## 1 Introduction  
Tissue ablation in human, for example, brains and eyes, requires the highest precision in the target tissues, reducing collateral damage to surrounding tissues, as far as possible. The viability of the remaining tissues is crucial to assess the effectiveness of ablation. Particularity, the mid-infrared laser at 6.45 \(\upmu\)m has tremendous potential as a high-precision surgical tool, as it offers minor collateral damage and a rapid ablation rate [1]. The penetration depth by 6.45 \(\upmu\)m laser amounts to several \(\upmu\)m, which approximates the single cell thickness and approaches the optimal value [2]. Especially, it is useful for ablation in ocular, neural, and dermal tissues [3]. Therefore, the development of the MIR laser at 6.45 \(\upmu\)m has become a perennial hot topic. Four ways are commonly used to obtain 6.45 \(\upmu\)m laser, including free electron lasers (FELs), gas Raman lasers, strontium vapor lasers, and solid-state lasers, based on nonlinear optical (NLO) frequency conversion techniques [1, 2, 4, 5]. However, the high cost and bulky structure of FELs limit their extensive application. Short operating life and tedious maintenance also hinder the development of gas Raman and strontium vapor lasers. In view of the precise and compact requirements of their application, solid-state lasers based on NLO frequency conversion techniques are an attractive means to generate high-energy MIR waves, such as optical parametric generation (OPG), the optical parametric amplifier (OPA), and the optical parametric oscillator (OPO). Among these general methods, OPO based on MIR crystals with good nonlinear properties and accessible pump lasers is a promising candidate for generating high-performance, long-wave MIR sources. At present, a variety of promising NLO crystals, which can be pumped by 1 \(\upmu\)m laser to obtain a 6.45 \(\upmu\)m laser, have been developed, including chalcogenides and phosphorus compounds, such as AgGaSe\({}_{2}\) (AGSe) [6], LiInS\({}_{2}\) (LIS) [7], LiInSe\({}_{2}\) (LISe) [8], BaGa\({}_{4}\)S\({}_{7}\) (BGS) [9], BaGa\({}_{4}\)Se\({}_{7}\) (BGSe) [10, 11], and CdSiP\({}_{2}\) (CSP) [12]. Among these NLO crystals, BGSe [13, 14], is a potential NLO crystal for generating MIR radiation by parametric down-conversion. It is a new positive biaxial selenide compound and belongs to the \(m\) monoclinic point group, space group \(P_{\text{c}}\), with \(a\) = 7.6252(15) A, \(b\) = 6.5114(13) A, \(c\) = 14.702(4) A, \(\beta\) = 121.24\({}^{\circ}\), and \(Z\) = 2. Moreover, BGSe crystal can operate under high pulse energy, owing to its excellent, intriguing characteristics. For example, firstly, it has high NLO coefficients with the two major nonlinear tensor elements of \(d_{16}\) = 31.5 and \(d_{23}\) =22.1 pm/V [15]. Secondly, we should note that BGSe crystal can be processed in a large aperture via the Bridgman-Stockbarger method, which is favorable for high pulse energy operation. Besides, it has an intrinsic wide bandgap of about 2.73 eV, which can help to enhance the damage threshold (a surface laser damage threshold of 557 MW/cm\({}^{2}\) at 1 \(\upmu\)m with 1 Hz and 5 ns is about 3.7 times that of AgGaS\({}_{2}\)[16]) and avert the nonlinear absorption effect (two-photon absorption, TPA) of economical 1 \(\upmu\)m lasers [16, 17]. Furthermore, theoretical modeling indicates that the chemical kinetics happen on the nanosecond (ns) time scale, guiding us to research ns pulsed 6.45 \(\upmu\)m MIR lasers with high energy and narrow linewidths [2, 18]. Several works on 6.45 \(\upmu\)m ns MIR generation, from a 1 \(\upmu\)m laser based on BGSe-OPO, were reported [19, 20]. For example, a ns BGSe-OPO pumped by Q-switched Nd:YLiF\({}_{4}\) laser at 1053 nm was reported. MIR idler wave tuning from 2.6 to 10.4 \(\upmu\)m was demonstrated with an angle-tuned, type-I (o \(\rightarrow\) e + e) phase-matching \(y\)-cut sample, but the output energy at 6.45 \(\upmu\)m was only about 13 \(\upmu\)[19]. Moreover, widely tunable MIR radiation from 2.7 to 17 \(\upmu\)m was demonstrated.