Phase-matching with a twist: Second-harmonic generation in birefringent periodically poled fibers

We show theoretically and experimentally that twisting a birefringent periodically poled fiber with an artificially induced chi((2)) results in second-harmonic phase-matchings that are not permitted in untwisted fibers. We further demonstrate that both the strengths and the spectral positions of the generated second-harmonic signals can be controlled by changing the amount of twist. Of particular interest is a type II phase-matched signal emerging through and greatly enhanced by the twisting.