Gas sensors based on tunable laser absorption spectroscopy

Staff: A. Vicet, M. Bahriz, Y. Rouillard

PhD students: T. Nguyen Ba (2011-2014)

Projects: NexCILAS (ANR blanc international 2011 – 2015), MIDAS (ANR P2N 2011 – 2015),

Other collaborations:

Ongoing changes in atmospheric composition due to both natural and anthropogenic influences affect not only climate and atmospheric processes but also have a negative impact on the air quality (hence human health), the hydrological cycle and ecosystems, and consequently on economic developments. Understanding and prediction of future changes in atmospheric composition requires reliable long-term estimates of sources and sinks appropriate to particular emission management scenarios with very high accuracy and precision observations of the abundance of key atmospheric species as well as their isotopes. A validation of numerical models requires a comprehensive comparison with multiple types of observations/measurements (from satellite-, aircraft-, balloon-platform, and observation network) with regional resolution and global coverage. This has been a key driving force for the development of new research infrastructures. Real time in situ network monitoring of changes in the composition of the atmosphere over extended periods of time requires the implementation of a robust, sensitive, selective, accurate and precise measurement infrastructure for monitoring trace concentration of atmospheric gases.

The need to have low cost, sensitive and selective sensors is therefore increasing to provide in-situ accurate and reproducible measurements. Here, we lead to the development of sensors dedicated to air monitoring.

The targeted species are methane, CO, CO2 (biomass regulation, heat capacitor estimation, municipal dumps, lagoons…) for industrial or environmental applications, but also ethylene for more specific biological and agri-food applications. Our sensors have to be compact, lightweight and versatile thus favoring in situ measurements.

We have investigated and developed different approaches based on the use of antimonide tunable semiconductor lasers: wavelength modulation spectroscopy and quartz enhanced photoacoustic spectroscopy (QEPAS). Tunable lasers are distributed feedback lasers emitting in the 2 to 3.5 µm range, based on InGaAsSb/Al(In)GaAsSb quantum wells.

The study and development of such setups is strongly connected to the use of high quality single frequency lasers. As a consequence, the study and development of such devices is part of the setup realization. These spectroscopic techniques are extremely selective and highly sensitive.

Our QEPAS setups allowed the detection of small amounts of methane and ethylene in the frame of several collaborations, using DFB lasers. A detection limit of 100 ppbv was obtained on methane using a laboratory QEPAS setup. Then, we have worked on more compact setups. With the compact demonstrator, we are able to detect a few ppmv of methane in a gas mixture. The detectivity is currently limited by the laser emitted power.

Our future work will deal with the miniaturisation of the concept, by replacing the quartz tuning fork by a special micro-resonator, designed specifically for the gas analysis application.

FIG 2. Compact QEPAS setup. Methane detection. Dimensions of the C-QEPAS (V1): 15x5 cm.