Orbital angular momentum exchange in an optical parametric oscillator
  We present a study of orbital angular momentum transfer from pump to down-converted beams in a type-II Optical Parametric Oscillator. Cavity and anisotropy effects are investigated and demostrated to play a central role in the transverse mode dynamics. While the idler beam can oscillate in a Laguerre-Gauss mode, the crystal birefringence induces an astigmatic effect in the signal beam that prevents the resonance of such mode. 
M. Martinelli, J.A.O. Huguenin, P. Nussenzveig, A.Z. Khoury