Thermal stability and spectroscopic properties of Er3+-doped niobium tellurite glasses for broadband amplifiers

Thermal stability and spectroscopic properties of Er3+-doped niobium tellurite glasses for broadband amplifiers

Materials Chemistry and Physics 90 (2005) 78–82 Thermal stability and spectroscopic properties of Er3+-doped niobium tellurite glasses for broadband ...

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Materials Chemistry and Physics 90 (2005) 78–82

Thermal stability and spectroscopic properties of Er3+-doped niobium tellurite glasses for broadband amplifiers D.D. Chena,∗ , Y.H. Liua , Q.Y. Zhanga , Z.D. Denga , Z.H. Jianga,b a

Key Laboratory of Specially Functional Materials and Advanced Manufacturing Technology, Ministry of Education, China and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, PR China b Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 210800, PR China Received 1 June 2004; received in revised form 22 September 2004; accepted 4 October 2004

Abstract Three Er3+ -doped tellurite glasses with compositions of 70TeO2 –30ZnO, 70TeO2 –20ZnO–10Nb2 O5 and 70TeO2 –20ZnO–5BaO–5Nb2 O5 have been investigated for developing fiber and planar broadband amplifiers and lasers. The optical spectroscopic properties and thermal stability of Er3+ -doped tellurite glasses have been discussed. The results show that the incorporation of Nb2 O5 increases the thermal stability of Er3+ -doped tellurite glasses significantly, Er3+ -doped niobium tellurite glasses 70TeO2 –20ZnO–10Nb2 O5 and 70TeO2 –20ZnO–5BaO–5Nb2 O5 exhibit the good thermal stability (T > 150 ◦ C), the large emission cross-section (>10 × 10−21 cm2 ) and broad full width at half maximum (∼65 nm), will be preferable for broadband Er3+ -doped fiber amplifiers. © 2004 Elsevier B.V. All rights reserved. Keywords: Glasses; Annealing; Differential scanning calorimetry; Photoluminescence spectroscopy; Luminescence

1. Introduction The current trend in advancing optical fiber networks is to increase the system capacity by transmitting multiple channels through a wavelength division multiplexing link. A wide bandwidth optical amplifier is vital for the development of multichannel transmission systems. Although tremendous progress in optical fiber network has been made, the limitation of the bandwidth of Er3+ -doped silica amplifier, so far, has hindered it from future dry fiber communications [1]. Achieving new amplifying wavelengths have been one of the main commercial driving factors up-to-now for the development of broadband amplifiers. Tellurite glasses have attracted a great deal of attention both in fundamental research and also in optical devices fabrication over the past several years. It is mainly because the optical glasses based on tellurite are showing the good transparency in the visible–infrared (0.35–6 ␮m), the relatively low phonon energy (∼750 cm−1 ) ∗

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0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.005

compared with other oxide glasses such as silicate and phosphate glasses, the large resistance against corrosion, the possibility to incorporate a large amount of rare-earth (RE) dopants, and the high refractive index [2–4]. In addition, tellurite glasses could be used in the production of optical fiber and planar waveguides [5]. These special optical properties encourage in identifying them as important materials for potential applications in high performance optics and laser technology and optical communication networks. In this paper, we report and discuss the optical spectroscopic properties and thermal stability of Er3+ -doped niobium tellurite glasses for a broadband fiber amplifier in the third telecommunications window. Three tellurite glasses compositions have been studied; the compositions and properties of these glasses are summarized in Table 1. The Judd–Ofelt intension parameters Ωt (t = 2, 4, 6) of Er3+ were calculated by using the Judd–Ofelt theory [6,7] and compared with silicate, phosphate and fluoride glasses. The results indicated that Er3+ -doped niobium tellurite glasses exhibit the good thermal stability, the large emission cross-section and the broad full width at half maxi-

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Table 1 Thermal properties of tellurite glasses Glass

Composition (mol%)

Tg (◦ C)

Tx (◦ C)

T (◦ C)

TZE TZBNE TZNE

70TeO2 70TeO2 70TeO2

333 397 393

440 – 551

107 – 158

mum (FWHM) are preferable for broadband Er3+ -doped fiber amplifiers.

2. Experimental details Tellurite glasses with the molar composition of 70TeO2 –30ZnO (TZE), 70TeO2 –20ZnO–10Nb2 O5 (TZNE) and 70TeO2 –20ZnO–5BaO–5Nb2 O5 (TZBNE) doped with 1 mol% of Er2 O3 were prepared using a conventional melt method from powders with reagent-grade TeO2 (99.99%), ZnO (99.5%), Ba(NO3 )2 (99.5%), Nb2 O5 (99.5%) and Er2 O3 (99.9%). The required amounts of raw materials were mixed and melted in a platinum crucible at 800 ◦ C for 30 min in an electric furnace, and then quenched on preheated stainless steel blocks and annealed at their glass transition temperature for 2 h before ramping down to room temperature. After annealed, all the samples were polished and cut at the size of 10 mm × 20 mm × 1.5 mm. Thermal analyses of tellurite glasses were determined using a Netzsch STA 449C Jupiter different scanning calorimeter (DSC) at a heating rate of 10 K/min from room temperature to 700 ◦ C. The absorption spectra were measured with a Perkin-Elmer Lambda 900 UV–visible-NIR spectrophotometer in the range of 350–1700 nm with the resolution of 1 nm. The fluorescence spectra in the range of 1450–1650 nm were obtained with a computer-controlled Triax 320 spectrofluorimeter (Jobin-Yvnon Inc.) upon excitation of a 980 nm laser dioder. Raman scattering spectra were obtained using a microscope spectrophotometer (model RM 2000, Renishaw, England) with the 514.5 nm laser exciting line and of 20 mW excitation. All Raman spectra were recorded at room temperature in the wavenumber range of 200– 1400 cm−1 .

Fig. 1. DSC traces of Er3+ -doped tellurite glasses.

nificantly. Nb2 O5 occurs in the glass network as two sites, [NbO4 ] and [NbO6 ], and forms the thermal stability Nb O bond [8]. Moreover, the electronegativity of Nb (XNb = 1.6) is large. This means that the structure rearrangement in the niobium tellurite glasses become difficult due to the large attraction between Nb5+ ion and the neighbor [TeO4 ] or [TeO3+δ ] units. As shown in Fig. 1, the peaks due to the glass crystallization are extremely weak. Particularly in the sample of TZBN, no obvious crystallization peak was detected under a heating rate of 10 K/min, indicates that the sample is more stable against devitrification and thus more suitable for fiber drawing. Raman spectra of tellurite glass and silicate glass with the composition of 70TeO2 –20ZnO–10Nb2 O5 and 70SiO2 –5CaO–25Na2 O glasses respectively are shown in Fig. 2. In the Raman spectra obtained for the tellurite glass 70TeO2 –20ZnO–10Nb2 O5 , the bands around 420, 670 and 750 cm−1 are assigned to stretching vibrations in TeO4 and TeO3 (and/or TeO3+1 ) ground, respectively [9].

3. Results and discussion Fig. 1 illustrates the DSC curves of all the three tellurite glasses, and the measured DSC data are also summarized in Table 1. The difference between the glass transition temperature (Tg ) and the onset crystallization temperature (Tx ), T = Tx − Tg , has been frequently used as a rough estimate of glass formation ability or glass thermal stability. To avoid any crystallization during fiber drawing, it is desirable for a glass host to have as large T as possible. It is noted that adding Nb2 O5 increase the thermal stability of tellurite glasses sig-

Fig. 2. Raman spectra of 70TeO2 –20ZnO–10Nb2 O5 and 70SiO2 –5CaO– 25Na2 O glasses.

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D.D. Chen et al. / Materials Chemistry and Physics 90 (2005) 78–82 Table 2 Judd–Ofelt intensity parameters of Er3+ -doped different glasses

Fig. 3. Absorption spectrum of Er3+ -doped TZBNE niobium tellurite glass.

In the Raman spectra of the silicate glass, four peaks at 560, 780, 950 and 1100 cm−1 are observed. The 560 cm−1 band is assigned to Si O Si bending vibration mode of connecting SiO4 tetrahedral. The 950 and 1100 cm−1 band are attributed to symmetric stretching vibration modes of the Si O bond in the tetrahedron, which contains two and one nonbridging oxygen, respectively [10–12] but the assignment of 780 cm−1 band is not known yet. Fig. 3 shows the absorption spectrum in the visible and infrared regions of Er3+ -doped TZBNE glass. All relevant internal 4f–4f electronic transitions of Er3+ ions in the range of 370–1700 nm have been observed and identified. The Judd–Ofelt theory was often used to calculate the spectroscopic parameters, such as strength parameters Ωt (t = 2, 4, 6), spontaneous emission probability, branching ratio, and radiation lifetime, of rare earths in various matrixes. The three parameters Ωt (t = 2, 4, 6) can be obtained experimentally from the measured absorption spectrum and the refractive index of the host material [13,14]. For the transitions between the states which meet the transition selective rules S = J = 0, J = 0, ±1, there exists the contribution of magnetic dipole transitions (Smd ). In the case of the 4 I13/2 → 4 I15/2 transition of Er3+ , the difference in the total angular momentum J = 1. To obtain wide and flat 1.55 ␮m emission spectra, it is effective to increase the relative contribution of the electric-dipole transition (Sed ) [15]. The Smd is independent of ligand fields and is characteristic to the transition determined by the quantum numbers, while that of the Sed is a function of the ligand fields. It is possible to increase the fraction of the electricdipole transition by modifying the composition and structures of the hosts. According to the Judd–Ofelt theory, the Sed of 4 I13/2 → 4 I15/2 transition of Er3+ is given by: Sed [4 I13/2 , 4I 15/2 ] = 0.0188 Ω2 + 0.1176 Ω4 + 1.4617 Ω6 [16,17], where the three coefficients Ωt s are the reduced matrix elements of the unit tensor operators provided in Refs. [9,10], and coefficients t (t = 2, 4, 6) are the intensity parameters. Table 2 shows comparisons of the Judd–Ofelt intensity parameters, Ωt (t = 2, 4, 6), of Er3+ in various glass hosts. According

Glass

Ω2 (×10−20 cm−2 )

Ω4 (×10−20 cm−2 )

Ω6 (×10−20 cm−2 )

TZE TZBNE TZNE Tellurite [5] Silicate [15] Phosphate [15] Aluminate [15] Germanate [15] Fluoride [15]

7.69 5.99 9.20 6.98 4.23 6.65 5.60 5.81 2.91

1.96 1.91 2.33 2.52 1.04 1.52 1.60 0.85 1.27

0.92 1.20 1.21 1.10 0.61 1.11 0.61 0.28 1.11

to previous studies [15], Ω2 is related with the symmetry of the glass hosts while Ω6 is inversely proportional to the covalency of Er O bond. The Er O bond is assumed to be related to the local basicity around the rare-earth sites, which can be adjusted by the composition or structure of the glass hosts [13]. The Ω2 values of tellurite glasses are larger than those of other glasses due to the large polarization of the Te4+ and the asymmetric of tellurite glasses. It should be mentioned here that the Ω6 values in tellurite-based glasses are comparable to those of fluoride glass and are larger than those in silicate, aluminate and germanate glass hosts. As shown in Table 2, with increasing Nb2 O5 content, the value of Ω6 of niobium tellurite glasses increases monotonically. On the basis of the electronegativity theory, the smaller the difference of electronegativity between cation and anion ions, the stronger the covalency of the bond. Since the values of electronegativity, for Ba, Nb and O elements, are 0.9, 1.6 and 3.5, respectively, the covalency of Nb O bond is stronger than that of Ba O bond, it is expected that the influence of the Nb O bond on the local ligand environments around Er3+ increase with an increase of Nb2 O5 content. Consequently, the covalency of the Er O bond decreases, and the value of Ω6 increase accordingly. Table 3 represents the calculated results of the electricdipole transition rates, Aed , and magnetice-dipole transition rates, Amd , branching ratios, β, and radiative lifetimes, τ rad , of Er3+ -doped tellurite glasses. It is found that tellurite glasses exhibit the large radiative transition probability (>300 s−1 ) for the 4 I13/2 level of Er3+ , which is much larger than that in silicate and phosphate glasses. The radiative transition rates for the 4 I13/2 level of TZNE and TZBNE are 354 and 351.2 s−1 , respectively, which is much higher than that of TZE, due to the higher J O parameters and refractive index, indicates an enhanced local field and a larger radiative transition rate of Er3+ in the niobium tellurite glasses. Fig. 4 shows the absorption cross-section and stimulated emission cross-section of Er3+ for the 1.55 ␮m transition in 70TeO2 –20ZnO–5BaO–5Nb2 O5 glass. The absorption cross-section was determined from the absorption spectra, and the stimulated emission crosssection was calculated from McCumber method [18] by σ e (λ) = σ a (λ) exp[(ε − h␥)/kT], where h is the Planck constant, k the Boltzmann constant, and ε the net free energy

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Table 3 Calculated spontaneous radiation transition rates, fluorescent branch ratio and radiative lifetime of Er3+ -doped tellurite glasses Glass

Initial level → end level

Average energy (cm−1 )

A (S−1 ) Aed

TZE

4I

TZNE

6527 10248 3721 12529 6002 2281 6531 10244 3713 12515 5984 2271 6535 10256 3721 12531 5996 2275

15/2

→ 4 I15/2 4I 4 11/2 → I13/2 4I 4 9/2 → I15/2 4I 4 9/2 → I13/2 4I 4 9/2 → I11/2 4I 4 13/2 → I15/2 4I 4 11/2 → I15/2 4I 4 11/2 → I13/2 4I 4 9/2 → I15/2 4I 4 9/2 → I13/2 4I 4 9/2 → I11/2 4I 4 13/2 → I15/2 4I 4 11/2 → I15/2 4I 4I → 11/2 13/2 4I 4 9/2 → I15/2 4I 4 9/2 → I13/2 4I 4 9/2 → I11/2 4I

TZBNE

13/2

→ 4I

11/2

required to excite one Er3+ from the 4 I15/2 state to 4 I13/2 state at temperature T. The stimulated emission cross-section for the 4 I13/2 → 4 I15/2 transition of Er3+ in different types of glass hosts is compared in Table 4. Since the stimulated emission cross-section is proportional to the host glass refractive index, σ e ∼ (n2 + 2)2 /n, it is apparent that Er3+ in tellurite

(S−1 )

209.6 238.2 33.74 381.0 87.2 0 259.2 303.6 40.9 365.9 110.0 0 263.2 302.4 42.0 438.1 110.0 0

Amd 93.2 0 26.6 0 0 5.5 92.0 0 26.1 0 0 5.4 90.8 0 25.9 0 0 5.3

β (%)

τ rad (ms)

100 79.8 20.2 80.4 18.4 1.2 100 81.9 18.1 76.0 22.9 1.1 100 81.7 18.3 79.1 19.9 1.0

3.30 3.35

(S−1 )

2.11

2.85 2.69 2.08

2.82 2.70 1.81

glasses are capable of providing large stimulated emission cross-section at 1.55 ␮m bands. Fig. 4 illustrates the emission spectra of Er3+ in the three different tellurite glass. The gain bandwidth of an amplifier is determined largely by the width of the emission spectrum and the stimulated emission cross-section. A figure-of merit (FOM) for bandwidth as the product σ e × FWHM is also compared in Table 4. As the emission cross-section strongly depends on the multiplicity of the RE ion sites, Figs. 4 and 5 clearly indicates the apparent advantages of choosing tellurite glass hosts for designing broadband amplifiers. The Er3+ emission spectra in the tellurite glasses are significantly broader (FWHM ∼ 65 nm) than that in other glass studied as potential Er3+ -doped fiber amplifier (EDFA) hosts. The network modifiers, ZnO, BaO and Nb2 O5 , when present in the TeO2 glass network produce a range of structural sites for RE ions. These structural sites could be a variant of trigonal bipyramid for TeO4 , trigonal pyramid for TeO3 , and a polygonal structure for TeO3+δ [20],

Fig. 4. Absorption and stimulated emission cross-sections of Er3+ -doped TZBNE niobium tellurite glass. Table 4 The emission cross-sections and FWHM of Er3+ in various glass hosts Glass

nD

FWHM (nm)

σ e (×10−21 cm−2 )

FOM (σ e × FWHM)

TZE TZBNE TZNE Tellurite [5] Silicate [18] Phosphate [19]

2.08 2.10 2.15 2.00 1.585 1.569

72 65 65 65 40 37

10.12 10.96 12.20 7.95 5.5 6.4

728.64 712.4 793.0 516.75 220 236.8

Fig. 5. A 1.55 ␮m emission spectra of Er3+ -doped tellurite glasses.

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by comparison, in a silicate glass the main structural unit is SiO4 tetrahedral, considering the Raman spectra of the two glasses in Fig. 2. The sites to sites variation bring to an inhomogeneous broadening, which is clearly desirable for a broadband EDFA glass host. 4. Conclusions In summary, we investigated the optical spectroscopic properties and thermal stability of Er3+ -doped tellurite glasses with compositions of 70TeO2 –30ZnO, 70TeO2 – 20ZnO–10Nb2 O5 and 70TeO2 –20ZnO–5BaO–5Nb2 O5 for developing fiber and planar broadband amplifiers and lasers. Adding Nb2 O5 increases the thermal stability of Er3+ -doped tellurite glasses significantly, and the results also show that Er3+ -doped niobium tellurite glasses 70TeO2 –20ZnO–10Nb2 O5 and 70TeO2 –20ZnO–5BaO– 5Nb2 O5 exhibit not only the good thermal stability (T > 150 ◦ C), but also the large emission cross-section (>10 × 10−21 cm2 ) and broad full width at half maximum (FWHM ∼ 65 nm), which indicate that niobium tellurite glasses are preferable for broadband amplifiers. Acknowledgements We would like to acknowledge finical supported from Guangdong Natural Science Foundation (04020036)

and Guangdong Science and Technology Foundation (2004A10602002).

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