Heterodyne Detection of Sub-millimetre Radiation

Abstract

THE development of the CN maser¹ as a continuous wave source of coherent radiation at a wavelength of 0.337 mm has made possible the use of a heterodyne detection system for this wavelength. The advantages of heterodyne detection are well known². To use this technique a source of coherent radiation and a suitable detector are required. It is a general characteristic of infra-red and sub-millimetre wave detectors that their voltage output is proportional to the radiation power input so that they are equivalent to radio-frequency square law detectors. If a detector of this type is placed in a position at which two beams of radiation of similar wavelength interfere, the detector output will be proportional to (P₁ P₂)½ where P₁ and P₂ are the powers received from the first and second beams respectively. If P₁ is modulated in some way so that it contains components of frequency ν₁ differing slightly from the frequency ν₂ of P₂, the output from the detector will contain components alternating at the difference frequency (ν₁–ν₂). If the intensity of the unmodulated beam P₂ is greater than that of P₁, the output signal will be proportional to (P₁ P₂)½ and correspondingly greater than the signal which would be obtained if a weak beam P₁ only were received by the detector (this assumes, of course, that the relative phases of the two beams at the detector are adjusted for maximum intensity). Thus the difficulty of detecting a weak signal P₁ can be reduced by using a heterodyne technique. In some applications the difference frequency (ν₁–ν₂) may be relatively large, so that for this technique to be of practical value a detector with a wide frequency response is required. On the other hand, in some applications in spectroscopy or radiometry long integration times can be used to facilitate the detection of very weak signals. This is equivalent to a situation in which (ν₁–ν₂) is very small. In measurements of this type, therefore, the use of detectors with low frequency response can be satisfactory.