# Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 cm\({}^{-1}\) Shifts
###### Abstract  
A compact KTA cascaded Raman system driven by a passively Q-switched Nd:YAG/Cr4+:YAG laser at 1064 nm was demonstrated for the first time. The output spectra with different cavity lengths were measured. Two strong lines with similar intensity were achieved with a 9 cm length cavity. One is the first-Stokes at 1146.8 nm with a Raman shift of 671 cm\({}^{-1}\), and the other is the Stokes at 1178.2 nm with mixed Raman shifts of 234 cm\({}^{-1}\) and 671 cm\({}^{-1}\). At the shorter cavity length of 5 cm, the output Stokes lines with high intensity were still at 1146.8 nm and 1178.2 nm, but the intensity of 1178.2 nm was higher than that of 1146.8 nm. The maximum average output power of 540 mW was obtained at the incident pump power of 10.5 W with the pulse repetition frequency of 14.5 kHz and the pulse width around 1.1 ns. This compact passively Q-switched KTA cascaded Raman laser can yield multi-Stokes waves, which enrich laser output spectra and hold potential applications for remote sensing and terahertz generation.  
raman laser; KTA crystal; passively Q-switch Article  
Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 cm\({}^{-1}\) Shifts  
[PERSON], [PERSON], [PERSON], [PERSON], H. Passively Q-Switched KTA Cascaded Raman Laser with 234 and 671 cm\({}^{-1}\) Shifts. _Appl. Sci._**2021**, _11_, 6895. [[https://doi.org/10.3390/appl1156895](https://doi.org/10.3390/appl1156895)]([https://doi.org/10.3390/appl1156895](https://doi.org/10.3390/appl1156895))  
Academic Editor: [PERSON]  
Received: 4 July 2021  
Accepted: 20 July 2021  
Published: 27 July 2021  
**Published: 3 Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.**  
Received: 20 July 2021  
Submitted: 27 July 2021  
**Published: 3 Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.**  
Received: 20 July 2021  
Submitted: 27 July 2021  
**Published: 3 Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.**  
Received: 4 July 2021  
Accepted: 20 July 2021  
reported the second harmonic generation (SHG) laser converted from the first-Stokes in KTA Raman with the shift of 671 cm\({}^{-1}\)[24]. Our group also studied the multi-Stokes waves from cascade Raman conversion based on a KTA crystal [25]. From the main Raman shifts of 671 cm\({}^{-1}\) and 234 cm\({}^{-1}\) in the KTA crystal, the gains of high order Stokes waves can enrich laser spectral lines which cannot be directly produced from the laser crystal doped with rare earth ions [26]. In addition, via the processes of the SHG and sum frequency generation (SFG), the Stokes waves can produce visible light which has important applications in remote sensing, visual displays, and medical treatments [27; 28]. The simultaneous multi-order Stokes waves output has also been reported to be a prospective pumping source for the terahertz (THz) wave generation employing the nonlinear optics method [29].  
A solid-state laser based on Cr\({}^{4+}\):YAG passively Q-switching has the advantages of compactness and low cost for high pulse repetition frequency pulse laser operation [30; 31; 32]. Therefore, a passively Q-switched cascaded Raman laser could be a compact and efficient laser system for Stokes laser output. In this work, for the first time, multi-Stokes waves from passively Q-switched KTA cascaded Raman lasers were studied. A Nd:YAG/Cr\({}^{4+}\):YAG composite crystal was used as the saturable absorber. Under a pumping power of 10.5 W and pulse repetition frequency of 14.5 kHz (for the setup with a 5 cm length cavity) the largest average output power of 540 mW was realized with a pulse width of about 1.1 ns. The measured spectra show various Stokes lines in the range of 1050-1200 nm, which were converted via the Raman shifts of 234 cm\({}^{-1}\) and 671 cm\({}^{-1}\), and two strong lines were located at 1146.8 nm and 1178.2 nm.