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Fast-tuning VCSEL detects methane

07 Feb 2003

The 1.68 micron laser can measure gas concentrations every 200 ns.

A research team based in Austria and Germany has used a vertical-cavity surface-emitting laser (VCSEL) to detect methane gas.

Max Lackner and colleagues at Vienna University of Technology and the Walter Schottky Institut in Munich say that using a VCSEL has the advantage of a wide tuning range that can be scanned rapidly. This means that the emission can quickly scanned cover the entire absorption feature, so that the absorption signal can be discerned from the background attenuation (Measurement Science and Technology 14 101).

The team used a 1.68 micron-emitting indium phosphide VCSEL fabricated by the Walter Schottky spin-out company VertiLas to probe the second harmonic of a methane absorption. This long output wavelength is possible thanks to a novel device structure that incorporates a buried tunnel junction (BTJ).

Because the VCSEL is tuned on and off the absorption signal at a repetition rate of 5 MHz, the technique ought to be useful for studying fast processes in high-pressure environments like flames. These environments tend to broaden the absorption lines of a gas and this makes edge-emitting diodes unsuitable.

"VCSELs have got two outstanding properties that can be exploited for spectroscopy. They have a wide singlemode continuous tuning range, and they can also be tuned much faster than edge-emitting diode lasers," Lackner told Optics.org.

Until now, VCSELs have been considered as inferior sources for spectroscopy and have not been studied extensively in molecular studies. However, Lackner and colleagues now claim that their VCSELs are superior devices for applications like flame investigations, combustion dynamics and chemical kinetics, where species concentrations change quickly and absorption features are broad.

"Fast measurements are important in transient environments," said Lackner. In his setup, the 5 MHz tuning rate corresponds to being able to obtain a species concentration or temperature measurement every 200 ns.

The small cavity of a VCSEL means that it can be tuned quickly by altering the injection current. While edge-emitting diodes are also fast-tuned in this way, it tends to limit the frequency-shift possible and wide wavelength scans are normally performed by temperature tuning. External-cavity diode-laser systems permit an extended tuning range but require moving parts and are therefore not suited to fast repetition rates.

Lackner and colleagues have also used VCSELs tailored to other wavelengths to measure oxygen (760 nm), ammonia (1.54 micron), hydrogen chloride (1.8 micron) and water (1.8 micron).

Lackner says that their potentially low cost and power consumption could see VCSELs find applications in future spectrometers for specific target molecules.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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