Laser Heterodyning

The development of heterodyne detection from the radiowave to the optical region of the electromagnetic spectrum is briefly reviewed. Attention is focused on the submillimetre/far-infrared region. A simple phenomenological model for the transition from electric-field to photon-absorption detection is developed. The relative detected power at doubleand sum-frequencies is multiplied by an attenuating coefficient that depends on the incident photon energy and on the effective temperature of the system. Several recent developments in heterodyne detection are mentioned. 1. Brief history Heterodyne detection has a long and august history in the annals of electrical engineering, reaching back to the earliest years of the century. The term has its roots in the Greek words 'heteros' (other) and 'dynamis' (force). In 1902, Reginald Fessenden was awarded a United States patent [1] 'relating to certain improvements... in systems where the signal is transmitted by [radio]waves differing in period, and to the generation of beats by the waves and the employment of suitable receiving apparatus responsive only to the combined action of waves corresponding in period to those generated . . '. The subsequent realization that one of these waves could be locally generated (the development of the local oscillator or LO) provided a substantial improvement in system performance. Practical demonstrations of the usefulness of the technique were carried out between the 'Fessenden stations' of the U.S. Navy at Arlington (Virginia) and the Scout Cruiser Salem, between the Salem and the Birmingham (1910), and at the National Electric Signaling Company. In 1913, John Hogan provided an enjoyable account of the development, use, and performance of the Fessenden heterodyne signaling system in the first volume of the Proceedings of the Institute of Radio Engineers [2]. It was not long thereafter, in 1917, that Edwin H. Armstrong of the Department of Electrical Engineering at Columbia University carried out a thorough investigation of the heterodyne phenomenon occurring in the oscillating state of the 'regenerative electron relay' [3]. A major breakthrough in the field, the development of the superheterodyne receiver, was achieved by Armstrong in 1921 [4], and this famous invention is now used in systems as diverse as household radios and microwave Doppler radars. The prefix 'super' refers to 'the super-audible frequency that could be readily amplified'. In the succeeding years, the application of heterodyne and superheterodyne principles followed the incessant march toward higher frequencies that culminated in the remarkable developments in microwave electronics about the time of World