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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:3805 |
| Date | 11 February 2014 |
| Creators | Burger, Johan Petrus |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Johannesburg |
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