The detection and localization of gas releases, such as methane from leaking natural gas pipelines or nitrogen oxides from failing electrical equipment, require high sensitivity to the target gas and insensitivity to non-target gases. Infrared (IR) absorption spectroscopy gives highly specific characterization of the identity and amount of airborne chemicals. Traditionally, spectroscopy techniques for long-range chemical sensing separate the “sensitive to x gas” and “insensitive to x gas” atmospheric measurements by quickly switching back and forth between them to reduce errors caused by any atmospheric changes. Because the samples are not measured at the same time, it is impossible to ensure that all atmospheric conditions (e.g., contaminants or humidity) remain constant between the two measurements.

The WACS optical receiver has three major subcomponents: the angle-tunable beamsplitter and associated relay telescope to provide two angle-tuned optical paths, the Fabry-Pérot (F-P) etalon assembly that translates the angle tuning into spectral tuning, and the detector assembly that provides the four spectral measurements to yield two power-insensitive spectral ratios.

The Wide-Area Chemical Sensor (WACS) addresses the problem of temporal atmospheric variation by taking the measurements concurrently. This approach compensates for spectral drift, atmospheric contaminants, and atmospheric scintillation, while simplifying the technical requirements of the laser (e.g., stability, power).

The WACS system uses a spectrally self-referencing technique to decouple the target gas from the background atmosphere. The system consists of a reconfigurable 3.2 to 3.8-μm-wavelength laser source with 5GHz linewidth and ~30-MHz wavelength stability that is tunable to the target gas species, and a self-referencing detector that is tuned to the laser source. The laser operates at a 10-kHz repetition rate and an average transmit power of 3 mW. Any laser source with a linewidth commensurate with the target gas absorption width and with sufficient power could be substituted. WACS uses reference gas cells, such as methane buffered by nitrogen, and a low-concentration gas cell that contains the target species to calibrate the transmitter laser tuning and to verify gas sensitivity and specificity.

To detect and discriminate gas clouds, a single, relatively broadband laser source is transmitted through the atmosphere, and the laser's spectral deformation by the material it passes through is measured in a receiver. The deformations are matched to targeted materials, enabling the detection and discrimination of gas clouds at very low concentrations (~1 part per trillion by volume) over tens of kilometers. A second sensor not tuned to the particles of interest corrects for false alarms caused by atmospheric turbulence, variable atmospheric transmission, and other non-target gases.

The ability to separate light that “sees” the gas of interest from light that “does not see” requires a very narrow, tunable, and stable spectral filter; for this purpose, the WACS system uses a Fabry-Pérot etalon, a single lossless resonant cavity that consists of two partially reflective facing mirrors. The etalon transmission varies strongly with wavelength and with the angle at which the light strikes the etalon. All light encountering the cavity transmits through it or reflects off it; comparing these two light intensities enables measurement of the gas-induced absorption. This absorption varies over the bandwidth of the optical cavity's spectral response because of time-varying but spectrally constant losses caused by atmospheric scintillation and atmospheric contaminants.

The WACS technique enables the measurement of concentrations of specified target gases within the atmosphere at longer ranges than those achieved by other active optical techniques, while using a much lower-power laser source for a given range. Additionally, WACS is capable of at least five times the absolute sensitivity of other sensors at the longest ranges because of its ability to measure and remove the effects of spectral drift, atmospheric contaminants, and atmospheric scintillation.

For more information, contact Dr. Eric R. Statz at This email address is being protected from spambots. You need JavaScript enabled to view it.; 781-981-3784.


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This article first appeared in the December, 2018 issue of Tech Briefs Magazine.

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