Numerical Analysis on Thermal Blooming of Collimated Pulsed Laser Beams Propagating in the Atmosphere

Propagation of a collimated,pulsed laser Gaussian beam in an absorbing medium has been investigated theoretically.The energy absorbed by the medium of the propagation path causes heating,leading to an index of refraction change that causes self-defocusing,which induces thermal blooming.Thermal-blooming effects can limit the laser power that can be effectively propagated through the atmosphere.Numerical results for pulsed laser Gaussian beams are presented in form of irradiance profiles as functions of range.The pulse time is of the order of the hydrodynamic time,i.e.,the time required for a signal traveling at the sonic velocity to cross the beam.The long laser pulse is subjected to thermal blooming more than the short laser pulse,which is mainly subjected to the linear absorption.An initial Gaussian beam distribution is deformed into the dimple near the center with the peak irradiance near the edge of the beam.Finally,an approach adopting repetitively pulsed laser with optimized intervals between pulses is put forward,by which high average power can be transmitted.