In this work, we report the near-infrared emission properties of Tm3+-Er3+ codoped tellurite TeO2-WO3-PbO glasses under 794 nm excitation. A broad emission from 1350 to 1750 nm corresponding to the Tm3+ and Er3+ 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 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.
©2009 Optical Society of America
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 . 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 , and a significantly broadened emission at 1.5 μm (Er3+) [11,12] and 1.4 μm (Tm3+) . 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 4I11/2 to 4I13/2 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 ThermostarTM 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, 3F2,3, 3H4, 3H5, and 3F4 of Tm3+ ions, together with the Er3+ absorption bands from the 4I15/2 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.
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 4I9/2 (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 4I13/2→4I15/2 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 4I13/2→4I15/2 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 4I13/2→4I15/2 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)  and comparable to that reported in Tm3+-Er3+ codoped tellurite fiber . 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 .
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 4I13/2 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.
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 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.
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 4I9/2 (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 2H11/2→4I15/2 (530 nm), 4S3/2→4I15/2 (550 nm), and 4F9/2→4I15/2 (665 nm). The 2H11/2→4I15/2 transition is only observed at room temperature because 2H11/2 is populated from 4S3/2 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 4F9/2 level is due to the population of this level from the 4S3/2 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.
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 4I9/2 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 4I9/2 level, multiphonon relaxation occurs to level 4I11/2, and part of the excitation energy in the 4I11/2 level further relaxes, radiatively and nonradiatively to level 4I13/2 originating the 1535 nm emission. Under this excitation condition, excited state absorption (ESA) from 4I13/2 to 2H11/2 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 4I9/2 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 4I11/2 and 4I13/2. Process (3) is a nonresonant process (energy mismatch +2870 cm-1) in which one Er3+ ion relaxes from 4I11/2 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 4I13/2 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 4F9/2 level. This effect has been previously observed and attributed to process (5) which increases the population of level 4F9/2 [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.
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 4I13/2→4I15/2 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 4I13/2(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+):4I11/2→4F9/2(Er3+) process.
This work was supported by the Spanish Government MEC (MAT2005-06508-C02-02) and the Basque Country Government (IT-331-07).
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