Tuning the orbital angular momentum in optical vortex beams.

We introduce a method to tune the local orbital angular momentum density in an optical vortex beam without changing its topological charge or geometric intensity distribution. We show that adjusting the relative amplitudes a and b of two interfering collinear vortex beams of equal but opposite helicity provides the smooth variation of the orbital angular momentum density in the resultant vortex beam. Despite the azimuthal intensity modulations that arise from the interference, the local orbital angular momentum remains constant on the vortex annulus and scales with the modulation parameter, c = (a-b)/(a+b).