|Title:||Miniature fiber-tip Fabry-Perot interferometric sensors for pressure and acoustic detection|
|Subject:||Optical fiber detectors|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||xix, 155 p. : ill. ; 30 cm.|
|Abstract:||Pressure/acoustic detection has widespread applications in automobile, bio-medical, and oil/gas industries. Compared with their electro-mechanical counterparts, optical fiber pressure sensors (OFPSs) offer a number of advantages such as remote detection capability, light weight, electromagnetic immunity and corrosion resistance. Fabry-Perot interferometers (FPIs) with a deflectable diaphragm attached to a hollow-cavity at the end of a single mode fiber (SMF) represent an important class of OFPSs. These OFPSs are expected to have superior performance in terms of size and weight, detection sensitivity and bandwidth, and immunity to polarization fading. The objective of this dissertation is to improve the performance of the diaphragm-based OFPSs. Various types of OFPSs are reviewed and the emphasis is placed on the diaphragm based FPI sensors. Basic theories of FPIs, as well as the mechanical response of the diaphragm subjected to static and dynamic pressures, are outlined. These theories form the foundation of the diaphragm-based FPIs for pressure measurement and provide the guideline for evaluating the sensor performance in terms of pressure sensitivity, frequency response, and noise equivalent pressure level. A simple and low-cost technique for fabricating all-silica micro-cavities at the fiber end is developed. The technique involves splicing a hollow silica capillary to one end of a SMF and then melting the capillary to form a micro-cavity at the fiber end by using a fusion splicer. Experimental tests show that the micro-cavity has good mechanical strength and can sustain high temperature up to 1000 °C. With a pressure-assisted tapering process, the silica wall-thickness of the micro-cavity can be controlled and micro-cavities with the wall thickness from tens of micrometers to 2.2 μm have been practically made. The micro-cavity with the wall-thickness of 2.2 μm demonstrates a pressure sensitivity of ~315 pm/MPa, a low temperature sensitivity of 1.55 pm/°C, and a maximum working pressure up to 40 MPa.|
Ultra-sensitive fibre-optic FPI-based pressure/acoustic sensors with graphene diaphragms are developed. Graphene is the thinnest and strongest material known to us and it would represent the ultimate limit for two-dimensional diaphragm-based pressure sensors. A technique for transferring such thin graphene films is developed and used to build FPI sensors at the end of SMFs. With ~1 nm-thick and ~25 μm-diameter graphene as the diaphragm, a sensor demonstrates a pressure sensitivity of ~40 nm/kPa. This is a highly sensitive device considering the small size of the diaphragm, and the sensitivity is actually limited by the pre-stress in the diaphragm rather than the diaphragm thickness. With a larger (~125 μm in diameter) multiple-layer graphene film (~100 nm in thickness), the sensor demonstrates a pressure (acoustic) sensitivity up to 1100 nm/kPa, and this, to my knowledge, is the highest value reported to date for the same type of sensors with similar diaphragm sizes. The sensor exhibits a flat frequency response from 0.2 to 22 kHz and a noise equivalent acoustic pressure level of ~60 μ Pa/Hz1/2 at the frequency of 10 kHz. A fiber-tip pressure sensor based on the mechanical resonance of the graphene diaphragm is demonstrated for the first time to my knowledge. A poly(methyl methacrylate)-assisted graphene transferring technique is developed and the technique might be also useful for building other graphene based optical fiber devices. The resonator is excited and interrogated optically by using an all-fiber system and this type of sensors would have potential applications in mass, pressure and force detection. For pressure detection, a miniature micro-resonator with a beam-shape diaphragm demonstrates a pressure sensitivity of over an order of magnitude higher than that of a silicon cantilever. An additional advantage of the resonance detection scheme is that it needs no sealed cavity, making this type of pressure sensors inherently more immune to possible diaphragm fatigue.
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