This thesis reports my original research on the development and application of fiber grating-based acoustic sensors. This research covers both theoretical and experimental works, which include theoretical modeling of novel sensing schemes, design and fabrication of fiber grating-based sensors, development of a data acquisition system, proof of concept experiment, and application of fiber sensors in the mechanical engineering field. With regard to issues and potential related to the fiber lasers and fiber gratings sensors identified in the research, I propose several schemes and/or techniques to improve overall system performance. Particular focus is given to the enhancement of acoustic sensitivity that is severely limited by the high stiffness of glass fiber. The enhancement was achieved through the fiber grating and system design approaches. This thesis also implements optical fiber sensor technologies into an industrial application of marine structure.
Firstly, I propose and experimentally demonstrate a novel sensitivity enhancement scheme in a composite cavity fiber laser (CCFL) hydrophone. The scheme works based on partial cavity and pre-strained cavity sensing. In this scheme when only one cavity is made responsive and pre-strained, substantial wavelength/phase sensitivity can be attained with regard to the existing single cavity lasers. A CCFL sensing model was developed from multiple reflections theory. From the analysis of the model, the effects of design parameters such as the cavities length ratio, gratings reflectivity and gain toward the sensing performances were identified. The scheme was experimented using in-house fabricated CCFLs in an interferometric-based hydrophone system, and attained an excess sensitivity of 14 dB compared to the standard response, which is in close agreement with the theoretical expectation.
Secondly, I propose and experimentally demonstrate a novel multiple subchannels sensing concept to improve sensitivity (in terms of signal-to-noise ratio) of an intensity-type acoustic sensor interrogated by a broadband source. This scheme was realized using a multiple phase-shifted fiber Bragg grating (MPS-FBG) pair. In this scheme, the collective and simultaneous operation of the subchannels of MPS-FBG multiplies the total acquired signal power change at a particular channel, considerably enhancing sensitivity with regard to the normal FBG. From the intensity-sensing model developed from cross-correlation relations, the effects of FBG design parameters on the sensing performances were identified. Optimum MPS-FBG designs were sought through the transfer matrix analysis. In experiment, substantial sensitivity enhancement was achieved compared to the normal FBG, e.g. 20 dB using 17-phase shifted FBG. The improved sensitivity while retaining the system simplicity would be an attractive option for an economical coarse wavelength division multiplexed acoustic sensing system.
Thirdly, I develop and implement practical fiber grating-based sensor systems for an application in the mechanical engineering field, namely for failure monitoring of marine structures made of E-glass/vinylester composites. Two assessment techniques were implemented in this study: an embedded FBG strain sensor array to provide absolute interlaminar strain; and surface attached FBG acoustic sensors to provide an assessment technique based on the acoustic emission technique. Through the development of the system, a novel embedding procedure of optical fiber strain sensor in curve composites to achieve high survival rate during the vacuum infusion manufacturing process has been proposed. Progressive failure and structural strength of the structure were successfully identified and the result is comparable with those of the commercial piezoelectric sensors.
Throughout the course of this thesis, I have meticulously refined the procedures required for in-house manufacturing and successfully implemented the procedures in manufacturing of complex grating structures presented above. I have developed the required experimental platform including the digital signal processing technique used for data acquisition and demodulation.
I have also developed in-house grating-based devices analysis tools based on the transfer matrix method. The tools have been successfully applied to design and analyze complex gratings and fiber lasers in my experimental research mentioned above, e.g. designing apodized DFBFL and analyzing its performance in terms of the lasing efficiency and the higher order mode threshold.