Highly sensitive detection of methane by near-infrared laser absorption spectroscopy using a compact dense-pattern multipass cell

Highly sensitive detection of atmospheric methane (CH4) was performed by long optical pathlength absorption spectroscopy based on a novel compact dense-pattern multipass cell (DP-MPC) in conjunction with a fiber-coupled distributed feedback diode laser operating at 1.653 mu m. Wavelength modulation spectroscopy approach was used and a minimum detectable concentration (1 sigma) of 100 ppb was obtained with a lock-in time constant of 1 ms. A measurement precision of <79 ppb was achieved by average of five laser scans in 1 s. This newly developed DP-MPC realized 215 times multiple reflections between two very cheap and robust silver coated concave spherical mirrors separated by a distance of 12 cm (forming an absorption cell of 280 cm(3) volume), offering an effective optical path length of 26.4m, which is very suitable for applications to trace gas sensing in harsh environment, and weight-limited unmanned aerial vehicle (UAV) - or balloon-embedded field observations. (C) 2015 Elsevier B.V. All rights reserved.