Optical inhomogeneities in a photodissociation nanosecond-pulse iodine amplifier utilizing slow pumping by ultraviolet radiation

A study was made of the evolution of the refractive index gradient profile of the active medium in a nanosecond-pulse iodine amplifier with a pump duration t_p exceeding the period t_r of radial acoustic vibrations of the gas in the laser cell. The refractive index gradient was found to be made up of a slow component, monotonically increasing with time, and a rapidly oscillating component associated with the appearance of standing acoustic waves. The former is equivalent to a converging spherical lens and the latter to an aspheric lens. It is shown that only the first cylindrical vibrational mode is excited and that the adverse effects of the acoustic vibrations can be avoided if the pulse is amplified at the moment when the refraction of the aspheric lens vanishes. A comparison between the experimental data and calculations demonstrates that the monotonic component of the refractive index gradient can be explained by the radially inhomogeneous nature of the pumping. The calculated oscillatory component is of much lower intensity than that observed experimentally. This is explained by assuming that the source exciting the acoustic wave is concentrated in the region near the walls. The possibility is mentioned of using pump pulses of t_p > t_r duration in large-diameter amplifiers having output energies of the order of hundreds of joules.