Abstract

We report a high-sensitivity fiber-optic ultrasonic sensor system using a self-injection-locked distributed-feedback (DFB) diode laser where a $\pi$-phase-shifted fiber Bragg grating ($\pi \mathrm{FBG}$) serves as both the locking resonator and the sensing element in a fiber ring feedback loop. By controlling the delay time of the feedback light through a fiber stretcher, the laser wavelength is locked to an external cavity mode on the spectral slope of the $\pi \mathrm{FBG}$, and the ultrasound-induced wavelength shifts of the $\pi \mathrm{FBG}$ are converted to laser intensity variation. The ultrasonic sensing scheme simplifies the feedback control because the self-injection locking automatically pulls the laser wavelength to the $\pi \mathrm{FBG}$ resonant wavelength. In addition, it improves the detection sensitivity because the frequency noise of the DFB laser is drastically reduced. We show that the sensor system achieves a strain sensitivity of $78\mathrm{f\epsilon }/{\mathrm{Hz}}^{1/2}$ at around 200 kHz.

© 2019 Optical Society of America

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