A millimetre-wave dispersive fourier transform spectrometer

We describe a novel dispersive Fourier transform spectrometer developed for the precise determination of the optical constants of solids (metals, dielectrics and semiconductors), liquids and gases in the range 30 to 300 GHz, extendible to 1000 GHz. The target accuracy is ±5% in refractive and absorptive indices, at a resolution of 0.05 cm^–1. The methods of Gauss-Laguerre modal analysis for diffraction-limited beams have been used to design a compact polarising two-beam instrument of the Martin-Puplett type with adequate long-wavelength throughput while maintaining measurement precision at the shortest wavelengths. The complete optical system, with solid, liquid and gas cells, is contained on a table only 1300 mm x 650 mm and 300 mm deep. The optical apertures and sample sizes are 120 mm diameter.The major optical components are located by dowels on a jig-bored base-plate, thereby simplifying alignment and maintaining the required dimensional stability of the instrument. The broadband source is a mercury arc lamp, overlaid with solid-state noise diodes to reinforce power at the longer wavelengths, and the detector is a helium-cooled broadband InSb hot-electron bolometer. The polarising beam-splitter is a wire-wound grid of 10m tungsten wires spaced at 25m, which can be rotated to allow samples to be analysed in orthogonal polarisations. Linear phase modulation is introduced by a roof-mirror mounted on a 1-m precision lead-screw driven by a stepper-motor at up to 40 mm s^–1, which raises the sampling rate above the noise knee of the detector.