Fiber-Optic Ultrasonic Sensing via Quasi-Continuous Quadrature Frequency Modulation

In this work, we investigated the applicability and effectiveness of ultrasound detection using laser frequency modulated phase shifting interferometry. A quasi-continuous quadrature frequency modulation scheme is proposed to improve the time resolution and dynamic range. Based on a modulated grating Y-branch (MG-Y) laser, we achieve fast and discrete frequency switching with flat intensity output. Selective data sampling is used to eliminate the instability caused by the abrupt changes in drive currents, so as to obtain an effective frequency modulation rate of 10 MHz. Through numerical simulation, the demodulation errors induced by time delay are analyzed. The 5B-step phase shifting algorithm (PSA) shows a better performance. Simulation and experimental results demonstrate that this technique can be effectively used to demodulate extrinsic Fabry-Perot interferometric (EFPI) ultrasonic sensors. With programmable frequency modulation, essential data for signal recovery can be obtained in nanoseconds, which provides a novel perspective for high-speed and high-performance fiber-optic sensing.