27 Aug 2008
Carbon monoxide can now be detected by vertical-cavity surface-emitting lasers, thanks to a design that pushes emission out to 2.3 microns.
An InP-based VCSEL engineered to emit at 2.3 µm is allowing researchers in Germany to measure carbon monoxide (CO) in the parts-per-million (ppm) range (Optics Letters 33 1566).
"This is the first time that the VCSEL has been tested in an application and we have shown its suitability for gas sensing," Andreas Hangauer of Siemens told optics.org. "The VCSEL was manufactured by VERTILAS and the Technical University of Munich. Here at Siemens, we are working on a CO sensor prototype while VERTILAS plans to commercialize the 2.3 µm VCSEL."
VCSELs emitting up to 2.04 µm are commercially available and are ideal for gas sensing because they can be electrically pumped, singlemode, continuous-wave and operated at room temperature. Many important gases such as ammonia, methane and carbon dioxide can be detected using near-infrared wavelengths.
Extending the emission up to 2.3 µm is crucial as it opens the door to detecting CO at ppm sensitivity. According to Hangauer, only a slight modification to the VCSEL's active region was necessary to reach 2.3 µm.
"A V-shaped quantum well design was employed with 1 nm of pure InAs in the centre of the quantum well," he explained. "Otherwise the device is a typical buried tunnel junction InP VCSEL."
To highlight the VCSEL's suitability for gas sensing, the team used wavelength modulation spectroscopy (WMS). The set-up involved placing the VCSEL and an InAs photodetector side-by-side and using a spherical mirror to direct the laser light into the detector. The optical path length was around 40 cm.
"The laser wavelength is modulated sinusoidally at frequencies in the kilohertz range," explained Hangauer. "An absorption line converts the wavelength modulation to an amplitude modulation, which is recorded by the photodetector. A narrowband detection is possible, which allows for very selective and sensitive detection of the absorption signal with a good noise suppression. The method is also insensitive to unwanted light (such as sunlight) on the photodetector. A lock-in amplifier was used to perform standard second-harmonic detection."
Temperature tuning was used to vary the laser's wavelength and the researchers report that singlemode operation was maintained throughout. "We did a wavelength scan of 3.7 nm corresponding to a temperature variation of 15–37°C," commented Hangauer. "This was not the whole tuning range – at least 6 nm is likely."
Hangauer adds that the sensor prototype has already achieved a resolution of 4 ppm (standard deviation) for CO with a 10 cm optical path length and one second time resolution. "If multipass cells are used, the sensitivity increases by at least an order of magnitude," he concluded.
Jacqueline Hewett is editor of Optics & Laser Europe magazine.
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