A resonantly-pumped tunable Q-switched Ho:YAG ceramic laser with diffraction-limit beam quality Lei Wang,1 Chunqing Gao,1,* Mingwei Gao,1 Yan Li,1 Fuyong Yue,1 Jian Zhang,2 and Dingyuan Tang2 2
1 School of Opto-Electronics, Beijing Institute of Technology, Beijing 100081, China School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 22116, China * [email protected]
Abstract: We demonstrated a Q-switched Ho:YAG ceramic laser operating at 2097 nm. The Ho:YAG ceramic laser was resonantly pumped by a Tm:YLF laser at 1908 nm. The laser performance with two Ho-doping concentrations of Ho:YAG ceramics in a U-shaped resonator was studied. Different pump spots were investigated to obtain high extract efficiency. The wavelength of Ho:YAG ceramic laser was tuned from 2090.70 nm to 2098.10 nm. The Q-switched pulse energy were 9.6 mJ at a pulse repetition frequency (PRF) of 200 Hz and 10.2 mJ at a PRF of 100 Hz, respectively. The beam quality M2 factors were measured to be less than 1.1 in both directions. © 201 Optical Society of America OCIS codes: (140.3460) Lasers; (140.3540) Lasers, Q-switched; (140.3600) Lasers, tunable; (160.5690) Rare-earth-doped materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
O. L. Antipova, N. G. Zakharova, M. Fedorov, N. M. Shakhova, N. N. Prodanets, L. B. Snopova, V. V. Sharkov, and R. Sroka, “Cutting effects induced by 2 μm laser radiation of CW Tm:YLF and CW and Q-switcheded Ho:YAG lasers on ex-vivo tissue,” Med. Laser Appl. 26(2), 67–75 (2011). Y. Pan, Y. Wang, X. Li, X. Xiong, Y. Wang, S. Li, Q. Luo, J. Zhu, and W. Zhang, “Ho:YAG laser application in cerebellopontine angle tumor operation,” Proc. SPIE 5967, 59671R (2005). G. D. Spiers, R. T. Menzies, J. Jacob, L. E. Christensen, M. W. Phillips, Y. Choi, and E. V. Browell, “Atmospheric CO2 measurements with a 2 μm airborne laser absorption spectrometer employing coherent detection,” Appl. Opt. 50(14), 2098–2111 (2011). S. Haidar, K. Miyamoto, and H. Ito, “Generation of tunable Mid-IR (5.5-9.3μm) from a 2-μm pumped ZnGeP2 optical parametric oscillator,” Opt. Commun. 241(1–3), 173–178 (2004). E. Lippert, H. Fonnum, G. Arisholm, and K. Stenersen, “A 22-watt mid-infrared optical parametric oscillator with V-shaped 3-mirror ring resonator,” Opt. Express 18(25), 26475–26483 (2010). S. Lamrini, P. Koopmann, M. Schäfer, K. Scholle, and P. Fuhrberg, “Efficient high-power Ho:YAG laser directly in-band pumped by a GaSb-based laser diode stack at 1.9 μm,” Appl. Phys. B 106(2), 315–319 (2012). X. M. Duan, B. Q. Yao, C. W. Song, J. Gao, and Y. Z. Wang, “Room temperature efficient continuous wave and Q-switched Ho:YAG laser double-pass pumped by a diode-pumped Tm:YLF laser,” Laser Phys. Lett. 5(11), 800–803 (2008). P. A. Budni, C. R. Ibach, S. D. Setzler, E. J. Gustafson, R. T. Castro, and E. P. Chicklis, “50-mJ, Q-switched, 2.09-microm holmium laser resonantly pumped by a diode-pumped 1.9-microm thulium laser,” Opt. Lett. 28(12), 1016–1018 (2003). W. X. Zhang, J. Zhou, W. B. Liu, J. Li, L. Wang, B. X. Jiang, Y. B. Pan, X. J. Cheng, and J. Q. Xu, “Fabrication, properties and laser performance of Ho:YAG transparent ceramic,” J. Alloy. Comp. 506(2), 745–748 (2010). X. J. Cheng, J. Q. Xu, M. J. Wang, B. X. Jiang, W. X. Zhang, and Y. B. Pan, “Ho:YAG ceramic laser pumped by Tm:YLF lasers at room temperature,” Laser Phys. Lett. 7(5), 351–354 (2010). H. Chen, D. Shen, J. Zhang, H. Yang, D. Tang, T. Zhao, and X. Yang, “In-band pumped highly efficient Ho:YAG ceramic laser with 21 W output power at 2097 nm,” Opt. Lett. 36(9), 1575–1577 (2011). H. Yang, J. Zhang, X. Qin, D. Luo, J. Ma, D. Tang, H. Chen, D. Shen, and Q. Zhang, “Polycrystalline Ho:YAG transparent ceramics for eye-safe solid state laser applications,” J. Am. Ceram. Soc. 95(1), 52–55 (2012). L. Wang, C. Gao, M. Gao, and Y. Li, “Resonantly pumped monolithic nonplanar Ho:YAG ring laser with highpower single-frequency laser output at 2122 nm,” Opt. Express 21(8), 9541–9546 (2013).
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 254
14. N. P. Barnes, “Solid-state lasers from an efficiency perspective,” IEEE J. Sel. Top. Quantum Electron. 13(3), 435–447 (2007). 15. L. Zhu, M. Wang, J. Zhou, and W. Chen, “Efficient 1645 nm continuous-wave and Q-switched Er:YAG laser pumped by 1532 nm narrow-band laser diode,” Opt. Express 19(27), 26810–26815 (2011). 16. N. P. Barnes and B. M. Walsh, “Amplified spontaneous emission-application to Nd:YAG lasers,” IEEE J. Sel. Top. Quantum Electron. 35(1), 101–109 (1999).
1. Introduction High-energy Q-switched lasers with eye-safe wavelengths around 2 μm are widely used in fields of medicine, coherent Doppler Lidar, differential absorption Lidar, etc [1–3]. Moreover, high-energy 2 μm lasers are attractive candidates for pumping mid-infrared optical parametric oscillator to generate 3~5 μm lasers [4, 5]. Last decade, resonantly pumped Ho-doped laser has attracted great attentions due to its low quantum defect and high efficiency. Ho:YAG single crystal has been widely studied to obtain continuous-wave(CW) and Q-switched operation in 2 μm wavelengths [6–8]. Recently, more and more scientists are interested in Ho:YAG ceramic, whose thermomechanical properties are as good as Ho:YAG single crystal. Moreover, Ho:YAG ceramic has many advantages compared with Ho:YAG single crystal, such as ease of fabrication, short fabrication time, low cost, mass production, and feasibility of large size [9]. In 2010, X. J. Cheng et al. reported a Ho:YAG ceramic laser pumped by a Tm:YLF laser [10]. The maximum output power of 1.2 W at 2.09 μm was obtained with absorbed pump power of 5 W. In 2011, H. Cheng et al. demonstrated a high-power polycrystalline Ho:YAG ceramic laser in-band pumped by a Tm:fiber laser [11]. The maximum output power of 21.4 W was obtained under 35 W of incident pump power. In 2012, H. Yang et al. reported a Ho:YAG ceramic slab in-band pumped by a 1907 nm Tm:fiber laser and 20.6 W output power at 2097 nm was obtained [12]. In this paper, we demonstrate a Q-switched Ho:YAG ceramic laser at 2097 nm, resonantly pumped by a Tm:YLF laser. Up to 9.6 mJ Q-switched energy at 200 Hz was obtained from a 0.8 at.% Ho:YAG ceramic laser, with beam quality M2 factors less than 1.1 in both directions. To our knowledge, this is the first time to report a Q-switched Ho:YAG ceramic laser operating at 2097 nm with high energy and good beam quality. 2. Experimental setup Figure 1 shows the experimental setup of the Ho:YAG ceramic laser resonantly pumped by a 1908 nm Tm:YLF laser built by ourselves. The Tm:YLF laser is the same with the pump laser reported in [13].The beam from the Tm:YLF laser was then collimated and focused into the Ho:YAG ceramic by spherical lenses with focal lengths of −50 mm and 100 mm. The focal lengths of the spherical lenses were changed to investigate the influence of the pump beam spot size on the Ho:YAG laser efficiency. As for this reason, we changed the pump spot size to match the laser mode in the resonator.
Fig. 1. Experimental setup of the Ho:YAG ceramic laser pumped by a Tm:YLF laser.
Two kinds of Ho:YAG ceramics were used in the experiments, one with a Ho-doping concentration of 0.8 at.% and a length of 25 mm, and the other with 1.0 at.% Ho-doping
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 255
concentration and a length of 20 mm. The diameters of two Ho:YAG ceramics were 4 mm. Both ends of Ho:YAG ceramics were anti-reflection coated at 1908 nm and 2097 nm. The 45° dichroic mirrors (DM) has an anti-reflection coating at 1908 nm and high-reflection coating at 2097 nm. The transmittance of DM for the pump beam is about 96.4%. A 0.1-mm-thick uncoated etalon was inserted into the resonator to tune the wavelength of the Ho:YAG ceramic laser. For Q-switched operation, an acousto-optic modulator (AOM) (QSWITCH/33027-100-5-2-I-HGM-W, NEOS) was used inside the resonator. An U-shape resonator was applied to the Ho:YAG laser for avoiding the optical feedback of the pump laser. The total resonator length of the Ho:YAG ceramic laser was around 950 mm. The radius of curvature of the output mirror was 750 mm and the transmittance was 30% at 2097 nm. The high-reflective mirror has an anti-reflection coating at 1908 nm and high-reflection coating at 2097 nm. The radius of curvature of the high-reflective mirror was 500 mm. The laser beam radius in the middle of the Ho:YAG ceramic was calculated to be 423 μm by using the software of LASCAD with consideration of the thermal lens. And we also design other cavity using many other cavity mirror with different curvatures and output coupling. But the 45°DM mirror was damaged when we used lower output coupling and smaller resonator mode. We report the best result with the optimized cavity parameters. The temperature of the Ho:YAG ceramic was controlled at 20°C by using a thermal electric cooler (TEC). 3. Experimental results The CW output power and average output power of the 1.0 at.% Ho:YAG ceramic laser with the etalon inside the resonator as functions of the incident pump power are shown in Fig. 2. And the losses introduced by the etalon were about 1%. When the pump diameter was 294 μm, the maximum CW output power was 4.62 W with a slope efficiency of 41.3%. For 441 μm and 588 μm pump diameters, the maximum CW output power were 4.38 W and 4.17 W, respectively. In Q-switched operation, the maximum average output power of 1.90 W was obtained with the pump diameter of 588 μm when the pulse repetition frequency (PRF) was 200 Hz. And the maximum average output powers were 1.88 W and 1.70 W when the pump beam diameters were 441 μm and 294 μm, respectively. We also measured the output powers of the 0.8 at.% Ho:YAG ceramic laser with different pump diameters. The trend of experimental results of 0.8 at.% Ho:YAG ceramic laser is similar to that of 1.0 at.% Ho:YAG ceramic laser. Therefore, highest energy extract efficiency can be achieved with the biggest pump diameter.
Fig. 2. CW output power and average output power of 1.0 at.% Ho:YAG ceramic laser with different pump spot diameters. For the Q-switched operation, the pulse repetition frequency was 200 Hz.
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 256
The CW lasing performance of 0.8 at.% and 1.0 at.% Ho:YAG ceramics were compared when the pump diameter was 588 μm. Figure 3 shows CW output power of 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers with the etalon inside the resonator. For 0.8 at.% Ho:YAG ceramic, CW output power of 4.48 W was obtained with a slope efficiency of 41.5% with respect to the incident pump power. The maximum CW output power of 4.17 W was obtained from 1.0 at.% Ho:YAG ceramic with a slope efficiency of 39.2% with respect to the incident pump power. The pump absorption efficiencies for 0.8 at.% and 1.0 at.% Ho:YAG ceramic in the single pass pump scheme are 86.8%% and 86.4%, respectively. Therefore, the slope efficiencies for 0.8 at.% and 1.0 at.% Ho:YAG ceramic are 47.1% and 46.3% with respect to the absorbed pump power, respectively. Bigger resonator mode volume can reduce the risk of damaging optical components. To get good overlap between the pump mode and the resonator mode, we have increased the pump mode. But bigger pump mode volume also induces lower pump intensity, and increasing the pump threshold and decreasing the slope efficiency. Furthermore, 45° dichroic mirrors also lead to slope efficiency reduction due to immature coating process.
Fig. 3. CW output power of Ho:YAG ceramic laser verse incident pump power with the pump diameter of 588 μm.
We investigated the wavelength tuning of two Ho:YAG ceramic lasers by changing the angle of the intra-cavity etalon. The wavelength of the Ho:YAG ceramic laser was measured by using an EXFO WA-650 spectrum analyzer combined with a WA-1000 wavemeter with a resolution of 10 pm. Figure 4 shows the measured results of the output power at different wavelengths. For the 0.8 at.% and 1.0 at.% Ho:YAG ceramics, the incident pump power was kept at 16.67 W and 9.02 W, respectively. The laser wavelength of 0.8 at.% Ho:YAG ceramic was tuned from 2091.90 nm to 2098.19 nm. The maximum power was 4.51 W with the wavelength of 2097.56 nm. For the 1.0 at.% Ho:YAG ceramic, the tuning range was 2091.75 nm to 2098.51 nm, and the maximum output power was 1.19 W at 2097.61 nm. As mentioned above, the laser wavelength can be tuned from 2091nm to 2098nm with two Ho:YAG ceramic, when we changed the angle of F-P etalon. But the wavelength jumped to 2091nm if we continued to change the angle of etalon in the same direction. So in our experiments the tuning range limit is 7 nm by using intra-cavity etalon as the tuning element. If prisms or gratings are used the tuning range may become larger. The free-running wavelengths of the two Ho:YAG ceramic lasers were about 2097 nm without inserted etalon.
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 257
Fig. 4. Wavelength tuning of the Ho:YAG ceramic laser with two Ho-doping concentrations.
Output energy of the Q-switched Ho:YAG ceramic laser at different PRF with a pump diameter of 588 μm is shown in Fig. 5. At high PRF like 500 Hz and 1kHz, output energy of 0.8 at.% Ho:YAG ceramic laser was higher than that of 1.0 at.% Ho:YAG ceramic laser, but the energy extraction efficiency of two lasers were comparable. When the PRF decreased from 500 Hz to 300 Hz, the maximum output energy of 1.0 at.% Ho:YAG ceramic laser was close to that of 0.8 at.% Ho:YAG ceramic laser, and the slope efficiency of 0.8 at.% Ho:YAG ceramic laser shown a little saturation as pump power increased.
Fig. 5. Output energy of the Ho:YAG ceramic laser with pump diameter of 588 μm verse incident pump power.
Figure 6 shows output energies and pulse widths of the 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers at a PRF of 200 Hz, respectively. With the increase of the incident pump power, the output energy was gradually saturated. For the 0.8 at.% Ho:YAG ceramic laser, output energy of 9.6 mJ at 200 PRF with a pulse width of 83 ns was obtained. Output energy of 9.4 mJ was achieved from 1.0 at.% Ho:YAG ceramic laser with a pulse width of 85 ns. At a PRF of 200 Hz, the energy extract efficiency of 1.0 at.% Ho:YAG ceramic laser was 45.1%, which was higher than that of 0.8 at.% Ho:YAG ceramic laser (42.9%). We also measured output energy of 0.8 at.% Ho:YAG ceramic laser at 100 Hz PRF. When the pump power was 15.28 W, output energy of 10.2 mJ was measured. However, the 45° dichroic mirror was damaged with the increase of the pump power. According to the result of simulation by using
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 258
software of LASCAD, the mode size was about 344 μm at the 45° DM. So the maximum intra-cavity peak power density was about 112 MW/cm2 at the 45° DM.
Fig. 6. Output energy and pulse width of the Ho:YAG ceramic lasers at 200 Hz PRF with pump diameter of 588 μm verse incident pump power.
The extract efficiency ηs for a CW pumped Q-switched laser is given by [14]:
η s = Pav ( PRF ) / PCW = τ / τ Q [1 − exp(−τ Q / τ )].
(1)
where, τ is the effective upper level lifetime and τQ is 1/PRF, PCW is output power in CW operation, and Pav(PRF) is average output power in Q-switched operation. We measured average output power of 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers as functions of PRF (100 Hz to 5 kHz) at different incident pump power, and the experimental results is shown in Fig. 7.
Fig. 7. Average output power of 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers as functions of PRF (100 Hz to 5 kHz) at different incident pump power. The solid symbols are for 1.0 at.% Ho:YAG ceramic, and the hollow core symbols are for 0.8 at. % Ho:YAG ceramic.
Figures 8 and 9 show extract efficiency of 0.8 at.% and 1.0 at.% Ho:YAG ceramics as functions of PRF. We also measured the absorption pump power under lasing condition, and no absorption saturation was observed. Utilizing the formula (1), we fitted the experimental data to determine the effective upper laser level lifetime of 0.8 at.% and 1.0 at.% Ho:YAG
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 259
ceramics as functions of the incident pump power, as shown in Fig. 10. The effective upper laser level lifetime of 0.8 at.% Ho:YAG ceramic decreased from 5.78 ms to 3.44 ms when the pump power was increased from 8.76 W to 16.67 W. For 1.0 at.% Ho:YAG ceramic, the effective upper laser level lifetime decreased from 4.96 ms to 3.44 ms. The effective upper laser level lifetime of Ho:YAG ceramic is shorter than Ho radiative lifetime of 7 ms. Several explanations have been proposed for the reduction in effective lifetime, including energytransfer up-conversion (ETU) and amplified spontaneous emission (ASE) [14,15]. ETU is related to Ho-doping concentration, and the ASE effect depends on the average path length for the spontaneously emitted photon and pump power [16]. At low pump power, ETU plays a dominant role in the reduction of the effective lifetime, wherefore the effective lifetime of 0.8 at.% Ho:YAG ceramic is longer than that of 1.0 at.% Ho:YAG ceramic. However, with the increase of the pump power, the ASE effect is much stronger and likely to have a main influence on the effective lifetime. So the effective lifetime of 0.8 at.% and 1.0 at.% Ho:YAG ceramics are comparable at the pump power of 16.67 W.
Fig. 8. The extract efficiency of 0.8 at.% Ho:YAG ceramic lasers as functions of PRF.
Fig. 9. The extract efficiency of 1.0 at.% Ho:YAG ceramic lasers as functions of PRF.
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 260
Fig. 10. Effective upper laser level lifetime of 0.8 at.% and 1.0 at.% Ho:YAG ceramics as functions of incident pump power.
The beam quality of the output beam at output energy of 9 mJ was measured. Figure 11 shows the measured beam radii at different positions along the beam propagation and the picture inserted is a two-dimensional beam profile of the laser beam. By fitting the measured data with a hyperbolic curve, the M2 factors were calculated to be 1.085 and 1.052 in x and y directions, respectively.
Fig. 11. Beam propagation and intensity profile of Q-switched Ho:YAG ceramic laser. The inserted picture is the two-dimensional beam profiles of the laser beam.
4. Conclusion In summary, we reported a Q-switched Ho:YAG ceramic laser resonantly pumped by a Tm:YLF laser. The Q-switched pulse energy is 9.6 mJ at 200 Hz PRF and 10.2 mJ at 100 Hz PRF, respectively. The measured M2 factors were less than 1.1 in both directions. To the best of our knowledge, this is the first time to report a Q-switched Ho:YAG ceramic laser. By using injection-seeding technology, the Ho:YAG ceramic laser with high energy and good beam quality can be used to obtain single frequency pulse laser as light sources for coherent wind measurement Lidar. Acknowledgments The authors acknowledge much help from Prof. Yang Suhui and Doctor Li Jing from Beijing Institute of Technology. This work is supported by the National Natural Science Foundation of China (61378021, 61178027) and the Beijing Natural Science Foundation (4132036).
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 261
1 School of Opto-Electronics, Beijing Institute of Technology, Beijing 100081, China School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 22116, China * [email protected]
Abstract: We demonstrated a Q-switched Ho:YAG ceramic laser operating at 2097 nm. The Ho:YAG ceramic laser was resonantly pumped by a Tm:YLF laser at 1908 nm. The laser performance with two Ho-doping concentrations of Ho:YAG ceramics in a U-shaped resonator was studied. Different pump spots were investigated to obtain high extract efficiency. The wavelength of Ho:YAG ceramic laser was tuned from 2090.70 nm to 2098.10 nm. The Q-switched pulse energy were 9.6 mJ at a pulse repetition frequency (PRF) of 200 Hz and 10.2 mJ at a PRF of 100 Hz, respectively. The beam quality M2 factors were measured to be less than 1.1 in both directions. © 201 Optical Society of America OCIS codes: (140.3460) Lasers; (140.3540) Lasers, Q-switched; (140.3600) Lasers, tunable; (160.5690) Rare-earth-doped materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
O. L. Antipova, N. G. Zakharova, M. Fedorov, N. M. Shakhova, N. N. Prodanets, L. B. Snopova, V. V. Sharkov, and R. Sroka, “Cutting effects induced by 2 μm laser radiation of CW Tm:YLF and CW and Q-switcheded Ho:YAG lasers on ex-vivo tissue,” Med. Laser Appl. 26(2), 67–75 (2011). Y. Pan, Y. Wang, X. Li, X. Xiong, Y. Wang, S. Li, Q. Luo, J. Zhu, and W. Zhang, “Ho:YAG laser application in cerebellopontine angle tumor operation,” Proc. SPIE 5967, 59671R (2005). G. D. Spiers, R. T. Menzies, J. Jacob, L. E. Christensen, M. W. Phillips, Y. Choi, and E. V. Browell, “Atmospheric CO2 measurements with a 2 μm airborne laser absorption spectrometer employing coherent detection,” Appl. Opt. 50(14), 2098–2111 (2011). S. Haidar, K. Miyamoto, and H. Ito, “Generation of tunable Mid-IR (5.5-9.3μm) from a 2-μm pumped ZnGeP2 optical parametric oscillator,” Opt. Commun. 241(1–3), 173–178 (2004). E. Lippert, H. Fonnum, G. Arisholm, and K. Stenersen, “A 22-watt mid-infrared optical parametric oscillator with V-shaped 3-mirror ring resonator,” Opt. Express 18(25), 26475–26483 (2010). S. Lamrini, P. Koopmann, M. Schäfer, K. Scholle, and P. Fuhrberg, “Efficient high-power Ho:YAG laser directly in-band pumped by a GaSb-based laser diode stack at 1.9 μm,” Appl. Phys. B 106(2), 315–319 (2012). X. M. Duan, B. Q. Yao, C. W. Song, J. Gao, and Y. Z. Wang, “Room temperature efficient continuous wave and Q-switched Ho:YAG laser double-pass pumped by a diode-pumped Tm:YLF laser,” Laser Phys. Lett. 5(11), 800–803 (2008). P. A. Budni, C. R. Ibach, S. D. Setzler, E. J. Gustafson, R. T. Castro, and E. P. Chicklis, “50-mJ, Q-switched, 2.09-microm holmium laser resonantly pumped by a diode-pumped 1.9-microm thulium laser,” Opt. Lett. 28(12), 1016–1018 (2003). W. X. Zhang, J. Zhou, W. B. Liu, J. Li, L. Wang, B. X. Jiang, Y. B. Pan, X. J. Cheng, and J. Q. Xu, “Fabrication, properties and laser performance of Ho:YAG transparent ceramic,” J. Alloy. Comp. 506(2), 745–748 (2010). X. J. Cheng, J. Q. Xu, M. J. Wang, B. X. Jiang, W. X. Zhang, and Y. B. Pan, “Ho:YAG ceramic laser pumped by Tm:YLF lasers at room temperature,” Laser Phys. Lett. 7(5), 351–354 (2010). H. Chen, D. Shen, J. Zhang, H. Yang, D. Tang, T. Zhao, and X. Yang, “In-band pumped highly efficient Ho:YAG ceramic laser with 21 W output power at 2097 nm,” Opt. Lett. 36(9), 1575–1577 (2011). H. Yang, J. Zhang, X. Qin, D. Luo, J. Ma, D. Tang, H. Chen, D. Shen, and Q. Zhang, “Polycrystalline Ho:YAG transparent ceramics for eye-safe solid state laser applications,” J. Am. Ceram. Soc. 95(1), 52–55 (2012). L. Wang, C. Gao, M. Gao, and Y. Li, “Resonantly pumped monolithic nonplanar Ho:YAG ring laser with highpower single-frequency laser output at 2122 nm,” Opt. Express 21(8), 9541–9546 (2013).
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 254
14. N. P. Barnes, “Solid-state lasers from an efficiency perspective,” IEEE J. Sel. Top. Quantum Electron. 13(3), 435–447 (2007). 15. L. Zhu, M. Wang, J. Zhou, and W. Chen, “Efficient 1645 nm continuous-wave and Q-switched Er:YAG laser pumped by 1532 nm narrow-band laser diode,” Opt. Express 19(27), 26810–26815 (2011). 16. N. P. Barnes and B. M. Walsh, “Amplified spontaneous emission-application to Nd:YAG lasers,” IEEE J. Sel. Top. Quantum Electron. 35(1), 101–109 (1999).
1. Introduction High-energy Q-switched lasers with eye-safe wavelengths around 2 μm are widely used in fields of medicine, coherent Doppler Lidar, differential absorption Lidar, etc [1–3]. Moreover, high-energy 2 μm lasers are attractive candidates for pumping mid-infrared optical parametric oscillator to generate 3~5 μm lasers [4, 5]. Last decade, resonantly pumped Ho-doped laser has attracted great attentions due to its low quantum defect and high efficiency. Ho:YAG single crystal has been widely studied to obtain continuous-wave(CW) and Q-switched operation in 2 μm wavelengths [6–8]. Recently, more and more scientists are interested in Ho:YAG ceramic, whose thermomechanical properties are as good as Ho:YAG single crystal. Moreover, Ho:YAG ceramic has many advantages compared with Ho:YAG single crystal, such as ease of fabrication, short fabrication time, low cost, mass production, and feasibility of large size [9]. In 2010, X. J. Cheng et al. reported a Ho:YAG ceramic laser pumped by a Tm:YLF laser [10]. The maximum output power of 1.2 W at 2.09 μm was obtained with absorbed pump power of 5 W. In 2011, H. Cheng et al. demonstrated a high-power polycrystalline Ho:YAG ceramic laser in-band pumped by a Tm:fiber laser [11]. The maximum output power of 21.4 W was obtained under 35 W of incident pump power. In 2012, H. Yang et al. reported a Ho:YAG ceramic slab in-band pumped by a 1907 nm Tm:fiber laser and 20.6 W output power at 2097 nm was obtained [12]. In this paper, we demonstrate a Q-switched Ho:YAG ceramic laser at 2097 nm, resonantly pumped by a Tm:YLF laser. Up to 9.6 mJ Q-switched energy at 200 Hz was obtained from a 0.8 at.% Ho:YAG ceramic laser, with beam quality M2 factors less than 1.1 in both directions. To our knowledge, this is the first time to report a Q-switched Ho:YAG ceramic laser operating at 2097 nm with high energy and good beam quality. 2. Experimental setup Figure 1 shows the experimental setup of the Ho:YAG ceramic laser resonantly pumped by a 1908 nm Tm:YLF laser built by ourselves. The Tm:YLF laser is the same with the pump laser reported in [13].The beam from the Tm:YLF laser was then collimated and focused into the Ho:YAG ceramic by spherical lenses with focal lengths of −50 mm and 100 mm. The focal lengths of the spherical lenses were changed to investigate the influence of the pump beam spot size on the Ho:YAG laser efficiency. As for this reason, we changed the pump spot size to match the laser mode in the resonator.
Fig. 1. Experimental setup of the Ho:YAG ceramic laser pumped by a Tm:YLF laser.
Two kinds of Ho:YAG ceramics were used in the experiments, one with a Ho-doping concentration of 0.8 at.% and a length of 25 mm, and the other with 1.0 at.% Ho-doping
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 255
concentration and a length of 20 mm. The diameters of two Ho:YAG ceramics were 4 mm. Both ends of Ho:YAG ceramics were anti-reflection coated at 1908 nm and 2097 nm. The 45° dichroic mirrors (DM) has an anti-reflection coating at 1908 nm and high-reflection coating at 2097 nm. The transmittance of DM for the pump beam is about 96.4%. A 0.1-mm-thick uncoated etalon was inserted into the resonator to tune the wavelength of the Ho:YAG ceramic laser. For Q-switched operation, an acousto-optic modulator (AOM) (QSWITCH/33027-100-5-2-I-HGM-W, NEOS) was used inside the resonator. An U-shape resonator was applied to the Ho:YAG laser for avoiding the optical feedback of the pump laser. The total resonator length of the Ho:YAG ceramic laser was around 950 mm. The radius of curvature of the output mirror was 750 mm and the transmittance was 30% at 2097 nm. The high-reflective mirror has an anti-reflection coating at 1908 nm and high-reflection coating at 2097 nm. The radius of curvature of the high-reflective mirror was 500 mm. The laser beam radius in the middle of the Ho:YAG ceramic was calculated to be 423 μm by using the software of LASCAD with consideration of the thermal lens. And we also design other cavity using many other cavity mirror with different curvatures and output coupling. But the 45°DM mirror was damaged when we used lower output coupling and smaller resonator mode. We report the best result with the optimized cavity parameters. The temperature of the Ho:YAG ceramic was controlled at 20°C by using a thermal electric cooler (TEC). 3. Experimental results The CW output power and average output power of the 1.0 at.% Ho:YAG ceramic laser with the etalon inside the resonator as functions of the incident pump power are shown in Fig. 2. And the losses introduced by the etalon were about 1%. When the pump diameter was 294 μm, the maximum CW output power was 4.62 W with a slope efficiency of 41.3%. For 441 μm and 588 μm pump diameters, the maximum CW output power were 4.38 W and 4.17 W, respectively. In Q-switched operation, the maximum average output power of 1.90 W was obtained with the pump diameter of 588 μm when the pulse repetition frequency (PRF) was 200 Hz. And the maximum average output powers were 1.88 W and 1.70 W when the pump beam diameters were 441 μm and 294 μm, respectively. We also measured the output powers of the 0.8 at.% Ho:YAG ceramic laser with different pump diameters. The trend of experimental results of 0.8 at.% Ho:YAG ceramic laser is similar to that of 1.0 at.% Ho:YAG ceramic laser. Therefore, highest energy extract efficiency can be achieved with the biggest pump diameter.
Fig. 2. CW output power and average output power of 1.0 at.% Ho:YAG ceramic laser with different pump spot diameters. For the Q-switched operation, the pulse repetition frequency was 200 Hz.
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 256
The CW lasing performance of 0.8 at.% and 1.0 at.% Ho:YAG ceramics were compared when the pump diameter was 588 μm. Figure 3 shows CW output power of 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers with the etalon inside the resonator. For 0.8 at.% Ho:YAG ceramic, CW output power of 4.48 W was obtained with a slope efficiency of 41.5% with respect to the incident pump power. The maximum CW output power of 4.17 W was obtained from 1.0 at.% Ho:YAG ceramic with a slope efficiency of 39.2% with respect to the incident pump power. The pump absorption efficiencies for 0.8 at.% and 1.0 at.% Ho:YAG ceramic in the single pass pump scheme are 86.8%% and 86.4%, respectively. Therefore, the slope efficiencies for 0.8 at.% and 1.0 at.% Ho:YAG ceramic are 47.1% and 46.3% with respect to the absorbed pump power, respectively. Bigger resonator mode volume can reduce the risk of damaging optical components. To get good overlap between the pump mode and the resonator mode, we have increased the pump mode. But bigger pump mode volume also induces lower pump intensity, and increasing the pump threshold and decreasing the slope efficiency. Furthermore, 45° dichroic mirrors also lead to slope efficiency reduction due to immature coating process.
Fig. 3. CW output power of Ho:YAG ceramic laser verse incident pump power with the pump diameter of 588 μm.
We investigated the wavelength tuning of two Ho:YAG ceramic lasers by changing the angle of the intra-cavity etalon. The wavelength of the Ho:YAG ceramic laser was measured by using an EXFO WA-650 spectrum analyzer combined with a WA-1000 wavemeter with a resolution of 10 pm. Figure 4 shows the measured results of the output power at different wavelengths. For the 0.8 at.% and 1.0 at.% Ho:YAG ceramics, the incident pump power was kept at 16.67 W and 9.02 W, respectively. The laser wavelength of 0.8 at.% Ho:YAG ceramic was tuned from 2091.90 nm to 2098.19 nm. The maximum power was 4.51 W with the wavelength of 2097.56 nm. For the 1.0 at.% Ho:YAG ceramic, the tuning range was 2091.75 nm to 2098.51 nm, and the maximum output power was 1.19 W at 2097.61 nm. As mentioned above, the laser wavelength can be tuned from 2091nm to 2098nm with two Ho:YAG ceramic, when we changed the angle of F-P etalon. But the wavelength jumped to 2091nm if we continued to change the angle of etalon in the same direction. So in our experiments the tuning range limit is 7 nm by using intra-cavity etalon as the tuning element. If prisms or gratings are used the tuning range may become larger. The free-running wavelengths of the two Ho:YAG ceramic lasers were about 2097 nm without inserted etalon.
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 257
Fig. 4. Wavelength tuning of the Ho:YAG ceramic laser with two Ho-doping concentrations.
Output energy of the Q-switched Ho:YAG ceramic laser at different PRF with a pump diameter of 588 μm is shown in Fig. 5. At high PRF like 500 Hz and 1kHz, output energy of 0.8 at.% Ho:YAG ceramic laser was higher than that of 1.0 at.% Ho:YAG ceramic laser, but the energy extraction efficiency of two lasers were comparable. When the PRF decreased from 500 Hz to 300 Hz, the maximum output energy of 1.0 at.% Ho:YAG ceramic laser was close to that of 0.8 at.% Ho:YAG ceramic laser, and the slope efficiency of 0.8 at.% Ho:YAG ceramic laser shown a little saturation as pump power increased.
Fig. 5. Output energy of the Ho:YAG ceramic laser with pump diameter of 588 μm verse incident pump power.
Figure 6 shows output energies and pulse widths of the 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers at a PRF of 200 Hz, respectively. With the increase of the incident pump power, the output energy was gradually saturated. For the 0.8 at.% Ho:YAG ceramic laser, output energy of 9.6 mJ at 200 PRF with a pulse width of 83 ns was obtained. Output energy of 9.4 mJ was achieved from 1.0 at.% Ho:YAG ceramic laser with a pulse width of 85 ns. At a PRF of 200 Hz, the energy extract efficiency of 1.0 at.% Ho:YAG ceramic laser was 45.1%, which was higher than that of 0.8 at.% Ho:YAG ceramic laser (42.9%). We also measured output energy of 0.8 at.% Ho:YAG ceramic laser at 100 Hz PRF. When the pump power was 15.28 W, output energy of 10.2 mJ was measured. However, the 45° dichroic mirror was damaged with the increase of the pump power. According to the result of simulation by using
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 258
software of LASCAD, the mode size was about 344 μm at the 45° DM. So the maximum intra-cavity peak power density was about 112 MW/cm2 at the 45° DM.
Fig. 6. Output energy and pulse width of the Ho:YAG ceramic lasers at 200 Hz PRF with pump diameter of 588 μm verse incident pump power.
The extract efficiency ηs for a CW pumped Q-switched laser is given by [14]:
η s = Pav ( PRF ) / PCW = τ / τ Q [1 − exp(−τ Q / τ )].
(1)
where, τ is the effective upper level lifetime and τQ is 1/PRF, PCW is output power in CW operation, and Pav(PRF) is average output power in Q-switched operation. We measured average output power of 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers as functions of PRF (100 Hz to 5 kHz) at different incident pump power, and the experimental results is shown in Fig. 7.
Fig. 7. Average output power of 0.8 at.% and 1.0 at.% Ho:YAG ceramic lasers as functions of PRF (100 Hz to 5 kHz) at different incident pump power. The solid symbols are for 1.0 at.% Ho:YAG ceramic, and the hollow core symbols are for 0.8 at. % Ho:YAG ceramic.
Figures 8 and 9 show extract efficiency of 0.8 at.% and 1.0 at.% Ho:YAG ceramics as functions of PRF. We also measured the absorption pump power under lasing condition, and no absorption saturation was observed. Utilizing the formula (1), we fitted the experimental data to determine the effective upper laser level lifetime of 0.8 at.% and 1.0 at.% Ho:YAG
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 259
ceramics as functions of the incident pump power, as shown in Fig. 10. The effective upper laser level lifetime of 0.8 at.% Ho:YAG ceramic decreased from 5.78 ms to 3.44 ms when the pump power was increased from 8.76 W to 16.67 W. For 1.0 at.% Ho:YAG ceramic, the effective upper laser level lifetime decreased from 4.96 ms to 3.44 ms. The effective upper laser level lifetime of Ho:YAG ceramic is shorter than Ho radiative lifetime of 7 ms. Several explanations have been proposed for the reduction in effective lifetime, including energytransfer up-conversion (ETU) and amplified spontaneous emission (ASE) [14,15]. ETU is related to Ho-doping concentration, and the ASE effect depends on the average path length for the spontaneously emitted photon and pump power [16]. At low pump power, ETU plays a dominant role in the reduction of the effective lifetime, wherefore the effective lifetime of 0.8 at.% Ho:YAG ceramic is longer than that of 1.0 at.% Ho:YAG ceramic. However, with the increase of the pump power, the ASE effect is much stronger and likely to have a main influence on the effective lifetime. So the effective lifetime of 0.8 at.% and 1.0 at.% Ho:YAG ceramics are comparable at the pump power of 16.67 W.
Fig. 8. The extract efficiency of 0.8 at.% Ho:YAG ceramic lasers as functions of PRF.
Fig. 9. The extract efficiency of 1.0 at.% Ho:YAG ceramic lasers as functions of PRF.
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 260
Fig. 10. Effective upper laser level lifetime of 0.8 at.% and 1.0 at.% Ho:YAG ceramics as functions of incident pump power.
The beam quality of the output beam at output energy of 9 mJ was measured. Figure 11 shows the measured beam radii at different positions along the beam propagation and the picture inserted is a two-dimensional beam profile of the laser beam. By fitting the measured data with a hyperbolic curve, the M2 factors were calculated to be 1.085 and 1.052 in x and y directions, respectively.
Fig. 11. Beam propagation and intensity profile of Q-switched Ho:YAG ceramic laser. The inserted picture is the two-dimensional beam profiles of the laser beam.
4. Conclusion In summary, we reported a Q-switched Ho:YAG ceramic laser resonantly pumped by a Tm:YLF laser. The Q-switched pulse energy is 9.6 mJ at 200 Hz PRF and 10.2 mJ at 100 Hz PRF, respectively. The measured M2 factors were less than 1.1 in both directions. To the best of our knowledge, this is the first time to report a Q-switched Ho:YAG ceramic laser. By using injection-seeding technology, the Ho:YAG ceramic laser with high energy and good beam quality can be used to obtain single frequency pulse laser as light sources for coherent wind measurement Lidar. Acknowledgments The authors acknowledge much help from Prof. Yang Suhui and Doctor Li Jing from Beijing Institute of Technology. This work is supported by the National Natural Science Foundation of China (61378021, 61178027) and the Beijing Natural Science Foundation (4132036).
#197986 - $15.00 USD (C) 2014 OSA
Received 19 Sep 2013; revised 29 Nov 2013; accepted 18 Dec 2013; published 2 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000254 | OPTICS EXPRESS 261
