JP2011205171

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DESCRIPTION JP2011205171
A conventional ultrasonic wave receiver includes a condenser microphone and the like, but since
these use a diaphragm in principle, there is a limitation in a frequency band. An ultrasonic wave
propagating through a waveguide is in contact with a waveguide having an opening through
which the ultrasonic wave is incident and through which the ultrasonic wave incident on the
opening propagates, and a plane parallel to the propagation direction of the ultrasonic wave in
the waveguide. It is arranged at both ends of the propagation direction of the light wave crossing
the propagation medium part in the propagation medium part, and a light source for emitting the
light wave crossing the propagation medium part from the propagation medium part where part
is incident, and the direction different from the propagation direction of ultrasonic waves.
Ultrasonic wave reception comprising: a partial reflection member transmitting part of the light
wave and reflecting the remaining part light wave; and a photoelectric conversion part
converting the light wave transmitted through the partial reflection member and the propagation
medium part into an electrical signal Provide the [Selected figure] Figure 1
Ultrasonic wave receiver
[0001]
The present invention relates to an ultrasonic wave receiver having particularly high
performance with respect to the reception of ultrasonic waves, and particularly to an ultrasonic
wave receiver for gas having high performance when ultrasonic waves propagating through a
gas such as air are received. About.
[0002]
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A microphone is conventionally known as a device for receiving a sound wave.
Many microphones represented by dynamic microphones and condenser microphones use
diaphragms. In these microphones, the input sound wave vibrates the diaphragm, and the
vibration is taken out as an electric signal. However, in a microphone using a diaphragm, there is
a problem that the diaphragm has resonance characteristics and can be used only in a limited
frequency band.
[0003]
There are the techniques of Patent Documents 1 and 2 for microphones that do not use a
diaphragm. The technique described in Patent Document 1 detects a light wave reflected a
plurality of times by the reflecting member in order to measure the sound field between the
opposingly disposed reflecting members as shown in FIG. Is a microphone that measures
[0004]
As shown in FIG. 24, the technique described in Patent Document 2 uses an Fabry-Perot
interferometer to detect a change in frequency of laser light irradiated to the surface of an object,
thereby transmitting ultrasonic waves propagating through the object. Detect with high
sensitivity. Further, as shown in FIG. 25, in Patent Document 3, ultrasonic waves are taken into
the propagation medium portion with high efficiency from a medium with a very small acoustic
impedance such as air, or ultrasonic waves with high efficiency from the propagation medium
portion into air. A silica dry gel is described as a solid medium that can be emitted. Hereinafter, a
microphone that detects a sound wave using a light wave is referred to as an "optical
microphone".
[0005]
Patent Document 1: JP-A-2004-279259 Patent Document 2: JP-A-9-281086 Patent Document 2:
International Publication No. 2004-098234
[0006]
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In the optical microphone of Patent Document 1, the reflecting member also serves as an
opening through which a sound wave is input.
The distance between the reflecting members is 1 m or more, and the size of the reflecting mirror
is about 0.6 m × 0.5 m, which is a very large aperture. Therefore, the optical microphone of
Patent Document 1 uses sound in a wide area such as aircraft noise for measurement, and can
not be used as it is as an acoustic receiver with a small aperture as in the prior art.
[0007]
It is also conceivable to narrow the distance between mirrors and reduce the aperture, but the
sensitivity of the optical microphone largely depends on the product ΔnL of the light
propagation distance L and the change amount Δn of the refractive index of air caused by the
sound propagation. Therefore, if the distance between the mirrors is narrowed, the light
propagation distance L is shortened and sufficient sensitivity can not be obtained, and it is
difficult to operate as an acoustic receiver. Therefore, the optical microphone of Patent Document
1 can not measure the local sound pressure with a small aperture as in the conventional
microphone.
[0008]
In Patent Document 2, as shown in FIG. 24, by detecting a change in frequency of laser light
irradiated to the surface of the object using a Fabry-Perot interferometer, ultrasonic waves
propagating through the object are detected with high sensitivity. Do. However, this is to detect
the sound waves in the solid, not to detect the sound waves in the air. Applying this, it is also
conceivable to take in ultrasonic waves in air into solids and detect ultrasonic waves, but normal
solids differ greatly in acoustic impedance with air, and reflection at the air / solid interface is
Because it is large, it can not be efficiently incorporated inside.
[0009]
Then, an object of this invention is to solve the said subject and to provide the small-sized
ultrasonic wave receiver which detects the sound wave in the air, without using a diaphragm.
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[0010]
An ultrasonic wave receiver according to the present invention is an ultrasonic wave receiver for
receiving an ultrasonic wave propagating in a space filled with an environmental fluid, the
ultrasonic wave receiver having an opening through which the ultrasonic wave is incident, the
opening And a propagation medium portion in contact with a plane parallel to the propagation
direction of the ultrasonic wave in the waveguide, and a part of the ultrasonic wave propagating
in the waveguide being incident; A light source for emitting a light wave crossing the propagation
medium portion from a direction different from the propagation direction of the sound wave and
both ends of the propagation medium portion in the propagation direction of the light wave
crossing the propagation medium portion transmit part of the light wave A partial reflection
member for reflecting the remaining part of the light wave, and a photoelectric conversion part
for converting the light wave transmitted through the partial reflection member and the
propagation medium part into an electric signal, the propagation medium part, and the light
source , The partially reflecting member, and the light source A Fabry-Perot interferometer is
configured, and the density ρ1 of the propagation medium portion, the speed of sound C1 in the
propagation medium portion, the density 22 of the environmental fluid, and the speed of sound
C2 in the environmental fluid are (ρ2 / ρ1) <(C1 / C2). ) Meet the relationship <1
[0011]
According to the present invention, by disposing the propagation medium portion in the FabryPerot interferometer and detecting the output light thereof, the ultrasonic wave transmitted from
the environmental fluid to the propagation medium portion is detected.
This makes it possible to provide a small-sized, small-sized ultrasonic wave receiver without
using a diaphragm.
[0012]
The perspective view of the optical microphone by Embodiment 1 of this invention The xz
sectional drawing of the optical microphone by Embodiment 1 of this invention The figure which
shows the transmission characteristic of silica dry gel Another example of the optical microphone
of Embodiment 1 The figure which shows the condition where the spot diameter of a light wave
is larger than the wavelength of the ultrasonic wave in a perspective view propagation medium
part which shows The figure which shows a condition where the spot diameter of a light wave is
smaller than the wavelength of the ultrasonic wave in a propagation medium part Figure showing
an example of input / output response of Fabry-Perot interferometer by yz cross section of
optical microphone according to the figure Figure showing dependence on reflectivity of mirror
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of Fabry-Perot interferometer Another example of optical microphone according to Embodiment
1 Is a yz sectional view showing another embodiment of the optical microphone according to the
first embodiment, and yz is a sectional view showing the other embodiment of the optical
microphone according to the first embodiment. The yz sectional view which shows other
examples A perspective view which shows the other example of the optical microphone of
Embodiment 1 The figure which shows an example of the input-output response of reflection
type Fabry-Perot interferometer The light by Embodiment 2 of this invention The figure which
illustrates the design of the shape according to Embodiment 2 of the present invention. The
perspective view of the optical microphone according to the embodiment 3 of the present
invention. The cross section of the microphone shown in the embodiment of the optical
microphone according to the present invention. The yz cross section showing the design in the
example of the optical microphone according to the invention The figure showing the ultrasonic
signal detection result of the optical microphone according to the present invention The
measurement result of the output response to the sound pressure of the optical microphone
according to the present invention FIG. 2 shows an optical microphone according to FIG.
[0013]
Hereinafter, embodiments of an ultrasonic wave receiver according to the present invention will
be described with reference to the drawings.
[0014]
First Embodiment FIG. 1 shows a configuration of an ultrasonic wave receiver according to the
present embodiment.
The ultrasonic wave receiver has an opening 1 to which an ultrasonic wave 14 propagating from
an environmental fluid filling the space (external space) around the ultrasonic wave receiver is
incident, and the ultrasonic wave 14 incident from the opening 1 propagates Waveguide 10, a
propagation medium portion 2 in which a part of the ultrasonic wave 14 propagating in the
waveguide 10 propagates, a holding portion 7 for holding the waveguide 10 and the propagation
medium portion 2, and the propagation medium portion 2 Between the two partial reflectors 3a
and 3b disposed opposite to each other, the light source 4 for emitting the lightwave 8 toward
the partial reflector 3a, and the lightwave 8 transmitted through the partial reflector 3b as an
electric signal And an electric signal detection unit 6 electrically connected to the photoelectric
conversion unit 5 and detecting an output signal of the photoelectric conversion unit 5.
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In FIG. 1, the propagation direction of the light wave 8 is Y axis, the propagation direction of the
ultrasonic wave 14 is Z axis, and the direction orthogonal to the propagation direction of the
light wave 8 and the propagation direction of the ultrasonic wave 14 is X axis.
Here, "environmental fluid" indicates a fluid that exists outside the light micro vehicle. The
environmental fluid is, for example, air.
[0015]
The two partial reflection members 3a and 3b, the light source 4, and the electrical signal
detection unit 6 constitute a Fabry-Perot interferometer. FIG. 2 shows a view of the optical
microphone according to the present embodiment from the direction of the light source 4 (the
positive direction of the Y axis). As shown in FIG. 2, a part of the plane parallel to the propagation
direction of the ultrasonic wave of the waveguide 10 is in contact with the propagation medium
portion 2. Moreover, a part of the plane parallel to the propagation direction of the ultrasonic
wave of the waveguide 10 may be configured by the propagation medium portion 2. A part of the
ultrasonic wave propagating in the waveguide 10 propagates into the propagation medium
portion 2 by the refraction propagation phenomenon. The density 11 of the propagation medium
unit 2, the sound velocity C1 of the propagation medium unit 2, the density 22 of the
environmental fluid, and the sound velocity C2 of the environmental fluid satisfy the relationship
of (ρ2 / ρ1) <(C1 / C2) <1.
[0016]
As described in Patent Document 3, when satisfying the relationship of ((2 / ρ1) <(C1 / C2) <1, it
is possible to transmit ultrasonic waves from the environmental fluid to the propagation medium
unit 2 with high efficiency. it can. Therefore, the ultrasonic wave propagating through the
waveguide 10 can be efficiently taken into the propagation medium portion 2.
[0017]
(Silica-Dried Gel) The propagation medium portion 2 satisfying the relationship of (ρ2 / ρ1)
<(C1 / C2) <1 is preferably a silica-dried gel. Further, by using a hydrophobized silica dry gel, it is
possible to prevent deterioration with time due to moisture in the air. Since the silica dry gel has
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low physical strength, it is preferable to hold the propagation medium portion 2 by the holding
portion 7 as shown in FIG.
[0018]
FIG. 3 shows the results of measurement of transmission characteristics using a dried silica gel
having a density of 150 kg / m <3> and an acoustic velocity of 90 m / sec and a thickness of 5
mm. It can be seen from FIG. 3 that the transmission characteristics of the dried silica gel show
that the attenuation increases as the wavelength of light decreases. Therefore, the wavelength of
the light wave 8 emitted from the light source 4 is preferably at least 400 nm or more. When the
density of the dried silica gel is 50 kg / m <3> or less, the silica drying step produces white
turbidity in the drying step of the gel and the light transmittance is significantly reduced.
Therefore, the density of the dried silica gel is preferably 50 kg / m <3> or more.
[0019]
(Acoustic Horn) As shown in FIG. 4, when the acoustic horn 9 is disposed at the front end of the
opening 1 and the focusing end 9b of the acoustic horn is connected to the opening 1, the
ultrasonic waves 14 inputted from the opening end 9a of the horn are acoustic As the horn 9
propagates from the opening end 9a to the focusing end 9b, it is compressed to increase the
energy density, and ultrasonic waves of high energy density can be transmitted to the opening 1.
[0020]
(Operation of Optical Microphone) The operation of the optical microphone of the present
embodiment will be described.
The optical microphone of the present embodiment takes in the ultrasonic wave 14 propagated
from the environmental fluid into the propagation medium unit 2 and detects it as an optical
signal by a Fabry-Perot interferometer.
[0021]
When ultrasonic waves 14 propagate inside the propagation medium portion 2, compressional
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waves are generated in the propagation medium portion 2. When a compressional wave is
generated in the propagation medium unit 2, the refractive index changes Δnn in the portion
where the compressional wave is generated. At this time, there is a relationship of Δnn = P
(nn−1) / (Cn <2> ρn) between the sound pressure P of the ultrasonic wave propagating in the
propagation medium portion 2 and the refractive index change amount Δnn. Here, n n is the
density of the propagation medium portion 2, C n is the speed of sound of the propagation
medium portion 2, and nn is the refractive index of the propagation medium portion 2. Thus,
when the ultrasonic wave propagates in the propagation medium unit 2, the refractive index
changes according to the sound pressure of the ultrasonic wave.
[0022]
Due to the propagation of ultrasonic waves, the changing silica dry gel refractive index is larger
by one digit or more than the amount of change of the refractive index in air or water. For
example, when the amount of change in refractive index was measured using a dried silica gel
having a density of 150 kg / m <3> and a sound velocity of 90 m / sec, the change in refractive
index with respect to a sound pressure of 1 Pa was 1.1 × 10 <-7> Met. The value of this
refractive index change is very high compared to the refractive index change (2.0 × 10 <-9>) in
air or the refractive index change (1.5 × 10 <-10>) of water. Great. Accordingly, it can be seen
that the silica dry gel is a medium having a large amount of change in refractive index with
respect to sound pressure.
[0023]
When ultrasonic waves propagate inside the propagation medium unit 2, the refractive index has
a distribution in the order of the wavelength of the ultrasonic waves in the plane of the
propagation medium unit 2. FIGS. 5 and 6 show propagation of the light wave 8 so as to cross
the propagation medium portion 2 in which the ultrasonic wave 14 is propagating. Although FIG.
5 shows that the propagation direction of the ultrasonic wave 14 and the propagation direction
of the light wave 8 are orthogonal, at least the propagation direction of the ultrasonic wave 14
and the propagation direction of the light wave 8 may not be parallel.
[0024]
The difference in lightness of the propagation medium portion in FIG. 5 indicates that
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compressional waves are propagating. If the spot diameter of the light wave 8 incident from the
light source 4 is large, the light wave 8 propagates so as to include both a high lightness portion
(white portion) and a low lightness portion (black portion). That is, since the light wave 8
propagates simultaneously at the place where the refractive index changes positive and the place
where the refractive index changes negative, the total change in refractive index that can be
detected from the light wave 8 transmitted through the propagation medium unit 2 And become
smaller.
[0025]
Therefore, as shown in FIG. 6, it is preferable to make the spot diameter of the light wave 8
smaller than the wavelength Λ of the ultrasonic wave propagating through the propagation
medium portion 2. Thereby, the light wave 8 can propagate in any part where the refractive
index changes positively and where the refractive index changes negative. As a result, compared
with the case where the spot diameter of the light wave 8 is large, the sensitivity of the refractive
index change detected from the light wave 8 transmitted through the propagation medium
portion 2 is improved.
[0026]
Further, when the acoustic horn 9 is provided as shown in FIG. 4, the propagation direction of
the ultrasonic wave propagated from the environmental fluid can be aligned, so the ultrasonic
wave propagating in the propagation medium portion 2 is the z axis in FIG. It can be propagated
as a plane wave in the direction of. Therefore, the distribution of the refractive index can be
ignored in the y-axis direction, which is the propagation direction of the light wave 8.
[0027]
Next, a Fabry-Perot interferometer used for the optical microphone of the first embodiment will
be described.
[0028]
In the optical microphone of the first embodiment, two partial reflection members 3a and 3b are
disposed in the propagation medium portion 2 at both end faces in the direction in which the
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light wave 8 propagates.
There are a plurality of paths until the light wave 8 emitted from the light source 4 passes
through the partial reflection member 3 a, the propagation medium portion 2 and the partial
reflection member 3 b and propagates to the photoelectric conversion portion 5.
[0029]
As shown in FIG. 7, a part of the light wave 8 emitted from the light source 4 is transmitted
through the partial reflection member 3 a and propagates in the propagation medium portion 2.
Then, part of the light transmitted through the partial reflection member 3a is further
transmitted through the other partial reflection member 3b.
[0030]
Part of the light transmitted through the partial reflection member 3a is repeatedly reflected a
plurality of times between the partial reflection member 3a and the partial reflection member 3b,
and propagates through the propagation medium portion 2, and part of the light is partially
reflected It permeate ¦ transmits from the member 3b. In the optical microphone of the first
embodiment, a Fabry-Perot interferometer is configured by arranging the two partial reflection
members 3a and 3b so that the transmitted light of the partial reflection member 3b interferes.
[0031]
FIG. 8 shows the result of calculation of how the intensity of the output light changes due to the
change of the refractive index of the propagation medium part 2 in the Fabry-Perot
interferometer. In FIG. 8, the vertical axis is the output light intensity, and the horizontal axis is
the refractive index change amount. FIG. 8 shows the optical microphone shown in FIG. 7 in
which the wavelength of the light wave 8 is 633 nm, both the reflectances of the partially
reflecting members 3a and 3b are 50%, and the refractive index of the propagation medium
portion 2 before ultrasonic waves propagate. 07 is the result of calculating the output response
when the distance between the two partially reflective members 3 is 5 mm.
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[0032]
From this, it can be seen that the light intensity changes in accordance with the change in the
refractive index. Preferably, the operating point is a point at which the light intensity changes
linearly with the change in the refractive index. Adjustment of the operating point can be
performed by changing the distance between the two partially reflecting members 3 or changing
the refractive index of the propagation medium portion 2.
[0033]
FIG. 9 shows the result of calculation of how the output response changes when the reflectances
of the partially reflecting members 3a and 3b change. In FIG. 9, the vertical axis is the output
light intensity, and the horizontal axis is the refractive index change amount.
[0034]
FIG. 9 shows calculated results when the reflectances of the partially reflecting members 3a and
3b are changed to 30%, 50%, 70%, and 90%. From this, the higher the reflectance of the partial
reflection members 3a and 3b, the sharper the change in light intensity. If the operating point is
properly selected, a large change in output intensity with respect to the change in refractive
index can be obtained, and high sensitivity is obtained. It can be understood that However, as the
output response becomes sharper, the setting of the operating point becomes severer, and a
slight deviation from the optimum point makes it impossible to obtain a sufficient change in light
intensity. In consideration of the accuracy and sensitivity of operating point adjustment, the
reflectance of the partial reflection members 3a and 3b is preferably about 10% to 70%.
[0035]
As shown in FIG. 7, the partial reflection members 3a and 3b may be in contact with the
propagation medium portion 2, and as in FIGS. 10, 11 and 12, the partial reflection members 3a
and 3b are propagation medium portions It may be separated from 2.
[0036]
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As shown in FIG. 11, in addition to the propagation medium portion 2, a part of the holding
portion 7 may be present between the two partial reflection members 3.
However, in that case, it is necessary to use a transparent material such as an acrylic material or
glass as the holding portion 7.
[0037]
Further, the holding unit 7 does not necessarily have to hold the entire surface of the
propagation medium unit 2, and as shown in FIG. 7, FIG. 10 and FIG. The light is incident on the
propagation medium unit 2 without propagating through the holding unit 7. In this case, it is not
necessary to use a transparent material as the holder 7.
[0038]
Alternatively, the partially reflecting member 3 may be part of the holding portion 7 as shown in
FIGS. 7, 12 and 13. If film formation is performed on the surface of the propagation medium unit
2 so that partial reflection occurs on the surface of the propagation medium unit 2, the filmformed surface can be used instead of the partial reflection member 3.
[0039]
In the Fabry-Perot interferometer, the light wave incident on the photoelectric conversion unit 5
does not have to be necessarily transmitted through the partial reflection member 3b, and as
shown in FIG. 14, from the partial reflection member 3a to the light source 4 A propagating light
wave can also be incident. This is because the light wave propagating in the direction from the
partial reflection member 3a to the light source 4 is also the interference light of the light
reflected multiple times between the two partial reflection members 3a and 3b, similarly to the
light wave transmitted through the partial reflection member 3b. It is because there is. When this
is done, if the beam splitter 12 is used, the incident light and the interference light can be
separated, and the arrangement of the photoelectric conversion unit 5 becomes easy.
[0040]
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FIG. 15 shows changes in refractive index and intensity of output light. FIG. 15 shows the results
of calculation in the case where the reflectances of both the partially reflective members 3a and
3b are 50%. It can be seen from FIG. 15 that a change in light intensity corresponding to a
change in refractive index can be obtained as in the case of measuring the transmitted light of
the partial reflection member 3b. When measuring reflected light, it is also possible to make the
reflectance of the partial reflection member 3b 100%.
[0041]
The detection of ultrasonic waves using a Fabry-Perot interferometer is also described in Patent
Document 2, but this is a method in which reflected light from a subject is incident on the FabryPerot interferometer. When the ultrasonic wave is reflected by an object propagating, the
frequency of light changes due to the Doppler effect. This frequency change of light is detected
by a Fabry-Perot interferometer.
[0042]
In the optical microphone according to the present embodiment, the output of the Fabry-Perot
interferometer changes because the refractive index inside the interferometer changes as
ultrasonic waves propagate to the propagation medium portion 2 in the Fabry-Perot
interferometer. , Ultrasonic waves can be detected. Thus, no optical microphone has been
disclosed so far for detecting a change in refractive index associated with the propagation of
ultrasonic waves inside a Fabry-Perot interferometer.
[0043]
As described above, the ultrasonic waves propagated from the environmental fluid are taken into
the propagation medium portion 2 and the change in the refractive index caused by the
propagation of the ultrasonic waves in the propagation medium portion 2 is detected by the
Fabry-Perot interferometer. It can be detected by light.
[0044]
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Second Embodiment The configuration of the optical microphone in the present embodiment will
be described.
FIG. 16 is a cross-sectional view showing the difference between the optical microphone
according to the present embodiment and the optical microphone according to the first
embodiment. The difference between the optical microphone according to the present
embodiment and the optical microphone according to the first embodiment is the shape of the
waveguide 10 connected from the opening 1 and the shape of the propagation medium portion 2
constituting a part of the waveguide 10 is there.
[0045]
The optical microphone of this embodiment shows the shapes of the propagation medium
portion 2 and the waveguide 10 in FIG. In FIG. 17, a convergence point F at which the ultrasonic
waves converge is set inside the propagation medium unit 2.
[0046]
When an arbitrary point on the side surface where the propagation medium portion 2 and the
waveguide 10 are in contact is a point Pk, the distance Lk from the opening 1 to the point Pk and
the distance Lnk from the point Pk to the convergence point F are Lak / Design a curved surface
where Ca + Lnk / Cn is constant. Here, the speed of sound Ca of the ultrasonic wave propagating
in the waveguide 10 and the speed of sound Cn of the ultrasonic wave propagating in the
propagation medium portion 2 are Cn <Ca.
[0047]
In the side surface (P0, P1,... Pn...) Where a part of the ultrasonic wave incident from the opening
1 and propagating in the waveguide 10 is in contact with the propagation medium portion 2 and
the waveguide 10 It propagates while refracting and propagating in the part 2. If the side face
where the waveguide 10 and the propagation medium portion 2 are in contact is designed so as
to satisfy the above equation, from which point (P0, P1,... Pn...) On the side face of the
propagation medium portion 2 The ultrasonic waves transmitted into the propagation medium
portion 2 also reach the focusing point 11 equally in time. As a result, the ultrasonic waves
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propagating from the waveguide 10 into the propagation medium portion 2 can be focused on
the focusing point 11, and a large sound pressure can be obtained at the focusing point 11.
[0048]
The width of the waveguide 10 may be constant or may be designed to narrow as ultrasonic
waves propagate. Also, the end 13 of the waveguide 10 may be closed or open.
[0049]
At the convergence point F, a detection unit for detecting an ultrasonic wave is disposed. For
example, by disposing the photoelectric conversion unit 5 or the like at the convergence point F,
the ultrasonic wave taken into the propagation medium unit 2 is detected using a Fabry-Perot
interferometer as in the first embodiment. At this time, if the position where the light wave 8 is
made incident is the focusing point 11, ultrasonic waves can be detected with high sensitivity.
[0050]
Third Embodiment The configuration of the optical microphone in the present embodiment will
be described. FIG. 18 shows a perspective view of the optical microphone according to the
present embodiment. The optical microphone according to the present embodiment takes in the
ultrasonic wave input from the opening 1 into the propagation medium unit 2. At this time, the
propagation medium portion 2 is directly connected to the opening 1 without using the
waveguide 10. In the present embodiment, the refraction propagation phenomenon is not used.
[0051]
Any material which satisfies the relationship of (ρ2 / ρ1) <(C1 / C2) <1 if the density 11 of the
propagation medium part 2, the sound velocity C1 in the propagation medium part 2, the density
22 of the environmental fluid, and the sound velocity C2 in the environmental fluid For example,
since the acoustic impedance is small and the difference from the acoustic impedance of air
which is the environmental fluid is also significantly small as compared with a normal solid,
ultrasonic waves in the air can be efficiently taken into the propagation medium portion 2.
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[0052]
The ultrasonic waves taken into the propagation medium section 2 as described above are
detected using a Fabry-Perot interferometer as in the first embodiment.
Example 1 An example of the implementation of the optical microphone according to the first
embodiment of the present invention is shown below. Cross sections of the prototyped optical
microphone are shown in FIG. 19 and FIG.
[0053]
The horn 9 was produced by machining an aluminum plate. The length of the horn is Lhorn =
100 mm, the size of the opening of the horn is width Hin1 = 40 mm, the height Hin2 = 50 mm,
and the size of the opening of the focusing portion of the horn is width Hout1 = 4 mm, height
Hout2 = 5 mm did.
[0054]
The holding part 7 was produced by machining an acrylic plate. As for the size of the holding
portion 7, the size of the outer shape was set to La1 = 30 mm and La2 = 35 mm, respectively,
and to La3 = 15 mm. In addition, the thicknesses t1, t2 and t3 of the holding portion 7 for
holding the upper surface, the lower surface and the side surface of the propagation medium
portion 2 were all 5 mm. The width of the waveguide 10 was Lw = 4 mm.
[0055]
The partial reflection members 3a and 3b are both the same, and the size is 15 mm in length
Lm1 and width Lm2, and 3 mm in thickness Lm3. When the partial reflection members 3a and
3b are disposed, the partial reflection members 3a and 3b are inserted and fixed in the through
holes prepared in the support 7 in advance in accordance with the size of the partial reflection
members 3a and 3b.
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[0056]
In addition, the size of the propagation medium portion 2 is 20 mm in length Ln1 and width Ln2,
and 5 mm in thickness Ln3. From the above, the size of the opening 1 is 4 mm in width and 5
mm in height, and this is connected to the focusing end 9 b of the horn 9.
[0057]
As a material of the propagation medium portion 2, a silica dry gel having a density of 150 kg /
m <3> and a sound velocity of 90 m / sec was used. The silica dry gel can be produced, for
example, by the following method.
[0058]
First, a mixed solution of tetraethoxysilane (hereinafter abbreviated as TEOS), ethanol and
aqueous ammonia is prepared, and this is gelled to prepare a wet gel. The wet gel refers to a
state in which the pore portion of the dried gel is filled with liquid. The liquid portion of this wet
gel is replaced with liquefied carbon dioxide gas and removed by supercritical drying using
carbon dioxide gas to obtain a silica dry gel. The density of the silica dry gel can be adjusted by
changing the mixing ratio of TEOS, ethanol and aqueous ammonia, and the speed of sound
changes with the density.
[0059]
The silica dry gel is a material consisting of a fine porous structure of silicon oxide, and the
skeleton is hydrophobized. The size of the pores and the skeleton is about several nm. When the
solvent is dried directly from the state in which the liquid is contained in the pore portion of such
a structure, a large force by capillary action is exerted when the solvent is volatilized, and the
structure of the skeleton portion is easily broken. By using supercritical drying in which no
surface tension acts to prevent the breakage, a dried gel can be obtained without breaking the
silica skeleton.
[0060]
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As the light source 4, a He̶Ne laser light source having a wavelength of 633 nm was used. The
spot diameter is about 0.6 mm. When the sound velocity of the propagation medium unit 2 is 90
m / sec and the ultrasonic wave to be detected is 40 kHz, the wavelength Λ of the ultrasonic
wave in the propagation medium unit 2 is 2.25 mm, which is sufficiently smaller than this.
[0061]
As the partially reflecting members 3a and 3b, a partially reflecting mirror made of a dielectric
multilayer film having a reflectance of 56% at a wavelength of 633 nm was used, and the
reflecting surface was disposed to face the propagation medium portion 2. A non-reflection
coating was applied to the surface other than the reflection surface of the partial reflection
mirror.
[0062]
As the photoelectric conversion unit 5, a Si photodiode having a high light receiving sensitivity at
a wavelength of 633 nm was used.
[0063]
The light source 4, the propagation medium unit 2, the partial reflection members 3a and 3b, the
support unit 7, and the photoelectric conversion unit 5 are arranged to constitute a Fabry-Perot
interferometer, and the output of the photoelectric conversion unit 5 is an electric signal
detection unit I entered 6.
In the present embodiment, an oscilloscope is used as the electric signal detection unit 6.
[0064]
Subsequently, an ultrasonic wave was output from the tweeter into the air, and a wave receiving
experiment was conducted with the prototyped optical microphone.
[0065]
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From the tweeter, one 40-kHz sine wave signal and an ultrasonic signal with an amplitude of
13.8 Pa of the sound pressure are output.
In the oscilloscope, observation was performed after removing the DC component of the signal
using a high pass filter. The measurement results are shown in FIG. From this, the detection of
ultrasonic waves can be confirmed.
[0066]
Moreover, the same signal was input while changing the sound pressure, and the change of the
output signal was observed. The results are shown in FIG. From this, it can be confirmed that the
output changes in proportion to the sound pressure. From the above, the operation of the optical
microphone according to the present invention has been confirmed.
[0067]
Since the ultrasonic waves taken into the propagation medium unit 2 are detected by the FabryPerot interferometer without using the diaphragm, the frequency is not restricted due to the
resonance of the diaphragm. Therefore, a broadband ultrasonic wave receiver can be provided.
[0068]
Also, the optical microphone according to the present invention can produce the aperture 1 on
the order of millimeters, and can provide a small aperture ultrasonic wave receiver.
[0069]
In addition, when using a laser Doppler vibrometer as in Patent Document 1, the acousto-optic
modulator, the arithmetic unit, etc. need to be incorporated in the laser Doppler vibration system,
but the device becomes large. The microphone can also be provided in a small size since the
optical microphone according to the present invention can be configured from the partial
reflection members 3a and 3b, the propagation medium unit 2, the light source 4, the holding
unit 7, the photoelectric conversion unit 5, and the electric signal detection unit 6. Can.
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[0070]
According to the optical microphone of the present invention, the ultrasonic wave can be
received in a wide frequency band by detecting the ultrasonic wave without using the diaphragm.
[0071]
Reference Signs List 1 aperture 2 propagation medium portion 3a, 3b partially reflecting
member 4 light source 5 photoelectric conversion portion 6 electric signal detection portion 7
holding portion 8 light wave 9 horn 9a horn open end 9b horn focusing end 10 waveguide 11
focusing point 12 beam splitter 13 End 14 ultrasound
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