CW and mode-locked operation of Yb 3+ -doped Lu 3 Al 5 O 12 ceramic laser

Transcription

1 CW and mode-locked operation of Yb 3+ -doped Lu 3 Al 5 O 12 ceramic laser Hiroaki Nakao, 1,* Akira Shirakawa, 1 Ken-ichi Ueda, 1 Hideki Yagi, 2 and Takagimi Yanagitani 2 1 Institute for Laser Science, University of Electro-Communications, Chofugaoka, Chofu, Tokyo , Japan 2 Takuma Works, Konoshima Chemical Co., Ltd., 80 Kouda, Takuma, Mitoyo-gun, Kagawa , Japan *nakao@ils.uec.ac.jp Abstract: CW laser operation and first mode-locked laser operation of Yb:LuAG ceramic are reported. Efficient CW laser operation was obtained with maximum output power of 2.14 W and a 72% slope efficiency. Femtosecond mode-locked laser operation was achieved with pulse duration of 699 fs and a 200 mw average output power Optical Society of America OCIS codes: ( ) Lasers, solid-state; ( ) Laser materials; ( ) Ultrafast lasers. References and links 1. C. R. E. Baer, O. H. Heckl, C. J. Saraceno, C. Schriber, C. Kränkel, T. Südmeyer, and U. Keller, Frontiers in passively mode-locked high-power thin disk laser oscillators, Opt. Express 20(7), (2012). 2. C. R. E. Baer, C. Kränkel, O. H. Heckl, M. Golling, T. Südmeyer, R. Peters, K. Petermann, G. Huber, and U. Keller, 227-fs pulses from a mode-locked Yb:LuScO 3 thin disk laser, Opt. Express 17(13), (2009). 3. D. Bauer, I. Zawischa, D. H. Sutter, A. Killi, and T. Dekorsy, Mode-locked Yb:YAG thin-disk oscillator with 41 µj pulse energy at 145 W average infrared power and high power frequency conversion, Opt. Express 20(9), (2012). 4. O. Pronin, J. Brons, C. Grasse, V. Pervak, G. Boehm, M. C. Amann, V. L. Kalashnikov, A. Apolonski, and F. Krausz, High-power 200 fs Kerr-lens mode-locked Yb:YAG thin-disk oscillator, Opt. Lett. 36(24), (2011). 5. S. Ricaud, A. Jaffres, P. Loiseau, B. Viana, B. Weichelt, M. Abdou-Ahmed, A. Voss, T. Graf, D. Rytz, M. Delaigue, E. Mottay, P. Georges, and F. Druon, Yb:CaGdAlO 4 thin-disk laser, Opt. Lett. 36(21), (2011). 6. M. Tokurakawa, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, and A. A. Kaminskii, Diode-pumped sub-100 fs Kerr-lens mode-locked Yb 3+ :Sc 2O 3 ceramic laser, Opt. Lett. 32(23), (2007). 7. T. H. Geballe and G. W. Hull, Isotopic and Other Types of Thermal Resistance in Germanium, Phys. Rev. 110(3), (1958). 8. M. E. Wieser and T. B. Coplen, Atomic weights of the elements 2009 (IUPAC Technical Report), Pure Appl. Chem. 83(2), (2011). 9. K. Beil, S. T. Fredrich-Thornton, F. Tellkamp, R. Peters, C. Kränkel, K. Petermann, and G. Huber, Thermal and laser properties of Yb:LuAG for kw thin disk lasers, Opt. Express 18(20), (2010). 10. J. He, X. Liang, J. Li, H. Yu, X. Xu, Z. Zhao, J. Xu, and Z. Xu, LD pumped Yb:LuAG mode-locked laser with 7.63ps duration, Opt. Express 17(14), (2009). 11. A. A. Kaminskii, H. Rhee, O. Lux, H. J. Eichler, S. N. Bagayev, H. Yagi, K. Ueda, A. Shirakawa, and J. Dong, Stimulated Raman scattering in garnet Lu 3Al 5O 12 ceramics a novel host-materiel for Ln- and TM-lasant ions, Laser Phys. Lett. 8(6), (2011). 12. C. W. Xu, D. W. Luo, J. Zhang, H. Yang, X. P. Qin, W. D. Tan, and D. Y. Tang, Diode pumped highly efficient Yb:Lu 3Al 5O 12 ceramic laser, Laser Phys. Lett. 9(1), (2012). 13. T. Yanagitani and H. Yagi, J. P. Appl. Nos and (1998). 14. M. S. Akchurin, R. V. Gainutdinov, I. I. Kupenko, K. Yagi, K. Ueda, A. Shirakava, and A. A. Kaminskii, Lutetium Aluminum Garnet Laser Ceramics, Dokl. Phys. 56(12), (2011). 15. H. Nakao, A. Shirakawa, K. Ueda, A. A. Kaminskii, S. Kuretake, N. Tanaka, Y. Kintaka, K. Kageyama, H. Yagi, and T. Yanagitani, Investigation of the laser and optical properties of new laser materials, in 7th Laser Ceramics Symposium, paper I-6, Singapore, Nov. 15, (invited talk) 16. H. M. Pask, R. J. Carman, D. C. Hanna, A. C. Tropper, C. J. Mackechnie, P. R. Barber, and J. M. Dawes, Ytterbium-Doped Silica Fiber Lasers: Versatile Sources for the µm Region, IEEE J. Sel. Top. Quantum Electron. 1(1), 2 13 (1995). 17. C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, Q-switching stability limits of continuous-wave passive mode locking, J. Opt. Soc. Am. B 16(1), (1999). (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15385

2 18. M. J. Lederer, B. Luther-Davies, H. H. Tan, C. Jagadish, N. N. Akhmediev, and J. M. Soto-Crespo, Multipulse operation of a Ti:sapphire laser mode locked by an ion-implanted semiconductor saturable-absorber mirror, J. Opt. Soc. Am. B 16(6), (1999). 1. Introduction Recently advances of laser-diode (LD) pumped ultrashort pulsed solid-state lasers with Yb 3+ - doped gain materials are progressive [1 6]. Yb 3+ -doped gain materials have attracted attention for compact high power, highly efficient lasers due to their unique energy level scheme ( 2 F 5/2 2 F 7/2 inter manifold transition). The remarkable improvement of the InGaAs LDs enables the high power laser operation of Yb 3+ -doped gain materials based on direct diode pumping. For high power laser operation, gain materials with excellent thermal properties are required. However, the thermal conductivity decreases rapidly as an increase of the doping concentration due to decline of the mean free path of phonons caused by a large mass difference between a host atom and a dopant atom [7]. Compared with Y 3 Al 5 O 12 (YAG) and Lu 3 Al 5 O 12 (LuAG), the thermal conductivity of YAG is higher than that of LuAG. But in the case of YAG, the huge mass difference between Yb 3+ (173 g/mol) and Y 3+ (88.9 g/mol) exists [8]. In contrast, the atomic mass of Lu 3+ (175 g/mol) is very close to that of Yb 3+. So, by Yb 3+ -doping, the thermal conductivity of YAG decreases dramatically, but that of LuAG declines only moderately. Hence the thermal conductivity of Yb:LuAG can be higher than that of Yb:YAG for C Yb > 2-3% [9]. This makes Yb:LuAG an attractive gain material especially for thin-disk laser since it needs high doping. As the latest results regarding Yb:LuAG single crystal, a thin-disk laser with a 5 kw output power [9] and a mode-locked laser with a 7.63 ps pulse duration [10] were reported. Recently, growth of transparent LuAG ceramics have been successfully reported [11]. Most recently, continuous wave (CW) laser operation with a 7 W output power was reported by use of Yb:LuAG ceramics fabricated by the hot-press method [12]. In this paper, the thermal properties, the optical properties, CW laser properties and first mode-locked laser properties of Yb:LuAG ceramic fabricated by our vacuum sintering and nanocrystalline technology [13] are reported. For the CW laser operation, as high as a 72% slope efficiency and a 2.14 W output power were obtained. For the mode-locking, a 699 fs pulse duration and a 200 mw average output power were obtained at a pump power of a 3.86 W. 2. Thermal properties of LuAG ceramics Up to now, the thermal properties of Yb:LuAG and non-doped LuAG ceramics have not been reported. So this is the first report regarding the thermal properties of Yb:LuAG and nondoped LuAG ceramics. Non-doped LuAG ceramics were also fabricated by the same way as our Yb:LuAG ceramic. The average grain size of non-doped LuAG is 1-2 µm and the mechanical properties of non-doped LuAG ceramics are superior to that of single crystal LuAG [14]. The thermal conductivity K (W/mK) is described as K = α C p ρ. (1) The thermal diffusivity α (mm 2 /s) and the heat capacity C p (J/gK) were measured by the flash method (LFA447 NanoFlash, NETZSCH Inc.). ρ (g/cm 3 ) is the density and was measured by ourselves. Table 1 shows the results. The thermal conductivity of non-doped LuAG ceramic is 8.80 ± 0.06 W/mK and that of 10 at.% Yb:LuAG ceramic is 8.04 ± 0.03 W/mK. Yb:LuAG indicates high thermal conductivity even in highly doped case like single crystal [9]. The drop (8%) is a bit larger than that in single crystal, but the ceramics indicates higher thermal conductivities. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15386

3 Table 1. Thermal Properties of LuAG and Yb:LuAG Ceramics and Single Crystals α (mm 2 /s) C p (J/gK) ρ (g/cm 3 ) K (W/mK) LuAG ceramic 2.44 ± ± ± at.% Yb:LuAG ceramic 2.59 ± ± ± 0.03 LuAG single crystal 7.7 [9] 10 at.% Yb:LuAG single crystal 7.4 [9] 3. Optical properties of Yb:LuAG ceramic Figure 1(a) shows the absorption spectrum of the 10 at.% Yb:LuAG ceramic. The highest peak is at 968 nm (zero line; σ abs = cm 2 ) and the second highest peak is around 940 nm (σ abs = cm 2 ). The 940 nm absorption is very broad and suitable for LD pumping. The luminescence lifetime was measured by using a pulse-driven LD with a 10 µs pulse duration at a wavelength of 915 nm. The luminescence lifetime is 1.07 ms with a singleexponential decay. By use of the luminescence lifetime and the luminescence spectrum, the emission cross section of Yb:LuAG ceramic was calculated with the Füchtbauer-Ladenburg method and is shown in Fig. 1(b). The emission cross section of the Yb:YAG ceramic is also shown in Fig. 1(b) for the reference. The Yb:LuAG has luminescence peaks at 1030 nm (σ em = cm 2 ) and 1046 nm (σ em = cm 2 ). The FWHM of the main peak is 6.1 nm. The peak emission cross section of Yb:LuAG is 25% higher than that of Yb:YAG (σ em = cm 2 ). This is an additional advantage of Yb:LuAG in a thin-disk laser and amplifier. Fig. 1. (a) Absorption cross section spectrum of 10 at.% Yb:LuAG ceramic and (b) emission cross section spectra of Yb:LuAG and Yb:YAG ceramics. 4. CW laser operation For the CW laser operation, a non-coated 1.24 mm thick 10 at.% Yb 3+ -doped LuAG ceramic was used. Figure 2 shows the schematic of the CW laser experiment. The Yb:LuAG was placed in a plane-concave 2-mirror resonator with a ~100 mm resonator length. The plane mirror was a dielectric (HR) mirror and the concave mirror was an output coupler (OC) with a 100 mm radius of curvature (ROC) and different transmittances (1%, 3%, 5%). As a pump source, a fiber coupled LD operated at 940 nm with the core diameter of 100 µm and numerical aperture (NA) of 0.22 was used and focused in the Yb:LuAG with the spot (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15387

4 diameter of about 130 µm. The typical value of measured absorption efficiency was 73%. Figure 3 shows the output power and optical-to-optical efficiency against the absorbed pump power, and Fig. 4 shows the laser spectra. Since the Yb:LuAG ceramic used for CW laser operation was non-coated and was not arranged at the Brewster angle, multiple of the longitudinal modes of Yb:LuAG ceramic as an etalon appeared. With the 1% output coupling, the maximum output power of 1.73 W and a 45% slope efficiency were obtained. The lasing wavelength was always around 1049 nm. With the 3% output coupling, the maximum output power of 2.09 W and a 61% slope efficiency was obtained. The lasing wavelength was always around 1047 nm. These lasing wavelengths correspond to the emission peak at 1046 nm. With the 5% output coupling, the best result was obtained. The maximum output power of 2.14 W and the slope efficiency of 72% [15]. The wavelength of laser oscillation shifted from 1032 nm to 1047 nm as an increase of pump power. This phenomenon can be explained by the reabsorption loss of the Yb:LuAG. Figure 5 shows the small-signal gain of the Yb:LuAG with different pump densities. The blue, red and green curves indicate the small-signal gain profiles at the threshold absorbed pump powers for output coupling of 1%, 3% and 5%, respectively. The calculation of small-signal gain is based on [16]. At a lower pump density, the small-signal gain around 1030 nm is zero or less. With an increase of pump density, small-signal gain around 1030 nm increases. In the case of lower output coupling (1% and 3%), the small-signal gain around 1046 nm first reaches the threshold, and thus the lasing wavelength is always around 1046 nm. With the higher output coupling (5%), in the lower pumping regime, the small-signal gain around 1030 nm first reaches the threshold (Fig. 4(c), P abs = 470 mw). But, the increase of absorbed pump power elevates the temperature inside the gain medium and leads to the increase of the reabsorption loss that suppresses the growth of the small-signal gain around 1030 nm. Therefore, the small-signal gain around 1046 nm becomes equal (Fig. 4(d), P abs = 2.0 W) to and higher (Fig. 4(e), P abs = 3.9 W) than that of around 1030 nm. So, the lasing at 1047 nm appeared as an increase of the pump power. Fig. 2. Schematic of the CW laser experiment. Fig. 3. Output power and optical-to-optical efficiency against the absorbed pump power. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15388

5 Fig. 4. The laser spectra for (a) 1% OC, (b) 3% OC, (c) 5% OC with P abs = 470 mw, (d) 5% OC with P abs = 2.0 W and (e) 5% OC with P abs = 3.9 W. Fig. 5. Small-signal gain spectra at different pump densities. The threshold levels for the three output couplers are indicated by dashed lines. 5. Mode-locked laser operation Figure 6 shows the schematic of the mode-locked laser experiment. The non-coated 1.24 mm thick 10 at.% Yb:LuAG was arranged with the Brewster angle of 61.2 between the two HR mirrors with 100 mm ROC (M1 and M2). An SF10 prism pair with a 45 cm separation and a chirped mirror (CM) with a group delay dispersion (GDD) of 250 fs 2 was inserted for the dispersion compensation. The total negative GDD was ~-3000 fs 2 per a round trip. For modelocking, a semiconductor saturable absorber mirror (SESAM, BATOP GmbH) with modulation depth of 1% at 1030 nm, saturation fluence of 120 µj/cm 2, and relaxation time constant of 500 fs was used. The transmittance of OC was 1% with a wedge angle of 30 min. As a pump source, a broad stripe LD operating at 930 nm was used and focused in the Yb:LuAG ceramic with diameters of 20 (vertical) 90 (horizontal) µm in air. Since the laser mode diameter is calculated to be 50µm, a broad stripe LD can lead much better mode matching than that in fiber coupled LD used above. The laser beam was focused onto the (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15389

6 SESAM by a concave mirror with 300 mm ROC (M3). Figure 7 Shows the SHG autocorrelation trace and laser spectrum of the passively mode-locked Yb:LuAG ceramic laser. The input-output property is shown in Fig. 8. A self-q-switched mode-locked operation was observed in a lower pump power region, and then switched to a CW mode-locked operation at a pump power of 2.3 W. At a pump power of 3.86 W, the average output power of 200 mw, single-pulsed operation (Fig. 9) with the sech 2 -fit pulse duration of 699 fs and the spectral bandwidth of 1.7 nm were obtained. The time-bandwidth product was (Fourier transform limit = 0.315). The center wavelength was nm and the repetition rate was 96.6 MHz. When the pump power was further increased, the multi-pulsed operation was observed. The maximum average output power was 294 mw at a pump power of 5.5 W. When using a HR mirror and a 0.3% OC instead of the CM and 1% output coupler respectively, the shortest pulse duration was obtained. The pulse duration of 541 fs and a 2.5 nm spectral bandwidth was obtained with a 53 mw average output power. The timebandwidth product was Above 53 mw, multi-pulsed operation was obtained. The pulse duration in a single-pulsed operation is at present limited mainly by the SESAM we use. We tested SESAMs with deeper modulation depths for further shortening, but favorable results could not be obtained (only Q-switching, only CW, so on). The principal limitation of the pulse duration will be given by the relatively narrow gain bandwidth of Yb:LuAG (6.1 nm). In a solitary mode-locking, a larger intracavity pulse energy can lead to a shorter pulse duration and broader spectral bandwidth [17]. But there is a strong tendency of pulse splitting to two or more, since the broader pulse duration (narrower spectral bandwidth) in the multi-pulsed operation has a higher effective gain [18]. Kerr-lens modelocking enables shorter pulse generation near the gain bandwidth limit [4] or even below [6] and is the next objective. Fig. 6. Schematic of the passively mode-locked laser experiment. Inset: Pump beam profile at the focusing with the spot diameters of 20 (vertical) 90 (horizontal) µm in air. Fig. 7. (a) SHG autocorrelation trace. The red curve is the sech 2 -fit. (b) Laser spectrum. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15390

7 Fig. 8. Input-output property. 6. Conclusion Fig. 9. Oscillograms of mode-locked pulse train with P avg = 200 mw in the time scales of (a) 4 µs/div and (b) 2 ns/div. In conclusions, the thermal properties, optical properties, CW laser operation and first modelocked laser operation of Yb:LuAG ceramic were reported. Yb:LuAG ceramic indicates a high thermal conductivity even in the highly doped case. In the CW laser operation, maximum output power of 2.14 W and a 72% slope efficiency were obtained. In the modelocked laser operation, a maximum average power of 200 mw was obtained with a 699 fs pulse duration and shortest pulse duration of 541 fs was obtained with a 53 mw average power. Yb:LuAG ceramic is a promising material for thin-disk laser and amplifier due to its high thermal conductivity and large emission cross section. Acknowledgments This research was partly supported by Grant-in-Aid Scientific Research and the Photon Frontier Network Program of Ministry of Education, Culture, Sports, Science and Technology of Japan. (C) 2012 OSA 2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15391