JPS6118300A - Optical microphone - Google Patents

Abstract

PURPOSE:To attain ease of setting of an optimum operating point of a repetitive reflection interference system of a sound receiving section by providing a photodetector coupled optically to a radiating end of a feedback light from a flat sound receiving plate propagated through an optical fiber of an optical branching device. CONSTITUTION:The light of single wavelength irradiated from a continuous wavelength variable light source 30 is led to the optical branching device 32 and divided into two; reference light (j) and measured light (k). The reference light (j) is subject to photoelectric conversion by a reference light detector 33. On the other hand, the measured light (k) is propagated inside of the optical fiber 35 and led to the inside of the repetitive reflection interference system formed with the flat sound receiving plate 36 and the end face of the optical fiber 35. The light is reflected repetitively and the light fed back to the optical fiber 35 through the repetitive reflection interference system is led to the signal light detector 37 via the branching device 32 and subject to photoelectric conversion. The outputs of the photodetectors 33, 37 are amplified differentially by a differential amplifier 38 and the result is extracted from the output terminal 39.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to optical microphones and therefore to acousto-optic converters that produce changes in light intensity due to changes in sound pressure.

Conventional configurations and their problemsIn recent years, semiconductor lasers and optical fibers have entered the practical stage. Optical microphones have been proposed based on various conversion principles that utilize the unique characteristics of light for measuring and transmitting acoustic signals.

In general, such optical microphones are characterized by their resistance to electromagnetic induction of light and high-quality transmission. For example, in a broadcasting station studio.

This makes it possible to collect and transmit high-quality sound in factories and other places. In addition, if we focus on the characteristics of light such as environmental resistance, compactness, light weight, and safety, we can find
It will also be effective in disaster prevention fields such as tunnels and coal mines, and in medical fields such as blood pressure and heart sound measurements.

As a conventional optical microphone, an optical microphone that utilizes repeated reflection interference at a sound receiving section will be described below.

First, the principle of acousto-optical conversion by repeated reflection interference will be explained using FIGS. 1 and 2.

FIG. 1 is a block diagram of a repetitive reflection interference system. 1 is a first medium, 2 is a second medium arranged parallel to the first medium 1 with an interval β, and the opposing surfaces of the first medium 1 and the second medium 2 are both flat. It is.

3 is an air layer between the first medium 1 and the second medium 2. In the figure, boundary surface A and boundary surface B are the boundary surfaces between the first medium 1 and the air layer 3, and the boundary surfaces between the second medium 2 and the air layer 3, respectively. rAz rn are the amplitude reflection coefficients of the first medium 1 and the second medium 2 for the light waves traveling from the air layer 3 toward the medium, respectively. In addition, engineering i is the first medium 1
The IR is the light incident on the air layer 3, and the IR is the repeatedly reflected light inside the air layer 3.

is the feedback light from the air layer 3 to the first medium 1.

It is known that the system configured as described above forms a repeating reflection interference system.

The operation of the repeating reflection interference system configured as described above will be explained below.

When incident light Ii enters the air layer 3 perpendicularly to the boundary surface A through the first medium 1, a portion of the incident light beam Ii is reflected by the boundary surface A, and the remainder is transmitted and enters the air layer 3. do. The light incident on the air layer 3 is repeatedly reflected by the boundary surface and the boundary surface B, and the boundary surface M and the boundary surface B form a repeated reflection interference system. In the repeated reflection interference, a part of it becomes transmitted light each time it is reflected and is emitted out of the repeated reflection interference system.The feedback light IO is transmitted from the repeated reflection interference system to the first medium 1. It is a composite wave of transmitted light.The main wavelength of the incident light I is λ, and the total air refractive index is n.
Then, the optical path difference Δ in one round trip through the repeating reflection interference system is Δ=2nd ・・・・・・・・・・・・
It is given by (1). Further, the phase difference δ due to the optical path difference is given by: At this time, the feedback light beam is a composite wave of 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 squaring the absolute value of H.

Figure 2 shows the strong NrB of the repeated reflection interference system calculated based on equation (4) with rA=rB=r and r as a parameter.
In the case of , the zero point no longer appears in the intensity reflection coefficient lR12, but the shape of the curve will still be similar to these.

If the intensity reflection coefficient lR12 is nβ, and the change Δl in the interval β between the first medium 1 and the second medium 2 is a sufficient ratio than the wavelength λ, IRI may be practically assumed to have a linear relationship with Δβ.

A conventional optical microphone that utilizes the above-mentioned repeated reflection interference in its sound receiving section will be described below.

FIG. 3 is a system configuration diagram of a conventional optical microphone that utilizes repeated reflection interference in its sound receiving section.

10 is a helium-neon gas laser that oscillates with single mode linearly polarized light. 11 is a first beam splitter that splits the laser output light a into a reference light and a measurement light C;
12 is a gray filter disposed on the output side of the reference beam split by the first beam splitter 11; 13 is a photomultiplier tube for the reference beam that detects the light transmitted through the gray filter 12; and 14 is the first beam splitter. A polarizing plate 16 is placed on the output side of the measurement light C of the beam sinter 11, a % wavelength plate 16 is placed on the output side of the polarizing plate 14, and a second % wavelength plate 16 is placed on the output side of the % wavelength plate 16. Beam splitter, 17 is a lens arranged on the transmission optical axis of the second beam splitter 16, 18 is an optical fiber optically coupled to the lens 17, 19 is a sound receiving plate, 2Q is the second beam splitter 16 21 is a photomultiplier tube for signal light that detects the light transmitted through mask 20; 22 is for reference light; A differential amplifier 23 is electrically connected to the photomultiplier tube 13 and the signal light photomultiplier tube 21, and 23 is an output terminal of the differential amplifier 22.

Figure 4 shows the sound receiving section 1 of the optical microphone shown in Figure 3.
92L is a structural diagram. 24 is an air layer between the end face of the optical fiber 18 and the sound receiving plate 19; 26 is an optical fiber holder for the optical fiber 18; and 26 is a spacer for fixing the sound receiving plate 19 and the end face of 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 2ruIL and a thickness of 1571 m. The optical fiber 18 has a core diameter of 60 μm and a cladding outer diameter of 160 μm. The mutually opposing surfaces of the sound receiving plate 19 and the optical fiber 18 have an amplitude reflectance of 0.
．． Aluminum is deposited to make it 5. Spacer 26
has a small opening at the center corresponding to the core portion of the optical fiber 18, and prevents reflected light from the sound receiving plate 19 from entering the cladding portion. The air layer 24 is an optical fiber holder 26
It is opened to the outside through a minute gap between the optical fiber 18 and the optical fiber 18 .

The operation of the conventional optical microphone configured as described above will be explained below.

First, the light beam emitted from the helium/neon gas laser 1o is split into two by a first beam splitter 11 into a reference beam and a measurement beam C, and the reference beam passes through a gray filter 12 and is then photoelectron multiplied for the reference beam. It is detected by the tube 13, photoelectrically converted, and input to the differential amplifier 22. On the other hand, the measurement light is guided to the second beam splitter 16 via the polarizing plate 14 and the fox wave plate 16. The light transmitted through the second beam sinter 16 is mode-matched by a lens 17 and enters an optical fiber 18 . The light transmitted through the optical fiber 18 reaches the sound receiving plate 19, and the sound receiving plate 19 and the optical fiber 18
Repeated reflections occur between the end faces of the

The feedback light d that passes through the repeating reflection interference system formed by the sound receiving plate 19 and the end face of the optical fiber 18 and returns to the optical fiber 18 is propagated in the opposite direction through the optical fiber 18 again to the second beam splitter 16. The light then passes through a mask 20, is guided to a photomultiplier tube 21 for signal light, is photoelectrically converted, and is input to a differential amplifier 22. In the differential amplifier 22 , the output of the reference light photomultiplier tube 13 and the output of the signal light photomultiplier tube 21 are differentially amplified, and a 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 distance between the repeated 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 according to the relational expression. If the displacement of the sound receiving plate 19 is a tenth of the wavelength of the incident light f, the intensity of the feedback light may be considered to have a linear relationship with the displacement of the sound receiving plate 19. In this way, the displacement of the sound receiving plate 19 relative to the sound pressure is extracted from the output terminal 23. By making the optical fiber 18 long, the optical microphone with this configuration enables remote measurement using an optical system that does not include an electrical system, and has features such as electromagnetic induction resistance, high-quality transmission, environmental resistance, and small size.
It is possible to provide a microphone that is lightweight and has excellent safety.

However, in the conventional configuration, since a helium-neon gas laser is used as a light source and the wavelength is fixed, there is a problem in that it is difficult to set the optimum operating point of the repetitive reflection interference system in the sound receiving section 19a. 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 section 191L as a parameter. It is an input-output conversion diagram of a repeated reflection and interference system. The vertical axis is the intensity reflection coefficient IRI2 of the caressing light d. The horizontal axis is tan.

This is a case where the amplitude reflection coefficients of the end face of the λ sound receiving plate 19 and the optical fiber 18 are both r=o and s. p, , p2°P3 each indicate an operating point. In this way, the output waveform changes depending on the set value of the operating point. As a microphone, in order to set the optimum operating point with high conversion efficiency and low distortion,
The interval l of the repeating reflection interference system must be adjusted. For this purpose, the spacer must be manufactured with an accuracy of at least one wavelength λ. For example, in the case of a helium-neon gas laser, the value is λ = 6328 A
It requires about 60 degrees of accuracy compared to fx, and is extremely difficult.

Further, in order to improve the conversion efficiency, as is clear from FIG. 2, it is possible to increase r, but this poses the problem of increasing the difficulty level.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned conventional problems, and an object thereof is to provide an optical microphone that can easily set the optimal operating point of a repeating reflection interference system of a sound receiving section.

Structure of the Invention The present invention provides a continuously variable wavelength light source capable of continuously changing the wavelength at a single wavelength, an optical splitter optically coupled to the continuously variable wavelength light source, and an optical fiber optically coupled to an output end of the incident light from the light source and a plane sound receiving plate forming a repeating reflection interference system between the output end face of the optical fiber; and the optical splitter. By changing the wavelength of the light source with an optical microphone equipped with a photodetector optically coupled to the output end of the return light from the plane sound receiver transmitted through the optical fiber,
This makes it possible to easily set the optimum operating point of the repeated reflection interference system of the sound receiving section.

DESCRIPTION OF EMBODIMENTS FIG. 6 is a system configuration diagram of an optical microphone according to a first embodiment of the present invention. 30 is a continuously variable wavelength light source that uses a continuously variable wavelength laser. 31 is a condensing lens placed on the output side of the continuously variable wavelength light source 3o, 32 is an optical splitter placed on the output side of the condensing lens 31, and the output light of the continuously variable wavelength light source 30 is measured as the reference light j. The light splits into two.

an optical fiber coupling lens arranged on the light output side; 3;
5 is an optical fiber optically coupled with the optical fiber coupling lens 34; 36 is a flat sound receiving plate arranged parallel to the output end of the optical fiber 36 at a distance E; the movable part has a diameter of 2
B. A nickel metal thin film with a thickness of 2 μm is used.

37 is the feedback light m from the flat sound receiving plate 36 of the optical splitter 32;
That is, a signal light photodetector disposed on the side where the signal light p is diffracted and emitted; 38 is a differential amplifier electrically connected to the reference light photodetector 33 and the signal light photodetector 37; 39;
is the output terminal of the differential amplifier 38.

The operation of the optical microphone of this embodiment configured as described above will be explained below. First, the single wavelength light emitted from the continuously variable wavelength light source 30 is transmitted through the condenser lens 3.
1, the light is guided to a light splitter 32, where it is split into two, 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 optical fiber coupling lens 34, propagates inside the optical fiber 35, and enters the inside of the repeated reflection interference system formed by the flat sound receiving plate 36 and the end face of the optical fiber 36. be guided. There, the optical fiber 36 is repeatedly reflected and passed through a repeated reflection interference system.
The returned light propagates in the opposite direction inside the optical fiber 36 again, passes through the optical fiber coupling lens 34 and the optical splitter 32, is guided to the signal light photodetector 37, and is photoelectrically converted there. Ru. The outputs of the reference light photodetector 33 and the signal light photodetector 37 are differentially amplified by a differential amplifier 38 and taken out from an 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 repeated 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 according to the relational expression (4). If the displacement of the flat sound receiving plate 36 is sufficiently smaller than the wavelength of the light source, it may be assumed that the intensity of the feedback light m has a linear relationship with the displacement of the flat sound receiving plate 36. In this way, the displacement of the flat sound receiving plate 36 relative to the sound pressure is extracted from the output terminal 39.

As is clear from FIG. 6, the operating point of the repetitive reflection interference system is determined by the value of nl. Therefore, even if the interval l of the λ repeating reflection interference system is fixed, the optimum operating point can be set by changing the wavelength λ.

To set the optimum operating point of the repetitive reflection interference system, apply a sound pressure of a constant frequency to the flat sound receiving plate 36, and while observing the output waveform of the output terminal 39, determine whether the conversion efficiency is high and The wavelength of the continuously variable wavelength light source 300 is adjusted so that the distortion is reduced.

Gradually increase the input sound pressure and repeat the above operation. As described above, when the optimum operating point of the repetitive reflection interference system is set, the wavelength of the continuously variable wavelength light source 30 is fixed.

As described above, according to this embodiment, the continuously variable wavelength light source 3Q has a single wavelength and can continuously change the wavelength;
an optical splitter 32 optically coupled to the continuously variable wavelength light source 30; and the continuously variable wavelength light source 3 of the optical splitter 32.
Repeated reflection interference occurs between the optical fiber 35 that is optically coupled to the output end of the incident light from 0 and the output end face of the optical fiber 35 that is arranged parallel to and opposite to the output end face of the optical fiber 36. A flat sound receiving plate 36 forming a system is optically coupled to an output end of the optical splitter 32 for the return light from the flat sound receiving plate 36 that is transmitted via the optical fiber 36. A mechanical microphone equipped with a photodetector 37,
By changing the wavelength of the continuously variable wavelength light source, it becomes easy to set the optimum operating point of the repeating reflection interference system of the sound receiving section.

A second embodiment of the present invention will be described below 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 an optical splitter,
33 is a reference light photodetector, 34 is an optical fiber coupling lens, 36 is an optical fiber, 36 is a flat sound receiving plate, 37 is a signal light photodetector, 38 is a differential amplifier, 39 is an output terminal,
The above configuration is similar to the configuration shown in FIG. What is different from the configuration shown in FIG. 6 is that the continuously variable wavelength light source 3o in FIG.
This is the point formed by 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 explained below.

First, light emitted from a light source 40 having a continuous spectral distribution is converted into a parallel beam by a parallel beam forming lens 41 and guided to a Fabery-Perot etalon 42 . Of the light incident on the Fabry-Perot etalon 42, only light with a specific wavelength determined by the resonator length d is selected and emitted.

FIG. 8 is a diagram showing the filter effect of the Fapry-Perot etalon 42, FIG. 8(a) is a spectral distribution diagram of incident light on the etalon 42, FIG. 8(b) is a diagram of the configuration of the etalon,
FIG. 8(0) is a spectral distribution diagram of the emitted light that has passed through the etalon 42. The light emitted from the Fapry-Perot etalon 42 operates similarly to the embodiment in Box 1 below.

As described above, according to this embodiment, a continuous wavelength light source having a single wavelength and continuous wavelength is used as a light source having a continuous spectral distribution.
By forming a Fabry-Perot etalon and changing the resonator of the etalon, the same effect as in the first embodiment can be obtained for setting the optimum operating point of the repeating reflection interference system in the sound receiving section. .

In the first embodiment, the flat sound receiving plate 36 is made of a metal thin film, but the flat sound receiving plate 36 may be a sound receiving plate formed by metal vapor deposition on a polyvarnish film.

Further, in the second embodiment, the light source 40 having a continuous spectral distribution is a light emitting diode, but the light source 40 having a continuous spectral distribution may be a halogen lamp.

In addition, in the second embodiment, the Fapp IL-Perot etalon 42 is coated with a dielectric multilayer film and is made into an etalon with a limited wavelength range, so that a high-quality single wavelength can be obtained, and the above-mentioned optical The distortion of the target microphone can be reduced.

In the second embodiment, the Fabry-Perot etalon 42 has a structure in which a movable glass plate forming the etalon is held by a piezoelectric ceramic, and a voltage is applied to the piezoelectric ceramic to vary the resonator length d of the etalon. Therefore, it is possible to adjust the resonator d with high precision, and it is also possible to adjust the wavelength with high precision, that is, to set the optimal operating point of the repetitive reflection interference system.

Furthermore, by negatively feeding back the direct current component of the output from the output end 39 to the piezoelectric ceramic, fluctuations in the wavelength formed by the Fabry-Perot etalon 42 can be suppressed.

In addition, in the first and second embodiments, the flat T sound receiving pressure plate 36 and the optical fiber 35 have the same amplitude reflection coefficients on the surfaces where they face each other, as is clear from equation (4). By coating each of the microphones with a multilayer dielectric film, the dynamic range of the optical microphone can be expanded.

Effects of the Invention By using light to measure and transmit acoustic signals, the present invention enables remote measurement using an optical system that does not include an electrical system, and has excellent electromagnetic induction resistance, high quality transmission, environmental resistance, In addition to being compact, lightweight, and safe, the continuous wavelength variable light source makes it possible and easy to set the converter's optimum operating point, making the converter highly sensitive and By forming a repeating reflection interference system that can be set to a low distortion factor and have the same amplitude reflection coefficient, it is possible to realize an excellent optical microphone that can expand the dynamic range of the converter.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of the repetitive reflection interference system, Figure 2 is a diagram of its relationship with Figure 3, Figure 3 is a system configuration diagram of a conventional optical microphone, and Figure 4 is the conventional optical microphone shown in Figure 3. Fig. 5 is an input/output conversion diagram using the operating point of the repetitive reflection interference system in the sound receiving part as a parameter, and Fig. 6 is an optical microphone according to the first embodiment of the present invention. 7 is a system configuration diagram of the optical microphone in the second embodiment of the present invention, and FIG. 8 (2L) to
(C) is a diagram showing the filter effect of the etalon in FIG. 7. 3o... Continuously variable wavelength light source, 31...
Condensing lens, 32... Optical splitter, 33...
...Photodetector for reference light, 34°°"...Optical fiber, lens for coupling, 36...Optical fiber '4, 36-
°-Pa plane sound receiving plate, 37... Signal light photodetector, 38... Differential amplifier, 39... Output terminal, 4o... Light source with continuous spectral distribution, 41-=-... Parallel beam forming lens, 42...
... Fahri's own Perrault's etalon. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3 Figure 5 Figure 6 Figure 7 Figure 8 (ill, n (b)
(C), 2

Claims (10)

[Claims] (1) a continuously variable wavelength light source capable of continuously changing the wavelength at a single wavelength; an optical splitter optically coupled to the continuous wavelength variable light source; and the continuously variable wavelength of the optical splitter. A repeating reflection interference system is formed between an optical fiber optically coupled to an output end of incident light from a light source and an output end face of the optical fiber that is arranged parallel to and opposite to the output end face of the optical fiber. and a photodetector optically coupled to an output end of the return light from the flat sound receiving plate transmitted via the optical fiber of the optical splitter. Features an optical microphone. (2) The continuously variable wavelength light source is a light source formed by a light source having a continuous spectral distribution and a Fabry-Perot etalon with a variable resonator length. optical microphone. (3) The optical microphone according to claim 1, wherein the continuously variable wavelength light source is a continuously variable wavelength laser. (4) The optical microphone according to claim 1, wherein the flat sound receiving plate is a sound receiving plate made of a thin metal film. (5) The optical microphone according to claim 1, wherein the flat sound receiving plate is a sound receiving plate formed by metal vapor deposition on a polyester film. (6) The plane sound receiving plate and the optical fiber are each coated with a dielectric multilayer film so that the amplitude reflection coefficients of the surfaces facing each other are the same. Optical microphone as described in section. (7) The optical microphone according to claim 2, wherein the light source having a continuous spectral distribution is a halogen lamp. (8) The optical microphone according to claim 2, wherein the light source having a continuous spectral distribution is a light emitting diode. (9) The etalon is characterized in that the movable glass plate forming the etalon is held by a piezoelectric ceramic, and the resonator length of the etalon is made variable by applying a voltage to the piezoelectric ceramic. The optical microphone according to item 2. (10) The optical microphone according to claim 2, wherein the etalon is an etalon coated with a dielectric multilayer film to limit the wavelength range.