Radiation Resistance of Er-Doped Silica Fibers: Effect of Host Glass Composition

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 5, MARCH 1, 2013 749

Radiation Resistance of Er-Doped Silica Fibers:Effect of Host Glass Composition

Mikhail E. Likhachev, Mikhail M. Bubnov, Kirill V. Zotov, Alexander L. Tomashuk, Denis S. Lipatov,Mikhail V. Yashkov, and Alexey N. Guryanov

Abstract—With the aim to find the best-suited host glass forradiation-resistant erbium-doped fibers (EDF), radiation-inducedabsorption (RIA) is measured and analyzed after -irradiation to3.0–4.5 kGy in 18 silica optical fibers with various concentrationsof dopants: Al O , P O , GeO and Er O . The RIA depen-dence on the content of Al O , P O and Er O in singly dopedfibers is investigated. RIA of fibers codoped simultaneously withAl O and P O are found to be significantly lower than those ofsingly Al O -doped fibers. This is explained as the result of theformation of AlPO -joins in the silica network. GeO codoping ofAlPO -doped silica is shown to further reduce RIA. Nevertheless,the application of H -loading and photobleaching of RIA by980-nm laser radiation shows that the AlPO - and GeO -codopedsilica is outperformed by P-free Al O - and GeO -codoped silica,which is, therefore, concluded to be the best host glass compositionfor radiation resistant EDFs.

Index Terms—Erbium-doped fibers (EDF), optical fiber radia-tion effects, radiation-induced absorption (RIA), radiation resis-tance, space applications.

I. INTRODUCTION

I NTEREST in radiation resistance of erbium-doped silicafibers (EDF) is, primarily, due to their expected space appli-

cations (amplifiers for free-space optical inter-satellite links [1]and superluminescent sources for fiber gyroscopes [2], [3]). Atpresent, both commercially available and a majority of experi-mental EDFs demonstrate inadmissibly large radiation-inducedabsorption (RIA) at relevant doses and, therefore, cannot ensurea sufficiently long service life in space [3]–[6].In [7]–[9] we proposed a solution to the EDF radiation re-

sistance problem consisting in H -loading of the glass of a her-metically coated EDF. The underlying physics is rather straight-forward: hydrogen passivates radiation-induced color centers(RCC) in the fiber glass network and thereby suppresses RIA,while the hermetic coating prevents the diffusional escape ofthe H molecules from the fiber. By testing the lasing efficiencyof -irradiated Al O -doped silica EDFs, we concluded that

Manuscript received August 30, 2012; revised November 17, 2012; acceptedNovember 27, 2012. Date of publication December 20, 2012; date of currentversion January 23, 2013. This work was supported in part by Grant MK-1459.2011.2 of the President of the Russian Federation and in part by the Ministry ofEducation and Science of the Russian Federation.M. E. Likhachev, M. M. Bubnov, K. V. Zotov, and A. L. Tomashuk are with

the Fiber Optics Research Center of the Russian Academy of Sciences, Moscow119333, Russia (e-mail: [email protected]).D. S. Lipatov, M. V. Yashkov, and A. N. Guryanov are with the Institute

of Chemistry of High Purity Substances of the Russian Academy of Sciences,Nizhny Novgorod 603950, Russia (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JLT.2012.2233196

H -loading can strongly prolong their service life in space [9]. Itwas also found that the presence of H enhances the efficiencyof photobleaching of RIA by laser radiation at nm,which proved to be a more appropriate pumping wavelengththan nm. As the result, H -loading in combinationwith photobleaching was estimated to provide a gain in the EDFservice life in space as large as 30-fold [9].H -loaded hermetically coated fiber is a realistic and practi-

cable technology. It is based on a known technology of applyinga carbon layer, 50 nm in thickness, onto a fiber in the processof its drawing, overcoating the carbon layer with a high-temper-ature polymer and then loading such a fiber with H moleculesat pressure of just a few MPa and a temperature of 200 C[7]–[9]. Estimations show that hermeticity of such a coating willfully prevent the diffusional escape of H molecules from thefiber: the H concentration in the fiber will decrease two-fold at20 C in as many as 2 10 (!) years [8].Despite the strong gain due to H -loading, one can try to fur-

ther improve EDF radiation resistance by optimizing the hostglass composition. It is generally recognized that the EDF hostglass should contain aluminum to provide good active proper-ties (e.g., see [3], [10]). However, it is aluminum that is respon-sible for the high radiation sensitivity of EDFs [6], [11]. Phos-phorus can also act as a solvent of rare-earth ions in silica net-work, though less efficiently than aluminum [10]; however, itis also a dopant drastically increasing fiber radiation sensitivity[12]. The dependence of RIA on the Al O and P O concen-trations in alumosilicate and phosphosilicate glass fibers has notbeen so far investigated, and is one of the objectives of thispaper.According to previous papers [6] and [11], the contribution

of Er-related RCC does not show up on the background ofthe RCC associated with the host glass. In this paper, we,nevertheless, attempted to assess the Er-related contribution toRIA of EDFs.Up to now, radiation resistance of EDFs co-doped simultane-

ously with Al O - and P O has not been investigated, althoughsuch host glass is very promising for the following reasons.First, when present in silica together, Al and P atoms be-

come fourfold-coordinated and form P-O- Al bonds, sim-ilar by structure to the regular Si-O-Si bonds [13], [14].Thus, the resultant silica network can be imagined as doped withAlPO joins [13], [14]. Because such a network is similar tothe radiation-resistant undoped silica network, one may expecta much lower RIA in this ternary glass than in singly Al O -,or P O -doped silica. In this paper, we investigate, for the firsttime, radiation resistance of AlPO -doped silica fibers.

0733-8724/$31.00 © 2012 IEEE

750 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 5, MARCH 1, 2013

Second, despite the similarity to undoped silica,AlPO -doped silica efficiently dissolves rare-earth ions andthereby ensures excellent active properties of EDFs [15].Third, AlPO -doping almost does not increase the refractive

index of silica [13], which allows fabrication of large-mode-areaEDF with minimized nonlinear effects.As follows from the refractive index measurements [13] and

Raman spectroscopy [14], in case Al and P atoms are presentin silica in equal amounts, all of them form AlPO joins. If theamount of these atoms is unequal, only excess Al or P atoms areincorporated in the usual form (Al O , or P O ).Owing to the remarkable properties of Al O -and

P O -codoped silica, we have recently developed an effi-cient double-clad laser and a high-power amplifier based on aYb-free AlPO -doped EDF [16].One more dopant useful for the EDF host glass is GeO . It

was found in [5] and [17] that GeO codoping reduces RIA inrare-earth-doped fibers with alumosilicate host glass. That is tosay that germanium passivates, in someway, Al-related RCC. In[18] experimental data was presented testifying that germaniumpassivates P-related RCC as well. Thus, this dopant should betaken into account in the search for an optimum host glass forEDF.Thus, the purpose of this paper was to reveal the influence of

each of the possible dopants in EDFs—Al O , P O , AlPO ,GeO , and Er O —on RIA and to propose the best-suited hostglass composition for radiation resistant EDFs, taking into ac-count the feasible possibility to apply H -loading and photo-bleaching of RCC.RIA in optical fibers is a rather involved phenomenon and

depends, in the general case, not only on dose, but also on doserate. Thus, to describe the physical pattern of the RIA formationand decay and, as a result, to predict RIA at a small dose ratein space, it is usually necessary to carry out RIA measurementsimmediately in the process of irradiation. However, the dose-rate-dependent contribution to RIA in fibers doped with P Oand Al O is known to be extremely slight (see [12] and [6],respectively); for this reason, one may judge about radiationresistance of such fibers based on post-irradiation RIA only.

II. SAMPLES AND EXPERIMENTS

Three sets of optical fibers with various Er O , Al O , P O ,and GeO concentrations in the silica core and with undopedsilica cladding (F300 silica glass of Heraeus) were fabricated bythe MCVD-technique (Tables I, II and III). All the fibers weredrawn in a polymer coating, except carbon-coated fiber No. 9investigated previously in [9]. The cut-off wavelength was

m for all the fibers, except fibers No. 2 and 12 which hadm. The optical loss in all the fibers beyond the Er ab-

sorption bands near 980 and 1530 nm did not exceed 20 dB/km.The Al, P, and Ge atomic concentration profiles were

measured in fiber performs with the aid of a JSM5910L scan-ning electron microscope of JEOL equipped with an X-rayMicroanalyser of Oxford Instruments. The Er concentrationwas measured with a Camebax X-ray Microprobe of Cameca.The measured radial distributions of the atomic concentrationswere nearly step-like, except the narrow central dip area for

TABLE ISET 1 FIBER CHARACTERISTICS

TABLE IISET 2 FIBER CHARACTERISTICS

TABLE IIISET 3 FIBER CHARACTERISTICS

P and Ge concentrations. From the data measured far fromthe core center, the oxide molar concentration was calculated,the accuracy being better than 0.2 mol. %, taking into accountthe small noise-like concentration variations along the corediameter.For fibers doped with Al O and P O simultaneously, the

molar concentrations were calculated from the measured atomicconcentration in the following simple way. The AlPO concen-tration was calculated from the smaller atomic concentration (Alor P), whereas their difference was used to calculate the concen-tration of the oxide present in excess (Al O or P O ). In the‘fiber designation’ column of Tables I and II, the dopant presentin a smaller amount (P or Al) is indicated by inferior letters. Forexample, in fiber No. 2 (Table I), the measured P atomic concen-tration proved to be greater than the measured Al atomic con-centration. Therefore, the AlPO concentration of 13.6 mol.%was calculated from the Al atomic concentration in the assump-tion that all Al atoms were included in the AlPO joins (seeabove). Next, from the difference of the P and Al atomic con-centrations, the P O concentration of 4 mol.% was calculated.The dash in the Al O column of this fiber does not mean thatthere were no Al atoms in the core of this fiber, but the fact thatall of them were present in the form of AlPO , not Al O .

LIKHACHEV et al.: RADIATION RESISTANCE OF ER-DOPED SILICA FIBERS 751

The Er atomic concentration profiles were measured with aCamebax X-ray Microprobe of Cameca, which were also nearlystep-like. The accuracy of the resulting Er O concentrationwas better than 0.005 mol.%.The fibers of set 1 (Table I) were intended for the investiga-

tion of radiation resistance of Al O - and P O -codoped silicahost glass, including the effect of its additional GeO -codoping.The fibers of set 2 (Table II) were used to investigate the ef-

fect of P O and Al O concentrations on the value of RIA inthe cases of alumosilicate and phosphosilicate host glass. Twofibers of this set (No. 16 and 17)wereH -loaded immediately be-fore -irradiation (H pressure of10MPa, temperature of 100 C,and duration of 24 hours). The H -loaded fiber pieces are la-beled in the Figures by an additional mark “-H ”. Immediatelybefore the RIA measurements, the H -loaded pieces were ex-posed to 980-nm laser radiation, the input power in thefiber being100mWand the exposure duration being 3 hours. The purpose ofthis experiment was to assess the behavior of different host glasscompositions, when additional radiation hardening techniques(H -loading and photobleaching of RCC) are applied to EDFs.The two fibers of set 3 (Table III) were intended for the esti-

mation of the RIA share due to Er-associated RCC.GeO -dopedsilica was used as the host glass in these fibers, as more radia-tion resistant than P O - and Al O -doped silicas. Some piecesof these fibers were also H -loaded immediately before -irra-diation under the above conditions with the aim to assess thereaction of the Er-related RCC to hydrogen.The three fiber sets were -irradiated from a Co-source in-

dividually in the conditions indicated in Table IV. The Cosource was constituted by 77 identical parallel cobalt rods, 400mm in height and 9 mm in diameter. The rods were situated insuch a way that they formed the lateral surface of a cylinder,220 mm in diameter. They were initially housed in an under-ground led shield and were lifted up to the cobalt source roomto irradiate the samples. The fibers of each set were wound ona common plastic reel, 150 mm in diameter, which was posi-tioned in the cobalt room m away from the cylinder axis(for sets 1 and 2). The reel with the set 3 fibers was placed coax-ially inside the cylinder. The irradiated fiber lengths werem. We assess the dose-rate uniformity along the fiber lengthsand among the fibers in each of the three sets to be no worsethan 3%. The absolute value of dose rate was measured by aglass dosimeter calibrated for the Co gamma-rays in units ofabsorbed dose to water with guaranteed accuracy of 5%. Theirradiations were performed at room temperature.RIA was determined as the difference of the pre- and post-

-irradiation loss spectra measured by the widespread cut-backtechnique. The fiber length used in the measurements was in therange 1–20 m and was chosen so as to have the loss in the fiberpiece of no less than 1 dB (typically – dB), the accuracy ofthe loss measurements being better than 5%. It was impossibleto measure optical loss in the regions of the Er absorption bands(in the vicinity of 980 and 1530 nm), because the absorptionwas too large. Therefore, RIA was interpolated in these regions.In [9] we used interpolated RIA values to predict the gain effi-ciency of irradiated EDF lasers to obtain predictions very closeto the experimental values, which confirms correctness of theinterpolation.

TABLE IVIRRADIATION AND MEASURING CONDITIONS

Fig. 1. RIA spectra of fibers of set 1 measured 8 months after irradiation toa dose of 3, 0 kGy. Dots show RIA intrapolation in the regions of the erbiumabsorption bands.

The time lapse between the end of irradiation and the post-ir-radiation optical loss measurement was from 7 days to 8 months(Table IV). Before the post-irradiation loss measurements, thefibers were kept at room temperature.According to [3], the largest dose to be absorbed by EDFs in

a sputnik during 15-year mission is 2 kGy, a little less than thedoses used in this study.What is more significant is that the labo-ratory dose rates are much larger than those in space. Therefore,the post-irradiation loss spectra measured in a certain period oftime after the end of irradiation must be closer to those in spacethan spectra measured directly in the process of irradiation.The fibers of each set were irradiated and measured in iden-

tical conditions. However, in comparing fibers of different sets,we had to take into account the differences in doses and mea-surement times among the sets (Table IV). In addition, pieces offibers No. 1 and 9 of set 1 were also irradiated simultaneouslywith the fibers of set 2 as reference samples.

III. RESULTS AND DISCUSSION

A. Effects of AlPO Joins and GeO -Codoping

Fig. 1 shows RIA spectra of the fibers of set 1 (Table I), whichdiffered in the P O and Al O concentrations, some of whichcontained both the dopants (i.e., they were AlPO -doped), andsome were, in addition, codoped with GeO .Let us first consider the GeO -free fibers. Of these fibers,

Al O -free P O -doped fibers No. 1 and 2 as well as P O -ex-cess AlPO -doped fiber No. 8 feature the smallest RIA, theRIA of the three fibers being similar by shape and magnitude.Al O -doped EDFNo. 9 demonstrated the largest RIA, whereas


Al-excess AlPO -doped fiber No. 3 containing a greater amountof Al atoms than fiber No. 9 showed, nevertheless, a smallerRIA.The RIA shape of all the fibers of set 1 (Fig. 1) fall into

two types: it is determined either by the known Al-associatedRCC, or by the P-associated ones. The Al-associated RIA isknown to be due to the band tail of the radiation-induced alu-minum-oxygen-hole center (Al-OHC) peaking at 540 nm [19].One can see that the RIA of our P-free Al O -doped fibers andAl-excess AlPO -doped fibers is well described by this bandtail. The P-associated RIA is known to be due to the phos-phorus-oxygen-hole center (P-OHC), whose band tail stretchesfrom the visible region, and the P -center peaking at 1570 nm[20]. The RIA of our Al-free P O -doped fibers and P-excessAlPO -doped fibers is well described by these two RCC.As this takes place, we virtually do not see the presence of

the P-associated RCC in the RIA shape of the Al O -excessAlPO -doped fibers and the presence of the Al-associated RCCin the RIA shape of the P O -excess AlPO -doped fibers. Thisfact along with the fact that AlPO -doped fiber No. 3 containingmore aluminum showed a lower RIA than P-free Al O -fibersNo. 4 and 9 allows us to conclude that the Al and P atoms par-ticipating in the AlPO joins produce few, if any, RCC. In otherwords, AlPO -doped silica provides enhanced radiation resis-tance, which is one more remarkable property of this host glasscomposition.This conclusion is in full agreement with recent paper [21], in

which AlPO -doped silica ytterbium fibers have exhibited sig-nificantly weaker photo-darkening than ytterbium fibers withthe common host glass composition—P-free Al O -dopedsilica. In fact, photo-darkening is an allied phenomenon togamma-radiation-darkening, because both of them are due toRCC associated with the host glass [22].Radiation resistance of AlPO -doped EDF can be further en-

hanced by GeO -codoping, the effect demonstrated in [5] and[17] as applied to Al O host glass. In our comparison experi-ment we see that two identical fibers No. 5 and 4 differing onlyin the presence or absence of a small GeO admixture give no-ticeably different RIA values (Fig. 1). The smallest RIA of theset 1 fibers was measured in an AlPO -doped fiber with a rela-tively large GeO content (No. 6): it is almost an order of mag-nitude lower than that of GeO -free fiber No. 9.The above facts mean that, on the one hand, the radiation

hardening effect increases with increasing the GeO contentand, on the other hand, this effect is very efficient as applied toAl-excess AlPO -doped fibers as well as to Al O -doped fiberstested in [5] and [17]. Unfortunately, we did not have an appro-priate fiber to investigate the GeO codoping effect on the P-re-lated RCC; however, an earlier observation [18] testifies thatgermanium does suppress the P-related RCC, too.The question arises as to what is the physical mechanism of

RIA reduction in the presence of GeO . The main effect of ion-izing radiation on GeO -doped silica is known to be the dis-ruption of the germanium oxygen-deficient centers (Ge-ODC),which are always present in sufficient amounts in silica andact as donors of electrons [23]. The chief RCC in Al O - andP O -doped silica—Al-OHC, P-OHC and P —are hole-typedefects (see [19] and [20], respectively). Therefore, the elec-

Fig. 2. RIA spectra in the aluminosilicate and phosphorosilicate fibers of set 2measured 14–15 days after irradiation to a dose of 4.5 kGy.

trons emanating from Ge-ODC efficiently passivate the latterRCC. It should be remarked that adding GeO to undoped silica,on the contrary, increases its radiation sensitivity, which, how-ever, still remains essentially lower than the radiation sensitivityof GeO -doped phosphosilicate or alumosilicate glasses.Thus, it follows from the set 1 comparison experiment that the

best-suited host glass composition for radiation-resistant EDFis AlPO - and GeO -codoped silica. However, further experi-ments yielded an even better solution (see below).

B. RIA Dependence on P O , Al O , and Er OConcentrations

Fig. 2 gives a comparison of RIA in fibers of set 2 (Table II)with binary glass compositions and various P O and Al Oconcentrations (fiber No. 9 contained, in addition, Er O ).By comparing the curves of Al O -doped fibers in Fig. 2,

we cannot reveal a regular RIA dependence on the Al Ocontent. In fact, fibers No. 14 and 15 exhibit nearly the sameRIA, whereas their Al O content is strongly different (4.5 and1.7 mol.%, respectively). At the same time, the RIA differencebetween fibers No. 4 and 9 in Fig. 1 (4 and 6.5 mol.% Al O )is as big as 40–60%. Across the whole range of Al O concen-trations—from 1.7 to 11 mol.%—RIA varies by just 60%.As regards P O -doped fibers, RIA in the fibers of Fig. 2 also

varies only slightly (by no more than 60%) with varying theP O content considerably, from 4 to 12 mol.%. Note that theminimum RIA occurs in fiber No. 11 having a medium P Ocontent (6.5 mol.%).Therefore, one may conclude that the RIA dependence on

both Al O and P O concentrations in alumosilicate and phos-phosilicate fibers is intricate, non-monotonic and rather weak.We believe that this unusual RIA dependence on the dopant con-centration is due an intricate dependence of the amount of theRCC precursors in the glass network on the P O and Al Oconcentrations. Another factor that might be at work here is theRIA tendency for saturation at kilogray doses (e.g., see [24]).However, the previous investigations of alumosilicate and phos-phosilicate fibers show that RIA does continue to grow mono-tonically with dose at kilogray doses, but at a little lower ratethan at units of gray [9], [12]. For example, it was found in [9]


Fig. 3. RIA spectra in the germanosilicate Er-doped fibers of set 3 measured 14days after irradiation to a dose of 3.5 kGy. Dotted lines show RIA intrapolationin the regions of the erbium absorption bands.

that the RIA increase with dose in an alumosilicate EDF is welldescribed as dose to the power of 0.7 until, at least, 10 kGy.Thus, the RIA tendency for saturation is unlikely to explainthe nonmonotonicity of the dopant concentration dependence.However, for more in-depth analysis of the RIA dependence onthe P O and Al O concentrations, it would be interesting tocarry out RIA measurements immediately in the process of ir-radiation using a greater number of fibers with various dopantconcentrations.It is interesting to note that a repeat RIA measurement of

fibers performed 4 months after the irradiation showed the fol-lowing. RIA of phosphosilicate fibers No. 1, 11, and 12 re-mained virtually the same, as those depicted in Fig. 2, whichis in agreement with known fact that P-associated RCC featurehigh thermal stability [12]. At the same time, the RIA of alu-mosilicate fibers No. 13–15 proved to be 15–20% lower.Let us now consider the possible Er effect on RIA. The shape

and magnitude of the RIA of Er-containing fibers No. 7–9 inFig. 1 can be, apparently, fully explained by the RCC associatedwith Al and P. In fact, the curves of fibers No. 7 and 9 demon-strate a monotonic increase with decreasing wavelength, therate of this increase being typical for the aluminum-oxygen-holecenter (Al-OHC, see above). Note that RIA of fiber 7 is muchlower owing to a GeO admixture. The RIA curve of fiber 8is very close to that of fiber 2, which does not contain erbium.Thus, Er-related RCC, if any, are overshadowed by RCC asso-ciated with the host glass.To try to reveal the Er-contribution to RIA, which must

be very small, we performed a comparison experiment onthe fibers of set 3 (Table III, Fig. 3), which had the samegermanosilicate host glass composition (15 mol. % GeO ) andstrongly different Er contents (0.04 and 0.23 mol.% Er O ,respectively). The contribution of the Ge-associated RCC tothe RIA at 1300 nm under our experimental conditions can beassessed as dB/m (e.g., see [23] and references therein);therefore, a major share of the RIA of fibers No. 18 and 19is due to Er-associated RCC. In fact, the total RIA in thesefibers at nm is 0.18 and 0.44 dB/m, respectively.Hence, the Er-associated RIA amounts to 78% and 91% in the

Fig. 4. RIA spectra of EDFs of set 2 measured 7 days after irradiation to a doseof 4, 5 kGy. Dotted lines show RIA intrapolation in the regions of the erbiumabsorption bands.

respective fibers. We see that the Er contribution to RIA is no-ticeably greater for a greater Er O content (Fig. 3). However,it increases only threefold with increasing the Er O contentby a factor of .Let us try to compare the Er-associated RIA to the Al- and

P-associated RIA observed in Figs. 1 and 2. One can see that theEr-contribution to the RIA of fiber No. 19 (0.23 mol.% Er O )amounts to 3–4% of the RIA of alumosilicate fiber No. 4 withan Al O content rather typical for EDF (4 mol.%). At the sametime, in comparison with Er-free AlPO - and GeO -codopedfiber No. 6, which demonstrated the least RIA among the set 1fibers, the Er-contribution to the RIA of fiber No. 19, is onlytimes lower. Note, however, that the fibers of set 3 were irradi-ated to a little larger dose and the time lapse between the irradia-tion and the measurement was shorter (see Table IV); therefore,the Er-contribution to RIA is, in reality, somewhat smaller thanthe above estimations. Nevertheless, as one can see, Er-relatedRIA may not be negligibly small in EDFs with an optimizedhost glass composition, such as the glass composition of fiberNo 6.The fiber pieces irradiated in the presence of H showed

much smaller RIA (Fig. 3), which nearly coincided for boththe fibers. This means that this RIA is due to RCC in thehost glass, whereas Er-associated RCC have been passivatedvirtually completely. It is known that hydrogen incorporationinto the germanosilicate glass network causes not suppression,but formation of RCC, known as GeH color centers [23]. TheRIA demonstrated by the H -loaded pieces of fibers No. 18 and19 (Fig. 3) is, evidently, due to this GeH color center, whereasRIA in an H - and Er-free germanosilicate fiber containing15 mol.% GeO would be times smaller. Thus, our resultssuggest that the Er-associated RCC can be efficiently passivatedin the presence of hydrogen.

C. Most Radiation-Resistant EDF Design

Fig. 4 shows RIA in pieces of fibers No. 16 and 17 of set2: as-drawn pieces, those H -loaded before irradiation andthose exposed to 980-nm laser radiation after H -loading and-irradiation.


The as-drawn piece of AlPO - and GeO -codoped fiber No.17 showed lower RIA than that of Al O - and GeO -codopedfiber No. 16 (see Table II), which is in agreement with the aboveexperimental fact that AlPO -joins provide enhanced radiationresistance. Even a considerably higher GeO concentration infiber No. 16 (7.5 versus 3 mol.%) proved to be an insufficientfactor. Al O -doped EDF No. 9 showed much higher RIAowing, evidently, to the complete absence of GeO .The surprising thing is that the H -loaded piece of fiber No.

17 behaved noticeably worse than that of fiber No. 16 (Fig. 4).What this means is that H suppresses RCC in an AlPO -dopedEDF less efficiently than in an Al O -doped EDF. This unex-pected effect is, nevertheless, rather easy to explain. High radi-ation resistance of H -free AlPO -doped EDFs testifies that theAlPO -join ( P-O-Al ) is a rather rigid structure: on being dis-rupted by ionizing radiation, the bonds tend to recover in orderto restore the join as a whole. However, when there is an H mol-ecule (H atom or a proton) in the immediate vicinity of the join,then O- H, P- H, or Al-H bonds are likely to arise at the sites ofradiation-disrupted bonds. As a result, the join cannot restore it-self and becomes a disordered mixture of P and Al atoms proneto produce RCC.As was shown in [9], RIA can also be significantly reduced

via photobleaching, if the EDF is pumped at nm(not 1480 nm). It was also shown in [9] that photobleachingis especially efficient in H -containing EDFs. Note that thatH -concentration in fibers No. 16 and 17 by the beginningof the photobleaching experiment had decreased by an orderof magnitude with respect to its value before the irradia-tion. Photobleaching under the action of 980-nm radiation(Fig. 4) turned out to proceed more efficiently in the H -loadedAl O - and GeO -codoped EDF No. 16 than in the analogouspiece of AlPO - and GeO -codoped EDF No. 17. More-over, the H -loaded and photobleached piece of Al O - andGeO -codoped fiber No. 16 showed the least RIA of all thefibers tested in this paper (Figs. 1–4); therefore, this host glasscomposition may be considered as the most appropriate forradiation-resistant EDFs for space applications.As was shown above, GeO -codoping can play a positive role

by suppressing Al-related RCC and a negative one by formingGeH RCC in the course of fiber irradiation in the presence ofH . Thus, a question arises as to the role of GeO -codopingin the record-low RIA of the H -loaded and photobleachedpiece of fiber No. 16. From comparing Figs. 3 and 4, one mayconclude that in our case the RIA level due to the GeH RCCmust be times lower than the RIA in H -loaded fibers No.18 and 19 (Fig. 3), which contained 2 times more GeO (seeTables II and III). However, the latter RIA itself is lower thanthe RIA of the H -loaded and photobleached piece of fiber No.16. Consequently, the remaining RCC in the H -loaded andphotobleached piece of fiber No. 16 are still due to aluminum,not germanium, while the germanium atoms, apparently, con-tinue to foster suppression of Al-associated RCC. Nevertheless,further experiments should be aimed at more delicate optimiza-tion of the contents of each dopant (Er O , Al O , and GeO ),as well as of the H concentration.There is one point we would like to make regarding the

practical implementation of H -loading at high temperatures.

Estimations show that at 200 C, half of H molecules willoutdiffuse from a carbon-coated fiber already within 20 days[8], therefore, an alternative technology of fiber H -loading isneeded. An efficient solution could be as follows: an EDF witha usual polymer coating is accommodated aboard the spacecraftin a hermetic housing filled with H gas at a pressure of severalmegapascal (e.g., the fiber is threaded through a thin metal tubewhich is then hermetically sealed and pressurized). Clearly,such a housing can be made sufficiently light and compactand it is easy to ensure its hermeticity at the above values oftemperature and pressure.

IV. CONCLUSION

We have experimentally assessed the contribution of theEr-associated radiation color centers to the total RIA of alu-mosilicate EDFs to be as small as units of percent for the-radiation dose of kGy. In the case of host glass of higherradiation resistance (AlPO - and GeO -codoped silica), theEr-associated RIA can amount already to % of the totalRIA. The Er-associated RIA has been found to be nonlinearwith respect to the Er concentration: it increases only threefoldwith increasing the Er O concentration by a factor of .The Er-associated color centers can be passivated virtuallycompletely by dissloving molecular hydrogen in the fiber glass.RIA of alumiosilicate fibers has been found to change only

slightly (by less than 40%) with increasing the Al O concen-tration by a factor of (from 1.7 to 11 mol.%). Similarly, RIAin phosphosilicate fibers varies by just 60% in varying the P Oconcentration threefold, from 4 to 12 mol.%. In addition, theRIA dependence on the Al O and P O concentrations in thesesingly doped silica fibers was found to be non-monotonic: forboth the fiber types, RIA reached a minimum at some mediumconcentration in the above ranges.RIA of fibers simultaneously doped with Al O and P O , is

significantly lower than that of fibers doped with Al O only.Evidently, this is so, because Al and P atoms make AlPO joinsin the silica network, which produce much fewer radiation colorcenters than the same amount of aluminum atoms in a P-freefiber. RIA in Al O - and P O -codoped silica fibers with un-equal concentration of the dopants is due to excess P or Alatoms, unincluded in the AlPO joins. An Al O excess leadsto a larger RIA than a P O excess.Our experiments also confirm the earlier observation that

GeO codoping of alumosilicate fibers significantly lowersRIA. We have applied GeO codoping to an Al-excessAlPO -doped EDF to obtain a RIA about an order of magni-tude lower than that in a GeO -free alumosilicate EDF (6.5mol.% Al O ), the dose being kGy. Thus, using AlPO -and GeO -codoped silica with comparable Al O and P Oconcentrations as the host glass of EDFs, apart from excellentactive and waveguiding properties demonstrated elsewhere,will provide relatively high radiation resistance.However, the above host glass has not been found to be effi-

cient in combination with the other techniques to suppress RIA,namely, H -loading and photobleaching.It has been found that the best-suited EDF host glass design

for space applications from the standpoint of radiation resis-tance is P-free Al O -doped silica with GeO -codoping. With


this host glass composition, H -loading yields a much strongerRIA suppression than with AlPO -doped silica and much moreefficient photobleaching of RIA by 980-nm radiation.

ACKNOWLEDGMENT

The authors are grateful to Professor E. M. Dianov, the Di-rector of the Fiber Optics Research Center, for his continuousinterest and support of this work.

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Radiation Resistance of Er-Doped Silica Fibers: Effect of Host Glass Composition

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