1 Tunable mid-infrared laser sources operating at the transparency windows of 3-5 µ m in the atmosphere are more and more important for applications in laser remote sensing, laser target detecting, spectra analysis, atmospheric environment monitoring, space communication and applications in military. There are two effective methods to generate it: second harmonic generation (SHG) of CO 2 laser [1, 2] and optical parametric oscillator (OPO) [3][4][5][6][7][8][9][10]. The former has the limited tuning range at the atmospheric window, which is due to the limited tunable range of CO 2 laser. The latter is an effective tool to cover the 3-5 µ m range. Mid-infrared OPO especially pumped by around 1 µ m wavelength (such as Nd:YAG, Nd:YLF, Yb:YAG lasers) is more and more important with the development of growth technology of nonlinear crystals. The most important reason is that commercial Nd or Yb lasers, as a pumping source, have high output energy and good beam quality, which are widely distributed and used in mobile systems with compact design. In order to cover the ultra-broadband range of 3-5 µ m, the nonlinear crystal in OPO is required to have the large optical transmission and can be phase matched around 1 µ m. By far, commercial negative silver thiogallate crystal is one of the few crystals which can be pumped by commercially available 1.064 µ m Nd:YAG laser to achieve phase-matched down-conversion into the λ > 5 µ m region. AgGaS 2 (AGS) crystal has high nonlinear optical coefficient and high optical transmission from 0.5-12.0 µ m, which makes it realistic to generate infrared parametric radiation. That is why numerous experiments with one or multistage OPO/OPG pumped by ns and ps IR dye, Nd:YAG and another near IR solid state 1 The article is published in the original. lasers, so as by femtosecond Ti:sapphire and Cr:forsterite lasers, are carried out during last decade. Comparing with optical parametric superfluorescence, ultra-broadband tunable, mid-infrared OPO is a more challenging work due to the difficulty on its fabrication of ultrabroadband reflectivity coating.In this paper, the presented setup is a simplified version of our earlier one [7]. A coated prism as the output coupling device of OPO was used to replace the common cavity mirror. It was also functionalized a filter to separate the pump and ultra-broadband tunable idler at the same time. The problem on ultra-broadband reflectivity coating has been avoided. AgGaS 2 type-I singly resonant optical parametric oscillator (SRO OPO) with prism cavity pumped by a ns 1.064 µ m Nd:YAG laser was demonstrated experimentally. The ultra-wide tuning 2.35 to 5.27 µ m and maximum output energy 0.58 mJ per pulse at idle light were obtained.Due to the relatively low damage threshold of AGS crystal, how to lower the pump threshold was considered firstly. Threshold pump energy density is mainly determined as a function of cavity length, crystal physical length, pump spot size, pump pulse width as we discussed previously [7,11]. The influence of the parameters on th...
