Broadband fiber optic hydrophone sensors for sub-millimeter ultrasound resolutions

Broadband fiber optic hydrophone probes are an attractive solution for characterization of ultrasound fields with sub-millimeter resolution up to 100 MHz frequency range without need for complicated spatial averaging corrections. In this work, optical hydrophone sensors based on single mode straight - cleaved fibers and down-tapered coated fibers with sensitive dimensions of less than 10 mum have been reported. Thinly gold coated optical fiber sensors demonstrated a higher sensitivity and bandwidth performance than straight cleaved uncoated and down-tapered uncoated. The statistical analysis clearly indicates that the gold coated fibers with thickness of 30 nm provide a 26 dB improvement in sensitivity over that of the uncoated straight cleaved fibers, raising the overall sensitivity of the fiber optic hydrophone sensor to a value of -245 dB re 1 V /muPa (unprecedented 0.56 V/ MPa). Broadband testing of these sensors with a 10 MHz acoustic tone burst further indicates that the bandwidth of gold coated fiber sensor is as high as 60 MHz. Amplified optical source intensity noise limits the current intensity detection system up to 60 MHz.