Optical filters - Institut Fresnel, Marseille

Tracking a mystery

It is still not clear what influence storms in the upper atmosphere have on our planet. In order to gather new insights, the French national space research center CNES is sending a satellite into space at the end of 2019. This satellite is equipped with an innovative optical filter that was developed by Institut Fresnel in Marseille with the help of Bühler technology.

About 20 years ago, researchers discovered the existence of gigantic light events in the upper atmosphere. Until now, there has been no clear knowledge of what influence this light show and its energetic discharges have on Earth, and what relationship they have with terrestrial gamma ray flashes (TGFs) that occur in upper thunderclouds. Currently, the French national space research center CNES (Centre National d’Études Spatiales) is working on the first satellites for researching TLEs and TGFs. In 2019, the mission Taranis is expected to start. The satellite will fly over thousands of such light events and systematically record them. 

New insights thanks to innovative filter

In order to make the individual components of the light spectacle visible and to determine the material composition, the equipment used in the satellite includes measuring equipment as well as a telescope with a spectral filter. Very complex filters that make nitrogen visible, for example, are used for the Taranis project. The Marseille-based Institut Fresnel has developed and produced this filter.

Thanks to the excellent technology from Bühler, it was possible for us to manufacture such innovative filters for space research.

Julien Lumeau, Head of the Optical Thin Film Research Team at Institut Fresnel


layers required for a Taranis filter
It took Institut Fresnel more than 10 hours to fabricate the Taranis filter with about 150 layers.
Almost 200 people in 13 different teams are working in the field of optics and photonics on the development of new imaging systems and components at Institut Fresnel in Marseille.
Institut Fresnel as been working on the development of optical thin-film filters for over 35 years.

Upon entering the lab for the Institut Fresnel, everything is bright and spotlessly clean. “Cleanliness is one of our top priorities,” Lumeau explains. “If only one dust particle ends up in the filter, this could cover a light event and thereby distort thescientific work.” Access to the lab is only permitted with a full body safety suit. 

A total of five optical coating devices are in the lab. The three more recent are from Bühler. The oldest equipment is 35 years old. In 2012, a Helios coating system from Bühler was added. In 2015, a Syrus Pro 710 was added. Followed by a second Syrus Pro 710 in 2018. “The high performance and quality of the machines and the good relationship pushes us to invest in additional equipment from Bühler,” says Lumeau.

Every layer is unique

The Institut Fresnel has the privilege of being able to choose from various technologies to find those best suited to the respective project. In so doing, Lumeau works very closely with the experts from Bühler. “We propose new concepts, and then we develop a proof-of-concept with Bühler,” says Lumeau. The filter for the Taranis project was produced on the Bühler Helios coating machine.

The machine works under vacuum. The coating material is sputtered when bombarded with ions, and the knocked-out atoms condense on the glass substrate. Layer by layer, the different materials are applied. About 150 layers are required for a Taranis filter. Compared with antireflection coatings that are applied on prescription lenses, for which filters are made of only a few layers, this is quite a lot. The total coating time is therefore significantly longer, too. While this takes a few minutes for lense coating, for the Taranis filter it took more than 10 hours to fabricate. “For the light to be filtered at the exact wavelength, it is essential that every layer be applied at exactly the right thickness,” explains Lumeau. “This means in the nano range. The individual layers are much thinner than 1 micrometer. They are typically less than one-thousandth of the diameter of a strand of hair.”

Merging research and industry

With such dimensions in the nano range, it is no surprise that measurability is an important issue. “The technology has developed rapidly in the last few years. Today, coatings that looked impossible 15 years ago are now commonly fabricated,” says Lumeau. “The challenge is to further improve the precision of the layer thickness and reach atomic precision for each layer.” For this reason, the Institut Fresnel and Bühler are planning to work together on improving optical measuring systems (OMS). The goal is to create new methods for OMS that automatically perform the evaluations and are therefore much more precise than today’s systems.

The new OMS technique is not the only project that could be of interest to the optical industry. Institut Fresnel and Bühler are also working on developing a variable filter. This should allow several color values to be visible at the same time. “People no longer want to be able to capture only one spectral value with one filter, but instead several at the same time,” says Lumeau.

To make this possible, it is necessary for the individual layers of a filter to be of various thicknesses in different spots. “Bühler offers us the best technology on the market for manufacturing homogeneous complex filters,” Lumeau says. “Using this as the basis, we are developing a prototype process for variable filters.” Institut Fresnel, in turn, is making the results of its research available to Bühler. “We are developing a standard process from this and will offer it to our customers,” says Yvonne Bonnin-Degner, Area Sales and Service Manager for Bühler. “This is the perfect merging of research and industry,” adds Lumeau.

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