Abstract

We report the construction of an atmospheric methane measurement instrument based on a Zeeman-split IR He–Ne laser. The laser has a transverse magnetic field over ∼2/3 of its gain length and can oscillate at an (unsplit) frequency (2947.91 cm−1) centered on a methane absorption line, or on either of two frequencies split by ±0.055 cm−1 from the center, with low CH4 absorption. The laser is tuned to dwell sequentially at each frequency, giving two differential absorption measurements in each 46-ms tuning cycle. Atmospheric measurements are made using two multiple pass absorption cells, one with fast (0.75-s) and one with slow (5-s) flow response times. Fluctuations in ambient CH4 of ∼20-ppb (rms, 1-s averaging) are detected, with interference fringe effects the dominant noise source. The instrument has operated in a field experiment (NASA GTE/ABLE-3A) in Alaska.

© 1989 Optical Society of America

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