Abstract
We report the construction of an atmospheric methane measurement instrument based upon a Zeeman‐split IR HeNe laser. The laser has a transverse magnetic field over ∼2/3 of its gain length, and can oscillate at an (unsplit) frequency (2947.9 cm−1) centered on a methane absorption line, or at ±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. There are two atmospheric measurement channels, with fast (0.75 sec) and slow (5 sec) flow response times. Fluctuations in ambient CH4 of ∼20 ppb (RMS, 1 sec TC) are detected, with fringe effects the dominant noise source. The instrument has operated in a field experiment (NASA GTE/ABLE‐3A) in Alaska.
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