The numerical model of stimulated Raman scattering (SRS) taking into account spatial dynamics of amplitude-phase characteristics and consisting of non-linear partial parabolic differential equations is developed. A finite-difference numerical method was used to solve these equations. Results of simulation are in good agreement with experimental data.
Numerical model describing transient stimulated Raman scattering and taking into account diffraction was developed and Stokes wave evolution in compressed hydrogen was simulated to study space-time dynamics of amplitude-phase SRS characteristics. Space-time intensity and phase dependencies as well as spectrum and spatial coherence function of pump and Stokes waves were obtained. Considerable difference in mentioned characteristics was found out for transient and quasi-stationary stimulated Raman scattering modes. More complicated space-time dependencies are typical for transient mode in comparison with quasi-stationary mode. However, under quasi-stationary conditions Stokes wave phase varies in wider limits, which results in spatial coherency lowering. Module of spatial coherency function value lowers to threshold and then becomes stable as conversion coefficient increases. Presence of Stokes beam focusing is shown at stimulated Raman scattering, which can be explained by competition of strong Raman amplification and diffraction. Results of simulations are in good agreement with experimental data.
Transient SRS in mixture of two Raman active media (doubled gas mixtures) with pumping by ultrashort (2 ps) pulses was theoretically analysed. The numerical simulations show that there is strong energy exchange between direct Stokes waves and combination Stokes line during ultrashort pulse propagation in gas mixture. It is necessary to achieve the same phase and temporal dependencies of direct Stokes pulses for effective energy conversion in combination Stokes line. One-peak pulse generation in combination Stokes line with temporal compression is possible at pump intensity from threshold value up to doubled level. The decrease of duration of pulse by 12 times at conversion efficiency of 6% was shown. Increase of input pump energy causes destruction of Stokes pulses form and formation of multipeak Stokes pulses. The results show that the optimum ratio of gas pressure for transient SRS generation of sum and difference combination Stokes lines is proportional to the relation of the g/T2 of mixture components or to the relation of their Raman cross-sections. Our calculation is in good agreement with experimental results1.
The processes of transient SRS generation and amplification of continuous Stokes radiation were investigated by mathematical modeling. The detailed calculations of Stokes generation evolution in gaseous hydrogen, methane, crystal hydrogen and in barium nitride were carried out using high finesse resonators. It was shown, that the distinctive feature of resonator continuous SRS is the presence of relaxation oscillations at the initial moments of development of Stokes generation. The 100 times amplification fo the input pulsed Stokes signals with duration determined by the time characteristics of the resonator and SRS medium was shown. The results of numerical calculations correspond to the results of experiments.
The concept for a new approach to generating and receiving optical system for sodium guide star excitation based on solid-state laser technology and the stimulated Raman scattering/amplification in compressed gases is presented. Requirements to laser system for laser guide star generation are analyzed. The most promising SRS media are chosen.