Abstract

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 ${\omega _3} \gt {\omega _2}$ 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 $1.55\,\,\unicode{x00B5}{\rm m}$, to generate radiation with wavelengths longer than $2\,\,\unicode{x00B5}{\rm m}$, are discussed for silica and fluoride fibers.

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