Passive quadrature demodulation of an ultrasonic fiber-optic interferometric sensor using a laser and an acousto-optic modulator

Abstract

We propose and demonstrate the use of a single-frequency laser and an acousto-optic modulator (AOM) for quadrature demodulation of fiber-optic ultrasonic sensors whose spectrum features sinusoidal fringes. The light from the laser is split into two channels, before it is combined into the fiber leading to the sensor, with an AOM in one of the channels introducing a frequency shift to the light. Thus, the light in the fiber contains two wavelengths whose difference is designed to be an odd number times a quarter of the free spectral range of the sensor, so that at least one of them is located on the spectral slope of the fringes for sensitive ultrasonic detection without the need to tune the laser wavelength. The intensities of the light in the two channels are sinusoidally modulated at two different frequencies much higher than the ultrasonic frequency, and the ultrasound signal is encoded into the amplitude of the intensity modulations. The optical signals from the two channels are separated in the frequency domain, and the ultrasound signals are detected by simple envelope detectors. Using a low-finesse Fabry–Perot interferometric sensor formed by two weak chirped fiber Bragg gratings written on a coiled bend-insensitive fiber, we demonstrate that this method can perform ultrasound detection, even when the spectrum of the sensor experiences large environmental drifts.