Atmospheric methane measurement instrument using a Zeeman-split He-Ne laser

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 CH(4)) 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 CH(4)) 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.