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
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