Methane sensing in the mid-IR using short wave IR photon counting detectors via non-linear interferometry

We demonstrate a novel MIR methane sensor shifting measurement wavelength to SWIR (1.55 μ 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 μ 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 (∼1.65 μ m) integrated path differential absorption direct sensing at high (>10 −4 ) non-linear gain.