When Dutch researcher Paul Crutzen won the 1995 Nobel Prize in Chemistry for discovering the ozone decay threat, the Netherlands government established a firm policy to study atmospheric gases.
Climate change is similarly motivating, given the country is two thirds below sea level, explains Harry Förster, national projects coordinator at the Netherlands Space Office (NSO) in The Hague. ‘Our government has stated, “We will do everything to keep our feet dry”,’ he says. The country’s policy makers therefore want the best information from scientists, who in turn want good atmospheric data from satellite instruments.
That led to the NSO’s involvement in the Global Ozone Monitoring Experiment (GOME) instrument, on the second European Remote Sensing (ERS-2) satellite launched in 1995. It scanned strips across the planet recording absorption spectra in reflected and scattered sunlight over wavelengths from 240 to 790nm, identifying atmospheric gases, resolving 90 to 120km by 60 to 130km areas. In decades since, NSO has invested in several instruments improving these capabilities. The latest is the TROPOspheric Monitoring Instrument (Tropomi), which launched on the European Space Agency (ESA) Sentinel-5 Precursor satellite in October 2017.
Tropomi’s gratings are so important, Förster explains, that getting them right took a two-year development project before work on the rest of the instrument could start. Typically, incoming light immediately hits the surface of a conventional grating and reflects at several angles, dispersing towards detectors. By contrast, the immersed gratings that Tropomi uses are designed so that light first travels through the grating’s bulk material, before hitting the grating at its rear surfaces. The light then travels back through the bulk material, dispersing all the while. Then, when the dispersed light leaves the bulk material, it refracts again. This gives much better dispersion, Förster says, which helps lower the instrument’s volume. To produce the specific geometry involved using highly advanced silicon manufacturing, he adds, in collaboration with leading lithography tool supplier ASML, in Veldhoven, Netherlands.
Diffraction gratings are a core technology for Massachusetts-based imaging spectrometer designer and manufacturer Headwall Photonics. David Bannon, the company’s chief executive officer, explains that Headwall is able to make aberration-corrected image sensors using convex diffraction gratings built with precise groove profiles and depths. ‘In any airborne or space-borne application aberration-corrected instruments eliminate the inherent keystone and smile aberrations typically seen in most imaging spectrometers,’ Bannon says.