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JPS6118300

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DESCRIPTION JPS6118300
[0001]
FIELD OF THE INVENTION The present invention relates to optical microphones, and thus to an
acoustic-to-light converter that produces a change in light intensity due to a change in sound
pressure. The configuration of the conventional example and its problems In recent years,
semiconductor lasers, optical fibers, etc. have come into practical use. Proposals have been made
for optical microphones based on various conversion principles in which characteristics unique
to light are used to measure and transmit acoustic signals. Generally speaking, the characteristics
of such an optical microphone, focusing on the resistance to electromagnetic induction and high
quality transmission of light, include a high environment of electromagnetic induction generated
from lighting 2 peripheral devices, power supply lines, etc., and long signal transmission In
environments that require a road, for example, a studio in a broadcast station. In a factory etc.,
high quality sound collection and transmission become possible. In addition, focusing on
characteristics such as environmental resistance, small size, lightness and safety of light, it is also
effective in the field of disaster prevention such as chemical and petroleum plants, tunnels and
coal mines, and medical fields such as blood pressure and heart sound measurement. It is to
become. Hereinafter, as a conventional optical microphone, an optical microphone using
repeated reflection interference in the sound receiving unit will be described. First, the principle
of sound-to-light conversion by repetitive reflection interference will be described with reference
to FIGS. 1 and 2. FIG. FIG. 1 is a block diagram of a repetitive reflection interference system.
Reference numeral 1 is a first medium, and 2 is a second medium disposed in parallel to the first
medium 1 with a spacing ?, and the opposing surfaces of the first medium 1 and the second
medium 2 are both flat. It is. Reference numeral 3 denotes an air layer between the first medium
1 and the second medium 2. In the same figure, the interface A and the interface B are the
interface between the first medium 1 and the air layer 3 and the interface between the second
medium 2 and the air layer 3, respectively. rAzrn is an amplitude reflection coefficient of the first
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medium 1 and the second medium 2 to the light wave traveling from the air layer 3 toward the
medium, respectively. Also, light i is incident light on the air layer 3 from the first medium 1, IR is
repetitively reflected light in the air layer 3, light i. Is a feedback light from the air layer 3 to the
first medium 1. The system configured as described above is known to form a repetitive
reflection interference system. The operation of the repetitive reflection interference system
configured as described above will be described below. When incident light Ii enters the air layer
3 perpendicularly to the boundary surface A via the first medium 1, the incident light weight is
partially reflected at the boundary surface A, and the remaining part is transmitted and enters
the air layer 3 Do. The light incident on the air layer 3 is repeatedly reflected at the boundary
surface and the boundary surface B, and the boundary surface and the boundary surface B form
a repetitive reflection interference system. The repetitive reflection interference is partially
transmitted light (outgoing from the repetitive reflection interference system) each time the
reflection is repeated.
The feedback light IO is a composite wave of light transmitted from the repetitive reflection
interference system to the first medium 1. Assuming that the wavelength of the incident light I is
? and the total air refractive index is n, the optical path difference ? for one round trip of the
repetitive reflection interference system is ? = 2nd... (1) Given by Further, the phase difference
? due to the optical path difference is given by At this time, a feedback light weight which is a
composite wave of the transmitted light from the repetitive reflection interference system to the
first medium 1. The amplitude reflection coefficient R of is given by Therefore, its intensity
reflection coefficient lR12 is given by the square of the absolute value of H. In FIG. 2, in the case
of strong NrB of the repetitive reflection interference system calculated with rA = rB = r and r as
a parameter based on the equation (4), no zero point appears in the intensity reflection
coefficient lR12, but still The shape of the curve is similar to these. If the change .DELTA.l of the
distance .beta. Between the medium 1 and the second medium 2 with respect to n.beta. With
respect to the intensity reflection coefficient lR12 is sufficiently larger than the wavelength
.lambda., IRI may be practically in a linear relationship with .DELTA..beta. Hereinafter, a
conventional optical microphone using the above-described repetitive reflection interference as a
sound receiving unit will be described. FIG. 3 is a system configuration diagram of a conventional
optical microphone using repeated reflection interference as a sound receiving unit. 10 is a
helium neon gas laser oscillating with single mode linear polarization. 11 is a first beam splitter
that splits the laser output light a into reference light and measurement light C, 12 is a gray filter
disposed on the emission side of the reference light split by the first beam splitter 11; 13 is a
reference light photomultiplier tube for detecting light transmitted through the gray filter 12, 14
is a polarizing plate disposed on the emission side of the measurement light C of the first beam
splitter 11, 16 is an emission of the polarizing plate 14 % Wavelength plate disposed on the side,
16 a second beam splitter disposed on the exit side of the% wavelength plate 16, 17 a lens
disposed on the transmission light axis of the beam second splitter 16, 18 a lens Optical fiber
optically coupled to 17; 19 a sound receiving plate; 2Q a mask of the second beam splitter 16
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disposed on the side from which the feedback light d from the sound receiving plate 19, that is,
the signal light e is diffracted and emitted , 21 is the mass 20 is a signal light photomultiplier
that detects light transmitted through 20, 22 is a differential amplifier electrically connected to
the reference light photomultiplier 13 and the signal light photomultiplier 21, and 23 is a
differential It is an output terminal of the amplifier 22. FIG. 4 is a structural view of a sound
receiving unit 192L of the optical microphone shown in FIG. Reference numeral 24 denotes an
air layer between the end face of the optical fiber 18 and the sound receiving plate 19, reference
numeral 26 denotes an optical fiber holder of the optical fiber 18, and reference numeral 26
denotes a spacer for fixing the end faces of the sound receiving plate 19 and the optical fiber 18
in parallel.
The sound receiving plate 19 is a mica thin plate sound receiving plate having a movable part
diameter of 2 ruIL and a thickness of 1571 m. The optical fiber 18 is an optical fiber having a
core diameter of 60 ?m and a cladding outer diameter of 160 ?m. The mutually facing surfaces
of the sound receiving plate 19 and the optical fiber 18 are deposited by aluminum so as to have
an amplitude reflectance of 0.5. The spacer 26 has a small opening at the center corresponding
to the core of the optical fiber 18, and prevents the reflected light from the sound receiving plate
19 from entering the cladding. The air layer 24 is open to the outside through a minute air gap
between the optical fiber holder 26 and the optical fiber 18. The operation of the conventional
optical microphone configured as described above will be described below. First, the light beam
emitted from the helium-neon gas laser 1o is split into two by the first beam splitter 11 into the
measurement light C and the reference light passes through the gray filter 12 and the
photoelectron multiplication for reference light The light is detected by the tube 13 and
photoelectrically converted and input to the differential amplifier 22. On the other hand, the
measurement light is guided to the second beam splitter 16 through the polarizing plate 14 and
the wedge wavelength plate 16. The light transmitted through the second beam splitter 16 is
mode-matched by the lens 17 and enters the optical fiber 18. The light transmitted through the
optical fiber 18 reaches the sound receiving plate 19 and is repeatedly reflected between the end
surfaces of the sound receiving plate 19 and the optical fiber 18. The feedback light d that has
passed through the repetitive reflection interference system formed by the sound receiving plate
19 and the end face of the optical fiber 18 and is fed back to the optical fiber 18 propagates the
optical fiber 18 in the reverse direction again to produce the second beam splitter 16 Further,
the light passes through the mask 20 and is guided to the signal light photomultiplier 21 for
photoelectric conversion and input to the differential amplifier 22. In the differential amplifier
22, the output of the reference light photomultiplier 13 and the output of the signal light
photomultiplier 21 are differentially amplified, and the final output is taken out from the output
terminal 23. When sound pressure is applied to the sound receiving plate 19, the sound receiving
plate 19 is displaced, and the interval ? of the repetitive reflection interference system formed
by the sound receiving plate 19 and the end face of the optical fiber 18 changes. The intensity of
the feedback light d changes in accordance with the equation of If the displacement of the sound
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receiving plate 19 is sufficiently larger than the wavelength of the incident light f, the intensity of
the feedback light may be in a linear relationship with the displacement of the sound receiving
plate 19. Thus, the displacement of the sound receiving plate 19 relative to the sound pressure is
taken out from the output terminal 23. In the optical microphone having such a configuration, by
making the optical fiber 18 longer, remote measurement with an optical system not including an
electric system becomes possible, and the electromagnetic induction resistance, the high quality
transmission property, the environmental resistance, the small size, A lightweight, safe
microphone can be provided.
However, in the above-described conventional configuration, since the wavelength is fixed using
a helium neon gas laser as the light source, it is difficult to set the optimum operating point of
the repetitive reflection interference system in the sound receiving unit 19a. It was FIG. 6 shows
the relationship between the input waveform, that is, the vibration waveform of the sound
receiving plate 19 and the output waveform, that is, the intensity change of the feedback light d,
using the operating point of the repetitive reflection interference system in the sound receiving
unit 191L as a parameter. It is an input-output conversion figure of repetition reflection and an
interference system. The vertical axis represents the intensity reflection coefficient IRI2 of the
return light d. The horizontal axis is the moon. In this case, both the amplitude reflection
coefficients of the end faces of the ? receiving plate 19 and the optical fiber 18 are r = o and s.
p, p2 and p3 indicate the operating points, respectively. Thus, the output waveform changes
according to the set value of the operating point. As the microphone, the interval l of the
repetitive reflection interference system has to be adjusted in order to set an optimal operating
point with high conversion efficiency and small distortion. For this purpose, the spacer must be
manufactured with at least one precision of the wavelength ?. The value is extremely difficult
because, for example, in the case of a helium-neon gas laser, an accuracy of about 60 persons of
? = 6328 A fx is required. In addition, to improve the conversion efficiency, as is clear from FIG.
2, r should be increased, but at that time there was a problem that the degree of difficulty further
increases. SUMMARY OF THE INVENTION The object of the present invention is to solve the
above-mentioned conventional problems, and it is an object of the present invention to provide
an optical microphone capable of easily setting the optimum operating point of the repetitive
reflection interference system of the sound receiving unit. The present invention provides a
continuous wavelength tunable light source capable of changing wavelengths at a single
wavelength and continuously, an optical splitter optically coupled to the continuous wavelength
tunable light source, and the above optical splitter. An optical fiber optically coupled to an output
end of incident light from a light source, a flat sound receiving plate forming a repetitive
reflection interference system between the output end face of the optical fiber, and the above
light splitter An optical microphone comprising: a light detector optically coupled to an output
end of feedback light from the planar sound reception transferred via an optical fiber; and
receiving light by changing the wavelength of the light source It is possible to easily set the
optimum operating point of the repetitive reflection interference system of the sound part.
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Description of the embodiment Fig. 6 is a system configuration diagram of an optical microphone
in the first embodiment of the present invention. A continuous wavelength variable light source
30 uses a continuous wavelength variable laser. Reference numeral 31 denotes a condenser lens
disposed on the exit side of the continuous wavelength variable light source 3o, and reference
numeral 32 denotes an optical splitter disposed on the exit side of the condenser lens 31. The
output light of the continuous wavelength tunable light source 30 is measured as reference light
j Divide into 2 to light.
An optical fiber coupling lens disposed on the light emitting side, 35 is an optical fiber optically
coupled to the optical fiber coupling lens 34, 36 is disposed in parallel at an emitting end of the
optical fiber 36 with an interval E. In the flat-surfaced sound receiving plate, a movable thin film
made of nickel having a diameter of 2 B and a thickness of 2 .mu.m is used. Reference numeral
37 denotes a light detector for signal light disposed on the side from which the feedback light m
from the flat sound receiving plate 36, that is, the signal light p, of the optical splitter 32 is
diffracted and emitted, and 38 denotes a light detector 33 for reference light and the signal light.
A differential amplifier electrically connected to the photodetector 37 is an output terminal of the
differential amplifier 38. First, the operation of the optical microphone of the present
embodiment configured as described above will be described below. Light of a single wavelength
emitted from the continuous wavelength variable light source 30 passes through the condenser
lens 31 to be an optical splitter. It is guided to 32 and divided there into a reference light j and a
measurement light. The reference light j is photoelectrically converted by the reference light
photodetector 33. On the other hand, the measurement light passes through the light 7 Iber
coupling lens 34 and propagates inside the optical fiber 35, and is formed inside the repetitive
reflection interference system formed by the planar sound receiving plate 36 and the end face of
the optical fiber 36. Led. Therefore, the light that has been repeatedly reflected, passed through
the repetitive reflection interference system, and returned to the optical fiber 36 is again
transmitted in the reverse direction inside the optical fiber 36 to form the optical fiber coupling
lens 34 and the optical splitter 32. Then, it is led to the signal light photodetector 37 where it is
photoelectrically converted. The outputs of the reference light photodetector 33 and the signal
light photodetector 37 are differentially amplified by the differential amplifier 38 and taken out
from the output terminal 39. Now, when sound pressure is applied to the flat sound receiving
plate 36, the flat sound receiving plate 36 is displaced, and the interval E of the repetitive
reflection interference system formed by the flat sound receiving plate 36 and the end face of the
optical fiber 35 changes. Therefore, the intensity of the feedback light m changes in accordance
with the relational expression (4). If the displacement of the flat receiving plate 36 is sufficiently
smaller than the wavelength of the light source, the intensity of the feedback light m may be in a
linear relationship with the displacement of the flat receiving plate 36. In this way, the
displacement of the planar sound receiving plate 36 with respect to the sound pressure is taken
out from the output terminal 39. The operating point of the repetitive reflection interference
system is determined at a value of n1, as is apparent from FIG. Therefore, even if the interval l of
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the ? repetitive reflection interference system is fixed, setting the optimum operating point is
possible by changing the wavelength ?. The setting of the optimum ░ operating point of the
repetitive reflection interference system applies a sound pressure of constant frequency to the
flat sound receiving plate 36 and observes the output waveform of the output terminal 39, while
the conversion efficiency is thick from the output waveform and The wavelength of the
continuous variable wavelength light source 300 is adjusted to reduce distortion.
Gradually increase the input sound pressure and repeat the above operation. As described above,
when the setting of the optimum operating point of the repetitive reflection interference system
is completed, the wavelength of the continuous wavelength variable light source 30 is fixed. As
described above, according to the present embodiment, the continuous wavelength variable light
source 3Q capable of changing the wavelength continuously at a single wavelength, and the
optical splitter 32 optically coupled to the continuous wavelength variable light source 30. An
optical fiber 35 optically coupled to an output end of the incident light from the continuous
wavelength variable light source 30 of the optical splitter 32, and an optical fiber 36 disposed in
parallel to face the output end face of the optical fiber 36 A flat sound receiving plate 36 forming
a repetitive reflection interference system with the emitting end face of 35, and a feedback from
the flat sound receiving plate 36 of the light branching device 32 transmitted through the optical
fiber 36 Dynamic microphone with a light detector 37 optically coupled to the light emitting end,
by changing the wavelength of the continuous wavelength variable light source, optimum
operation of the repetitive reflection interference system of the sound receiving unit Setting of
points becomes easy. Hereinafter, a second embodiment of the present invention will be
described with reference to the drawings. FIG. 7 is a system configuration diagram of an optical
microphone showing a second embodiment of the present invention. In FIG. 7, 31 is a condenser
lens, 32 is a light branching device, 33 is a light detector for reference light, 34 is a lens for fiber
coupling, 36 is an optical fiber, 36 is a flat sound receiving plate, 37 is a signal light The
photodetector 38 is a differential amplifier, 39 is an output terminal, and the above is the same
as that shown in FIG. What differs from the configuration of FIG. 6 is that the continuous
wavelength variable light source 3 o in FIG. 6 is formed by a light source 40 having a continuous
spectrum distribution, its parallel beam forming lens 41 and an etalon 42 of Fabry-Perot. . A light
emitting diode was used as the light source 4o having a continuous spectral distribution. The
operation of the optical microphone of the second embodiment configured as described above
will be described below. First, light emitted from a light source 40 having a continuous spectrum
distribution is collimated by a collimated beam forming lens 41 and guided to Faber-Perot's
etalon 42. As for the light incident on the Fabry-Perot etalon 42, only light of a specific
wavelength determined by its resonator length d is selected and emitted. Fig. 8 is a diagram
showing the filter effect of the Fabry-Perot etalon 42, Fig. 8 (a) is a spectrum distribution of
incident light of the etalon 42, Fig. 8 (b) is a block diagram of the etalon, Fig. 8 (0) is a spectral
distribution diagram of the outgoing light that has passed through the etalon 42. FIG.
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The light emitted from Fapri-Perot's etalon 42 operates in the same manner as the box 1
embodiment. As described above, according to the present embodiment, a continuous wavelength
light source of a single wavelength and a continuous wavelength is formed by a light source
having a continuous spectrum distribution and an etalon of Fabry-Perot to change the resonator
of the etalon. The same effect as that of the first embodiment can be obtained for setting the
optimum operating point of the repetitive reflection interference system of the sound receiving
unit. In the first embodiment, the flat sound receiving plate 36 is a metal thin film, but the flat
sound receiving plate 36 may be a sound receiving plate formed by metal deposition on a varnish
film. Although the light source 40 having a continuous spectrum distribution is a light emitting
diode in the second embodiment, the light source 40 having a continuous spectrum distribution
may be a halogen lamp. Further, in the second embodiment, the hap IL,... Perot's etalon 42 is
coated with a dielectric multi-layered film to obtain a single wavelength of high quality by
making the etalon with a limited wavelength range, the above optical Microphone distortion can
be reduced. In the second embodiment, the Fabry-Perot etalon 42 holds the movable glass plate
forming the etalon with a piezoelectric ceramic, applies a voltage to the piezoelectric ceramic,
and varies the resonator length d of the etalon. Thus, the resonator d can be adjusted with high
accuracy, and wavelength adjustment, that is, setting of the optimum operating point of the
repetitive reflection interference system can be performed with high accuracy. Furthermore, by
negatively feeding back the direct current component of the output of the output terminal weight
39 to the piezoelectric ceramic, it is possible to suppress the fluctuation of the wavelength
formed by the Fabry-Perot etalon 42. Also, in the first and second embodiments, as is apparent
from the equation (4), the flat-T sound receiving pressure plate 36 and the optical fiber 35 have
the same amplitude reflection coefficient on the surfaces facing each other. As described above,
the dynamic range of the optical microphone can be expanded by applying a multilayer film
coating of each dielectric. Effect of the Invention By using light for measurement and
transmission of an acoustic signal, the present invention enables remote measurement with an
optical system not including an electrical system, and is resistant to electromagnetic induction,
high quality transmission, environment resistance, In addition to the small size, light weight, and
excellent safety, by making the light source continuous wavelength variable, it is possible and
easy to set the optimum operating point of the converter, therefore the sensitivity of the
converter can be increased. An excellent optical microphone capable of expanding the dynamic
range of the converter can be realized by forming a repetitive reflection interference system
which can be set to a low distortion factor and has the same amplitude reflection coefficient.
[0002]
Brief description of the drawings
[0003]
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1 is a block diagram of a repetitive reflection interference system, FIG. 2 is a relationship diagram
with FIG. 3, FIG. 3 is a system block diagram of a conventional optical microphone, and FIG. 4 is a
conventional optical microphone of FIG. Fig. 5 is an input / output conversion diagram using the
operating point of the repetitive reflection interference system in the same sound receiving unit
as a parameter, and Fig. 6 is an optical microphone according to the first embodiment of the
present invention. FIG. 7 is a system block diagram of the optical microphone in the second
embodiment of the present invention, and FIGS. 8 (2L) to 8 (C) are diagrams showing the filter
effect of the etalon of FIG. ?.
3 o иииииииии Continuous wavelength variable light source, 31
иииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииииии Photo detector for reference light ░ "Optical phi,
bar coupling lens, 36 ...... Optical phi '4 и 36-?-Flat plane sound receiving plate, 37 ...
Photodetector for signal light, 38 и и и и и и и и и и и и Differential amplifier, 39 и и и и и и и и output terminal,
4 o и и и и и и light source with continuous spectrum distribution, 41-=-и и parallel beam forming lens,
42 и и и? ? Fafri's own Eylon of Perot. Name of Agent Attorney Nakao Toshio and others 1
person Figure 1 Figure 2 Figure 3 Figure 5 Figure 6 Figure 7 Figure 8 (ill, n (b) (C), ?
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