Properties of laser radiation scattering by a laser-induced spark plasma revisited after 40 years

Experimental studies of a laser-induced spark produced in air by 1.05-μm, 100-ns pulses with spatial TEM₀₀, TEM₀₁ and TEM₀₂ modes are described. It is found that when the spark is observed at an angle of 90° to the laser beam axis, the scattered radiation has the maximal intensity outside the beam waist. The intensity ratio of the scattered laser radiation for two orthogonal polarisations is ~100, and the spatial structures of their depolarisation considerably differ. These properties are explained by using a model of the Fresnel reflection from the spherical front of the plasma — undisturbed gas interface.