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ESA Mis­sion FORUM: “A ther­mo­me­ter tur­ned satel­lite with extreme precision”

Fraun­ho­fer IOF rese­ar­chers deve­lop inno­va­tive dia­mond struc­tures for cli­mate change research

Mea­su­ring how Earth is losing its cool – that is the under­ta­king of the FORUM mis­sion. The Euro­pean Space Agency’s (ESA) satel­lite mis­sion, sche­du­led for 2027, aims to ana­lyze Earth’s heat bud­get in order to bet­ter under­stand glo­bal warm­ing and Earth’s cli­mate sys­tem. For the spec­tro­me­ter on board the satel­lite, rese­ar­chers from Jena have deve­lo­ped a novel dia­mond struc­ture that enables pre­cise mea­su­re­ments on the far-infrared spec­trum. The first flight hard­ware has now been delivered.

Earth is start­ing to sweat – lite­rally. The years 2018 and 2022 were among the war­mest since record­kee­ping began. An important piece of the puz­zle in the study of glo­bal warm­ing is the radia­tion of heat from Earth to space. “Cli­mate change is cau­sed by an imba­lance in Earth’s radia­tion bud­get,” explains Dr. Falk Eilen­ber­ger, a rese­ar­cher at Fraun­ho­fer IOF. “The pro­blem: Green­house gases reduce the amount of heat that is radia­ted from Earth to space. The result: It’s get­ting pro­gres­si­vely war­mer down here. Howe­ver, this pro­cess is extre­mely com­plex and is influen­ced by fac­tors such as the dis­tri­bu­tion of the gases, cloud for­ma­tion and curr­ents in the atmo­sphere,” the rese­ar­cher continues.

To bet­ter under­stand these com­plex mecha­nisms, the Euro­pean Space Agency (ESA) plans to launch the FORUM mis­sion in 2027. A satel­lite will be used to record Earth’s radia­tion bud­get with pre­cis­ion to a local level. “Figu­ra­tively, that means FORUM is a ther­mo­me­ter tur­ned satel­lite with extreme pre­cis­ion,” Eilen­ber­ger illus­tra­tes. A spec­tro­me­ter is employed on the satel­lite to func­tion as a ‘ther­mo­me­ter’. It records Earth’s heat radia­tion on the far-infrared spec­trum – that is, from about 10 to 100 micro­me­ters (µm). The key com­po­nent here is the spectrometer’s beam splitter.

Etched dia­mond struc­ture acts as a beam split­ter in the interferometer

This is where the Fraun­ho­fer Insti­tute for Applied Optics and Pre­cis­ion Engi­nee­ring IOF and the Insti­tute of Applied Phy­sics (IAP) at Fried­rich-Schil­ler-Uni­ver­si­tät Jena play a cru­cial role: For this very beam split­ter, rese­ar­chers from both insti­tu­tes have deve­lo­ped and manu­fac­tu­red an inno­va­tive dia­mond micros­truc­ture for the high-pre­cis­ion mea­su­re­ment of spec­tra in the far-infrared range. A spe­cial tech­no­logy is used for this, in which micro­sco­pic pyra­mids are etched into a diamond.

“For the extreme range to be recor­ded by FORUM, we need a beam split­ter that is trans­pa­rent over the entire spec­tral range,” explains Falk Eilen­ber­ger, who heads the Depart­ment of Micro- and Nanos­truc­tu­red Optics at Fraun­ho­fer IOF. He con­ti­nues: “There is no opti­cal mate­rial that has this pro­perty – except dia­mond.” Accor­din­gly, the rese­ar­chers used a dia­mond about the size of a cre­dit card (~43mm x 64 mm). Its shiny (and later addi­tio­nally coa­ted) sur­face acts as a beam split­ter. And here the next chall­enge awaits, because: “Only one sur­face of the dia­mond is allo­wed to shine,” Eilen­ber­ger points out. »So, our job was to make the second sur­face antireflective.”

Con­ven­tio­nal anti­re­flec­tion pro­ces­ses, such as those used for eye­glass len­ses, are unsui­ta­ble for this appli­ca­tion because they con­sist of lay­ers of dif­fe­rent mate­ri­als and are not trans­pa­rent over the entire spec­tral range. Because of this, the rese­ar­chers have deve­lo­ped a spe­cial etching pro­cess to place the neces­sary struc­tures into the dia­mond. To do this, they drew inspi­ra­tion from nature – spe­ci­fi­cally, from the eye of the moth.