Phase-coherent synthesis and precision frequency measurements in the far infrared

A short review of the basic concepts and problems in frequency multiplication into the infrared region of the electromagnetic spectrum is given. Results obtained with a multiplier chain starting from 5 MHz are reported. The carrier collapse is found at a frequency of about 15 THz. The present state of technological development is illustrated and the inherent limitations of this technique are indicated. The need for more spectral pure sources than those now available and/or different approaches is underlined in connection with the increasing thrust towards the measurement of visible frequencies.