ALL-OPTICAL HYDROPHONE THAT IS NOT SENSITIVE TO
TEMPERATURE OR STATIC PRESSURE
The invention relates to the general field of underwater acoustic
transducers or hydrophones. It relates more particularly to optical fiber laser
based optical hydrophones.
5 As regards the production of underwater acoustic transducers, the use
of optical fibers is a known solution which exhibits recognized advantages
which include firstly the small bulk of a hydrophone produced using such
technology, as well as the possibility of producing an assemblage of
hydrophones on the basis of one and the same fiber by multiplexing on this
10 same fiber the pressure variation information detected by the various
hydrophones forming this assemblage, each hydrophone being associated
with a given wavelength.
However, in order to obtain sensitivity sufficient to be capable of
listening to a very low level of acoustic signal, typically a level below sea
15 state noise 0, on the Knudsen scale, it is known that it is necessary to amplify
the deformation of the optical fiber induced by the pressure wave by means
of an appropriate acousto-mechanical device.
There currently exist two main classes of optical fiber hydrophones
capable of listening to a noise level below sea state noise 0 and therefore
20 potentially usable in hydrophone arrays or in antennas using hydrophones for
underwater applications: optical fiber coil interferometric hydrophones and
optical fiber laser cavity hydrophones.
In optical fiber coil interferometric hydrophones, the optical quantity
25 measured is the optical phase variation aggregated over the total length of an
optical fiber (20 to 100m typically), coiled on a compliant mandrel configured
in such a way that its diameter varies under the effect of the acoustic
pressure to be measured, thereby inducing a dynamic variation in the length
of the optical fiber wound thereon, an air mandrel for example.
30 The dynamic variation in the length of the optical fiber is manifested by
a variation in the phase of the signal transported by the fiber. Thus, for a
standard single-mode silica optical fiber, it may be shown that the relative
variation of the phase of the signal is equal to about 0.78 times the relative
deformation of the length of the fiber. This phase variation may be measured
with the necessary precision by placing the hydrophone in the arm of an
unbalanced optical fiber interferometer of Michelson type for example.
5 This type of hydrophone exhibits the main drawback of not being able
to be sufficiently miniaturized. Indeed, in order to transmit the information
relating to the pressure measurements carried out by each hydrophone
constituting an array, various interrogation and multiplexing techniques well
known to the person skilled in the art may be implemented. Their object is to
10 make it possible to link, with the aid of a single optical fiber, all the associated
hydrophones within one and the same array to the system charged with
utilizing these measurements. However, the multiplexing on a single optical
fiber of a large number of hydrophones requires that their insertion losses
along the optical fiber be low, typically less than IdB. This implies that, in
15 each hydrophone, the optical fiber is coiled on a mandrel exhibiting a
minimum diameter, so as to limit to the minimum the losses related to the
curvature of the fiber. Now, even by using microstructured optical fibers with
a very low admissible radius of curvature (Fiber for FTTH applications), good
performance in terms of sensitivity and insertion loss are difficult to achieve
20 with a winding diameter of substantially less than 15 or 20mm.
Moreover, insofar as each hydrophone constituting the array
measures a simple pressure value, it is necessary to introduce into the
hydrophone array, one or more optical fiber coil reference sensors, not
subjected to the acoustic pressure, so as to circumvent by subtraction the
25 variations in the intrinsic sensitivity of the optical fiber coil to static pressure
and to temperature. This procedure exhibits the drawback of having to add
further sensors and of limiting the domain of use of the hydrophone in
immersion (limited dynamic range of the sensor).
30 Optical fiber laser cavity hydrophones make it possible to produce
hydrophones of high sensitivity, that is to say capable of listening to low-level
sea noise. Their implementation makes it possible to employ techniques of
coherent optical processing to precisely measure the frequency variations of
the optical signal conveyed by the fiber. Such a variation is here consequent
35 upon the reception of an acoustic signal by a Bragg grating inscribed in an
active optical fiber, an Erbium or ErbiumNttrium doped optical fiber for
emission around 1.5 pm for example.
In a known manner a Bragg grating includes, in proximity to its center,
a phase jump of substantially equal to n so as to constitute a monofrequency
5 laser cavity, the optical pumping of the cavity being achieved by way of a
diode (pumping diode) which may be sited remotely a long distance away via
a standard optical fiber. The emission frequency of the laser cavity thus
constituted depends on the spacing of the Bragg grating and the central
phase of the phase shift. Therefore, if the laser cavity is uniformly deformed,
10 the emission frequency of the laser will vary in the same proportions with a
coefficient of 0.78 related to the elasto-optical properties of the silica
constituting the fiber.
It is thus possible to wavelength multiplex several hydrophones having
laser cavities exhibiting different operating wavelengths, lying in the
15 amplifying band of the doping material of the active optical fiber: about 40
hydrophones, for example, in the case of an Erbium doped fiber, for
wavelengths lying on the 100GHz ITU grid of band C of Erbium.
However, the axial deformation induced by an acoustic pressure
applied directly to the external surface of the laser cavity is not sufficient to
20 obtain hydrophone sensitivity compatible with a level of less than sea state
noise O., having regard to the PSD (power spectral density) of the intrinsic
noise of the laser (of the order of 20 H Z /to ~Ik H z) which limits the noise
of the interrogation system. It is indeed possible to show that it is necessary
to have a hydrophone sensitivity of greater than at least 100 or 110 dB HzIPa
25 in order to be capable of listening to a sea state noise 0 on the Knudsen
scale in a band of about 10Hz to 10 kHz.
Therefore, it is necessary to amplify the deformation of the cavity
induced by the acoustic pressure by means of an acousto-mechanical
device. Having regard to the intrinsic sensitivity of the laser cavity to a
30 deformation the objective to be achieved for the acousto-mechanical device
is of the order of a nanostrainlpa with a stable response in a broad frequency
band. Several known configurations make it possible to achieve these
values. Nonetheless, in all these configurations the hydrophone retains an
intrinsic sensitivity to temperature at least equal to that of the laser cavity
4
together with sensitivity to static pressure, without being associated with a
voluminous hydrostatic filter.
Thus, in the current state of the prior art, no technical solution
5 belonging to one or the other of these two classes makes it possible to
ensure, in an intrinsic manner, the insensitivity of an optical fiber hydrophone
to variations in static pressure and to variations in temperature. Now, the
operation of a hydrophone under varied environmental conditions (variation
of immersion and of temperature, acceleration), is actually possible only if its
10 behavior is actually independent of immersion depth, stated otherwise of
static pressure, of temperature and of accelerations.
Moreover, in the case of arrays of sensors it is also very important to
have acoustic sensors exhibiting- stable interrogation wavelengths, so as to
be able to access the various sensors via a single optical fiber by
i s multiplexing and to carry out the wavelength multiplexing and demultiplexing
of the information regarding pressure transmitted by the various sensors by
simply using optical fiber passive components.
An aim of the invention is to propose a structure making it possible to
20 produce an optical hydrophone which is intrinsically insensitive to variations
in static pressure and in temperature, and very good hydrophone sensitivity.
Another aim of the invention is to propose a structure exhibiting a low
volume. Yet another aim is to propose a structure making it possible to
produce an optical hydrophone capable of operating satisfactorily in a broad
25 frequency band and up to very great immersions with a very low sensitivity to
accelerations.
For this purpose the subject of the invention is a laser optical cavity
hydrophone of the type comprising a Bragg grating optical fiber element
forming the laser cavity. Said hydrophone comprises a mechanical structure
30 forming a cavity filled with a fluid, and inside which the optical fiber element is
placed along the longitudinal axis of the cavity, the mechanical structure
furthermore comprising:
- a substantially cylindrical hollow rigid body forming the cavity inside
which is placed the optical fiber element;
- two end caps configured and designed to seal the ends of the rigid
body and traversed by the optical fiber element, said optical fiber element
being fixed to the end caps at the level of the points of traversal so as to be
permanently under tension.
5 According to the invention, the two end caps are configured, having
regard to the material from which they are made, so as to exhibit a
longitudinal deformation when they are subjected:
- to variations in the pressure exerted by the exterior medium in which
the hydrophone is immersed; the deformation of the end caps giving rise to a
10 variation in the length of the opticai fiber element;
- to variations in the temperature of the exterior medium in which the
hydrophone is immersed; the deformation of the end caps giving rise to a
variation in the length of the optical fiber element which compensates
variations of the emission frequency of the laser cavity as a result of these
15 temperature variations.
- a through orifice configured to allow communication between the
cavity and the exterior medium and to achieve equilibration of the static
pressure between the exterior medium and the fluid contained in the cavity.
20 According to a preferred embodiment, the rigid body and the two end
caps are configured in such a way that the hydrophone exhibits symmetry
planes arranged so as to minimize its sensitivity to parasitic accelerations.
According to a particular embodiment, the dimensions of the rigid
25 hollow body are defined in such a way that the mechanical cavity formed can
accommodate the optical fiber element and that, having regard to their
dimensions, the end caps undergo, under the action of the dynamic
variations in the pressure exerted by the exterior medium, a deformation
sufficient to induce on the optical fiber element a variation in length making it
30 possible to obtain the desired sensitivity of the hydrophone to dynamic
variations in pressure.
According to another particular embodiment, the dimensions, length
and cross-section Shale, of the through orifice are determined in such a
35 way that, having regard to the viscosity of the fluid contained in the
hydrophone cavity, the latter exhibits, having regard to its volume V,,"ity, a
law of variation of its sensitivity to pressure variations, as a function of the
frequency of these variations, exhibiting a given span of frequencies of
, variation of the pressure a substantially constant sensitivity, this span of
5 frequencies being situated before the frequency F2 corresponding to the
mechanical resonance peak of the cavity.
According to a variant of the previous embodiment where the fluid
contained in the cavity is hardly if at all viscous, the dimensions of the
10 through orifice are determined in such a way that, the span of frequencies of
variation of the pressure for which the sensitivity of the cavity of the
hydrophone is substantially constant lying between the frequency F2 and a
resonance peak (51) at a frequency F1 equal to the Helmholtz frequency fH
of the cavity defined by:
where c represents the velocity of the acoustic waves in the fluid
considered,
the frequency fH is as low as possible.
20 According to another variant of the previous embodiment where the
fluid contained in the cavity is viscous, the dimensions of the through orifice
are determined in such a way that, the span of frequencies of variation of the
pressure for which the sensitivity of the cavity of the hydrophone is
substantially constant lying between the frequency F2 and a frequency F1
25 equal to the cutoff frequency f, of the cavity defined by the relation:
where p represents the density of the fluid, c the velocity of the sound waves
in the fluid and Rhole the radius of the orifice,
the frequency fp is as low as possible.
30
According to a particular embodiment, the two end caps closing the
cavity constituted by the rigid body have a hollow cylinder shape open at one
end, said cylinder, configured to be inserted into the cavity so as to seal the
end thereof and exhibiting a substantially cylindrical side-wall and an elastic
end-wall in the shape of a disk, perpendicular to the longitudinal axis of the
cavity, and sealing the end of the end cap which is inserted into the
cylindrical body, the elastic end-wall being configured to deform under the
5 action of the variations in the pressure exerted by the exterior medium; the
side-wall comprising at least two segments:
- a first segment B for which the thickness of the material constituting
the wall is determined in such a way that the side-wall of the end cap comes
into tight (leaktight) contact with the internal wall of the rigid body, so as to
10 ensure a rigid link between the rigid body and the end cap;
- a second segment C for which the thickness of the material is
defined in such a way that the side-wall of the end cap can deform freely in
the longitudinal direction under the action of the temperature variations.
15 According to a variant of the previous embodiment, the side-wall of the
end cap furthermore comprises an end-segment A whose thickness defines a
shoulder which abuts the end of the rigid body when the end cap is put in
place on the latter.
20 According to a particular embodiment, the dimensions of the cavity
and the nature of the materials constituting the rigid body and the end caps
are determined so as to obtain the highest possible frequency F2.
According to a particular embodiment, the cavity accommodating the
25 optical fiber element contains an open foam, said foam being soaked with the
fluid contained in the cavity, the material constituting the foam being defined
so as to damp the natural resonant modes of the optical fiber element.
According to a particular embodiment, the through orifice is a circular
30 orifice made through the wall of the rigid body.
According to another particular embodiment, the through orifice is a
circular orifice made longitudinally in the thickness of the side-wall of an end
cap.
35
According to another particular embodiment, the rigid body is made of
titanium or glass.
According to another particular embodiment, the end caps are made of
5 PPO plastic or of polyoxymethylene (POM) or else of polyformaldehyde.
According to another particular embodiment, the rigid body or the end
caps being made entirely or in part from a porous material, the through orifice
consists of the pores of the material.
10
The characteristics and advantages of the invention will be better
appreciated by virtue of the description which follows, description of a
particular, exemplary embodiment not limiting the scope of the invention,
which is supported by the appended figures which represent:
15
- Figure 1, a schematic illustration presenting the general structure of
the hydrophone according to the invention;
- Figure 2, a schematic illustration presenting the general structure of
the end caps constituting the ends of the hydrophone according to the
20 invention;
- Figures 3 and 4, illustrations presenting the behavior of the
hydrophone according to the invention under the action of the dynamic
variations in the pressure exerted by the exterior medium;
Figure 5, a curve presenting the form of the law of variation of the
25 sensitivity of a hydrophone according to the invention as a function of the
frequency of variation of the pressure exerted by the exterior medium;
- Figure 6, a curve presenting the form of the variation of the sensitivity
of the hydrophone according to the invention as a function of the
compressibility of the fluid contained in the cavity of the hydrophone;
30 - Figures 7 and 8, illustrations presenting the structure of the
hydrophone according to the invention in a particular embodiment.
The subsequent description presents the characteristics of the optical
hydrophone according to the invention, through a preferred embodiment,
35 taken here as exemplary embodiment not limiting the scope or extent of the
invention. The structure of this particular embodiment as well as the account
of its operating principle is notably illustrated by Figures 1 to 6.
The hydrophone according to the invention comprises a protective
5 mechanical structure defining a cavity 11 inside which a Bragg grating optical
fiber element 12 is placed. In addition to a protective function, this cavity also
has the function, in a known manner, of amplifying the axial deformation
imposed on the optical fiber element 12 by the dynamic pressure exerted by
the exterior medium.
10
In a known manner, the optical fiber element 12 constituting the Bragg
grating optical fiber monofrequency laser cavity, which forms the sensitive
part of the hydrophone, fulfills three distinct functions:
- a pressure sensor function utilizing the variation in emission
15 frequency of the laser cavity associated with an axial elastic deformation, by
stretching and/or retraction, of the optical fiber cavity which constitutes the
main element of the sensor;
- a function of transmitting the luminous signal carrying the
measurement;
20 - a multiplexing function by virtue of the wavelength selectivity given by
the Bragg grating constituted by the optical fiber element 12, the active part
of the hydrophone.
Hereinafter in the text, insofar as the characteristics of the optical fiber
monofrequency laser cavity used are in the known domain, the technical
25 characteristics of the mechanical structure forming the cavity 11 of the
hydrophone according to the invention enclosing the optical fiber element 12
are more particularly described.
From a general point of view, as illustrated by Figures 1 and 2, the
30 optical hydrophone according to the invention thus consists of a mechanical
structure comprising a rigid body of cylindrical shape 13 closed on each side
by an end cap 14 or 15.
Generally within the framework of the invention, the notion of cylinder
is considered in its most general definition. Here we consider the
35 mathematical definition of the cylinder, the latter being able for example to be
a right prism in the case of a polygon-shaped base or of a cylinder of
revolution in the case of a circular base.
- To minimize the sensitivity to parasitic transverse accelerations, it is
however preferable to use a cylinder of revolution for its axisymmetry which
5 confers a certain insensitivity to accelerations of this type. Moreover, on
account of the existence of a symmetry plane of the cylinder, perpendicular
to the fiber, the use of a cylinder of revolution also makes it possible to
minimize the sensitivity to parasitic axial accelerations.
10 In a preferred embodiment, the rigid body is therefore an axisymmetric
right cylinder whose generating straight line is the axis of the optical fiber.
Alternatively, in another embodiment, the rigid body is a hyperboloid or
a generalized ellipsoid. Moreover, as in the case of the cylinder of revolution,
15 an axisymmetric geometry makes it possible to minimize the sensitivity to
parasitic transverse accelerations.
Thus, if we define the orthonormal frame (Oxyz) where 0 is the center
of the fiber and (Oz) is the axis of the fiber, the generic equation which
defines a hyperboloid or an ellipsoid of revolution is then obtained:
20
x2 + y2 = f (z)
with f is a strictly positive function.
25 It is then possible to minimize the sensitivity to parasitic axial
accelerations if z = 0 is a symmetry plane. Therefore, the function f must be
an even function.
The end caps 14 and 15 are configured so as to form two pistons
30 located at the two ends of the cylindrical body whose displacement,
deformation, under the action of the variations in the pressure exerted by the
medium, causes the volume of the cavity 11, as well as the length of the
Bragg grating optical fiber element 12 housed in this cavity, to vary.
The optical fiber element 12, forming the active part of the
35 hydrophone, is placed along the central axis of the cylindrical body 13
defining the cavity 11. Thus it enters and exits the cavity 11 by passing
through the walls of the end caps 14 and 15, to which walls it is fixed in such
a way that when the hydrophone is assembled, it undergoes a pre-tension. In
this way, the deformation (the displacement) of the end caps 14 and 15
5 under the action of the pressure exerted by the medium gives rise
systematically to a variation in the length of the optical fiber element 12
enclosed in the cavity 11.
The device according to the invention also comprises one or more
through orifices 16 whose function is to place the interior medium contained
10 in the cavity 11, delimited by the cylindrical body 13 and the two end caps 14
and 15, permanently in contact with the exterior medium. The orifices 16
advantageously allow the pressures prevailing inside and outside the cavity
11 to balance so that the device is thus insensitive to static pressure.
Therefore the static pressure being identical inside and outside the cavity 11,
15 the pistons formed by the end caps 14 and 15 do not experience its
influence. Therefore, the Bragg grating optical fiber element 12 does not
experience on the part of the end caps any stress consequent upon the static
pressure exerted on the latter.
According to the invention, the dimensional characteristics of the
20 cavity 11 formed as well as the size and the arrangement of the end caps 14
and 15 and communication orifices 16 are determined so as to carry out the
following functionalities:
- sufficient mechanical amplification of the deformation caused by the
dynamic pressure exerted by the exterior medium on the optical fiber element
25 12, to achieve the hydrophone sensitivity aimed at;
- low-pass hydrostatic filtering intended to place the laser cavity,
consisting of the optical fiber element 12, in equi-pressure with the pressure
of the medium surrounding the hydrophone, for the pressure variations of
frequency below a given cutoff frequency;
30 - compensation of the variations in the emission frequency of the
optical fiber laser cavity 12 with temperature;
- the symmetry planes of the device as a whole make it possible to
minimize the sensitivity to parasitic accelerations.
Therefore, according to the invention, the cylindrical body 13, consists
of a cylindrical tube made of a rigid material rendering it hardly deformable
under the effect of pressure.
In a preferred embodiment, it is made of titanium, titanium being
5 chosen for its rigidity and its low thermal expansion. Alternatively it can also
be made of any appropriate rigid material, compatible with the filling fluid.
Of rigid structure, the cylindrical body 13 thus advantageously takes
only a negligible part in the net deformation. It thus takes no part whatsoever
in the amplification of the deformation exerted by the structure on the optical
10 cavity constituted by the optical fiber element 12, this amplification being
ensured by the end caps 14 and 15.
The thickness E of the wall of the cylindrical body 13, generally
constant, is also defined so as to ensure the necessary rigidity having regard
15 to the maximum pressure that the body has to undergo. Its length L is for its
part imposed by the length of the optical fiber portion that it has to
accommodate, this length being, preferably, sufficient such that the optical
fiber element 12 is fixed to the end caps 14 and 15 in a zone 18 or 19 of
simple luminous conduction, which does not correspond to the Bragg grating
20 forming the laser cavity, the Bragg grating, of length I,, thus not being fixed
directly to the end caps 14 and 15 by its ends.
The interior diameter D of the cylindrical body 13 is for its part defined
as a function of the dimensional characteristics imposed on the end caps 14
25 and 15 so as to fulfill their function of amplification and compensation for the
effects of temperature variations on the behavior of the optical cavity formed
by the optical fiber element 12.
According to the invention, the end caps 14 and 15 are, for their part,
30 configured to fulfill various functions:
- a conventional function of sealing the cylindrical body 13 the object of
which is to hermetically close the cavity 11 in such a way that the interior
medium communicates directly with the exterior medium only via the orifices
16 made in the structure of the hydrophone;
- a function of keeping the optical fiber element housed in the cavity 11
tensioned, in particular the element 12 which forms the laser cavity;
- a function of modulating the tension applied to the optical fiber
element, as a function of the variations in the temperature of the medium;
5 - a piston function making it possible to vary the length of the optical
fiber element 12 housed in the cavity 11 under the action of the dynamic
pressure variations exerted on the hydrophone, a pressure increase being
manifested by a displacement of the end caps, or at least of the internal end
of the end caps, toward the interior of the cavity 11 and by a decrease in the
10 tension imposed on the optical fiber element 12 and a pressure reduction
being manifested by a reverse displacement and by an increase in the
tension imposed on the optical fiber element 12.
Therefore, to fulfill these functions, various embodiments of the end
15 caps 14 and 15 are conceivable, in particular embodiments implementing
end caps exhibiting a part, rigidly tied to the cylindrical body 13, intended to
keep the end cap in place on the latter, and a part which is mobile or free to
deform being able to move inside the cavity, toward the center of the cavity
or toward its end, while entraining in its motion the optical fiber element 12
20 which is attached to it.
For this purpose the end caps 14 and 15 can therefore, for example,
consist, at least in part, of an elastic material, a substantially less rigid
material than the material constituting the cylindrical body 13, of PPO plastic
of NorylG3 type for example or else of polyoxymethylene (POM) or of
25 polyformaldehyde, of DelrinG3 type for example.
Figure 2 presents the structure of the end caps implemented in the
preferred embodiment taken as example.
In this particular embodiment each end cap 14 or 15 is formed by a
30 hollow body comprising a wall 21. This wall 21 defines a cylindrical, in the
mathematical sense of the term, cavity 23. In the case where this cylindrical
cavity is a cylinder properly speaking, it exhibits a constant internal diameter
d. The cylindrical cavity 23 exhibits an axis of symmetry coincident with that
of the cylindrical body and of an elastic wall D 22, for example disk-shaped,
35 perpendicular to the axis of symmetry of the cylindrical cavity 11 and sealing
one of the ends of the cavity 23, the other end being open. The end cap thus
constituted is intended to be inserted into the cavity 11 by its end sealed by
the wall 22. The hollow body and the wall 21 are, for example cylindrical
bodies, in the mathematical sense of the term.
5 According to this embodiment, the wall 21 is a wall whose nonconstant
thickness essentially defines two distinct segments B and C for
which the end cap exhibits distinct exterior diameters. However, this wall
could also comprise just the free segment C.
The wall 21 of the end cap therefore comprises a first segment B
10 whose thickness is defined in such a way that when the latter is inserted into
the cavity 11 of the hydrophone its wall, at the level of the segment B, is
placed in tight, preferably leaktight, contact with the internal wall of the
cylindrical body 13, by tight keying for example, in such a way that the wall of
the end cap is kept in a fixed position in the cavity 11 of the hydrophone at
15 the level of the segment B.
It also comprises a second segment C whose thickness is smaller than
that of the segment B. In this way, the segment C not being in contact with
the internal wall of the cylindrical body 13, it appears free to expand or to
contract longitudinally, as a function of temperature variations notably.
20
According to a particular embodiment, the wall 21 can further comprise
an end-segment A whose thickness defines a shoulder which abuts the end
of the cylindrical body 13 when the end cap is put in place on the cylindrical
body 13.
25
It should be noted that the end caps can as a function of the geometric
shape of the rigid body 13 be of cylindrical, hyperboloid or ellipsoid shape on
condition however that they exhibit the 3 segments: the segment B in tight
and leaktight contact with the rigid body, the segment C free to expand with
30 temperature variations and whose projected length on the axis of the fiber is
equal to x and the wall 0, 22 perpendicular to the fiber.
Stated otherwise, the end cap comprises a part rigidly tied to the body
13. This part rigidly tied to the cylindrical body 13 comprises the parts A and
B but could equally well be made differently provided that the end cap is
35 rigidly tied to the cylindrical body 13.
The part of the end cap free to deform being able to move inside the
cavity, entraining in its motion the optical fiber element 12 comprises the part
C which is free to expand or to contract longitudinally as a function of
temperature variations. This part also comprises the membrane D.
5
Each of the two end caps must in fact exhibit a part rigidly tied to the
body 13 and a part which is mobile with respect to the body 13 comprising a
deformable wall able to be deformed when it is subjected to variations in the
pressure exerted by the exterior medium in which the hydrophone is
10 immersed; the deformation of the end caps giving rise to a variation in the
length of the optical fiber element 12. The deformable wall exhibits an
internal face 221 directed toward, that is to say facing, the interior of the
cavity and an external face 222 directed toward the exterior of the cavity. It is
arranged in such a way that its internal face 221 is mobile in translation with
15 respect to the rigid body 13 along the longitudinal axis of the cavity under the
effect of a variation in the temperature of the exterior medium in which the
hydrophone is immersed; giving rise to a variation in the length of the optical
fiber element 12 which at least partially compensates for the variations in the
emission frequency of the laser cavity as a result of these temperature
20 variations.
The material and the geometry of the mobile parts of each of the end
caps comprising the length of the mobile part of each of the end caps are
such that the deformation of the end caps under the effect of a temperature
increase gives rise to a decrease in the length of the fiber held between the
25 two end caps, the effect of which is to decrease or indeed cancel the
increase in the emission wavelength of the laser cavity due to the increase in
the optical index of the fiber under the effect of this temperature increase.
In the embodiment described previously the deformable wall is the
30 elastic wall 22. This wall is mobile in translation along the longitudinal axis
under the effect of temperature variations because the part C is able to
contract or expand freely in the longitudinal direction under the effect of
temperature variations, thereby giving rise to a displacement of the elastic
wall 22 in the longitudinal direction with respect to the body 13.
The end caps could be made differently. It would be possible to design
end caps exhibiting a part which is fixed with respect to the rigid body 13 and
a part which is mobile in translation with respect to the body 13 along the axis
of the cavity consisting of a deformable wall 22 perpendicular to the axis of
5 the cavity. The translational motion of the internal face 221 along the axis of
the cavity would then be engendered by the expansion and contraction of the
deformable wall.
From a dynamic point of view, the operation of the device according to
the invention comprises three states:
10 - A rest state, illustrated by Figure 1, for which the pressure inside the
cavity and the pressure of the exterior medium are identical. In this state, the
constraints exerted by the exterior medium and by the interior medium on the
end caps, on the walls 22 in particular, balance so that they do not undergo
any deformation (i.e. neither stretch nor compression).
15 - A first dynamic state, illustrated by Figure 3, for which the pressure
exerted by the exterior medium undergoes a fast increase giving rise to the
appearance of a resulting pressure, depicted by the arrows 31 in Figure 3.
The effect of this resulting pressure is to give rise to a deformation of the
elastic walls 22 of the end caps 14 and 15, which is manifested, in the case
20 of the preferred embodiment taken as example, by a bending Ax2 of the
elastic wall 22 of the end cap inside the cavity of the hydrophone. This
deformation, which induces a decrease in the volume of the cavity and the
consequence of which is to increase the internal pressure, continues until the
internal and external pressures balance again. In Figure 3, the initial position
25 of the end cap is represented by dashes.
- A second dynamic state, illustrated by Figure 4, for which the
pressure exerted by the exterior medium undergoes a fast decrease giving
rise to the appearance of a resulting pressure, depicted by the arrows 41 in
Figure 4. The effect of this resulting pressure is to give rise to a deformation
30 of the end caps 14 and 15 which is manifested, in the case of the preferred
embodiment taken as example, by a reverse bending Ax3 of the elastic wall
22 of the end cap. This deformation, which induces an increase in the volume
of the cavity and the consequence of which is to decrease the internal
pressure continues until the internal and external pressures balance again. In
35 Figure 4 the initial position of the end cap is represented by dashes.
According to the invention, the optical fiber element 12 is, as was
stated previously, fixed to the wall 22 of each of the end caps which closes
the cavity in such a way that, when the hydrophone is assembled, it
undergoes a pre-tension. The value of this pre-tension is defined in such a
5 way that when the end cap undergoes its maximum lengthening under the
action of an increase in the pressure of the exterior medium, which
lengthening is optionally aggregated, with the length variations due to
temperature, the optical fiber element 12 retracts but nonetheless remains
under the effect of a residual tension which keeps it rectilinear, under tension,
10 as illustrated by Figure 3. Thus when the resulting pressure varies between a
maximum value (positive) considered to the minimum value (negative)
considered while passing through a zero value (equilibrium of the pressures),
the optical fiber element 12 sees its length vary from a maximum value l3 to a
minimum value l2 while passing, at equilibrium, through a length II but always
15 remaining under tension, whatever the temperature considered in the
temperature range aimed at.
From a dimensional point of view, the device according to the
invention is defined so as to meet various requirements.
20 Thus, the length L of the cylindrical body 13 is determined, in a
preferred embodiment, at one and the same time by the length of the Bragg
grating optical fiber element 12 housed in the cavity, which length is imposed
by construction, and to a lesser extent, by the resonant frequency of the
cavity.
25
Thus again, the dimensions of the through orifice (of the orifices) 16,
orifice(s) put in place according to the invention so as to ensure the
insensitivity of the device to static pressure, are defined so as to have an
optimal span of frequency of use, that is to say a span of frequencies over
30 which the sensitivity of the device complies with the expected value.
In practice, the dimensions of the orifice 16 may be determined by
likening the latter to a cylinder of radius Rhole and of length lhole and by
considering the curve of variation of the sensitivity of a cavity to pressure
variationgas a function of the frequency of these variations.
As may be noted in the illustration of Figure 5, a cavity such as that
constituted by the device according to the invention, exhibits, under the effect
of variations in pressure and on account of the presence of the orifice 16 and
of the viscosity of the medium inside the cavity notably, one or two resonance
5 peaks 51 and 52 at frequencies fl and f2, these frequencies being defined at
one and the same time by the volume of the cavity and the dimensions (Rhole
and Ihole).
As shown by the two curves of Figure 5, two distinct behaviors at low
frequencies, that is to say faced with low-frequency variations of the pressure
10 exerted by the medium, are distinguished as a function of the viscosity of the
fluid contained in the cavity. The first behavior, illustrated by the dashed
curve, corresponds to that of only slightly viscous or non-viscous fluids, while
the second behavior, illustrated by the solid curve, corresponds to that of
viscous liquids. The first behavior is manifested by the presence of two peaks
15 51 and 52, while the second behavior is manifested by the presence of a
single peak 52.
As regards only slightly viscous fluids, the peak 51 indicates that the
behavior of the cavity at very low frequencies can be regarded as that of a
20 Helmholtz cavity. The resonance peak 51 appears for variations of low
frequencies in particular when the fluid inside the cavity is considered to be
non-viscous andlor when the hole is of relatively large diameter, of the order
of a mm for example. It is observed that, for these low frequencies, the cavity
has a behavior similar to a Helmholtz cavity and that the resonant frequency
25 corresponding to the peak 51 is close to the Helmholtz resonant frequency of
the cavity. Now, in the case of a Helmholtz resonator, the cavity is generally
modeled by a mass-spring system, the hole behaving as a mass and the
cavity playing the role of a stiffness. In accordance with this model, provided
that the cylinder is considered to be-rigid, the resonant frequency is then -
30 defined by the following relation:
Accordingly, the stiffness of a fluid filled cavity being defined, by
analogy with that of a beam, by the relation:
The expression for the Helmholtz frequency is:
10 As regards inore viscous fluids, the cavity at very low frequencies can
simply be regarded as a first-order system modeled by a Poiseuille flow
through the hole, this system satisfying the following relation:
15
in which Q represents the flowrate through the hole, r l the viscosity of the . .
fluid and AP the pressure difference between the interior and the exterior of
the cavity at the level of the hole.
The pressure inside the cavity then satisfies the differential equation:
20
dp n R h o l e 4 ~ 2 =
4 2 -+ n-RhOle PC
'flhole 'cavity 8~Lhole Gity
Po
The cutoff frequency f, of the system consisting of the hole, the through
orifice, is then defined by the relation:
25
As illustrated by Figure 5, in the case of a viscous fluid, an
intermediate frequency span is observed between the two resonance peaks
51 and 52, or between the frequency fp (not represented in the figure since it
is too close to zero for the scale used) and the frequency of the resonance
peak 52 for which the cavity exhibits a substantially constant non-negligible
sensitivity and in which the cavity may be used to serve as amplifier of the
5 inherent deformations of the fiber. Indeed, after the first peak 51, or the cutoff
frequency f, in the case of a viscous fluid, the orifice 16 behaves as if it were
sealed so that the variations in the exterior pressure are transmitted to the
interior medium only by the piston motions followed by the two end caps, or
more exactly by the elastic wall 22 of the end cap free to deform, which
10 motions are transmitted to the optical fiber element 12.
It should be noted that the second resonance peak corresponds here
to a purely mechanical resonance originating from the two end caps, the
frequency of this resonance therefore depending essentially on the geometry
I 5 of the end caps.
According to the invention, the geometry of the cavity is therefore
optimized to obtain a band of use which is as wide as possible. Stated
otherwise, the geometry of the cavity is optimized in such a way that the
20 frequency fl of the first peak, or the cutoff frequency f, in the case of a
viscous fluid, is as low as possible and that the frequency f2 of the second
peak is as high as possible.
Now, in addition to the parameters entering the expressions for fH and
f, (cf. relations 3 and 6), a certain number of parameters, such as the
25 geometric parameters of the cavity, of the piston and of the hole as well as
the material used for the piston or else the fluid inside the cavity, can alter
these resonant frequencies: Parametric studies conducted elsewhere on the
influence of the geometry of the hydrophone on the values of the two
frequencies, fl or fp on the one hand and f2 on the other hand, make it
30 possible to demonstrate diverse results.
It is thus noted that, for a fluid filling the cavity of given viscosity, the
diameter of the orifice 16 has no appreciable influence on the frequency f2 of
the second peak 52. On the other hand, it is noted that the frequency fl of the
first peak 51, just like the frequency fp, increases with the diameter of the
35 orifice. It is therefore advantageously possible to adjust the value of the
frequencies f l or fp, of the cavity by choosing the diameter of the orifice in an
appropriate manner.
It is thus also noted that, in contradistinction to what happens for the
orifice 16, the diameter of the cavity influences the values of the frequencies
5 fl or fp on the one hand and f2 on the other hand. It is noted more precisely
that the values of fl, or fp, and f2 vary in the same direction as a function of
the diameter of the cavity, toward values which are all the lower the greater
the diameter. It is thus advantageously possible to adjust the position of the
useful frequency span by choosing the diameter of the cavity in an
10 appropriate manner.
It is further noted that, as previously but in a lesser manner, the length
of the cavity simultaneously influences the values of the frequencies fl or fp,
and f2, the values of fl, or fp, and f2 varying toward values which are all the
lower the greater the length of the cavity.
15
It should be noted that other parameters can also influence, although
more slightly, the position on the frequency axis and the width of the
hydrophone's utilization band. It is possible to cite for example the elasticity
of the material used to make the end caps sealing the cavity of the device
20 according to the invention or else the nature of the fluid inside the cavity.
However, insofar as these parameters make it possible to adjust other
operating characteristics of the device, these are not taken into account in
adjusting the operating span of the device.
25 From the dimensional point of view also, the diameter, or more
generally the aperture, of the cylindrical body 13 determines the dimensions
of the end caps and in particular dimensions of the cylindrical wall 21 of an
end cap.
The dimensions of an end cap are, for their part, determined so as to
30 obtain the desired amplification of the longitudinal deformation imposed on
the optical fiber element by the pressure variations and to compensate for the
effects of the variations in temperature on the length I of the optical fiber
element.
In particular, the insensitivity of the device according to the invention to
35 temperature variations is achieved by altering the geometry of the end caps.
Indeed, the temperature variations cause on the one hand a variation in
index within the fiber 12 and on the other hand a variation in the length of the
end caps 14 and 15 from which the elongation or retraction of the fiber 12
originates. Therefore, a judicious choice of the dimensions of the end caps,
5 for a given end cap material, makes it possible to compensate exactly or
substantially for the variation in the emission frequency of the optical fiber
laser cavity 12 under the action of the temperature by varying the length of
this optical fiber element as a function of temperature.
Thus, for a temperature variation AT, the variation in the wavelength h
10 due to the variation in the index n of the optical fiber element 12 is defined by
the relation:
15 Now, the variation in the wavelength h as a function of the variation in
the length L of the fiber is defined by the relation:
20 Therefore, to compensate for the variation in index, the variation in the
length Lfibefro r a temperature variation AT must give rise to a variation in the
wavelength h opposite to the variation caused by the variation in the index n.
The variation in the length Lfiber is therefore thus defined by the following
equality:
25
-ufiber -- --.- 1 dn AT.
Lfiber Kn dT
Now, as regards the device according to the invention, if it is considered that
the cylindrical body 13 defining the cavity is rigid and hardly sensitive to the
30 temperature variations considered, it is advantageously possible to express
the elongation ALfiberu ndergone by the optical fiber element 12 housed in the
cavity, which element is secured to the end caps 14 and 15, as a function of
the variation in length of the end caps under the effect of the temperature
variation AT and therefore to dimension the end caps in an appropriate
manner so that their variation in length gives rise to the variation ALfiber in
length of the fiber making it possible to compensate exactly or substantially
for the variation in the index n. This may be achieved in several ways.
5 In the case of the preferred embodiment serving as support to the description
of the invention and illustrated by Figures 1 and 2 notably, the dimensioning
of the end cap is carried out by considering that ALfiber is defined by the
following relations:
In which atube and Upiston represent the coefficients of thermal expansion of the
materials constituting respectively the cylindrical body 13 and the end caps
14 and 15 or, at least, the free cross-section C, of the end caps and AT the
temperature variation. The lehgth x is the length of the mobile part which is,
15 here, the length of the free cross-section C.
Therefore, to achieve effective temperature compensation, the
dimensions of the end caps must be determined in such a way that the length
x is defined by the following relation:
It is recalled that Lfiberis the length of the fiber, that atube and apistonre present
the coefficients of thermal expansion of the materials constituting respectively
the cylindrical body 21 and the end caps 14, 15 or, at least, the free crosssection
C, n represents the index of the cavity, dnIdT the variation in the
25 index of the cavity per temperature unit.
It is noted that this length x is independent of AT and depends only on
the properties of the materials used and the length Lfiber of the fiber fixed
between the end caps.
Generally, the end cap can exhibit different shapes, but it is always
30 possible to find a material and a geometry of the mobile part of the end caps
which satisfies relation 9 in a wide temperature range. In the case of a
complex geometry, a three-dimensional thermal calculation by the finite
element procedure makes it possible to arrive at the optimal length of the
mobile part of the end caps.
5 It should be noted that from a practical point of view, in order to avoid
too great a piston length, it is advantageous to use, as stated previously, a
material having a low thermal expansion to make the cylindrical body 13 and
a strongly expanding material to make the end caps 14 and 15. It is possible
to choose for example titanium for the cylindrical body (at,b, = 8.6 ~.rm/m/K)
10 and DelrinB for the piston (apisto=n 90 pm/m/K). A length x equal to 4.7 mm is
then obtained, for a fiber length of 42.5 mm.
As stated previously, the device according to the invention is therefore
defined from a structural point of view, as a device comprising a cylindrical
body 13 of rigid material, with a low expansion coefficient and end caps 14
15 and 15 of material with a large expansion coefficient, these elements defining
a cavity in which the Bragg grating optical fiber element 12 is housed. Such
as is defined and independently of the fluid filling the cavity, the device is
advantageously hardly sensitive to temperature variations and to variations in
static pressure. It makes it possible advantageously to amplify the length
20 variations imposed on the optical fiber element 12 by the dynamic variations
in pressure. However, studies conducted elsewhere demonstrate that the
nature of the fluid contained in the cavity conditions the sensitivity of the
device.
In practice, the sensitivity of this hydrophone hardly depends on the
25 geometric parameters. It depends predominantly on the compressibility of the
fluid used. Therefore, it is thus possible to increase the sensitivity of the
hydrophone according to the invention by using a more compressible fluid
than water to fill the cavity of the hydrophone. It is thus possible, for example,
to choose an oil in place of water. The curve of Figure 6 gives the form 61,
30 for a dynamic variation in pressure of frequency equal to 1 kHz, of the
variation in the sensitivity of an optical hydrophone according to the invention
as a function of the compressibility of the fluid used to fill the cavity.
As also mentioned in the foregoing text, the optical fiber element 12
35 housed in the cavity is fixed to the end caps 14 and 15 in such a way as to
undergo a tension prestrain the value of which is defined such that whatever
the dynamic pressure variations imposed by the exterior medium and,
whatever the lengthening due to temperature variations, it is kept under
tension. Therefore the fiber element 12 behaves as a vibrating cord whose
5 resonant frequency depends on its free length and its lineal mass as well as
the prestrain applied to it.
The consequence of this prestrain is that natural resonant frequencies
whose values are directly dependent. on the pre-tension applied are
conferred on the fiber element 12. Now, within the framework of the
10 invention, the tension prestrain applied to the fiber depends directly on the
operating temperature range. Indeed, the temperature insensitivity of the
device according to the invention is obtained by varying the length of the fiber
element in such a way that the wavelength 3L remains constant. The variation
in the length of the fiber being itself induced by the deformation of the end
15 caps to which it is fixed under the action of temperature, the axial
deformation notably.
Therefore, the tensile force F applied to the fiber element 12 being
fixed the same holds for the natural modes of vibration of the fiber element
12. Now, one or more of these natural modes may correspond to resonant
20 frequencies lying in the span of use of the device so that for these
frequencies, the sensitivity of the device is impaired.
These natural modes may be determined in a known manner by
considering the tensile force F applied to the fiber element 12 to keep the
25 latter under pre-tension and the speed of propagation of a deformation wave
along the vibrating cord constituted by this element.
Indeed, the tensile force F applied to the fiber element 12 may be
defined by the following relation:
30
F=kx=-6ELS
I [I 21
where E represents Young's modulus for silica (the material constituting the
fiber), where S and I represent respectively the cross-section and the length
35 of the fiber, and where 6L represents the lengthening imposed on the fiber.
This lengthening is determined by taking into account the lengthening of the
fiber as a function of the temperature variation considered.
The propagation speed v of a wave in the vibrating cord constituted by
5 the fiber element 12, subjected to the tension force F, is moreover defined by
the expression:
10 Where p is the linear mass of the fiber.
The natural vibration modes of the cord are for their part obtained for
wavelengths such that:
where L represents the length of the fiber and n an integer
Therefore, the natural resonant frequencies of the vibrating cord
20 constituted by the optical fiber element 12 are defined by the following
relation:
2 5 Thus, for example, for an optical fiber element of length I = 42 mm
made from a silica fiber of diameter equal to 250 pm and exhibiting a relative
variation in length equal to 9.36 pm/m/"C, the fiber lengthening 6L obtained
for a temperature variation of 50°C is substantially equal to 23.4 pm and the
tension F to be applied iS substantially equal to 0.6 N. The application of this
30 tension itself induces the occurrence of natural resonant frequencies at the
level of the optical fiber element 12, the first of these natural frequencies
having a value substantially equal to 1500 Hz.
Therefore, if this value lies in the frequency span of use of the
hydrophone, this natural resonant mode will be excited as soon as a
transverse acceleration of identical frequency is exerted by the exterior
medium. Such an acceleration, due essentially to mechanical disturbances
5 caused, for example, by the motions of the acoustic antenna into which the
hydrophone is integrated, introduces a disturbance of the sensitivity of the
hydrophone.
Therefore, in such a configuration, it becomes necessary to damp the
10 natural vibration modes of the optical fiber element 12.
According to the invention, this damping may be obtained by altering
the viscosity of the fluid contained in the cavity of the hydrophone.
Alternatively, it can also be obtained by enclosing the fiber element 12
inside a metallic cylinder filled with gel and to center this cylinder inside the
15 cavity of the hydrophone, the cavity itself being filled with a fluid serving to
transmit the pressure variations imposed by the exterior medium.
Alternatively again, it can also be obtained by filling the cavity of the
hydrophone with the aid of an open foam, impregnated with fluid. The nature
of the foam is then determined in such a way that it damps the vibrations of
20 the optical fiber element 12 without substantially modifying the behavior of
the cavity in relation to the pressure variations.
Figures 7 and 8 present in a schematic manner the structure of an
alternative variant embodiment of the device according to the invention. In
25 this alternative, the cylindrical body 13 does not exhibit any orifice 16. On the
other hand, through orifices 71 are here made in the thickness of the end
caps 14 and 1 5.
With respect to the general embodiment presented previously and
illustrated by Figures 1 and 2, this embodiment makes it possible
30 advantageously on the one hand to leave the cylindrical body 13 intact from
any drilling and on the other hand to obtain orifices exhibiting a length lhole
that is greater than the thickness of the wall of the cylindrical body and
therefore to design hydrophones exhibiting a lower Helmholtz frequency than
in the previous embodiment (cf. relation [3]).
35
It should be noted that insofar as the presence of one or more through
orifices 16 corresponds to an essential characteristic of the invention, the
device according to the invention comprises such orifices whatever
embodiment is envisaged.
5 However instead of consisting of one or more holes made in the
cylindrical body 13 or in the end caps 14 and 15, the function of the through
orifices 16 may, in an alternative embodiment where the rigid body 13, or
else a part of the rigid body, is made of a porous material, be fulfilled by the
pores of the material.
10 Likewise, in an alternative embodiment where the end caps are made,
in part or entirely, from a porous material, the function of the through orifices
16 may be fulfilled by the pores of the material.

1. A laser cavity hydrophone of the type comprising an active
optical fiber element (12) with Bragg gratings inscribed in the optical
fiber forming the laser cavity, characterized in that it comprises a
mechanical structure forming a cavity (11) filled with a fluid, and
5 inside which the optical fiber element (12) is placed along the
longitudinal axis of the cavity, the mechanical structure furthermore
comprising:
- a substantially cylindrical hollow rigid body (13) forming the
cavity (1 1) inside which is placed the optical fiber element (12);
10 - two end caps (14, 15) configured and designed to seal the
ends of the rigid body (13) and traversed by the optical fiber element
(12), said optical fiber element (12) being fixed to the end caps (14,
15) at the level of the points of traversal so as to be permanently
under tension;
15 the two end caps (14, 15) each exhibiting a part rigidly tied to the rigid
body (13) and a mobile part comprising a deformable wall (22) able to
be deformed when it is subjected to variations in the pressure exerted
by the exterior medium in which the hydrophone is immersed; the
deformation of the end caps giving rise to a variation of the length of
20 the optical fiber element (12), the deformable wall (22) exhibiting an
internal face (221) directed toward the interior of the cavity and an
external face (222) directed toward the exterior of the cavity and
being arranged in such a way that its internal face (221) is mobile in
translation with respect to the rigid body (13) along the longitudinal
25 axis of the cavity under the effect of a variation in the temperature of
the exterior medium in which the hydrophone is immersed; giving rise
to a variation in the length of the optical fiber element (12) which at
least partially compensates the variations of the emission frequency
of the laser cavity as a result of these temperature variations,
30 - a through orifice (16) configured to allow communication
between the cavity (11) and the exterior medium and to achieve
equilibration of the static pressure between the exterior medium and
the fluid contained in the cavity (1 1).
2. The hydrophone as claimed in the preceding claim, in which
the deformable wall (22) is mobile in translation with respect to the
rigid body (13) under the effect of a temperature variation of the
5 exterior medium in which the hydrophone is immersed.
3. The hydrophone as claimed in the preceding claim, said end
caps exhibiting a hollow body shape open at one end, said cylinder
being configured to be inserted into the cavity so as to seal the end
10 thereof and exhibiting a side-wall (21) and an elastic end-wall,
perpendicular to the longitudinal axis of the cavity, and sealing the
end of the end cap which is inserted into the cylindrical body, the
elastic end-wall being said deformable wall (22); the side-wall (21)
comprising a segment C for which the thickness of the material is
15 defined in such a way that the side-wall (21) of the end cap (14, 15)
can deform freely in the longitudinal direction under the action of the
temperature variations.
4. The optical hydrophone as claimed in any one of the
20 preceding claims, characterized in that the rigid body (13) and the
two end caps (14, 15) are configured in such a way that the
hydrophone exhibits symmetry planes arranged so as to minimize its
sensitivity to parasitic accelerations.
25 5. The optical hydrophone as claimed in any one of the
preceding claims, characterized in that the dimensions of the rigid
hollow body (13) are defined in such a way that the cavity formed
(1 1) can accommodate the optical fiber element (12) and that, having
regard to their dimensions, the end caps (14, 15) undergo, under the
30 action of the dynamic variations in the pressure exerted by the
exterior medium, a deformation sufficient to induce on the optical
fiber element (12) a variation in length making it possible to .o btain
the desired sensitivity of the hydrophone to dynamic variations in
pressure.
35
6. The optical hydrophone as claimed in any one of the
preceding claims, characterized in that the dimensions, length lhol,
and cross-section Shale, of the through orifice (16) are determined in
such a way that, having regard to the viscosity of the fluid contained
5 in the hydrophone cavity, the latter exhibits, having regard to its
volume VmVit,, a law of variation of its sensitivity to pressure
variations, as a function of the frequency of these variations,
exhibiting, a given span (53) of frequencies of variation of the
pressure a substantially constant sensitivity, this span of frequencies
10 being situated before the frequency F2 corresponding to the
mechanical resonance peak (52) of the cavity (1 1).
7. The optical hydrophone as claimed in the preceding claim,
characterized in that in the case where the fluid contained in the
15 cavity is hardly if at all viscous, the dimensions of the through orifice
(16) are determined in such a way that, the span of frequencies of
variation of the pressure (53) for which the sensitivity of the cavity of
the hydrophone is substantially constant lying between the frequency
F2 and a resonance peak (51) at a frequency F1 equal to the
20 Helmholtz frequency fH of the cavity defined by:
where c represents the velocity of the acoustic waves in the fluid
considered,
the frequency fH is as low as possible.
8. The optical hydrophone as claimed in claim 6, characterized
in that in the case where the fluid contained in the cavity (11) is
viscous, the dimensions of the through orifice (16) are determined in
such a way that, the span of frequencies of variation of the pressure
30 (53) for which the sensitivity of the cavity (1 1) of the hydrophone is
substantially constant lying between the frequency F2 and a
frequency F1 equal to the cutoff frequency f, of the cavity defined by
the relation:
where p represents the density of the fluid, c the velocity of the sound
waves in the fluid and Rhole the radius of the orifice (16),
the frequency fp is as low as possible.
9. The optical hydrophone as claimed in any one of claims 1 to
8, characterized in that the side-wall (21) comprises a first segment B
for which the thickness of the material constituting the wall is
determined in such a way that the side-wall (21) of the end caps
10 comes into tight contact with the internal wall of the rigid body (13),
so as to ensure a rigid link between the rigid body (13) and the end
caps (14, 15).
10. The optical hydrophone as claimed in any one of the
15 preceding claims; characterized in that the side-wall of the end caps
furthermore comprises an end-segment A whose thickness defines a
shoulder which abuts the end of the rigid body (13) when the end
-..
caps (14, 15) is put in place on the latter.
20 11. The optical hydrophone as claimed in any one of claims 6
to 9, characterized in that the dimensions of the cavity (1 1) and the
nature of the materials constituting the rigid body (13) and the end
caps (14, 15) are determined so as to obtain the highest possible
frequency F2.
25
12. The optical hydrophone as claimed in any one of the
preceding claims, characterized in that the cavity (11)
accommodating the optical fiber element (12) contains an open foam,
said foam being soaked with the fluid contained in the cavity (1 I), the
30 material constituting the foam being defined so as to damp the
natural resonant modes of the optical fiber element (12).
13. The optical hydrophone as claimed ,in any one of the
preceding claims, characterized in that the through orifice (16) is a
circular orifice made through the wall of the rigid body (1 3).
5 14. The optical hydrophone as claimed in any one of claims 1 to
11, characterized in that the through orifice (1 6) is a circular orifice
made longitudinally in the thickness of the side-wall (21) of an end
cap (14, 15).
15. The optical hydrophone as claimed in any one of claims 1 to
11, characterized in that, the rigid body (13) or the end caps being
made entirely or in part from a porous material, the through orifice
(16) consists of the pores of the material.
16. The optical hydrophone as claimed in any one of the
preceding claims, in which the length x of the mobile part is defined
by the following relation:
x = Lfiber . (a,,, + K1n .- -d)wnh ere her is the length of
2(apisron - arl#be )
the fiber, atube and a,t, represent the coefficients of thermal
expansion of the materials constituting respectively the
cylindrical body (13) and the end caps (14, 15) or, at least, the
free cross-section C, n represents the index of the cavity, dn/dT
the variation in the index of the cavity per temperature unit.
17. The hydrophone as claimed in any one of preceding claims,
25 in which the end caps are dimensioned, having regard to the material
from which they are made, in such a way that the displacement of the
internal faces (21) of the deformable walls (22) of the end caps (14,
15), along the axis of the cavity, gives rise to a variation of the length
of the optical fiber element (12) which compensates substantially the
3 o variatiofs in the emission frequency of the laser cavity as a result of
said temperature variations.