Open Access
Add to BibSonomy
Abstract
Tm3+-doped fluorophosphate glasses with varying Ba(PO3)2 content were prepared by the melt quenching technique and their thermal and optical properties were investigated by studying differential scanning calorimetry, Raman spectra, fluorescence spectra, decay curves, transmission and absorption spectra. The Judd-Ofelt theory was applied to calculate the intensity parameters of the resultant glass. The glass forming criterion was obtained to be 146 °C. The gain coefficient and fluorescence lifetime of Tm3+-doped fluorophosphate glass with 20 mol% Ba(PO3)2 were 3.045 × 10−21 cm2 × ms and 0.406 ms, respectively, which are the highest value among the fluorophosphate glasses with similar components to the best of our knowledge. These results clearly indicate that the prepared fluorophosphate glass is an attractive candidate for 2 μm lasers and as a gain media for optical amplifier applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tm3+-doped 2 μm lasers have attracted much attention and have been extensively investigated for its useful applications in several fields, including but not limited in atmospheric gas detection, optical parametric oscillator pumping, eye security remote detection system, remote sensing [1–7]. Many reports about Tm3+-doped lasers have been published, in which host materials for the efficient performance of 2 μm fiber lasers are mostly focused on tellurite, germanate, silica and fluorophosphate glasses. For example, NP Photonics, a US company, has acquired laser emission with the central wavelength of 2 μm in Tm3+-doped germanate glass fiber [8]. Xin Wen et al. investigated the Tm3+-doped germanate glass fiber and obtained 2 μm laser output with an output power of 165 mW, and a slope efficiency of 17% [9]. However, nonradiative decay of germanate glass is very competitive because of their high phonon energiess, i.e. 900 cm−1 [10] Besides, the university of Southampton studied Tm3+-doped silica fiber, and obtained the laser emission with a central wavelength of 2 μm, the output power and slope efficiency are 51 mW and 30%, respectively [11]. Y. Jeong et al. obtained 2 μm laser output in Yb3+/Tm3+ co-doped silica fiber, with output power of 75 W and slope efficiency of 32% [12]. Up to now, although Tm3+-doped silica fiber is the glass medium with the largest output power of 2 μm laser, but its laser emission efficiency is low, its phonon energies are very high, extending to 1100 cm−1 [13] and the fluorescence lifetime of Tm3+ in 3F4 energy level is only about 340 ms, which makes it difficult to achieve narrow-pulse width Q output [14]. In addition, Richards et al. studied Tm3+-doped tellurite glass fiber and pumped it by Yb3+-doped silica fiber to obtain 2 μm laser output, which output power and slope efficiency are 67 mW and 10%, respectively [15]. In 2008, laser emission in 2 μm was demonstrated from a Tm3+-doped tellurite fiber with an output power of 280 mW and a slope efficiency of 76% [16]. In addition, in 2009, laser emission in the range of 1.910-1.994 μm was obtained in Tm3+/Yb3+ doped tellurite fiber, its maximum output power of 67 mW [17]. However, the thermal performance of tellurite glass limits the improvement of their output power [18]. In view of their attractive application, it is necessary to select the matrix with good performance and further investigate 2 μm optical properties. Nevertheless, the choice of glass composition with good thermal stability and excellent optical properties is a big problem. Among other gain medium, fluorophosphate (FP) glasses combined with the significant merits of fluoride and phosphate glass systems [19,20], have a lot of excellent properties, such as decreasing the hygroscopicity of phosphate glass and crystallinity of fluoride glass [21], low phonon energy and good thermal stability [22–24], high quantum efficiency and solubility of rare earth ions, wide tunable emission wavelength range and long fluorescence lifetime [25,26], which make it a promising matrix for Tm3+-doped laser material.
As a gain host for high power laser output, fluorescence lifetime and stimulated emission cross-section are two important parameters to evaluate glass gain performance. According to the previous research, the highest stimulated emission cross-section and gain coefficient of Tm3+-doped FP glass are 5.5 × 10−21 cm2 and 1.513 × 10−21 cm2 × ms, respectively [27]. However, since the gain coefficient of these FP glasses are far less enough to make them highly efficient for the diode-pump high power and short pulse laser applications [28], exploring new FP glasses system and further improving their gain coefficients are critical for the achievement of laser applications. To the best of our knowledge, there are only few reports regarding the correlation of the structural changes with their optical properties, especially the gain properties, of low and high fluoride based Tm3+-doped FP glasses. Hence, it is necessary to explore the effect of structural changes on optical and thermal properties by changing the relative content of phosphate and fluoride.
In this work, the differential scanning calorimetry (DSC) curves, gain coefficient, Raman spectra, fluorescence spectra, absorption spectra and infrared transmittance spectra of Tm3+-doped FP glasses with varying Ba(PO3)2 content were systematically studied. Besides, the J-O intensity parameters of FP glass with 20 mol % Ba(PO3)2 were also investigated.
2. Experimental details
A group of FP glasses with a molar composition (mol%) of X Ba(PO3)2 - (100-X) (36AlF3 – 8BaF2 – 20MgF2 - 7ZnF2 - 29LiF) - 1.5Tm2O3 (X = 20, 30, 40, 50 and 60) have been prepared, and labeled as FP20-FP60 in turn. The raw materials (≥99.99%) were weighted and mixed in an alumina crucible and then melted in an electric furnace at 980-1040 °C for 15 min under ambient atmosphere and then cast into a copper mold preheated at 350 °C. The molded samples were annealed at 400 °C (near the glass transition temperature (Tg)) before cooled down to room temperature. After annealing, all the glass samples were cut and precisely polished into the size of 15 mm × 10 mm × 2 mm for later test.
The DSC measurement was carried out on NETZSCH DSC 404 F3 under a N2 flow with heating rate of 10 °C/min. The absorption spectra were tested with a UV-VIS-NIR spectrophotometer (Jasco V-570, JPN). The infrared transmission spectra were recorded with a Fourier transform infrared spectra (FTIR, Bruker Vertex 70, DE) from 2300 to 3900 nm. Raman spectra were collected with a Jobin-Yvonne LabRam microscope with a 532 nm laser excitation in the range of 200-1400 cm−1. The emission spectra were measured with Edinburgh FLS920 upon the external excitation of 808 nm LD with the power of 1.2 W. InAs detector (Hamamatsu Photonics, P7165) and InGaAs detector (Edinburg, NIR301/2) are used to measured emission spectrum and decay curve, respectively. All the measurements were carried out at room temperature.
3. Results and discussion
Figure 1 shows the DSC curves of FP20-FP60 samples. And the characteristic temperature, including glass transition temperature (Tg), glass crystallization temperature (Tx), and ΔT (the temperature gap between Tx and Tg) of this series of samples are listed in Table 1. It can be found from Fig. 1 that the Tg of FP20 is 418 °C and ΔT is 146 °C, indicating that FP20 have relatively high thermal stability. The corresponding ΔT is much higher than those of FP glass (20-90 °C) [29], fluorogermanate glass (144 °C) [9], tellurate glass (102 °C) [30]. With the increase of Ba(PO3)2 the corresponding Tg increased gradually, which is related to the increased strength of the average chemical bond or the close-knit glass network caused by the introduction of Ba(PO3)2 content in this type FP glasses [31]. As for ΔT, it gradually enhanced up to a critical value of 205 °C when X = 40, whereas ΔT diminishes when X>40, which suggests that a small quantity of Ba(PO3)2 could connect the terminal groups of fluoride and strengthen the structure of glass network [32], but excess Ba(PO3)2 (>40 mol%) could form (PO3F)2- group that are inconsistent with the intrinsic network, leading to the instability of the glass structure. In all, this series of FP glasses in the paper show relative higher thermal stability that is greatly beneficial to optical fiber drawing.
Table 1. Characteristic temperatures of FP glasses with different Ba(PO3)2 content.
In order to explore the changes of glass structure, Raman spectra were used to examine the glass network, as shown in Fig. 2. The strongest peak D (located at around 1055 cm−1) arise from the symmetric stretching of O-P-O in P2(O,F)7 group [33]. Two small peaks C and E as the shoulders of the peak D at 1000 cm−1 and 1125 cm−1 are assigned to monophosphate group P(F,O)4 vibration and O-P-F stretching vibration in the (PO3F)2- group, respectively [34,35]. The other dominant peaks A and B at 356 and 745 cm−1 are associated with bending vibration of phosphate structural units and symmetric stretching of P-O-P in metaphosphate tetrahedron, respectively [35]. Apart from these pronounced peaks there is a less pronounced band in spectra. The bands cover a broad region (480-610 cm−1) originating from the vibration of [AlF4] vibration and R-F (R = Mg2+, Ca2+, Sr2+, Ba2+) bond vibration [36]. It can be found that the peak A increase with the increase of Ba(PO3)2 content. Obviously, the peak E have become stronger that comparisons to the main peak D, while, the peak C almost disappeared, which indicates that the glass structure changes from P(O,F)4 to P2(O,F)7 when Ba(PO3)2 is added into FP glasses. It is also noteworthy that the main peak D gradually enhanced up to a critical value of when X = 40 with the increase of Ba(PO3)2 content, whereas its intensity diminishes when X>40, which is in accordance with the changes of ΔT in Table 1.
Figure 3 depicts the absorption spectra of these glass samples in the wavelength region of 400-2200 nm. The absorption bands center at near 464 nm, 659 nm, 684 nm, 791 nm, 1210 nm and 1660 nm corresponding to the absorption from the ground state 3H6 to the excited states 1G4, 3F2 and 3F3, 3H4, 3H5, and 3F4, respectively. The intensity of absorption peaks decrease with the increase of Ba(PO3)2 content, indicating that the actual doping concentration of Tm3+ decrease with the addition of Ba(PO3)2 in the case glasses. This inference can be confirmed by the solubility of rare earth ions in this type glasses that can be estimated roughly by the integral absorption intensities [9,37], as shown the gradually decreasing integral absorption intensities of 3H6→3F4 transition in the inset of Fig. 3.
Fig. 3 Absorption spectra of FP20-FP60 samples. Inset shows the variation of integral absorption intensities of 3H6→3F4 transition with the change in Ba(PO3)2 content.
Judd-Ofelt theory is used to determine the important spectroscopic and laser parameters of rare earth doped glasses by many researchers [38–41]. Based on the absorption bands of Tm3+-doped these FP glasses, the experimental oscillator strength (fexp) of the transitions can be calculated by [42]
where m and e are the mass and charge of electron, c is the light velocity in the vacuum, N is the concentration of rare earth ions, is the integrated absorption coefficient, and d is the sample thickness.
The theoretical oscillator strength for an electric dipole transition within the 4fN configuration can be expressed as
The theoretical oscillator strength for a magnetic dipole transition can be written as
where m and c are same as in Eqs. (1), v is the wave number, h is the Planck’s constant, J is the angular moment quantum number of the initial state for a transition. Ωt (t = 2, 4, 6) are the Judd-Ofelt parameters; and are the reduced matrix elements for the electric and magnetic dipole transitions; n is refractive index of the host.
The calculation errors δ for the oscillator strengths were estimated by using the following equation
where N is the transition number involved in the Judd-Ofelt calculation, and are, respectively, the experimental and theoretical oscillator strengths of ith transition.
According to the J-O theory, the intensity parameters Ωt (t = 2, 4 and 6) can be calculated and listed in Table 2. The Ω2 increases with the increment of Ba(PO3)2 due to the sensitivity of the Ω2 parameter on the covalence [43] and the higher covalent character of bonding in oxide glasses compared to fluoride glasses. As a result, Ω2 of FP60 is large than that of FP20. In the case of a resonate pumping in the 3H4 state, FP60 have the highest stimulated emission cross-section for the 3F4→3H6 among all the prepared glasses since it is strongly affected by Ω2 parameter [44].
Table 2. J-O strength parameters (Ωt = 2,4,6, × 10−20 cm2) for Tm3+ in the studied FP glasses
While the Judd-Ofelt parameters are confirmed, the radiative transition rates (Arad), radiative transition lifetime (τR), and branch ratio (β) can be calculated from following equations [41]:
The value including spontaneous transition probability, branching ratios and radiative lifetimes are listed in Table 3. It is seen that τR of 3F4→3H6 transition is 7.2 ms,which is longer than that of silicate glass (5.21 ms) [45], germanate glass (1.12 ms) [46] and other FP glass (1.37 ms) [47]. The relatively longer radiation lifetime is beneficial to reduce the laser oscillation threshold [47]. It suggests that this Tm3+-doped FP glass might be a promising candidate for 2 μm laser or amplifier.
Table 3. Calculated line strengths for electric-dipole transition (Sed), Transition probabilities (Arad), branching ratios (β), and radiative lifetime (τR) of FP20.
Figure 4(a) presents the transmittance spectra of various Ba(PO3)2 content in this type FP glasses. It can be seen that the absorption peak central wavelength of OH- group moved from 3155 nm (FP20) to the longer wavelength 3704 nm (FP60) gradually in these FP glasses with the increase of Ba(PO3)2 content. This phenomenon is related to the vibration frequency of OH- group in this type glass. It is well established that phosphorus ions have a pair of lone electrons that can form a double bond with oxygen, and the hydrogen can form strong hydrogen bond with oxygen [48]. Herein, the phosphorus and hydrogen ions compete to combine with oxygen in this type glass, and the balance of this competitiveness tends to phosphorus with the increase of Ba(PO3)2 content. Thus, the weakening bond of hydrogen and oxygen reduce the vibration frequency of OH- group in this type glass, leading to the red shift of OH- group absorption.
Fig. 4 (a) Infrared transmission spectra of FP20-FP60 samples, (b) The variation of OH- absorption coefficient as a function of Ba(PO3)2 content.
In order to improve the excellent emission property in this type glasses, it is necessary to reduce the concentration of OH- that plays a role of quenching centers in the energy transfer processes of Tm3+ ions in FP glasses [29]. Herein, the corresponding OH- absorption coefficients are also calculated according to the following equation [49]
where(cm−1), L, T0 and T are the absorption coefficient, thickness of the sample, the transmission at 2631 nm and the transmission near 3200 nm, respectively. Obviously, as shown in Fig. 4(b), increases with the increment of Ba(PO3)2 in this type glass, which is due to the increased hygroscopicity of FP glass with the increase of Ba(PO3)2 content in the case glasses that lead to larger value. This value of FP20 (0.36 cm−1) sample is lower than that of silicate glass (1.7 cm−1) [50] and tellurite glass (0.5 cm−1) [51].
Figure 5 represents the emission spectra of Tm3+-doped FP samples in the range of 1500-2100 nm pumped by 808 nm. The strong emission band peaking at around 1850 nm was ascribed to the 3F4→3H6 transition of Tm3+ ions, and their intensity decreased with the addition of Ba(PO3)2 content. Compared with fluoride, oxide has the relative higher phonon energy that lead to the relatively low luminescence efficiency in this glass. Meanwhile, the OH- quenching effect on fluorescence become larger with the increase of Ba(PO3)2, as mentioned in Fig. 4, resulting in the declining fluorescence intensity.
According to the absorption spectra, the absorption cross-section σabs can be calculated by using Beer-Lambert equation [52]
where N is the concentration of Tm3+ ion, l is the thickness of the FP glass and is the absorptivity from absorption spectra.
The emission cross-section σem can be deduced from the absorption cross-section by using McCumber formula [53]
where σabs represents the absorption cross section, and are the partition function of the lower and the upper states, respectively, T is the temperature (here is the room temperature), k is the Boltzmann constant and λ is the wavelength of peak absorption. EZL is the energy corresponding to the peak wavelength of the absorption,.
The calculated stimulated emission cross-sections of Tm3+ for 1800 nm transition are shown in Table 4. In Fig. 6, the maximum value of emission cross-section is 10.04 × 10−21 cm2, which is larger than that of Tm3+-doped ZBLAN glass (2.41 × 10−21 cm2) [54] and silicate glass (3.89 × 10−21 cm2) [46]. The higher stimulated emission cross-section is beneficial for the determination of laser action [55].
Table 4. The calculated emission cross-section , measured lifetime and of 3F4→3H6 transition of FP20-FP60 samples.
The fluorescence decay curves of the 3F4 excited state, measured by monitoring the 3F4 → 3H6 transition, are shown in Fig. 7 for the samples of FP20-FP60. The measured fluorescence lifetime values (τm) of 3F4 excited state have been determined by single exponential fitting and recorded in Table 4. It was found that with the increase of Ba(PO3)2 content, the lifetime of 3F4 decreased. The maximum lifetime can be determined to be 0.406 ms, which is large than that of silicate glass (0.250 ms) [56] and tellurite glass (0.226 ms) [57]. The value of is one of the important parameters for laser and optical amplifiers and should be as large as possible to attain high gain [58]. The maximum The value of is 3.045 × 10−21 cm2 × ms, which is twice bigger than other Tm3+-doped FP glass (1.513 × 10−21 cm2 × ms) [27]. Thus, the FP20 glass is an attractive candidate for 2 μm laser.
Based on the calculated σabs and σem, it is valuable to evaluate the wavelength dependence of net gain as a function of population inversion for the upper laser level so as to acquire the gain property quantitatively [59]. It is assumed that Tm3+ ions are either in the ground state(3H6) or in the upper level (3F4), the net gain G(λ) can be expressed by [60]
where P represents the population of the upper laser level, and N stands for the total Tm3+ concentration.
Figure 8 depicts the gain spectra with the various P ranging from 0 to 1 with the step of 0.1 were calculated for the 3F4→3H6 transition of the FP20 sample. The inset is the maximum gain coefficients of FP20-FP60 samples, the maximum gain coefficients decrease with the increment of Ba(PO3)2 content. When Ba(PO3)2 content reaches to 20 mol%, the maximum gain coefficient around 1840 nm is 4.18 cm−1. It is larger than that of silicate glass (1.5 cm−1) [60], and other Tm3+-doped FP glass (0.97 cm−1) [27]. It is expected that this kind of FP glass would be an appropriate host material to achieve infrared emission due to high gain properties.
Fig. 8 Gain coefficient curves for different values of the population inversion for the FP20 sample. The inset is the maximum gain coefficients of FP20-FP60 samples.
4. Conclusion
Tm3+-doped FP glasses modified with Ba(PO3)2 content were successfully prepared by a conventional melt quenching technique and their optical properties were studied. Raman analysis confirms the changes in glass structure by changing the relative content of phosphate and fluoride. The DSC results show the ΔT of the FP glasses are in the range of 146 to 205 °C, which indicates they have excellent thermal stability and suitable for fiber drawing. By increasing the Ba(PO3)2, absorption peak central wavelength of OH- group moved from 3155 nm (FP20) to the longer wavelength 3704 nm (FP60) gradually and the OH- absorption coefficient increased from 0.39 cm−1 to 1.05 cm−1 but still in low level. The fluorescence spectra and fluorescence lifetimes indicate these FP glasses have the outstanding emission properties. Under the investigation of and G (λ), they are consistent with each other very well in deceasing trend with increase of Ba(PO3)2. Among these FP glasses, FP20 shows high gain coefficient (3.045 × 10−21 cm2 × ms), good thermal stability (146 °C), longer fluorescence lifetime (0.406 ms) and lower hydroxyl absorption coefficient (0.39 cm−1) than other Tm3+-doped FP glasses. Thus, the FP20 glass is an attractive candidate for 2 μm lasers and optical amplifier applications.
Funding
National Natural Science Foundation of China (NSFC No. 61605106); CAS Light of West China Program, Special Research Projects of Department of Education of Shaanxi Provincial (No. 18JK0707); Starting Grants of Shaanxi Normal University (No. 1112010209, 1110010717); Fundamental Research Funds For the Central Universities (GK201802006, 2016CSY024); China Postdoctoral Science Foundation (Grant 2017M620383), The Science and Technology Innovation Commission of ShenZhen (JCYJ20170818141407343).
References
1. T. Feng, F. P. Yan, Q. Li, W. J. Peng, S. Y. Tan, S. C. Feng, P. Liu, and X. D. Wen, “A stable wavelength-tunable single frequency and single polarization linear cavity erbium-doped fiber laser,” Laser Phys. 23(2), 025101 (2013). [CrossRef]
2. S. D. Jackson, A. Sabella, A. Hemming, S. Bennetts, and D. G. Lancaster, “High-power 83 W holmium-doped silica fiber laser operating with high beam quality,” Opt. Lett. 32(3), 241–243 (2007). [CrossRef] [PubMed]
3. B. D. O. Richards, Y. H. Tsang, D. J. Binks, J. Lousteau, and A. Jha, “CW and Q-switched 2.1 µm Tm3+/Ho3+/Yb3+-triply-doped telluritefibre lasers,” Proc. SPIE Int. Soc. Opt. Eng. 7111, 711105–711109 (2008).
4. A. S. Kurkov, E. M. Dianov, O. I. Medvedkov, G. A. Ivanov, V. A. Aksenov, V. M. Paramonov, S. A. Vasiliev, and E. V. Pershina, “Efficient silica-based Ho3+ fibre laser for 2 μm spectral region pumped at 1.15 μm,” Electron. Lett. 36(12), 1015–1016 (2000). [CrossRef]
5. Y. J. Shen, B. Q. Yao, X. M. Duan, G. L. Zhu, W. Wang, Y. L. Ju, and Y. Z. Wang, “103 W in-band dual-end-pumped Ho:YAG laser,” Opt. Lett. 37(17), 3558–3560 (2012). [CrossRef] [PubMed]
6. G. X. Chen, Q. Y. Zhang, G. F. Yang, and Z. H. Jiang, “Mid-infrared emission characteristic and energy transfer of Ho3+-doped tellurite glass sensitized by Tm 3+.,” J. Fluoresc. 17(3), 301–307 (2007). [CrossRef] [PubMed]
7. M. Wang, L. Yi, G. Wang, L. Hu, and J. Zhang, “2 μm emission performance in Ho3+-doped fluorophosphate glasses sensitized with Er3+ and Tm3+ under 800 nm excitation,” Solid State Commun. 149(29-30), 1216–1220 (2009). [CrossRef]
8. J. Wu, Z. Yao, J. Zong, and S. Jiang, “Highly efficient high-power thulium-doped germanate glass fiber laser,” Opt. Lett. 32(6), 638–640 (2007). [CrossRef] [PubMed]
9. T. Wei, Y. Tian, F. Chen, M. Cai, J. Zhang, X. Jing, F. Wang, Q. Zhang, and S. Xu, “Mid-infrared fluorescence, energy transfer process and rate equation analysis in Er3+ doped germanate glass,” Sci. Rep. 4(1), 6060 (2015). [CrossRef] [PubMed]
10. D. D. Martino, L. F. Santos, A. C. Marques, and R. M. Almeida, “Vibrational spectra and structure of alkali germanate glasses,” J. Non-Cryst. Solids 293-295(1), 394–401 (2001). [CrossRef]
11. D. C. Hanna, M. J. McCarthy, I. R. Perry, and P. J. Suni, “Efficient high-power continuous-wave operation of monomode Tm-doped fibre laser at 2 μm pumped by Nd:YAG laser at 1.064 μm,” Electron. Lett. 25(20), 1365–1366 (1989). [CrossRef]
12. Y. Jeong, P. Dupriez, J. K. Sahu, J. Nilsson, D. Y. Shen, W. A. Clarkson, and S. D. Jackson, “Power scaling of 2 μm ytterbium-sensitised thulium-doped silica fibre laser diode-pumped at 975 nm,” Electron. Lett. 41(4), 173–174 (2005). [CrossRef]
13. S. A. Brawer and W. B. White, “Raman spectroscopic investigation of the structure of silicate glasses. I. the binary alkali silicates,” J. Chem. Phys. 63(6), 2421–2432 (1975). [CrossRef]
14. S. D. Jackson and T. A. King, “Efficient gain-switched operation of a Tm-doped silica fiber laser,” IEEE J. Quantum Electron. 34(5), 779–789 (1998). [CrossRef]
15. B. Richards, S. Shen, A. Jha, Y. Tsang, and D. Binks, “Infrared emission and energy transfer in Tm3+, Tm3+-Ho3+ and Tm3+-Yb3+-doped tellurite fibre,” Opt. Express 15(11), 6546–6551 (2007). [CrossRef] [PubMed]
16. B. Richards, Y. Tsang, D. Binks, J. Lousteau, and A. Jha, “Efficient 2 μm Tm3+-doped tellurite fiber laser,” Opt. Lett. 33(4), 402–404 (2008). [CrossRef] [PubMed]
17. B. Richards, Y. Tsang, D. Binks, J. Lousteau, and A. Jha, “2 μm Tm3+/Yb3+-doped tellurite fibre laser,” J. Mater. Sci. 20(1), 317–320 (2009).
18. M. Pollnau and S. D. Jackson, “Mid-infrared fiber lasers,” In Solid-state mid-infrared laser sources. Springer, Berlin, Heidelberg, 225–261 (2003).
19. Q. Mi, “Partial dispersions of optical FP glasse,” J. Non-Cryst. Solids 52(1), 347–356 (1982). [CrossRef]
20. N. Rigout, J. L. Adam, and J. Lucas, “Chemical and physical compatibilities of fluoride and fluorophosphate glasses,” J. Non-Cryst. Solids 184(1), 319–323 (1995). [CrossRef]
21. R. C. Kwong, M. R. Nugent, L. Michalski, T. Ngo, K. Rajan, Y. J. Tung, M. S. Weaver, T. X. Zhou, M. Hack, M. E. Thompson, S. R. Forrest, and J. J. Brown, “High operational stability of electro phosphorescent devices,” Appl. Phys. Lett. 81(1), 162–164 (2002). [CrossRef]
22. M. R. Ozalp, G. Özen, A. Sennaroglu, and A. Kurt, “Stimulated and spontaneous emission probabilities of Tm3+ in TeO2-CdCl2 glass: the role of the local structure,” Opt. Commun. 1(1-6), 281–289 (2003). [CrossRef]
23. B. Karmakar, P. Kundu, and R. N. Dwivedi, “IR spectra and their application for evaluating physical properties of fluorophosphate glasses,” J. Non-Cryst. Solids 289(1–3), 155–162 (2001). [CrossRef]
24. S. Ronchin, R. Rolli, M. Montagna, C. Duverger, V. Tikhomirov, A. Jha, M. Ferrari, G. C. Righini, S. Pelli, and M. Fossi, “Erbium-activated aluminum fluoride glasses: optical and spectroscopic properties,” J. Non-Cryst. Solids 284(1–3), 243–248 (2001). [CrossRef]
25. W. Seeber, S. Barth, F. Seifert, H. Ebendorff-Heidepriem, and D. Ehrt, “New Yb-doped fluoride phosphate laser glass-structural investigations using probe ions,” J. Lumin. 72(74), 449–450 (1997). [CrossRef]
26. M. Weber, J. Lynch, D. Blackburn, and D. Cronin, “Dependence of the stimulated emission cross section of Yb3+ on host glass composition,” IEEE J. Quantum Electron. 19(10), 1600–1608 (1983). [CrossRef]
27. R. Li, C. Tian, Y. Tian, T. Wei, B. Li, X. Jing, F. Ruan, and F. Wang, “Mid-infrared emission properties and energy transfer evaluation in Tm3+-doped fluorophosphate glasses,” J. Lumin. 162, 58–62 (2015). [CrossRef]
28. C. Hönninger, R. Paschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. M.S. Biswal, J. Nees, A. Braun, G. A. Mourou, I. Johannsen, A. Giesen, W. Seeber, and U. Keller, “Ultrafast ytterbium-doped bulk lasers and laser amplifiers,” Appl. Phys. B 69(1), 3–17 (1999).
29. N. Rigout, J. L. Adam, and J. Lucas, “Chemical and physical compatibilities of fluoride and fluorophosphate glasses,” J. Non-Cryst. Solids 184(1), 319–323 (1995). [CrossRef]
30. J. Yuan, S. X. Shen, W. C. Wang, M. Y. Peng, Q. Y. Zhang, and Z. H. Jiang, “Enhanced 2.0 μm emission from Ho3+ bridged by Yb3+ in Nd3+/Yb3+/Ho3+ triply doped tungsten tellurite glasses for a diode-pump 2.0 μm laser,” J. Appl. Phys. 114(13), 133506 (2013). [CrossRef]
31. R. Lebullenger, L. A. O. Nunes, and A. C. Hernandes, “Properties of glasses from fluoride to phosphate composition,” J. Non-Cryst. Solids 284(1), 55–60 (2001). [CrossRef]
32. M. Wang, L. Yi, Y. Chen, C. Yu, G. Wang, L. Hu, and J. Zhang, “Effect of Al(PO3)3 content on physical, chemical and optical properties of fluorophosphate glasses for 2 μmapplication,” Mater. Chem. Phys. 114(1), 295–299 (2009). [CrossRef]
33. J. J. Videau, J. Portier, and B. Piriou, “Raman spectroscopic studies of fluorophosphate glasses,” J. Non-Cryst. Solids 48(2–3), 385–392 (1982). [CrossRef]
34. D. Ehrt, “Structure and properties of fluoride phosphate glasses,” Proc. SPIE 1992, 1761 (1992).
35. J. Y. Ding, P. Y. Shih, S. W. Yung, K. L. Hsu, and T. S. Chin, “The properties and structure of Sn□Ca□P□O□F glasses,” Mater. Chem. Phys. 82(1), 61–67 (2003). [CrossRef]
36. L. Koudelka, J. Klikorka, M. Frumar, M. Pisárčik, V. Kellö, V. D. Khalilev, V. I. Vakhrameev, and G. D. Chkhenkeli, “Raman spectra and structure of FP glasses of (1-x)Ba(PO3)2-x LiR-AlF6,” J. Non-Cryst. Solids 85(1–2), 204–210 (1986). [CrossRef]
37. Y. G. Choi, K. H. Kim, and J. Heo, “Spectroscopic Properties of and Energy Transfer in PbO–Bi2O3–Ga2O3 Glass Doped with Er2O3,” J. Am. Ceram. Soc. 82(10), 2762–2768 (1999). [CrossRef]
38. M. Shojiya, Y. Kawamoto, and K. Kadono, “Judd–Ofelt parameters and multiphonon relaxation of Ho3+ions in ZnCl2-based glass,” J. Appl. Phys. 89(9), 4944–4950 (2001). [CrossRef]
39. Y. Zhang, B. Chen, S. Xu, X. Li, J. Zhang, J. Sun, X. Zhang, H. Xia, and R. Hua, “A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the β-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation,” Phys. Chem. Chem. Phys. 20(23), 15876–15883 (2018). [CrossRef] [PubMed]
40. G. Sui, B. Chen, J. Zhang, X. Li, S. Xu, J. Sun, Y. Zhang, L. Tong, X. Luo, and H. Xia, “Examination of Judd-Ofelt calculation and temperature self-reading for Tm3+ and Tm3+/Yb3+ doped LiYF4 single crystals,” J. Lumin. 198, 77–83 (2018). [CrossRef]
41. Y. Zheng, B. Chen, H. Zhong, J. Sun, L. Cheng, X. Li, J. Zhang, Y. Tian, W. Lu, J. Wan, T. Yu, L. Huang, H. Yu, and H. Lin, “Optical transition, excitation state absorption, and energy transfer study of Er3+, Nd3+ single doped, and Er3+/Nd3+ codoped tellurite glasses for mid-infrared laser applications,” J. Am. Ceram. Soc. 94(6), 1766–1772 (2011). [CrossRef]
42. Y. Zheng, B. Chen, J. Sun, L. Cheng, H. Zhong, L. I. Xiangping, and L. Huang, “Composition-dependent optical transitions and 2.0 μm emission properties of Ho3+ in xGeO2-(80–x)TeO2-9.5ZnF2-10NaF-0.5Ho2O3 glasses,” J. Rare Earths 29(10), 924–928 (2011). [CrossRef]
43. R. Reisfeld and C. K. Jørgensen, “Excited state phenomena in vitreous materials,” Handbook on the physics and chemistry of rare earths 9, 1–90 (1987).
44. A. Kermaoui and F. Pellé, “Synthesis and infrared spectroscopic properties of Tm3+-doped phosphate glasses,” J. Alloys Compd. 469(1), 601–608 (2009). [CrossRef]
45. X. Wang, K. Li, C. Yu, D. Chen, and L. Hu, “Effect of Tm2O3 concentration and hydroxyl content on the emission properties of Tm3+-doped silicate glasses,” J. Lumin. 147(3), 341–345 (2014). [CrossRef]
46. B. Peng and T. Izumitani, “Optical properties, fluorescence mechanisms and energy transfer in Tm3+, Ho3+, and Tm3+-Ho3+ doped near-infrared laser glasses, sensitized by Yb3+,” Opt. Mater. 4(6), 797–810 (1995). [CrossRef]
47. Q. Zhang, G. Chen, G. Zhang, J. Qiu, and D. Chen, “Infrared luminescence of Tm3+/Yb3+codoped lanthanum aluminum germanate glasses,” J. Appl. Phys. 107(2), 285 (2010). [CrossRef]
48. G. X. Cao, H. F. Hu, and F. X. Gan, “Study on the Optical Properties of Fluorophosphate Glass,” Journal of Zhengzhou University 34(4), 90–93 (2013).
49. R. Wang, X. Meng, F. Yin, Y. Feng, G. Qin, and W. Qin, “Heavily erbium-doped low-hydroxyl fluorotellurite glasses for 2.7 μm laser applications,” Opt. Mater. Express 3(8), 1127–1136 (2013). [CrossRef]
50. X. Liu, X. Wang, L. Wang, P. Kuan, M. Li, W. Li, X. Fan, K. Li, L. Hu, and D. Chen, “Realization of 2 μm laser output in Tm3+-doped lead silicate double cladding fiber,” Mater. Lett. 125, 12–14 (2014). [CrossRef]
51. K. Li, G. Zhang, X. Wang, L. Hu, P. Kuan, D. Chen, and M. Wang, “Tm3+ and Tm3+-Ho3+ co-doped tungsten tellurite glass single mode fiber laser,” Opt. Express 20(9), 10115–10121 (2012). [CrossRef] [PubMed]
52. K. F. Li, G. N. Wang, J. J. Zhang, and L. L. Hu, “Broadband 2μm emission in Tm3+-Ho3+ co-doped TeO2-WO3-La2O3 glass,” Solid State Commun. 150(39–40), 1915–1918 (2010). [CrossRef]
53. D. E. Mccumer, “Theory of photon-terminated opticalmasers,” Phys. Rev. 134, A299 (1964).
54. J. L. Doualan, S. Girard, H. Haquin, J. L. Adam, and J. Montagne, “Spectroscopic properties and laser emission of Tm doped ZBLAN glass at 1.8 μm,” Opt. Mater. 24(3), 563–574 (2003). [CrossRef]
55. F. Cornacchia, A. Toncelli, and M. Tonelli, “2μm lasers with fluoride crystals: Research and development,” Prog. Quantum Electron. 33(2–4), 61–109 (2009). [CrossRef]
56. X. L. Zou and H. Toratani, “Spectroscopic properties and energy transfers in Tm3+ singlyand, Tm3+/Ho3+ doubly-doped glasses,” J. Non-Cryst. Solids 195(1–2), 113–124 (1996). [CrossRef]
57. H. Lin, X. Wang, L. Lin, C. Li, D. Yang, and S. Tanabe, “Near-infrared emission character of Tm3+-doped heavy metal tellurite glasses for optical amplifiers and 1.8 µm infrared laser,” J. Phys. D Appl. Phys. 40(12), 3567–3572 (2007). [CrossRef]
58. K. Linganna, K. Suresh, S. Ju, W. T. Han, C. K. Jayasankar, and V. Venkatramu, “Optical properties of Er3+-doped K-Ca-Al fluorophosphate glasses for optical amplification at 1.53 μm,” Opt. Mater. Express 5(8), 1689–1703 (2015). [CrossRef]
59. X. Wen, G. Tang, Q. Yang, X. Chen, Q. Qian, Q. Zhang, and Z. Yang, “Highly Tm3+ doped germanate glass and its single mode fiber for 2.0 μm laser,” Sci. Rep. 6(1), 20344 (2016). [CrossRef] [PubMed]
60. M. Li, G. Bai, Y. Guo, L. Hu, and J. Zhang, “Investigation on Tm3+-doped silicate glass for 1.8 mm emission,” J. Lumin. 132(7), 1830–1835 (2012). [CrossRef]
