D.Ing. (Electrical and Electronic Engineering) / The subject of this research work was the reflective fibre optic ring interferometer utilizing allow coherence length source. The interferometer consists of an interrogating fibre connected to a fibre ring via an input/output splitting coupler, and a reflector. The fibre ring is formed between one input and one output leg of a fibre coupler. The reflector can be of any type, including a fibre loop mirror, Faraday rotator mirror, a silvered fibre end-face or multilayer dielectric mirror deposited on the fibre end face. A theoretical treatment was developed for the interferometer. It was shown that the interferometer had a reflectance vs nonreciprocal phase shift characteristic, similar to that of a Fabry-Perot interferometer. Upon phase modulation of the interferometer quadrature terms were obtained at harmonics of the modulation frequency. It was shown that by decreasing the finesse of the sensor a quadrature phase tracker could be employed. The dependence of the sensor's reflectance, finesse and sensitivity on various system parameters were simulated with the aid of the abovementioned theory. A high finesse interferometer was proposed. It utilizes a fibre doped amplifier which would increase the sensitivity above that of a Sagnac interferometer. Intermediate and low finesse versions of the reflective ring interferometer were applied as gyroscopes...