Strain-Controlled Phase Matching of Optical Harmonic Generation in Microfibers

Nonlinear parametric processes in optical fibers, such as harmonic generation and four-wave mixing, provide great opportunities to expand advanced fiber-based technologies of lasers, communications, and sensors. Unfortunately, the efficiencies of these processes are significantly degraded by phase mismatching due to natural material dispersion, which hinders their applications. Though phase-matching techniques based on multimode operation, birefringence, and self-phase modulation have been reported, they suffer from low efficiency and poor flexibility. Here, we propose a simple strategy to fine-tune phase matching of harmonic generations by applying a mechanical strain over an optical microfiber. The structure and refractive index of the microfiber are modified by the strain, which adjusts the effective refractive indices of pump and signal modes for phase matching. The results indicate that the second- and third-harmonic generations from the microfiber both exhibit expected modulation of sinc2 function with respect to the applied strain. In the condition of phase matching, the maximum efficiency of harmonic generation is improved by around 8 times. The strain-controlled phase-matching method represents an effective and amenable strategy to manipulate nonlinear processes and motivate all-fiber nonlinear optics.