We develop a novel ultrasonic sensor system using a fiber ring laser (FRL) to detect acoustic emissions. The sensor
system incorporates two fiber Bragg gratings (FBGs) in the FRL cavity, a short and strong FBG as the sensing element
and a long and weak FBG as the adapting element. The reflection spectra of both FBGs are matched such that the
reflection peak of the long FBG is positioned at the linear slope of the short FBG’s reflection spectrum. Ultrasonic waves
impinging onto the FBGs are to modulate the FRL cavity loss, which leads to laser intensity variations that can be
detected directly by photodetectors. The two FBGs are placed side-by-side in close proximity so that the sensor system is
able to adapt to the ambient temperature drift. We demonstrate that the ultrasonic sensor system can operate normally
within approximately 15ºC temperature change. In addition, the performance of signal-to-noise ratios is investigated as a
function of the FRL cavity loss. The proposed temperature-insensitive sensor system is attractive in practical applications
where temperature change is unavoidable.
In this paper, we present a temperature-insensitive refractive index sensor based on π-phase-shifted Bragg gratings fabricated on side-hole fibers processed by wet chemical etching technique. The reflection spectrum of the π-phase shifted gratings on etched side-hole fiber features two notches with large spectral separation, which was used for refractive index (RI) detection in our application. The relative spectral notch separation exhibited a RI sensitivity of −278.5 pm/RIU (RIU: RI unit). Theoretical simulation obtained the temperature sensitivity of −0.00241 pm/°C, and experimental results also showed little sensitivity to temperature of our RI sensor.