Open Access
Add to BibSonomyAbstract
Structural, optical, and spectroscopic properties of novel Tm3+:Lu2O3 ceramics are studied. The average grain size is determined to be ~0.54-0.56 μm. The absorption spectra show good opportunities for diode pumping at 796 nm and 811 nm. The ceramics have high mid-IR transmittance of up to 7 μm. Strong luminescence lines are measured at 1942 nm, 1965 nm, and 2066 nm. CW laser operation at 2066 nm with an output power of up to 26 W and a slope efficiency of 42% is obtained. Q-switched operation with a pulse duration of 100-150 ns and a repetition rate of 5-10 kHz is achieved.
©2012 Optical Society of America
1. Introduction
Highly efficient high-power 2-μm lasers are very attractive for many applications, such as medical surgery, atmospheric wind lidar, gas detection, material processing, and pumping of mid-IR optical parametric oscillators [1,2]. Tm-doped materials (crystals or silica glass fibers) pumped by commercially available 800 nm laser diodes enabling generation at 1.8-2.1 μm provide a good opportunity for creation of such laser sources [3,4]. The efficient cross-relaxation populating the upper laser level with quantum efficiency close to 2 leads to a high overall efficiency of the lasers. Tm-doped sesquioxides (Tm:Lu2O3 and Tm:Sc2O3), having an extraordinary long-wavelength 2-μm luminescence band and a high thermal conductivity, open up new opportunities for highly efficient laser operation at around 2.1 μm under direct diode pumping at around 800 nm [2,5–7]. Diode-pumped 2.1-μm lasers based on sesquioxides could become an alternative to the laser-pumped Ho:YAG lasers for several applications [6,7].
Transparent laser ceramic materials are attracting great interest as substitutes for single crystals [8,9]. Recently, the high-quality sesquioxides ceramics doped by Nd3+, Yb3+ or Ho3+ ions have demonstrated good potential for efficient laser oscillation [10–16]. This paper is devoted to the investigation, for the first time to the best of our knowledge, of structural, optical, and spectroscopic properties of the novel Tm:Lu2O3 ceramics and the possibility for lasers based on these ceramics to generate both CW and Q-switched 2066-nm radiation.
2. Optical and luminescence properties of Tm: Lu2O3 ceramics
Tm:Lu2O3 ceramics (Fig. 1a), produced by Konoshima Chemical Co. with 2 at.% of Tm doping concentration, have low scattering and absorption losses (outside any absorption band): the extinction coefficient at 840 nm was estimated to be less than 3 × 10 −2 cm−1. The grain structure of the ceramic material was studied with both a scanning electron microscope (Jeol, model JSM–6490, Japan) and an atomic force microscope (NT-MDT, model Solver PRO, Russia) (Figs. 1b and 1c). The grain sizes were calculated using a chord length measurement. The grain size distribution is well approximated by a log-normal distribution function. The average grain size was determined to be about 535-565 nm.
Fig. 1 Photograph of a Tm:Lu2O3 ceramic disc (a); structure of the Tm:Lu2O3 ceramics recorded using a JSM - 6490 scanning electron microscope (magnification x104) (b), and a 3-dimensional AFM-image of the ceramic surface after etching (c).
The absorption spectrum of the ceramics was measured using a Perkin-Elmer Lambda 9 spectrophotometer (PerkinElmer Inc.) with a 0.2-nm wavelength step. Several absorption lines were recorded. They correspond to the transitions from the ground state 3H6 to levels 3F2 and 3F3, 3H4, 3H5, 3F4 (Fig. 2a ). More accurate measurements of the absorption in the near 800-nm pump band (with a 0.05-nm wavelength step) and determination of the absorption cross-section showed similarity to data published for a Tm: Lu2O3 crystal (Fig. 2b) [2,5]. Maxima of the absorption lines (most appropriate for diode pumping) were at 796 nm and 811 nm.
Fig. 2 Absorption cross-section of Tm:Lu2O3 ceramics and corresponding transition lines (a), and the absorption in the near 800-nm pump band (red line) in comparison with the absorption of the Tm:Lu2O3 crystal (blue dotted line shows results published in [2,5]) (b).
The mid-IR transmission of the ceramic material (studied using a Varian FTS-7000 Fourier transform spectrometer) extended up to 7 μm.
The 2-μm fluorescence spectra of the Tm:Lu2O3 ceramics (a polished disc 3 mm in thickness) under diode pumping at 796 nm were measured using a monochromator (LOMO, model MDR-41, Russia). The emission cross-section spectrum calculated from the fluorescence spectrum (via the Füchtbauer-Ladenburg equation [17]) was found to be similar to the spectrum of the Tm:Lu2O3 crystal (Fig. 3 ).
Fig. 3 Emission cross section of Tm:Lu2O3 ceramics (red curve) and Tm:Lu2O3 crystal (blue curve, data from [2]).
To determine the lifetime of the 3F4 and 3H4 manifolds, the Tm:Lu2O3 ceramic disc was pumped by 10-ns pulses from a Ti:Sapphire laser at 796 nm. The non-exponential decay of fluorescence at wavelengths 400-1200 nm (excluding pump wavelengths) was measured. This dynamics (corresponding to depopulation of the 3H4 manifold of Tm3+ ions due to cross relaxation (3H4;3H6)→(3F4,3F4), radiative and nonradiative relaxations [3,6]) can be associated with two characteristic decay times: 3.4 ± 0.3 μs (at the initial stage) and 37.8 ± 0.7 μs (at the final stage). The decay time of fluorescence at a wavelength of ~2 μm (corresponding to lifetime of the 3F4 upper laser level) was determined to be 3.7 ± 0.2 ms. Similar results for the lifetimes were obtained by measuring the kinetics of the refractive index change in the Tm:Lu2O3 ceramic disc (pumped by 10-ns pulses from a Ti:Sapphire laser at 796 nm) using a Jamin-Lebedev interferometer [18]. The determined lifetimes of the 3F4 and 3H4 manifolds of the ceramics are comparable to those for the Tm:Lu2O3 crystal [6].
The refractive index of the ceramics was measured using a PhE-102 spectroscopic ellipsometer (Micro Photonics Inc.) at wavelengths ranging from 300 nm to 1100 nm. It was found to be ~1.94 at 800 nm. The approximation of the wavelength dependence of the refractive index (by Cauchy-Sellmeier formula [19]) and its extrapolation to longer wavelengths gave a value of the refractive index of ~1.92 at 2066 nm. The refractive index value is close to that for the Lu2O3 ceramics and crystals with different dopants [10,20].
3. Laser experiments
3.1 CW regime
A compact linear cavity was chosen for the laser experiments (Fig. 4 ). Tm:Lu2O3 (2 at.% of Tm) ceramic rods 3 mm in diameter and with three different lengths of 11.6 mm, 10.3 mm, and 8.4 mm were used. Both facets in all the rods had an antireflective (AR) coating for the pump and lasing wavelengths at 811 nm and 2066 nm, respectively. The ceramic rod was wrapped with indium foil and fixed in a Cu radiator. The temperature of the radiator was kept at 7.5° C by external water.
Fig. 4 Schematic of a Tm: Lu2O3 ceramic laser pumped at 811 nm with a cavity formed by two mirrors (M1,2); an acousto-optic modulator (AOM) was used to obtain the Q-switched oscillation.
A fiber-coupled diode laser (JENOPTIK, Germany, model JOLD-140-CAXF-6A) was used as the pump laser. The tuning range of the central wavelength of the pump radiation (varied by diode temperature) was 808–811 nm. A two-lens telescope at different magnifications (1:1; 1:1.5; 1:2) was used for image transfer of a pump beam from a 600-μm fiber output into the ceramic rod.
The incoupling mirror M1 had high transmission at 811 nm and high reflection at 2066 nm. The output coupler (mirror M2) was partially transmitting at 2066 nm and highly reflective at 811 nm, leading to a double pass of the pump radiation through the rod. Different output coupling transmissions Toc = 6%, 9%, 11% at the laser wavelength were available. The radius of curvature of the output coupler was also varied (R = 98 mm, 200 mm, 299 mm, ∞).
The output-coupler parameters (Toc, R), the cavity length, the pump beam diameter, and the waist position in the ceramics, as well as the pump wavelength, were optimized to achieve maximum powers of the multimode beam for both the CW and Q-switched regime.
The pump beam waist diameter (inside the active medium) was ~600 μm for laser experiments with the 8.4-mm and 10.3-mm ceramic rods. The output power for the CW regime at 2066 nm grew with increasing pump power and rolled over at the absorbed pump powers of ~65 W and ~75 W (with a sharp drop in the slope efficiency) for the 8.4-mm and 10.3-mm ceramic rods, respectively (Fig. 5 ). The thermal stress-induced damage of both the 8.4-mm and 10.3-mm rods was observed at ~80 W and ~90 W of incident pump power (corresponding to 70.2 W and 82.5 W of absorbed pump power), respectively.
Fig. 5 Output power at 2066 nm (solid lines) and slope efficiency (dashed lines are β-spline approximations) of the ceramic laser with a plane output coupler (Toc = 6%) versus double-pass absorbed power of the pump at 811 nm for different lengths (8.4, 10.3 and 11.6 mm) of the Tm:Lu2O3 rod.
A 900 μm waist diameter of the pump beam was chosen for experiments with the 11.6-mm ceramic rod. The slope efficiency of the CW generation at 2066 nm was slightly lower than in the experiment with the 10.3 mm rod, and the optical-to-optical conversion efficiency in the laser based on the longer rod was even lower than in experiments with the 8.4 mm rod. However, the highest output power was achieved in the laser based on the 11.6-mm rod without any decrease in the slope efficiency (up to ~80 W of the absorbed pump power).
The transmitted power after single-pass pump propagation through the ceramic rod was measured; the double-pass absorbed pump power was calculated assuming the unsaturated absorption in the rod. According to our estimations, about 87.8%, 91.6%, and 94.2% of the pump power (twice propagating through the Tm:Lu2O3 element) were absorbed during laser operation in the 8.4-mm, 10.3-mm, and 11.6-mm active rods, respectively. Thus, the slope efficiency with respect to the absorbed pump power reached 42% for the 10.3-mm rod. Note that the obtained result is comparable to that for the Tm:Lu2O3 single-crystal laser [2,5].
The maximum CW-oscillation power of 26 W with a ~32.6% optical-to-optical conversion efficiency was achieved in experiments with optimized parameters: 11.6-mm active rods, a plane output coupler with Toc = 6%, and a 15-mm cavity length.
The Tm:Lu2O3 laser was operating in a smooth CW regime beginning with the output power of 10 W; for lower powers the laser generated a spike train (with a random pedestal for output power of 3-10 W). Spectral analysis (made with a MDR-24 monochromator) showed that the central wavelength of laser operation was ~2066 nm, and the linewidth was ~4 nm in our experiments with 10.3-mm rod (Fig. 6 ). An additional operation line centered at ~2089 nm was registered in the spectrum of the laser based on the 11.6-mm Tm:Lu2O3 ceramic rod (with the same plane output coupler). The oscillation dynamics measured after the monochromator (within the narrow linewidth) always contained the spikes with 100-% modulation.
The CW laser operation in the ceramics was also obtained under diode pumping at 796 nm. The laser threshold was approximately the same for diode pumping at 796 nm and 811 nm.
3.2 Q-switched regime
For Q-switched laser operation, a quartz acousto-optic modulator (AOM) was placed between an output coupler M2 (with transmission Toc = 11%) and an active element. The physical cavity length was increased to 6 cm. The pulse repetition rate of the Q-switched laser oscillation was 5-10 kHz (it was the best operational range of the AOM used) with the pulse duration of 100-150 ns (Fig. 7 ). The average power of the Q-switched radiation reached 7 W (corresponding to 96% of CW power in the same laser cavity); corresponding peak power was 14 kW (for a pulse duration of 100 ns at 5 kHz repetition rate). The instability of pulse power and energy was less than 5% at maximum average power.
4. Summary and conclusion
The novel Tm:Lu2O3 ceramics have demonstrated promising optical and spectroscopic properties for 2-μm laser generation: high near-IR transmittance with absorption bands suitable for diode pumping at 796 nm or 811 nm, and extended luminescence band up to 2.1 μm. Laser operation of the diode-pumped ceramics at 26 W power and ~32.6% optical-to-optical conversion efficiency has been achieved in the CW regime at 2066 nm. The Q-switched generation of 100-150 ns pulses at 5-10 kHz repetition rate has been realized using an acousto-optic modulator. The novel ceramic laser demonstrates good scalability for efficient CW or Q-switched generation of high-power beams at 2066-nm wavelength under diode pumping at 811 nm or 796 nm.
Acknowledgments
The authors would like to thank the “Konoshima Chemical Co.” team for fabrication of the novel Tm:Lu2O3 ceramics, and Prof. K.-I. Ueda and Prof. A.A. Kaminskii for helpful discussions. The research was supported by the Russian Academy of Science and the Russian Ministry of Education and Science through the “Nonlinear-optical methods and materials for novel laser systems” program and contract No. 14.740.11.0071, and the Russian Foundation for Basic Research through grants No. 10-02-90049 and 11-02-97111_r.
References and links
1. E. C. Honea, R. J. Beach, S. B. Sutton, J. A. Speth, S. C. Mitchell, J. A. Skidmore, M. A. Emanuel, and S. A. Payne, “115-W Tm:YAG Diode-Pumped Solid-State Laser,” IEEE J. Quantum Electron. 33(9), 1592–1600 (1997). [CrossRef]
2. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, “2 µm Laser Sources and Their Possible Applications,” in Frontiers in Guided Wave Optics and Optoelectronics, B. Pal, ed. (InTech, 2010), pp. 471–500.
3. B. M. Walsh, “Review of Tm and Ho Materials: Spectroscopy and Lasers,” Laser Phys. 19(4), 855–866 (2009). [CrossRef]
4. P. F. Moulton, G. A. Rines, E. V. Slobodtchikov, K. F. Wall, G. Frith, B. Samson, and A. L. G. Carter, “Tm-Doped Fiber Lasers: Fundamentals and Power Scaling,” IEEE J. Sel. Top. Quantum Electron. 15(1), 85–92 (2009). [CrossRef]
5. P. Koopmann, S. Lamrini, K. Scholle, P. Fuhrberg, K. Petermann, and G. Huber, “Efficient diode-pumped laser operation of Tm:Lu2O3 around 2 μm,” Opt. Lett. 36(6), 948–950 (2011). [CrossRef] [PubMed]
6. P. Koopmann, R. Peters, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and highly efficient laser operation of thulium-doped Lu2O3 around 2 μm,” Appl. Phys. B 102(1), 19–24 (2011). [CrossRef]
7. P. Koopmann, S. Lamrini, K. Scholle, P. Fuhrberg, K. Petermann, and G. Huber, “Long Wavelength Laser Operation of Tm:Sc2O3 at 2116 nm and Beyond,” in Conference “Advanced Solid-State Photonics 2011” (Istanbul, Turkey, 2011), paper ATuA5.
8. V. Lupei, A. Lupei, and A. Ikesue, “Single crystal and transparent ceramic Nd-doped oxide laser materials: a comparative spectroscopic investigation,” J. Alloy. Comp. 380(1-2), 61–70 (2004). [CrossRef]
9. K. Ueda, J.-F. Bisson, H. Yagi, K. Takaichi, A. Shirakawa, T. Yanagitani, and A. A. Kaminskii, “Scalable Ceramic Lasers,” Laser Phys. 15(7), 927–939 (2005).
10. K. Takaichi, H. Yagi, A. Shirakawa, K. Ueda, S. Hosokawa, T. Yanagitani, and A. A. Kaminskii, “Lu2O3:Yb3+ ceramics – a novel gain material for high-power solid-state lasers,” Phys. Status Solidi A 202(1), R1–R3 (2005). [CrossRef]
11. A. Ikesue, Y. L. Aung, T. Taira, T. Kamimura, K. Yoshida, and G. L. Messing, “Progress in ceramic lasers,” Annu. Rev. Mater. Res. 36(1), 397–429 (2006). [CrossRef]
12. M. Tokurakawa, K. Takaichi, A. Shirakawa, K. Ueda, H. Yagi, S. Hosokawa, T. Yanagitani, and A. A. Kaminskii, “Diode-pumped mode-locked Yb3+:Lu2O3 ceramic laser,” Opt. Express 14(26), 12832–12838 (2006). [CrossRef] [PubMed]
13. M. Tokurakawa, A. Shirakawa, K.-I. Ueda, H. Yagi, S. Hosokawa, T. Yanagitani, and A. A. Kaminskii, “Diode-pumped 65 fs Kerr-lens mode-locked Yb3+:Lu2O3 and nondoped Y2O3 combined ceramic laser,” Opt. Lett. 33(12), 1380–1382 (2008). [CrossRef] [PubMed]
14. M. Tokurakawa, A. Shirakawa, K. Ueda, H. Yagi, M. Noriyuki, T. Yanagitani, and A. A. Kaminskii, “Diode-pumped ultrashort-pulse generation based on Yb3+:Sc2O3 and Yb3+:Y2O3 ceramic multi-gain-media oscillator,” Opt. Express 17(5), 3353–3361 (2009). [CrossRef] [PubMed]
15. J. Sanghera, J. Frantz, W. Kim, G. Villalobos, C. Baker, B. Shaw, B. Sadowski, M. Hunt, F. Miklos, A. Lutz, and I. Aggarwal, “10% Yb3+-Lu2O3 ceramic laser with 74% efficiency,” Opt. Lett. 36(4), 576–578 (2011). [CrossRef] [PubMed]
16. G. A. Newburgh, A. Word-Daniels, A. Michael, L. D. Merkle, A. Ikesue, and M. Dubinskii, “Resonantly diode-pumped Ho3+:Y2O3 ceramic 2.1 µm laser,” Opt. Express 19(4), 3604–3611 (2011). [CrossRef] [PubMed]
17. B. F. Aull and H. P. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]
18. A. Zinoviev, R. Soulard, O. Antipov, R. Moncorge, and E. Ivakin, “Dynamics of Refractive Index Changes in Tm-doped Crystals Tm:YAG and Tm:YLF, and Ceramics Tm:Lu2O3,” in Conference on Lasers and Electro-Optics/Europe-2011 (22–26 May 2011, Munich, Germany), paper CA.P.22.
19. M. Born and E. Wolf, Principles of Optics (Pergamon Press, 1984), p. 121.
20. O. Medenbach, D. Dettmar, R. D. Shannon, R. X. Fischer, and W. M. Yen, “Refractive index and optical dispersion of rare earth oxides using a small-prism technique,” J. Opt. A, Pure Appl. Opt. 3(3), 174–177 (2001). [CrossRef]
