Design and fabrication of an optical fiber interrogation instrumentation system
Creators: name: Vahesan, Srirajasingam
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2007
Subjects: subject: Engineering, Electronics and Electrical.

description: The design and fabrication of an optical Fiber Interrogation Instrumentation System is studied in this thesis. The objective is to design a Fiber Bragg Grating Interrogation (FBGI) instrumentation system using a Fiber Fabry Perot Tunable Filter (FFP-TF). An FBGI instrumentation system is designed to monitor the back-reflected wavelength shifts from a Distributed Fiber Bragg Grating (DFBG) network and to interpret those wavelength shifts into environmental temperature, strain and pressure variations. This thesis encapsulates the complete system development lifecycle, inclusive of an applied case study of an FBGI Instrumentation system for monitoring the movements of bedsore patients. Specifically, in this thesis, an overview of Fiber Bragg Grating based fiber-optic sensors and their applications are presented, a novel design and implementation of an FBG interrogation system is described, which includes a detailed study on embedding techniques of FBGs in carbon composite, and an analysis of the test results from different embedding configurations is included. The discussion combines the novel approaches taken to resolve multiple peaks and peak-splitting phenomena observed in the reflected spectrum. A novel approach to remove the undesired temperature effect during strain sensing has been illustrated using neutral layer embedding of temperature FBG sensors. This thesis proposes the system architectures for the designs of a single-board microprocessor and a DAQ card based interrogation system using FFP-TF scanning technology. A 16-bit microprocessor ADSP-2181 from Analog Devices and a Data Acquisition Card DAQ-6015 from National Instruments is used as the front-end signal-processing circuit. The electronics circuit is built on a 4-layer printed circuit board. By incorporating an optical switch in the design and implementing a multiplexing technique, more FBGs are detected. This novel approach is successful due to the quasi-static nature of the measured environmental changes. This thesis demonstrates a detailed analysis and experiments on the optical path of the design. These involve design and verification of a 980-nm laser pump circuit, design and experiments of an ASE source and an EDFA design where it is shown that EDFA design is necessary to provide the required amplification to the reflected signal spectrum. In addition, EDFA gain flattening, FFP-TF characteristics verification, optical-receiver design and the like are described. It is demonstrated that co-directional pumping is better than counter-directional pumping. In addition, the dependency of EDFA gain on the wavelength, the input power of the signal and the length of the erbium doped fiber is discussed. Novel approaches such as linear segmentation and look-up table implementation are proposed to compensate for the non-linear characteristics of the FFP-TF filter. An innovative coarse and fine sampling technique is implemented to improve the sampling speed while keeping the tuning speed of FFP-TF low in the range of 100--300 Hz to achieve better resolution of the FFP-TF output. Finally recommendations are suggested for future work. In general the performance can be improved by running the signal processing algorithms on an on-board high-speed processor.