Cost-effective assembly of a basic fiber-optic hydrophone for measurement of high-amplitude therapeutic ultrasound fields

TLDR

Fiber‑optic hydrophones offer larger bandwidth, higher spatial resolution, reduced directionality, and greater electromagnetic immunity than traditional PVDF hydrophones, though they typically have lower sensitivity. This work presents design, assembly, and operating procedures for a cost‑effective basic fiber‑optic probe hydrophone to guide groups building similar devices. The described system uses a 100‑µm multimode fiber, a 1‑W laser diode, a 2×2 coupler, and a 50‑MHz silicon p‑i‑n photodetector, with waveforms compared to a reference PVDF hydrophone and calibration methods discussed. The hydrophone’s voltage‑to‑pressure transfer factor is about 0.8 mV MPa⁻¹, and adjustments to optical components can substantially increase this value, with recommendations provided for component selection and routine use.

Abstract

Design considerations, assembly details, and operating procedures of one version of a cost-effective basic fiber-optic probe hydrophone (FOPH) are described in order to convey practical information to groups interested in constructing a similar device. The use of fiber optic hydrophones can overcome some of the limitations associated with traditional polyvinylidene difluoride (PVDF) hydrophones for calibration of acoustic fields. Compared to standard PVDF hydrophones, FOPH systems generally have larger bandwidths, enhanced spatial resolution, reduced directionality, and greater immunity to electromagnetic interference, though they can be limited by significantly lower sensitivities. The FOPH system presently described employs a 100-μm multimode optical fiber as the sensing element and incorporates a 1-W laser diode module, 2×2 optical coupler, and general-purpose 50-MHz silicon p-i-n photodetector. Wave forms generated using the FOPH system and a reference PVDF hydrophone are compared, and intrinsic and substitution methods for calibrating the FOPH system are discussed. The voltage-to-pressure transfer factor is approximately 0.8mV∕MPa (−302dBre1V∕μPa), though straightforward modifications to the optical components in the FOPH system are discussed that can significantly increase this value. Recommendations are presented to guide the choice of optical components and to provide practical insight into the routine usage of the FOPH device.