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Ming, letter 10: Title of invention
Optical fiber acoustic pressure sensor-
8. Detailed Description of the Invention i State of the State of the State--Mu-Replacement-Study 4
Schools--Ichi Yu-Yu "→ -1 1 The present invention relates to a device which greatly increases the
acoustic sensitivity per unit length of the optical fiber acoustic sensor Describe. Several types of
optical fiber acoustic sensors are described in the following document, using pressure induced
changes in the optical phase of the coherent light transmitted through the optical fiber There is.
An example of such a reference is as follows, as is U.S. Pat. Jay A. PCaro et al. 1 Fiber optic
acoustic sensor-Applied optics 1.6.1761. (1977) Identical 1 optical fiber to underwater 'turtle
story' Journal of a Stake Society America (J, Acoust, Soc, Am,), 62, A5, 1802-4 +, Nov, 1977 'same
as above Measurement of the sensitivity of optical fibers for optical fiber applied optics 1.8, 988
(1979) The acoustic sensitivity of such a device is proportional to the optical phase change Δφ
per unit pressure change ΔP of the unit fiber length Lfi. . For naked light 7 Ibar EndPage: 1 This
amount is given by Here, light propagation constant in β-fibers E = Young elastic modulus of
fiber material μ = Poisson's ratio of fiber material n = refractive index of fiber material pn + pu =
coefficient of relationship between strain of fiber material and light For most of glass If the
values of E and μ + pn + pu are greatly changed over a certain range, it does not occur. The
measured value of Δφ / Δp is about 2.6 × 101. It is / λrad / Pa. In this type of optical fiber, it
is shown that it can be used for acoustic detection such as underwater phone application as
underwater acoustic application. However, the sensitivity is practically limited. In the above-cited
eighth article by Pukao et al., When the protective plastic envelope remains on the fiber, its
coupling coefficient is one digit thicker than the uncoated fiber. This document brings to mind
the following: The plastic jacket is higher than the fiber (... Because it has Poisson's ratio and
compressibility, it extends more than a bare glass fiber and results in tensioning the glass fiber
and thereby increased responsiveness. In the present invention, an improved fiber optic acoustic
sensor is described in which the coil of the optical fiber is embedded in a mass of material having
a lower modulus than the material of the fiber.
This structure is very sensitive to the acoustic (i.e. pressure) of such a fiber optic ring sensor. The
basic principle of this improved structure is as follows. The harder material, in this case the fiber,
carries a large part of the Ls force being applied by pressure. And it is stressed much higher than
bare fibers that are actually under the same pressure. This ↓ 15, force increase causes higher
strain to increase once more than that of bare fibers and more than that of fibers coated easily
with-. The sensitivity of the fiber optic acoustic sensor described herein is due to the pressure
induced optical phase shift through the fiber. This phase shift has two main causes. Partly
pressure induced changes in fiber length and changes in refractive index within the fiber. The
change in length is explained by a simple elastic theory. The change in refractive index is
explained by the effect of the distortion-light relationship. And in isotropic materials such as
glass-even depending on both the axial and transverse strain components of the fiber. In a
cocoon fiber exposed to hydrostatic pressure, stress and cocoon are equal in all directions. The
effect on the dephasing of the change in length and the change in refractive index is opposite in
sign and equal in magnitude (the net effect is smaller than any of the above two terms since it
was found to be equal) . If buried in a mass of material with lower modulus of elasticity than the
fiber coil 応 力, the stress and strain in the fiber will vary significantly. The radial stress is
somewhat higher than that of the bare fiber, but not larger than that of the axial one. The most
important thing is that the axial stress can be greatly increased in the buried fiber. This means
that in any composite structure the force element (fiber) bears more force relative to the total
load, and, and then, the entire axial cart will layer pressure. This is because the cross-sectional
area of the fiber is multiplied. Therefore, much of the pressure applied to the axial force direction
of the buried fiber structure is transmitted from the mass of material in front of the fiber. So the
axial ratio, changes in force and length are much thicker than bare fibers. This increased axial
stress in the buried fiber and the corresponding increase in length change obviously increase the
pressure induced optical phase shift and the sound pressure sensitivity. However, there is a
second aspect of the sensitivity EndPage: 2 increase in the noon buried fiber.
Since the lateral strain in the buried fiber does not increase significantly, the dephasing due to
refractive index change does not increase as quickly as due to the change in length. The net
optical phase shift is the difference between both the change in length and the change in
refractive index. [In buried fibers this difference may even increase more quickly than by changes
in length alone. The improvement in sensitivity over uncoated fibers depends on the ratio of the
modulus of elasticity of the fiber to the modulus of the embedded material and the ratio of the
thickness to the radius of the fiber. Once the ratio of the 24th eye is high enough, it is almost
impossible to obtain the lessons of the sensitivity, but if these ratios are large, the improvement
of the acoustic sensitivity is as thick as the dog temperature. The sensitivity improvement also
depends on the value of the Poisson's ratio of the fiber and the buried material. Referring now to
FIG. 2, at 10, it is buried in a solid mass 12 made of a material having a lower modulus of
elasticity than the fiber material, eg light such as molten hydrofluoric acid '7 Ivar) The coil 11 of
the r-j bar is shown. The low modulus material may be, for example, a thermoset or heat fusible
material such as bite rubber. Plans have been made that the greatest increase in acoustic
sensitivity is obtained by the use of a buried material with low modulus and low Poisson's ratio.
This material should also be able to be molded around the fiber. Materials with low modulus also
tend to have high Poisson's ratio values, but many types of soft plastic can increase (or increase)
the sensitivity. Several boron rubber formulations and one polyurethane formulation are used to
achieve a 100-fold or greater sensitivity increase over bare fiber optics. The following is a trace
of some of the ingredients used. General Elect 1 Luk company Ha rubber) LTV 615) LTV 602,
Ferris di-through 1 mold compound (Fenis 5ee-ThruMo1dCompound) manufactured by JE Erler
エ イ ト Eight, and polyurethane PFL-1574 manufactured by Product Research & Chemical Co. In
some applications, fiber optic acoustic sensors must work under high static pressure. Under
these conditions, it may not be desirable to select the two softest investment materials for the
greatest increase in acoustic sensitivity. Soft materials with low modulus will experience large
static strain due to high static pressure.
Thus, the fibers embedded in this material will also be very distorted. Fiber breakage can be
experienced under such conditions. Moreover, the elastic properties of the investment material
alter the function of the static pressure, such as reducing the acoustic sensitivity of the sensor,
making the response non-proportional, and under large strains. In the case of such high static
pressure, a compromise can be made to choose a material waiting for a higher modulus as the
implantation material, 1 and its properties are not so affected by static pressure. The acoustic
sensitivity will be less if softer investment material is supported, but the fiber will be less likely to
break and the response of the acoustic sensor will depend on static pressure (01 In the case of,
etc., many more tl II like eg epoxy, polystyrene, polytetrafluoroethylene! A heat-resistant material
41 or a heat-resistant% 41 is suitable. Referring now to FIG. 3, at 20 a cylinder 22 with a fiber
optic coil 21 and a hollow core 28 is shown which has also been filled in a cylinder made of a
material having a lower modulus than the fiber material at 20. It is done. That is, the fiber can be
buried in the lower modulus hollow structure. And the shape of the structure is designed to
increase the stress D in some direction of the structure when exposed to isotropic pressure. For
example, when applied to an external pressure Pb'-thin-walled hollow sphere, the tangential
stress of the spherical shell is somewhat thicker than P. Similarly, when the pressure P is applied
to the outside of a thin-walled hollow cylinder closed at both ends by an end plate, the axial
stress of the wall can be greater than P. (10) EndPage: axial stress and optical phase induced by
pressure in 3 fibers in a structure where the fibers are embedded so that they run predominantly
in the direction of maximum stress The slippage can be increased somewhat more than if the
fibers are embedded in a uniform mass of the same elastic material. In hollow structures made of
such lower modulus materials, one example is a relatively thin-walled hollow sphere of lower
modulus material in which the fiber is embedded in the material It can mean circumferentially
wound ones. The second example is a relatively thin-walled hollow cylinder made of an elastic
material closed at both ends by end plates, which are predominantly vertically wound, embedded
in the fiber or the material. It can. And the eighth example is a similar cylinder, but it could mean
that the fibers are wound circumferentially (r). .
4. Brief Description of the Drawings FIG. 1 is a view showing an embodiment of a conventional
optical fiber acoustic pressure sensor. FIG. 2 and FIG. 8 are views of one non-inventive
embodiment. 0υ1.0.20 ····························································································· Block material Patent
applicants Honeywell Inc. Attorney's attorney Eiji Matsushita 02) EndPage: 4