Optical far-IR wave generation : an ESA review study

A study initiated by the European Space Agency was recently concluded, which aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime for space applications. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and, possibly, miniaturization. As the title of the paper suggests, the baseline technologies considered from the start in this study revolved around optical schemes, such as photornixing, compared to all electronic techniques. However, emphasis was put on conducting a perusal of any scheme found in the literature and performing an overall comparison with a fixed set of criteria. To this end, a number of universities and research institutes were involved to allow for a sufficient coverage of all relevant technical and research areas and, thus, ensure a review of both current and emerging technologies in this field that would be as thorough and complete as possible. The review study combined published results and mathematical modelling in order to determine the theoretical performance limitations of a given technology and/or scheme. The basic requirements to help crystallize selection criteria were, broadly speaking: power performance, linewidth, tunability and space worthiness in the frequency range —1 to 3 THz. Some of the most significant fmdings will be presented together with the two teclmologies selected for medium to short term development. These are a scheme using advanced p-i-n photomixer superlattice structures and, reflecting recent results, THz Quantum Cascade Laser technology.