Journal of Alloys and Compounds 461 (2008) 617622

Spectroscopic properties of GeO2- aw

ng ad Insgzhochnoluly 20007

Abstract

Er3+-dope nvestEffects of G dopeincorporatio signglasses exhi whicglasses. In a GeOwith increas mighamplifier de 2007 Elsevier B.V. All rights reserved.

Keywords: Tellurite glasses; Rare-earth ions; Thermal analysis; Optical properties

1. Introdu

Telluritetonics appmost promifier (EDFAwide transmdurability,cal propert[14]. Thesstudy on thdoped withwith Er3+section and1.5-m (4Iof the mostglasses havmental rese

CorresponE-mail ad

qyzhang@scu

0925-8388/$doi:10.1016/jction

glasses are promising candidate materials for pho-lications and have been recognized as one of thesing materials for broadband Er3+-doped fiber ampli-). Tellurite glasses have been proven to possessission region (0.355m), good glass stability and

high refractive index (2), better non-linear opti-ies and relatively low phonon energy (800 cm1)e special features have attracted many researchers toe structure and optical properties of tellurite glassesrare-earth ions. Glass based on tellurium oxide dopedions by virtue of a high stimulated emission cross-

a broad full width at half maximum (FWHM) at13/2 4I15/2 transition), has been recognized as onepromising materials for broadband EDFA. Tellurite

e attracted a great deal of attention not only in funda-arch but also in optical devices fabrication over the

ding author. Tel.: +86 13302261009; fax: +86 20 87114834.dresses: zhaochun@scut.edu.cn (C. Zhao),t.edu.cn (Q.Y. Zhang).

past several years for their good electrical, optical and magneticproperties [5,6].

Although tremendous progress have been made, unfor-tunately, there are some difficulties such as relativelylow thermal stability and strong upconversion lumines-cence of tellurite glasses have so far been hindering theEr3+-doped tellurite glass devices from their full commer-cialisation. In this paper, the main object is to carry outa detailed study on effects of GeO2 and Nb2O5 contentson the thermal stabilities and spectroscopic properties ofEr3+-doped TeO2GeO2Nb2O5Na2OK2OZnO glasses toexamine their suitability as potential optical glasses for fiberamplifiers.

2. Experimental

2.1. Glass preparation

Tellurite glasses with molar compositions of xGeO2(70 x)TeO25K2O5Na2O10Nb2O510ZnO0.2Er2O3 (x = 0, 10, 20, 50 and 70, namelyG0, G1, G2, G3 and G4, respectively) and yNb2O5(14.7 y)Na2O10ZnO5K2O5GeO265TeO20.3Er2O3 (y = 0, 3, 5, 7 and 9, namely N0, N1,N2, N3 and N4, respectively) were fabricated by using reagent-grade Na2CO3,

see front matter 2007 Elsevier B.V. All rights reserved..jallcom.2007.07.072tellurite glasses dopedC. Zhao a,b, G.F. Yang a, Q.Y. Zha

a Key Lab of Special Functional Materials of Ministry of Education, anSouth China University of Technology, Guan

b College of Applied Physics, South China University of TeReceived 25 April 2007; received in revised form 13 J

Available online 27 July 2

d tellurite glasses TeO2GeO2Nb2O5Na2OK2OZnO have been ieO2 and Nb2O5 on the thermal stability and optical properties of Er3+-n of GeO2 or Nb2O5 increases the thermal stability of tellurite glassesbit the stimulated emission cross-section as great as 10.7 1021 cm2,ddition, the intensity of upconversion luminescence of the Er3+-dopeding GeO2 or Nb2O5 content. As a result, Er3+-doped tellurite glassesvices.nd Nb2O5-modifiedith Er3+,

, Z.H. Jiang atitute of Optical Communication Materials,u 510641, Chinaogy, Guangzhou 510641, China07; accepted 21 July 2007

igated for developing 1.5-m fiber and planar amplifiers.d tellurite glasses have been discussed. It is noted that theificantly. Er3+-doped GeO2- and Nb2O5-modified telluriteh is significantly higher than that of silicate and phosphate2- and Nb2O5-modified tellurite glasses decreased clearlyt be a potential candidate for developing laser or optical

618 C. Zhao et al. / Journal of Alloys and Compounds 461 (2008) 617622

K2CO3, ZnO, Nb2O5, GeO2, TeO2, and high purity Er2O3 as the starting mate-rials. The glass samples were prepared using the conventional melt-quenchingmethod described anywhere.

2.2. Measurements

Thermal analysis of glasses were examined by a Netzsch STA 449C Jupiterdifferent scanning calorimeter (DSC) at a heating rate of 10 k min1 from roomtemperature (RT) to 700 C. Raman scattering spectra were recorded in the rangeof 2501000 cm1 using a microscope spectrophotometer (model RM 2000,Renishaw) with 514.5 nm laser as an excitation source and the working poweris 20 mW. The absorption spectra were obtained by a Perkin-Elmer Lambda-900 UV/VIS/NIR spectrophotometer ranging 3501700 nm. Fluorescence andupconversion luminescence spectra were measured on the TRIAX 320 typespectrometer with 977 nm laser diode (LD) as an excitation source. The lifetimesof Er3+: 4I13/2 level were measured by exciting the samples with the 977 nm LDand detected by a photon-counting R5108 photomultiplier tube. Fluorescencesignal is collected in a direction perpendicular to the exciting beam, and all thesamples were located at the same site in the process of measuring fluorescenceproperties. All the measurements were performed in the same condition at RT.

3. Results

3.1. Therm

The valund) for tellincrease mNb2O5 conglasses aretemperaturclearly seebetween TgTable 1. Asincrease froincreasingwith Nb2Odetected in

The therestimate ofof temperaa glass hosFig. 1, it isin tellurite

Table 1Density, refra

Sample D(g

G0 4G1 4G2 4G3 4G4 4

N0 4N1 4N2 4N3 4N4 4

Fig. 1. DSC curves of G0, G2, G4 (a) and N0, N2, N4 (b).

d under a heating rate of 10 K min1 in the samples of, and N2 indicating that these glasses are much preferableforming fabrication and crystal-free fiber drawing [2,7].

aman spectra

. 2(a and b) shows the Raman spectra of G0, G2, G4,d N0. The bands centered at around 462 cm1 areed to the stretching vibrations of Ge O Ge and Te O Tees. The bands centered at around 688 cm1 are orig-from vibration of the continuous networks composed

O4]4 tetragonal bipyramids and the bands centered at764 cm1 are contributed to [TeO3+1]4, [TeO3]2 and

] [811]. With the increase of GeO2 content, the sixfoldGeO6] of G0G4 glasses increase in the network structure.und that the maximum phonon energy (MPE) of G0G4s increased from 747 to 778 cm1 gradually. As for the-modified tellurite glasses, the number of the [TeO3+1]4nate polyhedra and [TeO3]2 trigonal pyramids increaseplacing Nb2O5 for Na2O in the network structure. It is

that the MPE of N0 is slightly lower than that of N4. Obvi-the MPE of the tellurite glasses studied increase with theoration of Nb2O5 or GeO2, which might be caused these of the upconversion intensity of Er3+-doped glasses.and discussion

al and physical properties of glasses

es of density () and refractive index (d-line index,urite glasses are shown in Table 1. Both and ndonotonically with decreasing GeO2 or increasingtent. The DSC curves of G0, G2, G4 and N0, N2, N4illustrated in Fig. 1 as examples. The glass transitione (Tg) and crystallization on set temperature (Tx) aren in Fig. 1. The values of Tx, Tg and the difference

and Tx (T = Tx Tg) of the glasses are given inshown in Table 1, Tg, Tx and T of G0G4 glassesm 340, 414 and 74 C to 531, 648 and 117 C with

GeO2 from 0 to 70 mol%. Tg values increase rapidly5 content. There is no obvious crystallization peakN0, N1, N2 samples.mal stability factor T is frequently used as a roughthe glass stability. To achieve a large working range

ture during sample fiber drawing, it is desirable fort to have as a large T as possible [5]. As shown inbenefit for glass stability to increase GeO2 contentglasses and there is no obvious crystallization peak

ctive indices and thermal properties of tellurite glasses

ensity/cm3)

Refractiveindices

Tg (C) Tx (C) Tx Tg (C)

.99 2.11 340 414 74

.85 2.05 407 477 70

.68 1.98 451 538 87

.30 1.86

.02 1.76 531 648 117

.70 1.94 262.7

.75 1.98 300.6

.78 2.00 317.1

.82 2.03 335.4 422.9 87.5

.86 2.05 352.8 460.0 107.2

detecteN0, N1for per

3.2. R

FigN4 anassignlinkaginatedof [Tearound[GeO6units [It is foglasseNb2O5coordiwith refoundously,incorpdecrea

C. Zhao et al. / Journal of Alloys and Compounds 461 (2008) 617622 619

Fig

3.3. Effect

Fig. 3 sN0, N2, NThe inhomsitions fromEr3+ ions.and 6) oftheory from4, 6) arethe increas

ig. 3.

. Th10

Table 2JuddOfelt in

Sample

G0G1G2G3G4

N0N1N2N3N4

Germanium [Silicate [11]Phosphate [11Tellurite [11]. 2. Raman spectra of G0, G2, G4 (a) and N0, N4 (b).

of GeO2 and Nb2O5 on JuddOfelt parameters

F

studied6.41 hows optical absorption spectra of G0, G2, G4 and4 glasses in the wavelength range of 3501700 nm.ogeneously broadened bands are ascribed to the tran-

the ground state 4I15/2 to the excited states ofJuddOfelt (JO) intensity parameters t (t = 2, 4each glass can be obtained according to the JO

the measured absorption spectra. The t (t = 2,listed in Table 2. Apparently, 2 increase withe of Nb2O5 or GeO2 content in tellurite glasses

increase froglasses whaluminate gvary from 10.89 10values are cthan that oAccordingof the glass

tensity parameters of tellurite glasses

2 (1020 cm2) 4 (1020 cm2)6.41 1.636.82 1.58.3 2.017.86 2.638.98 2.09

6.62 1.756.85 1.916.96 1.898.22 2.257.86 2.19

11] 5.81 0.864.23 1.04

] 6.65 1.524.74 1.62Absorption spectra of G0, G2, G4 (a) and N0, N2, N4 (b).

e values of 2 in present glasses increase from20 to 8.98 1020 cm2 for G type of glasses and

20 20 2m 6.62 10 to 7.86 10 cm for N type ofich are larger than that in germanate, silicate, andlasses [11]. The values of 6 for G type of glasses.44 1020 to 1.18 1020 cm2 and increase from

20 to 1.13 1020 cm2 for N type of glasses. Theseomparable to the 6 of fluoride [12] glass but largerf silicate, aluminate and germanate glasses [13].to previous studies, 2 is related to the symmetryhost. The more the asymmetric of the glass host, the

6 (1020 cm2) (107)1.44 1.30.81 1.31.08 1.40.95 1.21.18 1.1

0.89 1.10.96 1.31.0 1.21.05 1.21.13 1.4

0.28 0.61 1.11 0.64

620 C. Zhao et al. / Journal of Alloys and Compounds 461 (2008) 617622

larger the 2 is. The increase of GeO2 or Nb2O5 contributesto increase the asymmetric of the glass host which results inthe increase of 2. 6 is inversely proportional to the cova-lency of Er O bond. The covalency of Er O bond is attributedto be related with the local basicity around the rare-earth sites,which can be adjusted by the composition or structure of theglass hosts. With the substitution of GeO2 for TeO2 or Nb2O5for Na2O, more and more nonbridge oxygen ions, which tendto coordinate with Er3+, will contribute to coordinate with glassformer cation ions. According to the electronegativity theory, thecovalency of the bond will become stronger with the decreaseof the difference of electronegativity between cation and anionions. Since the values of electronegativity, for Te, Ge, Na, Nband O elements, are 2.1, 1.8, 0.9, 1.6 and 3.5, respectively, thecovalency of Te O and Nb O bond are stronger than Ge O andNa O, respectively. As a result, the covalency of Te O, Nb Obonds are stronger than those of the Ge O and Na O bond,respectively. Consequently, the value of 6 decreases with thesubstitution of GeO2 for TeO2 and increase with the substitutionof Nb2O5 fglasses is lphosphate

3.4. Effectcross-sectio

Fig. 4 illdoped telluat 977 nm e1532 nm, wis found thawith decreaTable 3 shoEr3+: 4I13/2FWHM decfor TeO2, aconcentratiwith increaGeO2 contfor wavelen

ormor G0

andut 5nium

abtions aa() exp[( h)/kT], where h is the Planck constant,oltzmann constant, and is the net free energy required

ite one Er3+ from the 4I15/2 to 4I13/2 at temperature T.s absorption cross-section which has been determined by:2.303OD()/Nl, where N is the concentration of rare-

l the length of absorption and OD() is the absorptionty. Table 3 shows the peak emission cross-sections (pe )of Er3+: 4I13/2 4I15/2 transition in glass samples. Fig. 6

d lifetime of 4I13/2 level of Er3+ (f) and FWHM.or Na2O. The covalency of Er O bond in the presentower than that in germanate, silicate, aluminate andglasses [4].

of GeO2 and Nb2O5 on emission spectra andns

ustrates the normalized fluorescence spectra of Er3+-rite glasses in the wavelength range 14401650 nmxcitation. All samples exhibit broad bands peaked athich assign to the 4I13/2 4I15/2 transition of Er3+. Itt the FWHM of fluorescence spectra become broadersing GeO2 or increasing Nb2O5 content. Fig. 5 andw the compositional dependence of FWHM of the 4I15/2 transition in various glasses systems. Therease from 52 to 46 nm with the substitution of GeO2nd it vary from 38 to 45 nm with increasing Nb2O5on. Obviously, the FWHM increase monotonicallyse of Nb2O5 content but decrease with increase ofent. A larger value of FWHM could be interestinggth-division multiplexing applications. As shown in

Fig. 4. NLD (a) f

Fig. 5is abogerma

Theabsorpsectione() =k the Bto exca() ia() =earth,intensivalues

Fig. 5. Nb2O5 (a) and GeO2 (b) compositional dependence of measurealized fluorescence spectra of tellurite glasses excited by a 977 nm, G1, G2, G3 and G4; (b) for N0, N1, N2, N3, N4.

Table 3, the broadest FWHM value of these glasses2 nm for G0, which is larger than that in silicate,

glasses [14].sorption cross-sections are determined from the

spectra, and the stimulated emission cross-re calculated from McCumber method [15] by:

C. Zhao et al. / Journal of Alloys and Compounds 461 (2008) 617622 621

Table 3Emission parameters of Er3+ at 1.5-m emission in various glasses systems

Sample FWHM (nm) pe (1021 cm2) f (ms) p 28 3 p 21 2

G0 52 9.9 3.2G1 50 9.6 2.9G2 48 9.3 2.7G3 47 8.6 2.5G4 46 7.6 2.3

N0 38 7.2 3.6N1 39 7.6 3.4N2 42 8.1 2.9N3 43 9.1 3.0N4 45 10.7 2.7

Tellurite [15] 60 6.6 4Silicate [16] 45 5.5 10Phosphate [17] 37 6.4 8

illustrates the calculated absorption and emission cross-sectionsof G1 and N2 glasses in the wavelength range of 14201630 nm.Since the stimulated emission cross-section is proportional tothe host glass refractive index, e (n2 + 2)2/n [2], the pe valuemonotonically decreases from 9.9 1021 to 7.6 1021 cm2with increasing GeO2 content. The values of pe monotonicallyincrease up to the maximum (10.7 1021 cm2) with increas-ing Nb2O5 content. It is apparent that Er3+ in the glass with

Fig. 6. Calcuin the wavelen

more Nb2Ocross-sectio

3.5. EffectEr3+: 4I13/

To obtaconditionsmetastablesuccess of EFig. 5 illusfunction offrom 3.2 tothat decreato 9 mol%.with increa

red lve ded/or

ve deresceval

te thduct pemeasu

radiatiions anradiatiof fluo

Theevaluathe proand fplated absorption and emission cross-sections of G1 (a) and N2 (b)gth range of 14001650 nm.

e , FWHMwith the incontent. AsFWHM and G0, resand phosph

3.6. Effect

The freqglasses in tunder 977 nbands centwhich are4F9/2 4I1FWHM e (10 cm ) e f (10 cm ms)516 31.7480 27.8446 25.1402 21.5355 17.5

274 25.9296 25.8340 23.5392 27.3482 28.9

396 26.4247 55237 51.2

5 is capable of providing larger stimulated emissionn at 1.5-m.

of GeO2 and Nb2O5 on uorescence lifetime of2 level and amplify properties

in high population inversions under steady-stateusing modest pump powers, long lifetime of thestate of Er3+: 4I13/2 level is a critical factor in ther3+-doped fiber amplifier in optical communications.

trates the measured fluorescence lifetimes (f) as theGeO2 and Nb2O5. Clearly, the lifetime decreases

2.3 ms with increasing GeO2 from 0 to 70 mol%, andses from 3.6 to 2.7 ms when Nb2O5 increases from 0According to the Raman spectra, the MPE increasessing GeO2 or Nb2O5 content. The decrease of theifetime might be due to the enhancement of non-cay process. The interactions between Nb5+ and O2between Ge4+ and O2 ions contribute to the non-cay of the 4I13/2 level, which result in the decreasence lifetimes of Er3+: 4I13/2 level.

ues of FWHM pe and f pe are often used toe gain properties of EDFA [16]. Generally, the biggers, the better the properties are. FWHM, FWHM peare also compared in Table 3. The values of FWHM,

p p e and f e in tellurite glasses studied increasecrease of Nb2O5 content or the decrease of GeO2shown in Table 3, the maximum values of pe and

pe are 10.7 1021 cm2, 516 1028 cm3 for N4

pectively, which are much larger than that of silicateate glasses.

s of GeO2 and Nb2O5 on upconversion spectra

uency upconversion spectra of Er3+-doped telluritehe ranging 500750 nm have also been investigatedm LD excitation as shown in Fig. 7. Three intense

ered at 525, 547 and 657 nm have been observed,assigned to the 2H11/2 4I15/2, 4S3/2 4I15/2, and5/2 transitions of Er3+, respectively. The quadratic

622 C. Zhao et al. / Journal of Alloys and Compounds 461 (2008) 617622

Fig. 7. IR-to(x = 0, 1, 2, 3 aof the upconvlaser power a

dependencintensitiesb), indicatitwo-phono

It is notwith the coFig. 7(a), t6% of thaison withintensity dN3 and N4shown in Fby the concence intenhosts, whildeterminedthat the M

ing Nb2O5 or GeO2 content, which results in the promotion ofthe multi-phonon relaxation rate and decrease the upconversionemissions.

4. Conclusions

umm

TeOvelopand N+-do

e incbilit

2O5-10are s

s. Inn lum

witor Ntents.In sGeO2for deGeO2of Er3that thmal staor Nb10.7 whichglasseversioclearlyGeO2-be a podevice-visible upconversion luminescence spectra of Er3+-doped Gxnd 4) (a) and Ny (y = 0, 1, 2, 3 and 4) glasses (b). The dependencesersion green and red emissions intensity of G1 and N2 on pumpre shown in inset of (a) and (b), respectively.

es of the upconversion green- and red-emissionson pump power shown in the inset of Fig. 7(a andng that both the green- and red-emissions due ton absorption processes [19].ed that the green upconversion emission decreasesntent of Nb2O5 or GeO2 increasing. As shown in

he green intensity decreases to 70%, 41%, 9% andt in the G1, G2, G3 and G4 glasses in compar-the G0 glass, respectively,. Meanwhile, the greenecreases to 94%, 90%, 70% and 61% for N1, N2,

glasses compared with N0 glass, respectively, asig. 7(b). The red intensity is not affected apparentlycentration of GeO2 or Nb2O5. The green lumines-sity is mainly determined by the MPE [17] of glasse the red upconversion emission intensity is mainlyby ESA procedure [18]. Our investigation indicates

PE of host glass might be improved with increas-

Acknowled

Authors50602017)cial assista

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ped tellurite glasses have been discussed. It is notedorporation of GeO2 or Nb2O5 could increase ther-y of tellurite glasses significantly. Er3+-doped GeO2-modified tellurite glasses exhibit large pe (up to21 cm2) and FWHM pe (up to 482 1028 cm3),ignificantly higher than that of silicate and phosphateaddition, it is found that the intensities of upcon-inescence of Er3+-doped tellurite glasses decrease

h increasing GeO2 or Nb2O5 content. As a result,b2O5-modified Er3+-doped tellurite glasses might

ial candidate for developing laser or optical amplifier

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are grateful to the Project of NSFC (50472053,, DSTG (2006J1-C0491), NCET (04-0823) for finan-nce.

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Spectroscopic properties of GeO2- and Nb2O5-modified tellurite glasses doped with Er3+IntroductionExperimentalGlass preparationMeasurements

Results and discussionThermal and physical properties of glassesRaman spectraEffect of GeO2 and Nb2O5 on Judd-Ofelt parametersEffect of GeO2 and Nb2O5 on emission spectra and cross-sectionsEffect of GeO2 and Nb2O5 on fluorescence lifetime of Er3+: 4I13/2 level and amplify propertiesEffects of GeO2 and Nb2O5 on upconversion spectra

ConclusionsAcknowledgementsReferences