## 1 Introduction  
Deep ultraviolet (DUV) lasers are currently widely employed in various applications. For instance, a DUV laser at 260 nm has been applied as an \"external seed\" of a free-electron laser (FEL) with output wavelengths as short as 4.3 nm, which would make it possible to do the scientific research beyond the carbon K-edge [1]. In the industrial applications, laser machining of wide bandgap materials would be also beneficial from the DUV lasers owing to their characteristics of high photon energy [2]. In the past decades, with the fast development of the solid-state lasers and the nonlinear optical crystals, high-power DUV lasers at 266 nm/258 nm have been studied by many research groups fruitfully [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. These DUV lasers are usually generated by fourth-harmonics generation (FHG) of the near-infrared (NIR) laser at 1 \(\upmu\)m as shown in Figure 1. Until now, the highest average output power of these DUV lasers was 40 W at the wavelength of 266 nm, of which the fundamental light was a high-power Nd:Y\({}_{3}\)Al\({}_{5}\)O\({}_{12}\)(Nd:YAG) laser at 1064 nm and the nonlinear optical crystal was a CsLiB\({}_{6}\)O\({}_{10}\) (CLBO) [8]. In the scientific and industrial applications, the beam quality of the DUV laser is also an important parameter. Particularly, the beam quality of the DUV laser dramatically affects the laser machining results. The beam quality of the DUV laser is determined by that of the fundamental laser and the crystal for the frequency conversion. Recently, a high-power DUV laser (>10 W) at 258 nm was reported with a beam quality of M\({}^{2}\) < 1.5, which was generated by FHG of a Yb:YAG laser at 1030 nm using the CLBO crystal [13]. This laser would lead to a better laser machining result compared to the previous DUV lasers with larger M\({}^{2}\). In terms of the solid-state DUV laser at 193 nm, the laser machining as well as other applications such as mask-inspection or lithography require a high-power DUV laser output [4; 18; 19]. The highest output power of the continuous-wave (CW) 193-nm laser was 120 mW by stages of sum-frequency generation (SFG) between DUV lasers (wavelength longer than 193 nm) and corresponding NIR lasers [4]. For the pulsed lasers, a DUV laser at 193 nm with average power of 300 mW was reported and had already been applied to be the seed of a hybrid ArF excimer laser [2; 12]. By use of this hybrid ArF excimer laser schematics, both high power (>100 W) and high coherence could be obtained at the same time, which consists of a solid-state DUV laser seed and an ArF excimer amplifier [2]. A high-power solid-state DUV seeding laser with high coherence is beneficial to the final output of the hybrid ArF excimer laser, and it would also result in suppressing the amplified spontaneous emission (ASE) of the amplifier. SFG is the most popular method to generate the DUV laser at 193 nm. Typically, a DUV laser, of which the wavelength is usually from 200 nm to 300 nm, and a NIR laser play the roles of SFG lasers in the nonlinear crystals to generate the 193-nm DUV laser. So far, the highest output power at 193 nm by following the above SFG schematics was 1 W by two stages of SFG [20]. As aforementioned, there is still great demand for power scaling of the solid-state DUV laser at 193 nm. Thus, by the SFG schematics, the requirement is to increase both the DUV laser (wavelength longer than 193 nm) power and the NIR laser power at the same time. By choosing the appropriate high-power fundamental laser at 1 um as well as the nonlinear optical crystals, the average power of the DUV laser at the fourth harmonics of 1 um could be obtained to be at the 10-watts level in the previous reports. On the other side, the power scaling of the NIR laser is another aspect to increase the average power of the 193-nm DUV laser.