Fiber bragg gratings sensor system

Abstract

Fiber Bragg grating (FBG) is a key component in the optical sensing field. The unique advantages of FBG sensors make them the preferences for the applications that normal electrical sensors are unworkable. However, the complexity and cost of the sensing systems have limited the applications of FBG sensors. Various multiplexing and interrogation schemes have been explored to enhance the competitiveness of FBG sensing systems in the global sensor market. This thesis presents a number of techniques that may simplify the FBG sensor detection systems. Code division multiplexing (CDM) is one multiplexing technique for Bragg grating sensor systems. It uses the orthogonal property of codeword to resolve the reflections from the different grating sensors in a serial array. This enables the advantages of time division multiplexing (TDM) technique to be maintained and at the same time overcomes some limitations of current TDM schemes. However, many aspects related to the application of CDM in fiber sensor systems including code design, optimum system architecture and system performance were not investigated. In this thesis these problems are addressed through investigating the use of code-division multiplexing method for a proposed optical fiber sensor system. Effort has been made to study the performance of CDM technique for FBG sensor multiplexing. Tunable laser is a useful component for optical sensor interrogation. However, the drawbacks of high cost, low tuning speed and poor tuning repeatability have limited the usefulness of tunable laser in the practical sensing systems. In this thesis, two wavelength tuning s128 range is not required, vertical cavity surface emitting laser (VCSEL) driven in a special way can be used as a narrow band tuning laser source. The tuning properties of VCSEL as a wavelength tuning source are studied in this thesis. It is applied to a tilted FBG vibration sensing system for fast vibration detection. Experimental results show that the coupled power is about 15dB larger than the broadband ASE source and a better system performance is obtained. Vibration measurement up to 200Hz has been achieved. Another approach to achieve wavelength tuning light source for sensing applications is using a modulated laser pulse sequence to feed into a loop with single-sideband (SSB) modulator. The wavelength of pulse will shift after every loop. A series of pulses can be generated at the output port of the 3dB coupler and can be used as a wavelength tuning source to interrogate the wavelength shifts of the FBG sensors. The advantages of this scheme are stable wavelength output, high accuracy and easy tuning range adjustment by using different optical band pass filter. Its performance in a FBG sensing system is evaluated.