Open multipass absorption cell for in situ monitoring of stratospheric trace gas with telecommunication laser diodes

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-mum region) and water vapor (in the 1.39-mum 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. (C) 2002 Optical Society of America.