Low flow-noise conformal fiber optic hydrophone

A fiber-optic hydrophone having a pair of jacketed fiber optic windings fed in a concentric planar spiral configuration in a layer of polyurethane is provided. One of the fiber optic windings has a fiber with a bonded acoustically sensitive jacket thereby increasing its sensitivity to acoustic energy. The second fiber optic winding, the reference winding, has a unbonded jacket enclosing the fiber resulting in reduced sensitivity to acoustic energy. Sensitivity to vibrational energy; however, is not reduced. The combining of signals from the pair of fibers provides a vibration-canceled acoustic signal.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The invention relates generally to acoustic sensors and more particularly 
to fiber optic acoustic sensors. 
2. Prior Art 
The use of laser interferometers for detection of acoustic signals is 
known. Typical prior art devices have been configured by winding an 
optical fiber onto a pressure sensitive mandrill or by suspending a 
pressure sensitive fiber in an acoustic medium, typically in a spiral 
configuration. 
The limitations of the prior art devices include the requirement of 
suspending the array in a manner that does not restrict the acoustic 
response. This requirement greatly complicates suspension. Further, prior 
sensors are typically configured three-dimensionally, thereby further 
increasing suspension difficulties and limiting their use in moving flow 
fields. In the low wave number domain experienced by hull structures, 
pressure fluctuations in the turbulent boundary layer induce vibration in 
the hull structure resulting in vibrational noise. Prior sensors are 
sensitive to vibration induced noise and are therefore limited in overall 
sensitivity to acoustic energy signals due to the necessity of suppressing 
vibrational noise. Acoustic sensors used to quantify low wave number 
pressure components in a turbulent boundary have lacked sufficient 
sensitivity because of this required suppression of noise vibration 
induced. This lack of sensitivity is critical because the wavenumber 
characteristics of the turbulent boundary layer pressure fluctuations have 
large values at the structural wavenumber response of the hull of 
underwater vehicle. The hull structure in these circumstances is excited 
by the boundary layer pressure fluctuations and the resulting vibrations 
become a significant noise source. The vibration induced noise and 
impinging acoustic signals are in the same frequency range and therefore 
cannot be filtered by conventional means. Therefore, the presence of 
vibrational noise greatly reduces the effectiveness of prior hydrophone 
sensors in detecting and isolating acoustic energy signals from the 
acoustic medium. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a fiber optic 
hydrophone having improved sensitivity to acoustic signals. 
It is a further object of the invention to provide a fiber optic hydrophone 
having a reduced sensitivity to vibrational noise and in particular to 
flow noise. 
It is yet another object of the invention to provide a fiber optic 
hydrophone having a planar configuration. 
It is still a further object of the invention to provide a fiber optic 
hydrophone having an acoustically-transparent structure. 
It is a further object of the invention to provide a fiber optic hydrophone 
having a compliant material which may be molded to a ship hull or other 
structure. 
The invention is a fiber optic hydrophone having a pair of optical fibers 
each having acoustically sensitive jackets and wound in a planar coil 
embedded in a compliant sheet of acoustically transparent material. The 
windings form a single planar disk in which one of the optical fibers, the 
sensing fiber, is continuously bonded to its protective jacket while the 
other fiber, the reference fiber is enclosed within, but not bonded to, 
its protective jacket. The unbonded fiber is less sensitive to acoustic 
signals from the acoustic medium but remains sensitive to vibration 
induced medium. The entire assembly is further encapsulated in a 
polyurethane layer forming a large pancake sensor which may be molded to a 
ship hull or other shape as required. 
The resulting hydrophone detects acoustic energy by use of an electronic 
unit which detects the phase difference between the two optic fibers. 
Phase shifts caused by vibrations however, are not detected as each fiber 
is equally affected and there is no phase difference. This arrangement 
effectively cancels vibrational noise.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the hydrophone of the present invention, 
designated generally by the reference numeral 10, is shown with its major 
components. These components include a laser source 11, a fiber-optic 
sensor assembly 13, and an opto-electronic unit 15. Laser source 11 may be 
any available source providing coherent light. In the preferred 
embodiment, a laser emitting at a wavelength of 800 nM was used. A second 
embodiment used a laser emitting at a wavelength of 1300 nM was used. 
The heart of the invention lies in the planar fiber optic sensor assembly 
13. Energy going to the planar fiber optic sensor from a laser light 
source 11 is split into two parts by splitter 12, one part travelling 
through sensing loop 131 and a second part traveling through reference 
loop 132. Sensing loop 131 is formed in a spiral configuration encased in 
an elastomer material 133. The preferred elastomer material is 
polyurethane. However, any compliant and acoustically transparent material 
may be used. Reference loop 132 is also formed in a spiral configuration 
in the same elastomer material and is further arranged in concentric 
pattern with the sensing loops. This pattern allows the reference loop to 
be located adjacent to the sensing loop at all points along the spirals. 
By locating the loops in concentric spirals, vibrational inputs caused by 
vibration of the structure upon which the planar fiber optic sensor is 
mounted are sensed by both the sensing loop 131 and the reference loop 132 
equally. Effectively, both loops lie in approximately the same physical 
locations and therefore both are subject to the same vibrational inputs. 
Referring now to FIG. 2, a cross-section of sensing loop 131 is shown 
depicting the component layers. The center portion is the glass/silicone 
optical fiber 21. A bonding agent 22, is applied between the optical fiber 
and the acoustically sensitive jacket 23. The preferred jacket material is 
Hytrel. The result of this bonding is that the sensing arm is made highly 
sensitive to acoustic energy, while the reference arm, without bonding 
between the jacket and the optical fiber, is far less sensitive to 
acoustic and therefore, insulated from impinging energy. This is an 
important feature because the vibrational and acoustic signals are in the 
same frequency range and therefore cannot be filtered by conventional 
means. 
It is well known that the shape of acoustically sensitive materials is 
altered by pressure fluctuations induced by impinging acoustic energy. 
When the acoustically sensitive jacket is bonded to the fiber of sensing 
loop 131, the shape change is more fully transferred to the underlying 
fiber and, thus, the acoustically sensitive fiber changes shape under 
pressure. The change in the strain within the glass core causes changes in 
the index of refraction of the glass core and the length of the fiber, 
resulting in a difference in optical path length (or phase) between light 
travelling through the two loops. The unbonded jacket of reference loop 
132 does not effectively transmit the acoustic energy to the acoustically 
sensitive fiber of the reference loop. However, both the sensing and the 
reference loops are equally affected by vibrational input and, therefore, 
the light traveling through the two loops experiences identical phase 
shifts due to vibration. The phase difference of the light traveling 
through the two loops, is therefore, directly related to the acoustic 
energy level. Signals from the two loops are then recombined, obtaining a 
phase modulated signal through the Doppler effect. The electro-optic unit 
15 in the preferred embodiment was designed by the Naval Research 
Laboratory and fabricated by Optech, Inc. using a synthetic heterodyne 
demodulation technique for interrogating the phase modulated signal. The 
unit comprises an optical phase detector, photo-voltage converter and a 
pre-amplifier. 
Operation of the hydrophone may be seen by reference to FIG. 3, wherein a 
block diagram depicts the functional components of the hydrophone. The 
laser source provides a coherent light signal to a splitter which sends 
part of the signal to the bonded sensor loops and part to the unbonded 
reference loop. The output signal is recombined, the phase shift between 
the loops is detected, thus isolating the acoustic signals from the 
vibration noise. The signal is further processed to provide a voltage 
conversion which is proportional to real time acoustic pressure at the 
sensor. These signals can be used either independently or in acoustic 
arrays. 
The advantages of the present invention are numerous. The planar hydrophone 
provides a compliant disk which can be formed to fit the shape of a ship's 
hull or any other desired geometric shape. The acoustically transparent 
elastomer which encapsulates the array of fiber optic windings protects 
the array and allows direct attachment to the hull without reducing 
acoustic sensitivity. 
Also the acoustically transparent elastomer permits layering of multiple 
hydrophones. This feature permits installation the present invention over 
a conventional hydrophone or sonar array. Alternatively, multiple planar 
fiber optic hydrophones may be installed in layers. This layering is 
useful for spatial filtering or array-like beamforming, or simply 
redundancy to improve reliability. 
Additionally, the hydrophone provides a unique structure in that the 
sensing and reference loops are everywhere collocated by use of the 
concentric spiral windings. The collocating of fiber optic loops and the 
bonded and unbonded jackets on the fiber optics provide a unique result. 
Because both sensing and reference loops are encased concentrically within 
the same elastomer material, both receive identical vibrational inputs, 
but only the sensing loop only is affected by acoustic signals. The 
detection process only measures phase differences, thus effectively 
cancelling any vibration-induced signals. The result occurs even though 
the vibration and acoustic inputs are in the same frequency range. 
Although the invention has been described relative to a specific embodiment 
thereof, there are numerous variations and modifications that will be 
readily apparent to those skilled in the art in the light of the above 
teachings. It is therefore to be understood that, within the scope of the 
appended claims, the invention may be practiced other than as specifically 
described.