Thermal focusing and defocusing of a laser beam in an absorbing dielectric is studied when the laser operates in the TEM01 mode. The paraxial-ray approximation is employed to obtain an analytical solution of the wave equation. When the refractive index decreases with increasing temperature, the electromagnetic energy converges in the x direction and diverges in the y direction; the reverse is the case when the refractive index increases with increasing temperature. The authors have also studied the geometric-optics self-focusing of laser beams operating in the (i) doughnut mode and (ii) mixed TEM00 and TEM10 cylindrical modes oscillating in phase opposition. In both cases, the energy of the beam concentrates around a circular ring of maximum irradiance.
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