High-intensity narrow light pulse produced by self-focusing in laser spark

The shape of the laser light pulse transmitted through the spark produced in air by the same light pulse is investigated. A single-mode and single-frequency ruby laser is used. A narrow spike, overriding the amplitude of the original light pulse, is observed in the transmitted light. The spike is explained as the result of self-focusing taking place in the plasma. The amplitude and the time of the appearance of the spike, i.e., of the self-focused pulse, is investigated independently of the different experimental parameters. The light scattered by the spark is also observed; furthermore, clear experimental evidence of the self-focusing taking place during the cascade ionization process is presented