  We demonstrate a novel MIR methane sensor shifting measurement wavelength to
SWIR (1.55$\mu$m) by using non-linear interferometry. The technique exploits
the interference effects seen in three-wave mixing when pump, signal, and idler
modes make a double pass through a nonlinear crystal. The method allows sensing
at wavelengths where detectors are poor ($>$3$\mu$m) and detection at
wavelengths where photon counting sensitivity can be achieved. In a first
experimental demonstration, we measured a small methane concentration inside a
gas cell with high precision. This interferometer can be built in a compact
design for field operations and potentially enable the detection of low
concentrations of methane at up to 100m range. Signal-to-noise ratio
calculations show that the method can outperform existing short wavelength
($\sim$1.65$\mu$m) integrated path differential absorption direct sensing at
high ($>$$10^{-4}$) non-linear gain.
