Stimulated Raman scattering (SRS) in the crystalline Raman gain media is a well-established technique for extending the spectral coverage of lasers. However, as a third-order nonlinear process, the SRS suffers a relatively low nonlinear gain and consequently has a high threshold, specifically when operating in a continuous-wave (CW) scheme. The intra-cavity pump scheme, in which the Raman crystal is located within the fundamental laser cavity, is an effective alternative to achieve efficient CW Raman output with moderate primary pump power because the high circulating fundamental laser power in the cavity generates sufficient Raman gain. To date, the highest CW Stokes output power of end-pumped intra-cavity Raman lasers has been realized with the self-Raman scheme, in which the processes of lasing and SRS take place in one crystal to minimize insertion losses. However, intra-cavity Raman lasers with separate lasers and Raman gain media have the advantages of a more flexible output wavelength and distributed thermal load, which are helpful for power scaling. This study presents an efficient CW Nd∶YVO₄/KGW intra-cavity Raman laser. The output power of the CW Stokes wave at 1177 nm reaches 6.63 W under an incident laser diode (LD) pump power of 36.6 W, with the corresponding optical efficiency being 18.1%.

The experimental setup of the CW intra-cavity Raman laser is shown in Fig. 1. A 15 mm long a-cut Nd∶YVO₄ crystal and a 20 mm long N_p-cut KGW crystal serve as the fundamental laser and Raman gain media, respectively. The LD pump wavelength is 878.6 nm, and the pump beam radius at the laser crystal is 280 μm. The Nd∶YVO₄ crystal has a low doping atomic fraction of 0.2% to alleviate the thermal effect. The 1064 nm fundamental laser cavity is defined by a flat highly reflective (HR) mirror (M1) and a curved HR mirror (M2) with a radius of curvature of 100 mm. The M2 also has a transmissivity of 0.4% at a Stokes wavelength of 1177 nm. A flat dichroic mirror (M3) with HR coating at 1.15-1.18 μm and highly transmissive at 1064 nm is inserted into the cavity to make the Raman Stokes cavity with M2. The lengths of the fundamental and Stokes cavities are 50 mm and 22 mm, respectively.

First, the polarization direction of the linearly polarized fundamental frequency light generated by Nd∶YVO₄ is parallel to the N_m axis of the KGW crystal (E∥N_m). With this polarization, the Raman gain coefficient of the 901 cm^-1 Raman line is over two times larger than that of the 768 cm^-1 Raman line. The Stokes output power as a function of incident LD pump power is shown in Fig. 2. The SRS threshold is 7.5 W LD power, and the maximum Stokes output power reaches 6.63 W under the maximum pump power of 36.6 W. Only the first Stokes field at 1177.3 nm is observed during the entire process. The spectral linewidths of the fundamental laser and Stokes wave are 0.08 nm and 0.02 nm at the SRS threshold and are broadened to 0.3 nm and 0.2 nm, respectively, at the maximum power, as shown in Fig. 3. Because of the astigmatic thermal lens in the KGW crystal, the Stokes output beam profile becomes the Hermite-Gaussian (HG) mode at the maximum power, as shown in Fig. 4. We also attempt fundamental polarization parallel to the N_g axis of the KGW crystal. In this case, the laser output power and conversion efficiency are lower than those for E∥N_g. The Stokes output power under the same maximum pump power of 36.6 W is only 4.86 W. We find that the output wavelength contains both 1159 nm and 1177 nm components, which correspond to the 768 cm^-1 and 901 cm^-1 Raman shifts, respectively, when the pump power exceeds the SRS threshold of 7.5 W. The cascaded Raman Stokes light at 1171 nm and 1189 nm corresponded to the 89 cm^-1 Raman shift also occurs at higher pump power, as shown in Fig. 5. The multiline Stokes field decreases the effective Raman gain, whereas the cascaded Raman conversion decreases the interaction between the fundamental Stokes fields. Therefore, the E∥N_m arrangement, in which the 901 cm^-1 Raman shift dominates, is more suitable for efficiently generating high-power Stokes outputs with high spectral purity.

In conclusion, we present an efficient CW Nd∶YVO₄/KGW intra-cavity Raman laser. The effects of the fundamental laser polarization direction on the power, spectral mode, and transverse mode of the KGW Raman laser are investigated experimentally. When the fundamental polarization distribution is parallel to the N_m axis of the N_p-cut KGW crystal, the laser benefits from a higher Raman gain at 901 cm^-1 Raman shift. The 6.63 W CW Stokes output at 1177.3 nm is obtained under an incident LD pump power of 36.6 W, with corresponding optical and slope efficiencies of 18.1% and 24.7%, respectively.