Abstract:
The influence of nano-structured
overlays on the properties of fibre optic Long Period
Gratings (LPGs) has been investigated. The presence of overlays of thickness of order
100 m is shown to
significantly alter the response of the LPG to environmental
parameters, such as the surrounding refractive index. A LPG with a nano-structured
overlay was observed to exhibit enhanced sensitivity to refractive indices lower than
silica, and was observed to exhibit sensitivity to refractive indices greater than that of
the silica
cladding, not observed in the absence of the overlay. A theoretical model
predicts the observed behaviour. The observation was exploited to enhance the
sensitivity of a LPG based chemical concentration sensor.
The
response of the transmission spectrum of concatenated long period gratings
(LPGs) to ' the deposition of nano-structured overlays was also investigated.
Concatenated LPGs act as a Mach-Zehnder interferometer and the
phase of the
interference
fringes that appear within the LPGs° attenuation bands were shown to be
highly sensitive to the optical thickness of the overlay, for thicknesses of order 100
mn. Several devices were fabricated with
increasing path lengths to demonstrate
effect of overlay thickness and refractive index on the transmission spectrum of the
LPG.
