<title>Beam shape sensitivity of spatial scanning</title>

Abstract

The effects of optical beam shape variations upon the near-field transmission features of a thin nonlinear medium located at a beam waist have been investigated. This work is particularly relevant to the z-scan techniques, as well as to the aperture -scan techniques, for characterising nonlinear media. In the model that has been explored, a family of incident transverse beam shapes F(p,v) characterised by a shape parameter v has been introduced at the focusing lens. For v > 2, these transform to a diffraction pattern containing circular rings and expressible in terms of an ordinary Bessel function. For v < 2 there is only a central spot, expressible in terms of a modified Bessel function. The functions F(p ,v) conserve both the incident peak irradiance and the beam power as V changes. A simple cubic (Kerr) nonlinearity has been studied. It is shown that a basic trade-off exists between the width and height of the peak-valley separation in the normalised transmission curve.