Fundamental-mode third harmonic generation in microfibers by pulse-induced quasi-phase matching.

A scheme to enhance the fundamental-mode third harmonic generation efficiency in microfibers is presented. By introducing an appropriate counter-propagating pulse train, large propagation constant mismatch is partly overcome and nonlinear phase shifts could be corrected for, thus quasi-phase matching between the fundamental pump mode and the fundamental third harmonic mode is achieved, enabling the harmonic power to grow along the direction of propagation. Depending on the microfiber and pulse parameters, phase matching can enhance the conversion efficiency by several orders of magnitude with respect to the non-phase matched case. This scheme offers an alternative approach for harmonic generation and could potentially be applied to other small core waveguides.