Second harmonic generation in spliced poled fibers

Optical poling is a very simple technique for inducing a quadratic nonlinearity in germanium-doped silica fibers [1] , [2] . By injecting into the fiber fundamental frequency (FF) pulses together with a low power second harmonic (SH) seed, a multi-photon quantum interference process naturally generates a quadratic nonlinearity whose period satisfies the quasi-phase matching (QPM) condition. The nonlinear grating writing process takes a few hours before showing a saturation of the SH conversion efficiency, which is always lower than the best values achievable by resorting to thermal poling [3] . Also, the maximum length of the poled region exhibits saturation, and this hampers the SH generation [4] .