ISSN 1054660X, Laser Physics, 2009, Vol. 19, No. 11, pp. 2140–2143.
NOVEL LASER MATERIALS
© Pleiades Publishing, Ltd., 2009. Original Russian Text © Astro, Ltd., 2009.
Crystal Growth, Spectroscopic and Laser Properties of Tm:LuAG Crystal1 X. D. Xua, *, X. D. Wangb, Z. F. Linc, Y. Chenga, D. Z. Lia, S. S. Chenga, F. Wua, Z. W. Zhaoa, C. Q. Gaoc, M. W. Gaoc, and J. Xud a
Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800 China b Department of Applied Physics, University of Science and Technology of Suzhou, Suzhou, 215009 China c Department of OptoElectronics, Beijing Institute of Technology, Beijing, 100081 China d Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201800 China *email:
[email protected] Received June 5, 2009; in final form June 9, 2009; published online September 23, 2009
Abstract—Tm:Lu3Al5O12 (Tm:LuAG) crystal was grown by the Czochralski method. The segregation coeffi cient was measured by Inductively Coupled Plasma Atomic Emission Spectrometer. The cell parameters were analyzed with Xray powder diffraction experiments. The absorption and fluorescence spectra of Tm:LuAG crystal at room temperature were investigated. With a 20 W fibercoupled diode laser as pump source, the continuouswave (CW) laser action of Tm:LuAG crystal was demonstrated. The maximum output power at 2020 nm was obtained to be 3.04 W, and the slope efficiency was 25.3%. PACS numbers: 42.70.Hj, 81.10.Fq, 42.55.Xi DOI: 10.1134/S1054660X09210129 1
1. INTRODUCTION
2. CRYSTAL GROWTH
2μm wavelength Tm3+doped lasers have attracted a great deal of attention due to their important appli cations, such as gas sensing, wind measurement, iatri cal applications and pump source of Ho slab amplifier [1–9]. Lu3Al5O12 (LuAG) crystal is a good host mate rial due to its many attractive characteristics such as high thermal conductivity, excellent physical and chemical properties [10, 11]. The laser operation of Yb:LuAG [12, 13], Nd:LuAG [14], Tm:LuAG [15– 21], and Tm, Ho:LuAG [22, 23] crystals were reported. Compared with Tm:YAG crystal, the emis sion wavelength of Tm:LuAG is shifted slightly to longer wavelength, which is more closer to the optical transmission window [21]. A diodepumped, room temperature Tm:LuAG laser with a total opticalto optical efficiency of 0.073 and an opticaltooptical differential efficiency as high as 0.236 was demon strated in 1995 [15]. Wu et al., reported a Tm:LuAG laser with a maximum output power at 2.023 μm wave length of 4.91 W at room temperature in 2008 [17].
Tm:LuAG crystal boule was grown by the Czo chralski method [14]. The 99.999% grade raw materi als were appropriately predried and weighed according to the formula (Tm0.04Lu0.96)3Al5O12. After the com pounds were ground and mixed, they were pressed into pieces, and put into an aluminium crucible. The pieces were heated to 1300°C for 20 h. The charge was then loaded into an iridium crucible for crystal growth. During the growth process, pulling rate was 1– 3 mm/h and rotation rate was 15–30 rpm. High purity nitrogen gas was used as a protective atmo sphere. In order to prevent the crystal from cracking, the crystal was cooled to room temperature slowly after growth. Tm:LuAG crystal with dimensions of ∅25 × 65 mm3 was obtained, as shown in Fig. 1, and the crystal boule was primrose yellow color and free from cracks, inclusions and scattering centers. The sample was cut from the crystals adjacent the seed crystal position, and then was grinded to powder in an agate mortar for measurement. The powder was analyzed with (ICP–AES) for Tm3+ concentration. The segregation coefficient of Tm3+ in Tm:LuAG crys tal can be calculated by the following formula [15, 24]:
In this paper, the growth of the Tm3+ doped LuAG single crystal by Czochralski method was reported, the spectroscopic and laser properties of the crystal were also studied. 1 The article is published in the original.
km = ctop/c0,
(1)
where ctop is the Tm3+ concentration at the growth starting position in the crystals, and c0 is the initial Tm3+ concentration in the melt. The segregation coef ficient of Tm3+ in Tm:LuAG crystal was calculated to
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Intensity, arb. units 3500 3000 2500 2000 1500 1000 500 0 10 20 30 40 50 60 70 80 2θ, deg
Fig. 1. The photograph of asgrown Tm:LuAG crystal boule.
Absorption coefficient, cm−1 7 6 5 4 3 2 1 0 400 800 1200 1600 2000 Wavelength, nm
Fig. 2. Xray powder diffraction pattern of the Tm:LuAG crystal.
Intensity, arb. units 4000 3000 2000 1000 0 1400 1600 1800 2000 2200 Wavelength, nm
Fig. 3. Absorption spectrum of Tm:LuAG crystal.
Fig. 4. Fluorescence spectrum of Tm:LuAG crystal.
be 0.96, which suggests that the Tm3+ ion was easily doped into the LuAG lattice and could be doped with high concentration.
(full width at half maximum is 14 nm) with a maxi mum absorption cross section of about 4.37 × 10 ⎯21 cm2 at 786 nm.
The Xray powder diffraction of the Tm:LuAG crystal was measured by JUINER HÄJJ Camera, and the pattern is shown in Fig. 2. The result shows that the Tm:LuAG crystal crystallizes in the cubic with space group Ia3d and has the cell parameter: a = 1.1932 nm, V = 1.6998 nm3.
The room temperature emission spectrum was obtained under 790 nm excitation by a Nikon G250 monochromater and PbS detector. The decay time was measured by a Yocogawa DL1620 digital oscilloscope. All measurements were performed at room tempera ture. The emission spectrum ranging from 1300 to 2200 nm is presented at Fig. 3. The emission bands 3H of Tm3+. The correspond to the transition 3F4 6 3H tran stimulated emission crosssection of 3F4 6 sition calculated by the Fuechtbauer–Ladenbury method [25] is 2.55 × 10 ⎯21 cm2 at 1748 nm, 1.52 × 10 ⎯21 cm2 at 1882 nm and 1.87 × 10–21 cm2 at 2021 nm. 3H energy Fluorescence decay curve for the 3F4 6 transition is shown in Fig. 5. The fluorescence lifetime was measured to be 11.9 ms. The lifetime value deter mined in our study is somewhat higher than the value of 10.2 ms reported in [15] for the 3F4 lifetime of Tm:LuAG crystals with Tm3+ concentration varying from 2 to 10% and 7.1 ms reported in [10] for the 3F4
3. SPECTROSCOPIC CHARACTERIZATION Sample used in spectral analysis was cut from the boule with two surfaces perpendicular to the 〈111〉 axis and polished to spectral quality. The room tempera ture absorption spectrum of Tm:LuAG crystal was recorded using a Lambda 900 spectrophotometer (PerkinElmer Company) in the range 200–2000 nm, as shown in Fig. 3. From this figure, we can see that there are seven bands associated with Tm3+ transitions from the 3H6 ground state to 3P2, 1D2, 3F4, 3H5, 3H4, 3F –3F , 1G excited states, respectively. The Tm3+ 3 2 4 3H absorption band is comparatively broad 3H 6 4 LASER PHYSICS
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XU et al. Intensity, arb. units 0.008 0.006 0.004 0.002 0
0.01 0.02 0.03 0.04 0.05 Time, s
Fig. 5. Decay curves of the 3F4 manifold of the Tm3+ in Tm:LuAG crystal.
Tm:LuAG
LD Fiber
Laser beam
Input mirror Output mirror Coupler lens
Fig. 6. Experimental setup of a laserdiodepumped Tm:LuAG laser.
Output power, W 3.5 TmLuAG 12 mm η = 25.6% TmLuAG 8 mm 3.0 η = 25.3% 2.5 2.0 1.5 1.0 0.5 0
5
10 15 20 Pump power, W
Fig. 7. Fibercoupled laserdiodepumped lasing of Tm:LuAG crystal.
lifetime of Tm:LuAG crystal with the concentration 5%. 4. LASER PERFORMANCE OF Tm:LuAG The pump source used in our experiments was a 20 W fibercoupled laser diode with a center wave length of 793 nm, which was tuned by temperature to match the absorption peak of the laser media. The fiber core diameter was 200 μm and the numerical aperture is 0.22. The experimental setup is shown in Fig. 6. The resonant chamber consisted of one flat input mirror (antireflection coated at 785 nm on the entrance face, high reflection coated at 2020 nm and high transmittance coated at 785 nm on the other face), one concave output mirror (transmission at
2020 nm of 3.6%) with the curvature radius of 100 mm and one output coupler (OC). The length of the cavity was 130 mm. The two Tm:LuAG laser rods were cut parallel to the crystallographic 〈111〉 axis, with dimen sions of ∅5 × 8 mm3 and ∅5 × 12 mm3. They were antireflection coated at 785 nm and 2020 nm on each face. Figure 7 shows the fundamental output power ver sus the absorbed power of the Tm:LuAG crystal sam ples. The laser reached its threshold at an absorbed pump power of 1.88 W. For the sample with dimension of ∅5 × 8 mm3, at an incident pump power of 18.82 W, the laser produced an output power of 3.04 W at an emission wavelength of 2020 nm, and the slope effi ciency was 25.3%. For another sample with dimension of ∅5 × 12 mm3, at an incident pump power of 20.40 W, the laser produced an output power of 2.86 W at an emission wavelength of 2020 nm, and the slope efficiency was 25.6%. The laser output is lower for the sample with dimension of ∅5 × 12 mm3, which may be due to the reabsorption loss in longer sample. As can be seen from Fig. 7, the diodepumped Tm:LuAG laser showed output saturation tendency, often result ing from thermal effects in the laser medium, although the crystal was not cooled actively. Substantial power scaling of the Tm:LuAG laser will be possible by opti mizing the cavity design and the crystal absorption, using a suitable Tmdoping concentration crystal sam ple and active cooling in order to reduce the thermal population in the terminal laser level. 5. CONCLUSIONS Tm:LuAG crystal with dimensions up to ∅25 × 65 mm3 was grown successfully by the Czochralski method. The segregation coefficient of Tm3+ in the LuAG host lattice is equal to 0.96. The crystal crystal lizes in the cubic with space group Ia3d and has the cell parameter: a = 1.1932 nm, V = 1.6998 nm3. The absorption and fluorescence spectra of TmLuAG crys tal were investigated at room temperature. The absorption band at 786 nm has a full width at half max imum of 14 nm and the absorption cross section at 786 nm was calculated to be 4.37 × 10–21 cm2. The stimulated emission crosssection is 1.87 × 10–21 cm2 at 2021 nm and the fluorescence lifetime is 11.9 ms. The continuouswave laser operation of Tm:LuAG using diode laser pumping was demonstrated at room temperature. A slope efficiency of 25.3% was achieved at 2020 nm and a maximum output power of 3.04 W was achieved. REFERENCES 1. M. Schellhorn, S. Ngcobo, and C. Bollig, Appl. Phys. B 94, 195 (2009). 2. N. Coluccelli, G. Galzerano, D. Parisi, M. Tonelli, and P. Laporta, Opt. Lett. 33, 1951 (2008). LASER PHYSICS
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CRYSTAL GROWTH, SPECTROSCOPIC AND LASER PROPERTIES 3. S. S. Cai, J. Kong, B. Wu, Y. H. Shen, G. J. Zhao, Y. H. Zong, and J. Xu, Appl. Phys. B 90, 133 (2008). 4. C. T. Wu, Y. L. Ju, Z. G. Wang, Q. Wang, C. W. Song, and Y. Z. Wang, Laser Phys. Lett. 5, 793 (2008). 5. A. A. Kalachev, K. R. Karimullin, V. V. Samartsev, and V. A. Zuikov, Laser Phys. Lett. 5, 882 (2008). 6. X. M. Duan, B. Q. Yao, Y. J. Zhang, C. W. Song, L. L. Zheng, Y. L. Ju, and Y. Z. Wang, Laser Phys. Lett. 5, 347 (2008). 7. N. Coluccelli, G. Galzerano, P. Laporta, D. Parisi, A. Toncelli, and M. Tonelli, Opt. Express 14, 1518 (2006). 8. X. D. Wang, X. D. Xu, Z. W. Zhao, W. W. Xu, F. Wu, and J. Xu, Acta. Phys. Sin. 57, 5007 (2008). 9. B. Q. Yao, L. L. Zheng, X. M. Duan, Y. Z. Wang, G. J. Zhao, and Q. Dong, Laser Phys. Lett. 5, 714 (2008). 10. H. Kalaycioglu, A. Sennaroglu, A. Kurt, and G. J. Ozen, J. Phys.: Condens. Matter 19, 036208 (2007). 11. Y. Kuwanoa, K.Sudab, N. Ishizawab, and T. Yamadac, J. Cryst. Growth 260, 159 (2004). 12. T. Kasamatsu, H. Sekita, and Y. Kuwano, Appl. Opt. 38, 5149 (1999). 13. J. Dong, K. Ueda, and A. A. Kaminskii, Opt. Lett. 32, 3266 (2007).
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14. X. D. Xu, X. D. Wang, J. Q. Meng. Y. Cheng, D. Z. Li, S. S. Cheng, F. Wu, Z. W. Zhao, and J. Xu, Laser Phys. Lett. 19, 678 (2009). 15. N. P. Barnes, M. G. Jani, and R. L. Hutcheson, Appl. Opt. 34, 4290 (1995). 16. Y. F. Li, Y. Z. Wang, and Y. L. Ju, Laser Phys. 18, 722 (2008). 17. C. T. Wu, Y. L. Ju, Y. F. Li, Z. G. Wang, and Y. Z Wang, Chin. Opt. Lett. 6, 415 (2008). 18. K. Scholle, E. Heumann, and G. Huber, Laser Phys. Lett. 1, 285 (2004). 19. C. T. Wu, Y. L. Ju, Z. G. Wang, Y. F. Li, H. Y. Ma, and Y. Z. Wang, Laser Phys. Lett. 5, 510 (2008). 20. V. Sudesh and J. A. Piper, IEEE J. Quantum Electron. 36, 879 (2000). 21. J. D. Kmetec, T. S. Kubo, T. J. Kane, and C. J. Grund, Opt. Lett. 19, 186 (1994). 22. V. Kushawaha, Y. Chen, Y. Yan, and L. Major, Appl. Phys. B 62, 109 (1996). 23. V. Sudesh and K. Asai, J. Opt. Soc. Am. B 20, 1829 (2003). 24. X. D. Xu, Z. W. Zhao, J. Xu, and P. Z. Deng, J. Cryst. Growth 255, 338 (2003). 25. S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, IEEE J. Quantum Electron. 28, 2619 (1992).
