2.92-μm high-efficiency continuous-wave laser operation of diode-pumped Er:YAP crystal at room temperature

Mid-infrared lasers have attracted attention for application to the fields of medicine and industry. In this study, we demonstrate continuous-wave laser operation of a diodepumped 5 at % Er-doped YAlO3 (YAP) single-crystal lasing at 2.92 μm with near-quantumdefect slope efficiency at room temperature. A high slope efficiency of 31% is achieved with a maximum output power of 0.674 W for a cavity length of 18 mm and an output coupler transmittance of 2.5%. This efficiency is 94% of the theoretical quantum-defect efficiency. Our results indicate that Er:YAP lasers can potentially be utilized to realize high-power midIR lasing. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Er:YAP laser operating in the 3.0-µm spectral range via the application of a flash-lamp pumping scheme [12,13] and an Argon laser pumping scheme [14]. In addition, recent studies have reported on CW laser operation by diode-pumping Er:YAP at approximately 2.7 µm [15,16]. In 2018, Quan et al. [16] demonstrated a CW 10 at.% Er-doped YAP laser at a dual wavelength of 2710 and 2728 nm by diode-pumping at room temperature. The CW Er:YAP laser achieved a maximum output of 739 mW with the slope efficiency of 12.1%. However, high-output, high-efficiency, CW laser operation using Er:YAP at approximately 2.9 µm at room temperature has not been demonstrated previously. Therefore, in this study, we attempt to demonstrate high-output, high-efficiency CW laser operation using a diode-pumping Erdoped YAP at room temperature.
We report on a demonstration of 2.92-µm Er:YAP CW laser operation. When a 5 at.% Er:YAP crystal was pumped by an LD with a center wavelength of 976.2 nm at room temperature, CW lasing at a wavelength of 2920 nm was obtained for an absorbed pump power of 1.98 W. Further, a slope efficiency of 31% and an output power of 0.674 W was obtained with 3.49 W of absorbed pump power; these maximal values were obtained with a 2.5% output coupler (OC) transmittance. These experimental results indicate that the Er:YAP laser can find potential application as a mid-IR laser.

Optical properties of Er:YAP
In this study, a 5 at.% Er-doped YAP single-crystal (Crytur Co., Ltd.) was used for measuring the optical properties and for laser operation. The Er:YAP crystal was rectangular in shape with the aperture dimensions of 2 mm × 5 mm, and it was 8 mm in length. The aperture was uncoated, and the optical axis was aligned perpendicular to the "b" crystallographic axis. The absorption spectrum of Er:YAP in the range of 325-3200 nm was measured with a spectrophotometer (UV3600 Plus, SHIMADZU Co., Ltd.) at room temperature, as shown in Fig. 1. Note that Er:YAP exhibits no absorption around a wavelength of 3000 nm for laser emission with the 4 I 11/2 → 4 I 13/2 transition. On the contrary, absorption bands due to Er 3+ ions are observed in the range of 340-1660 nm. Each absorption band corresponds to the transitions between the Er 3+ ion energy levels [17]. The inset in Fig. 1 shows the absorption bands at around 1000 nm originating from the 4 I 15/2 → 4 I 11/2 transition of the Er 3+ ions when subjected to laser pumping. These separated peaks are due to the Stark effect [17,18]. In this study, we used a fiber-coupled LD (K976A02RN-9.00WN0N-10255I10ESM0, BWT BEIJING) with a center wavelength (λ center ) of 976.2 nm, spectral width of 0.4 nm, core diameter of 105 µm, and numerical aperture (NA) of 0.22 as the excitation source. The center wavelength overlapped one of the absorption wavelengths at 976 nm with an absorption coefficient of 1.67 cm −1 . Thus, laser oscillations with a wavelength of approximately 3 µm can be expected for this sample pumped by this LD. As shown in Fig. 2, luminescence at around 3 μm and 1.66 μm is observed for the 4 I 11/2 → 4 I 13/2 and 4 I 13/2 → 4 I 15/2 transitions, respectively. In our study, the fluorescence spectrum was measured by diode-pumping the Er:YAP with a LD, as mentioned earlier. We note from the figure that the spectrum exhibits peaks at around 1600 nm for the 4 I 13/2 → 4 I 15/2 transition and at 2800 nm for the 4 I 11/2 → 4 I 13/2 transition. The single exponential decays of fluorescence, which indicate the fluorescence lifetime, were measured with an infrared detector (C12492-210, HAMAMATSU) and analyzed using an oscilloscope with a frequency band of 500 MHz (TDS5054B, Tektronix). The lifetimes of the 4 I 13/2 → 4 I 15/2 and 4 I 11/2 → 4 I 13/2 transitions are 7.3 and 0.85 ms, respectively. A peculiarity of the Er 3+ -doped laser is that the lower-level lifetime is greater than that of the upper level [4,19]. Here, attention is required to compare spectroscopic parameters in YAP crystal because there are different absorption coefficients by the axes in the YAP crystal [20]. Thus, it is difficult draw comparisons spectroscopic parameters such as absorption coefficient and lifetime at this paper owing to the lack information on the crystal axis in Ref [15]. of 1 at.% Er:YAP and Ref [21]. of 10 at.% Er:YAP. We aim to measure the dopant dependence of the absorption coefficient and lifetime in future work.

Optimizat
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-temperature 2 crystal. Our res f an Er:YAP CW ssion cross se urements and higher-efficienc he Er 3+ conc ser has the po asers promising n Science (NIF (18H01204).