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JPH09178544

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DESCRIPTION JPH09178544
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
cylindrical optical fiber acoustic sensor, and more particularly to a high water pressure resistant
cylindrical optical fiber acoustic sensor that converts sound pressure into phase change of light
to detect underwater acoustic waves.
[0002]
2. Description of the Related Art For a general cylindrical optical fiber acoustic sensor, see G.I. F.
It is disclosed by the following document entitled "Theoretical Analysis of a Respiratory
Vibration-Type Optical Fiber Hydrophone: Theoretical Analysis of a Push-Pull Fiber-Optic
Hydrophon" by Mcdearmon. Literature name: Journal of Lightwave Technology, [vol. Lt 5], No. 5,
pp 647-652, May 1987 (US)
[0003]
The cylindrical optical fiber acoustic sensor disclosed in the above-mentioned document utilizes
respiratory vibration of two cylinders, and FIG. 5 shows a schematic cross-sectional view of this
sensor. In the figure, in the sensor, two cylinders of an outer cylinder 1 and an inner cylinder 2
having a predetermined length are arranged concentrically, lids 3 are attached to both ends of
the cylinder, and air chambers are provided by spaces equally spaced between the cylinders. It is
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formed as four. The optical fibers 5 and 5a are respectively wound around the inner surface of
the outer cylinder 1 and the outer surface of the inner cylinder 2 in a coiled configuration.
[0004]
Here, when sound pressure is applied to the sensor, a pressure difference occurs between the
inside and the outside of the cylinder, so that both cylinders vibrate in the opposite direction to
each other. At this time, since the optical fibers 5 and 5a wound around the two cylinders also
expand and contract in the opposite direction to each other, the phase of the laser light
propagating in the optical fibers 5 and 5a changes. Therefore, the sound pressure is detected
with high sensitivity by a known method using interference of light with the phase change
amount of the laser light proportional to the sound pressure.
[0005]
SUMMARY OF THE INVENTION The above-mentioned conventional cylindrical fiber optic
acoustic sensor has an air chamber formed by closing and sealing the space between the inner
and outer cylinders with a lid. It is not a balanced cylindrical optical fiber acoustic sensor.
Therefore, when it is used in water, there is a problem that the water pressure resistance is
generally low and it can not be used, for example, in deep water as currently required.
[0006]
SUMMARY OF THE INVENTION A high water pressure resistant cylindrical optical fiber acoustic
sensor according to the present invention comprises a double cylindrical structure in which two
inner and outer cylindrical bodies are combined in a concentric manner. A cylindrical optical
fiber acoustic sensor comprising: a lid for closing a gap formed by two cylinders at each end
thereof and an optical fiber wound along the side of each cylinder, the lid comprising The cover
is formed on the lid on one side of the opening and has an opening for equalizing the hydrostatic
pressure of the inside of the gap and the periphery thereof. Here, the diameter of the opening
should be a diameter that does not pass sound waves at frequencies higher than the Helmholtz
resonance frequency in the Helmholtz resonator constituted by the opening and a gap
corresponding to the cavity. is necessary.
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[0007]
In the present invention, since the lid in one side of the lid used to seal the gap between the two
cylinders at both ends of the cylinder is provided with an orifice-like opening, this sensor is used.
In the case of grounding or floating in water, the inside of the gap and the outer periphery
thereof communicate with each other through the opening, and the inside of the gap and the
outer periphery thereof become equal in hydrostatic pressure. A sensor is configured. In this
case, the opening and the gap simulate the relationship between the hole and the cavity of an
acoustic device (musical instrument etc.), and this structure constitutes a kind of Helmholtz
resonator for sound waves (sound). Therefore, sound waves of frequencies below the Helmholtz
resonance frequency determined by the diameter of the opening pass through the opening, but
sound waves of frequencies above the Helmholtz resonance frequency do not pass through the
opening. Therefore, in this configuration, if a sound wave of a frequency higher than the
Helmholtz resonance frequency is used as a probe of the sensor, "hydrostatic pressure + sound
pressure of this sound wave" is applied to the outside of the sensor, but only hydrostatic pressure
is applied to the gap. I can only join. Accordingly, pressure unbalance occurs between the inside
and the outside of the cylinder, and the inner and outer cylinders vibrate with respiration by the
sound pressure of this sound wave.
[0008]
DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] FIG. 1 is a schematic
sectional view showing a first embodiment of a high water pressure resistant cylindrical optical
fiber acoustic sensor (hereinafter referred to as a sensor) according to the present invention. In
FIG. 1, two cylinders of an outer cylinder 1 and an inner cylinder 2 having a predetermined
length are arranged concentrically, lids 3 are attached to both ends of the cylinders, and gaps by
gap spaces equally spaced between each cylinder It is formed to be six.
[0009]
The optical fibers 5 and 5a are respectively wound in a coil shape on the outer surface of the
outer cylinder 1 and the outer surface of the inner cylinder 2 to constitute an interferometer. And
in the case of this embodiment, it has a sensor structure where the orifice 7 which leads to the
gap part 6 is provided in any one lid among the two lids 3 which are upper and lower sides as an
opening. With this structure, when the sensor is installed or floated, for example, in the water,
the gap 6 is filled with water through the orifice 7, so the gap 6 is filled with the same medium as
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the medium around the sensor and with the same hydrostatic pressure. It can be satisfied.
[0010]
Next, the operation will be described. When the orifice 7 is provided on the lid 3 on one side, and
this sensor is installed or floated in water, for example, the gap 6 is filled with the same liquid as
the outer periphery, so the Helmholtz determined by the diameter of the orifice 7 Sound waves
having a frequency lower than the resonance frequency pass through the orifice 7. Therefore,
although "the hydrostatic pressure + the sound pressure of this sound wave" is applied to the gap
6, the same "hydrostatic pressure + the sound pressure of this sound wave" is applied to the
liquid (water) outside the sensor. And the pressure on the outside are balanced, and the inner and
outer cylinders do not vibrate by breathing with this sound wave. However, this configuration
provides a pressure balance structure that does not collapse under hydrostatic pressure.
[0011]
Therefore, in this configuration, if a sound wave of a frequency higher than the Helmholtz
resonance frequency is used as a probe of the sensor, this sound wave does not pass through the
orifice 7, so "hydrostatic pressure + sound pressure of this sound wave" is applied outside the
sensor. However, only the hydrostatic pressure is applied to the gap 6. Accordingly, pressure
unbalance occurs between the inside and the outside of the cylinder, and the inner and outer
cylinders vibrate with respiration by the sound pressure of this sound wave. Therefore, it
functions as an acoustic sensor as described in the prior art. In other words, the lid 3 may be
provided with an orifice 7 having a diameter that does not allow the passage of sound waves
having a frequency higher than the Helmholtz resonance frequency. Thus, the pressure-balanced
dual-cylindrical optical fiber acoustic sensor having the structure in which the lid 3 has the
orifice 7 and the gap portion 6 between the inner and outer cylinders is filled with liquid has
high water pressure resistance. It will be applicable even in deep water.
[0012]
As described above, according to the first embodiment, the lid for sealing the gap between double
cylinders of the cylindrical optical fiber acoustic sensor is provided with an opening formed by
an orifice, and the above-mentioned gap is filled with the liquid. By making it possible to match
the hydrostatic pressure in the gap with the hydrostatic pressure outside the sensor, thereby
obtaining a cylindrical optical fiber acoustic sensor (hydrophone) with high water pressure
resistance. it can.
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[0013]
Second Embodiment FIG. 2 is a schematic sectional view showing a second embodiment of the
high water pressure resistant cylindrical optical fiber acoustic sensor according to the present
invention.
In FIG. 2, two cylinders of an outer cylinder 1 and an inner cylinder 2 having a predetermined
length are arranged concentrically, lids 3 are attached to both ends of the cylinders, and gaps by
gap spaces equally spaced between each cylinder It is formed to be six. Further, in the present
embodiment, the optical fibers 5 and 5a are respectively wound around the inner side surfaces of
the outer cylinder 1 and the inner cylinder 2 in a coil shape to constitute an interferometer. And
although the orifice 7 is provided in the lid 3, the configuration and structure thereof are the
same as those described in the first embodiment.
[0014]
And in this embodiment, the optical fibers 5 and 5a are each coiled around the inner side surface
of the outer cylinder 1 and the inner cylinder 2 so as to constitute an interferometer. The other
configurations, operation and effects and advantages to advantages are the same as in the first
embodiment described above, and thus detailed description will be omitted.
[0015]
[Third Embodiment] FIG. 3 is a schematic cross sectional view showing a third embodiment of the
high water pressure resistant cylindrical optical fiber acoustic sensor according to the present
invention.
In FIG. 3, two cylinders of an outer cylinder 1 and an inner cylinder 2 having the same
predetermined length are arranged concentrically, lids 3 are attached to both ends of the
cylinders, and gaps by equally spaced gap spaces between the cylinders It is formed to be a part
6. Further, in the present embodiment, two optical fibers 5 and 5b and two optical fibers 5a and
5c are wound in the form of a coil on the inner side and the outer side on the inner and outer
surfaces of the outer cylinder 1 and the inner cylinder 2, respectively. The structure is configured
to construct an interferometer as shown in FIG. And although the orifice 7 is provided in the lid ¦
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cover 3, the structure and the structure are the same as that of what was demonstrated in 1st
Embodiment.
[0016]
Here, the structure of the interferometer of FIG. 5 just described will be described. As seen in FIG.
5 as an example, the optical fibers 5 and 5b provided on the inner side and the outer side of the
outer cylinder 1 and the optical fibers 5a and 5c provided on the inner side and the outer side of
the inner cylinder 2 respectively Each of the optical paths is separately connected to form two
optical paths, and both ends of the optical paths are connected to two couplers 8 and 8a. In this
configuration, for example, the laser light propagated in one optical fiber from the left in the
figure is split into two optical paths by the coupler 8, combined into one optical path by the
coupler 8a, and again to the right by one optical fiber It is designed to be taken out. In the case of
this interferometer, when a sound wave arrives at the sensor and the two cylinders breathe and
vibrate, the four optical fibers 5 to 5 c also vibrate, and the wave fronts of the laser beams of the
two optical paths shift. It is used to observe the interference wave in the emitted laser light. In
the first and second embodiments, illustration and description are omitted, but a two-element
interferometer having a smaller number of elements than the interferometer in FIG. 5 is formed.
[0017]
In the present embodiment, the optical fibers 5, 5a, 5b, 5c are optionally coiled around the inner
and outer surfaces of the outer cylinder 1 and the inner cylinder 2 to form an interferometer
with four elements. It is characterized in that the measurement sensitivity by the interferometer
is higher than in the first and second embodiments described above. However, since the other
configurations, actions, effects, and advantages and advantages are the same as those of the first
and second embodiments, the detailed description thereof will be omitted.
[0018]
As described above, according to the present invention, the gap formed by the double cylindrical
structure in which the two inner and outer cylinders are combined concentrically and formed by
the two inner and outer cylinders is formed. The lid on one side of the lid of a cylindrical optical
fiber acoustic sensor comprising a lid closing each of its ends at its opposite ends and further
comprising an optical fiber wound along the side of each cylinder Since an opening is provided to
equalize the hydrostatic pressure between the inside of the part and the outer peripheral part,
the hydrostatic pressure in the gap and the hydrostatic pressure outside the sensor can be made
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to coincide, so high sensitivity and high water resistance A cylindrical fiber optic acoustic sensor
can be obtained.
[0019]
Brief description of the drawings
[0020]
1 is a schematic cross-sectional view showing a first embodiment of a high water pressure
resistance cylindrical optical fiber acoustic sensor according to the present invention.
[0021]
2 is a schematic cross-sectional view showing a second embodiment of the high water pressure
resistance cylindrical optical fiber acoustic sensor according to the present invention.
[0022]
3 is a schematic cross-sectional view showing a third embodiment of the high water pressure
resistance cylindrical optical fiber acoustic sensor according to the present invention.
[0023]
4 is a schematic explanatory view showing an interferometer configuration of the embodiment of
FIG.
[0024]
5 is a schematic cross-sectional view showing a conventional cylindrical optical fiber acoustic
sensor.
[0025]
Explanation of sign
[0026]
Reference Signs List 1 outer cylinder 2 inner cylinder 3 lid 44 air chamber 5, 5a, 5b, 5c optical
fiber 6 gap 7 orifice 8 8a coupler
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