The LiB₃O₅(LBO) was inserted into the branch of the L-shaped BaGa₄Se₇(BGSe) optical parametric oscillator (OPO) to improve the conversion efficiency for the first time. When the pump laser energy is 115 mJ (1.06 μm), the idler light (3.5 μm) energy was 16.18 mJ, corresponding to the conversion efficiency of 14.06%, and the slope efficiency was 18.4%, which was the highest conversion efficiency of BGSe OPO pumped by 1 μm laser. The signal, idler, and pump wave waveform in BGSe L OPO cavity with and without LBO crystals was simulated, and the output waveform of idler light was given. Compared with traditional OPO cavities, L-type OPO cavities (with frequency doubling crystals) suppress the inverse conversion under high-energy pumping conditions, achieving higher idle frequency light conversion efficiency.

  Objective  The mid-infrared (IR) coherent sources in the 3-5 μm have always been intensively demanded for a wide range of scientific and technological applications in remote sensing, spectrum analysis, materials diagnostics, aerospace fields, etc. Optical parametric oscillation is an attractive approach, especially when high energy and average power are demanded simultaneously. However, there is reverse conversion in the OPO cavity.When the pump energy is high, the signal light and idle frequency light generated are also strong. At this time, the signal light and idle frequency light will be converted to the pump light, which seriously affects the conversion efficiency of OPO. In addition, due to the high intensity of signal light in the cavity during the reverse conversion, it is easy to damage the nonlinear crystal or its coating.Therefore, how to suppress reverse conversion in the OPO cavity under high-energy pumping conditions and improve the conversion efficiency of OPO has always been the focus of research.

  Methods  To suppress the inverse conversion in the OPO cavity, we proposed a method of inserting a frequency doubling crystal into the L-type OPO cavity to suppress the signal light intensity (Fig.1). All three mirrors of the L-shaped cavity are coated with a high-reflection coating for the signal laser, and crystals are inserted in the L-branch to achieve intracavity frequency doubling of the signal laser. When the energy density of the signal laser in the OPO cavity is high, the signal laser is converted into red light by the frequency doubling crystal and output from the L branch. At the same time, the signal laser is attenuated, reverse conversion is suppressed, and the efficiency of idle laser conversion is improved.

  Results and Discussions  The idler laser energy was 16.18 mJ at a pump energy of 115 mJ, corresponding to an optical-to-optical conversion efficiency of 14.06% and a slope efficiency of 18.4% (Fig.2). It is the highest conversion efficiency for BaGa₄Se₇ (BGSe) OPO pumped by a 1.06 μm laser, to the best of our knowledge. The energy density of the three waves at the output of the OPO cavity is simulated. The simulation results show that the optical-to-optical conversion efficiency of the idler laser with the LiB₃O₅ (LBO) inserted in the cavity is 1.20 times higher than that without LBO in the cavity at a pump energy of 80 mJ (Fig.3). The OPO output wavelength could be tuned by adjusting the angle of the BGSe crystal (Fig.5). When the θ angle of the crystal is changed, the experimental peak wavelength agrees well with the theoretical simulation curve, and the measured \Delta \lambda _2/\Delta \mathrm\theta is −231.81 nm/(°) . When changing the φ angle of the crystal, the measured \Delta \lambda _2/\Delta \varphi of −6.25 nm/(°) deviates from the theoretical value of −1.25 nm/(°) because the incident direction of the pump laser is difficult to exactly coincide with the θ=56.3° line of BGSe.

  Conclusions  The conversion efficiency of idler light in OPO cavity was improved by inserting a signal laser frequency doubling crystal into the L-shaped OPO cavity for the first time. When the pump energy is 115 mJ, the 16.18 mJ of the idler laser energy was obtained in BGSe OPO. The optical-to-optical conversion efficiency was 14.06%, and the slope efficiency was 18.4%, which is the highest conversion efficiency of BGSe OPO pumped by a 1.06 μm laser. The output wavelength of BGSe OPO with high conversion efficiency can also be tuned.