FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

More documents

Recommendations

Info

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

LED’s are fabricated both as edge and surface emitters. An example of surface emission is shown in Fig. 2.50. A well is etched in the substrate to within EPOXY= d FIBER ELECTRICAL N CONTACT N RECOMBINATION GE~SUBSTRATE LAYER (N OR P) Fig. 2.50 INSULATING / LAYER A surface-emitting LED with an etched well. approximately 1 micron of the recombination layer. This puts the surface close to the recombination layer and reduces the tendency for the light generated in the recombination layer to be reabsorbed before it can escape from the crystal. The optical fiber into which the light is being coupled is epoxied to the LED is also shown. This arrangement is satisfactory for a multimode optical fiber but, for coupling into singlemode fiber, edge emitters mounted in the same manner as diode lasers are more desirable. The waveguide character of the heterojunction structure leads to improved coupling efficiency and greater directionality, that is, it confines the emitted light to a narrower beam. In this case, the ends are cleaved at an angle several degrees from the normal to the surface of the recombination region in order to breakup optical standing waves and thus extend the region of spontaneous emission. In the discussion so far, the production of photons by electron-hole recombination has been considered. The reverse can also occur. A photon can be absorbed and thus produce an electron-hole pair. This phenomenon occurs in photodetectors and will be considered next. 2.4 PHOTODETECTORS The simpleat type of photodiode is the homojunction or p-n diode as shown in Fig. 2.51. The most 1 >DEpLET10t4 REGloN L- 1- ABSORPTION REGION -1 successful photodetectors employ silicon ( Si ) although galium arsenide (GaAs) is sometimes used. When the device is reverse (back) biased as shown, the electric field is not uniform. It peaks around the p-n junction as shown the bottom of Fig. 2.51. Thus , electron-hole pairs formed by the absorption of photons in this region are swept away (depleted) , electrons going to the n side and holes to the p side. This region of increased electric field is known as the depletion region. A S shown, the depletion and absorption regions do not necessarily coincide, the absorption region tending to be larger. Electron-hole pairs, formed by the absorption of photons from the depletion region, randomly diffuse, often recombining to produce photons. A depletion region may exist even without a reverse (back) bias, but then it is narrow. However, there are circumstances where unbiased operation is important, such as for low electrical power operation. Also with a bias, a “’dark field” current (dark photocurrent) flows due to thermally generated electron-hole pairs even in the absence of light. Removing the reverse (back) bias eliminates the “dark field” current. Operation with zero bias that is, without a bias supply, is known as photovoltaic operation. In order to make the depletion region as large, or larger than, the absorption region, the arrangement shown in Fig. 2.52 is used. Here a wide re- P-REGION LIGHT 3~ FDEPLETIONREGIONfi INTRINSIC REGION l—ABSORPTION~ REGION R 1-,1~1 BIAS SUPPLY b N-REGION ● OUTPUT Fig . 2.52 A positive-intrinsic-negative (PIN) photodiode with bias supply. gion with little or no dopant is placed between heavily doped n-type and p-type regions on opposite ends. An undoped semiconductor is referred to as an intrinsic semiconductor, therefore the broad lightly-doped region is called the intrinsic or i-type region, or simply the i-region. Such photodiodes are known as poaitive-intrinsic-negative or PIN diodes. The corresponding electric potential curve is also shown in Fig. 2.52. The highly doped n- and p-type regions at each end have low resistivity and therefore make good electrical contact. The resistivity of the i-region is often so high that even without a reverse (back) bias the depletion region extends half way through the i-region. The voltage required to extend the depletion region completely through the i-region ia called the ‘punchthrough voltage.” Fig. 2.51 The electric field and regions of a p-n (homojunction) photodiode with bias supply. When considering fiberoptic microbend sensors, reference will be made to dark field operation. A current exists in a reverse-biased photodiode even with no incident light. This current is called the dark field current and results from the thermally gen-
crated electron-hole paira that are driven by the bias voltage. Thus, the amount of dark current depends on the temperature of the photodiode, the energy gap, and the geometry of construction. Silicon photodiodes have been manufactured with very low dark currents. In order to insure that nearly all of the photons are absorbed (high quantum efficiency) the width of the i-region should exceed that of the absorption region by a factor of 2 or 3. However, the photodiode should be as thin as possible for fast response. Thus, high quantum efficiency and fast response represent design tradeoffs. Photodiodes such as those shown in Fig. 2.53 are known as avalanche photodiodes (APD). Here a highly-doped layer of p-type material is sandwiched between the i- and n-regions. This results in a region of high electric field just before the positive contact. In this arrangement, an electron freed in the i-region drifts toward the positive electrode. When it enters the high field region it speeds up achieving sufficient kinetic energy to produce another electron-hole pair if it collides with the lattice. The new carriers generated in this manner can likewiae produce additional carriers. Thus, a “primary” electron freed in the i-region can free numerous “secondary” electrons in the high field region. The resultant devices exhibit high quantum efficiency. An example of one type of APD construction is shown in Fig. 2.54. The temperature dependence of APD’s is greater than that of either p-n or PIN photodiodes. hi~ w kDEpLETION REGION+SECONDARY ELECTRON PRODUCTION REGION LIGHT P I P N 34 OUTPUT -111~+ BIAS SUPPLY Fig. 2.53 Field regions in an avalanche photodiode (APD) with bias supply. v ‘k;:;;:;LL /,;;;; ,,//:: 4’, *;, P-.~ A L!IGH! ,,, ,;;; ::,, ,,, ,,,,,:< ,, R b OUTPUT P INTRINSIC — BIAS .—– SUPPLY + N ELECTRICAL CONTACT Fig. 2.54 [ r * I ,’,, , ,,’ ,~,,,’, T The physical construction of an avalanche photodiode (APD). 2-23
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: I in one portion of the recombinati
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
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
show all

FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

Create successful ePaper yourself

Delete template?

Save as template?