Ultra-compact TDLAS humidity measurement cell with advanced signal processing

In this paper, tunable diode laser absorption spectroscopy humidity measurements with an ultra-compact measurement cell are presented. The optical path length is 2 cm. The system uses a vertical cavity surface emitting laser at 1.854 μm. The main limiting factor of the humidity resolution is not the noise but interference fringes produced by reflecting surfaces. Next to the system setup, a novel rejection method to eliminate these fringes, based on Fourier domain analysis of the absorption line, is described. In contrast to other fringe rejection methods, the presented method is able to handle fringes, whose free spectral range is in the range of the half width of the absorption line. The achievable humidity resolution for the presented cell is below 0.25 % relative humidity at room temperature.