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Abstract
Broadband emission was obtained over 2.6 to 4.1 μm (Pr3+: 1G4→3F4, 3F3) in AlF3-based glass samples doped with different concentrations of praseodymium and 1 mol% ytterbium using a 976 nm laser pump. An efficient energy transfer process from Yb3+: 2F5/2 to Pr3+: 1G4 was analyzed through emission spectra and fluorescence lifetime values. The absorption and emission cross-sections were calculated by Füchtbauer-Ladenburg and McCumber theories and a positive gain can be obtained when P>0.3. To the best of the authors’ knowledge, this work represents the first report of broadband mid-infrared emission of Pr3+ in an AlF3-based glass. The results show that praseodymium doped AlF3-based glass sensitized by ytterbium could be a promising candidate for fiber lasers operating in mid-infrared region.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared (MIR) lasers in the range of 2-5 μm have attracted significant scientific interest as their wavelength range coincides with minimum attenuation in the atmospheric transmission window. In addition, the presence of many absorption peaks for gas and organic molecules in the same wavelength region guarantees the application of MIR lasers in many fields, including aerospace communication [1], atmospheric monitoring [2], spectroscopy [3] and national defense [4]. There are two distinct technological approaches to the implementation of MIR lasers. The first one is based on nonlinear optical effects, including optical parametric oscillation (OPO) [5] and difference frequency generation (DFG) [6]. However, this form of laser usually suffers from significant complexity, low electro-optical efficiency, and a complicated OPO crystal preparation process. Alternatively, MIR lasers can also be realized directly through gain materials including quantum well (QW) semiconductors [7] and transition metal (TM) doped II-IV semiconductors [8]. QW lasers are high-beam-quality devices, while TM-doped II-IV semiconductor lasers suffer from dramatically reduced laser output efficiency at higher temperatures [9]. Solid state lasers based on these materials have been extensively studied and have already been commercialized.
However, compared with the above techniques, rare earth (RE) doped MIR fiber lasers exhibit significant advantages including a greater spectral range, better pump efficiency, higher transmittance, stability, improved portability and easier-integration, etc. [10]. Fluoride and chalcogenide glasses are known as suitable materials for MIR fiber lasers. Of particular note very recently, there has been some progress in lasing beyond 5 μm, that has been achieved in chalcogenide glass fibers thanks to its lower phonon energy, greatly expanding the potential applications of RE doped fiber lasers [11,12]. At present, most 2-5 μm MIR fiber lasers use fluoride glass fibers doped with Er3+, Dy3+ and Ho3+ to produce an output in the range of 2.7-3.9 μm in fluorozirconate and fluoroindate materials, which are easy to fabricate and possess wide transmission windows and higher rare earth solubility than chalcogenide glasses, moreover, they also show high transmittances in this region compared to chalcogenide [13]. ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) glass is one of the most widely studied host materials since 1985 [14,15]. Some significant research advances have been made in RE-doped ZBLAN materials in the past few years [13]. In recent years, fluoroindate material with its lower phonon energy has attracted attention for example the demonstration of a 197 mW fiber laser at the wavelength of 3.9 μm [16]. However, generally speaking, fluorozirconate and fluoroindate fibers suffer from severe deliquescence, which limits their applications in many fields.
AlF3-based glasses possess much higher chemical and thermal stability, exhibit superior moisture resistance performance compared to ZBLAN glass [17,18], and have been employed in VIS-NIR-MIR lasers. In 2010, a visible yellow laser output was realized in Dy3+-doped AlF3-based glass fibers [19]. In 2000, Nd3+-doped AlF3-based glass fibers were utilized for a 1.3 μm fiber amplifier [20]. Very recently, some authors of our group successfully demonstrated a ∼2.86 μm MIR laser in Ho3+/Pr3+ co-doped AlF3-based glass fibers with output powers up to 1 W [21,22], indicating that AlF3-based glass has great potential in the field of mid-infrared lasers.
RE-doped fluoride fiber lasers operating at ∼3.5 μm usually require a complex setup, and are limited by the pump efficiency and pump source availability [23,24]. Therefore, it is necessary to develop novel rare-earth doped materials that could be excited by commonly available commercial laser devices operating at ∼808 nm or ∼980 nm to produce an intense ∼3.5 μm MIR emission.
In this study, intense 2.6-4.1 μm broadband emission peaked at 3.46 nm is realized in Pr3+/Yb3+ co-doped AlF3-based glass pumped by a 976 nm laser diode, demonstrating that there is an efficient energy transfer process from Yb3+ to Pr3+. The calculated results using Judd-Ofelt and Füchtbauer-Ladenburg theories give a more detailed analysis for this material.
2. Experiments
The AlF3-based glass compositions used in the experiment can be expressed in terms of molecular ratio as 30AlF3-10BaF2-19CaF2-(9.5-x-y)YF3-12.5SrF2-3.5MgF2-3LiF-10ZrF4-2.5PbF2-xPrF3-yYbF3 (x=0, 0.1, 0.2, 0.3, 0.5, 1, 2, 3; y=0, 1). Using high-purity and dehydrated fluorides, the AlF3-based glasses were fabricated using a conventional melt-quenching technique. The mixture was heated and melted in a platinum crucible at 900 °C for 60 minutes in a glove box with ultra-dry N2 to reduce the content of hydroxyl impurities. The melt was then cast onto a brass plate preheated at 370 ℃ and then annealed for 3 hours. Glass samples were then cut and polished with dimensions 10×10×2 mm for subsequent measurements.
Absorption and transmission spectra in the range of 200−2500 nm and 2500−9000 nm were recorded using a Perkin Elmer Lambda 750 UV–VIS–NIR spectrophotometer and Perkin Elmer Fourier-transform infrared (FTIR) spectrometer, respectively. Fluorescence spectra were measured using computer controlled Zolix Omni-λ300i monochromators and spectrographs which were equipped with InGaAs and InSb detectors to suit the wavelength range in use. The pump source was a commercial 976 nm multimode fiber laser (BWT, K976A02RN). The luminescence decay curves were measured using an optical parametric oscillator (Horizon II OPO) with a pulse width of 6 µs and repetition rate of 10 Hz. A spectrometer (Synerjy 1000M) and a digital phosphor oscilloscope (DP04104B) were used to detect and record the fluorescence decay curves. All measurements were conducted at room temperature.
3. Results and discussions
AlF3-based glass samples doped with 1 mol% Yb3+ and with different concentrations of Pr3+ were synthesized. The absorption and transmission spectra of these glasses are shown in Fig. 1. The absorption peaks in Fig. 1(a) correspond to the transitions from ground level to excited energy levels of Yb3+ and Pr3+, and the overlapping peak near 980 nm indicates that a ∼980 nm LD could be used as the pump.
Fig. 1. (a) The absorption spectra, and (b) transmission spectrum of Pr3+/Yb3+ co-doped AlF3-based glasses. Inset: an enlargement spectrum from 2500 nm to 3500 nm.
As shown in Fig. 1(b), the average transmittance is ∼92% for wavelengths shorter than 5 μm, and the cutoff wavelength is 9 μm, proving that the AlF3-based glass can be utilized for mid-infrared applications. The inset shows the spectral region near 3μm, and the very weak absorption peak of OH- is evidence that the OH- content of the AlF3-based glass was very low. The absorption coefficient of OH- calculated by αOH-=ln(T0/T)/l is 0.0078 cm-1, lower than that of fluorozirconate (0.031cm-1) [25] and fluoroindate (0.06 cm-1) [26], where T0 is the maximum transmittance, T the transmittance at λ∼3 μm and l the thickness of the glass sample.
The energy level diagram of Pr3+ and Yb3+ is shown in Fig. 2.
Although Pr3+ ions can be pumped to the 1G4 level by ground state absorption (GSA2), the efficiency is very low [27], while Yb3+ ions can efficiently absorb ∼980 nm photon energy through the GSA1 process. By introducing Yb3+, the photon energy can be transferred to Pr3+ ions through the energy transfer (ET1) process. After being pumped to 1G4, several down-conversion emissions from the 1G4 level to lower levels occurred, emitting photons at 1.1, 1.3, 1.8, 2.1, 2.9 and 3.5 μm. Meanwhile, the excited Pr3+ ions can absorb ∼980 nm photon energy and transit to 3P0 and 1D2 levels by excited state absorption (ESA1, ESA2) and energy transfer processes (ET2, ET3) [28]. Then the populations on higher levels will relax to the 1G4 level, resulting in more efficient emissions.
To evaluate the ET1 efficiency from Yb3+ to Pr3+, the luminescence decay curves of Yb3+:2F5/2 were recorded with pumping by a 976 nm LD, as shown in Fig. 3(a). One of the typical curves of 1Yb-0.3Pr is shown in the inset.
Fig. 3. (a) The dependence of Yb3+:2F5/2 lifetime and energy transfer efficiency on Pr3+ concentration. Inset: The luminescence decay curve of Yb3+:2F5/2 in 1Yb-0.3Pr sample. (b) The lifetime of Pr3+: 1G4→3H4, 3H5, 3H6 and 3F2.
The energy transfer efficiency can be calculated using the following equation:
where τ1Yb-xPr is the luminescence lifetime of Yb3+:2F5/2 and the Pr3+ concentration is x. It can be seen that the lifetime of the 2F5/2 level decreases as the Pr3+ concentration increases, and that the ET1 efficiency increases to its maximum value of 89% at 3 mol%, proving the efficiency of the ET1 process. Figure 3(b) shows the lifetimes of the transitions from 1G4 to 3H4, 3H5, 3H6 and 3F2 levels in Pr3+ ions. The decreases can be attributed to the energy transfer up-conversion (ETU1, ETU2, and ETU3) and cross-relaxation (CR) processes between the Pr3+ ions [29], which would increase accordingly as a result of the higher doping concentration and the shorter distance between ions. As shown in Fig. 2, these processes together lead to a reduction of the populations on the 1G4 level. Furthermore, the Pr3+ wide ground state absorption from 3H4→3H5 (GSA3) in the range of 2.5-5.7 μm can also cause a depopulation effect on the 1G4 level [30].Compared with Yb3+, Pr3+ has weak absorption peaks at 976 nm, as shown in Fig. 4(a). It is crucial to introduce Yb3+ as a sensitizer to enhance the pump efficiency due to its strong absorption in this wavelength region. Using a 976 nm LD as the pump source, the MIR emission spectra of Pr3+ single-doped and Pr3+/Yb3+ co-doped AlF3-based glasses were recorded and are shown in Fig. 4(b). The 2.9 and 3.5 μm MIR emission peaks can be attributed to the transitions of 1G4→3F3 and 1G4→3F4, respectively. Figure 4(b) shows that the 2.6-4.1 μm MIR emission intensity is greatly improved by a factor of 16 due to the ET1 process from the Yb3+ ions to Pr3+ ions, clearly proving that 976 nm is a suitable pumping wavelength in Pr3+/Yb3+ co-doped glasses. These energy transfer processes greatly enhance the populations on the Pr3+: 1G4 level and are therefore of great benefit in the generation of intense MIR emissions.
Fig. 4. (a) Absorption spectra near 980 nm in Yb3+ and Pr3+ single-doped AlF3-based glasses. (b) The ∼3.5 μm (from 2.6 to 4.1 μm) mid-infrared emission spectra of Pr3+/Yb3+ co-doped and Pr3+ single-doped AlF3-based glasses.
Figure 5 shows the emissions in the MIR (1G4→3F3, 3F4) and near-infrared (NIR) range (1G4→3H4, 3H5, 3H6, 3F2). It is worth noting that the characteristics of the radiative transitions mentioned above show evidence of concentration quenching. For NIR emissions at 1.1 and 2.1 μm, the concentration needed for maximum intensity is 0.1 mol%, while that for the emission at 1.32 μm is 0.5 mol%. As for the emission near 1.8 μm, it has almost equal intensities when the Pr3+ concentrations are 0.2 to 0.5 mol%. For the MIR emissions, the concentration needed for the maximum intensity is 0.3 mol%. Though 3.5 μm emission (1G4→3F4) is easily affected by GSA3 process, an efficient emission could be achieved with an appropriate doping concentration [31]. Compared with ZBLAN glass (2.8-3.95 μm [30]; 3-3.9 μm [31]), the transitions from 1G4→3F3 and 3F4 in AlF3-based glass have a wider width.
Fig. 5. The (a) Mid-infrared, and (b) near-infrared emission spectra of Pr3+/Yb3+ co-doped AlF3-based glasses with different concentration of Pr3+.
Figure 6 shows normalized mid-infrared emission spectra of AlF3-based glasses with different RE dopants. Several RE ions can generate radiative emissions corresponding to the transitions of Er3+:4I11/2→4I13/2, Dy3+:6H13/2→6H15/2, Ho3+:5I6→5I7, Pr3+:1G4→3F4, Er3+:4F9/2→4I9/2, Ho3+:5I5→5I6. These transitions have small energy gaps, thus when the phonon energy of the host material is relatively large, it will result in a high non-radiative decay rate, according to the modified non-radiative decay theory proposed by V. Dijk and M. Schuurmans [32]. Consequently, it is useful to note that it is possible to suppress the non-radiative relaxation process by reducing the phonon energy of the host material.
Fig. 6. The normalized mid-infrared emission spectra of AlF3-based glasses with different rare earth dopants in the region of 2500-4200 nm.
The emission results indicate that AlF3-based glass with low phonon energy (615 cm-1) is a suitable host for doping in order to emit photons in the MIR region. It is also worth noting that the emission band of Pr3+ can cover the range of 2.6 to 4.1 μm, which is much wider than that of other RE ions. This broad bandwidth could be a valuable asset in the implementation of wavelength-tunable MIR lasers.
Judd-Ofelt (J-O) theory is usually applied to evaluate the emission properties of RE dopants and the nature of host matrixes [33]. The J-O parameters of Pr3+ ions Ω2,4,6 were calculated to be 0.20×10−20, 3.96×10−20 and 5.32×10−20 cm2, respectively. Other radiative properties including radiative transition probabilities, energy level lifetimes and fluorescence branch ratios were also calculated. The results of transitions from 1G4 to lower levels are shown in Table 1.
Table 1. Radiative properties of transitions from 1G4 to lower levels.
Based on the above experimental data and calculation results, the emission and absorption cross-sections were calculated using Füchtbauer-Ladenburg and McCumber theories [34,35], as shown in Fig. 7. The calculated peak values of the emission and absorption cross-sections are 3.21×10−21 cm2 and 3.89×10−21 cm2, respectively, showing the emission cross-section of Pr3+ near 3.5 μm in AlF3-based glass material is much higher than that of ZBLAN material doped with Er3+ or Ho3+. This confirms the potential of Pr3+ in AlF3-based glass materials as a new approach to implementing MIR lasers [36,37].
Fig. 7. (a) The absorption and emission cross-sections of Pr3+:1G4→3F3, 3F4 in AlF3-based glass. (b) The gain spectra of Pr3+:1G4→3F3, 3F4 in AlF3-based glass.
The gain properties of laser medium can be estimated from its gain coefficient, which can be derived from the emission and absorption cross-sections. The gain coefficient of Pr3+/Yb3+ co-doped AlF3-based glass was calculated and the resulting values versus wavelength are shown in Fig. 7(b). A positive gain coefficient can be obtained when P≥0.3 (P = the population of the upper energy levels/the population of total energy levels) beyond 3659.4 nm, indicating that it requires a relatively low pump power threshold to generate laser output in a Pr3+/Yb3+ co-doped AlF3-based glasses fiber.
4. Conclusion
In summary, 2.6-4.1 μm MIR emissions were successfully achieved in Pr3+/Yb3+ co-doped AlF3-based glasses under excitation by a 976 nm LD. The optimal Pr3+ concentration was experimentally determined to be 0.3 mol% for MIR emission. Based on the measured luminescence decay curves and lifetimes, the transition mechanism and energy transfer efficiency were ascertained. After calculating the emission and absorption cross-sections, the gain coefficient was also calculated. The results demonstrate that Pr3+/Yb3+ co-doped AlF3-based glass shows good potential for use in 2.6-4.1 μm MIR fiber lasers.
Funding
National Natural Science Foundation of China (61935006, 62005060, 61905048, 62090062, 61805074); National Key Research and Development Program of China (2020YFA0607602); Shenzhen Technical Project (JCYJ20190808173619062); 111 project to the Harbin Engineering University (B13015); Heilongjiang Touyan Innovation Team Program.
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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