JPH06339193

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DESCRIPTION JPH06339193
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
optical fiber acoustic sensor for converting the sound pressure of an optical fiber hydrophone for
detecting underwater acoustic waves into a phase change of light and a method of manufacturing
the same.
[0002]
2. Description of the Related Art Two types of optical fiber acoustic sensors will be described
below as conventional examples. (A) First, conventionally, as a first prior art in such a field, for
example, “DA Brown, T. Hofler, SL Garrett“ High-Sensitivity, Fiber-Optic, Flexural Disk
Hydrophone with Reduced Acceleration Response ” ”Fiber and Integrated Optics. Vol. 8, pp.
169-191, 1989].
[0003]
FIG. 4 is a block diagram of a high-sensitivity optical fiber hydrophone using the bending of the
conventional diaphragm. As shown in FIG. 4, this hydrophone comprises a cylindrical case 1, a
diaphragm 2 provided at both ends of the case 1, and an optical fiber 3 adhered in a spiral shape
on the diaphragm 2.
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[0004]
(B) Next, conventionally, as a second prior art in such a field, for example, “Sato et al., Pressure
balancing of an optical fiber acoustic sensor using a circular diaphragm, as the first document,
the 13th ultrasonic wave Proceedings of the symposium on the basics and applications of
electronics, p. 229, November 30, 1992, "High-water-resistant high-fiber hydrophone with high
water resistance using bending of the diaphragm", as the second document, “A. Dandridge, A. B.
Tveten, T. G. Gialloenzi“ Homodyne Demodulation Scheme for Fiber Optic Sensor Using Phase
Generated Carrier ”Journal of Quantum Electronics. Ｖｏｌ． ＱＥ−１８． No. 10, 1982 "has
been disclosed.
[0005]
As shown in FIG. 5, since the orifice 4 is provided in the housing 1 and the inside of the housing
1 is filled with the liquid, the pressure balance is statically maintained between the inside and the
outside of the housing 1 Because the pressure resistance is high. When the diaphragm 2 vibrates
due to acceleration, the acceleration sensitivity is reduced by utilizing the fact that the sign of
distortion is reversed on the front and back of the diaphragm 2.
[0006]
In addition, there is a passive homodyne method (see the above-mentioned second document) as
one of the interferometry methods for detecting the phase change of the laser beam. In this
method, assuming that the output of the O / E converter is O by modulating the phase of the
interference light in a sine wave, O = A + B cos [C cos ωC t + Φ (t)] (1). Here, A, B, and C are
constants, ω c is a modulation angular frequency, t is time, and ((t) is a phase change of
interference light including an acoustic signal.
[0007]
Therefore, when the above equation (1) is expanded, O = A + B {[JO (C) + 2.SIGMA. (-1) K J2 K (C)
cos 2k.omega.Ct] cos .PHI. (T) -2.SIGMA. (-1) K J2K-1 (C) ) Cos (2 k + 1) ω C (t)} sin ((t) (2)
[0008]
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The spectrum of the above (2) is shown in FIG.
In FIG. 6, the horizontal axis represents the angular frequency ω c, the vertical axis represents
the spectrum, a represents the upper side band of the primary wave, and b represents the lower
side band of the secondary wave. Taking the amplitudes of the primary wave and the secondary
wave from the above equation (2), -BJ1 (C) sin ((t) (3a)-BJ2 (C) cos ((t) (3b) , -DΦ (t) / dt BJ1 (C)
cos ((t) (4a) dΦ (t) / dt BJ2 (C) sin ((t) (4b) above (3a), (4b), and (4b) 4a) and (3b) are multiplied
and -dΦ (t) / dt B2 J1 (C) J2 (C) sin 2Φ (t) (5a) dΦ (t) / dt B2 J1 (C) J2 (C) cos 2cos (T) (5b)
Subtracting these gives dd (t) / dt B2 J1 (C) J2 (C) (6)
[0009]
By integrating this, it is possible to detect the phase change Φ (t) of the interference light
including the acoustic signal.
[0010]
However, as described above, in the case of (1) the optical fiber hydrophone using bending of the
diaphragm, the sensitivity is further improved by lengthening the optical fiber bonded to the
diaphragm. , The diameter of the diaphragm becomes large, and the sensor becomes large.
[0011]
(2) In the passive homodyne system, as shown in FIG. 6, high frequency components of the lower
sideband of the secondary wave overlap the upper sideband of the primary wave, and distortion
occurs in the signal.
When time division multiplexing is performed with interference methods other than passive
homodyne, frequency aliasing occurs due to sampling, and the signal is distorted.
The present invention realizes a compact, high-hydraulic pressure-resistant optical fiber acoustic
sensor with a structure that reduces acceleration sensitivity in order to eliminate the problems
described above, and a cylindrical optical fiber that can reduce signal distortion due to frequency
aliasing. It aims at providing an acoustic sensor.
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[0012]
SUMMARY OF THE INVENTION In order to achieve the above object, the present invention
relates to (A) a cylindrical optical fiber acoustic sensor for converting the sound pressure of an
optical fiber hydrophone into a phase change of light. An optical fiber coil in which an optical
fiber is wound one by one is formed on the outer side, and a lid is attached to both ends of the
cylinder with a soft material interposed therebetween to detect respiratory vibration of the
cylinder.
[0013]
Further, in a method of manufacturing a cylindrical optical fiber acoustic sensor for converting
the sound pressure of an optical fiber hydrophone into a phase change of light, an optical fiber
with an adhesive attached to the outside of the cylinder is wound to form an optical fiber coil.
The optical fiber coil attached with the adhesive wound around a cylinder in which the adhesive
does not adhere is adhered to the inside of the cylinder to form an optical fiber coil.
[0014]
(B) In an optical fiber acoustic sensor with a pressure balance structure that converts the sound
pressure of an optical fiber hydrophone into a phase change of light, one optical fiber is wound
on each of the inside and the outside of a cylinder, and an orifice is formed on both ends of the
cylinder. It forms and detects the respiration vibration of the said cylinder.
Furthermore, in an optical fiber acoustic sensor with a pressure balance structure that converts
the sound pressure of an optical fiber hydrophone into a phase change of light, Hertzholm
resonators are formed on the inside and the outside of a cylinder, and the frequency at which the
cylinder vibrates is band limited. It is something like that.
[0015]
According to the present invention, as in the above (A), in the cylindrical optical fiber acoustic
sensor for converting the sound pressure of the optical fiber hydrophone into the phase change
of light, one each is inside and outside of the cylinder. An optical fiber coil is formed by winding
an optical fiber, and both ends of the cylinder are covered with a soft material and a lid is
attached so as to detect respiratory vibration of the cylinder. Since the structure is wound on the
outside, the sensitivity can be improved.
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[0016]
In addition, when it is desired to increase the length of the optical fiber, it is also possible to wind
them one on top of the other, so that the sensor does not become large and it is possible to
obtain a small-sized and highly sensitive optical fiber acoustic sensor.
(2) Since this optical fiber acoustic sensor detects only the respiratory vibration generated only
by the sound pressure, the acceleration sensitivity can be suppressed.
[0017]
(3) The optical fiber wound around a cylinder in which the adhesive does not adhere is adhered
to the inside of the cylinder, so that the manufacture is easy.
Also, as in the above (B), in other words, in an optical fiber acoustic sensor that converts the
sound pressure of an optical fiber hydrophone into a phase change of light, the optical fiber is
wound one inside each of the inside and outside of the cylinder with a lid The Helmholtz
resonator is constructed by providing an orifice in the cylinder to fill the cylinder with liquid, and
the pressure balance is maintained against hydrostatic pressure, and when the cylinder breathes
and vibrates under sound pressure above the Helmholtz resonance frequency Since the
difference in length of the two optical fibers is changed, in addition to the above effects, (1) all
the sensors in the sensor are statically pressure-balanced, so the water pressure resistance is
high. .
[0018]
(2) Further, if a Helmholtz resonator is provided outside the cylinder, a frequency band higher
than the resonance frequency fOA of the outer Helmholtz resonator is not detected, so that signal
distortion due to frequency aliasing can be reduced. In other words, for the problem that signal
distortion occurs due to frequency aliasing, a Helmholtz resonator is also applied to the outside
of the cylinder, and pressure balance is maintained between the inside and the outside of the
cylinder above the resonant frequency of the outside Helmholtz resonator. Configure to Thus, the
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high frequency component of the acoustic signal is blocked by the acoustic sensor unit of the
hydrophone.
[0019]
Embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a view showing the structure of a cylindrical optical fiber acoustic sensor showing a first
embodiment of the present invention. As shown in FIG. 1, the optical fiber coil 12 is wound on
the outside of the cylinder 11 of the cylindrical optical fiber acoustic sensor 10, and the optical
fiber coil 13 is wound on the inside of the cylinder 11. The lengths of the optical fiber coil 12
and the optical fiber coil 13 are approximately the same. Lids 15 and 17 are attached to both
ends of the cylinder 11 with soft ring-shaped plates 14 and 16 of rubber or the like interposed
therebetween, and the inside of the cylinder 11 is made an air chamber.
[0020]
The vibration model of the cylinder thus obtained is shown in FIG. Here, the radius of the
cylinder 11 is r0 and the thickness is 2 h. When the length L0 optical fiber is wound on the
outside and the inside of the cylinder 11, the number of turns N1 and N2 of the outside and the
inside optical fiber coils 12 and 13 are respectively N1 = L0 / [2π (r0 + h)] (7 ) N2 = L0 / [2π
(r0-h)] (8)
[0021]
When the cylinder 11 receives sound pressure and breathes and vibrates (primary mode), the
radius r0 of the cylinder 11 changes. Assuming that the axial average of this variation is Δr, the
optical fiber length L1 of the outer optical fiber coil 12 and the optical fiber length L2 of the
inner optical fiber coil 13 when the cylinder 11 breathes and vibrates are L1. = 2? (R0 + h-? R)
N1 (9) L2 = 2? (R0-h-? R) N2 (10)
[0022]
From the above equations (7) to (10), the difference in length L1-L2 of the two optical fibers is
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L1-L2 = 2hL0? R / (r0 2-h2), which is proportional to? R. The above is the operation principle of
the optical fiber acoustic sensor. As the whole optical fiber hydrophone, a laser beam is passed
through two optical fiber coils 12 and 13, and the phase difference of the laser beam generated
by L1-L2 is detected using an interference method.
[0023]
FIG. 3 is a perspective view showing a method of manufacturing a cylindrical optical fiber
acoustic sensor according to a first embodiment of the present invention. As shown in FIG. 3A, an
optical fiber with an adhesive is wound around a degradable cylinder 21 made by combining
rods 22 to form an optical fiber coil 13. On the other hand, an optical fiber with an adhesive
attached to the cylinder 11 is wound to form an optical fiber coil 12. The cylinder 21 is inserted
into the cylinder 11.
[0024]
Next, as shown in FIG. 3B, when the adhesive is cured, the cylinder 21 is disassembled and
removed, and the optical fiber coil 13 is fixed inside the cylinder 11 and the optical fiber coil 12
is fixed outside the cylinder 11 The cylinder 10 is obtained. Then, as shown in FIG. 3C, the lid 15
is adhered to the lower end of the cylindrical body 10 via the soft ring plate 14, and similarly, the
lid 17 is formed on the upper end of the cylindrical body 10 via the soft ring plate 16. To make
the inside of the cylinder 11 an air chamber.
[0025]
FIG. 7 is a cross-sectional view showing the structure of a cylindrical optical fiber acoustic sensor
with a high-frequency band limiting function according to a second embodiment of the present
invention. The optical fiber coil 32 is wound on the outside of the cylinder 31, and the optical
fiber coil 33 is wound on the inside of the cylinder 31. The lengths of the optical fiber coil 32
and the optical fiber coil 33 are approximately the same. A lid 34 provided with an orifice 35 is
attached to the end of the cylinder 31, and the inside of the cylinder 31 is filled with a liquid.
[0026]
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The vibration model of the cylinder is shown in FIG. The radius of the cylinder 31 is r0 and the
thickness is 2 h. When an optical fiber of length L0 is wound on the outer side and the inner side
of the cylinder 31, the number of turns N1 and N2 of the outer and inner optical fiber coils 32,
33 respectively become the above-mentioned formula (7) and formula (8). When the cylinder 31
receives sound pressure and vibrates in respiration (primary mode), the radius r0 of the cylinder
31 changes. Assuming that the axial average of this amount of change is Δr, the optical fiber
length L1 of the outer optical fiber coil 32 and the optical fiber length L2 of the inner optical
fiber coil 33 when the cylinder 31 breathes and vibrates are as described above. Formulas (9)
and (10) are expressed.
[0027]
From the above equations (7) to (10), the difference L1-L2 of the lengths of the two optical fibers
becomes L1-L2 = 2 hL0? R / (r0 2-h2), which is proportional to? R. Laser light can be passed
through the two optical fiber coils 32 and 33, and the phase difference of the laser light
generated by L1-L2 can be detected using an interference method. The Helmholtz resonator is
composed of the orifice 35 attached to the lid 34 and the liquid inside the cylinder 31, and the
pressure balance is maintained between the inside and the outside of the cylinder 31 against
hydrostatic pressure, so high water pressure resistance is obtained. When receiving sound
pressure higher than the Helmholtz resonance frequency, the cylinder 31 breathes and vibrates.
[0028]
Therefore, an HPF (high-pass filter) having a cutoff frequency near the Helmholtz resonance
frequency is configured. Thus, in an optical fiber acoustic sensor that converts the sound
pressure of an optical fiber hydrophone into a phase change of light, an optical fiber coil is
wound one each on the inside and the outside of the lidded cylinder, and an orifice is provided on
the lid to form an internal cylinder. Since the Helmholtz resonator is configured by filling the
liquid, the pressure balance is maintained against the hydrostatic pressure, and when the
cylinder vibrates under the sound pressure above the Helmholtz resonance frequency, the length
of the two optical fibers The difference in height changes and underwater sound waves can be
detected.
[0029]
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FIG. 9 is a cross-sectional view showing a structure of a band-limited cylindrical optical fiber
acoustic sensor according to a third embodiment of the present invention. In this embodiment,
the cylindrical optical fiber acoustic sensor shown in the second embodiment is housed in a
housing 46 provided with an orifice 47, and the housing 46 is filled with a liquid.
[0030]
The Helmholtz resonance frequency fOA at the orifice 47 provided in the housing 46 and the
cavity between the housing 46 and the cylinder 41, and the Helmholtz resonance frequency f0B
at the cavity in the cylinder 41 and the orifice 45 provided on the lid 44 of the cylinder 41 It is
assumed that f0B <fOA. Since the sound wave of f> fOA does not pass through both the orifice 45
of the lid 44 of the cylinder 41 and the orifice 47 of the housing 46, where the frequency of the
sound wave is f, the cylinder 41 does not vibrate.
[0031]
The sound wave of f0B <f <fOA does not pass through the orifice 45 of the lid 44 of the cylinder
41 but passes through the orifice 47 of the housing 46, so that the cylinder 41 vibrates. Since
the sound wave of f <f0B passes both the orifice 45 of the lid 44 of the cylinder 41 and the
orifice 47 of the housing 46, the pressure balance is maintained and the cylinder 41 does not
vibrate. Therefore, a BPF (band pass filter) is configured. The same effect can be obtained as f0B>
fOA.
[0032]
Thus, Hertzholme resonators can be configured on the inside and the outside of the cylinder so
as to band-limit the frequency at which the cylinder vibrates. Here, for the problem that signal
distortion occurs due to frequency aliasing, a Helmholtz resonator is also applied to the outside
of the cylinder, and pressure balance is maintained between the inside and the outside of the
cylinder above the resonance frequency of the outside Helmholtz resonator Let's do it. Thereby,
the high frequency component of the acoustic signal is blocked by the acoustic sensor unit of the
hydrophone.
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[0033]
In the above embodiment, one layer of optical fiber coil is formed on each of the inner and outer
sides of the cylinder, but if it is necessary to increase the length of the optical fiber, it is possible
to wind them one on top of the other. By configuring in this way, the sensor can not be large, and
it is possible to obtain a small-sized and highly sensitive optical fiber acoustic sensor. The present
invention is not limited to the above-described embodiment, and various modifications are
possible based on the spirit of the present invention, and they are not excluded from the scope of
the present invention.
[0034]
As described above in detail, according to the present invention, the following effects can be
achieved. According to the first invention, (1) the optical fiber is wound around the inside and the
outside of the cylinder, so that the sensitivity can be improved.
[0035]
In addition, when it is desired to increase the length of the optical fiber, by overlapping and
winding, the sensor does not become large and it is possible to obtain a small-sized and highly
accurate optical fiber acoustic sensor. (2) Since this optical fiber acoustic sensor detects only the
respiratory vibration generated only by the sound pressure, the acceleration sensitivity can be
suppressed.
[0036]
(3) Since the optical fiber wound in a cylinder in which the adhesive is prevented from adhering
is adhered to the inside of the cylinder, manufacture is easy. According to the second aspect of
the invention, furthermore, (1) the pressure balance is statically maintained in all of the sensors,
so the water pressure resistance is high.
[0037]
(2) Since a frequency band higher than the resonance frequency fOA of the outer Helmholtz
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resonator is not detected, signal distortion due to frequency aliasing can be reduced.
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