- Email: [email protected]
Characterization of broadband amplified spontaneous emission of erbium-doped tellurite fiber with D-shape cladding
Materials Letters 58 (2004) 3532 – 3535 www.elsevier.com/locate/matlet
Characterization of broadband amplified spontaneous emission of erbium-doped tellurite fiber with D-shape cladding Junjie Zhang*, Shixun Dai, Shunguang Li, Shiqing Xu, Guonian Wang, Lili Hu Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China Received 27 November 2003; received in revised form 24 May 2004; accepted 28 May 2004 Available online 2 August 2004
Abstract In this presentation, we report the results of spectral characteristics of amplified spontaneous emission (ASE) from a newly Er3+-doped tellurite fiber with D-shape cladding. When pumped at 980 nm, an erbium ASE source that has nearly a flatten FWHM bandwidth of 100 nm is obtained in the D-shape cladding erbium-doped tellurite fiber with 30–60 cm length. The changes in ASE with regard to pumping power and fiber lengths were measured. Output power up to 2.0 mW is obtained with a total pump power of 660 mW. D 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 42.81Bm; 91.60.Ki Keywords: Broadband ASE; Erbium-doped tellurite fiber; D-shape cladding
1. Introduction Because of its excellent optical and chemical properties, tellurite fibers are now promising candidates for use in fabricating novel optical amplifiers as a means of extending the transmission bandwidth of optical fibers beyond the range available from conventional Er3+-doped silica fiber, such as erbium-doped tellurite fiber amplifier (EDTFA) for the C-band and L-band [1–3], thulium-doped fiber amplifier (TDFA) for the S-band [4,5], and praseodymium-doped fiber amplifiers (PDFA) for the 1.3-Am band [6]. These fibers can offer broad amplification bandwidth and high radiative transition efficiencies [1–6]. Methods for broadening as well as flattening the amplification bandwidth of Er3+-doped tellurite fibers (EDTF) are very important in increasing the transmission capacity of wavelength-division multiplexed telecommunication systems. Therefore, great efforts have been focused mostly on amplification characteristics of tellurite-based amplifiers [1–6].
* Corresponding author. Tel.: +86 21 59914297; fax: +86 21 59914516. E-mail address: [email protected] (J. Zhang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.083
On the other hand, erbium-doped fibers (EDF) have recently been the focus of intensive research in ASE sources as well as optical amplifiers [7–9]. In general, an incoherent broadband optical source with low spectral ripples and high power intensity in ASE spectrum is required for better performance. The ASE from Er ions in silica glass covers the spectral range of 1520–1560 nm for the transition from 4I13/2to 4I15/2. Its effective bandwidth, however, could be narrower due to nonuniform nature of the ASE and external filters are often required for spectral uniformity. In order to obtain broadband output spectrum of ASE ranging from C-band to L-band in Er3+-doped silica fiber, several approaches have been proposed [7–10]. However, these configurations seem a little complicated because its experimental setups are always based on two stages or two-pump sources (LDs). EDTF has much better gain flatness and broader amplification range compared with silica-based fiber [1–3]. Therefore, EDTF as a kind of Er3+-doped fiber can be also used as ASE sources which can make the whole configuration simple [11]. For example, Thorlabs has recently produced a broadband ASE source with bandwidth up to 80 nm, which was constructed from a single Er3+-tellurite fiber pumped by a single LD. To our knowledge, few experimental inves-
J. Zhang et al. / Materials Letters 58 (2004) 3532–3535
tigations on the characterization of ASE from Er3+-doped tellurite fibers have been carried out. In order to couple high-power laser diode (LD) pump light effectively, optimization of fiber cross-section shapes and parameters is very important and necessary. In this letter, we report the successful fabrication of new Er3+doped tellurite glass fiber (EDTF) with D-shape cladding and also present the amplified spontaneous emission (ASE) spectrum of this kind of fiber with regard to pumping power and fiber lengths.
2. Experimental procedure The core and cladding glass compositions were based on TeO2–ZnO–La2O3–Li2O glass system. Er3+-doped tellurite glass was prepared by a conventional melt-quenching method. The anhydrous powders of TeO2, ZnO, La2O3, Li2O, and Er2O3 with more than 99.9% purity were used as raw materials. Accurately weighted 25-g batches were thoroughly mixed and moved into platinum crucibles. The batches were melted in the temperature range between 800 and 850 8C for 15 min in an electric furnace under air atmosphere. The melts were cast into preheated brass molds and then the obtained glasses were annealed at their glasstransition temperatures determined by differential thermal analysis (DTA). Fibers were fabricated from the same glasses and the fiber preform was prepared by the suction casting technology [12]. The procedure is as follows. The cladding glass melt was cast in a stainless-steel mold, which had been preheated to just below the glass-transition temperature, with a hole in the bottom; the core glass melt then was immediately cast above the cladding melt. By raising the mold, a part of the cladding glass melt flowed out of the bottom and a part of core glass melt was filled into the center of the cladding glass. A preform with different core/ cladding ratios can be obtained by controlling the time interval. Then the preformed rod, together with its mold, was annealed near the glass-transition temperature for about 2 h. The core glass is surrounded by a similar tellurite glass, which has lower refractive index. The core is uniformly
Fig. 1. Cross section of Er3+-doped tellurite fiber with D shape cladding.
3533
Fig. 2. Experimental setup.
doped with 5600 ppm Er2O3 having a fluorescence decay time of 3.3 ms. The cladding was coated with a lowrefractive index (n=1.52) polymer (DSM) outer cladding. The resulting fiber had a cladding diameter of 125 Am, a core diameter Dc20 Am, and a numerical aperture (NA) of 0.2. Fig. 1 shows the cross section of fiber. The average background loss was measured using the cutback method and was found to be 3.5 dB/m at 1310 nm. The ASE measuring experimental setup is shown in Fig. 2. A diode laser (Limo Fb101c765, Germany) capable of providing 15 W of pump light at 980 nm which has fibercoupled output is employed as the pump sources. The spot size at the tail fiber of the pump source has a diameter of 200 Am. The beam of the pumping laser is coupled into the EDTF using a precise positioning stage (M-561D, Newport); a high reflectivity mirror at 980 nm was located at the end of fiber. The fiber ends were terminated with small-size metal amalgamation (SMA) connectors and the end faces were polished. The output power of ASE spectrum from EDTF was measured with a power meter (Lxlightwave 0MM-681013). The ASE spectra of fibers were measured with Spectrometer (Jobin Yvon Triax550).
3. Results and discussion Fig. 3 shows the output ASE spectrum of a 30-cm length Er3+-doped tellurite fiber compared with the fluorescence spectrum of Er3+-doped core glass (size: 10101 mm).
Fig. 3. Comparison of ASE spectrum of the tellurite fiber and fluorescence spectrum of core glass.
3534
J. Zhang et al. / Materials Letters 58 (2004) 3532–3535
The shape of ASE spectrum in fiber emission is significantly broader and flattener than that in glass, especially at L-band range. The ASE band of the proposed fiber, at least 150 nm wide that is much broader than that of silica-based, covers over C-band and L-band from 1450 to 1650 nm. The full width at half maximum (FWHM) bandwidth is about 100 nm in fiber, which is nearly two times than that of core glass (55 nm). The ASE spectrum in fiber shows a peak emission near 1557-nm wavelength, whereas spectrum in glass shows higher emission in 1530 nm which corresponds to the transition between the lowest stark levels of the 4I13/2 and 4I15/2. From Fig. 3, we can see that broad and flatten bandwidth ASE source can be obtained by using Er3+-doped tellurite glass fiber with D-shape cladding. By the way, the total output power from ASE sources of 30-cm length fiber is measured to be about 2.0 mW when the pump power is around 460 mW. These values correspond to the launched powers, but not exact input power into the fiber, because of the high coupling losses between the silica and tellurite fibers. The 2.0-mW out power is less than that obtained from commercial ELED sources or rare-earth-doped silicabased fiber ASE. Note that the very only simple pumping scheme was adopted here, and the coupling loss is estimated to be very high. We anticipate significant increases in intensity with optimization of the pumping configuration as well as the fiber parameters. The ASE spectra from fiber with 50-cm length were measured at the different pump power. The results are shown in Fig. 4. Three spectra exhibit similar features. However, these features were noticed to shift to longer wavelengths with increasing fiber length. Meanwhile, the resonance lines were observed at higher pump power of 660 mW. At this condition, the ASE spectrum abruptly becomes narrow. This phenomenon is clearly due to successive back reflection from the ends of tellurite fiber sample, the large refractive index difference between the tellurite fiber (c1.9) and the spliced silica fibers enhancing Fresnel reflection [9].
Fig. 5. The ASE spectra of Er3+-doped tellurite fibers with the fiber length of 30, 45 and 55 cm, respectively.
Samples of three different lengths, from 30 to 55 cm, were tested under the same pump power of 460 mW. The results are shown in Fig. 5. All the output ASE spectra are found to exhibit very similar features. However, these features were also noticed to shift longer wavelengths with increasing fiber length. The shapes of ASE spectra apparently become broad and flatten by increasing the fiber length. This is attributed to typical characteristics of a threelevel system, mainly the re-absorption of emission by the ground state. We can expect that fibers with longer length at the proposed Er3+ doping concentration are more suitable to achieve a flatter broadband ASE source. For rare-earth-doped silica fiber, because the modified chemical-vapor deposition (MCVD) method is employed to fabricate the silica fibers, even the losses of the rare-earthdoped silica fiber can be reduced to a very low value. Because of the low loss, it is possible to match the fiber length to the optimum absorption of the pump light. On the contrary, for our Er: tellurite-glass fiber with D-shape cladding, because the oxides used as the raw materials in our experiments were not superpure, and because the suction casting technique was employed for fabricating the fibers, the fibers are unable to avoid being polluted during the process of the fiber fabrication. Therefore, it is not surprising that our fibers have relatively high losses. Now, such improvement as using superpure starting materials and so on is being applied to decrease the losses of the fiber. On the other hand, the coupling efficiency in our experiment was only about 20% between the laser diode and the fiber. We anticipate that the total losses of the fiber can be reduced from ~3.5 to 0.1–0.5 dB/m, and the coupling efficiency of the fiber source can reach 30–50%.
4. Conclusion Fig. 4. The ASE spectra of Er3+-doped tellurite fibers for various pumping power.
In conclusion, we have successfully fabricated Er3+doped tellurite fibers with D shape cladding and observed a
J. Zhang et al. / Materials Letters 58 (2004) 3532–3535
very broad amplification bandwidth when pumped with 980-nm LD. An erbium ASE source that has nearly a flatten FWHM bandwidth of 100 nm is realized in the D shape cladding erbium-doped tellurite fiber with 30–60 cm length. The ASE source with 2 mW of output power is obtained under the pump power of 460 mW. The very broad amplification bandwidth of the erbium ASE spectra indicates that such kind of fiber source presented here is not only worthy of investigation, but also a very promising candidate for use in fabricating novel optical amplifiers.
References [1] A. Mori, Y. Ohishi, S. Sudo, Electron. Lett. 23 (1997) 863. [2] H. Ono, A. Mori, K. Shikano, M. Shimizu, IEEE Photonics Technol. Lett. 14 (2002) 1070.
3535
[3] H. Ono, A. Mori, K. Shikano, M. Shimizu, IEEE Photonics Technol. Lett. 14 (2002) 1073. [4] N. Mira, S. Shaoxiong, J. Animsh, Appl. Opt. 39 (2000) 4979. [5] S. Shen, M. Naftaly, A. Jha, OFC TuQ6-1, 2001. [6] S. Man, H. Liu, Y. Wong, E. Pun, P. Chung, LEOS 1 (1998) 196. [7] S. Aozaso, T. Sakamoto, T. Kanamori, Hoshino, K. Kobayashi, M. Shimizu, IEEE Photonics Technol. Lett. 12 (2001) 1331. [8] H. Jeong, K. Oh, S.R. Han, T. Morse, Chem. Phys. Lett. 367 (2003) 507. [9] W. Huang, H. Tam, P. Wai, X. Dong, M. Hai, J. Xie, Chin. Phys. Lett. 19 (2002) 1307. [10] P. Espindola, G. Ales, J. Park, T. Strasser, Electron. Lett. 34 (2000) 1263. [11] S. Marjanovic, J. Toulouse, H. Jain, C. Sandmann, V. Dierolf, A. Kortan, N. Kopylov, R. Ahrens, J. Non-Cryst. 322 (2003) 311. [12] T. Kanamori, Y. Terunuma, Y. Nishida, K. Hoshino, K. Nakagawa, Y. Ohishi, S. Sudo, J. Non-Cryst. Solids 213 and 214 (1997) 121.
Characterization of broadband amplified spontaneous emission of erbium-doped tellurite fiber with D-shape cladding Junjie Zhang*, Shixun Dai, Shunguang Li, Shiqing Xu, Guonian Wang, Lili Hu Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China Received 27 November 2003; received in revised form 24 May 2004; accepted 28 May 2004 Available online 2 August 2004
Abstract In this presentation, we report the results of spectral characteristics of amplified spontaneous emission (ASE) from a newly Er3+-doped tellurite fiber with D-shape cladding. When pumped at 980 nm, an erbium ASE source that has nearly a flatten FWHM bandwidth of 100 nm is obtained in the D-shape cladding erbium-doped tellurite fiber with 30–60 cm length. The changes in ASE with regard to pumping power and fiber lengths were measured. Output power up to 2.0 mW is obtained with a total pump power of 660 mW. D 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Kf; 42.81Bm; 91.60.Ki Keywords: Broadband ASE; Erbium-doped tellurite fiber; D-shape cladding
1. Introduction Because of its excellent optical and chemical properties, tellurite fibers are now promising candidates for use in fabricating novel optical amplifiers as a means of extending the transmission bandwidth of optical fibers beyond the range available from conventional Er3+-doped silica fiber, such as erbium-doped tellurite fiber amplifier (EDTFA) for the C-band and L-band [1–3], thulium-doped fiber amplifier (TDFA) for the S-band [4,5], and praseodymium-doped fiber amplifiers (PDFA) for the 1.3-Am band [6]. These fibers can offer broad amplification bandwidth and high radiative transition efficiencies [1–6]. Methods for broadening as well as flattening the amplification bandwidth of Er3+-doped tellurite fibers (EDTF) are very important in increasing the transmission capacity of wavelength-division multiplexed telecommunication systems. Therefore, great efforts have been focused mostly on amplification characteristics of tellurite-based amplifiers [1–6].
* Corresponding author. Tel.: +86 21 59914297; fax: +86 21 59914516. E-mail address: [email protected] (J. Zhang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.083
On the other hand, erbium-doped fibers (EDF) have recently been the focus of intensive research in ASE sources as well as optical amplifiers [7–9]. In general, an incoherent broadband optical source with low spectral ripples and high power intensity in ASE spectrum is required for better performance. The ASE from Er ions in silica glass covers the spectral range of 1520–1560 nm for the transition from 4I13/2to 4I15/2. Its effective bandwidth, however, could be narrower due to nonuniform nature of the ASE and external filters are often required for spectral uniformity. In order to obtain broadband output spectrum of ASE ranging from C-band to L-band in Er3+-doped silica fiber, several approaches have been proposed [7–10]. However, these configurations seem a little complicated because its experimental setups are always based on two stages or two-pump sources (LDs). EDTF has much better gain flatness and broader amplification range compared with silica-based fiber [1–3]. Therefore, EDTF as a kind of Er3+-doped fiber can be also used as ASE sources which can make the whole configuration simple [11]. For example, Thorlabs has recently produced a broadband ASE source with bandwidth up to 80 nm, which was constructed from a single Er3+-tellurite fiber pumped by a single LD. To our knowledge, few experimental inves-
J. Zhang et al. / Materials Letters 58 (2004) 3532–3535
tigations on the characterization of ASE from Er3+-doped tellurite fibers have been carried out. In order to couple high-power laser diode (LD) pump light effectively, optimization of fiber cross-section shapes and parameters is very important and necessary. In this letter, we report the successful fabrication of new Er3+doped tellurite glass fiber (EDTF) with D-shape cladding and also present the amplified spontaneous emission (ASE) spectrum of this kind of fiber with regard to pumping power and fiber lengths.
2. Experimental procedure The core and cladding glass compositions were based on TeO2–ZnO–La2O3–Li2O glass system. Er3+-doped tellurite glass was prepared by a conventional melt-quenching method. The anhydrous powders of TeO2, ZnO, La2O3, Li2O, and Er2O3 with more than 99.9% purity were used as raw materials. Accurately weighted 25-g batches were thoroughly mixed and moved into platinum crucibles. The batches were melted in the temperature range between 800 and 850 8C for 15 min in an electric furnace under air atmosphere. The melts were cast into preheated brass molds and then the obtained glasses were annealed at their glasstransition temperatures determined by differential thermal analysis (DTA). Fibers were fabricated from the same glasses and the fiber preform was prepared by the suction casting technology [12]. The procedure is as follows. The cladding glass melt was cast in a stainless-steel mold, which had been preheated to just below the glass-transition temperature, with a hole in the bottom; the core glass melt then was immediately cast above the cladding melt. By raising the mold, a part of the cladding glass melt flowed out of the bottom and a part of core glass melt was filled into the center of the cladding glass. A preform with different core/ cladding ratios can be obtained by controlling the time interval. Then the preformed rod, together with its mold, was annealed near the glass-transition temperature for about 2 h. The core glass is surrounded by a similar tellurite glass, which has lower refractive index. The core is uniformly
Fig. 1. Cross section of Er3+-doped tellurite fiber with D shape cladding.
3533
Fig. 2. Experimental setup.
doped with 5600 ppm Er2O3 having a fluorescence decay time of 3.3 ms. The cladding was coated with a lowrefractive index (n=1.52) polymer (DSM) outer cladding. The resulting fiber had a cladding diameter of 125 Am, a core diameter Dc20 Am, and a numerical aperture (NA) of 0.2. Fig. 1 shows the cross section of fiber. The average background loss was measured using the cutback method and was found to be 3.5 dB/m at 1310 nm. The ASE measuring experimental setup is shown in Fig. 2. A diode laser (Limo Fb101c765, Germany) capable of providing 15 W of pump light at 980 nm which has fibercoupled output is employed as the pump sources. The spot size at the tail fiber of the pump source has a diameter of 200 Am. The beam of the pumping laser is coupled into the EDTF using a precise positioning stage (M-561D, Newport); a high reflectivity mirror at 980 nm was located at the end of fiber. The fiber ends were terminated with small-size metal amalgamation (SMA) connectors and the end faces were polished. The output power of ASE spectrum from EDTF was measured with a power meter (Lxlightwave 0MM-681013). The ASE spectra of fibers were measured with Spectrometer (Jobin Yvon Triax550).
3. Results and discussion Fig. 3 shows the output ASE spectrum of a 30-cm length Er3+-doped tellurite fiber compared with the fluorescence spectrum of Er3+-doped core glass (size: 10101 mm).
Fig. 3. Comparison of ASE spectrum of the tellurite fiber and fluorescence spectrum of core glass.
3534
J. Zhang et al. / Materials Letters 58 (2004) 3532–3535
The shape of ASE spectrum in fiber emission is significantly broader and flattener than that in glass, especially at L-band range. The ASE band of the proposed fiber, at least 150 nm wide that is much broader than that of silica-based, covers over C-band and L-band from 1450 to 1650 nm. The full width at half maximum (FWHM) bandwidth is about 100 nm in fiber, which is nearly two times than that of core glass (55 nm). The ASE spectrum in fiber shows a peak emission near 1557-nm wavelength, whereas spectrum in glass shows higher emission in 1530 nm which corresponds to the transition between the lowest stark levels of the 4I13/2 and 4I15/2. From Fig. 3, we can see that broad and flatten bandwidth ASE source can be obtained by using Er3+-doped tellurite glass fiber with D-shape cladding. By the way, the total output power from ASE sources of 30-cm length fiber is measured to be about 2.0 mW when the pump power is around 460 mW. These values correspond to the launched powers, but not exact input power into the fiber, because of the high coupling losses between the silica and tellurite fibers. The 2.0-mW out power is less than that obtained from commercial ELED sources or rare-earth-doped silicabased fiber ASE. Note that the very only simple pumping scheme was adopted here, and the coupling loss is estimated to be very high. We anticipate significant increases in intensity with optimization of the pumping configuration as well as the fiber parameters. The ASE spectra from fiber with 50-cm length were measured at the different pump power. The results are shown in Fig. 4. Three spectra exhibit similar features. However, these features were noticed to shift to longer wavelengths with increasing fiber length. Meanwhile, the resonance lines were observed at higher pump power of 660 mW. At this condition, the ASE spectrum abruptly becomes narrow. This phenomenon is clearly due to successive back reflection from the ends of tellurite fiber sample, the large refractive index difference between the tellurite fiber (c1.9) and the spliced silica fibers enhancing Fresnel reflection [9].
Fig. 5. The ASE spectra of Er3+-doped tellurite fibers with the fiber length of 30, 45 and 55 cm, respectively.
Samples of three different lengths, from 30 to 55 cm, were tested under the same pump power of 460 mW. The results are shown in Fig. 5. All the output ASE spectra are found to exhibit very similar features. However, these features were also noticed to shift longer wavelengths with increasing fiber length. The shapes of ASE spectra apparently become broad and flatten by increasing the fiber length. This is attributed to typical characteristics of a threelevel system, mainly the re-absorption of emission by the ground state. We can expect that fibers with longer length at the proposed Er3+ doping concentration are more suitable to achieve a flatter broadband ASE source. For rare-earth-doped silica fiber, because the modified chemical-vapor deposition (MCVD) method is employed to fabricate the silica fibers, even the losses of the rare-earthdoped silica fiber can be reduced to a very low value. Because of the low loss, it is possible to match the fiber length to the optimum absorption of the pump light. On the contrary, for our Er: tellurite-glass fiber with D-shape cladding, because the oxides used as the raw materials in our experiments were not superpure, and because the suction casting technique was employed for fabricating the fibers, the fibers are unable to avoid being polluted during the process of the fiber fabrication. Therefore, it is not surprising that our fibers have relatively high losses. Now, such improvement as using superpure starting materials and so on is being applied to decrease the losses of the fiber. On the other hand, the coupling efficiency in our experiment was only about 20% between the laser diode and the fiber. We anticipate that the total losses of the fiber can be reduced from ~3.5 to 0.1–0.5 dB/m, and the coupling efficiency of the fiber source can reach 30–50%.
4. Conclusion Fig. 4. The ASE spectra of Er3+-doped tellurite fibers for various pumping power.
In conclusion, we have successfully fabricated Er3+doped tellurite fibers with D shape cladding and observed a
J. Zhang et al. / Materials Letters 58 (2004) 3532–3535
very broad amplification bandwidth when pumped with 980-nm LD. An erbium ASE source that has nearly a flatten FWHM bandwidth of 100 nm is realized in the D shape cladding erbium-doped tellurite fiber with 30–60 cm length. The ASE source with 2 mW of output power is obtained under the pump power of 460 mW. The very broad amplification bandwidth of the erbium ASE spectra indicates that such kind of fiber source presented here is not only worthy of investigation, but also a very promising candidate for use in fabricating novel optical amplifiers.
References [1] A. Mori, Y. Ohishi, S. Sudo, Electron. Lett. 23 (1997) 863. [2] H. Ono, A. Mori, K. Shikano, M. Shimizu, IEEE Photonics Technol. Lett. 14 (2002) 1070.
3535
[3] H. Ono, A. Mori, K. Shikano, M. Shimizu, IEEE Photonics Technol. Lett. 14 (2002) 1073. [4] N. Mira, S. Shaoxiong, J. Animsh, Appl. Opt. 39 (2000) 4979. [5] S. Shen, M. Naftaly, A. Jha, OFC TuQ6-1, 2001. [6] S. Man, H. Liu, Y. Wong, E. Pun, P. Chung, LEOS 1 (1998) 196. [7] S. Aozaso, T. Sakamoto, T. Kanamori, Hoshino, K. Kobayashi, M. Shimizu, IEEE Photonics Technol. Lett. 12 (2001) 1331. [8] H. Jeong, K. Oh, S.R. Han, T. Morse, Chem. Phys. Lett. 367 (2003) 507. [9] W. Huang, H. Tam, P. Wai, X. Dong, M. Hai, J. Xie, Chin. Phys. Lett. 19 (2002) 1307. [10] P. Espindola, G. Ales, J. Park, T. Strasser, Electron. Lett. 34 (2000) 1263. [11] S. Marjanovic, J. Toulouse, H. Jain, C. Sandmann, V. Dierolf, A. Kortan, N. Kopylov, R. Ahrens, J. Non-Cryst. 322 (2003) 311. [12] T. Kanamori, Y. Terunuma, Y. Nishida, K. Hoshino, K. Nakagawa, Y. Ohishi, S. Sudo, J. Non-Cryst. Solids 213 and 214 (1997) 121.