## 4 DUV Laser Generation Based on Diamond Raman Laser
We first used a Yb:YAG laser at the wavelength of 1030 nm as the pump of the diamond Raman laser, which is the proof-of-principle experiment. For this proof-of-principle experiment, the Raman laser setup is depicted in Figure 4, of which is a short Raman laser cavity with a small piece of diamond inside it pumped by a 1030 nm laser [52]. The 1030 nm laser is from a 10 kHz Yb:YAG CTR amplifier with the average power of 27 W. The diamond is made by CVD process with a dimension of 6 \(\times\) 6 \(\times\) 1.5 mm\({}^{3}\) and with anti-reflection (AR) coating of the pump, the 1 st and the 2 nd Stokes wavelengths. The beam radius of the 1030 nm pump laser is about 700 \(\upmu\)m [52]. The diamond crystal is pumped in the direction of \(<\)110\(>\). The Raman laser cavity is a short and linear cavity with the length of 25 mm including an input concave mirror (M1) with R = 75 mm and an output coupler (OC) with R = 50 mm. The coating of M1 is high transmission (HT) at 1030 nm and high reflection (HR) at the 1 st (1194 nm) and the 2 nd Stokes wavelength (1420 nm) of 1030 nm. The OC is HR for both 1030 nm and 1194 nm. The transmission of OC at 1420 nm was tried with three different values of 5%, 20%, and 50%. Two reflection mirrors (BB1-E04, Thorlabs Inc., Newton, NJ, USA) were applied to filter out the fundamental light, and the 2 nd Stokes wavelength of 1420 nm was detected. Figure 4: Experimental setup of the diamond Raman laser pumped by the Yb:YAG laser (HT: high transmission; HR: high reflection; OC: output coupler). Figure 3: Conceptual setup of the 193 nm laser generation based on the diamond Raman laser. The Raman laser achieved lasing at the wavelength of 1420 nm when the pump laser power was above the threshold power. As shown in Figure 5, the threshold power was 1.25 W and 1.5 W for the 5% and 50% OC, respectively. The threshold power for 20% OC was between 1.25 W and 1.5 W. During the cavity alignment to achieve the 2 nd Stokes wavelength, the AR coating of the front surface of the diamond was easily damaged by the reflected pump beam back from the OC when the pump laser (1030 nm) power was higher than 3 W. This was because of the imperfect design of the current Raman laser cavity. Not only the AR coating of the diamond surface but also the coating of the input mirror M1 was damaged by the returned pump laser. The values of the intensity of the returned pump laser at M1 and the front surface of the diamond were estimated to be 0.6-1 J/cm\({}^{2}\). The pump power was limited to 3 W to prevent the damages inside cavity for 5% and 20% OC. For the 50% OC, the pump power was carefully increased to about 4 W during the experiment, which was still at a lower power level in order to avoid the damage of the M1 and diamond. The HT coating of the pump beam at OC or the change of the radius of the curvature of could solve this issue to realize the higher power operation. At the pump power of 3 W, the output power of the Raman laser was 130 mW, 293 mW and 325 mW using 5%, 20%, and 50% OC, respectively. By use of the 50% OC, the highest average power of this Raman laser was 0.586 W when the pump laser was 4.2 W as shown in Figure 5, with the slope efficiency of 23% which was comparable to the previous reported results. The spectrum of the generated 2 nd Stokes after the two reflection mirrors was measured by a spectrometer (NIRQuest512, Ocean Optics Inc., Largo, FL, USA) as shown in Figure 6.  
Figure 5: Output power of the 2 nd Stokes (1420 nm) vs. Pump power with 5% and 50% OC. Figure 6: Spectrum of the diamond Raman laser at the 2 nd Stokes wavelength of 1420 nm. The pulse duration of the second Stokes light was shortened to 5 ns as shown in Figure 7a, a single-shot picture from the oscilloscope, which is a common phenomenon in Raman laser [44] while the pulse duration of the pump laser was 10 ns. The beam profile shown in Figure 7b demonstrated a good beam quality as one of the merits of Raman laser, which would be beneficial to generate a 193-nm DUV laser with a high quality beam.