No abstract

The physical model of random lasing (RL) based on the concept of feedback distributed laser (FDL) are developed. It based on similarity of RL formation from scattered waves in a random laser and from reflected waves in FDL. In contrast to FDL the RL spatial feedback distribution along the radiation trajectories is random. All radiation trajectories interrelated in the tangle by scattering can be represented as the imaginary onedimensional chain including all enabled scatterers and keeping radiation under any scattering event. Output intensity of this chain is described by the expression being similar to one for the single-pass amplifier with length determined by concentration of scattering particles and gain. The expression adequately describes the main RL behaviors: narrowing of radiation spectrum under pump intensity and scatterer concentration increase.

Numeric simulation of light passing through the scattering medium by the Monte-Carlo method was carried out. The influence of multiple scattering on the light beam attenuation in an absorbing media was investigated. The additional absorption appearance was shown to be initiated by multiple scattering. The experimental evidence for this absorption is abrupt increasing of beam attenuation at high particle concentrations. The behaviour of the additional absorption was investigated. The results of numeric calculation are in good agreement with experimental results.

The arficle is devoted to the problem of random laser parameters optimisation. The investigated random laser was made of R6G solid solution in polymer with embedded scattering particles. We optimised efficiency ofthe positive feedback formned in different ways: by multiple scattering within sample volume and by the sample surface reflection. The total feedback of the random laser can be varied by the medium thickness change. We also demonstrated that the raising of the medium surface reflectivity can cause extra increasing of the efficiency of the positive feedback. According to the conclusions we proposed methods of the random laser efficiency improvement. These methods allow uipto 6 times increasing ofthe random laser efficiency.

Monte-Carlo simulation of random lasing under diffusive regime of light propagation in active random medium was carried out. Computing has been carried out for active media being suspension of dielectric particles in rhodamine 6G solution. Dependence of emitted photons density distribution in active media, their total quantity and spectrum on active medium length in a pump beam direction. Spatial distributions of pump radiation and amplification in a sample have been taken into account. It has been shown that quantity of emitted photons non-steadily depends on sample length and the maximal density of emitted photons is located at a depth near front (pump) sample side. Optimal active medium length is exists and maximizes total number of emitted photons and narrows spectrum of emission. Computing results are conformed to experiments.