Rigorous characterization of fiber-optic hydrophone responses using a discretized-domain-based rigid-piston model and a broadband photoacoustic pulse

Fiber-optic hydrophones are one of the most fundamental platforms widely used for measurement and evaluation of ultrasound waves in a non-destructive manner. Therefore, there is the need for rigorous modeling of their output responses which enables their extensive use with exact prediction of measured outputs. We characterize fiber-optic hydrophone responses rigorously by devising a modified rigid piston model that accommodates a discretized-domain analysis. Importantly, optical sensing conditions are taken into account accurately including a cross-sectional geometry of fiber and an optical power distribution. Then, we experimentally validate calculation results by utilizing a broadband photoacoustic pulse that effectively works as a normally incident delta-mimicked pulse. This reveals that an edge wave generated from the rim of fiber predominantly determines an output response. Finally, our verified modeling approach is applied, as an example, to design an improved structure adopting an eccentric core fiber that minimizes the edge wave-induced distortion and achieves a significantly flattened frequency response (variation less than 3 dB) over the range of sub-500 MHz.