Cascaded χ(2) nonlinearity in QND measurement

QND measurement schemes often use Kerr nonlinearity to couple the intensity fluctuations of the signal beam to the phase fluctuations of the probe beam by means of the cross-phase modulation effect. Such schemes use materials with intrinsic χ(3)nonlinearity. However, it has already been shown that Kerr-effect-like correlation between two light waves may be achieved through cascaded χ(2): χ(2) processes. The value of the nonlinearity induced by cascading χ(2) can, in many cases, be much higher than fibre χ(3).