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A fiber-optic Fabry-Perot interferometric chitosan membrane hydrophone for in vivo ultrasound measurements is proposed. The hydrophone is based on a thin chitosan film acting as a low-finesse Fabry-Perot cavity that is formed at the tip of hollow core fiber. Chitosan membrane provides maximum acoustic impedance matching for in vivo ultrasound measurement and optimizes the matched-loading condition due to its permeable property. The transduction mechanism is based on acoustically induced mechanical deformation of the chitosan sensing interferometer, which exhibits a voltage sensitivity of 0.5 mV/MPa or -306 dB re 1 V/μPa without the use of filtering and external preamplifiers. The sensor shows frequency response from 1 to 20 MHz in the presence of acoustic amplitude level up to 4 MPa with a minimum detectable pressure of 40 kPa. The wideband sensitive response, biocompatibility, and easy functionalization of chitosan membrane suggest the possibility for accurate and reliable measurements of exposure levels encountered in in vivo ultrasound measurements and may find applications as an alternative to piezoelectric hydrophone for ultrasound characterizations.