Three-photon spontaneous downconversion in highly nonlinear germania-silica optical fiber waveguides

Three-photon spontaneous parametric downconversion (TPSPDC) is a challenging problem in nonlinear quantum optics. A highly doped germania-silica optical fiber is a good candidate for the appropriate nonlinear medium, because of the big interaction length and tight field confinement. A principal condition for TPSPDC is the exact phase-matching between the pump and signal fiber modes.