Phase optical components for the generation of free-space quasimodes

An iteration method is proposed for the calculation of the transmission function of phase optical devices generating narrow weakly diverging beams (quasimodes). Such beams have a narrow Bessel spectrum, i.e., they represent superpositions of a small number of free-space Bessel modes. It is shown that the beams generated by an axicon and a binary phase device with its transmission described by sgn [J₀(x)] are examples of such quasimodes. The phase devices designed by the proposed method can generate beams whose diameter increases from zero to a desired value at the selected distance.