Abstract

Most of lasers have a fundamental mode that is Gaussian in shape. Although many applications benefit from such a profile, many other applications need a different shape. In this paper we describe the conversion of a Gaussian beam into another profile using a simple diffraction optics known as π-circular phase aperture that introduces a π phaseshift in its central portion of radius b. Figure 1 shows that the far-field pattern is made up of a doughnut profile having a central depression whose depth can be continuously ajusted by varying parameter Δ = b /W, where W is the incident beam width. This property may be useful for optical trapping of small particles which are caged by a central dip or for cooling and trapping of atoms. Another property of the π-plate is shown in figure 2 where θ_out(θ_in) is the angular divergence of the diffracted field (incident Gaussian beam). This property is used to transform an elliptic beam, as emitted by a laser diode, into a circular beam thanks a π-phase slit, oriented along the minor axis that is assumed to be vertical, which spreads out more in the horizontal direction that in the vertical direction.

© 2001 EPS