A hydrophone interlink that connects hydrophone elements and allows a sin sensing fiber to transition between hydrophone elements. The interlink has an outer-structure and a contained element. The outer-structure connects the hydrophone elements and has at least one turn such that the distance traveled along the turns exceeds the linear distance between hydrophone elements. The outer-structure material and shape allow temporary interlink stretching and compression during passes through handling sheaves, with memory to allow the interlink to return to its original shape. The outer-structure contains a groove on either end to transition the sensing fiber between the hydrophone elements and the contained element. The contained element is open cell foam that fills the hollow core of the outer-structure. The sensing fiber transitions from a first hydrophone element to immediately enter the feed at the interlink first outer-structure end, whereupon the fiber transitions to the open cell foam and follows the interlink outer-structure structure while remaining on the foam.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates generally to connecting hydrophone elements 
in linear arrays, and more particularly to a means for connecting 
hydrophone elements to pass a sensing material, such as optical fiber, 
from one hydrophone element to another without adversely affecting the 
sensing material or measurement sensitivity. 
(2) Description of the Prior Art 
Hydrophone is a generic term describing a transducer that detects or 
monitors underwater sound. Hydrophones are typically pressure-actuated 
sensors and form the basis of sonar systems. Fiber-optic hydrophones 
employ fiber-optic cabling to sense pressure generated by acoustic 
wavefronts. Acoustic wavefront pressure produces measurable phase 
differences in the light waves guided by optical fiber. 
A fiber-optic hydrophone typically includes a hollow, air-backed element 
known as a mandrel, with optical fiber wound on the mandrel surface. The 
advantages of multiple, smaller, interconnected elements when compared to 
a single larger element, are presented in U.S. Pat. No. 5,317,544, and 
such advantages include increased sensitivity and system robustness during 
deployment. Although prior art discusses the need to connect multiple 
mandrel-wound hydrophones in series with a single fiber, the interlink's 
design and material is often ignored. U.S. Pat. No. 5,317,544 mentions a 
means for compliantly connecting adjacent hydrophone components, while 
U.S. Pat. No. 5,475,216 claims a neoprene spacer, and U.S. Pat. No. 
5,155,548 describes a spacer preferably formed of neoprene. Neoprene 
spacers or interlinks induce undesirable phase noise in the sensing fiber. 
There is currently not a hydrophone interlink that allows a hydrophone 
array to pass through large bends across small diameter handling sheaves 
during array deployment, without placing excessive stress on the interlink 
or sensing fiber; and, there is not an interlink that additionally couples 
the sensing fiber to a structure such that the fiber is impervious to the 
structure's mechanical resonances. 
What is needed is an interlink that is flexible during deployment, but 
during post-deployment (i.e., operation), ensures minimal fiber stretching 
from mechanical resonances of the interlink structure. 
SUMMARY OF THE INVENTION 
It is a general purpose and object of the present invention to provide a 
hydrophone interlink that connects two hydrophone elements while allowing 
a single sensing fiber to transition between the two hydrophones. It is a 
further object that such interlink be flexible during deployment to 
protect the sensing fiber as the interlink passes through small diameter 
handling sheaves. It is yet a further object that such interlink, during 
the post-deployment phase, ensures minimal sensing fiber stretching along 
the interlink from hydrophone array noise sources or the interlink 
structure's mechanical resonances, as such noise sources cause phase 
changes that interfere with hydrophone element signals. 
Other objects and advantages of the present invention will become more 
obvious hereinafter in the specification and drawings. 
The objects are accomplished with the present invention through a 
hydrophone interlink that allows a single sensing fiber to transition 
between hydrophone elements. The interlink has an outer-structure and a 
contained element. The outer-structure connects the hydrophone elements 
and has at least one turn such that the distance traveled along the turns 
exceeds the linear distance between hydrophone elements. The 
outer-structure material and shape allow temporary interlink stretching 
and compression during passes through handling sheaves, with memory to 
allow the interlink to return to its original shape. The outer-structure 
contains a groove on either end to transition the sensing fiber between 
the hydrophone elements and the contained element. The contained element 
is open cell foam that fills the hollow core of the outer-structure. The 
sensing fiber transitions from a first hydrophone element to immediately 
enter the feed at the interlink first outer-structure end, whereupon the 
fiber transitions to the open cell foam and follows the interlink 
outer-structure while remaining on the foam. At the second interlink end, 
the sensing fiber utilizes the feed at the interlink outer-structure 
second end to transition to a second hydrophone element. A groove may be 
created to guide the sensing fiber along the open cell foam.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown an interlink 10 with a helical 
spring outer-structure 12 for connecting hydroacoustic elements. FIG. 1 
represents an interlink connecting optical fiber hydrophone elements known 
as mandrels. The helical spring outer-structure 12 provides flexibility 
for bending in handling systems without damaging the interlink, with 
memory to return the interlink to the original shape after passing through 
the handling system. FIG. 1 also shows feeds 14, 16 at either interlink 
end. The interlink 10 additionally comprises a hollow, circular core that 
is formed by the helical outer-structure 12, the core being filled with 
open cell foam 18. At the interlink ends, the feeds 14, 16 transition the 
optical fiber 20 between the hydrophone elements 22, 24 and the open cell 
foam 18. 
FIG. 1 shows a first hydrophone hydrophone element 22 with optical fiber 20 
wound directly on the mandrel surface. With optical fiber 20 wound from 
left to right as shown, as the end of the first hydrophone hydrop hone 
element occurs, the optical fiber winding angle 26 changes to allow the 
interlink feed 14 to accept the optical fiber 20. The feed 14 transitions 
the optical fiber to the open cell foam 18 contained within the helical 
spring outer-structure 12, and the optical fiber 20 continues winding 
along the open cell foam 18. Although the optical fiber 12 follows the 
outer-structure 12 between the feeds 14, 16, the optical fiber 20 does not 
contact the interlink outer-structure 12 in the interlink region between 
the feeds 14, 16. In the interlink region between the feeds 14, 16, the 
optical fiber 20 contacts only the open cell foam 18. Although not 
indicated in FIG. 1, a groove may be created in the open cell foam 18 to 
guide the optical fiber 20 along the foam 18. 
As the second interlink end is encountered, the optical fiber 20 is wound 
directly from the open cell foam 18 into the outer-structure feed 16 at 
the interlink end connected to the second hydrophone element 24, and onto 
the second hydrophone element 24. The optical fiber winding angle 28 is 
again adjusted to achieve the desired fiber winding for the second 
hydrophone hydrophone element 24. 
For optical fiber acoustic applications, the interlink outer-structure 12 
can be constructed of a relatively non-compliant material such as 
polycarbonate. The outer-structure interlink material selection is 
application dependent and should provide stability to protect the sensing 
fiber within the feeds, flexibility to expand or contract when required, 
and imperviousness to undesired effects (e.g., response to acoustic 
pressure). By filling the otherwise hollow outer-structure with open cell 
foam 18 and winding optical fiber 20 along the foam 18, the fiber is 
decoupled from the more rigid outer-structure 12 that can impose undue 
strain on the fiber 20 due to the outer-structure's mechanical resonances. 
The combination of more rigid outer-structure 12 and contained open cell 
foam 18 therefore provides a desirable rigid connection between hydrophone 
elements 22, 24 without adversely affecting the optical fiber 20 that is 
wound on the open cell foam 18. 
Referring now to FIG. 2, there is shown a longitudinal cross-sectional view 
of FIG. 1 to detail the connections between the hydrophone elements 20, 22 
and the interlink 10. In the preferred embodiment shown in FIG. 2, the 
interlink outer-structure 12 is connected to the hydrophone elements 22, 
24 at either end, while the open cell foam 18 is not connected to the 
hydrophone elements 22, 24. 
FIG. 2 displays a helical spring outer-structure 12 whose hollow interior 
internal circumference is slightly larger than the outer hydrophone 
hydrophone element circumference. The hydrophone element ends are inserted 
into the hollow interlink outer-structure ends and secured with epoxy or 
other bonding agent or process. Alternately, depending upon the interlink 
material and design, screws may be used to affix the interlink 
outer-structure 12 to the hydrophone elements 22, 24. The open cell foam 
18 is not secured to the interlink outer-structure 12 or the hydrophone 
elements 22, 24 and maintains its positioning using the interlink 
outer-structure 12 and hydrophone elements 22, 24 for support. 
The interlink of FIG. 1 and FIG. 2 is compatible with continuous fiber 
winding and assembly as the optical fiber 20 is wound directly from a 
first hydrophone element 22, through the interlink feed 14 to the open 
cell foam 18, across the open cell foam 18, through the second interlink 
feed 16, and to the second hydrophone element 24, with only a change in 
winding angle. The outer-structure 12 single piece construction 
additionally simplifies automated assembly and reduces material costs. The 
helical spring outer-structure 12 also provides offsetting stretching and 
compression effects while the interlink bends. 
The advantages of the present invention over the prior art are that: The 
present invention provides an interlink that rigidly connects hydrophone 
elements, yet provides sufficient flexibility during deployment for large 
bends across small diameter handling sheaves; however, the sensing fiber 
is not coupled to a rigid structure, and therefore the sensing fiber is 
not subjected to a rigid interlink's undesirable mechanical resonances. 
What has thus been described is an interlink for connecting hydrophone 
elements that allows a single sensing fiber to transition from a first 
hydrophone element, across the interlink, to a second hydrophone element, 
while providing a secure connection between hydrophone elements and 
decoupling the sensing fiber from rigid interlink material. The interlink 
has an outer-structure and a contained element. The outer-structure has a 
first end connected to a first hydrophone element, and a second end 
connected to a second hydrophone element. Between the outer-structure ends 
is at least one turn, and the distance traveled along the interlink turns 
is greater than the linear distance between the outer-structure ends. The 
outer-structure contains open cell foam in its hollow core, and the 
outer-structure contains a feed at either end to transition sensing fiber 
between the open cell foam and the hydrophone elements. Sensing fiber 
transitions from a first hydrophone element, into a feed embedded in the 
first outer-structure end, and onto the foam. The sensing fiber continues 
winding along the foam until the second interlink end is reached, 
whereupon the feed at the second outer-structure end transitions the 
sensing fiber to the second hydrophone element. The interlink turns and 
material provide flexibility for large bends through small handling 
sheaves, and elasticity to return to original form. A groove may be 
created in the foam to guide the sensing fiber. The sensing fiber is 
decoupled from the rigid interlink structure and its undesirable 
mechanical resonances. 
Obviously many modifications and variations of the present invention may 
become apparent in light of the above teachings. For example: Interlinks 
may have many designs, and although a helical spring was shown, various 
shapes may be used. There are many spring types and springs of the same 
type have different spring constants or stiffness. The interlink 
outer-structure may have a cross-section that is not round. Foams other 
than open cell foam may be contained within the outer-structure, and the 
foam may optionally be secured to the hydrophone elements or the 
outer-structure. The sensing fiber may be guided along the foam with a 
groove. The interlink may have any number of turns, depending upon the 
application and desire for flexibility. The interlink may be connected to 
the hydrophone elements in a variety of manners. Although the application 
shown included fiber-optic hydrophones, the same interlink may be used to 
connect hydrophone elements other than fiber-optic elements, where 
flexibility, elasticity, and the other interlink characteristics are 
desired. Multiple interlinks can connect multiple hydrophones in series. 
In light of the above, it is therefore understood that within the scope of 
appended claims, the invention may be practiced otherwise than as 
specifically described.