Minimum acoustic noise in the sea (solid curve), with contributions from the acoustic background (dashed curve) and the thermal agitation of water molecules (dotted curve) (reproduced after Ref. 10, as adapted from Ref. 11). The markers denote the hearing threshold of killer whales (after Ref. 13) measured with behavioral responses (×) and auditory evoked potentials (+). The noise is in units of μPa/Hz1/2 (left axis), while the orca audiogram is in units of μPa (right axis).
Exploded view of the sensor structure showing (a) the silicon-based sensor chip, (b) the brass backchamber, and (c) the fiber bundle with four SMFs, each with a mirror at its tip.
A cross-section of the sensor chip showing details of the channel structures and the three structural levels of the sensor chip. Scanning-electron micrographs of the three layers are included.
Diagram showing the steps of the sensor-chip fabrication.
The height profile through the center of a fabricated circular diaphragm, measured with an optical profilometer. The three-dimensional profile is shown as an inset.
Photograph of the packaged hydrophone.
Equivalent circuit formed by various lumped elements describing the sensor acoustics and the interfacing with optoelectronics. The circuit is superimposed upon a faded drawing of the sensor structure for reference. (The sensor drawing is not to scale.)
(a) Calculated response of sensor 1, (b) calculated noise spectrum (solid) showing contributions from radiation resistance (dashed), hole resistance (dotted), and channel resistance (dashed-dotted), and (c) calculated noise spectrum (solid) showing contributions from the noise coupling from sensors 2 (dashed) and 3 (dotted), and optoelectronic noise (dashed-dotted).
(a) Calculated MDP with Wenz’s minimum sea noise shown for reference, and (b) a better optimized MDP when the two parallel sensors are non-operational, so that crosstalk is reduced.
(a) Linearity of sensor response with respect to diaphragm displacement, showing the normalized linearities of the diaphragm displacement (solid), FP response (dashed), and power coupled back into fiber (dotted). (b) THD with respect to the pressure amplitude for sensors 1 (solid), 2 (dashed), and 3 (dotted).
Reynolds number at different frequencies for the annular channels of sensors 1 (solid), 2 (dashed), and 3 (dotted).
Setup used to characterize the optical hydrophone (HO), showing: the DSA, the reference hydrophone (HR), the preamplifier of the reference hydrophone (A1), the AP, the power amplifier of the projector (A2), the water-filled tube (WT), the IS, the laser (L), the optical circulator (C), the PD, and the feedback between the reference-hydrophone output and the projector input (F).
(a) Measured average frequency response of the optical hydrophone (solid) and result for one measurement (dotted). (b) Measured average noise of the optical hydrophone (solid) and optoelectronic noise (dotted). (c) Measured average MDP of the optical hydrophone (solid), with the minimum sea noise included for reference (dashed).
Measured coherence between the signals from the optical hydrophone and reference hydrophone (dashed), between the noise from the optical hydrophone and reference hydrophone (dotted), and between the noise from the reference reflector and reference hydrophone (solid).
Measured relative power spectrum of optical hydrophone for an acoustic wave at a constant frequency of 500 Hz and amplitude of 1.9 Pa, yielding a THD of −36 dB.
Structural dimensions of the optimized sensor.
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