Intensity-demodulated fiber-ring laser sensor system for acoustic emission detection

Abstract

Abstract:We theoretically and experimentally demonstrate a fiber-optic ultrasonic sensor system based on a fiber-ring laser whose cavity consisting of a regular fiber Bragg grating (FBG) and a tunable optical band-pass filter (TOBPF). The FBG is the sensing element and the TOBPF is used to set the lasing wavelength at a point on the spectral slope of the FBG. The ultrasonic signal is detected by the variations of the laser output intensity in response to the cold-cavity loss modulations from the ultrasonicallyinduced FBG spectral shift. The system demonstrated here has a simple structure and low cost, making it attractive for acoustic emission detection in structure health monitoring.

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

“…We have previously developed a model [10] to simulate a FRL sensor based on a FBG and a tunable optical filter where the EDF is described by a two-level model [13]- [15]. Such model is applied to simulate the operation of the FRL sensor system studied here.…”

“…The third peak at 40 kHz is the response to the ultrasonic wave, which is the signal peak. The last peak at 80 kHz is believed to be the second-order harmonic generation originating from the nonlinear nature of the laser operation [10].…”

“…However, the wavelength dependence of the gain medium is weak and difficult to be tuned, significantly limiting the ultrasonic detection sensitivity. To overcome this problem, a narrow tunable optical bandpass filter (TOBPF), whose wavelength is tuned to be on the slope of the reflection spectrum of an FBG, is added into the laser cavity [10]. The FRL lasing wavelength is determined by the TOBPF and its cold-cavity loss is modulated by the ultrasonically-induced spectral shift of the FBG.…”

Multiplexed fiber-ring laser sensors for ultrasonic detection

“…However, due to the high cost of tunable lasers, multiplexed ultrasound detection based on FBGs, in which each sensor requires a tunable laser at a unique wavelength, has rarely been demonstrated and may not be practically feasible. Recently, we have proposed and demonstrated a potentially low-cost fiber-optic ultrasonic sensor system for acoustic emission detection by incorporating a narrow-band tunable optical band-pass filter (TOBPF) and a sensing FBG inside the laser cavity of a fiber-ring laser (FRL) that employs an erbium-doped fiber (EDF) as the gain medium [8]. The sensor system has a simple structure and its operation does not require external tunable lasers.…”

“…The proposed multiplexed sensor system also has a simple structure and only uses standard fiber-optic components. Each channel of the multiplexed system detects the ultrasonic signal using the same principle as in a single-point FRL sensor system demonstrated in [8]. The sensors are demodulated directly from the variations of the laser output power by a photodetector (PD).…”