FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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following is obtained: L-BWP= [(SX/c)d2n/dA2]2+[(nA/*c)pi]2 [ 1 1’2 (6.12) where L-BWP is the optical fiber length-bandwidth product. Many of theae parameters are fixed by the optical fiber manufacturer. In addition, the modal dispersion caused by a fiber can be expressed in terms of the risetime for a step input optical signal. This can be empirically determined for typical high performance commercially available optical fiber. The parameters given in Eqa. (6.10) and (6.11), and therefore (6.12), contribute to the fiber riaetime modal dispersion coefficient used in Eq. (6.13) below. The portion of the fiber risetime due to modal dispersion (intramodal and intermodal) ia given empirically for a typical high-performance commercially available optical fiber, as: where L-BWPO = FIBER RISETIME DUE TO MODAL DISPERSION L-BWP = L eff = where L= 530 = trmo(cable) = 530/L-BWpo ‘s (6.13) (L-Bwp)/Leff = Optical 3-dB bandwidth of the fiber. Length-bandwidth product, MRz-km. L x = Effective length of fiber. Actual length of fiber. 0.5 < x < 1 Short lengtha, < 1 km, x = 1 Long lengths, x = 0.7 or 0.8. Fiber risetime modal dispersion coefficient for typical high-performance commercially available optical fiber. The portion of the fiber risetime due to material dispersion is given aa: where FIBER RISETIME DUE TO MATERIAL DISPERSION ‘rma(cable) = 1.1 MSL (6.14) M = Material dispersion coefficient, given in nslnm-km and as shown in Fig. 6.17. S = Spectral bandwidth of source, nm L = Link length, km A typical curve for the value of M, the material dispersion coefficient, as a function of wavelength for two specific glasses is shown in Fig. 6.17. RECEIVER RISETIME, tr(rcvr), due to the photodetector and its aaaociated electrical circuits. The receiver risetime la normally given by the manufacturer or can be constructed or eatimated from the manufacturer’s data. Normally a Gauasian distribution of the risetimes is assumed, and thus for a set of sequential components the overall risetime for the system is given as: ‘r(sya) = [t2*(xmtr) + 2 ‘r ma(cable) + ‘r(rcvr) 2 r mo(cable) + 1/2 (6.15) However, the total risetime for the link, ‘r(sys)~ cannot exceed the maximum allowable risetime for the link. In digital systems, the allowable risetime is limited by the requirement to prevent the bit error rate (BER) due to interaymbol (interpulse) interference from exceeding a prescribed value. In analog systems the frequency response at high frequencies must be sufficient to prevent distortion of the base band aignals. For example, in the nonreturn-to-zero (NRZ) method of signal representation (code), ‘he ‘r(sys) must be less than 0.7 times the bit interval, expressed as the reciprocal of the bit rate. If the bit rate ia 7.0 Mb/see, the maximum value tr(sys) can have ia is 100 nsec for NRZ coding. For return-to-zero (RZ) coding the factor is 0.5, in which case, for the same bit rate of 7.0 Mb/see, the maximum allowable tr(sya) is 71.4 ns. The following is given as an example of a riaetime budget for a typical fiberoptic link: LINR DESCRIPTION <strong>FIBEROPTIC</strong> LINK RISETIME BUDGET Data rate, Rw Link length, L Length-bandwidth-product, L-BWF Light source Operating wavelength,a Light source spectral width, S COMPONENT RISETIMES Transmitter [tr(xmtr)] (Manufacturer) *O ns Fiber modal dispersion risetime [trmo(cable)] 7.0 Mb/see 1.5 km 50 MHz-km LED 0.830 Pm 0.020 pm 0.25 (L-BWo) = (L-BWP)/Lx = 50/1.5°”8 = 36.1 MRz-km 015 010 ‘rmo(cable) = 530/(L-BWPo) = 530/36.1 = 14.7 ns Fiber material dispersion risetime M = 75 ns/m-km (Manufacturer) [t_(cable)] ‘rma(cable)=l- lMsL=( 101)(75)(0”02)(1-5)=2”5ns Receiver risetime [tr(rcvr)] (Manufacturer) 0.05 0 /00 900 1100 1: ) WAVELENGTH(nm) Fig. 6.17 The material dispersion coefficient versus wavelength for two types of glasses. 6-8 ‘r(rcvr) LINR RISETIME = 375/Belec = 375/50 = 7.5 ns 56.3 Substituting the above risetimes in Eq. (6.15) the overall system (link) risetime is calculated to be 26 ns.
MAXIMUM ALLOWABLE RISETIME = 0.7/RN~ = 100 nS <strong>FIBEROPTIC</strong> LINR OPTICAL POWER BUDGET (Fig. 6.16) RISETIME BUD(ZT MARGIN = 7fI ns LINK DESCRIPTION The 74 ns margin between the maximum allowable risetime based on the required signaling rate and Link length, L 1.5km Data Rate, R 7.0 Mblsec the system risetime for all the components can be used Operating wavelength 0.830 nm in many ways. For example, the cable could be made longer, the signaling rate could be increased, or a cheaper cable with a greater material or modal dispersion per unit length could be used. However, other budgets, such as the power budget, may place limits on theae changes. Perhaps the one single fact to remember in regard to risetime is that it represents reciprocal dollars, i.e., the longer the risetime, the cheaper the component. Therefore, except for a power safety margin, all allowable risetime should be used up for the performance requirement in a given application. 6.2.3.2 Optical Power Budget Analysis The optical power requirement of a given fiberoptic telemetry system is also a matter of distribution of power over a serial or a parallel set of fiberoptic channels. For example, if there are star couplers, there must be sufficient input power to satisfy the input power requirement for each parallel output channel. Serially connected components, such as cables, splices, and couplers, will each have an inser - tion loss. Repeaters will insert a gain. The losses and gains are added to obtain the source-to-receiver overall loss. The optical power dissipation that can be allowed is the difference between the sum of the transmitter and repeater optical power outputs and the required receiver (photodetector) optical power input in order to keep the bit error rate (BER) below, or signal-to-noise ratio (S/N) above, a specified value. A typical allowable optical power loss between transmitter and receiver for various data rates at an allowable BER of 10-9 is shown in Fig. 6.18. Normally one TRANSMITTER POWER, pT Light source type Average source optical power Fiber coupling loss Average launched optical power, REQUIRED RECEIVER POWRR, pREQ Detector type Required bit error rate, BER Receiver bandwidth Receiver sensitivity Link margin ‘REQ ALLOWABLE LINK LOSS OVER MARGIN, PAL pm = PT - PmQ = -14 -(-48) = 34 dB LINR LOSS COMPUTATION LED 0.1 mW -10 dBm -4 dB -14 dBm PIN-FET 1(3-9 50 MHz -55 dBm + 7 dB -48 dllm Contributor Unit Quantity Total Recr coupling 3.0 dB 1 3.0 dB Connectors 2.0 2 4.0 Splices 0.5 4 2.0 Cable 5.0 dB/km 1.5 km 7.5 Splitters Margin 2.0 dB 2.0 Total link loss 18.5 dB REMAINING POWER MARGIN (Allowable - Total link loss) = 34- 18.5 = 15.5 dB = II g n Fig. 6.18 o -lo -20 -30 -40 -50 -60 -70 t LASER-LAUNCHED POWER RANGE LED-LAUNCHED --..POWER RANGE _______ -14 I -80 -901 1 1 1 1 10 100 1000 DATA RATE (Mb/s) BER =10-9 optical power loss versus data rates for the fiberoptic data link used in the optical power budget analysis in Section 6.1.3. would not use up all the available power, but maintain a safety margin of a few dB to insure satisfactory performance. The allowable loss is then distributed over the fiberoptic link components as shown in Fig. 6.16 and given in the following example: Since a link margin of 7 dB and a link loss computation margin of 2 dB have already been allowed, the entire 34 dB may be used up. The loss in the link of the given design is 18.5 dB. Therefore, there is an excess total available optical power margin of 15.5 dB. It may be cheaper to use up this excess margin by selecting cheaper cable components with greater losses or it may be cheaper to choose a lower-power source. Link power is like dollars, it is best to get by with the least power, low power sources being cheaper. However, link loss is more like reciprocal dollars, the greater the power loss, the cheaper the components. Therefore, there is a trade-off to be made in order to minimize the overall cost of a fiberoptic link. 6.2.3.3 Cost Budget Analysis The cost of a fiberoptic telemetry system will be based on judicious maximal use of available risetime and minimal use of power as well as many other factors. Fiberoptic cable designs, repeater spacing, multiplexing schemes, allowable BERs, cable routings, the use of overhead versus subterranean cables, availability of components, simplicity of design and fabrication, and ease of maintenance, are just a few of the many factors that will enter into the overall cost of the system. 6.2.4 Fiberoptic Telemetry System Specific Configurations An example of a set of the final design para- 6-9
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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
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6=0 REFERENCE PLANE that propagate
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in the right center of Fig. 2.13. T
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The vertical height of the various
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60 40 20 10.C ~8 x z 6 n u 4 % c 62
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: 1.5 z o GeOz 0 ~ =1.49 u w > ~1.4
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tension it collapses to eliminate t
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MAXIMUM TENSILE STRENGTH (GN/m2 = l
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mately a 0.5 electron-volt increase
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I in one portion of the recombinati
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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
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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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Page 65 and 66: A simplified design of the cladding
Page 67 and 68: Referring once more to Fig. 5.35, i
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Page 75 and 76: Fig. 5.60 lists several sources of
Page 77 and 78: CHAPTER 6 FIBEROPTIC SENSOR ARRAYS
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Page 81 and 82: 1 The preceding discussion applied
Page 83: SIMPLEX FIBEROPTIC TRANSMITTER FIBE
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Page 101 and 102: M Mach-Zehnder interferometer. An i
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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.
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FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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