Some aspects of scaling the orbital angular momentum of light with conical diffraction

Abstract

The mechanisms leading to scaling the orbital angular momentum (OAM) from a Gaussian beam with no OAM up to integers 1, 2… have been pointed out in the cascaded conical diffraction process. The study is illustrated with two different biaxial crystals: KGd(WO2)4 and Bi2ZnOB2O6 (non-degenerate cascade). Scaling the OAM is the repetition of the following basic stage: the polarization of the highest OAM state generated by a previous crystal is changed with a quarter-wave plate in order that the OAM increment through the next biaxial crystal is maximized under a projection operator. This latter is constituted from a quarter-wave plate and a linear polarizer. The image of the field at the output of a crystal is repeated through the following one with a two-lenses afocal telescope, preventing its natural spread. The OAM is visualized by interference patterns with a reference beam and by a cylindrical lens. All the experimental patterns (intensity, phase, OAM) are quite well described by a full numerical model. The role of each crystal of the cascade in the OAM increment is simply exhibited in the Fourier space.