Normalization of the scattered light from an isolated defect illuminated by a Gaussian beam

Abstract

The intensity distribution of the beam from a laser operated in the zero order configuration for the transverse electromagnetic field (TEM/sub 00/ mode) is Gaussian in a plane perpendicular to the axis of the beam. In some applications it is desirable to have a uniform intensity over a certain region in space. For example, when a Gaussian beam is incident on a smooth surface containing small isolated defects, the light scattered by a defect will depend on the position of the defect relative to the center of the beam. In the past, several techniques have been devised to convert a Gaussian intensity profile into a uniform intensity over a specified region in space. In the present work a different approach is taken. A method of normalization is described which makes direct use of the Gaussian intensity distribution of the TEM/sub 00/ mode. By this method the amount of light scattered by a defect can be normalized to the value which would be observed if the defect were located at the center of the beam, for a defect small in size compared with the 1/e/sup 2/ diameter of the Gaussian profile. The normalization requires that three measurements be made of the scattering by the defect for an arbitrary location of the defect relative to the center of the Gaussian beam, and that the 1/e/sup 2/ radius of the Gaussian beam and the interval between adjacent illuminated spots on the surface are known. Experimental data verifying the theory were obtained from isolated defects on a superfinished spherical surface.