Fabrication and Characterization of Solution Doped ... - IEEE Xplore

Proc. of 2014 IEEE 5th International Conference on Photonics (ICP), Kuala Lumpur, 2-4 Sept. 2014

Fabrication and Characterization of Solution Doped Gallium and Barium Preforms S.Z. Muhd Yassin, M.I. Zulkifli, K.A. Mat-Sharif, M.H. Safar

Nasr Y.M. Omar, S. M. Aljamimi, Z. Yusoff, H.A. Abdul-Rashid

Photonics Laboratory Telekom Malaysia Research and Development Cyberjaya, Selangor, Malaysia [email protected]

Faculty of Engineering Multimedia University Cyberjaya, Selangor, Malaysia

Abstract—using standard MCVD with solution doping technique. The maximum concentrations of dopants that could be incorporated into silica were found to be 1.4 and 3.0 mol% for barium oxide and gallium oxide, respectively. At these concentrations, the final preforms showed reduced transparency in the core region indicating phase separation. Both of these dopants behavior in silica was discussed and compared with aluminum oxide being the typical dopant used in rare earth doped-silica fibers. The alumina-doped preform was observed to show opacity in the core region when the concentration of aluminum oxide was 10 mol%. Index Terms—Solution doping; rare earth; fiber fabrication

I. I NTRODUCTION Silica glass has been an outstanding host for optical fibers in communication technology due to its robustness, stable chemical properties and cheap productions. In principle, silica fibers can be doped with any network former/modifier materials [1]. The addition of these materials to silica changes its structure, which in turn alters its optical, mechanical, and chemical properties. To date, various dopants have been incorporated into silica fibers in order to meet numerous applications requirements. For instance, germanium is doped into silica to increase the refractive index of the core for typical low loss transmission fibers [2]. Other dopant materials have not matched the low loss performance of germanium dopedsilica even though several attempts have been made using alkali and alkaline earth metals [3]. Nevertheless, these attempts have shed light into other properties such as scattering, absorptions and glass composition that are beneficial for other applications. In rare earth (RE) doped fibers, glass modifiers are used to enhance the solubility of RE ions in silica and to reduce RE ions clustering [4]. Since RE precursors exhibit low vapor pressure at room temperature, the fabrication of RE dopedsilica fibers involves an add-on platform to the standard MCVD method such as solution doping and vapor phase techniques [5], [6]. Using this additional platform, the modifier precursor which exists in a similar phase as the RE precursor can be incorporated simultaneously with the RE ions. Aluminum has been successfully used in erbium-doped fiber amplifiers (EDFA) [7] as well as in various types of laser fibers (e.g., ytterbium, thulium and neodymium doped fibers) [8]. c 978-1-4799-4883-3/14/$31.00 2014 IEEE

It is a common knowledge that RE ions shows different spectroscopic properties such as absorption/emission crosssection and fluorescence lifetime in different glass/modifier environments [9]. This encourages the search for other types of modifiers that can further improve the performance of RE ions. This work focuses on the fabrication and characterization of gallium and barium doped-silica preforms for RE doped fiber applications. Using solution-doping technique with MCVD, the maximum amount of dopant that can be incorporated into silica was determined for both materials. The behavior of these doped glasses was compared with standard aluminum dopedsilica preforms. II. M ETHODOLOGY A. Preform Fabrication A standard MCVD with solution-doping technique [5] was used for the fabrication of three preforms each doped separately with aluminum, gallium and barium. Two passes of silica soot were deposited on the walls of a 25/19 (OD/ID) Heraeus F300 quartz substrate tube at optimized temperature of 1700 and 1750◦ C for first and second pass respectively. Three near aqueous solutions containing aluminum chloride (AlCl3 .6H2 O), barium chloride (BaCl2 .H2 O) and gallium nitrate (Ga(NO3 )3 .H2 O) were prepared for the solution doping process. The aqueous solution concentration is chosen near the saturation points. The details of each solution and soaking process are listed in Table I. B. Preform Characterization The refractive index profile (RIP) of each preform was measured using Photon Kinetics PK 104 refractive index profiler. The measurement was done transversely along 16 cm of each preform with a centimeter intervals. Dopants concentrations were determined by energy-dispersive X-ray spectroscopy (EDS). EDS analyses were carried out on polished metalcoated preform slices having a thickness of 5 mm. The metal coating was used to minimize the charging effect during EDS analysis. EDS analyses for dopants profiles were performed at approximately the same axial positions as the RIP scans in

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order to find the correlation between the concentration of the dopant and the refractive index difference. TABLE I: Dopant AlCl3 .6H2 O BaCl2 .H2 O Ga(NO3 )3 .H2 O

Concentration (M) 3.35 1.45 2.45

Soaking time (min) 90 90 90

III. R ESULTS AND D ISCUSSIONS Figure 1 illustrates the RIPs of the fabricated preforms at one axial position. As can be observed from the figure, the gallium doped preform exhibits large burn-off in the central core region indicating that Ga2 O3 suffers significant vaporization during the collapse stage. It seems that the very high collapse temperatures are also high enough to vaporize appreciable amounts of Ga2 O3 as Ga2 O. The barium doped preform, however, shows little burn-off in the central core region which indicates that BaO experiences less volatilization than Ga2 O3 . On the other hand, the aluminum doped preform shows no signs of Al2 O3 vaporization which is consistent with previous reports [3], [7]. It is worth notifying the slight difference is observed in the core thickness for each dopant type. The thickness obtained for all the preforms is an interplay between the concentration and type of dopant inside the core. Figure 1 and 4 shows that the largest core is obtained by BaO followed by Al2 O3 and later Ga2 O3 doped preform.

high temperatures encountered during sintering and collapse may then be enough to cause some metal diffusion into silica and hence producing regions of metal-rich silicates. The slow cooling of the produced materials as the burner moves away from these regions promotes solidification to a crystalline phase causing opacity of the core [10]. Another explanation is the formation of metal-rich silicates by phase separation. This may take place when the binary oxide mixture (MO/SiO2 ) encounters a suitable temperature during sintering or collapse provided that the metal concentration is high enough [10]. The highest opacity was observed for the aluminum-doped preform followed by barium and then gallium. The opacity of the core region is known to change the optical properties and chemical durability of glass [11]. Figure 3(d) shows an example of light scattering caused by the Al-rich opaque core. The scattering effect illuminates the core when injected with a 605 nm red laser at the preform’s end. The first few centimeters of the core are transparent (cf. Figure 3(c)) and show no sign of light scattering (cf. Figure 3(d)).

Fig. 2: Longitudinal refractive index of Ga, Al and Ba dopedsilica preforms

Fig. 1: Refractive index profiles of gallium, barium and aluminum dopedsilica preforms fabricated using nearly saturated salt solutions Figure 2 shows the longitudinal refractive index profiles of the fabricated preforms. The average index difference values of about 0.0100, 0.0088, and 0.0149 were obtained for barium, gallium and aluminum doped-silica preforms, respectively. Figure 3 illustrates the appearance of all fabricated preforms. As can be seen from the figure, all preforms show opaque core regions with distinctive colors for each dopant material (i.e. milky white for alumina, light blue for barium and purplish blue for gallium). This may be attributed to the deposition of metal-rich material in the tiny spaces between silica soot particles during the solution doping process. The

Fig. 3: Appearance of the gallium (a), barium (b) and aluminum (c) dopedsilica preforms; and (d) the effect due to the injection of a 605 nm laser at the preform’s end Figure 4 portrays the concentrations of barium and gallium oxides obtained from EDS measurements; and were found to be 1.4 and 3.0 mol%, respectively. It should be pointed out

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that such relatively low concentrations were enough to result in opacity of the core. The immiscibility zone for the binary system of silica and barium oxide lies between near zero to 30 mol% of barium oxide [11]. It seems that the nature of the MCVD-solution doping process limits the dopant content to the lower end of the immiscibility zone owing to the soot and solution characteristics [10]. However, barium oxide having concentration >30 mol% has been successfully doped into silica using other methods [12]. To the best of our knowledge, there are no reports in the literature concerning the phase diagram of the Ga2 O3 /SiO2 binary system.

Fig. 4: Concentrations of barium and gallium oxides as determined by EDS The concentration of Al2O3 in the core can be estimated from the knowledge that the refractive index of silica changes by 1.7×10−3 per mol% of alumina [13]. The highest index difference for Al-doped preform is 0.018 implying an aluminum oxide concentration of 10.5 mol%. This indicates that solution-doping method is suitable for doping high amounts of alumina into silica preforms.

R EFERENCES [1] S. Sudo et al., Optical fiber amplifiers: materials, devices, and applications. Artech house Boston and London, 1997. [2] J. Palais, Fiber Optic Communications, 5th ed., 1998. [3] D. S. Homa, “Synthesis and characterization of alkali/alkaline earthdoped fiber optic silica preforms,” Ph.D. dissertation, New Brunswick, 2001. [4] T. S. Izumitani, “Optical glass,” Optical Glass by TS Izumitani Tokyo: Kyoritsu Shuppan, Ltd., 1986, vol. 1, 1986. [5] S. Aljamimi, Z. Yusoff, H. Abdul-Rashid, K. Anuar, S. Muhd-Yasin, M. Zulkifli, S. Hanif, and N. Tamchek, “Aluminum doped silica preform fabrication using MCVD and solution doping technique: Effects of various aluminum solution concentrations,” in Photonics (ICP), 2013 IEEE 4th International Conference on. IEEE, 2013, pp. 268–271. [6] J. Townsend, S. Poole, and D. Payne, “Solution-doping technique for fabrication of rare-earth-doped optical fibres,” Electronics Letters, vol. 23, no. 7, pp. 329–331, 1987. [7] R. P. Tumminelli, B. C. McCollum, and E. Snitzer, “Fabrication of highconcentration rare-earth doped optical fibers using chelates,” Lightwave Technology, Journal of, vol. 8, no. 11, pp. 1680–1683, 1990. [8] S. Poole, “Fabrication of al2 o3 co-doped optical fibres by a solutiondoping technique,” in ECOC, vol. 88, no. 292, 1988, pp. 433–436. [9] J. Nilsson, W. Clarkson, R. Selvas, J. Sahu, P. Turner, S.-U. Alam, and A. Grudinin, “High-power wavelength-tunable cladding-pumped rareearth-doped silica fiber lasers,” Optical Fiber Technology, vol. 10, no. 1, pp. 5–30, 2004. [10] D. A. Simpson, “Spectroscopy of thulium doped silica glass,” Ph.D. dissertation, Melbourne, 2008. [11] F. Tang, P. McNamara, G. Barton, and S. Ringer, “Multiple solutiondoping in optical fibre fabrication II–rare-earth and aluminium codoping,” Journal of Non-Crystalline Solids, vol. 354, no. 15, pp. 1582– 1590, 2008. [12] V. McGahay and M. Tomozawa, “The origin of phase separation in silicate melts and glasses,” Journal of non-crystalline solids, vol. 109, no. 1, pp. 27–34, 1989. [13] P. Dragic, C. Kucera, J. Furtick, J. Guerrier, T. Hawkins, and J. Ballato, “Brillouin spectroscopy of a novel baria-doped silica glass optical fiber,” Optics express, vol. 21, no. 9, pp. 10 924–10 941, 2013.

IV. C ONCLUSION The maximum concentrations of gallium and barium oxides that can be incorporated into silica using MCVD-solution doping technique has been determined. It has been found that Al is still being the best available material for achieving higher dopant concentrations. For RE doped fiber applications, the low phonon energy of the environment surrounding the RE plays a major role in quenching the non-radiative decay [9]. The phonon energy of the environment is influenced by the number as well as the mass of the metal ions doped inside the core region. From the results of this study, there seems to be a tradeoff between these two since gallium and barium which are heavier than aluminum cannot be doped in similar amounts as aluminum. The optical performance of such doped preforms has yet to be established. ACKNOWLEDGEMENT We would like to thank Telekom Malaysia for providing us the fund (RDTC 130825/26) for the research work done in this article 118