Study On Cascaded Raman Frequency-mixing Laser Based On The Pure YVO₄ Crystal

Stimulated Raman scattering is an important nonlinear optical frequency conversion method,which can realize the flexible conversion of laser wavelength by Raman shift and has received increasing attention in the field of optical frequency conversion.In recent years,yttrium vanadate（YVO₄）has been widely used in the generation of near-infrared and visible laser due to its excellent Raman performance.The cascaded Raman system driven by the Neodymium-doped solid laser combined with the second-order nonlinear optical frequency conversion will provide the possibility to realize visible laser output which cannot be directly obtained by traditional solid-state lasers.These visible lasers can meet the application requirements of laser medical treatment,laser display,multi-spectral imaging and biophotonics.Neodymium-doped yttrium aluminate（Nd:YAlO₃）crystal is an ideal laser material due to its large natural birefringence and natural advantages in efficient frequency conversion.Therefore,in this paper,the acousto-optic Q-switched Nd:YAlO₃ laser was used as the pump source to intracavity drive an a-cut pure YVO₄ crystal for the research of Raman laser performance.Then,through the coating design of the cavity mirror,the fundamental,first-Stokes and second-Stokes waves simultaneously resonate in the laser cavity,and the laser output in visible waveband of 540～670 nm had been realized.The detailed research contents are as follows:1.The research on the acousto-optic Q-switched Nd:YAlO₃/YVO₄Raman laser for the first-Stokes and second-Stokes waves.By setting the polarization direction of Nd:YAlO₃ parallel to the z-axis of YVO₄,the X（ZZ）X Raman configuration was satisfied,and the fundamental wavelength at 1080 nm converted into the first-Stokes wave at 1195 nm and the second-Stokes wave at 1337 nm with the 890 cm^-1 Raman shift of YVO₄ crystal.At an absorbed pump power of 6.64 W and a pulse repetition frequency of 25 kHz,the first-Stokes with output power of1.24W and conversion efficiency of 18.7%was achieved.And then using the output mirrors with a transmittance of 84.9%at 1337 nm,the second-Stokes wave with the output power of 352 mW and the conversion efficiency of 5.3%was obtained.Due to the large transmittance at 1137 nm,the conversion efficiency of the second-Stokes wave was low.If the transmittance is further optimized,a higher output result can be obtained.2.The research on the acousto-optic Q-switched Nd:YAlO₃/YVO₄Raman laser for the intracavity frequency conversion.By using the non-linear frequency conversion crystal,the laser output in visible waveband was realized by the intracavity second-harmonic generation and sum-frequency generation between different wavelengths.For the second-harmonic generation of first-Stokes wave,the non-critical phase-matching LBO and the critical phase-matching BBO were used as the nonlinear optical crystals.At an absorbed pump power of 5.96 W and a pulse repetition frequency of 10 kHz,the 597 nm orange laser with the output power of 545 mW and 523 mW were obtained with conversion efficiencies of 9.1%and 8.8%,respectively.Furthermore,the BBO was used for selective frequency mixing laser research,and then the second-harmonic generation and sum-frequency generation between different wavelengths were realized by small angle tuning.At an absorbed pump power of 5.96 W and a pulse repetition frequency of 10 kHz,by fine-tuning the BBO phase-matching angle in the range of 2.3°,the average output powers for 540,567,597,631 and 668 nm laser were 800,340,460,190 and 326 mW,and the corresponding conversion efficiencies were 13.4%,5.7%,7.7%,3.2%and 5.5%,respectively.