Abstract:
Fibre
optic chemical sensing has been demonstrated using side-polished single mode
optical bre, evanescently coupled to chemically sensitive Langmuir-Blodgett overlay
waveguides. The sensors exhibit a channel dropping response centred on a wavelength
dependent upon the thickness and refractive index of the overlay waveguide. It has been
shown that
chemically sensitive organic dyes prove to be suitable materials for forming
the
overlay waveguide whereas the Langmuir-Blodgett deposition technique provides the
required overlay thickness control.
A brief
optic pH sensor has been demonstrated by
depositing a merocyanine dye onto a
side-polished optical fibre which exhibits a change in transmission of 9.7 ± 0.8 dB pH" at
a
working wavelength of 750 n and 4.2 ± 0.5 dB pH`1 at 780 nm, with a shift in the
channel
dropping centre wavelength of 18.8 ± 0.8 n pH`1. The response time of the
sensor to a
step change in pH has been measured to be ~ 20 s.
The
advantage of being able to chemically engineer the organic materials used to form
the
overlay waveguides has been demonstrated by manufacturing a second pH sensor
designed to operate at longer wavelengths using a second, modified merocyanine dye.
The overall
wavelength response of this second sensor to pH has been shown to be non-
linear but a
sensitivity of 23.3 ± 0.3 n pH`l is observed over a linear region. Finally, i
order to
investigate the non-linear response, a computer model based on the Kramers-
Kronig relations has been devised and shows good agreement with experimental values.
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