Interrogation of fiber Bragg grating sensors with a tunable laser source

Abstract

The main objective of this project is to investigate the use of tunable laser for the interrogation of fiber Bragg grating (FBG) sensors and sensor arrays. Two of the most common techniques for wavelength, detection: i.e. the direct detection and first derivative techniques are studied. The effect of a particular kind of noise. i.e. the unwanted tnterferometric signal on the system performance is investigated and methods for enhancing the wavelength detection accuracy are examined. It is found that the wavelength detection accuracy can be enhanced by using digital filtering and proper wavelength modulation. In a single FBG system that uses the direct detection technique, the wavelength detection error is reduced by using a suitable digital filter after photodetection when the system is working in a scanning mode. The experimental results show that the RMS strain error was about 2 ue in a single FBG sensor when a 51st order FIR digital filter is used when the system is working in a scanning mode. However, when the system is working in a lock-in mode, the frequency of unwanted interferometric signals are always in the low frequency region and are therefore difficult to be minimized by use of the low-pass filtering technique. On the other hand, a system that uses the first derivative technique, the wavelength detection accuracy can be improved by proper modulation of the laser wavelength. I have found that the wavelength detection accuracy depends on both the amplitude of the wavelength modulation and the path difference between the signal wave and other parasitic reflected waves. Generally speaking, the wavelength detection error reduces as the distance between the FBG and the reflection point increases, or when the amplitude of wavelength modulation increases. When the system is working in a scanning mode. the combination of wavelength modulation and digital filtering can reduce the wavelength detection error significantly when the amplitude of wavelength modulation is properly selected. Computer simulation is carried out to study the performance of a time-division-multiplexed (TDM) tree type FBG array as a function of sensor number, amplitude of wavelength modulation and the time delay between adjacent sensing channels. It is found that a 20 FBG array of about 1 ue resolution can be realized by using an optical pulse modulator with extinction ratio of -35 dB for first derivative technique. The maximum wavelength detection error for the direct detection technique with the same set of parameter is 7 ue. Experiemtal study is carried out on a 2-FBG and a 4-FBG TDM array. For the 2-FBG sensor array, measurement accuracy in term of RMS strain errors of 8.77 ue and 1.2 ue are obtained for the direct detection technique in scanning mode and the first derivative technique in lock-in mode, respectively. For the 4-FBG sensors array, the strain errors are 9.28 ue and 0.74 ue for the direct detection technique in scanning mode and first derivative technique in lock-in mode, respectively. When the first derivative technique is applied to the 4-FBG sensor array working in scanning mode, the RMS strain error , when proper wavelength modulation and digital filtering are applied, is found to be 3.98 ue for a wavelength modulation amplitude of 10 pm.