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PROJECTSIN FIBEROPTICS Applications Handbook

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Fh,- - trE l.--t l- l- -l H TABLEOFCONTENTS E Page Preface I 0.0 Primer in FiberOptics 3 1.0 HandlingFibers, Numerical Aperture 25 2.0 FiberAttenuation 32 3.0 Single-ModeFibers I 37 4.0 Single-ModeFibers II 43 5.0 CouplingFibers to Semiconductor Sources 47 6.0 Connectorsand Splices 54 7.0 Componentsfor Fiber Communication 62 8.0 FiberOptic CommunicationLink 70 9.0 MultimodeIntensity Sensors 75 10.0 Single-ModeInterferometric Sensors 82 References 89 EquipmentList 90

I I j PROJECTSIN FIBER OPTICS

PREEACE Projects in Fiber Optics (Newport Model #FKP)is a setof laboratoryequipment containing the hardwareneeded to complete a seriesof projectswhich will provide students, engineen and scientistswith an introductionto the hands-on E experience neededto master the basic conceptsand labora- tory techniquesof opticalfiber technology.The projects cover a wide rangeof applicationsin both communications and sensorsand cover the use of both multimode and single- mode fibers.Because this is a new and rapidly expanding technology,the educationof most engineersdoes not include coursesin fiber optics. Projects in Fiber Optics hasbeen developedby the technical staff of Newport Corporation in order to bridge the gap between current college courseoffer- ingsand today'srapidly expandingtechnology. This companionapplications handbook begins with a Primer in Fiber Opticswhich outlines,at an elementary level.the physicsand opticsbackground required to under- standthe field of fiber optics.The handbookthen givesa completedescription of each of the projectswhich are to be performedwith the equipmentin Projects in Fiber Optics. (A completelist of the projectsis given in Table I.) At the end of the handbookis a list of referenceswhich may be consideredfor classroomuse. This handbookwill serveas a TABLE I supplementto the material coveredin a comprehensive LIST OF PROJECTS classroomor independentstudy of fiber optics. Eachproject description contains a statementof pur- 1. Handlingfibers, numerical aperture poseoutlining what is to be accomplishedin that project,a 2. Fiber attenuation sectionreviewing the relatedbackground and theory,sug- 3. Single-modefibers I gestedreferences, and a completestep-by-step instruction set 4. Single-modefibers Il which will guidethe experimenterthrough the laboratory 5. Couplingfibers to semiconductorsources exercise.The materialcontained in eachproject description 6. Connectorsand splices will allow the studentto focuson "what's happening"in each 7. Componentsfor fiber communication laboratoryexercise. 8. Fiber optic communicationlink Theseprojects can be usedin structuringa courseat 9. Multimodeintensity sensors either the sophomoreor upper-divisionlevel for engineering 10. Single-modeinterferometricsensors or ptrysicalscience students. A knowledgeof ray opticsfrom higtrschoolphysics or from a collegefreshman physics course (not requiringcalculus) will be a sufficientbackground for a sophomorecourse using Projects in Fiber Optics. A simpli- fied descriptionof the propagationof modesat the levelof the Primer in Fiber Opticsmay be usedin Projects#3, #4, and #10. For an upper-divisioncourse for engineeringand physi- cal sciencemajors at a four-yearschool, the instructormay want to give a more completetheoretical development of the propagation of electromagneticwaves in optical fiber wave- guides.The backgroundfor sucha coursemight includea knowledgeof Maxwell'sequations, wave equations, propaga- tion of electromagneticwaves in dielectricmedia, and an introduction to waveguides.However, the instructor may chooseto includean introductionto someof this materialin the fiber opticscourse itself. Althoughmany of the studentsin thesecourses will be =- electricalengineering or electronictechnician students, the | intent ofthe projectsis to concentrate properties on the of the A-t opticalfibers and opticalcomponents being studiedand to keepthe amountof electricalequipment used and the J- amount of wiring done in the laboratoryto a minimum. The i problems of designingand building transmitters and receivers J for optical fiber systemsis best left to another course. n TheseProjects in Fiber Optics will provide a well- -lA grounded introduction to fiber optic technology.The techni- cal staff of Newport Corporation is availableto provide A technicaladvice in implementingthese projects. a a ACKNOWLEDGEMENTS :l The technicalstaff of NewportCorporation would like = to thank Dr. DonaldC. O'Shea,School of Physics,Georgia ) Instituteof Technology,for co-authoringthe Primer in Fiber -l= Optics.Thanks also to Dr. O'Sheaand to Dr. Rick Feinbers, Department of Physics,Western Washington University, f"or -A their commentson the instruction setsin this manual. | = ) 4 :l H

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O.I HISTORYOF FIBEROPTICS -200 The conceptof optical goes communications far back meterdistance into history.The sendingof messagesby light is certainlyas Parabolic old asthe first signalfires or smokesignals, and hascontin- diaphragm Reflector ued, in more recenthistory, in the useof signallamps for lightbeam communicationbetween ships at pat- sea.However, the first Speaker ents for an opticalcommunications system were filed in 1880. At that time, AlexanderGraham Bell obtainedpatents on the photophoneand demonstratedcommunication on a beam of light at a distanceof 200 meters.The photophone,shown in Fig.0.l, useda photosensitiveselenium cell to detectvaria- Figure 0.1. Schematic diagram of Alexander Graharn tionsin the intensityof a beam of light. However,all of these Bell's photophone. methodsmentioned here depend on the atmosphereas the transmissionmedium, and anyonewho hasever driven on a foggyday knows how unreliablethat is. A waveguidemade of a non-conductingmaterial which Light transmitslight (a dielectric),such as glassor plastic,would Source providea much more reliabletransmission medium, because it is not subjectto the variationsof the atmosphere.The guid- ing of light by a dielectricmedium is alsonot a new idea.In 1870,John Tyndall showed that light could be guidedwithin a streamof water.Tyndall's experiment is illustratedin Fig.0.2. By 1910,Hondros and Debyehad developeda theory of dielectricwaveguides. The brealthrough which has madethe optical fiber waveguidethe leadingcontender as the transmission mediumof choicefor current and future communications Stream of water systems was triggered by two events. The first was the demonstrationof the first operatinglaser in 1960.The second was a calculation,in 1966,by a pair of scientists,Charles Kao GuidedLight Ray and GeorgeA. Hockham,speculating that optical fiber waveguidescould compete with the existing coaxial cables Figure 0.2. Tyndall's experiment showing that a stream usedfor communicationsif fibers could be madethat would of water will guide a beam of light. transmit l% of the light in them over a distanceof I kilometer (km). lt is important to note that at that time the light energy which was transmittedwould be down to l% of its initial va-lueafter only 20 meters in the best existing fibers and that no materials expert was on record predicting that the required highquality transmissioncould be achieved. Many researchgroups began to activelypursue this s possibility,however. ln 1970,Corning GlassWorks investi- E gated high-silicaglasses for fibers and was the first to report a I transmissiongreater than l% over a distanceof I km. This group later increasedthe transmissionto greater than 40"/" = over I km. Today,transmissions in the range of 95-96%over I km are beingachieved. For comparison,if oceanwater had c an opticaltransmission of. -79"/o througheach km of depth, .U' one could seeto the bottom of the world'sdeepest oceans .9 E with the naked eye. The progressin high-transmissionfibers (t) is tracedin Fig.O.3. The achievementof low-losstransmission, along with the additionaladvantages of largeinformation carrying capacity,immunity from electromagneticinterference, and smallsize and weight,has created a new technology.Optical 0.01 1966

Figure 0.3. Progress in optical fiber transmission. The last two data points represent results near the theoreti- cal limits at 0.85 and 1.55 lm. 3

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fiber hasbecome the medium of choicefor communications - applications.For example,the TAf8 (Tians-AtlanticTele- phone #8)system, scheduled for completionin 1988,will a be a 6500-kmall-fiber link which will bring trans-Atlantictel- ephonecapacity to the equivalentof 20,000voice channels. I Comparethis with TATI, completedin 1955,which carried 50 voice channelsover coaxialcable. Also, Pacific Bell has - announcedplans to convertall of its cablesto opticalfiber. They plan to haveall long-distancecalls carried by fiber by 2005and plan to havefully convertedto an all-fibersystem I by 2025.Optical fiber is alsobeing usedextensively in Local a Area Networks(LAN's), which are usedfor voice or data com- t municationswithin or betweenbuildings. Many new build- ingsare now being built with fiber installedin their a frameworkfor future LAN use. Opticalfiber is alsoused in sensorapplications, where - the high sensitivity,low loss,and electromagneticinterfer- enceimmunity of the fiberscan be exploited.Optical fibers I are versatileand sensorscan be designedto detectmany physicalparameters, such as temperature, pressure, strain, and electricaland magneticfields, using either the power t transmissionproperties of multimodefibers phase or the sen- t sitiveproperties of single-modefibers. Another applicationof opticalfiber is beam delivery for medicaluses. Lasers are now being investigatedfor usein surgeryand diagnosticsand opti- t cal fiber is being usedto deliverbeams to siteswithin the humanbody.

0.2 GEOMETRICALOPTICS AND - FIBER OPTICS To understandwhat is occurring in theseprojects in = fiber optics,it is necessaryto understandsome basiccon- cepts physics. of opticsand This sectionis intendedto intro- L duce theseideas for those who may not have studiedthe field and to review theseideas for thosewho have.The ! student-experimenteris urged to seekadditional information in the referencesat the end of this handbook. i 0.2.T LIGHT AS AN ELECTROMAGNETIC FIELD 400nm 700nm The light by which we seethe world aroundus is a part I of the range,or spectrum,of electromagneticwaves that extendsfrom radio frequenciesto high power gammaradia- I tion. (Fig.0.4)These waves, a combinationof electricand Ultra-E Infrared magneticfields, which can propagatethrough - g a vacuum, a violet haveas their most distinguishingfeatures their wavelength

and frequencyof oscillation.The rangeof wavelengthsfor t visiblelight is from about 400 nanometers(nm) to about 700 nm. (A nanometeris one billionth of a meter.)In most of the - work done in the field of fiber optics,the most usefulsources Figure 0.4. The electromagnetic energy $pectrum. of electromagneticradiation emit just outsidethe visiblein I the near infraredwith wavelengthsin the vicinity of 800 to I 1500nm.

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It can be difficult to follow what happensin an optical E if the progressof light through the systemis fiber system depicted in terms of the wave motion of the light. For the sim- plest casesit is easierto think of light traveling as a seriesof rays propagating through space.The experience of seeingthe Esun'srays streaming through the cloudson a partially cloudy day provides a familiar example of light as a collection of tr rays. l- In a vacuum, light travels at approximately 3x108 metersper second.In materialmedia, such as air or water of glass,the speedis reduced.For air, the reduction is very tr small;for water,the reductionis about 25%;in glass,the E reductioncan vary from 30% to nearly 50%. 0.2.2 LIGHT IN MATERIALS In most cases,the resultsof the interaction of an elec- tromagnetic wave with a material medium can be expressed E in terms of a single number, the index of refraction of the medium. The refractive index is the ratio of the speedof light in a vacuum,c, to the speedof light in the medium,v, n = c/v. (0-l) Sincethe speedof light in a medium is alwaysless than it is in a vacuum, the refractive index is always greater than one. In air, the value is very closeto one; in water, it is about 4/3 (n = 1.33);in glasses,it variesfrom about 1.44to about1.9. There are somequalifications to the simplepicture pre- sented here. First, the refractive index varies with the wave- length of the light. This is called wavelength dispersion, which will be discussedin Project #8. Second,not only can the mediumslow down the light, but it can alsoabsorb some of the light asit passesthrough. DecreasingIncreasing Increasing Density RefractiveIndex ln a homogeneousmedium, that is,one in which the (7\, Temperatuie refractive index is constant in space,light travels in a straight Flzl-r t i i line. Only when the light meetsa variationor a discontinuity d-o

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object that would normally be absorbedby the road is bent l- toward the observer.The result is that someonelooking down J the road will seea reflection, called a mirage, of a distant = object on the road, as if it were reflected in a pool of water. This gradual bending of light by a refractive index gradient J = is used in fiber optics to increasethe information carrying capacity of fibers and to provide a very compact lens for - fiber optic systems. = lf the change in refractive index is not gradual,as in the I caseof the refractive index gradient, but is, instead,an abrupt l- change like that between glassand air, the direction of light is Figure 0.6. Geometry of reflection and refraction. governed by the Laws of GeometricalOptics. If the angle of - I incidence,0,, of a ray is anglebetween an.incident ray and a -1 line perpendicularto the interfaceat the point where the - light ray strikesthe interface(Fig.0.6), then: -'J = l. The angleof reflection,0,, also measured with .1 respectto the same perpendicular,is equal to the angle of = incidence: .J 0, = 0i (0-2) E 2. The angle of the transmitted light is given by the F relation: e n, sin (0) = ni sin (0r) (0'3) t The first of these relations is known as the Law of Reflection; the secondas the Law of Refraction or -/ Snell's Law. =

It is usefulto refer to a material whose refractive index -i is greaterthan anotheras being opticallydenser and one = whose refractive index is lessas being rarer. Thus, light travel- / ling into an optically denser medium would be bent toward r= the normal,while light enteringan opticallyrarer medium would be bent away from the normal.In Fig.0.7, a seriesof l- rays in a densemedium are incident on an interface at differ- ent anglesof incidence. Ray #l is refracted at the interface to - I a rarer medium according to Snell'sLaw Ray #2 is incident at an angle such that the refracted angle is 90o. Ray #3 is inci- -- h dent at an evenlarger angle. lf the angleof incidenceof Ray #3 in insertedinto Snell'sLaw the sineof the angleof trans- -- missionwill be foundto be greaterthan one! This can not I happen.Instead, all of the light is reflectedback into the inci- -< dent medium.There is no light transmittedinto the second = medium. The light is said to be totally internally reflected. I Figure 0.7. Geometry of total internal reflection. Ray #l For all anglesof incidence greater than a critical angle, total E is at less than the critical angle. Ray #2 is at the critical internal reflection will occur.This critical angle occurs at the angle incidenceat which the transmittedray is .1 angle. Ray #3 is totally internally reflected. of = refracted along the surfaceof the interface (the caseillus- -a trated by Ray #2). Setting the angle of transmissionequal to h 90o,the criticalangle, 0",it, is found from Eq. 0-3to be:

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o;td In the ray picture, the concept of total internal reflec- tion makes the interface look like a perfect mirror. If this proc- essis examined in terms of wave propagation,theory predicts and experiment confirms that a weak electromagneticfield exists in the rarer medium, but it decaysrapidly with distance from the interface and no light energy is transmitted to the rarer medium. This field is called an evanescent field. How- ever,iI another optically densematerial were located very close to the material in which the total internal reflection were occurring (within a wavelength or so)some of the light energ5rcould be coupled out of the first medium acrossthe small gap and into the seconddense medium. This processis called fnrstrated total internal reflection since the usual reflection is frustratedby the location of the material next to the interface. Frustratedtotal internal reflection is responsible lor the operation of a component in fiber optic systemscalled a bidirectional coupler.This component is studied in Project f7 and used in subsequentprojects.

O23 UGIITIN OPTICAL FIBERS Once you understandtotal internal reflection, you undentand the illuminated stream shown in Fig 0.2 of Sec. dor O.f . Light traveling through the water is reflected off of the surface of the water-air interface and trapped inside the stream.The same thing will happen to a glassrod or thread. Optical fibers are a little more complicated than this, however. If one were to use a fiber consistingof only a single strand of glassor plastic, light could be lost at any point where the fiber touched a surface for support. Thus, the amount of light that could be transmitted would be depen- dent on the methods used for holding the fiber. The output of the fiber would also be affectedby any movement of the fiber during its use.To eliminate these problems,the central light- carrying portion of the fiber, called the core, is surroundedby a cylindrical region, called the cladding (Fig 0.8). The clad- ding is then coveredwith a protective plastic jacket. Sincethe refractive index difference between the core and the cladding is lessthan in the caseof a core in air, the critical angle is much bigger for the clad fiber. The index of the cladding, n",, is still lessttnn the index of the core, n"o,",because total inter- nal reflection will occur only when n"o,")n",.Looking at a clcrg€oction of the fiber in Fig.0.8, one seesthat the cone of ray$ that will be acceptedby the fiber is determined by the trerence between the refractive indices of the core and clad- dng. The fractional refractive index difference is given by A = (n"o,"-nd)/n""," (0-5) Figure 0.8. Step-index fiber. The refractive index profile is shown at the right. The geometry for derivation of the Becausethe refractive index of the core is a constant numerical aperture is given. and the index changesabruptly at the core-claddinginter- face,the type of fiber in Fig. 0.8 is called a step.index fiber.

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-a, ..""dsd r d The definition of the critical angle can be usedto find the size of the cone of light that will be acceptedby an optical j fiber with a fractionalindex difference,A. In Fig.0.8 a ray is drawn that is incident on the core-claddinginterface at the critical angle.If the coneangle is 4, then by Snell'sLaq ni sin 0. = n"o,"sin 0, = ["o,"sin (900-0.,') = Il"o," CoS (0.,1) = = n""."/ l-rin1o;.; d From Eq. 0-4, sin (0*'J = n.1/n"o,",so

n,sinQ= (0-6) The numerical aperture, NA, is a measureof how d much light can be collected by an optical system,whether it is an optical fiber or a microscopeobjective lens or a photo- graphic lens. It is the product of the refractive index of the 1 incidentmedium and the sineof the maximum ray angle. d NA = tri sin (0."J (0-7) In most cases,the light is incidentfrom air and n' = l. In this case,the numerical aperture of a step-indexfiber is, from 4 Eqs.0-6 and 0-7, e NA= (0-8)

When A((1, Eq. 0-8can be approximatedby NA= 4e, It€"-JF*4 = n"o,"/2a (0-e)

in which A((l is referredto as the I The condition weakly-guiding approximation. The NA of a fiber will be measuredin Proiect #1. c In Fig.O.9, two rays are shown. One, the axlal ray, travels along the axis of the fiber; the other, the marginal path near the critical angle for the core- 4 ray, travels along a cladding interface and is the highest-angleray which will be e propagatedby the fiber. At the point where the marginal ray hits the interface the ray has traveled a distanceLr, while the . axial ray has traveled a distanceL'. From the geometry, it can 4 be seenthat sino = ["1/It"o,"=Lt/Lz (0-10) 4 The length L, is a factor I\1/n"0."larger than L' in the Figure 0.9. The geometry for derivation of the differen- 1 caseshown in the figure. For any length of fiber L the addi- -J tial delay of step.index fiber. a tional distancetraveled by a marginal ray is dL = (n"o," - n",) L/n",. (0-ll) = F,a < 7f-/ J

4,4 ) gl _d, Eq. 0-11can be simplified to 6L : LA. The additional time it takes light to travel along this marginal ray is 6t = 6L/v = LA n"o,"/c. (0-12) Therefore,a pulse with a length t representingone bit of information will be lengthened to t + dt. This differential time between axial and marginal rays will causea pulse to smear and thereby limit the number of pulsesper secondthat could be sent through a fiber and distinguishedat the far end' In such a case,the systemmay be limited not by how fast the sourcecan be turned on and off or by the speedof response of the detector,but by the differential time delay of the fiber. This smearingof pulsescan be remediedthrough the useof graded-indexor single-modefibers. Earlier, it was noted that light rays can be deflectedby variations in the refractive index of a medium as well as by encountering an abrupt interface between two indices.There are a number of methods of creating controlled index gradi' ents.Some involve introducing impurities into thin layersof glassas they are laid down on a substrate.This is not a contin- uous processsince the refractive index within each layer is nearly constant.The resulting variation of refractive index in a fiber resemblesthat of a seriesof concentric tree rings rather than a smoothchange in the index.Other techniques involve the removal of material from the baseglass by some type of chemical method. Fiberswhose cores have such an index gradient are called graded.index fibers' In our discus- sion, we will not make a distinction between these processes, but insteadassume that the graded'index profiles are smooth and exactly conform to theory. In real graded-indexoptical fibers this may not be correct and such departuresfrom the ideal gradientwill affecttheir performance. Once the refractive index gradient can be controlled through manufacturing processes,it is up to the designer of optical fibers to determine the most useful refractive index profiles, n(r), the variation of index with radial distancein the core. This usually fits a power law profile given by n2(r)= n"'[ - 2L(r/afl' (0-13) where rg is the index of refraction at the center of the core, and A is the fractional index difference defined earlier in Eq. G5, but with nonow substituting for n"o,".The parameter, o. is the exponent of the power law and determinesthe shapeof the graded-indexprofile. When a = oo, th€profile is that of a stepindex fiber. For a = 2, the profile is parabolic (Flg.O.l0). This is the profile found in most telecommunica- Figure 0.10. Graded-index fiber. The refractive index tions gnded-index fibers,because this profile eliminates the profile is shown at the right. Diverging rays are refo' differential time delay between axial and marginal rays.The cused at a point further down the fiber. numerical aperture of a graded-indexfiber is the same as that ol a step-indexfiber only for rays entering on the fiber axis. For rays entering at other points on the core, the local numerical aperture is lessbecause the local index, n(r), must be used in Eq. 0-8. In the caseof the parabolic graded-index fiber, the total amount of light which can be collected is one half of that which can be collected by a step-indexfiber with the same A. r ) 4d f Without plunging into the mathematicsneeded to la prove that graded-indexfibers with a parabolic profile remove l'' differential delay, it is possibleto get a qualitative feel of why the smearingof light pulsesin sucha fiber would be reduced. l Insteadof the rays bouncing off of the core-claddinginterface I as in step-indexfibers, rays follow a gently curving path in graded-indexfibers. In those with a parabolic profile, this path t/l is sinusoidal.That is,the path can be describedas a sinefunc- tion in space.It would seem that light paths which have a l I large radial amplitude are still longer than the path down the axis of the fiber. But becauseof the refractive index gradient, l the velocity of the light at the center of the fiber is smaller than the velocityof the light near the edgeof the core. : Although the light that travels near the edge of the fiber has I to go farther, it travels faster and arrives at the end of the l fiber at the same time as the light travelling down the center 'J of the fiber.If the lengthof the fiber is L and the speedof light down the centerof the fiber is v = c/no,then the time for a -l pulse to travel to the end of the fiber is t = L/v = n"L/c, For l light travelling a sinusoidalpath, the length traveled will be IJ L" and the time to travelto the end of the fiber ist=n(r,z)L" / c. The product of the geometrical path and the refractive l index is calledthe opticalpath length.If the opticalpath a length, n(r)L, is the same for all paths,there will be no differ- J ential delay in the time the rays take to travel through a fiber. ,' For all optical path lengths to be equal, the profile must be l parabolic(a =2\ J l 0.2.4GRADED.INDEX LENSES Y-J One thing to note in Fig.O.lO is that a fan of rays (a) injectedat a point in a graded-indexfiber spreadsout and - , then recrossesthe axis at a commonpoint just asrays from a :J smallobject are reimagedby a lens.The distanceit takesfor a H ray to traverseone full sinepath is calledthe pitch of the fiber. The pitch :/ lengthof the is determinedby A, the fractional I index difference. lf a parabolicgraded-index fiber is cut to a length of one 3a quarterof the pitch of the fiber,it can serveas an extremely compactlens (sometimes called a GRIN lens, for GRaded- 7 INdex)for fiber applications(Fig.0.ll). By positioningthe - output of a fiber at the faceof the short fiber length,light a (b) from the lenswillbe collimated,just as divdrginglight at the = focalpoint of a lensis collimated.Because its propertiesare ) Figure nl 0.1l. Graded.index (GRIN) lens. (a) 0.25 pitch lens. setby its length,this graded-indexlens is referredto as a r= O) 0.29 pitch lens. quarter-pitch or 0.25 pitch lens. -J ) t-

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.J n 10 d In some cases,it is not collimation of light which is requfue4 but focusingofthe fiber output onto a small detec- tor or focusingof the output of a sourceonto the core of a fiber. The easiestway of accomplishingthis is to increasethe length of the GRIN lens slightly to 0.29 of a pitch (Fig.0.1l). This enables the fiber optic system designer to move the sourceback from the lens and have the transmitted light refo- cus at some point beyond the lens.This is particularly useful for coupling sources to fibers and fibers to detectors. Both 0.25 and 0.29 pitch GRINlenses will be used in Projects #5, ?r819,and lO. O.3 WAVE OPTICSAND MODESIN OPTICAT FTBERS Although the ray picture of light propagation through a fter is easy to depict, it does not reveal some of the interest- ing pmpertles of light in optical fibers,particularly in those fters where the core size is on the order of the wavelength of ltght Gt.T WAVEFIELDS IN A FIBER The laws governing the propagation of light in optical fterr are Maxwell's equations,the same laws that describe the pmpagation of light in a vacuum or any medium. When inlormation about the material constants, such as the refrac- tive indices,and the boundary conditions for the cylindrical geometry of core and cladding is incorporated into the equa- tiong therymay be combined to produce a wave equation that can be solved for those electromagneticfield distributions that will propagatethrough the fiber. These allowed distribu- tions of the electromagneticfield acrossthe fiber are referred to as the modes of the fiber. They are similar to the modes found in microwave cavities and laser cavities.When the number of allowed modesbecomes large, as is the casewith largediameter-core fibers, the ray picture we have used gives an a@uate description of light propagation in fibers. The description of the modesthat propagatein a fiber b bund by solving the wave equation in cylindrical coordi- n't6 br the electric field of the light in the fiber. The cylin- Figure 0.12. Coordinate system for modes in an optical &ical oordinate systemfor a fiber is illustrated in Fig.0.l2. fiber. The rolutions, which are found to be harmonic (consistingof ire and cosine functions)in spaceand time. are of the form E(r,O,z)= f(r) cos(at-Bz + y) cos(qO), (0-14) where cois the frequency of light in radians/sec(a =2rw, wherc v is the linear frequency in Hertz, cycles/sec),B is the propagadon conatant, expressedin radians per unit dis- tancg y is a phaseconstant to provide the correct amplitude at time t = 0 and position z = 0, and q is an integer.The - J -t

parameter,B, is important for specifyinghow light propagates J in a fiber.In the ray opticsdescription, B is the projectionof the propagationvector on the z axis,where the magnitudeof J the propagationvector is k = 2nnl1",l" being the wavelength of light in vacuum.It is important to make the distinction J between the magrrittrdeof thE propaga-tionvEctor, k, and the propagationConstant, B, which is the z-componen\of the propagation J -- ' vector,in order to avoidlater confusiein. Sbtutionrfgr f- f(r), and q are obtained by substituting Eq.0-f 4 into the waveequation. The solutionswill depend J -r- | on the particularfiber geometryand index profile,including both the core and the cladding,under consideration.The n step-indexprofile is one of the few refractiveindex profilesfor /'1 /) which exactsolutions may be obtained.For this casethe solu- - ,/_ tions for f(r)are Besselfunctions. (Many people are unfamiliar

l'': ' I' with Besselfunctions, since most rarely get beyondtrigono- . .,) ,-; -l metric functionsor, perhaps,hyperbolic functions. While trig )' functionsare familiar from high schoolmathematics, it is (^ I hard to find a sinewave in everydaylife. The motion of a gui- tar string is fairly sinusoidalbut hardto seebecause of the high frequencyof vibration.However, Bessel functions are easilyfound. All that is requiredis a surfacethat can move a freely and a cylindricalboundary to that surface.Anyone's e morning cup of coffeewill suffice.Just tap the sideof the cup and watch the circularwaves generated on the surface:Bes- f sel functions!) quantity which modesof An important in determining = an electromagneticfield will be supportedby a fiber is a lokt parameter called the characteristic waveguide parameter or the normalized wavenumber, or, simply, the V-number = of the fiber.It is written as V : kr'a'NA, (0-15) =

where k, is the freespace wavenumber, 2nl1", a is 5 I the radiusof the core,and NA is the numericalaperture of the fiber. - When the propagationconstants (B's) of the fiber modes are plottedas a functionof the V-number(remember, each V-numberrepresents a particularwavenumber-core diame- ter-NAproduct), it is easyto determinethe number of modes that can propagatein a particularfiber. In Fig. 0.13, sucha a no(1- A)kf plot is givenfor someof the lowestorder modes.The number of propagatingmodes is determinedby the numberof curves V-number that crossa verticalline drawn at the V-numberof the fiber. Notethat for fiberswith V(2.405, only a singlemode will - Figure 0.13. I-ow order modes of an optical fiber. Plot of propagatein the fiber.This is the single-moderegion. The the propagation constant in a fiber (B)as a function of = wavelengthat which V 2.405is calledthe cut-off wave- - V-number of a fiber. Each V-number represents a differ- length, denotedby 1",because, for a particularproduct of ent fiber configuration or a different wavelength of light core diameterand NA, asthe wavelengthof radiationis in a given fiber configuration. [after D.B. Keck in Funda- increased,this is the wavelengthat which all higher-order E mentals of Optical Fiber Communication, 2nd Edition, modesare cut off and only a singlemode will propagatein - M. K. Barnoski, ed. see references, Copyright @ the fiber.A fiber which propagatesonly the HE" mode is said rl Academic Press, l98ll to be a single-mode fiber. For example,the NewportF-SV fiber hasa core diameterof 4 um and an NA of 0.11.Accord- = ing to Eq. 0-14,this fiber hasa V-numberof 2.19for 633 nm light, putting it well insidethe single-moderegion. An experi- E! a ment with this fiber will be done in Project #3.

I

t

=

-

12 2 g In the weakly-guiding approximation (A<node,so n > L For the azimuthalnodes, m > 0. The lowestorder HE,, mode consistsof two LPo,modes with polarizations at right anglesto one another. Fig. O.l5 showsthe propagation con- dants of these modes as a function of V-number.(Compare &% this figurewith the exactsolutions in Fig. 0.13.) When the V number is greaterthan 2.405(the value at which the fint zero of the zero-orderBessel function occurs), the next linearly-polarizedmode, LP,,, can be supportedby try the fiber,so that both the LPo,and LP,,modes will propagate. For a fiber with a V-numberof 3.832(corresponding to the LPoz LPzr first zero of the first-orderBessel function), two more linearly- Figure 0.14. Irradiance patterns for some low order lin- polarizedmodes can propagate:the LP, and the LPo,modes. early polarized modes. By changingthe positionand angleof the input beam inci- dent on a low-Vnumber multimode fiber, individual linearly- polarizedmodes can be launchedin the fiber and observedat the output.The propagationof individualmodes in sucha IoKt fiber will be observedin Project #4. This will help overcome one of the difficultiesof the conceptof modesin opticalfibers, which is understandingwhat they are and how they differ hom one another.

0.3.2 MODLS IN MULTIMODE FIBERS The multimodefibers used for telecommunications may havea = 25 Fm and NA = 0.20or a = 50 Fm and NA = 0.30,so that for 633nm light, the V-numberwill be about 50 or 150,respectively. This meansthat a largenumber of modes will be supportedby the fiber. The amount of light carried by each mode will be determined by the input, or no('l-a)kr launch,conditions. For example,if the angularspread of the rays from the source is greater than the angular spreadthat can be acceptedby the fiber (the NA of the input radiation is V-number than the NA of the fiber)and the radius the input Sfeater of Figure 0.15. Iow order linearly polarized modes of an beam is greaterthan the radius fiber, core of the then the optical fiber. Compare with Figure 0.13. fiber is said to be overfilled (Fig. 0.f 6 (a)).That is, some of the light which the sourcewill be putting into the fiber cannot be propagatedby the fiber. Conversely,when the input beam NA is lessthan the fiber NA and the input beam radiusis less

13

"f, ) J -l j than that of the fiber,the fiber is underfilled (Fig.0.l6 (b)) J J and only low-ordermodes (low-angle rays in the ray picture) -t will be excitedin the fiber.These two distributionswill yield J differentmeasured attenuations, with the overfilledcase hav- -{J i ing a higher lossthan the underfilledcase. In the ray picture, J the higher-orderrays will spendmore of the time near the 4 core-claddinginterface and will havemore of their evanes- I cent field extendinginto the fiber cladding,resulting in -tJ higherattenuation. Also, if the fiber undergoesbending, the I raysat high anglesto the fiber axis may no longer satisfythe - critical anglecondition and not be totally internallyreflected. J Sincepower from thesemodes will radiateinto the cladding x and increasethe attenuation,they are referredto as radia- a ) tion modes. There is anotherclass of modescalled leaky a modes. Thesemodes have part of their electromagnetic energydistribution inside the core and part of their energy I distributionin the cladding,but none of their energydistribu- J I tion is actuallyat the core-claddinginterface. The energyin 2 the core "leaks" into the claddingby a processknown, from - quantummechanics, as tunneling. Leaky modes are not true : Figure 0.16. Launching conditions in a multimode opti- guidedmodes, but may not be fully attenuateduntil the light '- cal fiber. (a) Overfilled. (b) Underfilled. hastraveled long distances. After light hasbeen launched into a fiber and hasprop- f 4 agateda considerabledistance (which may be severalkilome- ters),a distribution powerwithin the of the fiber ) of core ! developsthat is essentiallyindependent of further propaga- - tion distance.This is calleda stable mode distribution. To l generatean approximationof a stablemode distributionthat E will not be sensitiveto smallbends and twistsin the fiber ori- I entation,even with only a shortlength of fiber,a technique Y calledmode filtering is used.Mode filtering may be accom- I plishedthrough the useof mode scrambling. Modescram- = blingis doneby bendingthe fiberin a seriesof corrugations, as shownin Fig. 0.17.The effectof thesebends is to couple l- out the lightin the radiationand leakymodes and a portion Figure 0.17.Mode scrambler for optical fibers. The of the light in the higher-orderallowed modes and distribute ) bends tend to couple out higher-order and radiation l- the remaininglight amongthe guidedmodes of the fiber,pro- modes and to distribute the light into a distribution ducingan approximationof the stablemode distribution. -i of modes that will remain stable over long distances. l- Modescrambling permits repeatable, accurate measurements of fiber attenuationto be madein the laboratory,even with ts short lengthsof fiber.It will be usedin severalof the projects in thismanual. l- 0.3.3 POLARIZATION OF WAVES - The electromagneticfield is a vector quantity.Both the = electricand magneticfield componentsare vectorsat right anglesto eachother and both are,in most cases,mutually E perpendicularto the propagationvector of the light, asshown in Fig. 0.18. Oncethe propagationvector whose z-compo- Y nent is B and whosedirection along the light ray is known, information aboutthe electricfield is all that is requiredto I 2r fully definethe field in the medium (the magneticfield can F{ \ then be determinedfrom this information).The directionof { the electricfield determinesthe polarizationof the wave. = Figure 0.18. Components of an electromagnetic field. J T

I

I r

T

rrl t4 4 I In many light sources,the polarizationof the light var- ies in a randommanner and thesesources are saidto be randomly polarized. Other sources,such as the output of many lasers,are linearly polarized.When light is linearly polarized, the electricfield vectormaintains a constantori- entationin space,as depicted in Fig. 0.19a.Since the light field is a vector,it can be resolvedinto its componentsalong two perpendicularaxes. If there is a time lag betweenthe two components,which can be translatedinto a phasedelay, then other formsof polarizationare created.For example,if the time differencebetween two orthogonalpolarizations is l,/4 of a cycle (whichcorresponds Io 1/4 ol a wavelength),the phasedifference between the two componentsis 90o.The electricfield vectorof the waveis the resultantof the two components,and the electricfield vectortraces out an ellipse in space(Fig. 0.19b). For this reason,it is calledelliptically Figure 0.19. Forms of polarization of light. (a) Linear polarized light. As a specialcase, if the the two components polarization. (b) Elliptical polarization. (c) Circular are equaland out of phaseby 90o,the wavewould be circu- polarization. larly polarized asshown in Fig. 0.19c. In the caseof optical fibersthe polarizationof light transmittedthrough them may be preservedor it may be scrambledto yield randomlypolar- izedlight. depending on the fiberthat isused. Whenlight interacts with a material,the electronsin NoLisht I the materialare set in motion.While most of the light istrans- ScatteredAlong I mittedby the medium,a smallportion of the lightis scattered PolarizationDirectionI by the electronsand by defectsin the medium.In randomly polarizedlight, the lightis scattered in all directions.When the lightin the mediumis linearly polarized, however, there is littlelight scattered along the directionof the polarization vector.Most of the lightis scattered in or nearthe planeper- pendicularto the polarizationdirection, as shown in F Fig. 0.20.This means that if we sendlinearly polarized light LightStrongly Scatteredr to througha mediumthat changes the polarizationdirection, PolarizationDirection but doesnot scrambleit, we canfollow the orientationof the polarizationthrough the medium(Fig.0.2l). Thisis doneas part of Project #4. PolarizationDirection In a perfectlysymmetric, circular fiber, the two polar- Figure 0.20. Scattering of light by a linearly-polarized izedcomponents of the HE,,mode (the LP,,, modes with field (dipole). Light is scattered strongly at right angles orthogonalpolarizations) travel at the samevelocity, since to the dipole direction, but is not scattered at all along thel' haveidentical propagation constants. lf the fiber is not the direction of the electric field vector. perfectlysymmetric, then the fiber will be birefringent, sincethe two polarizationcomponents have different propa- gationconstants. For example,fibers with ellipticalcores will createa birefringencein which the slowand fast axes are alongthe majorand minor axesof the ellipse,respectively. Thisellipticity can be eitheraccidental, due to errorsin man- ufacture,or intentional,as part of the fiberdesign. If the bire- fringenceis to be controlled,it is mostoften createdby buildingstress regions into the fiber,as shown in the "bow-tie" birefringent fiber illustratedin Fig. 0.22. Here the slow axis is parallelto the high stressaxis of the bow tie (parallelto the bow tie) and the fastaxis is perpendicularto the high stressaxis. If lightis launchedwith a linearcomponent along each of the opticalaxes, the differencein propagationconstants causesthe resultantvector of the two polarizationsto vary

l5

a .4 ''11 L

the two com- periodically with distancealong the fiber' When F light is linearlv polarized'As the ;;;;;i;ar! in phase,the oYt phase'the -J iigiiipilp"g"tes and the componentsC9 9f = back to lin- p6turiruiion state goesfrom linear to elliptical and two polarization I i"i phasedifference of l80o' When the by launching lin- = .o*po-n*tt", " are made equal in amplitud.e polariza- pof"tit"d light at 45o to the opticalaxes' the ,J to t= "".fVti*"piogt"ttes from linear, to elliptical' to circular' is at an angle of ;Iffi.;i and back to linear in a plane that J This - giFi" ttt" plane of the original linear polarization' continues along r"qt"tt." alternating polarization states I "f Lo'over which the >r in" length of tne tiuer' The distance' { "niit" is known asthe ritut", through an entire.360o -! ;;l*it;i"" defocusingand i""i f""gtft of the fiber. (iust as the.alternate T to a pitch length' I ;"f";G; by a graded-indexfiber gives rise The beat length is ) a birefrinlent fiber causesa beat length) = bY = related to-the birefringence' dn D'to*-Iltu't' (0-16) 2 Lo = 2n/6[], - observedvisu- where 6B=2rbn/)'.'This beat lengthcan be into the ) ;iffi*.light from a helium-neonlaser is launched - at 45o to the fiblr with itJ directionof polarizationoriented in this section' a i"ti of the fiber. As was discussedearlier e, "*it illuminatedby linearly-polarizedlight in which scatteringfrom centers Figure 0.21. Scattering of lightbl a medium .) dis' variesfromzerotoamaximumastheangleofobservation F inE airection of linear polarization changeswith direction to perpendicular - fto. along the polarization I weak scattering since the observa- fiber' the tance. W represents "uii"tto it. Thus, as [g[t progressesthrough a birefringent J direction; S f iio" ait is along the polarization at right angleswill vary with the to amount of light scatteredout "tiinstrong scattering since it is perpendicular of a polariza- I represents tiut" of polaiization at each point' In the c-ase J direction' described I the polarization iion-pt"t"tuing fiber with thi launch condition and back ltt" polirization goesfrom linear to circular in "U".[, fnit'it slightly different from what was illustrated I rotatinglinear polar- "g"i".fig.0.if , since-tnaiwas for the caseof is linear' ization. However,at points where the polarization I (dependingon the ine scatteredlight is weaker or stronger polarization ait".iio" of obiervation) than at the circular - repetifiol$istances for scattered I ;;t-id;;. By measuringthe the fiber can be determined' iig":it"ii"iil,", the beailengttrof I Tiris will be done as part of Project #4' light is launchedwith the When linearly-polarized i fast or slow axis of a I polarization'hiil;l;;i;i;c"n"" vector parallel to either the til"t, theoutput polarization will stillbe "polarization- I iinluifvpor"tized despite fiber bending'.Th.is Stress pi"t"tuing" fiber provides reduced sensitivity to environmen- this will not Regions iui"f""tt]for othir launch conditions' however' the polarization U" t.u". fntt"ad, the fiber will act to change polarized light will of the light; the actual effect on the input the beat length' and U" a"t"t"*in"d by-Polarization-preserving the launch conditions' have applica- ine fiUer length. fibers light must tions wherever the polarization of the transmitted include fiber U" rtuUt" and well-defined' These application polarization'preserving fiber gyro- Figure 0.22. Cross section of a - interferometric sensors(studied in Project #10)' core provide an asym' fib"er.-tt*iw Stress regions outside the scopes,and heterodyne detection systems' tftut birefringence in the fiber core' Ellip- tical iibers ".,rJ""will also have the same effect' O.3.4 COHERENCE OF WAVES The ou@ut of many lasersis highly monochromatic. Ideally,the light from the laser is a single color; in actuality, it consistsof light within a small band of wavelengths,A,l. There are a number of ways of describingthe degreeof monochro- maticity in addition to the wavelengthbandwidth, Al. It can be characterizedas a frequency bandwidth, Au.The two are related by Lv = LIc/)( orA,l = Lvc/v2. (0-17) The smaller the bandwidth, the more monochromatic the light source. Another way of describing the monochromaticity of the sourceis through its coherence length. II a sourcewere totally monochromatic, the output would consistof an elec- tromagnetic lield of constant amplitude that oscillatesfor an infinitely long time with a single output frequency and wave- length. Any deviation in the amplitude or shortening of the length of oscillation resultsin an increasein the bandwidth of the sourceor, alternatively, a decreasein the coherence bngth of the radiation. One may think of this description as baim of waveswhose lengths representthe monochromati- city of the source.The relation between the coherencelength andthe bandwidth is l-=c/Lv=)'2/A)'. (0-18) O.35 INTERFERENCEOFWAVES When two electromagnetic fields are present in the same place at the same time, their result may be added vectorially, since the fields are vectors. If two fields are linearly polarized in the same direction, the fields may be added point-by-point as scalars.If two electric fields with the 4".A"=nn, same amplitude are in phase the sum will be twice as large; if they are 180' out of phase,the fields will cancel point by point and resultant field is zero, as shown in Flg. 0.23. The phenomenon of two or more electromagnetic fields summing frfo vv to a resultant is called inter{erence. When the fields are $ve ConstructiveInterference in phase, it is called conshuctive interference, and when the fields are l80o out of phase, it is called destuctlve interfer- ence. In many optics arrangementsthe path lengths alterna- tively vary through constructive and destructive interfer- ence, producing a series of dark and light fringes. From the patterns ++= fringe and their changesas physical conditions in the interfering paths are changed,it is possible to measure extremely small changesin distances and refractive indices. In all of this we have the same polarization direction. If the VM fields have polarizations at right anglesto each other, the DestructiveInterference resultant would be the vector addition of the two fields, but no constructive or destructive interference would occur and no fringes would be visible. A fuller explanation of interfer- Figure 0.23. Illustrations of the interference of two ence and interference fringes is available in a number of wave8. elementaryoptics texts. r I ''lf One geometry that exhibits interference is the - Mach-Zehnder inter{erometer, shown in Fig.0.24' The I sourceis collimated by lens L' and the collimated beam is l divided by beamsplitter BS,.A beamsplitter is a component t that transmits a fraction of the light incident on it and reflects the rest, assumingthat there is no absorption by it. In most lr cases.the ratio of transmissionto reflection is 50/50. The split mirrors M' and M, to be recom- l beams are then redirectedby I bined at a secondbeamsplitter BSr.Their interference is seen on the screenS. If there is any changein opticalpath length l I introduced into either path, or arm, of the interferometer,the fringe patternwill changein a mannerthat will, in most l cases,enable an observer to measurethat change.In Project #10, a fiber optics version of the Mach-Zehnder I interferometer will be constructedand examined. In this I Figure 0.24. Mach.Zehnder interferometer. L: collimat' device,the mirrors will be replacedby optical fibers and l ing lens; BS: beamsplitter; M: mirror. beamsplitterBS, will be replacedwith a componentcalled a I bidirectional coupler. J I 0.4 TRANSMITTINGPOWER THROUGH OPTICAL FIBERS Il ! 0.4.T IJOSSESIN FIBERS I : In all of the above discussion,it has been assumedthat light travels down the fiber without any lossesbeyond the Il thosefrom radiationand leaky modesand somehigher'order modesthat are coupledout into the cladding. j When light is transmittedthrough an absorbing I medium, the irradiance falls exponentially with the distance l of iransmission.This relation, called Beer's Law, can be I expressedas I(z) = 119;exP(-fz) (0-le) lI where I(z) is the irradiance at a distancez from a point z = 0, -I and I is the attenuation coefficient, expressedin units r reciprocal to the units of z. In some fields of physicsand l chemistry,where absorption by a material has been carefully I measured,the amount of absorption at a particular wave- can be used l length for a specific path length, such as I cm, I to measurethe concentration of the absorbing material in l a solution. I Although the absorption coefficient can be expressed in units of reciprocal length for exponential decay,in the field l I of fiber optics, as well as in most of the communications field, 'l (dB stand for the absorption is expressedin units of dB/km a decibels,tenths of a logarithmic unit). In this case,exponen' I I tial decay is expressedusing the base l0 instead of the basee I ( = 2.7182818...)as t I(z) = I(0)lQ-Fz/ro), (0-20) l i I I

I I

a I i i

7 I l8 - I where z is in kilometers and | - is now expressedin decibels per kilometer (dBlkm). Thus, a fiber of one kilometer length with an absorption coefficient of l0 dBlkm permits l(z)/l(0) = lQ-{totzto- g.l0 or l0% of the input powerto be transmitted through the fiber. Proiect #2 involves the measurementof the attenuation in an optical fiber. RayleighScattering The lossesin fibers are wavelength dependent.That is, E light of different wavelengthsintroduced into the same fiber S+ I will suffer different amounts of loss.Fig.0.25 showsthe c attenuation in dB/km of a typical optical fiber as a function of €q Infrared Absorption wavelength. c o Tail Although the exponential dependencewas described Ez for absorption losses,the same mathematicscan be used for other sourcesof lossesin fibers.Optical transmissionlosses in fibers are due to severalmechanisms. First, optical fibers are limited in the short wavelength region (towardthe visible and ultraviolet) by absorption bands of the material and by scat- 0 tering from inhomogeneitiesin the refractive index of the 0.7 0.8 1.0 1.1 1.2 1.3 1.4 fiber. These inhomogeneitiesare due to thermal fluctuations Wavelength(pm) when the fiber is in the molten state.As the fiber solidifies, Figure 0.25. Attenuation of an optical fiber as a function ttresefluctuations causerefractive index variations on a scale of wavelength. gnaller than the parabolic variation that is imposed upon gndeGindex fibers.Scattering off of the inhomogeneitiesis known as Rayleigh scattering and is proportional to l-4, wtrere I is the wavelength of the light. (Ihis samephenome- non is responsiblefor the color of the sky. The stronger scattering of light at shorter wavelengthsgives the sky its blue color.) In the long wavelength region, infrared absorption bandsof the material limit the long wavelength end of the radiation spectrum to about 1600nm. Thesetwo mechanisms are the ultimate limit for fiber losses.The highest quality fibers are sometimescharacterized by how closely they approachthe Rayleighscattering limit, which is about0.17 dB/km at 1550nm. At one time metal ions were the major sourceof absorption by impurities in optical fibers.It was the elimina- tion of these ions that produced low-lossoptical fibers.Today, the only impurity of consequencein optical fibers is water in the form of the hydroxyl ion (OH-),whose absorption bands at 950, 1250,and 1380nm dominatethe excessloss in today's fibers.They are evident in the absorption spectrum shown in Fig.0.25.

0.4.2 LIGHT SOURCES FOR OPTICAL FIBERS Although light of many wavelengthsand degreesof coherencemay be transmitted by an optical fiber, there are a number of sourcesthat are fairly convenient and efficient in coupling light into a fiber. lc =

The smallred indicatorlights that we seein smoke detectorsand electronic panel lights are light emitting diodes (LED's).The name is quitedescriptive, since these devicesare nothingmore than specialsemiconductor diodes that emit light. They are made of semiconductors,such as gal- - lium arsenide,to which smallamounts of atomicimpurities havebeen added to raisethe conductivity.The carrier of elec- trical currentis either an electronor a hole (theabsence of an electron).The materialin which electronsare the major car- - rier of current is calledn-type material and the materialin which holesare the major carrier is calledp-type.A diodeis createdwhen piecesof n-type and p-type material are con- structednext to one another,as in Fig. 0.26' The interface planebetween them is calledthe junction.When a voltageis - ipplied acrossthe diodejunction so that the diodeconducts, it emitslight which is radiationresulting from the recombina- tion of electronsand holes.This radiationis called,appropri- Diodebase ately, recombination radiation. The amount of light output - is proportionalto the number of electron-holepairs that Figure 0.26. Construction of a simple semiconductor recombinein the diodeand this is proportionalto the diode diode. Light emitting diodes and injection laser diodes current.Therefore, the opticalpower-current curve of an LED have a similar basic construction, although the actual will be a straightline. The wavelengthof the emittedradia- structure of the laser device is considerably more tion in an LED dependson the differencesbetween the ener- e complicated. giesof the electronsin the n-typematerial and holesin the p-typematerial. The bandwidthof the radiationis broad com- paredto that of lasersources' Although the construction of current injection laser - diodes (lLD's)is much more elaboratethan LED'sthe two are asbeing similar.Both in the simplestillus- shownin Fig. 0.26 F tration and in the basicprinciples of operation,these two devicesare similar.Current is injectedinto the diodeby - applyinga voltageacross the diode.However, the current densitiesare considerablygreater in an ILD than thosein an LED.Instead of electron-holepairs recombining spontane- - ously asin an LED,in an ILD this enormouscurrent flow stim- creatinga more powerful ulatesthe pairsto emit coherently, G output with a narrowerbandwidth. This processis called stimulated emission. The optical power'current curve of - the ILD is differentfrom that of the LED in that the current

must reacha thresholdvalue before lasing can occur'The out- a put then increasesrapidly in proportionto the current in excessof the thresholdcurrent. The stimulatedprocess just describedis enhancedby the surfacesof the semiconductor crystalthat serveas partially reflectingmirrors to redirect part of the laseroutput back into the junction region.These = mirrors alsocause the output of the ILD to be partially colli- mated,although diffraction of the light by the edgesof the t junction regioncauses the light to be directedinto a fan- inapea beam with a divergencewhich is typically about 15o - by 30o.The largerdivergence angle is in the directionper- pendicularto the junction plane,as shown in Fig. 0.26. ] rl

4 a

-

1

a

a

20 t il In contrast to the solid state semiconductormedium of the ILD, the lasing medium of the helium-neon (HeNe)laser is a mixhrre of helium and neon gaseswhich is excited by an electrical current, creating a light-emitting dischargesimilar to those seenin neon signs.The differencebetween the neon sign and the HeNe laser is the proportions of the gas mixture, a niurow dischargepath in the glasstube, and reflecting end mirrors, as shown in Fig. 0.27. The output wavelength of the HeNe laser is usually in the red at 633 nm, although outputs at other wavelengthsin the visible and infrared can be obtained by using different sorts of mirrors with higher reflectivities at the allowed wavelengths.The output is more Highreflection Glassenvelope 0utoutmirror highly collimated than the output of the ILD. For a typical mrrr0r (50/otransmission) HeNe laser,the beam divergenceis about I milliradian (mrad) or 0.06o. Figure 0.27. Construction of a Helium.Neon laser tube. The polarization of radiation in a fiber optics system dependson the type of sourcethat is used.Some HeNe lasers poss€ssa high degree of linear polarization; others are ran- domly polarized. Their polarization is usually determined by the details of the laser construction. The output of an LED is randomly polarized, while that of an ILD is polarized parallel to the plane of the p-n junction. The polarization of a source can be checked by observing the variation in power on a detector as a polarizer is rotated in front of the source.A lin- early polarized sourcewill show large variations in the trans- mifted power as the polarizer is rotated, while randomly polarized or circularly polarized light will show little or no variation. Separatingcircularly from randomly polarized light requiresthe use of an optical component known as a waveplate. There are other sourcesthat might be consideredfor use in fiber optics system:the sun, tungsten lamps,fluores- cent light neon lamps,electric arcs,etc. However,most of these sourcesare extended sources.That is, they have a large emissionarea comparedto the sourcesalready discussed.To introduce light from these sourcesinto a fiber requiresthat some optical systembe constructedto refocusthe sourceonto the fiber end. The larger and more divergent the source,the more difficult it is to couple light into the system.

0.4.3 COUPLINGSOURCES TO FIBERS One objective in any fiber optics systemis to insert as much power into the systemwith as little lossas possible.This allows the use of lower power sourcesin a system,reducing the costand enhancingthe reliability,since the sourcedoes not have to be operatednear its maximum rated power. Attention paid to coupling a sourceto a fiber or a fiber to other componentswill be repaidin a more reliableand cheaper system.([.osses which occur in fiber{o{iber coupling and splicing will be discussedand measuredin Project #6.)

2l

-.,.",,,rt-/ l* E

-t The direction of the radiation that is emitted from a E since sourcemust be consideredin the field of fiber optics, J that radiation has to be collected and focusedonto a fiber ,- Sourcescan range from isotropic (emitting in all direc- end. J tions)to collimated(emitting in only one direction)'In E general,the angulardistribution of the sourcecan be J expressedas b (0-22) B(g) = B" (cos0)', 0<0*"*, I where 0.", is the maximum angle from the normal at which ts the light is emittedand is determinedby the geometryof the I .J source.If m = 1 in Eq. 0-22,the sourceis calleda t< Lambertian source. Many non-lasersources closely approx- t m is very J imate Lambertian sources.For a collimated source, II sourcemay be considered large. For intermediate cases,the I The angular distribution of -- to 6e a partially collimated source' - an LED and and ILD will be measuredin Project #5' The ability of a fiber to accept radiation can be charac- terizedby its NA. We can describethe rangeof anglesinto E which a iource emitsby a similar NA' The definitionof the as easilydetermined as l- maximum angleof the sourceis not L the maximum angleof a fiber with its critical angle,since the -at light may be emittedinto a distributionof anglesthat does a not havea preciselydefined cut off' the light from the sourceis so divergent h In somecases, rr and the sourceis so large that the sourcemust be reimaged a on the fiber end faceby a short focallength lens'For such t- source,the lens is overfilled and the marginal rays,those at the edgeof the cone of light, are determinedby the sizeof the lensthlt is used.In that case,the NA of the sourceis given by = (0-23) NA"*t"na"a= nsin0, ts = r = radiusof the lensand d = image Figure 0.28. Calcutation of the NA of an extended where 0 tan-rr/d, with asshown in Fig.0.28. source. distance, = For collimatedlaser sources, the lensis usuallyunder- source'The light comesto a filled if it is placedclose to the E focusat the focalpoint of the lens.The beam then hasa diver- gence half-anglethat is approximately equal to the ratio of h Ihe beam waiit radius before the lens, ro,to the focal length of the lens.Thus, the NA of the beam is given by k NAu.u'= nsin(r"/f). (0-24) There are four parameterswhich affect the efficiency of ,l source-fibercoupling, the NAs of the sourceand the fiber and the dimensionsof the sourceand the fiber core.It is possible to showthat the productof the sourcediameter and the NA ol = the sourceis a constantno matter what the focallength of the imaginglens may be. By comparingthis valueto the product a of ttie fiber core diameter times the fiber NA, it is possibleto whether a lensmay be chosenthat can imagethe i determine !

I I

I

a t

I

I

I

I

22 I sourceonto the fiber core without overfilling the fiber. Over- filling is marked by a source NA which is larger than the fiber NA. If the diameter-NAproduct of the source is larger than that of its fiber counterpart, reducing a source NA to fit a fiber NA will not increasethe coupling since that action enlarges the diameter of the sourceimage on the fiber face. Thus, a careful considerationof the diameter-NAproducts will keep someonefrom trying to do the impossible.This same approach can be applied also to coupling between fibers of different sizesand NAs. Further details of coupling will be dis- cussedand illustrated in Project #5. 0.5 APPLICATIONS Most of the applicationsof fiber optics systemsfall into one of three categories:communications, sensors, and power distribution. In this section,each will be describedbriefly. By far the most extensiveuse of fiber optics is in the field of communications.It encompassesshort links between computersand telecommunicationsdevices, the so-called local area networks (LAN's)and longer distanceconnections that include those between the metropolitan areasof the NortheasternUnited Statesand those between America and the Europeancontinent. A fiber opticscommunication link will be constructedin Project #8. When informationis sentthrough a fiber opticssystem, it is encoded on the light wave by changing the light irradi anceas a functionof time. This processof varying the light level with time is called modulation. There are two types of modulation:analog and digital.Analog modulationconsists of changingthe light levelin a continuousmanner, while with digital modulation,the informationis encodedthrough a seriesof pulsesseparated by spaces,as shown in Fig. 0.29. Figure 0.29. Two types of signal modulation. (a) analog. The absenceor presenceof a pulseat somepoint on the (b) digital. streamof pulsesrepresents one element,or bit, of information. The performanceof a systemusing analog modulation is determined by how faithfully it reproducesthe signal and by the smallestsignal that can be transmitted,which is lim- ited by random or extraneousnoise in the system.Part of this is due to the type of detector that is usedto convert the modu- latedlight signalback into an electricalsignal and part is due to the systemitself. The ratio of the detectedsignal to the smallestsignal which can be distinguishedfrom the noiseis called the signal-to-noise ratio (SNR).This will be discussed as part of Project #8. In the caseof digital systems,the faith- ful reproduction of signal level is not required, which makes suchsystems superior in the presenceof noisesources. All that is requiredis that pulsesbe transmittedwith sufficient power for the detector and electronicsto determine the pres- enceor absenceof the pulse.Performance in digitalsystems is given in terms of the bit error rate (BER),the fraction of bits sent that are determined to be in error when compared with the original digital information.BER's of lessthan 10-e are generally required for a fiber optic digital communication link to be considereda high-quality system.

23

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Another application involves the use of optical fiber Il sensorsto measurephysical parameters. Because of their small diameter,sensors made of optical fibers can be fit into al tight geometrieswhere conventional sensorswould be too large. Also,because the fiber medium is non-conducting, Il fiber sensorscan be used in dangerouscircumstances, such as explosive atmospheres.Sensors can be used to measurephys- l I ical parameterssuch as temperature and pressureand engi- neering information such as liquid levelsand distances.Four l different types of multimode intensity sensorswill be I explored in Project #9 and in Project #10 a single mode I interferometricsensor will be constructed. Personswho have not studied fiber optics tend to think T of them as optical water hoses.But as we have seen,the I launchingconditions, the fiber NA, the mode distributionin l the fiber, and the fiber absorption and scatteringlosses all can a contribute to reducing the usefulnessof a fiber as a conductor l of optical power.There are, however,certain fields where I the transmissionof optical power by optical fibers has l proved useful. J In the field of medicine,the ability to insertoptical fibersinside small hollow tubesthat are pushedthrough small l incisionsin the body hasprovided a numberof successfulsur- I gical proceduresthat do not call for massivecutting of tissues aI and yet still provide treatment for diseasedparts of the body from the output of the optical fiber. Parallelto the power car- l rying opticalfiber there is usuallya secondtube with many ,J strandsof opticalfiber arrangedin a precisemanner that con- l duct illuminatinglight to the locationof the treatmentand ld carry an imageof the treatmentsite back to the surgeon. Many of the treatmentsare still in the experimentalphase. bal One of the most sought after products is an optical fiber-that would carry large amounts of long wavelength infrared radia- Ef tion from a carbondioxide laser.The focusedoutput of this laser makesan ideal surgicalscalpel, but in the near term lirl there are no fibers of sufficient flexibility, low cost, and low absorption at the CO, laser wavelength that this specificappli- l cationwill becomewidespread. t- Thereare a number of applicationsin the field of mate- l rial processingwhere the deliveryof laserpower to a location would be an ideal methodof operation.In dustyand difficult environments,the replacement f of multiplelens-based optical LI power delivery systemswith fiber-basedsystems is useful becauseof the reductionin down time and maintenance.Usu- f ally, the divergent output of the fiber must be refocusedby a lens to produce the required irradiance to heat treat, melt, or = vaporizean areaof the materialbeing processed. Depending on the wavelengthof the radiationbeing usedand the type of an fiber employed,there are maximum values of power that can be delivered by such systems. l 3 .l{ f JI J f f I

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J- 24

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I.O PROJECT#I: HANDLING FIBERS, N[UMERICALAPERTURE

In this first project,you will learn how to preparefiber endsfor usein the laboratory.You will be ableto observethe geometryof a fiber and you will measurethe numericalaper- ture (NA)of a telecommunications-gradefiber. The methodwhich is presentedfor determiningthe NA of a fiber is especiallyillustrative of what is to be learned. Another techniquefor measuringa fiber'sNA, which is one often usedin standardpractice, will be usedin Project #3. I.I FIBER GEOMETRY An opticalfiber is illustratedin Fig. 1.1.It consistsof a core, with refractive index n.".",of circularly-symmetriccross sectionof radiusa, and diameter2a, and a cladding,with refractive index n",,which surroundsthe core and has an outer diameter of d. Typical core diametersrange from 4-8 pm (l pm = I micrometer= 10-6m) for single-modefibers to 50- 100gm for multimodefibers used for communicationsto 200- 10fi) pm for large-corefibers used in power transmission applications.(See Section 0.5 for a discussionof theseappli- cations.)Communications-grade fibers will haved in the rangeof 125-140pm, with somesingle-mode fibers as smallas 80 pm. In high-qualitycommunications fibers, both the core Figure l.l. Geometry of an optical fiber, showing core, and the claddingare madeof silicaglass, with smallamounts cladding, and jacket. of impuritiesadded to the core to slightlyraise the index of refraction.There are alsolower-quality fibers available which havea glasscore surroundedby a plasticcladding, as well as someall-plastic fibers. The latter havevery high attenuation coefficients(Section 0.4.1) and are usedonly in applications requiringshort lengthsof fiber. Surroundingthe fiber will generallybe a protective jacket.This jacketmay be madefrom a plasticand havean outsidediameter of 500-1000prm. However, the jacketmay alsobe a very thin layer of varnishor acrylatematerial. I.2 FIBER MECHANICALPROPERTIES Beforemeasuring the NA of a fiber,it will be necessary to preparethe endsof the fiber so that light can be efficiently coupledin and out of the fiber.This is done by usinga scribe- and-breaktechnique to cleavethe fiber.A carbideor dia- Carbideor mond bladeis usedto start a smallcrack in the fiber,as Pull diamondblade Pull illustratedin Fig. 1.2. Evenlyapplied stress, applied by pull- a- -t ing the fiber,causes the crackto propagatethrough the fiber z- and cleaveit acrossa flat crosssection of the fiber perpendic- I I ular to the fiber axis. t In theory,the breakingstrength of glassfibers can be Tipof crack very large,up to about 725 kpsi (where I kpsi = 1000pounds/ propagatesasfiber ispulled sq.inch) or 5 GPa(where I Pa= I Newton,/sq.meter and 1 GPa= 1O'gPa).However, because of inhomogeneitiesand flaws,fibers do not exhibit strengthsanywhere near that Fiber

Figure 1.2. Scribe-and-break technique of fiber cleav- ing. A carbide blade makes a small scribe, or nick, in the fiber. The fiber is pulled to propagate the scribe through the fiber. ) A

value.Before being wound on a spool,a fiber is stretched l over a pair of pulleys,which apply a fixed amountof strain (stretchingper unit length).This processis calledproof-test- fJ ing. Typicalcommercial fibers may be proof-testedto about 50 kpsi (345MPa), which is equivalentto about a one pound -al load on a 725-pmOD fiber.When a crack is introduced,this is reducedeven further in the neighborhood of the crack.Frac- al ture occurswhen the stressat the tip of the crackequals the theoreticalbreaking strength, even while the averagestress l in the body of the fiber is still very low.'The crackcauses I sequentialfracturing of the atomicbonds only at the tip of the r

fn : = ) !! 26 )

3A I

agatingparallel to the z-axis.When a planewave is incident on the end faceof a fiber,then we can be surethat all of the light launchedinto the fiber hasthe sameincident angle, 0. in Fig.r.4. If the fiber end faceis then rotatedabout the point O in ( Fig. 1.4, we can then measurethe amountof light accepted by the fiber asa functionof the incidentangle, 0.. 100 Fig. 1.5 showsthe light acceptedby a NewportF-MLD fiber asa functionof acceptanceangle using the methodjust 70 described.The point where the acceptedradiation has fallen CU to a specifiedvalue is then usedto definethe maximum inci- Industries dent anglefor the acceptancecone. The Electronic 30 Associationuses the angleat which the acceptedpower has =an fallen to 5% of the peak acceptedpower asthe definitionof 6ZU the experimentallydetermined NA.3 The 5% intensitypoints o are chosenas a compromiseto reducerequirements on the = power levelwhich hasto be distinguishedfrom background Srn noise.o Notethat in Fig. 1.5, the radiationlevels were mea- suredfor both positiveand negativerotations of the fiber and OF the NA wasdetermined using one half of the full angle any = betweenthe two S%-intensitypoints. This eliminates ^o smallerrors resulting from not perfectlyaligning 0. = 0 to the .t plane wavelaser beam. The NA obtainedin this testcase was 5 0.29,which compareswell with the manufacturer'sspecifica- l- tion of NA = 0.30. I.4 REFERENCES -0.3 -0.2 -0.1 0 0.1 0.2 0.3 L: l. D. Kalish,et al., "Fiber Characterization'Mechani- Sin0 cal", in OpticalFiber Communications,S. E. Miller and A. G' tr Chynoweth,eds., Academic Press (New York) 1979'p. 406 of the datataken in the measurementof Figure 1.5.Plot "Fiber 2. D. Glogeand W. B. Gardner, DesignConsider- the NA of the NewPortF'MLD fiber. tr ations",in OpticalFiber Communiccfibns,S. E. Miller and A' , G. Chynoweth,eds., Academic Press (New York) 1979,p. 152 4.3.2,EIA' Engi- l-ilI 3. EIA StandardRS-455-47, Section neeringDept. (Washington D.C.) 1983 4. D. L. Franzenand E' M. Kim, "lnterlaboratorymea- tr comparisonto determinethe radiationangle (NA) surement of graded-indexoptical fibers", Applied Optics20, p.1220 F (le8l)

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27 l -/ I.5 PARTSLIST

Cat# Description Qty. F-MLD5O 100/140MM fiber, 50 meters XSN.22 2x2 Breadboard u-l301P I mw HeNelaser 807 Laser mount 340C Clamp 4l Short rod 815 Power meter F-CL1 Fiber cleaver . FK-BLX Balldriver set SK-25 7+20Screw kit SK48 &32 Screw kit RSX-2 Rotation stage MPH-I Micro-seriesholder MSP-1 Micro-seriespost VPH-2 Post holder SP-2 Post FP-1 Fiber positioner

Additionalequipment required: Mettrylenechloride for strippingfiLer jacket. (Many comm€rciallyavailable paint thinners contain methylene chlorideand workwell for this purpose.Methylene chloride is toxic andcan be absorbedthrough the skin.Any methylene chloridewhich gets on yourskin should be immediately washedoff.) Single-edgerazor blades may alsobe usedto li stripfiber jackets. A microscopefor viewingthe cleavedends of fibersis essential.If yo$r laboratoryis not alreadyequipped with an adequatehigh-powgqmicroscope, you maywish to consider usingthe NewporttMLl InspectionMicroscope for inspect- ing fiberends. You may alsowish to usethe F BK2fiber breakerin placeof tlre F-CLIfiber cleaver. . r.6 NSTRUCTIONSET

1.6.T PREPARING FIBER ENDS l. Remove -l-l/2 inchesof fiber jacket froma -2 meter segmentof F-MLD fiber; by dipping the fiber end in methylene chloride and letting it soak for - 3 minutes.(Some people prefer to use a single-edgerazor blade held at a low angle to do the stripping of the fiber jacket. This.requires some practice,but goesmuch faster once you are usedto it') 2. Usethe F-CLl Fiber Cleaverto cleavethe stripped end of the fiber.The cleavershould be placedon the top of the tablewith the bladepointing up.Draw the fiber over the bladewith a light motion. Be surethat the fiber is normal to the blade.You should not attemptto cut the fiber with the cleaver.You are only startinga smallnick which will propa- gatethrough the fiber when you pull it. Gently,but firmly, pull ScribeMark the fiber to cleaveit. 3. Checkthe quality of the cleaveby examiningit under a high-powermicroscope. Carefully examine the end faceof the fiber.The end faceshould appear flat and should be freeof defects,as in Fig. 1.6a.However, chips or cracks which appearnear the peripheryof the fiber are acceptableif o they do not extendinto the centralregion of the fiber.Some poorly cleavedfiber endsare illustratedin Fig. l.6b and c. The problemsassociated with the poor cleavesare discussed (a) in Step4. 4. If the inspectionof the fiber end facein Step3 does not show that the end facehas been properly cleaved, you shouldconsider the followingcommon sourcesof error: There are two principalreasons for obtaininga bad Cracksrunning cleave,1) a poor scribeand 2) a non-uniformpull of the fiber. intothe core A scribewhich is too deepmay causean irregular cleaveand may causemultiple cracks to propagatethrough the fiber (Fig.l.6b). A scribewhich is too shallowwill be the sameas no scribeat all and the fiber will break randomly. If the pull which propagatesthe crackthrough the fiber is not uniform, and especiallyif it includestwisting of the fiber,irregularities may showup on the fiber end faceor a lip may be formed on the end of the fiber,as in Fig.l.6c. If the fiber end is cleavedat an angle,the fiber was (b) probablyscribed at an angleother than 90o acrossthe fiber axis,although this, too, can be causedby a non-uniformpull of the fiber.ffhis will not be a problemif 1'ouhave chosen to usethe F-BK2Fiber Breaker,but will haveto be consideredif you are usingthe F-CL1Fiber Cleaver.See the note after Step5.) 5. Onceyou havea fiber segmentwith two wellcleaved ends,you may look at the geometryof the fiber,as it was describedin the introduction.View a fiber end asyou did in Step3. Usean incandescentlamp to illuminatethe far end of the fiber.You will be ableto seethe light shiningthrough the centralportion of the fiber.This is the fiber core.The region surroundingthe core is the fiber cladding.You will not be able to seethe fiber jacket,because you havestripped that away from the end of the fiber. (c) Figure 1.6. Cleaved fiber ends. (a) good cleave. (b) cracked fiber. (c) Side view of a lip on the end of a fiber. E

=

29

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NOTE: The F-CLI Fiber Cleaveris much more depen- I

dent on operatorskill than is the F-BK2which you may z chooseto useas an option in placeof the F-CLI. In that case, a follow the directionsthat are includedwith the F-BK2Fiber Breaker and cleavethe ends of the fiber. (Youwill still need to E usethe F-CLI in Project#4, so you may want to gain some experiencewith it at this time. ts

1.6.2 MEASURING NUMERICAL APERTURE I l. Bolt the Model 807 LaserMount to the Model340C Clamp, usingT/4-20 bolts from the SK-25Screw Kit. Placethe -h 340CClamp on the Model4l ShortRod. Mount this on the l L922 Breadboard.Place the ModelU-l30lP HeNeLaser into ) ts the 807 Mou,nt.Tighten the set screw.Do not overtighten as this will damagethe laser.Plug the laser power supply into a 1l0V wall outlet.Plug the cord from the laserhead into the , powersupply. Note that the plug from the laserhead to its power supply can only be inserted one way. The laser is E turned on at the key switch on the lront of the powersupply. The combinationof the 807 Mount and the 340CClamp will .v align the laserparallel to a line of bolt holeson the tableif the Model4l ShortRod is properly mountedon the table. J Check J the laseralignment with the line of bolt holesand adjustthe Model 41 Short Rod, if necessary. Y 2. Mount the RSX-2Rotation Stage to the breadboardso that the beam from the HeNelaser passes over the center hole of the rotation stage.The RSX-2Rotation Stagewill have l- to be placedat an angleto the line of bolt holesin order to i bolt it into placeas instructed, as shown in Fig 1.7. Mount the = Figure 1.7. Laboratory set.up for determination of fiber MPH-I Micro-SeriesPost Holder on the rotationstage. Use an NA. 8-32screw to mount the MPH-I. Thel/4-20 threadedhole F in the bottom of the MPH-I is usedonly asa through hole. Placethe MSP-IMicro.Series Post in the MPH-I, asshown l. in Fig. 1.7. 3. Prepare -2 a fiber segment, meterslong, with a Ir good cleaveat eachend face.The FPH-SFiber Holder comes as part of the FP-l Fiber Positioner.Insert one end of the fiber 2 into an FPH-SFiber Holder gou will needto havestripped at least3 " of the jacketfrom the fiber in order to do this and the followingstep) and placethis holderinto its FP-l Fiber Posi- -

F

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"rftff k tioner. which has been post-mounted on the RSX_2rotation stage,using the MPH seriespost holder and the MSp series post. 4. Extend the tip of the fiber and orient the Fp_l posi_ E tioner so that the fiber tip is at the center of rotation of the stage.This is a critical step if an accuratevalue for the fiber NA is to be obtained. 5. Re-checkthe alignment of your light_launchingsys_ tem by making sure that the tip of the fibei remains at the center of the laser beam as the stageis rotated. This set up achievesplane-wave launthing into the end of your fiber. E 6. Mount the far end of the fiber in an FpH-SFiber Holder (taken from an Fp-l Fiber positioner)and the Fp-l Fiber Fositioner.You can get a quick approximate measureof the fiber's NA with a 3x5 card placed i distance,L, away from the laser in a darkeded room, as shown in Fig. l.g. Measure l. the width, w, on the card of the spot out of th6 fiber and the dislance,L, from the fiber to the card. The NA of the fiber is approximately sin-t(Vzw/L).This is a quick method which is used when only an approximate measu.ementof a fiber's NA b needed. 7. Mount the detector head of the Model gl5 power Figure 1.8. Approximate measure of the NA of a fiber. Meter so that the output beam from the liber is incident on the detector head, as shown in Fig.l.?. Make a hood of alu- minum foil to keep stray room light off of the detector.you will find this to be necessary,because the power levels obtained in plane-wavelaunching are low. Block the laser beam and note the power measuredby the power meter. This determinesthe stray light seen by the metei. you will need to subtract this amount from all of your data. 8. Measurethe power acceptedby the fiber as a func_ tion of the incidentangle of the plane-wavelaser beam. Use both positive and negative rotation directions to compensate for any remainingerror in laser-fiber E' alignment. 9. Plot the power received by the detector as a function of the sineof the acceptanceangle. Semi-log paper is recom- mended.Measure the full width of the curve it the points F where the received power is at 5% of the maximum intensitv. The half-widthat this intensityis the experimentallydeter_ mined numerical aperture of the fiber. Compareyour results with the resultsof Step6 and Fig l.S.

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-/ - *-!- PROJECT#22 _= 2.O / ATTENUATION E FIBER :: .J :. J In this exercise,you will measureone of the most - per length' important fiber parameters,the attenuation unit The : of a multimode communications-gradeoptical fiber' I "cutback techniquedemonstrated here is called the -l method" and is generallyused for this measurement' t< that the condi- You will alio be introducedto the way I can affect -l tions under which light is launchedinto the fiber .T You learn about mode scramblingand this measurement. I g"n"tate a desirabledistribution of light in the fiber' -a iiot lo - 2.T MEASUREMENTOF OPTICAL FIBER I ATTENUATION l- of the Section 0.4.1 containeda detaileddescription ._) for the |- Iossmechanisms in optical fibers' An expression in a fiber after amount of optical power which still remains / 0-20 as it n"t ptop"iated i distance,z, was given in Eq' = -) : 10-02/to) (2-1) t- I(z) I(0) bs and ) The length of the fiber, z, is given in kilometers' b theattenuatio-ncoefficient,|,isgivenindecibelsperkilo. meter (dB/km). ) to Y decausethe designersof fiber optic systemsneed = propa- I know how much light will remain in a fiber after specifi F satinq a qiven distance,one of the most important In prin' I :;iffit;;t optical fiber is the fiber's attenuation' fiber mea- h ciple, the fiber attenuationis the easiestof all generallyused is rut"rn"ntt to make. The method which is is to I called the "cutback method."l All that is required .I- of fiber' I tuun.it power from a sourceinto a long Iength using a detec- ) *"urur" the power at the far end of the fiber / off a ra tor with a linear response'and then, after cutting by the J length of the fiber, measurethe power transmitted - length of -! ,no?t"t length. The reasonfor leaving a short sure that iino ut the input end of the systemis to make ) to the loss of the H the lossthat is measuredis due solely : fiberandnottolosswhichoccurswhenthelightsource.is illustration F .""pf"J," itte fiber. Fig. 2'f showsa,schematic shown in of the measurementryit"*' flhe mode scrambler Section 0'3'2') v the figure was discussedin f as 2.Cutback and measure the transmissionthrough the fiber is written ! outPutPower, P; Detector tI T : PrlPi, (2-2) J Coupler 1.Measure P1(final f Mode Fiber' -output where we have substitutedP, (initial power) and Scrambler Pr for Spool Power, power) for l(0) and I(z),respectively' A logarithmic result - I given by I ih" lott in decibels(dB), is I for cutback Figure 2.1. Schematic of laboratory set-up ? : -10 (2-3) -Jtnoa of determining fiber attenuation' L (dB) log (PrlPJ. ! as a The minus sign causesthe loss to be expressed I then positivenumber' Ttis allows lossesto be summed and I I ! a I ! I I T I I 32 I \ subtractedfrom an initial power when it is also expressed logarithmically.[n working with fiber optics,you will often find powersexpressed in dBm, which means"dB with respectto I mW of opticalpowerJ'Thus, e.g., 0 dBm : I mW 3 dBm : 2 mW, and -10 dBm : 100 ltW. Note that when lossesin dB are subtractedfrom powersin dBm, the result is in dBm. For example,an initial power of +3 dBm minus a loss of 3 dB resultsin a final power of 0 dBm. This is a shorthandway of saying'An initial power of 2 mW with a 50% loss resultsin a final power of I mW'l The attenuationcoefficient, I, in dB/km is found by dividing the loss,L, by the length of the fiber, z. The atten- uation coefficientis then given by

f(dB/km) : (l/z) [-10 log (PrlPt). (2-4)

The total attenuationcan then be found by multi- plf ing the attenuationcoefficient by the fiber length, giving a logarithmic result,in decibels(dB), for the fiber loss.

2.2 PRACTICAL PROBLEMS The cutback method works well for highJossfibers, with f on the order of 10 to 100dB/km. However,mean- ingful measurementson low-lossfibers are more difficult. The highest-qualityfibers will have losseswhich are on the order of 1 dB/km or less,so that cuttinga full I km from the fiber will result in a transmittedpower decreaseof less than 20%,putting greater demands on the measurement system'sresolution and accuracy. There is also an uncertainty due to the fact that the measuredloss will dependon the characteristicsof the way in which light is launchedinto the fiber.The launchcondi- tions which result in an overfilledor underfilled fiber were discussedin Section 0.3.2. When a fiber is overfilled,many high-orderand radiationmodes are launched.These modes are more highly attenuatedthan are low-ordermodes. When a fiber is underfilled,mostly low-ordermodes are launchedand lower lossesoccur. The solutionto this problem is to attempt to generate what is known as the stablemode distribution(also dis- cussedin Section 0.3.2) as quickly as possibleafter launch- ing. Fig. 2.2 comparesthe transmissioncharacteristics of the stabledistribution with those of the overfilledand underfilledlaunch conditions.The stablemode distribution may be achieved,even in a short length of fiber,by using mode scrambling(Section 0.3.2) to induce coupling be- tween the modesshortly after the the light is launched. Mode scramblinggenerates an approximationof a stabledistribution immediately after launch and allows repeatablemeasurements, which approximatethose that would be found in the field, to be made in the laboratory. Fig. 2.2 comparesthe optical power in a fiber as a function propagation for the three types of launch condi- of distance Figure 2.2. Cornparison of attenuation characteristics and stabledistribution. The tions: overfilled,underfilled, of various launch conditions. slope of the curve at large distancesis equal to the attenua-

33

J *) .,.9

: tion coefficient. It is the fact that the mode scrambling Y generates a stable distribution immediately after the source f that allows a short cutback length to be used in the cutback a method of measuring attenuation. ftt 2.3 REFERENCE : . l. D. Marcuse,Principles of Optical Fiber Measure- tY menfs,Academic Press (New York) 1981, p. 226236 ,.If 2.4 PARTSLIST a Cat# Description Qty. xsN-22 2x2 Breadboard 1 mw HeNe laser E u-I301P 807 Laser mount ..-f 340C Clamp 4l Short rod :r'.Ll 815 Power meter F-916 Fiber coupler ({o lens) M-20X 20X Objective lens 7Y F-CLI Fiber cleaver .f FK-BIJ( Balldriver set _4 SK-25 lz+-20Screw kit J VPH-2 Post holder F sP-2 Post FP-I Fiber positioner :I FM-I Mode scrambler F-MLDSOO 100/140MM fiber, 500 meters 5 Additional equipment required: Methylene chloride for stripping the jacket from the fiber. (Many commercially 1 available paint thinners contain methylene chloride and work well for this purpose. Methylene chloride is toxic and 5.a can be absorbed through the skin. Any methylene chloride 'f gets your should be washed off immediately.) which on skin .- Microscope slide cover glass for monitoring the positioning of the fiber in the F-916Fiber Coupler. f

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34

I\$- 2.5 INSTRUCTIONSET l. Prepareboth ends of the 500 meter fiber spool which has been provided, as you learned to do in Project #1 (Section 1.6.1,Steps f -3). This fiber is the Newport F-MLD-500fiber with a 100-pmcore and a 1407m OD. You may have to use some care in freeing the end of the fiber which was the start of the winding onto the spool. (Ihis end will be referred to as the far end of the fiber.) 2. Place the cleaved far end of the fiber in an FPH-S holder which has been removed from an FP-l Fiber Posi tioner and insert this into the post-mountedFP-1. Also, post mount the detector head of the Model 815 power meter. Align the detector head with the fiber end so that you will be able to measure the output power. The laboratory set-up for this project is shown in Fig. 2.3. 3. The use of the F-916Fiber Couplerto couple light from a HeNe laser into a fiber is illustrated in Fig. 2.4. Align the coupler and the HeNe laser so that the laser beam shinesalong the axis of the F-916Fiber Coupler. Mount a 20X microscopeobjective in the F-916.Place the Figure 2.3. Laboratory set-up for determining fiber deaved front end of the fiber into the fiber chuck from the attenuation using the cutback method. F-916and insert this into the coupler. Carefully align the fiber to maximize the light launchedinto the fiber, using the power meter to monitor the launchedpower. Use a microscopeslide cover glassin the path of the laser beam Beam reflected to look at the Fresnelreflection from the fiber end face. from Focusthe Fresnelreflected beam by adjustingthe z com- Fiberend ponent of the fiber position,as defined in Fig. 2.4; this is done by turning the z adjustment knob on the fiber positioner.When this reflectionis focused,the fiber end face is in the focal plane of the coupler'smicroscope objec- tive lens. 4. Positionthe FM-l Mode Scramblerat a convenient place near the launch end of the fiber, as shown in Fig. 2.3. 5. Rotatethe knob of the FM-l counter-clockwiseto Figure 2.4. Coupling of HeNe laser light into a fiber fully separate the two corrugated surfaces.The FM-l Mode using the F-916 Fiber Coupler. Scrambleris illustratedin Fig. 2.5. Placethe fiber between the two corrugated surfacesof the Mode Scrambler.Leave the fiber jacket on to protect the fragile glass fiber. Rotate the knob clockwise until the corrugated surfacesjust con- Adiustment tact the fiber. Examine the farjield distribution of the out- Knob put the fiber. Rotate the knob further clockwise and Slotwith of Corrugations notice the changesin the distributionas the amount of bending of the fiber is changed.Since a narrow collimated HeNe beam is being used to launch light into the fiber, the original launcheddistribution will be underfilled.When the distributionof the output just fills the NA of the fiber, an approximation of the stable distribution has been achieved. Do not add any more bending than is necessaryto accom- plish this, since that will result in excessloss. This launch- ing and mode scramblingset-up should not be changed

Figure 2.5. Model FM-l Mode Scrambler.

35

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again during the remainder of the exercise. It 6. Measure the power out of the far end of the fiber. I Note the exact length of the fiber. It will be part of the I information on the label of the spool. 7. Break off the fiber -2 meters after the mode I I scrambler (see Fig. 2.1) from the launching set-up. (Be sure to note on the spool how much fiber you have removld, so I I that other people using the same spool in the future will be able to obtain accurate results.)Cleave the broken end I of the fiber and measure the output from the cutback t segment. 8. Calculatethe fiber attenuation, using Eq. 2-4, and compare this with the attenuation written in the fiber speci- fication on the spool. Your value is probably somewhat I , higher than the specification.Why? ftIINT: Go back and I look at Fig.0.25. Remember,the HeNe laser operatesat a I wavelengthof 633nm.) T I I I J I l I'l

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J ! 36 e 3.0 PROJECT#3: SINGLE.MODEFIBERS I

In this project you will begin the first of two exercises which considerthe modal propertiesof fibers and the prop- erties of single-modefibers. You will learn to couple laser light into a 4-pm diameter core single-modefiber. You will then measurethe far-fieldpower distributionof the fiber as a function of angle and determine how well it fits a the- oretical model which will be describedin the following discussion.

3.T SINGLE.MODEFIBERS In the first two projects, some of the properties of multimode fibers were explored. The properties of multi- mode fibers are easily described in terms of the paths of ligbt rays propagating down the fibers. This ray picture of light propagation is adequate for describing large'core- diameter fibers with many propagating modes, but it fails for small-core-diameterfibers with only a few modes or with only a single mode. l-or fibers of this type, it is neces- sary to describe the allowed modes of propagation of light in the fibers. A detailed description of the propagation characteris' tics of an optical fiber can be obtained by solving Maxwell's equations for the cylindrical fiber waveguide. This leads to knowledge of the allowed modes which may propagate in the fiber. When the number of allowed modes is very large, the mathematicsbecomes very complex; this is when the ray picture is used to describe the waveguide properties. The solution of Maxwell's equations for the allowed modes of a fiber was outlined in Section 0.3.1. In that section, it was found that an important quan- tity in characterizing a fiber waveguide is a quantity which is called the V-numberof the fiber. RecallingEq. 0-15,it was written as

V: kr'a'NA, (3-l) where k, is the free-spacewavenumber,2r/)to (\" is the wavelength of the light in free space),a is the radius of the core, and NA is the numerical aperture of the fiber. The V-number can be used to characterize which guided modes are allowed to propagate in a particular waveguide struc' ture. When V <2.405, only a single mode, the HErl mode, may propagate in the waveguide. This is the single-mode regime. The wavelength at which V is equal to 2.405 is called the "cut-off wavelength," (denoted by tr.) because that is the wavelength at which the next higher-order mode is cut off and no longer propagates. The Newport F-SVfiber has a core diameter of 4 pm and an NA of 0.11.Therefore, according to Eq. 3-1,this fiber has a V-numberof 2.19 for 633 nm light, putting it well within the single-mode regime. This is the fiber which you will be using in this project. 3.2 GAUSSIANAPPROXIMATION In waveguidesin which the diameter of the core is extremely large compared to the wavelength of the light

37 71-:-

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(for example, the HeNe laser tube described in Section 0.4.2 and Fig. 0.27), the lowest order mode has an irradi- ance pattern which is Gaussian. That is, the irradiance as la a function of distance from the beam axis has the form 4 I(r) : I(0)exq Cr/roY, (3-2) ,- where I(0) is the irradianceat the center of the beam and ro is a measureof the radius of the beam, the radiusat h which the irradianceis l/e2 of that at the center of the beam. Fig. 3.1 showsthe irradianceof a Gaussianbeam. The HErr mode of a fiber is very closeto a Gaussian -l mode when the light is near the cut-offwavelength'l Fig' 3.2 showsthe shapeof the fundamentalHErr mode near I the cutoff of the next-higher-ordermode (that is, with V only slightly lessthan 2.405),as a function oI r/a, where r is tire riaiat position and a is the core radius.While the red line in the figure representsthe actual distributionof the mode, the black line is a Gaussian.The two curvesare Figure 3.1. Irradiance of a Gaussian beam. quite similar and the exact solution near the cut-offwave- length is often approximatedby a Gaussian.In the caseof =2.4 diameter,the 'l V a pirabolic profile fiber with an infinite core Giussian function is the exact solutiop for the fundamental -? 0.9 mode. showsthe exact modal distributionalong ts 0.8 Fig. 3.3 with thJGaussianapproximation for a longer wavelength, .E 0.7 further from cutoff. It can be seenthat the Gaussianapprox- @ EQ 0.6 imation is not as good as one gets away from the cut-off EX wavelength.However, the qualitativeshape of the exact t:E U.c Eits solution curve is still not too far from Gaussian' irb n4-" this Gaussian oZ In this proiect,you will be exploring E- Gaussian fiber. _- s 0.3 approximationfor a single-mode 0.2 3.3 COUPLINGTO A SINGLE.MODEFIBER 0.1 CouplingIight into a multimode fiber is relatively 0 easy.However, maximizing the coupling to a single-mode -2 -1.5 -1 -0.5 0 0.5 1 1.5 is much more difficult' In addition to very precise fiber - ila alignment of the fiber to the incoming beam, it is necessary to match the incident electromagneticfield distributionto Figure 3.2. Comparison of the exact mode field inten' L that of the mode which will be propagatedby the fiber' sity and its Gaussian approximation near the cut'off The mode profile of the HErr mode of a step-indexsingle- wavelength (V=2.4). mode fiber can be approximatedby a Gaussiandistribution l- y=1.8 a 1/e2 spatialhalf-width given by2 1 with b 0.9 wo : & (0'65+1.619 V-t 5 +2.879V-6), (3-3) )- 0.8 where a is the fiber core radius.For example,when 0.7 Y:2.405, the Gaussianspot size is approximately10% in this case,the L = larger than the core diameter.Therefore, o 0.6 incident light should be focusedto a spot size which is -E I ER 0.5 1.1 times the fiber core diameter at the fiber end face' _ra -(€ is a plot of the normalizedradius of the Gaussian 6tr Fig. 3.4 Lo 0.4 disiribution as a function of the V-number.It can be seen l- oZ (as\ E- 0.3 that, for a fiber of given radius,as V becomessmaller o As the wavelength .- = becomeslonger) the spot size increases. '{ 0.2 0.1 L 0 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 L ila Figure 3.3. Comparison of the exact mode field v iniensity and its Gaussian approximation further from the cut-off ruavelength (V=f .8). b

38 a 7 increases,the electromagneticfield of the mode is lesswell 5.5 confined within the waveguide.For this reason,single- mode fibers are designedso that the cutoff wavetengtnis not too far from the wavelength of the light intended for 4.5 use with the fiber. Typically,\ will be about g0-g0% of the wavelength at which the fiber is designedto be used. 3.4 REFERENCES g3 = 2.5 l. L. B. Jeunhomme, Singte-ModeFiber Optics, princi. ples and Applications, Marcel Dekker (New york) lgg3, 2 p. 16 .,Loss 1.5 2. ibid, p. 18, or D. Marcuse, analysisof single_ --- mode fiber splices,"Bell System Technical Jouinal 56. 70J (1e77) 0.5 1 1.5 V-number 3.5 PARTSLIST Figure 3.4. Mode field radius as a function of Cat# Description ety. V.number. FSV-20 41125SM fiber, 20 meters )(st*22 2x2 Breadboard L.Ll30lP I mw HeNe laser 807 Laser mount 340C Clamp 4l Short rod 815 Power meter F-916 Fiber coupler (w/o lens) M-2OX 20X Objective lens . F-CLI Fiber cleaver FK.BTX Balldriver set SK-25 %-20Screw kit SK48 &32 Screw kit RSX-2 Rotation stage MPH.I Micro-seriesholder MSP-I Micro-seriespost

Additional equipment required: Razor blades and tape to construct the slit mask for the power meter detector. Methylene chloride for stripping the F-SVfiber. (Many com- mercial paint strippers contain methylene chloride and work well for this purpose. Methylene chtoride is toxic and cT !e absorbed through the skin. Any methylene chloride which gets on your skin should be waihed oif immediately.) Microscope slide cover glass for monitoring the positioning of the fiber in the F-916Fiber Coupler.

39 F

3.6 INSTRUCTIONSET

3.6.T OPTIMIZING SINGLE.MODE COUPLING l. UsingEq. 3-1,confirm that V:2.1g at a wave_ lengthof 633 nm for the F-SVfiber (NA:0.11). UseEq.3_3 to find the spot size of the fiber mode for this V_number. (Spotdiameter : 2wo : 1.2 x 2a.l 2. . When couplinglight into a single_modefiber, one beginsby usinga microscopeobjective lens to focus the collimatedbeam from a laserto a small spot. The diameter, d,, of the spot sizeat the waist of the focusedlaser beam can be determinedfrom the focal length,f = g.3mm, of the microscopeobjective lens,and thg diameter,d, of !!re laser the rear focal plane of the FjSule 3.5. Coupling !"uT-1, objectivetens,-iisihg of HeNe taser tight into a dr= $"flnd. (d may be found from single.modefiber. the divergenceof the laser, which is 1.3mrad, and the distancefrom the laserto the objectrJg_]srSLs,using an equationfrom Gaussianoptics, = d do!l+(20ld)r. In this equation,do is the diameterof the laser beamat the output of the taser,0.63 mm for the laser which you are using,z is the lasey-toJensdistance, and g is Rear the beamdivergence of 1.3mrad or .0013in the equation.) focal 3. If your calculationsin StepsI and 2 show that Plane !r=2wo, then optimizedcoupling should be obtainedwhen the input beam is focusedon the fiber core, as shown in Fig..3.5l d,r12wo, I .lf then the value of d, must be adjusted x: by changingthe distancebetween the F-gl6 coupler and the laser.For example,moving the laser further from the objectivelens causesthe input beam diameter,d, to become larger due to the beam divergenceof the laser.This, in turn, causesd, to become smaller, 4. Mount the HeNe laser and the F_916Fiber Coupler, with the 20X objectivelens, so that the laser beam is paral_ lel to the lens axis and strikesthe lens at its center,as is illustratedin Fig. 3.5. Be sure to place the lens/fiber_ LJ positioner / I assembly,which forms the top of the coupler,so / l++l that the rear focal plane of the microscopeobjective lens is / directly over the pivot of the tilt platform. The rear focal Mouning'/"" plane Threads of the M-20X objectivelens is r/2,, in fiont of the lens, as shown in Fig. 3.6.

Figure 3.6. Microscope objective lens, showing the rear focal plane.

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\ 5. Cut a segment of F-SVfiber of about two metersin length and cleaveboth ends as you did in Project #l (Sec. tion 1.6.1,Steps 1.3).(Chemical stripping in methylene chlorideis recommended.You will not be ableto strip this fiber with arazor blade, if you have been using that method.) Mount one end of the fiber in the fiber holder from the F-916coupler and insertit into the F-916.Use the microscopeslide cover glassto monitor the Fresnelreflec- tion from the fiber end face,as you did in Project #2 (Sec. tion 2.5, Step 3), and bring the end of the fiber into the focal plane of the objective lens by rotating the coarse adjustmentknobs on the FP-l positionerof the F-916coup- ler.This will assurethat the z-componentof the fiber align- ment is approximately correct and will assurethat the laser beam is at leaststriking the fiber end face,if not the core. 6. Adjust the x and y componentsof the fiber align- ment, using the fine adjustmentknobs on the tilt stageof the F-916Fiber Coupler,to achievethe maximum coupling of the laser beam into the fiber.You may wish to refer again to the F-916instruction sheet to understandthe prin- ciple of operationof the tilt stageadjustment. Monitor the output power using the Model 815 PowerMeter. For some- one who has never done single-modecoupling before,a total loss (from laser output to fiber output) of 3 dB (50% loss;see Section 2.1) would be consideredto be a good result,but with experienceone should be able to consist- Figure 3.7. Laboratory $et-up for examining the ently achievelosses of lessthan 2 dB ( < 37% loss). Gaussian approximation for a single-mode fiber.

3.6.2 GAUSSIAN APPROXIMATION l. Mount the far end of the fiber on the RSX-2rota- tion stagein exactly the same way that the front end of the DetectorHead fiber was mounted in Project#l (Section 1.6.2, Steps 3, ofModel 815 4). The experimentalset-up is shown in Fig. 3.7. This time, PowerMeter however,you will actually be measuringthe far-fielddistri- bution of the fiber output. (Ihe far-fieldof a fiber is usually said to start at a distancez":(2a)2/|t from the end of the fiber.The far field is, effectively,the region where the fact that the core diameter is non-zeroplays no role in the energy distribution.) 2. Mask the detectorhead, as shown in Fig. 3.8, and post-mountit on the table a few inchesaway from the fiber end face.Two razor bladestaped together with a spacingof about .040" betweenthe blade edgesmakes a good mask. Be sure that the tape that is used is not transparentto the laser light. 3. When you rotate the tip of the fiber by turning the rotation stage,you will be scanningthe far-fieldpower distributionof the fiber acrossthe detector.Measure the power receivedby the detectoras a function of the angular Figure 3.8. Masking of the detector with a slit made position of the fiber. Use both positiveand negativeangular from two razor blades.

41

- .I l deviations to compensatefor any angular misalignment' I 4. Plot the far-field distribution of the fiber as a func- -t tion of angle. Fig. 3.9 shows a plot of data taken in New- a port's laboratory. Find the maximum recorded output power from your data. Call this value I(0). Now find the points 1.I where the output power is I(0)e-2.Measure the full width between these two points as shown in Fig. 3.9. Take half :If of that full width and call it 0". Plot a Gaussiancurve on graph with your experimentaldata. Use Eq. 3.2, the same f t r and do for ro. Compare the two plots and Gaussian / \ tloye{ substituting 0 for assessthe validity of the Gaussianapproximation for this f fiber at this wavelength. (Most single-modefibers have a 0 step-indexprofile. ln that case,the near-fielddistribution J -8 -6 -4 -2 0 2 4 6 8^ (scanningas a function of positionacross the end face of .I I (deg.) v the fiber) and the far'field distribution(scanning as a func' tion of angle far away from the fiber end face) will have .tI Figure 3.9. Plot of experimental data taken in the same form, with r/a and sind/NA substitutingfor each Nervport's laboratory. I(O) and 0o found from this data other as the scalingvariables. We are then able to examine f are indicated. iI the Gaussianapproximation by looking at the far-field distri- -I the distribu- bution of the fiber output, since it will mimic I tion as a function of positionwithin the fiber.) ; I ,I

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4.O PROJECT#42 llo Kt F SINGLE.MODEFIBERS II In Project#3 you had an introduction to single-mode fibers.In Project#4, you will continue to look at propaga- tion in fibers which have a small V-number.This time you will start with a fiber which has a V-numberwhich is slightly greaterthan 2.405.Such a fiber is a multimode Efiber, but the number of allowed modes is small enough so that they may be individually identifiedwhen the output of n^(1 the fiber is examined.Following that, you will look at a highly birefringentsingle-mode fiber, the polarization- preservingfiber, and measureits beat length. V-number E4.I FIBERSWITH V > 2.405 Figure 4.1. Low order linearly polarized modes of an optical fiber. Compare with Fig. 0.f 3. (this is a repeat In Section 0.3.1, it was seen that if the V-numberof of Fig. O.15) a fiber is lessthan 2.405,then only a single mode may propagatein the fiber waveguide.This single mode is the E HE,, mode, or, in the linearly-polarizedmode theory for weakly-guidingwaveguides (also discussed in Section 0.3.1),the LPorlinearly-polarized mode. When V > 2.405,other modes may propagatein the fiber waveguide,as shown in Fig. 4.1 (which is a repeat of Fig. 0.f 5). The first such linearly-polarizedmode, which comesin at V:2.405, is the LPtt mode,the next-lowest order mode in the weakly-guidingapproximation. When V is just slightly greater than 2.405,only the LPo,and the LP11modes may propagate.However, when V:3.832, two more linearly-polarizedmodes are now allowedto propagate.These are the LP, mode and the LPor LPrr LPormode. The electromagneticfield distributionsof these modes are shown in Fig. 4.2 (which is a repeat of Fig. 0.f4). If we have a fiber with the proper V-number,these modes can be selectivelylaunched by varying the position and angle at which a tightly-focusedbeam of the proper wave- length is projectedonto the fiber core. When this is done, the near{ield of the fiber output can be inspectedand the field distributionsof the individual modes can be identified. ttr Newport'sF-SS fiber is designedto be a single-mode fiber at a wavelengthof 1300nm. It has an NA of 0.ll and m, a core radius of 4 pm. At the 1300nm wavelength,it has a *v F V-numberof 2.13. However,at a wavelengthof 633 nm, it has a V-numberof 4.39. TheseV-numbers can be verified LPoz LPzt : : rf-{ by substitutinga: 4 pm, NA 0.11and \ 633nm into it can be seenthat Eq.0-15.By examinationof Fig.4.l, Figure 4.2. Irradiance pattern of some low order LPo, lj this fiber shouldallow the LP6r,LPrl, LPrt,and linearly polarized modes. (Ihis is a repeat of Fig. 0.14) linearly-polarizedmodes to propagateat the 633 nm HeNe laser wavelength. In this project, you will use HeNe laser light to selec- tively launch different linearly-polarizedmodes in a length Hof Newport F-SSfiber.

43 POLARIZATION.PRESERVING FIBERS - The mode which propagatesin a single_modefiber, I the.fundamental a HE,, mode, is actually a d"egeneratecombi- nation of two orthogonally-polarizedc'omporients. In a perfectly symmetric, : circular fiber, these componentstravel I at the same velocity; that is, they have identical propaga_ tion constants. If the fiber is not perfectly symmetric,then al the two polarization components have different propagation constants.The differencein their propagation l constants,08, I is known as the birefringence of tne iiler. The properties of birefringent fibers were discussedin detail in iection 0.3.8. l If _r= light is launchedwith a linear componentalong each of the optical -J axes,the differencein iropagation constantscauses I the net vector sum of thepoiarizations to vary periodically with distancearong the fiber, as discussed in Section 0.3.8. -l This . sequenceof alternatingpolarization states con_ tinues along f the entire length of tt l tim.. The distance,Lo, t- over which the polarizationrotates through an entire 360. is known as the beat length of the fiber. fni, i, related to l the birefringenceby (repeatingEq. 0_16) 3 = Lp:2t/60 (4-l) ) f As was discussed-inSection 0.8.3, this beat length can be observed visually when visible laser radiation is launched f into a birefringentfiber with its direction of polar_ I ization oriented at an angle of 45. with respectto the principal axis l of the fiber.scattering centeri in tiners emit J dipole radiation. Sincethe radiation-froma dipole is zero along-its vibration axis, each time that the ligirt becomes /J linearly polarized,there will be no scattering"detectable when the fiber is observedwith the line of s'ightalong the 3 vibration = axis. This allows a direct measuremlnt of Lo. Polarization-preservingfibershaveapplications . wherever the polarization J of the transmittedlight must be stable and well-defined.These applications inciude fiber J interferometric ,a sensors,fiber gyioscopes,and heterodyne detectionsystems. >3

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-{ ."J .a- -) F ) *F ) *u -J 44 *f-r -j F, 'l h 4.3 PARIS LIST Cat# Description Qty. XSN-22 2x2 Breadboard t u-1301P I mw HeNe Laser I 807 Laser mount I 340C Clamp I 4l Short rod I F-916 Fiber coupler (w/o lens) I EM-2OX 20X Objective lens I F-CLI Fiber cleaver I tr I FK-BTX Balldriver set SK-25 %-20Screw kit I i/ VPH-2 . Post holder 2 SP-2 Post 2 ?c:k,?.1-'\ FP-1 Fiberpositioner 1 ,*-, '" FS$20 8/125SM fiber, 20 meters I F-SPV-IO Polar-preserv. fiber, l0 m )-*-:;; E 1 M4OX 40XObjective lens . MH-2PM Optics holder I FK-POL Polarizer,sheet I Additional equipment needed: Millimeter scale for measurementof beat length. Methylene chloride for chemi- cal stripping of fibers. (Many commercial paint strippers contain methylene chloride and work well for this purpose. Methylene chloride is toxic and can be absorbed through the skin. Any methylene chloride which gets on your skin should be washed off immediately.) 4.4 INSTRUCTIONSET

4.4.I OBSERVING FIBER MODES L Prepare both ends of a segment of F-SSfiber approximately 2 meters in length as you did in Project #l (Section 1.6.1, Stepa l-3). 2. Couple the HeNe laser beam into the fiber using the F-916coupler as described in Project #3 (Section 3.6.f)' optimizing the coupling efficiency. 3. Insert the far end of the fiber into a post-mounted FP-l fiber positioner. Insert the 40X microscope objective into the MH-2PM Optics Holder and post mount this assem' order bly. The laboratory set-up for this part of the project is Figure 4.3. Iaboratory set'up for observing low shown in Fig.4.3. ,nodes in a multimode fiber with low V'number. 4. Use the microscope objective to image the output end of the fiber on a convenient near-by wall. The farther the imaging distance,the larger the image will be. Three meters is a convenient working distance.

L= { q

5. Examine this projectionof the near-fielddistribu- a tion of the fiber. Changethe x-y adjustmentof the fiber ) position in the F-916.This has the effect of changingthe I position and angle of the launch of the focused laser beam j into the fiber. Notice how this causesthe projectionof the a near-fielddistribution of the fiber output to change. ) 6. Sketchthe near-fieldimages which you are able to - obtain. Comparethem with the LP,- mode distributions shown in Fig. 4.2. ldentify the patternswhich appear to be J II pure LP,- modesand those which are combinationsof two i or more LP,- modes. J

I 4.4.2 BEAT LENGTH OF A BIREFRINGENT FIBER J I l. Cleaveboth ends of a -l-2 meter segmentof F-SPVfiber. This fiber requireschemical stripping in methy- -a b lene chloride. 2. Be sureof the orientationof the polarizationof the HeNe laser beam. Use the FK-POLSheet Polarizer to l- check the laser'spolarization axis. A method for determin- ing the polarizationaxis of the sheet polarizer is given in l- Section 9.6.2, Step l. Rotatethe polarizer in front of the laser beam. When the power through the polarizer is a > maximum,the plane of polarizationof the laseris parallel to that of the sheetpolarizer. b 3. Substitutean FPH-Schuck from an FP-l Fiber Positionerinto the F-916Fiber Coupler.Couple the HeNe l- laser light into the polarization-preservingfiber using the F-916coupler, as you did in Project#3 (Section 3.6.1). 4. Insertthe far end of the fiber segmentinto the a original chuck from the F-916.Loosen the s.etscrew on the knob of the FPH-Jand orient the chuck so that the major J and minor axesof the slightlyelliptical output spot are alignedwith the 0' and 90' marks on the chuck.Tighten - the set screw.The reasonfor doing this is so that you can now placethis end of the fiber back into the F-916coupler Figure 4.4. Laboratory set.up for orienting lr the with the fiber axes at a known orientation with respectto pohriation axes of the polarization fiber with the the polarizationof the laser beam. The arrangementof on the chuck from the F.916 I Sraduations Fiber your equipmentshould be like that shownin Fig. 4.4. Coupler. 5. Placethe fiber end which you have just oriented -1 ts in the chuck from the F-916coupler back into the F-916. Orient the fiber at 45" with respectto the planeof polariza- tion of the laserin the F-916coupler. h 6. Align the fiber to the laser beam as you did in Project#3 (Section 3.6.1). Maximize the coupledpower. = 7. Chemicallystrip a sectionof the middleof the fiber. Use a magnifying lens in a darkenedroom to observe l- the beatsin the fiber. You will be able to see alternating light and dark sectionsof the fiber. Measurethe beat ,- length.The beat lengthof this fiber is -2 mm. 8. Calculatethe birefringence,6B. Calculate the a refractiveindex differencebetween the fast and slow axes, Y 6n:6B(X"/22). 9. Watch what happensif you change the orientation ,- of the fiber axes to the plane of polarizationof the laser - beam. Give a qualitativedescription of the results. Y

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-- 5.0 PROJECT#52 COUPLINGFIBERS TO SEMICONDUCTORSOURCES

ln this project, you will learn to couple semiconductor sources,i.e., injection laser diodes(lLD's) and light-emitting diodes(LED's), to optical fibers.ILD's and LED's are the sourcesgenerally used with optical fibers in communica- tions and sensorapplications. You will also experimentally determinethe electricaland optical characteristicsof these sourcesand learn the differencebetween them. The coupling will be achievedusing a 0.29-pitch graded-index(GRIN) rod lens. GRIN-rodlenses have become widely acceptedfor use in fiber optic applicationsbecause of their small size,convenient focal lengthsand working distances,and high-qualityimages with low distortions. The sourceswhich you will be using are infrared devices,with the ILD emitting at approximately780 nm and the LED centeredat about 830 nm. Sincethese devices emit invisible radiation,proper safeguardsmust be used to ensurethat the possibilityof injury is eliminated.Never look directly into a laser beam or its reflection. 5.I TYPESOF SOURCES TWotypes of semiconductorlight sourcesare used in fiber optic systems.These are light-emittingdiodes (LED's) and injection laser diodes(lLD's). The theory of semicon- ductor deviceswas outlined in Section 0.4.2 and is detailed elsewhere.r'2In this project, we will be concernedwith the coupling of these devicesto optical fibers. A light sourcemay be characterizedby the distribu- tion of power emitted from its surfaceamong all of the possibleray directions.Sources are generallydivided into two types, dependingon the radiation distribution.These two types are Lambertiansources and collimatedsources. A lambertian source is one which emits light in all direc- tions from each differentialsource element.A surface- emitting LED closely approximatesa Lambertiansource. A sourcewhich emits light only into a very narrow range of anglesabout the normal to its surfaceproduces a collimated beam. The output of a HeNe laser approximatesa colli- mated beam. In general,the angular distributionof the source brightnesscan be expressedas (recallingEq. 0-22)

B(d): 3" (cos0)', 0 <0^u*, (5-l)

where 0-u* is the maximum angle from the normal at which light is emitted and is determinedby the geometry of the source.For a diffusesource, m:1. For a collimatedsource, m is large. For intermediatecases, the sourcemay be called a partially collimatedsource. The ILD is a specialcase. The far-fielddistribution of the radiation from an ILD diverges in a fan-shapedpattern with angleswhich are typically on the order of 15'x30'. This is becausethe smallemittance area of these devices(on the order of I pm on a side) causesthe collimation of the far-fielddistribution of the

47

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- radiationto be limited by diffractionat the output.Fig. 5.f showsthe output radiationcharacteristics, in polar coordi- - nates,for two sources,one with m: I (typicalof an LED) and one with m:20 (typicalof an ILD). a There are other propertieswhich distinguishlight- emitting diodesfrom laserdiodes. These include the optical power-currentcurves which are characteristicof the devices at and the polarizationof the output beam.These properties were discussedin Section 0.4.2. -

5.2 COUPLINGEFFICIENCY ar The amountof light energywhich can be coupled into a fiber is dependenton the NA of the fiber.Since a fl 90" fiber will acceptonly thoselight rayswhich are contained within a cone definedby the fiber'sNA and core diameter, ttt Ul=1 couplingloss will occur for sourceswhich havean angular Figure 5.1. Polar plot of radiation patterns from emissioncone largerthan the acceptancecone of the typical ILD and LED sources. - fiber'sNA. In somecases the fiber will be butt-coupledto the source.Butt-coupling is definedas coupling by placingthe flat fiber end directlyagainst the source,without the aid of any lenssystem. Butt-coupling cannot be achievedwhen the sourceis mountedin a packagewith a coveringwin- dow glass.lf the fiber is directlybutt-coupled to the light a source,the ratio of the poweraccepted by the fiber to the 20 power emittedby the sourcecan be shownto be' J 18 - P,/P":0.5 (m+ l)[o+(a/2)] NA', (5-2) I 16 where a is the index profileof the fiber.(ln Section 0.2.3, - it wasstated that a parabolicgraded-index would accept only one-halfas much light asa step-indexfiber. The factor a ^12 a/ a t2 is a mathematicalexpression of this fact.)The coup- CD ling efficiencyto either a graded-index(a:2) or a step- - 6to index (c = o) fiber is proportionalto the squareof the -9 numericalaperture and increaseswith increasingdirection- ality (increasingm) of the source.The couplingloss in dB a will be -10 log,o(P,/P"). Fig. 5.2 showsthe theoretical couplingloss as a functionof fiber NA for somevalues I of nr.

For optimum couplingefficiency, one needsto match I the sourcediameter-NA product to the fiber core diameter- NA product.This wasdiscussed in detail in Section 0.4.3. I

0.1 0.2 0.3 0.4 0.5 0.6 5.3 LENSCOUPLING USING A I LENS FiberNA GRIN.ROD This projectuses a graded-index(GRIN)-rod lens Figure 5.2. Plot of coupling loss as a function to facilitatesource-to-fiber coupling. Most optical devices fiber NA for various valuesof m, using of usedin fiber-opticsystems employ lenses, and for most f Equation 5-2. of thesedevices, GRIN-rod lenses have advantages over tl conventionallenses.

f a

f

t

:

=

48 :

\ ";il The GRIN-rodlens, which was describedin Section 0.2.4, is a glassrod, 1.0to 3.0 mm in diameter,with a radically-dependentindex of refraction. This index of refrac- tion is a maximum on the axis of the rod lens,and can be expressedas

n(r): n"(l-Ar'/2), (5-3) where n. is the index of refraction at the lens axis and VA is referred to as the quadratic gradient constant.(Ihis is really a restatementof Eq. 0-13,with A = 2Ala'.) By far the most popular choice of GRIN-rodlens length is one-quarterpitch. This is becausea beam travels exactly one quarter of a sinusoidalperiod in that distance. Therefore,a collimated beam incident on one end of the lens will be focusedto a point on the oppositeend of the lens.Conversely, any point sourceat the surfaceof a quarter-pitch lens will become a collimated beam at the far end, aswas seen in Fig.0.lla. The quarter-pitchlens will be usedin Projects #7, #9, and #10. Also widely usedis the 0.29-pitchlens (which was illustratedin Fig. O.llb), which is providedfor usein this project. This lens is used to couple a laser diode to a fiber or a fiber to a detector.The lens which you will be using hasn":1.599 andVf = 0.332mm'. Sincethe lengthof this lensis slightly more than one quarterpitch, the light from a point sourcewill be converted to a converging beam, rather than a collimated beam. Table 5-l gives examplesof the relationshipsbe- TABLE5.7 tween the working distances,I, and 1,,and the beam mag- nification,M, for the 0.29-pitchlens at a wavelengthof 0.83 WORKING DISTAIICES AND MAGNIFICATION pm. l, is the working distancefrom the sourceto the lens, OF THE O.2g.PITCHGRIN.ROD LENS while l, is the working distancefrom the lens to the receiv- lt (mm) lz (mm) M ing fiber. The table may be usedto optimize laser and fiber working distances.For example,a typical laser diode output 0.50 3.33 1.96 may have a beam divergencecone with hallangle of about 1.00 2.05 l.3l l5o at the half-powerpoints in the direction perpendicular 1.50 t.42 0.98 to the diode junction. Therefore,the ea power point for the 2.00 1.04 0.78 Gaussianoutput beamwill be at sin e -0.4. Sincethe nu- merical aperture of a typical multimode communications fiber is -0.2, a magnificationof about2 will optimizethe laser-fibercoupling. If the physicaldimensions of the device permit, l, and l, can now be adjustedto fit the required magnification. Note that the magnification in the table is the image size magnification; the beam divergencewill be reducedby the same factor.The laser diode provided for use in this project has a diode-to-windowdistance of ap- proximately 2.0 mm. Becauseof this, you will not be able to achievea magnificationof 2.0.The resultis that the coupling losswill be about 4 dB when the laser-fibercoup- ling is optimizedusing the 0.29-pitchGRIN-rod lens.

49 ) E f 5.4 REFERENCES - l. H. Kressel and J. K. Butler, Semiconductor Lasers 7 and Heterojunction LED's, Academic Press(New York), 1977 2. S. M. Sze, Physicsof SemiconductorDeuices, John IT Wiley & Sons (New York), 1969 3. M. K. Barnoski, in Fundamentak of Optical Fiber Communicstions, 2nd Edition, M. K. Barnoski, ed', Aca- xf demic Press(New York), 1981,P. 158 IF 5.5 PARIS LIST I Cat# Description QtY' 100/140MM fiber, 50 meters T F-MLDS0 -r XSN-22 2x2Breadboard 815 Powermeter f F-CLI Fiber cleaver FK-BIX Balldriver set 1 SK-25 l/+20Screw kit BS-05 7+20Thumbscrew kit 1 F B-l Base 2 VPH-2 Post holder 3 l Sp-2 Post FP-l Fiber Positioner I coupler 2 3 F-925 GRIN-rodlens fiber FK-ILD Laser diode assemblY 1 FK-LED Light-emitting diode assy' I I 3 FK-DRV Driver circuit FK-GR29 NSGSLW-1.&0.29lens 2 I FK-POL Polarizer, sheet I 1 F-IRCI IR PhosPhor card fT 2 CA-2 Universal clamP MH-2PM Optics holder 2 I Additionalequipment required: Digital voltmeter for monitoringILD ani LED currents.Methylene chloride for strippingthe jacket from fibers.(Many cornmercially avail' aUte'paintstrippers contain methylene chloride and work E well ior this puipose'Methylene chloride is toxic and can be absorbedthrbugh the skin' Any methylenechloride T, which getson youi skin shouldbe washedoff immediately') 1 I 5 I J !

{ tI I

Itf T l f 4 50 d 5.6 INSTRUCTIONSET CAUTION: READ THESE WARNINGS BEFORE PRO- CEEDING WITI{ THIS PROJECT. The LED and ILD devicesprovided for use in this project are infrared deviceswhich emit radiation which can damagethe human eye even though it is invisible. Proper precautionsmust be taken to ensure that the beams cannot enter the eye. This means knowing exactly what the beam path is at all times, including the possibility of specular reflections.The laser output is current limited to meet Class I U.S.Federal Laser SafetyRequirements. The output is limited so that not more than 35 pw of optical power will passthrough a 7 mm diameter aperture at a distanceof 20 cm from the source. Removal of the calibration labels on the FK-DRVdriver circuit will invalidate the ClassI certification. CAUTION: Use of controls or adjustmentsor performance of proceduresother than those specifiedherein may result in hazardous radiation exposure. CAUTION: The use of optical instrumentswith this product will increase eye hazard. The FKJLD diode should be used only with the FK- DRV driver with the same serial number. They are not to be interchangedwith other similar modules.Use of any other driver could destroy the laser and could cause hazard- ous radiation to be emitted. Use of any other driver will invalidatethe ClassI certification. Also, it is important to remember that semiconductor infrared sourcesare highly sensitivedevices. When going through the instruction set, check and double-checkto be sure that all connectionshave been properly made, and carefully follow all directionsfor device operation.A wrong connectioncan causethe catastrophicfailure of either the ILD or the LED. Before each use visually inspectthe laser diode in the FK-ILD assemblyto check for damageto the diode. SERVICE: Do not attempt to replace the diode in the FK-ILD assembly.This will invalidate the ClassI certifica- tion. Any repair to either the FK-ILD or FK-DRVmust be performed at the factory by Newport Corporation. 5.6.T INJECTIONLASER DIODE l. The FK-ILD Laser Diode Assemblyis mounted in a threadedplate. lnsert this plate into an MH-2PMOptics Holder. Post mount this assembly using two SP-2Posts and the CA-l UniversalClamp. Mount this on a B-l Base.Secure the base to the table using two t/a-20 thumbscrews from the BS-05Thumbscrew Kit. The proper set-upcan be seen in Fig. 5.3. Figure 5.3. Laboratory set-up for coupling semi- conductor aources to optical fibers.

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following connections only when the - 'r- 2. Make the a/' diode power supply is off. The driver which you will be taserW 2 LEDI, / power ILD and LED is illustratedin Fig' 5'4' a Diode Mrt Max using to the pffr, ,/ Connectthe ILD to the driver circuit. Attach a voltmeter in acrossthe output monitor pins of the circuit, as shown F at this point may be used LaserDiode Current Fig. 5.4. The voltage measured the ILD. The monitor volt- to monitor the current through a LEDAudio Input age to laser current calibrationis 70 mV per mA' The cur- LEDDriver rJnt in the diode should never exceed 100 mA. Typical I 0utputCurrent operatingcurrent is 65 mA, with a typical thresholdcurrent LaserDiode LED (LaserDiode oi SOrnA. It is very easy to blow out an ILD by exceed- Monitor Monitor Current0utput ing current specifications' - 0utput 0utput end) fromopposite 3. Placethe detector head of the 815 power meter directly in front of the laser window. Increasethe diode I Figure 5.4. Laser Diode'LED Driver, Model FK-DRV. .urr"nt to the operatingcurrent listed for the device' Moni- tor the output power as the current is increased'Make note v of the powlr obtainedwhen the listed optimum operating current is reached. - 4. Reducethe current through the laser to zero' Now, the coupled output slowly increasethe current, recording L power as a function of diode current. Thke data between t current : zero and the listed operatingcurrent' AdjustSet Y Screw 5. Plot the results.Draw a line along the rise in power abovethe onset of lasing' Extend this line down itrrougtrthe current axis. Comparethe current at this point a with the listed thresholdcurrent. This is the techniqueused \ to determinethe laser'sthreshold current. I 6. The F-lRCl IR PhosphorCard may be used to view in the path of the - the laser output. Placethe phosphorcard a beam at a convenientviewing distance.Measure the widths perpendicularto the width of the of the beam parallel and - diode junction. Using this and the distancefrom the device, calculatethe divergenceof the beam. The manufacturer - specifiesa divergenceof about 15'x30" for this laser' 7. Placea polarizing sheetwith a known polarization axis in the laser beam and determine the plane of polariza- I tion of the laser outPut. Fisure 5.5. Placing a 0.29'pitch GRIN-rodlens in GRIN-rodlens into the groove I of the Model F-925 GRIN'Rod Lens 8. Placethe 0'29-pitch th6 V-groove in Fig. 5.5. The lens should Coupler. of the F-925coupler, as shown extend out of the coupler - I mm toward the ILD' Insert a ; I cleavedsegment of F-MLDfiber into the FP'l of the coupler the FpH-Sttotaet and couple the laser output into the using I fiber through the GRIN-rodlens. 9. Optimize the coupling and determinethe coupling loss,using the power coupledinto the fiber and the power I out of the ILD which you measuredin Step 3. You will get the best coupling with the laser window as closeto the I lens as possible.With the Sharp LI020MC Iaserdiode' which is used here, and the F-MLDfiber, a coupling loss of I about 4 dB should be obtained'

52

.,,ali 10. Reduce the laser current to zero. Disconnect tae diode from the circuit before turning off any other part of the circuit.

5.6.2 UGIIT.EMITTING DIODE l. Postmount the FK-LED Light-EmittingDiode As- sembly in the same way in which you mounted the ILD. (SeeFig. 5.3) 2. Connectthe LED to the driver circuit. Connecta voltmeter across the LED monitor pins. The monitor voltage to LED current calibration is 50 mV per mA. Tirrn the cur- rent up to 100 mA and record the power out of the device. The LED driver circuit has a current limiter at 120 mA. 3. Reduce the LED current to zero. Record the power out of the coupled LED as a function of current from zero to approximately 110mA (10% over the optimum current). The data should provide a good fit to a straight line, a characteristic typical of LED's. 4. Placethe F-lRCl IR PhosphorCard in the path ol the LED output. The LED has a microlens over the semi- conductor chip; all of the output power will not be accepted by the lens, and the output will appear to be better colli- mated than might be expected from the discussion of Sec- tion 5.1. However,you will still seea markedcontrast to the output of the ILD. 5. Placethe polarizer used previouslyin the LED output beam. Confirm that the LED output is unpolarized. 6. Couplethe LED to the fiber using the F-925GRIN lens coupler as you did in Steps 8 and 9 of the previous section.Calculate the coupling loss using the power coupled into the fiber and the power out of the LED which you measuredin Step 2. r

LENS LENS 6.0 PROJECT#6: ,rrr, CONNECTORSAND SPLICES __\.r ,an-*- I v-- In addition to the need for coupling LED and ILD sourcesto fibers,as was explored in the previousproject, we also need to look at the problem of connectingfibers to fibers.Both mateable-demateableconnectors and perma- nent spliceswill be studiedin this project. You will begin this project by exploring the effectsof misalignmentson the coupling of two fibers.This will give you an understandingof the loss mechanismsseen in con- nectorsand splices.You will then go on to couple fibers together using both connectorsand splices. 6.I FIBER COUPLINGCOMPONENTS FibersSit in V-Groove Both permanent splicesand connectorswhich can be repeatedlycoupled and uncoupledare used for coupling fibers to fibers.Each has its own area of applications.The connectorfinds applicationin the connectionof fiber cables to transmitterand receivermodules, and in the connection to devicesin optical communicationssystems. Permanent splicesare required for joining sectionsof fiber in long distancecommunications links as well as for the construc- tion of integratedfiber sensorsystems. Connectorsuse alignment techniqueswhich fall into two generalcategories,l which are shownin Fig. 6.1. Lensedsystems (Fig. 6.fa) image the core of one fiber (c) onto the core of the secondfiber. Other techniquesuse butt-couplingof fibers to achievethe connection.Butt- Figure 6.1 Schematic depiction of connector types. coupledconnectors can be further divided into sub- a) Lens type. b) Precision v.groove type. c) Snug-fitting categories.In some, precisiongrooves are used to align the tube type used in this project. fibersto each other (Fig. 6.lb). In others,including the connectorsused in this project, snug-fittingtubes are used to align precision-machinedferrules with respectto each other (Fig. 6.lc). Typical insertion lossesfor connections between multimode fibers are lessthan 1.0 dB. Fibersplicing is the permanentjoining of two fibers to each other. The best resultsare obtainedfrom thermal fusion of the two fibers.The fibers are aligned,either manu- ally with precisionstages under a microscopeor with the aid of microprocessorcontrol. The fibers are then heated and fusedtogether as shown in Fig. 6.2. This fusion is typically done with a carefully regulatedelectric arc or with a microtorch. Skilled operatorscan achievelosses of lessthan 0.1 dB. The secondmethod of splicingis a preci- sion aligned groove techniquewhich will be used in this project. The fibers are insertedin a groove formed by an arrangementof four glassrods which have been fused Precision Stage together as shown in Fig. 6.3. Index-matchingepoxy is used to fix the splice in place. Untrainedoperators can routinelyachieve splice losses on the order of 0.1-0.3 dB. When it is necessaryto keep splice lossesas low as possi- Electric ble, the thermal fusion splice techniqueis the best choice, ArcWelder but the epoxy splicesare easierand can be made with little practice. Figure 6.2. Fiber splices by thermal fusion. Fusion welder using electric arc. Microtorch may also be used.

54

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1- Permanentsplices generally have Iower lossesthan connectorssince they do not need to be designedfor re- peatedcoupling and decoupling.Also, connectorsrequire mating hardwareattached to the fiber, making them more complicated.Both splicesand connectorsmust be able to Ebe done in the field by personnelwho may not be highly tr trained.This dictatesa designrequiring a minimum of fiber handling.The resultingjoints must be mechanicallyrugged, l--i- environmentallyinsensitive, and long lived. tr 6.2 ALIGNMENT LOSSES The principal sourceof loss in both connectorsand splicesis fiber-to-fiberend face misalignment.There are three types of misalignmentloss which may occur individu- ally or in combination.These are lateral misalignment, axial separation,and angular misalignment.These misalign- problem is to ments are illustratedin Fig. 6.4. The design EndCross Section E minimize these losseswhile making the devices,as far as (Enlarged) possible,self-aligning. The first of theseloss mechanisms, lateral misalign- Figure 6.3. Precision groove splice used in this ment, is the largestcontributor to the total lossin a fiber project. connection.Lateral misalignment is the failureof the cross sectionsof the two fiber coresto perfectlyoverlap (Fig. 6.4a).This may be due to toleranceson the fiber diameter,eccentricity of the fiber outsidediameter or of the core with respectto the fiber axis, or machinetolerances in E the constructionof the connector. Axial separationcontributes to the connectionloss when the end surfacesof the two fibers do not come into contactwith each other (Fig. 6.4b). In connectors,a slight F separationwill generallybe designedinto the device so that the fibers are not damagedby butting againsteach other. This type of loss dependsupon the numerical aper- ture of the fibers,since a large divergencewill result in lessof the beam from the first fiber being acceptedby the second. The third lossmechanism, angular misalignment(Fig. 6.4c), generallydoes not contribute significantlyto connec' l* -rl tion losses,both becausemanufacturing tolerances virtually eliminatethis misalignmentin connectorsand splicesand becausethe fiber connectionitself is more tolerant of angu- = lar misalignments. Theoreticalloss curves for these three misalignments when graded-indexfibers are coupledtogether are shown in Fig. 6.5, 6.6, and 6.7. In the caseof step-indexfibers, the lossfrom offset misalignmentcan be found by simply E calculatingthe overlap area of the coresbecause the irradi- ance is constantover the face of a step'indexfiber. In the caseof graded-indexfibers shown here (Fig. 6.5), the calcu- lation is much more complicatedbecause the irradiance variesas a function of position within the fiber core. The lossfrom the separationof two fiber ends (Fig. 6.6) is found by comparingthe area of the receivingfiber with the area of the beam from the first fiber at a distancez from its end face.The loss from angular misalignment(Fig. 6.7) has the form as the offset loss,with sin O/NA replacingz/a same Fisure 6.4. Misalignments of fiber cores in as the scaling factor. fib"er-fiber connectlions. a) Lateral offset. b) Axial E separation. c) Angular offset.

rl l tDl i I -) F i

As an example of the tolerances which are allowable I in making a fiber-to-fiber connection, 20 consider a step-index fiber with core diameter : 50 pm and NA : 0.20. Losses I J of 0.1 dB occur for approximately I pm lateral offset, 4 prm At 1A axial separation, or 0.5' angular misalignment. The prob- ; I @ lem is much more difficult when single-mode fibers are 9p used. If a single-mode fiber with core diameter : 4 p.m 6- a q and NA : 0.10 is used,then 0.1 dB lossesoccur for 0.1 pm ,ig lateral offset,0.7 y.m axial separation,and 0.2' angular misalignment.In each case,these are only the misalign- I ment losses.Fresnel reflection losseswill contribute about 4 I 0.2 dB loss per fiber face in dry connections.Imperfections .J in fiber end preparation may also contribute to the loss. 0 The losseswhich have been described here are for I 0 .4 .8 1.2 1.6 2.0 the butt-coupling types of connectors. Connectors which LatefilOffset (x/a) use lens systemsto minimize the connection loss are useful { i Figure 6.5. Lateral offset loss for graded-index in eliminating lateral offset and axial separation losses,but fibers. Note that the offset has been normalized to are very highly sensitive to angular misalignment. the fiber cone radius. I 6.3 REFERENCE i l. M. K. Barnoski, in Fundqmentals of Optical Fiber -a Communications,2nd Edition, M. K. Barnoski,ed., Aca- I demic Press(New York), 1981,p.177 I 6.4 PARIS LIST I

Cat# Description Qty. I F-MLD-50 100/140MM fiber, 50 meters XSN-22 2x2 Breadboard a u-1301P 1 mw HeNelaser 807 Lasermount t 340C Clamp 4l Short rod I 815 Powermeter F-916 Fibercoupler (w/o lens) I 048121620 M-2OX 20XObjective lens F-CLI Fibercleaver AxiatSeparation = 0.30) I f trun FK-BLX Balldriverset SK-25 Z+-20Screw kit Figure 6.6. Axial separation loss for graded-index - SK-08 &32 Screwkit -! fibers. Note that the separation has been normalized RSX-2 Rotationstage to the fiber core radius. NA=0.30 was used in the MPH-I Micro-seriesholder I calculation. MSP-I Micro-seriespost VPH-2 Post holder a SP-2 Post FP-1 Fiberpositioner 3 L 20 FM-I Mode scambler I F-CC-140 Connector halves (F-MLD) 10 --I =to F-CA-001 In-lineadapter 5 @ F-TKIE pkgs. o 20 epoxy t r12 F-TKIL 160lapping sheets rf @ o F-TKlP Polishingfixture ! ,i6 F-SK-C Splicefixture F-SK-S l0 PSILightlinker modules t *5

a

0 .4 .8 1.2 1.6 2.0 I Angulargx5s1 /SU-q\ \NA/ Figure 6.7. Angular misalignment loss for graded- -= index fibers. Note that the angle has been normalized to the fiber NA. _= 56 E

- Additional equipmentrequired: Sheet of plate glass for use in polishing connectors.Methylene chloride for strippingjacket from fibers.(Many commercialpaint strip- pers contain methylenechloride and work well for this purpose.Methylene chloride is toxic and may be absorbed through the skin. Any methylene chloride which gets on your skin should be washedoff immediately.) 6.5 INSTRUCTIONSET

6.5.T FBER ALIGNMENT LOSSES l. Preparetwo segmentsof F-MLDfiber, eachabout 1-2 metersin length, as you did in Project#1 (Section 1.6.1,Steps l-3). Couplelight from the HeNelaser into one of the fibers using the F-916Fiber Coupleras you did in Project#2 (Section 2.6, Step 3). Insert the other end of this fiber into a post-mountedFP-I. Measurethe output power from this fiber segment. Figure 6.8. Laboratory set-up for measuring fiber 2. Insert one end of the secondfiber segmentinto an alignment losses. FPH-Sholder and place this in an FP-1Fiber Positioner which has been post-mountedon the RSX-2rotation stage, as you did in Projects #l and #3. fihe laboratory set-up for this project is shown in Fig. 6.8.) Placethe far end of this fiber in another post-mountedFP-l so that the power coupledthrough the two fibers can be measuredwith the 815 PowerMeter. 3. Extend the fiber ends so that the fibers can be butt-coupledover the center of the rotation stage.Centering of the fibers on the stagecan be checkedby rotating the stageand adjustingthe fibers until the ends do not move relative to each other as the stageis turned. Adjust the x-y-z of the fibers to achievemaximum throughput in the butt coupling. 4. Measurethe power transmittedthrough the second fiber segment.Calculate the loss of the butt coupling of the fibers,using the power out of the first fiber measuredin Step I and the power coupled into the secondfiber as measurein this step.A loss of lessthan 0.25 dB is excellent for this dry connection.A measurementin Newport'slab during the developmentof this project yielded a loss of 0.27dB. 5. Measurethe excessloss as a function of lateral misalignmentas follows:Measure the power coupledinto the secondfiber and calculatethe total coupling loss as a function of position.After finding this total coupling loss, subtractthe loss measuredin Step4 from it. There is no 20 direct calibrationfor measuringthe offset from the FP-1,so it is best to calibratefrom the rotation of the adjusting EIO screws.These are 80 thread/inch screws,which meansthat a give a one turn of the screwwill 0.0125inch or 0.3175mm J12 of offset.(Be sure to move in one direction only so as to a- eliminatebacklash in the movement.If you wish to go in u8

0 .4 .8 1.2 1.6 2.0 LateralOffset (x/a) Figure 6.9. Experimental and theoretical lateral offset losses.

JI

J : =

the other direction,reset the systemand start over again.) = 14 Normalizethe offset to the fiber core radiusby dividing the offsetby the fiber core radius.This will allow you to plot 4 12 the loss as function of offset expressedin core radii. Plot the excessloss which you have obtainedas a function of 1 10 lateral offset.Fig. 6.9 showsthe loss,obtained in the I Newport lab using the F-MLDfiber, plotted againstthe I

curve shown in Fig. 6.5. The deviation from the theoretical I EX curve is due to the fact that the laser-fibercoupling under- J q - fills the fiber, as was discussedin Section 0.3.2. This causes -: J^ : @ more of the light to concentratedtowards the center of the -J

>< fiber core, reducingthe misalignmentloss. ut - q 6. Realignthe fiber ends and measurethe excessloss ) as a function of longitudinalseparation. The screw at the J back of the FP-l which controlsthis separationis also an I S0-pitchscrew which gives0.0125 inch or 0.3175mm per -:J turn. Normalizethe separationto the fiber core radiusand plot the results.The resultsfound in the Newport lab are .J 048121620 J shown in Fig. 6.f0. They are plotted with the theoretical AxiatSeparation = 0.30) : I trun curve of Fig. 6.6. The deviation has the same origin as ) discussedin the previousstep. Figure 6.10. Experi-.rrt.] and theoretical fiber separation losses. 7. Realignthe fiber ends once more and measurethe -j excessloss as a function of angular misalignmentusing the - RSX-2Rotation Stage to change the angular orientationof I the fiber ends.Normalize the sine of the angle of misalign- ment to the numerical aperture of the fiber.The results 20 obtainedin the Newportlab are shownin Fig.6.ll. The theoreticalcurve is from Fig. 6.7. The deviation at low anglesis for the reasondiscussed above. At high angles, - IO light is being launchedinto the cladding of the second fiber. Some of this light reachesthe detectorand decreases CO ra the apparentmisalignment loss. a

JR 6.5.2 CONNECTOR ASSEMBLY AND TEST 1. You may use the two fiber segmentswhich were )< - uJ4 used in the alignment loss study,above, for this exercisein connectorassembly. Cleave the fibers so that there is about 1/q-3/ainch of bare fiber extendingbeyond the jacket at a- the end of eachsegment which will be insertedinto the connector.End face inspectionis not required;this step is - simply to avoid having any jagged edge remain when the 0 .4 .8 1.2 1.6 2.0 fiber is broken off to this particular length and will improve sino\ Angularottset""""'\ / fiber insertion into the connector. NA/ 2. Removethe clip from the center of one of the Figure 6.11. Experimental and theoretical angular packetsof the F-TKIE Epoxy. Knead the epoxy until it is misalignment losses. thoroughly mixed. Cut a corner from the packet and squeezeout the.epoxy. 3. With a toothpick or other small probe, apply the epoxy to the length of the bared fiber. 4. Insert the fiber into each connectorhalf (Fig. 4 6.12). A slight rotation of the connectormay help "thread" 1 the fiber through the precisionhole in the connector.Use epoxy to fill the connector.(These connectors are designed to be used with cabled fibers.Filling of the connector halveswith epoxy accommodatesthe use of bare fibers - q ConnectorHalf insteadof the cabled fibers.) Figure 6.12. Inserting the fiber into the connector half.

!=

b 56 l't -.! .

\ .;d 1 I

5. Using the same small probe, place a small drop of epoxy onto the face of each of the connector halveswhere the fiber exits the precisionhole. Allow the epoxy to fully cure. flhis requiresabout 5 minutes.Check the unused remnant to tell when the epoxy is cured.) 6. Be sure that the polishing fixture is clean. If not, clean it with water and a lint-freewipe. Clean a sheet of plate glasswith water and a lint-freecloth. 7. Placethe 60 pm grit lapping sheet on the glass plate. (lf a few drops of water are placed on the glassfirst, the surfacetension will hold the sheet in place.)Screw the connectorhalf onto the fixtureuntil fingertight (Fig. 6.f 3). Do not overtighten. 8. Put a small puddle of water on the lapping film and begin the rough grind. Use a figure-Smotion (Fig. 6.14) in this and each of the subsequentgrinding and pol- ishing steps.Grind until the epoxy bead and excessmetal are removed.Exert only light pressureon the fixture. You will feel a definite bottoming-outeffect when the full face of the fixturecomes into contactwith the lappingfilm. This Figure 6.13. Screw the connector half onto the signifiesthat all of the excessmaterial has been removed. polishing fixture. About 20 to 30 strokesshould completethe rough grind. 9. Rinsethe polishing fixture with water and wipe dry. This is an important step which preventscarrying over contaminationfrom the larger to the smaller grits. Most poor-qualityfinishes in the final fiber surfaceare due to lack of care in this step. Be sure to repeat this at each step of the grindingand polishing. 10. Next,use the 9 ,lrmintermediate polish. Keep the lapping film wet and use a figure-Sstroke. Downward pres- sure may be required during this polish to overcomehydro- planingon the water.About 20 strokesshould be sufficient. Don't forget to rinse thoroughly between steps. 11.Repeat step 10,using the I pm grit and then the 0.3 micron grit final polish.Rinse and cleanthe connector between each step. 12. Inspectthe finishedfiber usingthe F-ML1Inspec- tion Microscopeor other suitablehigh-power microscope. 13. When two connector halveshave been com- pleted,slip the full sleeveover the end of one of the halves. Screwthe connector half onto the inline adapter.Screw the secondhalf onto the in-lineadapter; the connectorhalf will pressfit into the full sleeveas you tighten the connector. 14. The testingof the connectorto determinethe connectionloss is similar to the measurementused in the Figure 6.14. Use a Figure-8 motion in the polishing fiber attenuationproject (Project #2). With the connector of the fiber. mated, couple the He-Nelaser output into the fiber. Mea- sure the power through the connector.Demate the connec- tor and measurethe power from the first connectorhalf. Determine the connector loss from this data. Mate and dematethe connectorseveral times to check the repeatabil- ity of the loss figure.

59 17 r

15. The lossin a connectoris dependenton the modal distributionof light within the fiber core.If lower- order modesare preferentiallylaunched, the power in a graded-indexfiber will be concentratedtoward the center _ __I of the giving Sleeve core, lower connectionlosses. If higher-order J modesare launched,the light will tend to be more toward the perimeterof the core,yielding more sensitivityto align- I ment losses.Use the FM-l ModeScrambler as was donein ,\J\ Project2 (Section 2.5, Steps 4 and 5) to generatean r approximationof a stabledistribution. This will resultin lossmeasurements which will better approximatethose Alignmenl which would resultin actualfield practice. Guide 6.5.3 SPLICE ASSEMBLY AND TEST - 1. Preparetwo segmentsof F-MLD fiber, each about I meter in length. - Figure 6.15. Contents of the TRW Optasplice". 2. Identify the component parts of the TRW Optasplice'" (Newport module Model # F-SK-S).The parts I containedin the plasticenvelope are: one fiber alignment guide, which comes in a protectiveplastic tube, two silicone rubber strain reliefs,and one stainlesssteel sleeve (Fig. 6.15). I 3. Placethe alignment guide into the splice fixture (F-SK-C)(Fig. 6.16). j 4. Slidethe stainlesssteel sleeve onto one of the F-MLDfiber segmentsto be spliced.Slide one of the sili- cone rubber strain reliefsonto each of the fibers.The ta- pered end of the strain relief leadsup the fiber, away from

the fiber end to be spliced(Fig. 6.17). I 5. Cleaveboth of the fiber ends which are to be joined together.When the cleaveis completed, fiber each a should have about 7/16-l/2 inch of bare fiber extending out of the fiber jacket. Coupleone of the fibers to be splicedto the HeNe laser,using the F-916Fiber Coupleras I you did in Project#2 (Section 2.6, Step 3). Inspectionof the cleavedfiber ends will not be necessary.Simply go on t Figure6.16. Placement of the alignment guide tn to Step 6. the splice fixture. 6. Measurethe optical power coming from the first t fiber. Placethe two fibers in the dry fiber alignment guide (no epoxy has yet been applied),one at each end of the ; guide, as in Fig. 6.18. Insert the fibers until they butt to- TaperedEnd gether at the middle of the guide. Measurethe optical , of StrainRelief power coming from the far end of the secondfiber. The loss which is now measuredwill be Fresnel-reflectionlim- G- ited, since the splice is still dry, but it will give you a good indication of the quality of the fiber ends. If the measured loss is not lessthan about 0.5 dB, it is a good indication that the fiber ends have not been properly cleaved.The other major contributor to spliceloss is fiber eccentricity.If - rotating the fiber changesthe measuredsplice loss,then TaperedEnd problem t fiber eccentricityis a in this splice.If the splice 1 ol StrainRelief loss is too high and eccentricityis not a problem, return to Step 5 and recleavethe fibers. Figure 6.17. Positioning of the sleeve and strain relief a on the fiber before splielng. J

-

-

=

= 60 I

\ -{ 7. Now is a good time to return to Step 6 and gain experiencein handling fibers in this alignment guide before you make the permanentsplice. 8. Remove the fibers from the alignment guide. Mix the epoxy to be used,as you did in Step 2 of the connector procedure, above. 9. Coat the bare ends of the fibers with epoxy. A toothpick or the end of a small paper clip makesa good applicator. The epoxy will both wick into the alignment guide and be carried into the guide by the fibers when they are inserted. 10. Again insert the fibers into the alignment guide as you did in Step 5. Minimize the splice lossby again rotating the fibers, if necessary.The epoxy will be cured and the splice may be handled in about l0 minutes.The curing of the epoxy can be checkedby testingthe remnant of the mixed epoxy which was not used in the splice. Figure 6.18. lnsertion of the fiber ends into the 11. After the epoxy is cured, remove the alignment alignment guide. guide from the splice fixture. Slide the rubber strain reliefs down the fiber and onto the alignment guide substrate. Note that the flat side of the strain relief sits on the align- ment guide substrateand that the tabs on the substratefit into the notchesin the strain relief (Fig. 6.19). 12. Slide the stainlesscover over the splice.Use the crimp tool on the back side of the splice fixture to complete the splice,as shown in Fig. 6.20. Crimp at the red bands on the cover.The permanently splicedfiber is now complete. FlatSide 13. Measurethe power through the completed of splice. StrainRelief Calculatethe splice loss again. Comparethis with the value Downon found in Step 6. Substrate 14. Lossesin permanent splice are sensitiveto the way in which light is launchedinto the fiber in the same Figure 6.19. Placing strain reliefs on the alignment way that connectorsare, as discussedin Step 15 of the guide substrate. connectorassembly procedure. Use the FM-l Mode Scram- bler to generatean approximationof a stablemode distribu- tion as you did in Step 15 of the connectorassembly and remeasurethe splice loss. 15. When you find that you can make a good splice using the multimode F-MLDfiber, you may wish to try splicingthe F-SVsingle-mode fiber which you used in Proj- ect #3. Typical high-qualitysplices will have -0.5 dB loss for the single-modefiber, comparedto -0.2 dB for the multimode fiber.

Figure 6.20. Crimping the completed splice in the crimp tool at the back of the splice fixture.

6t -

= 7.O PROJECT#72 = COMPONENTSFOR = FIBER COMMUNICATION =

= In this project,the componentswhich are neededto build a fiber data distributionsystem will be explored. ==- Fiber optic componentsfor data distributionare of impor_ short-haulapplications, !ll:9. i" suchas localarea networks = (LAN's),which provideinformation transfer within struc_ S turessuch as officebuildings and manufacturingfacilities, and in suchvehicles as aircraftand ships.These compo- =- nents are alsoused in longer-haulsystems in order to fully _1 utilizethe bandwidthof the opticalfiber. = Currentdesign trends are towarddevice miniaturiza_ tion and integration at the systemlevel. These result in = smallersize, lower weight,lower power consumption,and higherreliability. Although a large numberof components =E havebeen inventedand usedin fiber systems,this project will concentrateon two devices, the fusedbidirectionai = couplerand the wavelengthdivision multiplexer (.WDM). -' which are representativesof this larger group.This will _- give you the opportunityto learn aboutparimeters which q are common to almostall fiber devices,such as excessloss, ) splittingratio, directivity, and crosstalk. = 7.1 MULTIMODEBIDIRECTIONAL d COUPLERS ) The need for devicesto combinelight from several I fibersinto a singlefiber or to split the light from one fiber ) into two or more fibershas been recognizedever since F opticalfibers were first consideredas a communications medium. 2x2 (2 fibers ,A in, 2 fibers out) bidirecti;;"i # coupler,r'2which is a devicefor splittingthe light in one ] input fiber into two output fibers,is illustratedschematically !J in Fig.7.l. In fiber optic systemsthey play a role which is equivalentto that of beam splittersin conventional ,= opticsor Ttaps - in electronics.Bidirectional couplers play I a criticalrole in fiber optic instrumentation,high_data-rate .) LAN's,and communicationssystems. Star couplers are ts nxn (n fibersin, n fibersout) device3 of the samebasic ) construction. -, Bidirectionalcouplers can be classified into two broad ) Fiberstwisted and fused to form fused biconical raper categories,those that useconventional optical beam split- =- ting techniques'andthose that couple betweenthe 7.1. Schematic illustration "n"rgy J Jigrre of the fused two fibers using the overlap of the evanescentwaves (Sec_ ,a biconical taper bidirectional coupler. - tion 0.2.2) of the light in the claddingof the fibers.Several | designsusing beam splittingtechniques have been devel- r-1 €1 oped.These have not found widespieaduse because of the problemof maintainingthe alignmentof discretemicro- - ). optic componentsunder extremeconditions. Ho*"u"r,1f,i, 4 type of device has an advantage over the evanescenrwave J couplerin that the modal contentof the input fiberrruvr isrr pre-vrL V, servedin the output fibers. _J When the cores of two fibers are closetogether, the #, energyis sharedbetween the two overlappingevanescent I wavesand energyis transferredfrom one fiber to the other. El For this reason, -- energy coupling between two fibers by this | technique is known as evanescentfield couplinq.The eva_ -1 btr nescentfields will penetratesignificantly into tn1 .iuOOing

62 J ) --.J a, l I only for the highest-orderrays, i.e., those nearestthe criti- cal angle for total internal reflection.Because of this, the coupling obtainedusing this techniqueis highly dependent on the modal content of the input fiber. If the light in the input fiber is carried in predominantlyhigher-order modes, more of the light will be coupledto the secondfiber. If the light is in predominantlylower-order modes, less light will be coupled.Bidirectional couplers which use this technique actually achievetheir specifiedcoupling ratio when light is evenly distributedamong the allowed modes of the input fiber. Light which comes out of Port 2 (usingthe notation of Fig. 7.1) will be in mostly high-ordermodes, while the light remaining in the input fiber and coming out of port 4 - will be in mostly low-ordermodes. This will also have an effect on the performanceof any other bidirectionalcoup- lers which are now connectedin serieswith the first coup- ler since the distancebetween couplersis not likely to be great enough to allow the light to be redistributedamong the modes.Couplers downstream from port 2 will split off more than the specifiedpower, while couplersdownstream from Port 4 will split off lessthan the specifiedpower. [This discussionof the modal dependenceof the evanescentwave coupling mechanismapplies to multimode couplers.Although they are harder to fabricate,single- mode couplersare simpler to understand,because they have only a single mode involved in the coupling.you will have a chanceto work with single-modebidirectional coup- lers in Project#10.1 Becauseof the exponentialdecay of the evanescent field, the two fiber coresmust be brought in closeproximity for electric field overlap and optical power coupling to occur effectively.The most commonly employedtechnique for fabricatingthe coupler is called the fused biconical taper technique. In this technique,the two fibers are twisted together and then heatedto about 1500.C.While the fibers are being heatedthey are subjectedto a tension which causesthem to stretch.This stretchingincreases the coupling between the two coresby two mechanisms.The first is due to the fact that when the fiber is stretched,the fiber cladding is taperedand becomesthinner, causingthe two coresto come closer together,thereby increasingthe evanescentfield overlap.The secondcomes about because of the thinning of the core itself, which causesa decrease in the V-numberof the fiber (seeSection 0.3.1). This causesthe fields' exponentiallydecreasing evanescent wavesto decreaseless rapidly as a function of radius,and this also increasesthe field overlap. In practice,the stretchingis continueduntil the desiredcoupling ratio is achieved.When the heating is stopped,the core separationis permanently locked in place. Couplersmade by the fusedbiconical taper techniqueare extremely insensitiveto temperaturechanges. The bidirectionalcoupler which resultswas shown in Fig.7.l. Light which is input at Port 1 may either come out of the same fiber at Port 4 or be coupledto the second fiber and emerge at Port 2. A small amount of light may be returned to the input port of the secondfiber, port I 3, I

I 63

) -

becauseof backscatteringand reflectionsdue to the con- E striction at the taper.The coupling ratio, or splitting ratio, is definedas the power coupledacross to the second fiber dividedby the total powerthrough the coupler Pulsed (Pr/Pr+Pn).Typical coupling ratios for commerciallyavail- LASCT F able devicesare 3 (50%),6 (25%),and l0 dB (10%).The

exce$s loss is defined as the total output power divided by t the total input power (P,+P,/P,).Early couplersexhibited lossesin the rangeof 2-3 dB. Mostcouplers today exhibit t lossesin the rangeof 0.5-1.0 dB. The directivity of the coup- ler is definedas the power returnedto the input of the second fiber dividedby the input power (P,,/P,).This ratio is typically -40 Detector Amplifier about dB for good couplers. In communicationsapplications, the bidirectional I- coupler is more often used as a tap. Its applicationis in Figure 7.2. Schematic diagram of an Optical Time Domain Reflectometer. tapping information off of a main data bus or in inserting , information onto the bus. In instrumentationapplications, the major use is in Optical Time Domain Reflectometers3 (OTDR's),which measurethe lossesin fiber optic systems. In essence,the OTDR is a one-dimensionaloptical radar = which providesan echo scan of the entire length of an optical fiber (Fig. 7.2). lt operatesby periodicallylaunching short-durationpulses from a laser diode into one end of the >. fiber being testedand measuringthe time-dependentchar- acteristicsof the light which is reflectedand/or backscat- t- tered to the same fiber end. The reflectedsignal, measured at the launch end of the fiber, is used to determinethe LogP(t), dB location and magnitudeof discontinuitiesin the fiber.A time-of-flightmeasurement allows the OTDR to determine the location of faults in the system.The bidirectionalcoup- ler directsthe pulse into the fiber and directsthe backscat- tered signal to the detector.A typical OTDR output is shown in Fig. 7.3. The slope is twice the attenuationcoefficient of the fiber. Faultsin the fiber, such as connectors,splices, or l- breaksare seen as abrupt changesin the amount of light returned to the detector. f-

7.2 WAVELENGTHDIVISION MULTIPLEXER - The wavelength division multiplexel (WDM) pro- .. vt vides the equivalentof multiple transmissionlines in a 2 single fiber by sendingsignals at various wavelengthsin Distance,m the same fiber. The WDM enablesan optical fiber to be used more effectively.It allows a greater use of the band- - Figure 7.3. L,ogarithmic plot of an OTDR signal. width of the fiber, allows bidirectionalcommunications, and When the optical porfler detected is plotted as a allows the simultaneoustransmission of differenttypes of function of distance to the scattering center in the signals,such as analog and digital data. fiber, the slope of the signal will be twice the A WDM device consistsof two or more input fibers, attenuation coefficient of the fiber. Also shown is each carrying a signal at a different wavelength,an output the effect produced by a connector and the effect of the reflection at the far end of the fiber. fiber which is connectedto the transmissionlink, and , wavelength-selectiveoptical components,such as gratings, 2 prisms,or thin-film filters,for combining the signalsat differentwavelengths onto the output fiber. A simple two- channelWDM, using two quarter-pitchgraded-index (GRIN)- tt rod lensesand a wavelengthselective filter, is shown in 4 Fig.7.4a. (SeeSections 0.2.4 and 5.3 for an introduction to GRIN-rodlenses.) Signals carried on two different wave- lengths,\t and \2, are input from Fibers I and 2, respec-

=

64 b J tively. The quarter-pitchlens collimatesthe input beam at the point where it seesthe filter and the secondlens refo- cusesthe beam into the output fiber. The wavelengthselec- tive filter reflects\, and passes\, to be focusedat the output, Fiber 3, which is placed symmetricallyabout the lens axis with respectto Fibers 1 and 2. Demultiplexingof the signalsat the receiver is accomplishedby simply revers- ing the path directions.Two-way transmission can be Source#1 accomplishedby replacingone of the sources,for example, Source#2, with a detector,as shown in Fig. 7.4b. The Figure7.4a WDM then insertsX, onto the transmissionlink and receivesXr. The WDM at the far end will send \, and receivetr,. In this project, you will constructa two-channelWDM of the type describedabove. The wavelengthselective filter is not used in WDM's that have higher channel count, becausean N-channelWDM would require N-l filters.This WDM would require the cascadingof severalfilters and this would greatly increasethe lossesin the device.WDM's of Source#1 Filterreflects L1 Detector#2 this type generallyuse a prism or a grating.A designfor a andpasses \2 five-channelWDM using a quarter-pitchGRIN-rod lens and Figure7.4b a reflectivediffraction grating is shown in Fig. 2.5. WDM devicesof this type allow more of the total bandwidth of the optical fiber to be used (seeSection 8.1.2). WDM's {igttt 7.4. _Interferelgg filter type wavelength employing gratingsare now availablewith more than division multiplexer (WDM). a) for two.channel ten channels. transmission. b) for two.way transmission. The two most important parametersin characterizing a WDM are the insertion lossand the crosstalk.If the power to the device from the ith source is P, and the total power from all sourcescombined on the output fiber is pr, then the insertion loss is defined as P',/DP,.Typical insertion lossesare about l-2 dB, dependingon the type of wave- length selectionused, but may be as high as 8-10dB when LED sourcesare used in a grating WDM device.The GlassWedge crosstalk, or isolation, is determinedafter the signalsare FiberArray demultiplexedat the receiver end. The crosstalkfrom chan- nel i into channelj is defined as the power receivedfrom 6 =l-A source i divided by the power receivedfrom sourcej when ;l both are measuredby the detector for channetj. Typical SLL valuesfor the crosstalkin availablemultiplexers is generally 1 lessthan -20 to -30 dB. WDM OUI 7.3 REFERENCES \r+\2 GRINLens l. M. K. Barnoski,in Fundamentalsof Optical Fiber ReflectionGrating Communications,2nd Edition, M. K. Barnoski,ed. AcademicPress (New York), 1981,p.334 Figrye, !.5. Fivelchannel grating WDM using a 2. S. D. Personick,Fiber Optics, kchnology and graded-index (GRIN) rod lens. Applications,Plenum Press(New York), 1985,p.107 3. ibid.,p.236 4. ibid.,p.110

65 1r'

7.4 PARTSLIST

Cat# Description Qty. F-MLD-s0 l00ll40 MM fiber, 50 meters XSN-22 2x2 Breadboard u-l30lP I mw HeNe laser 807 Lasermount 340C Clamp 4l Short rod 815 Powermeter F-916 Fiber coupler (w/o lens) M-2OX 20XObjective lens FK-BtX Balldriver set F-CL1 Fiber cleaver SK-25 %-20Screw kit B$05 Z+20Thumbscrew kit B-1 Base 2 VPH-2 Post holder 10 SP-2 Post l0 SP-3 Post 2 CA-2 Universal clamp 2 FP-I Fiber positioner 4 FM-I Mode scambler I F-925 GRIN-lensfiber coupler 2 FK-ILD LaserDiode assembly FK-LED Light-emittingdiode assy. FK-DRV ILD LEDDriver circuit FK-GR29 NSGSLW-1.&0.29lens 2 F-IRCI IR phosphor card I MH-2PM Optics holder 4 F-TKIE 20 pkgs. epoxy F-SK-C Splicefixture I F-SK-S l0 TRW OptaspliceModules FK-BTC ADCbirdirectional coupler FK-MUX Multiplexerassy. base 2 FK-GR25F NSG0.25-p lens w/ filter 2 I i FK-GR2sP NSG0.25-p lens w/o filter 2 FPH-S Fiber chuck 2 MPC Collar 2

Additional equipment required: Methylene chloride for stripping the jacket from fibers. (Many commercial paint strippers contain methylene chloride and work well for this purpose.Methylene chloride is toxic and can be absorbed .t through the skin. Any methylenechloride which gets on your skin should be washed off immediately.) Glycerin (glycerol) for index matching. 7.5 INSTRUCTIONSET

7.5.T BIDIRECTIONAL COUPLER l. Remove a section of the outer jacket from the cabled fiber pigtail of the FK-BTC bidirectional coupler at Port l. Cleave the end of the fiber. (You can actually use any of the four ports of the coupler, but using port I will allow you to use the notation of the previous discussion.)

66 E

2. Splicea one-meterlength of F-MLDfiber to the cleaved fiber as you learned to do in Project #6 (Section 6.5.3). This will make it much easierto handle the input to the coupler. However, the splice must be done properly, or there may be significantmode coupling at the splice and the following exercisewill be meaningless. 3. Couple HeNe laser light to the fiber at Port #2 as you did in Project #2 (Section 2.5; Step #3). The labora- tory set-upfor this exerciseis shown in Fig. 7.6. Arrange the input so that low-order modes are preferentially launched into the input fiber. You can check this by visually examining the output from Port #4. (Low-ordermodes will be confined to low output angles.Arrange the launch condi- tions so that the output cone at Port #4 is as narrow as possible.)Measure the outputs at Ports#2, #3, and #4. 4. Changethe launch conditionsso that high-order Figure 7.6. [.aboratory set-up for studying the modes are preferentially launched (large output cone). bidirectional coupler. Again, measure the output powers. 5. Return to the launch conditionsof Step #3. Use the FM-l mode scrambler as you did in Project #2 (Section 2.5, Step 5) to generate an approximation of a stable distri- bution in the launch fiber. Measure the output powers. 6. Put some index matching fluid such as glycerin on the fiber ends at Ports #2 and #4. Remeasurethe returned light at Port #3. Index matching will reducethe light returned to Port #3 by Fresnel reflections from the far end of the fiber at Ports #2 and #4. 7. Compare the results of Steps#3-#6. What condi- tions are assumedwhen the manufacturer quotes a 3 dB splitting ratio and a directivity of -40 dB?

7.5.2 WAVELENGTHDIVISION MULTIPLEXER l. Prepare four lengths of F-MLD fiber, each about I to lr/z meters long as you did in Project #l (Section Fiberslay parallel inslot 1.6.1, Steps l-3). Constructan array of two fibers as fol- lows: Lay two lengths in a single FPH-Sfiber chuck (Fig. 7.7a) so that they are parallel and so that the fiber end faces are co-planar (Fig. 7.7b). Mix some of the two-part Fiberend faces must epoxy (F-TKIE) and place a drop at both ends of the fiber becoolanar chuck (Fig. 7.7c). Make sure that the epoxy does not run (b) over the end faces of the fibers or over edge of the chuck, becausethis chuck will have to be inserted into an FP-l fiber positioner. Allow the epoxy to fully cure. Beadof epoxy 2. Repeat Step I with the other two lengths of fiber and a secondFPH-S chuck.

Figure 7.7. Construction of a two.fiber array. (a) I^ay two cleaved fibers in an FPH.S Fiber Chuck. (b) Fibers must lay parallel in the chuck with co-planar end faces. (c) Apply a bead of epoxy at each end to hold fibers in place. Do not allow epoxy to run over the edge of the chuck or over the end faces of the fibers.

67

L- rr. Its .-J'

You will now assemblethe lensesfor the WDM Y Undcoated 3. a--- device.You will need two of the GRIN-rodlenses as shown which 't- t in Fig. 7.8a. One is coated with an interferencefilter putr"i th" light from the ILD and reflectsthe light from the ModelNo. FK-GR25P The other does not have this - LeO 6,loa"t No. FK-GR2SF). _- Anti-reflection (Model No. FK-GR25P).Both are coated coatedends interferencefilter on one end with an anti-reflectioncoating. Place one of J Interference E a-- filter each of these lenseson an FK-MUX Multiplexer Assembly coatedend Base,with the antlreflection coated end (markedwith a < I- small black dot) toward the outsideof the assemblybase as ModelNo. FK-GR2SF in Fig. 7.8b. Epoxy these in place,making sure that there is a film of epoxy between the two lensesfor index match- -_t (a) 1 ing- purposes. -l +. fne procedurelisted in Step #3 for assemblingthe g Iensesof the WDM was quite involved. However,you now -J need to repeat the procedureof Step#3 in order to assem- - ble the lensesfor the wavelengthdemultiplexer device' 5. Prepareanother l to l-l/2 meter length of F-MLD J -F fiber and couple the output of the ILD into it as you did in (Section5.6.1, Steps 1,2,8, and 9)' The pre- Project#5 ,- noted in that instruction set still apply here' This I "uuiiontfiber will be fiber #2 of Fig.7.4a. Use the 815 PowerMeter to measurethe amount of power coupledinto this fiber' ltt 6. Couplethe light from the LED into one of the fibers which you have epoxied into one of the FPH-SFiber fr (Section 5.6'2, Steps I Fisure 7.8. (a) GRIN'rod lenses used in the Chucksas you did in Project #5 'the the 815 asiembly of WDM device. (b) Assgqrbly of the and 6). tfrii fiUer will be fiber #1 of Fig. 7.4. Use -J GRIN.rod lenses on the FK'MUX Multiplexer power meter to measurethe amount of power coupled into be fiber #3' I Assembly Base. iiUer#1. The other fiber in the samechuck will )- 7. Postmount one of the WDM assemblybases with lensesepoxied in place. Use the SP-3Post so that the the YJ lensescan be aligned at the proper height with respectto the fibers in the FP-l Fiber Positioner.Place the chuck with fibers #1 and #3 in a post-mountedFP-l Fiber Positioner lr- (seeFig. 7.9). J S. Couplelight from fiber #l to fiber #3 by aligning ..-ta- the fibers with the Iensesas shown schematicallyin Fig' 7.10. When the two fibers are Iocatedsymmetrically about -J the center line of the lens, LED light from fiber #1 will be { ---J reflectedfrom the interferencefilter, which is now located >. .D between the two GRIN-rodlenses, and will be focusedinto I fiber #3. lr. 9. Place fiber #3 in an FPH-SFiber Chuck' Hold this chuck in an MPC Collar and post-mountthe assembly' - Positionthe 815 PowerMeter so that you can measurethe - coupled' por//er. '1..J 10. Measurethe power in fiber #3 and calculatethe loss of the WDM for this channel. insertion "J 11. OnceStep l0 is completed,place the fiber into 3- which you have coupled light from the ILD in an FPH-S n Fisure 7.9. Construction of the WDM device. chuck ind place this in an FP-1Fiber Positioner'Post mount ,/ Ar:rangement of the components is in an the arrangementof equipmentshown e orientition which agrees with that shown this FP-1,completing schematically in Fig. 7.4a. in Fig. 7.9. t= i - tai

l-

U-l I E= 4 I 68 lJ t\-I \ --!

12. Couplelight from flber #2 into fiber #3 as shown in Fig. 7.11. Adjust fiber #2 until the maximum coupling is achieved.Flber #2 will be symmetricallyacross the lens center line from fiber #3 at the oppositeend of the lens assembly.Do not move the position of fiber #3, as this will reducethe coupling from fiber #l into fiber #3. Measure Fiber#1 the ILD power coupled into fiber #3 and calculatethe inser- tion loss for this channel. NOTE: If you are also going to do Project#8 as part of this series,you may wish to leave off at this point and do the rest of the constructionat the beginning of the next session. Fiber#3 13. Using one of the spliceswhich you used in Proj- ect #6 (Section 6.5.3), dry splice (no epoxy) fiber #3 to one of the fibers which have been epoxied into the other FPH-S Figure 7.10. Coupling light from fiber #l to fiber chuck. Measure power the optical for each channel coupled #3. When the fibers are placed symmetrically about through the splice. the center line of the lens, light out of fiber #l will 14. RepeatSteps 7 and 8 for this end of the link. You be reflected by the interference filter and be are now putting together the demultiplexer,so fiber #3 is focused into the core of fiber #3. the input, with channelsI and 2, and fiber #1 is the output (the arrows indicating the light path are reversedin Fig. 7.10). Measurethe LED power in fiber #l and calculatethe insertion loss of the demultiplexerfor this channel. 15. RepeatStep ll for the demultiplexer.The fibers with the demultiplexedsignals may be placed in the re- maining FPH-SFiber Chucks.Hold these in two post- mounted MPC Collarsto couple to the 815 PowerMeter. Measurethe ILD power in tiber #2 and calculatethe de- multiplexer loss for this channel.The arrows indicatingthe Fiber#2 Fiber#1 light path are reversedin Fig. 7.ll for the demultiplexer. The completedmultiplexer-demultiplexer system is shown \ in Fig. 7.12. 16. Measurethe ILD power in fiber #l and the LED power in tiber #2. Calculatethe crosstalkin each channel using the definition given in Section 7.2. Fiber#3 Figure 7.11. Coupling light from fiber #2 to fiber #3. Fiber #2 is placed symmetrically across the center line of the lens from fiber #3 at the opposite end of the lens assemblv

a Figure 7. 12. Completed multiplexer-demultiplexer system.

69 tr- 8.0 PROJECT#8: FIBER OPTIC COMMUNICATIONLINK

Previousprojects have concentratedon the basic rh propertiesof fibers,the coupling of optical power into opti- cal fibers,and fiber optic components.In this project,the techniquesand devicesexamined in precedingprojects will ,,b be utilized to constructa fiber optic communicationlink, as an illustrationof the most important and widely used appli- cation of fiber optic technology.Communication links are constructedto acceptdata in either digital or analog form. This project will take two analog inputs,wavelength divi- sion multiplex them onto a single fiber, and demultiplex them at the receiver end of the link.

8.T DATA COMMUNICATIONLINKS -t A schematicillustration of the analog communication link which will be constructedin this project is shown in Fig. 8.1. The outputs from a light-emittingdiode (LED)and + \2 DEMUX an injection laser diode (lLD) are both launchedonto the same fiber using a wavelengthdivision multiplexer (WDM), which was built during Project#7. The audio output signal of two radios,which are tuned to two differentstations, will be the modulation sources(see Section 0.5) for the two Figure 8.1. Schematic of communications link to be diodes.At the receiverend of the link, a demultiplexer built in this project. (DEMUX),which is a device identical to the WDM, but with light paths reversed,will separatethe two signalsand direct them to their respectivedetectors. The output of the two detectorswill drive the speakersof the radios.In this way, you will be able to hear the resultsof sendingtwo signals over a fiber optic communicationlink and will also be able l-{ to listen to the crosstalkto gain a qualitativeas well as a I quantitativeappreciation of this important link parameter. lr- Another option in building the link is to exchangepositions provide two-way of one of the sourceswith its detectorto a - communicationlink.

8.T.T SYSTEM LOSSES In order to determine the performancecharacteristics of a fiber communicationslink, severalfactors must be € taken into account.These include sourcepower characteris- ) tics, intrinsic fiber transmissionlosses, coupling and connec- ts- tion losses(all of which have been treated in previous projects), and the receiversensitivity. The soulce character- -:-= istiis (Project #5) include total power output, wavelength dependence,and NA and area mismatchwith the fiber. 4, € The fiber loss (Project #2) is the attenuationof the fiber in dB/km at the particular wavelengthbeing used multiplied by the lengthof the link, in kilometers.At eachfiber joint b- in the link, whether it be a connector or a splice (Project .../ #6), there will be a loss which must be included.The total ts lossin the link will be the sum of all of these lossesplus _-/ the loss due to the quantum efficiencyof the photodetector. 5.r (fhe quantum efficiency of a detector is the fraction of given pro- incident photons at a wavelengthof light which -!= duce eleitron-holepaiis.r The quantum efficiencyis repre- =

U 70 I {t

F / >-

I sented by =-l 4.) For the silicon detector used for this project, TABLE 8-I l_ the quantum efficiencyis 0.62 (representinga loss of Power - Loss Budget : 2 dB) at a wavelength of 90d nm. I When all of the lossesin the link have been esti- Source: ILD LED :a mated, they are summed to give a total system loss and are Powerfrom source I then subtracted from the input power to find the received 0.0dBm +7.0dBm !!. power at the end of the link. Such a calculationis called Losses: - I the power budget ol the link. This is usually done in tabular -Source.fiber connection, 4.0dB 18.0dB L. form as part of the formal design of a fiber optic communi- WDMfilter. 0.5dB 1.5dB i--t cation link. An estimate of the power budget for the data WDM-fiberconnection_ 0.5dB 0.5dB I - link describedabove is shown in Thble 8-1. The estimated Fibertransmission (0.5 km) 1.8dB 1.8dB - valuesin the table may be slightly different than those . Fiber-DEMUXconnection 0.5dB 0.5dB l_ which you actually find in practice in this project. flhe DEMUXfiltea 0.5dB 0.5dB -3 term dBm used in the table means"dB wiitr r"tpeit to Fiber-detector(4) 2.0dB 2.0dB I mW of optical power," as was discussedin Section 2.1.1 l_ " Total losses 9.8dB 24.8dB f{ 8.1.2sysrEM BANDwTDTH Detected power -9.8 dBm -17.8dBm l-- When the modulationof the light transmittedby the f{ link varies at a low rate, the detectorcan faithfully repro- L- duce the signal.However, when the modulatedlight varies l-5 at a high rate, the detectormay not be able to trick the variations. l_ - This frequencyresponse limitation may be due H to ordinary circuit limitations(capacitance and inductance I of the circuit) or to intrinsic electron-holerecombination -:-1 times.of the semiconductor - device.The bandwidth of a detector I is a measureof the modulationrate to which it !-f can respond.The 3-dB bandwidth is defined as the modu- I " lation rate at which the detectoris only 50% efficientin f--- respondingto the variation in the modulatedsignal.z The i-I detectorb"andwidth can be relatedto an equivalentrise !-- time, which is the time it takes for the detectorto go from i-I zero current to maximum current when a pulse of iight is I incident on it. The rise time of the detectorused in this - project is lessthan I nsec. I W" also need to calculatean equivalentrise time and bandwidth for = - the fiber. This rise time has two factors,the | modal dispersionand the material dispersionof the fiber. f=Z Modal dispersionis due to the differentialtime delay I " discussedin Section 0.2.3. Even when a graded-index ts- fiber is used,small variancesin the index profile from the i't ideal parabolicprofile will result in some differentialtime l_ delay.The modal dispersionterm is found from the l{ bandwidth-lengthproduct of the fiber. For the Newport I F-MLDfiber, which is the fiber used here, this product is = 300 MHz-km. The fiber bandwidth may be founa ty diviA- I ing the bandwidth product by the fiber link length. An equivalentrise time for : - the fiber is approximately0.35 | divided by the fiber bandwidth.For a I km link using the =i F-MLDfiber, this rise time is -1.2 nsec. - | There is another type of dispersionwhich is due to l-- the fact the index of refraction of the core of the fiber, d I n.o.",of Eq. 0-12is wavelengthdependent. When the signal t- carried by the fiber has a spreadof wavelengths,6t of Eq. l-= 0-12 will be non-zero err"n lo, rays traveling th" ,urn" puih. l_ This type of differential delay is due to the wavelength - dispersionof the index of refraction,which was discussed L_ in Section O.2.2;it takes into accountthe wavelength dependentdelays in the fiber. (you will also see this called l--: --1I

-{ I i'-5 l- 7l i- I - z maf€rid dLJpenion in some fiber optics literature.) A typical .JJ nalue of the wavelength dispersion for fiben used in the 75G900 nm window is about 100 psec of delay difference - per nanometer of spectral width per kilometer of fiber length. For an LED with a spectral width of 50 nm, this :t 'Y resultsin a delay of about 5 nsec/km. For laser sources, the delay is essentiallyzero, except when it is comparedto 2- the rise time requiredfor the highest-speeddata links. In a cascade-connectedsystem, the cumulativerise 2 time is found by multiplying the squareroot of the sum of r{ the squaresof the individual rise times by l.l. Thus, when the LED sourceis used,we have :J 1.lx[(l nsec)2+(1.2nsec)2+(5 nsec)2]r/2 : 5.7 nsec. (8-l) -lfy

In the caseof the ILD source,we find a rise time of y 1.7 nsecby replacingthe 5 nsecterm by zero. The 3 dB bandwidth of the systemcan then be found by dividing 2 H 0.35 by the resultingsystem rise time. This gives 6l MHz in -) the caseof the LED sourceand 206 MHz in the caseof the ILD source.From this, one can readily see why laser lr.< sourcesare used for high-speeddata communication. :..l In the analog data link which you will be construct- -t ing, the bandwidth of the systemwill not be a limiting l .1 factor.The ILD and LED sourceswill be driven at audio >< frequencies,which have maximum frequencieson the order of tens of kilohertz. Therefore,the bandwidth requiredby :/ the signal will be ordersof magnitudeless than the system bandwidth which we just calculated. = l-< 8.2 DATA TYPES J Data communicationslinks use two data forms, as was discussedin Section 0.5. Digital communicationsends = data in the form of a binary code of I's and 0's, and, after H the signal is received,it is decoded.Analog communication, which is used in this project, sendsdata as a continuously J varying signal,and the signal level itself is the message =rH which is being transmitted.

8.2.T DIGITAL SIGNALS :a For many transmissionapplications, a binary signal is generated.That is, the optical power coupled into the fiber = makes transitionsonly between two discretelevels, with those transitionsoccurring at only well-defined,periodically :l spaced,points in time. The signal sourcemay also produce another periodic output, known as a clock, which is syn- = chronouswith the data signal. The quality of the receivedsignal is generallyex- = pressedin terms of error rates.In each time slot, there is a chance that the slot will contain a pulse when the signal did not, or vice versa.The most common approachto J expressingthe error rate is to statethe averageratio of the number of errors to the number of transmittedpulses. This is the bit-error-rate(BER). State-olthe-art digital systems will have a BER of lessthan l0-s. t 8.2.2 ANALOG SIGNALS - Y bt Analog signalstake on a continuum of levels.The lf which will be used in this project are analog signals ll signals with direct intensity modulation of the sources.Transmis- J , sion fidelity requirementsare generallymuch greater for :l analog signalsthan for digital signals.These requirements h.f

E1:l 72 si) are expressedin terms of the signal-to-noiseratio (SNR). Typical SNR requirements are 50-70 dB for analog signals. The SNR can be determined by comparing the power re_ ceived in the messagesignal to the noise power generated in the detector and amplifier. For a given noise level at the detector, there is a received power which gives ".uly" 9f SNR: l. This power is defined as the noise-equivalent power (Nne;,r which is an often quoted parameter characterizing receivers. Since the noise at the detectormay be proportional to the system bandwidth, the NEp is usually expressedas watts per root hertz. A typical silicon p-in detectorwhich would be used in a 0.8 micron communicationslink would have an NEp on the order of 10-13watts,/Hz-t/2 for a l0 MHz bandwidth system. This project does not require a detector with that low of an NEP value, becausethe messagewill be at audio frequenciesand will require a bandwidth of onlv a few kilohertz 8.3 REFERENCES l. S. D. Personick,Fiber Optics, Tbchnologyand Applications,Plenum press(New york), 19g5,p.73 2. ibid.,p.53 3. S. D. Personick,in Fundamentals of Optical Fiber Communications,2nd Edition, M. K. Barnoski,ed.. Aca_ demic Press(New York), 1981,p.2gl 8.4 PARTSLIST

Cat# Description Qty. F-MLD-50 100/140MM fiber, 50 meters 1 F-MLD-500 100/140MM fiber, 500meters I (optional) XSN-22 2x2 Breadboard 1 F-CL1 Fibercleaver I 815 Powermeter I FK-BLX Balldriverset 1 SK-25 %-20Screw kit I BS45 %-20Thumbscrew kit I B-l Base 2 VPH-2 Post holder T2 SP-2 Post t2 FP-I Fiberpositioner 4 FPH-S Fiberchuck 2 MPC Collar 2 F-925 GRINlensfiber coupler 2 FK-ILD Laserdiode assembly I FK-LED Light-emittingdiode assy. I FK-DRV ILD LED Driver circuit I FK-GR29 NSGW-l.8-0.29 lens 2 F-IRC1 IR phosphor card I CA-2 Universalclamo 2 MH-2PM Optics holder + SP-3 Post 2 FK-MUX Multiplexerassy. base 2 FK-GR25F NSG0.25-p lens w/ filter 2 FK-GR25P NSG0.25-p lens w/o filter 2 FK-DET Detectorassembly 2 FK-SPKR Speakeroutput 2 F-TKIE Epo>ry I F-SK-S Splices I F-SK-C Splice fixture I

IJ h E

I

Additional equipmentrequired: Methylenechloride a for stripping fiber jackets.(Many commercialpaint strippers contain methylenechloride and work well for this purpose. I Methylenechloride is toxic and can be absorbedthrouBh - the skin. Any methylenechloride which gets on you. ,fin - should be washedoff immediately.)

a 8.5 INSTRUCTIONSET NOTE: I Time constraintsmay dictatethat part of the con- I struction done as part of Project#7 be left until the start of this project.A convenientbreaking point is indicatedin I I that instructionset. l. Disconnect the dry splicebetween the multiplexer a and demultiplexerwhich you made as part of Project#7. Splicethe 500 m spool of fiber which you used in project I #2 onlo the output of the WDM. Make a permanentepoxy you splice as did in Project#6 (Section 6.5.3). (lf you have I chosennot to use the 500 meter spool of fiber, go on to - Step 3, using the multiplexer output and demultiplexer input fibers.) r 2. Measurethe power out of the end of the spool of fiber and calculatethe loss of the transmissionfiber in your : link. (Usean averagevalue from other spliceswhich you

have made for the insertion lossof the splice.) I 3. Splicethe end of the fiber spool to the input fiber of the demultiplexer.Use the power out of the demulti- plexer and the known demultiplexerlosses, from project #7, to find the insertion loss of this last splice. - 4. Completea power budget for your link similar to the one in the introduction. 5. Placea pre-recordedtape in eachtape deck.The ear ,a jack output of the tape deck runs directlyto the modulation - : - * -' inputsof the driver Y;111 ;;:;-;:, 1-,", circuit.The current of both laserand LED - must be setso that the power output of eachdevice is about project #5 half-way.upthe curves which you generatedin I (Section 5.6.1, Step #5 and Section 5.6.2, Step #3).This will allow the tape deck input to modulatethe outputsof the al laserand LED without distortion. 6. Postmount each of the detectors.Run the output a of the detectorsto the two speakers. 7. Mount each of the demultiplexeroutputs in an FPH-SFiber Chuck. Hold each FPH-Sin a post-mounted - MPC Collar. 8. Mount each FK-DETDetector Assembly in a post- - mounted MH-2PMObjective Lens Holder,in the same man- ner as the FK-ILD and FK-LEDassemblies. Plug the output of each FK-DETassembly into an FK-SPKROutput Speaker. 9. Coupleeach fiber output to an FK-DETdetector Figure 8.2. Photograph of laboratory set.up of the assembly.The completedlaboratory set-upis shown in analog communications g.l. link. Compare with ['ig. Fig. 8.2. t 10. Powerup the sourcesone at a time and listen to 4 the sound from each of the speakers.Power up both ,,. sourcestogether and listen to each speakerseparately. How doeswhat you hear correlatewith the crosstalkfigures ta calculatedin Project#7? 1 11. Another project which you may wish to try is a two-way communicationslink. This can be accomplishedby - interchangingthe fiber to one of the detectorswith the corresponding fibea which is the input fiber to the WDM h for the same channel. [Changingthe connectionsin this completedlink to get two-way communicationwill be a project in itself.l =

I 74 DT 9.0 PROJECT#9: MI.JLTIMODEINTENSITY SENSORS

The projects which you have undertaken up to this point have explored the basic properties of fibers, compo- nents for fiber communication, and an analog communica- tion link. Another area of application in which fibers are being greatly used is the area of sensors.In this project, you will be looking at some examples of sensorswhich exploit the optical propertiesand tight-guidingcapabilities of multimode fibers. You will see examples of a variety of sensors,measuring a number of different physical parame- ters. What these fiber sensorshave in common is that they all measure the intensity of the light returned by the fiber from a discretesensor element. Single-mode fiber sensors, which measurethe phasechanges of the light carried by a single-modefiber, will be studied in project #10. The multimode sensorswhich are illustratedin this proiect represent only a small sample of the many intensity sensorswhich have been designed or are in use. Instructors of advanced students may choose to design and construct senson other than those presented here. 9.I FIBER OPTIC SENSORS The properties of optical fibers allow innovative ap- proachesto the designof optical sensors.Because optical fibers are non-conducting, fiber optic sensorsare immune to electromagneticand radio frequencyinterference and are safe in explosiveenvironments. The low attenuation coefficientsof fibers allow sensitiveelectronic equipment to be remotely located. The availability of fiber optics gives the systemengineer great flexibil.ityin sensordesign. Fiber optic sensorsare used in applications ranging from simple countersand limit switchesto measurementsrequiring high precision and accuracy. Fiber optic sensorscan be classifiedinto two general categories,intensity sensorsand phasesensors. These two classesoI sensorsdiffer not only in construction,but also in sensitivity,dynamic range, and signal transmissionand detectionschemes. Phase sensors use single-modefibers and employ interferometrictechniques to extract phase information from the sensor.They will be explored in proj- ect #10, "Single-ModeInterferometric Sensorsl' Intensity sensorsgenerally use multimode fibers and small transducerswhich can make measurementsat specific points along the light path. (A transducer is a device which convertsone physicalparameter into another,mak- ing the measurementof the first parametereasier. For example,the mercury in a thermometer is a transducer that convertstemperature into a column height which can be more easily measured.The transducerin a fiber optic intensity sensor converts a physical parameter into a changein the amount of light which is transmitted.)Optical power is transmitted to the sensor,the physical parameter ry lrr L

to be measuredcauses the transducerto changethe amount of light passedby the sensor,and the power is then returned to the detector. Depending on the type of intensity modulation used,multimode intensity sensorsmay be fur- ther subdividedinto two major groups,hybrid sensorsand = internal effect sensors.Hybrid sensorstreat the fiber as a light pipe, transmittingthe light to a remote sensor,which ,- is generally a miniaturized device at the end of the fiber. Hybrid sensorsallow a broad range of modulationschemes, h Figure 9.1. Liquid level sensor. d" is the critical angle which includes practically all conventional non{iber optical for total internal reflection at a glass-air interface in sensors,limited only by the imaginationof the system the glass fiber. designer.Internal effect sensors,on the other hand, use the fiber itself as the transducer,with the parameterbeing measuredcausing a modulation of the light-guidingproper- b ties of the fiber. The remainder of this discussionwill focuson the - fiber optic intensity sensorsthat will be examined in the | = losin2 zrtS/f laboratory project.The first three referenceswhich appear I I in the list following this introductioncite a large amount of %--+--- recent developmentalwork in fiber optic sensorsand fur- 17'."41 i l-,/,/./l i ther information can be gained from them.r-3 l:/,/,al ----.<,rl D- A 9.2 HYBRID SENSORS The simplestof sensorsacts as an on-off switch to l- detect the presenceor absenceof a stimulusat the sensor - site. An example of this, shown in Fig. 9.1, is a liquid level sensorwhich detectsthe presenceor absenceof liquid in - the gap between two fiber ends.As shown in the figure, the fiber ends are taperedso that the angle of the fiber end E is greater than the critical angle for total internal reflection of the central ray of the beam transmittedby the fiber. F-. Random When a liquid is presentin the gap between the fiber ends, Polarization glass-air internal reflection the interfaceat which total = occursis eliminatedand the index of the glassis nearly Figure 9.2. Pressure sensor using the photoelastic matched by the liquid. When this happens,the light is no l= effect. Unpolarized light is polarized, at an angle of longer totally internally reflectedand optical power is trans- 45' with re$pect to the stress axis, S. Stress causes mitted from one fiber end to the other. birefringence which causes the polarization of the Another, more sophisticatedhybrid sensoris the l-r light to change. Analyzer, A, selects the perpendicular pressuresensor, illustrated in Fig. 9.2, which usesthe component of the beam. photo-elasticeffect to monitor the pressurein a glasstrans- Fr ducer.aPressure applied to the glasscauses stress-induced birefringence,resulting in a change of the polarizationof the beam transmittedthrough the transducer.The polarizer, P in the figure, causesthe light incident on the glassto be - polarizedat an angle of 45" with respectto the applied stress,S. The stress-inducedbirefringence causes the plane l- of the polarizationto change from linear to elliptical.A secondpolarizer, A, which acts as an analyzer selectsthe I E- component of the transmittedbeam perpendicularto the original linear polarization.For the configurationshown, - the opticalpower transmittedby the sensorist e I | : I" sin2(rtSlf), (e-l) b - Ii

L

l--

E-

b ro

.. where t is the thicknessof the glasstransducer, S is the applied stress,and f : 2tSo,where Sois the stressrequired Reflected to make the transmittedpower through the transducergo light returned from a maximumto a minimum.It is calledthe material to detector fringe value. Typicalvalues for f are in the range of 0.2-0.3 MPa-m.This type of sensorhas been used to measureDres- suresof up to 21 MPa(3,000 psi) with a resolutionof better thanAP,/P: l0-a. The previoustwo examplesuse high-qualitylowJoss multimode transmissionfibers so that remote measuremenrs can be made at long distances.Some other fiber sensors make use of lower-technologyfiber bundles.An example of a proximity sensorwhich usesa bifurcated(y-branched) fiber bundleis shownin Fig. 9.3. Opticalpower, which, in this case,can be from a white-lightsource, is launchedin one arm of the fiber bundle. It is reflectedfrom a surfaceat the output end of the fiber, and part of that reflectedlight Figure 9.3. Proximity sensor using a bifurcated fiber is acceptedby the secondarm of the bundle and is trans- bundle. mitted to a detector.The amount of light returned to the detectordepends on the distancebetween the end of the bundle and the surfacebeing monitored. 9.3 INTERNAL EFFECT SENSORS MovableSurface Internal effect sensorsmake use of modulation schemeswhich perturb the fiber itself,so that the fiber is both the transmissionmedium and the transducer.The modulationeffects which may be used in these sensors ,,fiberdyne" includesthe microbendingloss mechanism,the modal intensity modulation effect,in which mode filtering is usedto detect redistribution of the optical power due to Displacementcauses mode coupling causedby mechanicaldeformation of the corrugationstobend fiber fiber, and internally generatedthermal radiation.2 As an example of the sensorswhich can be designed, Figure 9.4. Microbend strain sensor. Structural Fig. 9.4 showsa displacementsensor which makesuse of strain causes corrugations to bend the fiber, the microbendingphenomenon.6 Optical power is coupled causing loss. from guided modesto the cladding when the fiber is bent. The displacementsensor has a fiber placed between two corrugatedplates and the optical lossis measuredas a function of the displacementof the two corrugatedplates with respectto each other. The examplesof multimode fiber optic intensity sen- sorswhich have been discussedhere are, of course,only a small part of the large number of fiber optic sensorswhich have been investigatedor developed. 9.4 REFERENCES l. T. G. Giallorenzi,et al., "Optical fiber sensor technologyj'IEEE Journal on Quantum ElectronicseE-IS, 626 (1e82) 2. S. K. Yao and C. K. Asawa,"Fiber optic intensity sensors,"IEEE Journal on SelectedAreas in Communica- tions SAC-I, 562 (1983)and referencestherein. 3. Proceedingsof the SplE, Fr'berOptic ond laser Sensors,YoL412 (1983),Vot. 478 (1984),Vot. 566 (1935), Vol. 586 (1985) r:l- t- L. t-{ I L- F{" LI i- 77 I " 4. E. Hecht and A. Zajac, Optics. Addison_Wesley (Menlo Park), 1974,p.260 5. ,,Multimode W. B. Spillman,Jr., fiber-opticpressure I basedon I :e^lsor the photoelasticeffect,,, Optics Letters Z, 388(r982) I 6._C.K. yao, I . Asawa,S. K. R. C. Stearns,N. L. Mota, and J. W Downs, ..High sensitivityfiber optic strain a sensor I ror measunngstructural distortion," Electronics 362(rs82) Letters 1g, a I 9.5 PARTSLIST I Cat# Description I Qty. F-MLD-50 - 1001140MM fiber, 50 meters I I XSN-22 2x2 Breadboard u-1301P 1 I mw HeNelaser a 807 I Lasermount I 340C Clamp - 41 I - Short rod 1 dl5 Powermeter I F-CL1 - Fibercleaver I F-916 Fiber coupler (w/o lens) 1 M-2OX I 20XObjective tens I FK-BLX Balldriverset I SK-25 b %-20Screw kit I VPH-2 Postholder 7 SP-2 Post 7 l- FP-1 Fiberpositioner FM-1 J Modescambler t= F-TKIE 20 pkgs. epoxy F-TKIL 160 Lappingsheets ,- FK-LS LiquidJevelsensor assy. FK-PS Pressuresensor assy. base I FK-GT GIasstransducer A = FK-GR25P NSG0.25-p lens w/o filter 2 FK-POL l- Polarizer,sheet I 421-0.5 Translationstage I MPC - Collar 3 rl I I l-l Bifurcatedfiber bundle I t- Addi,ional equipmeatrequired: Laboratory weight set tor.^. the.. pressure sensor.Methylene chloride for stripping fiber jackets-(Many )- comm".iiully availablepaint strippers contain methylenechloride and work well for this purpose. Methylene - chloride is toxic and can be absorbedthrough = ,t-t"ttl. Any methytenechtoride *hich;;;;;n your skin should be washedoff immediately.) l-

9.6 INSTRUCTIONSET l- -'- 9.6.1 LIQUID_LEVEL SENSOR 14 Strip jacket the from the end of two lengthsof F-MLD Iiber. Lay the fiber in the grooves of ine Iiquidlevel a- sensorinserts. There is a recessedarea at the end of the insert into which jacketed the end of the fiber may be set F for strain relief. ? FiSy19.5. Epoxy the fibers into the v_groove of the 2. Use the angled pieces F-TKIE epoxy to mount the fibers in the of the FK-LS Liquid Level"Sensor. grooves g.5. Pr as shown in Fig. Be sure that this includes part jacketed of the fiber in the recessedarea. When .3. the epoxy is cured,potish the fiber ends. I tn9 same generaltechnique you used lLse to-poUrf,tn. \ fibers in the project -l I connectorhalvei in fO-lSttion e.S.Z, ts Step-s6-ll); this time, however,you ao noitu"" a polish_ ing fixture to hold the pieces. !- 78 L.

F I-

\-t f,-. t= 4. When the polishingis complete,place the insert on the sensorassembly base. Slide the insertsagainst the tr rail so that the fibers will be aligned with each other. Adjust the spacingbetween the fiber ends to about 1,/zmm. The actual spacingrequired depends on the surfacetension and viscosityof the liquid which you have chosento use in your experiment.The trade-offis between the fact that the F spacingmust be large enough so that surfacetension will not causea bead of liquid to remain betweenthe fiber tr ends when the liquid level subsides,and the fact that you also want to keep the spacingsmall to maximize the optical coupling betweenthe fibers. 5. Couplelight from the HeNe laser into one of Ethe fibers.Record the optical power through the sensor. The completelaboratory set-up for this sensoris shown in Figure 9.6. Laboratory set-up of the liquidJevel Fig. 9.6. aensor. 6. Immersethe sensorin a liquid bath or place a drop of liquid betweenthe fibers in the gap in the sensor, Polarizing E(Glycerin,water, Sheet or alcohol all work well. Acetonewill tr eventuallydissolve the epoxy.)Record the power through the sensor. -rPolarized r 7. Calculatethe ratio of power betweenthe on and lightpassed off statesof the sensor.

9.6.2 PRESSURE SENSOR 1. Determinethe optical axis of the FK-POLPolariz- GlassSheet ing Sheet.The following simple method can be used to E (a) determinethe optical axis of a polarizer.When viewed at tr an angle of -56', the glare from a sheet of glasswill be Polarizing polarizedin a plane parallelto the glasssheet. (See p.244 r Lightbeam sheet r of Reference4, above,for a descriptionof this phenome- Polarized non.) When viewed through the polarizer,the glare will be light a maximum when the optical axis'is parallel to the plane blocked of polarization(Fig. 9.7a) and the glare will be a minimum when the optical axis is perpendicularto the plane of polar- F ization (Fig. 9.7b). Glass = br 2. Cut two piecesfrom the FK-POLPolarizing Sheet (b) r{ so that the axis of polarizationis at an angle of 45' to the Figure 9.7. Determination the sidesof the rectanglewhich you cut and so that the pieces of optical axis of a polarizer. parallel plane l_ fit the ends of the FK-GTGlass Tiansducer. a) Optical axis to the of polarization. b) Optical axis perpendicular to the plane 3. Epoxy the polarizersto the glasstransducer. Use a of polarization. trr< thin film of epoxy; be sure that there are no bubblesin the epoxy. If the polarizationaxes of the polarizer and analyzer FK-GTGlass Transducer l_ are crossedwith respectto each other, the sensorwill give a sine2stress dependence, as was seen in Eq. 9-1. If the polarizationaxes are parallel,the sensorwill give a cosine2 dependence.Crossed polarizers would be preferredin a - commercialdevice, because the highestpressures would Ftrl give the largestsignals, and a large amplificationcould be used to detect small stressesin a dark background,but parallel polarizersare recommendedfor a student labora- F tory, becausethis makesthe optical alignment of the sensor easier. 4. Glue two l,/+-pitchGRIN-rod lenses (Model # F FK-GR2SP)into the grooves in the pressuresensor assembly base (seeFig. 9.8). Be sure that the ends of the lensesdo b-i- not extend into the central portion of the sensor,which will f-{ be occupiedby the glasstransducer. I AssemblyBlock 01FK.PS

=3F Figure 9.8. Placement of the two l/4-pitch GRIN.rod lenses in the FK-PS Pressure Sensor Assemblv. l-.., 79 z 5. Preparetwo lengthsof F-MLDfiber. CoupleHeNe laser light into one of the fibers using the F-916Fiber Coup- ler. Mount each fiber in an FP-l Fiber positioner,as shown in the laboratory set-upof Fig. 9.9. The sensorassembly should be mounted on poststo provide the proper height for coupling light through the sensor.Maximize the fiber-to- fiber coupling through the assembly. 6. Placethe glasstransducbr in the assembly.Again maximize the fiber-to-fibercoupling. 7. Bring the lever arm down on top of the transducer. Adjust the set screw so that the lever arm is horizontal when it restson the glass.Add weight to the end of the lever arm. Monitor the optical power through the sensoras a function of applied weight. Calculatethe pressurein the glassfrom the applied for,ce,the cross-sectionalarea of the Figure 9.9. Laboratory set-up of the pr,essure sensor. glass,and the mechanicaladvantage of the lever arm. 8. Plot the power transmittedby the sensoras a function of applied pressure.Compare this with a cosine2 curve (assumingthat you used parallel polarizers).Find the pressureat which the transmittedpower goesthrough a minimum. Use this to calculatethe fringe constantof the glassfrom Eq. 9-1.

9.6.3 PROXIMITY SENSOR l. Insert an 8-32 set screw into each of three Sp-2 posts.Screw an MPC collar onto each set screw.Insert each of the three ends of the 777-1dual fiber bundle into each collar.Gently tighten each set screw.Do not overtighten. The laboratory set-upfor this sensoris shown in Fig. g.l0. 2. Mount one of the single ends in a VpH-2 post holder and illuminate this end with a HeNe laser.This sensorcan also be constructedusing a whiteJight sourceif you have a high-powerlamp which can be focusedonto the end of the fiber bundle. 3. Fix a VPH-2post holder onto a 420-0.5translation stageand place the post with the common end of the fiber bundle into this holder.Set up a card so that the HeNe output Figure 9.10. Laboratory set.up of the proximity of the fiber bundle shineson the card or mount the transla- sensor. tion stageso that the output shineson a wall. In either case, the fiber bundle needsto be mountedso that it is able to actuallymake contactwith the surfaceto be measured. 4. Light reflectedby the surfacebeing monitored will be reflectedback into the fiber bundle and will go to the secondsingle end. Mount this end in a VpH-2 post holder and couple the output light onto the detectorof the gl5 Power Meter. 5. Measurethe reflectedoutput as a function of dis- tance,d, of the surfaceof the common end of the bifur- cated fiber bundle from the monitored surface.Also plot this power as a function of l/dz. 6. Find the point where a linear l/d2 dependence begins.This is the distanceat which the finite diameter of I I

1

80 , i the fiber core is no longer significantin determiningthe ReflectingSurface amount of reflected ligtrt accepted by the fiber bundle. Why does the amount of reflected power accepted by the fiber \ /v.. bundle drop from a maximum as you g"f ,r".y close to the \\ I i,.i't surface?IHINT Draw the light cone from the fiber bundle , / ,tl. incident on the surfacebeing monitored and the cone of acceptanceof the neighboringfiber which detectsthe light as in Fig.9.ll. Vary the distancebetween the fiber end faces and the reflectingsurface.) Fiber 7. Estimatethe positionalresolution of this sensor. from Receivingfiber Source 9.6.4. MICROBEND DISPLACEMENT SENSOR l. The FM-l mode scramblerwhich you used in Projects #2 and #6 may be used ur un of a dis_ placementsensor. Rotate the knoL''on the"*urnple FIiI-l to fullv separatethe corrugated surfaces. . _-2, Launch light into a segmentof F-MLDfiber using the HeNe laser and the F-916coupler. The laboratory set_up for this sensoris shown in Fig. g.12. 3. Placethe jacketedfiber in the slot betweenthe corrugated surfaces. 9all. 4. Rotate the knob until the corrugated surfacesjust !'igure lrawing showing Iight emitted by one fiber, reflected by Erontact the fiber. Note the knob position. Each major gradu- the near-bysor{ace, and received by- a neighboring fiber. Repeat ation on the knob representsa 25 micron displacemenl;the this drawing for different fiber.to.surface smaller graduationsmark l2.S micron displacements. distances 5. Record the output from the fiber as a function of displacement.The fiber has a soft buffer,so some period of stabilization may be needed after you set the displicement F for the transmitted power to come to equilibrium. In an actual sensorsystem, the knob-controlleddisplacement r would be replaced by having the corrugated iurfaces at- tached to two surfaceswhose relative position is to be F measured. r==

F Figure l<, 9.12. Use of the FM.l Mode Scrarnbler as a l_ fiber optic microbend displacement $ensor. -- r

T

Lr=l

>7 - tslL-, L-. < 81 a IO.O PROJECT#IO: SINGLE-MODE INTERFEROMETRIC SENSORS

I The previousproject investigatedmultimode intensity sensors.This project will look at single-modeinterferometric sensors.These sensors differ from multimode sensors,not only in the type of fiber used,but in the fact that they ! detect phasechanges rather than the intensity of the trans- mitted light. Thesephase changes are causedby the effects b which any of a variety of physical parameters may have on the fiber itself. In the courseof this project, you will constructa fiber interferometerand use it to measure single-mode ! changesin temperatureand pressure.You will see that this techniqueallows for the constructionof sensorswhich have high sensitivityand high resolution.

I IO.T INTERFEROMETRICSENSORS I Interferometricsensors are sophisticatedsensors which detect the influence of physicalperturbations on the phaseof the light propagatingin a single-modefiber. These sensorsoffer the potential of extremely high sensitivity, - but also require the best single-modefiber technologyavail- able for their construction.The most common phasesen- j sorsuse the Mach-Zehnderinterferometer. The geometry of the Mach-Zehnderinterferometer was describedin Section --l 0.3.5. A fiber optics version is illustratedin Fig. 10.1. The Beamsplitter is replacedby a bidirectional first beamsplitterof Fig. 0.24 a coupler and the light is launchedinto two single-mode fibers of equal lengths.These two fibers replacepaths #l - and #2 and the mirrors of Fig. 0.24. One of the fibers servesas the referencearm, which is kept isolatedfrom - external perturbations,while the other fiber servesas the sensorarm of the interferometer,which is exposedto the Figure 10.1. Mach-Zehnder fiber optic interferometer perturbationto be measured.The external perturbation configuration. inducesthe phaseshift in the sensingarm by means of a change in the optical path length, which is causedby a _= change in the index of refractionof the glassof the fiber fiber. The phaseshift and/or a change in the length of the l- is detectedwhen the two beamsare recombinedat the receiver end of the sensor.This resultsin fringes which can l- be detectedand counted.Sensors using the Mach-Zehnder configurationcan be constructedto measurea wide variety of physicalparameters, including stress,strain, acoustic _2 waves,magnetic field, and temperature.Single-mode optical fibers are employedin sensorsusing other types of interfer- l- ometers,also. The best known of these is probably the u fiber gyroscope for measuring rotation rates, which uses a ft Sagnacinterferometer. I When the Mach-Zehnderinterferometer has been constructed,the light from the two fibers interferesto form a seriesof bright and dark fringes.A change in the phase of the light in the sensorfiber with respectto the phaseof E the light in the referencefiber appearsas a displacementof the fringe pattern; a phasechange of 2zrradians causes a

*

82 a_ Ir

cd displacementof one fringe. The magnitudeof the change in the physical parameter to be measured can be deter- F mined directly by counting the fringe displacement. The phase of a light wave which travels a distance, L. in an optical fiber is given by A:BL, where B is the propa- gation constant of the light in the fiber. Changing any physi- cal parameter of the fiber's elvironment causesa phase F change given by

6d:B6L+168. (10-l)

The first term on the right hand side of the equation Eis due to a change in the length of the fiber, while the second term is due to a change in the propagation constant. The length, L, now representsthe length over which the F physical change affects the fiber. The quantity which we actually wish to determine is the phase change per unit fiber length per unit of physical stimulus, (6d)/SL, where S is the stimulus.The magnitudeof the stimuluscan then be measuredby counting the shift of the fringes for a fiber of Eknown interactionlength. rO.2 INTERFEROMETRICTEMPERATURE SENSOR As an example,consider the effect of a temperature Fchange,dT, which affectsa length, L, of the fiber in the sensor arm of the interferometer. There are two effects tr which occur: the changeof length due to thermal expan- sion or contraction,and the change of the propagation constantdue to the temperaturedependence of the index of refraction.Thus, Eq. l0-l becomes2

6dl(6TL)= (2r/)) [(n/L)(dLl6T)+ (6n/6T)], (10-2) Fwhere = -t = _ r< 6Ll6T [L(I,)_ L(I2)]/[Tr Tr] tr and 6n/6T = [n(I,) _ n(I)]/[T, _ Tr].

= For the caseof a fusedsilica fiber and a HeNe laser source,the following valuesfor pure silica might be used:

1- l/L(6Ll6T)= 5 x l0-zl"C, F---^ 6n/6T:l0xlO-e,/"C, lr< n : 1.456, 5 and \ : 632.8x lO-sm. F This gives

F 6dl(6TL) : 107 radians,/"C-m, (10-3) = which correspondsto 17 fringesper oC per meter of fiber. I This final value must be taken as an order of magni- tude calculation,since both the thermal expansioncoeffi- cient and the temperaturedependence of the index of refraction can vary greatly for multi-component glasses =a such as those in optical fibers (6n/6T may even be negative L-, <' 83 r' in some cases).However, this does show that even using a resolution of one fringe as a very conservative estimate yields a resolution of -0.06 oC-m for a single-modeinter- ferometric temperature sensor. rO.3 INTERFEROMETRICPRESSURE SENSOR As a second example, consider the effect of a pres- sure, P, on a length of fiber, L. flhe following derivation2 is quite involved. Studentswho have not had an upper' division course in solid-statephysics at the level of Kittel3 should proceedto Eq. l0-9.] If the pressureis isotropic, then any component of the stresscan be written as q = -p. The z-component of the strain can then be written as

e": -P(l-2p.)/E, (10-4)

where p is Poisson'sratio and E is Young'smodulus. The first term of Eq. l0.l becomes

B6L= Be"L: -f,$-2ry)LP/8. (10-5)

The second term of Eq. lGl reflects the strain-optic effect where the strain changes the refractive index of the fiber. (Ihere is also a contribution from the change of diameter of the fiber due to the pressure,but this term is negligible compared to the others.)

L60: 116B74n,un. (10-6)

The change in the index of refraction may be calcu' lated from the change in the optical indicatrix, 6(1/n2).This change in the refractive index is found to be

fn - -rl2n3f (1/nz) : -1/zn3e,(2pr2+prr), (10-7)

where p11and pt, are elementsof the strain-optictensor. Combining these last three equations yields

r/zn3(2p,r+p,,)].(10-8) 6dl(6PL) = (-2r}t) (l-21r)/E [n -

Again assuminga HeNe source and a pure silica fiber, we have

n : 1.456, E:7xl07N/m2, ptt : 0.121, PP:0'270' o'17' -r' ': and

I : 632.8x l&s m.

This resultsin

84 6dl(6PL) : -4.09 x l}s radian,/Pa-m, (10-9) F which correspondsto a fringe shift for each 154 kPa-m or 22.3 psi-m. Again, this is really an order of magnitude cal- culation, since, for multi-component glasses,Young's modu- lus may vary by more than 20%, and Poisson'sratio may vary by a factor of 2 from that used in this example. rO.4 POLARIZATION PROBLEMS The polarization of the light traveling in the interfero- metric sensor is important becauseif the polarizations of the two output beams are not plane parallel, sharp fringes will not be seen.In the worst case,when the two polariza- tions are orthogonal to each other, no interference will occur at all (Section 0.3.5). When long lengths of fiber are used in the interferom- eter, polarization-preserving fiber of the type investigated in Project #4 is used. This assuresthat the output beam will be plane polarizedin the same direction as at the input and that sharp fringes will be obtained.When short lengths of fiber, such as the approximately two meters required for this project, are used in the interferometer, standard single- mode fiber may be used. If plane-polarizedlight is launched in the fiber, the light will stay pretty well in its original plane-polarizedmode as long as the perturbationswhich would cause mode coupling are kept to a minimum. The output ends of the fibers can then be manipulatedto cause the polarizations of the output beams to be parallel. F This last statement may sound highly qualitative, but this approach works well for shorter fiber lengths, and avoidsthe need for handling and aligning the polarization- preserving fibers. rO.5 REFERENCES l. L. B. Jeunhomme, Single-ModeFiber Optics, Princi- ples and Applications, Marcel Dekker (New York), 1983 p.251 2. G. B. Hocker,"Fiber-optic sensing of pressureand temperaturei'Applied Optics 18, 1445(1979) E 3. C. Kittel, Introduction to Solid State Physics,Fourth Edition, John Wiley & Sons (New York), l97l 10.6 PARTSLIST

Cat# Description Qty F-SV-20 4/125SM fiber, 20 meters XSN-22 2x2Breadboard u-l30lP I mw HeNelaser F 807 Lasermount 340C Clamp 4l Short rod F-916 Fiber coupler (w/o lens) F M-2OX 20XObjective lens FK-BLX Balldriverset F-CLI Fiber cleaver F-. H' SK-25 %-20Screw kit I MPH-I Micro-seriespost holder

F-i r-L !a 85 z l- L

MSP-I Micro-seriespost MM-I Mirror mount FK-BSC Beamsplittercube - F-925 GRIN-rodlens coupler 2

FK-GR25P GRIN-rodlens 0.25-pitch 2 br FK-PS Pressuresensor assy.base I FK-GT Glasstransducer 2 E F-5068 SM DirectionalCoupler I

Additional equipmentrequired: Methylene chloride l- for stripping fibers.(Many commercial paint stripperscon- tain methylenechloride and work well for this purpose. f* Methylenechloride is toxic and can be absorbedthrough

the skin. Any methylene chloride which gets on your skin l- should be washedoff immediately.)Laboratory weight set for pressuresensor. I

rO.7 INSTRUCTIONSET l-

rO.7.I SETTING UP THE INTERFEROMETER h l. Couplethe output of a polarizedHeNe laser to any L

one of the four fibers of an F-506Bsingle-mode bidirectional E coupler,using the F-916coupler as you learned to do in Project#3 (Section 3.6.f). The fibersused in this project l- may be strippedusing methylenechloride. L 2. Cleavethe two output ends coming from the bidi- rectional coupler. 3. Cleavethe two pigtailsof the output end of the F-5068Bidirectional Coupler so that the two pigtailsare of b equal length. [Remember,in order to obtain sharp fringes, the total optical path lengthsof the two arms of the inter- b ferometer,from the point where the sensorand reference beamsare split to the point where they are recombined, need to be equal to within the coherencelength of the = laser (Section 0.3.4). The coherencelength of the U-1301P HeNelaser is 20 cm.l l- 4. Mount the MM-l Mirror Mount on the MSP-IMicro- SeriesPost and place this in the MPH-I Micro-SeriesPost l- Holder. Placethe FK-BSCBeamsplitter Cube on top of the place E Figure 10.2. Laboratory set-up of the Mach-Zehnder MM-I. The beamsplittercan be held in with double- interferometer. sided tape. Adjust the height of the beamsplitterso that it matchesthe height of the F-925GRIN-Rod Lens Coupler. |l- 5. Placean FK-GR25PGRIN-rod lens in each of the (This in Fig. Insert one of F-925couplers. was shown 5.5.) E the fibers of the interferometerinto each of the F-925coup- lers and adjustthe fiber positionso that the beam out of I the GRIN-rodlens is well collimated.Arrange the F-925 couplersso that the beamscross at the center of the beams- plitter. Adjust the beamsplitterso that the output beams b from the two arms of the interferometeroverlap. The labo- ratory set-upof the interferometeris shown in Fig. 10.2. /

- Y

/

a-

Y

{11 86 ,1 E

6. Two beamswill be coming from the beamsplitter at right anglesto each other. Either oi these beamsmay be F used for viewing fringes.Block the unwantedbeam and place a card in the path of the beam which you will be using at a distancewhich gives convenientviewing of the interferencefringes. Back the fibersin the F-g25couplers away from the GRIN-rodlenses. This will give an adequate beam expansionso that the interferencefrinqes can be viewed easily. 7. If fringeswith good contrastare not obtained, recheckyour alignment of Step5. If the beamsdo not coincideproperly at both the point where they crossat the center of the beamsplitter and where they overlap at the card, fringeswill be difficult to observe.If you see many fine fringesinstead of a bull's-eyepattern, you are off to the side of the interferencepattern. Adjust the fiber in one of the F-925couplers to find the center of the pattern.

IO.7.2 TEMPERATURE SENSOR l. In the discussionabove, it was found that the fringe displacementof the interferometeris highly tempera- ture dependent.You can get a good feeling for this by trying a simple qualitativedemonstration. After the interfer_ ometer has been set up, place your hand around one of the fiberswithout touching it. The heat from your hand will causea rapid displacementof the fringes,even though you are not in direct contact with the fiber. As you perfoim this and the following exercise,be aware of the environmental conditionsof your interferometer,especially if there is an air-conditioneroutlet near your apparatus. H 2. A simple quantitativeexperiment to calibratethe temperaturesensitivity of the interferometercan be done using ice. Before doing the experiment,use the data from Eq. 10-3to calculatethe expectedfringe displacement F between a typical room temperatureand 0"C. you should get a value of about g fringes for an interactionlength of about (about = I inch 21/z cm} 3. Get some ice cubesand allow them to stand until L" they have become wet to the touch. If they feel dry, they I'J may be at a temperaturewhich is significintly lessthan 0"C. If they feel wet, then their surfaceshould be right at 0"C. l_ 4. Lay one of the ice cubeson a straight section of E one of the fibers on the breadboard.Because of the large l- thermal massof the breadboard,it may take 30-60,".ond, for the fiber to come into thermal equilibrium with the ice. This will ensurethat you have plenty of time to count F--_ the lr{ fringes as they change. 5. Comparethe number of fringes which you counted with the value that you got when you did the calculationin 5 Step 2. How good is the "order of magnitude',calculation leading to Eq. l0-3?

F> r{

>l- -{

-$I -i -!l t- -l b-- = 87 ,/ 10.7.3 PRESSURE SENSOR l. Set up the FK-pS pressuresensor assembly (which you used in Project #9) at the edge of the breadboard. Since you will not be coupling light through it, you will not need to raise it up on posts as you did before. FK-GT 2. You will use two pieces of the glass GlassTransducers transducer, FK-GT,from the supplies for project #9. However, this time you will not be passing light through them to measure pressure.Lay one of the pieces of glass on its side in the assembly,as shown in Fig. 10.3. Cut two thin slices from a soft eraser.Lay one on top of the glass. place one of the fibers-over this. Lay the second piece of eraser on top of the fiber and then place the second piece of glass on top of the whole assembly. 3. Bring the lever arm of the FK-pS down on top of this stack and adjust the screw so that the arm is level.leveL = 4. Now add weight to the lever arm and measurethe I fringe displacementas a function of added weisht. i 5. Calculate,using Eq. l0-9, the expected"fringedis- I placement for the amount of weight whiih you aaaJa. i Even.though you are Figure 10.3. Assembling the pneasure sensor for this using a soft rubber miterial around I experiment. the fiber, you will probably find that the best agreement g between your calculation from Eq. l0-g and yoir experi- i mental result is obtained when you assume that the iorce is 1 concentrated on the fiber and not distributed over the full - area of the glass transducer. I I I I

J- J I 1 fl I l I I I I

la l J l F 1 .4J J a'l 's I tf l r_lI I I.^ J t!!

FrJ 88 J

aI REFERENCES

The lists of referenceswhich are included in this section are representative of materials that are available but are not intended to be complete. The following are books which you may wish to con_ sider as texts for a fiber optics coursei M. K. Barnoski, ed., Fundamentals of Optical Fiber Communicqtions,2nd Edition, Academicpress (New York), l98l S. D. Personick,Fiber Optics, kchnology and Applications, Plenum press (New york), lggf The following are texts which you may wish to use as optics referencebooks in a fiber optics course: F. A. Jenkinsand H. E. White, Fundamentalsof Optics,Fourth Edition, McGraw-Hill(New york), 1976 E. Hecht and A. Zajac, Optics, Addison-Wesley(New York),1974 M. Born and E. Wolf, principles of Optics,6th Edition, Pergamon Press(Oxford), lg80 You may wish to use the following as supplementary referencesin a fiber optics course: L. B. Jeunhomme, Single-ModeFiber Optics, Principles and Applications, Marcel Dekker (New York), 1983 H. Kressel and J. K. Butler, .Semrconductorlnsers and Heterojunction LED's, Academic press (New york). 1977 D. Marcuse, Theory of Dielectric Waueguides,Aca- demic Press(New York), 1974 D. Marcuse,Principles of Optical Fiber Measurement, AcademicPress (New york), l98l J. E. Midwinter, Optical Fibers for Transmission,John Wiley and Sons(New york), l97g S. D. Personick, Optical Fiber Transmission Systems, Plenum Press(New York), lg8l A. W. Snyder and J. D. Love, Optical Waueguide Theory,Chapman and Hall (New york), l9g3 S- M. Sze, Pltysicsof Semiconductor Deuices, John Wiley & Sons(New York), lg69 ,,Fiber S. K. Yao and C. K. Asawa, optic intensity sensors,"IEEE Journal on SelectedAreas in Communica_ tions S4C-1,562 (lgS3),and referencesthere-in.

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Equipment List t

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Model # Description L Qty. Model # Description Qty. B-l Base 2 FPH.S Chuck, fiber 2 J 8S.05 Kit, thumbscrew, | / 4-2O I xsN-22 Breadboard,2x2 I CA.2 Clamp, universal 2 M-20X Lens, objective,20x I t F.5068 Coupler, directional, SM I M-40X Lens, objective,40x I F-916 Coupler, fiber, w/o lens I MH.2PM Holde4 lens, objective 4 :. F.925 Coupler, fiber, GRIN.lens 2 MM.T Mount. mirror I F.CA.00l Adapter, in-line D MPC Collar 3 F.CC-140 Connector hatves, (for F-MLD) l0 MPH.I Holder, micro-series F.CLI Cleaver, fiber MSP.I Post, micro-series E F.IRCI Card, IR phosphor RSX-2 Stage, rotation F-MLD.50 Fiber, 100/140 MM. 50 m SK-08 Kit, screw, S-32 F.MLD.sOO Fiber, 100/140 MM, 500 m SK.25 Kit, screw, l/4-2O I L F.SK-C Fixture, splice SP-2 Post t2 F.SK.S Splice, fiber, l0 count sP-3 Post 2 F-SPV-10 Fiber, pol.-preserving, l0 m u-l30lP laser, HeNe, I mw, polarized I F.SS.20 Fiber, 8/125 SM, 20 m I VPH-2 Holde4 post t2 -r F-SV.20 Fiber,4/125 SM, 20 m I 340C Clamp I F.TKIE Epoxy, 20 pkgs. I 4l Rod, short I tDa F.TKIL Lapping Sheets, 160 count I 42t-O.5 Stage,translation I F.TKIP Fixture, polishing I 777-l Bundle, fibe4 bifurcated I ,- FK.BLX Set, balldriver I 807 Mount. laser I FK.BSC Cube, beamsplitter I 815 Meter, power I FK.BTC Coupler, biconical, MM I FKP-TEXT Applications Handbook I FK.DET Assy., detector , FK.DRV Driver, ILD.LED, + Iaser I Options - FK-GR25F Lens, GRIN,w/ filter(2 count) I FK.GR2sP Lens, GRIN,w/o filter (2 count) 2 F.MLT Microscope, inspection I FK.GR29 Lens,GRIN,0.29-pitch (2 count) I FP3-FHI Adapter, bare fiber (for F.MLI) I FK.GT Transducer, glass 2 FP3-FPH Adapter, fiber chuck (for F-MLI) I FK-LED Assy., = light.emitting diode I FP3.CA3 Adapter, SMA (for F-MLf ) I FK-LS Assy., sensor, liquid-levet I F.BK2 Breaker. fiber I FK.MUX Base,assy., multiplexer 2 = FK-POL Polarizer, sheet I FK-PS Base,assy., pnesaure aengor I NOTE: Model numbers with FK.prefix are available only FK'sPKR Speaker, output 2 with Projects in Fiber Optics. They are not listed in FK.TAPE Thpe deck, multipler

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U.S. Headquarten: European'Headquarters: Switzerlandl Canada: ""^ t Newport Corporation Federal Republic of Germany , ,' NewportlnstrumentsAG Newport Instruments Canada P.O.Box 8020/ 18235Mt. Baldy Circle Newport GmbH Telephone:0l -740 -2283 Telephone:41G567-0390 Fountain Valley, CA 92728-8020 Telephone:06151-1540 Facsimile:0f -740-2503 Facsimile:416-567-0392 Telephone:714-963-981 I Facsimile:06151-15450 Netherlands: Japan: Facsimile:7 14-963-2015 UnitedKingdom: ,, Newport B.V. K. K. Newport Newport Ltd Telephone:03402-50588 Telephone:0G359-0270 Telephone:0582-769995 Facsimile 03402-50577 Facsimile:0&3594280 Facsimile:0582-762655 t