Theoretical and Experimental Investigation of an Intensity-Demodulated Fiber-Ring-Laser Ultrasonic Sensor System

We theoretically and experimentally investigate the performance of an ultrasound detection system based on a fiber ring laser (FRL) whose cavity includes a pair of fiber Bragg gratings. The ultrasonic detection is achieved by the FRL power variations in response to the ultrasound-induced cold-cavity loss modulation of the FRL. The effects of key FRL parameters, including pump power, laser cold-cavity loss, and laser cavity length, on the system signal-to-noise ratio (SNR) performance have been investigated. It is found that the maximum SNR is achieved when the frequency of the ultrasound is the same as that of the FRL relaxation oscillation (RO). Harmonic generations are more prominent when the ultrasonic frequency is at the RO frequency and highly dependent on the strength of the ultrasonic signal. The analysis provides a useful tool for the understanding and optimization of such ultrasonic sensor systems.