Optical and physical properties of Er3+-doped oxy-fluoride tellurite glasses

Optical and physical properties of Er3+-doped oxy-fluoride tellurite glasses

Optical Materials 33 (2011) 389–396 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Op...

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Optical Materials 33 (2011) 389–396

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Optical and physical properties of Er3+-doped oxy-fluoride tellurite glasses E.F. Chillcce a,⇑, I.O. Mazali b, O.L. Alves b, L.C. Barbosa a a b

Instituto de Física GlebWataghin, Universidade Estadual de Campinas-UNICAMP, Campinas, SP 13083-970, Brazil Instituto de Química, Universidade Estadual de Campinas-UNICAMP, Campinas, SP, Brazil

a r t i c l e

i n f o

Article history: Received 10 July 2010 Received in revised form 21 September 2010 Accepted 27 September 2010 Available online 20 October 2010 Keywords: Oxy-fluoride tellurite glass Optical and physical properties Thermal stability Rayleigh scattering loss Erbium-ion emission cross section properties

a b s t r a c t In this manuscript we present the effects of ZnF2 concentration on the optical and physical properties of Er3+-doped oxy-fluoride tellurite glasses (7500 ppm Er2O3-(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%). In general, as the concentration of ZnF2 increases: (i) the thermal stability, the optical transparency window, and the lifetime of the 4I13/2 level increase; (ii) the density, the linear refractive index, the Rayleigh scattering loss, and the OH ion concentration decrease; and (iii) the Er3+ ion emission cross section spectrum is quenched and the bandwidth is reduced. The 4I13/2 level lifetime increase may be associated with the luminescence re-absorption and the radiative transition (4S3/2 ? 4I13/2) processes, since both processes may contribute to the 4I13/2 level population. The Er3+ ion emission cross section spectrum (at around 1550 nm) of oxy-fluoride tellurite glass containing 30 mol% of ZnF2 was very similar to those of Fluoride glasses. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Tellurite glasses present interesting and important optical properties such that interest in their potential applications has increased considerably over the last few years. Indeed, numerous devices including optical amplifiers (based on Raman and Brillouin effects), planar waveguides and nanowires have been fabricated using tellurite glasses [1–6]. One of the main problems concerning the use of this kind of material in optical fiber applications has been its high optical attenuation (>1 dB/m), although an optical fiber based on TeO2-Bi2O3-ZnO-Na2O glass has been recently reported with an attenuation of 20 dB/km [7]. On the other hand, rare earths show very high solubility in tellurite glasses [8], and this allows the material to be co-doped with several rare earths simultaneously [9]. In this context, tellurite fibers co-doped with Er3+ and Tm3+ ions show broadband emission spectra with bandwidths of 160 nm around the 1550 nm band [10]. Unfortunately, the lifetime of the 4I13/2 ? 4I15/2 transition correlated with this broadband is associated with energy transfer from (Er3+) 4I13/2 and (Tm3+) 3F4 levels [11], and the lifetime is short in glasses codoped with these rare earths. Although the deleterious effect of this energy transfer mechanism cannot be avoided, the lifetime of the 4I13/2 ? 4I15/2 transition may be increased in order to enhance the efficiency of optical fiber devices within the optical

⇑ Corresponding author. Tel.: +55 19 3521 4135; fax: +55 19 3521 5428. E-mail address: [email protected]fi.unicamp.br (E.F. Chillcce). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.09.027

communication window. Indeed, optical fiber amplifiers with long 4 I13/2 level lifetimes could be used as active systems because they present low power saturation [12], avoiding up-conversion processes. The lifetime of the 4I13/2 level may be increased by reducing the OH ions content in the tellurite glass matrix. This OH ions reduction was made possible by injecting oxygen gas during the melting process [13–15]. Since many of the OH overtones around the 1550 nm band coincide with the 4I13/2 ? 4I15/2 transition of Er3+ions, the depletion of OH ions causes a reduction in the energy transfer between (Er3+) 4I13/2 and OH. An alternative method to increase the lifetime of the 4I13/2 level involves the introduction of high concentrations of ZnF2 in the glass matrix. It was originally reported that tellurite glasses doped with high concentrations of ZnF2 presented long lifetimes of transitions between Nd3+-ion levels [16,17]. Subsequently, it was found that the lifetimes of transitions between Er3+ ion levels in Er3+-doped tellurite glasses were also long due to the high concentrations of ZnF2 [18,19]. In general, the lifetime of the 4I13/2 level is mainly influenced by the following mechanisms: (i) the up-conversion processes related to the pump laser power [20], (ii) the red-shift or the re-absorption processes related to the fiber length [21], and (iii) the band broadening (or narrowing) caused by the change in the environment of the Er3+ ions distributed within the glass structure [13]. It is well-known that Er3+-doped oxy-fluoride tellurite glasses containing high concentrations of ZnF2 present: (i) a low density, (ii) a low linear refractive index, (iii) a low OH ions concentration, (iv) a long 4I13/2 level lifetime, and (v) a wider transparency

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window (between 300 and 3000 nm) in comparison with that of the oxide tellurite glasses [16–18]. Hence Er3+-doped oxy-fluoride tellurite glasses could be used in optical fiber amplifiers, since such fibers could also be co-doped with Tm3+-ions, aiming at providing broadband emission spectra and the longest possible 4I13/2 level lifetime. In this study, the optical and physical properties of Er3+-doped oxy-fluoride tellurite glasses (ZnF2 concentrations ranging from 5 to 30 mol%) have been determined. The effects of ZnF2 on thermal stability, density, optical attenuation, Rayleigh scattering loss, linear refractive index, Raman spectrum, luminescence, lifetime, and Judd–Ofelt parameters have been investigated in order to evaluate the quantum efficiency of the glasses.

Fig. 1. Experimental set-up used to measure the luminescence and the lifetime. ‘‘FT” represents the filter types, ‘‘D” an InGaAs detector, and ‘‘OSA” an Optical Spectrum Analyzer used in this experiment.

2. Experimental The Er3+-doped oxy-fluoride tellurite glass samples employed in this study were produced from batches of powdered components (purities 99.999 wt.%) mixed in the proportions 7500 ppm Er2O3(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%. In each case, a 30 g sample of powder was transferred to a platinum crucible and then heated at 800 °C for 50 min in a Politron radio frequency induction furnace. The molten glass sample was then poured onto a stainless steel plate to solidify, and finally annealed at 300 °C under an atmosphere of air (1 h) in an electrical furnace (Lindberg 894 model). Thin (optically transparent) samples of the glasses, obtained from the bulk material with a diamond polisher (0.25 lm particle diameter), were used to determine the physical and optical properties. The thermal characterization of the Er3+-doped oxy-fluoride tellurite glasses was carried out by using a Differential Thermal Analyzer (Shimatzu DTA-50 model). In this case, approximately 100 mg glass samples (glass grains with diameters varying from 50 to 240 lm) inside alumina crucibles were heated from 25 to 700 °C, with a heating rate of 15 °C/min. The density was measured according to Archimedes principle, where the weight of a volume of distilled water equivalent to that of the glass sample was obtained to a precision of 0.0001 g. Linear refractive index measurements (0.001 precision) were carried out using a prism coupler (Metricon 2010 model). The linear refractive index of the glass samples was measured at wavelengths 632.8, 1305.4 and 1536 nm. UV–VIS-NIR transmittance spectra (200 to 3200 nm) were acquired using an spectrophotometer (Perkin Elmer Lambda 9 model, wavelength resolution 1 nm), whilst transmittance spectra in the IR region (from 1500 to 5000 cm1) were obtained with an spectrophotometer (ABB FTLA 2000 model, with an ATR accessory and energy resolution 1 cm1). Raman spectra were acquired using an spectrophotometer (Raman Systems R-2001 model, energy detection from 100 to 2600 cm1; energy resolution 1.37 cm1) which incorporates a 785 nm pump laser (500 mW). The experimental set-up shown in Fig. 1 was employed to measure luminescence spectra within the range of 350–1750 nm as well as the lifetime of the Erbium 4I13/2 level. The arrangement that comprises the optical fiber, the glass sample and the lenses was mounted on the micro-position stages (ThorLabs MBT610 model), helping to reduce the luminescence re-absorption processes that may influence the shape of the luminescence spectrum. The beam that emerges from the pigtail fiber of the 980 nm pump laser (Opto-Link PL420 model, power 120 mW, with a single mode silica fiber (SMSF; 8 lm core diameter)) is focused on the edge of the glass sample by using an X10 objective lens L1 (0.1 NA). The distance between the SMSF and lens L1 was optimised in order to generate the most intense green-light emission of the pump laser beam in the focus region.

Two types of spectrophotometers were employed in order to acquire the luminescence spectra of Er3+ ions within the range of 350–1750 nm. An optical spectrum analyser (‘‘OSA”; Yokogawa AQ-6315A model, wavelength resolution 1.4 nm; bandwidth resolution 10 nm) was used for the data acquisition within the range of 900–1750 nm. In this case, the luminescence that emerged from the edge of the glass sample (perpendicular to the pump laser beam) was collected and then collimated by an X20 objective lens L2 (0.54 NA), and finally focused on the core of a multimode silica fiber (MMSF; 62.5/125 lm core/external diameters) by an X20 objective lens L3 (0.40 NA). Various types of filters (labelled ‘‘FT” in Fig. 1) were introduced into the light path in order to suppress the laser lines and to reduce the dynamic range effect of the OSA. On the other hand, another spectrophotometer (Ocean Optics USB2000 model, wavelength resolution 0.4 nm) was used to acquire the luminescence spectra within the range of 350– 1025 nm. An optical fiber that had been adapted to the spectrometer was used to collect the luminescence of the Er3+ ions, and it was then positioned directly in front of the objective lens L2 (see Fig. 1). In order to measure the lifetime of the Erbium 4I13/2 level, a mechanical chopper was positioned very close to the fiber end (between the SMSF fiber end and lens L1). The luminescence of the Er3+ ions was guided through the MMSF fiber, and the fiber end was connected to an InGaAs detector (labelled ‘‘D”, Thorlabs PDA400 model) via an FC/PC connector similar to that of OSA. This InGaAs detector allowed the acquisition of luminescence data (used to estimate the lifetime), using an oscilloscope (Tektronix TDS1012 model).

3. Results Fig. 2 shows DTA curves of the Er3+-doped oxy-fluoride tellurite glass samples for ZnF2 concentrations that vary from 5 to 30 mol%. From Fig. 2 it is possible to observe the glass transition temperature (Tg), the onset crystallization temperature (Tx), and the melting temperature (Tm), obtained by analyzing the endothermic and exothermic events. Table 1 summarizes the Tg, Tx and Tm values for diverse concentrations of ZnF2. From Table 1 (or Fig. 2) it is possible to observe that the Tg decreases as a consequence of the ZnF2 increase. Also, it is clearly observed that the glasses with ZnF2 concentrations of 10, 20 and 30 mol% show two melting temperatures, while the glasses with ZnF2 concentrations of 25 and 30 mol% show two onset crystallization temperatures. Table 2 shows the thickness, density, Fluor concentration and Er3+ ion concentration for the studied oxy-fluoride tellurite glasses. The Fluor concentrations (mg-F/g-Glass) were obtained by chemical analysis and used to calculate the Er3+ ion concentrations in the

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Fig. 2. DTA curves of the oxy-fluoride tellurite glasses. The events related to glass transition (Tg), onset crystallization (Tx), and melting temperature (Tm) were identified by observing the endothermic and exothermic processes.

Table 1 The thermal parameters and the thermal stability of oxy-fluoride tellurite glasses as a function of the ZnF2 concentration. ZnF2 (mol%)

Tg (°C)

Tx (°C)

Tm (°C)

Tx–Tg (°C)

5 10 15 20 25 30

317 317 313 313 303 302

415 426 428 437 425; 455 428; 488

629 607; 658 654 592; 642 602 602; 627

98 109 115 124 122 126

Table 2 Sample thickness, density, Fluor concentration, and Er3+-ions concentration of oxyfluoride tellurite glass samples with compositions 7500 ppm Er2O3-(80x)TeO2xZnF2-20ZnO, where x = 5, 10, 15, 20, 25, and 30 mol%. ZnF2 (mol%)

Thickness (mm)

Density (g/cm3)

Fluor concentration (mg-F/g-Glass)

Er3+-ions concentration (1020 ions/cm3)

5 10 15 20 25 30

2.33 2.31 2.51 2.18 2.50 2.46

5.520 5.519 5.452 5.441 5.425 5.394

2.94 5.09 10.4 14.46 26.58 34.24

1.35 1.30 1.64 1.39 1.69 1.30

oxy-fluoride tellurite glasses. The linear refractive indices (n) of Er3+-doped oxy-fluoride tellurite glasses, measured at 632.8, 1305.4, and 1536 nm, showed a monotonic dependence on the ZnF2 concentration within the range of 5–30 mol% (Fig. 3A). For all glass samples, the measured linear refractive indices (at the three wavelengths) were fitted using the modified Sellmeier equation n2 ¼ 1 þ A0 k2 =ðk2  B0 Þ þ C 0 k2 [22]. From these curves (for the glass containing 30 mol% of ZnF2, Fig. 3B) the parameters A0, B0 and C0 were obtained, and then the linear refractive indices within the range of 350–1750 nm were estimated. The transmittance spectra of the Er3+-doped oxy-fluoride tellurite glass samples (Fig. 4A) showed absorption bands corresponding to the transitions between the ground level (4I15/2) and the excited levels of the Er3+ ions (2K15/2, 2G9/2, 2G11/2, 2H9/2, 4F3/2, 4F5/ 4 2 4 4 4 4 4 2, F7/2, H11/2, S3/2, F9/2, I9/2, I11/2 and I13/2). Although the intrinsic UV absorption of oxy-fluoride tellurite glass generally quenches the transition between the 4I15/2 ground level and the 2K15/2 excited level, at high ZnF2 concentrations the UV absorption is reduced. Consequently, the UV transmission edge is shifted to shorter wave-

Fig. 3. (A) Linear refractive indices of Er3+-doped oxy-fluoride tellurite glasses with compositions 7500 ppm Er2O3-(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%, measured at 632.8, 1305.4, and 1536 nm. (B) Linear refractive indices of Er3+-doped oxy-fluoride tellurite glass containing 30 mol% of ZnF2, fitted using the Sellmeier model.

lengths, allowing this transition to be observed. The inset of Fig. 4A displays the transmission spectra of Er3+-doped oxy-fluoride tellurite glass samples containing ZnF2 concentrations of 5 and 10 mol%. The broadband absorptions at 1850, 2100 and 2300 nm, corresponding to the OH ion vibrations were almost quenched when the ZnF2 concentration exceeded 15 mol%. Likewise, the OH ion absorption bands around 3000 nm also decreased when the ZnF2concentration increased. The absorption cross-section bands of Er3+ ions (Fig. 4B) tend to be quenched when the concentration of ZnF2 increases, but the quenching effect on the 4G11/2 and 2H11/2 hypersensitive bands is greater than that on the non-hypersensitive bands. Fig. 4C shows that while the 4I11/2 band in the absorption cross section spectra were clearly quenched when the ZnF2 concentration was greater than 15 mol%, only a modest ZnF2-dependent quenching of the 4 I13/2 band was apparent. Fig. 5 shows the Rayleigh scattering loss spectra (ranging from 1000 to 3000 nm) of the oxy-fluoride tellurite glasses. The Rayleigh scattering loss spectra were derived from the transmittance spectra of Fig. 4A by using the equation aScattering ðkÞ ¼ RnðkÞ=k4 , where R is a constant and n(k) is the refractive index that depend on the wavelength (k). The Rayleigh scattering loss presents a decreasing behaviour when the ZnF2 concentration increases. It is possible to observe that the scattering loss, at 1550 nm, decreases from 2300 to 160 dB/km when the ZnF2 concentration increases from 5 to 30 mol% respectively. The IR absorbance spectra of the oxy-fluoride tellurite glass samples (Fig. 6A) revealed OH ions absorption bands within the range of 2000–3600 cm1. The absorption bands at 2250 and

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Fig. 5. The Rayleigh scattering loss spectra of oxy-fluoride tellurite glasses dependent on the ZnF2 concentration and the wavelength.

Fig. 4. (A) The transmittance, and (B and C) the absorption cross section spectra of oxy-fluoride tellurite glasses with compositions 7500 ppm Er2O3-(80x)TeO2xZnF2-20ZnO, where x = 5, 10, 15, 20, 25, and 30 mol%.

3000 cm1 decreased when the concentration of ZnF2 increased from 5 to 30 mol%, confirming the reduction in the OH ions concentration under such conditions. In Fig. 6B the Raman spectra of the Er3+-doped oxy-fluoride tellurite glass samples are compared with Er3+-doped oxy-fluoride tellurite-silica (‘‘OFTS”; 7500 ppm Er2O3-40TeO2-14SiO2-45ZnF21Al2O3) and with Er3+-doped tungsten-tellurite (‘‘TT”; 7500 ppm Er2O3-70TeO2-19WO3-7Na2O-4Nb2O5). In the Er3+-doped oxy-fluoride tellurite glasses, the bands (around 440 and 670 cm1) corresponding to the respective Te-O-Te stretching and TeO4 structural unit vibrations were quenched when the ZnF2 concentration increased. Vibrations of TeO3+1 and/or TeO3 structural units gave rise to a more intense band around 760 cm1, and the maximum of this

Fig. 6. (A) The IR absorbance, (B) the Raman spectra of Er3+-doped oxy-fluoride tellurite glasses with compositions 7500 ppm Er2O3-(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%. The Raman spectra of Er3+-doped oxyfluoride tellurite-silica (OFTS; 7500 ppm Er2O3-40TeO2-14SiO2-45ZnF2-1Al2O3) and Er3+-doped tungsten-tellurite (TT; 7500 ppm Er2O3-70TeO2-19WO3-7Na2O4Nb2O5) glasses are also shown (B).

band was displaced from 744 to 772 cm1 when the ZnF2 concentration was increased from 5 to 30 mol%. Analogous bands around 440, 670 and 760 cm1 were observed in Er3+-doped OFTS and in Er3+-doped TT glasses, while an additional band at 910 cm1 (corresponding to the vibrations of the WO6 structural unit [23]) was observed in the TT glass spectrum. In contrast, bands at around

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950, 1005 and 1100 cm1, corresponding to the 4S3/2 ? 4I13/2 transition in Er3+ ions, were observed in all glasses except for the sample of Er3+-doped TT. These bands appeared because the pump laser excitation wavelength (785 nm) is approximately equal to the 4S3/2 ? 4I13/2 transition emission wavelength (850 nm). Moreover, the bands were more intense when the ZnF2 concentration increased, and this may be related to an increase in the radiative transition probability of the 4S3/2 ? 4I13/2 transition. Fig. 7A displays the luminescence spectra of an Er3+-doped oxyfluoride tellurite glass (containing 30 mol% of ZnF2) measured within the range of 350–1750 nm, with or without filters. The

Fig. 7. The luminescence spectra of oxy-fluoride tellurite glasses with compositions 7500 ppm Er2O3-(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%, showing: (A) the spectra of glass containing 30 mol% of ZnF2 obtained with (IR and RG1550BP) or without a filter; (B) The spectra of glasses measured using a spectrophotometer (Ocean Optics, USB2000 model) without a filter; and (C) the spectra of glasses measured using an OSA spectrophotometer without a filter.

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spectrum obtained without a filter showed two high intensity luminescence bands corresponding to transitions: 4I13/2 ? 4I15/2 (labelled 4I13/2) and 4I11/2 ? 4I15/2 (labelled 4I11/2). A magnified version of this spectrum (inset of Fig. 7A) shows three low intensity luminescence bands (within the range of 450–900 nm) associated with the transitions generated by the up-conversion processes, namely, 4S3/2 ? 4I15/2 (4S3/2), 4F9/2 ? 4I15/2 (4F9/2), and 4S3/2 ? 4I13/2. These low intensity luminescence bands were also observed in Er3+-doped oxy-fluoride tellurite glass containing 25 mol% of ZnF2, but were not observed in glasses containing 20% mol or less. The low intensity of these bands may be related to the OSA dynamic range influence. The OSA proved to be efficient within the range of 900–1750 nm. However, within the range of 350– 1025 nm it was necessary to use an Ocean Optics USB2000 spectrophotometer. The remaining spectra (Fig. 7A) were obtained using a combination of filters: an RG1000 and a Green-band filter (labelled ‘‘IR” in Fig. 7A); and an RG1000 and a 1550 nm band pass (labelled ‘‘RG1550BP”). The labelled IR filter transmits light from around 1300 to 1750 nm, and the RG1550BP completely suppresses the luminescence bands from 350 to 1500 nm. The studied luminescence spectra (within the range of 500– 1020 nm) of all oxy-fluoride tellurite glasses are depicted in Fig. 7B, from which it can be observed that the band intensity increases when the ZnF2 concentration increases from 5 to 30 mol%. The transitions 2H11/2 ? 4I15/2 (2H11/2), 4S3/2 ? 4I15/2 (4S3/2), 4 F9/2 ? 4I15/2 (4F9/2), 4I9/2 ? 4I15/2 (4I9/2), 4S3/2 ? 4I13/2, and 4I11/2 ? 4 I15/2 (4I11/2) were clearly observed for all the studied glass samples. The 4S3/2 transition showed the highest luminescence intensity in comparison with the other transitions. It should be pointed out that, in order to avoid the saturation of the detector, the intensity range of the USB2000 spectrophotometer was limited. Thus, the luminescence of the 4S3/2 transition in the glass sample (with 30 mol% of ZnF2) appears to be lower than that of the glass containing 25 mol% of ZnF2. The transitions 4I9/2 ? 4I15/2 (4I9/2) and 4S3/2 ? 4I13/2 that correspond to the luminescence bands at 800 and 850 nm, respectively, were most intense in oxy-fluoride tellurite glasses with high ZnF2 concentrations. Interestingly, these bands appear much less intense in oxy-tellurite glasses. In contrast, an 825 nm luminescence band associated with radiative decay and previously attributed to a 4I9/2 ? 4I15/2 transition [18] was not observed in the luminescence spectra of the oxy-fluoride tellurite glasses. The luminescence bands within the range of 950–1700 nm (Fig. 7C) were associated with 4I11/2 and 4I13/2 transitions. The intensity of the 4I11/2 transition increases when the ZnF2 concentration increases, while that of the 4I13/2 transition only increases when the ZnF2 concentration is >10 mol%. However, the intensity of the 4I11/2 transition is only greater than that of the 4I13/2 transition in glasses containing >20 mol% of ZnF2. In order to determine the lifetime of the 4I13/2 level it was necessary to eliminate influences of the 4S3/2, 4F9/2, 4S3/2 ? 4I13/2, and 4 I11/2 transitions using the filters RG1550BP and IR as described above. The use of the filters resulted in the reduction of the luminescence intensity (of the 4I13/2 transition) from 2378 pW (without filter) to 692.7 pW (with IR filter), or to 187.3 pW (with RG1550BP filter). Despite this shortcoming, it was still possible to measure the lifetime of the 4I13/2 level as a function of the ZnF2 concentration, and the results are depicted in Fig. 8A. When the IR filter is used, the lifetime of the 4I13/2 level corresponds to the emission around 1530 nm, and when the RG1550BP filter is used, the lifetime of the 4I13/2 level corresponds to the emission around 1550 nm. In both cases, the lifetime increased from 3 to 5.5 ms when the ZnF2 concentration was raised from 5 to 30 mol%. However, small differences (<6%) in absolute value were observed when the IR and RG1550BP filters were used. This finding is similar to that

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4. Discussion

Fig. 8. (A) the 4I13/2 level lifetimes in oxy-fluoride tellurite glasses with compositions 7500 ppm Er2O3-(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%, obtained using IR and RG1550BP filters. (B) The Judd–Ofelt parameters of the oxy-fluoride tellurite glasses. (C) The measured and calculated 4I13/2 level lifetimes and the quantum efficiencies of the oxy-fluoride tellurite glasses.

previously reported for tellurite glasses doped with a 7500 ppm Er2O3 [24]. Fig. 8B shows the Judd–Ofelt parameters (O2, O4 and O6) obtained by using the oscillator strength equations and the absorption cross section spectra (see Fig. 4B and C). These parameters were used to calculate the 4I13/2 level lifetime (sc) for each ZnF2 concentration [25,26], and then the calculated lifetime was compared with the measured lifetime (sm) (Fig. 8C). Additionally, the quantum efficiency related to the probability of the spontaneous transition between 4I13/2 and 4I15/2 levels was derived using the equation g ¼ sm =sC . As observed in Fig. 8C, the quantum efficiency rises from 47% to 61% when the ZnF2 concentration increases from 5 to 30 mol%.

The physical and optical properties of the oxy-fluoride tellurite glasses such as the thermal parameters, the density, the linear refractive index, the optical transparency window, the Rayleigh scattering loss, the OH ions concentration, and the bond binding (between structural units) were influenced by the ZnF2 concentrations within the range of 5–30 mol%, as were the absorption and emission spectra of the Er3+ ions and the lifetime of the Erbium 4 I13/2 level. The ZnF2 concentration increase in oxy-fluoride tellurite glasses was favourable to the thermal stability increase (see Table 1). This proves that it is possible to draw optical fibers from oxy-fluoride tellurite glasses without crystallization. The density reduction (see Table 2) of oxy-fluoride tellurite glasses with high ZnF2 concentrations may be due to the low density of ZnF2 and ZnO. An increase in the ZnF2 concentration in oxyfluoride tellurite glasses increases the F/O ratio, and it may also induce the transformation of some ZnF2 molecules into ZnO molecules during the melting process in an atmosphere of air. In this study the decreases in the linear refractive index (Fig. 3A) and the shifts in the UV absorption edge (Fig. 4A) are similar to those reported by Bürger et al. [27], when the concentration of ZnO in TeO2-ZnO glasses was increased. On the other hand, the optical transparency window increase was due to the displacement of the UV absorption tail (from 350 to 310 nm), which in turn was influenced by the increase in the ZnF2 concentration. In oxy-fluoride silicate glasses, an increase in the F/O ratio shifts the UV absorption edge to longer wavelengths [28]. Hence the blue shift of the UV absorption tail observed in oxy-fluoride tellurite glasses is mainly related to the increase in the ZnO concentration. The quenching of the 4G11/2 and 2H11/2 hypersensitive absorption bands (Fig. 4B) indicates that the environment of Er3+ ions is influenced by the ZnF2 concentration increase. These hypersensitive absorption bands were correlated with the Judd–Ofelt O2 parameter which decreased when the ZnF2 concentration increased, indicating that some Er-O covalent bonds had been replaced by Er-F ionic bonds. Consequently, the oxy-fluoride tellurite glass samples with high ZnF2 concentrations tend to be ionic, generating low O2 values in comparison with those of oxide glass, as previously reported by Reisfeld and Jorgensen [29] for Er3+-doped fluoride solutions. The scattering loss (Fig. 5) decreases when the ZnF2 concentration increases. This proves that the optical transparency increases when the ZnF2 concentration increases. Consequently, this positive effect may favour the use of these glasses in optical fiber amplifiers. In the Raman spectra (Fig. 6B), the peak associated with TeO3+1/ TeO3 structural units was displaced from 744 to 772 cm1, when the ZnF2 concentration was increased from 5 to 30 mol%, and its intensity was greater than that of the peak at 670 cm1 (TeO4). This indicates that the TeO3+1 and the TeO3 increase when the ZnF2 concentration increases. This is in agreement with the results reported for TeO2-ZnO glasses [27]. The increase in the lifetime of the 4I13/2 level observed when ZnF2 concentrations increased was a consequence of decreasing OH ion concentrations, corroborating the results presented by Jha et al. [13]. In optical communications, the 4I13/2 ? 4I15/2 (1550 nm) and 4I15/2 ? 4I13/2 (1530 nm) transitions are particularly important for the amplification processes. Hence the absorption and emission cross section spectra of these bands were analyzed in detail. The cross section spectra (Fig. 9) were derived considering the luminescence spectra (Fig. 7C), the 4I13/2 level lifetime values (Fig. 8A), and the equation of Miniscalco and Quimby [30]. The emission cross section spectra of the 4I13/2 ? 4I15/2 transition

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P10 mol% (Fig. 8A). On the other hand, the radiative transition (4S3/2 ? 4I13/2) also contributes to the population of the 4I13/2 level. The lifetime of this radiative transition (calculated by using the Judd–Ofelt parameters) varies from 2 to 3 ms as a result of the increase in the ZnF2 concentration. Moreover, this transition may also influence the lifetime of the 4I13/2 level. Fig. 9C displays emission cross section spectra of the 4I13/2 ? 4 I15/2 transition in Er3+-doped (Al/P silica, fluoride and tellurite) [2], in Er3+-doped oxy-fluoride tellurite (with 5 and 30 mol% of ZnF2), and in Er3+-doped OFTS glasses. The emission cross section spectra in the OFTS and in the oxy-fluoride tellurite (containing low ZnF2) glasses were broader in comparison with those in the Al/P silica and in the Fluoride glasses. Such increases in bandwidth occur due to changes in the environment of Er. These Er environment changes could be associated with the Er-F ionic and/or the Er-O covalent bonds as well as the Er sites of the TeO4, TeO3+1 and TeO3 structural units. In contrast, oxy-fluoride tellurite glasses with high ZnF2 concentrations presented narrowing bands because the O2 parameter was reduced as a consequence of the ionic behaviour of the glass. In this context, the oxy-fluoride tellurite glass containing 30 mol% of ZnF2 presented a narrow emission spectrum that was very similar to that of the Fluoride glass. 5. Conclusions When the concentration of ZnF2 in 7500 ppm Er2O3-doped TeO2-ZnO-ZnF2 glasses increases, the thermal stability increases, the optical transparency window broadens, and the lifetime of the Erbium 4I13/2 level increases too. On the other hand, oxy-fluoride tellurite glasses with higher concentrations of ZnF2 present reduced densities, lower linear refractive indices, lower Rayleigh scattering losses, and lower OH ion concentrations. The decreases in the Judd–Ofelt O2 parameter when the ZnF2 concentrations increase indicates that 7500 ppm Er2O3-doped TeO2-ZnO-ZnF2 glasses have ionic behaviours. The emission cross section spectra of Er3+ ions of oxy-fluoride tellurite glasses containing high concentrations of ZnF2 present narrow bandwidths that are very similar to those of Fluoride glasses. Acknowledgements Fig. 9. (A and B) The emission and absorption cross section spectra of the 4 I13/2 ? 4I15/2 transition in oxy-fluoride tellurite glasses with compositions 7500 ppm Er2O3-(80x)TeO2-xZnF2-20ZnO, where x = 5, 10, 15, 20, 25 and 30 mol%. (C) The emission cross section spectra of the 4I13/2 ? 4I15/2 transition in Er3+-doped Al/P silica, fluoride, tellurite, oxy-fluoride tellurite (with 5 and 30 mol% of ZnF2) and oxy-fluoride tellurite-silicate (OFTS) glasses. The spectra indicated by (*) are from reference [2].

The authors would like to thank the agencies CNPq, FAPESP and CePOF-Unicamp for their financial support. Dr. Enver F. Chillcce is very grateful to Eng. A. C. da Costa (IFGW-Unicamp) and Mr. Renato L. de Sousa (IFGW-Unicamp) and Mr. Arthur S. Kafunya for their technical support. References

(Fig. 9A) were quenched by higher concentrations of ZnF2, and the bandwidths (full width at half height- FWHH) were 52.4, 50.8, 50.0, 48.4, 47.2 and 47.1 nm, respectively, for glasses containing 5, 10, 15, 20, 25 and 30 mol% of ZnF2. These values do not agree with those of Nazabal et al. [18] who reported bandwidths varying from 74.8 to 77.5 nm, for ZnF2 concentrations between 7 and 35 mol%. The emission cross section spectra maxima (Fig. 9B) were lower than those of the absorption cross section spectra, for all concentrations of ZnF2 P 10 mol%, and this could be attributed to the re-absorption processes between the 4I13/2 ? 4I15/2 and 4I15/2 ? 4 I13/2 transitions [31]. In this study, the luminescence spectra of Fig. 7 were obtained using a 980 nm pump laser focused on the edge of the glass sample. This was necessary to avoid the re-absorption processes, as proposed by Martin et al. [31]. However, it is likely that such re-absorption processes lead to the increase in the lifetime of the 4I13/2 level for ZnF2 concentrations

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