Ammonia gas is invisible, but it can certainly be smelt. Not a pleasant experience, however – the biting smell of ammonia brings tears to your eyes. But lasers that emit infrared radiation can render this basic gas of the chemical industry visible to measurement devices. They are therefore used for online process control, monitoring the concentration of ammonia and other gases. In conventional lasers, lead salt generates the laser beam. The disadvantage of this technology is the need for extensive and expensive cooling with liquid nitrogen. As a result, these systems are quite large in size. With quantum cascade lasers, measurement systems can be designed to be cheaper and more compact since they only need a tiny Peltier cooling element.
“Fraunhoffer quantum cascade laser spectrometers can also measure a range of other gases that absorb infrared light,” notes Thomas Beyer of the Fraunhofer Institute for Physical Measurement Techniques IPM. “These include oxides of nitrogen, carbon monoxide and hydrocarbons. Thanks to the high radiation power of these lasers, gases can be measured over long distances and without direct contact.”
The core of the laser comprises hundreds of layers of different semiconductor materials. Their composition, thickness, and sequence determine the specific wavelengths at which the laser is set. The infrared beam is generated by electrons, which penetrate the layered structure in waterfall-like cascades. Scientists at the Fraunhofer Institute for Applied Solid State Physics IAF and the Institute for Technical Physics at Wurzburg University develop and manufacture these components. Funded by the German Federal Ministry of Education and Research BMBF, this project also involves a work group at the Technical University of Darmstadt, which investigates the characteristic features of the lasers. The IPM manufactures optical modules which are specially adapted to the lasers. Together with the corresponding detectors they are integrated into the new measuring systems.
“"We can expect the quantum cascade lasers to become indispensable for industrial measurement applications,” Thomas Beyer, head of the project, optimistically concludes. For more details, he can be contacted at +49 7 61/88 57-3 50, email: [email protected].