| A directly bonded fused-silica grism (prism + grating) has its grating at the inner (joined) surface. (Photo copyright Fraunhofer IAO) |
Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF (Jena, Germany) have developed an optics bonding technique that defies the inhospitable conditions in space and does not affect measurements of the satellite spectrometer it is being used for. The researchers are working together with the German Aerospace Center (DLR) and the European Space Agency (ESA); the bonding technique was used to assemble a grism (grating plus prism) for use in spectroscopic detection of methane and nitrous oxide from space. (Conventional optical adhesive was not an option, as it absorbs light, thereby distorting the measurement result, is sensitive to radiation, and ages too fast.)
"We combine the optical elements with each other at the atomic scale, namely via oxygen bridges," says Gerhard Kalkowski, a scientist at the IOF. "In this way, we will provide the key for high-resolution systems made of prism-grating structures to also be able to be used in space in the future."
The technology, which is already used for silicon wafers, is called hydrophilic bonding. In this process, oxygen and hydrogen atoms are bonded to the wafer's surface. By pressing the surfaces together at elevated temperatures in a vacuum, rigid (covalent) oxygen bonds form between the two parts. The researchers have now successfully transferred this technology to transparent silica glass. The advantages: the oxygen bridges firmly connect the grating and prisms together, and the radiation in space cannot damage them. In addition, there is no intermediate layer, as in the case of adhesive, which would distort the measurements of the spectrometer.
Among other things, the challenge was to accurately position the grating and prisms in relation to each other. The scientists thereby produce a mechanical edge on the grating which precisely corresponds to the grating history. "The orientations deviate by only about an arc minute, as required," Kalkowski says.
Initial prototypes successfully passed tests done by the ESA. The researchers presented their technology at the Berlin Air Show ILA (June 1 to 4, 2016; Berlin, Germany). In a further step, the scientists are working on complex prism-grating structures.
Source: https://www.fraunhofer.de/en/press/research-news/2016/May/measure-greenhouse-gases-from-space.html