Spectral Technologies and Systems

Electrically actuated micro-optical choppers are perfectly suited for ultra-fast, digital modulation of broadband, thermal infrared sources. Even highly optimized micro-thermal infrared emitters, designed for application in miniaturized absorption spectrometers are limited by their thermal heat capacity to modulation frequencies of several 10 Hertz. Higher actuation frequencies lead to a strong decrease in modulation contrast and limit the usability in spectroscopic applications. Fraunhofer ENAS developed a micro-optical shutter that realizes modulation frequencies of more than 1000 Hz, giving highly miniaturized broadband absorption spectrometers a new application perspective. Technically a subwavelength-structured, ultra-durable membrane, that is made of a stack of high tensile stress LP-CVD silicon nitride and aluminum, is electrically switched between a high reflective and a high transmissible operation state. The micro-optical shutter is working in the thermal infrared between 8 µm and 11 µm wavelength. It was successfully tested with more than 1.5 billion switching cycles without failure.