(a) Schematic of FPI sensor. (b) Phase (ITF) and its first derivative (ITF′). Operation at optimum phase bias point for the linear detection of a small acoustically-induced phase modulation is illustrated.
Effect of temperature on the wavelength ITF. A change in temperature from to produces a linear shift in the optimum bias wavelength from to .
Schematic of the FPI sensing structure deposited at the tip of the optical fiber.
(a) Schematic of the fiber-optic hydrophone system. The components contained in the dotted box form the interrogation unit shown in the photograph (b).
Iterative scheme to optimally bias the FPI in the presence of self-heating.
Comparisons of the outputs of (a) a 0.4 mm PVDF membrane hydrophone and (b) the fiber-optic hydrophone in response to a “shocked” 1 MHz toneburst. Insets show expanded timescale (in ).
Measured frequency responses of three typical fiber-optic hydrophones.
Directional response of a fiber-optic hydrophone. Response shown for frequencies: (a) 1–5 MHz, (b) 6–10 MHz, (c) 11–15 MHz, and (d) 16–20 MHz.
Change in optimum bias wavelength as a function of temperature change.
Experimental setup for making simultaneous pressure and temperature measurements in a HIFU field.
Comparison of temperature-time curves obtained by the fiber-optic hydrophone and the thin-film thermocouple.
Simultaneous acquisition of a temperature-time curve (top) and acoustic waveforms (lower row) captured at four different times during a 30 s insonation (cw). For the acoustic waveforms obtained at , 19, and 27 s, the waveform captured at (gray line) is also shown in order to illustrate the phase shift due to the thermally-induced change in sound speed.
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