Open Access
Add to BibSonomyAbstract
1.6 μm eye-safe emission of a diode pumped Er,Yb co-doped YAG ceramic laser is firstly demonstrated. Operation of the ceramic laser under different ceramic sample lengths, co-doping concentrations and control temperatures were experimentally investigated. A maximum output power of 222 mW was achieved at an absorbed pump power of 8.1 W, corresponding to a conversion efficiency of 2.74%. Laser emission at 1.05 μm for transition of the Yb3+ ions was also studied on the same ceramic samples. The results clearly show the existence of resonantly energy transfer from the Yb3+ ions to Er3+ ions.
© 2013 Optical Society of America
1. Introduction
Coherent light with wavelength around 1.5-1.6 μm has the features of eye safe, transparent in the atmosphere, and strong absorption by water molecules. They are attractive for many applications such as range-finding, environmental sensing, telecommunications, and medical surgery. The 4I13/2-4I15/2 transition of the Er3+ ions emits laser beams in the wavelength region. In addition, the nonlinear frequency conversion techniques, such as the stimulated Raman scattering [1,2], optical parametric oscillators [3,4] could also be used to generate coherent beam with these wavelengths. Comparing the above two approaches, generation of the light beams directly from the laser transition of the Er3+ ions has the advantages of high efficiency, high reliability, compact and simple setup. However, the Er3+ doped gain materials have the small absorption coefficients in the wavelength range of 900-1000 nm where high power laser diodes are easily available [5].
Different from Er3+, Yb3+ has a wider absorption band and a larger absorption cross-section around 900-1000 nm. Previous experiments have shown that if co-doping these two ions in the gain medium, the pump energy absorbed by the Yb3+ could be resonantly transferred to the Er3+ ions and excite it to the 4I11/2 level [6]. As most Er3+ in the 4I11/2 level will quickly relax nonradiatively to the 4I13/2 level, which decreases the back energy transfer from Er3+ to Yb3+, lasing can be achieved at the 1.6 μm by the transition of the Er3+ ions between the 4I13/2-4I15/2 levels. In 1995, Schweizer et al firstly demonstrated the 1.6 μm laser emission of an Er,Yb:YAG crystal laser [7]. Georgiou et al also realized high energy 1.65 μm output of an Er,Yb:YAG crystal pumped by qusi-cw laser diode arrays [8]. Recently, 1.6 μm laser emission had been obtained in diode-pumped Er, Yb co-doped YVO4 [9,10], YCOB [11], YAB [12], GAB [13] crystals.
Laser transparent ceramics have the advantages of easy to fabricate, feasible of large dimensions and composite structures. They have recently attracted considerable attention of research. Various rare-earth doped laser ceramics have been successfully fabricated and their laser performance were investigated. Zhou et al have first studied the optical properties of the Er,Yb co-doped YAG transparent ceramics [14]. However, to the best of our knowledge, so far no laser emission of the Er,Yb-codoped YAG ceramic has been reported. In this letter, we demonstrate the 1.6 μm emission of a diode pumped Er,Yb co-doped YAG ceramic laser. Operation of the ceramic laser under different ceramic lengths, co-doping concentrations and control temperatures were experimentally investigated. The maximum output power of 222 mW was achieved at the absorbed pump power of 8.1 W. Laser emission at 1.05 μm for transition of the Yb3+ ions was also studied on the same ceramic samples.
2. Material and experimental setup
The Er,Yb:YAG ceramics used were fabricated in-house using the solid-state reactive sintering under vacuum condition. The fabrication method was similar to that used to fabricate the Yb:YAG ceramics [15]. For comparison four YAG ceramic samples with different Er,Yb co-doping concentrations were fabricated. The samples (thickness = 3 mm) mirror polished on both surfaces were used to measure in-line optical transmittances by UV-Vis-NIR spectrophotometer (Cary 5000, Agilent). Figure 1 shows the transmission spectrum of Er,Yb:YAG ceramic over the wavelength range of 200–1900 nm at room temperature. The Er,Yb:YAG ceramics have an optical transmittance of about 84% in the nonabsorption regions. Figure 2 shows the absorption coefficient of the fabricated ceramic samples derived from the measured absorption of the samples. From Fig. 2 we can see that the absorption coefficient around the 900 nm varies with the Yb3+ concentrations, while the absorption coefficient around the 1500 nm varies with the Er3+ concentrations. The maximum absorption coefficient of the sample with 5% Yb3+ doping concentration at 940 nm is about 5.9 cm−1, and the absorption coefficient of the sample with 1 at.% Er3+ doping concentration at 1532 nm is about 1.74 cm−1.
The laser experiments were carried out on a plane-concave cavity laser with a cavity length of about 16 mm, as shown in Fig. 3. A flat mirror coated high transmission (HT, T = 96%) at 940 nm and high reflection (HR, R>99.8%) at 1.6 μm was used as the pump input mirror (IM). Two output couplers (OC) with the same radius of curvature of 100 mm but different transmissions at 1.6 μm were used. Reflectivity of both the output couplers at 940 nm is higher than 87%. The Er,Yb:YAG ceramics with different Er,Yb co-doping concentrations were studied. The ceramic samples were cut into dimensions of 3 × 3 × 5 mm3 and 3 × 3 × 3 mm3, respectively. Both facets of the cut ceramics were polished and anti-reflection (AR) coated at 940, 1600-1650 nm. The peripheral sides of the ceramics were wrapped with indium foil. The ceramic was mounted in a water-cooled copper block whose temperature was controlled at about 298K. The diode laser with the center wavelength at 940 nm was fibre-coupled. The delivery fiber has a core diameter of 100 μm and a numerical aperture of 0.22. The pump light was re-imaged into the Er,Yb:YAG ceramic with a spot size of about 160 μm in diameter by a simple telescopic lens system.
Fig. 3 Experimental setup for LD end-pumped Er,Yb:YAG ceramic laser: IM, input mirror; OC, output coupler.
3. Experimental results and discussion
First, the YAG ceramic with 1.0% Er, 5.0% Yb co-doping and 3 × 3 × 3 mm3 in size was investigated. Output couplers with either transmissions of T = 1.9% at 1615 nm & T = 2.3% at 1645 nm or T = 0.6% at 1615 nm & T = 0.4% at 1645 nm were used, respectively. The output powers and spectra of the 1.6 μm laser emissions are shown in Fig. 4. With both output couplers the laser threshold was around 1.0 W. Under low pump power, only the 1645 nm laser emission was detected, while as the pump power was increased dual-wavelength laser emission at both 1615 nm and 1645 nm was eventually achieved. The insets of the figure show the laser emission spectra measured at the maximum incident pump power of 9.4 W under each of the two output couplers. The ratio of the 1645 nm laser emission was larger when the output coupler with larger transmission was used. Using the output coupler with T = 0.6% at 1615 nm & T = 0.4% at 1645 nm, only 90 mW output power was obtained, and the laser tended to oscillate at dual wavelength. A maximum output power of 135 mW was obtained at an incident pump power of 9.4 W when the output coupler with T = 1.9% at 1615 nm & T = 2.3% at 1645 nm was used. Therefore, for the further experiments we have only used the output coupler with T = 1.9% at 1615 nm & T = 2.3% at 1645 nm.
Fig. 4 Output powers and spectra (inset) for 1.6 μm laser versus incident pump power using different output couplers.
We further investigated the laser operation under different Er,Yb:YAG ceramic sample lengths (3 mm and 5 mm) and co-doping concentrations (1.0%Er & 8.0%Yb, 1.0%Er & 5.0% Yb, 0.5%Er & 5.0%Yb and 2.0%Er & 5.0%Yb). The results are shown in Fig. 5. Laser emission had only been achieved on the 1.0%Er, 5.0%Yb, and 0.5%Er, 5.0%Yb co-doped YAG ceramic samples. When the incident pump power is below 7 W, the sample with low Er3+ doping concentration (0.5%Er, 5.0%Yb) and short length (3 mm) had the lowest output power. Samples with longer length and higher Er -doping concentration had larger output power. However, high Er-doping concentration could result in strong up-conversion and reabsorption which is unfavorable for the 1.6 μm laser emission with quasi-three-level system. Despite that higher output power was obtained at pump power blow 7W on samples with 1%Er, 5.0%Yb co-doping, the output power became saturated as the pump power increased above 7W. Therefore, for high Er-doping concentrations sample, the Er3+ up-conversion and reabsorption became serious with the increase of the 1.6 μm laser oscillation. The sample with 0.5%Er, 5.0%Yb co-doping and 5 mm length had achieved the maximum output of 197 mW under an incident pump power of 9.4 W. The absorbed pump power was mainly determined by the Yb doping concentration and the sample length. Under the maximum incident pump power of 9.4 W the absorbed pump power for the 3 mm and 5 mm samples were 6.8 W and 8.1 W, respectively.
Fig. 5 Output powers of the 1.6 μm laser versus incident pump power using different Er,Yb:YAG ceramic samples.
The operation of the 0.5%Er, 5.0%Yb co-doped YAG ceramic (5 mm in length) under different cooling temperatures of the sample was also investigated. Figure 6 shows the output power of the laser at 1.6 μm as a function of the absorbed pump power when the cooling temperature of the ceramic was set as 278, 283, 290, and 298 K. The output power increased with the reduction of cooling temperature. The output power increased from 197 mW at 298 K to 222 mW at 278 K with the absorbed pump power of 8.1W, corresponding the conversion efficiency of 2.74%.
Fig. 6 Output powers of the 1.6 μm laser versus absorbed pump power with different control temperature of ceramic.
We had also studied laser oscillation of the Er,Yb co-doped YAG ceramic samples under the Yb3+ transitions. To this end the laser cavity mirrors were replaced with those suitable for the diode pumped Yb:YAG lasers. Only the ceramic samples with 3 mm length and different co-doping concentrations were investigated. Again a hemispherical cavity with the same cavity length and an output coupler with 6% transmission at 1.05 μm were used. The results are shown in Fig. 7. Laser emission had only obtained on the sample with 0.5%Er, 5.0%Yb co-doping. A maximum output power of 1.02 W at 1.05 μm was obtained at an incident pump power of 9.4 W, which is much lower than that obtained on a Yb:YAG ceramic laser with the same Yb3+ doping concentration. The worse performance of the Er,Yb:YAG ceramic laser comparing with that of the Yb:YAG ceramic laser at 1.05 μm could be explained as a result of the resonantly energy transfer of the Yb3+ ions to the Er3+ ions in the co-doped ceramics, which is an serious loss mechanism to the laser transition of the Yb3+ ions.
Fig. 7 Output power of the 1.05 μm laser versus incident pump power using different Er,Yb:YAG ceramic samples.
4. Conclusion
In conclusion, we have demonstrated an diode-end-pumped Er,Yb co-doped YAG ceramic laser at 1.6 μm. Operation of the ceramic laser under different ceramic sample lengths, Er,Yb co-doping concentrations and ceramic sample cooling temperatures were experimentally investigated. A maximum output power of 222 mW was achieved at an absorbed pump power of 8.1 W, corresponding to a conversion efficiency of 2.74%. We will try to increase overall conversion efficiency by improving of the optical quality and co-doping concentrations of our ceramic in the further work. Laser oscillation at 1.05 μm could also be achieved on the Er,Yb codoped ceramic sample, but with degraded performance as compared with the Yb:YAG ceramic lasers of the same Yb concentration. The result clearly show the existence of resonantly energy transfer from the Yb3+ ions to the Er3+ ions inside the co-doped ceramic samples.
Acknowledgments
This work was supported by the National Science Foundation of Zhejiang Province under Grant no. LY12F05003 and LQ13F050004, High-level Talent Innovation Technology Project Funding of Wenzhou, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References and links
1. Y. T. Chang, K. W. Su, H. L. Chang, and Y. F. Chen, “Compact efficient Q-switched eye-safe laser at 1525 nm with a double-end diffusion-bonded Nd:YVO4 crystal as a self-Raman medium,” Opt. Express 17(6), 4330–4335 (2009). [CrossRef] [PubMed]
2. X. H. Chen, X. Y. Zhang, Q. P. Wang, P. Li, Z. J. Liu, Z. H. Cong, L. Li, and H. J. Zhang, “Highly efficient double-ended diffusion-bonded Nd:YVO4 1525-nm eye-safe Raman laser under direct 880-nm pumping,” Appl. Phys. B 106(3), 653–656 (2012). [CrossRef]
3. H. Y. Zhu, G. Zhang, H. B. Chen, C. H. Huang, Y. Wei, Y. M. Duan, Y. D. Huang, H. Y. Wang, and G. Qiu, “High-efficiency intracavity Nd:YVO4\KTA optical parametric oscillator with 3.6 W output power at 1.53 μm,” Opt. Express 17(23), 20669–20674 (2009). [CrossRef] [PubMed]
4. H. T. Huang, D. Y. Shen, J. L. He, H. Chen, and Y. Wang, “Nanosecond nonlinear Čerenkov conical beams generation by intracavity sum frequency mixing in KTiOAsO4 crystal,” Opt. Lett. 38(4), 576–578 (2013). [CrossRef] [PubMed]
5. N. W. H. Chang, N. Simakov, D. J. Hosken, J. Munch, D. J. Ottaway, and P. J. Veitch, “Resonantly diode-pumped continuous-wave and Q-switched Er:YAG laser at 1645 nm,” Opt. Express 18(13), 13673–13678 (2010). [CrossRef] [PubMed]
6. Y. J. Chen, Y. F. Lin, X. H. Gong, Q. G. Tan, Z. D. Luo, and Y. D. Huang, “2.0 W diode-pumped Er:Yb:YAl3(BO3)4 laser at 1.5-1.6μm,” Appl. Phys. Lett. 89(24), 241111 (2006). [CrossRef]
7. T. Schweizer, T. Jensen, E. Heumann, and G. Huber, “Spectroscopic properties and diode pumped 1.6μm laser performance in Yb-codoped Er: Y3A15O12 and Er: Y2SiO5,” Opt. Commun. 118(5-6), 557–561 (1995). [CrossRef]
8. B. I. Galagan, B. I. Denker, V. V. Osiko, and S. E. Sverchkov, “Efficiency of population of the 4I13/2 level of the Er3+ ion and the possibility of lasing at 1.5μm in Yb, Er:YAG at high temperatures,” Quantum Electron. 37(10), 971–973 (2007). [CrossRef]
9. Y. H. Tsang and D. J. Binks, “Record performance from a Q-switched Er3+:Yb3+:YVO4 laser,” Appl. Phys. B 96(1), 11–17 (2009). [CrossRef]
10. M. J. Wang, L. Zhu, J. Zhou, W. B. Chen, and D. Y. Fan, “Performance of an actively Q-switched Er3+:Yb3+:YVO4 laser,” Laser Phys. Lett. 10(8), 085806 (2013). [CrossRef]
11. P. A. Burns, J. M. Dawes, P. Dekker, J. A. Piper, H. Jiang, and J. Wang, “Optimization of Er, Yb:YCOB for CW laser operation,” IEEE J. Quantum Electron. 40(11), 1575–1582 (2004). [CrossRef]
12. Y. J. Chen, Y. F. Lin, X. H. Gong, J. H. Huang, Z. D. Luo, and Y. D. Huang, “Acousto-optic Q-switched self-frequency-doubling Er:Yb:YAl3(BO3)4 laser at 800 nm,” Opt. Lett. 37(9), 1565–1567 (2012). [CrossRef] [PubMed]
13. K. N. Gorbachenya, V. E. Kisel, A. S. Yasukevich, V. V. Maltsev, N. I. Leonyuk, and N. V. Kuleshov, “Highly efficient continuous-wave diode-pumped Er, Yb:GdAl3(BO3)4 laser,” Opt. Lett. 38(14), 2446–2448 (2013). [CrossRef] [PubMed]
14. J. Zhou, W. X. Zhang, T. D. Huang, L. Wang, J. Li, W. B. Liu, B. X. Jiang, Y. B. Pan, and J. K. Guo, “Optical properties of Er,Yb co-doped YAG transparent ceramics,” Ceram. Int. 37(2), 513–519 (2011). [CrossRef]
15. D. W. Luo, J. Zhang, C. W. Xu, X. P. Qin, D. Y. Tang, and J. Ma, “Fabrication and laser properties of transparent Yb:YAG ceramics,” Opt. Mater. 34(6), 936–939 (2012). [CrossRef]
