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Fabrication and properties of 10 at.% Yb:Y3Sc1.5Al3.5O12 transparent ceramics
Optical Materials 88 (2019) 339–344
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Fabrication and properties of 10 at.% Yb:Y3Sc1.5Al3.5O12 transparent ceramics
T
Yagang Fenga,b, Zewang Hua,b, Xiaopu Chena,b, Cong Chena, Yangcheng Ouc, Yang Liuc, Wenxue Lic, Jiang Lia,∗ a
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China c State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200062, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Yb:Y3Sc1.5Al3.5O12 ceramics Fabrication Spectroscopic properties Laser performance
10 at.% Yb:Y3Sc1.5Al3.5O12 (Yb:YSAG) transparent ceramics were fabricated by the solid state reactive of highpurity Y2O3, α-Al2O3, Yb2O3 and Sc2O3 powders, utilizing MgO and tetraethoxysilane (TEOS) as the sintering aids. The Yb:YSAG ceramic sample sintered at 1820 °C for 10 h has a dense and homogenous microstructure with average grain size of 26.9 μm, whose in-line transmittance reaches 83.2% at 1100 nm (2 mm thick). The absorption and emission cross sections are calculated to be 5.09 × 10−21 cm2 at 941 nm and 12.33 × 10−21 cm2 at 1031 nm, respectively. The full width at half maximum (FWHM) of the main emission peak centered at 1031 nm is 12.4 nm, which is about 1.3 times larger than that of the 10 at.% Yb:YAG ceramics. By using the three-mirror resonator, the Yb:YSAG ceramic laser output power of 1.67 W at 1031 nm and slope efficiency of 13.1% were obtained with an output coupler (OC) of T = 5%.
1. Introduction In the last decades, the researches of Yb3+ doped transparent laser ceramics have attracted much attention since the presentation of laserdiodes (LDs), as the ceramics have the equivalent or even better properties in contrast with crystals [1–7]. The Yb3+ doped gain media have several advantages of high quantum efficiency, no excited state absorption, no upconversion and concentration quenching. In addition, the relatively broad absorption peak of Yb3+ centered at 940 nm can well coupling with the InGaAs laser diode, which makes it possible to obtain highly efficient laser operation under the direct diode pumping [8–11]. Among many laser hosts, the yttrium aluminum garnet (YAG) has been proved to be an excellent laser gain medium due to its high thermal conductivity, good mechanical and optical properties [12–15]. In 2003, Takaichi et al. successfully fabricated the Yb:YAG ceramics and firstly achieved the laser output with a maximum output power of 345 mW and slope efficiency of 26% [16]. Then the Yb:YAG transparent ceramics have been greatly developed for utilizing both in the high power laser and ultra-fast laser fields [17–22]. For the Yb:YAG ceramics used in the ultra-fast pulse laser filed, the relatively sharp and narrow emission bandwidth at around 1030 nm makes it very challenging to obtain sub 100 fs pulse laser [23].
∗
According to the mode-locking laser operation, a shorter pulse laser can be obtained by compressing a broader emission bandwidth in a suitable laser cavity [24]. In order to solve the above problem, one effective way is using disordered garnet hosts which are obtained by substituting some of the lattice sites with other ions, and the ions have the similar chemical and physical properties. Then the disordered crystal field would strongly broaden both the emission and absorption lines, which is benefit for the generation of ultra-fast pulse laser [25–30]. Several disordered garnet laser materials have been reported, such as Yb:LuxY3xAl5O12 (Yb:LuYAG), Yb:GdxY3-xAl5O12 (Yb:GYAG), Yb:Y3ScxAl5-xO12 (Yb:YSAG), and so on [31–34]. Concerning the Yb:YSAG ceramics, they are formed by using the Sc3+ ions partly substitute the Al3+ ions in the YAG lattice to occupy the octahedron site. The modified crystal field causes a large splitting of the sublevel in the upper (2F5/2) and lower (2F7/2) manifolds of Yb3+, and this results in the FWHM of emission peak of Yb:Y3ScAl4O12 centered at 1031 nm is nearly 1.5 times larger than that of the Yb:YAG centered at 1030 nm [35,36]. The remarkable broaden of the emission bandwidth is very useful in the mode-locking laser operation and some significant research findings have been reported. In 2004, Saikawa et al. firstly fabricated the 15 at.% Yb:Y3ScAl4O12 ceramics and measured the laser performance. A 580 fs short pulse laser
Corresponding author. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.optmat.2018.11.057 Received 7 October 2018; Received in revised form 20 November 2018; Accepted 29 November 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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powders are in fine dispersity and homogeneity. The primary particle sizes of the Yb2O3, Y2O3, Sc2O3 and α-Al2O3 powders are 150 nm, 80 nm, 230 nm and 350 nm, respectively. Fig. 2 shows the FESEM micrograph of the ball-milled powder mixture. It reveals that the ball-milling process can efficiently break the large agglomerations of the Yb2O3, Y2O3 and Sc2O3 powders. Fig. 3 shows the XRD pattern of the 10 at.% Yb:YSAG ceramics sintered at 1820 °C for 10 h. The standard YAG pattern (JCPDF#33-0040) is displayed under the Yb:YSAG ceramics pattern. It reveals that all the peaks of Yb:YSAG ceramics match well with the standard diffraction patterns of cubic YAG phase. Compared with the pure YAG patterns, the diffraction peaks of the Yb:YSAG ceramics shift to the low angle, which is attributed to the bigger radius of the Sc3+ (0.074 nm) than that of the substituted Al3+ (0.053 nm) in the octahedron site. The calculated lattice parameter of the sample is 1.2224 nm by using cell refinement through MDI Jade 6.5 software, which is in the middle of the parameters of YAG (1.2002 nm) and Y3Sc2Al3O12 (1.2271 nm) [39]. Fig. 4 displays the in-line transmittance of 10 at.% Yb:YSAG ceramics with thickness of 2.0 mm. Besides, the photograph of the sample is inserted into Fig. 4. The in-line transmittances of the sample are 83.2% at 1100 nm and 79.9 at 400 nm, respectively, which is among the highest value to the best of our knowledge. And the decrease of transmittance at the short wavelength can be attributed to the Mie scattering caused by the small residual pores in the sample. Fig. 5 shows the FESEM photograph of thermally etched surface of the 10 at.% Yb:YSAG transparent ceramics. It can be seen that the sample has a nearly full dense structure and the grain boundaries are clean. The average grain size estimated by the intercept method is about 26.9 μm. Fig. 6 shows the absorption and emission spectra of the 10 at.% Yb:YSAG transparent ceramics measured at room temperature. The absorption spectrum reveals that there are four main peaks which are located at 913, 941, 968, 1030 nm, respectively, and they are attributed to the radiation transition of the Yb3+ ions from the low 2F7/2 to the high 2F5/2 energy level. The emission spectrum was measured by motivating the sample through a 915 nm fiber laser. It can be seen that there are four main emission peaks located at 944, 969, 1031, 1050 nm, respectively. The 941 nm absorption peak and 1031 nm emission peak shift to the long wavelength compared with the Yb:YAG ceramics, whose absorption and emission wavelengths are 940 nm and 1030 nm. As it is attributed to larger splitting of the sublevels of the upper (2F5/2) and the lower (2F7/2) manifolds of Yb3+ caused by the introduction of the Sc3+ [36]. From the above absorption spectrum, the absorption cross section can be calculated by the following formulas:
with output power of 62 mW was obtained through the passive modelocking laser operation. Besides, under the CW laser operation, the maximum output power of the gained laser is 810 mW with a slope efficiency of 72% [33,35]. And then, the passive mode-locking laser operation was demonstrated on the 5 at.% Yb:Y3(Sc0.5Al0.5)5O12 ceramics, and a 280 fs pulse laser with an average output power of 62 mW was achieved [37]. In 2017, a passively mode-locked pulse laser of 96 fs at 1052 nm with a maximum average output power of 51 mW was reported in the 10 at.% Yb:Y3ScAl4O12 ceramics by Ma et al. [23]. Recently, Pirri et at. investigated the laser performance of 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics. An output laser with maximum power of 6.3 W and the corresponding slope efficiency of 67.8% were realized by pumping in a Quasi-CW scheme at 936 nm, and they obtained the broadest tuning range which was from 991.5 nm to 1073 nm (81.5 nm) in the ever reported Yb:YSAG ceramics [36]. In this work, we fabricated the 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics by the solid state reaction with TEOS and MgO as sintering aids [38]. Microstructure, optical and laser properties were evaluated in detail. The optical properties of 10 at.%Yb:YSAG and 10 at.% Yb:YAG ceramics were compared. The CW laser outputs operated both in plano-plano resonator and three-mirror resonator as well as wavelength tuning of the non-coated Yb:YSAG ceramics were realized. 2. Experimental The 10 at.% Yb:Y3Sc1.5Al3.5O12 transparent ceramics were fabricated by the solid state reaction method combined with vacuum sintering. High-purity commercial powders were used as the primary powders. Y2O3 (99.99%, Yuelong New Materials Co., Ltd., Shanghai, China), α-Al2O3 (HFF-5, Fenghe Ceramics Co., Ltd., Shanghai, China), Yb2O3 (99.99%, Zhongkai New Materials Co., Ltd., Jining, China) and Sc2O3 (99.99%, Jingyun Material Technology Co., Ltd., Shanghai, China) were weighed in stoichiometric ratio as Yb0.3Y2.7Sc1.5Al3.5O12. And then all the powders were added into the corundum crucible with MgO (99.998%, Alfa Aesar, Tianjin, China) and TEOS (99.999%, Alfa Aesar, Tianjin, China) as well as anhydrous ethanol. The mixture was ball milled for 12 h with a speed of 130 r/min. Then the slurry was dried at 70 °C for 2 h, sifted with a 200-mesh screen and calcinated at 600 °C for 4 h. The calcinated powders were dry pressed into a disk with diameter of 18 mm at 46 MPa, and further cold isostatically pressed at 250 MPa to increase the density. Then the specimens were vacuum sintered at 1820 °C for 10 h, and then annealed at 1450 °C for 20 h in air to eliminate the oxygen vacancies. Finally, the transparent Yb:YSAG ceramics were mirror-polished into 2.0 mm thickness for measurement. The relative densities of the ceramics were calculated by the Archimedes method. The phase composition of the sintered sample was identified by the X-ray diffraction (Cu Kα1 radiation (λ = 0.15405 nm), XRD, Bruker D8 Focus, Germany) in the range of 2θ = 15-75° using nickel-filtered Cu-Kα radiation. The micrographs of the starting powders and the ceramics were observed by a field emission scanning electron microscopy (FESEM, S-4800, Hitach, Japan). And before the SEM observation, the ceramics were thermally etched at 1450 °C for 3 h in air. The average grain size of the sintered sample was calculated by the linear intercept method (more than 200 grains were counted). The in-line transmittance and absorption spectrum of the mirror-polished sample were measured by a UV-VIS-NIR spectrophotometer (Cary5000, Varian, USA). The emission spectrum excited by a 915 nm laser beam was measured by a low temperature absorption spectrometer (FLS-980, Edinburgh Instruments, UK) at room temperature.
α=
−2. 303lg(I/Io) L
σabs =
α N
(1)
(2)
where α is the absorption coefficient, lg(I/I0) is the optical density which can be obtained by the spectrophotometer; and L is the thickness of the sample; N is the Yb3+ ions concentration of the 10 at.% Yb:YSAG ceramics, which is 1.309 × 1021 ions/cm3. Taking these data measured by the instrument into the above formulas, the calculated absorption cross section spectrum is showed in Fig. 7(a). The absorption cross section spectrum of the 10 at.% Yb:YAG ceramics is also showed for comparison. The absorption cross section of the Yb:YSAG ceramics is 5.09 × 10−21 cm2 at 941 nm, which is smaller than that of the Yb:YAG ceramics (6.53 × 10−21 cm2 at 940 nm). The FWHM of the 941 nm peak of the Yb:YSAG ceramics is 22.0 nm, which is larger than that of the Yb:YAG ceramics (17.8 nm at 940 nm). The bigger FWHM is suitable for the direct laser diode pump, because there will be more tolerance on the accuracy of wavelength control of the pump diode. The emission cross section can be estimated by the reciprocity method using formula (3) and (4):
3. Results and discussion Fig. 1 shows the FESEM micrographs of the starting powers which have been calcined at 800 °C for 4 h to eliminate the adsorbed moistures and organic matters. It can be seen that the Yb2O3 and Y2O3 and Sc2O3 powders consist of micro-meter scale agglomerations. The α-Al2O3 340
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Fig. 1. FESEM micrographs of the starting powders (a) YbO3; (b) Y2O3; (c) Sc2O3; (d) α-Al2O3.
Fig. 2. FESEM micrograph of the ball-milled powder mixture.
Fig. 4. In-line transmittance of the 10 at.% Yb:YSAG transparent ceramics with thickness of 2.0 mm. The inset shows the photograph.
Fig. 3. XRD pattern of the 10 at.% Yb:YSAG transparent ceramics.
σem (λ ) = σabs (λ )
Za =
Zl c c exp((h − h )/kT) Zu λZL λ
∑ dia exp(−Eia/kB T ) i
Fig. 5. FESEM micrograph of the thermally etched surface of the 10 at.% Yb:YSAG transparent ceramics.
(3)
determined from the absorption and emission spectra which is 0.96; λZL is the wavelength of the zero phonon line which is 968 nm for the 10 at. % Yb:Y3Sc1.5Al3.5O12 ceramics at room temperature; h is the Planck
(4)
where Zl and Zu is partition function of the upper and lower energy level, respectively, and the Zl/Zu can be calculated by the energy level 341
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calculated FWHM of the Yb:Y3Sc1.5Al3.5O12 ceramics is 12.4 nm, which is 1.3 times larger than 9.5 nm of the Yb:YAG ceramics and nearly same as that of the reported Yb:Y3ScAl4O12 ceramics (12.5 nm) [33]. The laser performance of the 10 at.% Yb:YSAG ceramics with a size of Φ16 mm × 2.0 mm was measured by the CW operation. Both the plano-plano resonator and the three-mirror laser resonator were used to investigate the spectrum and realize the laser output. Fig. 8 shows the experimental setup of the Yb:YSAG ceramics laser test. A fiber coupled diode laser emitting at 974 nm with the maximum output power of 32 W was used as the pump source. The core diameter and numerical aperture (NA) of the fiber laser diode were 100 μm and 0.22,respectively. By using a couple of lenses, the pump beam was imaged into the Yb:YSAG ceramics with a spot diameter of 100 μm. Through the diode temperature controlling methods, the central output wavelength of pump source was stabilized at 974 nm. A flat dichroic mirror M1 (antireflection-coated from 940 nm to 976 nm, high reflection-coated from 1020 nm to 1120 nm) on the entrance side and an output coupler (OC) with different transmittances (optical transmittance over 90% from 1020 to 1120 nm) were used in both the plano-plano resonator and three-mirror resonator. A folding mirror M2 (high-reflection-coated from 1020 nm to 1120 nm) with a curvature of 300 nm was employed in the three-mirror resonator. A dispersive prism inserted at M2 and OC was used into the three-mirror resonator as the adjustable element to measure the frequency tunability. The 10 at.% Yb:YSAG ceramics were wrapped with indium foil and mounted in a water-cooling copper heat sink to remove the thermal loads during the laser experiment, and the temperature was kept at 14 °C. Laser performance of the 10 at.% Yb:YSAG ceramics was obtained from the plano-plano resonator with laser emitting at 1031 nm. Fig. 9 shows the laser output power versus the absorption pump power with different OCs of T = 2% and 5%. It shows that when the pump power is below 6 W, the output powers of the both OCs are nearly the same with laser thresholds as 0.53 W. Then increasing the pump power, the output power of the OC with T = 2% is slightly larger than that of the OC with T = 5%. When the pump power is below 10 W, both of the output powers improve nearly linear with the increase of the pump power. The saturation effect can be found with further increasing the pump power. This is because the absorption of 10 at.%Yb:YSAG ceramics at 974 nm is not strong enough and the length of gain media is only 2 mm. When T = 2%, a maximum output power of 1.27 W with the slope efficiency of 10.8% is obtained under the pump power of 14 W. In contrast, when the pump power is 13.6 W, it is 1.2 W and the slope efficiency is 9.8% for the OC with T = 5%. The typical output spectrum inserted at Fig. 9 was measured by using a fiber optical spectrum analyzer with resolution of 1 nm. There is no observable change for the different OCs, and the central wavelength is 1031 nm which is in agreement with the result of the emission spectrum. Fig. 10 shows the laser performance obtained by using the same OCs in the three-mirror resonator. The laser thresholds are 1.93 W for the both OCs, which is larger than that in the plano-plano resonator. The saturation trend can be also observed when the pump power is larger than 14 W. When the pump power is 15.1 W, a maximum output power of 1.67 W with the slope efficiency of 13.1% is obtained by using the OC with T = 5%. At the same maximum pump power, it is 1.36 W and the slope efficiency is 10.6% for the OC with T = 2%. And the central wavelength of the laser spectrum is also 1031 nm. Because of the better dissipation and less mechanical disturbs for the three-mirror laser cavity, it possesses a more stable configuration. And a Spiricon M2-200 laser beam analyzer was used to measure the output beam quality with a T = 5% OC in the three-mirror resonator. Fig. 11 reveals the laser beam quality and the M2 factors in x and y directions, and they are fitted to be about 1.139 and 1.208, respectively, indicating the TEM00 mode has very good beam quality closed to the diffraction limit. Referring to the reported results, the beam distortion is mainly caused by the inhomogeneous change of the refractive index of the 10 at.% Yb:YSAG ceramics and the diffraction impact under
Fig. 6. Absorption and emission spectra of the 10 at.% Yb:YSAG transparent ceramics measured at room temperature. The emission spectrum was excited by a 915 nm fiber laser.
Fig. 7. (a) Absorption cross section and (b) emission cross section spectra of the 10 at.% Yb:YSAG transparent ceramics. The emission cross section was estimated by the reciprocity method.
constant of 6.62606896(33) × 10−34 J·s; c is light speed of 3.0 × 108 m/s; k is 1.3806505(24) × 10−23 J/K; T is the Kelvin's temperature and chose as 300 K. The calculated emission cross section is showed at Fig. 7(b). The value of the emission peak at 1030 nm is 12.33 × 10−21 cm2, which is smaller than that of the Yb:Y3ScAl4O12 ceramics (14.20 × 10−21 cm2) and the Yb:YAG ceramics (17.16 × 10−21 cm2) [18,33]. Besides, the 342
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Fig. 8. Experimental setup for a CW Yb:YSAG transparent ceramic laser pumped by a fiber-coupled laser diode in (a) plano–plano laser resonator, (b) three-mirror laser resonator. M1, resonator mirror; M2, folding mirror, R = −500 mm; OC, output coupler.
Fig. 10. Laser performance of the 10 at.% Yb:YSAG transparent ceramics obtained in the three-mirror laser resonator: average output power versus pump power with OC of T = 2% and 5%; Inserted picture is the typical output spectrum.
Fig. 9. Laser performance of the 10 at.% Yb:YSAG transparent ceramics obtained in the plano-plano laser resonator: average output power versus the pump power with OC of T = 2% and 5%; The inserted picture is the typical output spectrum.
high power pump [22]. In the three-mirror resonator system with an OC of T = 5%, an intra-cavity fused silica prism with the optimum transmission efficiency was inserted between the M2 and OC at Brewster's angle. And the wavelength tuning of the laser was realized, which is showed in Fig. 12. It can be seen that the tuning range of the 10 at.% Yb:YSAG ceramic laser is from 1021 nm to 1040 nm (19 nm) and the main peak is located at 1031 nm. It is regarded that a suitable prisms can efficiently improve the wavelength tuning capacity of the 10 at.% Yb:YSAG ceramics, and a broader as well as smooth tuning spectrum can be expected. On the other hand, the insufficient tuning range in short-wavelength is mainly attribute to the limited high-reflection coating bandwidth from 1020 nm to 1120 nm of the laser cavity mirrors. By using appropriate laser mirrors or 940 nm off-axis laser diode pump source, it can be expected to obtain the tuning laser below 1021 nm. 4. Conclusion
Fig. 11. Laser beam quality measurement of the 10 at.% Yb:YSAG transparent ceramics operated in three-mirror laser resonator by recording the laser beam radii at different distances.
Utilizing the commercial oxide powders as raw materials, 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics with a dense and homogeneous structure 343
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Fig. 12. Tuning curve obtained for the 10 at.% Yb:YSAG ceramic laser in the three-mirror resonator with an OC of T = 5%.
were successfully fabricated by solid state reactive sintering at 1820 °C for 10 h. The in-line transmittance of the ceramic sample is as high as 83.2% at the wavelength of 1100 nm. The emission cross section (12.33 × 10−21 cm2) of the 10 at.% Yb:YSAG ceramics at the room temperature is similar with that of the reported 15 at.% Yb:Y3ScAl4O12 ceramics, and the FWHM of the sample at 1031 nm (12.4 nm) is approximately 1.3 times larger than that of the 10 at.% Yb:YAG ceramics at 1030 nm. The laser outputs of the non-coated 10 at.% Yb:YSAG ceramics were realized by the CW laser operation with the plano-plano resonator and three-mirror resonator, respectively. When the pump power is 15.1 W, A maximum output power is 1.67 W with a slope efficiency of 13.1% in the three-mirror resonator. By inserting an intracavity dispersive prism in the three-mirror resonator, the wavelength tuning which is from 1021 to 1040 nm was obtained. Optimizing the quality of laser resonant cavity, the further improvement of the laser performance can be expected. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0310500), the National Natural Science Foundation of China (Grant No. 61575212) and the Key research project of the frontier science of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC022). References [1] A. Ikesue, Y.L. Aung, Synthesis and performance of advanced ceramic Lasers, J. Am. Ceram. Soc. 89 (2006) 1936–1944. [2] A. Ikesue, Y.L. Aung, Ceramic laser materials, Nat. Photon. 2 (2008) 721–727. [3] G. Boulon, Fifty years of advances in solid-state laser materials, Opt. Mater. 34 (2012) 499–512. [4] A. Ikesue, Y.L. Aung, T. Taira, T. Kamimura, K. Yoshida, G.L. Messing, Progress in ceramic lasers, Annu. Rev. Mater. Res. 36 (2006) 397–429. [5] J. Dong, A. Shirakawa, K. Ueda, Laser-diode pumped heavy-doped Yb:YAG ceramic lasers, Opt. Mater. 32 (2007) 1890–1892. [6] J. Kong, J. Lu, K. Takaichi, T. Uematsu, K. Ueda, D.Y. Tang, D.Y. Shen, H. Yagi, T. Yanagitani, A.A. Kaminskii, Diode-pumped Yb:Y2O3 ceramic laser, Appl. Phys. Lett. 82 (2003) 2556–2558. [7] T. Yanagida, Y. Fujimoto, H. Yagi, T. Yanagitani, Optical and scintillation properties of transparent ceramic Yb:Lu2O3 with different Yb concentrations, Opt. Mater. 36 (2014) 1044–1048. [8] Y.S. Wu, J. Li, Y.B. Pan, J.K. Guo, B.X. Jiang, Y. Xu, J. Xu, Diode-pumped Yb:YAG ceramic laser, J. Am. Ceram. Soc. 90 (2007) 3334–3337.
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Fabrication and properties of 10 at.% Yb:Y3Sc1.5Al3.5O12 transparent ceramics
T
Yagang Fenga,b, Zewang Hua,b, Xiaopu Chena,b, Cong Chena, Yangcheng Ouc, Yang Liuc, Wenxue Lic, Jiang Lia,∗ a
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China c State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200062, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Yb:Y3Sc1.5Al3.5O12 ceramics Fabrication Spectroscopic properties Laser performance
10 at.% Yb:Y3Sc1.5Al3.5O12 (Yb:YSAG) transparent ceramics were fabricated by the solid state reactive of highpurity Y2O3, α-Al2O3, Yb2O3 and Sc2O3 powders, utilizing MgO and tetraethoxysilane (TEOS) as the sintering aids. The Yb:YSAG ceramic sample sintered at 1820 °C for 10 h has a dense and homogenous microstructure with average grain size of 26.9 μm, whose in-line transmittance reaches 83.2% at 1100 nm (2 mm thick). The absorption and emission cross sections are calculated to be 5.09 × 10−21 cm2 at 941 nm and 12.33 × 10−21 cm2 at 1031 nm, respectively. The full width at half maximum (FWHM) of the main emission peak centered at 1031 nm is 12.4 nm, which is about 1.3 times larger than that of the 10 at.% Yb:YAG ceramics. By using the three-mirror resonator, the Yb:YSAG ceramic laser output power of 1.67 W at 1031 nm and slope efficiency of 13.1% were obtained with an output coupler (OC) of T = 5%.
1. Introduction In the last decades, the researches of Yb3+ doped transparent laser ceramics have attracted much attention since the presentation of laserdiodes (LDs), as the ceramics have the equivalent or even better properties in contrast with crystals [1–7]. The Yb3+ doped gain media have several advantages of high quantum efficiency, no excited state absorption, no upconversion and concentration quenching. In addition, the relatively broad absorption peak of Yb3+ centered at 940 nm can well coupling with the InGaAs laser diode, which makes it possible to obtain highly efficient laser operation under the direct diode pumping [8–11]. Among many laser hosts, the yttrium aluminum garnet (YAG) has been proved to be an excellent laser gain medium due to its high thermal conductivity, good mechanical and optical properties [12–15]. In 2003, Takaichi et al. successfully fabricated the Yb:YAG ceramics and firstly achieved the laser output with a maximum output power of 345 mW and slope efficiency of 26% [16]. Then the Yb:YAG transparent ceramics have been greatly developed for utilizing both in the high power laser and ultra-fast laser fields [17–22]. For the Yb:YAG ceramics used in the ultra-fast pulse laser filed, the relatively sharp and narrow emission bandwidth at around 1030 nm makes it very challenging to obtain sub 100 fs pulse laser [23].
∗
According to the mode-locking laser operation, a shorter pulse laser can be obtained by compressing a broader emission bandwidth in a suitable laser cavity [24]. In order to solve the above problem, one effective way is using disordered garnet hosts which are obtained by substituting some of the lattice sites with other ions, and the ions have the similar chemical and physical properties. Then the disordered crystal field would strongly broaden both the emission and absorption lines, which is benefit for the generation of ultra-fast pulse laser [25–30]. Several disordered garnet laser materials have been reported, such as Yb:LuxY3xAl5O12 (Yb:LuYAG), Yb:GdxY3-xAl5O12 (Yb:GYAG), Yb:Y3ScxAl5-xO12 (Yb:YSAG), and so on [31–34]. Concerning the Yb:YSAG ceramics, they are formed by using the Sc3+ ions partly substitute the Al3+ ions in the YAG lattice to occupy the octahedron site. The modified crystal field causes a large splitting of the sublevel in the upper (2F5/2) and lower (2F7/2) manifolds of Yb3+, and this results in the FWHM of emission peak of Yb:Y3ScAl4O12 centered at 1031 nm is nearly 1.5 times larger than that of the Yb:YAG centered at 1030 nm [35,36]. The remarkable broaden of the emission bandwidth is very useful in the mode-locking laser operation and some significant research findings have been reported. In 2004, Saikawa et al. firstly fabricated the 15 at.% Yb:Y3ScAl4O12 ceramics and measured the laser performance. A 580 fs short pulse laser
Corresponding author. E-mail address: [email protected] (J. Li).
https://doi.org/10.1016/j.optmat.2018.11.057 Received 7 October 2018; Received in revised form 20 November 2018; Accepted 29 November 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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powders are in fine dispersity and homogeneity. The primary particle sizes of the Yb2O3, Y2O3, Sc2O3 and α-Al2O3 powders are 150 nm, 80 nm, 230 nm and 350 nm, respectively. Fig. 2 shows the FESEM micrograph of the ball-milled powder mixture. It reveals that the ball-milling process can efficiently break the large agglomerations of the Yb2O3, Y2O3 and Sc2O3 powders. Fig. 3 shows the XRD pattern of the 10 at.% Yb:YSAG ceramics sintered at 1820 °C for 10 h. The standard YAG pattern (JCPDF#33-0040) is displayed under the Yb:YSAG ceramics pattern. It reveals that all the peaks of Yb:YSAG ceramics match well with the standard diffraction patterns of cubic YAG phase. Compared with the pure YAG patterns, the diffraction peaks of the Yb:YSAG ceramics shift to the low angle, which is attributed to the bigger radius of the Sc3+ (0.074 nm) than that of the substituted Al3+ (0.053 nm) in the octahedron site. The calculated lattice parameter of the sample is 1.2224 nm by using cell refinement through MDI Jade 6.5 software, which is in the middle of the parameters of YAG (1.2002 nm) and Y3Sc2Al3O12 (1.2271 nm) [39]. Fig. 4 displays the in-line transmittance of 10 at.% Yb:YSAG ceramics with thickness of 2.0 mm. Besides, the photograph of the sample is inserted into Fig. 4. The in-line transmittances of the sample are 83.2% at 1100 nm and 79.9 at 400 nm, respectively, which is among the highest value to the best of our knowledge. And the decrease of transmittance at the short wavelength can be attributed to the Mie scattering caused by the small residual pores in the sample. Fig. 5 shows the FESEM photograph of thermally etched surface of the 10 at.% Yb:YSAG transparent ceramics. It can be seen that the sample has a nearly full dense structure and the grain boundaries are clean. The average grain size estimated by the intercept method is about 26.9 μm. Fig. 6 shows the absorption and emission spectra of the 10 at.% Yb:YSAG transparent ceramics measured at room temperature. The absorption spectrum reveals that there are four main peaks which are located at 913, 941, 968, 1030 nm, respectively, and they are attributed to the radiation transition of the Yb3+ ions from the low 2F7/2 to the high 2F5/2 energy level. The emission spectrum was measured by motivating the sample through a 915 nm fiber laser. It can be seen that there are four main emission peaks located at 944, 969, 1031, 1050 nm, respectively. The 941 nm absorption peak and 1031 nm emission peak shift to the long wavelength compared with the Yb:YAG ceramics, whose absorption and emission wavelengths are 940 nm and 1030 nm. As it is attributed to larger splitting of the sublevels of the upper (2F5/2) and the lower (2F7/2) manifolds of Yb3+ caused by the introduction of the Sc3+ [36]. From the above absorption spectrum, the absorption cross section can be calculated by the following formulas:
with output power of 62 mW was obtained through the passive modelocking laser operation. Besides, under the CW laser operation, the maximum output power of the gained laser is 810 mW with a slope efficiency of 72% [33,35]. And then, the passive mode-locking laser operation was demonstrated on the 5 at.% Yb:Y3(Sc0.5Al0.5)5O12 ceramics, and a 280 fs pulse laser with an average output power of 62 mW was achieved [37]. In 2017, a passively mode-locked pulse laser of 96 fs at 1052 nm with a maximum average output power of 51 mW was reported in the 10 at.% Yb:Y3ScAl4O12 ceramics by Ma et al. [23]. Recently, Pirri et at. investigated the laser performance of 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics. An output laser with maximum power of 6.3 W and the corresponding slope efficiency of 67.8% were realized by pumping in a Quasi-CW scheme at 936 nm, and they obtained the broadest tuning range which was from 991.5 nm to 1073 nm (81.5 nm) in the ever reported Yb:YSAG ceramics [36]. In this work, we fabricated the 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics by the solid state reaction with TEOS and MgO as sintering aids [38]. Microstructure, optical and laser properties were evaluated in detail. The optical properties of 10 at.%Yb:YSAG and 10 at.% Yb:YAG ceramics were compared. The CW laser outputs operated both in plano-plano resonator and three-mirror resonator as well as wavelength tuning of the non-coated Yb:YSAG ceramics were realized. 2. Experimental The 10 at.% Yb:Y3Sc1.5Al3.5O12 transparent ceramics were fabricated by the solid state reaction method combined with vacuum sintering. High-purity commercial powders were used as the primary powders. Y2O3 (99.99%, Yuelong New Materials Co., Ltd., Shanghai, China), α-Al2O3 (HFF-5, Fenghe Ceramics Co., Ltd., Shanghai, China), Yb2O3 (99.99%, Zhongkai New Materials Co., Ltd., Jining, China) and Sc2O3 (99.99%, Jingyun Material Technology Co., Ltd., Shanghai, China) were weighed in stoichiometric ratio as Yb0.3Y2.7Sc1.5Al3.5O12. And then all the powders were added into the corundum crucible with MgO (99.998%, Alfa Aesar, Tianjin, China) and TEOS (99.999%, Alfa Aesar, Tianjin, China) as well as anhydrous ethanol. The mixture was ball milled for 12 h with a speed of 130 r/min. Then the slurry was dried at 70 °C for 2 h, sifted with a 200-mesh screen and calcinated at 600 °C for 4 h. The calcinated powders were dry pressed into a disk with diameter of 18 mm at 46 MPa, and further cold isostatically pressed at 250 MPa to increase the density. Then the specimens were vacuum sintered at 1820 °C for 10 h, and then annealed at 1450 °C for 20 h in air to eliminate the oxygen vacancies. Finally, the transparent Yb:YSAG ceramics were mirror-polished into 2.0 mm thickness for measurement. The relative densities of the ceramics were calculated by the Archimedes method. The phase composition of the sintered sample was identified by the X-ray diffraction (Cu Kα1 radiation (λ = 0.15405 nm), XRD, Bruker D8 Focus, Germany) in the range of 2θ = 15-75° using nickel-filtered Cu-Kα radiation. The micrographs of the starting powders and the ceramics were observed by a field emission scanning electron microscopy (FESEM, S-4800, Hitach, Japan). And before the SEM observation, the ceramics were thermally etched at 1450 °C for 3 h in air. The average grain size of the sintered sample was calculated by the linear intercept method (more than 200 grains were counted). The in-line transmittance and absorption spectrum of the mirror-polished sample were measured by a UV-VIS-NIR spectrophotometer (Cary5000, Varian, USA). The emission spectrum excited by a 915 nm laser beam was measured by a low temperature absorption spectrometer (FLS-980, Edinburgh Instruments, UK) at room temperature.
α=
−2. 303lg(I/Io) L
σabs =
α N
(1)
(2)
where α is the absorption coefficient, lg(I/I0) is the optical density which can be obtained by the spectrophotometer; and L is the thickness of the sample; N is the Yb3+ ions concentration of the 10 at.% Yb:YSAG ceramics, which is 1.309 × 1021 ions/cm3. Taking these data measured by the instrument into the above formulas, the calculated absorption cross section spectrum is showed in Fig. 7(a). The absorption cross section spectrum of the 10 at.% Yb:YAG ceramics is also showed for comparison. The absorption cross section of the Yb:YSAG ceramics is 5.09 × 10−21 cm2 at 941 nm, which is smaller than that of the Yb:YAG ceramics (6.53 × 10−21 cm2 at 940 nm). The FWHM of the 941 nm peak of the Yb:YSAG ceramics is 22.0 nm, which is larger than that of the Yb:YAG ceramics (17.8 nm at 940 nm). The bigger FWHM is suitable for the direct laser diode pump, because there will be more tolerance on the accuracy of wavelength control of the pump diode. The emission cross section can be estimated by the reciprocity method using formula (3) and (4):
3. Results and discussion Fig. 1 shows the FESEM micrographs of the starting powers which have been calcined at 800 °C for 4 h to eliminate the adsorbed moistures and organic matters. It can be seen that the Yb2O3 and Y2O3 and Sc2O3 powders consist of micro-meter scale agglomerations. The α-Al2O3 340
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Fig. 1. FESEM micrographs of the starting powders (a) YbO3; (b) Y2O3; (c) Sc2O3; (d) α-Al2O3.
Fig. 2. FESEM micrograph of the ball-milled powder mixture.
Fig. 4. In-line transmittance of the 10 at.% Yb:YSAG transparent ceramics with thickness of 2.0 mm. The inset shows the photograph.
Fig. 3. XRD pattern of the 10 at.% Yb:YSAG transparent ceramics.
σem (λ ) = σabs (λ )
Za =
Zl c c exp((h − h )/kT) Zu λZL λ
∑ dia exp(−Eia/kB T ) i
Fig. 5. FESEM micrograph of the thermally etched surface of the 10 at.% Yb:YSAG transparent ceramics.
(3)
determined from the absorption and emission spectra which is 0.96; λZL is the wavelength of the zero phonon line which is 968 nm for the 10 at. % Yb:Y3Sc1.5Al3.5O12 ceramics at room temperature; h is the Planck
(4)
where Zl and Zu is partition function of the upper and lower energy level, respectively, and the Zl/Zu can be calculated by the energy level 341
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calculated FWHM of the Yb:Y3Sc1.5Al3.5O12 ceramics is 12.4 nm, which is 1.3 times larger than 9.5 nm of the Yb:YAG ceramics and nearly same as that of the reported Yb:Y3ScAl4O12 ceramics (12.5 nm) [33]. The laser performance of the 10 at.% Yb:YSAG ceramics with a size of Φ16 mm × 2.0 mm was measured by the CW operation. Both the plano-plano resonator and the three-mirror laser resonator were used to investigate the spectrum and realize the laser output. Fig. 8 shows the experimental setup of the Yb:YSAG ceramics laser test. A fiber coupled diode laser emitting at 974 nm with the maximum output power of 32 W was used as the pump source. The core diameter and numerical aperture (NA) of the fiber laser diode were 100 μm and 0.22,respectively. By using a couple of lenses, the pump beam was imaged into the Yb:YSAG ceramics with a spot diameter of 100 μm. Through the diode temperature controlling methods, the central output wavelength of pump source was stabilized at 974 nm. A flat dichroic mirror M1 (antireflection-coated from 940 nm to 976 nm, high reflection-coated from 1020 nm to 1120 nm) on the entrance side and an output coupler (OC) with different transmittances (optical transmittance over 90% from 1020 to 1120 nm) were used in both the plano-plano resonator and three-mirror resonator. A folding mirror M2 (high-reflection-coated from 1020 nm to 1120 nm) with a curvature of 300 nm was employed in the three-mirror resonator. A dispersive prism inserted at M2 and OC was used into the three-mirror resonator as the adjustable element to measure the frequency tunability. The 10 at.% Yb:YSAG ceramics were wrapped with indium foil and mounted in a water-cooling copper heat sink to remove the thermal loads during the laser experiment, and the temperature was kept at 14 °C. Laser performance of the 10 at.% Yb:YSAG ceramics was obtained from the plano-plano resonator with laser emitting at 1031 nm. Fig. 9 shows the laser output power versus the absorption pump power with different OCs of T = 2% and 5%. It shows that when the pump power is below 6 W, the output powers of the both OCs are nearly the same with laser thresholds as 0.53 W. Then increasing the pump power, the output power of the OC with T = 2% is slightly larger than that of the OC with T = 5%. When the pump power is below 10 W, both of the output powers improve nearly linear with the increase of the pump power. The saturation effect can be found with further increasing the pump power. This is because the absorption of 10 at.%Yb:YSAG ceramics at 974 nm is not strong enough and the length of gain media is only 2 mm. When T = 2%, a maximum output power of 1.27 W with the slope efficiency of 10.8% is obtained under the pump power of 14 W. In contrast, when the pump power is 13.6 W, it is 1.2 W and the slope efficiency is 9.8% for the OC with T = 5%. The typical output spectrum inserted at Fig. 9 was measured by using a fiber optical spectrum analyzer with resolution of 1 nm. There is no observable change for the different OCs, and the central wavelength is 1031 nm which is in agreement with the result of the emission spectrum. Fig. 10 shows the laser performance obtained by using the same OCs in the three-mirror resonator. The laser thresholds are 1.93 W for the both OCs, which is larger than that in the plano-plano resonator. The saturation trend can be also observed when the pump power is larger than 14 W. When the pump power is 15.1 W, a maximum output power of 1.67 W with the slope efficiency of 13.1% is obtained by using the OC with T = 5%. At the same maximum pump power, it is 1.36 W and the slope efficiency is 10.6% for the OC with T = 2%. And the central wavelength of the laser spectrum is also 1031 nm. Because of the better dissipation and less mechanical disturbs for the three-mirror laser cavity, it possesses a more stable configuration. And a Spiricon M2-200 laser beam analyzer was used to measure the output beam quality with a T = 5% OC in the three-mirror resonator. Fig. 11 reveals the laser beam quality and the M2 factors in x and y directions, and they are fitted to be about 1.139 and 1.208, respectively, indicating the TEM00 mode has very good beam quality closed to the diffraction limit. Referring to the reported results, the beam distortion is mainly caused by the inhomogeneous change of the refractive index of the 10 at.% Yb:YSAG ceramics and the diffraction impact under
Fig. 6. Absorption and emission spectra of the 10 at.% Yb:YSAG transparent ceramics measured at room temperature. The emission spectrum was excited by a 915 nm fiber laser.
Fig. 7. (a) Absorption cross section and (b) emission cross section spectra of the 10 at.% Yb:YSAG transparent ceramics. The emission cross section was estimated by the reciprocity method.
constant of 6.62606896(33) × 10−34 J·s; c is light speed of 3.0 × 108 m/s; k is 1.3806505(24) × 10−23 J/K; T is the Kelvin's temperature and chose as 300 K. The calculated emission cross section is showed at Fig. 7(b). The value of the emission peak at 1030 nm is 12.33 × 10−21 cm2, which is smaller than that of the Yb:Y3ScAl4O12 ceramics (14.20 × 10−21 cm2) and the Yb:YAG ceramics (17.16 × 10−21 cm2) [18,33]. Besides, the 342
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Fig. 8. Experimental setup for a CW Yb:YSAG transparent ceramic laser pumped by a fiber-coupled laser diode in (a) plano–plano laser resonator, (b) three-mirror laser resonator. M1, resonator mirror; M2, folding mirror, R = −500 mm; OC, output coupler.
Fig. 10. Laser performance of the 10 at.% Yb:YSAG transparent ceramics obtained in the three-mirror laser resonator: average output power versus pump power with OC of T = 2% and 5%; Inserted picture is the typical output spectrum.
Fig. 9. Laser performance of the 10 at.% Yb:YSAG transparent ceramics obtained in the plano-plano laser resonator: average output power versus the pump power with OC of T = 2% and 5%; The inserted picture is the typical output spectrum.
high power pump [22]. In the three-mirror resonator system with an OC of T = 5%, an intra-cavity fused silica prism with the optimum transmission efficiency was inserted between the M2 and OC at Brewster's angle. And the wavelength tuning of the laser was realized, which is showed in Fig. 12. It can be seen that the tuning range of the 10 at.% Yb:YSAG ceramic laser is from 1021 nm to 1040 nm (19 nm) and the main peak is located at 1031 nm. It is regarded that a suitable prisms can efficiently improve the wavelength tuning capacity of the 10 at.% Yb:YSAG ceramics, and a broader as well as smooth tuning spectrum can be expected. On the other hand, the insufficient tuning range in short-wavelength is mainly attribute to the limited high-reflection coating bandwidth from 1020 nm to 1120 nm of the laser cavity mirrors. By using appropriate laser mirrors or 940 nm off-axis laser diode pump source, it can be expected to obtain the tuning laser below 1021 nm. 4. Conclusion
Fig. 11. Laser beam quality measurement of the 10 at.% Yb:YSAG transparent ceramics operated in three-mirror laser resonator by recording the laser beam radii at different distances.
Utilizing the commercial oxide powders as raw materials, 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics with a dense and homogeneous structure 343
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Fig. 12. Tuning curve obtained for the 10 at.% Yb:YSAG ceramic laser in the three-mirror resonator with an OC of T = 5%.
were successfully fabricated by solid state reactive sintering at 1820 °C for 10 h. The in-line transmittance of the ceramic sample is as high as 83.2% at the wavelength of 1100 nm. The emission cross section (12.33 × 10−21 cm2) of the 10 at.% Yb:YSAG ceramics at the room temperature is similar with that of the reported 15 at.% Yb:Y3ScAl4O12 ceramics, and the FWHM of the sample at 1031 nm (12.4 nm) is approximately 1.3 times larger than that of the 10 at.% Yb:YAG ceramics at 1030 nm. The laser outputs of the non-coated 10 at.% Yb:YSAG ceramics were realized by the CW laser operation with the plano-plano resonator and three-mirror resonator, respectively. When the pump power is 15.1 W, A maximum output power is 1.67 W with a slope efficiency of 13.1% in the three-mirror resonator. By inserting an intracavity dispersive prism in the three-mirror resonator, the wavelength tuning which is from 1021 to 1040 nm was obtained. Optimizing the quality of laser resonant cavity, the further improvement of the laser performance can be expected. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0310500), the National Natural Science Foundation of China (Grant No. 61575212) and the Key research project of the frontier science of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC022). References [1] A. Ikesue, Y.L. Aung, Synthesis and performance of advanced ceramic Lasers, J. Am. Ceram. Soc. 89 (2006) 1936–1944. [2] A. Ikesue, Y.L. Aung, Ceramic laser materials, Nat. Photon. 2 (2008) 721–727. [3] G. Boulon, Fifty years of advances in solid-state laser materials, Opt. Mater. 34 (2012) 499–512. [4] A. Ikesue, Y.L. Aung, T. Taira, T. Kamimura, K. Yoshida, G.L. Messing, Progress in ceramic lasers, Annu. Rev. Mater. Res. 36 (2006) 397–429. [5] J. Dong, A. Shirakawa, K. Ueda, Laser-diode pumped heavy-doped Yb:YAG ceramic lasers, Opt. Mater. 32 (2007) 1890–1892. [6] J. Kong, J. Lu, K. Takaichi, T. Uematsu, K. Ueda, D.Y. Tang, D.Y. Shen, H. Yagi, T. Yanagitani, A.A. Kaminskii, Diode-pumped Yb:Y2O3 ceramic laser, Appl. Phys. Lett. 82 (2003) 2556–2558. [7] T. Yanagida, Y. Fujimoto, H. Yagi, T. Yanagitani, Optical and scintillation properties of transparent ceramic Yb:Lu2O3 with different Yb concentrations, Opt. Mater. 36 (2014) 1044–1048. [8] Y.S. Wu, J. Li, Y.B. Pan, J.K. Guo, B.X. Jiang, Y. Xu, J. Xu, Diode-pumped Yb:YAG ceramic laser, J. Am. Ceram. Soc. 90 (2007) 3334–3337.
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