Spectroscopic devices, systems, and methods for optical sensing of molecular species

A spectroscopic method for detecting one or more gas species of interest in a gas sample, the method comprising:

• transmitting an interrogation light beam from a coherent light source through a sample cell containing one or more gas species of interest to be detected, and a reference gas at below atmospheric pressure, said reference gas comprising at least one of the one or more gas species of interest to be detected in the sample cell, said coherent light source being disposed within a light source module comprising said coherent light source and a light source housing, said reference gas being disposed within a detector module comprising a detector module housing, a detector, and said reference gas, said sample cell being disposed between said light source module and said detector module;
• detecting the intensity of the interrogation light beam after passing through the reference gas and the sample cell;
• generating a direct absorption spectroscopy (DAS) signal based on the detected light intensity;
• generating a wavelength modulated spectroscopy (WMS) signal based on the detected light intensity;
• and
• generating calibration information by setting the direct absorption spectroscopy (DAS) signal as the constant reference signal for the wavelength modulated spectroscopy (WMS) signal.