## 2 DUV Laser Generated by FHG of the 1-um NIR Laser
Many kinds of crystals could be used for FHG such as BBO, CLBO, KBe\({}_{2}\)BO\({}_{3}\)F\({}_{2}\) (KBBF), YAl\({}_{3}\) (BO\({}_{3}\))\({}_{4}\) (YAB), NaSr\({}_{3}\)Be\({}_{3}\)B\({}_{3}\)O\({}_{5}\)F\({}_{4}\) (NSBBF) etc. as shown in Table 1. For FHG from 515 nm to 258 nm, the efficient nonlinear coefficient (d\({}_{\rm eff}\)) of the BBO crystal is the highest one among these crystals [15]. However, the BBO crystal also has the largest walk-off angle which will lead to a decline of the DUV \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Wavelength** & **FHG Crystal** & **Power/Energy** & **Repetition Rate** &
\begin{tabular}{c} **NIR to FHG** \\ **efficiency (\%)** \\ \end{tabular} & **M\({}^{2}\) (DUV)** & **Reference** \\ \hline
266 & BBO & 14.8 W & 100 kHz (\(\sim\)ns) & \(\sim\)10 & \(>\)1.5 & [3] \\
257 & BBO & 3.2 W & 30 kHz (\(\sim\)ns) & 14.5 & \(>\)1.1 & [4] \\
257.7 & BBO & 2.74 mJ & 1 kHz (\(\sim\)ps) & 9.7 & \(<\)3 & [5] \\
257 & BBO & 1.1 W & 14.5 kHz (\(\sim\)ns) & \(\sim\)31 & & [6] \\
258 & BBO & 4.6 W & 796 kHz (\(\sim\)fs) & \(\sim\)5.8 & 1.3 \(\times\) 1.6 & [7] \\
266 & CLBO & 40 W & 7 kHz (\(\sim\)ns) & \(<\)10 & \(>\)10 & [8] \\
266 & CLBO & 20.5 W & 10 kHz (\(\sim\)ns) & 14 & \(>\)10 & [9] \\
266 & CLBO & 28.4 W & 10 kHz (\(\sim\)ns) & \(<\)22 & \(>\)6.24 & [10] \\
257.5 & CLBO & 6 W & 100 kHz (\(\sim\)ps) & 10 & \(>\)1.5 & [11] \\
258 & CLBO & 3 W & 6 kHz (\(\sim\)ns) & 35 & \(<\)1.5 & [12] \\
258 & CLBO & 10.5 W & 10 kHz (\(\sim\)ns) & 31 & \(<\)1.5 & [13] \\
266 & KBBF & 7.86 W & 10 kHz (\(\sim\)ns) & \(<\)10 & \(>\)2 & [14] \\
266 & YAB & 5.05 W & 65 kHz (\(\sim\)ns) & 6.3 & & [15] \\
266 & LBO(SFG) & 3.3 W & 1 MHz (\(\sim\)ns) & 14 & & [16] \\
266 & NSBBF & 280 \(\upmu\)J & 10 Hz (\(\sim\)ps) & 36 & & [17] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical results of the solid-state DUV laser at 258 nm & 266 nm (NIR: near-infrared; DUV: deep ultra-violet; FHG: fourth-harmonics generation; BBO: \(\beta\)-BaB\({}_{2}\)O\({}_{4}\); CLBO: CsLiB\({}_{6}\)O\({}_{10}\); KBBF: KBe\({}_{2}\)BO\({}_{3}\)F\({}_{2}\); NSBBF: NaSr\({}_{3}\)Be\({}_{3}\)B\({}_{3}\)O\({}_{5}\)F\({}_{4}\); YAB: YAl\({}_{3}\) (BO\({}_{3}\))\({}_{4}\); SFG: sum-frequency generation). laser beam quality comparing to use the crystal with a smaller walk-off angle such as CLBO. LBO is also an excellent nonlinear crystal which is always applied to the SHG from the NIR to the green. The DUV laser is usually realized by two stages of SFG in the LBO crystals since there is no phase-matching angle for LBO to convert the green laser at 515 nm or 532 nm to the DUV directly by SHG [16]. Recently, the NSBBF crystal has also shown its potential to obtain a high conversion efficiency from 532 nm to 266 nm. The walk-off angle and d\({}_{\mathrm{eff}}\) of the NSBBF are almost similar to those of CLBO [17]. The conversion efficiency from fundamental NIR to DUV is from 5% to 36% according to the reported results in Table 1. Increasing the conversion efficiency will reduce the burden on the design and setup of the fundamental laser amplifier as well as the electric power consumption. A good beam quality of the fundamental laser will also contribute to the increasing of the conversion efficiency as well as the appropriate power intensity of the NIR and green laser in the SHG and FHG process, respectively. The highest conversion efficiencies from NIR to green and green to DUV laser by use of CLBO crystal are 70% and around 50%, respectively [12], which corresponds to the conversion efficiency of 35% from fundamental to DUV. In this report, the average output power of this DUV laser was 3 W which was limited by the fundamental NIR laser power. Recently, a DUV laser with 10.5 W average power output was reported, and the conversion efficiency was 31% from fundamental to DUV [13]. Its beam quality was the best for the 258 nm/266 nm DUV laser with the average power higher than 10 W, of which the M\({}^{2}\) is less than 1.5.