Open Access
Add to BibSonomyAbstract
Effects of multiple drawing operations on As38Se62 and Ge10As22Se68 chalcogenide microstructured optical fibers (MOF) are investigated. Fabrication of small-core single-mode chalcogenide MOF’s with 3 rings of holes necessitates a two-step drawing operation which may conduct to additional optical losses, as compared to single-step processes. Thus, glasses with high stability against crystallization are required. With this respect, Ge10As22Se68 single-mode microstructured optical were obtained with optical losses equal to 1 dB/m at 1.55 µm and lower than 1 dB/m at 3.0µm. Core diameter is as small as 4-6 µm.
©2012 Optical Society of America
1. Introduction
Chalcogenide glasses are known for their large transparency window and their high nonlinear optical properties. Indeed, they can be transparent from the visible region up to the mid infrared, depending on the glass composition [1]. Another remarkable property of chalcogenide glasses is their strong optical non linearity. The nonlinear refractive index of sulfur-based glasses is more than 100 times larger than that of silica. The nonlinear index of selenium-based glasses can be more than 1000 times larger than that of silica [2,3]. This property is an important requirement for nonlinear applications. In addition, small-core fiber geometries (diameter smaller than 5 µm) can enhance dramatically the intrinsic nonlinear optical properties of chalcogenide glasses. Such capability can be utilized for signal regeneration in telecommunication [4], for supercontinuum generation [5–7] and for light conversion to the mid infrared by using Raman shifting [8–11]. However, it is a challenge to obtain such small-core fiber with low optical losses. Besides, for most of these applications, single-mode waveguiding is needed. The first technique consists in the preparation of classical step-index fibers (SIF) with a core-clad structure [12,13]. The second route consists in the design of a microstructured optical fiber (MOF), where the guiding properties are mainly defined by the d/Λ ratio (d: diameter of the holes, Λ: distance between the holes) [14]. Firstly developed in silica this new design of fiber spreads to other glassy matrix [15]. Since the first attempt in 2000 of manufacturing an holey chalcogenide fiber, which at last did not exhibit light guiding [16], chalcogenide MOF with light propagation [6,17,18] and small mode area [19] have been obtained. The common method to prepare MOF is the stack-and-draw technique. This method is widely used for silica MOF [14]. In 2008, it has been shown that optical losses in chalcogenide fibers were due essentially to the presence of scattering defects at the interface between capillaries [20]. So, to avoid interfaces defects, MOFs have been prepared by a newly established casting method [21]. Thanks to this technique, suspended-core chalcogenide fibers with low optical losses have been elaborated [9,22]. However, despite their very small mode area (5-1 µm2), they still exhibit a multimode behavior, which is not suitable for telecom applications.
The aim of our study is to obtain a single-mode MOF combining a small effective area (< 2 µm), a very high nonlinear coefficient γ = 2πn2/λAeff > 10000 W−1.Km−1, and optical losses in the 1 dB/m range. Obtaining such a fiber is critical, because two drawing operations are needed which may induce strong additional optical losses [19,23]. Thus, glasses with high stability against crystallization must be utilized. The glass composition of chalcogenide MOF is often close to As2Se3 [21,23]. In our study, the effect of a second drawing operation is investigated with As38Se62 glass. To obtain improved transmission, a new composition from the Ge-As-Se glassy system is studied and compared with As38Se62. In a second stage, the realization of a small-core chalcogenide MOF, based on Ge10As22Se68, with low optical losses is described.
2. Elaboration of high-purity As-Se and Ge-As-Se glasses and single-index fiber fabrication
The glass rods were fabricated by melting high-purity (germanium), arsenic and selenium together with magnesium metal (Mg) and tellurium tetrachlorides (TeCl4), at 870°C in a vacuum-pumped silica ampoule introduced in a rocking furnace. Magnesium and tellurium tetrachlorides act as oxygen and hydrogen getters, respectively [24,25]. After 10 hours of treatment, the melt was cooled to 700°C and quenched by immersion of the ampoule in water for about 2s. This step is followed by annealing above Tg (Tg = 165°C and 180 C° for As38Se62 and Ge10As22Se68 respectively)
Then, the glass rods were purified by several distillations steps. In the first step, the glasses were distilled under dynamic vacuum. They were heated progressively from 360°C to 460°C to leave behind oxide impurities formed by reaction with magnesium and eliminate gaseous byproducts due to the reaction with TeCl4. In the second step, the glasses were distillated under static vacuum in order to eliminate possible remaining oxides. Finally, the distillates were melted again at 870°C for 10 hours, cooled to 650°C, quenched by immersion of the ampoule in water for about 2s and then annealed above Tg.
In order to evaluate the chemical purity of the glasses, the purified rods were drawn directly as fibers. The optical losses of fibers have been measured on a Bruker Tensor 37 FTIR apparatus with a cooled MCT detector. The standard cut-back technique was used to determine the attenuation (dB/m). The result is presented in Fig. 1 .
The high purification process used for the fabrication of AsSe and GeAsSe glasses results in low material losses, that is below 1 dB/m in the 1.5 µm - 9 µm spectral range, except around 4.55 µm where the presence of the Se-H absorption band induces 2.5 dB/m optical loss.
3. Impact of temperature-induced crystallization on the absorption losses of As-Se and Ge-As-Se single-index fibers
In previous works, in small core As38Se62 fibers which requires a two-step drawing process, it has been observed losses as high as 15 dB/m [23]. To quantify further this phenomenon, the impact of above-Tg heating on an As38Se62 fiber has been analyzed by using the experimental setup described in Fig. 2 . A 1.20-meter long single-index uncoated fiber was placed on a heating plate, with a thermal contact between the fiber and the plate of 20 cm in length. A thermocouple was placed in the middle of the heating plate to measure the temperature of the fiber (accuracy +/− 5 °C). The optical transmission of fibers upon heating, with respect to room temperature transmission, were measured by the Bruker Tensor 37 FTIR spectrometer described above. For each measurement, the temperature was held for 15 min to reach equilibrium. The reference spectrum is recorded before heat treatment (Sref). The reported transmission in Fig. 3 , corresponds to the ratio Smes/Sref.
Three spectral ranges, with distinct evolution of transmission as a function of temperature, are observed: 1-3 µm, 3-8 µm, and beyond 8 µm. For temperatures up to 163 °C, that is lower than the vitreous transition of the glass (Tg = 165°C), the transmission window decreases progressively at wavelengths greater than 8 µm. Vinh Q. Nguyen and al. already observed this phenomenon with chalcogenide glass fibers [26–28]. This decrease of transmission is due to the increase of temperature-dependent multiphonon absorption. After treatment, when the fiber temperature is back to room temperature, the optical transmission at long wavelengths is almost back to normal (black curve in Fig. 3a). At short wavelengths, no loss of transmission is observed in that same temperature range. However, when the temperature is closed to, or higher than, Tg, optical losses occur. Contrary to multiphonon absorption, these losses are still present after heat-treatment. Indeed, the final measurement at room temperature presents remaining losses in the 1 - 3 µm range. These losses are attributed to scattering phenomena due to the presence of crystals in glasses treated above Tg So, one can conclude thatAs38Se62 glass is suitable for preparing single mode fibers with core-diameter greater than 10 µm by means of only one drawing operation, but this composition is not stable enough to undergo a second drawing step for obtaining small-core fibers.
Then we search for a new glass composition, stable enough against crystallization to endure several drawing operations. We choose the Ge-As-Se glass system. Indeed, Ge-As-Se glass compositions are suitable for fiber optics [29]. Moreover, A. Prasad et al. [30] showed that this system presents compositions with a strong non-linear refractive index. Thus, the Ge10As22Se68 glass composition was selected for its thermal and non-linear properties. The refractive index of this composition is equal to 2.618 at 1.55 µm to be compared with 2.818 for As38Se62 (n = 2.618 at 1.55µm).
The same thermal treatment as for As38Se62 was performed to evaluate the ability of this glass composition to tolerate multiple drawing steps.
Figure 3b shows the transmission of Ge-As-Se glass fiber as a function of temperature, from room temperature to 231°C. In the short-wavelength cut-off region, the transmission loss is due to free electrons in the mobility edge which absorb sufficient phonon energy to be promoted into the conduction band, across the optical gap [26–28]. Besides, in a semiconductor material, as GeAsSe glass, the free-electron concentration increases with temperature. Beyond 8 µm, as for As-Se, transmission losses due to multiphonon absorption depend on the temperature of the fiber, and consequently they are not permanent. The black curve which corresponds to the transmission at room temperature, after the fiber was heat-treated, proves this reversibility.
However, in this Ge-As-Se glass fiber, a small permanent decrease of the transmission – from 100% to 95% is observed after thermal treatment. This damage of the uncoated single-index fiber is attributed to a small oxidation of the surface. Indeed, a Ge-O absorption band, appears at 9.5 µm. However, the decreasing of the transmission is less important than in the As-Se fiber. So, no strong crystallization has been observed in this composition.
These results are confirmed by Differential Scanning Calorimetry (DSC) measurements which were performed on both AsSe and GeAsSe glass after one drawing step. Indeed, any crystallization cannot be observed in the Ge10As22Se68 glass while a small exothermic peak resulting from crystallization was observed for the As38Se62 composition.
4. Germanium-based optical glass fibers with small core
4.1 Fiber elaboration
To reach a core-diameter smaller than 5 µm, fabrication of the fiber necessitates a four-step process. First, the jacket tube is prepared from unpurified GeAsSe glass rod using the rotational casting method [23]. The outer diameter of this item is 16 mm while the inner diameter is 6 mm. In the second step, a high purity GeAsSe preform with 3 rings of holes is elaborated using the casting method [21]. In the third step, the preform is reduced in size to a “cane” using a fiber drawing tower (first drawing operation). The outer diameter of the cane is selected to provide a close fit to the inner diameter of the jacket tube (≈6 mm). Finally, the cane is inserted into the jacket tube and this assembly is drawn down to form the final fiber (second drawing operation). Controlling carefully the drawing speed and the pressure applied in the holes of the microstructure, different diameters and geometries can be obtained as reported in Table 1 . Figure 4 presents a cross-section of the GeAsSe fiber with external diameter of 125 µm, core size of nearly 4 µm and d/Λ ratio = 0.4. Different fiber losses are less or equal to 1 dB/m which is suitable for telecom functions [22,31].
Table 1. Characteristic Geometries of Small-Core Ge-As-Se Microstructured Fibers
Fig. 4 SEM micrographs of the GeAsSe small-core fiber at two different scales. Fiber diameter = 125 µm, core diameter = 4 µm, d/Λ = 0.4.
4.2 Single mode propagation
In this measurement, the geometry of the 6.5 µm core fiber corresponds to a single mode fiber at λ = 1.55µm [19]. Near-field observation of the monochromatic guided beam (λ = 1.55µm) is visualized at the fiber output thanks to an IR camera as shown in Fig. 5 . A GaSn alloy was applied on the surface of the fiber to inhibit cladding mode guidance. The IR beam observed at the output of the fiber shows a single Gaussian profile, indicating single mode behavior. The 4 µm core fiber (d/Λ ratio = 0.4) presents also a single mode behavior. However the 3.8 µm core fiber (d/Λ ratio = 0.5) exhibits a multimode propagation (profiles not shown).
Fig. 5 Near field observation of the 1.55-µm beam at the output of the small-core Ge-As-Se microstructured fiber.The experimental profile (black curve) fits a Gaussian curve (red curve).
4.3 Optical losses
Optical losses are measured at wavelengths from 1 to 4.5 µm for a 6.5-µm core-diameter fiber, in order to facilitate the injection of the IR beam in the fiber. At the output, the signal is collected thanks to cooled InSb detector. The results are shown in Fig. 6 , together with the optical absorption of bulk glass, for comparison. For the 4-µm core fiber, the core is too small to allow efficient light injection from the FTIR light source, so that optical losses cannot be measured on a broad spectral range. However, losses can be obtained by using a 1.55-µm laser diode. They are equal to 1dB/m.
Fig. 6 Material losses of the GeAsSe glass (black curve) and attenuation of the GeAsSe small core microstructured fiber (grey curve). Picture: cross-section of the MOF used for this measurement.
From Fig. 6, it is observed that optical losses increase by less than 1dB/m in the range 1-4 µm, for the microstructured fiber obtained by multiple glass-casting steps as compared to the bulk glass. The fiber spectrum is limited to λ = 5 µm due to the detection range of the InSb detector.
Above 3 µm, a part of the MOF optical losses is attributed to guiding losses. They have been calculated at different wavelengths by the multipole method [32,33]. While at 1.55 µm, guiding losses are as low as 0.03 dB/m, they increase up to 2.2 dB/m at 4.5µm.
So, overall optical losses for the fiber are close to the material ones (difference is less than 1 dB/m at short wavelengths). This demonstrates that the multiple casting steps implemented in the fabrication of the preform have little effect on the final optical losses of the fiber. This is because Ge10As22Se68 glass composition does not crystallize upon the various heating steps needed to reach small-core fibers. This glass composition is consequently suitable for preparing small-core MOF with single-mode propagation and low optical losses.
5. Conclusion
Our objective was to obtain a microstructured optical fiber for telecom applications with the following requirements: a high nonlinear refractive index, a core diameter under 5 µm, optical losses less than 1 dB/m and single mode propagation. In previous work, small-core AsSe single-mode fiber was obtained with optical losses as high as 15 dB/m. Other AsSe fibers were obtained with lower losses but they presented a multimode light guidance. In this work, evolution of optical transmission in a single-index As38Se62 glass fiber as a function of temperature has been investigated. It was shown that, after a first drawing operation, this glass crystallizes upon heating above Tg. Thus, this composition cannot undergo the two drawing operations necessary to obtain fibers with a core less than 5 µm in diameter. Thus, we have selected a new glass composition, Ge10As22Se68. Changes in transmission of a GeAsSe single index fiber as a function of temperature have been also investigated and no crystallization or additional losses were observed. Thus, this composition was selected to elaborate small core MOF. Fibers presenting single mode propagation and losses equal to 1 dB/m at 1.55 µm have been thus obtained. Besides, in a 6.5 µm core fiber, the optical losses have been measured below 1.5 dB/m in the 1.5 – 4 µm spectra range, with a minimum around 0.7 dB/m at 2,5 µm.
Acknowledgments
The authors acknowledge the French Délégation Générale pour l’Armement for financial support.
References and links
1. J. A. Savage and S. Nielsen, “Chalcogenide glasses transmitting in the infrared between 1 and 20 μ — a state of the art review,” Infrared Phys. 5(4), 195–204 (1965). [CrossRef]
2. J. M. Harbold, F. O. Ilday, F. W. Wise, and B. G. Aitken, “Highly Nonlinear Ge-As-Se and Ge-As-S-Se Glasses for All-Optical Switching,” IEEE Photon. Technol. Lett. 14(6), 822–824 (2002). [CrossRef]
3. P. Houizot, C. Boussard-Plédel, A. J. Faber, L. K. Cheng, B. Bureau, P. A. Van Nijnatten, W. L. M. Gielesen, J. Pereira do Carmo, and J. Lucas, “Infrared single mode chalcogenide glass fiber for space,” Opt. Express 15(19), 12529–12538 (2007). [CrossRef] [PubMed]
4. L. Fu, M. Rochette, V. Ta’eed, D. Moss, and B. Eggleton, “Investigation of self-phase modulation based optical regeneration in single mode As2Se3 chalcogenide glass fiber,” Opt. Express 13(19), 7637–7644 (2005). [CrossRef] [PubMed]
5. M. El-Amraoui, J. Fatome, J. C. Jules, B. Kibler, G. Gadret, C. Fortier, F. Smektala, I. Skripatchev, C. F. Polacchini, Y. Messaddeq, J. Troles, L. Brilland, M. Szpulak, and G. Renversez, “Strong infrared spectral broadening in low-loss As-S chalcogenide suspended core microstructured optical fibers,” Opt. Express 18(5), 4547–4556 (2010). [CrossRef] [PubMed]
6. J. S. Sanghera, I. D. Aggarwal, L. B. Shaw, C. M. Florea, P. Pureza, V. Q. Nguyen, F. Kung, and I. D. Aggarwal, “Nonlinear properties of chalcogenide glass fibers,” J. Optoelectron. Adv. Mater. 8(6), 2148–2155 (2006).
7. D.-I. Yeom, E. C. Mägi, M. R. E. Lamont, M. A. F. Roelens, L. Fu, and B. J. Eggleton, “Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires,” Opt. Lett. 33(7), 660–662 (2008). [CrossRef] [PubMed]
8. P. A. Thielen, L. B. Shaw, P. C. Pureza, V. Q. Nguyen, J. S. Sanghera, and I. D. Aggarwal, “Small-core As-Se fiber for Raman amplification,” Opt. Lett. 28(16), 1406–1408 (2003). [CrossRef] [PubMed]
9. M. Duhant, W. Renard, G. Canat, T. N. Nguyen, F. Smektala, J. Troles, Q. Coulombier, P. Toupin, L. Brilland, P. Bourdon, and G. Renversez, “Fourth-order cascaded Raman shift in AsSe chalcogenide suspended-core fiber pumped at 2 μm,” Opt. Lett. 36(15), 2859–2861 (2011). [CrossRef] [PubMed]
10. A. Tuniz, G. Brawley, D. J. Moss, and B. J. Eggleton, “Two-photon absorption effects on Raman gain in single mode As2Se3 chalcogenide glass fiber,” Opt. Express 16(22), 18524–18534 (2008). [CrossRef] [PubMed]
11. O. P. Kulkarni, C. Xia, D. J. Lee, M. Kumar, A. Kuditcher, M. N. Islam, F. L. Terry, M. J. Freeman, B. G. Aitken, S. C. Currie, J. E. McCarthy, M. L. Powley, and D. A. Nolan, “Third order cascaded Raman wavelength shifting in chalcogenide fibers and determination of Raman gain coefficient,” Opt. Express 14(17), 7924–7930 (2006). [CrossRef] [PubMed]
12. R. E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Large Raman gain and nonlinear phase shifts in high-purity As2Se3 chalcogenide fibers,” J. Opt. Soc. Am. B 21(6), 1146–1155 (2004). [CrossRef]
13. J. Nishii, T. Yamashita, and T. Yamagishi, “Chalcogenide glass fiber with a core-cladding structure,” Appl. Opt. 28(23), 5122–5127 (1989). [CrossRef] [PubMed]
14. J. C. Knight, T. A. Birks, P. S. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21(19), 1547–1549 (1996). [CrossRef] [PubMed]
15. X. Feng, A. K. Mairaj, D. W. Hewak, and T. M. Monro, “Nonsilica Glasses for Holey Fibers,” J. Lightwave Technol. 23(6), 2046–2054 (2005). [CrossRef]
16. T. M. Monro, Y. D. West, D. W. Hewak, N. G. R. Broderick, and D. J. Richardson, “Chalcogenide holey fibres,” Electron. Lett. 36(24), 1998–2000 (2000). [CrossRef]
17. L. Brilland, F. Smektala, G. Renversez, T. Chartier, J. Troles, T. Nguyen, N. Traynor, and A. Monteville, “Fabrication of complex structures of Holey Fibers in Chalcogenide glass,” Opt. Express 14(3), 1280–1285 (2006). [CrossRef] [PubMed]
18. J. Le Person, F. Smektala, T. Chartier, L. Brilland, T. Jouan, J. Troles, and D. Bosc, “Light guidance in new chalcogenide holey fibres from GeGaSbS glass,” Mater. Res. Bull. 41(7), 1303–1309 (2006). [CrossRef]
19. F. Désévédavy, G. Renversez, L. Brilland, P. Houizot, J. Troles, Q. Coulombier, F. Smektala, N. Traynor, and J.-L. Adam, “Small-core chalcogenide microstructured fibers for the infrared,” Appl. Opt. 47(32), 6014–6021 (2008). [CrossRef] [PubMed]
20. L. Brilland, J. Troles, P. Houizot, F. Desevedavy, Q. Coulombier, G. Renversez, T. Chartier, T. N. Nguyen, J.-L. Adam, and N. Traynor, “Interfaces impact on the transmission of chalcogenides photonic crystal fibres,” J. Ceram. Soc. Jpn. 116(1358), 1024–1027 (2008). [CrossRef]
21. Q. Coulombier, L. Brilland, P. Houizot, T. Chartier, T. N. N’guyen, F. Smektala, G. Renversez, A. Monteville, D. Méchin, T. Pain, H. Orain, J.-C. Sangleboeuf, and J. Trolès, “Casting method for producing low-loss chalcogenide microstructured optical fibers,” Opt. Express 18(9), 9107–9112 (2010). [CrossRef] [PubMed]
22. S. D. Le, D. M. Nguyen, M. Thual, L. Bramerie, M. Costa e Silva, K. Lenglé, M. Gay, T. Chartier, L. Brilland, D. Méchin, P. Toupin, and J. Troles, “Efficient four-wave mixing in an ultra-highly nonlinear suspended-core chalcogenide As38Se62 fiber,” Opt. Express 19(26), B653–B660 (2011). [CrossRef] [PubMed]
23. M. D. Nguyen, S. D. Le, L. Brilland, Q. Coulombier, J. Troles, D. Méchin, T. Chartier, and M. Thual, “Demonstration of a low loss and ultra highly nonlinear AsSe suspended core chalcogenide fiber,” in ECOC (Torino, 2010), pp. 1–3.
24. D. Lezal, J. Pedlikova, J. Gurovic, and R. Vogt, “The preparation of chalcogenide glasses in chlorine reactive atmosphere,” Ceramics-Silikàty 40(2), 55–59 (1996).
25. W. A. King, A. G. Clare, and W. C. Lacourse, “Laboratory preparation of highly pure As2Se3 glass,” J. Non-Cryst. Solids 181(3), 231–237 (1995). [CrossRef]
26. I. D. Aggarwal, P. C. Pureza, F. H. Kung, J. S. Sanghera, and V. Q. Nguyen, “Very large temperature-induced absorptive loss in high Te-containing chalcogenide fibers,” J. Lightwave Technol. 18(10), 1395–1401 (2000). [CrossRef]
27. V. Q. Nguyen, J. S. Sanghera, P. C. Pureza, and I. D. Aggarwal, “Effect of heating on the optical loss in the As-Se glass fiber,” J. Lightwave Technol. 21(1), 122–126 (2003). [CrossRef]
28. V. Q. Nguyen, J. S. Sanghera, F. H. Kung, I. D. Aggarwal, and I. K. Lloyd, “Effect of temperature on the absorption loss of chalcogenide glass fibers,” Appl. Opt. 38(15), 3206–3213 (1999). [CrossRef] [PubMed]
29. G. E. Snopatin, V. S. Shiryaev, V. G. Plotnichenko, E. M. Dianov, and M. F. Churbanov, “High-Purity Chalcogenide Glasses for Fiber Optics,” Inorg. Mater. 45(13), 1439–1460 (2009). [CrossRef]
30. A. Prasad, C.-J. Zha, R.-P. Wang, A. Smith, S. Madden, and B. Luther-Davies, “Properties of GexAsySe1-x-y glasses for all-optical signal processing,” Opt. Express 16(4), 2804–2815 (2008). [CrossRef] [PubMed]
31. S. D. Le and D. M. Nguyen, “42.7 Gbit/s RZ-33% Wavelength Conversion in a Chalcogenide Microstructured Fiber,” in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2012), paper OTh4H.4.
32. T. P. White, B. T. Kuhlmey, R. C. McPhedran, D. Maystre, G. Renversez, C. M. de Sterke, and L. C. Botten, “Multipole method for microstructured optical fibers. I. Formulation,” J. Opt. Soc. Am. B 19(10), 2322–2330 (2002). [CrossRef]
33. B. T. Kuhlmey, T. P. White, G. Renversez, D. Maystre, L. C. Botten, C. M. de Sterke, and R. C. McPhedran, “Multipole method for microstructured optical fibers. II. Implementation and results,” J. Opt. Soc. Am. B 19(10), 2331–2340 (2002). [CrossRef]
