D-Shaped Fiber Bragg Grating Ultrasonic Hydrophone With Enhanced Sensitivity and Bandwidth

In this paper, we demonstrate a miniature flexible FBG-based underwater ultrasonic sensor with enhanced pressure sensitivity and bandwidth using a side-polished configuration of ~54 μm depth and ~15 mm length. The opto-mechanical responses of the sensor to acoustic-induced strain in axial and radial receiving modes are investigated and characterize the gain sensitivity dependent on the wavelength/Bragg length ratio. Then, ultrasonic testing in a water tank exhibited a maximum radial acoustic pressure-induced wavelength shift of 91.29 nm/MPa and a noise equivalent acoustic signal level of ~49.6 mPa/Hz
^1/2
in the range of 40-120 kHz, as well as a good gain flatness of less than 9.4 dB. This is significantly superior to FBG-based hydrophones previously ever reported to our knowledge. The proposed technique greatly simplifies the miniaturized integration of all-optical ultrasonic detectors and could, therefore, be further improved for ultra-high sensitive hydrophones in optoacoustic imaging.