Note: Descriptions are shown in the official language in which they were submitted.
P/8699~P48/CA
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Improvements Relatin~ to Transducers
This invention relates to transducers (e.g.
hydrophones) and relates more specifically to transducers
comprising optical fibres forming part of an array.
Arrays o~ large area hydrophones require large numbers
o~ individual transducers.
Large area hydrophones are currently in use or
proposed for sonar systems which need large filled apertures
and which required good rejection at high acoustic
wavenumbers in order to reject flow noise~
Optical flbre transducers for use in hydrophones are
known, for example in~our co-pending patent application No.
GB2126820A.
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This invention was intended to provide a transducer
elQment useable in an array. Although GB2126820A d als
mainly with multiplexed systems it was intended to produce
an element useable in any system whether multiplexed or not.
The invention provides a transducer comprising a
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plurality of coils of optical fibre optically linked to form
a single light path and encapsulated together within a solid
material.
This provides a transducer element with improved
rejection at high acoustic wavenumbers.
; The invention will now be descri*ed by way of example
only with reference to the accompanying diagrammat:ic figures
in which~
Figure 1 shows a known transducer element;
~: Figure 2 shows an array of transducer elements;
Figure 3 shows a first transducer element according to
the~invention;
Figure 4 shows a second transducer element according
to the:invention.
Referring to Figure 1 a known optical fibre pressure
~ sensor eLement is shown. This comprises a coil of optical
; ~: fibre 1 encapsulated with an encapsulating material 2 such
as an epoxy resin. Where the sensor is to be included in a
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hydrophone array extra encapsulating material can be added
to overpot the coil and form a square tile 3. Optical
signals can pass into the coil along ~ree end lA o~ the
optical fibre 1, and out of the coil along free end lB of
the optical fibre 1. As is well known, such a sensor
operates by changes in acoustic pressure altering the
optical path length of the fibre coil and thus altering the
time taken ~or light to pass through the coil.
Such a sensor element can be produced by winding the
optical fibre onto a mandrel and then removing the mandrel
and encapsulating the coil in epoxy resin.
Although a hydrophone array can be formed by a
plurality of such sensor elements linked in optical series
they give poor rejection at high acoustic wavenu~bers.
A typical array of hydrophone sensor elements 3 is
shown in Figuxe 2 where a plurality of sensor element 3 are
towed~behind a ship 14 on a buoyant cable 15. In order to
reduc~ the loads on the cable 15 the hydrophones 3 are
constructed to be neutrally buoyant. The cable 15 carries a
fibre op~ic cable to link all of the coils 1 as well as
securing the sensor elements 3 to the ship 14.
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It has been realised that the rej~ction of low noise
by a pressure sensor element could be i.mproved by replacing
the single coil with several smaller coplanar coils as shown
in Figure 3~
Four coils 4 are arranged in a coplanar group with
their axes of symmetry 4E parallel to form a single sensor
element. The coils 4 are arranged in a sguare array in a
:plane perpendicular to their axes of symmetry 4E. Each coil
4 consists of a coiled optical fibre 4A encapsulated by an
epoxy resin 4B~ Of course another suitable encapsulating
material could be used instead of epoxy resin. All ~our
coils 4 are linked to a splicer box 5 by free ends o~ their
respective coiled optica} fibre 4A. A free end 4C of one o~
the coiled optical fibres 4A in~one o~ the coils 4 acts as
an optical ~input for the sensor element, and a ~ree end 4D
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: ~ ` of the coiled optical fibre 4A in another one of the coils 4
: a~ts as an optical output for the sensor element.
Al1 four coils 4, the splicer box 5 and a11 of the
free ~ibre ends are encapsulated in suitable material 6 to
form a ~quare tile 7.
The input and output fibre ends 4C and 4D are each
covered by a layer of armour 8 to protect them, the ends 8A
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of this armour g are also encapsulated by the elastomeric
material 6.
Tha splicer box 5 contains couplers to link all four
coils 4 together optically in series and allows any strain
acting on the tile 7 to be relieved without placing strain
on the optical fibres 4A by allowing relative movement of
~ibres 4A and a reflective splice as described in
GB2126820A. Alternatively the reflective splice could be
incorporated outside the structure shown.
It has also been reaLised that another method of
improving the rejectîon of flow noise: would be to replace
the siDgle: coil: with a plurality of paraLlel coils:as shown~
,
in~Figure 4.
: Four :coils: 10 are arranged side by side with their
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axes of symmetry lOF ~ in paralLeL ~and coplanar to form a
single sensor element.
All four coiLs 10 are formed by a single optical fibre
lOC which:links~:them into~a slngle optical struc~ure and has
a first free end lOD ~orming:an optical input line, and a
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second free end loE forming an optical output lin , both
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; ~ free ends lOD and lOE are protected by armoured sheaths 11.
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The coils 10 and sheath 11 are encapsulated in an
acoustically compatible material 12 to form a square tile
~3.
Although four coils are shown in both examples any
number could be used.
The coils in the example of Figure 4 can be formed by
winding an optical fibre onto a single mandrel and then
splittiny the mandrel to produce four separate coils linked
by a single fibre.
The use or not of a splicer box is an option whose
desirability depends on the application for which the
hydr~phone is to be used, thus the example in Figure 4 could
use separate coils linked~by~a splicer box, similarly the
example shown in Figure~3 could be formed by one continuous
fibre by winding an optical fibre onto a mandrel as
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described above.
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In all cases the mandrels could be left within the
coils and enoapsulated with them.
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The systems shown are transmissive with separate input
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and output fibres, the invention would be equally applicable
to reflective systems having a single input and output
~ibre.
In all cases the encapsulating material forming the
square tiles 13 could contain internal sti~ening to ensure
that all resonance frequencies of the structure are ~utside
the operating band of the hydrophone system~
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