JP2008275515

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DESCRIPTION JP2008275515
An object of the present invention is to provide a vibration detection device capable of improving
detection sensitivity when vibration detection is optically performed. A laser beam Lout from a
light source 10 is separated into two light paths (a reference light path and a reflected light path)
in an interferometer. In addition, interference fringes are formed by causing the reference light
reflected by the reflection plate 141 in the reference light path and the reflection light reflected
by the diaphragm 131 and the half mirror 142 in the reflected light path to interfere with each
other. Then, the vibration of the vibrating film 131 is detected based on the interference pattern.
Thereby, the vibration of the vibrating film 131 is optically detected. In addition, the reflected
light is made to be multi-reflected between the vibrating film 131 and the half mirror 142 in the
reflected light path. The optical path difference between the reference light and the reflected
light becomes large, and the displacement of the vibrating film 131 is amplified and detected.
[Selected figure] Figure 1
Vibration detection device
[0001]
The present invention relates to a vibration detection device that optically detects displacement
of a vibrating body.
[0002]
In recent years, recording methods using SACD (Super Audio Compact Disc) and 24 bit-96 kHz
sampling have been used, and high sound quality has become mainstream.
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In such a flow, the microphone device of the conventional analog system has a limitation in
recording of the high frequency of 20 kHz or more, so let's record the content by making use of
the reproduction of the high frequency which is the feature of the above recording method And
when it was, it was a bottleneck.
[0003]
In addition, with regard to the dynamic range, it has not reached 144 dB which is possible by the
24-bit bit recording which is the feature of the above recording system, and the wide dynamic
range has not been fully utilized.
[0004]
Furthermore, at the recording site, in the conventional analog microphone device, noise increases
due to the long distance routing with the analog cable, and phantom power must be supplied to
the condenser microphone from the mixing console. It has been an obstacle to all digitization in
recording and production systems.
[0005]
Therefore, in recent years, several digital microphone devices have been proposed.
For example, in Patent Document 1, a digital audio signal is obtained by performing signal
conversion and digital signal processing of interference fringes caused by displacement of a
microphone diaphragm in an interferometer such as the Mach-Zehnder method. It has been
proposed to obtain an output.
Further, for example, in Patent Document 2, in the Michelson-type interferometer, a change in
interference fringes caused by displacement of the microphone diaphragm is converted by a
photoelectric conversion element and a bit stream signal is binarized by this value. It has been
proposed to have a diaphragm driving means for moving the microphone diaphragm as a
feedback path for constructing a so-called delta sigma (delta sigma) modulator.
[0006]
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JP-A-10-308998 JP-A-11-178099
[0007]
In Patent Document 1 described above, a digital audio signal is output by detecting the vibration
of the diaphragm using a Mach-Zehnder interferometer or a Michelson interferometer.
[0008]
On the other hand, in Patent Document 2 described above, a ΔΣ (delta sigma) modulator
including a diaphragm is configured.
Therefore, it is considered that a 1-bit digital audio signal can be obtained with a simple
configuration by the action of the ΔΣ modulator, and noise reduction of the audio signal in the
audible band can be achieved using the noise shaving effect. .
[0009]
However, in these patent documents 1 and 2, since the wavelength of a laser beam is about 0.6
micrometer, there existed a problem that detection sensitivity could not be improved very much.
Therefore, although it is effective when the vibration of the diaphragm is large, it is effective to
detect the vibration of the diaphragm when it is applied to a high sensitivity microphone for
which detection of vibration of several pm to several tens of pm is required. It was difficult and
there was room for improvement.
[0010]
The present invention has been made in view of such problems, and an object thereof is to
provide a vibration detection device capable of improving detection sensitivity when optically
detecting vibration.
[0011]
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The vibration detection device of the present invention comprises a light source for emitting a
laser beam, an interferometer, and a detection means.
Here, the interferometer includes a vibrator and a first reflector capable of reflecting laser light,
and a second reflector capable of at least partially reflecting laser light, and is emitted from a
light source. Separates the laser beam into the first and second optical paths and advances the
reference light reflected by the first reflector in the first optical path, and the vibrator and the
second reflector in the second optical path And the reflected light that has been multi-reflected
between them to interfere with each other to form an interference pattern. Further, the detection
means detects the vibration of the vibrator based on the formed interference fringes.
[0012]
In the vibration detection device of the present invention, laser light emitted from the light
source travels while being separated into two optical paths (first and second optical paths) in the
interferometer. At this time, the reference light reflected by the first reflector in the first light
path and the reflected light reflected by the vibrator and the second reflector in the second light
path interfere with each other to form interference fringes. Be done. Then, the vibration of the
vibrator is detected based on the interference fringes. Here, since the reflected light is multiply
reflected between the vibrating body and the second reflector in the second light path, the
displacement of the vibrating body is added for the number of reflections, and the reference light
and the reflected light are added. Thus, the displacement of the vibrating body is amplified and
detected.
[0013]
In the vibration detection device of the present invention, the second reflector may be configured
by a half mirror that partially reflects and partially transmits the laser light.
[0014]
In this case, when the reflected light is constituted by a plurality of types of reflection
components having different numbers of reflections due to multiple reflections between the
oscillator and the second reflector, among the plurality of types of reflection components having
different numbers of reflections It is preferable to set the optical path length in the second
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optical path by the reflection component of the desired number of reflections to be equal to the
optical path length of the first optical path.
In this configuration, the visibility (intelligibility) of the interference fringes formed due to
interference is maximized, and thus selective interference between the reflection component of
the desired number of reflections and the reference light becomes possible. . Note that "equal" is
not limited to the case of being literally equal, but also means the case of being substantially
equal due to manufacturing variations and the like.
[0015]
Further, when the reflected light is constituted by a plurality of types of reflection components
having different numbers of reflections due to multiple reflections between the vibrator and the
second reflector, interference fringes due to the plurality of types of reflection components
having different numbers of reflections It is preferable to set the optical path length of the first
optical path so that the intelligibility peaks do not interfere with each other. In such a
configuration, it is easy to independently detect the intelligibility peak of each interference fringe.
[0016]
According to the vibration detection device of the present invention, the laser light from the light
source is split into two light paths (first and second light paths) in the interferometer, and the
light is reflected by the first reflector in the first light path. The interference light is formed by
causing the reference light and the reflected light reflected by the vibrator and the second
reflector to interfere with each other in the second optical path, and the vibration of the vibrator
is detected based on the interference fringes. Therefore, the vibration of the vibrating body can
be detected optically. Further, since the reflected light is multiply reflected between the vibrating
body and the second reflector in the second optical path, the optical path difference between the
reference light and the reflected light is increased, and the vibration is caused. Body
displacement can be amplified and detected. Therefore, it is possible to improve the detection
sensitivity when performing vibration detection optically.
[0017]
Hereinafter, embodiments of the present invention will be described in detail with reference to
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the drawings.
[0018]
First Embodiment FIG. 1 shows a configuration of a vibration detection apparatus (optical
microphone apparatus 1) according to a first embodiment of the present invention.
The microphone device 1 outputs an audio signal Sout using a diaphragm (a diaphragm 131
described later) that vibrates according to the sound wave Sw, and the laser light source 10, the
diaphragm 131, the reflection plate 141, and the half A Michelson interferometer including a
mirror 142 and a detection unit for outputting an output signal (audio signal Sout) which is a
digital signal are provided.
[0019]
The laser light source 10 emits a laser light Lout, and for example, a self-pulsation type
semiconductor laser with low coherence is used. In order to reduce the coherence, a
semiconductor laser modulated at a high frequency may be used.
[0020]
The lens 11 is a lens (collimator lens) for collimating the laser beam Lout from the laser light
source 10.
[0021]
<Interferometer Configuration> The interferometer includes a polarization beam splitter 12, a
vibrating film 131, a reflector 141, a half mirror 142, three λ / 4 plates 151 to 153, a beam
splitter 16, and two polarizations. It is comprised from board 171,172.
[0022]
The polarization beam splitter 12 transmits two light paths, that is, a reflected light path (first
light path) on the vibrating film 131 side and a reference light path on the reflecting plate 141
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side (laser light Lout emitted from the laser light source 10 and passing through the lens 11).
And the second optical path).
Specifically, although the details will be described later, in this polarization beam splitter 12, the
P-polarization component p0 of the laser light Lout travels to the reflected light path side, and
the S-polarization component s0 of the laser light Lout travels to the reference light path side. It
is set.
The laser beam Lout is separated into approximately 50% each by the P-polarization component
p0 and the S-polarization component s0.
[0023]
The vibrating film 131 is displaced according to the sound wave Sw, and is formed of a vibrating
film or the like on the surface of which gold is vapor-deposited, as in the case of, for example, a
capacitor microphone. The vibrating film 131 is configured to be able to reflect the laser beam
Lout (specifically, the S polarization component s0) with a high reflectance, and is accommodated
in the microphone capsule 13 as shown in FIG. It is supposed to be Further, as shown in FIG. 1,
the distance between the vibrating film 131 and the polarization beam splitter 12 is previously
set to L1. A detailed configuration example of the microphone capsule 13 will be described later.
[0024]
The reflection plate 141 is configured to be able to reflect the laser light Lout (specifically, the Ppolarization component p0), which is the reference light, with a high reflectance. The distance
between the reflection plate 141 and the polarization beam splitter 12 is set to L0 as shown in
FIG. 1, and the distance L0 can be adjusted as described later.
[0025]
The half mirror 142 is disposed on the reflection light path, specifically, between the polarization
beam splitter 12 and the vibrating film 13. The distance between the half mirror 142 and the
vibrating film 131 is set to be L2 in advance as shown in FIG. The half mirror 142 partially
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reflects and partially transmits the laser light Lout (specifically, the S-polarization component s0)
(for example, 50% and 50% of the laser light Lout) Thus, as shown in FIG. 1, multiple reflection of
the laser beam Lout is possible between the vibrating film 131 and the half mirror 142 (multiple
reflected light Lr is generated). .
[0026]
The λ / 4 plate 151 is disposed on the reflected light path, specifically, between the polarization
beam splitter 12 and the half mirror 142. The λ / 4 plate 152 is disposed on the reference light
path, specifically, between the polarization beam splitter 12 and the reflector 141.
[0027]
As described later, the beam splitter 16 divides the S-polarization component s1 (reflected light)
and the P-polarization component p1 (reference light) of the laser light Lout incident through the
polarization beam splitter 12 into an optical path and polarization of the polarizing plate 171
side. The light path on the side of the plate 172 is separated by about 50% and advanced.
[0028]
The polarizing plates 171 and 172 are polarizing plates having polarization axes in directions
respectively inclined 45 degrees from the polarization direction of the incident S-polarization
component s1 (reflected light) and the polarization direction of the P-polarization component p1
(reference light).
Although the details will be described later according to such a configuration, in these polarizing
plates 171 and 172, the S polarization component s1 and the P polarization component p1
interfere with each other to form interference fringes. The λ / 4 plate 153 is disposed on the
optical path between the beam splitter 16 and the polarizing plate 171.
[0029]
With such an arrangement, in the interferometer of the present embodiment, the laser light Lout
emitted from the laser light source 10 is split into two light paths (first and second light paths)
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and travels. Specifically, the polarization beam splitter 12, the λ / 4 plate 151, the half mirror
142, the vibrating film 131, the half mirror 142, the λ / 4 plate 151, the polarization beam
splitter 12, the beam splitter 16, the polarization plates 171 and 172, and λ Second light path
(reflected light path) passing through the 1⁄4 plate 153, the polarization beam splitter 12, the λ
/ 4 plate 152, the reflection plate 141, the λ / 4 plate 152, the polarization beam splitter 12, the
beam splitter 16, the polarization plate 171, It travels separately to a first optical path (reference
optical path) passing through 172 and λ / 4 plate 153. At this time, light reflected by the
vibrating film 131 through the λ / 4 plate 151 in the reflection light path (S-polarized
component s1, reflected light) and reflected by the reflection plate 141 through the λ / 4 plate
152 in the reference light path Interference light (P-polarized light component p1, reference
light) interferes with each other in the polarizing plates 171 and 172 to form interference
fringes.
[0030]
<Structure of Detection Unit> The detection unit includes two photoelectric conversion elements
181 and 182 and a digital signal processing unit 19.
[0031]
The photoelectric conversion elements 181 and 182 detect interference fringes formed on the
polarizing plates 171 and 172, perform photoelectric conversion, and output output signals Sx
and Sy, respectively.
The photoelectric conversion elements 181 and 182 are configured by, for example, PD (Photo
Diode).
[0032]
The digital signal processing unit 19 AD (analog / digital) converts the output signals Sx and Sy
output from the photoelectric conversion elements 181 and 182, and outputs an output signal
(audio signal Sout) that is a digital signal. . Such digital counting method will be described in
detail later.
[0033]
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Next, with reference to FIG. 2 and FIG. 3, a detailed configuration example of the microphone
capsule 13 shown in FIG. 1 will be described. FIGS. 2 and 3 show cross-sectional configurations
of the microphone capsules 13A and 13B, which are detailed configuration examples of the
microphone capsule 13. FIG.
[0034]
The microphone capsule 13A shown in FIG. 2 includes a housing 130, a diaphragm 131, a back
electrode 132, a back plate 133, and a transparent member 134, and functions as an
omnidirectional microphone capsule. The vibrating film 131 is disposed on the side (front side)
on which the sound wave Sw is incident, and the back electrode 132 is disposed on the back side.
The back plate 133 is not provided with an opening or the like for sealing the microphone
capsule, but a part of the back plate 133 is a transparent member 134 made of glass, transparent
resin or the like on which an anti-reflection (AR) film is formed. It has become. With such a
configuration, in the microphone capsule 13A, the laser light Lout is transmitted through the
transparent member 134 on the back side without interfering with the incidence of the sound
wave Sw while maintaining the sealed structure for forming the omnidirectional microphone
capsule. Thus, it is possible to make the light enter the vibrating membrane 131.
[0035]
On the other hand, the microphone capsule 13B shown in FIG. 3 includes the housing 130, the
diaphragm 131, the back electrode 132, the back plate 133, and the opening 135, and functions
as a unidirectional microphone capsule. ing. In this microphone capsule 13B, an opening for
obtaining appropriate directivity by displacing the diaphragm 131 in a part of the back plate 133
by the difference between the sound pressure applied to the front of the diaphragm 131 and the
sound pressure on the back side. The portion 135 is provided, and the laser beam Lout can be
incident on the vibrating film 131 through the opening 135. With such a configuration, in the
microphone capsule 13B, the laser light Lout is made to enter the diaphragm 131 without
blocking the incidence of the sound wave Sw by using the opening portion 135 for forming a
single directional microphone capsule. Is possible.
[0036]
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Here, the vibrating film 131 corresponds to one specific example of the vibrator in the
present invention, the reflecting plate 141 corresponds to one specific example of the first
reflector in the present invention, and the half mirror 142 is the present invention.
Corresponds to one specific example of the second reflector in the above. The photoelectric
conversion elements 181 and 182 correspond to one specific example of the two photoelectric
conversion elements in the present invention, and the photoelectric conversion elements 181
and 182 and the digital signal processing unit 19 are one of the detection means in the
present invention. The digital signal processing unit 19 corresponds to a specific example of the
"graphic generation means" and the "counter" in the present invention, corresponding to the
specific example.
[0037]
Next, the operation of the microphone device 1 of the present embodiment will be described in
detail.
[0038]
First, the basic operation of the microphone device 1 will be described with reference to FIGS. 1
to 4.
[0039]
In the microphone device 1, as shown in FIG. 1, the laser light Lout is emitted from the laser light
source 10, and the laser light Lout is collimated by the lens 11 and then enters the polarization
beam splitter 12.
Then, the incident laser beam Lout is separated and travels by about 50% each to the reflected
light path (second light path) on the vibrating film 131 side and the reference light path (first
light path) on the reflective plate 141 side.
Thereby, the laser beam Lout is separated into the P-polarization component p0 traveling in the
reflection light path and the S-polarization component s0 (reference light) traveling in the
reference light path. That is, the polarization beam splitter 12 reflects the light of the S
polarization component and transmits the light of the P polarization component.
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[0040]
Here, when passing through the λ / 4 plate 151, the P polarization component p0 changes from
linear polarization to circular polarization, and after being reflected by the vibrating film 131, it
becomes circular polarization in the opposite direction, and passes through the λ / 4 plate 151
again. Is converted to the S-polarization component s1 (reflected light). Since the S-polarized
component s1 is reflected by the polarization beam splitter 12 as described above, it travels in
the direction of the beam splitter 16 along the reflected light path. On the other hand, the Spolarization component s0, which is the reference light, changes from linear polarization to
circular polarization when passing through the λ / 4 plate 152, and then becomes circular
polarization in the opposite direction when reflected by the reflection plate 141. By passing
through, it is converted into a P-polarization component p1. Then, the P-polarization component
p1 travels in the direction of the beam splitter 16 along the reference optical path because it
passes through the polarization beam splitter 12 as described above. At this time, the Spolarization component s1 and the P-polarization component p1 traveling on the same optical
path (the reflection optical path and the reference optical path) do not interfere with each other
because the polarization directions thereof are different by 90 degrees.
[0041]
Next, the S polarized light component s1 and the P polarized light component p1 traveling in the
reflected light path and the reference light path are separated by the beam splitter 16 into about
50% each in the light path on the polarizing plate 171 side and the light path on the polarizing
plate 172 side. To reach the polarizing plates 171 and 172, respectively. At that time, since the
λ / 4 plate 153 is inserted in the middle of the light path on the polarizing plate 171 side, the S
polarized light component s1 and the P polarized light component p1 reaching the vibrating
plate 171 and the S polarized light reaching the vibrating plate 172 The components of the
component s1 and the P-polarized component p1 are 90 degrees out of phase with each other.
The polarizing plates 171 and 172 respectively have polarization axes in directions inclined 45
degrees from the polarization direction of the S-polarization component s1 and the polarization
direction of the P-polarization component p1, respectively, so the phases of the S-polarization
component s1 and the P-polarization component p1 Even in the case of the present embodiment
in which the S polarization components s1 and the P polarization component p1 of the reference
light interfere with each other in the polarizing plates 171 and 172, interference fringes are
formed.
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[0042]
Next, the interference fringes formed on the polarizing plates 171 and 172 are detected by the
photoelectric conversion elements 181 and 182, respectively. Here, since the phases of the Spolarization component s1 and P-polarization component p1 reaching the diaphragm 171 and
the S-polarization component s1 and P-polarization component p1 reaching the diaphragm 172
differ from each other by 90 degrees, photoelectric conversion is performed. In the elements 181
and 182, the interference fringes are detected with the phases being shifted by 90 degrees. Then,
the interference fringes detected by the photoelectric conversion element 181 are converted into
an electrical signal and output as an output signal Sx, while the interference fringes detected by
the photoelectric conversion element 182 are also converted into an electrical signal and output
as an output signal Sy Ru.
[0043]
Next, in the digital count 19, assuming that the output signals Sx and Sy from the photoelectric
conversion elements 181 and 182 are X and Y signals, for example, a circular or arc-shaped
Lissajous figure as shown in FIG. It is supposed to generate. Specifically, assuming that the
amplitudes of the interference light from the two optical paths are A and B, the optical path
difference is ΔL, and the wavelength is λ, the intensities Ix and Iy of the interference light are
respectively (1) to (3) below. It is expressed as a formula. The output signals Sx and Sy output
signals X and Y corresponding to the intensities Ix and Iy of the interference light, respectively,
and a DC component signal CX corresponding to A <2> + B <2> which is a DC term of the light
intensity. The x and y signals are obtained by canceling CY and passing an amplifier (not shown)
having a gain G ′ corresponding to the light intensity gain G represented by the following
equation (4). Thus, the (x, y) signal is obtained from the (X, Y) signal by performing the following
equations (5) and (6). Ix = A <2> + B <2> + 2AB cos θ (1) Iy = A <2> + B <2> +2 AB sin θ (2) θ =
(2π × ΔL) / λ (3) G = 1 / (2 AB) ... (4) x = (X-CX) x G '= cos theta (5) y = (Y-CY) x G' = sin theta
(6)
[0044]
Then, from the movement of the signal point (x, y), a Lissajous figure moving on the
circumference centered on the central point C is obtained from the movement of the signal point
(x, y) by the calculation of the equations (5) Be At this time, the detection points (for example,
signal point P0 in the figure) detected by the photoelectric conversion elements 181 and 182 are
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one point on this circumference, and are displaced on the circumference according to the
displacement of the vibrating film 11 become. Therefore, the angle θ is uniquely determined by
the values of x and y in the angular range (range of −π / 2 to + π / 2) by θ = arctan (y / x),
and exceeds the upper limit of this range In the case, 1 is added to the accumulator value, and 1
is subtracted if the lower limit is exceeded. Then, the counted number is output as an audio
signal Sout of a digital signal which is information of the angle θ.
[0045]
Next, with reference to FIG. 5 to FIG. 7 in addition to FIG. 1 to FIG. 4, the operation of the
characteristic part of the present invention (multiple reflection between the diaphragm 131 and
the half mirror 142) will be described in detail. .
[0046]
First, in FIG. 1, the intensity I of the interference fringes due to the interference between the
reference light and the reflected light is expressed as the following equation (7) according to the
equations (1) to (3) described above.
Further, since ΔL in the equation (3) represents the displacement of the optical path difference
between the reference light and the reflected light, the displacement ΔL of the optical path
difference is δ, which is the displacement of the diaphragm 131 by the sound wave Sw.
Assuming that the incident angle of the laser beam Lout with respect to the vibrating film 131 is
θ, the following equation (8) is given. I = A <2> + B <2> +2 AB cos ((2π × ΔL) / λ) (7) ΔL = 2
× δ × cos θ (8)
[0047]
Here, in the interferometer of the present embodiment, a part of the reflected light reflected by
the vibrating film 131 is reflected by the half mirror 142 and returned in the direction of the
vibrating film 131, as shown in FIG. Multiple reflected light Lr is generated. Therefore, the
incident angle to the diaphragm 131 due to such multiple reflected light Lr can be θ1 (incident
angle at the first reflection), θ2 (incident angle at the second reflection),. The displacement ΔL
of the optical path difference is expressed by the following equation (9). According to the
equation (9), when the incident angle is around 0 ° (when the laser beam is incident almost
perpendicularly to the vibrating film 131), cos θ 1 = cos θ 2 =. The displacement ΔL of the
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difference is expressed by the following equation (10), which is approximately equal to the
product of the number of multiple reflections n between the vibrating film 131 of the laser light
Lout and the half mirror 142 (the displacement ΔL of the optical path difference is
Approximately n times the size). Accordingly, it can be seen that the optical path difference is
increased due to the multiple reflection between the vibrating film 131 and the half mirror 142,
and the displacement of the vibrating film 131 is also amplified and detected. Δ L = 2 × δ ×
(cos θ 1 + cos θ 2 + ... + cos θ n) (9) Δ L 2 2 × δ × n (10)
[0048]
Further, when using a laser light source 10 that emits low coherence light, such as a selfpulsation type semiconductor laser, in particular, the visibility (intelligibility) of the interference
fringes can be referred to, for example, as shown in FIG. The optical path difference between the
light and the reflected light is maximized when the optical path difference is zero and is rapidly
reduced when the optical path difference occurs. The visibility (intelligibility) of the interference
fringes is defined as the following equation (11), where Imax is the maximum value of the
intensity I of the interference fringes and Imin is the minimum value of the intensity I of the
interference fringes. Visibility (intelligibility) of interference fringes = (Imax-Imin) / (Imax + Imin)
(11)
[0049]
Here, assuming that the number of multiple reflections between the oscillation film 131 of the
laser light Lout and the half mirror 142 is n as described above, interference formed by the
interference between the n-time reflection light reflected n times and the reference light The
distance L0 between the beam splitter 12 and the reflection plate 141 when the visibility of the
stripes is maximized is L1, the distance between the beam splitter 12 and the diaphragm 131 as
described above, and the diaphragm 131 and the half mirror 142 And the distance between L
and L is expressed by the following equation (12). Therefore, for example, when the interference
light peak and the side peak of the interference light peak when the visibility of the interference
fringe is maximum are expressed as the distance L0 between the beam splitter 12 and the
reflector 141 on the horizontal axis, for example, It becomes like FIG. 6 (A) and (B).
Ｌ０＝Ｌ１＋（ｎ−１）×Ｌ２ …（１２）
[0050]
That is, for reflected light in the reflected light path, a plurality of types of reflection components
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(for example, n = 1, n = 2, n = in FIG. 6) having different numbers of reflections due to multiple
reflection between the diaphragm 131 and the half mirror 142 Since each reflection component
(3),... Is included, if the distance L0 between the beam splitter 12 and the reflection plate 141 is
set to L0 expressed by the above (12), In other words, if the distance L0 (and the distances L1
and L2) is set so that the optical path length in the reflected light path by the reflection
component of the desired number of reflections and the optical path length of the reference
optical path become substantially equal, While selective interference between the reflected
component and the reference light is possible, the visibility of the interference fringes due to the
interference is maximized.
[0051]
For example, as shown in FIG. 6A, when L2 >> d (d: the distance between the peaks of the
intelligibility of the individual interference fringes), the interference fringes are clearly identified
by a plurality of types of reflection components having different numbers of reflections. Since the
power peaks appear at positions separated from each other with respect to the distance L0, they
do not affect each other due to interference between the clearness peaks of the interference
fringes.
However, for example, as shown in FIG. 6B, when L2 ≒ d, the peaks of the intelligibility of
interference fringes due to a plurality of types of reflection components having different
numbers of reflections appear at positions close to each other with respect to the distance L0. As
a result, it becomes difficult to detect selective interference between the reflection component of
the desired number of reflections and the reference light, because the peaks of the intelligibility
of interference fringes interfere with each other as it is.
[0052]
Therefore, in such a case, for example, as shown in FIG. 7, by setting the distance L2 such that L2
≒ (d / n), interference fringes due to a plurality of types of reflection components having
different numbers of reflections can be obtained. Interference effects between intelligibility peaks
are minimized. As a result, it becomes easy to independently detect the peak of the intelligibility
of each interference fringe, so that the detection accuracy of the displacement of the diaphragm
131 is improved.
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[0053]
Thus, in the microphone device 1 of the present embodiment, the laser beam Lout emitted from
the light source 10 is split into two light paths (reference light path and reflected light path) by
the polarization beam splitter 12 in the interferometer, and S It travels as wave component s0
and P wave component p0. At this time, the reference light (P wave component p1) reflected by
the reflecting plate 141 in the reference light path and the reflected light (S wave component s1)
reflected by the diaphragm 131 and the half mirror 142 in the reflected light path interfere with
each other. Interference fringes are formed on the polarizing plates 171 and 172. Then, based on
the interference fringes, the light source conversion elements 181 and 182 and the digital signal
processing unit 19 detect the vibration of the diaphragm 131 as the quantized audio signal Sout.
Here, since the reflected light is multi-reflected between the vibrating film 131 and the half
mirror 142 in the reflected light path, the optical path difference between the reference light and
the reflected light becomes large. The displacement is amplified and detected.
[0054]
As described above, in the present embodiment, the laser light Lout from the light source 10 is
separated into two optical paths (reference optical path and reflected optical path) in the
interferometer, and the reference light and reflection reflected by the reflector 141 in the
reference optical path In the optical path, the reflected light reflected by the vibrating film 131
and the half mirror 142 is made to interfere with each other to form an interference pattern, and
the vibration of the vibrating film 131 is detected based on the interference pattern. Can be
detected optically. Further, since the reflected light is multiply reflected between the vibrating
film 131 and the half mirror 142 in the reflected light path, the optical path difference between
the reference light and the reflected light is increased, and The displacement can be amplified
and detected. Therefore, it is possible to improve the detection sensitivity when performing
vibration detection optically.
[0055]
In addition, it can be realized with a small and simple configuration using a Michelson
interferometer. Therefore, it is possible to miniaturize the vibration detecting device (microphone
device) that optically detects digital vibration.
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[0056]
In addition, since non-contact sensing can be performed by light, the size and lightness of the
vibrating film 131 can be freely selected, and the dynamic range and frequency characteristics
can be changed to conventional analog methods such as dynamic method and capacitor method.
It can be scaled up.
[0057]
Furthermore, since the digital signal can be directly taken out by counting the interference
fringes, the S / N ratio can be easily reduced and noise reduction of the audio signal Sout to be
output can be realized by raising the angle detection accuracy. .
In addition, since digital signals can be obtained directly from the microphone device 1, digital
transmission can be easily realized, and even in the case of drawing long lines from the
microphone device 1, the influence of noise or the like can be eliminated.
[0058]
In the present embodiment, as an example of the second reflector capable of at least partially
reflecting the laser light Lout, a half mirror 142 capable of partially reflecting and partially
transmitting the laser light Lout. For example, as shown in FIGS. 8A and 8B, a total reflection
mirror for totally reflecting the laser light Lout (total reflection mirrors 143A and 143B shown in
FIG. 8A). Alternatively, the interferometer may be configured using the total reflection mirrors
144A and 144B or the like shown in FIG. In such a configuration, the decrease in light intensity
at the time of reflection is suppressed, so that the detection accuracy of the interference fringes
can be improved and the detection accuracy of the diaphragm 131 can be improved in addition
to the effects in the above embodiment. It becomes.
[0059]
Second Embodiment Next, a second embodiment of the present invention will be described. The
same components as those in the first embodiment are denoted by the same reference numerals,
and the description thereof will be appropriately omitted.
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[0060]
FIG. 9 shows a configuration of a vibration detection apparatus (microphone apparatus 1A)
according to the present embodiment. In this microphone device 1A, an interferometer is
configured by a Mach-Zehnder interferometer. Specifically, the microphone device 1A includes a
laser light source 10, a Mach-Zehnder interferometer, and a detection unit including two
photoelectric conversion elements 181 and 182 and a digital signal processing unit 19. Further,
this Mach-Zehnder interferometer is composed of a beam splitter 161, two reflecting mirrors
145 and 146, three prisms 111 to 113, a corner cube prism 114, and a beam splitter 162.
[0061]
The beam splitter 161 splits the laser light Lout emitted from the laser light source 10 into a first
light path OP1 (reference light path) on the prism 111 side and a second light path OP2
(reflected light path) on the reflection mirror 145 side. It is for.
[0062]
The reflection mirror 145 is disposed on the optical path OP2, and reflects the laser light Lout
traveling on the optical path OP2 to the side of the prism 112.
[0063]
The prism 111 is disposed on the optical path OP1, and reflects the laser beam Lout (reference
light) traveling from the side of the beam splitter 161 on the optical path OP1 to the side of the
corner cube prism 114, and a corner on the optical path OP1. The laser light Lout (reference
light) traveling from the cube prism 114 side is reflected to the reflection mirror 146 side.
[0064]
The prism 112 reflects the laser beam Lout reflected by the reflection mirror 145 to the side of
the prism 113 and the vibrating film 131, and the beam splitter 162 reflects the reflected light
multiply reflected by the vibrating film 131 and the prism 113 as described below. It reflects on
the side of the
[0065]
The prism 113 is a reflective surface by metal deposition or the like on the surface on the
vibrating film 131 side, and is for multiple reflection of the laser beam Lout traveling through the
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optical path OP2 between the vibrating film 131 and the prism 113. It is.
[0066]
The corner cube prism 114 is disposed on the optical path OP1 and is for reflecting the laser
light Lout (reference light) reflected by the prism 111 and for causing the laser light Lout
(reference light) to travel to the side of the prism 111 again.
The corner cube prism 114 is arbitrarily displaceable as indicated by the arrow in FIG. 9,
whereby the optical path length of the reference optical path is arbitrarily adjusted as in the first
embodiment. It can be done.
[0067]
The reflection mirror 146 is disposed on the optical path OP1 and reflects the laser light Lout
(reference light) reflected by the prism 111 to the side of the beam splitter 162.
[0068]
The beam splitter 146 splits the reference light incident from the optical path OP1 and the
reflected light (multiple reflected light) incident from the optical path OP2 into an optical path on
the photoelectric conversion element 181 side and an optical path on the photoelectric
conversion element 182 side to travel. belongs to.
[0069]
Here, the corner cube prism 114 corresponds to a specific example of the first reflector in
the present invention, and the prism 113 corresponds to a specific example of the second
reflector and the total reflection mirror in the present invention. It corresponds.
[0070]
With such an arrangement, in the interferometer of the present embodiment, the laser beam Lout
emitted from the laser light source 10 is split into two optical paths OP1 and OP2 by the beam
splitter 161 and travels.
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Specifically, the first light path (reference light path) passing through the beam splitter 161, the
prism 111, the corner cube prism 114, the prism 111, the reflecting mirror 146 and the beam
splitter 162, the beam splitter 161, the reflecting mirror 145, the prism 112, The light beam
travels while being separated into a second optical path (reflected optical path) passing through
the diaphragm 131, the prism 113, the prism 112, and the beam splitter 162.
At this time, the reflected light reflected by the vibrating film 131 and the prism 113 in the
reflected light path and the reference light reflected by the corner cube prism 114 in the
reference light path interfere with each other in the beam splitter 162 to form interference
fringes.
Thus, based on the interference fringes, the photoelectric conversion elements 181 and 182 and
the digital signal processing unit 19 detect the vibration of the diaphragm 131 as the quantized
audio signal Sout as in the first embodiment.
[0071]
Further, since the reflected light is multi-reflected between the vibrating film 131 and the prism
113 in the reflected light path, the optical path difference between the reference light and the
reflected light becomes large, whereby the displacement of the vibrating film 131 Amplified and
detected
[0072]
Therefore, also in this embodiment, the same effect can be obtained by the same action as that of
the first embodiment.
That is, it is possible to improve the detection sensitivity when optically performing vibration
detection.
[0073]
Further, since the Mach-Zehnder interferometer is used as the interferometer, return light of the
laser light Lout is not generated with respect to the laser light source 10 without using expensive
optical components such as a wavelength plate and a polarization beam splitter. It is possible to
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avoid the noise generation in the laser light source 10 at low cost.
[0074]
Although the present invention has been described above by the first and second embodiments,
the present invention is not limited to these embodiments, and various modifications are
possible.
[0075]
For example, even if the count number of angle division is increased with respect to an angle
uniquely determined in the range of-(π / 2) <θ <+ (π / 2) of the Lissajous figure described in
the above embodiment. Good.
When configured in this manner, it is possible to improve the detection sensitivity by increasing
the angular resolution.
[0076]
Moreover, although the semiconductor laser was mentioned as a light source which emits the
laser beam Lout in the said embodiment and demonstrated, you may make it use a gas laser,
fixed laser, etc. besides this.
[0077]
Further, in the above embodiment, as an example of the vibration detection device of the present
invention, the vibration body is the vibration film (vibration film 131) that vibrates according to
the sound wave, and the vibration of the vibration film 131 is detected as the audio signal Sout.
Although the optical microphone device has been described, the vibration detection device of the
present invention is not limited thereto, and may be configured to detect another vibration.
[0078]
Furthermore, in the above embodiment, the case where digital detection is performed using the
digital count unit 19 as the signal Sout in which the vibration of the vibrating film 131 is
quantized, but the vibration of the vibrating film is directly output as an analog signal You may
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Specifically, for example, by using the output signals Sx and Sy from the photoelectric conversion
elements 181 and 182 in a region where the change in interference light intensity changes
linearly, it is possible to obtain an electric signal substantially proportional to the diaphragm
displacement. This signal may be output as an analog audio signal as it is.
According to a well-known method, the optical path length on the reference light side can be
moved by a piezo element or the like, and the DC component of the output signal can be
controlled to the position of the linear region by applying negative feedback to the piezo element
is there.
[0079]
BRIEF DESCRIPTION OF THE DRAWINGS It is a figure showing the whole structure of the
vibration detection apparatus which concerns on the 1st Embodiment of this invention.
It is sectional drawing showing an example of a detailed structure of the microphone capsule
shown in FIG.
It is sectional drawing showing the other example of a detailed structure of the microphone
capsule shown in FIG.
It is a figure showing an example of the Lissajous figure created in the digital signal processing
part shown in FIG.
It is a characteristic view which shows the general relationship of the optical path difference and
visibility in an interferometer.
It is a characteristic view showing the relation between the interference light peak and visibility
by a plurality of kinds of reflection components in a 1st embodiment. It is a characteristic view
for explaining interference of interference light peaks by a plurality of kinds of reflection
components. It is sectional drawing showing the aspect of the multiple reflection which concerns
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on the modification of 1st Embodiment. It is a figure showing the whole structure of the
oscillation detection instrument concerning a 2nd embodiment.
Explanation of sign
[0080]
1, 1A: microphone device, 10: laser light source, 11: lens, 111 to 113: prism, 114: corner cube
prism, 12: polarization beam splitter, 13, 13A, 13B: microphone capsule, 130: housing, 131:
Vibrating film, 132: back electrode, 133: back plate, 134: transparent member, 135: opening,
141: reflecting plate, 142: half mirror, 143A, 143B, 144A, 144B: total reflection mirror, 145,
146: reflection Mirror, 151, 152, 153 ... λ / 4 plate, 16, 161, 162 ... beam splitter, 171, 172 ...
polarizing plate, 181, 182 ... photoelectric conversion element, 19 ... digital signal processing
unit, Sw ... sound wave, s0, s0 s1 ... S polarization component, p0, p1 ... P polarization component,
OP1, OP2 ... optical path, Sx, Sy ... from the photoelectric conversion element Output signal, Sout:
voice signal, Lout: laser light, Lr: multiple reflection light, L0: distance between polarization beam
splitter and reflector, L1: distance between polarization beam splitter and diaphragm, L2: half
Distance between mirror and diaphragm, d distance between diffracted light peaks, C center
point P0 signal point Pa to Pd reference point E to H reference line.
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