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In-fiber strain characterization of fiber-opticconnector assemblies by Bragg grating sensors

Patricia F. Mead and Keita Broadwater

In-fiber Bragg grating sensors were used to study mechanical strain in optical fibers that were terminatedin standard-termination and ribbon connectors. Our findings indicate that terminated sensors experi-ence a compressive strain whose magnitude depends on the cure profile of the epoxy encapsulant used inthese connectors. Anneal treatments on these connectors generally reduce the mechanical stress byinducing stress relaxation in the encapsulant layer. These experiments demonstrate the viability ofin-fiber sensors to characterize fiber-optic connector assemblies during and following termination.© 2000 Optical Society of America

OCIS codes: 060.2370, 060.2340, 060.2280, 060.2270, 120.4290, 120.6810.

1. Introduction

As commercial and government use of optical sys-tems increases, one area of concern is the reliability offiber-optic components.1,2 It is desirable to optimizehe optical and mechanical performance of fiber-opticomponents over the long and short term. This isrue for benign environments but is also a major con-ern in harsh environments such as in space or oceannvironments where repairs and upgrades are expen-ive, difficult, or impractical to make.The fiber-optic connector ~FOC! is a passive optical

component that is used in many applications andwhose function is to physically and optically link twofiber sections. As opposed to a fiber splice, this link-age is temporary and should be repeatable as manytimes as possible without degradation of the opticalsignal. Reliability concerns for fiber connectorshave been discussed in the literature. Some of theprevalent issues have included poor adhesion to theepoxy encapsulant, voids in the encapsulant, noncon-centric ferrule geometry, fiber cracking, fiber piston-ing, and fiber buckling.1–4 These conditions canventually result in failure of the connector, and

The authors are with the Computer Aided Life Cycle Engineer-ing Electronic Products and Systems Center, Department of Me-chanical Engineering, University of Maryland, College Park,Maryland 20742. P. F. Mead’s e-mail address is ashaki@eng.umd.edu.

Received 31 January 2000; revised manuscript received 21 June2000.

0003-6935y00y285101-08$15.00y0© 2000 Optical Society of America

many of these problems are initiated during the con-nector termination process or because of stresses oc-curring in the field.

To understand why and how the conditions listedabove can lead to failure of the connector, it is impor-tant to understand the mechanics present in a con-nector. Although there are many designs andmanufacturers of fiber connectors, most connectorsshare a few basic characteristics, as indicated in Fig.1. A FOC is essentially a cylindrically shaped com-posite structure. Most connectors contain an opticalfiber, surrounded by an aligning ferrule, and an ex-ternal casing. In many cases, an adhesive epoxy isused to maintain positional stability of the fiber.Typical physical properties and dimensions for thesecomponents are listed in Table 1. From these val-ues, it can be seen that the FOC is a complex systemof dissimilar members that respond differently to ex-ternal stimuli such as mechanical stress and temper-ature. Therefore use of different materials in theconnector design will affect its performance and reli-ability. For example, ferrule construction generallyincludes use of stainless steel, zirconia, or plastics.1–4

Differences in the material properties of these mate-rials can affect the performance of the ferrule struc-ture while in physical contact with another connectorferrule. This is due to variations in the load-bearingcapacity of the ferrule. As a result of this, coupledwith the ferrule deformation qualities, important ef-fects can be observed in the connector’s optical andmechanical performance.5

Several analytic and finite-element models of theFOC have been developed to evaluate the mechanicalperformance under various external stresses.5–9

1 October 2000 y Vol. 39, No. 28 y APPLIED OPTICS 5101

r

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5

There are, however, little or no in situ experimentaldata documenting the stresses experienced by fibersthat have been installed in connectors. The focus ofthe present study is to characterize the mechanicalstrain in optical fibers following their terminationinto commercial FOC assemblies. This study is be-ing used to develop optimal termination proceduresand to improve the connector yield. Future studieswill emphasize in-the-field performance and long-term reliability as it relates to measured strain in thefiber.

A. Termination

Termination is the process whereby a fiber is in-stalled permanently inside of a connector. The ter-mination procedure will depend on the type andgrade of the fiber and the connector components be-ing used. The intended application for the FOC alsoinfluences the termination procedure. Thus forhigh-performance applications such as military,space, or aviation, the FOC will use high-quality ma-terials, and the termination procedure must be awell-controlled process.

The termination process can be divided into fivebasic categories:

1. preparation, inspection, and cleaning of thematerials ~fiber cable, epoxy, and connector!;

2. insertion of the epoxy and fiber inside the fer-ule;

3. curing of the epoxy bonding agent;4. polishing of the fiber–ferrule endface; and5. testing and inspection.

Fig. 1. Diagram of a connector cross section ~not to scale!.

Table 1. Typical Material Prope

Section~material!

Layer Thickness~mm! Melting Point ~°C!

Ferrule ~steel! 1170–1190 1250Ferrule ~zirconia! 1170–1190 2500Bonding agent

~epoxy!1–7 Glass transition

temperature,Tg 5 70–130

Optical fiber~silica!

62.5–70 1996

102 APPLIED OPTICS y Vol. 39, No. 28 y 1 October 2000

Depending on the design and end use for the connec-tor, some categories in the termination procedure canbe modified or left out altogether. For example,many low-cost connector designs do not include theepoxy bonding agent. In other designs, a solder isused to fix the fiber in the connector.

In practice, the termination procedure is optimizedto achieve an acceptable yield. It is often found, forexample, that the epoxy cure procedure is optimizedto achieve an acceptable glass transition or fibermovement performance; and the actual cure proce-dure could be a multistep procedure involving manytemperature steps, controlled thermal ramp rates,and well-defined dwell times at each temperature.Nevertheless, for most termination sessions, someconnectors still fail to meet requirements for manyreasons, some of which are related to the proceduresused and can be avoided by careful examination of thetermination procedure.1

B. Bragg Grating Fiber Sensors

To obtain an accurate experimental evaluation of thestresses present inside the connector, we decided touse Bragg grating and in-line fiber etalon optical fibersensors. We are assessing the advantages andshortcomings of these two sensor types through ourcurrent experimental program. However, thepresent study focuses on the Bragg grating sensor.

There are several positive qualities that make useof the Bragg grating sensor ideal for this investiga-tion.10,11 For example, the procedure used in writ-ing the grating structures does not affect the shape ofthe fiber, and the final state of the fiber is approxi-mately the same as would be the case for a standardfiber termination procedure. Also, because Bragggrating sensor strain measurements can be recordedwith a reflected optical signal, sensor measurementscan be made throughout the entire fiber terminationprocedure, including epoxy cure and fiber polish.Therefore the Bragg grating sensor is compatiblewith standard FOC termination procedures; and thissensing technique, as applied to FOC’s, can be de-scribed as in situ and nondestructive.

The gratings are written onto the fiber with a pho-tolithography technique. The reflection and trans-mission spectrum for a grating is a complexrelationship that includes several parameters, in-

f Fiber Connector Components

Young’s Modulus~Pa! Poisson’s Ratio

Coefficient ofThermal Expansion

~1026y°C!

20 3 1010 0.3 10.620 3 1010 0.22 10.13.1 3 109 0.33 Below Tg 5 50–90

Above Tg 5 90–160

7.2 3 1010 0.2 0.5

iauti

stSts

cluding grating length, mechanical strain, and tem-perature. In general, there is a relative maximum~minimum! in the reflection ~transmission! spectrum,and this is defined as the Bragg wavelength of thegrating. A typical transmission spectrum for a grat-ing written onto a single-mode fiber ~SMF! is shownn Fig. 2. Because of its sensitivity to externallypplied strain and temperature, Bragg sensors aresed as strain or temperature sensors. The equa-ion describing the shift in the Bragg wavelength thats due to an applied temperature and strain is12

DlBraggylBragg 5 KseL 1 KHeH 1 KTDT, (1)

where lBragg is the Bragg wavelength, KS is the axialstrain-optic coefficient, KH is the transverse strain-optic coefficient, KT is a thermal coefficient, DT is thechange in temperature, eH is the transverse strain,and eL is the axial strain. Van Steenkiste andSpringer12 show that these coefficients are obtainedfrom the Pockel’s constants and the thermo-optic co-efficient. Note that when Eq. ~1! is applied, an es-tablished value of 0.732 3 1026yme for the axialtrain-optic coefficient and 20.387 3 1026yme for theransverse strain-optic coefficient can be used for theMF.12,13 If there is a known relationship betweenhe transverse strain and the axial or longitudinaltrain, Eq. ~1! can be reduced to

DlBraggylBragg 5 KLSeL 1 KTDT, (2)

where KLS is the overall sensitivity to longitudinal

strain.Currently we are capable of producing Bragg grat-

ings of various gauge lengths from 1 mm to 2 cm.Because the typical length of a ferrule is 1 cm, thismakes use of Bragg gratings viable in a FOC envi-ronment. The demodulation scheme used in ourstudies is illustrated in Fig. 3. Briefly, light from abroadband erbium-doped fiber amplifier source iscoupled into the Bragg grating with a 2 3 1 coupler.The reflected or transmitted signal from the gratingis observed with an optical spectrum analyzer ~OSA!

Fig. 2. Bragg grating spectrum ~a! before termination and ~b! aspectrum profile.

at the appropriate output port. In reflection mode,the Bragg wavelength appears as a wavelength peak.Alternately, when operating in the transmissionmode, the Bragg wavelength appears as a dip in theoptical spectrum. There is no observed improve-ment in the sensing quality of the Bragg grating inthe reflection or transmission mode. When choosingbetween the two methods, our main consideration issimplicity and convenience of the experimental setup.

The OSA used in this study has a resolution of 0.1nm. When it is used to read out the SMF Bragggrating sensors, it will yield an error of 100 me. Inthis initial study, to gauge the effectiveness and use-fulness of the sensor configuration, this error wasdeemed acceptable. In future studies, sensor read-out systems with better resolution will be used.

2. Study

Three commercially available FOC structures arecurrently being studied: fiber-collimated ~FC!,standard-termination ~ST!, and ribbon connectors.The FC and ST connectors share the same basic in-ternal structures illustrated in Fig. 1. These includea precision ferrule, an epoxy adhesive, a casing, andthe fiber. A schematic of the ribbon connector isshown in Fig. 4. It consists of a molded plastic fer-rule with precision holes for 12 optical fibers; and likethe FC and ST connectors, the ribbon connector alsouses an encapsulant epoxy for positional stability.

In constructing the FOC sensors used in this study,we fabricated a Bragg grating onto a fiber segment.This sensor can have a gage length from 2 to 10 mm.

ermination. The Bragg grating produces the narrow dip in the

Fig. 3. Demodulation setup for Bragg sensors.

fter t

1 October 2000 y Vol. 39, No. 28 y APPLIED OPTICS 5103

Bf

b

5

The grating is then terminated into the connector ofinterest following as closely as possible a conven-tional fiber termination procedure. First, the grat-ing is located as best as possible at the endface of theconnector ferrule as shown in Fig. 5. Second, twocure profiles for the adhesive epoxy were used, de-pending on the epoxy type. A room-temperaturecure for 24 h was performed for connectors that usedTra-Bond F113 epoxy, and a thermal cure at 135 °Cfor 35 min was performed for connectors that usedTra-Bond F253 epoxy. Overall, our termination pro-cedure is identical to conventional procedures withone exception. Optical inspections of the FOC’swere not made following termination. Specifically,we did not test the FOC’s for acceptable insertion orreturn loss.

To interpret the strain in these environments fromthe sensor output, the nature of the strain environ-ment in the connector during the cure had to beknown. If there was a known relationship betweenaxial and transverse strain, such as in the case of anaxial load, then Eq. ~2! could be utilized. OtherwiseEq. ~1! would have to be used. It was realized thatstrain occurring in the fiber connector during ter-mination arose from two sources. One was fromthe solidification of the epoxy and the other was dueto thermally induced stresses from the high-temperature cure.

Epoxies used as adhesives in connector assembliesare thermosets, polymers that start as fluids andthrough exothermic chemical reaction transform intoelastic solids. In the solidification process, stressescan build up because of viscoelastic flow, thermalgradients, reaction exotherms, and cure shrinkage.Studies evaluating the stresses arising from thesolidification of epoxy have been published.14–17

These studies included a finite-element analysis of

Fig. 4. Schematic diagram of the 12-fiber MT ribbon connectorferrule.

Fig. 5. Cross section of the connector with the preferred sensorlocation.

104 APPLIED OPTICS y Vol. 39, No. 28 y 1 October 2000

complex mold geometries to predict solidificationstresses in the epoxy.14,16 However, there remainsto be developed a simple analytical model to describequantitatively the stresses that occur in an epoxyused in a very thin adhesive layer ~1–7 mm!, which isthe case under study. Efforts are currently beingmade to develop such a model.

To approximate the thermomechanical strain con-tribution, finite-element models of the connectorssubjected to temperature loads identical to the onesexperienced during the curing of the connectors wereevaluated. This model assumed linear elasticityand a perfect bond at all interfaces between materi-als. The temperature in which the cure starts is thezero-stress condition of the assembly. In this model,the thin epoxy layer of 0.5 mm was used. The strainresponses of the structure in the fiber core, the loca-tion where the fiber sensor would be placed, wereevaluated. A graph of the radial and axial strainsalong the core of the terminated fiber is illustrated inFig. 6.

When we evaluated the sensor response to termi-nation, two contributions were known: the strainthat was due to epoxy solidification and the strainthat was due to thermomechanically inducedstresses. However, only the latter contribution iswell understood. Therefore results from the finite-element model were applied to Eq. ~1! to predict the

ragg sensor response to thermomechanical stressrom epoxy cure.

A. Standard-Termination Connector Study

Bragg grating sensors were written onto eight SMF’sand then installed in ST fiber connectors. TheBragg wavelength was observed before and after ter-mination. Thermal and room-temperature epoxycures were performed during termination with theprocedure outlined above. Two important parame-ters from the study are listed in Table 2: ~1! theefore and after values of lBragg for the eight FOC’s

and ~2! the shift in the Bragg wavelength ~DlBragg!following termination.

Note that every termination produced a negativeshift in the Bragg wavelength of the sensors; thiscorresponds to a compressive strain on the sensors.

Fig. 6. Finite-element model results for the strain componentsalong the axis of an optical fiber that was embedded in a connectorand thermally cooled at 112 °C.

amIcat

crtfilr

Bmswg2sc~wsptbst

Table 2. Results of the ST Connector Experimenta

Also there is a difference in magnitude for the ambi-ent and thermally cured connectors. As shown inTable 2, ambient cures resulted in Bragg shifts thatwere usually less than thermal cures. The averageBragg shifts for the ambiently and thermally curedconnectors were 20.45 and 21.2 nm, respectively.This difference in magnitude was expected becausethe thermally cured connectors have strain contribu-tions from both epoxy solidification and thermal cool-ing effects whereas the ambiently cured connectorsexperience only strain from epoxy solidification.However, the standard deviations for ambient andthermal cures were 0.2 and 0.8 nm, respectively. Inconsideration of the OSA error, two of the connectorsensors, 5 and 8, may have experienced little or nostrain.

We believe that large scatter in the measured val-ues may be due to a few factors including poor controlof the grating location inside the connector, varia-tions in the gage length of the sensor, and pistoningof the fiber. Previously reported analytical as wellas our own finite-element models for the structureunder study have shown that the strain on the fibervaries along the length of the ferrule.8,9 These mod-els indicate that the axial and shear stresses in cy-lindrically shaped composite structures experiencehigh gradients near the cylinder’s ends. Becauseour sensors were placed at the ferrule endface, thesensor signal would likely be affected by these gradi-ents. However, the gradients occur in lengths thatare small compared with the gage length of the sen-sor; this suggests that any contribution to the strainresponse of the sensor would be relatively small. Toassess the relation between stress magnitude andsensor position, use of high-precision reflectometry~HPR! is being evaluated ~see Subsection 2.B!. Anlternative explanation for the large variance in theeasured strain is the possibility of fiber pistoning.

n a recent study, it was observed that pistoningould occur in connectors that have been left in stor-ge for several months. If pistoning has occurred,he measured strain may be different from the ideal

GratinglBragg

~nm!lBrag

~nm

1 1540.2 1539.2 1540.1 1539.3 1540.0 1537.4 1534.0 1537.5 1539.8 1539.6 1539.4 1538.7 1540.0 1538.8 1539.0 1538.Ambient averageAmbient standard

deviationThermal averageThermal standard

deviation

aBragg wavelength shifts, computed strain, and type of epoxy c

ase in which the fiber endface is flush with the fer-ule surface. In addition to these causes of varia-ion, there may be other uncontrolled and unforeseenactors that introduce sources of mechanical loadingn the termination steps. Further causes that mayead to variation in the strain of the sensors are cur-ently being investigated.

To further illustrate the behavior of a terminatedragg sensor, the before and after profiles of a ter-inated sensor are shown in Fig. 2. The figure

hows the transmitted signal from a Bragg grating,hich is a dip located at the Bragg wavelength of therating. The shift from its initial wavelength is1.3 nm. However, it is of interest to not only ob-

erve the shift in the Bragg wavelength, but also thehange in the overall spectral structure. In Fig. 2~a!before termination! the dip occurs only at the Braggavelength; in Fig. 2~b! ~after termination! the dip is

urrounded by perturbations, seen as smaller dips,eaks, and waves. There are a few explanations forhese phenomena that are being weighed. One isirefringence, the tendency of light in a fiber to beeparated into polarized components because ofransverse strain in an optical material.18 The sec-

ond is due to strain being unevenly distributed overthe gauge length of the fiber sensor, as shown in Fig.6. This can also result in multiple Bragg peaks.19

More investigation into the effects of nonuniformstrain is being carried out and will be reported infuture publications.

B. High-Precision Reflectometry

To accurately locate the sensor position within theconnector, HPR was utilized. A reflectometer is adevice that shines light down an optical fiber andmeasures the magnitude and location of any reflectedsignal with micrometer accuracy. Reflections can bedue to faults or cracks in the fiber, as well as reflec-tions from a grating and from the endface of a fiber.Therefore this technology can be used to determinethe location of a sensor, with respect to the endface ofthe fiber, and the sensor gauge length. We evalu-

Dl~nm! Type of Cure

20.6 Ambient20.3 Ambient22.1 Thermal22.1 Thermal20.1 Thermal21.4 Thermal21.3 Thermal20.2 Thermal20.45

0.2

21.20.8

re shown.

g

!

68997078

ure a

1 October 2000 y Vol. 39, No. 28 y APPLIED OPTICS 5105

csleacsshSetcd

nsssfcdiA

ar

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ated the gauge length and location of sensors 1, 5, and7 from Table 2 using a HP 8504B precision reflecto-meter. We terminated sensor 1 using an ambientcure and sensors 5 and 7 using thermal cure proce-dures. Measured results from the reflectometrystudy are shown in Figs. 7~a!, 7~b!, and 7~c!, whichorrespond to sensors 1, 5, and 7, respectively. Asuggested, the plots can be used to establish gaugeength and position for the terminated sensors. Thendface of the connector is indicated by the large peakt the right of each graph, and the adjacent signalorresponds to the beginning and end of the gratingtructure. As indicated in Figs. 7~a! and 7~b!, sen-ors 1 and 5 are located at the tip of the ferrule andave gauge lengths of 0.7 and 0.8 mm, respectively.ensor 7 is located approximately 0.5 mm from thendface and is approximately 1 mm long. The facthat sensors 1 and 5 are less than 1 mm is an indi-ation that a portion of the grating has been removeduring the connector polish procedure.A comparison of the Bragg shift of the three termi-

ated sensors shows that the magnitude in shift forensor 5 ~0.1 nm! is considerably lower than those forensors 1 ~0.6 nm! or 7 ~1.3 nm!. Moreover, the mea-ured strain for sensor 5 is uncharacteristically lowor a thermally terminated connector. Because theure procedure of sensor 5 is identical to the proce-ure for sensor 7, differences in the strain character-stics could be related to the grating location.dditional studies documenting the grating position

Fig. 7. HPR data of three terminated Bragg gratings. ~a! 0.7-mmragg grating at connector endface ~sensor 5!. ~c! 1-mm-long B

sensor 7!.

106 APPLIED OPTICS y Vol. 39, No. 28 y 1 October 2000

nd the measured strain are needed to establish theelationship between grating position and strain.

C. Ribbon Ferrule Study

A second study was conducted on a 12-fiber ribbonferrule as shown in Fig. 4. Sensors were writtenonto a 12-fiber ribbon cable that was then terminatedin the ribbon connector. The sensors were placed atthe endface of the ribbon ferrule, and the cure profilewas 135 °C for 20 min. As was previously done, theBragg wavelengths of the sensors were monitoredbefore and after termination. In addition, three sub-sequent anneal treatments were performed on theribbon connectors following termination. The firstanneal profile was 155 °C for 20 min, and the secondand third profiles were 155 °C for 1 h. The Braggwavelengths of the 12 sensors in the connector wererecorded before and after each anneal. In addition,the lapsed time between the termination and the firstanneal treatment was 2 weeks, the time between thefirst anneal and the second anneal was a week, andthe time lapse between the second and final annealwas 2 days. Results of this study are summarized inTables 3 and 4 and in Figs. 8 and 9.

Table 3, supplemented by Fig. 8, documents theBragg wavelength shifts throughout the experimentfor 8 of the 12 terminated sensors. The remainingfour sensors were damaged during the terminationprocess. The second column of Table 3 representsthe Bragg shift resulting from termination; the other

g Bragg grating at connector endface ~sensor 1!. ~b! 0.8-mm-longgrating set approximately 0.5 mm from the connector’s endface

-lonragg

ttndl

Table 4. Drift in Fiber Sensors between Termination and AnnealTable 3. Results of Ribbon Connector Experimenta

columns are shifts resulting from the subsequent an-neals.

These sensors experienced compression as a resultof termination; each anneal resulted in additionalstrain, either compressive or tensile ~in Fig. 8, a pos-itive value indicates a compressive shift and a nega-tive value indicates a tensile shift!. However, theBragg shifts seen in the sensors began to get smaller,so that the overall strain in the sensors approached aconstant value.

Although the magnitudes of the induced Braggshifts of the sensors are not uniform, it is clear thatover time they decrease in magnitude. After thefirst anneal, an average Bragg shift of 1.5 nm is ob-served with a standard deviation of 0.4 nm. By thethird anneal, the average shift drops to 0.1 nm witha standard deviation of 0.2 nm. These values arenear the floor of the OSA resolution. Overall theseresults assert that annealing connectors serve tosteady the strain experienced by fibers after termi-nation.

In addition to the Bragg wavelength shifts thatwere observed following the anneal treatments, therealso was an observed shift between anneals for eachsensor. Table 4 and Fig. 9 illustrate this drift. Thesecond column of Table 4 is the drift that occurredbetween the termination and the first anneal ~2-week

Ribbon Position

Anneal ~nm!

FirstDlBragg

SecondDlBragg

ThirdDlBragg

FourthDlBragg

R2 21.9 20.4 0.1 20.2R3 20.8 20.6 0.5 20.1R5 22.0 20.3 0.2 20.3R6 21.4 20.1 0.1 20.2R7 21.3 21.3 20.1 20.2R8 21.9 20.8 0.0 20.2R9 21.2 20.3 0.0 0.0R12 21.3 0.4 20.2 20.3

Average magnitude 1.5 0.5 0.1 0.2Standard deviation 0.4 0.5 0.2 0.1

aBragg shifts after termination and subsequent anneals areshown.

Fig. 8. Bragg shifts of sensors in ribbon ferrule after terminationand subsequent anneals.

ime period!; the third and fourth columns representhe drifts occurring between the first and second an-eals ~1 week! and the second and third anneals ~2ays!, respectively. Table 4 shows that drifts inBragg following termination can be rather large.

However, the drift magnitude is smaller with eachsubsequent anneal and eventually reaches the de-fined noise level. From these results, we concludethat the drift is most likely due to changes in thestrain state of the connector between anneals, possi-bly because of stress relaxation in the epoxy layer.Also, annealing helps to steady the strain state of theconnector during storage periods.

3. Conclusions

Bragg grating sensors have been terminated in STand ribbon connector structures by use of standardtermination procedures, and these sensors have thenbeen used to record fiber strain resulting from thetermination process. Our results indicate that ter-minated fibers experience a compressive strain, andfibers that have been terminated by an ambient cureprofile generally experience less induced strain ascompared with thermally cured samples. Thisstrain has contributions from the solidification of theepoxy and thermomechanical stresses arising fromcooling of a thermally cured connector assembly. Itwas also observed that the fiber strain following ter-

Fig. 9. Bragg shifts of sensors embedded in ribbon ferrule. Eachmeasurement number represents a step in the experiment, start-ing with termination and ending with the third anneal.

Treatments

Ribbon Position

Drift ~mm!

First Second Third

R2 0.4 20.2 0.0R3 20.8 0.4 0.0R5 20.5 0.2 0.0R6 20.6 0.1 0.1R7 20.3 20.1 0.2R8 0.2 0.2 0.1R9 20.8 0.1 20.4R12 21.1 20.3 0.1

Average magnitude 0.6 0.2 0.1Standard deviation 0.5 0.2 0.2

1 October 2000 y Vol. 39, No. 28 y APPLIED OPTICS 5107

7. E. Suhir, “Pull testing of a glass fiber soldered into a ferrule:

5

mination can change during dormant storage peri-ods, possibly because of stress relaxation of theencapsulant epoxy. However, the degree of straininstability during storage can be reduced when ananneal treatment is applied. Finally, results fromHPR experiments point to measurable variations inthe location and gage length of our sensors. Theresults of this study should be considered qualitativeand indicative of general trends. However, use ofin-fiber sensors shows promise as an in situ tool forassessing several parameters associated with the fi-ber connector termination process.

The authors thank John Kolasinski and KeithKienzle at NASA Goddard Space Flight Center forproviding guidance and equipment used in thisstudy. In addition the authors thank the facultyand students of the University of Maryland SmartMaterials and Structures Research Center for valu-able help and advice during this study. Thisresearch has been sponsored by NASA grantNG560 and National Science Foundation grantECSN9702329.

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108 APPLIED OPTICS y Vol. 39, No. 28 y 1 October 2000

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