Abstract
D.Ing.
This work comprises a study of reflective interferometric fibre optic sensors. The
use of Bragg gratings, multilayer quarter-wave stacks, and rugate mirrors for fibre optic
sensing applications is discussed in this work. Rugate mirrors are presented in more detail
since they form an important part of the research. The control system for an electron
cyclotron resonance plasma enhanced chemical vapour deposition reactor was developed
for the growth of inhomogeneous dielectric filters. The system is intended to control the
growth of films of the required refractive index profile for optical applications on different
substrates including fibre ends. The system also includes an automatic microwave tuner.
Rugate mirrors deposited directly on optical fibre were used in a dual cavity Fabry-
Perot interferometric strain sensor. It was found by computer simulation that reflectances
of 40% for the two outer mirrors and 68% for the centre mirror allow the interferometer
to have maximum fringe amplitude. The sensor was evaluated experimentally as a strain
gauge. The maximum strain applied to the sample was about 0.12% and the corresponding
phase change was about 800 radians. The discrepancy between the readings of this sensor
and those of a resistive strain gauge, applied to the same structure, did not exceed 5%.
Two novel fibre optic sensors were developed during this work: a merged Sagnac-
Michelson interferometric sensor for distributed disturbance detection, and a disturbance
location sensor using modified Sagnac and Mach-Zehnder interferometers. Both are
intended for distributed impact location along the fibre. The magnitude of impact can also
be measured with these sensors.
The merged Sagnac-Michelson interferometric sensor uses two light sources and a
frequency selective mirror to separate the Michelson and Sagnac signals. Birefringence in
the fibre was used to bias the Sagnac interferometer to have a sine response. Computer
simulations yielded the optimal biasing conditions: the state of linear polarisation of the
input light should be rotated by r/4 with respect to the horizontal axis; birefringence in the
Sagnac loop must provide retardation of ff/2 with the optical axes rotated by K/4 with
respect to the horizontal axis. We verified the sensor concept experimentally. The
discrepancy between measured and actual locations of disturbances applied to the fibre did
not exceed 2.5 m for a 200 m long sensing loop.
The sensor using the modified Sagnac and Mach-Zehnder interferometers makes
use of phase modulation and synchronous detection to obtain the sine-biased Sagnac
signal. A modified Sagnac interferometer configuration that incorporates an additional
coupler and a mirror, allows separation of the Sagnac and Mach-Zehnder signals.
Operation of the new configuration was verified experimentally in the system with a
sensing fibre of 100 m long. The discrepancy between measured and actual locations of
disturbances applied to the fibre did not exceed 2 m.