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Abstract
In this paper, the glass network of a newly developed bismuthate glass was adjusted and analyzed by changing the Al2O3 content, thus effectively increasing the doping concentration of Tm3+. The fundamental physical and thermal properties including density, molar volume, refractive indices, and characteristic temperatures were systematically investigated, suggesting the B2O3/Al2O3 anomaly ratio of the prepared host glass is 35%/10%. Various glass network units were found in the host glasses, so that the flexibility of the glasses was enhanced, which is favorable for highly and homogeneously doping of Tm3+ ions. A highly Tm3+-doped bismuthate glass with a concentration of 20.5 × 1020 ions/cm3 was prepared without quenching. Radiative parameters of the presented glass were determined from absorbance spectra. Moreover, relatively large emission cross-section (5.29 × 10−21 cm2) and gain coefficient (10.87 cm-1) were achieved in the prepared highly Tm3+-doped bismuthate glass. Finally, the microparameters for energy transfer processes were calculated by a spectral overlap method. Results show that the presented highly Tm3+-doped bismuthate glass has ideal potential for high gain fibers in ∼2 µm band.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, fiber laser around ∼2 μm has become a hot topic due to their applications in laser surgery, mid-infrared generation, and optical sensors [1–3].
Among rare-earth (RE) ions, Tm3+ not only has a broadband ∼2 μm emission (3F4 → 3H6) but also has a strong absorption at about 790 nm (3H6→3H4), which means Tm3+ can be pumped by available laser diodes (LD) effectively. Moreover, a high quantum efficiency up to 200% can be achieved in this pump scheme due to the cross-relaxation (3H4 + 3H6 → 3F4 + 3F4) [4,5]. All these features make Tm3+ very suitable active ions for ∼2 µm band fiber lasers. Glasses with higher Tm3+ doping concentration and less clustering are intensely desired to fabricate high gain Tm3+-doped fibers, which is favorable for the high-repetition-rate pulse laser operation by shortening the laser cavity [6]. Moreover, high Tm3+ concentration enables efficient two-for-one (two excited thulium ions into the 3F4 manifold for one pump photon) cross-relaxation processes, high quantum efficiency, large pump absorption, high output power, and the reduction of adverse effects of optical nonlinearities [7,8].
The choice of host glass is essential to synthesize heavily Tm3+ doped glasses. It is known that only low Tm3+ doping concentration can be achieved in silica glass for its intrinsic network structure, and excessive amounts of Tm3+ may cause concentration quenching and photodarkening [3,5]. As alternatives to pure silica glass, multi-component glass systems in which much higher RE ions doping concentrations (up to 1020 ions/cm3) can be achieved, have drawn much attention recently [9,10]. Lots of multi-component glass systems have been studied for the fabrication of highly Tm3+-doped glasses or fibers, such as silicate glass, germanate glass, and tellurite glass. For example, in 2015, Y. Lee et al. reported a Tm3+-doped SiO2-A12O3-BaO-ZnO-La2O3 glass fiber with a doping concentration of 8.35 × 1020 ions/cm3 and a gain of 5.8 dB/cm per unit length [3]. Compared with silicate glass, heavy metal oxide glasses offer lower phonon energies, higher refractive indices, higher infrared transparency as well as excellent RE ion solubility. In 2010, H. Gebavi et al. investigated the spectroscopic measurements of highly Tm3+-doped tellurite glass (TeO2-ZnO-Na2O), and obtained the optimum emission intensity at ∼2 µm while the Tm3+ doping concentration is 6.84 × 1020 ions/cm3 with a quantum efficiency of 24% [11]. In 2016, W. Xin et al. successfully fabricated a single mode Tm3+-doped germanate glass (BaO-Ga2O3-GeO2-La2O3-Y2O3) fiber with a doping concentration of 7.6 × 1020 ions/cm3 and a laser output power of 165 mW [6]. In 2021, L. Tu et al. demonstrated another highly Tm3+-doped germanate glass (BaO-Ga2O3-GeO2-Nb2O5, 6.84 × 1020 ions/cm3) through controllable structural tailoring by tuning the components of glass [7]. Bismuthate glasses were recently investigated a lot due to their wide infrared transmission range, good mechanical strength, and low cost. However, very few works of highly Tm3+-doped bismuthate glasses have been reported so far.
Dependence of the solubility and dispersity of RE ions on the glass network structure has been demonstrated in previous reports [12]. To obtain highly Tm3+-doped bismuthate glasses, adding modifier oxide (such as Al2O3) is helpful to tune the ligand field environment around the RE ions by adjusting its chemical composition in glass [4,13]. It has been reported that adding Al2O3 into the glass network can enhance the solubility of RE ions and reduce the clustering between adjacent RE ions [8,10]. Furthermore, the addition of Al2O3 is also favorable for the thermal, mechanical, chemical stabilities, and luminescence properties of glass [10,13]. Hence, a study about Tm3+-doped bismuthate glass with the addition of Al2O3 can be helpful to improve the RE ions doping concentration for high gain fibers in ∼2 μm band.
In this study, a highly Tm3+-doped bismuthate glass with a doping concentration of 20.5 × 1020 ions/cm3 was successfully fabricated based on the Al2O3 content adjustment. Physical and thermal characteristics of prepared bismuthate host glasses were analyzed, indicating the B2O3/Al2O3 anomaly ratio of the prepared host glasses is 35%/10%. Raman and FT-IR spectra showed there are various glass network units that form a compact glass structure for the highly and homogeneous doping of Tm3+ ions. Moreover, radiative, spectroscopic, and gain properties of the highly Tm3+-doped bismuthate glass were systematically studied. Note the prepared glass possesses a high gain coefficient (10.87 cm-1), higher than heavily Tm3+-doped multicomponent germanate glasses. Finally, energy transfer processes were quantitatively analyzed by a spectral overlap method.
2. Experimental
Bismuthate glasses with molar compositions of 50Bi2O3-(45-x)B2O3-xAl2O3-5BaF2 (where x = 2, 6, 10, 14, and 18, named as BBA1 to BBA5) and 50Bi2O3-35B2O3-10Al2O3-5BaF2-yTm2O3 (where y = 2, 4, 6, 8, and 10, denoted as BBAT1 to BBAT5) were synthesized by melt-quenching method. The B2O3 was introduced by HBO3 while raw materials are Bi2O3 (99.99%), HBO3 (99.99%), Al2O3 (99.99%), BaF2 (99.99%), and Tm2O3 (99.99%). Raw materials (10 g) were mixed homogeneously and melted in an alumina crucible at 1200 °C for 40 min. While preparing the glass samples, crucible lids were used to cover the alumina crucible to reduce the vaporization of molten glasses as much as possible. The samples were formed by casting in a mold and annealed for 3 h at 370 °C. The prepared glasses were polished for the optical measurements.
Densities were measured by the Archimedes method. Refractive indices at room temperature were measured by a prism coupler of Sairon Tech-SPA4000 TM. Thermal properties were studied by a differential scanning calorimeter (DSC). The Raman spectra were recorded with Renishaw invia Raman microscope under 488 nm excitation. Fourier transform infrared spectra (FT-IR) were carried out by a Spectrum Two FT-IR Spectrometer (PerkinElmer). Absorbance spectra were obtained by a Perkin-Elmer Lambda 900 UV-VIS-NIR spectrophotometer. Fluorescence spectra were measured on a Triax 320 type spectrometer (Jobin-Yvon Corp.) with 808 nm LD as a pump source. All the measurements were performed at room temperature.
3. Results and discussion
3.1 Density, index, and thermal property
Densities (ρ) and molar volume of BBA host glasses were given in Fig. 1. The molar volume is calculated as ${V_\textrm{m}} = \frac{M}{\rho }$, where M is the molecular (formula) weight of the glass. It is worth noting that the density of BBA host glasses increases in general with the increase of Al2O3 content by substituting B2O3, however, the density decreases abnormally while the content of B2O3/Al2O3 ratio is 35 mol%/10 mol%. This boron aluminum anomal phenomenon can be explained as follows. When the content of Al2O3 is small, Al3+ ions exist as tetrahedral [AlO4] units. Although, as glass network formers, the volume of [AlO4] is larger than that of [BO3], Al3+ ion has a larger molecular weight than B3+ ion. And the influence of volume change of the glass network is relatively weaker than its mass change so that the density of bismuthate glasses increases with the increase of the Al2O3. However, there are enough [AlO4] units existing in the glass network when Al2O3 content reaches 10 mol%, resulting that the influence of volume change of glass network is relatively stronger than its mass change, which causes a sudden reduction of density as shown in Fig. 1. Furthermore, when the Al2O3 content is more than 10 mol%, free oxygen in the glass network is insufficient. So that Al3+ ions exist as octahedral [AlO6] units which are network modifiers filling in the network gap of presented bismuthate glasses, making the glass network compact as well as causing the glass density to increase again. In addition, a similar phenomenon can be found while discussing the relationship between molar volume and B2O3/Al2O3 ratio.
The refractive indices of BBA host glasses, which are fitted using the second-order Sellmeier equation, are given in Fig. 2. Sellmeier equation can be expressed as:
Fig. 2. Dispersion curves of BBA host glasses fitted by Sellmeier equation, inset shows the measured index and fitting curve of BBA3 glass.
Furthermore, the nonlinear refractive index n2 is estimated as follows [14]:
Table 1. Optical parameters of BBA host glasses.
The DSC curves of BBA host glasses are displayed in Fig. 3, in which glass transition temperature Tg, crystallization onset temperature Tx, as well as ΔT (Tx - Tg), have been marked. One can find that the Tg of BBA host glasses increases with the augment of Al2O3 content, and the largest Tg is obtained when Al2O3 content is 10 mol%. However, the value of Tg decreases with further augment of Al2O3 content. This phenomenon is due to the transformation from [AlO4] to [AlO6] of Al3+ with the augment of Al2O3 content in the glass network. The large Tg (405 ℃) of BBA3 glass indicates it has a relatively higher damage threshold when transmitting high power laser [4]. Therefore, BBA3 glass was selected for further Tm3+-doping experiments and characterization due to its large Tg value. It is known a large ΔT represents good thermal stability [17]. The ΔT of BBA3 is 174℃, which is larger than that of tellurite glass (TeO2-Ga2O3-ZnO, 111℃) [16] and other bismuthate glass (Bi2O3-GeO2-Ga2O3-Na2O, 135℃) [18].
3.2 FT-IR and Raman spectra
The FT-IR spectra of BBA host glasses are displayed in Fig. 4 (a). Note that there are seven absorption peaks located at 500, 710, 760, 875, 1020, 1190, and 1340 cm-1, respectively. Among them, the IR absorption at 500 cm-1 is due to the vibration of [BiO6], [BO4], and [AlO6] groups [19]. The absorption at 710 cm-1 corresponds to the bending vibration of B-O-B in trigonal coordination [BO3] and the symmetric stretching vibration of Bi-O bond in [BiO3] [19,20]. Moreover, the absorption at around 760 cm-1 is caused by the stretching vibration of Al-O bond in [AlO4] units, and the absorption at 875 cm-1 is pertaining to the octahedral [BiO6] group [20]. The absorption at 1020 cm-1 is because of the stretching vibration of tetrahedral coordination [BO4] and B-O-Bi bonds [20,21]. Furthermore, the broad absorption band in the range of 1190-1300 cm-1 corresponds to the antisymmetric stretching vibration of B-O bond in [BO3] unit [19,20,22]. It is worth noting that there are many structural units including [BiO3], [BiO6], [BO3], [BO4], [AlO4], and [AlO6] in the prepared BBA host glasses so that the flexibility of the network structure of BBA glass was enhanced considerably. These various units enable the structural basis for the realization of high concentration and homogeneous doping of RE ions.
In order to analyze energy transfer characteristics, Raman spectra of the BBA host glasses were measured and presented in Fig. 4 (b). The peaks at 130 cm-1 and 184 cm-1 correspond to the Bi3+ ions [4,23]. The band at around 375 cm-1 is because of the Bi-O-Bi vibration in [BiO6] groups [23,24], while the band near 600 cm-1 is assigned to the Bi-O- stretching vibration in [BiO6] groups [25]. Furthermore, the wide vibration band at about 1200 cm-1 may be caused by the interaction of the stretching vibration of Bi-O- and B-O- bond in [BiO3] and [BO3] units [23,25], as well as the symmetric stretching vibration of B-O bond in [BO4] units [26]. Furthermore, Raman spectra of all the samples are deconvoluted using Gaussian fitting to measure the exact Raman peak positions as shown in Fig. 5. It was found that the maximum phonon energy of BBA1-5 glasses are 1329 cm-1, 1302 cm-1, 1150 cm-1, 1137 cm-1, and 1135 cm-1, respectively. It can be derived that the maximum phonon energy of BBA host glasses becomes smaller with the decrease of B2O3 content in the glass.
Fig. 5. (a), (b), (c), (d), and (e) are the deconvoluted Raman spectra of BBA1-5 host glasses, (f) maximum phonon energy dependence of Al2O3 concentration.
3.3 Absorbance, transmittance spectra, and Judd-Ofelt analysis
Figure 6 (a) gives the absorbance spectra of Tm3+-doped bismuthate glasses. The absorption bands at 480, 689, 793, 1210, and 1720 nm correspond to absorption transitions from the ground state 3H6 to 1G4, 3F2+3, 3H4, 3H5, and 3F4 levels of Tm3+, respectively. Note the absorption intensity increases gradually with the increase of Tm3+ concentration, and there is no shift in absorption peak, suggesting homogeneous incorporation of Tm3+ ions in the bismuthate glass network. Moreover, the transmittance spectrum of BBAT3 glass is presented in Fig. 6 (b). The broad absorption near 3 μm is corresponding to the stretching vibration of free OH- groups, and the OH- absorption coefficient (αOH) can be calculated using the following equation [6]:
where L is the thickness of the sample, T is the transmission at 3000 nm, T0 is the transmission at 2400 nm. The OH- absorption coefficient of core glass is 0.65 cm-1, lower than that of Tm3+ doped lead silicate glass (1.04 cm-1) [2].Judd-Ofelt (J-O) theory is widely used for determining the spectroscopic properties of RE doped glasses [27,28]. J-O parameters Ω2, 4, 6 of BBAT4 and other glasses were listed in Table 2 (J-O parameters were calculated by using absorption bands of 1G4, 3H4, 3H5, and 3F4 level from the ground state). Usually, a larger Ω2 value suggests higher covalency and symmetry [7]. Note the Ω2 of BBAT4 glass is larger than silicate, ZBLAN, and borate glasses, but smaller than other bismuthate glass (Bi2O3-Ge2O3-Ga2O3-Na2O). Moreover, the spectroscopic quality factor (χ = Ω4/Ω6) is often used to characterize the intensity of transition of Tm3+ [29,30]. Note BBAT4 glass possesses a relatively larger χ value, indicating a higher laser efficiency can be obtained in the prepared BBAT4 glass.
Table 2. J-O intensity parameters in different Tm3+ doped glasses.
Table 3 summarizes the radiative transition probabilities (Arad), fluorescence branching ratios (β), and radiative lifetimes (τrad) values of Tm3+ in BBAT3 glass. It is found that the τrad value of Tm3+: 3F4 is 3.15 ms, larger than that of Bi2O3-Ge2O3-Ga2O3-Na2O glass (2.51 ms) [4].
Table 3. Some radiative properties of BBAT4 glass.
3.4 Fluorescence emission
Figure 7 gives the fluorescence emission spectra of BBAT glasses under the pump of 808 nm LD, in which two obvious emission peaks at 1470 nm (3H4 → 3F4) and 1830 nm (3F4 → 3H6) can be observed. The inset shows the dependence between emission intensity and Tm2O3 concentration. Note the ∼2 µm emission strengthens gradually with the augment of Tm2O3 concentration, which is because of the shortening of the distance between Tm3+ ions that strengthens the cross-relaxation process. It is observed the strongest emission around ∼2 μm is obtained at 8 mol% Tm2O3 concentration, however, the reduction of emission intensity is found as Tm2O3 was further added due to concentration quenching [4]. Therefore, the doping concentration of Tm2O3 for BBAT4 glass reached 8 mol% (20.5 × 1020 ions/cm3) without quenching, which is larger than that of BaO-Ga2O3-GeO2-Nb2O5 glass (9.8 × 1020 ions/cm3) [7]. Besides, it is worth noting the 1.8 μm fluorescence band redshifted because of the self-absorption, which is due to the overlap between the emission band and the absorption band of 3F4 ↔ 3H6 transition [34]. However, there is no obvious red shift of 1.47 μm fluorescence band, because no obvious absorption around this band can be found in the absorbance spectra of BBAT glasses.
Fig. 7. Fluorescence spectra of the BBAT glasses, inset shows the ∼2 μm emission intensity dependence of Tm2O3 doping concentration.
Moreover, effective fluorescent bandwidth (Δλeff) can be determined by:
where I(λ) is the emission intensity, Imax is the peak emission intensity. The obtained Δλeff value of BBAT4 glass is 242 nm, which is higher than that of phosphate glass (P2O5-K2O-CaO-Al2O3, 230 nm) [35] and other bismuth glasses (Bi2O3-GeO2-Ga2O3-Na2O, 237 nm) [4]. The relatively large Δλeff indicates BBAT4 glass might be a promising gain medium for broadband amplifier.3.5 Cross-sections and gain coefficient
The absorption and emission cross-sections of Tm3+: 3F4 ↔ 3H6 transition in the BBAT4 glass are given in Fig. 8 (a). The absorption cross-section (σabs) is obtained by [36]:
where N is the Tm3+ doping concentration in BBAT4 glass, OD(λ) represents the optical density derived from absorption spectrum, d is the thickness of BBAT4 glass. Emission cross-section (σemi) is calculated by [37]:Fig. 8. Calculated (a) absorption and emission cross-sections, (b) gain coefficient of the Tm3+: 3F4 ↔ 3H6 transition in BBAT4 glass.
Supposing that Tm3+ ions are either in 3H6 level or in 3F4 laser level, gain coefficient G(λ) around 2 µm can be calculated by [5]:
where N is the Tm3+ doping concentration and P is the population inversion assigned to the concentration ratio of Tm3+ in the 3F4 level. Figure 8 (b) displays the gain coefficient of Tm3+: 3F4 ↔ 3H6 transition in BBAT4 glass with various P values. It is found that the maximum gain coefficient is 10.87 cm-1 at 1864 nm, larger than that of highly Tm3+-doped germanate glass (9.8 × 1020 ions/cm3, 6.31 cm-1) [7]. Additionally, it is observed that a positive gain can be obtained while P is more than 0.05, indicating only small pump power is needed for laser operation in the prepared BBAT4 laser glass [5].3.6 Energy transfer microparameters
Figure 9 (a) displays the simplified energy level diagram and some energy transfer channels of Tm3+. Under 808 nm LD excitation, Tm3+ in 3H6 level are pumped to the 3H4 level, and some energy can be transferred to the adjacent 3H4 level by energy migration (EM:3H6 + 3H4 → 3H4 + 3H6). Thereafter, 3F4 level is populated by cross-relaxation process (CR: 3H4 + 3H6 → 3F4 + 3F4), the ideal quantum efficiency is up to 200%. Meanwhile, some Tm3+ ions relax radiatively to 3F4 level with 1.47 μm emission. Finally, ∼2 μm emission is observed for Tm3+: 3F4 → 3H6 transition.
Fig. 9. (a) The energy level diagram of Tm3+. The emission cross-section of donor and absorption cross-section of acceptor in (b) EM process and (c) CR process, with one- and two-phonon emission sidebands of donor.
Moreover, a quantitative investigation of energy transfer processes in the prepared BBAT4 glass is determined by Dexter's theory. The energy transfer probability between the donor (D) and acceptor (A) can be described as [38]:
where $|{{H_{DA}}} |$ is the matrix element of the perturbation Hamilton between initial and final states, $S_{DA}^N$ is the overlap integral between the m-phonon emission sideband of donor and the k-phonon absorption sideband of acceptor, N is the total phonons ($N = m + k$).The m-phonon emission sideband $\sigma _{emi({m - phonons} )}^D$ and k-phonon absorption sideband $\sigma _{abs({k - phonons} )}^D$ can be determined by [38]:
The phonon emission sidebands of EM and CR processes of Tm3+ are given in Fig. 9 (b) and 9 (c).
Furthermore, if only the m-phonon emission process is considered (N = m, k = 0), hereafter, the energy transfer coefficient can be expressed as [38]:
The energy transfer microparameters are determined and summarized in Table 4. Note the EM process is a resonant process that hardly needs phonon involved. The energy transfer coefficient CD-A is 2.76 × 10−40 cm6/s, and one-phonon assistance has a contribution ratio of 94.906% in the CR process. It is found CD-D is considerably larger than CD-A so that the hopping model can be used. The energy transfer rate WET can be derived by:
where ND is the concentration of Tm3+. It can be derived the energy transfer rate WET of BBAT4 glass is 214.8 × 10−20 cm3/s.Table 4. Microparameters of Tm3+ in BBAT4 glass.
4. Conclusions
In conclusion, the structure of bismuthate glass was adjusted and controlled by changing the B2O3/Al2O3 ratio to obtain highly Tm3+-doped laser glass. Density and molar volume characterization showed that the B2O3/Al2O3 anomaly ratio is 35%/10%. The presented bismuthate host glasses exhibited large refractive indices (nF = 2.0270 - 2.0803), high nonlinear refractive indices (5.30 - 7.06 × 10−15 cm2/W), and good thermal stabilities. FT-IR and Raman measurements indicated that the prepared bismuthate glasses have various glass network units, enabling the structural basis for the realization of high Tm3+ ions doping concentration. Judd-Ofelt analysis showed a relatively large radiative lifetime (τrad = 3.15 ms) and spectral quality factor (χ = 1.18) have been realized in Tm3+-doped BBAT4 glass. Moreover, fluorescence spectra showed that the strongest emission intensity was achieved while the doping concentration is 20.5 × 1020 ions/cm3, which is the record highest doping concentration to the best of our knowledge. For BBAT4 glass, good gain property is demonstrated by the high gain coefficient (10.87 cm-1). Finally, the energy transfer processes were discussed and microparameters were calculated. Results show the highly Tm3+-doped bismuthate glass is a promising gain material for high-gain fiber lasers in ∼2 μm band.
Funding
Department of Science and Technology of Jilin Province (20200401053GX, 20200404163YY).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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