Remote determination of oxygen and water at millimetre wave frequencies using a fibre optic communications network

Abstract

This paper describes the development of a remote millimetre wave (MMW) spectrometer capable of operation in the 57–66, 114–128 and 171–189 GHz bands. A 9.5–10.5 GHz signal from a yttrium iron garnet (YIG) source is carried via an infrared (IR) laser down a 1 km fibre-optic cable using a high-speed communications modulator. The 6th harmonic of the transmitted microwave signal is generated directly with an active sextrupler, which permits working in the 57–66 GHz band. For operation at 114–128 and 171–189 GHz, the 57–66 GHz output from the sextrupler is doubled or tripled by a further harmonic generator. Absorption line strength measurements and hence sample concentration determinations are undertaken using a Fabry–Perot cavity absorption cell. The spectroscopic data are recovered from the remote spectrometer by transmitting the rectified signal back over a further fibre-optic cable. Also described are the methods of cavity stabilisation and control across this fibre optic network. Oxygen determinations in the 57–66 and 114–128 GHz bands are performed to evaluate the performance of the spectrometer. A determination of water vapour in air at atmospheric pressure, at 183 GHz, is also presented, over a range of ∼5×10 −5 to ∼0.025 volume fraction in air.