damage of optical fibers under wet environments

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Damage of Optical Fibers Under Wet Environments

R. El Abdi &A.D. Rujinski &M. Poulain &I. Severin

Received: 26 July 2009 / Accepted: 19 October 2009 / Published online: 6 November 2009# Society for Experimental Mechanics 2009

Abstract Besides signal transmission for telecommunica-tions, fibers are used in an increasing number of devices. A

number of applications relate to devices exposed to severewet environment (hot water, chemical attacks). It is thecase for the sensors used in nuclear plants, high energy

physics or plasmas devices. However, reliability issuesmust be addressed for optical fiber sensors operating undersevere conditions such as harsh chemical solutions. The

purpose of this work is to study the mechanical behaviorand aging of fibers exposed to hot water action, tohydrofluoric acid vapours (HF) and to tetramethoxysilane(TMOS) for different durations. Dynamic fatigue tests wereimplemented using a two-point bending testing device ortensile test set-up. Standard fibers tested immediately after

exposure show a broader distribution of fiber strengthaccompanied by the drastic decrease of the failure stress. Insome particular cases, the gain compared to as receivedfibers can be positive. Polymer reacts with different wetenvironments, which induces viscosity changes. This isconsistent with SEM observations.

Keywords Optical fibers . Aging . Epoxyacrylate coating .

Severe environments . Fiber strength

Introduction

The availability of silica fibers enlarges the field of thepossible applications [1]. Passive and active applicationswere foreseen at the early stage of development of fiberoptics [2]. A large group of applications encompassesoptical fiber sensors, remote chemical analysis, thermalmeasurements and thermal imaging, reflectometry, opticalinstrumentation and also laser power delivery. Mostmedical applications of fibers, with their specific require-ments, are related to this group. They could expand verysignificantly when technologic progress will lead todisposable fibers at reduced cost.

Active fibers are doped fibers from which laser effect

or amplification may be obtained. The large interactionlength of the fiber and the high energy density which may

be used for optical pumping are the main featuressupporting the development of the active fiber devices.Another attractive point is the possibility of generatingshort wavelength signals using IR laser diodes as pumpsources. Powerful laser fibers may be built in this waywith a very good beam quality, which is more difficult toachieve in semiconducting lasers. Cost aspects andreliability still limit the practical applications, but signif-icant changes are likely to occur in coming years. Allsolid state and compact fiber lasers could replace the large

and noisy gas lasers.Choosing the optimum optical fiber for a specific use

may be an obvious task or a subtle exercise of balancebetween various parameters: attenuation, reliability, toxic-ity, availability and cost. Fibers from different materials can

be used depending on choice criteria.Silica fibers offer a set of unique advantages: lowest

attenuations, good mechanical and chemical resistance,transparency range extending from 300 nm to 2 m. Silicafibers for telecommunications are inexpensive but price

R. El Abdi (*) : A.D. Rujinski : I. Severin

Laboratoire Larmaur, University of Rennes1, Campus de Beaulieu,CS 74205-35042 Rennes, Francee-mail: [email protected]

A.D. Rujinski : I. SeverinPolitehnica University of Bucharest, Fac. IMST,313 Splaiul Independentei,cod 06042 Bucharest, Romania

M. PoulainLab. Matriaux Photoniques,University of Rennes1, Campus de Beaulieu,CS 74205-35042 Rennes, France

Experimental Mechanics (2010) 50:12251234

DOI 10.1007/s11340-009-9310-1


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increases when special size and NA (sinus of themaximum angle of total reflection at the core/claddinginterface) are needed, requiring to manufacture special

preforms. Special silica fibers have been developed forUV transmission, either doped with OH or made fromfused quartz.

As a general rule, the life time of the fiber depends mainly

on chemical durability and applied stress. Failure corre-sponds to breaking which is known to happen as crack

propagation from a surface flaw. Real fibers have surfaceflaws resulting from the processing. The initial intrinsicstrength of a fiber is related to chemical composition, andultimately to the chemical bond energy. Current values are4 GPa for silica fibers and less than 1 GPa for fluoride andsulfide fibers. Initial strength decreases versus time when thefiber is subject to stress corrosion, or more simply, under a

permanent stress in humid environment. This phenomenonhas been extensively studied in silica fibers [3].

An external polymeric coating is applied to protect fiber

from scratches, to limit chemical attack of water and toincrease mechanical strength. Usual coatings are epoxya-crylate resins, but other polymers such as silicones and

polyimide may be used. In spite of their hydrophobicproperties, fluorinated polymersPTFE, FEPdo notmake an efficient barrier against hydrolysis and are lessfavorable for mechanical properties.

As the main interest of fibers lies in their flexibility, theywill be moved and bent during operation. The minimum

bending radius must be defined. It depends on fibermaterial and diameter: It is smalla few millimetersforsilica fibers, but is larger for fluoride, chalcogenide and

polycrystalline fibers.The aging of the fibers used for laser transmission also

depends on the evolution of the internal and ends defectsunder laser irradiation. Finally this aging effect is mini-mized if defect density is low enough, and if waterconcentration and residual stress are controlled.

Apart from the worsening of the surface flaws, waterhas little influence on oxide glasses which are insolubleand remain transparent. Things are different for someglasses such as phosphate and halide glasses. Thechemical action of water can weaken the fiber and reducetransmission if fiber ends and outer surface are corroded.

Direct contact with liquid water has to be avoided andvarious solutions have been successfully experienced forharsh environments.

In practice, optical fiber aging depends on variousfactors that may decrease effective fiber strength: appliedstress, temperature, water or chemical solutions. Severaloptical fiber manufacturers want to obtain the fiber

behavior submitted to severe wet environment to analysethe coating reliability and the evolution of the fiber strengthduring the fiber life.

The aim of this work is to study the silica optical fibersbehavior when these fibers are in contact with severeenvironments such as hot water, exposure to acid vapoursor in tetramethoxysilane solution and to analyze thestrength evolution and the life duration.

Tested Fiber

In this study, the monomode silica optical fibers are used,and the fiber diameter is equal to 250 m. A fiber silicacore has 125 m in diameter. Two epoxy-acrylate polymercoatings (0.2 NA Acrylate coupler fibers) with a thick of62.5 m are used. The epoxy-acrylate coating is the mostused in standard optical fibers. It protects the fiber but itremains permeable to water. The internal layer is soft with alow glass transition temperature and is applied onto theglass fiber surface. It ensures protection against micro-

bending and damping of the external stresses. The external

layer has a higher Tgand protects the fiber against physicalaggression. To comply with process requirements, the UV-

polymerisation of the acrylate resins is rapidly performed,in line, and offers an excellent adhesion and a large range ofelasticity (Young modulus).

Hot Water Effects

Aging Testing

Fibers subjected to aging were plunged into large tanks

containing deionised hot-water at 65C and 85C fordifferent durations ranging between 3 to 70 days. Eighteento 20 samples per series, each fiber of 1 meter in length,were carefully arranged floating into the hot water tanks,so as to subject no stress and the temperature along themto be constant all over the aging treatment. After theenvisaged aging duration, the series of aged fibers wereremoved from hot water and simply laid to dry into thelaboratory environment on absorbent paper for at least3 days.

Static Bending Test

The static fatigue parameters were measured by a staticbending test which was implemented accordingly to theinternational standard IEC 793 [4]. The aged and subse-quently dried one meter in length sample fibers weresubjected to bending stresses by winding around analumina mandrel with calibrated size in diameter. Theconstant level of applied stress can be varied by the properchoice of the mandrel size. The failure time is measured,this being the time required for the intrinsic strength of the

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fiber to degrade until it equals the applied stress through theproper mandrel diameter. The technique to monitor the timeto failure is the optical detection of the ceramic mandrel

presence in a special holder. Thus, the energy deliveredthrough fiber breaking pushed out the holder the mandrel,initially in a vertical stable position, the time to failure

being directly recorded with an accuracy of 1 s. the testingsetup consists of a large number of vats containing 16holders each.

The testing environmental conditions during static fatiguemeasurements have slightly ranged between 18.5 and 20.5C,in temperature and to 30 to 45% in a relative humidity.

Static Fatigue Testing Results

Optical fiber was wound on alumina mandrel. The appliedstress on the fiber depends on the mandrel diameter accord-ingly to the Mallinder and Proctor relation [5,6], as follows:

sE0:" 1a0:"

2

1

where : applied stress (GPa); E0: Young modulus (=72 GPafor the silica); : relative deformation of the fiber; =0.75 ;with : non-linearity elastic parameter (=6).

The relative deformation of the fiber depends on themandrel calibrated diameter, as follows:

" dglass

fdfiber2

with f the mandrel diameter (in m); dglace = dsilica core=

125 m, the fiber diameter; dfiber = 250 m, the fiberdiameter, including the double layer polymer coating.Fibers were subjected to aging in hot water at 65 and

85C for long durations and then winded on 2.4 mmdiameter mandrel in order to be tested. The use of equations(1) and (2) leads, in the case of usual telecommunicationfiber, to the corresponding stresses of 3.76 GPa. Anoscillating behaviour for the aging temperature of 65and85C was noticed (Fig. 1). For the aging temperature of65C, maximum values exceeding just a few the non agedfailure time (39.22 h for 4 days in hot water as compared to33.40 h) are noted. For higher aging temperature (85C),

seeming to damage the polymer coating more and moreseverely with the aging duration exposure, a drasticdecrease of the failure time may be observed immediately,with some oscillations around very low failure durationsand very severe decrease when exceeding 50 days of aging.

Structural RelaxationDiscussion

The cyclic evolution of the fiber failure time subjected tostatic fatigue was normally unexpected taking into accountthe fiber strength decrease induced in time by waterchemical action. This behaviour in static fatigue conditions

may present some practical consequences suggesting that ina wet environments, in some particular cases (temperatureand exposure duration), the as aged fiber lifetime hasexceeded the expected one, respectively the non aged fiberfailure time.

As already know, the microcracks smoothing due to thecuring effect of water molecules on silica resulted in astress intensity factor decrease and thus in a fiber failurestrength increase. But the smoothing effect might not be theonly responsible for an oscillatory process of the fiberfailure time while increasing the aging duration.

Failure time versus aging duration

0

10

20

30

40

0 10 20 30 40 50 60 70

Aging duration (days)

Failure

time(hours)

Aged at65C

Aged at85C

Fig. 1 Failure time versus aging duration at 65 and 85C

Faceplates

Controlblock

FiberPiezo

electricsensor

Steppermotor

Faceplates

Fiberdg

df dc

d

FiberFig. 2 Two point testing bench

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The cyclic evolution of failure time versus aging duration instatic fatigue should be explained through the cyclic changesat the fibers polymer interface due to the structural relaxation.As previously reported [7], the cyclic evolution was noticedfor longer aging duration, with the first maximum after3 months of exposure. Our series of experiments haveshowed that the oscillatory process appeared for all the tested

fibers inside the as previously reported period too.The structural relaxation is responsible for the oscillatory

evolution off the fiber failure time versus the aging durationthat means that a hydrated silica layer is formed on theglass polymer interface, when the fiber is aged in water.This layer inhibited in some extends, the micro-surfaceflaws. In a first phase, this layer retards the water diffusiontowards the glass surface. As the layer grows, the fiber isreinforced when subjected to static stress, but as containingwater, when the layer reaches a critical thickness, diffusionthrough polymer coating tends to decrease the layerthickness to a minimum value, accelerating the structural

relaxation accompanied by the fiber effective stress de-crease. So, once the cyclic finishes, a new process initiationallows the water attack of the glass-polymer interface.

Exposure to Hydrofluoric Acid Vapours

Silica exhibits a remarkable high chemical durability, asexemplified by an extensive use in chemistry laboratoriesfor beakers, crucibles and reaction tubes. However it has

been demonstrated that silica is sensitive to environmentalaction, including water, but also methanol and other

reagents, through the classical stress corrosion phenome-non. Considering the numerous application fields of theoptical fibers, one may wonder to what extent moreaggressive chemical reagents may influence mechanicalreliability.

This is the case for very alkaline and fluorinating media[8]. Silica is quite stable in most acids and, as an example

immersion into hot sulphuric acid is used for polymercoating stripping [913]. While surface flaws may be easilyinduced on the pristine fiber once the polymer coating isremoved, fiber strength is not much affected by this acidetching if fiber is handled properly [10]. On the contrary,fluorinating reagents severely attack silica [14], the intensereaction between silica and hydrofluoric acid being largelyused in electronics.

The merged effects of stress corrosion and chemicaldamage are studied hereafter. For this purpose, fibers have

been exposed to fluorinating reagents, and their mechanicalstrength has been compared to that of untreated fibers.

Experimental Procedure

Chemical attack by hydrofluoric acid vapours (HF)

Series of 30 samples of 20 cm in length were carefullyarranged on a rigid bar and their upper extremity pastedwith adhesive tape. This bar was exposed to hydrogen

HF exposure, testing immediately

0

20

40

60

80

100

7 7.5 8 8.5

ln (F, MPa)

Failurecumulative

probability(%)

15 min

45 min90 min

60 min

120 min

150 min

non-aged

Fig. 3 Weibull plots recorded from fibers exposed to HF vapours fordifferent times (in min) and tested immediately

HF exposure, dried 3...5 days

0

20

40

60

80

100

7.8 8 8.2 8.4 8.6 8.8

ln (F, MPa)

Failurecumulative

probability(%)

15 min

45 min

90 min60 min

120 min

150 min

Fig. 4 Weibull plots of fibers exposed to HF vapours for differenttimes (in min) and dried in ambient environment prior to test

Table 1 Physical properties of tetramethoxysilane used product

Chemical name Tetramethoxysilane

Formula C4H12O4Si

Molecular weight 152.22

Boiling point 122C (760 mm Hg)

Flash point 26C

Color and appearance Colorless transparent liquid

Density (25/25C) 1.032

Refractive index 1.3688 (20C)

Min. purity 98.5% by GC

Package 180 kg/drum

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fluoride vapours produced through the interaction between40% aqueous hydrofluoric acid (HF) and concentratedsulphuric (H2SO4) acid. Sulphuric acid absorbs water andHF molecules that are less soluble in H2SO4than in watergo to the vapour phase. The fiber was subjected to gaseousHF into a transparent hermetic disposal for durationsranging from 15 to 150 min.

A first series of treated fiber packages was testedimmediately; the second one was dried in air in thelaboratory environment (20C, 3040% RH) for 35 days

prior to testing. Each series consisted in multiple packagesof 30 samples; each package was exposed to HF fordifferent durations, as previously mentioned. For safetyreasons, the chemical container was put into an extractorhood.

Dynamic fatigue measurement using a two-point bending

testing apparatus

While bending method does not replace tensile testing as afiber strength measurement technique, it presents attractivefeatures and advantages, providing valuable informationabout flaw size distribution [15]. In our case, the ease andthe duration of the testing manipulations and the smalleffective length of the fiber sample made the bending testthe most appropriate choice for investigation.

The as-received fibers and those exposed to fluorinatedvapours for different durations (15, 45, 60, 90, 120,150 min) were put subsequently through dynamic testsusing a two-point bending testing device [16,17] (Fig.2).The fiber package was cut into two parts, leading tosamples of 10 cm in length that were carefully bent and

placed between the grooved faceplates of the apparatus.

Special care was required to avoid the fiber slipping duringthe faceplates displacement and to maintain the fibers endsin the same vertical plan. The faceplates are broughttogether by a computer-controller stepper motor which ishalted when the fiber fracture is detected by an acousticsensor (Fig. 2). A series of 30 samples were tested for afaceplate constant velocity of 400 m/s that is a convenientvalue for our investigation. The measurements were

performed in the ambient environment (20C, 3040%RH), temperature and relative humidity being recorded foreach measurement series.

The stress to fracture was calculated from the distanceseparating the faceplates. The failure stress was measuredfor each fiber, and then the results were treated through astatistical approach using the Weibull theory.

During the two point bending tests and at breaking time,the stress applied to the fiber was deduced using thedistance valuedbetween the two faceplates and the relationdefined by Griffioen [18] i.e.:

s E0:"

1

a00:"

2

3

The parameteris not equal to of equation (1), butdefined by:

a00

3

4a

1

4 4

where E0 is the silica Young modulus, is the appliedstress (GPa), is a non linearity elastic parameter (=6) andis the strain in the fiber:

"1:198 df

ddc2dg

5

0

20

40

60

80

100

1.6 1.62 1.64 1.66 1.68 1.7 1.72 1.74 1.76

Ln (F) (GPa)

Failurecumulative

probability(%).

50mm/min AR

50mm/min 4h

150mm/min AR

150mm/mn 4h

300mm/min AR

300mm/mn 4h

500mm/min AR

500mm/min 4h

50mm/min 4d

150mm/min 4d

300mm/min 4d

500mm/min 4d

Fig. 5 Aged fibers during 4 h(4 h) and during 4 days (4 d) inTMOS solution (17C) fordifferent tensile test velocities(in mm/min)AR means AsReceived fibers

Table 2 Median fiber strength for aging in cold TMOS solution (17C) during 4 h for different tensile test velocities

50 mm/min 150 mm/min 300 mm/min 500 mm/min

As received 4 h As received 4 h As received 4 h As received 4 h

Median strength (GPa) 5.038 5.086 5.274 5.368 5.401 5.418 5.442 5.454

Gaina (%) 0.95 1.76 0.30 0.21

aThe gain is calculated as the percentage of the improvement of the strength of aged fibers compared to that of non aged fibers

Exp Mech (2010) 50:12251234 1229


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where df is the polymer coating diameter, dc is the fiberdiameter, 2dgis the total depth of the 2 grooves and dis thedistance between the two faceplates (Fig. 2).

The statistical Weibull law gives a relationship betweenthe probability Fof fiber rupture with a length L and theapplied stress :

Ln 1L

Ln 11F

m Ln s Ln so 6

where mis a size parameter and

is a scale parameter.

The evolution ofLn 1L

Ln 11F

n oh i(Failure cumulative

probability in (%)) according to Ln(F) is called Weibulldiagram (Fis the failure stress).

Results and Discussions

Fibers in harsh conditions

In previous experiments, the fiber was exposed to adifferent fluorinated atmosphere consisting in the vapour

phase originating from heated ammonium hydrogenofluor-ide NH4HF2 [19]. From the observations gained in thisstudy, the duration treatment was adjusted to 19 h. Theamount of acids was chosen taking into consideration thecontainer volume of 2 litres: approximately 5 g of 40%aqueous HF and 5 ml of 9597% H2SO4.

After 19 h exposure, fibers appeared extremely damagedand the colour of the coating turned to brown. Moreover,treated fibers had lost elasticity and could be bended

permanently, making mechanical tests impossible as no

fracture could be registered.Further examination showed that glass fiber was largely

or even entirely destroyed, so that the fiber diameter wasclose to zero while the polymeric coating remained as anempty shield.

As this 19 h duration was obviously excessive, shorterexposure times were applied for the following experiments.

Influence of exposure into hydrofluoric acid vapours

Less severe chemical attack was implemented reducingexposure time, typically from 15 to 150 min. The dynamic

testing using the two-point bending apparatus (Fig.2) wereperformed (faceplate velocity of 400 m/s).

Various series of fibers were subject to HF attack. Afterexposure, some series were immediately tested and someother series of samples were dried at room atmosphere for

35 days then tested. The resulting Weibull plots (Failurecumulative probability according to the failure stress F)are shown in Figs. 3 and 4.

As already noticed during previous manipulations andtests, the fibers exposed to HF vapours were much more

brittle then the non aged fibers (Figs.3,4). In spite of greathandling care, more than half of the prepared samples werelost because they broke. In many cases, just touching the

bench faceplates or trying to bend the fiber to put it in thebench lead to fiber fracture.

In the case of the standard fibers tested immediately afterexposure (Fig.3), fiber strength showed a broader distribu-

tion and mean failure stress decreased drastically, reachingonly 50 to 75% of the origin strength. In some case thedistribution was bimodal, meaning that more than one singletype of defects is present on the fiber surface (Fig.3). Takinginto account fiber position during vapours exposure into thedisposal and the use of one single package for two differenttesting, no difference was observed according to the sample

position (upper or lower) along the fiber package. Howeverthe colour change of the polymer coating turning to brownwas more evident for the lower extremity of the package.During measurements, some handling difficulties due to theHF absorption into the polymer coating were encountered:

Table 3 Median fiber strength for aging in cold TMOS solution (17C) during 4 days for different tensile test velocities


As received 4 d As received 4 d As received 4 d As received 4 d


Gain* (%) 2.36 1.91 1.44 2.25

Twopolymercoatings

Fig. 6 Fracture morphology of as received silica optical fiber (tensiletest velocity of 50 mm/min)

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apart from acidic smell, fibers left some kind of paste onapparatus faceplates that had to be cleaned frequently, whileset-up had to be recalibrated. This observation is consistentwith the assumed change of polymer Tg.

It seems that the failure stress Fincreases according to

the exposure time. For a small exposure time (15 min) theobtained results are not far away from the one of non agedfibers (Fig.3). On the other hand, this failure stress rapidlydecreases for 45 min of exposure time, then increases withexposure time. One has noted many deposits at externalsurface of fiber aged during 150 min, and these deposits arelikely to have grown from the vapour phase that contained amixture of inorganic species (SiF4, HF, H2O) and organicfrom polymer coating. The involved chemical processesmay be complex and these deposits lead to the increasing offailure stress in bending test.

Drying prior to test has also led to a broader distribution

of the experimental values of the exposed standard fibers(Fig. 4) by comparison to the as-received fibers (Fig. 3).But it was steeper than that of the fibers exposed to HF andtested immediately. These dried fibers also show a decreaseof failure stress, down to 25 to 50% of the origin strength.The most obvious bimodal distribution appeared for theexposure duration of 15 min, while distribution appearedroughly mono-modal, although broader, for the othertreatment durations.

Fiber in Tetramethoxysilane Solution (TMOS)

The tetramethoxysilane solution TMOS is used in roomtemperature vulcanizing silicon rubber as a cross-linkingagent, and also used as reagent for ketal synthesis,deposition frosting of glass, co catalyst with CsF forMichael additions. It is used in manufacturing chemical

resistant coating and heat resistant coating. In some cases,Tetramethoxysilane solution, used as sol-gel material isoften employed to improve the use of optical fibers inspectroscopic detection [20,21].

On the other hand, water is the major factor limitingfiber strength through the well known stress corrosionmechanism. Incorporating colloidal silica in the externalcoating could be one way to reduce this effect. This may beachieved using tetramethoxysilane (TMOS) that formssilica by hydrolysis.

The aim of this paragraph is to study the evolution of thesilica optical fibers strength when these fibers have been

exposed to TMOS in various conditions. The results willalso give information about the influence of similarchemical reagents.

Table1 gives the typical physical properties of the usedproduct.

Fibers are put in containers filled with TMOS solutionthe aged during several durations. Others fibers are put incontainers TMOS solution which are then posed on hotwater in thermal enclosures maintained at constant temper-ature during different aging durations. After aging in hotTMOS solution, the fibers are then dried during one day atambient environment (17C, 3040% RH) then subjected to

the dynamic tensile test. The obtained results are thencompared with those of as received fibers (reference fibers).

The strength of aged fibers during 4 hours in cold TMOSsolution (17C) appreciably increases (Fig. 5). The maxi-mum gain is about 1.76% (the gain is calculated as the

percentage of the improvement of the strength of agedfibers compared to that of non aged fibers) (Table 2).Table3 shows the maximum gain become 2.36% when theaging duration in cold TMOS solution is equal to 4 days.

(a) (b)Fig. 8 (a) Fiber aged in coldTMOS solution during 4 days

(17C) (tensile test velocity of50 mm/min) and (b) polymerdeterioration with externalmicrocraks

Fig. 7 Fiber aged in cold TMOS solution during 4 h (17C) (tensiletest velocity of 50 mm/min)

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Table 5 Median fiber strength for aging in hot TMOS solution (40C) during 16 h for different tensile test velocities


As received 16 h As received 16 h As received 16 h As received 16 h40C 40C 40C 40C


Gain* (%) 2.12 3.73 4.96 4.67

0

20

40

60

80

100

1.58 1.6 1 .62 1 .64 1 .66 1 .68 1.7 1.72

Ln (F) (GPa)

Failurecumulativ

e

probability(%).

50mm/min AR

50mm/min 16h/40C

150mm/min AR

150mm/min 16h/40C

300mm/min AR

300mm/min 16h/40C

500mm/min AR

500mm/min 16h/40C

Fig. 10 Aged fibers during 16hours in TMOS solution at 40C(16 h/40C) for different tensiletest velocities (in mm/min)ARmeans As Received fibers

Table 4 Median fiber strength for aging in hot TMOS solution (65C) during 4 h for different tensile test velocities


As received 4 h As received 4 h As received 4 h As received 4 h

65C 65C 65C 65C


Gain* (%) 7.83 9.76 10.73 9.75

0

20

40

60

80

100

1.45 1.5 1.55 1.6 1.65 1.7 1.75

Ln (F) (GPa)

Failurecumulative

probability(%).

50mm/min AR

150mm/min AR

300mm/min AR

500mm/min AR

50mm/min 4h/65C

150mm/min 4h/65C

300mm/min 4h/65C

500mm/min 4h/65C

Fig. 9 Aged fibers during 4 h inTMOS solution at 65C (4 h/65C) for different tensile test ve-locities (in mm/min)ARmeans As Received fibers

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Figure5shows that the increase in the aging duration (from4 h to 4 days) led to an increase in the average fiberresistance.

As fiber surface has determined fracture to a large extent,external coating appears critical. This coating is polymericin most cases, and modern optical fibers are coated by twodifferent layers, a soft coating at glass surface and a hard

coating at external surface (Fig.6). The coating first makes aprotection against scratches that occur in normal handling; italso fills the surface flaws gluing in some a way the two sidesof the micro cracks and finally, it reduces water activity atglass surface. A non aged broken fiber is given in Fig.6. Thecrack propagation is not perpendicular to the fiber axis butthe microcrack is propagated with an angle of 45 and thisindicates a brittle fracture of the fiber core.

When the fiber is aged in cold TMOS solution during4 h (17C) (Fig.7), small epoxy chips are present on the endof the broken fiber.

No microscopic crack is present on the external surface

of the fiber after the dynamic tensile test. For a higherexposure time (4 days), [Fig. 8(a)] a large circular crack

between the fiber core and the coating can lead to the corepull out. Polymer deterioration is observed and externalmicrocraks appear [Fig.8(b)].

On the other hand, one observes a decreasing of the fiberstrength as soon as the TMOS temperature increases(Fig.9) and the gain loss can reach10.73% (Table4). If

the TMOS temperature decreases from 65C to 40C evenif the aging duration increases (from 4 h to16 h), thestrength decrease is lower than the one obtained when theaging temperature was of 65C (Fig. 10 and Table5).

When the TMOS solution is heated at 40C, a highdamage of the fiber core is observed [Fig. 11(a)]. The fibercore is broken and the two polymer coatings are separate.

This shows severe attack of the warm TMOS solution. Onthe polymer external surface [Fig. 11(b)], the dense andcontinuous network of microscopic cracks appears andquickly weakens the fiber resistance.

When the aging temperature of TMOS solution increases(from 40 to 65C) core and polymer damage is accentuated(Fig. 12). The fiber core is much deteriorated (several

pieces are removed), the polymer is torn with large cracks:the fiber is very damaged.

Conclusion

Monomode optical fibers, aged in hot water at differenttemperatures and durations were tested in static conditions.All static fatigue testing showed an oscillatory evolution ofthe fiber failure time versus aging duration.

The cyclic evolution may be explained by the structuralrelaxation phenomenon at the glass polymer coatinginterface. So, water has presented, in the same time, oppositeeffects on the fiber interface. Faced to water, the polymercoating is permeable, having a sponge type behaviour that hasfavoured exchanges during aging. On one hand, water has acorrosive action on the glass interface, tending to promote the

surface micro-cracks and on the other hand, it accelerates thestructural relaxation phenomenon.

On the other hand, the chemical attack by hydrofluoric acidvapours (HF) has evidenced the merged effects of thechemical damage in fluorinated environment on mechanicalreliability of monomode optical fiber. While standard fiberwas more severely damaged by hydrofluoric acid vapourexposure, the epoxy-acrylate coating appeared not efficient to

protect from severe chemical damage. Consequently, it doesnot ensure the mechanical stability in extreme conditions.

Fig. 12 Fiber aged in cold TMOS solution during 4 h at 65C (tensiletest velocity of 50 mm/min)

(a) (b)Fig. 11 Fiber aged in coldTMOS solution during 16 h at40C (tensile test velocity of50 mm/min)

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Drying into laboratory environment the processedstandard fibers has lead to the failure stress decrease, downto 25 to 50% of the origin strength.

Lastly, TMOS solution effect on epoxy-acrylate fiber isdisastrous when this solution is warm. SEM observationsare consistent with the mechanical measurements. Fibercore is broken and multiple cracks are generated on external

polymer coating which reduces the mechanical fiberresistance. For this environment the use of hermetic opticalfibers is advised. Such fibers are designed to improve aging

behaviour and to avoid diffusion through glass surface.Fabrication process includes in-line deposition of a thinlayer of diamond like carbon, a few hundred in thicknessand firm polymer coatings.

References

1. Poulain M (1998) Fluoride glass fibers: applications and pros-

pects. Proc SPIE 3416:2122. Kapany NS (1967) Fiber optics. Principles and applications.

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