Use of skew rays in multimode fibers to generate speckle field with nonzero vorticity

Abstract

Beams at the output of an axially symmetric multimode fiber were produced with mostly positive or negative values of angular index m through skew illumination of the input end of the fiber. In this way rays with predominantly positive or negative helicity were excited inside the fiber. The output cw speckle field at the wavelength lambda = 632.8 nm contained large densities N(+) and N(-) of wave-front dislocations, or optical vortices, of both positive and negative sign. Vorticity was defined as V = N(+) - N(-) , was expressed analytically through the Wigner function of the field, and was found experimentally to be in good agreement with theory.