1. 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 low-loss band of silica-based optical fibers. A logical extension of EDFAs would be the addition of other rare-earth ions such as Tm³⁺ [1-3]. The ³H₄→³F₄ transition of Tm³⁺ 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 [6-15]. 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 °C) 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 Er³⁺ ions. To overcome these drawbacks different compositions have been studied. Tellurite glasses containing WO₃ and PbO have the advantages of a higher glass transition temperature (380 °C), higher phonon energies (≈925 cm^-1), higher linear and non linear refractive indexes [16], and a significantly broadened emission at 1.5 μm (Er³⁺) [11,12] and 1.4 μm (Tm³⁺) [17]. The stronger covalent bonding of the WO₃ 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 ⁴I_11/2 to ⁴I_13/2 levels under 980 nm excitation more efficient and thus enhances the optical pumping efficiency for the 1.5 μm emission of Er³⁺ 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 TeO₂ and WO₃ 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 Tm³⁺-Er³⁺ codoped tellurite TeO₂-WO₃-PbO glasses for different Tm³⁺ and Er³⁺ 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 Tm³⁺ and Er³⁺ ions is evidenced by both the temporal behavior of the near-infrared luminescence and the effect of Tm³⁺ codoping on the visible upconversion of Er³⁺ ions in the codoped samples.

2. Experimental techniques

The glasses with mol% composition 50TeO₂-30WO₃-20PbO (TWP) were prepared by melting 10 g batches of high-purity TeO₂ (Sigma-Aldrich 99.995), WO₃ (Aldrich 99.995), and PbO (99.999 Aldrich) reagents in a platinum crucible placed in an electrical Thermostar^TM furnace at variable temperatures between 710 and 740 ° C during 30-45 min. The melts were stirred with a platinum rod and then poured onto a preheated brass plate, annealed 15 min at 390-400 ° C, and further cooled at a 3 ° C/min rate down to room temperature. The codoped samples were prepared with 0.3 wt% Er₂O₃ (6.5×10¹⁹ Er³⁺ ions/cm³) and different Tm₂O₃ concentrations (0.1, 0.3, 0.4, 0.5, and 0.7 wt%) which correspond to 2.1×10¹⁹, 6.4×10¹⁹, 8.5×10¹⁹, 1.1×10²⁰, and 1.5×10²⁰ Tm³⁺ ions/cm³ respectively and with 0.1 wt% of Er₂O₃ (2.1×10¹⁹ Er³⁺ ions/cm³) and 0.3 and 0.5 wt% of Tm₂O₃ respectively. The optical measurements were carried out on polished planoparallel glass slabs of about 2 mm thickness.

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.

3. 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% Tm₂O₃ and 0.3 wt% Er₂O₃. The spectrum is characterized by the transition bands from the ³H₆ ground state to the different higher levels ¹G₄, ³F_2,3, ³H₄, ³H₅, and ³F₄ of Tm³⁺ ions, together with the Er³⁺ absorption bands from the ⁴I_15/2 ground state to the higher levels. Energy levels higher than ¹G₄ 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 Tm³⁺ and Er³⁺ concentrations. The integrated absorption coefficient for different absorption bands shows a linear dependence on concentration, which indicates that the relative concentrations of Tm³⁺ and Er³⁺ ions are correct.

 

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.

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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 ³H₄ (Tm³⁺) and ⁴I_9/2 (Er³⁺) levels. Figure 2 shows the emission spectra for the Er-single doped sample and the codoped samples with 0.1 wt% of Er₂O₃ and 0.5 wt% of Tm₂O₃ and with 0.3 wt% of Er₂O₃ and 0.3, 0.5, and 0.7 wt% of Tm₂O₃, normalized to the Er³⁺ emission. The emission spectra of the codoped samples shows the ³H₄→³F₄ and ⁴I_13/2→⁴I_15/2 transitions of Tm³⁺ and Er³⁺ ions respectively, together with the short wavelength tail of the Tm³⁺ emission corresponding to the ³F₄→³H₆ 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 ⁴I_13/2→⁴I_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 Er₂O₃ and different Tm₂O₃ concentrations the ³H₄→³F₄ transition of Tm³⁺ ions increases with the increasing concentration of Tm³⁺ ions. The intensity balance between the ³H₄→³F₄ and ⁴I_13/2→⁴I_15/2 transitions depends on the Tm³⁺ concentration becoming nearly equal to unity for a sample doped with 0.1 wt% of Er₂O₃ and 0.5 wt% of Tm₂O₃. 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 Tm³⁺-Er³⁺ codoped tellurite fiber [2]. The ³H₄→³F₄ emission in Tm³⁺ 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 0.4×10^-20 cm² which is twice the one in ZBLAN glass. An extensive presentation of the spectroscopic properties of the ³H₄→³F₄ emission of Tm³⁺ ions in TWP glasses was given by the authors in a separate paper [18].

 

Fig. 2. Room temperature emission spectra of Tm³⁺ and Er³⁺ in the codoped samples together with the emission spectrum of Er³⁺ ions in the single doped glass.

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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 Tm³⁺ were measured at 1450 nm, whereas for Er³⁺ the emission was monitored at 1550 nm. Figure 3(a) shows the lifetime values of the ⁴I_13/2 level in a single doped sample with 0.3 wt% of Er₂O₃ and in the codoped samples with the same Er₂O₃ concentration. The time dependent behavior of the Er³⁺ 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 Er³⁺ fluorescence from the codoped samples shows a non-exponential behavior, and a shortening of the lifetime as Tm³⁺ concentration increases as compared with the single doped sample. This behavior is attributed to the additional probability of relaxation by nonradiative energy transfer to Tm³⁺ ions.

 

Fig.3. (a) Lifetimes of the ⁴I_13/2→⁴I_15/2 emission (red) and Er³⁺-Tm³⁺ energy transfer efficiency (blue) as a function of Tm³⁺ concentration including zero. (b) Logarithmic plot of the fluorescence decays of the ⁴I_13/2→⁴I_15/2 emission as a function of Tm³⁺ concentration.

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The Er³⁺→Tm³⁺ energy transfer efficiency has been estimated from the lifetime values in the single doped and codoped samples according to the expression,

where τ _Tm-Er and τ _Er are the Er³⁺ lifetimes monitored at 1550 nm, with and without Tm³⁺ ions respectively. The lifetime values for the codoped samples correspond to the average lifetime defined by $〈\tau 〉=\genfrac{}{}{0.1ex}{}{\int I\left(t\right)\mathit{dt}}{I_0}.$ Fig 3(a) also shows the Er³⁺ → Tm³⁺ transfer efficiencies for the codoped samples with 0.3 wt% of Er₂O₃ as a function of Tm³⁺ concentration. The transfer efficiency increases up to 80% for the sample with the highest concentration of Tm³⁺ ions.

On the other hand, the emission spectra obtained under excitation at 770 nm where only Tm³⁺ ions absorb show in addition to the Tm³⁺ emission, the Er³⁺ emission band which indicates the presence of Tm³⁺→Er³⁺ energy transfer processes. A further evidence of this energy transfer can be obtained from the lifetimes of the ³H₄ level of Tm³⁺ ions in the presence of Er³⁺ ions. As can be seen in Fig. 4, there is a shortening of the lifetimes of Tm³⁺ as compared with the values in the single doped samples with the same Tm³⁺ concentration.

 

Fig 4. Lifetimes of the ³H₄ level of Tm³⁺ ions in single doped samples (red) and in the codoped samples (blue).

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To further investigate the energy transfer mechanisms, we have measured the visible emission spectra of Er³⁺-doped and Er³⁺-Tm³⁺ codoped samples at room temperature under excitation at 794 nm within the ⁴I_9/2 (Er³⁺) and ³H₄ (Tm³⁺) levels. No upconversion emission was observed in the Tm³⁺ single-doped sample. Figure 5 shows the upconversion emission spectra for the samples with 0.3 wt% of Er₂O₃ and 0, 0.3, 0.5, and 0.7 wt% of Tm₂O₃. The observed emissions correspond to transitions ²H_11/2→⁴I_15/2 (530 nm), ⁴S_3/2→⁴I_15/2 (550 nm), and ⁴F_9/2→⁴I_15/2 (665 nm). The ²H_11/2→⁴I_15/2 transition is only observed at room temperature because ²H_11/2 is populated from ⁴S_3/2 via a fast thermal equilibrium between both levels. As can be observed the green emission reduces the intensity as Tm³⁺ concentration increases. The weak red emission from the ⁴F_9/2 level is due to the population of this level from the ⁴S_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 Tm³⁺ concentration. The addition of Tm³⁺ into the Er³⁺-doped glass affects the green and red emission intensities, which indicates the presence of efficient energy transfer between both ions.

 

Fig. 5. Upconversion emission spectra of Er³⁺ ions in the single doped and codoped samples.

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The observed behaviour of the near infrared and visible luminescence for the Tm³⁺-Er³⁺ codoped samples could be explained on the basis of the energy level diagram of Tm³⁺ and Er³⁺ ions shown in Fig. 6. In the first step the laser excitation populates the ³H₄ level of Tm³⁺ and the ⁴I_9/2 level of Er³⁺. After excitation in the ³H₄ level the relaxation of Tm³⁺ ions from ³H₄→³F₄ yields the emission at 1470 nm, whereas in the case of Er³⁺ ions after excitation in the ⁴I_9/2 level, multiphonon relaxation occurs to level ⁴I_11/2, and part of the excitation energy in the ⁴I_11/2 level further relaxes, radiatively and nonradiatively to level ⁴I_13/2 originating the 1535 nm emission. Under this excitation condition, excited state absorption (ESA) from ⁴I_13/2 to ²H_11/2 can occur. Tm³⁺ ions in the ³F₄ level relax to the ground state emitting photons of around 1820 nm. There are several energy transfer processes previously reported to explain Tm³⁺↔Er³⁺ energy transfers in tellurite glasses [19-22]:

The first energy transfer process, which is a near-resonant energy transfer (energy mismatch +150 cm^-1), depopulates ³H₄ level and populates the ⁴I_9/2 level. Process (2) with an energy mismatch of +180 cm^-1 promotes Er³⁺ ions to the first excited state. These processes can explain the presence of the Er³⁺ emission when we only excite Tm³⁺ ions and the reduction of the lifetimes of level ³H₄ in the presence of Er³⁺ ions. Processes (3) and (4) are related to the depopulation of levels ⁴I_11/2 and ⁴I_13/2. Process (3) is a nonresonant process (energy mismatch +2870 cm^-1) in which one Er³⁺ ion relaxes from ⁴I_11/2 level to ground state and transfers its energy to a Tm³⁺ ion in the ground state which is in turn promoted to the level ³H₅ from which it decays nonradiatively to level ³F₄. This process should reduce the intensity of the 1535 nm emission and enhance the 1820 nm emission from Tm³⁺ ions. Process (4) depopulates the ⁴I_13/2 level of Er³⁺ ions and can also reduce the lifetime of the 1535 nm emission of Er³⁺ 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 ⁴F_9/2 level. This effect has been previously observed and attributed to process (5) which increases the population of level ⁴F_9/2 [20-22]. 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 Tm³⁺ and Er³⁺ ions. Solid lines represent the absorption and near infrared emissions and dashed lines the possible cross-relaxation process.

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4. Conclusions

The near-infrared emission of Tm³⁺ and Er³⁺ ions in codoped tellurite glasses of composition 50TeO₂-30WO₃-20PbO obtained under 794 nm excitation shows a broadband luminescence in the 1350-1750 nm range corresponding to the ³H₄→³F₄ and ³F₄→³H₆ emissions of Tm³⁺ and the ⁴I_13/2→⁴I_15/2 emission of Er³⁺ 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 Tm₂O₃ and 0.1 wt% of Er₂O₃ which suggests that these glasses can be promising materials for broadband light sources and broadband amplifiers for WDM transmission systems.

The Tm³⁺-Er³⁺ energy transfer processes reduce the lifetimes of ⁴I_13/2(Er³⁺) and ³H₄(Tm³⁺) levels in the codoped samples. The Er³⁺→Tm³⁺ transfer efficiency reaches 80% for the highest Tm³⁺ concentration. However, this efficient energy transfer reduces, as expected, the Er³⁺ emission efficiency at 1535 nm if compared to the one of the single doped sample. The upconverted emission of Er³⁺ ions in the codoped samples also evidences the presence of energy transfer between Tm³⁺ and Er³⁺ ions. The addition of Tm³⁺ reduces the upconverted green emission due to energy transfer between Er³⁺ and Tm³⁺ whereas the red emission is slightly enhanced due to the cross-relaxation ³F₄→³H₆(Tm³⁺):⁴I_11/2→⁴F_9/2(Er³⁺) process.