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Ho3+-codoped TeO2–WO3–ZnO glasses for 1.47 μm amplifier
Spectrochimica Acta Part A 72 (2009) 734–737
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic properties and energy transfer of Tm3+ /Ho3+ -codoped TeO2 –WO3 –ZnO glasses for 1.47 m amplifier Ganxin Chen a,b , Qinyuan Zhang a,∗ , Yun Cheng b , Chun Zhao a , Qi Qian a , Zhongmin Yang a , Zhonghong Jiang a a b
MOE Key Lab of Specially Functional Materials and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, PR China Department of Communication and Control Engineering, Hunan Institute of Humanities Science and Technology, Hunan 417000, PR China
a r t i c l e
i n f o
Article history: Received 17 January 2008 Received in revised form 7 November 2008 Accepted 15 November 2008 Keywords: Tm3+ Amplifier Tungsten tellurite glass
a b s t r a c t We report on spectroscopic properties and energy transfer of Tm3+ /Ho3+ -codoped tungsten tellurite glasses for 1.47 m amplifier. Fluorescence spectra and the analysis of energy transfer indicate that Ho3+ is an excellent codopant for 1.47 m emission. Comparing with other tellurite glasses, the radiative lifetime of the 3 H4 level of Tm3+ in tungsten tellurite glass is slightly lower, but the spontaneous emission probability, stimulated emission cross-section and the figure of merit for bandwidth are obviously larger. Although the pump efficiency of tungsten tellurite amplifier is ∼50% less than that of fluoride glass, the figure of merit for bandwidth is approximately three times larger in tungsten tellurite glass than in fluoride glass. The results indicate that Tm3+ /Ho3+ -codoped tungsten tellurite glass is attractive for broadband amplifier. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In order to meet the rapidly increasing demand for transmission capacity, many efforts have been devoted to expand the existing window to the S band (1460–1530 nm) using Tm3+ -doped fiber amplifiers [1–9]. Tellurite glasses have approved as promising candidates of the host material for S band optical fiber amplifier [8]. Previous studies on tellurite glasses for 1.47 m emission focus on alkali tellurite glasses of doping Tm3+ or co-doping Tm3+ with other rare earth ions, e.g. Ho3+ , Tb3+ , Nd3+ , Dy3+ , Eu3+ , etc. [2–8]. Only a few papers pay attention to tungsten tellurite glasses for 1.47 m [9]. Tungsten tellurite glass has a larger vitrification scale and the content of WO3 or ZnO can be adjusted from 0% to 40% [10]. Comparing with alkali tellurite glasses, tungsten tellurite glass has several advantages as a host for 1.47 m amplifier. Its transition temperature (362–398 ◦ C) [11] is higher than that of alkali tellurite glasses (290–300 ◦ C) [12]. The higher transition temperature is benefit to design fibers for chemical process sensing. There is no alkali metal oxide, such as Na2 O, in tungsten tellurite glass which raises more doubts about the long-term durability of such fibers in aggressive environments [13]. In this work, Tm3+ /Ho3+ -codoped tungsten tellurite glasses are prepared and their spectroscopic properties and energy transfer for evaluation of potential amplifier performance at 1.47 m are
∗ Corresponding author. Tel.: +86 20 87113681. E-mail address: [email protected] (Q. Zhang). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.11.014
investigated. The addition of Ho3+ in Tm3+ -doped tellurite glass is aimed to effectively realize the population inversion between the 3 H and 3 F level of Tm3+ for 1.47 m emission [14]. 4 4
2. Experimental procedures TeO2 -based glasses with the molar compositions of 70TeO2 –20WO3 –10ZnO doped with 0.5 mol% Tm2 O3 and x mol% Ho2 O3 (x = 0, 0.05, 0.1, 0.15 and 0.3) were prepared by using reagent-grade TeO2 (99.99%), WO3 (99.99%), ZnO (99.99%), Ho2 O3 (99.99%) and Tm2 O3 (99.99%). Batch materials of 15 g were mixed well and melted at about 850 ◦ C for 20 min with a platinum crucible. Melts were thermally quenched by casting the melt into a preheated stainless steel mold, and then annealed at a temperature close to the vitreous transition temperature for 2 h before ramping down to room temperature. All the obtained samples were cut into specimens of 10 mm × 10 mm × 2 mm and optically polished for the measurement of the absorption and emission spectra. Absorption spectra of rare-earth ions doped samples were determined by a PerkinElmer Lambda 900/UV/VIS/NIR spectrophotometer in the spectral range of 400–2300 nm at room temperature. The fluorescence spectra in the range of 1300–2200 nm were obtained by using a computer-controlled TRIAX 320 fluorescence spectrometer (Jobin-Yvnon Corp.) with a 808 nm laser diode (LD) as pump source. The fluorescence lifetimes were obtained from the first e-folding time of decay curves by using a computer-controlled digitizing oscilloscope.
G. Chen et al. / Spectrochimica Acta Part A 72 (2009) 734–737
735
Table 1 Judd–Ofelt intensity parameters ˝t (t = 2, 4, and 6), calculated lifetime of the 3 H4 level rad , spontaneous emission probability Ar , the stimulated emission cross-section of peak wavelength e , the full-width at half-maximum of 1460 nm band eff and FOM for bandwidth ( e × eff ) of Tm3+ in different hosts. Glass a
Tungsten tellurite Lithium tellurite [18] Sodium tellurite [2] Oxyhalide tellurite [2] Barium tellurite [19] ZBLAN [20] a
˝2 (10−20 cm2 )
˝4 (10−20 cm2 )
˝6 (10−20 cm2 )
rad (s)
4.8 3.79 3.38 2.51 5.7 2.2
1.51 1.1 2.17 1.71 1.4 1.6
1.42 0.98 1.17 1.46 1.6 1.3
305 350 347 471 333 1310
Ar (s−1 ) 3278 2857 2882 2123 3003 763
e (10−21 cm2 ) 3.7 3.5 2.6 2.4 3.2 1.8
eff (nm) 110 109 105 105 – 76
e × eff (10−21 cm2 nm) 407 382 273 252 – 137
This work.
3. Results and discussion 3.1. Absorption spectroscopy and Judd–Ofelt analysis The absorption spectra of Tm3+ /Ho3+ -codoped tungsten tellurite glasses recorded at room temperature in the UV–vis and IR regions are shown in Fig. 1. The band assignments are indicated. The inset in the figure is the amplified spectrum in the range of 540–750 nm. The radiative transition within the 4fn configuration of a rareearth ion can be analyzed with Judd–Ofelt theory [15,16]. The Judd–Ofelt intensity parameters ˝t (t = 2, 4, and 6) are determined by fitting with a least-squares approach. In the fitting progress, no less than four absorption bands are needed to obtain experimental oscillator strengths. However, there are only two accurate bands located at 790 and 1700 nm, respectively. Because of the transition of Tm3+ :3 H6 → 3 H5 fits the magnetic dipole transition rulers (J = J + 1, J and J are the total angular momentum of 3 H6 and 3 H5 levels, respectively), the oscillator strengths of the band located at 1211 nm should involve electric and magnetic dipole radiations. The bands located at 661 and 687 nm are partially superposed in Fig. 1. Therefore, deconvolution in the spectral region 540–750 nm was made by using the least-squares approach assuming that the two bands are Gaussian shape. By comparing, four bands located at 661, 687, 793 and 1700 nm are selected to calculate parameters ˝t . The four bands located at 486, 539, 644 and 890 nm are used to calculate the Judd–Ofelt intensity parameters of Ho3+ . The calculated values of ˝2 , ˝4 and ˝6 for Ho3+ in the tellurite glasses are 5.26 × 10−20 , 2.28 × 10−20 and 2.18 × 10−20 cm2 , respectively. The values are comparable to those found by Reisfield [17]. The parameters ˝t (t = 2, 4, and 6), spontaneous emission probability Ar , fluorescence branching ratios ˇ and radiative lifetime rad have been calculated. The ˇ of the transitions of Tm3+ from the ground state 3 H6 to the excited states 3 H4 , 3 F4 and 3 H5 are
0.88, 0.08 and 0.03, respectively, which are almost similar to the cases in fluoride and other tellurite glasses [4,8,18]. Table 1 summarizes the ˝t values, rad and Ar of the 3 H4 level of Tm3+ in different hosts. The value of ˝2 in tungsten tellurite is comparable to that found in barium tellurite glass, but it is larger than those in alkali tellurite, oxyhalide tellurite and ZBLAN glasses. The value of ˝2 is strongly depended on the host composition and increases with the increasing WO3 amount in TeO2 –WO3 glasses while the values of ˝4 and ˝6 are slightly changed [9]. Therefore, the additions of heavy metal oxides, for example WO3 , in Tm3+ -doped tellurite glass maybe enhance the value of ˝2 . The total rad of the 3 H4 level of Tm3+ can be expressed as [2]: (3 H4 ) = rad /ˇ(3 H4 ), where (3 H4 ) and ˇ(3 H4 ) represent the radiative lifetime and fluorescence branching ratio of Tm3+ :3 H4 → 3 H6 transition, respectively. The rad of the 3 H4 level of Tm3+ in tungsten tellurite glass is close to that in barium tellurite glass, but it is lower than those in alkali tellurite glasses and oxyhalide tellurite glass. It is probably due to the higher line strength Sed of the Tm3+ :3 H6 → 3 H4 transition. According to Judd–Offelt theory [9], the higher line strength Sed of the Tm3+ :3 H6 → 3 H4 transition can be expressed as follows: Sed = 0.2187˝2 + 0.0944˝4 + 0.5758˝6 , where the three coefficients of ˝t ’s are the reduced matrix elements of the unit tensor operators, U(t) . Because of a few changes of ˝4 and ˝6 , the larger value of ˝2 in tungsten tellurite glass indicates the higher line strength. The higher line strength induces a higher spontaneous emission probability which leads to a lower radiative lifetime. It is noted that the addition of ZnO in Tm3+ -doped TeO2 –WO3 glass has an obvious impact on the values of ˝t , especially for ˝2 . The calculated ˝2 of the TeO2 –WO3 –ZnO is about 2.0 times smaller than that of the 70TeO2 –30WO3 glass [11], which reveals that the covalent effect of the ternary tellurite glass is relatively lower that in the TeO2 –WO3 glass [21,22]. ˝4 and ˝6 are related to the covalence of the lattice site of rare earth ions and less sensitive to the environment of rare-earth ions than that of ˝2 [23,24]. The larger the value of ˝6 , the weaker will be the covalence between rare-earth ions and anions. The relatively smaller value of ˝6 indicates that the covalence between rare-earth ions and anions in the TeO2 –WO3 –ZnO ternary glass is weaker than that in the TeO2 –WO3 binary glass. 3.2. Emission spectroscopy
Fig. 1. Absorption spectra of 70TeO2 –20WO3 –10ZnO–0.5Tm2 O3 –xHo2 O3 (mol%) glasses.
Fig. 2 shows the fluorescence spectra of Tm3+ /Ho3+ -codoped tungsten tellurite glass in the range of 1300–2200 nm under excitation of 808 nm LD at the room temperature. The emission peaks at 1.46, 1.8 and 2.0 m are assigned to the transitions of Tm3+ :3 H4 → 3 F4 , Tm3+ :3 F4 → 3 H6 and Ho3+ :5 I7 → 5 I8 , respectively, as shown in Fig. 3. The population of 3 F4 level mainly originates from the transitions of Tm3+ :3 H4 → 3 F4 and the cross-relaxation (CR) mechanism Tm3+ :3 H4 + Tm3+ :3 H6 → Tm3+ :3 F4 + Tm3+ :3 F4 . The energy stored in 5 I7 level of Ho3+ is due to the depopulation of the 3 F4 level of Tm3+ by energy transfer (ET). From Fig. 2, it can be observed that the concentration of Ho2 O3 has very little effect on the emission intensity of the peak at 1.47 m. The emission intensity of the peak at 1.8 m decreases obviously with increasing
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G. Chen et al. / Spectrochimica Acta Part A 72 (2009) 734–737
Fig. 2. The fluorescence spectra of 70TeO2 –20WO3 –10ZnO–0.5Tm2 O3 –xHo2 O3 (mol%) glasses. The inset shows the dependence of the lifetime of Tm3+ :3 F4 on the concentration of Ho2 O3 .
Fig. 4. Absorption and emission cross-sections of Tm3+ and Ho3+ in tungsten tellurite glass (B) Tm3+ :3 H4 → 3 F4 , (C) Tm3+ :3 H6 → 3 F4 , (D) Tm3+ :3 F4 → 3 H6 , (E) Ho3+ :5 I7 → 5 I8 .
Ho2 O3 concentration, while that at 2.0 m increases significantly. The results give an evidence of Tm3+ :3 F4 → Ho3+ :5 I7 ET which has little effect on the population of the 3 H4 level but obviously depopulate the 3 F4 level of Tm3+ . The inset of Fig. 2 shows the dependence of Ho2 O3 concentration on the lifetime of 3 H4 level of Tm3+ when Tm2 O3 concentration is fixed at 0.5 mol%. It is evident that the Ho2 O3 concentration only has slight effect on the lifetime of 3 H4 level of Tm3+ . These indicate that Ho3+ is an excellent deactivor which induce a population inversion between the 3 H4 and 3 F4 level [4].The full-width at half-maximum (FWHM) of 1.47 m band is ∼110 nm for the glass which is larger by ∼50% than that of fluoride glass [20], but resembles those of other tellurite gasses [4,25,26]. The pump efficiency is dominated by the (n2 eff )−1 term (eff is effective line width) and therefore a fluoride amplifier will be ∼50% more pump efficient than tellurite amplifier at peak wavelength [2].
where I0 and I are the intensities of incident and transmitted light, respectively, N is the concentration of Ho3+ , and I is the thickness of the samples. The peak values of Tm3+ :3 H6 → 3 F4 and Ho3+ :5 I8 → 5 I7 absorption cross-sections are about 6.0 × 10−21 and 8.05 × 10−21 cm2 , respectively. The stimulated emission crosssection e () can be calculated from the fluorescence spectra utilizing the following relationship [1]:
3.3. Absorption and emission cross-sections, a () and e () The absorption cross-section a () can be calculated from measured absorption spectra by using Beer–Lambert equation [24]: a () =
2.303 log(I0 /I) Nl
(1)
Fig. 3. Partial energy levels diagram of Tm3+ and Ho3+ ions in tungsten tellurite glass.
e () =
4 Ar g() 8cn2
(2)
where g() is the normalized line-shaped function obtained from the measured fluorescence spectra, c is the speed of light, n is the refractive index. The emission cross-section of Tm3+ :3 H4 → 3 F4 transition has a maximum of 3.7 × 10−21 cm2 at 1460 nm. This value is higher than those of other tellurite glasses (Table 1) and almost twice as large as the one of gallate–bismuth–lead glass (1.9–2.2 × 10−21 cm2 ) [1] or fluorite glass (2.1 × 10−21 cm2 ) [26], but close to that of lead–niobium–germanate glass (3.5 × 10−21 cm2 ) [27]. From Table 1 and Eq. (1), we can find that the larger stimulated emission cross-section of Tm3+ in tellurite glass is mainly due to the high refractive index of host glass and high spontaneous transition probability [24]. The figure of merit (FOM) for bandwidth is defined as the product of the stimulated emission cross-section and full-width at half-maximum ( e eff ) [20]. The FOM for bandwidth of Tm3+ in different hosts are also shown in Table 1.The FOM for bandwidth in tungsten tellurite is obviously higher than those of other tellurite glasses and almost three times larger than that of fluoride (ZBLAN) glass. Fig. 4 shows the absorption and emission cross-sections of Tm3+ and Ho3+ in tungsten tellurite glass. The energy difference of the average centers between the Tm3+ :3 H4 → 3 F4 emission and Tm3+ :3 H6 → 3 F4 absorption spectra is approximately 967 cm−1 . Therefore, the cross-relaxation among Tm3+ ions needs to involve the compensation for the energy gap with the lattice vibration [8,14]. On the other hand, there is a few overlap between the Tm3+ :3 H4 → 3 F4 emission and Tm3+ :3 H6 → 3 F4 absorption spectra, which provides the possibility that the resonant energy transfer among Tm3+ ions leads to cross-relaxation in tungsten tellurite glass. The peak wavelengths of the Tm3+ :3 F4 → 3 H6 emission and Ho3+ :5 I8 → 5 I7 absorption cross-sections are located at 1800 and 1950 nm, respectively, as shown in Fig. 4. The energy difference is estimated as about 427 cm−1 from the emission to absorption peak. Especially, there is a large overlap between the Tm3+ :3 F4 → 3 H6 emission and Ho3+ :5 I8 → 5 I7 absorption cross-sections, and this
G. Chen et al. / Spectrochimica Acta Part A 72 (2009) 734–737
indicates the existence of resonant energy transfer between Tm3+ and Ho3+ ions. The effect of this large overlap on the 3 F4 level of Tm3+ will be discussed in the following sections. 3.4. Energy transfer between Tm3+ and Ho3+ in tungsten tellurite glass There is energy transfer originated from Tm3+ :3 F4 → Ho3+ :5 I7 which has been proved by the emission spectra of tungsten tellurite. In order to obtain a precise description on the energy transfer between Tm3+ (3 F4 ) and Ho3+ (5 I7 ), the direct energy transfer constant CDA for Tm3+ :3 F4 → Ho3+ :5 I7 and the backward energy transfer constant CAD for Ho3+ :5 I7 → Tm3+ :3 F4 need to be calculated. Where D and A are donor and acceptor, respectively. Dexter s model for dipole–dipole interaction was employed to get CDA and CAD defined as follows [28]: CDA = CAD =
6 RDA
(3)
D 6 RAD
(4)
A
where D and A are the total lifetime of the donor (Tm3+ ) and acceptor (Ho3+ ), respectively, both measured in single-doped samples. The critical radii RDA and RAD are calculated utilizing the extending integral method [29]: 6 RDA
=
6 = RAD
6cD
low gD
up
(2)4 n2 gD 6cA
gAlow
D A emis ()abs () d
(5)
A D emis ()abs () d
(6)
up
(2)4 n2 gA
up
low and g are the degeneracy of the respective lower where gD D up and upper levels of donor, and corresponsively, gAlow and gD are those of acceptor. emis () and abs () are the stimulated emission and absorption cross-section of donor or accepter, respectively, and were calculated by using McCumber relation [24]. The energy transfer constants are obtained as follows (in cm6 s units): CDA = 103 × 10−40 , CAD = 5.35 × 10−40 . The ratio of CAD /CDA is about 0.05, which indicates that only 5% of the excited Tm3+ ions settle in the 3 F4 level due to the existence of backward energy transfer. This is similar to that of fluorozirconate glasses [28]. The Ho3+ shows a larger quenching effect on the 3 F4 level of Tm3+ , which further proved that it is a good codopant for optical amplifier at 1.47 m.
4. Conclusion We report on spectroscopic properties and energy transfer of Tm3+ /Ho3+ -codoped tungsten tellurite glasses for 1.47 m ampli-
737
fier. The absorption spectra have been analyzed by using the Judd–Ofelt theory. Fluorescence spectra and the analysis of energy transfer indicate that Ho3+ is an excellent codopant for 1.47 m emission. Comparing with other tellurite glasses, the rad of the 3 H4 level of Tm3+ in tungsten tellurite glass is slightly lower, but the Ar , e and FOM for bandwidth are obviously larger. Although the pump efficiency of tungsten tellurite amplifier is ∼50% less than that of fluoride glass, the e , eff and FOM for bandwidth of the former are at least 50% larger than those of the latter, respectively. These characteristics indicate that Tm3+ /Ho3+ -codoped tungsten tellurite glass is attractive for broadband amplifier [2,3]. Acknowledgement This project was financially supported by NSFC (Grant No. 50602017). References [1] J.H. Song, J. Heo, J. Appl. Phys. 93 (2003) 9441. [2] E.R. Taylor, L.N. Ng, N.P. Sesions, J. Appl. Phys. 92 (2002) 112. [3] E.R. Taylor, L.N. Ng, J. Nilson, R. Caponi, A. Pangano, M. Potenza, B. Sordo, Photo. Technol. Lett. 16 (3) (2004) 777. [4] S. Tanable, Proc. SPIE 4282 (2001) 85. [5] S. Shen, A. Jha, E. Zhang, S. Wilson, J. Lumin. 126 (2007) 434. [6] D.H. Cho, Y.G. Choi, K.H. Kim, Chem. Phys. Lett. 322 (2000) 263. [7] Y.G. Choi, D.H. Cho, K.H. Kim, J. Non-Cryst. Solids 276 (2000) 1. [8] Y. Ohishi, T. Fumita, H. Yamauchi, T. Suzuki, Proc. SPIE 6014 (2005) 601417. [9] G. Özen, A. Aydinli, S. Cenk, A. Sennaro˘glu, J. Lumin. 101 (2003) 293. [10] G.D. Jiang, Glass Handbook, Chinese Construction Industry Publisher, Beijing, 1985 (Chapter 8). [11] S. Cenk, B. Demirata, M.L. Övecoglu, G. Özen, Spectrochim. Acta A 57 (2001) 2367. [12] V.K. Tikhomiron, A.B. Seddon, D. Furniss, M. Ferrari, J. Non-Cryst. Solids 326–327 (2003) 296. [13] C.J. Hill, A. Jha, J. Non-Cryst. Solids 353 (2007) 1372. [14] H. Yamauchi, G.S. Murugan, Y. Ohishi, J. Appl. Phys. 96 (2004) 7212. [15] B.R. Judd, Phys. Rev. 127 (1962) 750. [16] G.S. Ofel, J. Chem. Phys. 37 (1962) 511. [17] R. Reisfield, R. Hormadaly, J. Chem. Phys. 64 (1976) 3207. [18] H. Lin, X. Wang, L. Lin, C. Li, D. Yang, S. Tanable, J. Phys. D: Appl. Phys. 40 (2004) 3567. [19] J.S. Wang, E. Snitzer, E.M. Vogel, G.H. Sigel, J. Lumin. 60–61 (1994) 145. [20] M. Naftaly, S. Shen, A. Jha, Appl. Opt. 39 (2000) 4979. [21] A.K. Singh, S.B. Rai, V.B. Singh, J. Alloys Compd. 403 (2005) 97. [22] E. Rukmini, C.K. Jayasankar, Opt. Mater. 4 (1995) 529. [23] S. Tanabe, T. Ohyagj, N. Soga, T. Hanada, Phys. Rev. B 46 (1992) 3305. [24] G.X. Chen, Q.Y. Zhang, G.F. Yang, Z.H. Hong, J. Fluorensc. 17 (2007) 301. [25] S. Ohara, N. Sugimoto, Y. Kondo, K. Ochia, Y. Kuroiwa, Y. Fukasawa, T. Hirose, H. Hayashi, S. Tanabe, Proc. SPIE 4645 (2002) 8. [26] R.M. Percival, D. Szebesta, S.T. Davey, Elect. Lett. 28 (20) (1992) 1866. [27] R. Balda, L.M. Lacha, J. Ferna’ndez, J.M. Ferna’ndez-Navarro, Opt. Mater. 27 (2005) 1771. [28] L.D.D. Vila, L. Gomes, L.V.G. Tarelho, S.J.L. Ribeiro, Y. Messaddeq, J. Appl. Phys. 95 (2004) 5451. [29] L.D.D. Vila, L. Gomes, C.R. Eyzaguirre, E. Rodriguez, C.L. Cesar, L.C. Barbosa, Opt. Mater. 27 (2005) 1333.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Spectroscopic properties and energy transfer of Tm3+ /Ho3+ -codoped TeO2 –WO3 –ZnO glasses for 1.47 m amplifier Ganxin Chen a,b , Qinyuan Zhang a,∗ , Yun Cheng b , Chun Zhao a , Qi Qian a , Zhongmin Yang a , Zhonghong Jiang a a b
MOE Key Lab of Specially Functional Materials and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, PR China Department of Communication and Control Engineering, Hunan Institute of Humanities Science and Technology, Hunan 417000, PR China
a r t i c l e
i n f o
Article history: Received 17 January 2008 Received in revised form 7 November 2008 Accepted 15 November 2008 Keywords: Tm3+ Amplifier Tungsten tellurite glass
a b s t r a c t We report on spectroscopic properties and energy transfer of Tm3+ /Ho3+ -codoped tungsten tellurite glasses for 1.47 m amplifier. Fluorescence spectra and the analysis of energy transfer indicate that Ho3+ is an excellent codopant for 1.47 m emission. Comparing with other tellurite glasses, the radiative lifetime of the 3 H4 level of Tm3+ in tungsten tellurite glass is slightly lower, but the spontaneous emission probability, stimulated emission cross-section and the figure of merit for bandwidth are obviously larger. Although the pump efficiency of tungsten tellurite amplifier is ∼50% less than that of fluoride glass, the figure of merit for bandwidth is approximately three times larger in tungsten tellurite glass than in fluoride glass. The results indicate that Tm3+ /Ho3+ -codoped tungsten tellurite glass is attractive for broadband amplifier. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In order to meet the rapidly increasing demand for transmission capacity, many efforts have been devoted to expand the existing window to the S band (1460–1530 nm) using Tm3+ -doped fiber amplifiers [1–9]. Tellurite glasses have approved as promising candidates of the host material for S band optical fiber amplifier [8]. Previous studies on tellurite glasses for 1.47 m emission focus on alkali tellurite glasses of doping Tm3+ or co-doping Tm3+ with other rare earth ions, e.g. Ho3+ , Tb3+ , Nd3+ , Dy3+ , Eu3+ , etc. [2–8]. Only a few papers pay attention to tungsten tellurite glasses for 1.47 m [9]. Tungsten tellurite glass has a larger vitrification scale and the content of WO3 or ZnO can be adjusted from 0% to 40% [10]. Comparing with alkali tellurite glasses, tungsten tellurite glass has several advantages as a host for 1.47 m amplifier. Its transition temperature (362–398 ◦ C) [11] is higher than that of alkali tellurite glasses (290–300 ◦ C) [12]. The higher transition temperature is benefit to design fibers for chemical process sensing. There is no alkali metal oxide, such as Na2 O, in tungsten tellurite glass which raises more doubts about the long-term durability of such fibers in aggressive environments [13]. In this work, Tm3+ /Ho3+ -codoped tungsten tellurite glasses are prepared and their spectroscopic properties and energy transfer for evaluation of potential amplifier performance at 1.47 m are
∗ Corresponding author. Tel.: +86 20 87113681. E-mail address: [email protected] (Q. Zhang). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.11.014
investigated. The addition of Ho3+ in Tm3+ -doped tellurite glass is aimed to effectively realize the population inversion between the 3 H and 3 F level of Tm3+ for 1.47 m emission [14]. 4 4
2. Experimental procedures TeO2 -based glasses with the molar compositions of 70TeO2 –20WO3 –10ZnO doped with 0.5 mol% Tm2 O3 and x mol% Ho2 O3 (x = 0, 0.05, 0.1, 0.15 and 0.3) were prepared by using reagent-grade TeO2 (99.99%), WO3 (99.99%), ZnO (99.99%), Ho2 O3 (99.99%) and Tm2 O3 (99.99%). Batch materials of 15 g were mixed well and melted at about 850 ◦ C for 20 min with a platinum crucible. Melts were thermally quenched by casting the melt into a preheated stainless steel mold, and then annealed at a temperature close to the vitreous transition temperature for 2 h before ramping down to room temperature. All the obtained samples were cut into specimens of 10 mm × 10 mm × 2 mm and optically polished for the measurement of the absorption and emission spectra. Absorption spectra of rare-earth ions doped samples were determined by a PerkinElmer Lambda 900/UV/VIS/NIR spectrophotometer in the spectral range of 400–2300 nm at room temperature. The fluorescence spectra in the range of 1300–2200 nm were obtained by using a computer-controlled TRIAX 320 fluorescence spectrometer (Jobin-Yvnon Corp.) with a 808 nm laser diode (LD) as pump source. The fluorescence lifetimes were obtained from the first e-folding time of decay curves by using a computer-controlled digitizing oscilloscope.
G. Chen et al. / Spectrochimica Acta Part A 72 (2009) 734–737
735
Table 1 Judd–Ofelt intensity parameters ˝t (t = 2, 4, and 6), calculated lifetime of the 3 H4 level rad , spontaneous emission probability Ar , the stimulated emission cross-section of peak wavelength e , the full-width at half-maximum of 1460 nm band eff and FOM for bandwidth ( e × eff ) of Tm3+ in different hosts. Glass a
Tungsten tellurite Lithium tellurite [18] Sodium tellurite [2] Oxyhalide tellurite [2] Barium tellurite [19] ZBLAN [20] a
˝2 (10−20 cm2 )
˝4 (10−20 cm2 )
˝6 (10−20 cm2 )
rad (s)
4.8 3.79 3.38 2.51 5.7 2.2
1.51 1.1 2.17 1.71 1.4 1.6
1.42 0.98 1.17 1.46 1.6 1.3
305 350 347 471 333 1310
Ar (s−1 ) 3278 2857 2882 2123 3003 763
e (10−21 cm2 ) 3.7 3.5 2.6 2.4 3.2 1.8
eff (nm) 110 109 105 105 – 76
e × eff (10−21 cm2 nm) 407 382 273 252 – 137
This work.
3. Results and discussion 3.1. Absorption spectroscopy and Judd–Ofelt analysis The absorption spectra of Tm3+ /Ho3+ -codoped tungsten tellurite glasses recorded at room temperature in the UV–vis and IR regions are shown in Fig. 1. The band assignments are indicated. The inset in the figure is the amplified spectrum in the range of 540–750 nm. The radiative transition within the 4fn configuration of a rareearth ion can be analyzed with Judd–Ofelt theory [15,16]. The Judd–Ofelt intensity parameters ˝t (t = 2, 4, and 6) are determined by fitting with a least-squares approach. In the fitting progress, no less than four absorption bands are needed to obtain experimental oscillator strengths. However, there are only two accurate bands located at 790 and 1700 nm, respectively. Because of the transition of Tm3+ :3 H6 → 3 H5 fits the magnetic dipole transition rulers (J = J + 1, J and J are the total angular momentum of 3 H6 and 3 H5 levels, respectively), the oscillator strengths of the band located at 1211 nm should involve electric and magnetic dipole radiations. The bands located at 661 and 687 nm are partially superposed in Fig. 1. Therefore, deconvolution in the spectral region 540–750 nm was made by using the least-squares approach assuming that the two bands are Gaussian shape. By comparing, four bands located at 661, 687, 793 and 1700 nm are selected to calculate parameters ˝t . The four bands located at 486, 539, 644 and 890 nm are used to calculate the Judd–Ofelt intensity parameters of Ho3+ . The calculated values of ˝2 , ˝4 and ˝6 for Ho3+ in the tellurite glasses are 5.26 × 10−20 , 2.28 × 10−20 and 2.18 × 10−20 cm2 , respectively. The values are comparable to those found by Reisfield [17]. The parameters ˝t (t = 2, 4, and 6), spontaneous emission probability Ar , fluorescence branching ratios ˇ and radiative lifetime rad have been calculated. The ˇ of the transitions of Tm3+ from the ground state 3 H6 to the excited states 3 H4 , 3 F4 and 3 H5 are
0.88, 0.08 and 0.03, respectively, which are almost similar to the cases in fluoride and other tellurite glasses [4,8,18]. Table 1 summarizes the ˝t values, rad and Ar of the 3 H4 level of Tm3+ in different hosts. The value of ˝2 in tungsten tellurite is comparable to that found in barium tellurite glass, but it is larger than those in alkali tellurite, oxyhalide tellurite and ZBLAN glasses. The value of ˝2 is strongly depended on the host composition and increases with the increasing WO3 amount in TeO2 –WO3 glasses while the values of ˝4 and ˝6 are slightly changed [9]. Therefore, the additions of heavy metal oxides, for example WO3 , in Tm3+ -doped tellurite glass maybe enhance the value of ˝2 . The total rad of the 3 H4 level of Tm3+ can be expressed as [2]: (3 H4 ) = rad /ˇ(3 H4 ), where (3 H4 ) and ˇ(3 H4 ) represent the radiative lifetime and fluorescence branching ratio of Tm3+ :3 H4 → 3 H6 transition, respectively. The rad of the 3 H4 level of Tm3+ in tungsten tellurite glass is close to that in barium tellurite glass, but it is lower than those in alkali tellurite glasses and oxyhalide tellurite glass. It is probably due to the higher line strength Sed of the Tm3+ :3 H6 → 3 H4 transition. According to Judd–Offelt theory [9], the higher line strength Sed of the Tm3+ :3 H6 → 3 H4 transition can be expressed as follows: Sed = 0.2187˝2 + 0.0944˝4 + 0.5758˝6 , where the three coefficients of ˝t ’s are the reduced matrix elements of the unit tensor operators, U(t) . Because of a few changes of ˝4 and ˝6 , the larger value of ˝2 in tungsten tellurite glass indicates the higher line strength. The higher line strength induces a higher spontaneous emission probability which leads to a lower radiative lifetime. It is noted that the addition of ZnO in Tm3+ -doped TeO2 –WO3 glass has an obvious impact on the values of ˝t , especially for ˝2 . The calculated ˝2 of the TeO2 –WO3 –ZnO is about 2.0 times smaller than that of the 70TeO2 –30WO3 glass [11], which reveals that the covalent effect of the ternary tellurite glass is relatively lower that in the TeO2 –WO3 glass [21,22]. ˝4 and ˝6 are related to the covalence of the lattice site of rare earth ions and less sensitive to the environment of rare-earth ions than that of ˝2 [23,24]. The larger the value of ˝6 , the weaker will be the covalence between rare-earth ions and anions. The relatively smaller value of ˝6 indicates that the covalence between rare-earth ions and anions in the TeO2 –WO3 –ZnO ternary glass is weaker than that in the TeO2 –WO3 binary glass. 3.2. Emission spectroscopy
Fig. 1. Absorption spectra of 70TeO2 –20WO3 –10ZnO–0.5Tm2 O3 –xHo2 O3 (mol%) glasses.
Fig. 2 shows the fluorescence spectra of Tm3+ /Ho3+ -codoped tungsten tellurite glass in the range of 1300–2200 nm under excitation of 808 nm LD at the room temperature. The emission peaks at 1.46, 1.8 and 2.0 m are assigned to the transitions of Tm3+ :3 H4 → 3 F4 , Tm3+ :3 F4 → 3 H6 and Ho3+ :5 I7 → 5 I8 , respectively, as shown in Fig. 3. The population of 3 F4 level mainly originates from the transitions of Tm3+ :3 H4 → 3 F4 and the cross-relaxation (CR) mechanism Tm3+ :3 H4 + Tm3+ :3 H6 → Tm3+ :3 F4 + Tm3+ :3 F4 . The energy stored in 5 I7 level of Ho3+ is due to the depopulation of the 3 F4 level of Tm3+ by energy transfer (ET). From Fig. 2, it can be observed that the concentration of Ho2 O3 has very little effect on the emission intensity of the peak at 1.47 m. The emission intensity of the peak at 1.8 m decreases obviously with increasing
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Fig. 2. The fluorescence spectra of 70TeO2 –20WO3 –10ZnO–0.5Tm2 O3 –xHo2 O3 (mol%) glasses. The inset shows the dependence of the lifetime of Tm3+ :3 F4 on the concentration of Ho2 O3 .
Fig. 4. Absorption and emission cross-sections of Tm3+ and Ho3+ in tungsten tellurite glass (B) Tm3+ :3 H4 → 3 F4 , (C) Tm3+ :3 H6 → 3 F4 , (D) Tm3+ :3 F4 → 3 H6 , (E) Ho3+ :5 I7 → 5 I8 .
Ho2 O3 concentration, while that at 2.0 m increases significantly. The results give an evidence of Tm3+ :3 F4 → Ho3+ :5 I7 ET which has little effect on the population of the 3 H4 level but obviously depopulate the 3 F4 level of Tm3+ . The inset of Fig. 2 shows the dependence of Ho2 O3 concentration on the lifetime of 3 H4 level of Tm3+ when Tm2 O3 concentration is fixed at 0.5 mol%. It is evident that the Ho2 O3 concentration only has slight effect on the lifetime of 3 H4 level of Tm3+ . These indicate that Ho3+ is an excellent deactivor which induce a population inversion between the 3 H4 and 3 F4 level [4].The full-width at half-maximum (FWHM) of 1.47 m band is ∼110 nm for the glass which is larger by ∼50% than that of fluoride glass [20], but resembles those of other tellurite gasses [4,25,26]. The pump efficiency is dominated by the (n2 eff )−1 term (eff is effective line width) and therefore a fluoride amplifier will be ∼50% more pump efficient than tellurite amplifier at peak wavelength [2].
where I0 and I are the intensities of incident and transmitted light, respectively, N is the concentration of Ho3+ , and I is the thickness of the samples. The peak values of Tm3+ :3 H6 → 3 F4 and Ho3+ :5 I8 → 5 I7 absorption cross-sections are about 6.0 × 10−21 and 8.05 × 10−21 cm2 , respectively. The stimulated emission crosssection e () can be calculated from the fluorescence spectra utilizing the following relationship [1]:
3.3. Absorption and emission cross-sections, a () and e () The absorption cross-section a () can be calculated from measured absorption spectra by using Beer–Lambert equation [24]: a () =
2.303 log(I0 /I) Nl
(1)
Fig. 3. Partial energy levels diagram of Tm3+ and Ho3+ ions in tungsten tellurite glass.
e () =
4 Ar g() 8cn2
(2)
where g() is the normalized line-shaped function obtained from the measured fluorescence spectra, c is the speed of light, n is the refractive index. The emission cross-section of Tm3+ :3 H4 → 3 F4 transition has a maximum of 3.7 × 10−21 cm2 at 1460 nm. This value is higher than those of other tellurite glasses (Table 1) and almost twice as large as the one of gallate–bismuth–lead glass (1.9–2.2 × 10−21 cm2 ) [1] or fluorite glass (2.1 × 10−21 cm2 ) [26], but close to that of lead–niobium–germanate glass (3.5 × 10−21 cm2 ) [27]. From Table 1 and Eq. (1), we can find that the larger stimulated emission cross-section of Tm3+ in tellurite glass is mainly due to the high refractive index of host glass and high spontaneous transition probability [24]. The figure of merit (FOM) for bandwidth is defined as the product of the stimulated emission cross-section and full-width at half-maximum ( e eff ) [20]. The FOM for bandwidth of Tm3+ in different hosts are also shown in Table 1.The FOM for bandwidth in tungsten tellurite is obviously higher than those of other tellurite glasses and almost three times larger than that of fluoride (ZBLAN) glass. Fig. 4 shows the absorption and emission cross-sections of Tm3+ and Ho3+ in tungsten tellurite glass. The energy difference of the average centers between the Tm3+ :3 H4 → 3 F4 emission and Tm3+ :3 H6 → 3 F4 absorption spectra is approximately 967 cm−1 . Therefore, the cross-relaxation among Tm3+ ions needs to involve the compensation for the energy gap with the lattice vibration [8,14]. On the other hand, there is a few overlap between the Tm3+ :3 H4 → 3 F4 emission and Tm3+ :3 H6 → 3 F4 absorption spectra, which provides the possibility that the resonant energy transfer among Tm3+ ions leads to cross-relaxation in tungsten tellurite glass. The peak wavelengths of the Tm3+ :3 F4 → 3 H6 emission and Ho3+ :5 I8 → 5 I7 absorption cross-sections are located at 1800 and 1950 nm, respectively, as shown in Fig. 4. The energy difference is estimated as about 427 cm−1 from the emission to absorption peak. Especially, there is a large overlap between the Tm3+ :3 F4 → 3 H6 emission and Ho3+ :5 I8 → 5 I7 absorption cross-sections, and this
G. Chen et al. / Spectrochimica Acta Part A 72 (2009) 734–737
indicates the existence of resonant energy transfer between Tm3+ and Ho3+ ions. The effect of this large overlap on the 3 F4 level of Tm3+ will be discussed in the following sections. 3.4. Energy transfer between Tm3+ and Ho3+ in tungsten tellurite glass There is energy transfer originated from Tm3+ :3 F4 → Ho3+ :5 I7 which has been proved by the emission spectra of tungsten tellurite. In order to obtain a precise description on the energy transfer between Tm3+ (3 F4 ) and Ho3+ (5 I7 ), the direct energy transfer constant CDA for Tm3+ :3 F4 → Ho3+ :5 I7 and the backward energy transfer constant CAD for Ho3+ :5 I7 → Tm3+ :3 F4 need to be calculated. Where D and A are donor and acceptor, respectively. Dexter s model for dipole–dipole interaction was employed to get CDA and CAD defined as follows [28]: CDA = CAD =
6 RDA
(3)
D 6 RAD
(4)
A
where D and A are the total lifetime of the donor (Tm3+ ) and acceptor (Ho3+ ), respectively, both measured in single-doped samples. The critical radii RDA and RAD are calculated utilizing the extending integral method [29]: 6 RDA
=
6 = RAD
6cD
low gD
up
(2)4 n2 gD 6cA
gAlow
D A emis ()abs () d
(5)
A D emis ()abs () d
(6)
up
(2)4 n2 gA
up
low and g are the degeneracy of the respective lower where gD D up and upper levels of donor, and corresponsively, gAlow and gD are those of acceptor. emis () and abs () are the stimulated emission and absorption cross-section of donor or accepter, respectively, and were calculated by using McCumber relation [24]. The energy transfer constants are obtained as follows (in cm6 s units): CDA = 103 × 10−40 , CAD = 5.35 × 10−40 . The ratio of CAD /CDA is about 0.05, which indicates that only 5% of the excited Tm3+ ions settle in the 3 F4 level due to the existence of backward energy transfer. This is similar to that of fluorozirconate glasses [28]. The Ho3+ shows a larger quenching effect on the 3 F4 level of Tm3+ , which further proved that it is a good codopant for optical amplifier at 1.47 m.
4. Conclusion We report on spectroscopic properties and energy transfer of Tm3+ /Ho3+ -codoped tungsten tellurite glasses for 1.47 m ampli-
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fier. The absorption spectra have been analyzed by using the Judd–Ofelt theory. Fluorescence spectra and the analysis of energy transfer indicate that Ho3+ is an excellent codopant for 1.47 m emission. Comparing with other tellurite glasses, the rad of the 3 H4 level of Tm3+ in tungsten tellurite glass is slightly lower, but the Ar , e and FOM for bandwidth are obviously larger. Although the pump efficiency of tungsten tellurite amplifier is ∼50% less than that of fluoride glass, the e , eff and FOM for bandwidth of the former are at least 50% larger than those of the latter, respectively. These characteristics indicate that Tm3+ /Ho3+ -codoped tungsten tellurite glass is attractive for broadband amplifier [2,3]. Acknowledgement This project was financially supported by NSFC (Grant No. 50602017). References [1] J.H. Song, J. Heo, J. Appl. Phys. 93 (2003) 9441. [2] E.R. Taylor, L.N. Ng, N.P. Sesions, J. Appl. Phys. 92 (2002) 112. [3] E.R. Taylor, L.N. Ng, J. Nilson, R. Caponi, A. Pangano, M. Potenza, B. Sordo, Photo. Technol. Lett. 16 (3) (2004) 777. [4] S. Tanable, Proc. SPIE 4282 (2001) 85. [5] S. Shen, A. Jha, E. Zhang, S. Wilson, J. Lumin. 126 (2007) 434. [6] D.H. Cho, Y.G. Choi, K.H. Kim, Chem. Phys. Lett. 322 (2000) 263. [7] Y.G. Choi, D.H. Cho, K.H. Kim, J. Non-Cryst. Solids 276 (2000) 1. [8] Y. Ohishi, T. Fumita, H. Yamauchi, T. Suzuki, Proc. SPIE 6014 (2005) 601417. [9] G. Özen, A. Aydinli, S. Cenk, A. Sennaro˘glu, J. Lumin. 101 (2003) 293. [10] G.D. Jiang, Glass Handbook, Chinese Construction Industry Publisher, Beijing, 1985 (Chapter 8). [11] S. Cenk, B. Demirata, M.L. Övecoglu, G. Özen, Spectrochim. Acta A 57 (2001) 2367. [12] V.K. Tikhomiron, A.B. Seddon, D. Furniss, M. Ferrari, J. Non-Cryst. Solids 326–327 (2003) 296. [13] C.J. Hill, A. Jha, J. Non-Cryst. Solids 353 (2007) 1372. [14] H. Yamauchi, G.S. Murugan, Y. Ohishi, J. Appl. Phys. 96 (2004) 7212. [15] B.R. Judd, Phys. Rev. 127 (1962) 750. [16] G.S. Ofel, J. Chem. Phys. 37 (1962) 511. [17] R. Reisfield, R. Hormadaly, J. Chem. Phys. 64 (1976) 3207. [18] H. Lin, X. Wang, L. Lin, C. Li, D. Yang, S. Tanable, J. Phys. D: Appl. Phys. 40 (2004) 3567. [19] J.S. Wang, E. Snitzer, E.M. Vogel, G.H. Sigel, J. Lumin. 60–61 (1994) 145. [20] M. Naftaly, S. Shen, A. Jha, Appl. Opt. 39 (2000) 4979. [21] A.K. Singh, S.B. Rai, V.B. Singh, J. Alloys Compd. 403 (2005) 97. [22] E. Rukmini, C.K. Jayasankar, Opt. Mater. 4 (1995) 529. [23] S. Tanabe, T. Ohyagj, N. Soga, T. Hanada, Phys. Rev. B 46 (1992) 3305. [24] G.X. Chen, Q.Y. Zhang, G.F. Yang, Z.H. Hong, J. Fluorensc. 17 (2007) 301. [25] S. Ohara, N. Sugimoto, Y. Kondo, K. Ochia, Y. Kuroiwa, Y. Fukasawa, T. Hirose, H. Hayashi, S. Tanabe, Proc. SPIE 4645 (2002) 8. [26] R.M. Percival, D. Szebesta, S.T. Davey, Elect. Lett. 28 (20) (1992) 1866. [27] R. Balda, L.M. Lacha, J. Ferna’ndez, J.M. Ferna’ndez-Navarro, Opt. Mater. 27 (2005) 1771. [28] L.D.D. Vila, L. Gomes, L.V.G. Tarelho, S.J.L. Ribeiro, Y. Messaddeq, J. Appl. Phys. 95 (2004) 5451. [29] L.D.D. Vila, L. Gomes, C.R. Eyzaguirre, E. Rodriguez, C.L. Cesar, L.C. Barbosa, Opt. Mater. 27 (2005) 1333.