Proposal for the generation of photon pairs with nonzero orbital angular   momentum in a ring fiber
  We present a method for the generation of correlated photon pairs in desired orbital-angular-momentum states using a non-linear silica ring fiber and spontaneous parametric down-conversion. Photon-pair emission under quasi-phase-matching conditions with quantum conversion efficiency $ 6\times 10^{-11} $ is found in a 1-m long fiber with a thermally induced $\chi^{(2)}$ nonlinearity in a ring-shaped core. 
Dalibor Jav\r{u}rek, Ji\v{r}\'i Svozil\'ik, Jan Pe\v{r}ina Jr
quant-ph physics.optics