## Abstract

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.

© 2002 Optical Society of America

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### Equations (12)

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(1)
$${f}_{c}=\frac{R_{0}{}^{2}}{4l+2\left({R}_{0}-{R}_{1}\right)}-\frac{{R}_{0}}{2}.$$
(2)
$$\mathrm{\alpha}=2h\left(\frac{2l}{{R}_{0}{R}_{1}}+\frac{1}{{R}_{1}}-\frac{1}{{R}_{0}}\right),$$
(3)
$${x}_{n}={x}_{0}cos\left(n\mathrm{\theta}\right)+{\left(\frac{d}{4f-d}\right)}^{1/2}\left({x}_{0}+2{\mathit{fx}}_{0}\prime \right)sin\left(n\mathrm{\theta}\right),$$
(4)
$$cos\left(\mathrm{\theta}\right)=1-\left(\frac{d}{2f}\right).$$
(5)
$$N\mathrm{\theta}=2\mathrm{\pi}M,$$
(6)
$$d=2f\left[1-cos\left(2\mathrm{\pi}\frac{M}{N}\right)\right].$$
(7)
$${x}_{n}=Asin\left(n\mathrm{\theta}+\mathrm{\Psi}\right),{y}_{n}=Bsin\left(n\mathrm{\theta}\right),$$
(8)
$${A}^{2}=\frac{4f}{4f-d}\left(x_{0}{}^{2}+{\mathit{dx}}_{0}{x}_{0}\prime +{\mathit{dfx}}_{0}{\prime}^{2}\right),{B}^{2}=\frac{4f}{4f-d}\left({\mathit{dfy}}_{0}{\prime}^{2}\right)$$
(9)
$$tan\left(\mathrm{\Psi}\right)=\frac{{\left(\frac{4f}{d}-1\right)}^{1/2}}{1+2f\frac{{x}_{0}\prime}{{x}_{0}}}.$$
(10)
$${x}_{0}\prime =tan\left(\mathrm{\alpha}\right)sin\left(\mathrm{\Phi}\right),{y}_{0}\prime =tan\left(\mathrm{\alpha}\right)cos\left(\mathrm{\Phi}\right).$$
(11)
$${x}_{N-1}^{\prime}=\frac{{x}_{0}}{f}+{x}_{0}^{\prime},{y}_{N-1}^{\prime}={y}_{0}^{\prime}.$$
(12)
$${x}_{N-1}^{\prime}=tan\left(\mathrm{\alpha}\prime \right)sin\left(\mathrm{\Phi}\prime \right),{y}_{N-1}^{\prime}=tan\left(\mathrm{\alpha}\prime \right)cos\left(\mathrm{\Phi}\prime \right).$$