A General Purpose Fiber-Optic Hydrophone Made of Castable Epoxy

The inner core was placed in a lucite mold which could then be filled with epoxy to form the capsule.
After the epoxy had cured, the capsule was removed from the mold and the two machine screws were
removed from the capsule. The capsule was then placed in a vacuum oven and heated for approximately
twelve hours at 500 C to remove the wax and post-cure the epoxy. Before encapsulation, both screw holes
were filled with epoxy, but at this stage only one hole was filled so that the other hole could be used to
connect the capsule to a vacuum rig for static pressure sensitivity calibration. A photograph of the
completed capsule is shown as Figure 7.
4. HYDROPHONE CALIBRATION AND COMPARISON TO THEORY
The results reported in this section were obtained by the comparison technique39 in air and in water
near atmospheric pressure in the calibrators which were small compared to the acoustic wavelengths of
interest since the hydrophone was compact'0 (i.e., small compared to the wavelengths of interest) and its
response is expected to be omnidirectional The hydrophone was tested in water over the range of
temperatures from 7.5 °C to 45 °C, at ambient pressure.
The acoustical and vibration sensitivities were measured by a fixed phase shift techniquel8 which
involved the adjustment of a sinusoidal excitation (acoustic pressure or acceleration) until the output of the
interferometer, observed on an oscilloscope, corresponded to an identifiable amount of optical phase shift
(integer multiples of 2it radians per half-cycle of excitation). This technique can provide results which
were accurate to better than and agreed with the Bessel function zero-crossing technique4'5 also to
better than 1 %. The results reported here are therefore demodulator independent. The normalized
sensitivity, M, was obtained from the measured sensitivities by subtracting 20 log10 (2nkL) from the
measured sensitivities expressed in decibels. The factor of two appears due to the "double pass" in a
Michelson interferometer. For this hydrophone, n = 1.48, A = 830 nm, and L = 9.0 m, which
corresponds to a normalization factor is 166 dB.
4 . 1 Comparison Calibration Standards
Measurements of acoustic pressure sensitivity in air were made between 60 Hz and 900Hz using a 1-
inch laboratory standard condenser microphone (Brüel & Kjer Model 4132, S/N 172894) as a transfer
standard. The calibration of this microphone with its associated pre-amplifier (Brüel & Kjer Model
2660), cable, and power supply (BrUel & Kjer Model 2807) was checked using a pistonphone (Brüel &
Kjer Model 4220) for acoustic calibration prior to and immediately after use. This calibration was verified
with a high precision reciprocity calibration of the 1-inch microphone using the BrUel & Kjer Type 4143
Reciprocity Calibration Apparatus and two additional BrUel & Kjar 1-inch reference microphones. The
measured sensitivity was 48.3 mV/Pa.
Measurements of acceleration sensitivity were made between 40 Hz and 1 kHz using a piezoelectric
accelerometer (BrUel & Kjar Type 4382) followed by a voltage pre-amplifier (Ithaco Model 1201). The
calibration of the accelerometer, cable, and amplifier were checked 159 Hz using a calibration exciter
(Brüel & Kjar Type 4294) which was itself check by comparison to the "chatter" method based on the
magnitude if the Earth's gravitational acceleration41. The two calibrations were found to agree to within
2%.
22 / SPIE Vol. 1367 Fiber Optic and Laser Sensors VIII (1990)