The Fabry-Perot Interferometer at Millimetre Wavelengths

Abstract

The design and operation of an interferometer of the Fabry-Perot type at wavelengths around 8 mm is discussed. The reflectors have been made using multiple quarter wavelength sheets of dielectric, and extremely sharp fringes have been obtained giving selectivity or Q factors exceeding 60 000. The effects of diffraction on the interferometer are discussed and wavelength measurements made at various positions. The frequency of the millimetre wave source has been stabilized to a high degree and the wavelength measurements at wider reflector spacings shown to agree with a substandard cavity wavemeter within the accuracy attempted of a few parts in 10⁵. Errors in the measured wavelengths at other positions of the interferometer do not exceed a few parts in 10⁴, the measured values being greater than the true value. Measurements of dielectric constants and dielectric loss have been made using the interferometer, and results are in agreement with values obtained by other methods. Since the interferometer is the free space analogue of the cavity wavemeter it is particularly suitable for measurements on materials of low loss tangent at millimetre wavelengths. Finally, the use of the interferometer for length measurements is mentioned, and also its possible use for the accurate determination of the velocity of electromagnetic waves.