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Optimization of draw processing parameters for As2Se3 glass fiber
Optical Fiber Technology 38 (2017) 46–50
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Regular Articles
Optimization of draw processing parameters for As2Se3 glass fiber ⁎
MARK
Dong Xu, Shixun Dai , Chenyang You, Yingying Wang, Xin Han, Changgui Lin, Yongxing Liu, Zijun Liu, Xunsi Wang, Yinsheng Xu, Feifei Chen Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo 315211, China
A R T I C L E I N F O
A B S T R A C T
Keywords: As2Se3 glass Draw processing parameters Minimum loss Crystallization
As2Se3 glass fibers measuring 250 μm in diameter were fabricated based on different draw processing parameters, including preform dropping temperatures (T1 = Tg + ΔT, ΔT = 20, 30, 40, 50 °C), fiber-drawing temperatures (T2 = Tg + ΔT, ΔT = 10, 20, 50 °C), and drawing speeds. Raman spectra indicated crystallization on fiber surface at high temperatures. After fiber drawing, oxygen was detected on fiber surface by energy dispersive X-ray spectra. High-quality fiber with minimum loss of 1.88 dB/m (at 9.05 µm) was achieved under optimal dropping temperature of 218.1 °C (Tg + 30 °C), fiber-drawing temperature of 208.1 °C (Tg + 20 °C), and drawing speed of 0.10 m/min.
1. Introduction Chalcogenide glasses (ChGs) received increasing attention because of their high linear and nonlinear refractive indices, low phonon energies, ability to be drawn into fiber, and exceptional transmission range spanning from visible to mid-infrared (IR) (up to 20 µm) [1–4]. ChGs, especially As2S3 and As2Se3, feature potential as low-loss optical fibers in IR laser-powered delivery, all-optical switching, thermal imaging, generation of mid-IR broadband supercontinuum (SC), and Raman fiber lasers and amplifiers [5–9]. In comparison with As2S3, As2Se3 glass features wider IR transmission range and higher nonlinearrefractive index, suggesting better prospective application in IR nonlinear optical fiber devices [8,10]. In the last decades, many researchers conducted systematic investigation on preparation of As2Se3 glass fibers, especially on purification technology [10–14]. Dianov et al. [11] produced an unclad As35Se65 fiber with minimal optical loss of 76–80 dB/km at near 4 µm. Devyatykh et al. [12] obtained the best reported minimum loss of 76 dB/km at 4.3 µm in unclad As2Se3 fibers obtained through singleand double-crucible methods. In 2002, Nauyen et al. [13] reported the presence of low Se–H impurity band at 0.2 dB/m in As–Se core/clad fibers manufactured by vacuum distillation. Shiryave et al. [10] demonstrated a remarkable minimum loss of 67 dB/km at 6–6.5 µm wavelength in a core-clad As40Se60/As38Se62 glass fibers. Danto et al. [14] also systematically studied effects of impurities on properties of As2Se3 glass fiber. Viscosity-temperature characteristics of ChGs varies at 10–20 °C,
⁎
which is narrower [15,16] compared with that of oxide-glass fibers (particularly silica fiber (> 100 °C) [17], phosphate fiber (20–30 °C) [18]). Metastable crystallization of As2Se3 fibers may occur at high drawing speed [19]; thus, drawing temperature and speed require accurate control during fiber preparation. However, no study systematically investigated influences of drawing processes on fiber quality. In this study, we fabricated five As2Se3 ChGs with nearly the same qualities and then drew them into fibers with the same diameter under different drawing processes. We explored effects of different preform dropping and fiber drawing temperatures and drawing speed on fiber properties. Results of this work will provide practical guidance for other ChG fiber drawing processes. 2. Experimental 2.1. Glass preparation Five As2Se3 glass rods with outer diameter of ∼16 mm were prepared as fiber preforms and designated as G0, G1, G2, G3, and G4. Total weight of As and Se raw materials (5 N purity) for melting glass rod measured 50 g. As2Se3 bulk glass was prepared by melt-quenching [20]. A specific purification setup was designed during melting. Raw materials doped with magnesium (20 ppm) as oxygen collector were inserted in a bent tube to operate glass distillation on a three-zone furnace, as shown in Fig. 1. Glasses were purified through a distillation process in which glass melt was distilled from section A to section B (Fig. 1a). Finally, the tube was sealed near the connection of two ampoules
Corresponding author at: Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China. E-mail address: [email protected] (S. Dai).
http://dx.doi.org/10.1016/j.yofte.2017.07.003 Received 14 May 2017; Received in revised form 2 July 2017; Accepted 18 July 2017 1068-5200/ © 2017 Elsevier Inc. All rights reserved.
Optical Fiber Technology 38 (2017) 46–50
D. Xu et al.
Fig. 1. Glass distillation process for melting As2Se3 glass: (a) Three-zone furnace setup. (b) Tube after distillation.
(Fig. 1b). Materials were melted in a rocked furnace at 750 °C for 14 h before subsequent cooling to 450 °C for 1 h and air-quenching. The obtained glass was annealed for 5 h at 5 °C above glass transition temperature (Tg) and cooled down at 3 °C/h to room temperature. Each glass rod was cut into 2 mm-thick disk and polished to mirror smoothness on both sides for measurement of optical properties.
Table 2 Thermal properties of As2Se3 glass. Tg (°C)
Tx (°C)
Ts (°C)
ΔT
α (1E-6/°C)(25–180 °C)
188.1
362.5
212.0
174.4
13.9
exhibited a Tg at 188.1 °C and a crystallization temperature (Tx) at 362.5 °C. Temperature difference ΔT (defined as ΔT = Tx − Tg) reached 174.4 °C (higher than 100 °C), implying excellent thermal stability for fiber drawing [21]. Thermal expansion coefficient (α) of As2Se3 glass totaled 1.39 × 10−5/°C at temperature range of 25–180 °C, and glass softening temperature (Ts) measured 212.0 °C. Fig. 2 displays five IR transmission spectra of slices from G0, G1, G2, G3, and G4 preforms. Samples exhibited the same transmission spectra, which showed an evident wide and flat feature with a transmission rate of 55% from 2.5 µm to 13 µm. Difference in baselines of these samples resulted from slight differences in polishing quality. Relative heights of all impurity bands are presented in inset spectra, especially those of hydroxide (2.9 µm) and Se–H (3.53, 4.12, and 4.57 µm) bands. These bands were mainly caused by reactions between atmospheric moisture and glass batch during glass synthesis. Further distillation and purification can effectively eliminate impurities [14]. To clarify whether crystallization exists under different drawing processes, we first measured XRD patterns of base glass sample and crystallized glass sample (heat-treated at 340 °C for 20 h), and results are shown in Fig. 3. Peaks at 18.1°, 31.0°, 42.8°, 51.0°, and 60.9° were ascribed to As2Se3 crystal phase and were confirmed by comparison with standard JCPDF card of No. 65-310 (As2Se3, monoclinic system). Fig. 4 shows effects of different drawing processes on optical loss spectra of obtained fibers. At 9.05 µm wavelength, lowest optical attenuation of all five fibers measured 26.24, 12.0, 5.65, 1.88, and 7.75 dB/m, respectively. Fiber-I was drawn at the highest temperatures of preform dropping (238.1 °C, Tg + 50 °C) and fiber drawing (238.1 °C, Tg + 50 °C); it presented the highest baseline loss among five fiber spectra. In comparison with other four fiber loss spectra and existing impurity absorptions of Se–H at 3.6 (weak), 4.13 (weak), and 4.57 µm (strong), As-O at 7.9 µm, and Se-O at 10.67 µm, Fiber-I possessed H2O (6.31 µm) and OH (2.92 µm) impurities. When preform dropping temperature decreased from 238.1 °C (Tg + 50 °C) to 218.1 °C (Tg + 30 °C), and drawing temperature was maintained at 208.1 °C (Tg + 20 °C), optical loss decreased gradually. Then, optical loss increased as preform dropping temperature decreased to 208.1 °C
2.2. Fiber drawing Five glass rods were drawn separately into fibers using a high-precision fiber drawing tower (SGC, Customized, UK) at 1 °C control temperature. Different temperatures of preform dropping and fiber drawing were designed according to characteristic glass temperatures, which were obtained by thermal measurements. Table 1 lists relative parameters of five different drawing processes. Final diameter of all fibers was controlled to 250 µm. During fiber drawing, heating chamber was maintained under continuous high pure nitrogen (99.999%) gas flow to avoid oxidation of preform surface, and the pressure is about 0.001 Mpa. 2.3. Optical and thermal parameters measurements Thermal properties were tested by differential scanning calorimetry (DSC) (TA, Q2000, USA) and thermo-mechanical analysis (TMA) (Netzsch, DIL402, GERMANY). IR spectra were obtained by Fourier transform IR (FTIR) spectroscope (Thermo Nicolet, Nexus380, USA) within 2.5–20 µm. Amorphous nature of glasses was examined through X-ray diffraction (XRD) with a power diffractometer (German Bruker D2) using Cu Ka radiation. Fiber transmission was evaluated by FTIR (Thermo Scientific, Nicolet5700, USA), and fiber loss was calculated using standard cut-back method. Raman spectra were acquired using a confocal microscopy laser Raman spectrometer (Renishaw InVia) under 785 nm excitation. Surface morphology of fibers was observed by Super Long Depth Of View Optical Microscope (Keyence, VHX-1000E, JAPAN). Chemical compositions of fibers were measured by energy dispersive X-ray spectroscopy (EDS) combined with scanning electron microscopy (Tescan, VEGA3 SB-Easy Probe, CZECH). All measurements were conducted at room temperature. 3. Results and discussion Table 2 shows DSC and TMA results of As2Se3 glass. The glass Table 1 Parameters of five different drawing processes. Process No.
Preform dropping temperature (°C)
G0 G1 G2 G3 G4
238.1 238.1 228.1 218.1 208.1
(Tg + 50) (Tg + 50) (Tg + 40) (Tg + 30) (Tg + 20)
Fiber drawing temperature (°C)
Preform feeding velocity (mm/min)
Fiber drawing velocity (m/min)
Sample
238.1 208.1 208.1 208.1 198.1
4.342 0.087 0.056 0.043 0.026
10.0 0.20 0.13 0.10 0.06
Fiber-I Fiber-II Fiber-III Fiber-IV Fiber-V
(Tg + 50) (Tg + 20) (Tg + 20) (Tg + 20) (Tg + 10)
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Fig. 2. IR transmission spectra of five slices from G0, G1, G2, G3, and G4 preforms (Samples thickness all measured 2 mm). Inset shows enlarged spectra from 2.5 µm to 6 µm.
Fig. 3. XRD patterns of JCPDF card of No. 65-310 (As2Se3, monoclinic system); As2Se3 base glass and crystallized glass obtained by heat treatment at 340 °C for 20 h.
Fig. 5. Optical micrographs of surfaces of Fiber-(a) I, (b) II, (c) III, (d) IV, and (e) V.
speed) [22], as manifested by distorted crystalline phases at high drawing speeds. (2) When preform dropping temperature was lower than Ts and fiber drawing temperature was at 198.1 °C (Tg + 10 °C, and at lower drawing rates), preform soaking time in drawing furnace was prolonged, and nucleation and crystal growth in fiber surface also possibly occurred. Fig. 5a–e show optical microscopic images of surfaces of Fiber-I, II, III, IV, and V under different drawing processes. Surface of Fiber-I (Fig. 5a) resulted in much more randomly distributed particles; this fiber presented the highest preform dropping (238.1 °C, Tg + 50 °C) and fiber drawing (238.1 °C, Tg + 50 °C) temperatures. Reduced number of particles appeared on fiber surface (Fig. 5b–d) as preform dropping temperature decreased from 238.1 °C (Tg + 50 °C) to 218.1 °C (Tg + 30 °C) at constant drawing temperature of 208.1 °C (Tg + 20 °C). Then, as preform dropping temperature decreased to 208.1 °C (Tg + 20 °C), the amount of particles increased to a certain extent (Fig. 5e). Fig. 5(d) shows that at dropping temperature of 218.1 °C (Tg + 30 °C) and drawing temperature of 208.1 °C (Tg + 20 °C), hardly any particles were detected on fiber surface. We also searched for crosssections of five fibers but did not detect any particles. Fiber surface quality exhibited a direct effect on optical loss, indicating that Fiber-IV, which manifested minimum surface defects, featured the lowest optical loss and showed good agreement with measurement of fiber loss
Fig. 4. Optical loss spectra of Fiber- I, II, III, IV, and V.
(Tg + 20 °C). (1) When preform dropping temperature was higher than Tg + 50 °C, glass melt easily reacted with remaining H2O in air, leading to excessive H2O and OH impurities. Crystallization possibly occurred on fiber surface at high fiber drawing temperatures (high drawing 48
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Fig. 6. Raman scattering spectra of: (a) As2Se3 base glass; (b) As2Se3 crystalline glass obtained by heat treatment at 340 °C for 20 h; (c) location A on surface of Fiber-I; (d) location B on surface of Fiber-I.
indicated in Fig. 4. Different from oxide glasses, chemical bonds in ChGs are relatively weak and induce structural instability or surface oxidation during fiber drawing. Therefore, further studies are required to understand how glass structure and chemical compositions evolve with different draw processing parameters. Raman spectra further confirm precipitation of crystalline phases in fiber surface. Fig. 6a shows Raman spectrum of As2Se3 base glass; peak-fitting results are also displayed in the same figure. Table 3 lists Raman bands assigned to As2Se3 glass. Fig. 6b presents Raman spectrum of crystalline As2Se3 glass obtained by 340 °C heat treatment for 20 h with peaks at 202, 216, 231, 248, and 273 cm−1, which are in good agreement with previous reported data in Ref. [23]. Modes at 202 and 216 cm−1 represent stretching modes equivalent to rigid sub-lattice of diatomic crystals [24]. The peak at 248 cm−1 indicates stretching mode of crystalline As2Se3, and other peaks were speculated as vibration of molecular clusters of As2Se3 [25]. Fig. 6c and d present Raman spectra of locations A and B (marked in Fig. 5a) on surface of Fiber-I, respectively. Raman spectra of A and B areas are similar to those of As2Se3 crystal (Fig. 6b) and glass (Fig. 6a), respectively, indicating that fiber surface defects mainly resulted from crystallization behavior during fiber drawing rather than air pollution or other external influences. Fig. 7 displays EDS measurements of oxygen content in Fiber-I, II, III, IV, and V under different drawing processes. For each sample, chemical compositions were measured in at least 10 different positions in areas ranging from 1 × 1 µm2 to 10 × 10 µm2, and results were averaged. Fiber-I yielded the highest preform dropping (238.1 °C, Tg + 50 °C), and fiber drawing (238.1 °C, Tg + 50 °C) temperatures and the highest oxygen content (11.45 mol%). Oxygen amount decreased
Fig. 7. Oxygen content of surface of Fiber-I, II, III, IV, and V.
from 9.36 mol% to 0 mol% when preform dropping temperature decreased from 238.1 °C (Tg + 50 °C) to 218.1 °C (Tg + 30 °C), and drawing temperature was constant at 208.1 °C (Tg + 20 °C). As preform dropping temperature decreased to 208.1 °C (Tg + 20 °C), oxygen amount elevated as a result of long heat-preservation time. 4. Conclusions As2Se3 glass fibers measuring in 250 μm diameter were fabricated through a high-precision fiber-drawing tower under different draw processing parameters, and their optical properties were characterized. Raman spectra and EDS analyses of fibers indicated crystallization on fiber surface; this phenomenon slightly changed chemical composition of fibers. High-quality fiber with minimum loss of 1.88 dB/m (at 9.05 µm) and smooth fiber surface was achieved under optimal preform dropping temperature of 218.1 °C (Tg + 30 °C), fiber drawing temperature of 208.1 °C (Tg + 20 °C), preform feeding speed of 0.043 mm/ min, and drawing speed of 0.10 m/min. We plan to reduce fiber loss through improved purification, composition optimization, and control of fiber drawing parameters in the future.
Table 3 Raman bands (in cm−1) and their assignments. Peak position (cm−1)
Assignments
Ref.
212 225 243 259
Interaction of the AsSe3 pyramids As–Se vibration in AsSe3 pyramidal units As–Se vibration in AsCh3 units and/or Se–Se chain As–Se vibration in AsCh3 units and/or Se–Se ring fracture Interaction of the AsSe3 pyramids
[26] [25] [27] [28]
273
[26]
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[13] V.Q. Nguyen, J.S. Sanghera, P. Pureza, F.H. Kung, I.D. Aggarwal, Fabrication of arsenic selenide optical fiber with low hydrogen impurities, J. Am. Ceram. Soc. 85 (2002) 2849–2851. [14] S. Danto, M. Dubernet, B. Giroire, J.D. Musgraves, P. Wachtel, T. Hawkins, J. Ballato, K. Richardson, Correlation between native As2Se3 preform purity and glass optical fiber mechanical strength, Mater. Res. Bull. 49 (2014) 250–258. [15] B. Giroire, Optimization of processing parameters for As2Se3 glass for low loss, high strength. Dissertations & Theses - Gradworks (2012). [16] G. Yang, T. Rouxel, J. Troles, B. Bureau, C. Boussard-Plédel, P. Houizot, J.C. Sangleboeuf, Viscosity of As2Se3 glass during the fiber drawing process, J. Am. Ceram. Soc. 94 (2011) 2408–2411. [17] S. Fujino, H. Ijiri, F. Shimizu, K. Morinaga, Measurement of viscosity of multicomponent glasses in the wide range for fiber drawing, J. Jpn. I. Met. 62 (1998) 106–110. [18] B.A. Sava, M. Elisa, L. Boroica, R.C.C. Monteiro, Preparation method and thermal properties of samarium and europium-doped alumino-phosphate glasses, Mat. Sci. Eng. B 178 (2013) 1429–1435. [19] P. Hari, P.C. Taylor, W.A. King, W.C. Lacourse, Metastable, drawing-induced crystallization in As2Se3 fibers, J. Non-Cryst. Solidss 227–230 (1998) 789–793. [20] X. Han, C. You, S. Dai, P. Zhang, Y. Wang, F. Guo, D. Xu, B. Luo, P. Xu, X. Wang, Mid-infrared supercontinuum generation in a three-hole Ge20Sb15Se65 chalcogenide suspended-core fiber, Opt. Fiber Technol. 34 (2017) 74–79. [21] Z. Yang, T. Luo, S. Jiang, J. Geng, P. Lucas, Single-mode low-loss optical fibers for long-wave infrared transmission, Opt. Lett. 35 (2010) 3360–3362. [22] D.W. Henderson, D.G. Ast, Viscosity and crystallization kinetics of As2Se3, J. NonCryst. Solids 64 (1984) 43–70. [23] R. Zallen, M.L. Slade, A.T. Ward, Lattice vibrations and interlayer interactions in crystalline As2S3 and As2Se3, Phys. Rev. B 3 (1971) 4257–4273. [24] G. Lucovsky, R.M. Martin, A molecular model for the vibrational modes in chalcogenide glasses, J. Non-Cryst. Solids 8–10 (1972) 185–190. [25] I. Abdulhalim, R. Beserman, Raman scattering study of laser induced structural transformations in glassy As2Se3, Solid State Commun. 64 (1987) 951–955. [26] W. Li, S. Seal, C. Rivero, C. Lopez, K. Richardson, A. Pope, A. Schulte, S. Myneni, H. Jain, K. Antoine, Role of S/Se ratio in chemical bonding of As–S–Se glasses investigated by Raman, X-ray photoelectron, and extended X-ray absorption fine structure spectroscopies, J. Appl. Phys. 98 (2005) 643–758. [27] R.P. Wang, D. Bulla, A. Smith, T. Wang, B. Lutherdavies, Structure and physical properties of GexAsySe1−x−y glasses with the same mean coordination number of 2.5, J. Appl. Phys. 109 (2011) 1270–1271. [28] X. Su, R. Wang, B. Luther-Davies, L. Wang, The dependence of photosensitivity on composition for thin films of Gex Asy Se1– x – y chalcogenide glasses, Appl. Phys. A 113 (2013) 575–581.
Acknowledgements The project was sponsored by the National Natural Science Foundation of China (No. 61435009) and K. C. Wong Magna Fund in Ningbo University. References [1] C.R. Petersen, U. Møller, I. Kubat, B. Zhou, S. Dupont, J. Ramsay, T. Benson, S. Sujecki, N. Abdelmoneim, Z. Tang, Mid-infrared supercontinuum covering the 1.4–13.3 μm molecular fingerprint region using ultra-high NA chalcogenide stepindex fibre, Nat. Photonics 8 (2014) 830–834. [2] J.S. Sanghera, L.B. Shaw, I.D. Aggarwal, Chalcogenide glass-fiber-based mid-IR sources and applications, IEEE J. Sel. Top. Quantum Electron. 15 (2009) 114–119. [3] F. Smektala, C. Quemard, L. Leneindre, J. Lucas, A. Barthélémy, C.D. Angelis, Chalcogenide glasses with large non-linear refractive indices, J. Non-Cryst. Solids 239 (1998) 139–142. [4] A. Zakery, S.R. Elliott, Optical properties and applications of chalcogenide glasses: a review, J. Non-Cryst. Solids 330 (2003) 1–12. [5] J.S. Sanghera, L.B. Shaw, I.D. Aggarwal, Applications of chalcogenide glass optical fibers, C. R. Chim. 5 (2002) 873–883. [6] S.D. Jackson, G. Anzueto-Sánchez, Chalcogenide glass Raman fiber laser, Appl. Phys. Lett. 88 (2006) 221106. [7] A.F. Kosolapov, A.D. Pryamikov, A.S. Biriukov, V.S. Shiryaev, M.S. Astapovich, G.E. Snopatin, V.G. Plotnichenko, M.F. Churbanov, E.M. Dianov, Demonstration of CO2-laser power delivery through chalcogenide-glass fiber with negative-curvature hollow core, Opt. Express 19 (2011) 25723–25728. [8] Y.N. Sun, S. Dai, P. Zhang, X. Wang, Y. Xu, Z. Liu, F. Chen, Y. Wu, Y. Zhang, R. Wang, Fabrication and characterization of multimaterial chalcogenide glass fiber tapers with high numerical apertures, Opt. Express 23 (2015) 23472–23483. [9] M. Asobe, Nonlinear optical properties of chalcogenide glass fibers and their application to all-optical switching, Opt. Fiber Technol. 3 (1997) 142–148. [10] V.S. Shiryaev, M.F. Churbanov, G.E. Snopatin, F. Chenard, Preparation of low-loss core–clad As–Se glass fibers, Opt. Mater. 48 (2015) 222–225. [11] E.M. Dianov, V.G.P.G. Devyatykh, M.F. Churbanov, I.V. Scripachev, Middle-infrared chalcogenide glass fibers with losses lower than 100 dB/km, Infrared Phys. 29 (1989) 303–307. [12] G. Devyatykh, E. Dianov, V. Plotnichenko, I. Skripachev, M. Churbanov, Fiber waveguides based on high-purity chalcogenide glasses, High-Purity Substances 5 (1991) 1–27.
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Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Regular Articles
Optimization of draw processing parameters for As2Se3 glass fiber ⁎
MARK
Dong Xu, Shixun Dai , Chenyang You, Yingying Wang, Xin Han, Changgui Lin, Yongxing Liu, Zijun Liu, Xunsi Wang, Yinsheng Xu, Feifei Chen Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo 315211, China
A R T I C L E I N F O
A B S T R A C T
Keywords: As2Se3 glass Draw processing parameters Minimum loss Crystallization
As2Se3 glass fibers measuring 250 μm in diameter were fabricated based on different draw processing parameters, including preform dropping temperatures (T1 = Tg + ΔT, ΔT = 20, 30, 40, 50 °C), fiber-drawing temperatures (T2 = Tg + ΔT, ΔT = 10, 20, 50 °C), and drawing speeds. Raman spectra indicated crystallization on fiber surface at high temperatures. After fiber drawing, oxygen was detected on fiber surface by energy dispersive X-ray spectra. High-quality fiber with minimum loss of 1.88 dB/m (at 9.05 µm) was achieved under optimal dropping temperature of 218.1 °C (Tg + 30 °C), fiber-drawing temperature of 208.1 °C (Tg + 20 °C), and drawing speed of 0.10 m/min.
1. Introduction Chalcogenide glasses (ChGs) received increasing attention because of their high linear and nonlinear refractive indices, low phonon energies, ability to be drawn into fiber, and exceptional transmission range spanning from visible to mid-infrared (IR) (up to 20 µm) [1–4]. ChGs, especially As2S3 and As2Se3, feature potential as low-loss optical fibers in IR laser-powered delivery, all-optical switching, thermal imaging, generation of mid-IR broadband supercontinuum (SC), and Raman fiber lasers and amplifiers [5–9]. In comparison with As2S3, As2Se3 glass features wider IR transmission range and higher nonlinearrefractive index, suggesting better prospective application in IR nonlinear optical fiber devices [8,10]. In the last decades, many researchers conducted systematic investigation on preparation of As2Se3 glass fibers, especially on purification technology [10–14]. Dianov et al. [11] produced an unclad As35Se65 fiber with minimal optical loss of 76–80 dB/km at near 4 µm. Devyatykh et al. [12] obtained the best reported minimum loss of 76 dB/km at 4.3 µm in unclad As2Se3 fibers obtained through singleand double-crucible methods. In 2002, Nauyen et al. [13] reported the presence of low Se–H impurity band at 0.2 dB/m in As–Se core/clad fibers manufactured by vacuum distillation. Shiryave et al. [10] demonstrated a remarkable minimum loss of 67 dB/km at 6–6.5 µm wavelength in a core-clad As40Se60/As38Se62 glass fibers. Danto et al. [14] also systematically studied effects of impurities on properties of As2Se3 glass fiber. Viscosity-temperature characteristics of ChGs varies at 10–20 °C,
⁎
which is narrower [15,16] compared with that of oxide-glass fibers (particularly silica fiber (> 100 °C) [17], phosphate fiber (20–30 °C) [18]). Metastable crystallization of As2Se3 fibers may occur at high drawing speed [19]; thus, drawing temperature and speed require accurate control during fiber preparation. However, no study systematically investigated influences of drawing processes on fiber quality. In this study, we fabricated five As2Se3 ChGs with nearly the same qualities and then drew them into fibers with the same diameter under different drawing processes. We explored effects of different preform dropping and fiber drawing temperatures and drawing speed on fiber properties. Results of this work will provide practical guidance for other ChG fiber drawing processes. 2. Experimental 2.1. Glass preparation Five As2Se3 glass rods with outer diameter of ∼16 mm were prepared as fiber preforms and designated as G0, G1, G2, G3, and G4. Total weight of As and Se raw materials (5 N purity) for melting glass rod measured 50 g. As2Se3 bulk glass was prepared by melt-quenching [20]. A specific purification setup was designed during melting. Raw materials doped with magnesium (20 ppm) as oxygen collector were inserted in a bent tube to operate glass distillation on a three-zone furnace, as shown in Fig. 1. Glasses were purified through a distillation process in which glass melt was distilled from section A to section B (Fig. 1a). Finally, the tube was sealed near the connection of two ampoules
Corresponding author at: Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China. E-mail address: [email protected] (S. Dai).
http://dx.doi.org/10.1016/j.yofte.2017.07.003 Received 14 May 2017; Received in revised form 2 July 2017; Accepted 18 July 2017 1068-5200/ © 2017 Elsevier Inc. All rights reserved.
Optical Fiber Technology 38 (2017) 46–50
D. Xu et al.
Fig. 1. Glass distillation process for melting As2Se3 glass: (a) Three-zone furnace setup. (b) Tube after distillation.
(Fig. 1b). Materials were melted in a rocked furnace at 750 °C for 14 h before subsequent cooling to 450 °C for 1 h and air-quenching. The obtained glass was annealed for 5 h at 5 °C above glass transition temperature (Tg) and cooled down at 3 °C/h to room temperature. Each glass rod was cut into 2 mm-thick disk and polished to mirror smoothness on both sides for measurement of optical properties.
Table 2 Thermal properties of As2Se3 glass. Tg (°C)
Tx (°C)
Ts (°C)
ΔT
α (1E-6/°C)(25–180 °C)
188.1
362.5
212.0
174.4
13.9
exhibited a Tg at 188.1 °C and a crystallization temperature (Tx) at 362.5 °C. Temperature difference ΔT (defined as ΔT = Tx − Tg) reached 174.4 °C (higher than 100 °C), implying excellent thermal stability for fiber drawing [21]. Thermal expansion coefficient (α) of As2Se3 glass totaled 1.39 × 10−5/°C at temperature range of 25–180 °C, and glass softening temperature (Ts) measured 212.0 °C. Fig. 2 displays five IR transmission spectra of slices from G0, G1, G2, G3, and G4 preforms. Samples exhibited the same transmission spectra, which showed an evident wide and flat feature with a transmission rate of 55% from 2.5 µm to 13 µm. Difference in baselines of these samples resulted from slight differences in polishing quality. Relative heights of all impurity bands are presented in inset spectra, especially those of hydroxide (2.9 µm) and Se–H (3.53, 4.12, and 4.57 µm) bands. These bands were mainly caused by reactions between atmospheric moisture and glass batch during glass synthesis. Further distillation and purification can effectively eliminate impurities [14]. To clarify whether crystallization exists under different drawing processes, we first measured XRD patterns of base glass sample and crystallized glass sample (heat-treated at 340 °C for 20 h), and results are shown in Fig. 3. Peaks at 18.1°, 31.0°, 42.8°, 51.0°, and 60.9° were ascribed to As2Se3 crystal phase and were confirmed by comparison with standard JCPDF card of No. 65-310 (As2Se3, monoclinic system). Fig. 4 shows effects of different drawing processes on optical loss spectra of obtained fibers. At 9.05 µm wavelength, lowest optical attenuation of all five fibers measured 26.24, 12.0, 5.65, 1.88, and 7.75 dB/m, respectively. Fiber-I was drawn at the highest temperatures of preform dropping (238.1 °C, Tg + 50 °C) and fiber drawing (238.1 °C, Tg + 50 °C); it presented the highest baseline loss among five fiber spectra. In comparison with other four fiber loss spectra and existing impurity absorptions of Se–H at 3.6 (weak), 4.13 (weak), and 4.57 µm (strong), As-O at 7.9 µm, and Se-O at 10.67 µm, Fiber-I possessed H2O (6.31 µm) and OH (2.92 µm) impurities. When preform dropping temperature decreased from 238.1 °C (Tg + 50 °C) to 218.1 °C (Tg + 30 °C), and drawing temperature was maintained at 208.1 °C (Tg + 20 °C), optical loss decreased gradually. Then, optical loss increased as preform dropping temperature decreased to 208.1 °C
2.2. Fiber drawing Five glass rods were drawn separately into fibers using a high-precision fiber drawing tower (SGC, Customized, UK) at 1 °C control temperature. Different temperatures of preform dropping and fiber drawing were designed according to characteristic glass temperatures, which were obtained by thermal measurements. Table 1 lists relative parameters of five different drawing processes. Final diameter of all fibers was controlled to 250 µm. During fiber drawing, heating chamber was maintained under continuous high pure nitrogen (99.999%) gas flow to avoid oxidation of preform surface, and the pressure is about 0.001 Mpa. 2.3. Optical and thermal parameters measurements Thermal properties were tested by differential scanning calorimetry (DSC) (TA, Q2000, USA) and thermo-mechanical analysis (TMA) (Netzsch, DIL402, GERMANY). IR spectra were obtained by Fourier transform IR (FTIR) spectroscope (Thermo Nicolet, Nexus380, USA) within 2.5–20 µm. Amorphous nature of glasses was examined through X-ray diffraction (XRD) with a power diffractometer (German Bruker D2) using Cu Ka radiation. Fiber transmission was evaluated by FTIR (Thermo Scientific, Nicolet5700, USA), and fiber loss was calculated using standard cut-back method. Raman spectra were acquired using a confocal microscopy laser Raman spectrometer (Renishaw InVia) under 785 nm excitation. Surface morphology of fibers was observed by Super Long Depth Of View Optical Microscope (Keyence, VHX-1000E, JAPAN). Chemical compositions of fibers were measured by energy dispersive X-ray spectroscopy (EDS) combined with scanning electron microscopy (Tescan, VEGA3 SB-Easy Probe, CZECH). All measurements were conducted at room temperature. 3. Results and discussion Table 2 shows DSC and TMA results of As2Se3 glass. The glass Table 1 Parameters of five different drawing processes. Process No.
Preform dropping temperature (°C)
G0 G1 G2 G3 G4
238.1 238.1 228.1 218.1 208.1
(Tg + 50) (Tg + 50) (Tg + 40) (Tg + 30) (Tg + 20)
Fiber drawing temperature (°C)
Preform feeding velocity (mm/min)
Fiber drawing velocity (m/min)
Sample
238.1 208.1 208.1 208.1 198.1
4.342 0.087 0.056 0.043 0.026
10.0 0.20 0.13 0.10 0.06
Fiber-I Fiber-II Fiber-III Fiber-IV Fiber-V
(Tg + 50) (Tg + 20) (Tg + 20) (Tg + 20) (Tg + 10)
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Fig. 2. IR transmission spectra of five slices from G0, G1, G2, G3, and G4 preforms (Samples thickness all measured 2 mm). Inset shows enlarged spectra from 2.5 µm to 6 µm.
Fig. 3. XRD patterns of JCPDF card of No. 65-310 (As2Se3, monoclinic system); As2Se3 base glass and crystallized glass obtained by heat treatment at 340 °C for 20 h.
Fig. 5. Optical micrographs of surfaces of Fiber-(a) I, (b) II, (c) III, (d) IV, and (e) V.
speed) [22], as manifested by distorted crystalline phases at high drawing speeds. (2) When preform dropping temperature was lower than Ts and fiber drawing temperature was at 198.1 °C (Tg + 10 °C, and at lower drawing rates), preform soaking time in drawing furnace was prolonged, and nucleation and crystal growth in fiber surface also possibly occurred. Fig. 5a–e show optical microscopic images of surfaces of Fiber-I, II, III, IV, and V under different drawing processes. Surface of Fiber-I (Fig. 5a) resulted in much more randomly distributed particles; this fiber presented the highest preform dropping (238.1 °C, Tg + 50 °C) and fiber drawing (238.1 °C, Tg + 50 °C) temperatures. Reduced number of particles appeared on fiber surface (Fig. 5b–d) as preform dropping temperature decreased from 238.1 °C (Tg + 50 °C) to 218.1 °C (Tg + 30 °C) at constant drawing temperature of 208.1 °C (Tg + 20 °C). Then, as preform dropping temperature decreased to 208.1 °C (Tg + 20 °C), the amount of particles increased to a certain extent (Fig. 5e). Fig. 5(d) shows that at dropping temperature of 218.1 °C (Tg + 30 °C) and drawing temperature of 208.1 °C (Tg + 20 °C), hardly any particles were detected on fiber surface. We also searched for crosssections of five fibers but did not detect any particles. Fiber surface quality exhibited a direct effect on optical loss, indicating that Fiber-IV, which manifested minimum surface defects, featured the lowest optical loss and showed good agreement with measurement of fiber loss
Fig. 4. Optical loss spectra of Fiber- I, II, III, IV, and V.
(Tg + 20 °C). (1) When preform dropping temperature was higher than Tg + 50 °C, glass melt easily reacted with remaining H2O in air, leading to excessive H2O and OH impurities. Crystallization possibly occurred on fiber surface at high fiber drawing temperatures (high drawing 48
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Fig. 6. Raman scattering spectra of: (a) As2Se3 base glass; (b) As2Se3 crystalline glass obtained by heat treatment at 340 °C for 20 h; (c) location A on surface of Fiber-I; (d) location B on surface of Fiber-I.
indicated in Fig. 4. Different from oxide glasses, chemical bonds in ChGs are relatively weak and induce structural instability or surface oxidation during fiber drawing. Therefore, further studies are required to understand how glass structure and chemical compositions evolve with different draw processing parameters. Raman spectra further confirm precipitation of crystalline phases in fiber surface. Fig. 6a shows Raman spectrum of As2Se3 base glass; peak-fitting results are also displayed in the same figure. Table 3 lists Raman bands assigned to As2Se3 glass. Fig. 6b presents Raman spectrum of crystalline As2Se3 glass obtained by 340 °C heat treatment for 20 h with peaks at 202, 216, 231, 248, and 273 cm−1, which are in good agreement with previous reported data in Ref. [23]. Modes at 202 and 216 cm−1 represent stretching modes equivalent to rigid sub-lattice of diatomic crystals [24]. The peak at 248 cm−1 indicates stretching mode of crystalline As2Se3, and other peaks were speculated as vibration of molecular clusters of As2Se3 [25]. Fig. 6c and d present Raman spectra of locations A and B (marked in Fig. 5a) on surface of Fiber-I, respectively. Raman spectra of A and B areas are similar to those of As2Se3 crystal (Fig. 6b) and glass (Fig. 6a), respectively, indicating that fiber surface defects mainly resulted from crystallization behavior during fiber drawing rather than air pollution or other external influences. Fig. 7 displays EDS measurements of oxygen content in Fiber-I, II, III, IV, and V under different drawing processes. For each sample, chemical compositions were measured in at least 10 different positions in areas ranging from 1 × 1 µm2 to 10 × 10 µm2, and results were averaged. Fiber-I yielded the highest preform dropping (238.1 °C, Tg + 50 °C), and fiber drawing (238.1 °C, Tg + 50 °C) temperatures and the highest oxygen content (11.45 mol%). Oxygen amount decreased
Fig. 7. Oxygen content of surface of Fiber-I, II, III, IV, and V.
from 9.36 mol% to 0 mol% when preform dropping temperature decreased from 238.1 °C (Tg + 50 °C) to 218.1 °C (Tg + 30 °C), and drawing temperature was constant at 208.1 °C (Tg + 20 °C). As preform dropping temperature decreased to 208.1 °C (Tg + 20 °C), oxygen amount elevated as a result of long heat-preservation time. 4. Conclusions As2Se3 glass fibers measuring in 250 μm diameter were fabricated through a high-precision fiber-drawing tower under different draw processing parameters, and their optical properties were characterized. Raman spectra and EDS analyses of fibers indicated crystallization on fiber surface; this phenomenon slightly changed chemical composition of fibers. High-quality fiber with minimum loss of 1.88 dB/m (at 9.05 µm) and smooth fiber surface was achieved under optimal preform dropping temperature of 218.1 °C (Tg + 30 °C), fiber drawing temperature of 208.1 °C (Tg + 20 °C), preform feeding speed of 0.043 mm/ min, and drawing speed of 0.10 m/min. We plan to reduce fiber loss through improved purification, composition optimization, and control of fiber drawing parameters in the future.
Table 3 Raman bands (in cm−1) and their assignments. Peak position (cm−1)
Assignments
Ref.
212 225 243 259
Interaction of the AsSe3 pyramids As–Se vibration in AsSe3 pyramidal units As–Se vibration in AsCh3 units and/or Se–Se chain As–Se vibration in AsCh3 units and/or Se–Se ring fracture Interaction of the AsSe3 pyramids
[26] [25] [27] [28]
273
[26]
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