Fig 1: Strong an well separated infra-red absortion bands of gas species at the operating wavelengths of IR microcavities make them suitable for gas detection in automotive application.
Optical gas monitoring requires light sources emitting at wavelengths above 3 µm since, in the applications, most of the gas of interest (CO, CO2, NOx, HC1, SO2, N2O, NH3, CH4....) have their fundamental absorption lines between 3 and 6 µm (Fig. 1). We report here on resonant microcavity light sources emitting at room temperature between 2 µm and 6 µm. The emitter combines a CdxHg1-xTe light emitting heterostructure, grown by molecular beam epitaxy, and two evaporated YF3/ZnS multilayered Bragg mirrors. After the epitaxial growth of the active layer, the top mirror is evaporated. Then the substrate is thinned and chemically etched to allow the deposition of the second mirror. The obtained emitter can be optically pumped by a commercial low-power and low-cost III-V laser diode (0.8-0.9 µm), without the need of temperature regulation. As compared to an unprocessed sample (without microcavity), this device shows a drastic line narrowing (10 fold), a 3.3 fold intensity increase at 3.327 µm, a 2.4 fold angular spread decrease, and a 100 times better temperature stability (0.02 nm/°C). This technique, using a resonant microcavity emitter with evaporated mirrors, allows one to tailor the emission wavelength and the line width, and thereby to fit the absorption band of a specific gas. The absorption coefficient in these mid-IR bands is so high that band integrated gas detection can be considered with optical power in the µW range.
An experimental demonstration of gas measurement has been made with CH4 by using IR microcavities emitting at 3.3 µm (Fig. 2). A two-wavelength differential set-up is used: one wavelength is tuned on the gas absorption band and the other is chosen outside this band. With a 10 cm long optical path in the gas, we can measure the gas concentration in a range extending from 100% down to a few tens of ppm. Since this device can be produced at a very low cost and downsized to less than 1 cm3, automotive applications are currently under evaluation through a GROWTH project of the European Commission with Renault, EADS and Delphi.
 E.Hadji, E.Picard, Les Techniques de l’Ingénieur RE-7, 2002
Maj : 27/09/2016 (68)