**1. Introduction**

Widely tunable mid-infrared radiation source operating in 3~5 μm region has been applied to numerous frontier applications, including remote sensing, molecular spectroscopy and atmosphere environmental monitoring [1–3]. OPOs pumped by a Q-switched laser are commonly used in high energy, widely tunable mid-infrared pulse generation. The nonlinear crystal is of great significance to the output performance OPO. Non-oxide crystals usually have a wider transparent range than oxide crystals which have unneglectable multi-phonon absorption over 4 μm [4]. Therefore, non-oxide crystals have advantages in mid-infrared wave generation especially in the range over 4 μm. Due to the defects from the grown process and the limit of the bandgap, the transmittance in visible to nearinfrared range of some non-oxide crystals is relatively low. It leads to a limitation on the pump wavelength of non-oxide crystal-based OPOs. Non-oxide crystals used for 1 μm laser pumped OPOs are of great interest, including AgGaS2 [5], HgGa2S4 [6], LiInSe2 [7], LiGaS2 [8], BaGa4S7 [9], and CdSiP2 [10]. High-quality non-oxide crystals which can be used for a commercial 1 μm laser pumped OPO are still urgently needed.

The newly developed mid-infrared crystal BaGa4Se7 (BGSe) exhibits a wide bandgap (2.64 eV), wide transparent range (0.47–18 μm) and high laser damage threshold (557MW/cm2), which is beneficial for OPOs pumped by an economical 1064 nm laser [11]. Generation of high energy mid-infrared laser has been reported in both 3~5 μm mid-wave mid-infrared [12–14] and 8~14 μm long-wave mid-infrared range [14–16]. The maximum energy of 21.5 mJ/pulse has been reported using a large-size BGSe of 10 × <sup>10</sup> × 16 mm3 [17]. However, due to the relatively

**Citation:** He, Y.; Yan, C.; Chen, K.; Xu, D.; Li, J.; Zhong, K.; Wang, Y.; Yao, R.; Yao, J.; Yao, J. High Repetition Rate, Tunable Mid-Infrared BaGa4Se7 Optical Parametric Oscillator Pumped by a 1 μm Nd:YAG Laser. *Appl. Sci.* **2022**, *12*, 7197. https:// doi.org/10.3390/app12147197

Academic Editor: Edik U. Rafailov

Received: 21 June 2022 Accepted: 14 July 2022 Published: 17 July 2022

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low thermal conductivity [18] and the increasing damage probability under high repetition rate laser pumping [19–21], reports on the high average power BGSe-OPO pumped with 1064 nm laser are rare.

In this paper, we demonstrated a widely tunable BGSe optical parametric oscillator with a repetition rate up to 250 Hz. Pumped by a side-pumped Q-switched Nd:YAG laser, the BGSe-OPO reached a maximum output average power of 250 mW at 4.06 μm and an optical-to-optical conversion efficiency of 11.1%, which is the highest in the current results regarding 1064 nm laser pumped high repetition rate mid-infrared source based on BGSe crystal. Using a pump double-pass configuration, the OPO threshold was reduced to 2.65 mJ/pulse with a cavity length of 30 mm. Tunable mid-infrared output from 3.42 μm to 4.73 μm is achieved by changing the critical phase-matching condition. The pulse width of the generated mid-infrared wave was measured to be 13.6 ns and the linewidth at 4061 nm is estimated to be 3.92 nm.
