Proposal for Second-Harmonic Generation Based on Mode Coupling in Coaxial Optical Fiber

We propose the quasi-phase-matching (quasi-PM) conditions for second-harmonic generation, which can be realized through the mode coupling in coaxial multilayer optical fiber. The coupling behavior between core and tube modes can be explained by the propagation of multiple supermodes with the modified propagation constants, which can be used to compensate for the index mismatch between fundamental and second-harmonic modes. Compared to periodically spatial modulation of quadratic nonlinearity, the advantage of these PM schemes is the simple multilayer structure and the available long-distance interaction in the coaxial fiber.