Remote spectroscopic monitoring of liquids via silica optical fibres

Abstract

This thesis describes advances in the modelling and characterisation of sampling techniques and extrinsic optical fibre sensors for remote or on-line emission spectroscopy of liquids, using the inelastic scattering processes of Raman and fluorescence spectroscopy.

A semiconductor laser has been used as an excitation source, coupled via an optical fibre to a measurement cell; the collected light has been analysed with a spectrograph based around a holographically-produced concave diffraction grating and a cooled CCD detector. Its design, construction, and performance are described. This equipment has been interfaced to a PC, which has been programmed to direct excitation light alternately between two measurement cells, and process the signals from them, to achieve optimum rejection of interference from the measured data.

A mathematical model of the scattered-light collection efficiency of a number of optical-fibre probes, including parallel-optical-fibre and single-fibre types has been derived, and shown to be valid within experimental error. The model variables include the optical fibre numerical aperture and core radius (both of which may be different for each fibre in the probe), axial separation of the probe fibres, and analyte refractive index and absorption. Several representative optical-fibre probe configurations are compared in the light of this new model.

The interface between optical-fibre probes and various novel light-guiding sampling arrangements has been investigated. Mathematical models of coupling between fibre probe and sample cell, and the signal enhancements achieved by using various cell configurations, have been derived. These have been compared with experimental results from Fresnel-reflection light-guiding cells, metallic-reflection light-guiding cells, and waveguiding cells based on low-refractive index polymer films and free-falling streams of analyte. The model includes sample absorption, and accounts for the reflectivity and surface quality of the cell walls, and it has been shown to be more complete and accurate than any previously published work on the topic. In particular, no previous references had been found on the use of the low-refractive index polymer Teflon-AF as a waveguide cladding, or the use of free-falling water streams for the enhancement of collected light in spectroscopic work.

The cell configurations will enhance the scattered light collection efficiency of any parallel-optical-fibre probe: by coupling a 100 mm long free-falling stream of an aqueous solution of fluorescent dye with a parallel-optical-fibre probe, the measured light collection efficiency of the fibre probe increased by 9 times in this work.

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