Parametric Amplification in Dual-Core Fibers Via Intermodal Four-wave Mixing

The four-wave mixing (FWM) process in silica fibers features the in-pair generation of signal and idler photons with frequencies symmetrically located on both sides of the pump frequency. The corresponding optical parametric amplification (OPA) gain spectrum is hence known for its frequency symmetric shape. In this work, we report a novel scheme of OPA in dual-core fibers, based on intermodal FWM and featuring an asymmetric gain spectrum, which results from the interplay between pump-power imbalance between the two cores and the frequency dependency of the coupling. We show analytically and numerically that signal and idler waves are naturally generated in two different supermodes. Our theory is based on the two linearly coupled nonlinear Schrödinger equations with frequency-dependent coupling $C(\omega)=\sum_{n=0}^{\infty}(\omega-\omega_{0})^{n} C_{n} / n !$ , where $C_{n}$ is the n-th coefficient of the Taylor expansion of the coupling strength around the pump frequency $\omega_{0}$ . By injecting unequal pump waves into the two cores, corresponding to asymmetric stationary solutions [1] with power $P_{1,2}=\frac{1}{2}(P_{p}\pm P_{d})$ -being $P_{p}$ the total pump power and $P_{d}=\sqrt{P_{p}^{2}-(2C_{0}/\gamma)^{2}}$ the power asymmetry parameter - then the evolution equations of signal and idler waves with frequencies $\omega_{s,i}=\omega_{0}\pm\Omega$ can be derived in the supermodes basis $E_{\pm}=(e_{s1}\pm e_{s2},e_{i1}^{*}\pm e_{i2}^{a*})^{T}$ :