Systems and methods of laser dispersion spectroscopy for in situ, quantitative and non-intrusive measurements of combustion parameters, such as temperature and gas concentrations are provided. The system includes a laser source configured to sweep across the spectral features of molecules, an RF generator configured to modulate a laser to generate a three-tone laser beam, a photodetector configured to detect the transmitted three-tone laser beam to generate a heterodyne beat note signal, and a lock-in amplifier configured to further process the beat note signal in order to measure a phase of the heterodyne beat note signal.