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

Abstract

A tunable diode laser system was developed for remote monitoring of ambient methane using fiber optically connected all-optical sensor heads. Experiments were performed to quantitatively evaluate the influence of selected system hardware and software configurations, with the further goal of enabling measurements of ambient methane concentrations over an intrinsically-safe fiber optic network at sufficient precision and sensitivity to detect unknown fugitive emission sources. The designed system could switch between the sweep integration (SI) and wavelength modulation spectroscopy (WMS) detection methods, which inferred the methane volume mixing ratio from an absorption or second-harmonic (2f) feature respectively. Signals controlling the laser injection current were optimized to balance trade-offs between measurement precision and system sensitivity, and fiber optic components were thermally stabilized to reduce system drift. Starting from this base system, experiments were performed to evaluate the effectiveness of theory-based and experimental calibration methods, software and dual laser approaches to estimating the absorption-free intensity, different signal processing approaches to suppress effects of residual amplitude modulation (RAM) of the laser output intensity, and methods to reduce system drift. Tests also considered effects of varying fiber lengths between the central laser control hardware and the remotely located optical sensor heads. Finally, long-term stability was evaluated by quantifying bias (drift) and precision uncertainty in tests up to 16 months after initial calibration. The best measurement performance was achieved using the WMS method with thermally stabilized optical components within the central control hardware combined with a theory-based calibration, automated daily calibration supported by concurrent softwarebased estimation of the absorption-free intensity, and pair-wise averaging of the 2f feature maxima in each sweep period to suppress effects of RAM. At an averaging time of 1 s and methane concentrations between 1.0 and 50 ppm v , the system implementing the 2f-WMS method surpassed the methane volume mixing ratio measurement targets with a precision of 1.6 ppm v (absorbance of 6.57×10 -7 cm -1 ) and lower detection limit (LDL) of 1.5 ppm v (6.13×10 -7 cm -1 ). In achieving these values, thermal stabilization was responsible for a 93% reduction in the precision uncertainty; adding the concurrent software-based absorption-free intensity estimate netted a further reduction of 73%; and implementing RAM suppression via pair-wise averaging of the resolved 2f features within each sweep period technique allowed a further reduction of 9%.3

Development of a Tunable Diode Laser Absorption Spectroscopy System for Quantification of Transient Emissions from Liquid Hydrocarbon Storage Tanks

“…Volume mixing ratios (VMRs) were obtained following a methodology adapted from (Schoonbaert, 2015) in which a four-dimensional look-up table (LUT) was used to compare measured and theoretical 2f peak heights for various temperatures, pressures, and corresponding oxygen mixing ratios as discussed in Section 2.2.3. For each measurement, the temperature and pressure from the cross-pipe cell were used to linearly interpolate the LUT to obtain a two-dimensional dataset of theoretical 2f peak heights and oxygen VMR.…”