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The intensity of the absorption bands due to the hydroxyl (OH) vibrations in the spectrum of tellurite glasses was decreased by using tellurium tetrachloride (TeCl4) as a dehydrating additive. This additive is incorporated with the initial reagents of the glass. During synthesis, it reacts with the OH groups present in the glass melt by releasing gaseous hydrochloric acid. The low chloride content remaining in the different glasses after their synthesis negligibly modifies their thermal properties, their stability against crystallization, and their refractive index. Its low impact on the refractive index of the glasses was assessed in the mid-infrared thanks to an enhanced prism coupling system using a supercontinuum laser as light source.
© 2016 Optical Society of America
1. Introduction
Tellurite glasses have interesting properties for optical applications requiring transparency in the mid-infrared to cover the second atmospheric transmission window (3 to 5 μm) [1, 2]. These glasses usually transmit light from 0.4 to 6.0 μm, they have a relatively high nonlinear refractive index (∼20 times higher than that of silica) and they possess proper chemical, thermal and mechanical properties to be drawn into optical fibers [1–5]. Thus, numerous works about the generation of nonlinear effects in the mid-infrared, such as supercontinuum, with tellurite fibers are reported in the literature [6–8]. The structure of tellurite glasses may also accept a high percentage of rare earth ions, which makes them appropriate to be employed as optical gain medium [9–11].
Despite the advantages of tellurite glasses, under normal conditions of synthesis, the extent of their transmission spectrum in the mid-infrared is limited by absorption bands ranging from 3 to 5 μm due to the vibrations of hydroxyl (OH) groups. These OH groups are incorporated inside the glass matrix during their synthesis through the initial reagents contamination by water or metal hydroxides, and through the moisture contained in the atmosphere of synthesis.
Several methods have been developed to attenuate these absorption bands. First, the glass synthesis can be performed under a dry oxygen atmosphere [3, 12, 13]. The low concentration of water vapor in the atmosphere of synthesis allows the decrease of the OH concentration inside the melted glass until an equilibrium is reached. Reactive gases (e.g. chlorine (Cl2) [14], carbon tetrachloride (CCl4) [15]) can also be introduced inside the atmosphere of synthesis and/or bubbled inside the glass melt to eliminate water and OH groups. However, this last method can be difficult to implement since these gases are corrosive and/or toxic.
Halogenated reagents, such as zinc fluoride (ZnF2) [12, 16] and lead chloride (PbCl2) [17], can also be included inside the glass composition. During synthesis, these reagents react with the OH groups contained in the glass melt to evacuate their hydrogen atoms under the form of hydrofluoric acid (HF) or hydrochloric acid (HCl). However, the incorporation of these halogenated reagents may modify the thermal properties of the glasses. Indeed, these last compounds have a high boiling point (≥950 °C) compared to the temperature of synthesis (∼800 °C), so they remain in the glass composition after synthesis. Their presence can reduce the stability of the glasses against crystallization and corrosion by moisture, which can affect their ability to be drawn into optical fibers [12, 16].
In this work, tellurium tetrachloride (TeCl4) was used as an additive to reduce the OH absorption bands in the optical spectrum of tellurite glasses with molar composition 78TeO2–22WO3. It reacts with the OH groups present in the glass melt and water according to the following reactions:
This dehydrating reagent possesses the advantage of being compatible with all glass compositions containing tellurium oxide because its decomposition produces TeO2 which is the main constituent of tellurite glasses. Unlike other chlorides or fluorides used for glass dehydration, TeCl4 only slightly modifies the thermal and the optical properties of the glass. Indeed, it has a low boiling point (380 °C) compared to the temperature of synthesis so that its mass fraction in the final glass is consequently low due to its evaporation during synthesis. However, since TeCl4 is a highly hygroscopic substance, it is essential to make its purification by vacuum sublimation prior to its use to ensure its effectiveness as a dehydrating reagent. The combination of a dry oxygen atmosphere during glass synthesis with sublimated TeCl4 as a dehydrating additive allows the decrease of the absorption losses related to the OH groups in the optical spectrum of tellurite-tungsten glasses without affecting their thermal properties, their stability against crystallization, and their optical properties. In order to precisely assess the effect of the TeCl4 incorporation on the refractive index of the glasses in the mid-infrared, a prism coupling set-up using a supercontinuum laser source coupled to a monochromator was developed.
2. Experimental
2.1. Purification of TeCl4
A vacuum system composed of two silica ampoules welded together and connected to a turbo-molecular pump was used for the purification of TeCl4 [Fig. 1(a)]. A batch of 100 g of TeCl4 (Alfa Aesar, 99.9 %) was loaded in the ampoule A and was then transferred to the ampoule B by sublimation under vacuum. This sublimation was performed by heating the ampoule A to a temperature of 160 °C while keeping a pressure smaller than (4 × 10−6) Torr during 48 h. Meanwhile, the ampoule B was kept at 100 °C to allow the solid condensation of TeCl4 while avoiding the condensation of volatile impurities. After the sublimation steps, the ampoule B was sealed and was stored in a glove box system with a dry atmosphere (water concentration ≤0.05 ppm).
Fig. 1 (a) Set-up for TeCl4 purification by sublimation under vacuum. (b) Furnace system and (c) thermal pattern used for the glass synthesis.
2.2. Fabrication of tellurite glasses with low OH contents
To prepare 78TeO2–22WO3 glasses with low OH content, tellurium (Sichuan Western Min-metals, 99.99 %) and tungsten oxides (Alfa Aesar, 99.9 %) were respectively pre-dried at ∼525 °C and ∼750 °C under vacuum (pressure ≤ 5 × 10−7 Torr) in silica ampoules for 70 hours. Then, the silica ampoules were sealed and introduced in the glove box system.
Inside the glove box, the ampoules were opened to weigh and to mix the oxides in accordance with the previously mentioned molar composition. The purified TeCl4 was incorporated to these powders to make different glasses with TeCl4 mass fractions of 0 %, 0.06 %, 0.6 %, 2.5 %, and 5.0 %. The powders were then placed in a platinum crucible (diameter: 20 mm, length: 80 mm) and were melted in an electric furnace [Fig. 1(b)] connected to the glove box system. For each glass, the synthesis was done by following a three-step thermal pattern designed to maximize the TeCl4 quantity able to react with OH groups contained inside the glass melt [Fig. 1(c)]. The first step of synthesis was done at 230 °C for 30 min to melt the TeCl4 in order to enhance its reactivity with the remaining water adsorbed on the powders. After this step, the powders were sintered together and were showing a brown-yellowish color. During the second step, the crucible was taken out of the furnace to be stored in the glove box while the temperature of the furnace was raised to 800 °C. When this temperature was reached, the crucible was reintroduced in the furnace for the third step of synthesis during which all the powders were quickly melted together in order to slow down the evaporation of the TeCl4 by entrapping it inside the glass melt. The glass melt was kept at 800 °C for 2 h to allow the completion of the OH removal process. During the entire synthesis process, a constant flow of dry oxygen pre-filtered by two moisture traps (CaSO4 filter and 5A molecular sieve) was kept inside the furnace to maintain a level of humidity between 0.1 and 1 ppm. The humidity of the atmosphere of synthesis was measured with a hygrometer (Kahn Cermet II) located at the gas output of the furnace. After synthesis, the glass melt was manually stirred once and was then poured in a platinum crucible at room temperature.
2.3. Glass characterization
Differential scanning calorimetry (DSC) measurements were acquired with a Netzsch DSC 404 F3 Pegasus calorimeter at a heating rate of 10 °C/min. The chemical composition of the glasses were analysed with a CAMECA SX-100 electronic microprobe. For optical characterization, glass samples were shaped into windows having two parallel polished surfaces and a thickness (L) around 2.5 mm. To avoid OH contamination on the glass surfaces due to polishing with water, the last polishing steps were performed by using isopropanol as lubricant. A FT-IR spectrometer (Perkin Elmer, Frontier) was used to measure the infrared transmission spectrum of each glasses and their respective spectrum was corrected to remove the contribution of multi-phonon absorption. The absorption coefficients (a0) were calculated with the Beer-Lambert law (a0 = log[I/I0]/L) and were converted in units of decibels per meter. The precision on the calculated values of absorption coefficients, which is mainly determined by the signal-to-noise ratio inside the FTIR transmission spectra, was evaluated as being ±2 dB/m.
The refractive indices of the glass samples were measured over the spectral range of 532 to 1538 nm with a prism coupler (Metricon, 2010/M). A homemade prism coupler was used for measuring the refractive index from 1.5 to 3.3 μm [Fig. 2]. This set-up uses a supercontinuum (SC) laser as light source (Le Verre Fluoré) that emits over the spectral range from 800 to 4000 nm and whose spectral content was selected by a monochromator. This last configuration gives the opportunity of measuring the refractive index at any wavelength over the spectral range of the SC laser source, and of facilitating optical alignment due to the superposition of all the spectral content into a single beam. These features are advantageous compared to that of other prism coupling systems using several laser sources at different wavelengths to cover the mid-infrared [18]. The experimental set-up was calibrated by using a sapphire sample and its accuracy and precision were verified by analyzing a fused silica window. Its precision on the refractive index measurement was evaluated as being within ±5×10−4 RIU. All refractive index measurements made on the tellurite glass samples were performed with a rutile (TiO2) prism. A three-term Sellmeier curve was fitted to the refractive index data according to the following equation:
where λ is the wavelength in micrometers and B1, B2, B3, C1, C2 and C3 are optical constants determined by the least square method [19].Fig. 2 Experimental set-up of the homemade prism coupler used to measure the refractive index of the glasses beyond 1538 nm (SL: supercontinuum laser source, OAPM: off-axis parabolic mirror, F: low-pass frequency filter, P: linear polarizer, OC: optical chopper, CM: convergent mirror (focal length= 500 mm), D: PbSe photodetector connected to a lock-in amplifier, GS: glass sample).
3. Results and discussion
The intensity of the OH absorption bands decreases as the TeCl4 mass fraction incorporated in the TeO2-WO3 glass is increased [Fig. 3(a)]. When 2.5 % (w/w) of TeCl4 is added, the absorption losses at 3.1 μm are reduced to a minimum level of 9 dB/m, which is about 10 times lower than when no TeCl4 is added and about 100 times lower than when the synthesis is carried out under ambient air. Further increasing the TeCl4 mass fraction beyond 2.5 % does not lead to additional attenuation of the OH absorption bands [Fig. 3(b)]. Since the optical absorbance of the OH bands is directly proportional to the OH concentration in the glass, these last results show that the 2.5 % and 5.0 % glasses possess similar OH contents.
Fig. 3 (a) Optical absorption of the OH bands (b) and absorption losses at 3.1 μm in the spectra of 78TeO2–22WO3 glasses with different initial TeCl4 mass fractions (the line is a guide for the eye).
The microanalysis of the glass samples indicates that their final molar fraction in chloride ions after synthesis is lower than that of their original composition [Table 1]. These differences of chloride content for each composition before and after their synthesis become particularly significant for glasses with initial TeCl4 mass fractions of 2.5 % and 5.0 %. Indeed, their final content in chloride ions is approximately 8 times lower than their initial compositions and their values are of the same order of magnitude as that of the glass having an initial TeCl4 mass fraction of 0.6 %. These last observations show that significant quantities of TeCl4 were decomposed or evaporated from the 2.5 % and 5.0 % glasses during their respective synthesis. The residual chloride content in the final glasses only slightly changes their chemical composition compared to the initial molar ratio of 78TeO2–22WO3 [Table 2].
Table 1. Comparison between the initial and the final chloride (Cl) molar fractions of the different 78TeO2–22WO3 glasses determined by microanalysis. The thermal parameters of these glasses measured by DSC, and their refractive index (n) measured at 1.54 μm.
Table 2. Chemical compositions of the different 78TeO2–22WO3 glasses determined by microanalysis after their respective synthesis.
The association of the results obtained by optical absorption spectroscopy and by micro-analysis suggest that for the glasses with initial TeCl4 mass fractions of 2.5 % and 5.0 %, a significant fraction of TeCl4 was eliminated from them without contributing to the OH removal process. During synthesis, excess TeCl4 can be evaporated from the glass melt because of its low boiling point (380 °C) or it can be converted into TeO2 by the oxidizing atmosphere. These two last processes reduce the TeCl4 concentration inside the glass melt and they consequently decrease the OH removal rate as the synthesis proceeds. As the TeCl4 comes to lower concentrations, the OH removal rate decreases more slowly and its value eventually becomes similar to that of the OH incorporation rate, which is mostly ruled by the moisture level of the furnace atmosphere. At some point during synthesis, the OH concentration inside the glass melt seems to reach a lower limit set by a quasi-equilibrium between the rates of OH removal and OH incorporation. Thus, the limiting factor for the OH removal process appears to be the humidity level of the atmosphere of synthesis. We consider that the lowest OH concentration achievable within the experimental conditions used in this work was likely reached for the glass samples with initial TeCl4 mass fractions of 2.5 % and 5.0 %.
DSC measurements show that the different transition temperatures (Tg: temperature of glass transition, Tx: onset of crystallization, Tc: peak of crystallization) of the different glass samples remain practically unchanged by the presence of the residual chloride ions [Table 1, Fig 4a]. The difference between Tg and Tx, which is considered as an approximate criterion to evaluate the stability of the glass against crystallization, is also unaffected by the remaining chloride ion content. Thus, the TeCl4 offers the opportunity to reduce the OH content in tellurite glasses without causing their destabilization against crystallization. This last feature is relevant to produce tellurite glasses with low propagation losses in the mid-infrared able to be drawn into optical fibers. Interestingly, the refractive indices of the glasses were also almost unchanged by the presence of the residual chlorides [Tables 1 and 3, Fig 4b]. This represents an additional advantage over other halogenated reagents used for glass dehydration, such as ZnF2, for which their incorporation inside tellurite glasses results in a significant decrease of the glass refractive index [16]. Also, in accordance with the Kramers-Kronig relations, the refractive index over the spectral range the OH bands should be modified as the intensity of these bands is reduced from one sample to the other [20]. However, these changes in refractive index were not high enough to be detected with the homemade prism coupling system.
Fig. 4 (a) DSC curves of the glass with inital TeCl4 content of 0 % and 5.0 % (w/w). (b) Refractive index dispersion of the glass initially containing 5.0 % (w/w) of TeCl4 (Sellmeier coefficients: B1=1.393, C1=0.06815 μm2, B2=2.035, C2 =0.008401 μm2, B3=3.828, C3 = 309.3 μm2).
Table 3. Comparison between the refractive indices of the 78TeO2–22WO3 glasses with initial TeCl4 content of 0 % and 5.0 % measured at different wavelengths with the prism coupling systems.
4. Conclusion
Tellurite-tungsten glasses with low OH contents were fabricated by using TeCl4 purified by vacuum sublimation as a dehydrating additive and by following a multi-step thermal pattern for their synthesis. The absorbance of the OH bands in their optical spectrum was gradually reduced as the TeCl4 mass fraction in their precursors was increased. The absorption losses at 3.1 μm were reduced to a minimum level of 9 dB/m for the glass sample initially containing 2.5 % (w/w) of TeCl4. The absorbance of the OH bands was not further reduced by increasing the TeCl4 mass fraction beyond 2.5 %. Indeed, the excess content in TeCl4 was eliminated from the glass melt without contributing to the OH decreasing process. The residual chloride content contained in the final glasses did not significantly modify their thermal properties, their stability against crystallization, and their refractive index. The quasi-constancy of the refractive index from one sample to the other was confirmed up to 3.3 μm in the mid-infrared with an improved prism coupling system using a supercontinuum laser as light source. To conclude, TeCl4 is a proper dehydrating reagent for the production of tellurite glasses with good optical transmission in the mid-infrared without reducing their potential to be processed into optical fibers.
Acknowledgments
We acknowledge financial support from the Canada Excellence Research Chairs (CERC), the Fonds de recherche du Québec - Nature et technologies (FRQNT), and the Canada Foundation for Innovation (CFI). We also thank Dr. Guillaume Marcotte for fruitful scientific discussions.
References and links
1. J. S. Wang, E. M. Vogel, and E. Snitzer, “Tellurite glass: a new candidate for fiber devices,” Opt. Mater. 3, 187–203 (1994). [CrossRef]
2. A. Jha, B. D. O. Richards, G. Jose, T. Toney Fernandez, C. J. Hill, J. Lousteau, and P. Joshi, “Review on structural, thermal, optical and spectroscopic properties of tellurium oxide based glasses for fibre optic and waveguide applications,” Inter. Mater. Rev. 57, 357–382 (2012). [CrossRef]
3. V. V. Dorofeev, A. N. Moiseev, M. F. Churbanov, G. E. Snopatin, A. V. Chilyasov, I. A. Kraev, A. S. Lobanov, T. V. Kotereva, L. A. Ketkova, A. A. Pushkin, V. V. Gerasimenko, V. G. Plotnichenko, A. F. Kosolapov, and E. M. Dianov, “High-purity TeO2-WO3-(La2O3, Bi2O3) glasses for fiber-optics,” Opt. Mater. 33, 1911–1915 (2011). [CrossRef]
4. M. D. O’Donnell, K. Richardson, R. Stolen, A. B. Seddon, D. Furniss, V. K. Tikhomirov, C. Rivero, M. Ramme, R. Stegeman, G. Stegeman, M. Couzi, and T. Cardinal, “Tellurite and Fluorotellurite Glasses for Fiber optic Raman Amplifiers,” J. Am. Ceram. Soc. 90, 1448–1457 (2007). [CrossRef]
5. M. Boivin, M. El-Amraoui, R. Vallée, and Y. Messaddeq, “Germanate-tellurite composite fibers with a high-contrast step-index design for nonlinear applications,” Opt. Mater. Express 4, 1740–1746 (2014). [CrossRef]
6. P. Domachuk, N. A. Wolchover, M. Cronin-Golomb, A. Wang, A. K. George, C. M. B. Cordeiro, J. C. Knight, and F. G. Omenetto, “Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter segments of highly nonlinear tellurite PCFs,” Opt. Express 16, 7161–7168 (2008). [CrossRef] [PubMed]
7. M. Liao, C. Chaudhari, G. Qin, X. Yan, T. Suzuki, and Y. Ohishi, “Tellurite microstructure fibers with small hexagonal core for supercontinuum generation,” Opt. Express 17, 12174–12182 (2009). [CrossRef] [PubMed]
8. I. Savelli, et al., “Mid-infrared 2000-nm bandwidth supercontinuum generation in suspended-core microstructured Sulfide and Tellurite optical fibers,” Opt. Express 20, 27083–27093 (2012). [CrossRef]
9. J. Carreaud, A. Labruyère, H. Dardar, F. Moisy, J.-R. Duclère, V. Couderc, A. Bertrand, M. Dutreilh-Colas, G. Delaizir, T. Hayakawa, A. Crunteanu, and P. Thomas, “Lasing effects in new Nd3+-doped TeO2-Nb2O5-WO3 bulk glasses,” Opt. Mater. 47, 99–107 (2015). [CrossRef]
10. M. J. V. Bell, V. Anjos, L. M. Moreira, R. F. Falci, L. R. P. Kassab, D. S. da Silva, J. L. Doualan, P. Camy, and R. Moncorgé, “Laser emission of a Nd-doped mixed tellurite and zinc oxide glass,” J. Opt. Soc. Am. B 31, 1590–1594 (2014). [CrossRef]
11. A. Mori, Y. Ohishi, and S. Sudo, “Erbium-doped tellurite glass fibre laser and amplifier,” Electron. Lett. 33, 863–864 (1997). [CrossRef]
12. J. Massera, A. Haldeman, J. Jackson, C. Rivero-Baleine, L. Petit, and K. Richardson, “Processing of Tellurite-Based Glass with Low OH Content,” J. Am. Ceram. Soc. 94, 130–136 (2011). [CrossRef]
13. H. Ebendorff-Heidepriem, K. Kuan, M. R. Oermann, K. Knight, and T. M. Monro, “Extruded tellurite glass and fibers with low OH content for mid-infrared applications,” Opt. Mater. Express 2, 432–442 (2012). [CrossRef]
14. P. Joshi, B. Richards, and A. Jha, “Reduction of OH− ions in tellurite glasses using chlorine and oxygen gases,” J. Mater. Res. 28, 3226–3233 (2013).
15. X. Feng, S. Tanabe, and T. Hanada, “Hydroxyl groups in erbium-doped germanotellurite glasses,” J. Non-Cryst. Solids 281, 48–54 (2001). [CrossRef]
16. M. D. O’Donnell, C. A. Miller, D. Furniss, V. K. Tikhomirov, and A. B. Seddon, “Fluorotellurite glasses with improved mid-infrared transmission,” J. Non-Cryst. Solids 331, 48–57 (2003). [CrossRef]
17. X. Feng, J. Shi, M. Segura, N. M. White, P. Kannan, W. H. Loh, L. Calvez, X. Zhang, and L. Brilland, “Halotellurite glass fiber with low OH content for 2–5 μm mid-infrared nonlinear applications,” Opt. Express 21, 18949–18954 (2013). [CrossRef] [PubMed]
18. A. H. Qiao, N. C. Anheier, J. D. Musgraves, K. Richardson, and D. W. Hewak, “Measurement of chalcogenide glass optical dispersion using a mid-infrared prism coupler,” Proc. of SPIE 8016, 80160F (2010).
19. L. E. Sutton and O. N. Stavroudis, “Fitting Refractive Index Data by Least Squares,” J. Opt. Soc. Am. 51, 901–905 (1961). [CrossRef]
20. R. D. L. Kronig, “On the theory of dispersion of X-Rays,” J. Opt. Soc. Am. 12, 547–557 (1926). [CrossRef]
