Research On The Theory And Preliminary Experiment Of Solid-state Raman Lasers Based On A Cascade Coupling Cavity

With the advantages of small size,high efficiency,long lifetime and good beam quality,all-solid-state lasers are a powerful method to enrich and expand the laser frequency range,and to promote the development of laser medicine,biology and military laser technology.In this study,the cascaded oscillation property and gain competition phenomenon of the Raman laser based on multi-order Stokes scattering are theoretically studied in detail,and the first-order Stokes Raman laser is experimentally and preliminary studied.The main purpose of this study is to establish a theoretical model of the multi-order Raman laser,and to analyze the gain competition among different Stokes lines by numerical simulation,which can provide a theoretical guidance for high-efficiency and high-power solid-state Raman lasers.The first main part of the work are carried out in theory.Based on the third-order nonlinear optical coupled-wave equations and the theory of nonlinear polarization,the principles of stimulated Raman scattering and high-order cascaded Raman scattering are explained,and the polarizability and gain coefficient of the stimulated Raman scattering are derived.The characteristics of solid medium for Raman scatterings are also summarized and compared in this study.On the other hand,based on the rate equation theory of four-level lasers,the rate equations of the first,second and third order Stokes Raman lasers and the frequency-doubled Raman lasers are established.By numerical simulation of the above rate equations based on the MATLAB software,the dynamical process of Stokes lights and the mechanism of energy transfer between different optical fields are analyzed,and then the mechanism of gain competition among multi-order and frequency doubled Raman lasers is explicated theoretically.At last,based on the matrix optics theory,the resonator is designed and optimized by MATLAB software,and the beam waist radius and stability factor of the cascaded Raman laser cavity are simulated under different cavity lengths and reflecting mirrors with various radius curvature.The beam spot sizes of the fundamental and Raman laser at different position in the cavity are also calculated.Therefore,the optimal cavity scheme for improving the output energy is finally obtained.The second main part is experimental study.An 880nm LD array is used as the pump source of the acousto-optic Q-switched Nd:YVO₄/YVO₄ Raman laser.The fundamental laser with a oscillating wavelength at 1064nm is adopted to pump the YVO4 Raman crystal in which the first-order Stokes Raman laser at 1176nm is generated.The impacts of the pump power and repetition rate on the average output power and pulse duration are studied using three groups of cavity mirrors with different radius of curvature.Under the LD pump power of 10.1W and t he repetition rate of 30kHz,a maximum Raman output power of 59mW at 1176nm is obtained.A minimum threshold of 2.145W is reached at the pump power of 2.1W and the repetition rate of 10kHz.Besides,the pulse duration can be stable around6ns when the pump level is higher than 6.98W.