QUANTUM-THEORY OF CROSS-PHASE MODULATION

The quantum noise characteristics imposed on an initially coherent signal light beam in propagation along an optical fibre in the company of a pump beam are obtained. The Kerr nonlinearity of the fibre affects the signal beam by its own self-phase modulation (SPM) and by cross-phase modulation (XPM) from the pump beam. It is found that XPM tends to destroy the well-known squeezing of the signal beam produced by SPM alone. The effects of both SPM and XPM are interpreted geometrically for short propagation distances. More generally, the quantum noise is evaluated analytically and numerically for arbitrary propagation distances with signal and pump beams in the forms of square pulses. The contrasting limits of zero group velocity dispersion and complete pulse walkthrough are treated separately.