Recovery of the absolute absorption line shape is demonstrated for methane with a 1650nm edge-emitting laser and for carbon dioxide with a 2004nm VCSEL. The potential for direct detection and wavelength modulation spectroscopy are explored.OCIS codes: (300.6260) Spectroscopy, diode lasers; (120.0280) Remote sensing and sensors; (280.4788) Optical sensing and sensors 1. Calibration-free wavelength modulation spectroscopy techniques Industrial gas measurement using tunable diode laser spectroscopy (TDLS) has witnessed significant advances in recent times. The advent of frequency-agile semiconductor lasers in mid-infrared region [1], has given new impetus to the extraction of concentration, pressure and temperature [2,3]. TDLS is ideal for high-specificity, real-time measurements and offers very good sensitivity particularly for mid-infrared fundamental absorption lines of gases. When the unavailability of mid-infrared sources precludes direct detection and one is constrained to probe the significantly weaker near-infrared overtones, one uses wavelength modulation spectroscopy (WMS) to achieve high sensitivity. WMS essentially involves the interaction of the spectrally-modulated output of a laser with a gas absorption line to give rise to signal components at various harmonics of the modulation frequency (f m ).For injection current-modulated laser diodes, the intensity modulation (IM) is separated from the wavelength modulation (WM) by a modulation frequency-dependent phase angle ψ 1 , and is predominantly linear. The IM and the WM components interact with a gas absorption line to give rise to various concentration-dependent IM-WM terms that become progressively smaller at increasingly higher harmonics of f m . Typically, one of these harmonics is recovered and the gas concentration and pressure are extracted from specific attributes of the detected signal. WMS signals are characterized by the modulation index defined as m = a/γ, where γ is the half-width-half-maximum value of the linewidth of the absorption feature and ais the WM amplitude. The m-value is a very important parameter because it determines the form and amplitude of the Fourier coefficients of the spectrally-modulated absorption line and therefore the strength of the harmonic signals. Edge-emitting lasers as well as VCSELs have been popular for WMS because of their unique and rather contrasting properties. VCSELs are particularly popular because of their lowpower operation, wide tunability, narrow linewidth and low intensity modulation properties.Traditional 1f or 2f WMS however requires signal calibration because the 1f and 2f signals are affected by variable system parameters such as laser intensity, optical throughput and variable gas pressure in industrial processes. Two recent calibration-free 1f WMS techniques namely the RAM method and the Phasor Decomposition (PD) method [3], that allows the recovery of the absolute gas absorption line from which gas parameters are conveniently extracted. These methods are particularly useful for edge-emitting ...