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Cost-effective assembly of a basic fiber-optic hydrophone for measurement of high-amplitude therapeutic ultrasound fields
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Image of FIG. 1.
FIG. 1.

Components comprising the FOPH system. A hermetically sealed, fiber-coupled laser diode with FC termination is mounted directly to a heatsink device, which provides electrical connection of the laser driver/temperature controller console to both the integrated thermoelectric cooler and the laser diode. The laser diode pigtail is coupled to Port 1 of the optical coupler via a FC-FC mating bracket. The fiber attached to Port 3 of the coupler is left bare with no connector so that the endface can be cleaved and positioned in the calibration tank using a fiber chuck that is mounted to a three-axis motorized positioning system. The signal reflected at the fiber endface is directed via the coupler to a photodetector connected to Port 2, and the resultant voltage is measured on an oscilloscope. The coupler’s fourth port can be attached to another photodetector for noise suppression purposes or can be otherwise terminated to minimize reflections. The grey arrows indicate the direction of travel of the optical signal during operation.

Image of FIG. 2.
FIG. 2.

Pressure wave forms collected using a PVDF membrane hydrophone with active diameter (left panels), compared to those collected using the fiber optic probe hydrophone system (right panels). In all panels, the acoustic source is a spherical shell transducer emitting a ten-cycle pulse. The transducer is driven using identical excitation conditions in (A) and (B), and then is also driven using identical higher-amplitude conditions in (C) and (D). No noise filtering is performed in any case, though the FOPH wave forms are averaged prior to recording [128-point averaging in (B), 16-point in (D)]. Though the FOPH wave forms are generally noisier, comparison of (C) and (D) demonstrates the ability of the FOPH system to more faithfully record the peak compressional pressures of shocked wave forms due to its enhanced bandwidth and smaller active aperture compared to those of the PVDF membrane hydrophone used.

Image of FIG. 3.
FIG. 3.

Wave forms generated under identical excitation conditions measured with the FOPH system using different fiber orientations with respect to the beam axis. (A) Standard orientation with the fiber endface normal to the beam axis. (B) Alternate orientation with the fiber mounted vertically in the calibration tank so that the endface is parallel to the beam axis. In both panels, the acoustic source is a annular array emitting a pulse. For these wave forms of limited nonlinearity, very similar results are generated independent of the fiber orientation, with the exception that the parallel orientation depicted in (B) reduces ringing observed in the tail of the pressure signal that arises due to reverberations from the fiber chuck apparatus.


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