## 4 DUV Laser Generation Based on Diamond Raman Laser  
In this section, a new method to generate the 193 nm DUV laser is introduced, which is based on the diamond Raman laser scheme. In terms of the above methods of the 193 nm generation with 258 nm and 1553 nm, there is a limitation for its power scaling. The power of 258 nm laser is improved to more than 10 W while the power of the 1553 nm laser is limited because of the lack of the amplification medium and methods. The output power of the current Er-doped fiber amplifier is less than 2 W which is limited due to the stimulated Brillouin scattering [42]. For the solid-state laser, the average output power was also no more than 1 W [43]. Hence, this longer wavelength is usually generated by use of OPO process pumped by the Nd- or Yb-doped lasers. In the past few decades, many Raman materials have been studied [44; 45; 46], which showed the high conversion efficiency to the first Stokes light from 1 \(\upmu\)m. The stimulated Raman scattering (SRS) laser is remarkable for the momentum conservation automatically determined by the scattered phonons, which will induce the Stokes light beam to be a high quality [22; 47]. Particularly, by use of the chemical vapor deposition (CVD) diamond, the diamond Raman laser depicted a high power result of 100 W [22] and a high slope efficiency of 84% approaching the quantum limit [23]. In detail, the most interesting advantages of diamond includes the high Raman gain (pumping at 1030 nm, \(\mathrm{g_{R}}\) = 13.5 \(\pm\) 2.0 cm/GW) and the large frequency shift of the 1 st order Raman mode (1332.5 cm\({}^{-1}\)) [22; 48], which could achieve a high-power operation and a sufficient frequency shift. Moreover, the high thermal conductivity (2200 W/m\(\cdot\)K), which is two to three orders of magnitude higher than conventional Raman materials, and the low thermal expansion coefficient of diamond (1.1 \(\times\) 10\({}^{-6}\) K\({}^{-1}\)) will be helpful for power scaling to a high level as well as its high damage threshold [45; 46; 47; 48]. Another important advantage of the diamond is the broad transmission band. Recently, the longest solid-state Raman laser wavelength was obtained in the 3-\(\upmu\)m region in a CVD diamond, showing a new pathway to the mid-IR laser and the possibility to cover from NIR to mid-IR wavelength [49]. Hence, by choosing a suitable Raman wavelength, a high-power 193 nm laser will be effectively generated by stages of SFG. Figure 3 demonstrates the conceptual setup of the high-power 193 nm laser by the diamond Raman laser. The idea is also basically based on two stages of SFG process. The first DUV wavelength is at 260 nm, which is the FHG of a 1040 nm laser. Meanwhile, the wavelength of the NIR laser is 1512 nm, which is the second Stokes wavelength of a 1078 nm laser and generated by a diamond Raman laser. Both the 1040 nm and 1078 nm laser come from one dual-wavelength Yb-doped fiber laser. Each laser is amplified by stages of fiber amplifier and Yb-doped materials amplifier, respectively. Figure 2: Schematic setup of 193 nm generation by frequency mixing of 1030-nm and 1553-nm lasers (NCPM: noncritical phase-matching). The Yb-doped materials amplifier could be a Yb:Lu\({}_{2}\)O\({}_{3}\)[50] or a Yb\({}^{3+}\):CaCdAlO\({}_{4}\)[51] laser at both wavelengths (1040 nm & 1078 nm). For the frequency conversion by nonlinear crystals to short wavelengths, the 1040 nm laser generates the green laser at 520 nm by SHG using the LBO crystal, and then the DUV laser at 260 nm will be generated by SHG from 520 nm by use of the CLBO crystal. On the other hand, the 1078 nm is the pump laser of the diamond Raman laser; its 2 nd Stokes wavelength of 1512 nm will be the NIR laser for the following SFG processes to 193 nm. Hence, a DUV laser at 222 nm will be achieved by SFG between the 260-nm laser and the 1512-nm laser. The 193-nm laser will be generated by the second stage SFG between the 222-nm laser and the residual of the 1512-nm laser.