Raman-induced polarization-dependent gain in parametric amplifiers pumped with orthogonally polarized lasers

Fiber-optic parametric amplifiers are often pumped using two orthogonally polarized lasers. We show that, contrary to a common belief, such amplifiers exhibit polarization-dependent gain (PDG) because of the contribution of Raman scattering to the underlying four-wave mixing (FWM) process. Such PDG results not only from Raman gain/loss on the signal/idlers imposed by the pumps, but also from variations of FWM efficiency caused by Raman-induced refractive index changes.