Spectroscopy and concentration quenching of the infrared emissions in Tm 3 +-doped TeO 2-TiO 2Nb 2 O 5 glass

In this work, we report the optical properties of Tm ions in tellurite glasses (TeO2-TiO2-Nb2O5) for different Tm 3+ concentrations ranging between 0.05 and 1 wt%. Judd-Ofelt intensity parameters have been determined to calculate the radiative transition probabilities and radiative lifetimes of excited states. The stimulated emission cross-sections of the infrared emissions at 1487 nm and 1800 nm have been determined from the line shape of the emission spectra and the lifetimes of levels H4 and F4 respectively. The emission spectra obtained under 793 nm excitation reveal the existence of energy transfer via cross-relaxation among Tm ions. As a result, the intensity of the infrared H4→F4 emission at 1487 nm decreases in relation to the one at 1800 nm, as concentration increases. The nonexponential character of the decays from the H4 level with increasing concentration indicates the presence of a dipole-dipole quenching process assisted by energy migration. The self-quenching of the F4→H6 emission at 1800 nm can be attributed to limited diffusion within the active centers. ©2007 Optical Society of America OCIS codes: (140.3380) Laser Materials; (160.5690) Rare earth doped materials; (300.6280) Spectroscopy, fluorescence and luminescence References and links 1. J.Y. Allain, M. Monerie, H. Poignant, “Tunable cw lasing around 0.82, 1.48, 1.88, and 2.35 μm in thulium doped fluorozirconate fiber,” Electron. Lett. 25, 1660-1662 (1989). 2. S. Tanabe, X.Feng, T. Hanada, “Improved emission of Tm-doped glass for a 1.4 μm amplifier by radiative energy transfer between Tm and Nd, Opt. Lett. 25, 817-819 (2000). 3. J. Wu, Z. Yao, J. Zong, and S. Jiang, “Highly efficient high-power thulium-doped germanate glass fiber laser,” Opt. Lett. 32, 638-640 (2007). 4. J.S. Wang, E.M. Vogel, and E. Snitzer, “Tellurite glass: a new candidate for fiber devices,” Opt. Mater. 3, 187-203 (1994). 5. R.A.H. El-Mallawany, Tellurite Glasses Handbook-Physical Properties and Data, (CRC Boca Raton, FL 2001). 6. S.Q. Man, E.Y.B. Pun, P.S. Chung, “Tellurite glasses for 1.3 μm optical amplifiers,” Opt. Commun. 168, 369-373 (1999). 7. M. Yamada, A. Mori, K. Kobayashi, H. Ono, T. Kanamori, K. Oikawa, Y. Nishida, Y. Ohishi, “Gainflattened tellurite-based EDFA with a flat amplification bandwidth of 76 nm,” IEEE Photon. Technol. Lett. 10, 1244-1246 (1998). 8. S. Shen, A. Jha, L. Huang, and P. Joshi, “980-nm diode-pumped Tm/Yb-codoped tellurite fiber for Sband amplification,” Opt. Lett. 30, 1437-1439 (2005). 9. Aiko Narazaki, Katsuhisa Tanaka, Kazuyuki Hirao, Naohiro Soga, “Induction and relaxation of optical second-order nonlinearity in tellurite glasses,” J. Appl. Phys. 85, 2046-2051 (1999). 10. S. Tanabe, K. Hirao, and N. Soga, “Upconversion fluorescences of TeO2and Ga2O3-based oxide glasses containing Er,” J. Non-Cryst. Solids 122, 79-82 (1990). #81812 $15.00 USD Received 3 Apr 2007; revised 11 May 2007; accepted 13 May 2007; published 17 May 2007 (C) 2007 OSA 28 May 2007 / Vol. 15, No. 11 / OPTICS EXPRESS 6750 11. Y. Ohishi, A. Mori, M. Yamada, H. Ono, Y. Nishida, and K. Oikawa, “Gain characteristics of telluritebased erbium-doped fiber amplifiers for 1.5 μm broadband amplification,” Opt. Lett. 23, 274-276 (1998). 12. A. Mori, “1.58-μm Broad-band erbium-doped tellurite fiber amplifier,” IEEE J. Lightwave Technol. LT-20, 822-827 (2002). 13. R. Balda, J. Fernández, M.A. Arriandiaga, J. Fernández-Navarro, “Spectroscopy and frequency upconversion in Nd doped TeO2-TiO2-Nb2O5 glass,” J. Phys.: Conden. Matter 19, 086223-086234 (2007). 14. I. Iparraguirre, J. Azkargorta, J.M. Fernández-Navarro, M. Al-Saleh, J. Fernández, and R. Balda, “Laser action and upconversion of Nd in tellurite bulk glass,” J. Non-Cryst. Solids 353, 990-992 (2007). 15. S. Kim and T. Yoko, “Nonlinear Optical Properties of TeO2-Based Glasses: Mox-TeO2 (M=Sc, Ti, V, Nb, Mo, Ta, and W) binary glasses,” J. Am. Ceram. Soc. 78, 1061-1065 (1995). 16. H. Lin, G. Meredith, S. Jiang, X. Peng, XT. Luo, N. Peyghambarian, and E. Y. Pun, “Optical transitions and visible upconversion in Er doped niobic tellurite glass,” J. App. Phys. 93,186-191 (2003). 17. M. E Lines, “Oxide glasses for fast photonic switching: A comparative study,” J. App. Phys. 69, 68766884 (1991). 18. M.A. Villegas and J.M. Fernández Navarro, “Physical and structural properties of glasses in the TeO2-TiO2Nb2O5 system,” J. Eur. Ceram. Soc. 27, 2715-2723 (2007). 19. H. Nasu, T.Uchigaki, K. Kamiya, H. Kanbara, K. Kubodera, “Nonresonant-Type Third-order Nonlinearity of (PbO,Nb2O5)-TiO2-TeO2 Glass Measured by Third-Harmonic Generation,” Jpn. J. Appl. Phys. 31 38993900 (1992). 20. B.R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127, 750-761 (1962). 21. G.S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37, 511-520 (1962). 22. W.T. Carnall, P.R. Fields, K. Rajnak, “Spectral Intensities of the trivalent lanthanides and actinides in solution. II. Pm, Sm, Eu, Gd, Tb, Dy, and Ho,” J. Chem. Phys. 49, 4412-4423 (1968). 23. M. Eyal, R. Reisfeld, A. Schiller, C. Jacoboni, and C.K. Jorgensen, “Energy transfer between manganese (II) and thulium (III) in transition metal fluoride glasses”, Chem. Phys. Lett. 140, 595-602 (1987). 24. M.J. Weber, “Probabilities for radiative and nonradiative decay of Er in LaF3”, Phys. Rev. 157, 262-272 (1967). 25. A. Brenier, C. Pedrini, B. Moine, J.L. Adam, and C. Pledel, “Fluorescence mechanisms in Tm singly doped and Tm, Ho doubly doped indium-based fluoride glasses”, Phys. Rev. B 41, 5364-5371 (1990). 26. M.J. Weber, D.C. Ziegler, and C.A. Angell, “Tailoring stimulated emission cross sections of Nd laser glass: Observation of large cross sections for BiCl3 glasses”, J. Appl. Phys. 53, 4344-4350 (1982). 27. M. Naftaly, S. Shen, and A. Jha, “Tm-doped tellurite glass for a broadband amplifier at 1.47 μm”, Appl. Opt. 39, 4979-4984 (2000). 28. J.L. Doualan, S. Girard, H. Haquin, J.L. Adam, J. Montagne, “Spectroscopic properties and laser emission of Tm doped ZBLAN glass at 1.8 μm,” Opt. Mater. 24, 563-577 (2003). 29. M.J. Weber, “Luminescence decay by energy migration and transfer: observation of diffusion-limited relaxation”, Phys. Rev. B 4, 2932-2939 (1971). 30. M. Yokota and O. Tanimoto, “Effects of diffusion on energy transfer by resonance”, J. Phys. Soc. Japan 22, 779-784 (1967). 31. A. I. Burshtein, “Hopping mechanism of energy transfer,” Sov. Phys. JETP 35, 882-885 (1972). 32. Y.S. Han, J. Heo and Y.B. Shin, “Cross-relaxation mechanism among Tm ions in Ge30Ga2As6S62 glass,” J. Non-Cryst. Solids 316, 302-308 (2003). 33. A. Sennaroglu, A. Kurt, and G. Özen, “Effects of cross-relaxation on the 1470 and 1800 nm emissions in Tm:TeO2-CdCl3 glass,” J. Phys. Condens. Matter 16, 2471-2478 (2004). 34. F. Auzel, G. Baldacchini, L. Laversenne, and G. Boulon, “Radiation trapping and self-quenching analysis in Yb, Er, and Ho doped Y2O3,” Opt. Mater. 24, 103-109 (2003). 35. R. Balda, J. Fernández, M.A. Arriandiaga, L.M. Lacha, and J.M. Fernández-Navarro, “Effect of concentration on the infrared emissions of Tm ions in lead niobium germanate glasses,” Opt. Mater. 28, 1247-1252 (2006). 36. F. Auzel, “A fundamental self-generated quenching center for lanthanide-doped high-purity solids,” J. Lumin. 100, 125-130 (2002).


Introduction
In the last years the rapid expansion of bandwidth requirements for telecommunications in the 1400-1600 nm low-loss optical fiber transmission window has generated considerable interest in Tm 3+ ions.The 3 H 4 → 3 F 4 transition at around 1470 nm allows a band extension in the spectral range corresponding to the S-band amplifier region, on the short wavelength side of the conventional erbium-doped fiber amplifier C band at 1530-1570 nm.On the other hand, the thulium emission around 1800 nm is of interest to extend lasing capability into the 1600-1900 nm atmospheric window [1][2][3].However, there are some problems to develop efficient amplifiers using Tm 3+ -doped glasses.Two important factors to be considered in developing more efficient optical devices based on rare-earth ions are the glass host and the active ions concentration.The host matrix should have a low phonon energy to minimize multiphonon relaxation rates because the energy difference between the 3 H 4 and the next lower-lying 3 H 5 is not large (≈4300 cm -1 ).To avoid this problem, glasses with low phonon energies such as fluorides, tellurites, chalcogenides, and heavy metal oxide glasses are required.
Tellurite glasses have been a subject of increasing interest for optoelectronics applications, especially because of their high refractive index and low phonon energies [4,5].Moreover, these glasses combine good mechanical stability, chemical durability, and high linear and nonlinear refractive indices, with a wide transmission window (typically 0.4-6 μm), which make them promising materials for photonic applications such as upconversion lasers, optical fibre amplifiers, non linear optical devices, and so on [6][7][8][9][10][11][12][13][14].Broadband Er-doped fiber amplifiers have been achieved by using tellurite-based fibers as the erbium host [11,12].Among the different compositions studied, niobic tellurite glasses have proven to possess a large transparency window as well as a high refractive index and high stability [15][16][17][18].Besides, the addition of TiO 2 produces a further increase of the linear and nonlinear refractive indices.The high linear index increases the local field correction at the rare earth site leading to large radiative transition probabilities, whereas the non-linear one enhances the optical nonlinearities [15,17,19].
The second factor to be considered is the ion concentration.When the activator ion concentration in glass becomes high enough, ions interact and ion-ion energy transfer occurs.The energy transfer processes reduce the lifetime and consequently the efficiency of the 3 H 4 level due to the well-known cross relaxation process ( 3 H 4 , 3 H 6 → 3 F 4 , 3 F 4 ) [4].In this process part of the energy of an ion in the 3 H 4 level is transferred to another ion in the ground state with both ions ending up in the 3 F 4 level.
In this work, we present together with the spectroscopic properties of Tm 3+ ions in niobic tellurite glass (80TeO 2 -5TiO 2 -15Nb 2 O 5 ), the effect of concentration on the 3 H 4 → 3 F 4 and 3 F 4 → 3 H 6 emissions for different Tm 3+ concentrations (0.05, 0.1, 0.2, 0.5, and 1 wt%) and at different temperatures between 10 K and 295 K.As the Tm 3+ ions concentration is increased both infrared emissions show concentration quenching.In the case of the 1487 nm emission the non-exponential character of the decays from the 3 H 4 level with increasing concentration, together with the dependence of the quenching rates on Tm 3+ concentration, indicate the presence of a dipole-dipole quenching process in the framework of a diffusion-limited regime.The average critical distance, which indicates the extent to which the energy transfer can occur, has been obtained at different temperatures and compared with other glasses.Concerning the 1800 nm emission, the analysis of the experimental decays as a function of concentration indicates that the self-quenching can be attributed to limited diffusion within the active centers.

Experimental techniques
The glass was prepared by melting a 10 g batch of high purity TeO 2 (Alfa 99.99), Nb 2 O 5 (Alfa 99.995), and TiO 2 (Sigma Aldrich 99.99) reagents and heating them in a platinum crucible placed in a Thermostar vertical furnace, at 780º C during 30 min in air atmosphere.The melt was stirred with a platinum rod and then poured onto a preheated brass plate, annealed 15 min at 410º C, and further cooled at a rate of 3º C/min down to room temperature.The glass was doped with 0.05, 0.1, 0.2, 0.5, and 1 wt% of Tm 2 O 3 (Alfa 99.999) with an accuracy of 1% for the less concentrated sample.The Tm 3+ concentrations are based on the dopant added.No post-melting analysis was made.These concentrations correspond to 0.8x10 19 , 1.6x10 19 , 3.2x10 19 , 0.8x10 20 , and 1.6x10 20 ions/cm 3 .Finally the samples were cut and polished for optical measurements.
Conventional absorption spectra were performed with a Cary 5 spectrophotometer.The steady-state emission measurements were made with a Ti-sapphire ring laser (0.4 cm -1 linewidth) in the 760-940 nm spectral range as exciting light.The fluorescence was analyzed with a 0.25 monochromator, and the signal was detected by a PbS detector.Lifetime measurements were obtained by exciting the samples with a Ti-sapphire laser pumped by a pulsed frequency doubled Nd:YAG laser (9 ns pulse width), and detecting the emission with an extended IR Hamamatsu R5509-72 photomultiplier.Data were processed by a Tektronix oscilloscope.

Absorption and emission properties
The room temperature absorption spectra were obtained for all samples in the 400-2000 nm range with a Cary 5 spectrophotometer.Its spectral resolution was 0.5 nm at wavelengths below 1100, and 2 nm, above.Figure 1(a) shows the room temperature absorption cross section as a function of wavelength.The spectrum is characterized by six bands corresponding to the transitions starting from the 3 H 6 ground state to the different higher levels 1 G 4 , 3 F 2 , 3 F 3 , 3 H 4 , 3 H 5 , and 3 F 4 .Energy levels higher than 1 G 4 are not observed because of the intrinsic bandgap absorption in the host glass.The integrated absorption coefficient for different absorption bands shows a linear dependence on concentration, which indicates that the relative concentrations of Tm 3+ ions are in agreement with the nominal values.Figure 1(b) shows the energy level diagram showing the positions of the J states of the Tm 3+ ions in this glass derived from the absorption spectrum.Data from the spectrum in Fig. 1(a), together with the value of the refractive index (n=2.191)have been used to calculate the radiative transition rates by using the Judd-Ofelt (JO) theory [20,21].To obtain the contribution to the integrated absorption coefficient corresponding to levels 3 F 2 and 3 F 3 a Gaussian fit method has been used to separate the overlapping peaks into two independent ones.The absorption bands measured are all dominated by electric dipole transitions except the transition 3 H 6 → 3 H 5 , which contains electric-dipole and magnetic-dipole contributions.The magnetic-dipole contribution, f md , can be obtained from the equation f md =nf´ [22], where n is the refractive index of the studied glass and f´ is a quantity calculated on the basis of energy-level parameters for lanthanide aquo ions.The electric dipole oscillator strength for this transition is then obtained by subtracting the calculated magnetic-dipole contribution from the experimental oscillator strength.By using a least squares fitting of calculated and experimental oscillator strengths, the JO parameters obtained for this glass are Ω 2 =4.09x10 -20 cm 2 , Ω 4 =1.36x10 -2 cm 2 , and Ω 6 =1.19x10 -20 cm 2 , with a root-mean-squared deviation equal to 3.88x10 -7 .These values are  in agreement with those previously reported in tellurite glasses [23].The error analysis of the measured quantities used in the JO calculation gives an accuracy of 5%.The radiative transition probabilities for all excited levels of Tm 3+ can be calculated by using the JO parameters.The radiative transition probabilities, the branching ratios, and the radiative lifetimes of some selected levels of Tm 3+ in TeO 2 -TiO 2 -Nb 2 O 5 (TTN) glass are shown in Table 1.
Table 1.Predicted radiative transition rates, lifetimes, and branching ratios of Tm 3+ in TTN glass.

Transitions
Energies (cm -1 ) A rad (s -1 ) τ rad (ms) β (%) The infrared emissions in the 1300-2200 nm spectral range were obtained for all samples at room temperature by exciting at 793 nm. Figure 2 shows the fluorescence spectra corresponding to the 3 H 4 → 3 F 4 and 3 F 4 → 3 H 6 transitions normalized to the 3 H 4 → 3 F 4 transition for the samples doped with 0.1, 0.2, 0.5, and 1 wt% of Tm 2 O 3 .The spectra show a strong emission band centered around 1487 nm which corresponds to the 3 H 4 → 3 F 4 transition together with a less intense emission band centered around 1800 nm and corresponding to the 3 F 4 → 3 H 6 transition.The peak position and the bandwidth do not change with Tm 3+ concentration.However, the ratio of the integrated emission intensity of transition 3 H 4 → 3 F 4 to that of transition 3 F 4 → 3 H 6 decreases with increasing Tm 3+ concentration.This reduction of the intensity of the 1487 emission with concentration has been attributed to cross relaxation between 3 H 4 → 3 F 4 and 3 F 4 → 3 H 6 transitions [4,25] The room temperature stimulated emission cross section of the 3 H 4 → 3 F 4 laser transition has been obtained by using the following expression [26], broadband amplifiers specially in the wavelength range that overlaps the conventional band of erbium doped fiber amplifiers.The maximum emission cross-section is 0.4x10 -20 cm 2 which is sligthly higher than the one found in other tellurite glasses and twice the one of ZBLAN glass [27].The gain bandwidth of an amplifier is determined by the width of the emission spectrum and the emission cross-section.Using the figure of merit (FOM) for bandwidth as the product of the stimulated emission cross-section and FWHM, this value is nearly three times larger than in fluoride glass.Assuming that the FOM for bandwidth is an indication of the achievable gain band, the obtained value for these niobic-tellurite glasses suggests that these glasses may provide extended short wavelength gain of the erbium-doped C band at 1530-1570 nm. Figure 3 shows the spectral overlap between the 3 H 4 → 3 F 4 and 4 I 13/2 → 4 I 15/2 transitions of Tm 3+ and Er 3+ ions respectively in TTN glasses.We have also obtained the room temperature stimulated emission cross section of the 3 F 4 → 3 H 6 laser transition by using expression (1).In this case, the branching ratio for an emission between the first excited level and the ground state is equal to unity.The maximum emission cross section is 0.92x10 -20 cm 2 .As for the 3 H 4 → 3 F 4 transition, this value is more than twice the one of ZBLAN glass [28].

Lifetimes
The lifetime values of the 3 H 4 level were obtained for different Tm 3+ concentrations as a function of temperature by exciting at 793 nm.The fluorescence lifetime at low concentration and temperature is equal to the calculated radiative lifetime (293.5 μs); however, as concentration increases, the lifetime decreases even at low temperature, which indicates the presence of nonradiative energy transfer processes.The fluorescence decays for the samples doped with 0.05, 0.1, 0.2, and 0.5% of Tm 2 O 3 can be described at all temperatures by an exponential function to a good approximation.For the sample doped with 1 wt% the decays become non exponential.As an example, Fig. 4 shows the logarithmic plot of the experimental decays of the 3 H 4 level at 295 K for the samples doped with 0.1, 0.5, and 1 wt%.Figure 5 shows the lifetime values of the 3 H 4 level between 10 K and 295 K for the samples doped with 0.1, 0.2, 0.5, and 1 wt%.The lifetime of the sample doped with 0.05% (not shown in the figure) is equal to the radiative lifetime at low temperature.However it shows a small decrease at room temperature probably related to the presence of some impurity ions.In fact, infrared transmission measurements reveal the presence of a band around 3 μm corresponding to OH -impurities.The lifetime values for the sample doped with 1 wt% correspond to the average lifetime defined by 0 ) ( The decays of the 3 F 4 level were obtained for all samples at 295 K by exciting at 793 nm.The decays show an initial rise, due to the lifetime of the 3 H 4 level, followed by the decay.The decays of the samples doped with 0.05, 0.1, 0.2, and 0.5 wt% can be described by an exponential function to a good approximation.However, the decay of the sample doped with 1 wt% deviates from a simple exponential.As an example, Fig. 6 shows the logarithmic plot of the experimental decays of the 3 F 4 level at 295 K for the samples doped with 0.1 and 1 wt%.The lifetimes decrease from 2.16 ms to 1.56 ms as concentration increases from 0.05 to 1 wt% of Tm 2 O 3 .

Concentration quenching of the 3 H 4 emission
As we mentioned in Section 3, the emission from the 3 H 4 level shows concentration quenching, and as concentration rises the 1487 nm emission becomes less intense whereas the relative intensity of the 1800 nm emission increases.This concentration quenching is also reflected by the decrease of the experimental lifetimes.This behavior has been previously observed in Tm 3+ -doped systems and attributed to cross-relaxation between Tm 3+ ions [4].In this process part of the energy of an ion in the 3 H 4 level is transferred to another ion in the ground state with both ions ending up in the 3 F 4 level ( 3 H 4 , 3 H 6 → 3 F 4 , 3 F 4 ).This process reduces the lifetime of the 3 H 4 level and consequently the efficiency of the 1487 nm emission.
The characteristic decay time of the 3 H 4 level should be governed by a sum of probabilities for several competing processes: radiative decay, nonradiative decay by multiphonon relaxation, and by energy transfer to other Tm 3+ ions.In these tellurite glasses nonradiative decay by multiphonon relaxation is expected to be small because the energy difference between 3 H 4 and 3 H 5 levels is 4240 cm -1 and the energy of the highest phonons is about 780 cm -1 .This corresponds to 5.4 phonons, which indicates that multiphonon relaxation process is weak and can be neglected in this case.Energy transfer processes such as cross-relaxation are generally described in terms of three limiting cases: (i) direct relaxation, (ii) fast diffusion, and (iii) diffusion limited relaxation [29].In the diffusion limited relaxation model, in the case of the dipole-dipole interaction, the quenching rate is given by, where τ R is the intrinsic decay time, K is a constant involving donor-donor and donor-acceptor transfer constants, and N A and N D are the acceptor and donor concentrations respectively.In our case the donor and acceptors are the Tm 3+ ions and the equation gives the quenching rate as a function of the square of concentration.
In the case of very fast diffusion, the decay of the donor fluorescence is purely exponential and the quenching rate shows a linear dependence on concentration.Figure 7 shows the quenching rate of the 3 H 4 level as a function of the square of Tm 3+ concentration at room temperature.The intrinsic decay time τ R corresponds to the lifetime of the lowest concentrated sample which is equal to the radiative lifetime (293.5 μs).As can be observed, in this concentration range the quenching rate shows a linear dependence on the square of concentration which indicates that the behavior is close to a dipole-dipole quenching mechanism in the framework of a limited-diffusion regime.The same behavior is observed at low temperature.Taking into account the existence of energy migration among donors, we have used the Yokota-Tanimoto and Burshtein expressions to fit the donor fluorescence decay for an energy transfer assisted by donor migration [30,31].The best agreement between experimental data and theoretical fit is obtained with the expression corresponding to the Burshtein model, ( ) where τ R is the intrinsic lifetime of donor ions, γ characterizes the direct energy transfer, and W represents the migration parameter.In the case of dipole-dipole interaction, γ is given by , where N is the concentration and C DA is the energy transfer microparameter.Figure 8 shows the fit for the sample doped with 1% wt.These results indicate that the electronic mechanism of energy transfer is a dipole-dipole interaction in the framework of a diffusion-limited regime.From the fitting in Fig. 8, the value obtained for the energy transfer microparameter is (1.14±0.07)x10 -3 cm 6 /s and the migration transfer rate was found to be (579±10) s -1 .
The value for the critical radius R 0 , which is defined as the distance at which the probability of the cross-relaxation process becomes equal to the intrinsic decay rate of the metastable level, can be calculated in terms of C DA and τ R from .The obtained value for the critical transfer radius in this glass is 8.3±0.02Å which means that energy transfer can occur among ions located within this distance.This value is larger than the one reported in the case of thulium chalcogenide glass (7.3 Å) [32] and much shorter than the 17.9 Å value recently reported for thulium doped TeO 2 -CdCl 2 glass [33].The critical distance decreases from 8.3 Å at 295 K to 7.0 Å at 10 K indicating that the cross-relaxation process is less efficient at low temperature.On the other hand the migration transfer rate increases from 116 to 579 s -1 as temperature increases from 10 K to 295 K.The increase of the migration transfer rate with temperature has been examined by Weber in Ref. 29.This rate depends on the frequencies, linewidths, and probabilities of the transitions involved in the process.At low temperature only the lowest Stark levels are populated and the number of resonant transitions giving rise to energy migration are reduced [29].

Concentration quenching of the 3 F 4 emission
Concerning the 1800 nm emission, the relative luminescence intensity increases whereas its lifetime decreases with concentration.In this case, in which level 3 F 4 is the first excited state, the quenching of luminescence when active ion concentration increases can not be due to cross-relaxation between various excited states, and it has been mainly considered as due to diffusion towards unidentified impurities (such as OH or other impurities present in the starting materials) [34].The problem can be separated into two cases: diffusion limited regime and fast diffusion.The first case is considered to occur when the order of magnitude for transfer probability between sensitizers and sensitizer to activator are the same.As we have seen, in the case of the diffusion limited situation the quenching rate is proportional to the square of concentration.As it is shown in Fig. 9 the quenching rate of level 3 F 4 shows a linear behavior as a function of the square of Tm 3+ concentration, which indicates that we are dealing with a diffusion limited regime.In such a case, and assuming a dipole-dipole interaction, the self quenching behavior can be described by [34], where τ w is the measured lifetime at low concentration (2.16 ms) and N 0 is a critical sensitizer concentration for self-quenching.The longer measured lifetime compared with the radiative one could be due to radiative trapping.Figure 10 shows the experimental values together with the fit to expression (4).The critical concentration is (3.08±0.17)x10 20ions/cm 3 .This value is an indication of the self-quenching.The obtained value for the critical concentration is higher than the values obtained in Tm-doped lead-niobium-germanate glass [35] and Er-doped phosphate glasses [36].As can be seen from Fig. 10, in these glasses the diffusion limited hypothesis gives a rather good description of the experimental results.

Conclusions
Absorption and luminescence measurements have been performed in Tm 3+ doped niobic tellurite glasses.The Judd-Ofelt intensity parameters and radiative transition rates have been calculated.The infrared emissions at 1487 and 1800 nm have been characterized for concentrations ranging from 0.05 to 1 wt% of Tm 2 O 3 .Fluorescence measurements show that the 1487 nm emission is broader by nearly 30 nm in this glass if compared to fluoride glass and the stimulated emission cross section is twice which makes these glasses attractive for broadband amplifiers specially in the wavelength range that overlaps the conventional band of the erbium doped fiber amplifier.
The 3 H 4 → 3 F 4 emission intensity at 1487 nm was found to decrease in relation to the 3 F 4 → 3 H 6 emission at 1800 nm as thulium concentration increases, due to the presence of cross-relaxation processes.An analysis of the fluorescence decays of the 3 H 4 → 3 F 4 emission as a function of concentration reveals that the electronic mechanism responsible for the ion-ion interaction is a dipole-dipole quenching process in the framework of a diffusion-limited regime.
The self-quenching of the 1800 nm emission can be attributed to limited diffusion within the active centers.This means that the probability for the diffusive steps between active centers is of the same order of magnitude than the one for quenching between impurities and centers.

)
$15.00 USD Received 3 Apr 2007; revised 11 May 2007; accepted 13 May 2007; published 17 May 2007 (C) 2007 OSA λ p is the peak fluorescence wavelength, β is the branching ratio for the transition, n is the index of refraction of the host matrix, c the velocity of light, τ R the radiative lifetime of the emitting level, and Δλ eff is the effective linewidth.The effective linewidth (105 nm) of the transition has been calculated by using the relation broader by nearly 30 nm than the one in fluoride glasses.This makes this niobic tellurite glass attractive for #81812 -$15.00USD Received 3 Apr 2007; revised 11 May 2007; accepted 13 May 2007; published 17 May 2007 (C) 2007 OSA

Fig. 4 .
Fig. 4. Logarithmic plot of the fluorescence decay of the 3 H 4 level obtained under excitation at 793 nm at room temperature for the samples doped with 0.1, 0.5, and 1 wt%.

Fig. 6 .
Fig.6.Logarithmic plot of the fluorescence decay of the 3 F 4 level obtained under excitation at 793 nm at room temperature for the samples doped with 0.1 and 1 wt%.The inset shows the rise times.

Fig. 7 .
Fig. 7. Quenching rates of the 3 H 4 emission as a function of the square of Tm 3+ concentration at room temperature.

Fig. 8 .
Fig. 8. Experimental emission decay curve of level 3 H 4 for the sample doped with 1 wt% of Tm 2 O 3 at room temperature and the calculated fit with equation (3) (solid line).

Fig. 9 .Fig. 10 .
Fig.9.Quenching rates of the 3 H 4 emission as a function of the square of Tm 3+ concentration at room temperature.