Abstract
M.Eng. (Electrical and Electronic Engineering)
An in-fibre Bragg grating is a period perturbation of the refractive index of the core of an
optical fibre, where the length of the grating is much longer than the period of the
grating. These gratings reflect light in a narrow band around a certain optical frequency.
This narrow band is shifted in wavelength when the element is strained and is an
absolute measure of the strain state of the grating.
One way to interrogate this type of sensor is by using it as a mirror of an optical fibre
laser, therefby forcing the laser emission band to coincide with the resonant frequency
of the grating. In this thesis a theoretical model was developed to describe the
operation of optically pumped Er+-doped optical fibre lasers. An explicit expression for
the ouput power for a Fabry Perot fibre laser, was derived for the first time. The intracavity
power propagation was also solved numerically.
An optical fibre laser consisting out of an Er+-doped optical fibre optically pumped at
980nm, was built. A multilayer dielectric stack mirror was deposited on the one fibre
endpoint of the laser. The other reflector is an in-fibre Bragg grating with a peak
reflectance of 94% at 1548.5nm and a bandwith of <O.3nm. The buildup of the output
power of the laser during the deposition, showed good correlation with the predicted
values. The length of the active fibre was also varied, and again the theory could
correctly predict the output power as well as the laser pump threshold.
A scanning type of Sagnac interferometer was investigated as a possible wavelength
demodulation system for the fibre laser sensor. With some alterations this system shows
merit as an inexpensive and effective way to monitor the wavelength shifts in in-fibre
Bragg gratings.