A gas sensor system includes a laser module, optical chamber module, and a gas sensor cell. The laser chamber module includes two laser light sources producing laser light emissions at wavelengths λ1 and λ2, respectively. Each beam path optionally includes an optical isolator. The beam paths enter the housing of the optical chamber module where they are combined into a third wavelength, λ. The housing of the optical chamber module includes an inlet and an outlet for passing a selected target gas. The gas sensor cell mates to the inlet of the housing. The target gas passes through the adjacent gas cell and into the optical chamber module through the inlet. The target gas exits the optical chamber module through the outlet. Within the optical chamber module, a nonlinear crystal receives the laser light emissions at wavelengths λ1 and λ2 and generates the third wavelength, λ3. The wavelength λ3 is selected to be at the mid-IR spectral absorption feature of the target gas, i.e. a strong absorption line in the vibrational spectrum of a specific chemical species that is targeted by the gas sensor. The third wavelength λ3 is generated by the interaction of the electric fields of the two laser emissions. The nonlinear optical crystal may be periodically poled, and configured to frequency-convert the first and second laser beams. The residual light from the two laser sources is removed from the mid-infrared beam with an absorptive and/or reflective optical filter.