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

Abstract

a b s t r a c tHighly sensitive detection of atmospheric methane (CH 4 ) 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 m. Wavelength modulation spectroscopy approach was used and a minimum detectable concentration (1 ) 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 26.4 m, 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.

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

“…Recently, new types of gas cells have been developed to achieve long optical path in a smaller volume, including modified MPCs [111][112][113][114][115][116], circular multi-reflection (CMR) cells [117][118][119][120][121][122][123][124], and HWG [32,63,69,[125][126][127][128]. Guo and Sun reviewed the progress in modified MPCs and CMR cells and compared them in terms of optical pathlength (OPL), volume, and path-to-volume ratio (PVR) [116].…”

Recent Developments in Modulation Spectroscopy for Methane Detection Based on Tunable Diode Laser

“…where, τ(υ) represents the transmittance of light, I o (υ) and I t (υ) represent the incident and transmitted intensities of a certain wavelength, respectively, σ(υ) represents the cross-section of gas absorption at a certain wavelength, x represents the gas concentration, and L represents the length of the path that the light travels through absorbing media, α(υ) represents the transmission coefficient. In absorption spectroscopy, the detection sensitivity is related to the length of the absorption path and the absorption line intensity of molecule, respectively [2]. According to differences in measurement area and monitoring technique, different frequency bands involving methane transitions from near infrared to mid infrared have been applied [30].…”

“…White cells [99], Herriott cells [100] and Chernin [101] are the three most common MPCs applied for detecting gas, and variations on these have been developed. Liu et al [2] studied a novel compact dense-pattern multipass cell (DP-MPC). The cell was used to detect atmospheric methane in TDLAS with WMS.…”

“…Due to global warming and climate change, the monitoring and detection of atmospheric gas concentration has come to be of great value. Although the average background level of methane (CH 4 ) (~1.89 ppm) in the earth's atmosphere is roughly 200 times lower than that of CO 2 (~400 ppm), the contribution of CH 4 to the greenhouse effect per mole is 25 times larger than CO 2 [1,2]. Therefore, a fast, accurate, and precise monitoring of trace greenhouse gas of CH 4 is essential.…”

Design of an Optical Fiber-Coupled Sensor for Ambient Methane Measurement

“…Similarly it is common to locate the photodetector immediately after the interrogation region [i.e. (Berman et al, 2012;Ren et al, 2014;Liu et al, 2015;Nwaboh et al, 2013;Bauer et al, 2014;Huang et al, 2014;Ye et al, 2015;Smith et al, 2013;Mappé et al, 2013;Brumfield et al, 2015;Krzempek et al, 2013;Leen et al, 2013;Rieker et al, 2014;Shemshad, 2015;Triki et al, 2015)]. However, while having a detector as part of the interrogation cell improves tolerance to beam deviation and dispersion due to the larger target area on a photodiode versus a collimating optic, eliminating the photodetector from an interrogation region placed in situ reduces the need for electrical components and cabling, and more importantly for this application, removes the flammability risk by ensuring the remote sensor head is intrinsically safe.…”