Introduction

Recently, Tm3+ doped glass has been widely investigated for mid-infrared fiber lasers around 2.0 μm since the transitions of Tm3+:3F4 → 3H6. The mid-infrared fiber lasers in the 2.0 μm have generated great interest for its potential applications in several fields, such as medical surgery, remote sensing, eye-safe laser radar, military and atmospheric pollution monitoring in the last decades1,2,3,4. Rare–earth thulium ions are often chosen to produce laser radiation at 1.8 μm through the transitions of Tm3+:3F4 → 3H6. In this paper, Tm3+ has been selected as active ions because the ground state of Tm3+:3H6 can be pumped directly by 808 nm commercial laser diodes. Laser emission at 1.8 μm strongly dependents on the efficiency of the energy transfer process among Tm3+ ions. In order to obtain high-power infrared emissions from Tm3+, the host glass plays an important role. Most researches in Tm3+ doped glass matrix for ~2.0 μm emission have focused on silica5,6,7,8, silicate9,10,11, fluoride12,13,14, fluorophosphates15,16,17, tellurite18,19,20,21 and germanate22,23,24,25 based glasses. Due to the energy cross-relaxation process, the host material should be chosen to have high solubility of the thulium ions. Compared with SiO2-based glass, GeO2-TeO2-based glass have the higher solubility for Tm3+ ions and the lower phonon energy (germanate-tellurite glass' is about 800–900 cm−1), which can avoid very competitive nonradiative decay for Tm3+ ions. High phonon energy leads to faster multi-phonon relaxation and thus results in lower cross-relaxation rate. By contrast with germanate-tellurite glass, fluoride glass also has very low phonon energy (around 500 cm−1) but the low mechanical strength and damage threshold limit its application in high power or energy fiber laser systems. Therefore, germanate-tellurite glass may have higher ions solubility and appropriate phonon energy for developing more efficient optical devices by comprehensive evaluation.

In this work, we report primarily a series of glass compositions in the Tm3+-doped germanate-tellurite (GT) glasses to study mid-infrared spectroscopic properties. We induce La2O3 to substitute for Nb2O5 to improve the thermal stability and anti-crystallization of this glass system, by reasons of the ratio of La3+ ions (0.12 nm) and coordination number (8) are higher than Nb5+ ions (ratio = 0.07 nm/coordination number = 6). The absorption and emission spectra of Tm3+-doped GT glasses were obtained. Results and analysis reveal that these germanate-tellurite glasses with good spectroscopic properties may provide a high gain as a good medium for high-power level laser.

Experiments and measurements

The gemanate-tellurite glass with molar composition of 70GeO2-10TeO2-10K2O-(9-x)Nb2O5-xLa2O3-1Tm2O3 (x = 1, 3, 5 and 7 mol. %) was investigated, which is hereafter denoted as GTLa-x glass. All the samples were prepared by traditional melt-quenching method with using high-purity of GeO2, TeO2, K2CO3, Nb2O5, La2O3 and Tm2O3 powder. Well-mixed 20 g batches of the samples were placed in a platinum crucible and heated with a SiC-resistance electric furnace at 1150°C for 30 min to melt. The melt was poured onto a preheated steel plate and annealed for 2 hours in a furnace around the glass transition temperature (Tg), after which it was allowed to cool slowly in the furnace to room temperature. The annealed samples were fabricated and polished to 10 × 10 × 1 mm for optical measurements.

The density was tested by Archimedes' liquid-immersion method in distilled water and the refractive index of the glass samples was measured by prism minimum deviation method. The DSC curve is recorded by NETZSCH STA 409PC. Furthermore, the absorption spectra were recorded with a Perkin-Elmer-Lambda 900 UV/VIS/NIR spectrophotometer in the range of 300–2000 nm and the emission spectra were measured with a Triax 320 type spectrometer (Jobin-Yvon Co., France) upon excitation at 808 nm. For the fluorescence lifetime measurements, the instrument applied was FLSP 920 fluorescence spectrophotometer (Edinburgh Analytical Instruments Ltd, UK). All the measurements were carried out at room temperature.

Results and discussion

Thermal property

Figure 1 shows the thermal property of GTLa-x glass from 300°C to 900°C. From the DSC curve of present glasses system, we can find out that no crystallization peak is apparent and the glass transition temperature Tg are 355°C, 459°C, 550°C, respectively. The Tg increase with the contents of La2O3 increase. It indicated the introduce La2O3 to substitute for Nb2O5 can improve the thermal stability and anti-crystallization ability of this glass system. We could conclude that thermal properties of the GTLa-x glass are good for fiber drawing from the analysis of DSC curve.

Figure 1
figure 1

DSC curve of GTLa-x glasses.

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Absorption spectrum

The absorption spectra were obtained at room temperature over a wavelength region of 300–2000 nm. The absorption spectra of the GTLa-x samples at room temperature is shown in Fig. 2 and the absorption bands centered corresponding to the transitions from the 3H6 ground state to the exited stated of 3F4 (1694 nm), 3H5 (1208 nm), 3H4 (792 nm), 3F2 + 3F3 (686 nm) and 1G4 (492 nm), respectively. The transitions to energy levels higher than 1G4 are not observed because of the intrinsic band gap absorption of host glass. The insert of the figure 2 is the transmittance curve of the GTLa-3 glass sample from 2.5 μm to 8 μm. We can find that the OH− concentration on present glass sample is lager and result in the lager absorption around 3 μm regions. As well known, OH− groups play an important role and the much OH− groups on glass host would decrease the performance of the 1.8 μm emission. So, improving the OH− concentration on present glass system is a crucial issue to obtain efficiently 1.8 μm emission.

Figure 2
figure 2

Absorption spectra of GT glasses.

The insert of figure is the transmittance curve of GTLa-3.

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Judd-Ofelt analysis

Judd-Ofelt (J-O) theory26,27 has been commonly applied to determine the important spectroscopic and laser parameters of rare earth ions doped glasses by many researchers. According to the J-O theory, based on absorption spectra, the measured oscillator strengths and some spectroscopic parameters of many trivalent rare earths in solids, such as intensity parameters Ωλ, radiative transition probability Arad and radiative lifetime τrad can be calculated. The theoretical (ftheor) and the measured (fexp) oscillator strengths of the GTLa-1 glass are shown in Table 1.

Table 1 Measured and calculated oscillator strengths of Tm3+ in GTLa-1 glass
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As is shown in Table 1, the root-mean-square error deviation of intensity parameters is 0.39 × 10−6, which indicates the validity of the J-O theory for predicting the spectral intensities of Tm3+ and the reliable calculation. The J-O intensity parameters are important for investigating the local structure and bonding in the vicinity of RE ions. As known large Ω2 parameter is related with the amount of the strong covalent bond between rare-earth ions and ligand anions, strongly depends on the asymmetry of the local environment at the Tm3+ ion site in the glass hosts and a strong polarizability of the anion, while the Ω6 parameter is related to the overlap integrals of 4f and 5d orbits28,29. Values of Ω4 and Ω6 also provide some information on the rigidity and viscosity of hosts30. In this work, the J-O intensity parameters Ωλ are compared with those of various Tm3+ doped glasses presented in Table 2. It can be found that the value of Ω2 becomes small when La2O3 increase but La2O3 < Nb2O5. When the contents of La2O3 approximate that of Nb2O5, the value of Ω2 increases slightly, while La2O3 > Nb2O5, the value of Ω2 drop down quickly. Similarly, Ω6 also changed with the contents of La2O3. These proved the change of asymmetry of the local environment at the Tm3+ ion site and found a variation in the covalent and bond strength of Tm-O in glass hosts. In table 2, Ω2 for GTLa-x glass is higher than fluoride glasses', because the electronegativity of oxide is smaller than that of fluorine. It means the asymmetry and covalent environment of the Tm-O band in GT glass are stronger than other host glasses'.

Table 2 Ωλ(λ = 2, 4, 6)(×10−21 cm2) of Tm3+-doped glass
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The radiative transition probabilities for Tm3+:3F4 → 3H6 level can be calculated by using J-O intensity parameters. The results of J-O analysis are shown in Table 3. The radiative transition probability Arad for the transition in GTLa-1 glass is 372.1 s−1 and this value is slightly higher than those in fluorophosphate glass16. Because radiative transition probability depends intensively on the refractive indices, obtaining of larger radiative transition probability is reasonable in GT glass with higher refractive index31 and larger values are beneficial in achieving intense near-infrared emission.

Table 3 The electric (Sed) and magnetic (Smd) dipole line strengths, radiative transition probability (Arad), branching ratio (β) and radiative lifetime (τrad) of Tm3+ in GT glass
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Infrared fluorescence spectra

The fluorescence spectra of the samples are measured as shown in figure 3 to investigate the near-infrared emission characteristics of the prepared Tm3+-doped GT glasses as a function of the La2O3 contents. Besides, the inset of figure 3 is the energy level diagram and energy transfer sketch map of Tm3+ ions pumped at 808 nm. The cross relaxation (CR) and energy migration (EM) into the 3H4 level are also indicated. The 3F4 → 3H6 fluorescence around 1.8 μm is obviously observed and the intensity is much larger than that of 1.47 μm corresponding to 3H4 → 3F4 transition for samples under the 808 nm diode laser. The emission at 1.47 μm due to the 3H4 → 3F4 transition is much lower in intensity relative to the 1.8 μm because the cross relaxation between Tm3+ ions is efficiency. Moreover, the intensity of 1.8 μm emission firstly decreased and then increased with the increment of La2O3 concentration. But the intensity of 1.8 μm emission slightly increased when the concentration of La2O3 more than the concentration of Nb2O5. The maximum fluorescence peak intensity for GT glass is observed around 1 mol. % La2O3.

Figure 3
figure 3

Fluorescence spectra of the GT glasses with Tm3+ ions.

The inset is the energy level diagram and energy transfer sketch map of Tm3+ when pumped at 808 nm.

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Absorption and stimulated emission cross section

According to the absorption spectra, the absorption cross-section (σabs) can be calculated by using Beer-Lambert equation

where N is the rare-earth ion concentration (ions/cm3), l is the sample thickness, I0(λ) is the incident optical intensity and I(λ) is the optical intensity throughout the sample.

The stimulated emission cross-section (σem) is calculated from the absorption cross-section by using McCumber formula equation33

where Zl and Zu are the partition functions of the lower and upper states, respectively, k is the Boltzmann constant, T is the temperature and λZL is the wavelength for the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets (so called ‘zero-phonon line’). In the following calculation, the ratio of the partition functions of the lower to the upper state Zl and Zu is equal to 13/9, for the reason that the values of the lower and upper states Zl and Zu simply become the degeneracy weightings of the two states in the high temperature limit. The zero-phonon line is assumed to the peak wavelength of the absorption, λZL = λpeak-abs.

Figure 4 shows the σabs and σem for optical transitions involving the ground and first excited states of Tm3+ ions of GTLa-1 glass. The calculated maximum σabs and σem for Tm3+ are 4.17 × 10−21 cm2 at 1700 nm and 14.6 × 10−21 cm2 at 1806 nm, respectively. Moreover, the product of the stimulated emission cross-section and the radiative lifetime (τrad), σem × τrad is another important parameter to characterize laser materials, which is the figure of merit (FOM) for amplifier gain28,34,35.

Figure 4
figure 4

The absorption and emission cross sections of GTLa-1 glass.

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Table 4 shows the emission cross-section, radiative lifetime and FOM gain (σem × τrad) of 3F4 → 3H6 transition of Tm3+-doped glasses. For laser glasses, it is generally desirable for the emission cross section to be as large as possible to provide a high gain36. This peak emission cross section had reported as 4.3 × 10−21 cm2 in silica glass by Turri32, 4.11 × 10−21 cm2 in fluorophosphate by Tian16 and 9.20 × 10−21 cm2 in tellurite by Balda21. It can be seen that the σem in GTLa-1 sample has a maximum value which is higher than that of Tm3+-doped other kinds of glasses. And the σem × τrad of Tm3+ doped GT glasses is much larger than other Tm3+-doped host glasses. The advantage of GT glass possessing larger emission cross section may prove it to be a promising laser glass with high gain to scale a high power level laser.

Table 4 Calculated emission cross-section σem, radiative lifetime τrad and σem × τrad of 3F4 → 3H6 transition of different Tm3+-doped host glasses
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Fluorescence lifetime

The fluorescence lifetime of Tm3+:3F4 becomes shorter with increasing of the La2O3 concentrations and the lifetime becomes longer when doped with 7 mol% La2O3, as shown in table 5. However, it indicated that the measured lifetime is much shorter than the calculated lifetime. It is due to nonradiative quenching9. The nonradiative decay originated from several mechanisms, such as energy transfer processers between the Tm3+ ions themselves, multiphonon decay, etc. Energy transfer processes between the Tm3+ ions themselves are cross-relaxation and energy migration. The cross relaxation energy transfer process between Tm3+ ions described in the introduction does not quench fluorescence by itself, but energy migration is a resonant nonradiative mechanism, which increases the probability of luminescence quenching by impurities. Generally, the relatively longer radiation lifetime is beneficial to reduce the laser oscillation threshold38. Therefore, this Tm3+-doped GT glass can be considered as an appropriate medium to achieve a 1.8 μm laser with high quality.

Table 5 Measured fluorescence lifetime of the Tm: 3H4 level and Tm: 3F4 level at 295 K of different Tm3+-doped host glasses
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Summary

Spectroscopic properties of 1.8 μm emission have been investigated in the Tm3+ -doped germanate-tellurite glasses (GeO2-TeO2-K2O-Nb2O5-La2O3). We investigate the effect of the spectroscopic properties for different compositions with varying Nb5+/La3+ ions ratio. On the basis of our experimental results, Judd–Ofelt intensity parameter, spontaneous transition probability, radiative lifetime and absorption cross-section, as well as stimulated emission cross-section are calculated and discussed. For the Tm3+-doped GT glass samples, the fluorescence measurement shows that the maximum value of 1.8 μm emission intensity occurs in GTLa-1 glass. The optimal concentration of La2O3 for a laser application has been determined, which is 1 mol.% La2O3. Results indicate that these germanate-tellurite glasses with good spectroscopic properties may provide a high gain as a good medium for mid-infrared laser.