There are a number of different methods which can be used to determine the concentration of gases or gas components. One of these is "Tunable Diode Laser Absorption Spectroscopy" (TDLAS) a laser spectroscopy method designed for mid-infrared range. The principle: laser diodes generate a beam whose emission frequency can be altered or adapted to suit the gas of investigation. The advantages of TDLAS: a very high measuring speed coupled with a low detection limit. This method is used in a number of applications including environmental engineering to determine both the level of pollutant concentration and the composition of the earth´s atmosphere.
In Project "MARS – Molded Anti-Reflex Structures", a compact and robust spot-gas-sensor for detecting methane gas, far out-performing current state of the art technology in terms of sensitivity, reliability and cost efficiency, is going to be developed. The core of the project is the development of infrared lenses with integrated anti-reflex structure, manufactured by precision glass molding technology.
For the optics used in the gas sensors materials which are transparent at infrared wavelengths are required. However, a high refractive index is one of the characteristics shared by these materials (e.g. germanium, silicon, zinc selenide, chalcogenide glass). A high refractive index is associated with strong reflections, which result in transmission losses.
As a rule, lenses are coated with anti-reflective materials in order to reduce reflection and transmission losses. However, lithographic processes are associated with additional costs, extended production time and, frequently, with the use of environmentally unfriendly substances. A single-layer coating also reduces reflection for only one wavelength and is effective at only one angle of incidence. Since gas detectors are required to have a broadband, anti-reflective effect regardless of the angle concerned, complex multiple coatings would have to be applied.
The engineers at the Fraunhofer IPT are pursuing a different path: instead of using anti-reflective coating, microstructures like those found on the eyes of nocturnal moths are generated to increase the transmission of the chalcogenide glass lenses. The structures are generated via precision glass molding. The precision glass molding in conjunction with the wear resistant coatings developed at the Fraunhofer IPT is better suited than conventional production processes such as grinding and polishing, particularly for tasks such as the manufacture of optical components with complex geometries and boasts a high level of scalability. There is a wide range of chalcogenide glasses from which the most suitable for precision glass molding of these microstructured optics will be selected.
This research project is funded by the German Federal Ministry for Economic Affairs and Energy.