Study of optical output couplers for submillimeter wavelength backward-wave oscillators (BWO's)

The machining of slow wave structures for high frequency BWO's is extremely difficult beyond l THz. Recently a microfabrication technique using photolithography and ion-beam assisted etching has been used to construct a prototype BWO operating at 200 to 265 GHz.The output coupler for such tubes remains a problem. Waveguides do not exist or are very lossy at the frequencies of interest (300 to 2000 GHz).This paper discusses several scaled experiments of optical output couplers for submillimeter BWO's. Various designs of planar antennas (Vivaldi horns) and lens-feed systems (hyperhemispherical lens) were constructed and tested between 20 and 100 GHz using a spectrum analyzer. The lens system was also tested at 337 GHz using a CO₂ pumped FIR laser.