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.

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The absorptance of metals is often low within the visible and near-infrared band at room temperature. Increasing the absorption of metals plays a vital role in reducing energy consumption and production cost. After irradiation by 10 ns linearly polarized pulses with fluence close to the zero-probability damage threshold, the surface of 45# steel samples exhibited four kinds of surface structures in the spot area. The samples’ absorptance is improved by 38% when a high-quality laser-induced surface structure (LIPSS) appears at the spot centre. With the increase of the number of pulses, LIPSS begin to melt down, which will decrease the surface absorptance due to the appearance of damage stripes. The relative absorptance of samples was measured by an integrating sphere system. The paper reports detailed experiments to show that LIPSS can improve samples’ absorptance significantly.