QUANTUM-THEORY OF CROSS-PHASE MODULATION

The quantum noise characteristics imposed on an initially coherent sig nal 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-ph ase 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 a lone. The effects of both SPM and XPM are interpreted geometrically fo r short propagation distances. More generally, the quantum noise is ev aluated analytically and numerically for arbitrary propagation distanc es with signal and pump beams in the forms of square pulses. The contr asting limits of zero group velocity dispersion and complete pulse wal kthrough are treated separately.