keywords dispersion spectroscopy; gas sensor; heterodyne; infrared; quantum cascade laser; spectroscopy; wavelength modulation spectroscopy; carbon monoxide; chemical sensors; dispersion; waves; gas detectors; heterodyning; infrared radiation; molecular spectroscopy; nanoelectronics; nanosensors; nitric oxid; nitrogen oxides; photoacoustic spectroscopy; quantum cascade lasers; spectroscopy; sulfur dioxide; absorption signals; anomalous dispersion; concentration ranges; higher frequencies; limit of detection; linear concentration; measurement techniques; wavelength modulation spectroscopy; modulation