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Twenty-five years of development and sophistication have taken place in Fourier transform spectroscopy since Fellgett drew attention to the multiplex advantage in 1952. Three important applications are reviewed. First, high resolution broadband submillimeter-wave spectroscopy at high wavenumber and spectroradiometric accuracy is discussed, with particular reference to its application in measuring the composition of the high atmosphere. Second, a rapid-scan technique applicable to the measurement of the emission from pulsed tokamak plasmas for diagnostic purposes is described, and finally some results using the technique of dispersive Fourier spectroscopy applied to solids and liquids are given.

The infrared spectra of H_2O and D_2O in the liquid state at ambient temperature (30°C) have been remapped in the spectral region between 10 and 330 μ. The major features observed were extremely intense absorption bands with maxima at 685 and 505 cm^−1 in H_2O and D_2O, respectively. These major bands are overlapped at the low-frequency ends by much less intense bands producing transmittance minima near 193 and 187 cm^−1, respectively. No evidence was obtained for the series of narrow bands recently reported by Stanevich and Yaroslavskii. Extinction coefficients have been determined for the range 170–50 cm^−1 and are compared with recent data; present data on linear absorption coefficients for H_2O in the range 1500–1100 cm^−1 are in fair agreement with the results of previous workers. The influence of temperature variations on the frequencies of infrared bands has been studied for all bands in the region between 4000 and 32 cm^−1. Theoretical interpretation of the results is discussed briefly.

An attempt has been made to Q-switch the cw 337-μm maser, but no large increase in peak power was observed. The gain factors of 337-μm radiation for pulsed and cw discharges have been measured and found to be 70% per meter and 10% per meter, respectively. It has also been shown that the pulsed maser is saturated in normal operation. Multiple pulse emission has been observed with the output delayed by times of the order 750 μsec after the applied electrical pulse.

SUB-MILLIMETRE waves are attractive for metrology over long distances, because they are not much scattered by fog or cloud1and can be specularly reflected from ordinary machined surfaces. A device using interference over a long path difference would give a simple method of measuring small movements of a distant body and could be developed for the precise measurement of distances. Interference between beams which have traversed a large path difference requires that the radiation be confined to a narrow bandwidth, and to this end we were interested in finding the effective line width of the continuous-wave 337µ maser2. A preliminary experiment was carried out by beating two masers together as shown in Fig. 1, using a Putley indium-antimonide detector3as the nonlinear mixing element.