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In this paper we introduce a novel tunable diode laser absorption spectroscopy (TDLAS) technique which allows for the simultaneous measurement of spatially distinct absorption paths. This method employs sheet forming optics to manipulate the beam of a diode laser into a sheet imaged by a linear array camera, thus generating path-integrated absorption measurements over a line. We demonstrate the method using scanned direct absorption of water vapor over well-established transition lines near 1388 nm. Measurements are demonstrated under controlled conditions at various pressures and room temperature in a low-pressure absorption cell. Initial attempts at evaluating the method in combustion environments identified several challenges that need to be resolved for the successful application of the technique. One of these challenges is due to interference due to the time-varying flame luminosity that reduces the SNR of the measured spectral absorbance. Another challenge is due to beam steering by a flame that deforms the shape and intensity distribution of the excitation laser sheet, thus introducing distortion of the measured absorption spectrum. Another challenge is due to the formation of etalon noise by the sheet forming optics that require careful optical alignments to minimize it. An evaluation of the light collection and noise characteristics of the linear array camera used in the study shows the adequacy of the device to generate acceptable absorption spectrum, from which an acceptable measure of temperature and pressure could be extracted at each pixel or groups of pixels (spatially average measurement). The approach holds the promise to generate high resolution spatial measurements along a line of the path-integrate thermodynamic state of the absorbing species. The method could be extended to other species, properties and absorption-based diagnostics, including reconstruction-based diagnostic methods. Fig. 1 (Left) Spatial distribution of the spectral absorption over a line at room temperature and 28.6 kPa. (Right) Line-averaged spectrum compared to simulated spectrum.

Planar Laser Absorption Spectroscopy for the Resolution of Simultaneous, Spatially-Distinct Absorption Paths