A Stable and Compact Mid-IR at 6.45 \(\upmu\)m Exceeding 6 mJ of Pulse Energy BaGa\({}_{4}\)Se\({}_{7}\) Optical Parametric Oscillator  
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Therefore, it can be said that OPO based on the different NLO crystal materials supplies a simple, stable and efficient method to obtain such high pulse energy and high average power lasers [9]. As the central module of the OPOs, the characteristics of NLO crystals are determining factors in the performance of nonlinear frequency conversion. ZnGeP\({}_{2}\) (ZGP) crystal is a crucial NLO crystal for MIR radiation. ZGP-OPO is an ideal way to obtain high-efficiency laser radiation at around 6 \(\upmu\)m in view of its excellent physical and chemical properties, high thermal conductivity, high nonlinear coefficient and high damage threshold [10]. However, ZGP crystal exhibits a severe absorption effect at a near-infrared wavelength of around 1 \(\upmu\)m and thus has to be pumped by laser radiation above a 2 \(\upmu\)m wavelength, whose pump sources with high performance are expensive and complicated. To date, with the rapid development of a variety of promising NLO crystals, such as AgGaSe\({}_{2}\) (AGSe) [11], BaGa\({}_{4}\)Se\({}_{7}\) (BGSe) [12] and CdSiP\({}_{2}\) (CSP) [13] crystals, a very straight possibility would be a mature 1 \(\upmu\)m pumped OPOs to create wavelengths above 6 \(\upmu\)m. Similarly, among these crystals, each has its own advantages and disadvantages. For example, AGSe crystal possesses wide transparent regions but has a low laser damage threshold, which restricts its capability to achieve a high pulse energy and high average power long wave MIR laser. Additionally, CSP crystal shows an excellent nonlinear effect and exhibits exceptionally high nonlinearity. For instance, with CSP-OPO synchronously pumped by a master oscillator power amplifier (MOPA) system at 1064.1 nm with 8.6 ps, an idler pulse energy up to 1.5 mJ has been achieved at 6.275 \(\upmu\)m with an optical-to-optical (o-o) conversion efficiency of 5% [14]. The idler wavelength is tunable over 486 nm. Unfortunately, when pumped by a 1064 nm laser, for type I phase-matching this exhibits idler wavelengths that are approximate to the upper-wave transparency limit which is essential to minimally invasive surgery in the spectrum wavelength near 6.45 \(\upmu\)m and reachable by working at 122 \({}^{\circ}\)C [15], which is inconvenient to use. On the other hand, BGSe crystal is a commonly available MIR NLO crystal due to large nonlinear coefficients (\(d_{16}\) = 31.5 and \(d_{23}\) = 22.1 pm/V) [16], wide transmission range (0.47-18 \(\upmu\)m), wide bandgap (2.64 eV), moderate birefringence (\(\Delta\)_n_ = 0.08 @ 1 \(\upmu\)m) and high laser damage threshold (557 MW/cm\({}^{2}\)) [17]. BGSe crystal is a part of the monoclinic class, point group \(m\), space group _Pc_ and is a positive biaxial crystal. This NLO crystal has good growth performance, and it is easy to obtain large-size and high-quality crystals, which are fit for the achievement of high average power and high pulse energy MIR waves. Recently, a tunable MIR (2.7-17 \(\upmu\)m) BGSe-OPO pumped by a 1064 nm laser was presented. With a high pump energy of 27 mJ from a Nd:YAG MOPA system, an output pulse energy of \(\sim\)1 mJ at 6.45 \(\upmu\)m with a pulse duration of \(\sim\)10 ns and a pulse repetition rate (PRR) of 10 Hz was obtained, having an o-o conversion efficiency of 4% [18]. However, the MOPA laser system was quite complicated, bulky and costly. With a model that was still very new, based on a BGSe-OPO pumped by a more simple and compact 1064 nm laser oscillator, we successfully scaled pulse energy at 6.45 \(\upmu\)m to 1.23 mJ under 58 mJ pump energy, with a pulse duration of 24.3 ns and a PRR of 10 Hz, corresponding to an o-o conversion efficiency of 2.1% [19], where an unfocused pump spot with a diameter of 2 mm and a BGSe crystal with the length of 18 mm were employed.