# Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 cm\({}^{-1}\) Shifts
## 3 Results and Discussion
Therefore, different cavity lengths also resulted in the difference of output spectra, which might be mainly attributed to the intensity distribution changing in the cavity. The pulse width and pulse repetition frequency with different incident pump powers were obtained using the free-space InGaAs photodetector and exhibited on the 500 MHz oscilloscope (DPO3052B model). In view of the fact that the measured multi-Stokes wavelengths were very close to each other, and they cannot be segregated in our investigation, only the whole temporal pulse profiles of the Stokes waves were provided. As shown in Figure 5, with the incident pump power changed from 3.9 W to 10 W, the pulse repetition frequency went up from 2.9 kHz to 14.5 kHz, and the pulse energy rose from 21 u] to 36 u] except for a small decline when the incident pump changed from 4.5 W to 5.2 W. The pulse  
Figure 3: Average output power versus incident pump power of the Raman lasers with the cavity lengths of 5 cm and 9 cm. repetition frequency was not as stable when the instability reached about 25%, which might be caused by the competition of multi-Stokes generation. The measured pulse trains and temporal pulse profiles at the incident pump power of 10.5 W are displayed in Figure 6. The pulse width was around 1.1 ns, and the pulse repetition frequency was 14.5 kHz. A smaller value of pulse width than 1.1 ns might be more accurate, since the critical time resolution of the used 500 MHz oscilloscope was limited. We also measured the pulse width of weak leaked fundamental laser at 1064 nm, and the obtained value was around 11 ns. Therefore, significant pulse shortening was  
Figure 4: Raman laser spectra measured under the incident pump power of 10.5 W with the cavity lengths of 5 cm and 9 cm. Figure 5: Pulse repetition frequency (PRF) and pulse energy versus incident pump power. detected for this KTA Raman laser which is similar to the fact that achieved in SrMoO\({}_{4}\) Raman laser [33]. The spectra of the weak visible light accompanying with the Stokes wave output have been measured using a fiber optic spectrometer (AvaSpec-3648 model). The measured spectra are shown in Figure 7. It can be seen that the main lines are located at 573.4 nm and 574.3 nm, and the former can be assigned to the SHG of 1146.8 nm, while the latter is from the SFG of 1146.8 nm and 1150.4 nm with the phase matching angle \(\theta\) of 82-83\({}^{\circ}\) in the x-z plane of KTA crystal. These weak visible lights possessed an output power of about a dozen milliwatts, which should be ascribed to the fact that the phase matching directions of SHG and SFG for the Stokes waves in the cavity were close to the x-axis. To our surprise, the SFG light (567 nm) of 1120.7 nm and 1146.8 nm was not observed. As known from the calculation, the SFG matching angle between these two wavelengths is closer to the x-axis of the KTA crystal compared with those for 573.4 nm and 574.3 nm light generation. The absence of 567 nm SFG light could be attributed to the light conversion from 1120.7 nm to 1150.4 nm due to high reflection of the resonant cavity for 1120.7 nm. Therefore, the 567 nm SFG light was negligible in contrast to the lines of 573.4 and 574.3 nm. Figure 6: Temporal pulse profiles and pulse trains under the incident pump power of 10.5 W.  
Figure 7: Spectra of weak visible light irradiated from the KTA crystal.