Thermal Stabilities in Er 3+ -Doped TeO 2 -B 2 O 3 (GeO 2 )-ZnO-K 2 O Glasses

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1 Improvement of I / I / Transition Rate and Thermal Stabilities in Er + -Doped TeO -B O (GeO )-ZnO-K O Glasses Doo Hee Cho, Yong Gyu Choi, and Kyong Hon Kim Spectroscopic and thermal analysis indicates that tellurite glasses doped with B O and GeO are promising candidate host materials for wide-band erbium doped fiber amplifier (EDFA) with a high 98 nm pump efficiency. In this study, we measured the thermal stabilities and the emission crosssections for Er + : I / I 5/ transition in this tellurite glass system. We also determined the Judd-Ofelt parameters and calculated the radiative transition rates and the multiphonon relaxation rates in this glass system. The 5 mol% substitution of B O for TeO in the Er + -doped 75TeO -ZnO- 5K O glass raised the multiphonon relaxation rate for I / I / transition from 96 s - to 7 s -, but shortened the lifetime of the I / level by % and reduced the emission cross-section for the I / I 5/ transition by %. The 5 mol% GeO substitution in the same glass system also reduced the emission cross-section but increased the lifetime by 7%. However, the multiphonon relaxation rate for I / I / transition was raised merely by s -. Therefore, a mixed substitution of B O and GeO for TeO was concluded to be suitable for the 98 nm pump efficiency and the fluorescence efficiency of I / I 5/ transition in Er + -doped tellurite glasses. Manuscript received March 6, ; revised October 5,. This work was supported by the Korean Ministry of Information and Communications. Doo Hee Cho (phone: , chodh@etri.re.kr), Yong Gyu Choi ( ygchoi@etri.re.kr), and Kyong Hon Kim ( kyongh@etri.re.kr) are with Basic Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon, Korea. I. INTRODUCTION A tellurite-based erbium doped fiber amplifier (T-EDFA) provides a wide amplifying bandwidth due to its high stimulated emission cross-section [], []. T-EDFA shows a broad bandwidth of over 7 nm with a medium population inversion in contrast to a silica-based EDFA that shows about a nm bandwidth. The EDFA for amplification of.5 µm signal can use both 98 nm and 8 nm optical pumping. Generally, the 8 nm pumping is suitable for high power EDFA because the ground state absorption to I / manifold has a high absorption cross-section relative to that of I /. However, the 8 nm pumping is a resonant pumping scheme that cannot provide full population inversion and good signal-to-noise ratio (S/N)[]. Thus, the 98 nm pumping scheme on T-EDFA is necessary for the wide bandwidth optical amplifier with a good noise figure. It has been reported that the small signal gain of T- EDFA with 98 nm pumping is significantly lower than that with 8 nm pumping []. This may be due to the low phonon energy of tellurite glasses. For Er + doped in silicate glasses, the lifetime of I / is less than µs due to the high multiphonon relaxation rate, and population feeding from I / to I / results dominantly from the multiphonon relaxation [5]. The highest phonon energy of silicate glasses is around cm -, while that of tellurite glasses is around 7cm - [6]. Therefore, the multiphonon relaxation rate of the I / I / transition of Er + in tellurite glasses is much smaller than that in silicate glasses, and the lifetime of I / for tellurite glasses is much longer than that for silicate glasses. This results in several undesirable effects on the 98 nm pumping efficiency in Er + -doped tellurite glasses. First, the accumulation of population in the ETRI Journal, Volume, Number, December Doo Hee Cho et al. 5

2 I / level becomes difficult because the branching ratio of the radiative I / I / transition is about four times smaller than that of the I / I 5/ transition [7]. Secondly, the cross relaxation of I / : I / F 7/ : I 5/ may depopulate the I / level. Finally, excited state absorption of I / F 7/ by the pumping light can also depopulate the I / level [8]. Consequently, the improvement in the multiphonon relaxation rate of I / I / in Er + -doped tellurite glasses can enhance the population accumulation in the I / level and the 98 nm pumping efficiency. We have already proposed that the Ce + - codoping can enhance the 98 nm pumping efficiency through the non-radiative energy transfer Er + : I /, Ce + : F 5/ Er + : I /, Ce + : F 7/ [9]. In this study, we propose another effective method to increase the 98 nm pumping efficiency. The addition of B O along with GeO into Er + -doped tellurite glasses is considered to increase the highest phonon energy of host glasses and the multiphonon relaxation rate of the I / I / transition, and then successfully to enhance the population accumulation in the I / level and the 98 nm pumping efficiency. increased, and glass transition temperature, T g, was decreased with substitution of B O and GeO for TeO. Table shows the changes of T g, T x, T c and T x - T g with the concentration of B O and GeO. The value of T x T g has been frequently used as a measure of thermal stability of the host glass for fiber drawing [], []. It is desirable for a host glass to have a large value of T x T g for an optical fiber drawing. As shown in Table, the value of T x T g was increased with substitutions of B O or GeO for TeO, and increased further with mixed substitutions of B O and GeO compared to the separate B O or GeO substitutions. Thermal stability of tellurite glasses may be improved with the addition of SiO, but SiO cannot use for substitution material because TeO SiO ZnO K O glasses were spontaneously phase separated even though glass melt was quenched by a brass plate. Since B O and GeO are glass network formers having a high phonon energy, their additions into tellurite glasses can improve the glass forming ability and thus enhance the thermal stability. Emission spectra of 75TeO -ZnO-5K O-.Er O, 6 TeO -5GeO -ZnO-5K O-.Er O, and 6TeO -5BO / - II. EXPERIMENTAL Glass samples were prepared by a conventional melting and quenching method. Starting materials were oxide and carbonate powders with more than 99.9% purity. Melting was carried out in air by using an electric furnace. Mixed powder was melted in an alumina crucible at 95 C for 6 minutes, and cast on a brass plate preheated at about C, then annealed at 8 C for minutes. Glass transition and crystallization temperatures were measured by differential scanning calorimetry (DSC) thermal analysis. A Ti: sapphire laser excited by Ar ion laser was used to pump the sample, and a monochromator and an InGaAs detector were used to measure the fluorescence spectrum. The fluorescence lifetimes for the I / and I / levels were measured with a Ti: sapphire laser, a chopper wheel, and the optical detector coupled with a HP 5 MHz digital oscilloscope. The wavelength of the excitation laser was tuned to 8 nm for the fluorescence spectrum and lifetime measurements. Absorption spectra were measured with a UV-Visible- NIR spectrophotometer. Heat flow ( a.u.) 75TeO ZnO5K O 6TeO 5BO /ZnO5K O Tc 6TeO 7.5GeO 7.5BO /ZnO5K O III. RESULTS AND DISCUSSION All the Er + -doped glass samples were of transparently pink color. DSC curves of 75TeO -ZnO-5K O, 6TeO -5BO / - ZnO-5K O, and 6TeO -7.5BO / -7.5GeO -ZnO-5K O glasses are shown in Fig.. Crystallization on-set temperature, T x, and crystallization peak temperature, T c, were significantly Tg Tx Temperature ( o c) Fig.. DSC curves for tellurite glasses. 5 Doo Hee Cho et al. ETRI Journal, Volume, Number, December

3 Table. T g, T x and T x -T g of tellurite glasses measured by DSC. Composition T g ( C) T x ( C) T c ( C) T x-t g 75TeO -ZnO-5K O 5±5 ± 76± 7 7TeO -5BO /-ZnO-5K O 5±7 55± 8± 5 65TeO -BO /-ZnO-5K O ±5 6± 9± 6 6TeO -5BO /-ZnO-5K O ±5 6± 9± 6 6TeO -5GeO -ZnO-5K O ±5 7± ± 5 6TeO -GeO -BO /-ZnO-5K O ±7 7± ± 6 6TeO -7.5GeO -7.5BO /-ZnO-5K O ±5 78± 9± 78 6 l / - l 5/ I / lifetime Emission intensity ( a.u.) l / - l 5/ 6TeO 5BO /ZnO5K O I/ Lifetime (µsec ) 8 I / lifetime I/ Lifetime ( msec ) I/ Lifetime (µsec) 6TeO 5GeO ZnO5K O 75TeO ZnO5K O 6 Wavelength ( nm ) Fig.. Emission spectra for Er + -doped tellurite glasses. 5 I / lifetime I / lifetime 5 5 BO / concentration (mol%) Fig.. The dependence of I / and I / lifetimes on B O concentrations in (75-x) TeO -xbo / -ZnO-5K O glasses. ZnO-5K O-.Er O glasses are shown in Fig.. The GeO substitution for TeO hardly changed the shape of emission spectrum of Er +, but the B O substitution almost elimi- I/ Lifetime (msec) 6 8 BO /concentration (mol%) Fig.. The dependence of I / and I / lifetimes on B O concentrations in 6TeO -(5-x) GeO -xbo / -ZnO-5K O glasses. nated the 98 nm emission ( I / I 5/ ) peak. The B O substitution was more effective than the GeO substitution for optical quenching of I / I 5/ transition. Figure shows the lifetime dependence of the I / and I / levels on the B O concentration in (75-x)TeO -xbo / -ZnO-5K O glasses. The lifetime of the I / level was remarkably shortened as the B O concentration increased, while that of I / showed a small change. Figure shows the lifetime dependence of the I / and I / levels on the B O concentration in 6TeO -(5-x)GeO - xbo / -ZnO-5K O glasses. As shown in Fig. and, the lifetimes for I / and I / of Er + in a 75TeO -ZnO-5K O glass were measured to be 9 µs and. ms, respectively. Those in a 6TeO -5GeO -ZnO-5K O glass were 6 µs and.5 ms, and those in a 6TeO -5BO / -ZnO-5K O glass were µs and.6 ms, respectively. The decreasing effect on the lifetime for I / level by the GeO substitution was much smaller than that by the B O substitution. The substitution of GeO for TeO increased the lifetime of the I / level, but the substitution of B O for TeO shortened it. ETRI Journal, Volume, Number, December Doo Hee Cho et al. 5

4 Table. Refractive indices and Judd-Ofelt intensity parameters of Er + in tellurite glasses. Composition n (5 nm) Ω ( - cm ) Ω ( - cm ) Ω 6 ( - cm ) rms (%) 75TeO -ZnO-5K O TeO -5BO /-ZnO-5K O TeO -5GeO -ZnO-5K O TeO -7.5GeO -7.5BO /-ZnO-5K O Measured decay rate (W m ) from the excited state energy level can be expressed as W m = W r + W mp + W et + W cr, () where W r is the radiative transition rate calculated from Judd- Ofelt analysis and W mp, W et and W cr are the rates of nonradiative transitions due to multiphonon relaxation, energy transfer, and cross relaxation, respectively []. Because the concentration of the Er + ion was kept low enough, W et and W cr can be neglected in our cases. Thus, W m can be written as W = = W + W τ m m r mp. () The Judd-Ofelt (J-O) approach for determining radiative and nonradiative transition rates of rare earth ion in glass has been widely used [7], [], []. In the J-O analysis, the line strength for an electric dipole transition is given by S ed N ( t) N ( aj; bj ') = e Ω f [ αsl] J U f [ α' S' L'] J ',() t t=,,6 (t ) where Ω t is J-O parameter and U is doubly reduced matrix element [], [5]. The J-O parameters are determined from absorption spectra and refractive indices. We calculated the values of Ω, Ω, and Ω 6 for our tellurite glass samples by using the procedure provided in []. In these calculations, the doubly reduced matrix elements from Weber [6] were used. The values of J-O parameters for the tellurite glasses are listed in Table. After obtaining J-O parameters, the radiative transition rate (W r ) can be determined by W 6π n aj; bj ') = ( χ ed Sed + χ md Smd ), () hε λ (J + ) r ( where the χ terms account for a local field correction and are given by χ ed = (n +) /9n and χ md = n [6], [7]. By using () and (), the multiphonon relaxation rates can be determined. The calculated radiative transition rates (W r ), measured lifetimes (τ m ), quantum efficiencies (η = τ m /τ r ), and multiphonon relaxation rates (W mp ) of Er + in the tellurite glasses are listed in Table. It can be seen from Table that the W mp for I / I / transition was increased significantly by the B O substitution; however, the change of that by the GeO substitution was small. The calculations for W mp of I / I 5/ transitions resulted in nearly in the tellurite glasses except for 6TeO - 5BO / -ZnO-5K O glass. On the other hand, an empirical rule for the multiphonon relaxation rate is written as W mp = W exp(-α E), (5) where W is a positive constant depending on host glasses, E is an energy gap between the excited state energy level of interest and immediately lower level [8]. It can be seen from (5) that the multiphonon relaxation rate is exponentially increased with decreasing E. And α is given by p α = ln, (6) hν g where hν is the phonon energy coupled to rare earth ions, p is the effective number of phonons participating in the multiphonon relaxation and g is electron-phonon coupling strength [9]. Therefore, it is considered from the above (5) and (6) that the multiphonon relaxation rate is increased and the measured lifetime is decreased by increasing the phonon energy of host glasses. B O, SiO and GeO are glass network formers having phonon energy higher than that of TeO. The addition of these formers into tellurite glasses increases the phonon energy of host glasses. The energy gap between I / and I / is about 6 cm -. The highest phonon energy of borate glass, germanate glass, and tellurite glass is approximately cm -, 9 5 Doo Hee Cho et al. ETRI Journal, Volume, Number, December

5 Table. Calculated radiative transition rates (W r ), measured lifetimes (τ m ), quantum efficiencies (η), and multiphonon relaxation rates (W mp ) of Er + in tellurite glasses. glass transition Wr (s - ) τ m (µs) η (%) W mp (s - ) I / I 5/ 6 I / I /, I 5/ I / I 5/ I / I /, I 5/ 95 7 I / I 5/ 5 I / I /, I 5/ I / I 5/ 96 I / I /, I 5/ * The composition of the glass is 75TeO -ZnO-5K O, the glass 6TeO -5BO /-ZnO-5K O, the glass 6TeO -5GeO -ZnO-5K O, and the glass 6TeO -7.5GeO -7.5BO /-ZnO-5K O. Table. σ a, σ e and σ e τ m for the Er + : I / I 5/ transition in the tellurite glasses at peak wavelength. Composition σ a (peak λ) σ e (peak λ) σ e τ m 75TeO -ZnO-5K O 8.55x - cm (5nm) 8.8x - cm (5nm).7x - cm s 6TeO -5BO /-ZnO-5K O 7.9x - cm (5nm) 7.8x - cm (55nm).8x - cm s 6TeO -5GeO -ZnO-5K O 7.8x - cm (56nm) 7.95x - cm (57nm).58x - cm s 6TeO -7.5GeO -7.5BO /-ZnO-5K O 7.8x - cm (5nm) 7.8x - cm (55nm).8x - cm s cm -, and 7 cm -, respectively [6]. Only ~ phonons need to be combined in borate glasses in order to bridge the energy gap between I / and I /, while approximately and 5 phonons need in germanate glasses and tellurite glasses, respectively. Thus, only in the case of the B O substitution into tellurite glasses did the W mp of the I / I / transition increase. On the other hand, the energy gap between I / and I 5/ is about 65 cm -, and thus the multiphonon relaxation rate is very small. The lifetime of the I / level is hardly affected by the change of the phonon energy in host glasses. Therefore, the decreasing effect on the I / lifetime by the B O substitution was small. In the case of the GeO substitution, the measured lifetime of the I / level increased. This is considered to result from the increase of intrinsic radiative lifetime (τ r = /W r ) with the GeO substitution as shown in Table. The intrinsic radiative lifetime is increased by lowering the refractive indices [], and the GeO substitution reduced the refractive indices as shown in Table. The peak wavelength of the emission cross-section for the I / I 5/ transition was shifted to a long wavelength side by the B O and GeO substitutions as shown in Table. The spectral gain profile of EDFA strongly depends on the difference spectrum of σ e - σ a []. The peak wavelength of σ e - σ a was also shifted to a long wavelength side by the B O and GeO substitution as shown in Fig. 5. Thus, the corresponding spectral gain profile is expected to shift to a long wavelength region. The fluorescence efficiency of an active ion in solids can be expressed in terms of a combination of various spectroscopic parameters such as absorption and emission cross-sections, excited state lifetimes, transition probabilities, and concentration of dopant ions []. The large emission cross-section and the long lifetime of the excited state energy level make the rare earth doped materials promising materials for fiber amplifier applications [], []. The absorption and emission cross-sections of Er + : I / I 5/ in 75TeO -ZnO-5K O and 6TeO -7.5 GeO -7.5BO / -ZnO-5K O glasses are also shown in Fig. 5. The absorption cross-sections were determined from absorption spectrum, and the emission cross-sections were calculated by McCumber theory []. According to McCumber theory, the emission cross-sections and the absorption cross-sections are related by ETRI Journal, Volume, Number, December Doo Hee Cho et al. 55

6 Cross section (x - cm ) absorption absorption difference emission 75TeO ZnO5K O difference emission 6TeO 7.5GeO 7.5BO /ZnO5K O Wavelength (nm) Fig. 5. The spectra of absorption cross-section (σ a ), emission cross-section (σ e ), and σ e σ a for the Er + : I / I 5/ transition in the tellurite glasses. the 6TeO -5GeO -ZnO-5K O glass was reduced merely by %. The GeO substitution reduced the emission crosssection, but extended the lifetime. Thus, the GeO substitution scarcely changed the value of σ e τ m in Er + -doped tellurite glasses. However, only the GeO substitution was not enough to raise the multiphonon relaxation rate of I / I / transition as shown in Table. Therefore, TeO -GeO -B O -MO- R O glass system is expected to be a suitable host glass for the tellurite-based erbium-doped fiber amplifier material with a high 98 nm pump efficiency, where M indicates a divalent metal such as Zn and Ba and R indicates an alkali metal. IV. CONCLUSIONS The substitutions of B O and GeO improved the thermal stability of tellurite glasses. The substitution of B O into tellurite glasses significantly raised the multiphonon relaxation rate of Er + : I / I /, which was thus expected to enhance the 98 nm pumping efficiency in T-EDFA. However, the B O substitution caused the reduction of the I / lifetime and the emission cross-section for Er + : I / I 5/. On the other hand, the GeO substitution into the tellurite glasses extended the I / lifetime and slightly reduced the emission crosssection, but the increasing effect on the multiphonon relaxation rate for the I / I / transition was small. Therefore, TeO - GeO -B O MO-R O glass system is expected to be a suitable host glass for the tellurite-based erbium-doped fiber amplifier material with a high 98 nm pump efficiency. σ e (ν) = σ a (ν)exp[(ε-hν)/kt], (7) where ε is the temperature dependent excitation energy, h is the Plank constant, k is the Boltzmann constant, and T is the temperature. The value of ε was determined by using the procedure provided in [5]. The emission cross-sections decreased with the B O and GeO substitutions for TeO as shown in Table. The decreasing effect on the emission cross-section by the GeO substitution was smaller than that by the B O substitution. The B O and GeO substitutions for TeO also lowered refractive indices of host glasses. The emission cross-section is expected to be decreased with lowering the refractive index of a host glass, because the stimulated emission cross-section owing to the electric dipole-moment transition of rare earth ions is increased as the refractive index of a host glass is raised [σ ~ (n +) /n] [6]. Often the value of σ e (emission cross-section) τ m (measured lifetime) is used as a material parameter of estimation for fiber amplifier applications [7], [8]. The value of σ e τ m for the Er + : I / I 5/ transition in the 6TeO - 5BO / -ZnO-5K O glass was reduced by about % compared to that in the 75TeO -ZnO-5K O glass, while that of REFERENCES [] A. Mori, Y. Ohishi, M. Yamada, H. Ono, Y. Nishida, K. Oikawa, and S. Sudo,.5 µm Broadband Amplification by Tellurite- Based EDFAs, Technical Digest of Conf. Optical Fibe- Comm.997 (OFC 97), Feb. 6-, 997, Dallas, Texas, US, PD [] Y. Ohishi, A. Mori, M. Yamada, H. Ono, Y. Nishida, and K. Oikawa, Gain Characteristics of Tellurite-Based Erbium-Doped Fiber Amplifiers for.5-µm Broadband Amplification, Opt. Lett., vol., no., 998, p.7. [] W.J. Miniscalco, Erbium-Doped Glasses for Fiber Amplifiers at 5 nm, J. Lightwave Technol., vol. 9, 99, p.. [] T. Nakai, Y. Noda, T. Tani, Y. Mimura, T. Sudo, and S. Ohno, 98 nm-pumped Er-Doped Tellurite-Based Fiber Amplifier, OSA TOPS, vol. 5, 998, p.8. [5] M.P. Hehlen, N.J. Cockroft, T.R. Gosnell, and A.J. Bruce, Spectroscopic Properties of Er + - and Yb + -Doped Soda-Lime Silicate and Aluminosilicate Glasses, Phys. Rev. B, vol. 56, 997, p.9. [6] R. Reisfeld and C.K. Jorgensen, in Handbook of Physics and Chemistry of Rare Earths, edited by K.A. Gschneidner, Jr. and L. Eyring, Elsevier Science Publishers B.V., 987. [7] X. Zou and T. Izumitani, Spectroscopic Properties of Er + - and 56 Doo Hee Cho et al. ETRI Journal, Volume, Number, December

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