Based on the mode-hopping of single mode diode laser,two different currents drive the laser alternately,the laser emits alternately two beam lights whose wavelength is differently,but closely.The first beam light's wavelength is at the absorption peak of methane.The second beam light's wavelength is a little shorter than the first one,which is not absorbed by methane,and it will serve as a reference light beam.The double beam will be converted into an electronic square wave after passing through the gas cel...