tc

suba,

Keywords:

Long-term reliability environmental testsAccelerated aging tests

talmaonion

passing through it had steady optical characteristics. Then we performed an accelerated aging test on

and removal of ber for splicing resulted in failure. We consider that the repetition of adhesion between

orks hent o

onnect

components with gel type RIMM has been conrmed.By contrast, single-mode hole-assisted ber (HAF), which has a

germanium-doped core with several air holes, has been investigat-ed in recent years because it has excellent bending loss character-istics [6]. This advantage has led to single-mode HAF beingintroduced as optical cord in central ofces [7] and as indoor

ty comparable toction.rtant in terastructur

provide the customer with a stable service. Therefore, wethe long-term reliability of solid type RIMM for eld instconnection. To ensure the long-term reliability of the material,we perform environmental and accelerated aging tests. The envi-ronmental tests we conducted are based on an IEC standard andthey conrm the optical performance. The accelerated aging testswere performed at 140 C on the index matching material itselfand on spliced bers passing through the index matching material.Also, as the worst condition, we undertook an accelerated aging

Corresponding author.E-mail address: saitou.koutarou@lab.ntt.co.jp (K. Saito).

Optical Fiber Technology 23 (2015) 1823

Contents lists availab

r

. costable over a wide temperature range [5]. Moreover, componentsconnected with gel type RIMM have passed environmental testsand have been in practical use for more than 20 years. Therefore,the long-term connection reliability of eld installable connection

we were unable to conrm long-term reliabilithat of gel type RIMM for eld installable conne

Stable optical characteristics are very imporealizing practical use because the network infhttp://dx.doi.org/10.1016/j.yofte.2015.01.0211068-5200/ 2015 Elsevier Inc. All rights reserved.rms ofe mustclarifyallablethose employing mechanical splices and eld installable connec-tors [24]. These eld installable connection techniques use refrac-tive index matching material (RIMM) to obtain good opticalperformance by eliminating the Fresnel reection caused by theair gap between bers. Gel type RIMM is currently used with eldinstallable connection techniques and is physically and chemically

because gel type RIMM would enter the air holes, and the spliceloss would degrade at the splicing point. So we have developed aconnecting component that is applicable to both SMF and single-mode HAF by using a solid type RIMM to prevent RIMM intrusion[9]. And we optimized the refractive index, refractive indextemperature coefcient, and hardness of RIMM [10,11]. However,1. Introduction

Fiber to the home (FTTH) netwJapan [1]. This widespread deploymwidespread use of eld installable cindex matching material and bers causes the splice to degrade. With this result, we used the Arrheniusmodel to estimate a median lifetime of about 68 years in a high temperature environment of 60 C. Thussolid type index matching material at a splice point is highly reliable over long periods under normalconditions of use.

2015 Elsevier Inc. All rights reserved.

ave spread throughoutf FTTH has led to theion techniques, such as

optical cable for use in restricted spaces, for example, for installa-tion in the space around a door [8]. However, the terminalconguration of single-mode HAF consists simply of a connectoror a fusion spliced single-mode ber (SMF). Field installableconnection techniques cannot be applied to single-mode HAFField installable connectionSolid refractive index matching materials

an index matching material attached to a built-in ber before splicing it in the worst condition, whichis different from the normal use conguration. As a result, we conrmed that the repeated insertionHigh reliability solid refractive index mainstallable connections in FTTH network

Kotaro Saito , Mitsuru Kihara, Tomoya Shimizu, KeiAccess Network Service System Laboratories, NTT Corporation, 1-7-1 Hanabatake, Tsuku

a r t i c l e i n f o

Article history:Received 10 October 2014Revised 19 January 2015Available online 21 February 2015

a b s t r a c t

We performed environmenrefractive index matchingenvironmental tests basedhigher than the specicat

Optical Fibe

www.elsevierhing materials for eld

ke Yoneda, Toshio KurashimaIbaraki 305-0805, Japan

and accelerated aging tests to ensure the long-term reliability of solid typeterial at a splice point. Stable optical characteristics were conrmed inan IEC standard. In an accelerated aging test at 140 C, which is very muchtest temperature, the index matching material itself and spliced bers

le at ScienceDirect

Technology

m/locate /yof te

test on index matching material attached to a built-in berbefore splicing, which is different from the normal use congura-tion. From the result we estimated the median lifetime andconrmed the reliability of RIMM for long-term use under normalconditions.

2. Refractive index matching material

Refractive index matching material is a fundamental elementof the eld installable splicing technique. RIMM matches therefractive indices of spliced bers to prevent any degradationof the insertion and return losses by Fresnel reection. Table 1provides general information about the gel type RIMM used withcurrent eld installable splicing components and solid typeRIMM, which is now being developed for practical use with aeld installable connector for employment on a customerspremises.

Gel type RIMM is a silicone oil compound, and it is easy toincorporate RIMM into a eld installable splicing componentbecause it is physically and chemically stable and can be simplydispensed onto a splicing component. Also splicing componentswith gel type RIMM have been in practical use for more than20 years, and its long-term reliability has been conrmed. Howev-er gel type RIMM cannot be applied to single-mode HAF because

So we cannot estimate when its optical property will fail and wecannot ensure long-term stability.

Therefore, our aim is to ensure the long-term reliability of solidtype RIMM at a splicing point by conrming its environmental testcharacteristics and by estimating its median lifetime.

3. Reliability tests

3.1. Reliability test method

Optical characteristics such as insertion loss and return loss areimportant at a splicing point. For practical use, a splicing pointrequires both good initial optical characteristics and long-termreliability. There are two ways to assess long-term reliability.One is to use an environmental test [12], the other employs anaccelerated aging test [13,14]. In the environmental test we evalu-ate whether the optical characteristics fall within certain criteriaunder the severe conditions specied in the standard. In the accel-erated aging test, we estimate the median lifetime of a sampleunder normal use conditions by imposing conditions that are muchworse than in the environmental test, and by causing failure. Inthis investigation we performed both environmental and accelerat-ed aging tests, and conrmed the long-term reliability of solid typeRIMM.

year

10 c

K. Saito et al. / Optical Fiber Technology 23 (2015) 1823 19the gel penetrates the air holes and the splice loss degrades whenwe splice bers through gel type RIMM [9].

On the other hand, solid type RIMM is an acrylic resin. The rstprototype of a solid type RIMM [9,10] was silicone resin, but acrylicresin is now used because it is easy to design as a RIMM in terms ofhardness and curing time. For example we changed the hardnessand curing time from 20 shore E and 1 h to 70 shore E and 24 h.Solid type RIMM is applicable to both SMF and single-mode HAFbecause it has no uidity and so does not seep into air holes. How-ever, it is not just long-term reliability that is needed if solid typeRIMM is to nd practical employment in splicing components.Only its temperature cycling characteristics have been conrmed[11], although some other environmental tests are available [12].Moreover, there is no long-term test data for splicing componentswith solid type RIMM, whereas gel type RIMM has a long history.

Table 1General information about gel type RIMM and solid type RIMM.

Gel type refractive index matching material

Material Silicone oil compoundPhoto RIMM

Optical fiber

Feature Used for eld installable connection with > 20Easy to incorporate in splicing component

Issue Inapplicable to hole-assisted ber

Table 2Test conditions and results.

Test Condition

Temp. cycling 40 C 70 C/6 h 10 cyclesTemp./Humid. Cycling 10 C 65 C, 93%RH @ 65 C/24 h High temp. 70 C 240 hLow temp 40 C 240 hThermal aging 85 C 336 h& &Thermal and humidity aging 60 C, 95%RH 336 h& &

Temp. cycling 40 C 75 C/8 h 42 cyclesSalt spray 35 C, 5% 24 h3.2. Environmental test

We spliced SMFs by using solid type RIMM in a eld installableconnector that had a built-in ber inside a ferrule and we splicedthe inserted ber by using a mechanical splice structure. We thenperformed an environmental test based on the IEC standard on the-se spliced SMFs samples [12]. Table 2 shows the test conditionsand results. The environmental test conditions and sample num-bers are listed here: temperature cycling: 40 C 70 C/6 h 10cycles, N = 12, temperature and humidity cycling: 10 C 65 C,93%RH @ 65 C/24 h 10 cycles, N = 12, high temperature:70 C 240 h, low temperature: 40 C 240 h, N = 12, thermalaging & thermal and humidity aging & temperature cycling:85 C 336 h & 60 C, 95%RH 336 h & 40 C 75 C/8 h 42 cycles, N = 12, salt spray: 35 C, 5% 24 h, N = 10.

Solid type refractive index matching material

Acrylic resin

RIMMOptical fiber

lifetime Applicable to hole-assisted ber

No long-term test data as splicing component

Criterion (dB) Sample no. Max. (dB)

50.3 12 0.04ycles 50.3 12 0.05

50.2 12 0.0550.3 12 0.0450.3 12 0.1150.2 10 0.02

The results showed that the maximum loss change of all thesamples met the design criterion. Thus we conrmed the realiza-

the stability of its physical property and the spliced ber passingthrough the RIMM was prepared to estimate the median lifetimeof solid type RIMM at a splice point in a practical use conguration.As a practical use conguration, we spliced SMF by using a eldinstallable connector in which RIMM was already attached to abuilt-in ber. We put each prepared sample in a heat chamber.After heating the samples for a certain period, we removed themfrom the heat chamber and measured their physical propertiesand optical characteristics such as incident light power P0, trans-mitted light power P1 and reected light power P2. As regardsthe RIMM itself, the heating temperature was 140 C, and the mea-sured physical properties were transmittance at 1310 nm, refrac-tive index at 589 nm, hardness at shore E [15], and adhesionforce [16]. In relation to the spliced bers, the heating tem-peratures were 110, 130 and 140 C and the measured optical char-acteristics were insertion loss at 1310 nm and return loss at1310 nm.

Fig. 2 shows the (a) transmittance, (b) refractive index, (c) hard-ness and (d) adhesion force results. There was no noticeable degra-dation in any of the samples over 30 days although there was aslight deterioration in adhesion force. Thus we conrmed that solidtype RIMM is a stable material even at an extremely high tem-perature of 140 C.

Fig. 3 shows the insertion and return loss results obtained at

Heat chamber

Refractive index matching material

P0 P1

P2

P0

P1

Refractive index matching material

Optical fiber

(a) Only RIMM (b) Spliced fiber with RIMM

Fig. 1. State of sample and ow of accelerated aging test.

20 K. Saito et al. / Optical Fiber Technology 23 (2015) 1823tion of stable optical performance in various environmental tests.

3.3. Accelerated aging test

We then performed an accelerated aging test to estimate themedian lifetime of solid type RIMM at a splice point. In an acceler-ated aging test performed on planar lightwave circuits, in whichoptical adhesive is used and plays the same function as solid typeRIMM, accelerated aging is achieved by heating and humidication[13,14]. In our case, we assumed that the splicing component wasused in an indoor environment, so we performed a heat acceleratedaging test. Fig. 1 shows the state of the sample and the ow of theaccelerated aging test. As for the state of the sample, we preparedtwo types; one was RIMM itself, the other consisted of splicedbers passing through RIMM. The RIMM was prepared to conrmFig. 2. Transmittance, refractive inde110, 130 and 140 C. We considered an insertion loss of more than0.5 dB and a return loss of less than 40 dB to constitute failure [17].All the insertion and return loss results were within the designedcriteria at 110, 130 and 140 C after 30 days under acceleratedaging conditions. We consider these results to mean that the beris attached tightly to the RIMM once we have spliced them, andthis attached condition is maintained because the RIMM is con-stantly subjected to a pressing force from the ber. As a result,good optical characteristics were obtained even if the adhesionforce of the RIMM degraded slightly.

We performed accelerated aging tests on two types of RIMMsample and on spliced bers passing through RIMM. We couldnot estimate the median lifetime because no failure occurred.However, the RIMM itself and RIMM at a splicing point both havehigh reliability, because there was no degradation under the severex, hardness, and adhesion force.

r Technology 23 (2015) 1823 21K. Saito et al. / Optical Fibecondition of 140 C, which is very much higher than the specica-tion test temperature. Therefore, in normal use they have stableoptical characteristics.

4. Long-term median lifetime prediction of RIMM

In the previous section we mentioned that we could not esti-mate the median lifetime of solid type RIMM at a splicing point,which simulated a normal use conguration, because there wasno degradation even under extremely severe conditions. Thus weperformed an accelerated aging test on a sample that did not simu-late a normal use conguration to conrm the failure mode.

Fig. 4 shows the state of the sample and ow for the medianlifetime prediction. We subjected a sample in which the RIMMadhered to a built-in ber to high temperature. After heating wespliced the ber and evaluated its optical characteristics. Herethe heating temperatures were 110, 130 and 140 C and the mea-sured optical characteristics were the insertion loss at 1310 nmand the return loss at 1310 nm for 5 samples at each acceleratedaging time. We believe that by subjecting the sample to accelerat-ed aging before splicing, the adhesion of the RIMM degrades to asmall degree and the bers are inadequately attached to the RIMMwith the slight pressing force imposed during splicing. We alsobelieve that the adhesion will degrade more noticeably if werepeatedly insert and remove the ber. So we also repeatedly per-formed a test where, after measuring the optical characteristicsonce, we removed the inserted ber, then reinserted it and

Fig. 3. Insertion and return losses at 110, 130, and 140 C.

(a) Fiber with RIMM before splicing

(b) Repeated splicing

P0 P1

P2

P0 P1

P2

Withdraw

Fig. 4. State of sample and ow for median lifetime prediction.

Tec22 K. Saito et al. / Optical Fibermeasured the optical characteristics. Fig. 5 shows the insertion andreturn loss results for an aged sample before splicing at 110, 130,and 140 C. From these results stable, we found that optical

Fig. 5. Insertion and return losses of aged samp

Fig. 6. Number of successful accelerated aging tests.

Fig. 7. Arrhenius plot of median lifetime of sample.hnology 23 (2015) 1823characteristics are obtained after 30 days at 110, and 130 C butthat the number of samples that failed in terms of insertion andreturn losses increased with aging time at 140 C. Fig. 6 showsthe number of samples where the repeated splice characteristicsare within the standard value against accelerated aging time. Asthe heating temperature became high, the sample number withinthe standard value decreased. So we consider that the adhesionof RIMM itself degrades and the adhesion of bers and RIMMdegrades by repeated splicing as we expected.

Then we estimated the median lifetime based on the Arrheniusmodel [13,14] by assuming that the median lifetime is dened asthe maximum number of elapsed days within which the character-istics of all ve samples are within the standard value at each tem-perature. Here, at 110 C we assumed a median lifetime of 60 daysto estimate the median lifetime under more severe conditions thanusual, although degradation did not occur after 60 days. Fig. 7shows an Arrhenius plot of the median lifetime of the sample.The plot shows the median lifetime at each temperature and thesolid line shows a linear approximation. The median lifetimeincreases as the temperature decreases. For example, this medianlifetime estimation indicates that failure will not occur for at least68 years in a 60 C environment.

Therefore, we prepared a sample that was different from thenormal use conguration, because degradation did not occur innormal use conguration even at an extremely high temperatureof 140 C. From the results we obtained for quickly aged samplesbefore splicing, failure occurredwhenwe repeated the splice opera-tion. We consider that by repeatedly inserting and removing theber, the adhesion of the RIMM itself degrades as does the adhesionbetween the RIMM and the bers. However, even though we set aharsh use conguration on purpose, we estimated the medianlifetime to be about 68 years at a high temperature of 60 C. Thus,

le before splicing at 110, 130, and 140 C.

there is no problem in a normal use environment, and solid typeRIMM at a splicing point has high long-term reliability.

5. Conclusion

We performed environmental and accelerated aging tests. Wethen estimated the median lifetime of solid type RIMM at a splicingpoint to ensure long-term reliability. In environmental tests, weconrmed that we realized stable optical characteristics underthe IEC standard condition. An accelerated aging test revealed thatRIMM itself and spliced bers passing through RIMM have steadyoptical characteristics at a high temperature of 140 C, which isvery much higher than the specication test temperature. Nextwe performed an accelerated aging test on RIMM attached to abuilt-in ber before splicing, which is unlike the normal use con-guration. As a result, repeated splicing resulted in failure. We con-sider that the repeated insertion and removal the ber causes theadhesion between RIMM and bers to degrade. And we estimatedthat the median lifetime for repeated splicing is about 68 years in ahigh temperature (60 C) environment. We showed that solid typeRIMM at a splice point has excellent long-term reliability for prac-tical use.

References

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[4] H. Aoyama, H. Tanaka, Y. Hoshino, Y. Oda, Optical wiring technology for homenetworks for a service-ready and low-cost FTTH service, NTT Tech. Rev. 3 (4)(2005) 3337.

[5] I. Sankawa, T. Satake, N. Kashima, S. Nagasawa, Fresnel reection reducingmethods for optical ber connector with index matching material, IEICE, B 67(12) (1984) 14231430.

[6] K. Nakajima, T. Shimizu, C. Fukai, T. Kurashima, M. Shimizu, Single-mode hole-assisted ber with low bending loss characteristics, in: Proc. IWCS09, 2009,pp. 264269.

[7] S. Aozasa, Y. Enomoto, H. Oohashi, Y. Azuma, Highly reliable optical berdistribution facilities in central ofce employing single-mode hole-assistedber cord, in: Proc. IWCS10, 2010, pp. 126132.

[8] K. Kuramoto, H. Minami, M. Ida, T. Shimizu, A. Daido, Y.Aoyagi, K. Takamizawa,Flexible optical wiring technology for existing buildings, in: Proc. IWCS12,2012, pp. 513516.

[9] Y. Abe, T. Hoshijima, T. Matsui, S. Tomita, Optical characteristics and reliabilityof mechanical splice utilizing solid refractive index matching material for hole-assisted ber connection, Photonics Technol. Lett. 21 (4) (2009) 194196.

[10] K. Saito, R. Koyama, Y. Abe, K. Nakajima, T. Kurashima, Mechanical splicecharacteristics of hole-assisted ber, J. Lightwave Technol. 30 (2) (2012) 267272.

[11] K. Saito, M. Numata, K. Nakajima, T. Kurashima, Field assembly splicetechnique with solid refractive index matching material, J. LightwaveTechnol. 32 (2) (2014) 344348.

[12] IEC 608741 Fibre optic interconnecting devices and passive components Connectors for optical bres and cables part 1: generic specication, 2011.

[13] H. Hanafusa, S. Sumida, N. Takato, Long-term reliability of silica-base planarlightwave circuit devices, Mat. Res. Soc. Symp. Proc. 531 (1998) 349358.

[14] H. Watanabe, N. Araki, H. Fujimoto, Highly reliable PON optical splitters foroptical access networks in outside environments, IEICE Trans. Electron. E93-C(7) (2010) 11801190.

[15] JIS K 6253, 2012.[16] JIS Z 0237, 2009.[17] ITU-T Recommendation G.671, 2009.

K. Saito et al. / Optical Fiber Technology 23 (2015) 1823 23

High reliability solid refractive index matching materials for field installable connections in FTTH network1 Introduction2 Refractive index matching material3 Reliability tests3.1 Reliability test method3.2 Environmental test3.3 Accelerated aging test

4 Long-term median lifetime prediction of RIMM5 ConclusionReferences