A two-mirror multipass absorption cell that is operated open to the atmosphere from a stratospheric balloon to monitor in situ methane (in the 1.65-µm region) and water vapor (in the 1.39-µm region) with telecommunication laser diodes is described. A small Cassegrain-type telescope is used to couple the cell simultaneously with two near-infrared InGaAsP laser diodes by means of optical fibers. The 1-m cell provides an absorption path length of 56 m. The optical cell was carefully designed to be free of incidental fringing in the 10-5 absorption range. It is used in combination with a dual-beam detector to obtain a detection limit of 10-5 absorption units, a large dynamic range of the measurements of many orders of magnitude, and a precision error in the concentration determination of a few percents. The optical arrangement of the cell and its ability to be used to detect in situ trace gas in the stratosphere, in severe environmental conditions, are exposed.
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