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Near-infrared emission from Pr-doped borophosphate glass for broadband telecommunication
Journal of Luminescence 135 (2013) 38–41
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Near-infrared emission from Pr-doped borophosphate glass for broadband telecommunication Qiuchun Sheng a,b, Xiaolin Wang a,b, Danping Chen a,n a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2012 Received in revised form 24 September 2012 Accepted 24 October 2012 Available online 1 November 2012
Broadband near-infrared emission from Pr-doped borophosphate glass was investigated. The emission band had three peaks centered at 1040, 1163, and 1470 nm with full widths at half maximum of 108, 147, and 205 nm, respectively. These figures cover the entire spectrum of fiber-optic communication bands. The emission lifetimes at 1040, 1163, and 1470 nm were 68, 321, and 68 ms respectively. The product st of stimulated emission cross-section (s) and lifetime (t) was 3.44 10 23 cm2 s. These results indicate that the Pr-doped glass can be used as an amplification medium for tunable lasers and broadband optical amplifiers for wavelength division multiplexing transmission system applications. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Borophosphate glass Pr-doped Near-infrared Broadband emission
1. Introduction The development of wavelength division multiplexing (WDM) networks is increasingly becoming important in enhancing information traffic in telecommunication networks. Among the equipment for WDM optical communication systems, broadband amplifiers and tunable lasers are pivotal devices because of the number of channels that depend on the gain bandwidth of amplifiers and laser sources [1–6]. Numerous researchers have exerted considerable effort toward achieving broadband amplification; in such initiatives, scholars use rare-earth and transition metal-doped amplifiers, such as Er-doped fiber amplifiers [7–10], and Tm-doped fiber amplifiers [3,11,12], as well as Ti3 þ :Al2O3 [13], Cr4 þ :YAG [14], and Ni2 þ -doped glass ceramics, for broadband tunable lasers [15]. Single rare-earth ions expand optical amplification bandwidths, however, this expansion is limited because the bandwidth of 4f–4f optical transition is narrow by nature, and these various amplifiers have a gain bandwidth of less than 100 nm [8,16–20]. Given that rare-earth ions easily exist in multivalent states, the valence state of rare-earth ions used in doping materials should be controlled [21]. Realizing broadband amplification with high gain efficiency via onefold wavelength pumping would mean revolutionary progress in WDM technology.
n
Corresponding author. Tel.: þ86 21 5991 1596. E-mail address: [email protected] (D. Chen).
Pr ions exhibit short broadband emissions at approximately 1330 nm in glasses and glass ceramics with full widths at half maximum (FWHM) of less than 100 nm [22–24]. However, to the best of our knowledge, no comprehensive investigation on broadband emission from Pr-doped borophosphate glasses has been conducted. In this letter, we report the characteristics of broadband near-infrared emission from Pr-doped P2O5–B2O3–Al2O3– Y2O3 (PBAY) glass. We realized efficient broadband emission in the near-infrared region (880–1700 nm) at room temperature.
2. Experimental details A PBAY glass system (12Y2O3–8Al2O3–15B2O3–65P2O5) was used as the host material of Pr ions. Analytical reagents of NH4H2PO4, H3BO3, Al(OH)3, and Y2O3 were used as raw materials. Glass samples were prepared by a conventional casting method. We tried 0.25, 0.5, 1.0, and 2.0 mol% Pr2O3 as dopant concentration and found that the optimum concentration of Pr2O3 used to dope the PBAY glass was 0.25 mol%. Raw materials of reagent grade were thoroughly mixed and melted in an alumina crucible in air atmosphere at 1400 1C for 2 h in an electric furnace. The melts were poured onto an annealed steel plate and then slowly cooled to room temperature. The sample was polished to optical quality before optical measurements were performed. The thickness of the samples was approximately 1 mm. Absorption spectra were recorded within a wavelength range of 400–2400 nm on a JASCO V-570 UV/VIS/IR spectrophotometer. The photoluminescence spectra were 850–1700 nm, as determined using a
0022-2313/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.10.040
Q. Sheng et al. / Journal of Luminescence 135 (2013) 38–41
TRIAX550 spectrophotometer with a PbS detector and a 445 nm laser diode (LD) as the excitation source. Emission decay curves were obtained with an FLSP920 (Edinburgh Instruments Ltd., UK). The refractive indices of the glass were measured by a prismcoupling method. All measurements were carried out at room temperature.
3. Results and discussion 3.1. Optical absorption spectra The optical absorption spectra of the glass samples in the 400– 2400 nm region are shown in Fig. 1. The difference in spectrum of absorptions between the Pr-doped and undoped glasses is also shown in the figure. Two visible absorption bands at approximately 445 and 580 nm, as well as two broadband near-infrared absorption bands centered at 1500 and 1900 nm in the difference spectrum can be attributed to the 3H4-3P2 (445 nm), 3H4-1D2 (586 nm), 3H4-3F3 (1498 nm), and 3H4-3F2 (1905 nm) transitions of Pr3 þ in the Pr-doped PBAY glass. All the transitions in the absorption spectra of Pr3 þ started from the ground state 3H4. These bands correspond to the 4f2-intra-configurational electric dipole transitions from the ground state 3H4 to excited states 3P2, 1 D2, and 3F2, 3. The wavelengths of the absorption peaks matched with the results in previous reports on Pr-doped glasses [25–27]. The band widths are due to the combination of inhomogeneous broadening, site-to-site variations in crystal field strength and
39
unresolved Stark splitting [28]. The intense absorption of the Prdoped PBAY at 445 nm indicates that this glass can be more efficiently pumped using an LD at 445 nm. The Judd–Ofelt (J–O) theory is commonly used to analyze the spectra of the rare earth doped matrix and the compositional dependence of the spectroscopic parameters. The calculated J–O intensity parameters of Pr-doped PBAY glass are compared with those of other hosts in Table 1. It is widely accepted that O4/O6 determines the spectroscopic quality of the host materials. From Table 1, the present glass has a relatively large O4/O6 value by comparison with other hosts. It indicates that the Pr-doped PBAY glass is a good matrix for broadband near infrared emission. 3.2. Emission spectra and energy level scheme The emission spectra of the Pr-doped PBAY glass are illustrated in Fig. 2. Under an LD excitation of 445 nm, we observed three broad emission bands centered approximately at 1040, 1163, and 1470 nm with FWHM values of 108, 147, and 205 nm, respectively, covering nearly the entire range (830–1700 nm) of practical fiber-optic communication bands in the Pr-doped PBAY glass. Considering the application as optical gain media for telecommunication, an emission wavelength to cover from S-band ( 1300 nm) to L-band ( 1650 nm) is desirable [32]. In the experiment, we placed an 808 nm filter between the glass sample and detector. The fluorescence spectrum was not a single band; hence, the emission spectrum was fitted tentatively by multipeak Gauss fitting [30]. Only the three-peak Gauss fitting was
Fig. 1. Absorption spectra of the (a) Pr-doped glass, (b) undoped glass, and (c) their difference.
Table 1 Comparison of J–O parameters of Pr3 þ ions in various glass hosts. Glass
O2 (10 20 cm2)
O4 (10 20 cm2)
O6 (10 20 cm2)
O4/O6
Reference
Borophosphate Fluorotellurite Borate Phosphate Fluorozirconate
3.12 3.57 0.77 4.26 2.16
7.21 6.60 3.84 4.33 1.41
5.81 5.18 3.58 6.27 1.13
1.24 1.27 1.07 0.69 1.25
Present work [22] [29] [30] [31]
40
Q. Sheng et al. / Journal of Luminescence 135 (2013) 38–41
Fig. 2. Infrared emission spectra of the PBAY glass sample under 445 nm excitation, and multi-peak fitting emission spectra.
The ground state 3H4 of Pr3 þ was first excited to higher state 3P2 when LD radiation was focused on the Pr-doped PBAY glass. The 3 P0, 1D2 levels of Pr3 þ presented just below the energy level of 3P2 of Pr3 þ [37,38]. First, the 3P2 state nonradiatively decays to 3P0, 1 D2, and 1G4 states. The near-infrared emission at approximately 1040 nm is due to the 3P2-3P0 transition under the abovementioned pumping source excitation. A phonon-assisted energy transfer occurred because the energy mismatch between the higher levels of Pr3 þ ions in the 3P2 state nonradiatively decays to 1D2 and 1G4 states. Consequently, the 1470 and 1163 nm emissions respectively associated with the Pr3 þ :1D2-1G4 (1470 nm) and 1G4-3H5 (1163 nm) transitions occurred. 3.3. Analysis of fluorescence decay curves
Fig. 3. Energy level scheme for the observed absorption as well as the excitation and emission bands of the Pr-doped PABY glass.
The fluorescence decay curves at 1040, 1163, and 1470 nm under an LD excitation of 445 nm are shown in Fig. 4. The decay curves are consistent with the first-order exponential decay with lifetimes of 68, 321, and 68 ms, correspondingly. Stimulated emission cross-section s as a function of wavelength to a first approximation coincided with the spontaneous emission spectrum 2
sðlÞ ¼ successful. The fluorescence spectrum was fitted into three Gaussian bands at approximately 1040, 1163, and 1470 nm. The available FWHM values were all above 100 nm. Such emissions can be partly characterized as the typical emission of the 3P0-1G4 (1040 nm), 1G4-3H5 (1163 nm), and 1D2-1G4 (1470 nm) transitions of Pr3 þ ions, which occupy high-field sites. However, the other Pr-doped glasses did not show broadband emission in the nearinfrared region possibly because Pr3 þ ions occupy low-field sites [33–36]. This result indicates that Pr-doped PBAY glasses can be used as host materials for potential broadband optical amplifiers in WDM. The energy level scheme for the observed absorption as well as the excitation and emission transitions of Pr3 þ ions is depicted in Fig. 3. The energy level structure and energy transfer processes of Pr3 þ excited by 445 nm LD radiation are also shown in the figure.
l gðlÞ 8pn2 t
ð1Þ
where l is the wavelength, g(l) represents the normalized spontaneous emission shape function, n is the host refractive index, and t denotes the emission lifetime. Assuming a Gaussianshaped emission band, stimulated emission cross-section s at the band center can be estimated by the following formula: 1=2 l0 2 ln2 sem ¼ ð2Þ 2 p 4pn ct Du where l0 is the band center wavelength and Dn is the FWHM of the emission expressed by the wave number. We derived s ¼7.82 10 20, 1.99 10 20, and 1.61 10 19 cm2, with l0 ¼1040, 1163, and 1470 nm, n ¼1.57, t ¼ 68, 321, and 68 ms, and Dn ¼ 1028, 1073, and 998 cm 1 ,respectively, for the Prdoped PBAY glass. The product of the stimulated emission
Q. Sheng et al. / Journal of Luminescence 135 (2013) 38–41
41
Fig. 4. Emission decay curves of PBAY. The monitoring wavelengths were 1040, 1163, and 1470 nm under 445 nm excitation.
cross-section and lifetime, st, is an important parameter for characterizing laser materials, because the laser threshold is inversely proportional to st [39]. The total st product of Pr3 þ in the PBAY glass was calculated to be 3.44 10 23 cm2 s. The st product of the glasses is approximately 24.6 times larger than that of Ti: Al2O3 (st ¼ 1.4 10 24 cm2 s) [40]. These results indicate that Pr-doped PBAY glasses can serve as optical gain media for tunable lasers and broadband optical amplifiers for WDM transmission system applications.
4. Conclusions In conclusion, broadband near-infrared emission from Pr-doped PBAY glass was investigated as a candidate for tunable lasers and broadband optical amplifiers for WDM transmission system applications. Intense broadband luminescence was observed at room temperature, and long fluorescence lifetimes at 1040, 1163, and 1470 nm were 68, 321, and 68 ms, respectively. This material yielded a large product of the stimulated emission cross-section with the lifetime. The total st product of the Pr3 þ in the PBAY glass was 3.44 10 23 cm2 s. The results show that Pr-doped PBAY glasses have potential applications in broadband telecommunication systems.
Acknowledgments The authors acknowledge the financial support provided by the National Natural Science Foundation of China (NNSFC) under Grant nos. 51272262 and 50911140475. References [1] C.H. Kim, Opt. Express 20 (2012) 3473. [2] M. Naftaly, S.X. Shen, A. Jha, Appl. Opt. 39 (2000) 4979. [3] X.S. Wang, Q.H. Nie, T.F. Xu, X. Shen, S.X. Dai, N. Gai, J. Rare Earths 26 (2008) 907. [4] F. Bulllesfeld, B. Drapp, V. Hagemann, M. Letz, U. Peuchert, S. Reichel, R. Sprengard, T. Kaelber, M. Brinkmann, B. Schreder, E. Pawlowski, S. Pucilowski, J.S. Hayden, Rare-Earth-Doped Mater. Devices Vii 4990 (2003) 77. [5] S.C. Kumar, O. Kimmelma, M. Ebrahim-Zadeh, Opt. Lett. 37 (2012) 1577. [6] J.W. Kim, J.K. Sahu, W.A. Clarkson, Opt. Commun. 285 (2012) 1362.
[7] J.C. Jasapara, M.J. Andrejco, A. DeSantolo, A.D. Yablon, Z. Varallyay, J.W. Nicholson, J.M. Fini, D.J. DiGiovanni, C. Headley, E. Monberg, F.V. DiMarcello, IEEE J. Sel. Top. Quant. 15 (2009) 3. [8] B.C. Jamalaiah, T. Suhasini, L.R. Moorthy, K.J. Reddy, I.G. Kim, D.S. Yoo, K. Jang, Opt. Mater. 34 (2012) 861. [9] Y.H. Liu, Y.J. Chen, Y.F. Lin, Z.D. Luo, Y.D. Huang, Opt. Mater. 30 (2008) 1883. [10] X.S. Wang, Q.H. Nie, L.R. Liu, T.F. Xu, X. Shen, S.X. Dai, X.H. Zhang, Chin. Phys. Lett. 23 (2006) 2553. [11] S. Aozasa, T. Sakamoto, T. Kanamori, K. Hoshino, K. Kobayashi, M. Shimizu, IEEE Photonics Technol. L 12 (2000) 1331. [12] G.F. Yang, Q.Y. Zhang, D.M. Shi, Z.H. Jiang, J. Am. Ceram. Soc. 90 (2007) 307. [13] S.G. Bartoshevich, V.D. Burlakov, V.V. Zuev, N.P. Nazarenko, A.N. Maltsev, G.A. Skripko, Appl. Opt. 31 (1992) 7575. [14] I.T. Sorokina, S. Naumov, E. Sorokin, E. Wintner, A.V. Shestakov, Opt. Lett. 24 (1999) 1578. [15] S.F. Zhou, J.H. Hao, J.R. Qiu, J. Am. Ceram. Soc. 94 (2011) 2902. [16] V.K. Tikhomirov, K. Driesen, C. Goerller-Walrand, M. Mortier, Opt. Express 15 (2007) 9535. [17] N.G. Boetti, J. Lousteau, A. Chiasera, M. Ferrari, E. Mura, G.C. Scarpignato, S. Abrate, D. Milanese, J. Lumin. 132 (2012) 1265. [18] J. Bromage, J.M. Fini, C. Dorrer, J.D. Zuegel, Appl. Opt. 50 (2011) 2001. [19] R.R. Thomson, N.D. Psaila, S.J. Beecher, A.K. Kar, Opt. Express 18 (2010) 13212. [20] C.B. Olausson, A. Shirakawa, M. Chen, J.K. Lyngso, J. Broeng, K.P. Hansen, A. Bjarklev, K. Ueda, Opt. Express 18 (2010) 16345. [21] Y.X. Zhuang, Y. Teng, J. Luo, B. Zhu, Y.Z. Chi, E. Wu, H.P. Zeng, J.R. Qiu, Appl. Phys. Lett. 95 (2009). [22] B. Zhou, L. Tao, Y.H. Tsang, W. Jin, E.Y.B. Pun, Opt. Express 20 (2012) 3803. [23] B. Zhou, E.Y. Pun, Opt. Lett. 36 (2011) 2958. [24] D.R. Simons, A.J. Faber, H. de Waal, Opt. Lett. 20 (1995) 468. [25] Y. Sun, M. Koshimizu, S. Kishimoto, K. Asai, J. Sol–Gel Sci. Technol. 62 (2012) 1. [26] Y.Y. Liu, J. Ren, Y. Tong, T.F. Wang, W.N. Xu, G.R. Chen, J. Am. Ceram. Soc. 95 (2012) 41. [27] V.K. Rai, D.K. Mohanty, J. Fluoresc. 21 (2011) 1455. [28] Z. Mazurak, S. Bodyl, R. Lisiecki, J. Gabrys-Pisarska, M. Czaja, Opt. Mater. 32 (2010) 547. [29] R. Reisfield, C.K. Jorgensen, Laser and Excited States of Rare Earths, 1, Springer-Verlag, Berlin, 1977. [30] M. Czaja, S. Bodyl, J. Gabrys´-Pisarska, Z. Mazurka, Opt. Mater. 31 (2009) 1898. [31] D.J. Vickers, R.A. Mcfarlane, OSA Proc. 13 (1992) 280. [32] S.L. Logunov, Appl. Opt. 40 (2001) 1570. [33] H.O. Hartley, Technometrics 3 (1961) 269. [34] M.G. Brik, A. Majchrowski, G.A. Kumar, K. Ozga, I. Sildos, A. Suchocki, A. Slezak, T. Lukasiewicz, I.V. Kityk, J. Rare Earth 27 (2009) 612. [35] Y. Arikawa, K. Yamanoi, T. Nakazato, E.S. Estacio, T. Shimizu, N. Sarukura, M. Nakai, T. Norimatsu, H. Azechi, T. Murata, S. Fujino, H. Yoshida, K. Kamada, Y. Usuki, T. Suyama, A. Yoshikawa, N. Sato, H. Kan, Rev. Sci. Instrum. 80 (2009) 113504-1–113504-4. [36] A.B. Seddon, D. Furniss, M.S. Iovu, S.D. Shutov, N.N. Syrbu, A.M. Andriesh, G.J. Adriaenssens, Proceedings of the XIII International Symposium on Non-Oxide Glasses and New Optical Glasses Pts 1 and 2, 2002, pp. 537–540. [37] Y. Katayama, S. Tanabe, Opt. Mater. 33 (2010) 176. [38] G. Lakshminarayana, J.R. Qiu, Physica B 404 (2009) 1169. [39] J. Ruan, Y.Z. Chi, X.F. Liu, G.P. Dong, G. Lin, D.P. Chen, E. Wu, J.R. Qiu, J. Phys. D: Appl. Phys. 42 (2009) 1. [40] G.F. Albrecht, J.M. Eggleston, J.J. Ewing, Opt. Commun. 52 (1985) 401.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Near-infrared emission from Pr-doped borophosphate glass for broadband telecommunication Qiuchun Sheng a,b, Xiaolin Wang a,b, Danping Chen a,n a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2012 Received in revised form 24 September 2012 Accepted 24 October 2012 Available online 1 November 2012
Broadband near-infrared emission from Pr-doped borophosphate glass was investigated. The emission band had three peaks centered at 1040, 1163, and 1470 nm with full widths at half maximum of 108, 147, and 205 nm, respectively. These figures cover the entire spectrum of fiber-optic communication bands. The emission lifetimes at 1040, 1163, and 1470 nm were 68, 321, and 68 ms respectively. The product st of stimulated emission cross-section (s) and lifetime (t) was 3.44 10 23 cm2 s. These results indicate that the Pr-doped glass can be used as an amplification medium for tunable lasers and broadband optical amplifiers for wavelength division multiplexing transmission system applications. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Borophosphate glass Pr-doped Near-infrared Broadband emission
1. Introduction The development of wavelength division multiplexing (WDM) networks is increasingly becoming important in enhancing information traffic in telecommunication networks. Among the equipment for WDM optical communication systems, broadband amplifiers and tunable lasers are pivotal devices because of the number of channels that depend on the gain bandwidth of amplifiers and laser sources [1–6]. Numerous researchers have exerted considerable effort toward achieving broadband amplification; in such initiatives, scholars use rare-earth and transition metal-doped amplifiers, such as Er-doped fiber amplifiers [7–10], and Tm-doped fiber amplifiers [3,11,12], as well as Ti3 þ :Al2O3 [13], Cr4 þ :YAG [14], and Ni2 þ -doped glass ceramics, for broadband tunable lasers [15]. Single rare-earth ions expand optical amplification bandwidths, however, this expansion is limited because the bandwidth of 4f–4f optical transition is narrow by nature, and these various amplifiers have a gain bandwidth of less than 100 nm [8,16–20]. Given that rare-earth ions easily exist in multivalent states, the valence state of rare-earth ions used in doping materials should be controlled [21]. Realizing broadband amplification with high gain efficiency via onefold wavelength pumping would mean revolutionary progress in WDM technology.
n
Corresponding author. Tel.: þ86 21 5991 1596. E-mail address: [email protected] (D. Chen).
Pr ions exhibit short broadband emissions at approximately 1330 nm in glasses and glass ceramics with full widths at half maximum (FWHM) of less than 100 nm [22–24]. However, to the best of our knowledge, no comprehensive investigation on broadband emission from Pr-doped borophosphate glasses has been conducted. In this letter, we report the characteristics of broadband near-infrared emission from Pr-doped P2O5–B2O3–Al2O3– Y2O3 (PBAY) glass. We realized efficient broadband emission in the near-infrared region (880–1700 nm) at room temperature.
2. Experimental details A PBAY glass system (12Y2O3–8Al2O3–15B2O3–65P2O5) was used as the host material of Pr ions. Analytical reagents of NH4H2PO4, H3BO3, Al(OH)3, and Y2O3 were used as raw materials. Glass samples were prepared by a conventional casting method. We tried 0.25, 0.5, 1.0, and 2.0 mol% Pr2O3 as dopant concentration and found that the optimum concentration of Pr2O3 used to dope the PBAY glass was 0.25 mol%. Raw materials of reagent grade were thoroughly mixed and melted in an alumina crucible in air atmosphere at 1400 1C for 2 h in an electric furnace. The melts were poured onto an annealed steel plate and then slowly cooled to room temperature. The sample was polished to optical quality before optical measurements were performed. The thickness of the samples was approximately 1 mm. Absorption spectra were recorded within a wavelength range of 400–2400 nm on a JASCO V-570 UV/VIS/IR spectrophotometer. The photoluminescence spectra were 850–1700 nm, as determined using a
0022-2313/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.10.040
Q. Sheng et al. / Journal of Luminescence 135 (2013) 38–41
TRIAX550 spectrophotometer with a PbS detector and a 445 nm laser diode (LD) as the excitation source. Emission decay curves were obtained with an FLSP920 (Edinburgh Instruments Ltd., UK). The refractive indices of the glass were measured by a prismcoupling method. All measurements were carried out at room temperature.
3. Results and discussion 3.1. Optical absorption spectra The optical absorption spectra of the glass samples in the 400– 2400 nm region are shown in Fig. 1. The difference in spectrum of absorptions between the Pr-doped and undoped glasses is also shown in the figure. Two visible absorption bands at approximately 445 and 580 nm, as well as two broadband near-infrared absorption bands centered at 1500 and 1900 nm in the difference spectrum can be attributed to the 3H4-3P2 (445 nm), 3H4-1D2 (586 nm), 3H4-3F3 (1498 nm), and 3H4-3F2 (1905 nm) transitions of Pr3 þ in the Pr-doped PBAY glass. All the transitions in the absorption spectra of Pr3 þ started from the ground state 3H4. These bands correspond to the 4f2-intra-configurational electric dipole transitions from the ground state 3H4 to excited states 3P2, 1 D2, and 3F2, 3. The wavelengths of the absorption peaks matched with the results in previous reports on Pr-doped glasses [25–27]. The band widths are due to the combination of inhomogeneous broadening, site-to-site variations in crystal field strength and
39
unresolved Stark splitting [28]. The intense absorption of the Prdoped PBAY at 445 nm indicates that this glass can be more efficiently pumped using an LD at 445 nm. The Judd–Ofelt (J–O) theory is commonly used to analyze the spectra of the rare earth doped matrix and the compositional dependence of the spectroscopic parameters. The calculated J–O intensity parameters of Pr-doped PBAY glass are compared with those of other hosts in Table 1. It is widely accepted that O4/O6 determines the spectroscopic quality of the host materials. From Table 1, the present glass has a relatively large O4/O6 value by comparison with other hosts. It indicates that the Pr-doped PBAY glass is a good matrix for broadband near infrared emission. 3.2. Emission spectra and energy level scheme The emission spectra of the Pr-doped PBAY glass are illustrated in Fig. 2. Under an LD excitation of 445 nm, we observed three broad emission bands centered approximately at 1040, 1163, and 1470 nm with FWHM values of 108, 147, and 205 nm, respectively, covering nearly the entire range (830–1700 nm) of practical fiber-optic communication bands in the Pr-doped PBAY glass. Considering the application as optical gain media for telecommunication, an emission wavelength to cover from S-band ( 1300 nm) to L-band ( 1650 nm) is desirable [32]. In the experiment, we placed an 808 nm filter between the glass sample and detector. The fluorescence spectrum was not a single band; hence, the emission spectrum was fitted tentatively by multipeak Gauss fitting [30]. Only the three-peak Gauss fitting was
Fig. 1. Absorption spectra of the (a) Pr-doped glass, (b) undoped glass, and (c) their difference.
Table 1 Comparison of J–O parameters of Pr3 þ ions in various glass hosts. Glass
O2 (10 20 cm2)
O4 (10 20 cm2)
O6 (10 20 cm2)
O4/O6
Reference
Borophosphate Fluorotellurite Borate Phosphate Fluorozirconate
3.12 3.57 0.77 4.26 2.16
7.21 6.60 3.84 4.33 1.41
5.81 5.18 3.58 6.27 1.13
1.24 1.27 1.07 0.69 1.25
Present work [22] [29] [30] [31]
40
Q. Sheng et al. / Journal of Luminescence 135 (2013) 38–41
Fig. 2. Infrared emission spectra of the PBAY glass sample under 445 nm excitation, and multi-peak fitting emission spectra.
The ground state 3H4 of Pr3 þ was first excited to higher state 3P2 when LD radiation was focused on the Pr-doped PBAY glass. The 3 P0, 1D2 levels of Pr3 þ presented just below the energy level of 3P2 of Pr3 þ [37,38]. First, the 3P2 state nonradiatively decays to 3P0, 1 D2, and 1G4 states. The near-infrared emission at approximately 1040 nm is due to the 3P2-3P0 transition under the abovementioned pumping source excitation. A phonon-assisted energy transfer occurred because the energy mismatch between the higher levels of Pr3 þ ions in the 3P2 state nonradiatively decays to 1D2 and 1G4 states. Consequently, the 1470 and 1163 nm emissions respectively associated with the Pr3 þ :1D2-1G4 (1470 nm) and 1G4-3H5 (1163 nm) transitions occurred. 3.3. Analysis of fluorescence decay curves
Fig. 3. Energy level scheme for the observed absorption as well as the excitation and emission bands of the Pr-doped PABY glass.
The fluorescence decay curves at 1040, 1163, and 1470 nm under an LD excitation of 445 nm are shown in Fig. 4. The decay curves are consistent with the first-order exponential decay with lifetimes of 68, 321, and 68 ms, correspondingly. Stimulated emission cross-section s as a function of wavelength to a first approximation coincided with the spontaneous emission spectrum 2
sðlÞ ¼ successful. The fluorescence spectrum was fitted into three Gaussian bands at approximately 1040, 1163, and 1470 nm. The available FWHM values were all above 100 nm. Such emissions can be partly characterized as the typical emission of the 3P0-1G4 (1040 nm), 1G4-3H5 (1163 nm), and 1D2-1G4 (1470 nm) transitions of Pr3 þ ions, which occupy high-field sites. However, the other Pr-doped glasses did not show broadband emission in the nearinfrared region possibly because Pr3 þ ions occupy low-field sites [33–36]. This result indicates that Pr-doped PBAY glasses can be used as host materials for potential broadband optical amplifiers in WDM. The energy level scheme for the observed absorption as well as the excitation and emission transitions of Pr3 þ ions is depicted in Fig. 3. The energy level structure and energy transfer processes of Pr3 þ excited by 445 nm LD radiation are also shown in the figure.
l gðlÞ 8pn2 t
ð1Þ
where l is the wavelength, g(l) represents the normalized spontaneous emission shape function, n is the host refractive index, and t denotes the emission lifetime. Assuming a Gaussianshaped emission band, stimulated emission cross-section s at the band center can be estimated by the following formula: 1=2 l0 2 ln2 sem ¼ ð2Þ 2 p 4pn ct Du where l0 is the band center wavelength and Dn is the FWHM of the emission expressed by the wave number. We derived s ¼7.82 10 20, 1.99 10 20, and 1.61 10 19 cm2, with l0 ¼1040, 1163, and 1470 nm, n ¼1.57, t ¼ 68, 321, and 68 ms, and Dn ¼ 1028, 1073, and 998 cm 1 ,respectively, for the Prdoped PBAY glass. The product of the stimulated emission
Q. Sheng et al. / Journal of Luminescence 135 (2013) 38–41
41
Fig. 4. Emission decay curves of PBAY. The monitoring wavelengths were 1040, 1163, and 1470 nm under 445 nm excitation.
cross-section and lifetime, st, is an important parameter for characterizing laser materials, because the laser threshold is inversely proportional to st [39]. The total st product of Pr3 þ in the PBAY glass was calculated to be 3.44 10 23 cm2 s. The st product of the glasses is approximately 24.6 times larger than that of Ti: Al2O3 (st ¼ 1.4 10 24 cm2 s) [40]. These results indicate that Pr-doped PBAY glasses can serve as optical gain media for tunable lasers and broadband optical amplifiers for WDM transmission system applications.
4. Conclusions In conclusion, broadband near-infrared emission from Pr-doped PBAY glass was investigated as a candidate for tunable lasers and broadband optical amplifiers for WDM transmission system applications. Intense broadband luminescence was observed at room temperature, and long fluorescence lifetimes at 1040, 1163, and 1470 nm were 68, 321, and 68 ms, respectively. This material yielded a large product of the stimulated emission cross-section with the lifetime. The total st product of the Pr3 þ in the PBAY glass was 3.44 10 23 cm2 s. The results show that Pr-doped PBAY glasses have potential applications in broadband telecommunication systems.
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