# Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 cm\({}^{-1}\) Shifts
## 3 Results and Discussion  
Table 1 lists Stokes wavelengths with contributed Raman shifts. It can be seen that there are many Stokes wavelengths ranging from 1050 to 1200 nm, which were yielded via two Raman shifts of 234 cm\({}^{-1}\) and 671 cm\({}^{-1}\). The fundamental and Raman laser oscillation shared the identical plane-concave linear cavity constituted by the input mirror (IM) and the output coupler (OC) mirror. The IM was high-transmission (HT, T \(>\) 98%) coated at 808 nm and high-reflection (HR, R \(>\) 99.9%) coated from 1000 to 1200 nm. The OC mirror with a radius curvature of 100 mm was HR coated for the range of 1050-1130 nm. The transmittance curve of the output coupler is displayed in Figure 2. With the wavelength larger than 1130 nm, the transmittance increased, reaching 1.5% at 1147 nm and 67% at 1182 nm. The KTA Raman laser output characteristics were optimized by adjusting the pump power and cavity length. The maximum power of the output was achieved under a pump power of 10.5 W. The data of the output power were detected using the thermal sensor power meter (PM310D, Thorlabs INC). Figure 3 exhibits the average output power for two cavity lengths of 5 cm and 9 cm, with respect to the incident pump power. It is shown that the Raman threshold for a 5 cm length cavity was around 3.4 W, and for a 9 cm length cavity it was around 4.9 W. For the two cavity length setups, the average output powers both raised with the increase of incident pump power, provided with the slope efficiency of 7.6% for the 5 cm length cavity and 8.0% for the 9 cm length cavity. A lower Raman threshold for 5 cm length cavity benefited from its low cavity loss with the short and compact laser \begin{table}
\begin{tabular}{c c} \hline \hline
**Wavelength** & **Contributed Raman Shifts** \\
**(nm)** & \(\omega_{\text{R1}}\) = 234 cm\({}^{-1}\), \(\omega_{\text{R2}}\) = 671 cm\({}^{-1}\) \\ \hline
1092.0 & \(\omega_{\text{R1}}\) \\
1120.7 & \(2\omega_{\text{R1}}\) \\
1146.8 & \(\omega_{\text{R2}}\) \\
1150.4 & \(3\omega_{\text{R1}}\) \\
1178.2 & \(\omega_{\text{R2}}\) + \(\omega_{\text{R1}}\) \\
1182.4 & \(4\omega_{\text{R1}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Stokes wavelengths with contributed Raman shifts. Figure 2: Transmittance curve of the OC mirror for different wavelengths. cavity, and the 5 cm length cavity is more suitable for high output power than the 9 cm length cavity with an increase of pump power. Under an incident pump power of 10.5 W, the obtained maximum output powers were 530 mW for the setup with 5 cm length cavity, and 320 mW for the setup with 9 cm length cavity. The power instability at the maximum output for these two length cavities were better than 5%. Figure 4 displays the laser spectra with the 5 cm and 9 cm cavity lengths, respectively. The data were measured at the pump power of 10.5 W using a grating monochromator (Omni-\(\lambda\)500 model, ZOLIX, Beijing, China). According to the wavelengths and Raman shifts shown in Table 1, the spectra lines can be identified. For the setup with the 9 cm length cavity, its spectrum contains two strong lines with similar intensity, which are the first-Stokes at 1146.8 nm with the Raman shift of 671 cm\({}^{-1}\), and the Stokes at 1178.2 nm with mixed Raman shifts of 234 cm\({}^{-1}\) and 671 cm\({}^{-1}\). Four low intensity lines were also detected. They are the fundamental wave at 1064 nm, the first, second and third-Stokes at 1092.0, 1120.7, and 1150.4 nm, respectively, with a Raman shift of 234 cm\({}^{-1}\). For the setup with 5 cm length cavity, due to the lower loss caused by the shorter cavity length, the low order Stokes were converted to higher order Stokes. The first-Stokes at 1092.0 nm and the second-Stokes at 1120.7 nm became negligible, while the intensity of 1178.2 nm was significantly enhanced, and more obvious third-Stokes (1150.4 nm) and fourth-Stokes (1182.4 nm) waves with Raman shift of 234 cm\({}^{-1}\) were also observed. However, the output was still dominated by the Stokes waves at 1178.2 nm and 1146.8 nm.