Fibre evanescent field absorption (FEFA): an optical fibre technique for measuring light absorption in turbid water samples

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doi:10.1071/MF03133

Fibre evanescent field absorption (FEFA): an optical fibre technique for measuring light absorption in turbid water samples

D. W. Lamb ^B ^F, Y. Bunganaen ^A, J. Louis ^A, G. A. Woolsey ^C, R. Oliver ^D, G. White ^E

^A School of Biological, Biomedical and Molecular Sciences, University of New England, Armidale, NSW 2351, Australia.
^B Farrer Centre, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia.
^C Division of Energy and the Environment, Department of Electronic & Electrical Engineering, University of Strathclyde, Glasgow G1 1XW, United Kingdom.
^D Murray–Darling Freshwater Research Centre, PO Box 921, Albury, NSW 2640, Australia.
^E School of Science and Technology, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia.
^F Corresponding author. Email: dlamb@une.edu.au





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Marine and Freshwater Research 55(5) 533–543 http://dx.doi.org/10.1071/MF03133
Submitted: 4 September 2003 Accepted: 29 April 2004 Published online: 5 August 2004

Abstract

An optical fibre technique for measuring the absorption of water-borne pigment in the present of significant suspended sediment concentration has been evaluated. Based on the absorption of the evanescent field of light propagating down a single glass (silica) fibre, the fibre evanescent field absorption (FEFA) technique has been demonstrated to be approximately 10-fold less sensitive to absorbing species than traditional bulk absorption methods. However, unlike traditional optical absorption measurements, the FEFA technique is insensitive to scattering by the suspended particles for particle concentrations expected in typical inland waters. A simple calculation estimates that this insensitivity persists for sediment concentrations up to 2000-fold those expected in Australian inland rivers. In addition to experimental results, a discussion of the potential operational use of this technique in measuring optical absorption properties of water containing suspended sediment is presented.

Keywords: evanescent field, optical fibre sensing, water color.