Polarization-Maintaining Fiber Loop Mirror Based Ultrasound Sensing Technique With Enhanced Signal-to-Noise Ratio

In this article, we propose and demonstrate a fiber optic ultrasound sensing technique using a polarization-maintaining fiber loop mirror (PMF-LM) to achieve an enhanced signal-to-noise ratio (SNR). The SNR performance of the ultrasound sensor is investigated both theoretically and experimentally. It is found that the dominating shot noise results in the higher SNR of the detected ultrasound signal when the laser wavelength is chosen near the valley point of the transmission spectrum, while the highest sensitivity occurs at the quadrature point. The SNR near the valley point is approximately 14 dB higher than that at the quadrature point, across a wide wavelength range of 1520–1570 nm. This sensor exhibits a broadband frequency response from 200 to 3000 kHz with an SNR exceeding 58 dB. Furthermore, the measured SNR at different ultrasound frequencies varies as the change of the incident power and the driving voltage of the PZT actuator was investigated. Thanks to the high NR, the calibration of the minimum detectable pressure (MDP) of the sensor is $36.47 \mu $ Pa/Hz $^{\text {1/2}}$ at 100 kHz, six times improvement over MDP at quadrature points. This approach opens up a novel pathway to enhance the performance of ultrasound sensors.