Fatah, Rebwar M. A.; (1991) Multiplexed microresonator sensors for optical fibre surveillance. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis we have developed a multiplexed sensor system suitable for optical fibre cable surveillance. To achieve an optimum multiplexed sensor system three technologies have been merged, silicon processing, lasers and optical fibres, and optical time domain reflectometry (OTDR). We have demonstrated that a silicon based sensor can be combined with a fibre optic cable without the involvement of any electronics for an all optical monitoring system. A silicon vibrating element produces an optical output that is a function of the measurand. This optical signal can then be converted to a digital bit stream and then processed using digital signal processing techniques. At resonance the device needs just enough optical power in order to over come its losses. The power required to sustain resonance can be less than 10μW. The resonator dimensions are comparable with that of the fibre optic cross section which makes it easy to fit the device into the end of the optical fibre. Silicon is a mature technology resulting in a low unit cost. Silicon as a material has suitable mechanical properties, including low hysteresis and reproducibility. Silicon historically has been well studied and well understood. Communicating with the sensor head optically has several advantages. The cost of installation is low because redundant fibres are present in installed cables. Optical signals propagating in fibres are immune to EMI. A two-source fibre optic system has been developed to study the vibration characteristics of silicon and silicon dioxide resonators. The properties of both metal coated silicon and its oxide are studied. It is concluded that the resonator beams are under stress. This is because during fabrication stress is induced in the material due to the thermal mechanical mismatch between silicon and its oxide, also between the metal and the silicon and its oxide. The pressure and temperature sensitivity of silicon and silicon dioxide resonators is theoretically and experimentally studied. It is found that silicon is more suitable as a pressure sensor and silicon dioxide as a temperature sensor. A potential of a 'self-excited' system for sensitivity and multiplexing has been demonstrated. In such a system, a single source has been used for activation and detection of the resonant frequency. An optical fibre system has been proposed for multiplexing resonator sensors. Sensors with the same resonant frequency are differentiated from each other by using long pulse optical time domain reflectometry (LPOTDR). In this system, the signal gives information about the location of the sensor and the effect of the measurand.

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