NOVELTY - The method comprises modulating a laser wavelength of a tunable laser at a modulation frequency to produce a wavelength-modulated laser beam with a laser frequency range including an absorption band of molecules of a molecular species in the gas medium, directing the wavelength-modulated laser beam that transmits through the gas medium, and measuring a transmission of the wavelength-modulated laser beam through the gas medium to obtain measurements of a signal at a harmonic frequency of the modulation frequency at different modulation depths in modulating the laser wavelength.    USE - The method is useful for optically sensing a gas medium based on wavelength modulation spectroscopy, where the gas medium is a mixture of a gas fuel and air in a premixing gas chamber in a gas turbine or engine.    ADVANTAGE - The method ensures accurate measurement of the gas medium based on wavelength modulation spectroscopy with high sensitivity, mitigates and controls gas turbine oscillations and/or nitrogen emissions during measurement, and reduces or minimizes background noise in signal detection.    DETAILED DESCRIPTION - The method comprises modulating a laser wavelength of a tunable laser at a modulation frequency to produce a wavelength-modulated laser beam with a laser frequency range including an absorption band of molecules of a molecular species in the gas medium, directing the wavelength-modulated laser beam that transmits through the gas medium, measuring a transmission of the wavelength-modulated laser beam through the gas medium to obtain measurements of a signal at a harmonic frequency of the modulation frequency at different modulation depths in modulating the laser wavelength, processing the measurements of the signal at the harmonic frequency of the modulation frequency at different modulation depths to obtain a total broadening of absorption spectrum of the molecular species in the gas medium, where the signal comprises a first signal at a second harmonic frequency of the modulation frequency, modulating the laser wavelength of the tunable laser to make the signal at the harmonic frequency to be sensitive to a concentration of the molecules of the molecular species, modulating a laser wavelength of a second tunable laser at a second modulation frequency to produce a second wavelength-modulated laser beam with a laser frequency range including the absorption band of molecules of the molecular species in the gas medium, directing the second wavelength-modulated laser beam through the gas medium, measuring a transmission of the second wavelength-modulated laser beam through the gas medium to obtain measurements of a second signal at a harmonic frequency of the second modulation frequency at different modulation depths in modulating the laser wavelength, modulating the laser wavelength of the second tunable laser to make the second signal at the harmonic frequency of the second modulation frequency to be sensitive to a pressure in the gas medium, and processing the measurements of the second signal to obtain additional information on the gas medium and to extract the concentration of the molecules of the molecular species and the pressure in the gas medium. The processing step is performed without prior knowledge of a composition of the gas medium and pressure broadening coefficients of the molecules of the molecular species of the gas medium. The method further comprises obtaining a concentration of the molecules of the molecular species and information on a pressure of the gas medium using the signal at the second harmonic frequency, obtaining information on a pressure of the gas medium and a concentration of the molecules of the molecular species using the second signal at the fourth harmonic frequency, monitoring a pressure oscillation from the measured transmission of the wavelength-modulated laser beam, and controlling injection of the gas medium and air into the premixing gas chamber based on the monitored pressure oscillation to stabilize the pressure in the premixing gas chamber. INDEPENDENT CLAIMS are included for:    (1) a method for performing wavelength modulation spectroscopy measurements; and    (2) a gas turbine system.    DESCRIPTION OF DRAWING(S) - The diagram shows a flow chart of a wavelength modulation spectroscopy measurement process.