sensors is increased, the second term in Eq. (6.6) becomes less significant. Eq. (6.6) does not apply to one senaor because then L = O, the second term in brackets becomes indeterminate, and in fact there is no array. In this case, only the risetime requirement will place a limit on the sampling period. The maximum permissible sampling rate PR~s is: requirement for any specific spatial relationship among the sensors. The minimum sampling period for each sensor tssen in the array will be: ‘asen = mtad (6.9) where m is the number of sensors in the array and tad is the analog-to-digital conversion time. This presumes a photodetector for each sensor output, a multiplexer (for polling each photodetector output), and a aingle A-D converter with its sample-and-hold and other associated circuitry aa shown in Fig. 6.8. Pluqs.= l/ts (6.7) where ts is defined in Eq. (6.6). The sampling rate can be arbitrarily reduced, but should not be less than twice the highest significant frequency component in the baseband signal in order to obtain reproduction of the baseband signal without significant distortion (Nyquist criterion). Another method for energizing an array of sensors is to connect the array to a pulsed electrical bus as shown in Fig. 6.7, rather than the pulsed optical ># I l--+-l EEk5?ltlzl ‘1 LENGTH L-~ WLSSDELECTRICALF 7’I w , EWAUY-SPACEDPJSES NANOFTCAL REluRssus SENSORS L-RARRAY OFMEWY SPACEDSENZORS WITNLSOAND INTSN.91v-M~L4T10N Fig. 6.8 A fiberoptic star-coupler-fed sensor random array telemetry system with multiple and single cable returns. Fig. 6.7 A fiberoptic lightfield pulsed bus-fed sensor array telemetry single optical return bus. electricalsystem with The random array of fiberoptic sensors may be energized instead by a direct-current electrical bus as shown in Fig. 6.9. In this case, there is a contin- bus just described. A light source, such as an LED, is optically connected to each sensor. Thia is the brightfield form of senaor energization. The return bus can be the same, as was shown in Fig. 6.6, and the analysis above still applies except that the propagation time in the electrical bus will be different. In this case, the electrical sampling pulae period will be: CONTFWOUS (DC) ELECTRICAL FEED BUS telec=l/2 [ts+[2+l.8/(m-l)]L/v+trc] (6.8) where the variables are as in Equation (6.6), v is the velocity of propagation in the electrical bus, and trc is the combined electrical and optical risetimes that will be discussed later in this chapter. Another configuration for energizing a randomly-distributed (non-linear) array of fiberoptic aensors is shown in Fig. 6.8. A continuous source of light (laser) energizes a star coupler that may be placed at the center of the array to reduce fiber requirements. The output of the coupler is fed to each fiberoptic sensor where it is modulated by the baseband signal. The senaor outputs are individually fed to an arrayof photodetector (PDs). In the arrangement shown in Fig. 6.8, the inputs to the photodetector (PD) array (one PD for each sensor) are time-division multiplexed, sampled, digitized, and telemetered as a digital data bit stream via a fiberoptic cable to another location for exploitation. In this case, there ia no 6-4 Fig. 6.9 A fiberoptic electrical direct-current busfed sensor array telemetry aystem with multiple and single cable return. uous-output light source (LED) for each sensor. The baseband-modulated optical output of each sensor is separately cabled to a photodetector array and processed there in a manner similar to the preceding arrangement that had the continuous optical feed as was shown in Fig. 6.8. J
1 The preceding discussion applied primarily to intensity-modulated lightfield and darkfield sensors. The outputs of arrays of other tYPes of sensors can also be telemetered to locations distant from the sensors. An interferometer sensor array configuration is shown in Fig. 6.10. The optical outputs of all fiber- (REPEAT OF R,G”T S,DE, Fig. 6.10 OPTICAL BuS mL——...-.! \ I COUPLERS ELECTROOPTIC PHOTODETECTORS INTEGRATED CHIP DETECTIONIFEEDBACK DIGITIZATION(MULTIPLEXER LED CLOCK L A fiberoptic interferometric star-couplerfed sensor array telemetry system with optical multiple cable and single cable return and integrated optical circuit chip. . . . There are many other variations and combinations of the basic fiberoptic sensor-array telemetry schemes shown here. The basic options include the selection of the type of telemetry link (to and from the sensor array, electrical, optical, or electrooptical); the type of fiber optic sensor (interferometric, intensity, phase, darkfield, brightfield); the type of coupling (star, tapped bus); the type of light sources (laser-powered or LED-powered bus, LED at each sensor); signal types (analog, discrete); multiplexing schemes; and many others. Each of these options will incur a different set of technical problems that require solution, a different set of costs, and a different set of performance characteristics. For example, frequencydivision multiplexing of fiberoptic sensor outputs can also be applied by energizing the common feed bus of the equally-spaced-sensor arrays discussed earlier with a constantly-changing-frequency pulse (frequency ramp). Thus, each fiberoptic sensor in the linear equallyspaced array will be fed a different frequency to be modulated by the baseband signal. The outputs can be demultiplexed with appropriate narrowband filters. 6.1.3 Fiberoptic Sensor Array Budgets Fiberoptic aensor array budgets may be prepared in much the same manner as for the telemetry budgeta to be described in Subsection 6.2.3. Risetime, power, cost, and other budgets for fiberoptic sensor arrays depend on the same set of factors as for the entire sensor-telemetry system. tin optical power budget for the sensor array shown in Fig. 6.20 is as follows: optic sensors are brought to a common point, an electrooptic chip. The integrated optical chips are not yet commercially available, however, they are under development. A single optical fiber is used to conduct the output of the electrooptic chip to a distant location. Ml signal processing is done at the array location on the chip. The output of each senaor is sent to the single integrated fiberoptic chip for processing via a separate fiber. If It should prove desirable to use less fiber, use can be made of the spatial separation between sensors and the outputs can be automatically time-division multiplexed provided the signal processing can be accomplished at each interferometric sensor as ahown in Fig. 6.11. In this arrangement, a single optical return bus is used to telemeter the outputs to a distant location. T!+AE Owwm MULTIPLEXER Fig. 6.11 0 ~-– LASER 1 I REFEREW3EARM ‘ II II POWER> ELECTRCAL BUS : L--—-.—— ...__._J ELECTROOPTCCH!P Tlt.lEDIvISlON MULTIPLEXER A fiberoptic interferornetric star-couplerfed equally spaced sensor array telemetry system with electrical outputs to a single electrical return bus. 6-5 SENSOR ARRAY POWER BUDGET OUTPUT POWER Laser output power Coupling loas in fiber pigtail (78% Coupling) AVERAGE POWER OUTPUT SYSTEM LOSS Star coupler insertion loss Star coupler splitting loss (1:60) Coupler insertion loss (2, ldB each) 3 dB coupler aplitting loss Fiber loss (300 m, 5 dB/km) Splicing loss (6, 0.5 dB each) TOTAL TOTAL POWEṚ 6.2 SYSTEM LOSS AVAILABLE MARGIN 37.3 - 29.3 = MARGIN AT EACH DETECTOR Antilog of 0.80 = FIBEROPTIC TELEMETRY SYSTEMS 7 mW = 38.5 dB VW - 1.2 dB 37.3 dB UW 2.0 dB 17.8 2.0 3.0 1.5 3.0 29.3 dB 8.0 dB VW 6.3 UW The properties of optical fibers and fibersensors have been discussed in detail in prior optic chapters where particular attention was given to construction (materials and geometry), principles of operation (light transmission properties of fibers), and relative advantages of fiberoptic sensors over other types of sensors. Various ways were discussed in which fiberoptic sensors can be designed to measure absolute magnitudes or relative changes of a physical parameter, develop an output signal that is a function of these absolute or relative values, and emit this signal in a form suitable for subsequent processing and transmission. In essence, the fiberoptic aensor is a transducer; it provides the transform that enables an on-
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 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 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: method of sensor energization will
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
show all

FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?