Modes coherence and photons in THz optical systems

The optical fields associated with THz measurement systems tend to be partially coherent, and as a consequence the analysis of THz systems is complicated. For example, in the case of a passive thermal imaging array it is necessary propagate the spatio-statistical properties of the field from a source through an optical system, and then to calculate how the radiation couples power into an imaging array, the individual elements which are also partially coherent. In this overview, the author briefly describes how calculations of this kind can be carried out entirely in terms of modes. Modal analysis is not only conceptually powerful, it is numerically efficient, and can be applied to a wealth of different kinds of systems and problems, including interferometers, pulsed spectrometers, and phased arrays. The scheme can be quantized easily such that the photon statistics, which at THz frequencies is often neither fully bunched nor fully Poisson, can be included for the purpose of determining sensitivities. The overall method is exceedingly powerful and has numerous implications for the way in which THz systems are modelled, experimentally characterised, and how images and spectra are recovered from data.