- Email: [email protected]
Fabrication and laser properties of transparent Yb:YAG ceramics
Optical Materials 34 (2012) 936–939
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and laser properties of transparent Yb:YAG ceramics Dewei Luo a, Jian Zhang b,⇑, Changwen Xu a, Xianpeng Qin a,c, Dingyuan Tang a, Jan Ma b a
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Temasek Laboratories@NTU, Nanyang Technological University, Singapore 639798, Singapore c Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b
a r t i c l e
i n f o
Article history: Available online 13 May 2011 Keywords: Yb:YAG ceramics Laser properties Reactive sintering
a b s t r a c t High optical quality transparent Yb:YAG laser ceramics have been successfully fabricated by a vacuum reactive sintering method. Commercial Al2O3 powder and co-precipitated Y2O3 and Yb2O3 powders were used as the raw materials. In-line transmittances at 1300 nm and 400 nm were measured to be 83.6% and 81.8% respectively for a 3 mm thick mirror polished Yb:YAG ceramics sample. Continuous wave (CW) lasing at the wavelength of 1030 nm was achieved when pumped by a 940 nm fiber coupled laser diode. A slope efficiency as high as 62.7% was obtained. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth doped Yttrium aluminum garnet (YAG) ceramics have been a popular laser material since 1995 when Ikesue et al. achieved the first CW laser oscillation using a Nd:YAG ceramic as the gain medium [1]. The polycrystalline laser ceramics hold many advantages over their single crystal counterparts such as significantly shorter fabrication periods, lower fabrication cost, higher doping concentration, and the ability to be fabricated to large size with complicated structures [2]. In recent years, ytterbium doped YAG (Yb:YAG) ceramics have also attracted great attention due to the advantages such as high quantum efficiency, long fluorescence lifetime, broad emission spectrum and so on. Especially there’s no concentration quenching effect in Yb:YAG ceramics, so the Yb doping concentration could be quite high [3]. These advantages make Yb:YAG a promising material for high power thin disk laser and ultra-short pulse laser applications [4]. Takaichi et al. demonstrated a diode end-pumped Yb:YAG ceramics laser for the first time in 2003 [5]. Dong et al. demonstrated a Yb:YAG ceramics microchip laser with a slope efficiency as high as 79% in 2006 [6]. Excellent tunable Yb:YAG ceramics laser was reported by Nakamura et al. in 2008 [7]. However, there is not many reports on the fabrication of transparent Yb:YAG ceramics . In this research, vacuum reactive sintering method was employed to fabricate high optical quality transparent Yb:YAG ceramics with various doping concentration. The commercial Al2O3 powder and co-precipitated Y2O3 and Yb2O3 powders were used as the raw materials. The optical properties and microstructure
⇑ Corresponding author. Tel.: +65 67906835. E-mail address: [email protected] (J. Zhang). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.017
of the fabricated Yb:YAG ceramic samples were investigated. Their lasing performance was also characterized when pumped by a fiber coupled laser diode. 2. Experiments 2.1. Fabrication of Yb:YAG ceramics The Al2O3 powder (99.99% purity, Shanghai Wusong Chemical Co. Ltd.) that we used was pure a-phase. The primary particle size of the Al2O3 powder was around 0.25 lm. A reverse strike coprecipitation (add salt solution into the precipitant solution) method using nitrates and ammonia was adopted to prepare the Y2O3 and Yb2O3 powders. The detailed synthesis process was similar to the method described by Zhang et al. in previous research [8]. The Y2O3 precursor was calcined at 1000 °C/3 h in air. The primary particle size of the powder was around 60–80 nm and the specific surface area was around 9.5–10.0 m2/g. The powders were weighed precisely according to the chemical stoichiometry composition, at Yb3+ doping concentrations of 1.0 at.%, 5.0 at.%, 8.0 at.%, 10.0 at.%, 15.0 at.% and 20.0 at.% respectively and mixed with 99.99% ethanol (analytical pure). The mixed slurry was then ball milled using a planetary milling machine for 15 h. High purity alumina balls (5 mm diameter, Tosoh, Japan) were used. Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99.999%) was used to introduce SiO2 as sintering aids. The TEOS was doped at 0.5 wt.%, which corresponding to a 0.14 wt.% of SiO2. The powder mixtures were dried at 120 °C for 24 h in oven and then sieved through 200-mesh screen. After removing organic components by calcining at 800 °C for 3 h, the powders were dry pressed in a stainless steel die at 15 MPa. The green body pellets were further cold isostatically pressed (CIPed) at 200 MPa. After CIP, the relative green body density was around 53%. The green bodies were then sintered at
D. Luo et al. / Optical Materials 34 (2012) 936–939
Fig. 1. Schematic diagram of the end-pumped Yb:YAG ceramic laser.
937
After grinded by a Hyprez horizontal grinding machine (Model: EHG-150, Engis, Japan) and then fine polished on both surfaces by a Hyprez precision lapping machine (Model: EJW400-IN-D, Engis, Japan), highly transparent Yb:YAG ceramic samples were obtained. The room temperature transmittance spectra were measured with a UV–VIS–NIR spectrometer (Carry 5000 Spectrophotometer, Varian, USA). The room temperature emission spectra were recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edison, USA) when samples were excited by a 940 nm laser diode. Scanning electron microscope (SEM, JSM-6360A, JEOL, Tokyo, Japan) was used to exam sample’s microstructure. 2.2. CW laser experiments
Fig. 2. Optical transmittance spectra of 1at.% doped Yb:YAG ceramics (3 mm thick). Inset: Yb:YAG ceramics before (green) and after (colorless) annealing process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
1770 °C for 8 h in a high temperature vacuum sintering furnace under 10 3 Pa vacuum condition. The sintered pellets were then annealed in air at 1400 °C for 35 h to completely remove internal stress and eliminate the oxygen vacancies.
An end-pump scheme shown in Fig. 1 was used for the laser experiment. A simple two-mirror cavity with a cavity length of 15 mm was set up. The input mirror was a plano mirror and the output coupler had a radius of curvature (ROC) of 50 mm. The pump source was a 25 W 940 nm fiber coupled laser diode (Limo-25-f100-dl940) with a fiber core diameter of 100 lm. The pump beam was focused on the ceramic sample with lens of a focal length of 30 mm. The ceramic sample was cut into the dimensions of 3 3 3.2 mm3 and antireflection coated on both (3 3 mm2) sides at the pump and lasing wavelength to reduce the reflection loss. The sample was wrapped with indium foil, placed in a copper holder which was water cooled at 18 °C. Two different output couplers with transmittances of 4.2% and 10.0% at the lasing wavelength respectively were used for the ceramic laser. 3. Results and discussion Fig. 2 shows the room temperature optical transmittance of a 1.0 at.% Yb:YAG sample with 3 mm in thickness. The photo of asfabricated Yb:YAG ceramics before (green) and after (colorless) annealed in air is shown in the inset of the figure. The higher the Yb doping concentration is, the longer the annealing time is needed to
Fig. 3. SEM photographs of 1.0 at.% (a) 8.0 at.% (b) 15.0 at.% (c) and 20.0 at.% (d) Yb:YAG ceramics.
938
D. Luo et al. / Optical Materials 34 (2012) 936–939
Fig. 4. The absorption spectra of fabricated Yb:YAG ceramics with different Yb doping concentration.
Fig. 5. The normalized emission spectra of fabricated Yb:YAG ceramics with different Yb doping concentration.
completely eliminate color center defects. For un-annealed sample, two broad absorption bands at 375 nm and 625 nm are appeared, which are ascribed to Re-F color centers formed during vacuum sintering process. This further confirms the presence of Yb2+ formed during the vacuum sintering. The above two absorption bands completely disappeared after the annealing process. For annealed sample, the in-line transmittance has reached 83.6% at 1300 nm and still remains 81.8% at 400 nm, indicating good optical quality of the ceramic samples. In order to exam their microstructure, the surfaces of samples were mirror-polished and then thermally etched at 1500 °C for 2 h to reveal its grain boundary. SEM photographs of surfaces of samples with different doping concentration are displayed in Fig. 3. They exhibit a pore-free structure. The average grain size decreased from 22.1 lm to 11.7 lm with the increasing of Yb doping concentration from 1.0 at.% to 20.0 at.%. The grain size could be further controlled by adjusting the sintering time, sintering temperature and the sintering aids during the reactive sintering process. Related research has also been reported by Lee et al. [9]. The absorption spectra from 810 nm to 1100 nm of the fabricated Yb:YAG ceramics with different doping concentrations are plotted in Fig. 4. All the samples have a very strong absorption peak located at 940 nm and two moderate absorption peaks located at 915 nm and 969 nm respectively, corresponding to the 2 F7=2 ? 2 F5=2 transition of Yb3+. The absorption coefficient at 940 nm increases from 1.06 cm 1 of the 1.0 at.% Yb doping concentration to 13.87 cm 1 of the 15.0 at.% doping concentration. Re-absorption at 1030 nm lasing wavelength is also clearly observed, which is attributed to the quasithree-level nature of ytterbium-doped gain media. The normalized room temperature emission spectra of samples with different Yb doping concentration are shown in Fig. 5. The full width at the half maximum (FWHM) of the 1030 nm wavelength emission peaks are 8.40 nm, 11.07 nm, 12.25 nm, 16.31 nm and 19.98 nm for samples with Yb doping concentration of 1.0 at.%, 5.0 at.%, 8.0 at.%, 15.0 at.% and 20.0 at.%, respectively. It is apparent that the emission spectra become broader with the increase of Yb doping concentration. The broad emission band is especially useful in mode-locked laser for ultra-short pulse generation. Besides the emission peak at 1030 nm, another emission peak at 1047 nm wavelength exhibits an increased intensity with higher Yb doping concentration. This could result a change of lasing wavelength and even dual-wavelength operation. Dong et al. [6] and Nakamura et al. [10] had also observed similar phenomenon previously. Fig. 6 shows the CW laser output power versus the absorbed pump power of samples with different Yb doping concentrations
Fig. 6. Laser performance of Yb:YAG with a 4.2% output coupler (a) and 10.0% output coupler (b).
D. Luo et al. / Optical Materials 34 (2012) 936–939
under two different cavity output couplings (Toc). The laser threshold for the 5.0 at.%, 8.0 at.%, and 15.0 at.% Yb:YAG ceramics were 1.31 W, 1.64 W and 2.08 W under Toc = 4.2% and 1.3 W, 2.16 W and 4.21 W under Toc = 10% , respectively. The output power increases with the increasing of the absorbed pump power. A maximum output power of 6.9 W was achieved on a 8.0 at.% sample under Toc = 4.2%. A slope efficiency as high as 62.7% was obtained for a 5.0 at.% Yb:YAG ceramics sample under Toc = 10%. No saturation was observed on all the Yb:YAG ceramic lasers. Similar results were also reported by Dong et al. previously [11]. We believe that with further optimization on the doping concentration and sample thickness, higher output power and slope efficiency can be realized. 4. Conclusions In conclusion, high optical quality transparent Yb:YAG laser ceramics with different doping concentrations varying from 1.0 at.% to 20.0 at.% have been successfully fabricated by vacuum reactive sintering method. The in-line transmittance of the fabricated ceramic sample has reached 83.6% at 1300 nm and still remains 81.8% at 400 nm, indicating good optical quality of the samples. SEM studies have shown that the average grain size of the ceramic samples decreases with the increase of Yb doping concentration. The FWHM of the emission peaks at 1030 nm wavelength become broader with the increase of Yb doping concentration. CW lasing of the ceramic sample was demonstrated using a simple two-mirror cavity under the end-pumping of 940 nm fiber coupled laser diode.
939
Slope efficiency as high as 62.7% was achieved for the 5.0 at.% Yb:YAG ceramic with 10% transmittance output coupler. Acknowledgement The project is sponsored by the Natural Science Foundation of China under the Contract Numbers of 60928010 and 50902139. References [1] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, J. Am. Ceram. Soc. 78 (4) (1995) 1033. [2] A. Ikesue, Y. Lin, J. Am. Ceram. Soc. 89 (6) (2006) 1936. [3] J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A. Kaminskii, Opt. Lett. 32 (2007) 1890. [4] E. Innerhofer, T. Südmeyer, F. Brunner, R. Häring, A. Aschwanden, R. Paschotta, C. Hönninger, M. Kumkar, U. Keller, Opt. Lett. 28 (2003) 367. [5] K. Takaichi, H. Yagi, J. Lu, A. Shirakawa, K. Ueda, T. Yanagitani, Phys. Status Solidi A. 200 (2003) R5–R7. [6] J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 89 (2006) 091114. [7] S. Nakamura, H. Yoshioka, Y. Matsubara, T. Ogawa, S. Wada, Opt. Commun. 281 (2008) 4411. [8] J. Zhang, L.Q. An, M. Liu, S. Shimai, S. Wang, J. Eur. Ceram. Soc. 29 (2009) 305. [9] S. Lee, S. Kochawattana, G.L. Messing, J.Q. Dumm, G. Quarles, V. Castillo, J. Am. Ceram. Soc. 89 (6) (2006) 1945. [10] S. Nakamura, Y. Matsubara, T. Ogawa, S. Wada, Jpn. J. of Appl. Phys. 47 (4) (2008) 2149. [11] J. Dong, K. Ueda, H. Yagi, in: Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD), 2008, Optical Society of America, Paper CThFF3, 2008.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and laser properties of transparent Yb:YAG ceramics Dewei Luo a, Jian Zhang b,⇑, Changwen Xu a, Xianpeng Qin a,c, Dingyuan Tang a, Jan Ma b a
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Temasek Laboratories@NTU, Nanyang Technological University, Singapore 639798, Singapore c Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b
a r t i c l e
i n f o
Article history: Available online 13 May 2011 Keywords: Yb:YAG ceramics Laser properties Reactive sintering
a b s t r a c t High optical quality transparent Yb:YAG laser ceramics have been successfully fabricated by a vacuum reactive sintering method. Commercial Al2O3 powder and co-precipitated Y2O3 and Yb2O3 powders were used as the raw materials. In-line transmittances at 1300 nm and 400 nm were measured to be 83.6% and 81.8% respectively for a 3 mm thick mirror polished Yb:YAG ceramics sample. Continuous wave (CW) lasing at the wavelength of 1030 nm was achieved when pumped by a 940 nm fiber coupled laser diode. A slope efficiency as high as 62.7% was obtained. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth doped Yttrium aluminum garnet (YAG) ceramics have been a popular laser material since 1995 when Ikesue et al. achieved the first CW laser oscillation using a Nd:YAG ceramic as the gain medium [1]. The polycrystalline laser ceramics hold many advantages over their single crystal counterparts such as significantly shorter fabrication periods, lower fabrication cost, higher doping concentration, and the ability to be fabricated to large size with complicated structures [2]. In recent years, ytterbium doped YAG (Yb:YAG) ceramics have also attracted great attention due to the advantages such as high quantum efficiency, long fluorescence lifetime, broad emission spectrum and so on. Especially there’s no concentration quenching effect in Yb:YAG ceramics, so the Yb doping concentration could be quite high [3]. These advantages make Yb:YAG a promising material for high power thin disk laser and ultra-short pulse laser applications [4]. Takaichi et al. demonstrated a diode end-pumped Yb:YAG ceramics laser for the first time in 2003 [5]. Dong et al. demonstrated a Yb:YAG ceramics microchip laser with a slope efficiency as high as 79% in 2006 [6]. Excellent tunable Yb:YAG ceramics laser was reported by Nakamura et al. in 2008 [7]. However, there is not many reports on the fabrication of transparent Yb:YAG ceramics . In this research, vacuum reactive sintering method was employed to fabricate high optical quality transparent Yb:YAG ceramics with various doping concentration. The commercial Al2O3 powder and co-precipitated Y2O3 and Yb2O3 powders were used as the raw materials. The optical properties and microstructure
⇑ Corresponding author. Tel.: +65 67906835. E-mail address: [email protected] (J. Zhang). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.017
of the fabricated Yb:YAG ceramic samples were investigated. Their lasing performance was also characterized when pumped by a fiber coupled laser diode. 2. Experiments 2.1. Fabrication of Yb:YAG ceramics The Al2O3 powder (99.99% purity, Shanghai Wusong Chemical Co. Ltd.) that we used was pure a-phase. The primary particle size of the Al2O3 powder was around 0.25 lm. A reverse strike coprecipitation (add salt solution into the precipitant solution) method using nitrates and ammonia was adopted to prepare the Y2O3 and Yb2O3 powders. The detailed synthesis process was similar to the method described by Zhang et al. in previous research [8]. The Y2O3 precursor was calcined at 1000 °C/3 h in air. The primary particle size of the powder was around 60–80 nm and the specific surface area was around 9.5–10.0 m2/g. The powders were weighed precisely according to the chemical stoichiometry composition, at Yb3+ doping concentrations of 1.0 at.%, 5.0 at.%, 8.0 at.%, 10.0 at.%, 15.0 at.% and 20.0 at.% respectively and mixed with 99.99% ethanol (analytical pure). The mixed slurry was then ball milled using a planetary milling machine for 15 h. High purity alumina balls (5 mm diameter, Tosoh, Japan) were used. Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99.999%) was used to introduce SiO2 as sintering aids. The TEOS was doped at 0.5 wt.%, which corresponding to a 0.14 wt.% of SiO2. The powder mixtures were dried at 120 °C for 24 h in oven and then sieved through 200-mesh screen. After removing organic components by calcining at 800 °C for 3 h, the powders were dry pressed in a stainless steel die at 15 MPa. The green body pellets were further cold isostatically pressed (CIPed) at 200 MPa. After CIP, the relative green body density was around 53%. The green bodies were then sintered at
D. Luo et al. / Optical Materials 34 (2012) 936–939
Fig. 1. Schematic diagram of the end-pumped Yb:YAG ceramic laser.
937
After grinded by a Hyprez horizontal grinding machine (Model: EHG-150, Engis, Japan) and then fine polished on both surfaces by a Hyprez precision lapping machine (Model: EJW400-IN-D, Engis, Japan), highly transparent Yb:YAG ceramic samples were obtained. The room temperature transmittance spectra were measured with a UV–VIS–NIR spectrometer (Carry 5000 Spectrophotometer, Varian, USA). The room temperature emission spectra were recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edison, USA) when samples were excited by a 940 nm laser diode. Scanning electron microscope (SEM, JSM-6360A, JEOL, Tokyo, Japan) was used to exam sample’s microstructure. 2.2. CW laser experiments
Fig. 2. Optical transmittance spectra of 1at.% doped Yb:YAG ceramics (3 mm thick). Inset: Yb:YAG ceramics before (green) and after (colorless) annealing process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
1770 °C for 8 h in a high temperature vacuum sintering furnace under 10 3 Pa vacuum condition. The sintered pellets were then annealed in air at 1400 °C for 35 h to completely remove internal stress and eliminate the oxygen vacancies.
An end-pump scheme shown in Fig. 1 was used for the laser experiment. A simple two-mirror cavity with a cavity length of 15 mm was set up. The input mirror was a plano mirror and the output coupler had a radius of curvature (ROC) of 50 mm. The pump source was a 25 W 940 nm fiber coupled laser diode (Limo-25-f100-dl940) with a fiber core diameter of 100 lm. The pump beam was focused on the ceramic sample with lens of a focal length of 30 mm. The ceramic sample was cut into the dimensions of 3 3 3.2 mm3 and antireflection coated on both (3 3 mm2) sides at the pump and lasing wavelength to reduce the reflection loss. The sample was wrapped with indium foil, placed in a copper holder which was water cooled at 18 °C. Two different output couplers with transmittances of 4.2% and 10.0% at the lasing wavelength respectively were used for the ceramic laser. 3. Results and discussion Fig. 2 shows the room temperature optical transmittance of a 1.0 at.% Yb:YAG sample with 3 mm in thickness. The photo of asfabricated Yb:YAG ceramics before (green) and after (colorless) annealed in air is shown in the inset of the figure. The higher the Yb doping concentration is, the longer the annealing time is needed to
Fig. 3. SEM photographs of 1.0 at.% (a) 8.0 at.% (b) 15.0 at.% (c) and 20.0 at.% (d) Yb:YAG ceramics.
938
D. Luo et al. / Optical Materials 34 (2012) 936–939
Fig. 4. The absorption spectra of fabricated Yb:YAG ceramics with different Yb doping concentration.
Fig. 5. The normalized emission spectra of fabricated Yb:YAG ceramics with different Yb doping concentration.
completely eliminate color center defects. For un-annealed sample, two broad absorption bands at 375 nm and 625 nm are appeared, which are ascribed to Re-F color centers formed during vacuum sintering process. This further confirms the presence of Yb2+ formed during the vacuum sintering. The above two absorption bands completely disappeared after the annealing process. For annealed sample, the in-line transmittance has reached 83.6% at 1300 nm and still remains 81.8% at 400 nm, indicating good optical quality of the ceramic samples. In order to exam their microstructure, the surfaces of samples were mirror-polished and then thermally etched at 1500 °C for 2 h to reveal its grain boundary. SEM photographs of surfaces of samples with different doping concentration are displayed in Fig. 3. They exhibit a pore-free structure. The average grain size decreased from 22.1 lm to 11.7 lm with the increasing of Yb doping concentration from 1.0 at.% to 20.0 at.%. The grain size could be further controlled by adjusting the sintering time, sintering temperature and the sintering aids during the reactive sintering process. Related research has also been reported by Lee et al. [9]. The absorption spectra from 810 nm to 1100 nm of the fabricated Yb:YAG ceramics with different doping concentrations are plotted in Fig. 4. All the samples have a very strong absorption peak located at 940 nm and two moderate absorption peaks located at 915 nm and 969 nm respectively, corresponding to the 2 F7=2 ? 2 F5=2 transition of Yb3+. The absorption coefficient at 940 nm increases from 1.06 cm 1 of the 1.0 at.% Yb doping concentration to 13.87 cm 1 of the 15.0 at.% doping concentration. Re-absorption at 1030 nm lasing wavelength is also clearly observed, which is attributed to the quasithree-level nature of ytterbium-doped gain media. The normalized room temperature emission spectra of samples with different Yb doping concentration are shown in Fig. 5. The full width at the half maximum (FWHM) of the 1030 nm wavelength emission peaks are 8.40 nm, 11.07 nm, 12.25 nm, 16.31 nm and 19.98 nm for samples with Yb doping concentration of 1.0 at.%, 5.0 at.%, 8.0 at.%, 15.0 at.% and 20.0 at.%, respectively. It is apparent that the emission spectra become broader with the increase of Yb doping concentration. The broad emission band is especially useful in mode-locked laser for ultra-short pulse generation. Besides the emission peak at 1030 nm, another emission peak at 1047 nm wavelength exhibits an increased intensity with higher Yb doping concentration. This could result a change of lasing wavelength and even dual-wavelength operation. Dong et al. [6] and Nakamura et al. [10] had also observed similar phenomenon previously. Fig. 6 shows the CW laser output power versus the absorbed pump power of samples with different Yb doping concentrations
Fig. 6. Laser performance of Yb:YAG with a 4.2% output coupler (a) and 10.0% output coupler (b).
D. Luo et al. / Optical Materials 34 (2012) 936–939
under two different cavity output couplings (Toc). The laser threshold for the 5.0 at.%, 8.0 at.%, and 15.0 at.% Yb:YAG ceramics were 1.31 W, 1.64 W and 2.08 W under Toc = 4.2% and 1.3 W, 2.16 W and 4.21 W under Toc = 10% , respectively. The output power increases with the increasing of the absorbed pump power. A maximum output power of 6.9 W was achieved on a 8.0 at.% sample under Toc = 4.2%. A slope efficiency as high as 62.7% was obtained for a 5.0 at.% Yb:YAG ceramics sample under Toc = 10%. No saturation was observed on all the Yb:YAG ceramic lasers. Similar results were also reported by Dong et al. previously [11]. We believe that with further optimization on the doping concentration and sample thickness, higher output power and slope efficiency can be realized. 4. Conclusions In conclusion, high optical quality transparent Yb:YAG laser ceramics with different doping concentrations varying from 1.0 at.% to 20.0 at.% have been successfully fabricated by vacuum reactive sintering method. The in-line transmittance of the fabricated ceramic sample has reached 83.6% at 1300 nm and still remains 81.8% at 400 nm, indicating good optical quality of the samples. SEM studies have shown that the average grain size of the ceramic samples decreases with the increase of Yb doping concentration. The FWHM of the emission peaks at 1030 nm wavelength become broader with the increase of Yb doping concentration. CW lasing of the ceramic sample was demonstrated using a simple two-mirror cavity under the end-pumping of 940 nm fiber coupled laser diode.
939
Slope efficiency as high as 62.7% was achieved for the 5.0 at.% Yb:YAG ceramic with 10% transmittance output coupler. Acknowledgement The project is sponsored by the Natural Science Foundation of China under the Contract Numbers of 60928010 and 50902139. References [1] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, J. Am. Ceram. Soc. 78 (4) (1995) 1033. [2] A. Ikesue, Y. Lin, J. Am. Ceram. Soc. 89 (6) (2006) 1936. [3] J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A. Kaminskii, Opt. Lett. 32 (2007) 1890. [4] E. Innerhofer, T. Südmeyer, F. Brunner, R. Häring, A. Aschwanden, R. Paschotta, C. Hönninger, M. Kumkar, U. Keller, Opt. Lett. 28 (2003) 367. [5] K. Takaichi, H. Yagi, J. Lu, A. Shirakawa, K. Ueda, T. Yanagitani, Phys. Status Solidi A. 200 (2003) R5–R7. [6] J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 89 (2006) 091114. [7] S. Nakamura, H. Yoshioka, Y. Matsubara, T. Ogawa, S. Wada, Opt. Commun. 281 (2008) 4411. [8] J. Zhang, L.Q. An, M. Liu, S. Shimai, S. Wang, J. Eur. Ceram. Soc. 29 (2009) 305. [9] S. Lee, S. Kochawattana, G.L. Messing, J.Q. Dumm, G. Quarles, V. Castillo, J. Am. Ceram. Soc. 89 (6) (2006) 1945. [10] S. Nakamura, Y. Matsubara, T. Ogawa, S. Wada, Jpn. J. of Appl. Phys. 47 (4) (2008) 2149. [11] J. Dong, K. Ueda, H. Yagi, in: Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, OSA Technical Digest (CD), 2008, Optical Society of America, Paper CThFF3, 2008.