Diaphragm based optical fiber pressure sensor for acoustic detection

Pao Yue-kong Library Electronic Theses Database

Diaphragm based optical fiber pressure sensor for acoustic detection

 

Author: Lai, Yiming
Title: Diaphragm based optical fiber pressure sensor for acoustic detection
Degree: M.Sc.
Year: 2016
Subject: Optical fiber detectors.
Detectors.
Hong Kong Polytechnic University -- Dissertations
Department: Faculty of Engineering
Pages: xiv, 106 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2894573
URI: http://theses.lib.polyu.edu.hk/handle/200/8513
Abstract: The main objective of this dissertation is to fabricate two types of graphene diaphragm based optical fiber acoustic sensors. And the fabricated sensors will be characterized by acoustic test. Comparing to typical electrical sensors, fiber optical sensors (FPOSs) exhibit some distinctive advantages. Therefore, such graphene diaphragm based FOPS has become popular in automobile, power electronics, oil extraction and biomedical industries. Two key components used in sensor fabrication are discussed, namely Fabry-Perot interferometer (FPI) and graphene diaphragm. FPI is used to fabricate the body of sensor and could be demodulated by two demodulation systems for different type of pressure. The mechanical properties of graphene diaphragm will be discussed and investigated by pressure response simulation. Then, two types of optical sensors will be fabricated with circular and long rectangular (bridge suspended) graphene diaphragm. And the procedure of fabrication is invented and discussed. The fabricated sensors will be then characterized by acoustic test. In terms of the test results the fabricated sensors with circular graphene diaphragm exhibit linear pressure response with pressure sensitivity of 0.478 m/Pa, signal to noise ratio (SNR) of 41.01 dB and minimum detectable pressure of 49.31 Pa/Hz1/2 at acoustic frequency of 10kHz. As for bridge suspended diaphragm sensor, its pressure response is nonlinear with pressure sensitivity of 23.94 m/Pa, signal to noise ratio (SNR) of 51.18 dB and minimum detectable pressure of 49.31 Pa/Hz1/2 at acoustic frequency of 10kHz. The stability of circular diaphragm sensor is fair, since its output voltage has a 2.9% decrease in 1 hour. And the stability of bridge suspended type sensor is poor, since its output has a 13.2% decrease in just 13 minutes. Basically, the fabricated sensor with bridge suspended diaphragm has better performance in acoustic detection, but the nonlinear and unstable nature will make its performance unpredictable. Two methods are developed to improve system stability. The first method is to construct a feed back control system to track the quadrature point (Q-point) of the acoustic detection system, and the output will only have a 0.7% decrease over 30 minutes with this feedback control. The second method is to use the Sagnac interferometer based demodulation system. With this new demodulation system, the output will have a 10% vibration around the initial value. Therefore, the feedback control is more effective for stabilization, and the Sagnac based system could be an optional way of doing stabilization if the feedback control is not available.

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