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Optical sensor cleans up car fumes

15 Dec 2004

Low-cost microcavity emitters have been combined with an infrared detector to create a real-time vehicle exhaust sensor.

A compact optical exhaust sensor could reduce fuel consumption and help improve air quality according to French scientists. Featuring a series of laser diode pumped microcavity emitters and a broadband infrared detector, their fast acting sensor can selectively monitor the concentration of gas species when the engine is running.

As Francois Levy from French research agency CEA-LETI explained at SPIE's recent Photonics in the Automobile event in Geneva, Switzerland, engine performance can be dramatically improved by controlling combustion parameters on a real-time basis.

Unfortunately, the chemical based emission sensors found on vehicles today are unsuitable for the task because they are too slow and provide poor selectivity between gas species. However, optical sensors could change all that.

"We have demonstrated already the ability to detect less than 50 ppms of methane," Levy told Optics.org. "And also the possibility to have sufficient response to make a measurement from cylinder to cylinder."

In partnership with automotive components supplier Delphi, car maker Renault and European technology developers EADS, CEA-LETI has come up with a fast acting, low-cost sensor that uses spectroscopy to monitor hazardous engine exhaust emissions.

The work forms part of the IMPECC (infrared microsystem for pollution emission control on cars) project supported by the European Commission. It is likely that from 2008 EU legislation will require all new vehicles to be fitted with a permanent emission control system.

The sensor determines gas concentration by passing infrared light, from a series of microcavity emitters, through a gas sampling chamber on to a detector. The final design will feature a microbolometer detector that can operate at room temperature. However, in their prototype version, the team use an MCT photoconductor detector cooled to -60 degC.

Each microcavity emitter consists of a heterostructure of cadmium, mercury and tellurium which emits light over the 3 to 5 µm range when pumped with a low power laser diode (830nm).

On its own, the emission spectrum of the photoluminescent heterostructure is too broad for use in gas analysis. However, by sandwiching the emitting structure between two dielectric mirrors the researchers have created an optical resonator that narrows the spectral emission.

Currently, the sensor operates at three wavelengths - 3.3 µm for hydrocarbon detection, 4.75 µm for carbon monoxide detection and 3.8 µm to provide a low absorption reference channel. The reference channel allows the team to make a differential measurement and correct for artefacts within the set-up such as detector drift.

Levy explains that although the IMPECC project is focused on the automotive sector, the sensors could be used elsewhere. "This kind of infrared source could also be applied to other fields, such as pollution or industrial control, security or even in the field of biomedical applications," he said.

The team is currently working on a smaller and more integrated design as part of the IMPECC 2 project which is due to deliver results at the end of 2005.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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