City Research Online

Abstract

A number of pressure sensing devices were constructed using the principle of frustrated total internal reflection (FTIR), where light is coupled between a critically cut fibre and a second optical element across an air gap through the evanescent wave created at the critically cut end of an optical fibre.
Initially a simple analogue device was designed and constructed. The characteristics of the sensor were obtained and compared with the predictions of a simple theoretical model. This showed that the use of the FTIR effect for detecting pressure was indeed valid, although certain problems did arise due to thermal effects. These being due to the thermal mismatch between the optical elements and the metallic body of the sensor.
A second sensor based on the FTIR effect was then designed and constructed in order to eliminate the thermal instabilities encountered in the analogue sensor. This sensor was digital in nature, where a quartz crystal cut into a double tuning fork structure, mounted upon a cantilever beam, was driven into resonance via the photovoltaic effect. This being a hybrid device, the power to drive the crystal was delivered via an optical fibre to a photo diode mounted at the sensor head. The voltage generated by the diode was amplified by a small transformer which drove the crystal electrically into resonance. The resonance frequency was optically detected via the FTIR effect. The change in the resonance frequency as function of applied pressure was measured, and graphs for its thermal response and long term stability were obtained.
An optical drive system using the photothermal effect to drive the quartz crystal into resonance was investigated. The results obtained were compared with a simple model. A sensor was constructed to use this effect to measure pressure, however, a number of problems were encountered in optically detecting the resonance frequency of the optically driven crystal. These are discussed in detail along with possible solutions.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QC Physics T Technology > T Technology (General) T Technology > TK Electrical engineering. Electronics Nuclear engineering

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