Emission of OAM entangled photon pairs in a nonlinear ring fiber   utilizing spontaneous parametric down-conversion
  We suggest the generation of photon pairs in a thermally induced nonlinear periodically-poled silica fiber by spontaneous parametric down-conversion. Photons are generated directly in eigenstates of optical angular momentum. Photons in a pair can be entangled in these states as well as in frequencies. We identify suitable spatial and polarization modes giving an efficient nonlinear interaction. By changing the pump field properties both narrow- and broad-band down-converted fields can be obtained. 
D. Jav\r{u}rek, J. Svozil\'ik, J. Pe\v{r}ina Jr
quant-ph physics.optics