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Fabrication and characterization of a water-free mid-infrared fluorotellurite glass Aoxiang Lin,1,2,* Aleksandr Ryasnyanskiy,1 and Jean Toulouse1,3 1
Center for Optical Technologies and Physics Department, Lehigh University, Bethlehem, Pennsylvania 18015, USA 2 State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics (XIOPM), Chinese Academy of Sciences (CAS), Xi’an 710119, China 3 e-mail:
[email protected] *Corresponding author:
[email protected]
Received November 29, 2010; revised January 18, 2011; accepted January 26, 2011; posted February 2, 2011 (Doc. ID 138728); published February 28, 2011 Using a physical and chemical dehydration technique and a high-pressure, ultradry O2 atmosphere in a semiclosed steel-chamber furnace, we fabricated a group of fluorotellurite glasses with a composition of ð90 − xÞTeO2 -xZnF2 -10Na2 O (mol.%, x ¼ 0–30). For x ¼ 30, no OH absorption was observed in the range of 0:38–6:1 μm. This is the first report of a water-free mid-IR fluorotellurite glass, to our knowledge, offering the common advantages of a robust oxide glass and an IR-transparent fluoride one. Besides optimized linear transmittance and absorption, the nonlinear refractive indices and Raman gain coefficients are reduced. These results are discussed in the context of mid-IR high-power laser generation and transmission. © 2011 Optical Society of America OCIS codes: 060.2390, 060.2290, 160.4670, 300.6340.
The Earth’s atmosphere allows transmission of optical signals in three IR windows covering 1–3, 3–5, and 8–14 μm. Glasses that can transmit in these ranges are therefore of considerable interest for meteorological, environmental, and military applications [1]. Among these three windows, the 3–5 μm mid-IR window presents the highest transparency and has therefore attracted the most attention for these potential applications. In addition to these, high-power laser applications require the development of new easily fiberized optical glasses possessing a high mid-IR transparency, a high optical damage threshold, and a small nonlinear refractive index in order to avoid self-focusing effects [2]. As traditional and well-known mid-IR glass materials, fluoride glasses (e.g., ZBLAN glass) possess a low phonon energy (≤550 cm−1 ) and a broad optical transmittance in the range of 0:3–7 μm [2]. However, they necessitate a complex fabrication route, are not easily fiberized, and are hygroscopic and very fragile in fiber form. Tellurite glasses and fibers are potentially strong competitors of fluorides, as they offer the chemical and physical stability of oxide glasses, are easily fiberized, and have low intrinsic loss and high optical damage threshold. Tellurite glasses have been developed that are transparent in the range of 0:38–6:1 μm, but they still suffer from the influence of residual OH groups that leads to two strong absorption bands at 3.4 and 4:4 μm, i.e., in the middle of their mid-IR transmission spectral range [3,4]. Consequently, removing these absorption bands is of great urgency if one is to take full advantage of their chemical/physical stability and mid-IR transmission. Although significant efforts have been made in the past ten years to fabricate tellurite glasses with low OH content [1–5], no one has yet reported water-free tellurite glasses and fibers. To the best of our knowledge, we are presenting in this paper the first evidence of a water-free mid-IR fluorotellurite glass with a composition of 60TeO2 -30ZnF2 -10Na2 O (TZNF-30, mol.%) , free of OH contamination and therefore transparent from the UV–visible (UV–VIS; 0:38 μm) 0146-9592/11/050740-03$15.00/0
to the mid-IR (6:1 μm), i.e., a new kind of water-free mid-IR glass with enhanced physical and chemical properties comparable to fluoride glasses. To prevent surface crystallization during fiber drawing and therefore enhance the strength of the fiber, telluritebased oxide glass and fibers with composition of 80TeO2 10ZnF2 -10Na2 O (TZN-80) were recently reported by our group [4], where we also presented a “physical dehydration (PDH)” technique to efficiently remove the molecular water present on grain surfaces and/or at interfaces in the starting powders. In the present study, we have further removed residual OH groups (also called “bound/ interstitial water” [1]) in the form of ½≡Te-OH, by introducing ZnF2 in the TZN-80 glass network to chemically dehydrate the deep water inside the glass melt according to Eqs. (1)–(3): ≡ Te–O–Te ≡ þH2 O↔2½≡Te-OH;
ð1Þ
2½≡Te-OH þ ZnF2 → ≡Te–O–Te ≡ þZnO þ 2HF↑; ð2Þ ZnF2 þ H2 O → ZnO þ 2HF↑:
ð3Þ
A group of fluorotellurite glasses with compositions ð90 − xÞTeO2 -xZnF2 -10Na2 O (mol.%, x ¼ 0–30, TZNF-x) have been synthesized and studied. By introducing large amounts of ZnF2 into the glass structure and using a highpressure ultradry O2 atmosphere during the melting process, we have optimized the glass synthesis processes and have produced OH-free or nearly OH-free fluorotellurite glasses. We designate the fabrication process to make water-free fluorotellurite glass as the “physical and chemical dehydration (PCDH)” technique. For optical glass fiber fabrication, the starting materials for glass melting should be of extremely high purity, and those used in our study are as follows: TeO2 (99.999%, Xinju, China), ZnF2 (99.995%, metals basis, Alfa Aesar), ZnO (99.9995%, metals basis, Alfa Aesar), Na2 CO3 © 2011 Optical Society of America
March 1, 2011 / Vol. 36, No. 5 / OPTICS LETTERS
(99.9999%, metals basis, Fluka), and O2 gas (99.993%, UN 1072, OX 4.3UH-T, H2 O < 3 ppm). The raw materials were weighed and mixed in a N2 glove box (H2 O < 1 ppm) and then melted in a semiclosed steelchamber furnace at 800 °C for 2 h under an O2 atmosphere with a positive pressure as high as 10 kPa. After annealing and optical polishing, a series of experiments were done to characterize the synthesized glasses: the linear refractive index n of these glasses was measured at 1550 nm using a Mach–Zehnder interferometer, the nonlinear refractive index n2 was measured by the degenerate four-wave mixing technique, and the Raman spectra were also measured at room temperature using an Ar-ion laser line at 488 nm and a double monochromator Raman spectrometer from Horiba Jobin Yvon. To emphasize the chemical dehydration effect of fluoride ZnF2 , Fig. 1 shows the contrast between the transmission spectra of TZN-80 treated by the PDH technique and that of TZNF-20 using the PCDH technique. With addition of ZnF2 and the corresponding removal of residual OH, the UV edge is found to be blueshifted by about 40 nm, the UV–VIS and near-IR transmission is found to increase, and the mid-IR transmission is found to significantly increase. As demonstrated in Fig. 2, the OH absorption of the glass disappears almost completely upon increasing the molar content of ZnF2 up to x ¼ 30 (TZNF30) and using our critically controlled melting conditions. Assuming the same Fresnel reflection for samples of different thicknesses, the absorption coefficient α was
Fig. 2. (Color online) Transmission and absorption spectra of TZNF-30 glass: (a) UV–VIS spectra and (b) mid-IR spectra.
calculated from the following equation, similar to the cutback method used in fiber optics [6–8]: 1 T α¼ · log 1 ; T2 L2 − L1
Fig. 1. (Color online) Transmission spectra contrast between TZN-80 glass and TZNF-20 glass to show the chemical dehydration effect by introducing ZnF2 : (a) UV–VIS + near-IR spectra and (b) mid-IR spectra.
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ð4Þ
in which L is the thickness and T is the transmittance of the tested glass sample. Compared with spectra reported earlier [1–5], the TZNF-30 glass presents the flattest spectrum of all in the mid-IR range, showing no observable OH absorption peaks at 3:4 μm (∼3000 cm−1 ) and 4:4 μm (∼2300 cm−1 ), and a much lower transmission loss and absorption coefficients, approximately one tenth of the lowest loss reported before, all due to the high purity of the starting materials. It is important to emphasize that the goal of our experiments was to make tellurite-oxide-based glass with the lowest OH/F content in order to preserve the intrinsic advantages of oxide glasses. Therefore, a 10 kPa ultradry O2 atmosphere was used in our experiments to maintain high oxidation conditions during melting, aiming to oxidize as much ZnF2 into ZnO as possible, as in silica-fiber fabrication, where SiCl4 is oxidized into SiO2 [6–8]. However, due to the presence of residual fluorine ions [1], the resulting glass is a chemically mixed fluorotellurite one rather than a pure tellurite oxide entirely free of fluorine. This may actually be desirable for high-power applications as is discussed below. To develop optical fibers especially for high-power laser generation and transmission in the mid-IR range, the fiberized glasses should have lower nonlinear refractive indices in order to avoid nonlinear effects, such as pulse broadening due to self-phase modulation, Raman wavelength conversion, and energy-transfer-induced loss. As shown in Fig. 3, ZnF2 is effective in decreasing the linear
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of Te–O bonds by fluorine and the formation of more TeO3þσ at the expense of the fully bonded glass composed of TeO4 units. In summary, we have reported a new (to our knowledge) kind of water-free mid-IR fluorotellurite glass with a composition of 60TeO2 -30ZnF2 -10Na2 O (TZNF-30, mol.%), fabricated under three critical conditions: (i) PCDH technique, (ii) a þ 10 kPa ultradry O2 oxidizing atmosphere during melting inside a semiclosed steelchamber furnace, and (iii) high-purity starting chemicals to remove transition-metal-impurity-induced loss in the mid-IR range. Combining the advantages of both oxide and fluoride glasses, and with a flat transmission from the UV–VIS into the mid-IR range, TZNF-30 is a promising glass for the development of high-power fiber laser sources in the range of 0:5–5 μm and can also be used more generally for applications requiring a broad transmission spectra range into the mid-IR. This work was performed at Lehigh University under a grant from the National Science Foundation (NSF) (grant No. DMR-0701526, “Glass Science, Processing and Optical Properties of Tellurite Fibers”). Aoxiang Lin’s work in China is being funded by grant No. 60907039 from the National Natural Science Foundation of China (NSFC) and by the West Light Foundation and the “Hundreds of Talents Program” from the Chinese Academy of Sciences.
Fig. 3. (Color online) Evolution with the concentration of ZnF2 of (a) the linear refractive index, (b) the nonlinear refractive index, and (c) the Raman gain coefficient relative to SiO2 .
and nonlinear refractive indices, in approximate accordance with Miller’s rule [9]. As mentioned in Ref. [3], a lower glass density resulting from the introduction of ZnF2 can also result in a decrease of the refractive index. Figure 3(c) shows the Raman gain coefficient (gR ) normalized by the ratio of the maximum Raman intensity of silica glass at 440 cm−1 to that of TZNF glasses at 665 cm−1 . Depending on composition, the Raman gain coefficient of tellurite glasses can be as high as 40 times that of silica (see also Ref. [10]). The arrows and molecular units indicated in Fig. 3(c) also indicate the molecular origin of the various spectral features: the peak observed at 455 cm−1 is attributed to bending vibrations and symmetric stretching of Te–O–Te linkages [11,12]. The 665 cm−1 peak is found to decrease relative to the 740 cm−1 peak with increasing ZnF2 , due to the breaking
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