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JP2011211311

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DESCRIPTION JP2011211311
An optical ultrasonic microphone capable of measuring a wide range of sound pressure up to an
ultrasonic region, having a small size, high sensitivity, and being resistant to environmental
fluctuations. An optical ultrasonic microphone according to the present invention includes an
acoustic waveguide (60) for taking in and propagating a sound wave from an opening (71), and
at least a part of a wall of the acoustic waveguide (60). A propagation medium 52 and a laser
doppler vibrometer head 8 are provided, and a sound wave traveling through an acoustic
waveguide 60 is taken into the photoacoustic propagation medium 52 and collected at the
acoustic focal point 57 to generate a change in refractive index with high efficiency. , It is
measured as a light modulation signal by the LDV head 8. When achieving further environmental
tolerance and measurement dynamic range, replace the LDV with a Mach-Zehnder interferometer
and optical heterodyne interferometry. [Selected figure] Figure 1
Optical ultrasonic microphone
[0001]
The present invention relates to a microphone using light. The present invention relates to an
optical ultrasonic microphone that receives ultrasonic waves propagating in a gas such as air,
and converts the received ultrasonic waves into electric signals using light.
[0002]
05-05-2019
1
Dynamic microphones or condenser microphones are widely used in the audio band as devices
for collecting and converting sound waves into electrical signals. In addition, piezoelectric
sensors are widely used in the ultrasonic region. These devices make use of the fact that the main
component of the energy of the sound wave propagates as a compressional wave of air, and
conduct the fine vibration that is excited in the vibration plate by making the sound wave
incident on the vibration plate electrically and electrostatically Or piezoelectrically converted into
electrical signals.
[0003]
In addition, an optical system such as a laser Doppler vibrometer (hereinafter referred to as an
LDV) that measures fine and high-speed vibration using monochromatic light represented by
laser light is widely used, and such an apparatus is Attempts have been made to collect sound
waves that have been used.
[0004]
Patent Document 1 discloses an optical microphone to which a diaphragm found in a normal
microphone and light measurement by light trigonometry are applied.
[0005]
Patent Document 2 discloses a laser Doppler microphone that measures sound pressure by
directly propagating laser light into a sound field and directly capturing, with an LDV, a change
in refractive index caused in the air by sound waves.
[0006]
The laser Doppler microphone in Patent Document 2 will be described with reference to FIG.
The laser Doppler microphone shown in FIG. 20 includes an LDV 121, a pair of reflecting mirrors
122 and 123, a cubic mirror 124, a sound field 126, and an operation unit 127.
The reflecting mirror 122 and the reflecting mirror 123 are disposed in parallel across the sound
field 126.
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The LDV 121 and the cubic mirror 124 are provided at both ends of a surface of the reflecting
mirror 123 that is orthogonal to the facing surface facing the reflecting mirror 122.
[0007]
The LDV 121 emits laser light toward the reflecting mirror 122 at an angle other than 90
degrees. The emitted laser light is repeatedly reflected a plurality of times by the reflecting
mirror 122 and the reflecting mirror 123, propagates along the laser light path 125, and reaches
the cubic mirror 124 provided at the end of the reflecting mirror 123. For the laser beam that
has reached the LDV 121, the LDV 121 and the calculation unit 127 measure the frequency
change of the returned laser beam and the amplitude and phase for each frequency change
amount.
[0008]
In FIG. 20, when no sound wave is present in the sound field 126, the laser light does not receive
the Doppler modulation, so the amount of change in the frequency of the laser light is zero.
However, when sound waves exist in the sound field 126, temporal fluctuations occur in the air
density of the sound field 126. This temporal fluctuation induces temporal fluctuation of the air
refractive index. This is the same value as the temporal expansion and contraction of the optical
length of the laser light path 125 which is the path length converted to the laser light
wavelength, and optically as if the cubic mirror 124 fluctuates temporally with respect to the
traveling direction of the laser light (time Motion that is equivalent to the
[0009]
Therefore, since the laser light is Doppler modulated, the amplitude and phase at the Doppler
frequency and each Doppler frequency are measured. By Fourier-transforming them into a realtime signal, the time variation of the air refractive index in the sound field 126 is reproduced. By
integrating it, the air refractive index at any time is calculated. As a result, sound waves in the
sound field 126 (average value of sound pressure on the laser light path 125) are measured.
[0010]
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3
The inventors of the present invention, in Patent Document 3, disclose an invention relating to a
gas ultrasonic transducer capable of transmitting and receiving ultrasonic waves in high
sensitivity and in a wide band by utilizing the refraction of ultrasonic waves in gas. In Non-Patent
Document 1, transmission and reception characteristics in the 500 kHz ultrahigh frequency
region are reported.
[0011]
FIG. 21 shows the ultrasonic transducer of the invention disclosed in Patent Document 3 and
Patent Document 4. As shown in FIG. 21, the ultrasonic transducer 101 according to the
invention of Patent Document 3 is provided at least on the front surface of the ultrasonic
transducer 102 and the ultrasonic transducer 102, and the environmental fluid 104 and the
ultrasonic transducer 102 are provided. And a propagation medium portion 103 which fills the
space between Here, environmental fluid indicates a fluid present outside the ultrasonic
wave receiver 101. The environmental fluid is, for example, air. Further, the interface between
the ultrasonic transducer 102 and the propagation medium unit 103 is defined as a first surface
area 111, and the interface between the propagation medium unit 103 and the environmental
fluid 104 is defined as a second surface area 112.
[0012]
The ultrasonic wave receiver 101 is surrounded by an environmental fluid 104. The ultrasonic
wave propagating in the ultrasonic wave propagation path 105 is received by the ultrasonic wave
receiver 101 from the environmental fluid 104 toward the ultrasonic wave receiver 101.
[0013]
The ultrasonic transducer 101 of Patent Document 3 transmits and receives ultrasonic waves
with high sensitivity by taking ultrasonic waves into a propagation medium portion of a
propagation medium with high efficiency from a medium with very small acoustic impedance
such as air. It is possible.
[0014]
In general, most of the ultrasonic waves are reflected at the interface of media having different
acoustic impedances, such as at the interface of gas and solid, so it is difficult to transmit with
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4
high efficiency and to receive with high sensitivity.
[0015]
As described above, in order to transmit the ultrasonic waves propagating in the gas with high
efficiency and to receive the ultrasonic waves with high sensitivity, the ultrasonic transducer 101
has a lower speed of sound velocity propagating inside thereof than the environmental fluid 104,
Uses a dry gel material (hereinafter referred to as "silica dry gel") composed of a silica skeleton
larger than the environmental fluid 104.
Silica dry gel is a material that can have various speeds of sound and density depending on the
manufacturing process.
For example, as the density 200 kg / m <3> and the sound velocity 150 m / s, the condition of
the propagation medium portion 103 capable of transmitting ultrasonic waves with high
efficiency is satisfied.
[0016]
The propagation medium portion 103 is made of silica dry gel, and as shown in FIG. 21, an angle
θ1 between the normal line of the second surface region 112 and the ultrasonic wave
propagation direction inside the propagation medium portion 103 and the inside of the
environmental fluid 104 The transmission / reception sensitivity of the ultrasonic transducer can
be improved by making the reflection of the ultrasonic wave in the second surface area 112
almost zero by appropriately selecting the angle θ2 with the ultrasonic wave propagation
direction in . In addition, since the transmission efficiency in the second surface area 112 is not
related to the frequency of the propagating ultrasonic wave, in principle a wide band
characteristic can be realized and various frequencies can be measured with high efficiency.
[0017]
Specifically, an ultrasonic wave is generated by giving an electric signal to the ultrasonic
transducer 102 from a drive circuit (not shown). Here, as shown in FIG. 21, the XYZ directions
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5
are set. The ultrasonic waves generated by the ultrasonic transducer 102 propagate from the
first surface area 111 to the second surface area 112 in the propagation medium portion 103 in
the positive direction of the Y axis. Then, the ultrasonic wave reaching the second surface area
112 changes the propagation direction according to the law of refraction, and propagates toward
the direction of the ultrasonic wave propagation path 105 (in this case, the opposite direction of
the arrow) to the fluid 104 To go.
[0018]
In the case of receiving ultrasonic waves, the ultrasonic waves propagating in the fluid 104 in the
surrounding space are refracted and transmitted to the propagation medium portion 103 when
reaching the second surface area 112, contrary to the case of transmission. It propagates inside
the propagation medium section 103 in the negative direction of the axis and reaches the
ultrasonic transducer 102. The ultrasonic wave that has reached the ultrasonic transducer 102 is
detected by a wave receiving circuit (not shown) in order to generate a potential difference
between the electrodes by deforming the ultrasonic transducer 102.
[0019]
In the ultrasonic transducer 101, even when the environmental fluid 104 is a medium having a
very small acoustic impedance (sound velocity of material x density of material) such as air, the
ultrasonic wave is incident from the environmental fluid 104 into the propagation medium
portion 103 with high efficiency. Alternatively, ultrasonic waves can be emitted from the
propagation medium portion 103 to the environmental fluid 104 with high efficiency.
[0020]
The ultrasonic transducer 101 is set to satisfy (高 く 2 / ρ1) <(C1 / C2) <1 in order to increase
the transmission efficiency of ultrasonic waves.
Here, the velocity of sound C1 in the propagation medium portion 103 of ultrasonic waves, the
velocity of sound C2 in the environmental fluid 104 of ultrasonic waves, the density 11 of the
propagation medium portion 103, and the density 22 of the environmental fluid 104 are shown.
[0021]
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In addition, θ1 can be calculated using (tan θ1) <2> = {(ρ2 / ρ1) <2>-(C1 / C2) <2>} / {(C1 /
C2) <2 using C1, C2, 11 and 22. It is set to satisfy> −1}.
[0022]
Further, θ1 and θ2 are set to satisfy (sin θ1 / C1) = (sin θ2 / C2).
[0023]
As shown in Patent Document 4, when the above equation is satisfied, the transmission efficiency
of ultrasonic waves in the second surface region 112 is approximately 1.
Therefore, it is possible to provide an ultrasonic transducer 101 capable of transmitting and
receiving ultrasonic waves with high efficiency.
[0024]
Patent Document 1: Japanese Patent Application Publication No. 2004-12421 Patent Document
2: Japanese Patent Application Publication No. 2004-279259 Patent Document 2: International
Publication No. 2004/098234 Patent Application Publication No. 2005/0139013
[0025]
"Acoustic properties of nanofoam materials and their application to ultrasonic sensors (general /
acoustic imaging) (Acoustic Properties of Nanofoam Material and its Applied Ultrasonic Sensors)"
Masahiko Hashimoto, Hidetoshi Nagahara, Takehiko Suginoi, The Institute of Electronics,
Information and Communication Engineers Technical Report of IEICE, Vol. 105, no. 619, US
2005-127 (P. 29-34).
[0026]
In the optical microphone disclosed in Patent Document 1, the mechanical resonance
characteristic of the diaphragm largely affects the frequency band, as in a normal microphone.
That is, the frequency lower than the mechanical resonance frequency of the diaphragm has
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7
relatively flat frequency characteristics.
However, above the resonance frequency, the sensitivity drops sharply, so the upper limit of the
measurable frequency band as the microphone is limited to around the resonance frequency.
Therefore, the characteristic guarantee upper limit frequency of the current condenser
microphone using the diaphragm is up to about 100 kHz, and in the case of more than that, the
piezoelectric type is used. Therefore, it is extremely difficult to construct a microphone having a
diaphragm whose band characteristics extend up to 100 kHz or more while maintaining
sufficient sensitivity.
[0027]
Moreover, since the laser doppler microphone disclosed in Patent Document 2 does not have a
diaphragm, there is no limitation of the high frequency region due to mechanical resonance. Also,
the high frequency limit in the vibration measurement of the LDV being used easily exceeds 1
MHz. However, since the change in refractive index related to the sound pressure of air is small,
an extremely long optical path is required to ensure sufficient sensitivity.
[0028]
In the example disclosed in Patent Document 2, an optical path length of 10 m or more is
required to obtain a sufficient S / N. Therefore, miniaturization of the measurement area is
extremely difficult. As a result, in the high frequency region, sound wave interference easily
occurs in the measurement region, and accurate measurement of the sound pressure becomes
difficult. This phenomenon corresponds to mechanical resonance in the diaphragm type, and is
called "cavity resonance".
[0029]
That is, the dimensions of the measurement range determine the upper limit of the microphone,
and the speed of sound of air is slower than the elastic wave velocity of a general diaphragm, so
if the measurement area and the diaphragm have the same area, the upper limit is The laser
05-05-2019
8
doppler microphone is lower. As described above, in the conventional optical microphone,
although the bandwidth of light measurement is sufficiently wide, the high frequency band is
limited by the mechanical resonance or cavity resonance to be used, and it is difficult to operate
particularly in the super high frequency region of 100 kHz or more Have the task of
[0030]
Furthermore, the point that an optical path length of 10 m or more is required poses a problem
different from the above. In general, air in free space has spatial / temporal inhomogeneity of
refractive index distribution due to temperature fluctuation or flow. Therefore, when using a light
interferometer such as LDV to capture sound waves as refractive index fluctuations on the optical
path, large spatial and temporal nonuniformities in the refractive index distribution due to long
optical path lengths are mixed in with refractive index fluctuations due to only sound waves. Do.
Therefore, there is also a problem that a signal to be originally excluded is mixed into the
measured sound signal, and correct sound measurement becomes difficult.
[0031]
By applying the configuration of Patent Document 2, it is composed of a mechanically strong
structure, the optical path is taken in a dilute gas atmosphere rich in fluidity such as vacuum or
helium, and high alignment accuracy is realized. Although it is possible in principle to solve this
problem, the prescription induces another problem that the price of the entire apparatus is
increased considerably.
[0032]
Further, in the ultrasonic transducer 101 of Patent Document 3, since there is no frequency
characteristic in taking in sound waves into the propagation medium portion 103 made of silica
dry gel, it is possible to take in sound waves in a wide frequency range. Although there is a need
for an ultrasonic transducer 102, such as a piezoelectric ceramic, to convert the captured sound
waves into an electrical signal.
[0033]
When the propagation medium portion 103 is made of silica dry gel and the ultrasonic
transducer 102 is made of piezoelectric ceramic, the acoustic impedance value is different from 2
digits to 3 digits. Most of the sound wave entering the element 102 is reflected at the interface
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9
(first surface region 111) of the propagation medium portion 103 and the ultrasonic transducer
102.
The reflected sound wave propagates in the propagation medium portion 103 in the reverse
direction and is partially emitted to the air, but the rest is propagated while repeating reflections
at the boundary (the second surface region 112) in the propagation medium portion 103 It
propagates in the medium section 103 and becomes a reverberation.
[0034]
The reflection phenomenon at the interface (the first surface region 111) of the propagation
medium portion 103 and the ultrasonic transducer 102 is piezoelectric as an element for
conversion to an electrical signal in the propagation medium portion 103 made of silica dry gel.
In a configuration in which materials having different acoustic impedances, such as ceramic, are
disposed, this is essentially unavoidable.
The reverberation associated with this reflection causes a reduction in S / N by superimposing on
a sound wave signal arriving later, and there is a problem that the frequency characteristic such
as an unnecessary resonance phenomenon is deteriorated.
[0035]
Furthermore, in the ultrasonic transducer 101 of Patent Document 3, there is also a problem that
the receiving sensitivity is low. The cause of this problem will be described below.
[0036]
The energy density of the ultrasonic wave that has propagated the environmental fluid 104
decreases when it is received by the ultrasonic transducer 101. This is the cause of the low
receiving sensitivity. The cause of the low receiving sensitivity will be described with reference to
FIG. In FIG. 21, the ultrasonic wave propagation path 105 is indicated by a solid arrow. As
described above, in order for the ultrasonic transducer 101 to receive ultrasonic waves with high
efficiency, (ρ2 // 1) <(C1 / C2) <1 and (tan θ1) <2> = {(ρ2 /) It is necessary to satisfy ρ1) <2>-
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10
(C1 / C2) <2>} / {(C1 / C2) <2> -1}, (sin θ1 / C1) = (sin θ2 / C2). At this time, in the path of the
ultrasonic wave propagating through the environmental fluid 104, the angle formed with the
normal to the second surface area 112 satisfies θ2.
[0037]
Therefore, in FIG. 21, the ultrasonic wave propagates in the range of the length (L 2 + L 3 + L 4)
of the fluid 104 toward the ultrasonic transducer 101, and further the ultrasonic wave
propagation path and the normal of the second surface area 112. It is assumed that the angle to
be formed satisfies θ2. Here, the range of the length L2 is a range parallel to the ultrasonic wave
propagation path 105 and means a range in which the ultrasonic waves do not reach the second
surface area 112 at all. The range of the length L3 is a range adjacent to the range of the length
L2 and parallel to the ultrasonic wave propagation path 105, which means a range in which the
ultrasonic waves can all reach the second surface area 112. . The range of the length L4 is a
range adjacent to the range of the length L3 and parallel to the ultrasonic wave propagation path
105, and means a range in which the ultrasonic waves do not reach all to the second surface area
112.
[0038]
As shown in FIG. 21, not all ultrasonic waves propagating in the range of the length (L2 + L3 +
L4) of the environmental fluid 104 are received by the ultrasonic transducer 101. The ultrasonic
waves propagating in the range of the length L3 reach the second surface area 112 and are
received by the ultrasonic transducer 101. However, the ultrasonic waves propagating in the
range of the length L2 and the range of the length L4 can not reach the second surface area 112
and are not received by the ultrasonic transducer 101.
[0039]
That is, a part of the ultrasonic waves (the ultrasonic waves propagating in the range of the
length L3) of the ultrasonic waves (the ultrasonic waves propagating in the range of the lengths
L2 + L3 + L4) that have propagated through the environmental fluid 104 The sound wave is
received by the sound wave transmitter / receiver 101.
[0040]
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11
Then, the ultrasonic wave propagating in the range of the length L3 of the environmental fluid
104 is transmitted through the second surface area 112 and detected by the ultrasonic
transducer 102 in the range of the length L1.
At this time, since L 3 << L 1, the ultrasonic waves received by the ultrasonic transducer 101 are
diffused in the second surface area 112 and reach the ultrasonic transducer 102. Accordingly,
when ultrasonic waves are received by the ultrasonic transducer 101, the energy density thereof
is reduced. Due to the decrease in energy density of the ultrasonic waves, the wave receiving
sensitivity of the ultrasonic transducer 101 is lowered.
[0041]
Due to the above reasons, the receiving sensitivity of the ultrasonic transducer 101 is low. That
is, since the length L3 of the propagation range of ultrasonic waves that can be received by the
second surface region 112 is smaller than the length L1 of the ultrasonic transducer 102, the
reception sensitivity of the ultrasonic wave receiver 101 is low. It has become a thing.
[0042]
The object of the present invention is made in view of the above-mentioned problems, and it is
possible to measure the sound pressure to the ultrasonic region far beyond the high frequency
limit of the conventional microphone, and to be highly resistant to environmental fluctuation and
high sensitivity. The present invention provides an optical ultrasonic microphone that achieves
high efficiency measurement.
[0043]
The microphone according to the present invention is a microphone for receiving a sound wave
propagating in a surrounding space filled with an environmental fluid, and the first sound wave is
incident, and the light is incident from the first wave opening. A photoacoustic propagation
medium having an acoustic waveguide through which sound waves propagate, and a
transmission surface, the transmission surface forming one surface of the acoustic waveguide
along the propagation direction of the ultrasonic wave. The transmission surface is configured
such that a part of the ultrasonic wave is transmitted from the transmission surface to the
propagation medium as it propagates through the waveguide and converges to a predetermined
convergence point, A photoacoustic propagation medium portion disposed with respect to a
waveguide, and a light wave emitted toward the convergence point, and the emitted light wave
05-05-2019
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propagating through the photoacoustic propagation medium portion is received and received
The sound pressure of the sound wave from the A detection unit, the density .rho.n of the
photoacoustic propagation medium portion, the speed of sound Cn in the photoacoustic
propagation medium portion, the density .rho.a of a gas filling the acoustic waveguide, and the
speed of sound Ca in the gas filling the acoustic waveguide Satisfy the relationship of (ρa / ρn)
<(Cn / Ca) <1 and set at an arbitrary position along the propagation direction of the ultrasonic
wave on the transmission surface from the first opening of the waveguide When the length of the
waveguide up to the point P is La and the length from the point P to the convergence point is Ln,
La / Ca + Ln / Cn is constant regardless of the position of the point P.
[0044]
According to the optical ultrasonic microphone of the present invention, the sound wave
propagating in the gas in the surrounding space is taken into the acoustic waveguide from the
opening, and the sound wave traveling from the acoustic waveguide to the inside of the
photoacoustic propagation medium is Mach-Zehnder By measuring with the use of an optical
interferometer, it is possible to measure up to a high frequency region that greatly exceeds the
limit due to the mechanical resonance of the conventional diaphragm, and the influence of the
reflection of the sound wave by the electroacoustic transducer such as the conventional
piezoelectric ceramic By realizing the small size and the high stability against fluctuations in the
surrounding environment, it is possible to provide an optical ultrasonic microphone capable of
more sensitive and accurate sound pressure measurement.
[0045]
A perspective view showing a schematic structure of the optical ultrasonic microphone 51 of the
first embodiment A portion of the optical ultrasonic microphone 51 shown in FIG. 1 at the center
of the focusing portion 77 and the acoustic waveguide member 56 in the X direction,
Propagation of sound waves in the optical ultrasonic microphone shown in FIG. 1 which is a
sectional view cut along a plane parallel to the YZ plane and which shows a part of the base of
the optical ultrasonic microphone shown in FIG. 1 and a part of the acoustic waveguide member.
-A schematic diagram for explaining refraction The schematic diagram for explaining the sound
wave convergence of the optical ultrasonic microphone in the first embodiment of the present
invention The sound showing the result of calculation experiment for the sound wave
propagation in the optical ultrasonic microphone shown in FIG. Pressure distribution diagram
Sound pressure distribution diagram showing the result of calculation experiment for sound
wave propagation in the optical ultrasonic microphone shown in FIG. 1 Sound pressure
distribution diagram showing the results of calculation experiment for sound wave propagation
in the microphone Sound pressure distribution diagram showing the results of calculation
experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. 1
Calculation for the sound wave propagation in the optical ultrasonic microphone shown in FIG.
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Sound pressure distribution chart showing experimental results Sound pressure distribution
chart showing experimental calculation results for sound wave propagation in the optical
ultrasonic microphone shown in FIG. 1 Graph showing the time waveform of the input sound
wave signal used in the experiments shown in FIGS. The sectional view showing the outline
device composition of optical ultrasonic microphone 51A concerning the modification of a 1st
embodiment The measurement of the equal phase side of the sound wave propagation of the
photoacoustic propagation medium part of the optical ultrasonic microphone in the 1st
embodiment of the present invention LDV output of the optical ultrasonic microphone 51 in the
vicinity of the contour line acoustic wave acoustic focus 57 showing the result Waveform
diagram showing an example of the waveform (amplitude measurement waveform) 81.
Configuration diagram showing the optical system structure of the optical sound pressure
measurement unit 1300 in the second embodiment of the optical ultrasonic microphone of the
present invention Second embodiment of the present invention Configuration of the device
showing the experimental device for demonstrating the principle of the optical ultrasonic
microphone in the form the time of the output signal from the digital oscilloscope 1408
measured by the experimental device for demonstrating the principle of the optical ultrasonic
microphone in FIG. Figure showing waveform and time waveform of input signal to speaker
1410 Configuration diagram showing optical system configuration of optical ultrasonic
microphone 1300 having wide measurement dynamic range and no signal distortion in the third
embodiment of the present invention 2 is a cross-sectional view of a conventional ultrasonic
transducer at 2 and FIG. 3 is a block diagram of a conventional optical microphone in Patent
Document 3
[0046]
The present inventors pay attention to the fact that the silica dry gel has a property close to
optical transparency, for example, the rate of change of the sound pressure and the refractive
index in the silica dry gel relative to the rate of change in air, I found that it was about one digit
higher.
[0047]
The rate of change of refractive index due to sound pressure usually increases in the order of
solid, liquid and gas, which is a very unique property not found in normal bulk materials.
[0048]
The present invention utilizes the basic principle of the interface phenomenon of an ultrasonic
wave receiver capable of propagating ultrasonic waves from a very low acoustic impedance
object such as gas to a solid with high efficiency, and a solid that satisfies these conditions. The
point is that an optical ultrasonic microphone is constructed in which the band characteristics
05-05-2019
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extend to an extremely high frequency region by using a phenomenon that the material
generates a very large refractive index change by the sound wave.
[0049]
In the present invention, a Mach-Zehnder interferometer is used to construct an entire optical
system with the shortest possible path length, and the refractive index fluctuation generated in
the photoacoustic propagation medium by the sound pressure is taken as the optical path
fluctuation. There is an advantage that it is possible to directly measure from amplitude
fluctuation of interference light or phase fluctuation of interference light.
[0050]
Hereinafter, the optical ultrasonic microphone of the present invention will be described with
reference to the drawings.
[0051]
First Embodiment FIG. 1 shows an optical ultrasonic microphone 51 according to a first
embodiment.
The optical microphone 51 according to the first embodiment includes an opening 71 of an
acoustic horn to which an ultrasonic wave propagating in the environmental fluid 14 is incident,
an acoustic waveguide 60 through which an ultrasonic wave incident from the opening 71
propagates, and an acoustic waveguide 60 and an LDV head 8 emitting laser light 58 toward an
acoustic focal point 57 where the ultrasonic waves propagating through the photoacoustic
propagation medium unit 52 converge. And an LDV arithmetic processing unit 9 for detecting an
ultrasonic wave from the reflected wave of the emitted laser beam 58.
[0052]
An outline of the operation of the optical microphone 51 will be described.
Ultrasonic waves enter the inside of the optical microphone 51 along the sound wave
propagation direction 55 from the environmental fluid 14 present outside the optical
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15
microphone 51.
The ultrasonic wave incident on the inside of the optical microphone 51 propagates through the
acoustic waveguide 60 and the photoacoustic propagation medium unit 52 and converges on the
acoustic focal point 57.
The LDV head 8 emits a laser beam 58 toward the acoustic focus 57.
The LDV head 8 receives the reflected wave of the emitted laser beam 58.
The LDV arithmetic processing unit 9 obtains, from the reflected wave received by the LDV head
8, the change in the refractive index of the photoacoustic propagation medium unit 52 due to the
ultrasonic wave converged on the acoustic focus 57.
The optical microphone 51 can detect an ultrasonic wave (for example, the sound pressure of the
ultrasonic wave) corresponding to the amount of change of the refractive index.
[0053]
Hereinafter, each part of the optical ultrasonic microphone 51 of FIG. 1 will be described in
detail.
[0054]
(Converging Unit 77) FIG. 2 is a cross section obtained by cutting a part of the optical ultrasonic
microphone 51 shown in FIG. 1 in a plane parallel to the YZ plane at the center of the converging
unit 77 and the acoustic waveguide member 56 in the X direction. Figure shows.
[0055]
The converging unit 77 includes a first opening 71 to which an ultrasonic wave propagating
from the outside is incident, and a second opening 63 connected to the acoustic waveguide
member 56 (acoustic waveguide 60).
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An inner space 70 through which the ultrasonic wave propagates is formed between the first
opening and the second opening.
[0056]
The ultrasonic waves propagating the environmental fluid 14 in the sound wave propagation
direction 55 are controlled in the propagation direction when propagating through the inner
space 70 after being incident from the first opening 71, and the sound pressure is enhanced (
Compressed).
The sound wave whose sound pressure has been increased by the converging unit 77 propagates
to the acoustic waveguide 60 connected to the converging unit 77.
[0057]
In the inner space 70, the cross-sectional area a7 of the plane orthogonal to the propagation
direction g7 gradually decreases along the propagation direction g7 in which the sound wave
propagates from the first opening 71.
More preferably, a converging portion 77 defining the shape of the inner space 70 so that the
cross-sectional area a7 decreases exponentially with respect to the propagation direction g7 from
the first opening 71 toward the second opening 63. The inner side surface of the light source is
configured to have a curved shape along the propagation direction g7.
[0058]
The width dimension of the converging portion 77 in the X direction may be constant, or the
width dimension may be gradually reduced.
When the width dimension in the X direction of the converging portion 77 is constant, the width
dimension in the Z direction is preferably configured to decrease exponentially with respect to
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the propagation direction g7. Also, the cross-sectional area a7 may be exponentially reduced by
decreasing the width dimension in the X direction and the width dimension in the Z direction of
the converging portion 77 in proportion to √e with respect to the propagation direction g7. As
the cross-sectional area a7 decreases exponentially in this manner, the reflection of the sound
wave at the converging portion 77 can be minimized, the sound wave can be compressed without
phase disturbance, and the sound pressure can be increased.
[0059]
The converging portion 77 has a length of, for example, 100 mm in the Y direction, and the first
opening 71 has a square shape having a length of 50 mm in each of the Z direction and the X
direction. Further, the end portion 72 is a square having a length of 2 mm in the X direction and
the Z direction. In the first embodiment, the lengths are changed in two directions, the Z direction
and the X direction. Assuming that the position of the opening 71 is the origin (0) in the Y
direction, the X direction and the Z direction of the inner space 70 at each position of the
position in the Y direction = 0 mm / 20 mm / 40 mm / 60 mm / 80 mm / 100 mm from the
origin (The length in the X direction and the length in the Z direction are the same at each
position. )は、
50.0mm/26.3mm/13.8mm/7.2mm/3.8mm/2.0mmである。
[0060]
According to the converging unit 77 having the above-described size, an effect of an increase in
the sound pressure of the sound wave of about 10 dB can be obtained as compared with the case
where the converging unit 77 is not provided. In addition, the shape of the sound pressure
waveform, which changes with time, hardly changes in the measurement results of the opening
71 and the end 72 and disturbs the sound wave propagating through the environmental fluid 14
(for example, air) Without the sound energy being compressed at the end 72.
[0061]
The converging portion 77 can be configured, for example, by machining an aluminum plate
having a thickness of 5 mm, which is a metal, into a predetermined shape by machining. The
convergence portion 77 may be formed of a material other than aluminum, as long as the
material can hardly transmit the sound waves propagating in the inner space 70 and can
05-05-2019
18
increase the density of the sound energy by the effect of the shape. For example, the convergence
portion 77 may be configured using a material such as resin or ceramic. Moreover, the
convergence part 77 does not need to have a horn-type external shape, and the internal space 70
should just have a horn shape which was mentioned above.
[0062]
(Acoustic Waveguide 60) Next, an acoustic waveguide 60 for propagating a sound wave in a
predetermined direction will be described. The acoustic waveguide 60 is constituted by an
acoustic waveguide member 56. The acoustic waveguide 60 is connected to the second opening
63 of the converging unit 77. A sound wave incident from the environmental fluid 14 and
propagating through the converging portion 77 is incident on the acoustic waveguide 60 from a
portion connected to the second opening 63.
[0063]
As shown in FIG. 2, the acoustic waveguide 60 reduces the cross-sectional area of the acoustic
waveguide 60 in a plane orthogonal to the propagation direction of the sound wave. Here, the
width dimension in the ZY plane changes depending on the position along the ultrasonic wave
propagation direction g6 parallel to the ZY plane. The width dimension in the X-axis direction of
the acoustic waveguide 60 is constant, for example, 2 mm. The width dimension in the X-axis
direction can also be designed to change.
[0064]
The reason why the acoustic waveguide 60 has such a shape will be described. The acoustic
waveguide 60 includes a transmission surface 61 in contact with the photoacoustic propagation
medium portion 52 and defined by an interface with the photoacoustic propagation medium
portion 52, and a waveguide outer surface 62 defined by the acoustic waveguide member 56.
There is. Further, the front and back sides in the X direction of the acoustic waveguide 60 are
also defined by the acoustic waveguide member 56. As the sound wave propagates in the
acoustic waveguide 60, it gradually infiltrates (propagates) from the transmission surface 61
where the photoacoustic propagation medium portion 52 and the acoustic waveguide 60 are in
contact to the photoacoustic propagation medium portion 52. At this time, in the transmission
surface 61, the propagation direction of the sound wave is refracted. The photoacoustic
05-05-2019
19
propagation medium portion 52 may be provided to constitute a part of the acoustic waveguide
60.
[0065]
As the sound wave propagates through the acoustic waveguide 60, as a result of the transmission
of at least a part of the sound wave from the transmission surface 61 to the photoacoustic
propagation medium portion 52, the energy of the sound wave propagating through the acoustic
waveguide 60 decreases. In order to compensate for the reduction in energy, the cross-sectional
area of the acoustic waveguide 60 in a plane orthogonal to the propagation direction of the
sound wave is reduced in order to compress the sound wave (increase the sound pressure of the
sound wave).
[0066]
Specifically, the acoustic waveguide 60, which is a space between the transmission surface 61
and the waveguide outer surface 62, has a shape in which the width perpendicular to the
propagation direction g6 in the YZ plane monotonically decreases with respect to the
propagation direction. Also, the waveguide end 64 of the acoustic waveguide 60 is closed. With
this shape, the sound wave can be efficiently refracted and transmitted to the photoacoustic
propagation medium portion 52 while keeping the energy density of the sound wave propagating
through the acoustic waveguide 60 constant. The specific operation until the sound wave
propagating through the acoustic waveguide 60 is incident on the photoacoustic propagation
medium portion 52 will be described later.
[0067]
(Photoacoustic Propagation Medium Portion 52) Next, the photoacoustic propagation medium
portion 52 defining the transmission surface 61 through which the sound wave passes from the
acoustic waveguide 60 to the acoustic propagation medium portion 52 will be described. The
photoacoustic propagation medium portion 52 is made of a material whose propagation velocity
of sound waves is slower than that of the environmental fluid 14. That is, assuming that the
velocity of the sound wave in the propagation medium is Cn and the sound velocity of the sound
wave in the environmental fluid 14 is Ca, (Cn / Ca) <1 is satisfied. Materials that satisfy this
condition include dried gels of inorganic acid compounds or organic polymers. It is preferable to
05-05-2019
20
use a silica dry gel as a dry gel of an inorganic acid compound. Hereinafter, a method for
producing a silica dry gel will be described.
[0068]
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 one
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.
[0069]
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.
[0070]
More preferably, the propagation medium of the photoacoustic propagation medium portion 52
is C n, the sound velocity of the acoustic fluid in the environmental fluid 14 is Ca, the density of
the propagation media is n n, and the density of the environmental fluid 14 is ρ a When (.rho.n /
.rho.a) <(Ca / Cn) <1 is satisfied.
[0071]
The propagation medium of the photoacoustic propagation medium portion 52 more preferably
has a density of the propagation medium of 100 kg / m 3 or more and a velocity of sound in the
propagation medium of 300 m / s or less.
05-05-2019
21
[0072]
The density n n of the silica dry gel constituting the photoacoustic propagation medium portion
52 used in the first embodiment is 200 kg / m <3>, and the speed of sound Cn in the silica dry
gel is 150 m / s.
These values are materials that satisfy the refraction propagation phenomenon shown in Patent
Document 1.
The density 空 気 a of air is 1.12 kg / m <3>, and the speed of sound Ca is 340 m / s at around
room temperature.
[0073]
Further, since the photoacoustic propagation medium unit 52 plays a role of propagating the
sound wave taken from the environmental fluid 14 to the acoustic focus 57, the sound wave
reaching the acoustic focus 57 is weakened when the internal loss is large. For this reason, the
photoacoustic propagation medium unit 52 is preferably made of a material having a small
internal loss. Silica dry gel is a material that meets the above-mentioned conditions of sound
velocity and density, and has a small internal loss.
[0074]
(Base 53) The base 53 supporting the photoacoustic propagation medium unit 52 will be
described. Since the silica dry gel which comprises the photoacoustic propagation medium part
52 has low density, mechanical strength is also low. For this reason, handling is difficult.
Therefore, a base 53 is provided to support the photoacoustic propagation medium portion 52
stably.
[0075]
FIG. 1 illustrates that a part of the base 53 located on the surface side of the photoacoustic
05-05-2019
22
propagation medium portion 52 is broken to expose the photoacoustic propagation medium
portion 52 for easy understanding. In fact, the surface of the photoacoustic propagation medium
portion 52 may be entirely covered with the base 53 except for the measurement through holes
53a described later. When the surface of the photoacoustic propagation medium portion 52 is
completely covered except for the measurement through holes 53a, it is possible to avoid the
incidence of sound waves other than the ultrasonic waves to be detected.
[0076]
As shown in FIG. 1, the base 53 covers the surface of the photoacoustic propagation medium
portion 52 (the back surface, the left side surface, the bottom surface, etc. of the photoacoustic
propagation medium portion 52 shown in FIG. 1). Further, the right side surface of the
photoacoustic propagation medium portion 52 shown in FIG. 1 is covered with the acoustic
waveguide member 56 and configured as a part of the acoustic waveguide 60. As a result, the
photoacoustic propagation medium portion 52 is held by the base 53 and the acoustic
waveguide member 56.
[0077]
The acoustic waveguide member 56 and the base 53 can be configured according to the shapes
shown in FIGS. 3 (a) and 3 (b). FIG. 3A is a perspective view showing a part of the base 53 of the
optical ultrasonic microphone 51 shown in FIG. FIG. 3B is a perspective view showing a part of
the acoustic waveguide member of the optical ultrasonic microphone shown in FIG.
[0078]
As shown in FIG. 3 (a), an aluminum member is used to form an acoustic waveguide member 56
defining an acoustic waveguide 60 including the waveguide outer surface 62. As shown in FIG.
On the other hand, as shown in FIG. 3B, a base 53 for holding the photoacoustic propagation
medium portion 52 is prepared. The exposed surface of the photoacoustic propagation medium
portion 52 held by the base 53 defines a transmission surface 61.
[0079]
05-05-2019
23
For example, a base 53 made of porous ceramic is formed. The base 53 is inserted into a mold in
which the surface defining the transmission surface 61 is made of fluorine resin or the like, and
the wet gel is introduced into the space. Thereafter, the liquid portion is replaced with liquefied
carbon dioxide gas and dried to obtain a member in which the photoacoustic propagation
medium portion 52 and the base 53 are integrated.
[0080]
As shown in FIG. 3B, both end portions of A and B of the base 53 holding the photoacoustic
propagation medium portion 52 and both end portions of C and D of the acoustic waveguide
member 56 shown in FIG. The acoustic waveguides 60 of which the transmission surface 61 is
defined by the photoacoustic propagation medium portion 52 are formed by respectively
bonding them with an adhesive such as epoxy resin.
[0081]
(Sound Wave Propagating Acoustic Waveguide 60) Next, the geometric shape of the acoustic
waveguide 60 and the photoacoustic propagation medium portion 52 defined by the acoustic
waveguide member 56 and the propagation of the sound wave will be described in detail.
[0082]
FIG. 4 is an enlarged view of a part of the acoustic waveguide 60 in the optical ultrasonic
microphone 51. As shown in FIG.
The propagation and refraction of sound waves will be described with reference to FIG.
[0083]
In FIG. 4, the transmission surface 61 and the waveguide outer surface 62 are shown by dotted
lines, and the perpendiculars of tangents at arbitrary points of the transmission surface 61 are
shown by dashed dotted lines.
Further, the propagation direction of the sound wave is indicated by an arrow 55a.
05-05-2019
24
[0084]
A sound wave incident from the opening 63 and traveling in the acoustic waveguide 60
propagates in the acoustic waveguide 60 while changing the traveling direction according to the
shape of the acoustic waveguide 60. The interior of the acoustic waveguide 60 is filled with the
environmental fluid 14.
[0085]
The component of the sound wave contacting the transmission surface 61, which is the interface
between the acoustic waveguide 60 and the photoacoustic propagation medium portion 52, is
incident on the transmission surface 61 at an angle θa with respect to the normal to the
transmission surface 61. As satisfying, the light is refracted and transmitted to the photoacoustic
propagation medium portion 52 at a constant angle θ n with the normal to the transmission
surface 61.
[0086]
The angle θ n in the propagation direction of the sound wave in the inside of the photoacoustic
propagation medium unit 52 is represented by Equation 1.
Here, when the relationship of (Cn / Ca) <1 is satisfied, the angle θn obtained by Equation 1
becomes a positive value, and the sound wave is refracted and transmitted into the photoacoustic
propagation medium portion 52.
[0087]
[0088]
In Equation 1, the velocity of the sound wave in the propagation medium is Cn, the sound
velocity of the sound wave in the environmental fluid 14 is Ca, the density of the propagation
medium is nn, and the density of the environmental fluid 14 is ρa.
05-05-2019
25
[0089]
On the other hand, the reflectance R at the interface between the acoustic waveguide 60 and the
photoacoustic propagation medium portion 52 is expressed by Equation 2.
[0090]
[0091]
In order to refract and transmit a sound wave from the acoustic waveguide 60 to the
photoacoustic propagation medium portion 52 as efficiently as possible, it is preferable that the
reflectance R be as small as possible.
When Cn, Ca, nn, ρa satisfy (ρn / ρa) <(Ca / Cn) <1, there always exist angles θa, θn where
the numerator of Equation 2 becomes zero.
That is, the reflectance R can be made zero.
[0092]
In the first embodiment, as described above, the density n n of the dried silica gel is 200 kg / m
<3>, the speed of sound Cn in the dried silica gel is 150 m / s, and the density a a of air is 1.12
kg. / M <3>, and the sound velocity Ca is 340 m / s near room temperature.
[0093]
Substituting these values into Equation 1, the angle θ n is approximately 26 degrees.
At this time, if the angle θa is about 89 degrees, the reflectance R is almost zero.
Therefore, under the conditions of the first embodiment, the sound wave is higher in the
direction in which the angle θ n becomes about 26 degrees by the sound wave entering the
transmission surface 61 at about 89 degrees with respect to the normal of the transmission
05-05-2019
26
surface 61 It permeate ¦ transmits the inside of the photoacoustic propagation medium part 52
by transmission efficiency.
[0094]
When the reflectance R is almost zero, the refraction angle θn is constant at about 26 degrees,
but by making the transmission surface 61 a curved surface, the sound waves transmitted to the
photoacoustic propagation medium portion 52 from different positions of the transmission
surface 61 Can be propagated toward a predetermined acoustic focus 57 to converge the sound
wave.
[0095]
Further, by bending the acoustic waveguide 60 along the transmission surface 61, as the sound
wave propagates through the acoustic waveguide 60, a part of the sound wave can be incident on
the transmission surface 61 at a constant angle θa.
By utilizing this phenomenon, the sound wave propagating through the acoustic waveguide 60 is
refracted and transmitted little by little to the photoacoustic propagation medium portion 52,
and the acoustic wave is converged to one point in the photoacoustic propagation medium
portion 52, thereby achieving high wave receiving sensitivity. To achieve.
[0096]
Further, the refraction angle θ n represented by Equation 1 and the reflectance R represented
by Equation 2 do not depend on the frequency of the sound wave.
Therefore, regardless of the frequency of the propagating sound wave, the sound wave can be
transmitted to the photoacoustic propagation medium unit 52 with high transmission efficiency.
Therefore, the optical ultrasonic microphone 51 of the first embodiment can detect a wide band
sound wave with high sensitivity. That is, according to the first embodiment, reception of
ultrasonic waves in a high frequency range and in a wide band, which was conventionally
difficult, becomes possible with high sensitivity, and a standard microphone having an effective
band of 100 kHz or more and high sensitivity It will be realized.
05-05-2019
27
[0097]
In the field of optical lenses, for example, Japanese Patent No. 2731389 discloses a structure for
focusing light emitted from the side surface of an optical waveguide. However, in general, in the
optical waveguide, light propagates while being repeatedly reflected at the boundary between the
cladding layer and the waveguide, whereas in the acoustic waveguide 60 of the first embodiment,
the sound wave is on the outer surface or side surface of the acoustic waveguide 60 It does not
reflect. For this reason, in the optical waveguide, it is important to propagate the sound wave
having the same phase in the first embodiment while the phase of the propagating light is not the
same.
[0098]
(Shapes of Transmission Surface 61 and Waveguide Outer Surface 62) Next, design of shapes of
the transmission surface 61 and the waveguide outer surface 62 defining the acoustic waveguide
60 will be described. The shapes of the transmission surface 61 and the waveguide outer surface
62 are designed in the following steps.
[0099]
First, from the size of the opening 63 of the acoustic waveguide 60, the length of the acoustic
waveguide 60 capable of efficiently incorporating the sound wave into the photoacoustic
propagation medium portion 52 is determined. Due to the length of the acoustic waveguide 60,
the transmission surface 61 is designed as a shape for focusing the sound wave. Thereafter, the
shape of the transmission surface 61 is designed in consideration of the determined shape of the
transmission surface 61 and the width required for the acoustic waveguide 60.
[0100]
The size of the opening 63 of the acoustic waveguide 60 is preferably half or less of the
wavelength of the received sound wave. If the width of the waveguide is larger than half of the
wavelength of the propagating sound wave, the sound wave is likely to be reflected inside the
acoustic waveguide 60, and the propagation of the sound wave is disturbed, and accurate
05-05-2019
28
measurement of the sound wave becomes difficult. It is.
[0101]
In the first embodiment, as an example, since the reception of sound waves up to a frequency of
80 kHz is considered, 2.0 mm smaller than 2.1 mm, which is a half wavelength of 80 kHz, is
used, and the opening 63 has one side It has a square shape of 2.0 mm. The end 72 of the
converging part 77 is designed to be equal in size to the opening 63.
[0102]
As the length of the acoustic waveguide 60 increases, the number of sound waves propagating
from the acoustic waveguide 60 to the photoacoustic propagation medium 52 increases.
Therefore, it is preferable that the sound wave propagating in the acoustic waveguide 60 has a
sufficient length so as to be refracted and transmitted to the photoacoustic propagation medium
portion 52.
[0103]
As described with reference to FIG. 21, in the ultrasonic wave receiver 101, the sound wave
propagating in the range of the length L3 is transmitted to the inside of the propagation medium
portion 103 through the second surface region 112. . The length L3 and the length L5 in FIG. 21
correspond to the length in the Z direction of the opening 63 in the YZ plane of the acoustic
waveguide 60 shown in FIG. 5 and the length in the YZ plane of the transmission surface 61. By
making the length of the transmission surface 61 in the YZ plane, ie, the length of the sound
wave propagation direction g6 in the acoustic waveguide 60 sufficiently long, the sound wave
can be sufficiently transmitted to the photoacoustic propagation medium portion 52, In the first
embodiment, it is possible to improve the receiving sensitivity, to reduce the influence of
reflection of the sound wave that can not be captured, and to improve the measurement
accuracy, and in the first embodiment, the photoacoustic propagation medium portion 52 in the
environmental fluid 14 Since the angle θa (FIG. 4) formed by the normal line and the sound
wave propagation direction 55a is about 89.3 degrees, the ratio of the length L2 to the length L1
is about L1 / L2 = 88. Therefore, ideally, the acoustic waveguide 60 preferably has a length of
about 90 times or more of the opening 63. In the first embodiment, the opening 63 of the
acoustic waveguide 60 is 2 mm, and the length of the acoustic waveguide 60 is set to 200 mm,
05-05-2019
29
which is 100 times the opening 63.
[0104]
Thus, the lengths of the opening 63 and the acoustic waveguide 60 are determined. The shape of
the transmission surface 61 and the shape of the outer surface of the waveguide are designed
based on the length of the acoustic waveguide 60. (Convergence of Sound Wave) FIG. 5 is an
enlarged view of the acoustic waveguide 60 and the photoacoustic propagation medium portion
52. As shown in FIG. The convergence of the sound wave in the optical ultrasonic microphone 51
in the first embodiment will be described with reference to FIG.
[0105]
An acoustic focus 57 for focusing the sound wave is set in the photoacoustic propagation
medium unit 52. The LDV head 8 is made to face the acoustic focus 57, and the LDV head 8 and
the LDV arithmetic processing unit 9 detect a sound wave using the laser beam 58.
[0106]
In FIG. 5, a point at the opening 63 of the transmission surface 61 is set as a start point P 0, and
points P 1, P 2, P 3,..., P n are sequentially arranged from the side closer to the opening 63 of the
transmission surface 61. The distance from the point P0 to the point P1 is La1, the distance from
the point P1 to the point P2 is La2,..., And the distance from the point Pn-1 to the point Pn is Lan.
Further, the distances between the points P1, P2, ... Pn and the acoustic focus 57 are Ln1, Ln2, ...,
Lnn, respectively.
[0107]
In order for the sound wave incident from the opening 63 to propagate in the acoustic
waveguide 60 and to be refracted and transmitted to the photoacoustic propagation medium
portion 52 to be focused at the acoustic focal point 57, it is necessary to satisfy the following
Equation 3 is there.
[0108]
05-05-2019
30
[0109]
The fact that the sound wave is focused on the acoustic focus 57 in the photoacoustic
propagation medium portion 52 means that the phase of the sound wave is aligned at the
acoustic focus 57.
That is, it means that the arrival time of the sound wave from the opening 63 to the acoustic
focus 57 is the same no matter which route it passes through.
[0110]
Specifically, in Equation 3, the left side (La1 / Ca) + (Ln1 / Cn) of the leftmost equal sign causes
the sound wave to propagate through the environmental fluid 14 by the distance La1, and the
photoacoustic propagation medium portion 52 By propagating by the distance Ln1, the time to
reach the acoustic focus 57 is shown.
Also, {(La1 + La2) / Ca} + (Ln2 / Cn), which is the right side of the leftmost equal sign, propagates
the sound wave by the distance (La1 + La2) in the environmental fluid 14, and the distance in the
photoacoustic propagation medium portion 52 By propagating Ln 2, the time to reach the
acoustic focal point 57 is shown.
[0111]
By the same procedure, at each point Pk, the time until the sound wave transmitted from the
acoustic waveguide 60 to the photoacoustic propagation medium portion 52 can reach the
acoustic focal point 57 can be obtained.
[0112]
Generalizing equation (3), the distance of the waveguide from the opening 63 of the acoustic
waveguide 60 to any point Pk along the propagation direction of the sound wave on the
transmission surface 61 is Lak, and from the point Pk to the photoacoustic propagation medium
Assuming that the distance to the acoustic focus F (57) different from the point Pk in the part 52
is Lnk, Equation 3 is (Lak / Ca) + (Lnk / for any k from 1 to n. It is expressed as a condition that
05-05-2019
31
Cn) is constant.
[0113]
The constant (Lak / Ca) + (Lnk / Cn) means that the time required from the opening 63 to the
acoustic focal point 57 at any position on the transmission surface 61 is constant, as described
above. It shows that there is.
In other words, by satisfying (Lak / Ca) + (Lnk / Cn), it means that the sound wave taken into the
photoacoustic propagation medium portion 52 converges on the acoustic wave focal point 57.
In the configuration in FIG. 1, the shape of the photoacoustic propagation medium unit 52 on the
side of the acoustic waveguide 60 satisfies (Lak / Ca) + (Lnk / Cn).
[0114]
FIG. 5 is a diagram for explaining that (Lak / Ca) + (Lnk / Cn) is constant. The sound wave
entered into the photoacoustic propagation medium portion 52 made of silica dry gel from an
arbitrary point on the transmission surface 61 of the acoustic waveguide 60 of the photoacoustic
propagation medium portion 52 starting from the opening 63 of the acoustic waveguide Even if
the transmission surface 61 satisfies (Lak / Ca) + (Lnk / Cn) at a constant level, it converges on
the acoustic focus 57. This is because, in the photoacoustic propagation medium portion 52, a
cylindrical (partially cylindrical) wavefront centered on the acoustic focus 57 is formed.
[0115]
Strictly speaking, it is considered more accurate to calculate the propagation distance of the
sound wave propagating in the acoustic waveguide 60 by using the central path of the acoustic
waveguide 60. However, as described below, the width dimension of the acoustic waveguide 60
is sufficiently smaller than its length. For this reason, the above-mentioned approximation has
practically sufficient accuracy.
05-05-2019
32
[0116]
(Light Source and Detection Unit) In FIG. 1, for convenience, only the propagation path of the
laser beam 58 in the air is described. A detection unit for detecting the ultrasonic wave
converged on the acoustic focus 57 will be described. In FIG. 1, the light source is configured by
an LDV head 8, and the detection unit is configured by an LDV arithmetic processing unit 9.
[0117]
By focusing the sound wave on the acoustic focus 57, a standing wave of a compressional wave
with a large amplitude is generated. As a result, a change in refractive index depending on the
sound pressure of the acoustic signal received by the converging unit 77 occurs in the acoustic
focus 57.
[0118]
The LDV head 8 emits (fires) the laser beam 58 toward the propagation medium unit 52. Further,
after being emitted, the LDV head 8 propagates the propagation medium portion 52 and receives
the reflected laser beam 58. The LDV arithmetic processing unit 9 converts the received laser
beam 58 into an electrical signal and performs signal processing.
[0119]
The laser beam 58 emitted from the LDV head 8 is frequency-modulated by the Doppler
frequency according to the speed of change of the refractive index. The modulation frequency is
detected by the LDV arithmetic processing unit 9 by a detection method such as self heterodyne
detection.
[0120]
For example, using the thickness information of the photoacoustic propagation medium portion
52 in the direction in which the laser beam 58 was emitted, twice the distance between the
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33
detecting portion and the measurement through hole 53a, and the velocity of the laser beam 58
emitted from the detecting portion The time from the emission of the light 58 to the reception of
the reflected wave is determined in advance. The amount of frequency modulation of the laser
beam 58 can be known from the difference between the determined time and the actually
measured time. The sound pressure of the sound wave can be determined from the amount of
frequency modulation.
[0121]
The optical ultrasonic microphone 51 according to the first embodiment propagates a sound
wave from the environmental fluid 14 having a very small acoustic impedance, such as gas, to
the solid with high efficiency, and converges the sound wave transmitted to the solid within the
solid. Energy density can be increased. Thereby, sound waves can be received with high
sensitivity.
[0122]
(Experimental Results) FIGS. 6 to 11 show a process in which the sound wave propagating
through the acoustic waveguide 60 of the optical ultrasonic microphone 51 of the first
embodiment is transmitted to the photoacoustic propagation medium portion 52 and is
converged to the acoustic focus 57. The results obtained by calculation experiments are shown. 6
to 11 are sound pressure distribution diagrams showing calculation experimental results for
sound wave propagation in the optical ultrasonic microphone shown in FIG. 6 to 11 show only
the acoustic waveguide 60 and the photoacoustic propagation medium portion 52 of the optical
ultrasonic microphone 51 in order to easily display the position and the phase of the sound
wave.
[0123]
6 to 11 show the propagation of sound waves over time. FIG. 6 shows the earliest in time, and
FIG. 11 shows the slowest state. The transmission surface 61 and the waveguide outer surface 62
which define the acoustic waveguide 60 shown in FIGS. 6 to 11 are designed such that the sound
wave propagating in the acoustic waveguide 60 converges on the acoustic focus 57 by the
above-described procedure. . The opening 63 of the acoustic waveguide 60 is at the top and the
closed end is at the bottom. The acoustic waveguide 60 is filled with an environmental fluid 14,
05-05-2019
34
here air.
[0124]
The time waveform of the input sound wave signal used for FIG. 12 at the experiment shown to
FIGS. 6-11 is shown in FIG. Specifically, the waveform of the sound wave incident from the
opening 63 is shown. The center frequency of the sound wave is about 40 kHz, and the sound
wave has a length of about 5 wavelengths.
[0125]
6 to 11, the sound pressure levels of the sound waves in the photoacoustic propagation medium
unit 52 and in the acoustic waveguide 60 are shown by shading of colors. Dark parts (black)
indicate sound pressure higher than atmospheric pressure, and light parts (white) indicate sound
pressure lower than atmospheric pressure. The same color, for example between black and black
or white and white, represents 40 kHz, ie corresponding to one wavelength of the sound wave.
[0126]
In FIGS. 6 to 11, although it is difficult to confirm because the acoustic waveguide 60 is very
narrow, in the acoustic waveguide 60, since the sound velocity of air is 340 m / s, the distance
between the same colors is That is, the distance of one wavelength is about 8.5 mm.
[0127]
On the other hand, since the speed of sound of the dried gel constituting the photoacoustic
propagation medium portion 52 is 150 m / s in the photoacoustic propagation medium portion
52, the distance between the same colors, that is, the distance of one wavelength is about 3 It will
be .75 mm.
[0128]
FIG. 6 shows the moment when three wavelengths of the sound wave propagate from the
opening 63 to the acoustic waveguide 60 and the peak of the fourth wave propagates from the
opening 63 into the acoustic waveguide 60.
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35
The portion of the acoustic wave propagated to the inside of the acoustic waveguide 60
propagates from the transmission surface 61 in contact with the acoustic waveguide 60 to the
photoacoustic propagation medium portion 52.
The portion shown by shading inside the photoacoustic propagation medium portion 52 is a
component of the sound wave refracted and transmitted from the transmission surface 61 to the
photoacoustic propagation medium portion 52.
[0129]
FIG. 7 shows a state slightly advanced in time from the state shown in FIG. Inside the acoustic
waveguide 60, sound waves propagate along the shape of the acoustic waveguide 60. Further, a
sound wave propagating inside the acoustic waveguide 60 is gradually refracted and transmitted
to the photoacoustic propagation medium portion 52, and propagates in the photoacoustic
propagation medium portion 52. As shown in FIGS. 6 and 7, the sound wave propagating through
the acoustic waveguide 60 propagates a longer distance from the opening 63 than the sound
wave propagating through the photoacoustic propagation medium unit 52. This indicates that
the sound velocity of air, which is the environmental fluid 14 of the acoustic waveguide 60, is
faster than the sound velocity of the drying gel, which is the propagation medium.
[0130]
Similarly, FIG. 8 also shows how, as part of the sound wave propagates through the acoustic
waveguide 60, it is refracted and transmitted to the photoacoustic propagation medium portion
52, and the sound wave propagates inside the photoacoustic propagation medium portion 52. .
Although the pattern indicated by black and white shades on the transmission surface 61 is bent
due to refraction and transmission, the pattern indicated by black and white shades in the
photoacoustic propagation medium portion 52 has a beautiful curve. It is getting worse. This
indicates that the phases of the sound waves propagating in the photoacoustic propagation
medium unit 52 are aligned.
[0131]
FIG. 9 shows a sound wave propagating near the near end of the acoustic waveguide 60 and a
05-05-2019
36
sound wave gradually converging toward the acoustic focal point 57 inside the photoacoustic
propagation medium portion 52.
[0132]
In FIG. 10, the propagation of the sound wave further progresses, and the sound wave
propagating inside the acoustic waveguide 60 reaches the end of the waveguide and is all
refracted and transmitted to the inside of the photoacoustic propagation medium portion 52. The
sound wave propagating inside is shown to be converging toward the acoustic focus 57 further.
[0133]
In FIG. 11, the first wave front of the sound wave propagated inside the photoacoustic
propagation medium portion 52 reaches the acoustic focal point 57.
As shown in FIG. 6F, the darker shades of black are darker, which indicates that the acoustic
wave converges at the acoustic focus 57 and the sound pressure is increased.
[0134]
Although specific numerical values are not shown in FIGS. 6 to 11, from experimental results,
when the change in sound pressure due to the sound wave from the atmospheric pressure inside
the acoustic waveguide 60 is about 4 Pa, The change in sound pressure from barometric
pressure was found to be about 34 Pa.
This indicates that the sound pressure of the sound wave has been increased by eight times or
more, and according to the first embodiment, it has become clear that the sound wave in the
environmental fluid can be observed with high sensitivity.
[0135]
As described above, according to the first embodiment, the reflection of the sound wave at the
interface different in acoustic impedance is suppressed by refracting the sound wave and
transmitting it from the environmental fluid 14 to the photoacoustic propagation medium unit
52, and the sound wave with high efficiency Can be transmitted to the photoacoustic propagation
05-05-2019
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medium unit 52.
[0136]
Further, the photoacoustic propagation medium portion 52 is disposed to constitute one surface
of the acoustic waveguide 60 filled with the environmental fluid 14, and as the acoustic
waveguide 60 propagates, part of the acoustic wave is transmitted to the photoacoustic
propagation medium portion 52. By designing the shape of the surface in contact with the
acoustic waveguide 60 so as to transmit light to and converge on the predetermined acoustic
focus 57, the phases of the sound waves transmitted to the photoacoustic Can be focused on the
acoustic focus 57.
Therefore, the acoustic wave can be converged using most of the acoustic wave incident from the
opening 63 of the acoustic waveguide 60, and the sound pressure of the received acoustic wave
can be increased. Thereby, a sound wave can be detected with high sensitivity.
[0137]
In the optical ultrasonic microphone 51 of the first embodiment, the end of the acoustic
waveguide 60 is closed, but the end may be open. FIG. 13 is a cross-sectional view showing a
schematic device configuration of an optical ultrasonic microphone 51A according to a
modification of the first embodiment. In the optical ultrasonic microphone 51A shown in FIG. 13,
the waveguide end 64A of the acoustic waveguide 60 is open. If the energy of the sound wave
propagating through the acoustic waveguide 60 is relatively high and it is not necessary to take
in all the energy, the portion of the sound wave propagating through the acoustic waveguide 60
that has not been transmitted to the photoacoustic propagation medium portion 52 terminates It
is preferable to remove it from the acoustic waveguide 60 so as not to be reflected and adversely
affected. According to the acoustic wave receiver 103, since the terminal end 64 of the acoustic
waveguide 60 is open, it is possible to remove the sound wave that has not been transmitted to
the photoacoustic propagation medium portion 52. Thereby, the target sound wave can be
accurately detected without disturbance of the received sound wave. In this case, the length of
the acoustic waveguide 60 may be shorter than the preferred length determined in relation to
the opening as described above.
[0138]
05-05-2019
38
FIG. 14 is a contour diagram showing the measurement results of the equiphase surface of the
sound wave propagation of the photoacoustic propagation medium portion of the optical
ultrasonic microphone in the first embodiment of the present invention. FIG. 14 shows the sound
wave propagation time measured by two-dimensional scanning with the LDV head 8 of the sound
wave propagation inside the photoacoustic propagation medium part 52 made of silica dry gel in
the optical ultrasonic microphone 51, as a result of the sound wave The situation of the
equiphase surface (wave front) 902 of propagation is shown.
[0139]
Here, a silica dry gel having a density of 270 kg / m 3 and an acoustic velocity of 145 m / s was
used as an example of the photoacoustic propagation medium portion 52. The incident angle at
each point in this case is 89.5 degrees, and the refraction angle is 26 degrees. The curved surface
was designed based on this sound velocity value. It can be observed that the sound wave
propagation direction 901 and the equal phase surface 902 propagate toward the sound wave
acoustic focus 57 as the silica dry gel 52 as a cylindrical wave, and the operation as the
theoretical design was confirmed.
[0140]
FIG. 15 is a time waveform diagram showing an example of the LDV output waveform 81
(amplitude measurement waveform) of the optical ultrasonic microphone 51 in the vicinity of the
sound wave acoustic focus 57. As shown in FIG. FIG. 15 shows the result when the broadband
tweeter emits a sound wave to be measured at a center frequency of 40 kHz and a drive signal of
one wavelength.
[0141]
In consideration of the center frequency of 40 kHz, both the width and the initial height (height
at the opening 54) of the acoustic waveguide 60 were 4 mm. The thickness of the dried silica gel
used as the photoacoustic propagation medium portion 52 is also 4 mm. For measurement of the
laser beam 58 reciprocating in the photoacoustic propagation medium portion 52, a heterodyne
type laser Doppler vibrometer (LDV head 8) using a He̶Ne laser with a wavelength of 633 nm
05-05-2019
39
is used as an example of a light source and light detection means It was.
[0142]
Modulation of light by sound waves is frequency modulation. An aluminum material is used for
the base 3. The laser beam 58 emitted from the He-Ne laser is a photoacoustic propagation
medium portion via the measurement through hole (a through hole formed in the acoustic focus
57) 53a of the base 53 located on the surface side of the silica dry gel 52 52, and penetrates the
photoacoustic propagation medium portion 52 in the thickness direction, propagates the light
path in reverse, reflects on the inner surface of the base 53 on the back side of the photoacoustic
propagation medium portion 52, and then propagates photoacoustics again. The light passes
through the medium portion 52 in the thickness direction, and is emitted from the through hole
53 a of the base 53 located on the surface side of the photoacoustic propagation medium portion
52, and returns to the LDV head 8. Therefore, the optical path for measuring the sound wave is 8
mm which is twice the thickness 4 mm of the photoacoustic propagation medium portion 52.
[0143]
From the results shown in FIG. 15, it can be seen that the receiver has extremely wide band
reception characteristics as in the first embodiment. From the waveform 81 of FIG. 15, the peak
displacement is about 5 nm. The converted sound pressure P is about 54.2 Pa. The input
conversion sound pressure at the end of the acoustic horn was about 25 Pa, and a convergence
effect of about twice was confirmed. Also in this case, the measured and converted sound
pressure and the input converted sound pressure sufficiently match in order, and by properly
calibrating the measured and converted sound pressure, Extremely accurate sound pressure
measurement is possible.
[0144]
In the first embodiment, by focusing the acoustic wave inside the photoacoustic propagation
medium unit 52, broadband reception can be performed with higher sensitivity.
[0145]
Second Embodiment The optical ultrasonic microphone of the second embodiment will be
05-05-2019
40
described with reference to FIGS. 16 to 18.
The difference between the second embodiment and the first embodiment lies in the
configuration of the optical system that optically detects the sound pressure at the acoustic focus
57. Hereinafter, this optical system is referred to as an optical sound pressure measurement unit
1300.
[0146]
In the first embodiment, the optical sound pressure measurement unit 1300 is configured of the
LDV head 8 and the LDV arithmetic processing unit 9. Although LDV is rich in versatility and
stability, it is difficult and expensive to miniaturize the LDV due to its configuration. Therefore, in
order to provide the optical ultrasonic microphone in a small size and at a low cost, it is desirable
to make the optical sound pressure measurement unit 1300 small and inexpensive. Hereinafter,
the configuration of the optical sound pressure measurement unit 1300 will be described. Also in
the second embodiment, the configuration described in the first embodiment and the ones that
are not modified will not be illustrated and described.
[0147]
FIG. 16 shows the optical system structure of the optical sound pressure measurement unit 1300
in the second embodiment of the optical ultrasonic microphone of the present invention.
[0148]
The monochromatic light source 1301 shown in FIG. 16 emits coherent light.
An optical system 1303 secures the flatness of the wavefront and expands or reduces the
monochromatic light 1302 emitted from the monochromatic light source 1301 to an appropriate
beam size. Thereafter, the monochromatic light 1302 is split by the beam splitter 1304 into two
monochromatic lights 1307 and 1308. Then, the monochromatic light 1307 travels to the
photoacoustic propagation medium unit 52, and the monochromatic light 1308 travels to the
plane mirror 1306.
05-05-2019
41
[0149]
The monochromatic light 1307 is made of a transparent base 1314 made of a material (for
example, a transparent reinforced acrylic plate) capable of transmitting the monochromatic light
1307 and capable of well constraining the acoustic vibration in the photoacoustic propagation
medium portion 52; The light passes through the acoustic focus 57 of the propagation medium
portion 52 and is reflected by the plane mirror 1310 and passes through the acoustic focus 57
again. Then, a part of the light passes through the beam splitter 1304, and the shape of the beam
is shaped by the focusing optical system 1311 and is led to the light intensity measuring
instrument 1312. The optical path through which the monochromatic light 1307 described
above passes is denoted as L11.
[0150]
The monochromatic light 1308 is reflected by the plane mirror 1306 and travels in the direction
of the beam splitter 1304 again, but a part thereof is reflected by the beam splitter 1340 and
then condensed by the condensing optical system 1311 in the same manner as before. It is led to
the intensity measuring device 1312. The light path through which the monochromatic light
1308 described above passes is denoted as L12.
[0151]
The schematic configuration of the optical ultrasonic microphone 1300 and the path through
which the monochromatic light 1302 passes have been described above. Next, the operation
principle of the optical ultrasonic microphone 1300 will be described with reference to FIG. The
wavefronts of the monochromatic light 1307 transmitted through the beam splitter 1304 and
the monochromatic light 1308 reflected by the beam splitter 1304 are sufficiently parallel, and
both monochromatic lights have high contrast (contrast is the maximum intensity fluctuation of
interference light with time average intensity Definition by dividing. Therefore, the entire optical
system is adjusted in optical axis so as to generate interference light having a real value of 0 to
2), and the reflection / transmittance of the beam splitter 1304 is selected.
[0152]
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42
Therefore, the intensity of interference light received by the light intensity measuring instrument
1312 fluctuates with high contrast depending on the difference in optical path length between
the optical paths L11 and L12 through which the two monochromatic lights pass. When there is
no time fluctuation of the optical path length difference, the output signal 1313 from the light
intensity measuring instrument 1312 shows a constant value regardless of time. However, the
refractive index of the acoustic focal spot 57 temporally fluctuates due to the temporal
fluctuation of the acoustic signal intensity collected to the acoustic focal spot 57 by the
photoacoustic propagation medium unit 52, and the optical path length difference temporally
fluctuates accordingly Therefore, the output signal 1313 fluctuates in time. Since the refractive
index fluctuation depends on the input sound pressure, the sound pressure fluctuation can be
observed by analyzing the output signal 1313. The above is the operation of the optical
ultrasonic microphone 1300.
[0153]
A variation of a possible apparatus configuration in the optical ultrasonic microphone 1300 of
the second embodiment and points to be considered in design will be described.
[0154]
In FIG. 16, the beam splitter 1304, the transparent base 1314, the plane mirror 1306, and the
plane mirror 1310 are all shown in contact with each other, but another optical medium such as
an air layer is sandwiched between them and they are separated from each other It goes without
saying that it may be arranged.
However, the stability of the interference light intensity in this configuration mainly depends on
how to suppress the refractive index fluctuation due to influences other than the acoustic signal
due to air fluctuation etc. which is suffered while the monochromatic light 1307, 1308 passes
through different optical paths. . Therefore, in order to obtain the highly stable output signal
1313, as shown in FIG. 16, while the insertion of the air layer and the mechanically unstable
optical medium is eliminated as much as possible, the optical paths L11 and L12 follow different
paths. It is desirable to configure the area as small as possible.
[0155]
Although the optical system 1303 and the condensing optical system 1311 are shown in FIG. 16,
the monochromatic light 1302 from the monochromatic light source 1301 has sufficient
coherency and a beam size required for measurement from the monochromatic light source
05-05-2019
43
1301. It is needless to say that it can be omitted when preparing from the point of emission.
[0156]
However, in the case where the shift of the focal position of the photoacoustic propagation
medium portion 52 is a problem in the measurable frequency band of the required acoustic
signal (ie, the photoacoustic propagation medium portion 52 produces chromatic aberration
to the acoustic signal. (If it has), the beam size of the monochromatic light 1302 needs to be
large enough to cover the positional variation of the acoustic focal spot 57 due to the aberration.
This is because, if an acoustic signal in a certain frequency band forms a focus outside the beam
spot of the monochromatic light 1302, the acoustic signal in that frequency band is not received,
so reception of the acoustic signal in the measurable frequency band of the acoustic signal This
is because the frequency dependency occurs in the characteristics. If frequency dependence
exists in the reception characteristics, so-called signal distortion occurs in which the similarity
between the time waveform of the acoustic signal at the acoustic focus 57 and the signal
waveform of the output signal 1313 can not be secured, which is not preferable in terms of
characteristics. Therefore, it is essential to avoid this problem in order to secure a wide acoustic
measurable frequency band, and it is necessary to insert an optical system 1303 and extend the
monochromatic light 1302 to have a certain beam size.
[0157]
The spectrum width required for the monochromatic light 1302 depends on the optical system
configuration of the optical ultrasonic microphone 1300. For example, when parallel flat glass is
inserted between the beam splitter 1304 and the plane mirror 1306, and the light paths L11 and
L12 are adjusted to be equal in a situation where no acoustic signal is input to the acoustic focus
57, monochromatic light 1302 is used. Can use broadband light or white light such as light
emitting diode light having a maximum intensity at a desired wavelength. As the spectral size of
the monochromatic light 1302 narrows, the physical size of the monochromatic light source
1301 increases and the cost increases. Therefore, when realizing the optical acoustic
measurement device in a small size and at low cost, the optical path L11 is as low as possible. It
is desirable to configure the optical system so that the optical path lengths of L., L12 are equal.
[0158]
05-05-2019
44
Also, although the transparent base plate 1314 is described as being made of a transparent
material so that the monochromatic light 1307 can be sufficiently transmitted, the small
openings are provided in both transparent base plates 1314 and the light path of the
monochromatic light 1307 is the small opening. It is needless to say that the transparent base
plate 1314 can be applied as an opaque one by being able to pass through. All configurations in
which the monochromatic light 1307 can pass through the transparent base 1314 are equally
operable if the above-mentioned requirements for the interference light intensity stability are
satisfied.
[0159]
Next, results of demonstrating the measurement principle of the optical ultrasonic microphone
1300 depicted in FIG. 16 will be described with reference to FIGS. 17 and 18.
[0160]
The optical ultrasonic microphone 1300 measures the refractive index fluctuation in the acoustic
medium generated by the sound pressure as the optical path length fluctuation by optical
interferometry.
Therefore, the optical interferometer shown in FIG. 17 was actually configured to demonstrate its
principle.
[0161]
FIG. 17 shows an experimental apparatus for demonstrating the principle of the optical
ultrasonic microphone in the second embodiment. In FIG. 17, laser light emitted from a He̶Ne
laser 1400 is split by a beam splitter 1401 into two paths L21 and L22. The laser beam in the
path L21 is turned back by the reflecting mirror 1402 and interferes with the laser beam in the
path L22 in the beam splitter 1405.
[0162]
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45
The laser light passing through the path L22 is folded back by the reflecting mirror 1403 and
passes through the acoustic medium 1404, and then interferes with the light ray of the path L21.
Then, the intensity of the generated interference light is output as an electrical signal by the
photodetector 1406. The electrical signal is output as an acoustic reception signal through
necessary signal processing such as removal of DC components included in the signal and
averaging processing by a digital oscilloscope 1408.
[0163]
An acoustic medium 1404 is inserted between the reflecting mirror 1403 and the beam splitter
1405 in the path L22. Furthermore, a horn 1409 is connected to the acoustic medium 1404 so
that an acoustic signal emitted from the speaker 1410 can be introduced to the acoustic medium
1404. In the present experimental apparatus configuration, since the optical path length
difference between the path L21 and the path L22 is measured as the interference light intensity
by the Mach-Zehnder interferometer, the interference light intensity is a refractive index change
generated by the acoustic signal introduced into the acoustic medium 1404 Depends on
[0164]
Next, the actual experimental situation will be described. In the experiment, as an input signal to
the speaker 1410, one sine wave having an amplitude of 0.5 V and a frequency of 40 kHz
generated by a function generator (not shown) was used. The distance from the front surface of
the speaker 1410 to the surface of the opening of the horn 1409 is 100 mm, and the acoustic
medium 1404 has a thickness of 5 mm (lateral direction with respect to the paper of FIG. 17) and
a square shape of 20 mm. The acoustic medium 1404 has an acoustic velocity of approximately
70 m with respect to a 40 kHz sine wave.
[0165]
FIG. 18 shows the measured results. FIG. 18A is a graph showing the time waveform of the
output signal from the digital oscilloscope 1408 measured by the experimental apparatus for
demonstrating the principle of the optical ultrasonic microphone of FIG. Further, FIG. 18B is a
graph in which a time waveform of an input signal to the speaker 1410 is described. In FIGS. 18A
and 18B, the horizontal axis has the same time width, and both signal wave fronts are arranged
05-05-2019
46
to coincide with each other.
[0166]
As can be seen from FIG. 18, the output signal to the speaker 1410 described in FIG. 18B is
converted into an acoustic signal, propagates in the air, is collected by the horn 1409, is
introduced into the acoustic medium 1404, and travels as a compressional wave. Accordingly,
the refractive index fluctuation generated at the intersection region of the acoustic medium 1404
and the optical path L22 is detected as an interference light intensity output from the digital
oscilloscope 1408. Therefore, it can be understood from this measurement result that the optical
acoustic measurement unit 1300 can actually function.
[0167]
Third Embodiment Next, an optical ultrasonic microphone of a third embodiment will be
described with reference to FIG.
[0168]
The optical ultrasonic microphone of the third embodiment also constitutes an optical system for
optically detecting the sound pressure at the acoustic focal point 57 as in the second
embodiment.
In the optical ultrasonic microphone 1300 described with reference to FIG. 16, the optical path
length difference between the two different paths L21 and L22 constituting the MichelsonMorley interferometer is converted to interference light intensity and measured.
[0169]
However, when the monochromatic light 1302 is coherent monochromatic light such as laser
light, the interference light intensity is converted to the wavelength of the monochromatic light
1302 and all optical path length variations having a difference of integer wavelengths have the
same intensity. An acoustic signal having a strong sound pressure corresponding to such an
optical path length difference fluctuation, or an output when the optical path length difference is
already a half odd wavelength in the absence of the acoustic signal, to give (ie, pitch jump) A socalled signal distortion occurs in which the similarity of the time waveform of the acoustic signal
05-05-2019
47
at the signal 1313 and the acoustic focus 57 is broken.
[0170]
This problem is caused by the fact that the absolute amount measurable range of the optical path
length difference variation amount of the interferometer constituting the optical ultrasonic
microphone shown in FIG. 16 is 1 wavelength or less, and has a wide measurement dynamic
range. This is a problem when constructing an optical acoustic microphone.
In order to solve this problem, it is necessary to use an interferometer having a large measurable
range of the optical path length difference. As described below, it is possible to construct an
optical ultrasonic microphone to which an interferometer having such a feature is actually
applied.
[0171]
The configuration of an optical ultrasonic microphone that can solve the above-described
problem will be described with reference to FIG. In addition, in the third embodiment, the
configuration appearing in the first embodiment and the one without change are omitted from
the illustration and the description.
[0172]
FIG. 19 shows an optical system configuration of an optical ultrasonic microphone 1300 having a
wide measurement dynamic range and no signal distortion in the third embodiment. In FIG. 19,
the same reference numerals as in FIG. 16 apply to the constituent elements applicable from the
device configuration in FIG. 16 without any change.
[0173]
In FIG. 19, reference numeral 1601 denotes a two-frequency linearly polarized laser light source,
which emits laser light 1602 composed of linearly polarized light whose polarization planes are
orthogonal to each other and whose frequencies are ω and ω + Δω.
05-05-2019
48
[0174]
In the following, in order to clarify the explanation, it is defined that one polarization plane of the
two linearly polarized light beams has a frequency ω parallel to the plane of FIG. 19 (therefore,
the other polarization plane is to the plane of FIG. Vertical with frequency ω + Δω).
[0175]
For the same purpose as the apparatus configuration shown in FIG. 17, the laser beam 1602 is
subjected to ensuring of the flatness of the wave front by the optical system 1303 and
enlargement / reduction to an appropriate beam size (the laser beam 1602 by the insertion of
the optical system 1303 Changes in polarization conditions are usually very small and negligible.
A part of both polarization components of the adjusted laser beam 1602 is reflected by the nonpolarization beam splitter 1603, and is guided to the light intensity measuring instrument 1605
by the condensing optical system 1604.
Since the reflection is performed on the non-polarization plane, the two polarization components
included in the laser beam 1602 immediately after reflection have rotation of the polarization
plane and linear polarization, and the amplitude intensity ratio of both components is the laser
beam 1602 before reflection. Is the same as
[0176]
A polarizing plate 1606 is inserted between the non-polarization beam splitter 1603 and the
focusing optical system 1604. The polarization axis of the polarizing plate 1606 is inclined 45 °
with respect to the paper surface of FIG. 19 and the polarization axis is equally 45 ° with
respect to two polarization planes in the laser beam 1602. Since both polarization planes are
projected to the polarization axis by the plate 1606, linearly polarized components having the
frequencies ω and ω + Δω, which were not dried due to the orthogonality of the polarization
planes, interfere with each other, and the generated difference frequency is generated. Beat light
of Δω is incident on the light intensity measuring instrument 1605. Therefore, the time
waveform of the reference beat signal 1607 is a sine wave of frequency Δω. Since the linear
polarization components having the frequencies ω and ω + Δω always pass through the same
05-05-2019
49
path, the reference beat signal 1607 does not include information on the acoustic signal.
[0177]
Next, the direction of the laser beam 1602 that has passed through the non-polarization beam
splitter 1603 will be described. The laser beam 1602 having passed through the non-polarization
beam splitter 1603 is subjected to path selection by reflection and transmission according to the
polarization plane direction by the polarization beam splitter 1608. Here, in order to simplify the
description, it is assumed that linearly polarized light having a polarization plane parallel to the
paper surface of FIG. 19 is completely reflected by the polarization beam splitter 1608.
Accordingly, only linearly polarized light having a polarization plane parallel to the paper surface
of FIG. 19 contained in the laser beam 1602 which has passed through the non-polarization
beam splitter 1603 is reflected toward the acoustic focal point 57 and a straight line having a
polarization plane perpendicular to the paper of FIG. The polarized light is completely
transmitted to the plane mirror 1617.
[0178]
Among the laser beams divided by the polarization plane direction as described above, the way of
the laser beam 1609 reflected in the acoustic focal point 57 direction will be described first. The
laser beam 1609 passes through the acoustic focus 57 twice while being reflected by the plane
mirror 1618, and therefore suffers an optical path length variation according to the refractive
index variation generated by the acoustic signal input to the photoacoustic propagation medium
unit 52. This point is the same as the configuration example shown in FIG. 17, but the
configuration in FIG. 19 is different in that it passes through the 1/8 λ wave plate 1610 twice in
the middle of the path. The 1/8 λ wave plate 1610 has the function of rotating the polarization
plane of the light passing therethrough by 45 °. Accordingly, the polarization plane of the laser
beam 1609 reflected by the plane mirror 1618 and returned back to the polarization beam
splitter 1608 is rotated by 90 ° and becomes perpendicular to the plane of FIG. 19 and passes
through the polarization beam splitter 1608 and the polarization plate 1613 Then, it is led to the
light intensity measuring instrument 1612 by the condensing optical system 1611.
[0179]
Next, the direction of the laser beam 1614 directed to the plane mirror 1617 will be described
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50
for the laser beam whose path is split by the polarization beam splitter 1608 in the polarization
plane direction. When the laser beam 1614 is reflected by the plane mirror 1617 and returns to
the position just before the reflection surface of the polarization beam splitter 1608 again, it
passes through the 1/8 λ wavelength plate 1615 twice and the polarization plane is rotated by
90 °. Parallel to each other. Therefore, the laser beam 1614 is reflected by the polarizing beam
splitter 1608, passes through the polarizing plate 1613, and is guided to the light intensity
measuring instrument 1612 by the focusing optical system 1611.
[0180]
As described above, the two laser beams 1609 and 1614 are integrated into a single laser beam
whose polarization planes are orthogonal to each other again immediately before the polarizing
plate 1613, but the polarizing axis of the polarizing plate 1613 is as shown in FIG. The two laser
beams 1609 and 1614 become coherent after passing through the polarizing plate 1614. The
laser beams 1609 and 1614 are inclined by 45 ° and function in the same manner as the
polarizing plate 1606 described above. Accordingly, the laser beams 1609 and 1614 interfere
with each other to become beat light of the difference frequency Δω, and a measurement beat
signal 1616 having a sine waveform having the same frequency is output from the light intensity
measurement device 1612.
[0181]
Unlike the reference beat signal 1607, the phase φ of the measurement beat signal 1616
depends on the optical path length difference of the path through which each of the two laser
beams 1609 and 1614 passes independently, and the monochromatic light wavelength λ = 2πc
/ ω (c The change of the optical path length difference of one wavelength in terms of the speed
of light corresponds to the change of 2π of the phase φ. The measurement of the phase φ is
performed by phase comparison between the reference beat signal 1607 output from the light
intensity measuring instruments 1605 and 1612 as an electrical signal and the measurement
beat signal 1616. The phase comparison can be performed with high accuracy by measuring the
phase difference of the measurement beat signal 1616 based on the reference beat signal 1607
using, for example, a lock-in amplifier (not shown).
[0182]
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51
Since the device configuration of FIG. 19 measures the phase φ which is a continuous quantity,
no signal distortion due to pitch jump, which is one of the problems in FIG. 16, occurs. The
measurement of the variation of the phase φ exceeding 2π, which is another problem, is
performed as follows. First, the phase φ0 in the state where there is no input of an acoustic
signal in the photoacoustic propagation medium portion 52 is measured and placed, and the
phase fluctuation Δφ (that is, φ = φ0 + Δφ) from that is constantly monitored to obtain a
phase φ exceeding 2π. It becomes possible to measure the fluctuation amount of As described
above, the phase fluctuation Δφ reflects the refractive index fluctuation at the acoustic focal
point 57 caused by the acoustic signal via the optical path length difference fluctuation, so that
the above-mentioned 2) can be obtained by analyzing the measured phase fluctuation Δφ. The
acoustic signal can be measured while solving one problem.
[0183]
In the optical ultrasonic microphone 1300 shown in FIG. 19, possible variations and
modifications in the apparatus configuration will be described. In the configuration of FIG. 19 as
well as in FIG. 16, the flatness of the wavefront where the light emitted from the dual-frequency
linearly polarized laser light source 1601 can generate interference light with sufficient contrast
as it is and the acoustic focus 57 If the beam size is sufficient to realize sufficient coverage, the
optical system 1303 and the focusing optical systems 1604 and 1611 can be omitted.
[0184]
It goes without saying that the existing dual-frequency laser light source is equally applicable to
the dual-frequency linear polarization laser light source 1601. For example, a linearly polarized
laser beam having a frequency difference Δω is generated by a two-frequency Zeeman laser or
an acousto-optic modulator (Acoustic-optic modulator), and two linearly polarized beams after
modulation so that their polarization planes are orthogonal to each other. A light source in which
the wave laser light is integrated into one light flux can be applied.
[0185]
Further, in FIG. 19, the polarization plates 1606 and 1613, the non-polarization beam splitter
1603, the polarization beam splitter 1608, the 1/8 λ wavelength plate 1610 and 1615, the
plane mirrors 1617 and 1618, and the photoacoustic propagation medium portion 52 are
05-05-2019
52
completely in contact. It goes without saying that similar functions can be produced even if
another optical medium such as an air layer is inserted between the elements. However, it is
better not to insert an air layer or a mechanically weak optical medium in order to achieve high
device stability that can measure only the refractive index variation generated in the acoustic
focus 57 and to miniaturize the entire device. .
[0186]
In FIG. 19, the polarization plane and polarization axis of the laser beam 1602, non-polarization
beam splitter 1603, polarization beam splitter 1608, and polarizing plates 1606 and 1613 are
set with reference to the paper of FIG. It is needless to say that the respective polarization planes
and polarization axes may be simultaneously rotated at an arbitrary angle around the optical
axis, as long as they do. Also, it goes without saying that the polarization selectivity of reflection /
transmission of the polarization beam splitter 1608 is similarly reversed.
[0187]
According to the optical ultrasonic microphone of the present invention, it is possible to receive
ultrasonic waves in a high frequency range and in a wide band, which was conventionally
difficult, and a standard microphone having an effective band of 100 kHz or more can be
realized.
[0188]
DESCRIPTION OF SYMBOLS 8 LDV head 9 LDV arithmetic processing part 14, 104 Environment
fluid 51, 51A Optical ultrasonic microphone 52 Photoacoustic propagation medium part 53 Base
53a Through hole for measurement 55, 901 Sound wave propagation direction 56 Acoustic wave
guide member 57 Acoustic focus 58, 1614, 1602, 1609 laser light 60 acoustic waveguide 61
transmission surface 62 waveguide outer surface 63 opening 64, 64 A waveguide end 70 space
71 opening 72 end 77 convergence 81 LDV output waveform 101 ultrasonic transducer 102
Ultrasonic transducer 103 Propagation medium portion 105 Ultrasonic wave propagation path
111 First surface region 112 Second surface region 121 LDV 122, 123, 1402, 1403 Reflector
124 Cubic mirror 125 Laser light path 126 Sound field 127 Arithmetic portion 902 Equal phase
surface 1300, 1600 optical Pressure measuring unit 1301 Monochromatic light source 1302,
1307, 1308 Monochromatic light 1303 Optical system 1304, 1401, 1405 Beam splitter 1306,
1310, 1617, 1618 Flat mirror 1311, 1604, 1611 Condensing optical system 1312, 1612, 1605
Light intensity measuring instrument 1313 output signal 1314 transparent base 1400 He-Ne
laser 1402 and 1403 reflector 1404 acoustic medium 1406 light detection unit 1407 digital
oscilloscope 1408 horn 1409 speaker 1602 dual frequency linear polarization laser light source
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1603 non-polarization beam splitter 1606 and 1613 polarization plate 1607 Reference beat
signal 1608 Polarization beam splitter 1610, 1615 1/8 λ wave plate 1616 Measurement beat
signal
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