| The passively Q-switched Raman laser pumped by laser diode(LD)can not only broaden the laser spectrum range,but also achieve the peak power laser pulse output with high optical conversion efficiency.It has a very broad application prospect in the fields of laser processing,laser communication,quantum computing and so on.The dual-wavelength peak power passively Q-switched Raman lasers with elliptically polarized state have shown more and more potential applications in material processing,molecular manipulation,optical array rotating laser to capture micro particles,generation of high order harmonics,supercontinuum,terahertz and new light sources.However,traditional passively Q-switched Raman lasers can only achieve linearly polarized laser output.Therefore,this paper proposes the use of passive Q-switched technology to generate peak power base frequency laser,combined with YVO4 Raman crystal to generate dual-wavelength Raman laser.By rotating a-cut YVO4 crystal,the influence of a-cut YVO4 crystal orientation on the properties of Raman laser,spectrum,elliptic polarization and other characteristics of passively Q-switched Raman laser was studied,and the method of regulating elliptic polarization by dual-wavelength Raman laser with high beam quality and peak power was obtained.During the study,a 1.8 mm-thick Cr,Nd:YAG crystal was used as the laser working medium,and a 2 mm-thick a-cut YVO4 crystal was used as the Raman crystal and the output coupling mirror to construct a passively Q-switched Raman laser.The cavity length of the resonator was 14 mm.When the incident pump power reaches 1.52 W,the dual-wavelength Raman output of 1166 nm and 1176 nm is produced by the Cr,Nd:YAG/YVO4 self-Q-switched Raman laser.The 1166 nm and 1176 nm Raman lasers are generated by using the 816 cm-1 and 890 cm-1 Raman frequency shift of a-cut YVO4 crystals,respectively,at 1064 nm fundamental frequency.By adjusting the crystal orientation and pump power of a-cut YVO4 crystal in the long axis direction of elliptically polarized fundamental frequency laser,the elliptically polarized dualwavelength Raman laser output with controllable ellipticity and azimuth angle is realized in a Cr,Nd:YAG/YVO4 self-Q-switched Raman laser.The results show that the ellipticity of the elliptically polarized Raman laser varies with the rotation of a-cut YVO4 crystal in sine square modulation,and increases with the increase of the incident pump power.The azimuth of the elliptically polarized Raman laser is the same as the rotation direction and velocity of the a-cut YVO4 crystal.As the a-cut YVO4 crystal rotates from 0° to 360°,the average output power,peak power and pulse energy of the Cr,Nd:YAG/YVO4 self-Q-switched Raman laser show the anisotropic behavior of sinusoidal modulation,while the pulse width and repetition frequency almost remain unchanged.Compared with the case that the crystal orientation[100],[010],[100],[010]of a-cut YVO4 crystal is parallel to the direction of the long axis of elliptically polarized fundamental frequency laser,when the crystal orientation[110],[110],[110],[110]of a-cut YVO4 crystal is parallel to the direction of the long axis of elliptically polarized fundamental frequency laser,the average output power,pulse energy and peak power of the dual-wavelength Raman laser generated by the Raman laser are increased by more than 40%.When the a-cut YVO4 crystal rotation angle is 135°,that is[110]parallel to the long axis direction of elliptically polarized fundamental frequency laser,the best laser performance is obtained by the Cr,Nd:YAG/YVO4 self-Q-switched Raman laser.When the incident pump power is 5 W,the average output power of the dual-wavelength Raman laser is 210 mW,and the corresponding Raman conversion efficiency is 4.2%.When the rotation angle of a-cut YVO4 crystal increases from 90°to 135°,the pulse energy of Raman laser increases from 2.8 μJ of the trough to 4 μJ of the peak,and the peak power increases from 0.9 kW of the trough to 1.3 kW of the peak at 5 W of the incident pump.The pulse width and repetition frequency were maintained at 3 ns and 52 kHz,respectively.The beam quality factor of dual-wavelength Raman laser is less than 1.32,which indicates that it has a high beam quality close to the diffraction limit.The research results of this paper provide a new idea and implementation method for the development of high beam quality and peak power passively Q-switched Raman lasers with elliptically polarized state.By choosing the crystal orientation of anisotropic Raman gain medium,the ellipticity and azimuth of the elliptic polarization state of the dual-wavelength Raman laser with peak power can be conveniently controlled,which provides a lot of flexibility for the practical application of the laser with peak power. |
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