A broadband, low noise heterodyne receiver for stratospheric measurements at 2.5 THz

The LHFT is currently working on a broadband heterodyne receiver for stratospheric measurements at 2.5 THz. Many molecules and radicals such as OH, H₂O and O₃ which are involved in the stratospheric ozone chemistry exhibit characteristic emissions in this spectral range. In order to measure these spectra and extract concentration data of the single components, a platform operating at high altitudes is necessary to overcome atmospheric absorption due to water vapor. Research at the LHFT includes the construction of a stable and compact local oscillator (LO) as well as the development of the receiver front end. In the LO subsystem an RF excited grating tunable sealed off CO₂ laser with an apodized out-coupling element will be used as a pump source. The compact and rugged design is realized by a slab discharge geometry and a metal tube housing. Long term stable gas composition for sealed off operation is optimized during the design phase of the laser by making use of tunable diode laser absorption spectroscopy (TDLAS). A methanol ring-laser with an output power of up to 49 mW at 2522.8 GHz (pump power 20 W) has been developed and is available as a very powerful FIR source. The optimization of the front end concentrates upon broadband and low noise performance. A mixer with newly developed substrate-less Schottky diodes followed by a broadband, noise matched IF-amplifier allow for low noise operation over a frequency range from 6 to about 13 GHz, thus covering emission lines of at least the three species mentioned above