为抑制光参量振荡器（Optical Parametric Oscillator，OPO）振荡过程中信号光和闲频光向泵浦光的逆转换，首次采用在L型OPO腔的支路中插入信号光倍频晶体LiB₃O₅的（简称LBO）的方式，实现了BaGa₄Se₇（BGSe） OPO闲频光的高转换效率输出，当泵浦激光（1.06 μm）能量为115 mJ时，闲频光（3.5 μm）能量为16.18 mJ，光光转换效率为14.06%，斜效率为18.4%，这是目前已知1.06 μm激光泵浦BGSe OPO最高的转换效率。模拟了不同泵浦能量下L型腔中有无LBO晶体时BGSe OPO腔内的三波波形，并给出了闲频光在实验中的输出波形。与传统OPO腔相比，所提出的L型OPO腔（含倍频晶体）在大能量泵浦条件下抑制了逆转换，可获得更高的闲频光转换效率。

Nonlinear frequency conversion of wavelength agile and high-power random fiber lasers can provide a promising way to generate continuous-wave (CW) visible and mid-infrared (MIR) light with unique properties such as the continuous modeless spectrum, low temporal/spatial coherence, and high temporal stability. Here, we report a dual-wavelength switchable and tunable random Raman fiber laser (RRFL) based on a phosphosilicate fiber that has two Raman gain peaks for the first time and demonstrate its superior capability to generate widely tunable CW visible and mid-infrared light via nonlinear frequency conversions. By using the combination of a tunable pump and two tunable gratings in Littrow configuration that can provide separated point feedback for the two Stokes wavelengths corresponding to silica- and phosphorus-related Raman peaks, the spectrum of an RRFL can be flexibly manipulated for the aim of nonlinear frequency conversions, including single-wavelength tunable emission at the 1.1 μm or 1.2 μm band for second-harmonic generation (SHG), dual-wavelength simultaneously tunable emission at the 1.1 μm and 1.2 μm bands for the sum-frequency generation (SFG), and dual-wavelength separation tunable emission for difference-frequency generation (DFG). As a result, with the combination of SHG and SFG in a periodically poled lithium niobate crystal array, we experimentally demonstrate the broadest tuning range (560–630 nm) of visible light generated from an RRFL, to the best of our knowledge. The tunable MIR light in the range of 10.7–12.3 μm is also demonstrated through DFG of an RRFL operating in separation tunable dual-wavelength emission mode in a BaGa4Se7 (BGSe) crystal, which is the first realization of >10μm CW DFG in the BGSe crystal. We believe the developed dual-wavelength switchable and tunable RRFL can provide a new compact, robust, and cost-effective platform to realize broadly tunable light in both the visible and MIR regions, which can also find potential applications in imaging, sensing, and temporal ghost imaging in various spectral bands.

The linewidth of the BaGa4Se7 (BGSe) optical parametric oscillator (OPO) was narrowed for the first time, to the best of our knowledge, by inserting a Fabry–Perot (FP) etalon into an L-shaped cavity. When a 15 mm long BGSe (56.3°, 0°) was pumped by a 1064 nm laser, the peak wavelength was ∼3529nm, and the linewidth was 4.53 nm (3.64cm-1) under type I phase matching. After inserting a 350 µm thick FP etalon, the linewidth was decreased to 1.27–2.05 nm. When the tilt angle of the etalon was 2.34°, the linewidth was 2.05 nm (1.65cm-1), and the peak wavelength was still ∼3529nm. When the tilt angle of the etalon was 3.90°, the peak wavelength was 3534.9 nm, and the linewidth was 1.27 nm (1.02cm-1), which was the narrowest linewidth of a BGSe OPO, to the best of our knowledge. The beam quality was also improved after inserting the FP etalon.

The temperature tuning of BaGa4Se7 (BGSe) was demonstrated for the first time, to the best of our knowledge. When the temperature of BGSe (56.3°,0°) was raised from 30°C to 140°C, the idler light under type I raised from 3637 nm to 3989 nm, the tunable range reached 352 nm, and Δλ2/ΔT reached 3.20 nm/°C. We calculated the phase matching curve of BGSe when ? and T took different values. The relationship between θ and Δλ2/ΔT was obtained by fixing ? at 0°. The maximum Δλ2/ΔT and its corresponding (θ,?) phase matching type were reported under different fixed λ2(3μm,3.2μm,…,5μm).

BaGa4Se7(硒镓钡，简称BGSe)与KTiOAsO4(砷酸氧钛钾，简称KTA)均可在1.06 μm激光泵浦下产生中红外激光。首先仿真计算出两种非线性晶体的相位匹配曲线，结果显示：切割角为(56.3°, 0°)的BGSe晶体在I类相位匹配条件下和切割角为(90°, 0°)的KTA在II-A类相位匹配条件下均可产生~3.5 μm的闲频光。然后理论计算出BGSe (56.3°, 0°, I类)的有效非线性系数为-11.9 pm/V，KTA(90°, 0°，II-A类)的有效非线性系数为-3.2 pm/V；在其他条件相同的情况下，15 mm长BGSe (56.3°, 0°, I类) 的OPO振荡阈值是20 mm长KTA (90°, 0°, II-A类) OPO振荡阈值的35.11%。最后通过实验验证BGSe (56.3°, 0°, I类, 15 mm) 的振荡阈值小于KTA(90°, 0°, II-A类, 20 mm)，输出的中红外激光能量大于KTA。因此，BGSe是一种极具应用前景的中红外非线性晶体。

We report an efficient mid-infrared extracavity optical parametric oscillator (OPO) based on the nonlinear crystal BaGa₄Se₇ pumped by a diode-side-pumped Q-switched Nd:Y₃Al₅O₁₂ (Nd:YAG) laser. The maximum pulse energy of 1.03 mJ at 4.25 μm is obtained with the repetition rate of 10 Hz and pulse width of 12.6 ns when the pump energy was 13.5 mJ, corresponding to an optical-to-optical conversion efficiency of 7.6% from 1.064 μm to 4.25 μm. The idler wave slope conversion efficiency was 12%. To the best of our knowledge, it is the highest reported conversion efficiency for the compact BaGa₄Se₇ OPO driven by the Nd:YAG laser.