Parametric amplification driven by orthogonal pump waves in birefringent fibers

All fiber-optic communication systems need optical amplifiers to compensate for fiber loss. One could use parametric amplifiers, which use four-wave mixing (FWM) to amplify the signals. Not only can parametric amplifiers provide broad-bandwidth, low-noise amplification at arbitrary wavelengths, they can also conjugate the signals and convert their wavelengths. For most signal frequencies the dominant FWM process is phase conjugation (PC), in which two pump photons, one from each pump wave, combine to produce one signal photon (s) and one idler photon (i). By tuning the pump frequencies judiciously, one can produce uniform gain over a practical bandwidth. However, the signal gain associated with parallel pump waves depends sensitively on the polarization of the input signal. Because transmission fibers are not polarization-maintaining, practical amplifiers must operate on signals with arbitrary polarizations. The properties of PC in such fibers should be intermediate between those of PC driven by parallel pumps, which was discussed in previous papers, and PC driven by orthogonal pumps, which is discussed in this paper.