Transparent Dy:Y2O3 ceramics with various doping concentrations were fabricated by spark plasma sintering (SPS). Their optical characteristics, including emission cross-sections and nonlinear absorption properties, were evaluated. The results show that Dy:Y2O3 ceramics are promising as a new candidate for 3-μm laser gain medium and passive Q-switch material. A passive Q-switching of a 2795 nm Er:YAP laser with a Dy:Y2O3 ceramic saturable absorber was performed to demonstrate the potential of this material.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The emission lines of Dy3+ ions around 3 and 4 μm [1,2] meet a lot of molecular absorption lines. This has great potential for mid-infrared (IR) laser applications, such as remote sensing of molecules [3,4], plasma diagnosis, and laser material processing . Dy-doped solid-state lasers at 3 μm have been realized in fluoride crystal and fibers with pumping at 1.1- or 1.3-μm wavelengths [6–8] and pumping at 2.8-μm wavelength from fiber lasers [9–11]. In addition, at 4 μm, Dy-doped lasers have been achieved with Dy-doped fluoride bulk crystals [12–14] with pumping at 1.7 or 1.3 μm. The lasing wavelength of Dy-doped lasers fills the gap in existing solid-state mid-IR lasers doped with erbium, holmium, or chromium ions around 3 μm. In the 4-μm band, Dy-doped lasers can be used as another approach for Fe:ZnS and Fe:ZnSe lasers [15,16], which require a cryogenic temperature of the gain medium to obtain a high lasing efficiency. Sesquioxide is one of the leading candidates for a mid-IR laser gain medium. Although the thermal conductivity of Y2O3 decreases with increasing dopant concentration, Y2O3 crystals still have an excellent thermal conductivity . Also this material has low phonon energy (∼591 cm−1) . These characteristics assist high-power and highly efficient laser operations in the mid-IR spectrum region. This material is chemically and mechanically stable because the melting point is higher than 2400°C. However, the fabrication of single crystal Y2O3 is difficult because of the high temperature required. Recent transparent ceramic technology is the solution to make this material. It can sinter the high-quality transparent bulk material by a temperature lower than the melting point. By using this technique, high-quality sesquioxide ceramics are now available and demonstrate the high-power and high-efficiency mid-IR laser performance with the dopant of Er ion [19–21]. Dy-doped sesquioxide materials have also been studied with ceramics and single crystals in recent years [22,23]. In , the optical properties of Dy-doped Y2O3 ceramics were reported in the visible range to 2.5 μm, and the possibility of laser oscillation in the visible range was evaluated by the spectroscopic method. In , the fabrication of Dy-doped Lu2O3 single crystal with optical and lasing properties in the visible range was demonstrated. However, there is no mid-IR lasing information in previous studies.
In this work, we fabricated Dy3+-doped Y2O3 ceramics by spark plasma sintering (SPS) and evaluated their optical properties as a new >3-μm mid-IR laser material. Additionally, we discuss the nonlinear absorption of Dy:Y2O3 ceramics around 3 μm for the first time. We also demonstrate the passive Q-switching behavior of a 2795 nm Er-doped yttrium aluminum perovskite (Er:YAlO3, Er:YAP) laser using a Dy:Y2O3 ceramic as a saturable absorber (SA).
2. Experimental methods
2.1. Sample preparation
Dy:Y2O3 ceramics with Dy3+ concentrations of 1, 3, and 5 at.% were fabricated by an SPS machine (LABOX-315, Sinter Land, Japan). Commercially available high-purity Y2O3 (99.99 wt% purity; Shin-Etsu Rare Earth, Japan) and Dy2O3 (99.9 wt% purity; Shin-Etsu Rare Earth, Japan) powders were used as starting materials. The powders were mixed to Dy3+ concentrations of 1, 3, and 5 at.% by ball milling with high-purity 5 mm diameter alumina balls in ethanol for 12 h. The mixture was dried in air and sieved through a 200-mesh screen. Then, the powder was poured into a graphite mold with an inner diameter of 10 mm. The mold was heated to 1200–1260°C for each specimen with a holding time of 1 h at the sintering temperature under a uniaxial pressure of 80 MPa in a vacuum. After the sintering, the samples were annealed in air at 1050°C for 10 h, and then mirror-polished on both sides using a diamond slurry for the measurement of optical properties. Detailed information about the fabrication technique was reported in a previous paper . Figure 1 shows a photograph of Dy-doped Y2O3 ceramic samples. The thicknesses of all samples were approximately 1 mm and the diameters were 10 mm. The translucency of this material in the visible region can be seen in Fig. 1.
2.2. Spectroscopic experiments
Transmittance was measured up to 5 μm. Two types of spectrometers (UV-3600Plus, Shimadzu and Frontier, Perkin Elmer) were used to measure the transmittance in different wavelength ranges. The scattering coefficients were estimated from the absorption baseline and Fresnel loss by power-law fitting. The lasing possibility of Dy:Y2O3 in the 3–4-μm wavelength region was evaluated by the spectroscopic method. The emission cross-sections were excited by a 450 nm laser diode (PL TB450B, OSRAM) and measured with an optical spectrum analyzer (OSA205C, Thorlabs). The Füchtbauer–Ladenburg method  with the fluorescence spectrum and a radiative lifetime calculated from the Judd–Ofelt theory [26,27] were used for delivering the emission cross-sections.
The fluorescence lifetime of the 3-μm emission was measured by an InAs detector module with a built-in preamp (C12492-210, Hamamatsu Photonics) with a 3.0–3.5-μm band-pass filter.
2.3. Tests of Dy:Y2O3 ceramic as an SA
The nonlinear absorption properties of the 1 at.% Dy:Y2O3 ceramic sample at a wavelength of 2795 nm were evaluated by a z-scan method with a custom-designed diode-pump continuous-wave Er:YAP laser  with a maximum power of 600 mW. A two-level saturable absorption model  was used for fitting. A passive Q-switching of an Er:YAP laser was set up with the 1 at.% Dy:Y2O3 ceramic sample as the SA. Figure 2 shows a schematic diagram of a passive Q-switched Er:YAP laser with the 1 at.% Dy:Y2O3 ceramic SA. The uncoated Dy:Y2O3 SA was set in the cavity between Er:YAP gain medium and output coupler (Transmittance = 2.5%) with the cavity length of 18 mm. A size of the pumping spot is 350 μm in diameter.
The Q-switched laser characteristics of output power and pulse waveform were measured with a commercial thermal sensor (3A, Ophir Optronics) and an MCT photo detector (HgCdTe, P VI-4TE-3-TO8-AL2O3-35, Vigo System).
3. Results and discussion
3.1. Optical properties of Dy:Y2O3 ceramics
Figure 3 shows the transmittance of the Dy:Y2O3 ceramic samples up to 5 μm. The theoretical transmittance (only considering the Fresnel reflection) of an undoped Y2O3 ceramic sample is shown as a dotted line for reference. Ground-state absorption bands of Dy3+ (6H15/2 →) were observed around 0.8 μm (6F7/2), 0.9 μm (6H5/2), 1.1 μm (6H7/2, 6F9/2), 1.3 μm (6H9/2, 6F11/2), 1.7 μm (6H11/2), and 2.8 μm (6H13/2) with several peaks. Figure 4 shows the wavelength dependence of the absorption coefficient. The absorption coefficient was proportional to the concentration of Dy3+ ions. This means that Dy3+ ions are well doped in the Y2O3 matrix. The absorption coefficients of the 5 at.% sample at 1.68 μm and 2.72 μm wavelengths were 6.6 cm−1 and 11.2 cm−1, respectively. Absorption cross-sections at 2719, 2780 and 2835 nm are 6.9 × 10−21, 5.9 × 10−21, and 3 × 10−21 cm2, respectively. These wavelengths are the optical pumping wavelength for mid-IR lasing. The 3 mm one-pass absorptions are 86.2% (1.7 μm) and 96.5% (2.8 μm) in the Dy:Y2O3 ceramics. The scattering coefficients of the 5 at.% sample were less than 0.08 cm−1 at 3 μm, which is the expected wavelength for mid-IR lasing. This estimation indicates that the expected optical losses without Fresnel reflection loss are lower than 2.4% at wavelengths around 3 μm in 3 mm long Dy:Y2O3 ceramics. A high pumping efficiency is available with a short optical pass length. The peak values of absorption coefficient at 1.3 μm wavelength in 3 at.% and 5 at.% samples could not be evaluated due to extinction saturation.
3.2. Emission properties of Dy:Y2O3 ceramics
Figure 5 shows the emission cross-section excited by a 450 nm laser diode. Emission peaks derived from 6H13/2 → 6H15/2 transitions were observed in the range of 2.6–3.4 μm with the highest cross-section of approximately 1.39×10−20 cm2 at 2843 nm and approximately 0.91×10−20 cm2 at 2779 nm in the 3 and 5 at.% samples, respectively. In spite of the fact that the emissions around 4.25 μm were expected in the radiative transition of 6H11/2 → 6H13/2 in Dy3+, there is no obvious emission around the 4-μm wavelength region from our measurement. This result indicates that 4-μm lasing would not be possible for a Dy:Y2O3 mediusm. This result is similar to that for Dy-doped CaF2 ceramics reported in our previous study . The host materials in which successful lasing at 4 μm with Dy dopant have been reported are include YLF  and PbGa2F4 , with phonon energies less than 495 cm−1, and chalcogenide glass . These materials have lower phonon energies than Y2O3 or CaF2 materials. It can be speculated that, for lasing at 4 μm, it is necessary to suppress the multi-phonon relaxation/non-radiative process in this transition with low phonon energy materials.
The fluorescence lifetime expresses the upper state lifetime of 3-μm lasing. The fluorescence lifetimes of the 1, 3, and 5 at.% samples were 27, 23, and 21 μs, respectively. These values are smaller than those reported for Dy:CaF2 single crystals and ceramics . In contrast to the result of CaF2, no strong concentration quenching due to Dy3+ clustering was observed in the Y2O3 ceramics. Er:ZBLAN fiber lasers [31,32] are suitable pumping sources for 3-μm Dy:Y2O3 ceramic lasers. The specific absorption peak of Dy:Y2O3 ceramics can be achieved by tuning the lasing wavelength . It covers wavelengths of 2.9 μm to 3.4 μm, which was estimated by the positive range of the gain coefficient, as shown in Fig. 6. This wavelength region is quite interesting for remote sensing of hydrocarbon and ammonia analysis .
3.3. Demonstration of Dy:Y2O3 as an SA
Characteristics of nonlinear absorption are important for laser materials as the laser gain medium and as an SA. The pump power dependence of transmittance in Dy:Y2O3 ceramic without Fresnel reflection loss is plotted in Fig. 7. The modulation depth (modulation ratios in total absorptivity) and non-saturable loss were 8.4% and 5.2%, respectively. This includes a scattering loss of about 1%. The total absorption was estimated as 60% of the modulation ratio in this sample. This property is preferable for mid-IR lasers. This value is better than our previous study in Dy:CaF2, which has a modulation depth of 5.1% and a non-saturable loss of 3%, respectively . In addition, these characteristics are better than commonly used SAs such as semiconductor layer SA and a monolayer graphene . The nonlinear absorption of Dy: Y2O3 ceramics exhibits full saturation of over 1 MW/cm2 intensity with a saturation intensity of 0.019 MW/cm2. This value is lower than that for semiconductor SA, monolayer graphene , and Dy:CaF2 ceramics in the mid-IR region .
After the evaluation of the nonlinear transmissivity of the Dy:Y2O3 ceramic SA, we demonstrated a passive Q-switching of an Er:YAP laser with this SA. Stable Q-switched laser emission at 2795 nm was observed within the absorbed pumping power from 4.7 to 8.8 W. The maximum average output power was 91 mW at a pump power of 8.8 W.
Figure 8(a) shows a typical temporal pulse train under 8.8 W pumping, which was the highest absorbed pump power under stable Q-switched operation. The maximum average output power was 91 mW with a 148.4 kHz repetition rate and a pulse energy of 0.61 μJ. Figure 8(b) shows the pulse waveform. The pulse duration in this condition was 267 ns full width at half maximum. The peak power of 2.3 W was calculated from these results. Figure 9 shows the relation between the pumping power and the pulse duration and repetition rate. From the experimental results, the pulse duration was shortened from 1534 to 267 ns with increasing pump power from 4.7 to 8.8 W. The repetition rate was proportional to the pump power and increased from 53.2 to 148.4 kHz. This relationship is reasonable from the passive Q-switching mechanism . This is the first demonstration of passive Q-switching using the Dy:Y2O3 ceramic SA in the 3-μm wavelength region. Although output performance of this scheme was relatively low, a higher pulse energy and peak power can be expected to be achieved with further optimization such as anti-reflection coating of the laser gain medium and SA and suitable cavity design to control the lasing beam intensity at the SA.
Dy:Y2O3 ceramic is a possible SA for various mid-IR solid-state lasers emitting around 3 μm such as [19,35,36]. In addition, a Dy:Y2O3 SA shows no absorption at the wavelength of the pump laser diode source of the Er-based lasers. This is a great advantage over SAs made of graphene, semiconductor, and Fe2+ dopant materials. It can support the simple cavity configuration such as a microchip for a Q-switched Er-doped solid-state laser without a special technique to avoid the pump light absorption in the SA. This is evident from the scaling of the evaluation of linear absorption and non-linear absorption as shown in Figs. 4 and 7.
In this study, we fabricated Dy:Y2O3 ceramics exhibiting a scattering coefficient of <0.08 cm-1 at the potential lasing wavelength around 3-μm by an SPS method. Their near-IR lasing possibility was evaluated based on the detailed optical properties and emission cross-section for various Dy3+ doping concentrations. As a result, we found that around 3-μm lasing is possible by the Dy-doped Y2O3 ceramics. It seems that 4 μm lasing is not possible due to the large non-radiative transition in this host material. A Dy-doped Y2O3 laser has the potential to fill the gap in existing solid-state mid-IR lasers around 3 μm. Our investigation of the nonlinear absorption properties of Dy:Y2O3 ceramics shows great potential as a practical SA at a wavelength of approximately 3 μm. As a demonstration, we have successfully applied the Dy:Y2O3 ceramics as an SA in a passively Q-switched Er:YAP laser at 2795 nm. Dy:Y2O3 ceramics are one of the candidates for the SA for various 2.5–3.2 μm solid-state lasers owing to their favorable SA properties in this wavelength range.
Nippon Sheet Glass Foundation for Materials Science and Engineering (R2-No.5); Amada Foundation (AF-2018228-C2, AF-2019221-B3); National Institute for Fusion Science (KBAH028, KLEH087, UFEX5003, ULHH040); Murata Science Foundation (H31助自009); Japan Society for the Promotion of Science (15KK024, 18H01204, 20K05374).
The authors declare no conflicts of interest.
1. M. R. Majewski, R. I. Woodward, J. Y. Carree, S. Poulain, M. Poulain, and S. D. Jackson, “Emission beyond 4 μm and mid-infrared lasing in a dysprosium-doped indium fluoride (InF3) fiber,” Opt. Lett. 43(8), 1926–1929 (2018). [CrossRef]
2. M. C. Falconi, G. Palma, F. Starecki, V. Nazabal, J. Troles, J. L. Adam, S. Taccheo, M. Ferrari, and F. Prudenzano, “Dysprosium-doped chalcogenide master oscillator power amplifier (MOPA) for mid-IR emission,” J. Lightwave Technol. 35(2), 265–273 (2017). [CrossRef]
3. F. Starecki, F. Charpentier, J. L. Doualan, L. Quetel, K. Michel, R. Chahal, J. Troles, B. Bureau, A. Braud, P. Camy, V. Moizan, and V. Nazabal, “Mid-IR optical sensor for CO2 detection based on fluorescence absorbance of Dy3+:Ga5Ge20Sb10S65 fibers,” Sens. Actuators, B 207(Part A), 518–525 (2015). [CrossRef]
4. S. Heinze, B. Vuillemin, and P. Giroux, “Application of ATR-FTIR spectroscopy in quantitative analysis of deuterium in basic solutions,” Analusis 27(6), 549–551 (1999). [CrossRef]
5. C. Frayssinous, V. Fortin, J. P. Bérubé, A. Fraser, and R. Vallée, “Resonant polymer ablation using a compact 3.44 μm fiber laser,” J. Mater. Process. Technol. 252, 813–820 (2018). [CrossRef]
6. L. F. Johnson and H. J. Guggenheim, “Laser emission at 3 μm from Dy3+ in BaY2F8,” Appl. Phys. Lett. 23(2), 96–98 (1973). [CrossRef]
7. B. M. Antipenko, A. L. Ashkalunin, A. A. Mak, B. V. Sinitsyn, Y. V. Tomashevich, and G. S. Shakhkalamyan, “Three micron laser action in Dy3+,” Kvantovaya Elektron (Moscow) 7, 983–987 (1980).
8. N. Djeu, V. E. Hartwell, A. A. Kaminskii, and A. V. Butashin, “Room temperature 3.4 μm Dy:BaYb2F8 laser,” Opt. Lett. 22(13), 997–999 (1997). [CrossRef]
9. M. R. Majewski and S. D. Jackson, “Tunable dysprosium laser,” Opt. Lett. 41(19), 4496–4498 (2016). [CrossRef]
10. R. I. Woodward, M. R. Majewski, G. Bharathan, D. D. Hudson, A. Fuerbach, and S. D. Jackson, “Watt-level dysprosium fiber laser at 3.15 μm with 73% slope efficiency,” Opt. Lett. 43(7), 1471–1474 (2018). [CrossRef]
11. V. Fortin, F. Jobin, M. Larose, M. Bernier, and R. Vallée, “10-W-level monolithic dysprosium-doped fiber laser at 3.24 μm,” Opt. Lett. 44(3), 491–494 (2019). [CrossRef]
12. N. P. Barnes and R. E. Allen, “Room temperature Dy:YLF laser operation at 4.34 μm,” IEEE J. Quantum Electron. 27(2), 277–282 (1991). [CrossRef]
13. M. C. Nostrand, R. H. Page, S. A. Payne, W. F. Krupke, and P. G. Schunemann, “Room-temperature laser action at 4.3–4.4 μm in CaGa2S4:Dy3+,” Opt. Lett. 24(17), 1215–1217 (1999). [CrossRef]
14. H. Jelínková, M. E. Doroshenko, M. Jelínek, J. Šulc, V. V. Osiko, V. V. Badikov, and D. V. Badikov, “Dysprosium-doped PbGa2S4 laser generating at 4.3 μm directly pumped by 1.7 μm laser diode,” Opt. Lett. 38(16), 3040–3043 (2013). [CrossRef]
15. S. B. Mirov, V. V. Fedorov, D. Martyshkin, I. S. Moskalev, M. Mirov, and S. Vasilyev, “Progress in Mid-IR lasers based on Cr and Fe-doped II–VI chalcogenides,” IEEE J. Sel. Top. Quantum Electron. 21(1), 292–310 (2015). [CrossRef]
16. H. Uehara, T. Tsunai, B. Han, K. Goya, R. Yasuhara, F. Potemkin, J. Kawanaka, and S. Tokita, “40 kHz, 20 ns acousto-optically Q-switched 4 μm Fe:ZnSe laser pumped by fluoride fiber laser,” Opt. Lett. 45(10), 2788–2791 (2020). [CrossRef]
17. J. H. Mun, A. Jouini, A. Novoselov, A. Yoshikawa, T. Kasamoto, H. Ohta, H. Shibata, M. Isshiki, Y. Waseda, and G. Boulon, “Thermal and Optical Properties of Yb3þ-Doped Y2O3 Single Crystal Grown by the Micro-Pulling-Down Method,” Jpn. J. Appl. Phys. 45(7), 5885–5888 (2006). [CrossRef]
18. C. Kränkel, “Rare-earth-doped sesquioxides for diode-pumped high-power lasers in the 1-, 2-, and 3-μm spectral range,” IEEE J. Sel. Top. Quantum Electron. 21(1), 250–262 (2015). [CrossRef]
19. L. Wang, H. Huang, D. Shen, J. Zhang, H. Chen, Y. Wang, X. Liu, and D. Tang, “Room temperature continuous-wave laser performance of LD pumped Er:Lu2O3 and Er:Y2O3 ceramic at 27 μm,” Opt. Express 22(16), 19495 (2014). [CrossRef]
20. T. Sanamyan, M. Kanskar, Y. Xiao, D. Kedlaya, and M. Dubinskii, “High power diode-pumped 2.7-μm Er3+:Y2O3 laser with nearly quantum defect-limited efficiency,” Opt. Express 19(S5), A1082–A1087 (2011). [CrossRef]
21. H. Uehara, S. Tokita, J. Kawanaka, D. Konishi, M. Murakami, S. Shimizu, and R. Yasuhara, “Optimization of laser emission at 2.8 μm by Er:Lu2O3 ceramics,” Opt. Express 26(3), 3497–3507 (2018). [CrossRef]
22. J. Shi, B. Liu, Q. Wang, H. Tang, F. Wu, D. Li, H. Zhao, Z. Wang, W. Deng, X. Xu, and J. Xu, “Crystal growth, spectroscopic characteristics, and Judd-Ofelt analysis of Dy:Lu2O3 for yellow laser,” Chin. Phys. B 27(7), 077802 (2018). [CrossRef]
23. Z. Hu, X. Xu, J. Wang, P. Liu, D. Li, X. Wang, J. Zhang, and D. Jun Xu, “Fabrication and spectral properties of Dy:Y2O3 transparent ceramics,” J. Eur. Ceram. Soc. 38(4), 1981–1985 (2018). [CrossRef]
24. H. Furuse, S. Nakasawa, H. Yoshida, K. Morita, T. S. Suzuki, B. N. Kim, Y. Sakka, and K. Hiraga, “Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering,” J. Am. Ceram. Soc. 101(2), 694–702 (2018). [CrossRef]
25. P. F. Moulton, “Spectroscopic and laser characteristics of Ti:Al2O3,” J. Opt. Soc. Am. B 3(1), 125–133 (1986). [CrossRef]
26. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]
27. G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37(3), 511–520 (1962). [CrossRef]
28. H. Kawase, H. Uehara, H. Chen, and R. Yasuhara, “Passively Q-switched 2.9μm Er:YAP single crystal laser using graphene saturable absorber,” Appl. Phys. Express 12(10), 102006 (2019). [CrossRef]
29. Q. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19(19), 3077–3083 (2009). [CrossRef]
30. H. Uehara, W. Yao, A. Ikesue, H. Noto, H. Chen, Y. Hishinuma, T. Muroga, and R. Yasuhara, “Dy-doped CaF2 transparent ceramics as a functional medium in the broadband mid-infrared spectral region,” OSA Continuum 3(7), 1811 (2020). [CrossRef]
31. H. Uehara, D. Konishi, K. Goya, R. Sahara, M. Murakami, and S. Tokita, “Power scalable 30-W mid-infrared fluoride fiber amplifier,” Opt. Lett. 44(19), 4777–4780 (2019). [CrossRef]
32. K. Goya, H. Uehara, D. Konishi, R. Sahara, M. Murakami, and S. Tokita, “Stable 35-W Er:ZBLAN fiber laser with Y2O3 end caps,” Appl. Phys. Express 12(10), 102007 (2019). [CrossRef]
33. R. I. Woodward, M. R. Majewski, D. D. Hudson, and S. D. Jackson, “Swept-wavelength mid-infrared fiber laser for real-time ammonia gas sensing,” APL Photonics 4(2), 020801 (2019). [CrossRef]
34. G. J. Spuhler, R. Paschotta, R. Fluck, B. Braun, M. Moser, G. Zhang, E. Gini, and U. Keller, “Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers,” J. Opt. Soc. Am. B 16(3), 376–388 (1999). [CrossRef]
35. H. Uehara, S. Tokita, J. Kawanaka, D. Konishi, M. Murakami, and R. Yasuhara, “A passively Q-switched compact Er:Lu2O3 ceramics laser at 2.8 μm with a graphene saturable absorber,” Appl. Phys. Express 12(2), 022002 (2019). [CrossRef]
36. D. W. Chen, C. L. Fincher, T. S. Rose, F. L. Vernon, and R. A. Fields, “Diode-pumped 1-W continuous-wave Er:YAG 3-μm laser,” Opt. Lett. 24(6), 385–387 (1999). [CrossRef]