Abstract:
This thesis describes the process and technique used to fabricate reproducible optical
fibre long period gratings (LPG) of various types. It explores how they can be
exploited for use as highly selective and sensitive sensors. A versatile method for
fabricating LPG sensors has been demonstrated. The single system has the capability
of fabricating LPGs of different configurations, such as uniform period, those
operating at the phase matching turning point (PMTP), as well as phase shifted
and chirped. LPGs were characterised for their sensitivities to temperature, axial
strain and surrounding refractive index. The gratings at the PMTP were found to
show higher sensitivities to external influences.
Novel sensing configurations that exploit the properties of LPGs were also constructed.
An LPG coated with a molecularly imprinted ceramic coating was demonstrated
to offer a selective method for porphyrin detection. A composite nanoscale
of a titanium oxide (TiO2) matrix and 5, 10, 15, 20 Tetrakis-(N-methyl-piridinium4-
yl)-21H, 23H-porphine tertakis (p-toluenesulfonate) [TMPyP] porphyrin template
film was deposited on the LPG via liquid phase deposition. Attempts to rebind porphyrins
to the matrix were carried out. The LPGs transmission spectrum exhibited
a higher sensitivity to the target TMPyP template than it did to other, structurally
similar porphyrins, showing high selectivity.
A continuously chirped long period grating (CCLPG) sensor for monitoring directional
flow and cure of an epoxy resin is also presented. The asymmetric properties
of the CCLPG were exploited to facilitate the measurement of the direction of the
flow. The CCLPG was also used to monitor changes in the refractive index of the
resin during its cure, showing close agreement with a fibre optic Fresnel refractometer.
