Effect of the solvent viscosity on the local irregular intensity fluctuations in high Fresnel number dye lasers

We study the influence of the solvent viscosity in the local irregular intensity fluctuations characteristic of high Fresnel number dye lasers. The relative amplitude of the fluctuations in a flashlamp-pumped dye laser is analyzed, We complete our previous work by using several solvents and different excitation energies above threshold. A decrease of the relative fluctuation amplitude is found as the solvent viscosity or the pumping energy increases. This effect is theoretically analyzed in the framework of the semiclassical Maxwell-Bloch equations, with a model based on the orientation of the dye molecules driven by the laser field in the solvent. These results point out the importance of the orientation of the dye molecules in the intensity fluctuation phenomenon.