Antireflection coated, wedged, single-crystal silicon aircraft window for the far-infrared

Operating far-infrared remote-sensing instruments from inside a pressurized cabin of an aircraft requires a window with high transmittance. Furthermore, the radiometric properties of the window, such as the transmittance and the emitted radiation (i.e., temperature distribution), have to be known. The design of a wedged, antireflection coated single-crystal silicon aircraft window, its modeled transmittance spectrum, and the applied coating technique are presented. Measurements of the window transmittance with the 2.5 THz heterodyne spectrometer TeraHertz OH-Measurement Airborne Sounder (THOMAS) and a Fourier-transform spectrometer are presented, showing a transmittance of about 90% around 84 cm^-1. The window was designed and built for the 2.5 THz OH-Sensor THOMAS, operated on the DLR research aircraft FALCON. The transmittance of 90% means a substantial improvement compared to the window used previously. With this new window, systematic errors in the measured atmospheric radiance could be lowered, making the retrieval of atmospheric parameters easier. Several successful flights with the new window up to an altitude of 43000 ft have already been performed