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

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

Journal of Alloys and Compounds 461 (2008) 617–622 Spectroscopic properties of GeO2- and Nb2O5-modified tellurite glasses doped with Er3+ C. Zhao a,b...

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Journal of Alloys and Compounds 461 (2008) 617–622

Spectroscopic properties of GeO2- and Nb2O5-modified tellurite glasses doped with Er3+ C. Zhao a,b , G.F. Yang a , Q.Y. Zhang a,∗ , Z.H. Jiang a a

Key Lab of Special Functional Materials of Ministry of Education, and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China b College of Applied Physics, South China University of Technology, Guangzhou 510641, China Received 25 April 2007; received in revised form 13 July 2007; accepted 21 July 2007 Available online 27 July 2007

Abstract Er3+ -doped tellurite glasses TeO2 –GeO2 –Nb2 O5 –Na2 O–K2 O–ZnO have been investigated for developing 1.5-␮m fiber and planar amplifiers. Effects of GeO2 and Nb2 O5 on the thermal stability and optical properties of Er3+ -doped tellurite glasses have been discussed. It is noted that the incorporation of GeO2 or Nb2 O5 increases the thermal stability of tellurite glasses significantly. Er3+ -doped GeO2 - and Nb2 O5 -modified tellurite glasses exhibit the stimulated emission cross-section as great as 10.7 × 10−21 cm2 , which is significantly higher than that of silicate and phosphate glasses. In addition, the intensity of upconversion luminescence of the Er3+ -doped GeO2 - and Nb2 O5 -modified tellurite glasses decreased clearly with increasing GeO2 or Nb2 O5 content. As a result, Er3+ -doped tellurite glasses might be a potential candidate for developing laser or optical amplifier devices. © 2007 Elsevier B.V. All rights reserved. Keywords: Tellurite glasses; Rare-earth ions; Thermal analysis; Optical properties

1. Introduction Tellurite glasses are promising candidate materials for photonics applications and have been recognized as one of the most promising materials for broadband Er3+ -doped fiber amplifier (EDFA). Tellurite glasses have been proven to possess wide transmission region (0.35–5 ␮m), good glass stability and durability, high refractive index (∼2), better non-linear optical properties and relatively low phonon energy (∼800 cm−1 ) [1–4]. These special features have attracted many researchers to study on the structure and optical properties of tellurite glasses doped with rare-earth ions. Glass based on tellurium oxide doped with Er3+ ions by virtue of a high stimulated emission crosssection and a broad full width at half maximum (FWHM) at 1.5-␮m (4 I13/2 → 4 I15/2 transition), has been recognized as one of the most promising materials for broadband EDFA. Tellurite glasses have attracted a great deal of attention not only in fundamental research but also in optical devices fabrication over the ∗

Corresponding author. Tel.: +86 13302261009; fax: +86 20 87114834. E-mail addresses: [email protected] (C. Zhao), [email protected] (Q.Y. Zhang). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.072

past several years for their good electrical, optical and magnetic properties [5,6]. Although tremendous progress have been made, unfortunately, there are some difficulties such as relatively low thermal stability and strong upconversion luminescence of tellurite glasses have so far been hindering the Er3+ -doped tellurite glass devices from their full commercialisation. In this paper, the main object is to carry out a detailed study on effects of GeO2 and Nb2 O5 contents on the thermal stabilities and spectroscopic properties of Er3+ -doped TeO2 –GeO2 –Nb2 O5 –Na2 O–K2 O–ZnO glasses to examine their suitability as potential optical glasses for fiber amplifiers. 2. Experimental 2.1. Glass preparation Tellurite glasses with molar compositions of xGeO2 –(70 − x)TeO2 –5K2 O–5Na2 O–10Nb2 O5 –10ZnO–0.2Er2 O3 (x = 0, 10, 20, 50 and 70, namely G0, G1, G2, G3 and G4, respectively) and yNb2 O5 –(14.7 − y)Na2 O– 10ZnO–5K2 O–5GeO2 –65TeO2 –0.3Er2 O3 (y = 0, 3, 5, 7 and 9, namely N0, N1, N2, N3 and N4, respectively) were fabricated by using reagent-grade Na2 CO3 ,

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K2 CO3 , ZnO, Nb2 O5 , GeO2 , TeO2 , and high purity Er2 O3 as the starting materials. The glass samples were prepared using the conventional melt-quenching method described anywhere.

2.2. Measurements Thermal analysis of glasses were examined by a Netzsch STA 449C Jupiter different scanning calorimeter (DSC) at a heating rate of 10 k min−1 from room temperature (RT) to 700 ◦ C. Raman scattering spectra were recorded in the range of 250–1000 cm−1 using a microscope spectrophotometer (model RM 2000, Renishaw) with 514.5 nm laser as an excitation source and the working power is 20 mW. The absorption spectra were obtained by a Perkin-Elmer Lambda900 UV/VIS/NIR spectrophotometer ranging 350–1700 nm. Fluorescence and upconversion luminescence spectra were measured on the TRIAX 320 type spectrometer with 977 nm laser diode (LD) as an excitation source. The lifetimes of Er3+ : 4 I13/2 level were measured by exciting the samples with the 977 nm LD and detected by a photon-counting R5108 photomultiplier tube. Fluorescence signal is collected in a direction perpendicular to the exciting beam, and all the samples were located at the same site in the process of measuring fluorescence properties. All the measurements were performed in the same condition at RT.

3. Results and discussion 3.1. Thermal and physical properties of glasses The values of density (ρ) and refractive index (d-line index, nd ) for tellurite glasses are shown in Table 1. Both ρ and nd increase monotonically with decreasing GeO2 or increasing Nb2 O5 content. The DSC curves of G0, G2, G4 and N0, N2, N4 glasses are illustrated in Fig. 1 as examples. The glass transition temperature (Tg ) and crystallization on set temperature (Tx ) are clearly seen in Fig. 1. The values of Tx , Tg and the difference between Tg and Tx (T = Tx − Tg ) of the glasses are given in Table 1. As shown in Table 1, Tg , Tx and T of G0–G4 glasses increase from 340, 414 and 74 ◦ C to 531, 648 and 117 ◦ C with increasing GeO2 from 0 to 70 mol%. Tg values increase rapidly with Nb2 O5 content. There is no obvious crystallization peak detected in N0, N1, N2 samples. The thermal stability factor T is frequently used as a rough estimate of the glass stability. To achieve a large working range of temperature during sample fiber drawing, it is desirable for a glass host to have as a large T as possible [5]. As shown in Fig. 1, it is benefit for glass stability to increase GeO2 content in tellurite glasses and there is no obvious crystallization peak Table 1 Density, refractive indices and thermal properties of tellurite glasses Sample

Density (g/cm3 )

Refractive indices

Tg (◦ C)

Tx (◦ C)

Tx − Tg (◦ C)

G0 G1 G2 G3 G4

4.99 4.85 4.68 4.30 4.02

2.11 2.05 1.98 1.86 1.76

340 407 451 – 531

414 477 538 648

74 70 87 – 117

N0 N1 N2 N3 N4

4.70 4.75 4.78 4.82 4.86

1.94 1.98 2.00 2.03 2.05

262.7 300.6 317.1 335.4 352.8

– – – 422.9 460.0

– – – 87.5 107.2

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

detected under a heating rate of 10 K min−1 in the samples of N0, N1, and N2 indicating that these glasses are much preferable for performing fabrication and crystal-free fiber drawing [2,7]. 3.2. Raman spectra Fig. 2(a and b) shows the Raman spectra of G0, G2, G4, N4 and N0. The bands centered at around 462 cm−1 are assigned to the stretching vibrations of Ge O Ge and Te O Te linkages. The bands centered at around 688 cm−1 are originated from vibration of the continuous networks composed of [TeO4 ]4− tetragonal bipyramids and the bands centered at around 764 cm−1 are contributed to [TeO3+1 ]4− , [TeO3 ]2− and [GeO6 ] [8–11]. With the increase of GeO2 content, the sixfold units [GeO6 ] of G0–G4 glasses increase in the network structure. It is found that the maximum phonon energy (MPE) of G0–G4 glasses increased from 747 to 778 cm−1 gradually. As for the Nb2 O5 -modified tellurite glasses, the number of the [TeO3+1 ]4− coordinate polyhedra and [TeO3 ]2− trigonal pyramids increase with replacing Nb2 O5 for Na2 O in the network structure. It is found that the MPE of N0 is slightly lower than that of N4. Obviously, the MPE of the tellurite glasses studied increase with the incorporation of Nb2 O5 or GeO2 , which might be caused the decrease of the upconversion intensity of Er3+ -doped glasses.

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Fig. 3. Absorption spectra of G0, G2, G4 (a) and N0, N2, N4 (b).

Fig. 2. Raman spectra of G0, G2, G4 (a) and N0, N4 (b).

3.3. Effect of GeO2 and Nb2 O5 on Judd–Ofelt parameters Fig. 3 shows optical absorption spectra of G0, G2, G4 and N0, N2, N4 glasses in the wavelength range of 350–1700 nm. The inhomogeneously broadened bands are ascribed to the transitions from the ground state 4 I15/2 to the excited states of Er3+ ions. Judd–Ofelt (J–O) intensity parameters Ωt (t = 2, 4 and 6) of each glass can be obtained according to the J–O theory from the measured absorption spectra. The Ωt (t = 2, 4, 6) are listed in Table 2. Apparently, Ω2 increase with the increase of Nb2 O5 or GeO2 content in tellurite glasses

studied. The values of Ω2 in present glasses increase from 6.41 × 10−20 to 8.98 × 10−20 cm2 for G type of glasses and increase from 6.62 × 10−20 to 7.86 × 10−20 cm2 for N type of glasses which are larger than that in germanate, silicate, and aluminate glasses [11]. The values of Ω6 for G type of glasses vary from 1.44 × 10−20 to 1.18 × 10−20 cm2 and increase from 0.89 × 10−20 to 1.13 × 10−20 cm2 for N type of glasses. These values are comparable to the Ω6 of fluoride [12] glass but larger than that of silicate, aluminate and germanate glasses [13]. According to previous studies, Ω2 is related to the symmetry of the glass host. The more the asymmetric of the glass host, the

Table 2 Judd–Ofelt intensity parameters of tellurite glasses Sample

Ω2 (×10−20 cm2 )

Ω4 (×10−20 cm2 )

Ω6 (×10−20 cm2 )

δ (×10−7 )

G0 G1 G2 G3 G4

6.41 6.82 8.3 7.86 8.98

1.63 1.5 2.01 2.63 2.09

1.44 0.81 1.08 0.95 1.18

1.3 1.3 1.4 1.2 1.1

N0 N1 N2 N3 N4

6.62 6.85 6.96 8.22 7.86

1.75 1.91 1.89 2.25 2.19

0.89 0.96 1.0 1.05 1.13

1.1 1.3 1.2 1.2 1.4

Germanium [11] Silicate [11] Phosphate [11] Tellurite [11]

5.81 4.23 6.65 4.74

0.86 1.04 1.52 1.62

0.28 0.61 1.11 0.64

– – – –

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larger the Ω2 is. The increase of GeO2 or Nb2 O5 contributes to increase the asymmetric of the glass host which results in the increase of Ω2 . Ω6 is inversely proportional to the covalency of Er O bond. The covalency of Er O bond is attributed to be related with the local basicity around the rare-earth sites, which can be adjusted by the composition or structure of the glass hosts. With the substitution of GeO2 for TeO2 or Nb2 O5 for Na2 O, more and more nonbridge oxygen ions, which tend to coordinate with Er3+ , will contribute to coordinate with glass former cation ions. According to the electronegativity theory, the covalency of the bond will become stronger with the decrease of the difference of electronegativity between cation and anion ions. Since the values of electronegativity, for Te, Ge, Na, Nb and O elements, are 2.1, 1.8, 0.9, 1.6 and 3.5, respectively, the covalency of Te O and Nb O bond are stronger than Ge O and Na O, respectively. As a result, the covalency of Te O, Nb O bonds are stronger than those of the Ge O and Na O bond, respectively. Consequently, the value of Ω6 decreases with the substitution of GeO2 for TeO2 and increase with the substitution of Nb2 O5 for Na2 O. The covalency of Er O bond in the present glasses is lower than that in germanate, silicate, aluminate and phosphate glasses [4]. 3.4. Effect of GeO2 and Nb2 O5 on emission spectra and cross-sections

Fig. 4. Normalized fluorescence spectra of tellurite glasses excited by a 977 nm LD (a) for G0, G1, G2, G3 and G4; (b) for N0, N1, N2, N3, N4.

Fig. 4 illustrates the normalized fluorescence spectra of Er3+ doped tellurite glasses in the wavelength range 1440–1650 nm at 977 nm excitation. All samples exhibit broad bands peaked at 1532 nm, which assign to the 4 I13/2 → 4 I15/2 transition of Er3+ . It is found that the FWHM of fluorescence spectra become broader with decreasing GeO2 or increasing Nb2 O5 content. Fig. 5 and Table 3 show the compositional dependence of FWHM of the Er3+ : 4 I13/2 → 4 I15/2 transition in various glasses systems. The FWHM decrease from 52 to 46 nm with the substitution of GeO2 for TeO2 , and it vary from 38 to 45 nm with increasing Nb2 O5 concentration. Obviously, the FWHM increase monotonically with increase of Nb2 O5 content but decrease with increase of GeO2 content. A larger value of FWHM could be interesting for wavelength-division multiplexing applications. As shown in

Fig. 5 and Table 3, the broadest FWHM value of these glasses is about 52 nm for G0, which is larger than that in silicate, germanium glasses [14]. The absorption cross-sections are determined from the absorption spectra, and the stimulated emission crosssections are calculated from McCumber method [15] by: σ e (λ) = σ a (λ) exp[(ε − hν)/kT], where h is the Planck constant, k the Boltzmann constant, and ε is the net free energy required to excite one Er3+ from the 4 I15/2 to 4 I13/2 at temperature T. σ a (λ) is absorption cross-section which has been determined by: σ a (λ) = 2.303OD(λ)/Nl, where N is the concentration of rareearth, l the length of absorption and OD(λ) is the absorption p intensity. Table 3 shows the peak emission cross-sections (σe ) values of Er3+ : 4 I13/2 → 4 I15/2 transition in glass samples. Fig. 6

Fig. 5. Nb2 O5 (a) and GeO2 (b) compositional dependence of measured lifetime of 4 I13/2 level of Er3+ (τ f ) and FWHM.

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

τ f (ms)

FWHM × σe (×10−28 cm3 )

σe × τf (×10−21 cm2 ms)

52 50 48 47 46

9.9 9.6 9.3 8.6 7.6

3.2 2.9 2.7 2.5 2.3

516 480 446 402 355

31.7 27.8 25.1 21.5 17.5

N0 N1 N2 N3 N4

38 39 42 43 45

7.2 7.6 8.1 9.1 10.7

3.6 3.4 2.9 3.0 2.7

274 296 340 392 482

25.9 25.8 23.5 27.3 28.9

Tellurite [15] Silicate [16] Phosphate [17]

60 45 37

6.6 5.5 6.4

396 247 237

26.4 55 51.2

Sample

FWHM (nm)

G0 G1 G2 G3 G4

σe (×10−21 cm2 )

4 10 8

illustrates the calculated absorption and emission cross-sections of G1 and N2 glasses in the wavelength range of 1420–1630 nm. Since the stimulated emission cross-section is proportional to p the host glass refractive index, σ e ∼ (n2 + 2)2 /n [2], the σe value −21 −21 monotonically decreases from 9.9 × 10 to 7.6 × 10 cm2 p with increasing GeO2 content. The values of σe monotonically increase up to the maximum (10.7 × 10−21 cm2 ) with increasing Nb2 O5 content. It is apparent that Er3+ in the glass with

p

p

more Nb2 O5 is capable of providing larger stimulated emission cross-section at 1.5-␮m. 3.5. Effect of GeO2 and Nb2 O5 on fluorescence lifetime of Er3+ : 4 I13/2 level and amplify properties To obtain high population inversions under steady-state conditions using modest pump powers, long lifetime of the metastable state of Er3+ : 4 I13/2 level is a critical factor in the success of Er3+ -doped fiber amplifier in optical communications. Fig. 5 illustrates the measured fluorescence lifetimes (τ f ) as the function of GeO2 and Nb2 O5 . Clearly, the lifetime decreases from 3.2 to 2.3 ms with increasing GeO2 from 0 to 70 mol%, and that decreases from 3.6 to 2.7 ms when Nb2 O5 increases from 0 to 9 mol%. According to the Raman spectra, the MPE increases with increasing GeO2 or Nb2 O5 content. The decrease of the measured lifetime might be due to the enhancement of nonradiative decay process. The interactions between Nb5+ and O2− ions and/or between Ge4+ and O2− ions contribute to the nonradiative decay of the 4 I13/2 level, which result in the decrease of fluorescence lifetimes of Er3+ : 4 I13/2 level. p p The values of FWHM × σe and τf × σe are often used to evaluate the gain properties of EDFA [16]. Generally, the bigger p the products, the better the properties are. FWHM, FWHM × σe p and τf × σe are also compared in Table 3. The values of FWHM, p p p σe , FWHM × σe and τf × σe in tellurite glasses studied increase with the increase of Nb2 O5 content or the decrease of GeO2 p content. As shown in Table 3, the maximum values of σe and p −21 2 −28 3 FWHM × σe are 10.7 × 10 cm , 516 × 10 cm for N4 and G0, respectively, which are much larger than that of silicate and phosphate glasses. 3.6. Effects of GeO2 and Nb2 O5 on upconversion spectra

Fig. 6. Calculated absorption and emission cross-sections of G1 (a) and N2 (b) in the wavelength range of 1400–1650 nm.

The frequency upconversion spectra of Er3+ -doped tellurite glasses in the ranging 500–750 nm have also been investigated under 977 nm LD excitation as shown in Fig. 7. Three intense bands centered at 525, 547 and 657 nm have been observed, which are assigned to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , and 4F 4 3+ 9/2 → I15/2 transitions of Er , respectively. The quadratic

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ing Nb2 O5 or GeO2 content, which results in the promotion of the multi-phonon relaxation rate and decrease the upconversion emissions. 4. Conclusions In summary, we concluded that Er3+ -doped tellurite glasses GeO2 –TeO2 –Nb2 O5 –Na2 O–K2 O–ZnO have been investigated for developing 1.5-␮m fiber and planar amplifiers. Effects of GeO2 and Nb2 O5 on the thermal stability and optical properties of Er3+ -doped tellurite glasses have been discussed. It is noted that the incorporation of GeO2 or Nb2 O5 could increase thermal stability of tellurite glasses significantly. Er3+ -doped GeO2 p or Nb2 O5 -modified tellurite glasses exhibit large σe (up to p 10.7 × 10−21 cm2 ) and FWHM × σe (up to 482 × 10−28 cm3 ), which are significantly higher than that of silicate and phosphate glasses. In addition, it is found that the intensities of upconversion luminescence of Er3+ -doped tellurite glasses decrease clearly with increasing GeO2 or Nb2 O5 content. As a result, GeO2 - or Nb2 O5 -modified Er3+ -doped tellurite glasses might be a potential candidate for developing laser or optical amplifier devices. Acknowledgements Authors are grateful to the Project of NSFC (50472053, 50602017), DSTG (2006J1-C0491), NCET (04-0823) for financial assistance. References

Er3+ -doped

Fig. 7. IR-to-visible upconversion luminescence spectra of Gx (x = 0, 1, 2, 3 and 4) (a) and Ny (y = 0, 1, 2, 3 and 4) glasses (b). The dependences of the upconversion green and red emissions intensity of G1 and N2 on pump laser power are shown in inset of (a) and (b), respectively.

dependences of the upconversion green- and red-emissions intensities on pump power shown in the inset of Fig. 7(a and b), indicating that both the green- and red-emissions due to two-phonon absorption processes [19]. It is noted that the green upconversion emission decreases with the content of Nb2 O5 or GeO2 increasing. As shown in Fig. 7(a), the green intensity decreases to 70%, 41%, 9% and 6% of that in the G1, G2, G3 and G4 glasses in comparison with the G0 glass, respectively,. Meanwhile, the green intensity decreases to 94%, 90%, 70% and 61% for N1, N2, N3 and N4 glasses compared with N0 glass, respectively, as shown in Fig. 7(b). The red intensity is not affected apparently by the concentration of GeO2 or Nb2 O5 . The green luminescence intensity is mainly determined by the MPE [17] of glass hosts, while the red upconversion emission intensity is mainly determined by ESA procedure [18]. Our investigation indicates that the MPE of host glass might be improved with increas-

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