A FAR-INFRARED HETERODYNE SIDE-BAND SPECTROMETER

We present recent experimental developments in far-infrared coherent spectroscopy. Most known far-infrared sidebands spectrometers use the direct detection scheme. The system developed at Lille operates in the heterodyne mode. This technique offers particular advantages when large detection instantaneous bandwidths are needed, as could be the case in time resolved spectroscopy. As a counterpart of a heavier electronic receiver system, the filtering constraints in the optical path are totally eliminated, leading to a potential lowering of the overall noise system. The receiver operates very close to physical limits. A carefully designed optical line, incorporating only reflective components, allows to keep optical losses at very low levels, on the whole operating range. By using particular laser injection coupling and power extraction schemes, reasonable short and long term stability has been achieved in sidebands power generation. Some spectroscopic results illustrating sensitivity and resolution capabilities are presented.