Dual-Core Optical Fibers for Efficient Mid-Infrared Generation via Third Order Frequency Mixing and Coupling-Length Phase Matching

Appropriately designed dual-core fibers using coupling-length phase matching (CLPM) allow for phase-matched frequency downconversion over wide frequency intervals using the third-order optical nonlinearity of glass. By tuning the distance between the two cores, CLPM allows continuously tunable phase matching for widely different wavelengths for the process in which a pump wave at a frequency $\omega_2$ generates or amplifies two waves with frequencies $\omega_3>\omega_2$ and $\omega_1 = 2 \omega_2 - \omega_3$. The intensity-dependent correction that accounts for nonlinear phase-modulation is derived in general. In addition, a specific CLPM configuration is found to be insensitive to phase-modulation, and can achieve 100\% theoretical quantum yield for pump wave injection in one core. Fiber-based frequency converters can thus be designed for large differences between pump wavelengths and generated wavelengths, with the phase-matched interaction enabling the use of meter-long fibers to compensate for low third-order susceptibilities. Examples of fiber designs for pump wavelengths at 1.3 and 1.55 $\mu$m, to generate radiation with wavelengths longer than 2 $\mu$m, are discussed for silica and fluoride fibers.