Research On Intracavity Solid-state Raman Laser And Synchronously Pumped Solid-state Raman Laser

Solid-state Raman lasers using crystals as Raman media are widely used in various fields because of their simple structure,high conversion efficiency and good stability.Compared with gas and liquid Raman media,solid Raman media have the advantages of large particle concentration,high Raman gain coefficient,good thermal conductivity and small volume.At the same time,the crystal Raman medium has a variety of Raman vibration modes that can achieve stimulated Raman conversion to generate coherent Stokes laser output.In addition to the Raman vibration mode with the largest Raman gain,it attracts intensive research interest to obtain coherent light by using the stimulated Raman scattering process of other modes.Through the experiments of multi-Raman mode solid-state lasers,stimulated Raman scattered laser of more wavelengths are realized,which further widen the coherent spectral range.In addition,pulses with shorter pulse width have been pursued intensively.Ultrashort pulses have been widely used in industrial,medical,military and information fields.In recent years,the generation of ultrashort pulses by synchronously pumped Raman lasers attracts intensive research interest.There are many experimental reports on this aspect.However,there are few theoretical studies on this aspect.LD-pumped actively Q-switched intracavity SrWO₄ Raman laser is studied experimentally in this paper.When there is only Nd:YAG laser crystal in the cavity,the average output power of the fundamental frequency light is 1.606 W with pulse repetition frequency of 30 KHz and pump power of 5 W,and the maximum conversion efficiency of pump light to fundamental frequency laser is 32.12%.When the pulse repetition frequency is 10 KHz and the pump power is 5 W,the maximum pulse energy is 116.4?J,and the minimum pulse width is 8 ns.After adding SrWO4 crystal into the cavity,the average output power of the fundamental frequency light is 0.92 W with 30 KHz and the pump power of 5 W,and the conversion efficiency is 18.4%.The highest conversion efficiency of 19.04%is obtained with the pump power of 4.5 W.When the pump is 5 W and the pulse repetition frequency is 10 KHz,the maximum average output power of Stokes laser at 1178 nm is 0.702 W,the conversion efficiency of pump light to Raman light is 14.04%,the slope efficiency is 17.25%,and the maximum pulse energy is 70.2?J.The theoretical study of the passively Q-switched multi-Raman modes solid-state laser is carried out in this paper.The vibration modes and corresponding relative Raman gains of common Raman crystals are analyzed,and the Raman modes that can achieve stimulated Raman scattering are summarized.Based on the passively Q-switched rate equations,the rate equations including two Stokes laser are deduced,which are named as Ramn laser 1 and Raman laser 2,correspondting to first Stokes laser of Raman mode with relatively lower Raman gain coefficint and that of Raman mode having highest Raman gain,respectively.The new rate equaitons are normalized.The coupling rate?of pump light and fundamental laser in the laser medium,saturable absorber bleaching factor?,normalized Raman gain coefficients M₁ and M₂ for Raman laser 1 and Raman laser 2,the loss ratio of Raman laser 1 to fundamental laser K₁,the loss ratio of Raman laser 2 to fundamental laser K₂ are obtained.Based on the data from experiments and references,the ranges of normalized parameters are estimated,and the effects of each normalized parameters on the output of two Raman lasers are studied.The effect of K₂/K₁ on the output is analyzed,and the effects of M₂,N,?,M₂/M₁ are also analyzed.It is found that K₂/K₁ plays a vital role in suppressing Raman laser 2 and maximizing the conversion efficiency of Raman laser 1.If K₂/K₁ is not large enough,it is difficult to achieve the high conversion efficiency of the secondary Raman mode.The larger the values of N,?,the easier it is to obtain a more efficient Raman laser 1 with the Raman laser 2 completely suppressed.The numerical simulation results are compared with experimental results in the literature,and some optimization suggestions are given for the experiment.The N can be increased with saturable absorber of smaller initial transmittance,the?can be increased by changing the cavity structure,and more efficient conversion of the Raman laser 1 can be obtained with larger N and?.At the same time,it is also possible to increase the Raman gain by choosing a reasonable Raman crystal of M₂/M₁ and long Raman crystal.The high efficiency Raman conversion of the mode with a smaller Raman gain can be achieved.The theoretical study of synchronously pumped ultra-short pulse Raman laser is carried out in this paper.The transiently stimulated Raman scattering coupled wave equations considering the group velocity dispersion are derived,and the finite difference method is used to convert partial differential equations to the ordinary differential equations on the space grid.The fourth-order Runge-Kutta algorithm is used for numerical simulation.The effects of cavity length detuning,output mirror reflectance and pump pulse width on Stokes pulse are analyzed.The pulse shape and movement trajectory of the Stokes pulse and the pump pulse before and after passing through the Raman crystal in the cavity are analyzed with output mirror reflectance of 90%for Stokes laser and the Raman cavity length detuning of+8?m,-30?m and-300?m,respectively.The phenomena are discussed,and the laser intensity and pulse width of the output Stokes laser and pump laser are given.It is obtained that when the cavity length is detuned by+8?m,the peak intensity of the output Stokes light is the strongest,and the pulse width is the narrowest.The pulse shape and movement trajectory of the pump pulse and Stokes pulse before and after passing through the Raman crystal are analyzed with output mirror reflectance of 80%for Stokes laser and the cavity length detuning of+8?m,-39?m,and-250?m,respectively.The laser intensity and pulse width of the output Stokes laser and the output pump laser are given.Compared the results with those of 90%reflectance,it is found that the pulse shape and movement trajectory are basically similar when the cavity length detuning is the same,but the Stokes peak intensity is greater and the pulse width is slightly wider for 80%reflectivity.The pulse shape and movement trajectory of the pump pulse and Stokes pulse before and after passing through the Raman crystal are analyzed with the cavity length detuning of+4?m and the pump pulse width of 1.92 ps,3.84 ps and 19.2 ps at the same pump energy,respectively.The intensity and pulse width of the output Stokes laser and the output pump laser are given.When the pump pulse width is different,the pulse shape and movement trajectory are basically similar.However,when the pump pulse width is 1.92 ps,the output Stokes laser peak intensity is the largest and the pulse width is the narrowest.This is because the narrower the pulse width of the pump,the greater the peak pump power for the same pump energy.The characteristics of transient stimulated Raman scattering can be better understood through theoretical research.Factors affecting the output Stokes laser intensity and pulse width are analyzed,and the experiment can be guided to get shorter pulse output.