## 3 DUV Laser at 193 nm
wavelength of DUV laser generated by SHG in BBO is limited to 205 nm by a phase matching condition although its absorption edge could reach down to 189 nm [40]. Nevertheless, the BBO crystal could be used in SFG process to generate the 193 nm laser. [PERSON] et al. reported a 193.4 nm laser by SFG between a 266 nm and a 708.6 nm laser with the output power 2.5 mW [19]. Even more than 20 years ago, femtoseconds 193 nm laser was generated in BBO by SFG between 258 nm and 774 nm [34]. Another drawback of BBO crystal is a large walk-off effect, which would deteriorate the SFG laser beam quality. LBO crystal could also be used for 193-nm laser generation if the wavelengths fulfilling the phase-matching condition. [PERSON] et al. reported the 193-nm laser generation between 213 nm and 2100 nm in LBO by SFG with the energy of 770 \(\upmu\)J [21]. LBO has a smaller walk-off angle leading to a better beam quality. However, the efficient nonlinear coefficient is smaller than that of the BBO or CLBO crystal, which limits the DUV laser power scaling. KABO is also an excellent nonlinear optical crystal for DUV laser generation. [PERSON] et al. reported a 193 nm laser with the average output power of 0.2 W which could run stable for 50 hours although KABO has the smallest d\({}_{\text{eff}}\) comparing to BBO, LBO, and CLBO [33]. For the CLBO crystal, [PERSON] reported a 120 mW DUV laser output at 193.4 nm by SFG between 234.1 nm and 1110 nm, which is the highest solid-state CW 193-nm laser until now [18]. Other results of 193 nm generation by SFG in CLBO also could reach to the average power from 100 mW to 300 mW [12; 31; 32]. CLBO has a larger d\({}_{\text{eff}}\) than LBO and a smaller walk-off angle than BBO, which means it could lead to a higher power DUV laser with better beam quality at the same time. Hence, the CLBO crystal is a suitable crystal for 193-nm laser generation. However, CLBO is more hygroscopic comparing to BBO or LBO crystals. Packaging the CLBO crystal into a cell, heating the crystal up to more than 150 \({}^{\circ}\)C, and purging it by use of noble gas is the solution to avoid these problems [41]. In addition, the previous CLBO crystal for FHG to DUV laser at 258 nm was also treated in the above conditions. In terms of the experimental setup, the two laser wavelengths for SFG should be chosen carefully in order to satisfy the phase-matching condition of the nonlinear optical crystals. The Nd:YAG, Yb:YAG, Nd:YVO\({}_{4}\) and Yb-doped fiber lasers could achieve both high power and high beam quality. According to the published results for generating 193 nm, these NIR lasers were used for the fundamental of a DUV laser with the wavelength longer than 193 nm such as 258 nm (4 th harmonics) [12], 213 nm (5 th harmonics) [21]. By use of nonlinear processes such as OPO, the 1-\(\upmu\)m NIR laser is converted to longer wavelengths [19; 21; 28], which extends the available laser wavelengths for SFG. Using the Er-doped fiber laser at 1550 nm together with the 1-\(\upmu\)m NIR laser, the final target of the 193 nm laser is achieved without involving the complex and unstable nonlinear OPO process [12; 31; 42]. Recently, 310 mW DUV laser at 193 nm was generated by stages of SFG in the CLBO crystals. The schematic setup for this 193 nm laser was shown in Figure 2. The 193-nm laser was generated by two stages of SFG process between a 258 nm laser generated by FHG of a 1030 nm laser and a 1553 nm laser. The 1030 nm laser was based on an Yb-doped hybrid amplifier and the 1553 nm laser was from an Er-doped fiber amplifier. The 1030 nm laser and the 1553 nm laser were synchronized by a commercial available delay/digital pulse generator. The LBO crystal was utilized for the SHG to 515 nm and the CLBO crystals were used for FHG to 258 nm, SFG to 221 nm, and SFG to 193 nm. The generated 193 nm laser was operated at 6 kHz with pulse duration of 4 ns. The average power was 310 mW and the linewidth was about 4 GHz, which satisfied the needs for injection-seeding the hybrid ArF excimer laser [2; 12].