FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

More documents

Recommendations

Info

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

CHAPTER 3 <strong>FIBEROPTIC</strong> COMPONENT INTERCONNECTION Optical power loss (attenuation) has been drastically reduced in optical fibers since 1970. A power loss of 0.2 dB per kilometer has been achieved and the prospect is good for another order of magnitude improvement to 0.02 dB per kilometer. If this occurs, approximately 50 kilometers of fiber would exhibit only a 1 dB leas. One consequenceof this progress inreducing attenuation in fiber is the increased importance of the attenuation associated with component-to-fiber, fiber-to-fiber, and fiber-to-component interconnections. Little is accomplished if 0.02 dB per kilometer attenuation is achieved in optical fibers and at the same time a number of Interconnections are required, each resulting in an appreciable fraction of a dB loss. In the case of fiberoptic sensora, where much shorter lengths of optical fiber are utilized than in communication systems, such as several hundred meters of fiber or less per sensor, and where the fiber used in most cases is not chosen for low 10SS, problems with interconnections may be less important although connection insertion losses can still be a large portion of the total loss in a fiberoptic sensor. Interconnections, especially singlemode fiber interconnections, are still required and therefore of importance. The current state of their development and manufacture will be considered in the following discussions. 3.1 ‘<strong>FIBEROPTIC</strong> CONNECTORS AND SPLICES Some of the uses of interconnections in the fabrication of fiberoptic sensors include joining sources and detectors to fiber , splitting the output of a source (especially laser diodes) among a number of sensors, beam splitting and combining of light in interferometers, and providing fiber-to-fiber interconnections. All interconnections must be designed taking reflection and consequent insertion losses into account, with the aim of minimizing the insertion losses. into which light is being introduced will be designated the “sink” fiber. In connectors and splices, power losses fall into two general classes: intrinsic and extrinsic. Intrinsic losses are due to variations or imperfections in the fiber that occur during the manufacturing process and are not mechanically or externally correctable. Extrinsic losses are those that occur after the manufacturing process and are mechanically or externally correctable, such as incorrect finishing of the fiber end-surfaces or incorrect mechanical mating of fibers. Some of these effects are shown in Fig. 3.1. Only the x INTRINSIC (I:C CORE AREA MISMATCH NUMERICAL APERTURE MISMATCH PROFILE MISMATCH EXTRINSIC ~ ~d -c END SEPARATION + p’- ANGULAR MISALIGNMENT -L / t LATERAL OFFSET Fig. 3.1 Some causes of intrinsic and extrinsic power losses in optical fiber interconnections. Interconnections can be grouped into three classes, namely (1) connectors (remountable interconnections between fibers or between a fiber and some component, such’as a source, a detector, or an integrated chip), (2) splices, (fusion joints or permanent joints between two fibers or a fiber and some optical component, and (3) couplers (connections that redistribute energy between two or more fibers). In the case of singlemode fibers, splices are relatively easy to form. Splices and connectors with less than one tenth dB insertion loss per splice can be achieved. Also, in the case of singlemode couplers, especially simple four-port couplers having two input ports and two output ports, losses of less than a dB have been achieved. Multimode connectors and couplers are now commerically available and their singlemode counterparts are just beginning to become available also. In the case of multimode connectors, the average loss is about 1 or 2 dB. Goals are set for less than 0.5 dB. For purposes of discussion, the fiber from which light is emerging will be designated the ‘source” fiber while the fiber 3-1 fiber cores are shown in these sketches. Intrinsic effects are shown on the left of Fig. 3.1. If the core areas of the source and sink fibers are not the same, the mismatch can result in a power loss. Differences in numerical aperture (NA) between the two fibers can also result in losses. For the case of graded index fibers, discussed earlier, a refractive index profile mismatch can lead to intrinsic losses. Losses occur only when directing light from a fiber of larger core or NA into a fiber of smaller core or NA. In these cases some of the light from the core of the source fiber will not be trapped in the core of the sink fiber. For the reverse, small-to-large core or NA, losses due to the mismatch do not occur. Examples of causes of extrinsic losses are shown in the right column of Fig. 3.1. If the light input to a sink fiber or output from a source fiber diverges, such as at cone angles of 15° to 20”, a core separation will allow some of the light emanating from the core of the source fiber to miss the core of the
sink fiber. Likewise angular misalignment can lead to a portion of the light from the source fiber entering the aink fiber at angles that will not allow trapping in the core. Finally, losses can occur due to lateral offset between two fibers because they are not properly aligned or their cores are not concentric with respect to the outer diameter of the fiber even when the outer surfaces of the cladding are properly aligned. In general, fibers are lined up by their outer surfaces. There are a number of other extrinsic effects that are not indicated here. The fiber end might not be smooth. ‘rhis can lead to scattering loaaea. The fiber ends may not be flat cauaing lensing effects to occur. Thus, it is esaential that care be taken in the manufacture or acquisition of optical fibers, connectors, and splicea in order to inaure that the intrinaic and extrinsic leases are or can be minimized. h effect that can be corrected easily ia reflection from the ends of both fibers due to refractive index difference between glass and air. For silicon dioxide (Si02) this reaulta in a 0.4 dB loaa. In order to correct thia it is only nesaary to employ an index-matching liquid or potting material between the ends of the two fibera being buttjoined. the same fiber. For a fiber with a 50-micron core it would be necessary to hold the dimensions to + 5 microns, but when dealing with singlemode fibers w~th a 5 micron core or less it is necessary to hold the diametera to within a half a micron. Differences in numerical aperature also need to be controlled accurately, however in general, refractive indices are being controlled within and among fibers to within a variation of only a few percent. Thua, the primary problem is the variation in the core diameter among fibera and within the aame fiber. The effect of the mismatch between either the core areas or the fiber numerical apertures is shown in Fig. 3.2. A problem exista when a source fiber with a 1.0 0.8 0.6 ~ 0.2 rn S 01 ~ 0.08 0.06 0.04 0.02 0.001 A I 1 1 I 1 1 2 4 6 8 10 12 14 PERCENT OF DIFFERENCE IN CORE DIAMETERS OR FIBERNA Fig. 3.2 Approximate losa in dB due to larger-tosmaller core diameter difference or fiber numerical aperture difference for two buttjoined optical fibers. larger core or larger numerical aperture ia joined to a aink fiber with a smaller core or a smaller numerical aperture. Furthermore, as their difference in numerical aperturea or core diameters is increased, the loss will increase. The curves in Fig. 3.2 ahow the optical power loaa in dB as a function of either the percentage difference in core diameters of larger source cores butt-joined to amaller aink corea, or source fibers with larger numerical apertures butt-joined to aink fibers with amaller numerical aperturea. These specific curves actually apply to step-index fibers but the general trenda shown are also true for graded-index fibers. A 10% mismatch in either the core diameters (larger to amaller) or the numerical aperturea (larger-to-amdler) would cauae approximately a 0.5 dB loss. For the larger multimode fibers it is not a difficult problem to maintain diameters to within 10% of each other or within 3-2 Fig. 3.3 The extrinsic leas due to end separation for atep-index fibers is shown in Fig. 3.3. The core diab 01 0.2 0.3 0.4 0.5 END SEPARATION (S/D) Variation of connector power loss with endseparation-distance-to-diameter ratio between two atep-index air-gap optical fiber ends for several valuea of numerical aperture (N.A.). meter ia indicated by D and the separation by S. The coupling leas in dB is plotted as a function of S/D. This effect is alao a function of numerical aperture. The greater the numerical aperture (NA) the greater the spreading of light from the source fiber and therefore the larger the percentage of light that will miss the core of the aink fiber. In thia figure, results are plotted for NA ranging from 0.15 to 0.50. For the fibera used in fiberoptic sensora the NA of Intereat is below 0.20 and in fact more nearly 0.15. In thia case, as can be seen in Fig. 3.3, a difference of 10% in the core diameters will produce only a couple of tentha of a dB loaa. Indeed, for NA = 0.15, an end separation of half the core diameter will produce about 0.7 dB coupling loaa. In the caae of aplicea there is no end separation therefore this losa does not occur. The effect of axial transverse (lateral) displacement of equal-diameter cores is ahown in Fig. 3.4. The fiber core diametera, D, and the transverse displacement, d, la shown. As can be seen, a 10% transverse displacement, which for ainglemode fibera can be 0.5 urn (micron) can reault in a 0.5 dB losa. When purchasing fiber, carefully apecified fiber dimensions are important, e.g., fiber outaide diameter should be maintained uniform to ~ 1% of some nominal value and corea should be concentric to within 0.5%. For an 80 urn fiber, a 1% variation in the diameter is 0.8 pm. It could lead to a 0.4 ~m transverse displacement, which for 5 ~m core could lead to d/D = 0.08 corresponding to a leas of approximately 0.4 dB. Another extrinsic effect, an axial angular
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: crated electron-hole paira that are
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

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

Delete template?

Save as template?