A method of fabricating a gas sensor on a substrate and a gas sensor fabricated on a substrate that includes optical and electronic components are described. The method includes fabricating a laser to output light over a range of wavelengths within a waveguide, fabricating a splitter to split the light output by the laser to a reference waveguide and to a detection waveguide, fabricating a reference cell to house the reference waveguide and a reference gas. An output of the reference waveguide is coupled to a first optical detector and an output of the detection waveguide is coupled to a second optical detector to identify or quantify an ambient gas.