Components for optical signal analysis and routing

Page-Jones, M.A., 1995, K2d..

Ph.D., London, King's College - 47-2271

Integrated optical sensing and communications is becoming ever more important, displacing conventional electronics in many applications. As a consequence there is a need to develop improved components, both for the construction and verification of optical systems. This thesis describes research into three such components.

An all fibre interferometer, capable of use as a Fourier Transform Spectrum Analyser, is described in three different versions. These represent a steady progression from a primarily bulk optical system, with fibre delay lines as the scanning elements, to a system which is fully optical fibre, except for the lens assembly used to launch the input light. In its final form this system could introduce an optical path difference of 3cm, and could sweep at up to 50 sweeps per second. During the course of this work a method for glue joining dissimilar optical fibres was developed.

The second line of research was into photoconductive three wave mixtures. This was originally done to assess the feasibility or otherwise of temperature measurement using the frequency shift and intensity of the Brillouin backscatter from an optical fibre using coherent detection. Two beams of light of different frequency impinge on a photo-resistive device, biased with an alternating microwave voltage at a frequency close to the optical difference. The resulting signal current contains a component at the difference of the optical beat and the microwave frequency. Because this work was performed with devices with non-ideal geometries and material properties, the sensitivity was not perhaps as high as had been hoped. However the system was capable of detecting stimulated Brillouin backscatter, and mixing it down to an intermediate frequency of a few hundred megahertz.

The third area to be covered was an all optical switch believed to be the first of its kind. The destination address, power supply and signal to be switched are all the same beam, impinging on a photorefractive crystal. Because the output is chosen by manipulation of the coherence of the input beam, such a switch could be programmed by a fibre interferometer similar to that described earlier in the thesis. However, for simplicity, and because the original crystal worked best in the green-blue region of the spectrum, the demonstration switch was actually programmed using a bulk optical interferometer.

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Please note that the information and ideas presented here are my own, and views expressed may not be those of the University, the Department, my current employers or indeed anyone remotely official

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