DESIGN AND NUMERICAL ANALYSIS OF A HIGHLY SENSITIVE ULTRASONIC ACOUSTIC SENSOR BASED ON π-PHASE-SHIFTED FIBER BRAGG GRATING AND FIBER MACH-ZEHNDER INTERFEROMETER INTERROGATION

A pi-phase-shifted fiber Bragg grating (pi-FBG) shows high sensitivity to the ultrasonic (US) wave as compared to the conventional FBG due to the strong slow-light phenomenon at the resonance peak. However, its sensitivity is limited by the interrogation schemes. A combination of pi-FBG and unbalanced fiber Mach-Zehnder interferometer (F-MZI) are theoretically analyzed and optimized for the highly sensitive acoustic sensor. The coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modelling of pi-FBG. For the optimized grating parameters of pi FBG, the proposed sensing system shows the high strain sensitivity of 1.2 x 10(8)/epsilon, the highest dynamic strain resolution of 4:1f epsilon/root Hz, and the highest wavelength shift resolution of 4.9 x 10(-9) pm. Further, the proposed sensing system strongly supports both time and wavelength division multiplexing techniques. Therefore, the proposed sensing system shows extreme importance in single as well as quasi-distributed US acoustic wave sensing networks.