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Abstract
In this paper, we demonstrated a linewidth-narrowed continuous-wave (CW) and acousto-optical Q-switched Ho:CaF2 laser for the first time. With a volume Bragg grating, a maximum CW output power of 6.94 W at 2100.5 nm, and slope efficiency of 57.9%, an FWHM linewidth of 0.31 nm was obtained. When absorbed pump power was 13.2 W, the maximum average output powers of 6.08 W, 5.9 W, and 5.71 W were achieved in a Q-switched Ho:CaF2 laser under pulse repetition frequencies of 10 kHz, 5 kHz, and 3 kHz, corresponding to the slope efficiencies of 51.2%, 49.6%, and 48.5%, respectively. The minimum pulse width of 54 ns was achieved at pulse repetition frequency of 3 kHz.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The laser radiation at 2-µm spectral region has high atmospheric transmission, and overlaps the absorption peaks of H2O and CO2 molecules etc. Therefore, holmium (Ho) lasers operating at 2 µm have many various technical applications such as medicine, lidar and chemical monitoring. Specially, they can be pumped the optical parametric oscillators (OPOs) to generate the mid-infrared (3-5 μm) and long-wave infrared (8-12 μm) laser radiation. Efficient continuous-wave (CW) or pulsed 2-µm laser actions based on the 5I7→5I8 transition of Ho ions have been widely investigated in different garnet [1,2], fluoride [3,4], vanadate [5,6], tungstate [7,8], aluminate [9] and silicate [10,11] hosts.
Apart from above materials, calcium fluoride (CaF2) crystal with short phonon spectra and weak multiphonon relaxation is another attractive host material for doping of Ho ions. Specially, it is very conducive to reduce the thermal effects in crystal because its negative refractive coefficient (−11.5 × 10−6 K−1) and high thermal conductivity (9.7 Wm−1K−1) [12]. In 2014, Šulc et al. demonstrated first 2-µm laser actions of Ho ions in CaF2 crystal [13]. A tunable range from 2016 nm to 2111 nm was achieved in a diode-pumped Tm, Ho:CaF2 laser. The maximum slope efficiency of Tm, Ho:CaF2 laser was only approximately 12% limited by complex energy resonance transfer and upconversion processes between Tm and Ho ions. In 2016, Jelínek et al. presented a cryogenic Ho:CaF2 laser pumped by a Tm-fiber laser [14]. At operating temperature of 83 K, the maximum CW output power of 2.37 W was obtained with the slope efficiency of 23%. In 2017, Jelínek et al. used experimental setup alike [14] to demonstrate a room temperature Ho:CaF2 laser at 2085 nm [15]. The slope efficiency of 41% and 53% with respect to the absorbed pump power was obtained in Ho:CaF2 laser under CW and free running pulsed regime, respectively. In 2017, Němec et al. reported a pulsed Tm fiber pumped tunable Ho:CaF2 laser at room temperature [16]. By using a Ho:CaF2 crystal with doping concentration of 1.5 at. %, the average output power of 2.1 W at 2113 nm and tunable range from 2073 to 2114 nm were obtained. Recently, a 1.5 W CW intra-cavity Tm:LuAG-Ho:CaF2 laser was reported with slope efficiency of 24.2% by our group [17]. Up to now, the literatures were focused on the CW or free running pulsed output performances of Ho:CaF2 laser. However, to our knowledge, there is no reported work on the Q-switched Ho:CaF2 laser.
For some applications, e.g., Doppler lidar and high resolution gas detection, the narrow linewidth of laser is necessary. Volume Bragg grating (VBG) is an excellent element to select a specific wavelength with narrow linewidth because its wavelength-dependent diffraction efficiency. In this paper, to the best of our knowledge, we demonstrated a first CW and Q-switched Ho:CaF2 laser linewidth-narrowed by a VBG. Under free running regime, the maximum output power of 7.17 W at 2085 nm was achieved with absorbed pump power of 12.8 W, corresponding to a slope efficiency of 59.8% with respect to absorbed pump power. With a VBG, maximum output power of 6.94 W and slope efficiency of 57.9% were obtained at output wavelength of 2100.5 nm with FWHM linewidth of 0.31 nm. Under Q-switching regime, when absorbed pump power was 13.2 W, the maximum average output powers of 6.08 W, 5.9 W and 5.71 W were achieved with slope efficiencies of 51.2%, 49.6% and 48.5% under pulse repetition frequencies (PRFs) of 10 kHz, 5 kHz and 3 kHz, respectively. The minimum pulse width of 54 ns, maximum single pulse energy of 1.9 mJ and maximum peak power of 35.3 kW were obtained with PRF of 3 kHz.
2. Experimental setup
A homemade unpolarized 20 W Tm-fiber laser wavelength-locked by a pair of FBG was used to pump the Ho:CaF2 crystal in this work. Its beam quality factor (M2) was 1.5 at maximum output level. The output spectrum of pump source was recorded by a spectral analyzer with resolution of 0.2 pm (Bristol 771A), as illustrated in Fig. 1. It has FWHM linewidth of 0.2 nm and central wavelength of 1943.5 nm which is close the an absorption peak of 1945 nm of Ho:CaF2 crystal (see Fig. 1). The Ho:CaF2 crystal with doping concentration of 1.0 at.% was used in this experiment. Its both ends were antireflection coated for 1.9~2.2 μm. The Ho:CaF2 crystal has dimensions of 3 × 3 mm2 (in cross section) and 20 mm (in length). It was wrapped in 0.05-mm-thick indium foil and mounted in a water-cooled copper heatsink. The water temperature was controlled at 15 °C.
Fig. 1 The absorption and emission spectra of Ho:CaF2 crystal [16] and output wavelength of Tm-fiber laser.
2.1 Continuous-wave operation
We used a simple linear cavity to investigate the CW output performances of Ho:CaF2 laser. The pump beam was collimated and focused into Ho:CaF2 crystal by a two aspheric lenses F1 and F2 with focal lengths of approximately 7 mm and 150 mm, respectively. The pump diameter in the middle of Ho:CaF2 crystal was approximately 300 μm which was measured by 90/10 knife-edge method. The pump Rayleigh range (zr = πω2n/λM2) of about 32 mm was calculated with ω = 150 μm, λ = 2.1 μm and n = 1.42. The 82% single-pass pump absorption of Ho:CaF2 crystal was measured under nolasing conditions. As illustrated in Fig. 2, the resonant cavity includes a VBG and an output coupler M1. A 5-mm-thick VBG (Optigrate Corp.) was used as the 0° dichroic mirror, which has 2100.5 nm diffraction wavelength and more than 99% diffraction efficiency with a FWHM spectral width less than 1 nm. The end faces of VBG were antireflection coated for 1.9~2.2 μm. It was embedded in a copper frame to ensure good thermal contact. The output coupler M1 was plano-concave mirror with 100 mm radius of curvature. The flat 45° dichroic mirror M was coated with a high reflectivity (R>99.7%) at 2.1 µm and a high transmittance (T>99.5%) at 1.94 µm. The physical cavity length was approximately 30 mm. The laser beam waist diameter of about 340 µm was calculated by ABCD matrix method. Hence the good overlap between the pump beam and Ho laser mode was achieved. In addition, the ABCD matrix method we also used to calculate that this cavity can endure a thermal focal length above 7 mm from Ho:CaF2 crystal.
2.2 Acousto-optic Q-switching operation
For Q-switching operation, the resonant cavity structure was changed to an L-shaped one. A 35 mm-long acousto-optic (AO) Q-switch (Gooch & Housego Corp.) was inserted to generate the Q-switched laser pulses. It has RF power of 20 W at frequency of 40.68 MHz and loss modulation of more than 45%. The resonant cavity includes a VBG, a 45° dichroic mirror M and an output coupler M1, as illustrated in Fig. 3. The parametric of VBG, mirrors M and M1 were same in 2.1 section. The physical length of whole cavity was extended to approximately 70 mm. The laser beam waist diameter of about 360 µm was calculated by ABCD matrix method. Meanwhile, we calculated that this cavity can endure a thermal focal length above 44 mm from Ho:CaF2 crystal.
3. Experimental results and discussions
3.1 Continuous-wave operation
A 30 W power meter (Ophir Optronics) was used to measure the output powers in this work. We used three output transmittances of 2%, 5% and 10% to study the CW output characteristics of Ho:CaF2 laser. Firstly, we used a conventional 0° dichroic mirror to replace the VBG, and measured the output powers of Ho:CaF2 laser under free running regime, as illustrated in Fig. 4(a). With transmittance of 5%, the Ho:CaF2 laser produced highest output power. At absorbed pump power of 12.8 W, the maximum output power of 7.17 W was obtained with slope efficiency of 59.8%. In the case of 2% transmittance, the output power and slope efficiency of Ho:CaF2 laser decrease to 6.28 W and 51.5% under absorbed pump power of 13.0 W, respectively. With transmittance of 10%, the slope efficiency of 42.9% and output power of 4.95 W were obtained when absorbed pump power was 12.9 W. The minimum pump threshold of 0.8 W was achieved under transmittance of 2%. It increases to 0.95 W and 1.42 W for transmittances of 5% and 10%, respectively. The maximum optical conversion efficiency was 56.0% under transmittance of 5%. To our knowledge, this is highest output power and conversion efficiency in reported Ho:CaF2 lasers. The output power stability of the CW Ho:CaF2 laser was also studied over a period of one hour. At maximum output level, the power fluctuation was approximately 2.0%. Next, we used the VBG as an element to narrow the output linewidths of Ho:CaF2 laser. The output powers of Ho:CaF2 laser with VBG is illustrated in Fig. 4(b). Alike to mirror conditions, the best output performance of Ho:CaF2 laser was achieved with transmittance of 5%. When the absorbed pump power was 12.8 W, the maximum output power of 6.94 W was reached with a slope efficiency of 57.9%. With transmittance of 2%, the output power and slope efficiency decrease to 5.95 W and 50.4%, respectively. In the case of transmittance of 10%, the output power of 4.78 W was obtained with slope efficiency of 40.0%. The pump thresholds slightly increase to 0.84 W, 0.99 W and 1.54 W under transmittances of 2%, 5% and 10%, respectively. This phenomenon was mainly caused by the diffraction loss of VBG. With the 82% single-pass pump absorption, the pump absorption coefficient of Ho:CaF2 crystal was calculated to be 0.86 cm−1. The thermal characteristics of Ho:CaF2 crystal were achieved from [12]. When the incident pump power was 15.7 W (12.9 W absorbed pump power), the thermal focal length of Ho:CaF2 crystal was numerically simulated to be 419 mm according to a thermal analysis model [18], which indicates that the cavity has excellent thermal stability.
Fig. 4 Output powers of Ho:CaF2 laser (a) with mirror and (b) with VBG under transmittances of 2%, 5% and 10%.
With mirror or VBG, under above three output transmittances, the output spectra of Ho:CaF2 laser at maximum output powers were measured by spectrum analyzer (Bristol 771A). Figure 5(a) illustrates the output spectra of Ho:CaF2 laser with mirror. With transmittance of 2%, the output wavelength was centered at 2113.2 nm with FWHM linewidth of 2.2 nm. When the output transmittance increases to 5%, the output wavelength and FWHM linewidth decrease to 2085.2 nm and 1.7 nm, respectively. With transmittance of 10%, the output central wavelength of 2085.2 nm and FWHM linewidth of 1.5 nm was observed. In contrast, the output central wavelengths of Ho:CaF2 laser with VBG were always located at 2100.5 nm, as illustrated in Fig. 5(b). Meanwhile, the narrow linewidths of 0.42 nm, 0.31 nm and 0.15 nm were achieved for transmittances of 2%, 5% and 10%, respectively. Furthermore, no obvious wavelength shift was observed in the VBG-locked regime. These experimental results indicate that the VBG has excellent stabilizing capability.
Fig. 5 The output spectrum of Ho:CaF2 laser with (a) mirror and (b) VBG under transmittances of 2%, 5% and 10%.
To give an approximate explanation of the output wavelengths of the Ho:CaF2 laser, the gain cross sections of Ho:CaF2 crystal were calculated by equation σg(λI) = βσemi(λI)−(1−β)σabs(λI), where β is the inversion ratio of Ho ions. As illustrated in Fig. 6(a), the maximum gain cross section was located at 2028 nm. The gain cross section gradually decreases with increasing of wavelength longer than 2028 nm. However, no 2028 nm wavelength was observed in the experiment, the 2113 nm wavelength with small gain cross section was firstly obtained with low output coupler transmittance of 2%. This phenomenon was mainly caused by the quasi-three-level properties of Ho:CaF2 laser. The pumping and lasing processes of Ho:CaF2 laser was illustrated in Fig. 6(b) according to the energy levels of Ho:CaF2 crystal at 10 K [19]. There are three pump wavelengths of 1939 nm, 1942 nm and 1944 nm at around 1.94 µm region, corresponding to transfers of 115.5 cm−1 → 5273, 5274cm−1, 158 cm−1 → 5308.5 cm−1 and 128 cm−1 → 5273, 5274 cm−1. The 2085 nm laser wavelength was related with the transfers of 5308.5 cm−1 → 513 cm−1 (2085 nm), 5274, 5273cm−1 → 475 cm−1 (2084 nm) and 5257 cm−1 → 455 cm−1 (2082 nm). The two transfers of 5274, 5273 cm−1 → 513 cm−1 (2100 nm) and 5257 cm−1 → 492 cm−1 (2099 nm) contribute to 2100.5 nm laser wavelength. For 2113 nm laser wavelength, only transfer of 5257 cm−1 → 513 cm−1 (2108 nm) can provide it. The Table 1 shows above processes for easy understanding. These processes indicate that the Ho:CaF2 laser belongs to a quasi-three-level system.
Fig. 6 The (a) gain cross section of Ho:CaF2 crystal at 300K and (b) pumping and lasing processes of Ho:CaF2 laser.
Table 1. Summary of pumping and lasing processes of Ho:CaF2 laser
3.2 Acousto-optic Q-switching operation
Based on the above experimental results, the Q-switched performances of Ho:CaF2 laser were demonstrated by an output coupler with transmittance of 5%. Firstly, with the Q-switch driver switched off, we recorded the output power of Ho:CaF2 laser under CW regime, as illustrated in Fig. 7. The pump threshold was 1.39 W. When the absorbed pump power was 13.2 W, the maximum output power of 6.45 W was reached with slope efficiency of 54.4% with respect to the absorbed pump power. Next, with the Q-switch driver switched on, the average output powers of the Q-switched Ho:CaF2 laser were recorded with PRF of 3 kHz, 5 kHz and 10 kHz, also as illustrated in Fig. 7. In the case of the PRF of 10 kHz, we obtained the maximum average output power of 6.08 W and a slope efficiency of 51.2% with respect to the absorbed pump power. With decreasing of PRF to 5 kHz, the average output power and slope efficiency decrease to 5.9 W and 49.6%, respectively. With the PRF of 3 kHz, the output power and slope efficiency were 5.71 W and 48.5%, respectively. The average output power stability of the Q-switched Ho:CaF2 laser was also investigated over a period of one hour. With PRF of 10 kHz, the power fluctuation was about 2.2% under maximum output level.
The output pulse profiles of Q-switched Ho:CaF2 laser were detected by an InGaAs photodiode with rise time of 28 ps (EOT Corp.) and recorded by a 350 MHz digital oscilloscope (Tektronix DPO 4034). The pulse widths, pulse energies and peak powers under different PRFs of 3 kHz, 5 kHz and 10 kHz were measured, calculated and illustrated in Fig. 8(a), Fig. 8(b) and Fig. 8(c), respectively. When absorbed pump power was 13.2 W, the minimum pulse widths of 54 ns, 79 ns and 118 ns were obtained with PRFs of 3 kHz, 5 kHz and 10 kHz, respectively. The maximum single pulse energies were 1.9 mJ, 1.2 mJ, and 0.6 mJ were achieved with PRFs of 3 kHz, 5 kHz and 10 kHz, respectively, corresponding to the maximum calculated peak powers of about 35.3 kW, 14.9 kW, and 5.2 kW. In addition, the temporal pulse profiles of minimum pulse widths are illustrated in Fig. 9. Finally, by using the 90/10 knife-edge technology, the M2 factors of Q-switched Ho:CaF2 laser with average output power of 6.45 W at PRF of 10 kHz were measured and estimated to be 1.2 and 1.1 for horizontal and vertical directions, respectively. For the Ho:CaF2 lasers at 2.1 µm, Table 2 summarizes the before presented output performances. As can be seen, the highest CW output power, slope efficiency and first Q-switched performances of Ho:CaF2 laser were reported in this work.
Fig. 8 The (a) pulse widths, (b) pulse energies and (c) peak powers of Q-switched Ho:CaF2 laser under different PRFs of 3 kHz, 5 kHz, 10 kHz.
Table 2. Summary of output characteristics of Ho:CaF2 laser at 2.1µm
4. Summary
In summary, we demonstrated an efficient linewidth-narrowed CW and Q-switched Ho:CaF2 laser resonantly pumped by a wavelength-locked Tm fiber laser at 1943.5 nm. Under CW regime, maximum output powers of 7.17 W at 2085 nm and 6.94 W at 2100.5 nm were achieved with mirror and VBG, corresponding to the slope efficiency of 59.8% and 57.9% with respect to absorbed pump power, respectively. Under Q-switching regime, when absorbed pump power was 13.2 W, the maximum average output powers of 6.08 W, 5.9 W and 5.71 W were achieved with slope efficiencies of 51.2%, 49.6% and 48.5% under PRFs of 10 kHz, 5 kHz and 3 kHz, respectively. The minimum pulse width of 54 ns, maximum single pulse energy of 1.9 mJ and maximum peak power of 35.3 kW were obtained with PRF of 3 kHz. As far as we know, this is highest output power in reported CW and Q-switched Ho:CaF2 lasers.
Funding
National Natural Science Foundation of China (NSFC) (51572053, 61378029, 61775053 and U1530152); Science Foundation for Outstanding Youths of Heilongjiang Province (JC2016016); Science Foundation for Youths of Heilongjiang Province (QC2017078); National Key Research and Development Program of China (2016YFB0701002).
References
1. Y. J. Shen, B. Q. Yao, X. M. Duan, G. L. Zhu, W. Wang, Y. L. Ju, and Y. Z. Wang, “103 W in-band dual-end-pumped Ho:YAG laser,” Opt. Lett. 37(17), 3558–3560 (2012). [CrossRef] [PubMed]
2. H. Chen, T. Zhao, H. Yang, L. Zhang, T. Zhou, D. Tang, C. Wong, Y. F. Chen, and D. Shen, “Energy level systems and transitions of Ho:LuAG laser resonantly pumped by a narrow line-width Tm fiber laser,” Opt. Express 24(24), 27536–27545 (2016). [CrossRef] [PubMed]
3. H. Fonnum, E. Lippert, and M. W. Haakestad, “550 mJ Q-switched cryogenic Ho:YLF oscillator pumped with a 100 W Tm:fiber laser,” Opt. Lett. 38(11), 1884–1886 (2013). [CrossRef] [PubMed]
4. J. W. Kim, J. I. Mackenzie, D. Parisi, S. Veronesi, M. Tonelli, and W. A. Clarkson, “Efficient in-band pumped Ho:LuLiF4 2 µm laser,” Opt. Lett. 35(3), 420–422 (2010). [CrossRef] [PubMed]
5. B. Q. Yao, Y. Ding, X. M. Duan, T. Y. Dai, Y. L. Ju, L. J. Li, and W. J. He, “Efficient Q-switched Ho:GdVO4 laser resonantly pumped at 1942 nm,” Opt. Lett. 39(16), 4755–4757 (2014). [CrossRef] [PubMed]
6. B. Q. Yao, Z. Cui, X. M. Duan, Y. Q. Du, L. Han, and Y. J. Shen, “Resonantly pumped room temperature Ho:LuVO4 laser,” Opt. Lett. 39(21), 6328–6330 (2014). [CrossRef] [PubMed]
7. X. Mateos, V. Jambunathan, M. C. Pujol, J. J. Carvajal, F. Díaz, M. Aguiló, U. Griebner, and V. Petrov, “CW lasing of Ho in KLu(WO4)2 in-band pumped by a diode-pumped Tm:KLu(WO4)2 laser,” Opt. Express 18(20), 20793–20798 (2010). [CrossRef] [PubMed]
8. V. Jambunathan, X. Mateos, M. C. Pujol, J. J. Carvajal, F. Díaz, M. Aguiló, U. Griebner, and V. Petrov, “Continuous-wave laser generation at ~2.1 µm in Ho:KRE(WO4)2 (RE = Y, Gd, Lu) crystals: a comparative study,” Opt. Express 19(25), 25279–25289 (2011). [CrossRef] [PubMed]
9. X. M. Duan, B. Q. Yao, X. T. Yang, L. J. Li, T. H. Wang, Y. L. Ju, Y. Z. Wang, G. J. Zhao, and Q. Dong, “Room temperature efficient actively Q-switched Ho:YAP laser,” Opt. Express 17(6), 4427–4432 (2009). [CrossRef] [PubMed]
10. B. Q. Yao, Z. P. Yu, X. M. Duan, Z. M. Jiang, Y. J. Zhang, Y. Z. Wang, and G. J. Zhao, “Continuous-wave laser action around 2-µm in Ho3+:Lu2SiO5,” Opt. Express 17(15), 12582–12587 (2009). [CrossRef] [PubMed]
11. X. T. Yang, B. Q. Yao, Y. Ding, X. Li, G. Aka, L. H. Zheng, and J. Xu, “Spectral properties and laser performance of Ho:Sc2SiO5 crystal at room temperature,” Opt. Express 21(26), 32566–32571 (2013). [CrossRef] [PubMed]
12. B. W. Woods, S. A. Payne, J. E. Marion, R. S. Hughes, and L. E. Davis, “Thermomechanical and thermo-optical properties of the LiCaAlF6:Cr3+ laser material,” J. Opt. Soc. Am. B 8(5), 970–977 (1991). [CrossRef]
13. J. Šulc, M. Němec, H. Jelínková, M. E. Doroshenko, P. P. Fedorov, and V. V. Osiko, “Diode pumped tunable lasers based on Tm:CaF2 and Tm:Ho:CaF2 ceramics,” Proc. SPIE 8959, 895925 (2014). [CrossRef]
14. M. Jelínek, V. Kubeček, W. Ma, B. Zhao, D. Jiang, and L. Su, “Cryogenic Ho:CaF2 laser pumped by Tm:fiber laser,” Laser Phys. Lett. 13(6), 065004 (2016). [CrossRef]
15. M. Jelínek, J. Cvrček, V. Kubeček, B. Zhao, W. Ma, D. Jiang, and L. Su, “Room temperature CW and QCW operation of Ho:CaF2 laser pumped by Tm:fiber laser,” Proc. SPIE 10238, 1023819 (2017). [CrossRef]
16. M. Němec, J. Šulc, M. Jelínek, V. Kubeček, H. Jelínková, M. E. Doroshenko, O. K. Alimov, V. A. Konyushkin, A. N. Nakladov, and V. V. Osiko, “Thulium fiber pumped tunable Ho:CaF2 laser,” Opt. Lett. 42(9), 1852–1855 (2017). [CrossRef] [PubMed]
17. X. M. Duan, X. S. Guo, B. Q. Yao, L. H. Zheng, and L. B. Su, “Efficient Ho:CaF2 laser intracavity-pumped by a Tm:LuAG laser in-band pumped at 1.6 µm,” Laser Phys. Lett. 15(9), 095802 (2018). [CrossRef]
18. B. Yao, Y. Tian, W. Wang, G. Li, and Y. Wang, “Analysis and compensation of thermal lens effects in Tm:YAP lasers,” Chin. Opt. Lett. 8(10), 996–999 (2010). [CrossRef]
19. M. Mujaji, G. D. Jones, and R. W. G. Syme, “Polarization study and crystal-field analysis of the laser-selective excitation spectra of Ho3+ ions in CaF2 and SrF2 crystals,” Phys. Rev. B Condens. Matter 46(22), 14398–14410 (1992). [CrossRef] [PubMed]
