Pump-power-dependent gain for small-signal parametric amplification in birefringent fibres

The nonlinear dependence of the parametric gain for small-signal amplificat ion is experimentally studied in a normally dispersive highly birefringent single-mode fibre. As a result of the pump-induced nonlinear phase shift, t he signal gain curve versus the pump power is bell shaped in the undepleted -pump regime with maximum gain attained away from perfect phase-matching. T his dynamics is quite different from the quasi-exponential gain measured fo r spontaneous parametric emission growing from noise. Moreover, the experim ental gain curves are more accurately predicted by modulation instability c omputations rather than four-wave mixing theory because of the influence of the non-phase-matched waves. (C) 2001 Published by Elsevier Science B.V.