JP2010245599A - Optical microphone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical microphone having flat frequency characteristics up to a high-frequency region. <P>SOLUTION: The optical microphone includes a prism 2, a metal layer 3, a sound reception layer 4, a light source 5, a light receiver 6, a light source driving circuit 8, and a detection circuit 9. Porous materials such as silica drying gel are used for the sound reception layer to detect a change in refractive index of the sound reception layer, which is caused by a sound pressure, using a change in surface plasmon resonance absorption. Thereby, a microphone having flat frequency characteristics to a high-frequency region can be provided, without causing reduction in sensitivity due to reduction in size. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a microphone using light. In particular, the present invention relates to an optical microphone that detects a refractive index change caused by a sound wave by surface plasmon resonance and has a flat frequency characteristic up to a high frequency region up to an ultrasonic region.

  As devices that collect sound waves propagating in the air and convert them into electrical signals, dynamic microphones and condenser microphones are widely used in the audible band, and piezoelectric sensors are widely used in the ultrasonic region. These devices make use of the fact that sound waves are fine vibrations of air, so that sound waves are incident on the diaphragm, and the minute vibrations excited by the sound by the sound are made conductive, electrostatic, or piezoelectric. Are converted into electrical signals.

  On the other hand, an optical system such as a laser Doppler vibrometer (hereinafter simply referred to as “LDV”) that measures fine and high-speed vibration using light typified by laser light is widely used. An attempt has been made to collect sound waves using various devices.

  The sound pressure conversion device described in Patent Document 1 discloses an optical microphone that applies an optical measurement by a diaphragm and an optical triangulation method found in a normal microphone.

  Further, Patent Document 2 discloses a laser Doppler microphone that measures sound pressure by directly propagating a laser beam in a sound field and directly capturing a refractive index change generated in the air by a sound wave with an LDV.

  Hereinafter, the structure and operation of the laser Doppler microphone in Patent Document 2 will be described with reference to FIG. In FIG. 6, 121 is an LDV, 122 and 123 are a pair of reflecting mirrors, 124 is a cubic mirror, 125 is a laser beam path, 126 is a sound field, and 127 is an arithmetic unit.

  In the configuration shown in FIG. 6, the pair of reflecting mirrors 122 and 123 are arranged in parallel, and the LDV 121 and the cubic mirror 124 are provided at both upper and lower ends of the reflecting mirror 123. Laser light is emitted from the LDV 121 toward the reflecting mirror 122 at an appropriate angle. The irradiated laser light propagates along the laser light path 125 while being reflected by the reflecting mirrors 122 and 123 a plurality of times, and reaches the cubic mirror 124 provided at the end of the reflecting mirror 123. The laser light incident on the cubic mirror 124 is reflected several times inside the cubic mirror and then emitted from the cubic mirror 124 in the direction in which the laser light is incident on the cubic mirror 124. While being reflected a plurality of times by 123, it propagates in the reverse direction through the laser beam path 125 and reaches the LDV 121. The laser beam that has reached the LDV 121 undergoes optical and electrical processing inside the LDV 121, and the vibration speed component is converted by the calculation unit 127.

  A Kretschmann type surface plasmon sensor using a surface plasmon resonance phenomenon is generally known as a method for detecting a refractive index change in a minute region (for example, Patent Document 3).

  The Kretschmann type surface plasmon sensor is a sensor that basically includes a prism, a metal layer provided on one surface of the prism, a light source, and a light receiver.

  The surface plasmon is generated when a linearly polarized light beam is incident on the interface between the prism and the metal layer at a specific angle greater than the total reflection angle, and an evanescent wave is generated. The surface plasmon is caused on the surface of the metal layer by the evanescent wave. Excited. Under specific conditions where light energy becomes surface plasmons, the intensity of reflected light is significantly reduced.

  There is a correlation between the refractive index of the sample attached to the interface opposite to the prism of the metal layer and the angle at which the surface plasmon is generated, and the refractive index of the sample attached to the metal layer by the change in the angle at which the surface plasmon is generated, Alternatively, the amount or the like can be detected.

  Patent Document 3 discloses a Kretschmann type surface plasmon sensor in which a functional layer capable of specifically binding a substance to be detected is provided on the surface of the metal layer opposite to the dielectric prism. In this surface plasmon sensor, the concentration of the substance to be measured and the reactivity with the functional layer are detected by detecting the change in the resonance absorption angle or reflected light intensity of the surface plasmon when a non-measurement substance is bound to the functional layer. Etc. can be quantitatively evaluated. However, conventional surface plasmon resonance has never been used to detect sound waves.

JP 2004-12421 A JP 2004-279259 A JP 2008-203187 A

  In the optical microphone disclosed in Patent Document 1, the mechanical resonance characteristic of the diaphragm greatly affects the frequency band as in the case of a normal microphone. That is, it has a relatively flat frequency characteristic at a frequency lower than the mechanical resonance frequency of the diaphragm, but the sensitivity rapidly decreases above the resonance frequency, so that the upper limit of the microphone is limited to the vicinity of the resonance frequency. For this reason, the high frequency characteristics of the current microphone using the diaphragm are limited to about 100 kHz.

  In order to configure a diaphragm type microphone so as to be compatible with a high frequency region of 100 kHz or more, it is necessary to form a very small diaphragm. This brings about a drastic increase in cost. Further, the sensitivity is lowered due to a decrease in the amount of displacement of the diaphragm accompanying the downsizing, and it is extremely difficult to provide an optical microphone for practical use.

  Further, since the laser Doppler microphone disclosed in Patent Document 2 does not have a diaphragm, there is no restriction on the high frequency range due to mechanical resonance. Moreover, the high frequency limit in the vibration measurement of the LDV being used easily exceeds 1 MHz.

  However, in practice, since the change in the refractive index with respect to the sound pressure of air is small, a very long optical path is required to ensure sufficient sensitivity. In the example disclosed in Patent Document 2, it is shown that an optical path length of 10 m or more is necessary to obtain a sufficient S / N. Therefore, it is extremely difficult to reduce the size of the measurement area, and the measurement area can be applied only to the collection of the averaged sound in a wide area. A light reflector is provided to ensure a long optical path length. Since air resonance (cavity resonance) corresponding to the interval between the reflecting plates is generated, it is extremely difficult to realize a flat frequency characteristic up to a high frequency region as in the case of the diaphragm type microphone.

  In other words, the conventional optical microphone provides a practical optical microphone that can handle high frequencies of 100 kHz or higher due to the mechanical resonance phenomenon, increased cost due to miniaturization, and decreased sensitivity, although the bandwidth of optical measurement is sufficiently wide. It was extremely difficult to do.

  An object of the present invention has been made in view of the above-described problems, and realizes an optical microphone that has flat frequency characteristics up to a high frequency region and does not decrease in sensitivity even when miniaturized.

  In order to solve the conventional problems, the present invention is configured as follows.

  The optical microphone of the present invention is an optical microphone that detects sound waves propagating in an ambient fluid by light, and includes a prism, a metal layer provided on one surface of the prism, and a surface of the metal layer opposite to the prism. A sound receiving layer that is exposed to surrounding fluid, a light source that emits a light beam at an angle greater than or equal to total reflection at an interface between the prism and the metal layer, a light source driving circuit that drives the light source, the prism, and the prism It comprises a light receiver that receives reflected light from the interface of the metal layer, and a detection circuit that converts a signal from the light receiver into a sound wave signal. With this configuration, it is possible to provide an optical microphone capable of detecting sound waves with flat frequency characteristics up to a high frequency region.

  The optical microphone of the present invention converts a sound wave propagating in the surrounding environment into a refractive index change in a sound receiving layer made of a porous material such as silica dry gel having a large refractive index change with respect to pressure, and this refractive index change is converted to surface plasmon. It is detected by resonance.

  Since surface plasmon resonance is a method for detecting a local change in refractive index, the sensitivity is constant regardless of the size of the microphone. That is, even if the microphone is downsized and can be used up to a high frequency range, the sensitivity does not decrease.

  In addition, the structure of the conventional microphone, such as a structure with a diaphragm, is simple because the metal layer and the sound-receiving layer made of silica dry gel are directly formed on the prism made of glass. Compared to the above, it is possible to stably provide a wide-band microphone that is easy to miniaturize and whose frequency band extends to a high frequency region at a low cost.

The perspective view of the optical microphone in this invention, Embodiment 1 Sectional drawing which shows the shape of the sound receiving layer in this invention, Embodiment 1 Sectional drawing of the optical microphone in this invention and Embodiment 1 Sectional drawing of the optical microphone in this invention and Embodiment 2 Sectional drawing which shows a part of optical microphone in this invention and Embodiment 3 The figure which shows the conventional optical microphone in patent document 2

A porous material such as silica dry gel has a change in refractive index per unit sound pressure that is about 1 to 2 digits higher than the change in refractive index in air. Usually, the change in refractive index due to sound pressure increases in the order of solid, liquid, and gas, and it was common knowledge that the solid is the smallest. Such characteristics are extremely unique properties that are not found in ordinary solid materials. Such unique properties are based on the fact that the present invention uses a porous material such as silica dry gel, which has a large refractive index change with respect to the sound pressure, as a conversion material between the sound pressure and the refractive index change. Detecting by surface plasmon resonance.

  The detection method based on surface plasmon resonance can directly detect a change in the refractive index of a very small region, and the detection sensitivity is independent of the microphone size and the optical path length. That is, even a small microphone corresponding to a high frequency region does not cause a decrease in sensitivity.

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
FIG. 1 is a perspective view showing a configuration of an optical microphone according to Embodiment 1 of the present invention. Here, the XYZ directions are set as indicated by arrows in FIG.

  1, an optical microphone 1 of the present invention includes a prism 2, a metal layer 3 formed on one surface 2c of the prism 2, a sound receiving layer 4 formed on the opposite surface of the metal layer 3 from the prism 2, and a prism. 2, a light source 5 that emits laser light 7 (emitted light 7 a) to a portion where the sound receiving layer of the metal layer 3 exists, a light receiver 6 that detects the laser light 7 (reflected light 7 b) from the metal layer 3, and a light source 5. And a detection circuit 9 for converting a signal from the light receiver 6 into a sound pressure. A fluid existing in the space around the optical microphone 1 is referred to as an environmental fluid 10. The wavefront of the sound wave propagating from the upper direction of the Z axis with respect to the opposite surface of the sound receiving layer 4 in contact with the metal layer 3 (hereinafter referred to as “wave receiving surface”) is referred to as a sound wave 11.

(Description of each component)
In the optical microphone 1, the configuration of at least the prism 2, the metal layer 3, and the sound receiving layer 4 is integrated. The opposite surface (sound receiving surface) of the sound receiving layer 4 in contact with the metal layer 3 is exposed to the environmental fluid 10 and is configured to receive the sound wave 11 (wave surface) propagating through the environmental fluid 10. Yes.

  The light source 5 is connected to a moving stage (not shown) so that the irradiation position and angle of the irradiation light 7a can be arbitrarily changed. Similarly, the light receiver 6 is connected to a moving stage (not shown) whose position and angle can be arbitrarily set so that the reflected light 7b can be received flexibly.

  The prism 2 is made of a material having transparency to a light beam, such as glass. Resin such as acrylic can be used in addition to glass.

  One surface 2a of the prism 2 is formed to be inclined at a certain angle with respect to the interface 2c so that the light beam 7a can be incident on the interface 2c between the prism 2 and the metal layer at an angle greater than the total reflection. If the incident surface of the light beam 7 and the interface between the prism 2 and the metal layer 3 are parallel, the light is refracted when light enters the prism having a high refractive index such as glass from an environmental fluid 10 such as air having a low refractive index. As a result, the incident angle of the transmitted light with respect to the interface between the prism 2 and the metal layer 3 is reduced, so that the light beam cannot enter the interface between the prism 2 and the metal layer 3 at an angle greater than the total reflection.

  The one surface 2b of the prism 2 through which the reflected light 7b of the prism 2 passes is formed substantially symmetrically with the light receiving surface 2a of the prism 2 with respect to the Y axis.

  In order to detect the reflected light 7b incident from the light source 5 and efficiently passing through the reflecting surface 2b and being detected by the light receiver 6, the above symmetrical shape with respect to the Y axis is suitable. Any shape such as a curved surface shape may be used as long as the reflected light 7b can be detected.

  The metal layer 3 is provided directly on one surface of the prism 2 and has a thickness in the Y direction of about 50 nm. This is a thickness at which surface plasmon resonance can be easily observed. The surface plasmon resonance is a phenomenon that can be observed remarkably when the metal layer is thin. When the thickness of the surface plasmon resonance is increased to the order of micrometers, the property as a bulk becomes strong, so that it is difficult to observe the surface plasmon resonance.

  Gold is mainly used for the metal layer 3 so as not to be deteriorated by oxidation or the like, but it may be any metal or alloy that excites surface plasmons, and silver, copper, platinum, and the like can also be used. The metal layer 3 is directly formed on one surface 2c of the cleaned prism by sputtering or vapor deposition.

  For the sound receiving layer 4 that plays a role of converting the sound wave propagating through the surrounding fluid 10 into a change in refractive index, a porous material such as a silica dry gel having a large change in refractive index with respect to pressure is suitably used. In addition, an organic dry gel can also be used.

Silica dry gel is a material whose properties can be changed in various ways depending on the production method. In this embodiment, a silica dry gel obtained by removing a liquid component from a wet gel formed by a sol-gel method by a supercritical drying method is used. The characteristics of the silica dry gel were a density of 110 kg / m ³ , a sound velocity of 50 m / s, a refractive index of 1.022 (−), and a refractive index variation with respect to pressure of 1.0 × 10 ⁻⁷ (Pa ⁻¹ ).

This silica dry gel contains not only pure silica (SiO ₂ ) but also some organic components, and is slightly translucent and translucent in the visible region. In the optical microphone of the present invention, it is not necessary to transmit light to the sound receiving layer, so that high transmittance is not necessary. By including an organic component, the refractive index change amount is increased.

  In the present embodiment, the shape of the sound receiving layer 4 in the XZ plane is an elliptical shape. The sound receiving layer receives a sound wave and deforms with a volume change to generate a refractive index change. When an external force such as a sound wave is received, a resonance vibration caused by the material and shape of the sound receiving layer may occur separately from the shape change synchronized with the sound wave. When such resonance vibration is in the measurement frequency range of the microphone, the flat frequency characteristic of the microphone is disturbed, which is not preferable.

  Resonance vibration has a particularly strong influence in the form of a square shape or a circular shape. Therefore, in this embodiment, an elliptical shape in which a frequency component due to the shape is difficult to occur is used. Also, as shown in FIG. 2 (a), the shape in the XZ plane direction is indeterminate, or as shown in FIG. Resonance vibration can be suppressed.

  In the present embodiment, the prism 2 and the metal film 3 are rectangular, and only the sound receiving layer 4 has a different shape. However, the prism and the metal film may have the same shape as the sound receiving layer.

  As described above, in addition to the method of making the shape capable of suppressing the resonance vibration, as a method of maintaining the flat frequency characteristics of the microphone, there is also a method of setting the resonance frequency higher than the measurement frequency by making the sound receiving layer very small. Is possible.

  When the above-mentioned silica dry gel with a sound velocity of 50 m / s is used, the sound wave receiving surface of the sound receiving layer is sufficiently reduced to a size of about 20 μm square so as to have resonance at 1.2 MHz or higher. Therefore, a flat frequency characteristic up to 1 MHz can be obtained.

  However, the microphone member having such a very fine size is difficult to handle, and it is difficult to accurately irradiate a light beam at an appropriate position for detecting a refractive index change by surface plasmon resonance. Usually, the sound receiving layer is formed with a size in the XZ plane direction of about 1 mm or more.

  Even when the sound receiving film is about 1 mm in size, if the sound receiving layer is divided into a large number in the XZ plane direction and the maximum size is 20 μm or less, the resonance frequency can be located at a high frequency. A microphone having a flat frequency characteristic up to a high frequency can be realized.

(Thickness of sound receiving layer)
In the present embodiment, the thickness of the sound receiving layer 4 in the Y direction is about 10 μm. Next, the reason why the thickness of the sound receiving layer 4 is 10 μm will be described. From the characteristics of the optical microphone, it is preferable to set it thin so as to have a resonance frequency in the high frequency region, as described above in the shape in the XZ plane direction.

  When the resonance frequency in the Y direction of the sound receiving layer 4 is calculated using a sound velocity of 50 m / s as the silica dry gel, the Y direction is generated when the thickness is ¼ of the wavelength (in the XZ direction). Is 1/2 of the wavelength), and the resonance frequency at a thickness of 10 μm is about 1.25 MHz.

  That is, in the range where the sound receiving layer 4 is thinner than 10 μm, the resonance frequency is set in a frequency region higher than the measurement region, so that flat characteristics are obtained in the measurement frequency range.

  On the other hand, restrictions on the thinning of the sound receiving layer 4 will be described.

  One is a limitation in the measurement method by surface plasmon resonance. Surface plasmon resonance is a detection method in which only a change of about one wavelength of light to be used is detected from the interface between the sound receiving layer 4 and the metal layer 3. In order to sufficiently obtain a change in the refractive index in this region as a change in surface plasmon resonance, it is preferable to secure a portion where the refractive index changes by about one wavelength. When the thickness is thinner than one wavelength, the detection area is decreased because the area detected by surface plasmon resonance is reduced. In this embodiment, since a visible region laser light source is used, a film thickness of 800 nm or more is required.

  Next, restrictions on thinning the silica dry gel from the manufacturing process will be described. The silica dry gel provided as the sound receiving layer 4 is formed on the metal layer 3 by a wet process after a metal layer is formed on the prism by sputtering or vapor deposition.

  The process for forming a dry gel will be described more specifically. First, a raw material liquid as a raw material for the dried gel is prepared, and this is dipped or spin coated to form a coating film on the metal layer of the prism. Thereafter, after undergoing processes such as gelation, solvent replacement, water repellent treatment, and washing, supercritical drying is performed.

  In this wet process step, that is, before the supercritical drying, it is necessary to always maintain a wet state. When the formed coating film or the gel film after gelation is dried, structural destruction occurs and desired characteristics cannot be obtained.

  In order to prevent drying in the process before supercritical drying, a certain film thickness of the wet gel is required. More specifically, when the film thickness after drying is about 3 μm or more, a silica dry gel layer can be formed almost stably.

  In addition to making the dried gel film as the sound receiving layer sufficiently thin as described above, the film thickness may be inclined and distributed as shown in FIG. This is an effect due to the indefinite shape, and is a method of preventing strong resonance.

  Even when the film thickness is not constant, the surface plasmon resonance observes only a change of 1 μm or less from the metal film, so that the measurement accuracy of the sound wave is not affected.

  The light source 5 can emit light having a single wavelength, and a gas laser, a semiconductor laser, or the like can be used. If the wavelength dispersion of the laser light is large, dispersion occurs at the angle at which plasmon resonance is excited, which is not preferable from the viewpoint of measurement accuracy. The light source 5 is connected to the light driving circuit 8 and emits laser light corresponding to the input signal of the light driving circuit.

  The light emitted from the light source 5 is s-polarized light having an electric field component perpendicular to the XZ plane parallel to the metal layer. Since surface plasmon resonance is not excited in p-polarized light (parallel to the XZ plane), reflected light is observed when p-polarized light is included even in a state where surface plasmon resonance is excited, which is preferable from the viewpoint of measurement accuracy. Absent.

  The light receiver 6 is configured to detect light reflected at the interface between the prism 2 and the metal layer 3. A type of light receiver having sensitivity to the frequency of light used for the light source 5 is selected. When light in the visible region is used, the light receiver 6, which can use a silicon photodiode or the like, detects the reflected light and sends it to the detection circuit 9.

  The detection circuit 9 calculates a change in the refractive index of the silica dry gel or the like used for the sound receiving layer based on the signal from the light receiver, and further calculates the sound from the correlation between the change in the refractive index of the sound receiving unit and the sound pressure. Convert to pressure. The detection circuit 9 is connected to a computer (not shown) and is stored as an acoustic signal.

(Operation of optical microphone 1)
The operation of the optical microphone 1 configured as described above will be described. For easier understanding, description will be made here with reference to FIG. 3 is a cross-sectional view of the XY plane in the Z direction of the optical microphone shown in FIG.

  First, as an initial setting of the optical microphone, a state is set in which reflected light from the interface between the prism and the metal film is hardly detected, that is, surface plasmon resonance is excited under the condition that no sound reaches the sound receiving layer.

More specifically, the light source has a He—Ne laser with a wavelength of 633 nm, the prism has a glass with a refractive index of 1.5 (−), the metal layer has a thickness of 50 nm, the sound receiving layer has a density of 110 kg / m ³ , and the sound velocity is 50 m. When a silica dry gel having a refractive index of 1.0 / (s) is used, the angle at which surface plasmon resonance is excited is 45.6 degrees.

Therefore, the angle θ _SPR formed by the laser beam 7a shown in FIG. 3 and the vertical direction of the interface between the prism and the metal layer is arranged to be 45.6 degrees. The light source 5 and the light receiver 6 use a so-called θ-2θ stage that controls the positions of the laser beams 7a and 7b so that the angle θ _SPR is always the same as the moving stage.

  The prism 2 is an isosceles triangle whose angle 2d facing the metal film in FIG. 2 is 90 degrees so as to transmit light without being refracted. The prism shape may be any shape as long as it can be incident on the interface 2c between the prism 2 and the metal film 3 at an angle greater than or equal to total reflection as described above. Since the incident angles on the metal interface are substantially the same, it is preferable because the adjustment of the angle and position can be easily adjusted intuitively.

In practice, the position of the light source 5 and the light receiver 6 (angle θ _SPR in FIG. 3) is changed while observing the light reception signal of the light receiver 6, and the light source, Fix the receiver. At this time, the integral part composed of the prism 2, the metal layer 3, and the sound receiving layer 4 is also fixed so as not to move. That is, in the state where the sound wave does not reach the sound receiving layer in the initial stage, the received light signal intensity is almost zero.

  A sound is transmitted to the portion of the sound receiving layer exposed to the surrounding fluid using a speaker, a tweeter, an ultrasonic layer transmitter, or the like to the optical microphone that has been initially set.

  The sound receiving layer that has received the sound wave undergoes deformation and volume change, and further changes the refractive index. When the refractive index of the sound receiving layer changes, the surface plasmon resonance angle changes, so that the received light signal intensity detected by the light receiver increases.

  When sound enters the sound receiving layer from a state where the light receiving intensity is substantially 0 in the initial state, the surface plasmon resonance condition changes, and the light receiving intensity at the light receiver 6 increases. From this received light intensity, the refractive index change amount of the silica dry gel, which is the sound receiving layer, can be calculated, and furthermore, the sound wave can be measured by converting the relationship between the sound pressure and the refractive index change amount to the sound pressure. It is.

As described above, the configuration of the optical microphone 1 of the present embodiment is that the light source 5 is a He—Ne laser with a wavelength of 633 nm, the prism 2 is glass with a refractive index of 1.5, the metal layer 3 is gold with a thickness of 50 nm, and the sound is received. When a silica dry gel having a refractive index of 1.022 (−) is used for the layer 4, the refractive index of the sound receiving layer 4 is 1.0 × 10 ⁻⁴ (in the vicinity of 45.6 degrees where the surface plasmon resonance is excited. -) When the degree changes, the angle at which surface plasmon resonance absorption is excited changes by about 0.01 degree, and the reflectivity of the light beam changes by about 1%.

Since the silica dry gel used as the sound receiving layer 4 has a refractive index variation of 1.0 × 10 ⁻⁷ (Pa ⁻¹ ), when it receives a sound pressure of about 1000 Pa, it exhibits surface plasmon resonance absorption. The angle changes by 0.01 degrees, and the reflectivity of the light beam changes by about 1%. By detecting the change in the intensity of the light beam with the light receiver 6, the sound wave 11 propagating through the environmental fluid 10 can be detected. By removing the direct current component of the light intensity signal detected by the light receiver 6 with a high-pass filter, the alternating current component corresponding to the frequency of the sound wave can be detected more clearly.

  In the present embodiment, the surface plasmon resonance is excited as an initial setting, but the initial setting may be set slightly larger or smaller than the angle θSPR.

  If the position where the received light signal intensity is 0 is set as the initial setting, the stability of the origin is good, but the received light signal intensity increases both when the sound pressure is high and when the sound pressure is low. Become.

  On the other hand, if the initial setting is shifted from the angle θSPR, there is a problem with the stability of the origin, but the sound pressure can be determined positively or negatively. It is also effective to set

    Since the optical microphone of the present invention is a system using surface plasmon resonance that detects a local refractive index change, it has an advantageous effect that the sensitivity does not decrease even if it is reduced in size. Since it has a diaphragm and has a simple and stable structure, it is easy to miniaturize and an optical microphone having a flat frequency characteristic up to a high frequency region can be realized.

  Further, high sensitivity is realized by using a silica dry gel having a refractive index change for a pressure larger than that of a gas such as air as a conversion material for changing sound pressure and refractive index.

  As described above, for the sound receiving layer that converts sound pressure into refractive index change, silica dry gel with a larger refractive index change amount than that of gas such as air is used, and the generated refractive index change is expressed by surface plasmon resonance. The optical microphone of the present invention that is detected by the above has an advantageous effect that the sensitivity does not decrease even if the size is reduced, and a flat frequency characteristic characteristic up to a high frequency region is realized.

  In the present invention, silica dry gel was used as the sound receiving layer. In addition, dry oxides of inorganic oxides such as titania, barium titanate, and alumina, or general thermosetting resins, thermoplastic resins, etc. For example, organic dry gels such as polyurethane, polyurea, phenol cured resin, polyacrylamide, or polymethyl methacrylate can also be used.

(Embodiment 2)
With reference to FIG. 4, the configuration and operation of the optical microphone according to the present invention and the second embodiment will be described. In FIG. 4, the same numbers are assigned to the same components as those in FIGS.

  The optical microphone in the present embodiment shown in FIG. 4 differs from the optical microphone in the first embodiment shown in FIG. 3 in the following two points. One point is that a linear light beam emitted from the light source 5 is diverged to be collimated, and the parallel light emitted from the beam expander is converged only in the XY plane of the prism (in the Z direction). The cylindrical lens 13 is provided between the light source 5 and the prism 2.

  The second point is that the light receiver 6 includes a plurality of light receiving elements 61. The light receiver of this embodiment has a structure in which a plurality of light receiving elements 61 are arranged one-dimensionally in a direction substantially parallel to the prism surface 2b through which reflected light is transmitted.

  The operation of such an optical microphone will be described. First, an initial setting, that is, a setting method in a state where no sound wave is received will be described.

  As shown in FIG. 4, when the light beam 7 emitted from the light source 5 passes through the beam expander 12 and the cylindrical lens 13, light having an angle range (θ1 to θ2, θ1 <θ2) to the interface between the prism and the metal layer. The beam will be incident.

At this time, θ1 and θ2 satisfy the relationship of θ1 < _θSPR <θ2 between the angle _θSPR for exciting the surface plasmon resonance.

  Such a light beam is incident on the interface between the prism and the metal layer at an incident angle having a width, and is reflected at the same angle as the incident angle. Reflected light is also obtained at an angle having a width, and is detected by a light receiving element that exists at a position corresponding to the angle of the reflected light among many light receiving elements arranged one-dimensionally.

  The light beam corresponding to the angle at which the surface plasmon resonance is excited has a reflected light intensity that is reduced to almost zero, so that the light receiving signal intensity of the light receiving element 61 arranged at a position corresponding to the angle of the light beam is substantially zero. Drop to.

  As an initial setting, it is preferable to adjust the position so that the light receiving element having a light receiving intensity of approximately 0 is approximately at the center of the one-dimensionally arranged light receivers. This makes it possible to detect sound waves over a wide sound pressure range.

  Next, the operation when receiving a sound wave will be described. When the sound receiving layer 4 receives sound waves, a refractive index change occurs, and the angle of the light beam excited by surface plasmon resonance changes. In other words, the position of the light receiving element 61 where the light receiving intensity becomes almost zero changes. From the position of the light receiving element where the light receiving intensity is reduced, it is possible to detect a change in the refractive index of the sound receiving layer 4 and further a sound pressure.

  According to the optical microphone of the present embodiment, a light receiver in which a large number of light receiving elements are arranged is required, resulting in a slight increase in cost. On the other hand, compared with the first embodiment, the measurement stability and the sign of sound pressure are positive and negative. There is an advantage that the measurement can be realized simultaneously.

  Furthermore, the resolution can be improved by moving the positions of the one-dimensionally arranged light receivers away from the prism 2, and the degree of freedom in measurement is high, for example, the amount of received light can be increased by being closer.

  As in the first embodiment, the light source has a He—Ne laser with a wavelength of 633 nm, the prism has a refractive index of 1.5 (−) glass, the metal layer has a thickness of 50 nm, the sound receiving layer has a refractive index of 1.022 ( -) Using a silica dry gel, the incident angle with respect to the interface between the prism 2 and the metal layer 3 is 45.3 to 46.0 degrees (θ1 = 45.3 degrees, θ2 = 49.0) by the beam expander 12 and the cylindrical lens 13. The operation of the optical microphone set to (degree) will be described. In the present embodiment, a light receiver in which about 1000 light receiving elements 61 are arranged in one dimension (in the XY plane of FIG. 4) is used as the light receiver 6. The length in the arrangement direction of the light receiving elements 61 of the light receiver 6 is about 10 mm, and the length in the arrangement direction per one element of the light receiving elements 61 is about 10 μm.

  The angle range of the reflected light corresponding to one light receiving element 61 is determined by the distance from the interface between the prism 2 and the metal layer 3 to the light receiver 6 and the length of the light receiving element 61 in the arrangement direction per one element.

In the present embodiment, since the distance from the interface between the prism 2 and the metal layer 3 to the light receiver 6 is 1 m and the width of the light receiving element 61 is 10 μm, the angle range of reflected light corresponding to one element of the light receiving element 61 Is 5.7 × 10 ⁻⁴ degrees.

As described in the first embodiment, in the vicinity of 45.6 degrees where the surface plasmon resonance is excited, when the refractive index of the sound receiving layer 4 changes by about 1.0 × 10 ⁻⁴ (−), the surface plasmon resonance The angle at which the absorption is excited varies by about 0.01 degrees.

In this embodiment, since the amount of change in the refractive index of the silica dry gel used as the sound receiving layer 4 is 1.0 × 10 ⁻⁷ (Pa ⁻¹ ), when a sound pressure of about 1000 Pa is applied, the surface plasmon The angle of resonance absorption changes by 0.01 degrees. This is because the received light intensity is almost 0, that is, about 17 light receiving elements for detecting dark lines move. Thus, the sound wave propagating through the environmental fluid 10 can be detected from the position of the light receiving element 61 where the received light intensity is substantially zero.

  Although this embodiment has been described using a light receiver of a type in which light receiving elements are arranged one-dimensionally, a light receiver of a type arranged in two dimensions may be used. In this case, since there are a plurality of light receiving elements corresponding to the same angle, the measurement stability can be further increased by a method such as averaging the light intensities of the light receiving elements corresponding to the same angle.

  In addition, the advantages of using the silica dry gel as a sound pressure and refractive index change conversion material in the optical microphone in the present embodiment will be quantitatively shown. Here, the sensitivity comparison with the structure in which the sound receiving layer 4 is not provided in FIG. 4, that is, when the refractive index change of air is directly measured is performed.

The amount of change in the refractive index with respect to the air pressure is about 2.0 × 10 ⁻⁹ (Pa ⁻¹ ). When a He—Ne laser (wavelength: 633 nm) is used as the light source and gold is used as the metal layer, the sensitivity is approximately 30% more than when detecting a change in the refractive index of air when silica dry gel is applied to the sound receiving layer. It improves to about 50 times.

  As described above, the optical microphone according to the second embodiment of the present invention uses a light receiver having a light receiving element that is incident in a light beam having an angular width at the interface between the prism and the metal layer and arranged in one dimension or more. By adopting the configuration, it is possible to provide an optical microphone that enables measurement with higher stability while maintaining a flat frequency characteristic up to a high frequency region.

(Embodiment 3)
The configuration and operation of the optical microphone according to Embodiment 3 of the present invention will be described with reference to FIG. The difference between the present embodiment and the first and second embodiments is that an acoustic horn 14 is provided in front of the sound receiving layer and on the side of the surrounding fluid, as shown in FIG. Is the same as The acoustic horn 14 is combined with a connection member to the prism 2 (not shown), and is integrated as the optical microphone 1.

  Since the sound wave propagating through the acoustic horn is larger in the acoustic horn inlet 14a than the outlet 14b in contact with the acoustic horn and the sound receiving portion, the sound wave is compressed by the shape effect and the sound pressure is increased. Since the refractive index change generated in the sound receiving layer is increased by the effect of sound wave compression, there is an advantageous effect that even smaller sounds can be detected.

  It is preferable to design the acoustic horn so that the opening area changes exponentially with respect to the Y direction in FIG. 5 because reflection of sound waves inside the acoustic horn is reduced.

(Embodiment 4)
The optical microphone of the present invention can also be used as an underwater optical hydrophone. When used as an underwater hydrophone, a water repellent treatment is essential for the silica dry gel used as the sound receiving layer. The optical microphone in this embodiment can be applied to any configuration shown in the first to third embodiments.

  Silica dry gel that is not subjected to water repellent treatment cannot exhibit the characteristics as a material having a large refractive index change, because countless fine pores are destroyed by the capillary force of water. Since the sound pressure in water is usually much higher than in the atmosphere, sufficient sensitivity can be ensured.

  The water-repellent treatment of the silica dry gel can be achieved by treating the catalyst in a wet gel state with ammonia water as a catalyst and alkoxysilane such as dimethyldimethoxysilane.

Similar to the comparison in the second embodiment, such an optical microphone is compared with a device configuration in which silica dry gel is not provided, that is, when a change in the refractive index of water is directly detected (change in the refractive index of water). In the case of using an amount of 1.5 × 10 ⁻¹⁰ Pa ⁻¹ ) and a silica dry gel as the sound receiving layer, a high sensitivity of about 250 times can be realized.

In the present invention, since the silica dry gel plays a major role as a conversion material for changing the sound pressure to change the refractive index, an example of a method for producing the silica dry gel used in the embodiment of the present invention will be described. .
1. Raw material preparation A small amount of triethylethoxysilane was added to tetraethoxysilane, ethanol and water were further mixed at a certain ratio, and the gel raw material solution mixed for about 1 minute in a beaker was kept at room temperature.
2. Coating Film Formation A substrate on which gold was sputtered on a glass prism was attached to an attachment having a V-groove on which the prism can be installed, and installed on a spin coater. After the prepared raw material liquid was dropped onto the gold film, the entire attachment was rotated at 1500 rpm for 30 seconds to shake off excess gel raw material liquid to form a thin gel coating film.
3. Gelation To prevent the coating film from drying, immediately place the attachment with the gel raw material film on the prism / gold film in a sealed container containing ammonia water and hold at 40 ° C for 24 hours. To complete the gelation.
4). Washing In order to remove unreacted raw material liquid and catalyst, the substrate was immersed in a sufficient amount of ethanol and held at 40 ° C. for 6 hours.
5). Hydrophobization The prism / gold / wet gel body after washing was dipped in a hydrophobization solution in which dimethyldimethoxysilane and aqueous ammonia were mixed, and the mixture was kept at 70 ° C. for 24 hours.
6). Solvent Replacement The prism / gold / wet gel body, which had been hydrophobized, was again immersed in a sufficient amount of ethanol and held at 40 ° C. for 6 hours.
7). Supercritical drying At 70 ° C. and 12 MPa, ethanol was fully substituted with carbon dioxide gas, and then the pressure was slowly decreased while maintaining the temperature, thereby completing the drying from the supercritical state to the gaseous state.

  Through the manufacturing process as described above, a silica dry gel layer having a density of 110 kg / m 3, a sound velocity of 50 m / s, and a refractive index variation of 1.0 × 10 −7 Pa −1 was obtained.

  The optical microphone of the present invention can realize a microphone having a flat frequency characteristic up to a high frequency range, which has been difficult in the past, and can be used for noise measurement, music use, various measurements using sound waves (distance measurement, velocity measurement, flow measurement) Etc.). Further, since the measuring unit main body does not use electricity, it is also suitable for a microphone used in an electromagnetic wave environment such as MRI.

DESCRIPTION OF SYMBOLS 1 Optical microphone 2 Prism 3 Metal layer 4 Sound receiving layer 5 Light source 6 Light receiver 7 Light beam 8 Light source drive circuit 9 Detection circuit 10 Ambient fluid 11 Sound wave (wavefront)
12 Beam expander 13 Cylindrical lens 14 Acoustic horn

Claims (10)

An optical microphone for detecting sound waves propagating through an ambient fluid by light,
Prism,
A metal layer provided on one surface of the prism;
A sound receiving layer provided on the surface of the metal layer opposite to the prism and exposed to surrounding fluid;
A light source that emits a light beam at an angle greater than or equal to total reflection at the interface between the prism and the metal layer;
A light source driving circuit for driving the light source;
A light receiver that receives reflected light from an interface between the prism and the metal layer;
A detection circuit for converting a signal from the light receiver into a sound wave signal;
Optical microphone consisting of The optical microphone according to claim 1, wherein the sound receiving layer is made of a dry gel of an inorganic oxide or an organic polymer. The optical microphone according to claim 2, wherein the inorganic oxide is silica. The optical microphone according to claim 2 or 3, wherein the solid skeleton of the dry gel is hydrophobized. The optical microphone according to claim 2, wherein the dry gel has a refractive index variation with respect to pressure of 2.0 × 10 ⁻⁹ Pa ⁻¹ or more. The optical microphone according to claim 1, wherein the light source emits a linearly polarized laser beam having an electric field component perpendicular to an interface between the metal layer and the sound receiving layer. 2. The optical microphone according to claim 1, wherein the detection circuit converts reflected light intensity into sound pressure. 2. The optical microphone according to claim 1, wherein the light source includes an optical system that converts the light beam into a light beam having an incident angle range greater than or equal to a certain angle toward an interface between the prism and the metal layer. The optical microphone according to claim 1, wherein the light receiver includes a plurality of light receiving elements. The optical microphone according to claim 1, further comprising an acoustic horn on the ambient fluid side of the sound receiving layer.

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