Abstract

In recent years, the formation of laser beams with phase singularities has attracted considerable interest. The field distribution of such beams can be described in terms of non-degenerate (p,l) La- guerre-Gaussian (LG) modes, having exp(ilϕ) spiral (helical) phase. Specific helical modes can be selected by inserting of spiral phase elements (SPE) into the laser cavity.¹ Alternatively, a helical beam can be obtained directly from a Gaussian beam, outside the laser cavity, by means of computer-generated holograms.² Another common technique for obtaining a beam with desired he- licity is to start with the fundamental Gaussian beam and convert it to a helical one by means of SPE with desired erp(ilϕ) helicity.³ However, the field distribution of the resultant beam does not represent a pure mode, whereby it contains a superposition of different LG modes. It was reported that the power contains less than 80% of the desired helical mode power, whereas the rest of the power is in other high order LG modes.³ With SPEs, the degradation in the efficiency is due to the different intensity distributions of a Gaussian and a desired helical beams.

© 2002 Optical Society of America