  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.
