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Metal-doped fibres for broadband emission: Fabrication with granulated oxides
Optical Materials 31 (2008) 247–251
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Metal-doped fibres for broadband emission: Fabrication with granulated oxides Martin Neff *, Valerio Romano, Willy Lüthy Institute of Applied Physics, University of Bern, CH-3012 Bern, Switzerland
a r t i c l e
i n f o
Article history: Received 6 March 2008 Received in revised form 4 April 2008 Accepted 8 April 2008 Available online 2 June 2008 PACS: 42.81.Bm 42.81.Wg 42.81.Cn 42.70. a
a b s t r a c t The goal of the present work is testing manufacturing of metal- or transition-metal-doped fibres with the technique of granulated oxides. Optical silica fibres are drawn from preforms assembled from silica tubes filled with granulated oxides of SiO2 doped with either V2O5, Cu2O, MnO, Bi2O3 or Bi2O3–Al2O3. The produced fibres are optically excited with either a N2 laser (337 nm) or an Ar+ laser (458–514.5 nm). All the investigated ions incorporated in the silica host show a broad fluorescence in the visible range between 500 nm and 600 nm with a FWHM between 100 nm and 150 nm. Bi2O3 also shows a broad fluorescence band in the near infrared range at 1425 nm with a FWHM of about 235 nm. With the admixture of Al2O3, this fluorescence band can be shifted to 1140 nm. The measured spectra are similar to published data even if other host materials than pure silica were used. The experiment shows that fibre manufacturing with granulated oxides leads to similar spectra as MCVD or Sol–Gel technique. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Metal-doped fibres Broadband emission Manufacturing of optical fibres Granulated oxides
1. Introduction Until 2005 all fibre lasers were based on rare earth-doped fibres. Rare earths are especially favourable due to their shielding of the 4f electrons resulting in a weak interaction with the host. This allows small bandwidths and high gain. Consequently, a lot of research has been made on rare earth-doped fibres. The bismuth-doped silica fibre is the first fibre laser that is not based on a rare earth-dopant [1–5]. What makes bismuth remarkable is the extent of its potential gain bandwidth extending from 1100 nm to beyond 1500 nm. As a consequence strong efforts are made to explore the potential of this laser for applications in telecommunications [6]. Based on the breakthrough with bismuth also other metalor transition-metal-doped fibres have regained much interest. Examples are vanadium [7,8], manganese [9–11] or copper [12– 14]. These fibres with unusual dopants are not commercially available. It is therefore, interesting to study possibilities of their fabrication. Recently, based on [15] a method has been developed to manufacture fibres by directly melting dry granulated oxides filled in silica tubes during the fibre drawing process [16,17]. This technique allows rapid production of doped fibres at low cost. It is not a
* Corresponding author. Tel.: +41 316318935. E-mail address: [email protected] (M. Neff). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.04.001
priori clear if this technique leads to results that are comparable with fibres produced by MCVD or Sol–Gel technique with respect to valence of the dopant and emission spectra. In our report we investigate silica fibres produced with granulated SiO2 and with either V2O5, Cu2O, MnO, Bi2O3 or Bi2O3– Al2O3. All these doped fibres are expected to emit spectra in the visible wavelength region. Manufacturing, excitation and emission properties of the fibres are described.
2. Experimental For the experiments several fibres are drawn. For the preforms silica tubes (HSQ 300) are used with an outer diameter of 21 mm and an inner diameter of 17 mm. A mixture of granulated silica with 200–400 lm grain size and oxides of the dopant is filled in the tube. The tube becomes the cladding of the drawn fibre while the mixture in the centre builds the core. The tube dimensions lead to fibres with a very large core compared to the cladding diameter. Before drawing the fibre, the preform is evacuated and heated for 1 h to 1300 °C. At 1300 °C the viscosity of silica is about 105 times higher than at the drawing temperature [18, p. 8]. Therefore, no modification of the preform geometry occurs. The preform cannot be rotated during the drawing process. We use a standard drawing tower equipped with a graphite furnace (Centorr) of 24 mm aperture. To obtain the best possible vitrification the fibre is drawn at a
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temperature of about 2000 °C. This high temperature leads to low viscosity of the silica and therefore, requires a very small drop (very small drawing force) at the beginning of the drawing process. A small drop is obtained if the preform is slowly (with 1 mm/min) inserted into the hot zone of the furnace. As soon as the weight of the drop is sufficient to overcome the viscosity of the SiO2, the drawing process starts. For our experiments only the first few meters of the fibre are used. This includes diameters from the drop with several mm down to the fibre with 100 lm. Since only few meters after the drop are needed for our investigations, most of the drawing tower facilities such as diameter measurement, coating applicator with its furnace, capstan and coiling apparatus are not used. During vitrification the granulated silica somewhat shrinks and the core-to-cladding ratio that was initially 17/21 in the preform is changed to about 5/7. In the drawn fibre this core-to-cladding ratio is maintained independent from the fibre diameter. In these fibres the losses have not been measured. But from rare-earth-doped fibres manufactured with the same technique we know that scattering losses are in the order of 1 dB/m [16]. The change of the refractive index given by the dopant in the core leads to a step index profile. For 10% aluminium in SiO2 the enhancement of the refractive index is about 10 2, but for the dopants V2O2, Cu2O, MnO, and Bi2O3 we have no information on their influence on the refractive index. 2.1. Vanadium-doped fibre The fibre is doped with 1 at% V with respect to Si. The used V2O5 has a grain size of 50–100 lm. No additional doping for wave-guiding is used. V2O5 is thermally decomposed at 1750 °C [19, pp. 4– 95]. Since fibre drawing is performed at higher temperatures we expect a mixture of different valences besides V5+. After drawing the fibre appears dark black showing also the occurrence of V valences of V4+ (deep black or blue) and/or V3+ (black) [19, pp. 4– 95]. A fibre length of 5 cm with a diameter of 600 lm is excited along the axis with the 337 nm emission of a pulsed nitrogen laser (Garching Instruments mod. SP1/II). The emission is coupled into the fibre with a Suprasil lens of 20 mm focal length. The fluorescence generated in the fibre core is measured from the side with a fibre-coupled spectrometer (AVS-USB2000). The resulting spectrum is shown in Fig. 1. The width of the fluorescence is about 125 nm at FWHM ranging from 475 nm to 600 nm. This result corresponds reasonably well to measurements performed at 77 K [7]. The measured life-
Fig. 1. Fluorescence spectrum of vanadium-doped silica fibre.
time is about 300 ls with a single exponential decay, about one order shorter than in experiments performed at 77 K [7]. In [7] a double exponential decay is described with lifetimes of s1 = 10 ms and s2 = 2.5 ms for a concentration of 1 at%. With the short time for vitrification of the preforms, the relatively high dopant concentrations and the lack of aluminium codoping it can be assumed that clustering of the dopant may occur. As yet, however, no experiments have been performed to clarify this point. 2.2. Copper-doped fibre The fibre is doped with 0.1 at% Cu+. Monovalent copper is introduced as Cu2O with 50–100 lm grain size. After drawing the preform, the fibre pieces with diameters between 100 lm and a few millimeters show a slight red colouring similar to the Cu2O used for doping. This supports the assumption that the added copper stayed monovalent. CuO would appear black [20] as well as the fine-dispersed metal. The Cu+ fibre is excited with the 337 nm emission of the N2 laser. The broadband fluorescence can easily be seen with the naked eye. The maximum intensity is found around 550 nm with an FWHM of about 110 nm (Fig. 2). This spectrum is similar to those presented in [12–14]. In [13] it is shown that the peak position is rather constant at 550 nm but the FWHM varies towards the long-wavelength side as a function of excitation wavelength and dopant concentration. According to [13] Cu+ shows three absorption bands between 250 nm and 350 nm in dependence of the dopant concentration. These absorption bands are assigned to the transitions from singlet 3d10 to the triplet 3d94s1 [13] and/or to the singlet 3d94s1 [14]. A description of Cu+:crystal or Cu+:glass spectroscopy is found in [21,22]. To measure the lifetime of the fluorescence, the laterally emitted light is collimated and dispersed in a prism. The short-wavelength part of the spectrum is blocked with an edge to eliminate pump-light. The long-wavelength part is detected with a photomultiplier (Hamamatsu R955) in connection with a digital oscilloscope (LeCroy 9310) terminated with 50 X. The temporal characteristic of the emission is shown in Fig. 3. Upon excitation with ns-pulses at 337 nm, the long-wavelength part of the emission reaches its maximum 7 ls after excitation. This delayed rise of the fluorescence is characteristic for a three-level-system [13,14]. The relaxation of the second level occurs with a lifetime of 49 ls (cf. Fig. 3). In contrast to [14] the lifetime is somewhat longer and no dual exponential characteristic is detected. The
Fig. 2. Fluorescence spectrum of copper-doped silica fibre.
M. Neff et al. / Optical Materials 31 (2008) 247–251
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2.4. Bismuth-doped fibre
Fig. 3. Fluorescence lifetime of the copper-doped silica fibre. Excitation with ns pulses at 337 nm.
A first fibre was drawn with 0.1 at% Bi3+ in silica glass. Besides visible emission it showed a strong emission band centred at 1425 nm with 235 nm FWHM. Comparing this result with [4] or [23] reveals differences in the NIR emission band. In [4,23] double-peaked emission at 1100 nm and 1400 nm was measured in glasses containing aluminium. From these results we assume that the content of Al3+ could be a possible reason for a two-peaked fluorescence. To test this assumption, two fibres are drawn from two different preforms: preform (a) doped with 0.1 at% Bi3+ and 1 at% Al3+, and preform (b) doped with 0.1 at% Bi3+ and no aluminium. Both samples are excited along the axis with an Ar+ laser. The fluorescence is detected with fibre coupled spectrometers (visible: AVS-USB2000, near infrared: AvaSpec-NIR256-1.7) at the rear end of the fibre. The results are shown in Figs. 5 and 6. For sample (b) the same spectra are measured as before. In sample (a) the near infrared spectrum changes (cf. Fig. 6). The admixture of aluminium produces two effects: the central wavelength of the fluorescence is shifted from 1425 nm to 1140 nm by 285 nm and the FWHM is narrowed by 75 nm. It is assumed that Al3+ with
exponential fit with 49 ls decay time cannot be distinguished from the measured curve. 2.3. Manganese-doped fibre Four silica fibres with different dopant concentrations have been drawn: 0.1 at% Mn2+, 1 at% Mn2+, 2 at% Mn2+ and 5 at% Mn2+. MnO with a grain size of 50 lm to 100 lm was used. The 2% and the 5% doped fibres show cracks in the silica cladding. It further seems that the cores of these two fibres are not completely vitrified; most of the dopant is still visible as a fine powder. Two 15 cm long fibres from the 0.1 at% Mn2+ and the 1 at% Mn2+ samples and a 5 cm sample with 2 at% Mn2+ are excited along the axis with an Ar+ laser. A broad fluorescence could be detected with a fibre-coupled spectrometer (AVS-USB2000) from the side of the fibre and also at the rear end of the samples. The measured fluorescence has a FWHM of 105 nm and is centred at 580 nm (Fig. 4). This is similar with the fluorescence of Mn2+ in Zn(PO3)2 glass [9]. In contrast to this report, however, no shift of the luminescence maximum has been measured as a function of the dopant level. From Fig. 2 in [9] it can be seen that in zinc metaphosphate glass the fluorescence maximum shifts from about 550 nm to 575 nm and then to 655 nm when the dopant concentration is 1%, 2.5% and 10%, respectively.
Fig. 4. Fluorescence spectrum of manganese-doped silica fibre.
Fig. 5. Fluorescence spectra of bismuth-doped silica fibre in the visible.
Fig. 6. Fluorescence spectra of bismuth-doped silica fibre in the near infrared.
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Fig. 7. Fluorescence lifetime of the bismuth-doped silica fibre in the visible.
Fig. 8. Fluorescence lifetime of the bismuth-doped silica fibre in the near infrared.
10-times the concentration of Bi3+ is sufficient to completely remove the sites responsible for 1425 nm emission. A lower Al3+ concentration is expected to allow both emissions: Si-sites leading to 1425 nm emission and Al-sites leading to 1140 nm emission. Differences are also seen in the visible part of the spectrum. Already with the naked eye it is seen that the fibre doped only with Bi3+ appears amber while the one co-doped with aluminium appears pink. In the visible part (cf. Fig. 5) the aluminium causes no shift of the spectrum but the width is increased by about 40 nm. The lifetimes of the fluorescence have been measured. The visible fluorescence band is excited with a N2 laser (337 nm), the band in the near infrared region is excited with the Ar+ laser (458– 514.5 nm). The fluorescence is measured with a photomultiplier (Hamamatsu R955 terminated 50 X). The decay time of the visible fluorescence for Bi3+ is 4 ls. The fluorescence of the Bi3+–Al3+ doped fibre shows a double exponential characteristic with decay times of 3 ls and 13 ls (Fig. 7). The lifetimes of the infrared bands show also a double exponential characteristic: for Bi3+ the decay times are 310 ls and 20 ls, for Bi3+–Al3+ 740 ls and 180 ls (Fig. 8). The fast components of the measured lifetimes are somewhat shorter than those reported in literature. In [23] a single exponential decay time of s = 549 ls at 1100 nm and 270 ls at 1350 nm are reported. Ref. [4] describes a single exponential decay time of 720 ls at 1140 nm.
3. Conclusion With the goal of testing manufacturing of metal- or transitionmetal-doped fibres with the technique of granulated oxides we have produced preforms doped with V2O5, Cu2O, MnO, Bi2O3 and Bi2O3–Al2O3. These preforms have been drawn to fibres in a standard drawing tower. The produced fibres have been optically excited with either a N2 laser (337 nm) or an Ar+ laser (458– 514.5 nm). The generated fluorescence has been measured and compared with data from literature. The five dopants all show fluorescence in the visible wavelength region, centred at 500–600 nm with a FWHM exceeding 100 nm. Strong fluorescence in the near IR range has been measured in Bi3+. The measured spectra are similar to published data even if other host materials than pure silica have been used. The experiments have shown that fibre manufacturing with granulated oxides leads to similar spectra as MCVD or Sol–Gel technique. Acknowledgments The authors thank T. Feurer for helpful discussions. L. Di Labio, R. Scheidegger, S. Scheidegger and M. Mühlheim we are grateful for their help in the fibre drawing team. This work was supported in part by the Swiss National Science Foundation under Project 200020-113269/1.
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References [1] E.M. Dianov, V.V. Dvoyrin, V.M. Mashinsky, A.A. Umnikov, M.V. Yashkov, A.N. Gur’yanov, Quantum Electronics 35 (12) (2005) 1083. [2] V.V. Dvoyrin, V.M. Mashinsky, L.I. Bulatov, I.A. Bufetov, A.V. Shubin, M.A. Melkumov, E.F. Kustov, E.M. Dianov, A.A. Umnikov, V.F. Khopin, M.V. Yashkov, A.N. Guryanov, Optics Letters 31 (20) (2006) 2966. [3] A.B. Rulkov, A.A. Ferin, S.V. Popov, J.R. Taylor, I. Razdobreev, L. Bigot, G. Bouwmans, Optics Express 15 (9) (2007) 5473. [4] I. Razdobreev, L. Bigot, V. Pureur, A. Favre, G. Bouwmans, M. Douay, Applied Physics Letters 90 (2007) 031103-1. [5] E.M. Dianov, A.V. Shubin, M.A. Melkumov, O.I. Medvedkov, I.A. Bufetov, in: Optical Fiber Communication and the National Fiber Optic Engineers Conference OFC/NFOEC, Anaheim, CA, March 25–29, 2007, p. 1, ISBN 155752-831-4. [6] J.R. Taylor, S. Popov, EP/F025785/1 grant of £423,469, Imperial College London, 1.1.2008–31.12.2010. [7] M. Kawashima, N. Oda, Y. Uchida, K. Matsui, Journal of Luminescence 87–89 (2000) 685. [8] M. Hughes, H. Rutt, D. Hewak, R.J. Curry, Applied Physics Letters 90 (2007) 031108. [9] M.C. Flores, U. Caldino, J. Hernández, E. Camarillo, E. Cabrera, H. del Castillo, A. Speghini, M. Bettinelli, H. Murrieta, Physica Status Solidi (c) 4 (3) (2007) 922.
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[10] I.E.C. Machado, L. Prado, L. Gomes, J.M. Prison, J.R. Martinelli, Journal of NonCrystalline Solids 348 (2004) 113. [11] L. Jiang, Physics and Chemistry of Glasses 45 (5) (2004) 291. [12] T. Lutz, C. Estournès, J.C. Merle, J.L. Guille, Journal of Alloys and Compounds 262–263 (1997) 438. [13] Y. Fujimoto, M. Nakatsuka, Journal of Luminescence 75 (1997) 213. [14] M.A. Garcia, E. Borsella, S.E. Paje, J. Llopis, M.A. Villegas, R. Polloni, Journal of Luminescence 93 (2001) 253. [15] J. Ballato, E. Snitzer, Applied Optics 34 (39) (1995) 6848. [16] R. Renner-Erny, L. Di Labio, W. Lüthy, Optical Materials 29 (2007) 919. [17] L. Di Labio, R. Renner-Erny, P. Blattnig, V. Romano, W. Lüthy, T. Feurer, F. Sandoz, in: Second EPS-QEOD Europhoton Conference, Pisa, 10–15 September, 2006 Europhysics Conference Abstract, 30J, ISBN 2-914771-41-X. [18] Hanauer Quarzglas und Quarzgut, Heraeus/Schott Publikation Q-A1/111. [19] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 79th ed., The Chemical Rubber Company, 1998–1999. [20] A. Hollemann, E. Wiberg, Lehrbuch der anorganischen Chemie, W. De Gruyter & Co., Berlin, 1964. p. 466. [21] L.G. DeShazer, in: P. Hammerling, A.B. Budgor, A. Pinto (Eds.), Tunable Solid State Lasers, Springer Series in Optical Sciences, 1985, p. 92. [22] S. Parke, R.S. Webb, Physics and Chemistry of Glasses 13 (6) (1972) 157. [23] T. Suzukia, Y. Ohishi, Applied Physics Letters 88 (2006) 191912.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Metal-doped fibres for broadband emission: Fabrication with granulated oxides Martin Neff *, Valerio Romano, Willy Lüthy Institute of Applied Physics, University of Bern, CH-3012 Bern, Switzerland
a r t i c l e
i n f o
Article history: Received 6 March 2008 Received in revised form 4 April 2008 Accepted 8 April 2008 Available online 2 June 2008 PACS: 42.81.Bm 42.81.Wg 42.81.Cn 42.70. a
a b s t r a c t The goal of the present work is testing manufacturing of metal- or transition-metal-doped fibres with the technique of granulated oxides. Optical silica fibres are drawn from preforms assembled from silica tubes filled with granulated oxides of SiO2 doped with either V2O5, Cu2O, MnO, Bi2O3 or Bi2O3–Al2O3. The produced fibres are optically excited with either a N2 laser (337 nm) or an Ar+ laser (458–514.5 nm). All the investigated ions incorporated in the silica host show a broad fluorescence in the visible range between 500 nm and 600 nm with a FWHM between 100 nm and 150 nm. Bi2O3 also shows a broad fluorescence band in the near infrared range at 1425 nm with a FWHM of about 235 nm. With the admixture of Al2O3, this fluorescence band can be shifted to 1140 nm. The measured spectra are similar to published data even if other host materials than pure silica were used. The experiment shows that fibre manufacturing with granulated oxides leads to similar spectra as MCVD or Sol–Gel technique. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Metal-doped fibres Broadband emission Manufacturing of optical fibres Granulated oxides
1. Introduction Until 2005 all fibre lasers were based on rare earth-doped fibres. Rare earths are especially favourable due to their shielding of the 4f electrons resulting in a weak interaction with the host. This allows small bandwidths and high gain. Consequently, a lot of research has been made on rare earth-doped fibres. The bismuth-doped silica fibre is the first fibre laser that is not based on a rare earth-dopant [1–5]. What makes bismuth remarkable is the extent of its potential gain bandwidth extending from 1100 nm to beyond 1500 nm. As a consequence strong efforts are made to explore the potential of this laser for applications in telecommunications [6]. Based on the breakthrough with bismuth also other metalor transition-metal-doped fibres have regained much interest. Examples are vanadium [7,8], manganese [9–11] or copper [12– 14]. These fibres with unusual dopants are not commercially available. It is therefore, interesting to study possibilities of their fabrication. Recently, based on [15] a method has been developed to manufacture fibres by directly melting dry granulated oxides filled in silica tubes during the fibre drawing process [16,17]. This technique allows rapid production of doped fibres at low cost. It is not a
* Corresponding author. Tel.: +41 316318935. E-mail address: [email protected] (M. Neff). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.04.001
priori clear if this technique leads to results that are comparable with fibres produced by MCVD or Sol–Gel technique with respect to valence of the dopant and emission spectra. In our report we investigate silica fibres produced with granulated SiO2 and with either V2O5, Cu2O, MnO, Bi2O3 or Bi2O3– Al2O3. All these doped fibres are expected to emit spectra in the visible wavelength region. Manufacturing, excitation and emission properties of the fibres are described.
2. Experimental For the experiments several fibres are drawn. For the preforms silica tubes (HSQ 300) are used with an outer diameter of 21 mm and an inner diameter of 17 mm. A mixture of granulated silica with 200–400 lm grain size and oxides of the dopant is filled in the tube. The tube becomes the cladding of the drawn fibre while the mixture in the centre builds the core. The tube dimensions lead to fibres with a very large core compared to the cladding diameter. Before drawing the fibre, the preform is evacuated and heated for 1 h to 1300 °C. At 1300 °C the viscosity of silica is about 105 times higher than at the drawing temperature [18, p. 8]. Therefore, no modification of the preform geometry occurs. The preform cannot be rotated during the drawing process. We use a standard drawing tower equipped with a graphite furnace (Centorr) of 24 mm aperture. To obtain the best possible vitrification the fibre is drawn at a
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temperature of about 2000 °C. This high temperature leads to low viscosity of the silica and therefore, requires a very small drop (very small drawing force) at the beginning of the drawing process. A small drop is obtained if the preform is slowly (with 1 mm/min) inserted into the hot zone of the furnace. As soon as the weight of the drop is sufficient to overcome the viscosity of the SiO2, the drawing process starts. For our experiments only the first few meters of the fibre are used. This includes diameters from the drop with several mm down to the fibre with 100 lm. Since only few meters after the drop are needed for our investigations, most of the drawing tower facilities such as diameter measurement, coating applicator with its furnace, capstan and coiling apparatus are not used. During vitrification the granulated silica somewhat shrinks and the core-to-cladding ratio that was initially 17/21 in the preform is changed to about 5/7. In the drawn fibre this core-to-cladding ratio is maintained independent from the fibre diameter. In these fibres the losses have not been measured. But from rare-earth-doped fibres manufactured with the same technique we know that scattering losses are in the order of 1 dB/m [16]. The change of the refractive index given by the dopant in the core leads to a step index profile. For 10% aluminium in SiO2 the enhancement of the refractive index is about 10 2, but for the dopants V2O2, Cu2O, MnO, and Bi2O3 we have no information on their influence on the refractive index. 2.1. Vanadium-doped fibre The fibre is doped with 1 at% V with respect to Si. The used V2O5 has a grain size of 50–100 lm. No additional doping for wave-guiding is used. V2O5 is thermally decomposed at 1750 °C [19, pp. 4– 95]. Since fibre drawing is performed at higher temperatures we expect a mixture of different valences besides V5+. After drawing the fibre appears dark black showing also the occurrence of V valences of V4+ (deep black or blue) and/or V3+ (black) [19, pp. 4– 95]. A fibre length of 5 cm with a diameter of 600 lm is excited along the axis with the 337 nm emission of a pulsed nitrogen laser (Garching Instruments mod. SP1/II). The emission is coupled into the fibre with a Suprasil lens of 20 mm focal length. The fluorescence generated in the fibre core is measured from the side with a fibre-coupled spectrometer (AVS-USB2000). The resulting spectrum is shown in Fig. 1. The width of the fluorescence is about 125 nm at FWHM ranging from 475 nm to 600 nm. This result corresponds reasonably well to measurements performed at 77 K [7]. The measured life-
Fig. 1. Fluorescence spectrum of vanadium-doped silica fibre.
time is about 300 ls with a single exponential decay, about one order shorter than in experiments performed at 77 K [7]. In [7] a double exponential decay is described with lifetimes of s1 = 10 ms and s2 = 2.5 ms for a concentration of 1 at%. With the short time for vitrification of the preforms, the relatively high dopant concentrations and the lack of aluminium codoping it can be assumed that clustering of the dopant may occur. As yet, however, no experiments have been performed to clarify this point. 2.2. Copper-doped fibre The fibre is doped with 0.1 at% Cu+. Monovalent copper is introduced as Cu2O with 50–100 lm grain size. After drawing the preform, the fibre pieces with diameters between 100 lm and a few millimeters show a slight red colouring similar to the Cu2O used for doping. This supports the assumption that the added copper stayed monovalent. CuO would appear black [20] as well as the fine-dispersed metal. The Cu+ fibre is excited with the 337 nm emission of the N2 laser. The broadband fluorescence can easily be seen with the naked eye. The maximum intensity is found around 550 nm with an FWHM of about 110 nm (Fig. 2). This spectrum is similar to those presented in [12–14]. In [13] it is shown that the peak position is rather constant at 550 nm but the FWHM varies towards the long-wavelength side as a function of excitation wavelength and dopant concentration. According to [13] Cu+ shows three absorption bands between 250 nm and 350 nm in dependence of the dopant concentration. These absorption bands are assigned to the transitions from singlet 3d10 to the triplet 3d94s1 [13] and/or to the singlet 3d94s1 [14]. A description of Cu+:crystal or Cu+:glass spectroscopy is found in [21,22]. To measure the lifetime of the fluorescence, the laterally emitted light is collimated and dispersed in a prism. The short-wavelength part of the spectrum is blocked with an edge to eliminate pump-light. The long-wavelength part is detected with a photomultiplier (Hamamatsu R955) in connection with a digital oscilloscope (LeCroy 9310) terminated with 50 X. The temporal characteristic of the emission is shown in Fig. 3. Upon excitation with ns-pulses at 337 nm, the long-wavelength part of the emission reaches its maximum 7 ls after excitation. This delayed rise of the fluorescence is characteristic for a three-level-system [13,14]. The relaxation of the second level occurs with a lifetime of 49 ls (cf. Fig. 3). In contrast to [14] the lifetime is somewhat longer and no dual exponential characteristic is detected. The
Fig. 2. Fluorescence spectrum of copper-doped silica fibre.
M. Neff et al. / Optical Materials 31 (2008) 247–251
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2.4. Bismuth-doped fibre
Fig. 3. Fluorescence lifetime of the copper-doped silica fibre. Excitation with ns pulses at 337 nm.
A first fibre was drawn with 0.1 at% Bi3+ in silica glass. Besides visible emission it showed a strong emission band centred at 1425 nm with 235 nm FWHM. Comparing this result with [4] or [23] reveals differences in the NIR emission band. In [4,23] double-peaked emission at 1100 nm and 1400 nm was measured in glasses containing aluminium. From these results we assume that the content of Al3+ could be a possible reason for a two-peaked fluorescence. To test this assumption, two fibres are drawn from two different preforms: preform (a) doped with 0.1 at% Bi3+ and 1 at% Al3+, and preform (b) doped with 0.1 at% Bi3+ and no aluminium. Both samples are excited along the axis with an Ar+ laser. The fluorescence is detected with fibre coupled spectrometers (visible: AVS-USB2000, near infrared: AvaSpec-NIR256-1.7) at the rear end of the fibre. The results are shown in Figs. 5 and 6. For sample (b) the same spectra are measured as before. In sample (a) the near infrared spectrum changes (cf. Fig. 6). The admixture of aluminium produces two effects: the central wavelength of the fluorescence is shifted from 1425 nm to 1140 nm by 285 nm and the FWHM is narrowed by 75 nm. It is assumed that Al3+ with
exponential fit with 49 ls decay time cannot be distinguished from the measured curve. 2.3. Manganese-doped fibre Four silica fibres with different dopant concentrations have been drawn: 0.1 at% Mn2+, 1 at% Mn2+, 2 at% Mn2+ and 5 at% Mn2+. MnO with a grain size of 50 lm to 100 lm was used. The 2% and the 5% doped fibres show cracks in the silica cladding. It further seems that the cores of these two fibres are not completely vitrified; most of the dopant is still visible as a fine powder. Two 15 cm long fibres from the 0.1 at% Mn2+ and the 1 at% Mn2+ samples and a 5 cm sample with 2 at% Mn2+ are excited along the axis with an Ar+ laser. A broad fluorescence could be detected with a fibre-coupled spectrometer (AVS-USB2000) from the side of the fibre and also at the rear end of the samples. The measured fluorescence has a FWHM of 105 nm and is centred at 580 nm (Fig. 4). This is similar with the fluorescence of Mn2+ in Zn(PO3)2 glass [9]. In contrast to this report, however, no shift of the luminescence maximum has been measured as a function of the dopant level. From Fig. 2 in [9] it can be seen that in zinc metaphosphate glass the fluorescence maximum shifts from about 550 nm to 575 nm and then to 655 nm when the dopant concentration is 1%, 2.5% and 10%, respectively.
Fig. 4. Fluorescence spectrum of manganese-doped silica fibre.
Fig. 5. Fluorescence spectra of bismuth-doped silica fibre in the visible.
Fig. 6. Fluorescence spectra of bismuth-doped silica fibre in the near infrared.
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Fig. 7. Fluorescence lifetime of the bismuth-doped silica fibre in the visible.
Fig. 8. Fluorescence lifetime of the bismuth-doped silica fibre in the near infrared.
10-times the concentration of Bi3+ is sufficient to completely remove the sites responsible for 1425 nm emission. A lower Al3+ concentration is expected to allow both emissions: Si-sites leading to 1425 nm emission and Al-sites leading to 1140 nm emission. Differences are also seen in the visible part of the spectrum. Already with the naked eye it is seen that the fibre doped only with Bi3+ appears amber while the one co-doped with aluminium appears pink. In the visible part (cf. Fig. 5) the aluminium causes no shift of the spectrum but the width is increased by about 40 nm. The lifetimes of the fluorescence have been measured. The visible fluorescence band is excited with a N2 laser (337 nm), the band in the near infrared region is excited with the Ar+ laser (458– 514.5 nm). The fluorescence is measured with a photomultiplier (Hamamatsu R955 terminated 50 X). The decay time of the visible fluorescence for Bi3+ is 4 ls. The fluorescence of the Bi3+–Al3+ doped fibre shows a double exponential characteristic with decay times of 3 ls and 13 ls (Fig. 7). The lifetimes of the infrared bands show also a double exponential characteristic: for Bi3+ the decay times are 310 ls and 20 ls, for Bi3+–Al3+ 740 ls and 180 ls (Fig. 8). The fast components of the measured lifetimes are somewhat shorter than those reported in literature. In [23] a single exponential decay time of s = 549 ls at 1100 nm and 270 ls at 1350 nm are reported. Ref. [4] describes a single exponential decay time of 720 ls at 1140 nm.
3. Conclusion With the goal of testing manufacturing of metal- or transitionmetal-doped fibres with the technique of granulated oxides we have produced preforms doped with V2O5, Cu2O, MnO, Bi2O3 and Bi2O3–Al2O3. These preforms have been drawn to fibres in a standard drawing tower. The produced fibres have been optically excited with either a N2 laser (337 nm) or an Ar+ laser (458– 514.5 nm). The generated fluorescence has been measured and compared with data from literature. The five dopants all show fluorescence in the visible wavelength region, centred at 500–600 nm with a FWHM exceeding 100 nm. Strong fluorescence in the near IR range has been measured in Bi3+. The measured spectra are similar to published data even if other host materials than pure silica have been used. The experiments have shown that fibre manufacturing with granulated oxides leads to similar spectra as MCVD or Sol–Gel technique. Acknowledgments The authors thank T. Feurer for helpful discussions. L. Di Labio, R. Scheidegger, S. Scheidegger and M. Mühlheim we are grateful for their help in the fibre drawing team. This work was supported in part by the Swiss National Science Foundation under Project 200020-113269/1.
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