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

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 ∼5x10-5 to ∼0.025 volume fraction in air