|Author:||Chan, Kok-cheung Peter|
|Title:||Multiplexing of fiber optic Bragg grating sensors|
|Subject:||Optical fiber detectors|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||ix, 129 leaves : ill. ; 30 cm|
|Abstract:||The main objective of this project was to develop a novel technique for multiplexing fiber Bragg grating sensors for strain measurements. Multiplexing is a very important issue for fiber Bragg grating sensors, as it allows them to be used for distributed sensing where their greatest impact is anticipated. Three types of multiplexed fiber Bragg grating sensor system prototypes were developed in this work. Most effort was devoted to a frequency-modulated continuous wave technique for multiplexing fiber Bragg grating sensors. A detailed mathematical analysis of the frequency-modulated continuous wave multiplexing technique was performed. It was identified that the technique can be used to multiplex up to 32 fiber Bragg grating sensors of the same nominal Bragg wavelength with a theoretical crosstalk performance of below -48 dB. This level of crosstalk corresponds to a wavelength detected error of well below 1 pm if fiber Bragg gratings having a bandwidth of around 0.2 nm are used. A few hundreds of sensors could be multiplexed by combining the frequency-modulated continuous wave technique with the well known wavelength-division-multiplexing technique. The practical factors which limit the performance, including the effect of biasing from the optimal working condition and the effect of non-ideal frequency sweeping intensity modulation, were investigated. The system performance, in terms of power budget and inter-sensor crosstalk for a serial and parallel architecture was also determined. A series of experiments were carried out to verify the principle of operation and to study the effects arising from the various practical performance limiting factors and from different network architectures. A three sensor system was experimentally demonstrated with - 30 dB crosstalk level and with 2 ue resolution in terms of root-mean-square strain value. The system performance was found to be limited by the residual amplitude modulation due to the non-ideal frequency response of the voltage-controlled oscillator. By minimizing the residual intensity modulation through the use of smaller frequency excursion, a fiber Bragg grating sensor system with sideline suppression ratio of -50 dB was demonstrated, indicating that the crosstalk level could be much lower than -30 dB if a multiple sensor system were to be implemented. The frequency-modulated continuous wave technique was combined with wavelength-division multiplexing technique for addressing fiber Bragg grating sensors in order to increase the multiplexing capacity. A six sensor system with three different Bragg wavelengths was constructed with the crosstalk level of about -30 dB. The fiber Bragg grating sensors were embedded inside a glass fiber composite sample and a glass fiber composite strengthened concrete sample for internal strain measurements. The strain values extracted from the embedded fiber Bragg grating sensors compare well with that measured by surface bonded electrical strain gauges. The fiber Bragg grating sensors showed the ability to measure internal strains of the sample which is a difficult task for external electrical strain gauges. Two other frequency-domain techniques (subcarrier intensity-modulated frequency-division-multiplexing and switching-based frequency-division-multiplexing) for addressing fiber Bragg grating sensors were also investigated. Both techniques can be used to multiplex fiber Bragg gratings of either identical or different Bragg wavelengths and can be combined with wavelength-division-multiplexing to increase the multiplexing capacity.|
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