Tunable diode lasers (TDL) near are used to measure high-resolution direct absorption and wave length modulation with second harmonic (WMS-) spectra at high pressures for two transitions near 3633.08 and , belonging to the vibrational band. Important factors influencing the design of a high-pressure TDL sensor and the variation of WMS- line shape with changes in pressure and laser parameters are discussed. Measurements of line strength and line broadening parameters are carried out for the transition in an atmospheric-pressure, high-temperature cell. A room-temperature high-pressure cell is then used to measure the pressure shift for both transitions. Deviation of the direct absorption and wavelength division spectroscopy (WMS) spectra from the Lorentzian profile is studied in a high-density () –Ar mixture. The WMS spectra are shown to be negligibly affected by non-Lorentzian effects up to and room temperature, in contrast with direct absorption. Measurements of concentration and temperature are carried out in nonreactive shock-tube experiments (, ) to validate the accuracy and precision of wavelength-modulation-spectroscopy-based sensing. time histories are then measured in heptane ignition experiments and compared with reaction kinetics mechanisms to demonstrate the use of this sensor in high-pressure combustion systems.
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