Efficient reduction of fringe noise in trace gas detection with diode laser multipass absorption spectroscopy

The combination of a diode laser source and a multipass cell is attractive for the detection of trace gases. The sensitivity achievable with this system, however, is often limited by etalon fringes arising from path length difference among the optical components. In this paper, we describe a sensitive detection method that minimizes the effect of the fringes. In accordance with the expected widths of the 2 f (second-harmonic of the modulation frequency) absorption signals of the target molecule, the base length of a multipass cell is chosen so that the free spectral range of the fringes becomes much narrower than the signal widths. Then, the 2 f signal is subjected to multistage smoothing to remove the fringe noise by exploiting its periodicity. As a demonstration of this method, trace moisture in nitrogen is detected using a near-infrared, distributed-feedback diode laser and a multipass cell of 20 m absorption length. The detection limit of the system is found to be about 2.3 ppb, two orders of magnitude better than the limit before fringe elimination.