  We report on experimentally observed addition, subtraction and cancellation
of orbital angular momentum (OAM) in the process of parametric four-wave mixing
that results in frequency up- and down-converted emission in Rb vapour.
Specific features of OAM transfer from resonant laser fields with different
optical topological charges to the spatially and temporally coherent blue light
have been considered. We have observed the conservation of OAM in nonlinear
wave mixing in a wide range of experimental conditions including a
non-collinear geometry of the applied laser beams and furthermore that the
coherent blue light accumulates the total orbital angular momentum of the
applied laser light. Spectral and power dependences of vortex and plane wave
front blue light beams have been compared.
