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JPS61150599

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DESCRIPTION JPS61150599
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
optical fiber hydrophone, and more particularly to an optical fiber for detecting an acoustic wave
in water by a 91 degree modulation method using optical fiber transmission loss characteristics.
Basically, there are an optical interference method and an optical intensity modulation method as
means for detecting underwater acoustic waves. The light intensity modulation method provides
a structure for causing light transmission loss by external pressure to a part of the light
transmission path, and applies an applied sound wave through a change in the light transmission
loss that occurs when an underwater sound wave is applied to this part The basic configuration
of such a light intensity modulation type hydrophone is a light emitting portion using a light
emitting element such as a laser, L.sub.ED (L ight Emission Diode) or an incandescent lamp, a
phototransistor, etc. A light receiving portion utilizing the light receiving element of the above, an
optical fiber forming an optical path by being interposed between the light emitting portion and
the light receiving portion, and the intensity of light propagating through the optical fiber
corresponding to the intensity of the applied underwater acoustic wave It consists of a light
intensity / f modulation structure to be changed. The loss addition function for forming the
optical transmission loss provided by the light intensity modulation structure also has two basic
functions, one of which is an optical fiber loss addition function and the other is a mechanical
loss addition function. The fundamental difference between these two loss-adding functions is 0
depending on whether the light propagation medium is continuous or discontinuous in the
modulation structure, ie whether the optical path medium in the modulation structure changes,
ie, the optical fiber loss The function is a continuous case where the propagation medium of the
light path does not change, and is usually used in the form of a microbend (m1 cro bend) type
hydrophone as described below. If this optical fiber is bent with a small radius of curvature, light
leakage from the central core to the outer cladding occurs, that is, transmission loss occurs, and
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the bending state is periodically bent. It is a no-idrophone that is configured as having its
sensitivity increased using the phenomenon that this transmission loss becomes large when it is
realized. Fig. 5 shows the basic structure of the microbend type nod idrophone. Fig. 5 is a
microbend type idrophone structure diagram. 01 pairs of pressure plates H1 and H2 are flat
plates each having a concavo-convex structure of a sinusoidal cross section. I hate the former
fiber L. As the uneven structure of this pressure plate, as shown in FIG. There are various types
such as wave-like ones, flat ones, alternately arranged round bars on WHI and H2, and triangular
ones, but the 0 pressure plate H1 is selected depending on the purpose of operation etc.
Normally, a diaphragm coupling post B is coupled via this to a diaphragm (not shown) to receive
a sound pressure P applied to the diaphragm, and the pressure plate H 2 is connected to an
optical fiber via a pressure plate mounting structure G. · Consideration is given to adding prestress by adding a force bias Q attached to the storage case (not shown) of idrophone to improve
the sound pressure sensitivity and the minimum detected sound pressure level. Under this
structure, the optical fiber L is given a microbend with a bending period to while holding the
force balanced with the pressure plates H1 and H2 and the king Kabias Q, the sound pressure P
Received As a typical structure of an optical fiber / S idrophone that utilizes 0 microbends to
extract sound pressure through the microbend change that occurs sometimes, in addition to the
contents described above, the optical fiber is drilled into a cylinder with a slit. There are some
types of wedges that use the above-mentioned pressure plate in a cylindrical shape, but in any
case, it is focused on features such as being able to take an arbitrary shape with a wide band and
light weight without receiving electromagnetic induction essentially. It is being used rapidly as a
senna recently.
[Problems to be Solved by the Invention] However, the light 7 Iber hydrophone which detects the
underwater sound wave based on the light intensity modulation method using the conventional
extraction of the optical fiber loss function has the following drawbacks. In other words, this
small fiber with a radius of curvature of several meters or so, which uses so-called microbends,
and idrophones utilize a flat plate having a concavo-convex structure having a cross section such
as a sine wave or an approximate sine wave 1 Due to the basic structure of holding the optical
fiber between the pressure plates of the pair, and winding the optical fiber in a cylinder with a
slit /% idrophone balances the internal and external pressure balance type with the liquid to be
filled inside, also When the hydrophone is an unbalanced type that does not balance internal and
external pressure, a portion of the optical fiber that crosses the gas such as air is generated,
which results in poor reproducibility of performance and is vulnerable to vibration and shock.
There is a drawback of being there. FIG. 6 is a detailed view of the portion A of the microbend
type idrophone of FIG. 5 and is a detail view of the portion A. Pressure plates H1 and H2 to
which 0 pressure bias is applied are mechanical due to the elasticity of the optical fiber L. When
the optical fiber L is made to follow the curved surface of the sine wave unevenness by closely
contacting the pressure plates H1 and H2 which are 0 rigid bodies which exist in a state capable
of extracting the change sound of the microbend by the sound pressure P while maintaining the
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dynamic balance. Clearly, it is impossible to extract the amount of change in microbends, so an
optical fiber L that can extract the amount of change in microbends due to sound pressure P
while maintaining mechanical balance with pressure plates H1 and H2 is shown in Figure 6 As is
apparent from the above, it is used in a state in which a highly mobile part is present in the gas
or liquid, which is not tightly attached to the pressure plates H1 and H2 or not pressed. The
microbend type optical fiber hydrophone extracts the change in the amount of microbend
corresponding to the level when 1 fP of sound is applied in the state of receiving a microbend
with a radius of curvature of several meters at most corresponding to the bending period. The
level of the sound pressure P is known, and when there is a portion of the optical fiber that can
move freely in gas or liquid, this directly causes the above-mentioned problems. The object of the
present invention is also to provide a light 7 Iva hydrophone which eliminates such a defect and
achieves a significant stabilization of the performance. [Means for Solving the Problems] The
sound receiving structure of the light 7 Iva hydrophone of the present invention. In the sound
receiving structure of an optical fiber hydrophone for detecting underwater sound waves by light
intensity modulation, Ii 4 has a structure in which at least a part of the inner and outer surfaces
of the main body cylinder has a periodic uneven shape in the circumferential direction. The
optical fiber is closely attached along the periodic concavo-convex shape and fixed in a coil
shape.
The present invention will now be described in detail with reference to the drawings. FIG. 1 is a
plan view showing the structure of a first embodiment of the sound receiving structure of the
light 7 Iber hydrophone of the present invention, and FIG. 2 is a sound receiving structure of the
light 7 Ivar hydrophone of the present invention shown in FIG. 1 is a perspective view showing
the structure of a first embodiment of the present invention. FIG. 1 shows the light emitting part
2 and the light receiving part 3 in addition to the plan view of the sound receiving structure 1,
and the optical fiber 11 is mounted on the elastic body cylinder 10 of the sound receiving part
structure 1ilt Both terminals are connected to the light emitting unit 2 and the light receiving
unit 3. In the sound receiving structure 1, at least a part of the inner and outer surfaces of the
elastic cylinder 10 made of a uniform material, in the case of this embodiment, a sinusoidal
uneven surface is provided on the outer surface, and slits formed along the periodic uneven
shape. The optical fiber 11 is fixed by burying a part or all of the optical fiber 11 or by fixing it
along a periodic uneven shape. In the case of the present embodiment, the optical fiber 11 using
a multimode fiber is fixed with a coating agent by causing the periodical irregular shape KG of a
sine wave provided on the outer surface of the elastic cylinder 10. Between the elastic cylinder
10 and the optical fiber 11 shown in FIG. 1, an adhesive (not shown) for fixing the both is
interposed. In such a sound receiving structure 1, since the optical fiber 11 is fixed along the
sinusoidal periodic unevenness provided on the elastic cylinder 10, the optical fiber 11 receives
microbends periodically repeated at the pitch t. A vibrating system is stably formed with the
elastic cylinder 10 in a stable state and in a state in which the whole is in close contact with the
elastic cylinder 10, and the sound pressure level is known through the amount of microbending
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that changes with the sound pressure. Each of the light emitting unit 2 and the light receiving
unit 3 utilizes the general configuration of this kind of optical fiber hydrophone, supplies laser
light from the light emitting unit 2 to the optical fiber 11 at a predetermined level, and receives
its output. In part 3, the transmission loss of the laser light in the optical fiber is known from the
level received, and the sound pressure level can be known. In this case, the periodic asperity
shape provided on the elastic cylinder also has the effect of providing the optical fiber 11 with a
bending bias corresponding to the conditions such as depth and pitch of the asperity beforehand,
that is, the above-mentioned caustic bias to improve sensitivity. . FIG. 3 is an explanatory view of
a change in the amount of optical fiber microbend for explaining the change in the amount of
microbending of the optical fiber at the time of the sound wave Omura in the first embodiment of
the sound receiving structure of the light 7 Iber hydrophone according to the present invention.
When a sound wave is incident on the elastic cylinder 10 shown by a solid line 0, the elastic
cylinder repeats contraction and expansion in the radial direction in accordance with the incident
sound pressure.
This force is converted into a circumferential force in the elastic cylinder. The oppositely directed
arrows shown in FIG. 3 indicate such circumferentially transformed compression and expansion
forces. The movement as shown by the elastic cylinder 1oa one-dotted line and dotted line by this
compression and expansion is repeated as the optical fiber (not shown) fixed in the form of a coil
along the periodic concavo-convex waveform. It can be regarded as a change of Bend 堵. The
transmission loss of the microbend type no idrophone in the case of using a 0 multimode 7 Iber
where the transmission loss becomes large corresponding to the change of the microbend will be
described in more detail as follows. The larger the ratio of change in the transmittance to the
applied sound pressure, the larger the change in the ratio of the transmittance of the 0 optical
fiber to the change in the applied force F to the optical fiber. P / dF can be expressed by the
following equation (1): OdP / dF = (dP / dx) (dx / dF) = (dP / dx) (A-Cm) (1) (1) ( 1) In the
equation, X is the bending amplitude of the optical fiber, A is the area of the pressure part, the
mechanical compliance of the CmVi optical fiber 0 so the minimum detection sound pressure and
sound l: E And compliance Cm Increase or increase dP / dx 0 If A and Cm are set and then
increase dP / dx after 0 and Am are set, then the 0 higher-order mode has a large loss, so dP / dx
is large Although the energy of the low-order mode is large in an ordinary optical fiber, the loworder mode must be changed to a high-order mode by means such as BJ. This means is the
above-described micropend and mode conversion to be described next. In mode conversion,
when waves of modes m and m + 1 propagate, a period in which they interfere with each other in
a constructive manner is proportional to a difference Δk between the wave numbers km and km
+ 1. When this interference period matches the bending period of the optical fiber, mode
conversion occurs. The receiving structure of the optical fiber · idrophone of this embodiment
shown in Fig. 1 and Fig. 2 is also an elastic having a periodic uneven shape and size in which
bending bit and te are set in consideration of the above-mentioned various conditions. 1ilj of the
optical fiber 11 using the body cylinder 10, i! The receiving structure 1 is formed. FIG. 4 is an
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explanatory view of the change of the microbend of the optical fiber for explaining the change of
the amount of microbend at the time of the sound wave incidence of the optical fiber in the
second embodiment of the sound receiving structure of the optical fiber hydrophone of the
present invention.
Fig. 4 shows the periodical concave and convex shape on the inner and outer surfaces, and the
convex part is an elastic body with a high bowing ratio, and the concave part is a solid line of
elastic cylinder 12 composed of an elastic material with a small bow The optical fiber is closely
adhered in a coil shape along the periodic concavo-convex shape on the side facing the incident
sound wave. In this state, when an incident sound wave is received, an IE right indicated by an
arrow. The expansion occurs, and the expansion and contraction movement of the radial
compression and expansion shown by the alternate long and short dash line and the dotted line
occurs correspondingly, which changes the amount of microbending of the optical fiber. In this
case, since the displacement of the convex portion is large and the displacement of the concave
portion hardly occurs, a change in the amount of microbending occurs, and this amount of
change can be obtained as much larger than in the case of FIG. The present invention is
characterized in that at least a part of the inner and outer surfaces has a structure having a
periodic uneven shape, and a light receiving structure in which an optical fiber is closely attached
along the periodic uneven shape and fixed in a coil shape. It has basic% characteristics, and
variations of the first and second embodiments described above can be considered in various
ways. For example, in the first embodiment shown by FIGS. 1.2 and 3, although the case where
the periodic concavo-convex shape is provided only on the outer surface of the storage cylinder
is described as an example, this is provided on both the inner and outer surfaces. It is obvious
that the same method may be carried out by using the change in the amount of microbending of
the optical fiber fixed along the periodical asperity shape of the outer surface, as shown in FIG. 4
and FIG. In the second embodiment, although the periodic concavo-convex shape is applied to the
inner and outer surfaces of the elastic cylinder, it is obvious that the same implementation may
be carried out as a structure limited only to the outer surface. And in the second embodiment, the
optical fiber is fixed to the surface of the elastic cylinder with an adhesive, but instead, the
portion of the elastic cylinder to be solidified is formed into a groove structure, Interfering as
fixing the fiber Absent. Furthermore, in the first and second embodiments described above, the
periodic uneven shape utilizes a sine waveform, but this may be another uneven shape, for
example, a circular, triangular or other arbitrary irregular uneven shape. All the above can be
easily implemented without impairing the subject matter of the present invention. As described
above, according to the present invention, in the sound receiving structure of the optical fiber
hydrophone for detecting underwater sound waves by the light intensity conversion method, at
least one of the inner and outer surfaces has a shape with irregularities in the circumferential
direction. An optical fiber having a sound receiving structure in which an optical fiber for
detecting sound is closely attached to and fixed to an outer surface of the optical fiber, the
reproducibility of performance is remarkably improved, and further the imaging resistance and
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impact resistance are remarkably improved.・ There is an effect that idrophone can be realized 0
[0002]
Brief description of the drawings
[0003]
FIG. 1 is a plan view showing a first embodiment of a sound receiving structure of an optical fiber
hydrophone according to the present invention, and FIG. 2 is a perspective view showing a first
embodiment of a sound receiving structure of an optical fiber hydrophone according to the
present invention. FIG. 3 is a diagram for explaining the change in the amount of micro-bending
of the optical fiber when the sound wave is incident on the optical fiber in the first embodiment
of the sound receiving structure of the optical fiber hydrophone of the present invention. FIG. 4
is an explanatory view of the change in the amount of optical fiber microbend for explaining the
change in the amount of microbend at the time of sound wave incidence of the optical fiber in
the second embodiment of the sound receiving structure of the optical fiber hydrophone of the
present invention; The figure is a microbend type hydrophone structure showing the basic
structure of the microbend type hydrophone, and FIG. 6 is a detail showing the structure of the A
part of the microbend type hydrophone of FIG. 5 in detail It is.
1 ..... sound receiving structure, 2 ...... emitting unit, 3 ..... light receiving unit, 10 ... ... elastic body
cylindrical, 11 ..... optical fiber, 12 .... Elastic cylinder. Eyebrows and beaks Fig.2 Fig.5 Fig.5
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