Researchers develop high-precision double slit for space spectrometer

ESA plans to launch its FLEX mission in 2025. Its purpose is to collect data on Earth's vegetation from space. For the spectrometer on board the satellite, researchers from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena have developed and manufactured a double-slit assembly with exceptional precision and two high-precision mirrors. A double split is provided SPIE Photonics West January 30 to February 1 in San Francisco.

How much light do plants emit and what can this light emission tell us about plant health? The ESA mission „Fluorescence Explorer Mission” (FLEX) will address this question starting in 2025. At the center of the satellite will be the „Fluorescence Imaging Spectrometer” (FLORIS for short).

However, unlike many spectrometers, FLORIS does not work with one light channel, but with two. „This requires two optical slits through which light can enter the instrument,” explains Falk Kemper. Kemper Fraunhofer is a researcher at the IOF and leads the FLEX program at the institute. „To create these two channels, you need a high-precision arrangement of two slits, called a double slit. This is what we developed and produced in Jena, high-precision mirrors for the spectrometer.”

Components for space spectrometers of exceptional precision

A special feature of the double-slit assembly is its exceptional precision: each slit of the double-slit assembly must be manufactured with a precise width of 85 (+/-1) micrometers, and is 44.15 millimeters long. „Slits that are too wide or too narrow would have directed too much or too little light into the detector, making it impossible to assess the light component of interest for the task,” Kemper explains.

Special requirements apply to the mirrors produced in Jena, which are intended to guide light onto the detector inside the spectrometer: they must have an rms (root mean square) hardness of 0.3 nanometers. „It roughly corresponds to the distance between one and two atoms,” Kemper explains. „The demands were incredibly high and at the limits of what was technically possible.”

A special lithographic structure for silicon-based double cleavage

Made on the basis of double-slit silicon. A special lithographic process chain was developed for production at the Fraunhofer IOF, in which lithographic structuring techniques were followed to etch the silicon wafers: „Our strategy was to cover the silicon wafers, form them and wet them in a time-controlled manner,” he says. The project manager explains the process. „Due to the very high precision required, many parameter iterations had to be done to get the production chain more stable and able to produce splits according to requirements.”

Finally, the slits were given a black coating to achieve specific optical reflectivity and optical density. „The black coating of the slits leads to a change in the geometry of the slits and must be retained accordingly when etching the slits.”

Robust mounting for use in space spectrometers

The device is designed so that the silicon is mounted on a double-split mechanical holder. There, the two brittle slits must be fitted very parallel to the holes in the holder, i.e., with an accuracy of less than 5 micrometers and a planarity of less than 10 micrometers. In addition, the two splices must be installed in such a way that they will not deform or break even when the flex satellite is subjected to strong vibrations, temperature fluctuations and accelerations during launch.

In order to ensure the necessary robustness of the assembly, researchers at the Fraunhofer IOF developed a special assembly concept, i.e. a strategy for combining different components together to form an assembly. „This was achieved using a combination of positive locking, clamping and bonding,” reports Kemper.

The dual-slit spectrometer allows for better and wider light analysis

The special design of the double-slit FLORIS spectrometer provides a decisive advantage for the purposes of the FLEX mission: „The double-slit allows two channels to be operated in the spectrometer: one for high resolution and one for low resolution,” says Falk Kemper.

A high-resolution channel can detect small differences in the wavelengths of light, while a low-resolution channel can capture wider parts of the light spectrum. The combination of the two channels enables a detailed analysis of light and therefore a detailed analysis of plant vegetation.

FLEX mission to monitor vegetation data on Earth

The Fluorescence Explorer mission, scheduled to launch in 2025, will provide global maps of plant fluorescence, i.e. light emission by plants. This data aims to provide information on photosynthetic activity and plant health and stress.

Such data are important not only for a better understanding of the global carbon cycle, but also for agriculture and future food security in the context of a growing world population. So far, it has not been possible to measure the photosynthetic activity of plants from space.