Study of broadband near-infrared emission in Tm 3 +-Er 3 + codoped TeO 2-WO 3-PbO glasses

In this work, we report the near-infrared emission properties of Tm 3+ -Er 3+ codoped tellurite TeO2-WO3-PbO glasses under 794 nm excitation. A broad emission from 1350 to 1750 nm corresponding to the Tm 3+ and Er 3+ emissions is observed. The full width at half-maximum of this broadband increases with increasing [Tm]/[Er] concentration ratio up to a value of ~ 160 nm. The energy transfer between Tm 3+ and Er 3+ ions is evidenced by both the temporal behavior of the near-infrared luminescence and the effect of Tm 3+ codoping on the visible upconversion of Er 3+ ions. ©2009 Optical Society of America OCIS codes: (160.5690) Rare earth doped materials; (300.6280) Spectroscopy, fluorescence and luminescence. References and links 1. H. Jeong, K. Oh, S.R. Han, and T.F. Morse, “Characterization of broadband amplified spontaneous emission from a Er-Tm co-doped silica fiber,” Chem. Phys. Lett. 367, 507-511 (2003). 2. Lihui Huang, Anismesh Jha, Shaoxiong Shen, Xiaobo Liu, “Broadband emission in Er-Tm codoped tellurite fibre,” Opt. Express 12, 2429-2434 (2004). 3. Z. Xiao, R. Serna, C.N. Afonso, “Broadband emission in Er-Tm codoped Al2O3 films: The role of energy transfer from Er to Tm,” J. Appl. Phys. 101, 033112-6 (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,


Introduction
In the last years, the rapid development of telecommunication and other data-transmitting services, demands to increase the transmission capacity of wavelength division multiplexing (WDM) systems.This requires broadband optical amplification beyond the conventional 1.5 µm window of Er-doped fiber amplifiers (EDFA) in order to fully utilize the 1.2-1.7 µm lowloss band of silica-based optical fibers.A logical extension of EDFAs would be the addition of other rare-earth ions such as Tm 3+ [1][2][3].The 3 H 4 → 3 F 4 transition of Tm 3+ ions at around 1470 nm will allow 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+L bands at 1530-1600 nm.
In order to achieve both practical gain and wide gain flatness, the choice of the host glass matrix is very important.Among oxide glasses, tellurite glasses have attracted a considerable interest especially because of their special properties.These glasses have smaller phonon energies than other oxide glasses such as silicate, phosphate, and borate glasses [4,5].Moreover, they 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 fiber amplifiers, non linear optical devices, and so on [6][7][8][9][10][11][12][13][14][15].Broadband Er-doped fiber amplifiers have been achieved by using tellurite-based fibers as the erbium host [11,12] and very recently, efficient laser emission around 2 µm has been demonstrated in a tellurite fiber [15].However, one drawback of tellurite glasses is the low glass transition temperature (290 ºC) which makes them liable to thermal damage at high pumping intensities.On the other hand, the phonon energy is relatively low (700-800 cm -1 ) resulting in efficient upconversion fluorescence which can be a loss source for the 1.5 µm emission of Er 3+ ions.To overcome these drawbacks different compositions have been studied.Tellurite glasses containing WO 3 and PbO have the advantages of a higher glass transition temperature (380 ºC), higher phonon energies (≈925 cm -1 ), higher linear and non linear refractive indexes [16], and a significantly broadened emission at 1.5 µm (Er 3+ ) [11,12] and 1.4 µm (Tm 3+ ) [17].The stronger covalent bonding of the WO 3 network increases the glass transition temperature and is responsible for the higher phonon energy of tungsten tellurite glasses as compared with tellurites.On the other hand, the addition of PbO increases the linear and nonlinear refractive indexes.The higher phonon energy of these glasses makes the non-radiative relaxation from 4 I 11/2 to 4 I 13/2 levels under 980 nm excitation more efficient and thus enhances the optical pumping efficiency for the 1.5 µm emission of Er 3+ ions.The high linear index increases the local field correction at the rare-earth site leading to large radiative transition probabilities.On the other hand, the presence of two glass formers, such as TeO 2 and WO 3 produces a more complex network structure with a great variety of sites for the RE ions which contributes to the inhomogeneous broadening of the emission bands.
In this work, we characterize the spectroscopic properties of Tm 3+ -Er 3+ codoped tellurite TeO 2 -WO 3 -PbO glasses for different Tm 3+ and Er 3+ concentrations by using steady-state and time-resolved laser spectroscopy.The study includes absorption and emission spectroscopy and lifetime measurements for the infrared fluorescence.The broad emission obtained from 1350 to 1750 nm with a full width at half-maximum of ~ 160 nm suggests that these glasses could be promising materials for broadband light sources and broadband amplifiers for wavelength-division-multiplexing (WDM) transmission systems.The energy transfer between Tm 3+ and Er 3+ ions is evidenced by both the temporal behavior of the near-infrared luminescence and the effect of Tm 3+ codoping on the visible upconversion of Er 3+ ions in the codoped samples.
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 an extended IR Hamamatsu R5509-72 photomultiplier and finally amplified by a standard lock-in technique.Upconversion emission was detected by a Hamamatsu R928 photomultiplier.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 a Hamamatsu R5509-72 photomultiplier.Data were processed by a Tektronix oscilloscope.All measurements were performed at room temperature.

Results and discussion
The room temperature absorption spectra were obtained for all samples in the 300-2000 nm range with a Cary 5 spectrophotometer.As an example, Fig. 1 shows the absorption spectrum of the codoped glass with 0.5 wt% Tm 2 O 3 and 0.3 wt% Er 2 O 3 .The spectrum is characterized by the transition bands from the 3 H 6 ground state to the different higher levels 1 G 4 , 3 F 2,3 , 3 H 4 , 3 H 5 , and 3 F 4 of Tm 3+ ions, together with the Er 3+ absorption bands from the 4 I 15/2 ground state to the higher levels.Energy levels higher than 1 G 4 are not observed because of the intrinsic bandgap absorption in the host glass.The spectra obtained for the other samples are similar, except for the band intensities, which are dependent on the Tm 3+ and Er 3+ concentrations.The integrated absorption coefficient for different absorption bands shows a linear dependence on concentration, which indicates that the relative concentrations of Tm 3+ and Er 3+ ions are correct.The near infrared emissions in the 1300-1750 nm spectral range were obtained for all samples at room temperature by exciting at 794 nm.At this wavelength we excite both the 3 H 4 (Tm 3+ ) and 4 I 9/2 (Er 3+ ) levels. Figure 2 shows the emission spectra for the Er-single doped sample and the codoped samples with 0.1 wt% of Er 2 O 3 and 0.5 wt% of Tm 2 O 3 and with 0.3 wt% of Er 2 O 3 and 0.3, 0.5, and 0.7 wt% of Tm 2 O 3, normalized to the Er 3+ emission.The emission spectra of the codoped samples shows 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, together with the short wavelength tail of the Tm 3+ emission corresponding to the 3 F 4 → 3 H 6 transition.This transition is not observed due to the upper limit of the detector at 1700 nm.The spectrum of the Er-doped glass corresponding to the 4 I 13/2 → 4 I 15/2 transition has the maximum at 1535 nm with a full width at half maximum (FWHM) of 80 nm.As can be seen in Fig. 2 the FWHM of the spectra of the codoped samples increases with increasing [Tm]/[Er] concentration ratio.Moreover, in the samples with a fixed concentration of 0.3 wt% of Er 2 O 3 and different Tm 2 O 3 concentrations the 3 H 4 → 3 F 4 transition of Tm 3+ ions increases with the increasing concentration of Tm 3+ ions.The intensity balance between the 3 H 4 → 3 F 4 and 4 I 13/2 → 4 I 15/2 transitions depends on the Tm 3+ concentration becoming nearly equal to unity for a sample doped with 0.1 wt% of Er 2 O 3 and 0.5 wt% of Tm 2 O 3 .The emission of this sample has a FWHM around 160 nm which covers bands S at 1440-1530 nm, C+L at 1530-1600, and U at 1600-1675 nm respectively.The FWHM of around 160 nm is much larger than that of the Tm-Er codoped silica fiber (90 nm) [1] and comparable to that reported in Tm 3+ -Er 3+ codoped tellurite fiber [2].The 3 H 4 → 3 F 4 emission in Tm 3+ single doped glass shows an effective linewidth of 102 nm which is broader by nearly 30 nm if compared to fluoride glasses and a maximum emision cross-section of 0.4x10 -20 cm 2 which is twice the one in ZBLAN glass.An extensive presentation of the spectroscopic properties of the 3 H 4 → 3 F 4 emission of Tm 3+ ions in TWP glasses was given by the authors in a separate paper [18].Characteristic decays of the codoped samples were obtained under laser pulsed excitation at 794 nm at two different emission wavelengths for all codoped samples.The lifetimes for Tm 3+ were measured at 1450 nm, whereas for Er 3+ the emission was monitored at 1550 nm. Figure 3(a) shows the lifetime values of the 4 I 13/2 level in a single doped sample with 0.3 wt% of Er 2 O 3 and in the codoped samples with the same Er 2 O 3 concentration.The time dependent behavior of the Er 3+ fluorescence from the codoped samples is shown in Fig. 3(b) together with the decay of the single doped sample.As can be seen, the Er 3+ fluorescence from the codoped samples shows a non-exponential behavior, and a shortening of the lifetime as Tm 3+ concentration increases as compared with the single doped sample.This behavior is attributed to the additional probability of relaxation by nonradiative energy transfer to Tm 3+ ions.On the other hand, the emission spectra obtained under excitation at 770 nm where only Tm 3+ ions absorb show in addition to the Tm 3+ emission, the Er 3+ emission band which indicates the presence of Tm 3+ →Er 3+ energy transfer processes.A further evidence of this energy transfer can be obtained from the lifetimes of the 3 H 4 level of Tm 3+ ions in the presence of Er 3+ ions.As can be seen in Fig. 4, there is a shortening of the lifetimes of Tm 3+ as compared with the values in the single doped samples with the same Tm 3+ concentration.To further investigate the energy transfer mechanisms, we have measured the visible emission spectra of Er 3+ -doped and Er 3+ -Tm 3+ codoped samples at room temperature under excitation at 794 nm within the 4 I 9/2 (Er 3+ ) and 3 H 4 (Tm 3+ ) levels.No upconversion emission was observed in the Tm 3+ single-doped sample.Figure 5 shows the upconversion emission spectra for the samples with 0.3 wt% of Er 2 O 3 and 0, 0.3, 0.5, and 0.7 wt% of Tm 2 O 3 .The observed emissions correspond to transitions 2 H 11/2 → 4 I 15/2 (530 nm), 4 S 3/2 → 4 I 15/2 (550 nm), and 4 F 9/2 → 4 I 15/2 (665 nm).The 2 H 11/2 → 4 I 15/2 transition is only observed at room temperature because 2 H 11/2 is populated from 4 S 3/2 via a fast thermal equilibrium between both levels.As can be observed the green emission reduces the intensity as Tm 3+ concentration increases.The weak red emission from the 4 F 9/2 level is due to the population of this level from the 4 S 3/2 through multiphonon relaxation.As can be seen in the inset of Fig. 5, there is a slight increase in the red emission intensity with increasing Tm 3+ concentration.The addition of Tm 3+ into the Er 3+ -doped glass affects the green and red emission intensities, which indicates the presence of efficient energy transfer between both ions.The observed behaviour of the near infrared and visible luminescence for the Tm 3+ -Er 3+ codoped samples could be explained on the basis of the energy level diagram of Tm 3+ and Er 3+ ions shown in Fig. 6.In the first step the laser excitation populates the 3 H 4 level of Tm 3+ and the 4 I 9/2 level of Er 3+ .After excitation in the 3 H 4 level the relaxation of Tm 3+ ions from 3 H 4 → 3 F 4 yields the emission at 1470 nm, whereas in the case of Er 3+ ions after excitation in the 4 The first energy transfer process, which is a near-resonant energy transfer (energy mismatch +150 cm -1 ), depopulates 3 H 4 level and populates the 4 I 9/2 level.Process (2) with an energy mismatch of +180 cm -1 promotes Er 3+ ions to the first excited state.These processes can explain the presence of the Er 3+ emission when we only excite Tm 3+ ions and the reduction of the lifetimes of level 3 H 4 in the presence of Er 3+ ions.Processes (3) and ( 4) are related to the depopulation of levels 4 I 11/2 and 4 I 13/2 .Process (3) is a nonresonant process (energy mismatch +2870 cm -1 ) in which one Er 3+ ion relaxes from 4 I 11/2 level to ground state and transfers its energy to a Tm 3+ ion in the ground state which is in turn promoted to the level 3 H 5 from which it decays nonradiatively to level 3 F 4 .This process should reduce the intensity of the 1535 nm emission and enhance the 1820 nm emission from Tm 3+ ions.Process (4) depopulates the 4 I 13/2 level of Er 3+ ions and can also reduce the lifetime of the 1535 nm emission of Er 3+ and consequently produce a reduction of the upconverted green emission due to the ESA process.
The enhancement of the red emission could be due to an increase of the energy transfer processes feeding the 4 F 9/2 level.This effect has been previously observed and attributed to process (5) which increases the population of level 4 F 9/2 [20][21][22].The energy mismatch of this process is +800 cm -1 , and thus this energy transfer process is likely to occur at room temperature.Fig. 6.Energy level diagram of Tm 3+ and Er 3+ ions.Solid lines represent the absorption and near infrared emissions and dashed lines the possible cross-relaxation process.

Conclusions
The near-infrared emission of Tm 3+ and Er 3+ ions in codoped tellurite glasses of composition 50TeO 2 -30WO 3 -20PbO obtained under 794 nm excitation shows a broadband luminescence in the 1350-1750 nm range corresponding to the 3 H 4 → 3 F 4 and 3 F 4 → 3 H 6 emissions of Tm 3+ and the 4 I 13/2 → 4 I 15/2 emission of Er 3+ ions, covering the complete telecommunication window of the wavelength-division-multiplexing (WDM) transmission systems.The FWHM and the relative intensity of the 1470 nm and 1535 nm emissions depend on the [Tm]/[Er] concentration ratio.A FWHM of about 160 nm is obtained by codoping the glass with 0.5 wt% of Tm 2 O 3 and 0.1 wt% of Er 2 O 3 which suggests that these glasses can be promising materials for broadband light sources and broadband amplifiers for WDM transmission systems.
The Tm 3+ -Er 3+ energy transfer processes reduce the lifetimes of 4 I 13/2 (Er 3+ ) and 3 H 4 (Tm 3+ ) levels in the codoped samples.The Er 3+ →Tm 3+ transfer efficiency reaches 80% for the highest Tm 3+ concentration.However, this efficient energy transfer reduces, as expected, the Er 3+ emission efficiency at 1535 nm if compared to the one of the single doped sample.The upconverted emission of Er 3+ ions in the codoped samples also evidences the presence of energy transfer between Tm 3+ and Er 3+ ions.The addition of Tm 3+ reduces the upconverted green emission due to energy transfer between Er 3+ and Tm 3+ whereas the red emission is slightly enhanced due to the cross-relaxation 3 F 4 → 3 H 6 (Tm 3+ ): 4 I 11/2 → 4 F 9/2 (Er 3+ ) process.

Fig. 1 .
Fig. 1.Room temperature absorption spectrum of the codoped sample with 0.5 wt% of Tm2O3 and 0.3 wt% of Er2O3 in TWP glass.

Fig. 2 .
Fig.2.Room temperature emission spectra of Tm 3+ and Er 3+ in the codoped samples together with the emission spectrum of Er 3+ ions in the single doped glass.

Fig 4 .
Fig 4. Lifetimes of the 3 H4 level of Tm 3+ ions in single doped samples (red) and in the codoped samples (blue).