High-power laser propagation: Thermal blooming

This paper presents a tutorial review of the self-induced thermal distortion of laser radiation propagating in absorbing media. The distortion of the laser beam is the result of heating of the path by absorption of a small fraction of the laser beam power by the medium which changes the index of refraction and therefore distorts the beam. Thermal-blooming effects can limit the laser powers which can be effectively propagated through the atmosphere, or in media which absorb laser power such as industrial or laboratory environments, liquid or gas cells, or even laser active media themselves. In this paper, we review the steady-state thermal blooming of CW beams including laboratory-simulation experiments and computer-code results. The thermal distortion of pulsed-laser radiation is also covered including single-pulse thermal distortion and the distortion of a train of laser pulses. In these discussions, we derive and identify the scaling laws and determine the important nondimensional parameters so that the results can be interpreted and applied to other propagation conditions. The thermal-blooming problem can be complicated by a number of circumstances, such as the geometry of the propagation path, and these special cases are also reviewed. Among those covered are: the influence of stagnation zones, transonic flow, the kinetic cooling effect, molecular and aerosol absorption and relaxation, laser-beam jitter, and atmospheric turbulence. In addition, the techniques utilized to minimize blooming, such as laser-beam shaping, and adaptive-optics phase correction, are also discussed.