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The present invention relates to an optical fiber hydrophone. The present invention relates to the
field of acoustic detection in seawater media, in particular to single-mode optical fibers operating
by elasto-optic effect using the interaction effect between the acoustic wave to be detected and
the single-mode optical fiber on which said acoustic waves act. It relates to the hydrophone. The
present invention relates to a hydrophone, in particular having a single mode optical fiber
immersed in water which is transmitted by sound waves. This propagation of sound waves
causes a pressure change in the propagation medium, resulting in geometrical and optical
parameter changes in the optical fiber due to the elasto-optic effect. A lightwave propagating in
the optical fiber undergoes a phase change which can be detected by interference using a second
single mode optical fiber forming a reference arm. The optical connection between the optical
fiber immersed in water and forming the measuring arm and the reference optical fiber is
achieved in an optical structure forming an interferometer designed according to the principle of
a Michelson interferometer. Michelson interferometers generally use an optical beam splitter,
such as a semi-transparent plate to provide two measuring arms terminated with monochromatic
optics, and an optical beam splitter (optical splitter) and light traveling back and forth along the
two measuring arms. It consists of photodetectors that are superimposed and collected via
splitters. Such a device makes it possible to detect a large number of physical quantities which
influence the propagation of light along the measuring arm. Whatever the propagation direction
of the light in each measurement arm, some of the physical quantities produce the opposite
effect of causing the same transmission delay. Other physical quantities produce non-reciprocal
effects that differentially affect transmission delay as a function of light propagation direction.
The two non-reciprocal effects that have been considered hitherto are the Faraday effect and the
relative inertia effect. The Faraday effect occurs when the measurement arm has a material
medium in which a magnetic field produces a preferred electron spin alignment. The relative
inertial effect used in link-type interferometers is called the Zachnank effect, and the
interferometer is then called the gyrometer. Reciprocal effects are not linked to the breaking of
space or symmetry of the material medium. This reciprocal effect is observed when the
measuring arm is an optical, thermal or mechanical stress source. Furthermore, light reflecting
systems based on the use of a photorefractive medium are known, which reflect the incident
wavefront in the form of a conjugate wavefront. Ordinary mirrors reflect light if light comes from
a temporary object that does not coincide with the object illuminating the mirror. On the
contrary, the photorefractive medium reflects a wave front having a conjugate space which
returns heteromorphic light from the conjugate space to the object.
If reciprocal effects are present and if such effects are not changed during the light round trip
and the photorefractive medium can be applied to these changes in effects, then this interactive
reflection is not sensitive to such effects. Guarantee. This tendency to offset the reciprocal effect
is not disadvantageous to the measurement of the non-reciprocal effect interferometer. Patent
FR-A-2460582 discloses the following prior art. That is, the device consists of a single-mode fiber
optic hydrophone operated by the elasto-optic effect, which comprises a single-mode laser source
coupled to the realized integrated optical circuit substrate, and two first integration leads. Means
for splitting light from the light source towards the waveguide, means for recombining light by
the two integrated optical waveguides, and a first single mode forming a measuring arm
immersed in the working medium exposed to the sound wave to be detected An optical fiber, and
a second single-mode optical fiber forming a reference arm, said two optical fibers having their
ends rigidly connected to one of the first and second destination waveguides respectively It is a
thing. The hydrophone also detects the light from the recombining means, an electrically
controlled phase modulator acting on one of the integrated optical waveguides to introduce a
phase displacement between the guided waves. And a device for processing a detection signal for
supplying a control signal to the phase modulator such that the phase displacement between the
two arms is maintained at the maximum sensitivity point independently of the sound wave to be
processed. The processor also provides a measurement signal that is characteristic of the sound
wave. Furthermore, European Patent Application No. 0'079 268 discloses a Michelson
interferometer in which the mirror is replaced by a conjugate mirror. This interferometer is
sensitive to non-reciprocal effects. This mirror is said to be a conjugate "," and in fact the incident
divergent wave in the medium is reflected in the form of a converging wave conjugate to the
incident wave. The Michelson interferometer of this light refracting mirror consists of a
monochromatic light source, a light beam splitter supplying two parts of light to the two
measuring arms terminated by the reflecting means, and two back and forth into the two
measuring arms And a photodetector that collects the two light portions in an overlapping
manner. The two light portions interfere in the photorefractive medium, and the reflector reflects
the first light portion that has passed through the medium towards the medium under normal
incidence. However, this second prior art device utilizing the conjugate wavefront of the first
incident wave to reach the photorefractive medium requires that the reflected wave be a
conjugate of the incident wave and thus that the medium not deform the wavefront of the
incident wave . Compared to the first prior art device, the hydrophone of the present invention
can constitute a bypass frequency filter to cancel out the low frequency interference results.
Furthermore, the hydrophone allows for self-alignment of the conjugated beams recombined in
the fiber without requiring alignment. It also allows remote delivery into the working medium
which can not be detected by conventional detection devices such as sonar. That is, according to
the present invention, it includes a system for transmitting and detecting light that incorporates a
monochromatic light source, detection means for detecting light, and a first splitter, wherein the
light source and the detection means The measuring head further comprising: a measuring head
connected to the splitter and immersed in a working medium receiving the acoustic wave to be
detected; and a single mode waveguide connecting the system to the measuring head Has a
second splitter / mixing means, a single mode optical waveguide forming a reference arm, a
photorefractive medium and a light reflecting means, and a single mode optical fiber forming a
measurement arm is in the working medium Immersed, a second splitter and mixing means
distributes the first and second portions of light to the measuring arm and the reference arm, the
two portions of light crossing in the photorefractive medium, the anti The means make it possible
to "reflect the first of the two light parts", the second splitter and mixing means make it possible
to recombine the light after it has crossed the measuring and reference arms, the first The
splitter provides an optical fiber hydrophone characterized in that the detection means detect
light according to the light path in the coupled waveguide. The invention will now be described
with reference to the accompanying drawings. The hydrophone of the present invention has an
interferometer structure and has a single mode optical fiber in one of the interferometer's optical
paths. This optical fiber is immersed in the sound pressure area to be detected assuming a
uniform pressure P and an angular frequency ?5. This sound wave region induces a refractive
index change ?n in the optical fiber refractive index n by the elastic optical effect. This refractive
index change is converted to a phase shift ?? which is a function of the refractive index change
?n, the immersed optical fiber length ? and the wavelength ? of the light wave. That is,
2?l?? (?5)-? 8n? Figure 1 shows a conventional interferometer with two arms. Like the
conventional Michelson interferometer, this interferometer has a monochromatic light source,
which emits light in the direction of the light splitting device 2 constituted, for example, by a
semi-reflecting plane plate. Incident light 11 on the semi-reflecting plate 2 is split into a first
transmitting portion 12 and a second reflecting portion 33. The transmission part 12 is focused
by the lens 4 onto the first front waveguide 6 and the waveguide 6 retransmits this light by its
output B. The reflective part 33 is reflected by the mirror 3 towards the lens 5 which focuses the
light on the input C of the two light guides 7.
The end of the waveguide 7 emits diverging light which meets the diverging light emitted by the
end B of the waveguide 6. The interference of the two light portions is detected by the light
detector 10, which provides a signal 5 (t) representative of the passage of the interference band.
The two measuring arms of the interferometer are formed by the elements 4 and 6 on the one
hand and the elements 3.5 and 7 on the other hand. The interferometer of FIG. 11 uses a concave
mirror 9 which reflects the light refracting medium 8 and the light portions circulated in the two
measuring arms from B to A and from D to C. The concave mirror 9 is arranged to receive a
spherical wavefront across the medium 8 such that the spherical wavefront is reflected under
normal incidence and in focus on the end B. The photorefractive medium 8 cooperates with the
concave mirror 9 to bombard the light from the end B so as to return the light having the
conjugated phase of the light from the end back to the end, and at the same time to diverge
there. In addition, light exiting end B and traversing the photorefractive medium 8 enters the
reflective surface of the mirror 9 perpendicularly, which reflects light towards the edge and
passes the medium 8 again. This light is considered as the pumping beam of the photorefractive
medium 8. The light from the end of the waveguide 7 constitutes a signal beam, which interferes
with the pump beam in the photorefractive medium 8. This interference spatially modulates the
refractive properties of the photorefractive medium to form a system of refractive index lines,
which can be thought of as a dynamic hologram of the light structures contained in the signal
beam. By receiving the bombarded light passing through the light refracting medium 8 after the
vertical reflection on the spherical mirror 9, the dynamic hologram diffracts the conjugated
restructure of the light coming out towards the end of the waveguide. If the light exiting in the
direction of the photorefractive medium is a traveling wave, then the conjugate reconstruction is
a reflected wave with a heteromorphic wave front with phase shift of opposite sign, the latter
being pumped as a reference It can be calculated considering the phase reference of the beam.
As is apparent from what has been described so far, the system of refractive index lines of the
photorefractive medium 8 acts in the same way as the polarizing mirror with respect to the end
of the second measuring arm of the interferometer. The device of the present invention uses a
Michelson interferometer, but the device is an optical fiber hydrophone, and this operation is
based on the average value of the detected signal in the photorefractive crystal. This device is
shown schematically in FIG. 2 and consists of two systems. The light transmission and detection
system 31 and the measuring head 32 immersed in the working medium 34 exposed to the
sound waves. In these two systems, they are connected by a waveguide element 14 which is, for
example, an optical fiber.
A measuring arm incorporating an optical fiber is connected to the measuring head, which is
immersed in the working medium 34 exposed to the acoustic waves. In particular, the device of
the invention may comprise a large number of elements. A laser 1 is provided, whose wavelength
corresponds to the spectral sensitivity region for the photoconductivity of the photorefractive
medium 8 used. In the case of materials such as bismuth oxide-silicon (B50), bismuth oxidegermanium (BGO) or barium titanate (BaTi03), the wavelength is between 575 and 450 nm,
making it possible to use argon and q krypton lasers Do. The beam splitter 15 enables the light
detector 19 to detect the interference of the two light portions received on the return path from
the measuring arm 18 and the reference arm 17. The photodetector 19 provides a signal
representative of the interference line. The single mode optical waveguide 14 maintains its
polarity and acts as a spatial filter. For example, this spatial filter can form a remote feed arm.
This filter can only hold the zero interference order since the waveguides used are for single
mode. Thus, this filter can remotely provide a complete interferometer. The advantage is that it
enables telemetry of the laser source. Furthermore, if the interferometer is made of a dielectric
material, it is not detected by conventional devices (sonar, lago) that are not laser sources.
Remote supply is also giving one advantage here. The splitter 16 splits the incident beam into
two light portions, which are transmitted by the two measurement and reference arms 18.17.
The splitter can be constituted by a splitter ink plate, an integrated optical splitter, or an optical
fiber coupler. Such a coupler can be formed, for example, from two optical fibers of fusion splice
type, the coupling being performed by evanescent waves. Two arms 17.18 are provided, one of
which can be made to be sensitive to pressure changes by being coated with a suitable material.
This covering of the measuring arm 18 is not necessary if the other arm is insensitive, ie if it is
inside the case 32 as shown in FIG. For example, the coating can be mostly made of a very elastic
material such as a comb, and the rubber compresses in the presence of sound waves, resulting in
the stretching of the optical fiber resulting in a phase change of the transmitted signal 4 Let
This optical fiber represents the 'sensor' arm of the interferometer and one is the reference arm.
The reference arm makes it possible to equalize the light path. If the light source has a sufficient
coherence length, it is possible to greatly reduce its length. Thus, for a measurement length of 10
meters, a reference arm with a length of 1 meter is conceivable for coherent lengths of at least 9
meters. The photorefractive material 20 acts as a working medium for the waves from the two
optical fibers. This material should be photoconductive and electro-optical. Photorefractive media
include bismuth oxide-silicon (BSO), bismuth-germanium oxide (BGO), barium titanate (BaT 103),
potassium niobate (KNbO3), barium niobate-strontium niobate (SBN), and potassium niobate
(KTN) The crystals can be pure or doped to be sensitive to long wavelengths (0,8 ?m or more). A
photorefractive medium is a photoactivatable medium such that incident photons generate
charge carriers, which can diffuse in the medium as the illumination alternates light and dark
areas. This medium is also electro-optical and it is possible to see the change in refractive index
generated by the internal electrolysis. Internal electrolysis itself results from the movement of
charge carriers. Based on these properties it is possible to optically condition the photorefractive
medium by providing interference of the signal beam (the target beam) and the bonding beam.
The system of bands forms a refractive index line, and the attribution line can generate a
conjugate signal beam by diffracting a bombarded beam. This is 4 when the pumping beam
which has traversed the medium is reflected back by the mirror which guarantees a return. ??
It is generated by interferometry. Optionally, a crystal or polaroid polarizer is placed between the
measuring arm, the reference arm and the photorefractive medium. Thus, the conjugate mirror
only works with polarization where the polarization direction is parallel to the crystal axis C.
Thus, polarizers are required when using multimode optical fibers. Also, if the optical fiber is
single moat, it does not maintain linear polarization. This interferometer consists of four waves,
each with a different function. Each of the interferometer's arms has two waves: a 'write
reference' wave and a 'readout' wave respectively on the outgoing and return paths of the
reference arm and on the outgoing and return paths of the measuring arm respectively.
'Objective' waves (signal waves) and conjugate waves are carried.
The reference wave and the target wave form a phase network in the photorefractive material.
The network is reread by the reflected 'reference' wave to become a 'readout wave'. This
rereading produces a spatially conjugate wave of the target wave, which, after being recombined
in the "sensor" optical fiber, interferes with the rereading wave and at the output of the
interferometer. The "wave" itself is "recombined into the reference fiber". When the optical length
of one of the two arms is reversible, the introduced phase difference changes the network drawn
in the conjugate mirror ", but the interference pattern at the output of the interferometer is not
changed. The apparatus of the present invention takes into account the time required for the
changes in the network drawn to use an interferometer with a time variable phase signal. The
reference arm provides the write reference and reread waves. The target beam is directed to the
crystal to interfere with the write reference beam. The reread wave is obtained by reflection on a
spherical mirror that reflects towards the photorefractive medium under a portion of its normal
incidence. It is also obtained by using an external oscillator. The partial oscillator has a phase
conjugation device that operates with four waves mixing in a photorefractive crystal, such as
barium titanate, without an external bombardment. This external oscillator is an oscillating
optical cavity placed in the center of the phase conjugation apparatus to provide conjugation of
the pink beam without an external energy source. The conjugate of the bombarded beam forms a
reread beam. This device has two aligned mirrors forming a resonant cavity and allows 10%
reflection of the incident wave in a Gaussian distribution. It can also be produced, for example, by
using a degenerate four-mode mixer which uses the photorefractive effect in nolium titanate
crystals. The crystal's angle, together with the auto-induction network formed by the spatial nonuniformity of the bombarded beam placed in the crystal, forms the oscillating optical cavity in
the manner described above. This automatic guidance network is different from the network
resulting from the bombardment and interference in the crystal of the target beam. The second
pomping him, usually required for four-wave mixing, is derived from the actual incident wave in
the crystal by '44 wave mixing 'coupled with the auto-induction cavity in the crystal. It is this
type of device which is considered in FIG. The angle of incidence of the target beam is such that
in the case of the reference beam the latter is not conjugated at the crystal angle. The reread
beam rereads the holocram formed between the reference beam and the target beat to provide a
conjugate beam.
This photorefractive medium 20 makes it possible, after interference in the photorefractive
medium 20, to "reflect" the portion of the light circulated in the forward and return paths of the
measuring and reference arms towards the beam splitter 1G. The photorefractive medium 20
cooperates with the bombarded light from the end of the reference arm to cause light having the
conjugated phase of the light emitted by the end of the measurement arm to diverge back
towards and simultaneously with the end of the measurement arm. Based on the properties of
such a photorefractive medium 20, the photorefractive medium is then optically conditioned by
providing interference of the signal beam and the bombardment beam. The system of the hand
forms a refractive index line which forms a conjugate signal beam by diffracting the bombarded
beam. This is formed according to a four-wave interference method, as the pumping beam is
reflected back to the medium by reflection at the angle of the crystal across the medium, thus
allowing the inversion of the bombarded beam. Thus, the light from the end of the measurement
arm constitutes a signal beam that interferes with the bombarded beam in the photorefractive
medium 20. This interference spatially modulates the refractive properties of the photorefractive
medium to form a system of refractive index lines which can be considered as the dynamic
holocram of the optical structure contained in the signal beam. After reflection at the angle of the
crystal, by receiving the bombarded light passing through the photorefractive medium 20, the
dynamic holocram diffracts the conjugate reconstruction of the light emerging from this end
towards the end of the measuring arm. If the light emerging from the end in the direction of the
photorefractive medium is a traveling electromagnetic wave, then the conjugate reconstruction is
the reflected electromagnetic wave associated with it, which is the wave front of the allomorph
with reflection of the sign of the phase shift. The latter can be calculated on the basis of the
phase reference of the pink-pink beam. Thus, the punt system of the photorefractive medium acts
in the same way as the deflecting mirror for the end of the measuring arm. This reflection
function does not give a specific condition to the signal beam, since the reflection of the
bombarded light by the angle of the crystal 20 preserves the shape of the wave front. Thus, with
the single mode waveguide 17 selected, the light source 1 must supply light of an appropriate
coherence length. For example, the light source l is a helium-neon laser, an alcon laser or a single
mode semiconductor (the present laser as a function of the sensitivity wavelength of the crystal.
The measurement is performed as follows. If a variable phase disturbance is applied to one of the
arms, the formed holocladus contains average phase information for the disturbed beam, ie direct
current, very low frequency components.
The alternating current component is quickly compared to the set-off (cut-off) frequency of the
holocram recorded in the crystal for recording. The phase of the conjugate beam is thus equal to
the mean value of the disturbance and opposite in polarity. When passing again through the
same medium, the beam is subject to phase displacement due to re-disturbance and only the
alternating current component is detected at the output. Thus the phase of the conjugate beam at
the output is (1-?. + (? 9 + ?, sin (I) t) = ?, equal to sin ? t (where ? is a continuously applied
phase displacement and ? 1 is a periodic phase displacement). If a disturbance is applied to the
reference arm, the following equation can be used: And at the output, ?-2?. + 2? + sin ? is
reread. Here again, the phase displacement between the two beams at the output is ? + Sin (uj).
For this reason, this result is a measure of the component of the signal having a high frequency
above the cutoff frequency of the signal (which varies over a few hours with Ins as a function of
the condition), and the low frequency component, in particular the DC current component not
exist. The stability of the DC current component makes it possible to dispense with the zero
calibration control conventionally required in this type of device. Thus, ?? is the phase
difference between the two waves from the measurement and reference arm after passing the
measurement and reference arm, and in the absence of ?nonreciprocal? disturbances, ?? is
Equal to O The sensitivity of the interferometer is very low if the phase difference ?? is
different from zero. This is true if you want to measure small acoustic signals. In order to
increase the sensitivity of the interferometer, it is possible to introduce a constant irreversible
bias in the phase of the two waves circulating in the opposite direction so as to displace the
operating point of the interferometer. For a function that varies according to the cosine function,
a high sensitivity point is obtained by an angle of (2K + 1) ? / 2 (where K is an integer).
Therefore, it is possible to choose a bias that introduces a phase change to the four waves. Thus,
phase modulation can be introduced onto the wave path. Thus, a phase modulation system can
be used to improve the response of the device. To provide this modulation, an integrated optical
modulator is conceivable in which two electrodes are arranged on both sides of the waveguide on
the electro-optical substrate. However, it may also be a hollow cylinder of piezoelectric material
on which the optical fibers constituting the measuring arm are attached. The cylinder, when
excited by the signal, expands and contracts, which results in the stretching of the optical fiber,
resulting in a phase change in the carried signal.
However, the periodic change to be measured must be slower than the modulation rate. In order
to extract periodical changes, only synchronous demodulation or heterodaton detection need be
performed. In each case, the phase modulation device is bifurcated into two equal parts arranged
symmetrically at the two ends of the light path in the measurement and reference arm and
excited with opposite polarity. This arrangement guarantees the complementary symmetry of the
phenomenon and reduces the second order error resulting from the possible non-linearity of the
modulator. The degree of this modulation must be relatively small in order not to interfere with
the holocram network. Thus, the recorded network is the average of the illumination at each
point S. In the crystal, the phase displacement is (?-? + sin ?t + KZ) (where K is the wave
vector of the lighting network and Z is the position in the crystal in the direction perpendicular to
the illumination line) at one point The average illumination is proportional to: That is,-10 Jo (2?
+) cos 2 Kz space change, ie, the value J to hold the term cos 2 Kz. It is necessary that (2?1) be
maintained as large as possible, and that the modulation degree be relatively small (ie, ?1 ?
0.5rd for J, (2?1) ? 0.8). The frequency f of this modulation must be well above the maximum
frequency of the signal to be detected, so that demodulation of the signal occurs by means of
heterofin detection at 2f. Thus, it is a sinusoidal modulation or square wave signal. The device
shown in FIG. 3 is a modification of the device of the present invention embodied as an
integrated optical device. The device comprises a light transmission and detection system formed
by the light source 1 and the detector 19 and a branching and mixing device embodied by an
integrated optical waveguide having a Y-shaped structure 24.25.21. The spatial filter is realized
by an optical fiber 24. The measuring head comprises a waveguide having a Y-shaped structure
35.26.27. The working medium is indicated at 20. The reference arm is constituted by the
waveguide 27, and the measurement arm is constituted by the optical fiber 18. The waveguide is
formed by integration in the substrate. The waveguide can be selected from materials such as
lithium niobate or lithium tantalate, in which titanium or niobium, respectively, is diffused to
form the waveguide. The substrate can be chosen from gallium arsenide such that the waveguide
can be realized by ion or proton implantation or barium titanate such that the waveguide is
formed by inversion of the method or region described above.
The modulator is branched into two modulators placed at both ends of the optical fiber. In
particular, the modulator used in the present invention can utilize various electro-optical effects
such as Pockels effect and Kerr effect. These two modulators, realized by an optical splitter
formed by interconnected single-mode waveguides to form a Y-shaped structure, are
interconnected by one of their branches and an optical fiber 14, a translucent plate Perform the
functions achieved in FIG. In this case, a second Y-shaped structure can be realized in the
photorefractive medium, so that the reflection of the waves obtained by coupling at the angle of
the medium as described above is used.
Brief description of the drawings
FIG. 1 shows a prior art Michelson interferometer.
FIG. 2 is a view showing a nod rophone of the present invention. FIG. 3 is a view showing a
modification of the nod rophone of the present invention. (Major reference numbers) 31 иии
Optical transmission and detection system 32 и и и Measurement head 19: light detector, 18:
measuring arm, 17: reference arm, 20: light refraction material.
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