Research On Wavelength-locked Laser Diode Resonance-pumped All Solid-state Continuous Wave Raman Lasers

Stimulated Raman scattering(SRS)is an efficient method for frequency conversion,which has been widely used to extend the spectral coverage of conventional solid-state lasers.The spectra obtained with SRS cover wide wavelength range from ultraviolet to near infrared regions.All-solid-state Raman lasers have the advantages of compactness,high conversion efficiency and good stability,which led to various applications in the areas of military,transportation,medicine,information,industry and agriculture,etc.All-solid-state Raman lasers operate at two modes:continuous-wave(CW)and pulsed modes.In the CW Raman lasers,severe thermal effect is the dominant factor limiting the improvement of laser performance.It has been demonstrated that thermal effect can be efficiently reduced by in-band pumping.However,pump absorption of in-band pumping is relatively low,which restrains the overall optical efficiency of Raman lasers.In this dissertation,firstly we will reduce the thermal effects by using in-band pumping and diffusion-bonded YVO4/Nd:YVO4/YVO4 crystal.Then a 878.9 nm wavelength-locked laser diode(LD)is introduced as the pump source,with the purpose of improving pump absorption of in-band pumping through the exact matching of LD narrow linewidth and pumped absorption band of laser crystal.On this basis,continuous-wave intracavity and self-conversion Raman lasers are studied experimentally and theoretically.The effects of crystals,resonant cavity structure and stability on the performance of CW Raman lasers are investigated in detail.The main contents are as follows:1.An efficient multi-wavelength,continuous-wave self-Raman laser at 1.lx ?m is demonstrated,with a composite YVO4/Nd:YVO4/YVO4 crystal pumped by a wavelength-locked 878.9 nm diode laser.The thermal effect is reduced by using in-band pumping and diffusion-bonded crystal,then pump absorption of in-band pumping is improved by using a wavelength-locked laser diode as the pump source.A maximum Raman output power of 5.3 W is achieved at the pump power of 26 W,corresponding to an optical conversion efficiency of 20%and a slope efficiency of 21%.The Raman threshold was only 0.92 W of the diode pump power.The Raman output power and efficiency are obviously higher than those under traditional 808 nm pumping.2.Continuous-wave all-solid-state composite YVO4/Nd:YVO4/YVO4 self-Raman laser in-band pumped by a wavelength-locked laser diode at 878.9 nm is theoretically investigated in detail.Considering the thermal lens effect in the laser crystal,cavity mode parameters are calculated for several output couplers with different radii of curvature,by employing the standard ABCD matrix approach and equivalent G parameter method.The impact of cavity structure on the output characteristics of the Raman laser is investigated by analyzing mode matching between the pump and the fundamental beams,as well as the fundamental intensities in the Raman crystal.This provides theoretical explanations for the experimental results,and based on the analysis above,laser cavity is optimized.3.A continuous-wave intracavity Raman laser employing composite YVO4/Nd:YVO4/YVO4as laser crystal and YVO4/BaWO4 as Raman medium is demonstrated.The composite YVO4/Nd:YVO4/YVO4 is in-band pumped by a wavelength-locked laser diode at 878.9 nm,with the purpose of reducing thermal effects and improving pump absorption simultaneously.The impact of crystal properties,resonator structure and stability on the performance of CW intracavity Raman lasers is investigated experimentally and theoretically.The results show that the resonator stability greatly affects laser performance due to the long cavity length of intracavity Raman lasers.By choosing the BaWO4 crystal with high Raman gain,we can not only obtain higher Raman conversion efficiency,but also reduce the thermal effect to a certain extent.Furthermore,the smaller the curvature radius of the output mirror in the plano-concave cavity structure,the greater the power density of the fundamental laser in the Raman crystal and the wider the dynamic stability region of the resonator,and hence the higher output power of the Raman laser can be achieved.Finally,by using 30-mm BaWO4 crystal as Raman medium,the highest Raman output of 2.86 W is obtained at a pump power of 25.1 W,corresponding to a diode-to-Stokes optical conversion efficiency of 11.4%.