An interrogator for fibre Bragg grating sensors using an ultrasonically induced long-period optical fibre grating

An ultrasonically induced long-period fibre grating (U-LPFG) functions as a tunable notch filter, since the pitch of refractive index modulation is changed by the driving frequency of the ultrasonic vibration propagating along a fibre. In this paper, we present the application of the U-LPFG for a fibre Bragg grating (FBG) sensor interrogator. If the U-LPFG is connected to the FBG sensor output and the ultrasonic frequency is swept, the FBG centre wavelength can be found as the ultrasonic frequency that gives a minimum output. The first half of the paper is devoted to the experimental investigation to reveal the optical transmission characteristics of the U-LPFG. We observed from our experiments that the travelling wave of flexural vibration is successfully excited on the fibre due to the remaining silicone coating at the fibre terminal. The rejection ratio is changed by the ultrasonic vibration amplitude, while the rejection band width depends on the length of the fibre where the vibration is excited. The wide tunable range of 160 nm (from 1490 to 1650 nm) is available for the ultrasonic frequency range of 300 kHz. We introduced several weighted mean methods to improve the accuracy of determining the centre wavelength. For an interrogator of an FBG temperature sensor, 7.4 degrees C in standard deviation was obtained.