JP2011205171A - Ultrasonic receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem wherein a condenser microphone or the like, included in conventional ultrasonic receivers, uses a diaphragm in principle, and thereby a frequency band is limited.SOLUTION: This ultrasonic receiver is equipped with: a waveguide having an opening for entering ultrasonic waves for transmitting the ultrasonic waves having entered in the opening; a transmission medium part contacting a surface in parallel to the transmission direction of the ultrasonic waves in the waveguide for entering a part of the ultrasonic waves transmitted through the waveguide; a light source for emitting optical waves crossing the transmission medium part from a direction different from the transmission direction of the ultrasonic waves; partial reflection members arranged at both ends of the transmission medium part in the transmission direction of the optical waves crossing the transmission medium part, transmitting a part of the optical waves, and reflecting the residual partial part of the optical waves; and a photoelectric conversion part for converting the optical waves having passed through the partial reflection members and the transmission medium part to an electric signal.

Description

  The present invention relates to an ultrasonic wave receiver that has a particularly high performance with respect to the reception of ultrasonic waves, and in particular, a gas ultrasonic wave receiver that has a high performance when receiving an ultrasonic wave propagating through a gas such as air. About.

  Conventionally, a microphone is known as a device for receiving a sound wave. Many microphones represented by dynamic microphones and condenser microphones use a diaphragm. In these microphones, an input sound wave vibrates the diaphragm, and the vibration is taken out as an electric signal. However, a microphone using a diaphragm has a problem that it can be used only in a limited frequency band because the diaphragm has resonance characteristics.

  For microphones that do not use a diaphragm, there are techniques of Patent Documents 1 and 2. The technique described in Patent Document 1 detects sound waves in a wide area by detecting light waves reflected a plurality of times by a reflecting member in order to measure the sound field between the reflecting members arranged opposite to each other as shown in FIG. It is a microphone that measures.

  As shown in FIG. 24, the technique described in Patent Document 2 uses an Fabry-Perot interferometer to detect a change in the frequency of laser light applied to the surface of the subject, thereby detecting ultrasonic waves propagating through the subject. Detect with high sensitivity. Also, as shown in FIG. 25, in Patent Document 3, ultrasonic waves are taken into a propagation medium part from a medium having a very low acoustic impedance such as air with high efficiency, or ultrasonic waves are emitted from a propagation medium part to air with high efficiency. Silica dry gel is described as a solid medium that can emit. Hereinafter, a microphone that detects sound waves using light waves is referred to as an “optical microphone”.

JP 2004-279259 A Japanese Patent Laid-Open No. 9-281086 International Publication No. 2004/098234

  In the optical microphone of Patent Document 1, the reflecting member also serves as an opening for inputting sound waves. The interval between the reflecting members is 1 m or more, and the size of the reflecting mirror is about 0.6 m × 0.5 m, which is a very large opening. Therefore, the optical microphone of Patent Document 1 uses a wide area sound such as aircraft noise for measurement, and cannot be used as it is as a conventional acoustic receiver having a small opening.

  Although it is conceivable to reduce the aperture by narrowing the mirror interval, the sensitivity of the optical microphone largely depends on the product ΔnL of the light propagation distance L and the change Δn in the refractive index of air caused by the sound propagation. Therefore, if the interval between the mirrors is narrowed, the light propagation distance L is shortened and sufficient sensitivity cannot be obtained, and it is difficult to operate as an acoustic receiver. For this reason, the optical microphone disclosed in Patent Document 1 cannot measure the local sound pressure with a small opening unlike the conventional microphone.

  In Patent Document 2, as shown in FIG. 24, an ultrasonic wave propagating through the subject is detected with high sensitivity by detecting a frequency change of the laser light irradiated on the subject surface using a Fabry-Perot interferometer. To do. However, this is to detect sound waves in the solid, not to detect sound waves in the air. Applying this, it is conceivable to detect ultrasonic waves by capturing ultrasonic waves in the air. However, ordinary solids have a very different acoustic impedance from air, and reflection at the air / solid interface is difficult. Because it is large, it cannot be efficiently taken into the interior.

  Accordingly, an object of the present invention is to solve the above-described problems and to provide a small-sized ultrasonic receiver that detects sound waves in the air without using a diaphragm.

The ultrasonic wave receiver of the present invention is an ultrasonic wave receiver that receives an ultrasonic wave propagating through a surrounding space filled with an environmental fluid, and has an opening through which the ultrasonic wave is incident. A waveguide through which the ultrasonic wave incident on the waveguide propagates, a propagation medium part in contact with a plane parallel to the propagation direction of the ultrasonic wave in the waveguide, and a part of the ultrasonic wave propagating through the waveguide is incident on the waveguide, A light source that emits a light wave that crosses the propagation medium part from a direction different from the propagation direction of the sound wave, and a light source that is disposed at both ends of the propagation medium part in the propagation direction of the light wave that crosses the propagation medium part, and transmits a part of the light wave. A partial reflection member that reflects the remaining part of the light wave, and a photoelectric conversion unit that converts the light wave that has passed through the partial reflection member and the propagation medium part into an electrical signal, the propagation medium part, and the light source And the partial reflection member and the light source Constitute a Fabry-Perot interferometer, the density [rho ₁ of the propagation medium portion, sound velocity C ₁ in the propagation medium portion, the density [rho ₂ of the environmental fluid, and the sound velocity C ₂ in the environmental fluid, (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) <1.

  According to the present invention, the ultrasonic wave transmitted from the environmental fluid to the propagation medium part is detected by arranging the propagation medium part in the Fabry-Perot interferometer and detecting the output light. Thereby, a small ultrasonic receiver with a small opening can be provided without using a diaphragm.

1 is a perspective view of an optical microphone according to Embodiment 1 of the present invention. Xz sectional view of the optical microphone according to the first embodiment of the present invention. Diagram showing the transmission characteristics of silica dry gel FIG. 7 is a perspective view showing another example of the optical microphone according to the first embodiment. The figure which shows the situation where the spot diameter of the light wave is larger than the wavelength of the ultrasonic wave in the propagation medium The figure which shows the situation where the spot diameter of the light wave is smaller than the wavelength of the ultrasonic wave in the propagation medium part Yz sectional drawing of the optical microphone by Embodiment 1 of this invention Diagram showing an example of input / output response of a Fabry-Perot interferometer Diagram showing dependency of Fabry-Perot interferometer on mirror reflectivity Yz sectional drawing which shows the other example of the optical microphone of Embodiment 1. FIG. Yz sectional drawing which shows the other example of the optical microphone of Embodiment 1. FIG. Yz sectional drawing which shows the other example of the optical microphone of Embodiment 1. FIG. Yz sectional drawing which shows the other example of the optical microphone of Embodiment 1. FIG. FIG. 7 is a perspective view showing another example of the optical microphone according to the first embodiment. Diagram showing an example of input / output response of a reflective Fabry-Perot interferometer Xz sectional drawing of the optical microphone by Embodiment 2 of this invention The figure explaining the design of the shape by Embodiment 2 A perspective view of an optical microphone according to a third embodiment of the present invention. The xz sectional view which shows the design in the Example of the optical microphone by this invention Yz sectional drawing which shows the design in the Example of the optical microphone by this invention The figure which shows the ultrasonic signal detection result of the optical microphone by this invention The figure which shows the measurement result of the output response with respect to the sound pressure of the optical microphone by this invention The figure which shows the optical microphone by patent document 1 The figure which shows the ultrasonic detection apparatus by patent document 2 Cross section of ultrasonic transducer according to Patent Document 3

  Hereinafter, embodiments of an ultrasonic receiver according to the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 shows the configuration of the ultrasonic receiver in the present embodiment. The ultrasonic receiver has an opening 1 into which an ultrasonic wave propagating from an environmental fluid filling a space (external space) surrounding the ultrasonic receiver is incident, and the ultrasonic wave incident from the opening 1 propagates. Between the waveguide 10 that propagates, the propagation medium portion 2 through which a part of the ultrasonic wave 14 that propagates through the waveguide 10 propagates, the holding portion 7 that holds the waveguide 10 and the propagation medium portion 2, and the propagation medium portion 2. Two partial reflection members 3a and 3b arranged so as to be opposed to each other, a light source 4 that emits a light wave 8 toward the partial reflection member 3a, and an optical signal that transmits the light wave 8 that has passed through the partial reflection member 3b A photoelectric conversion unit 5 that converts the signal to the photoelectric conversion unit 5, and an electrical signal detection unit 6 that is electrically connected to the photoelectric conversion unit 5 and detects an output signal of the photoelectric conversion unit 5. In FIG. 1, the propagation direction of the light wave 8 is the Y axis, the propagation direction of the ultrasonic wave 14 is the Z axis, and the propagation direction of the light wave 8 and the direction orthogonal to the propagation direction of the ultrasonic wave 14 are the X axis. Here, the “environmental fluid” refers to a fluid existing outside the optical microphone. The environmental fluid is, for example, air.

The two partial reflection members 3a and 3b, the light source 4, and the electric signal detector 6 constitute a Fabry-Perot interferometer. FIG. 2 shows a view of the optical microphone in the present embodiment viewed from the direction of the light source 4 (the positive direction of the Y axis). As shown in FIG. 2, a part of the surface of the waveguide 10 parallel to the ultrasonic wave propagation direction is in contact with the propagation medium portion 2. Further, a part of the surface of the waveguide 10 parallel to the ultrasonic wave propagation direction may be constituted by the propagation medium portion 2. A part of the ultrasonic wave propagating through the waveguide 10 propagates into the propagation medium part 2 due to a refraction propagation phenomenon.
The density ρ _{1 of} the propagation medium part 2, the sound speed C ₁ in the propagation medium part 2, the density ρ _{2 of} the environmental fluid, and the sound speed C ₂ in the environmental fluid are (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) < Satisfies 1 relationship.

As described in Patent Document 3, when the relationship of (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) <1 is satisfied, ultrasonic waves are transmitted from the environmental fluid to the propagation medium unit 2 with high efficiency. It can be transmitted. Therefore, the ultrasonic wave propagating through the waveguide 10 can be efficiently taken into the propagation medium unit 2.

(Silica dry gel)
The propagation medium part 2 that satisfies the relationship (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) <1 is preferably a silica dry gel. Further, by using a hydrophobized silica dry gel, deterioration with time due to moisture in the air can be prevented. Since the silica dry gel has low physical strength, it is preferable to hold the propagation medium part 2 by the holding part 7 as shown in FIG.

FIG. 3 shows the results of measuring the transmission characteristics using a silica dry gel having a density of 150 kg / m ³ and a sound velocity of 90 m / sec and a thickness of 5 mm. From FIG. 3, it can be seen that the transmission characteristics of the silica dry gel are more attenuated as the wavelength of light is shorter. Therefore, the wavelength of the light wave 8 emitted from the light source 4 is preferably at least 400 nm. When the density of the silica dry gel is 50 kg / m ³ or less, the silica dry gel becomes white turbid during the gel drying process, and the light transmittance is significantly reduced. Therefore, the density of the silica dry gel is preferably 50 kg / m ³ or more.

(Acoustic horn)
As shown in FIG. 4, when the acoustic horn 9 is disposed at the front end of the opening 1 and the focusing end 9b of the acoustic horn is connected to the opening 1, the ultrasonic wave 14 input from the opening end 9a of the horn opens the acoustic horn 9. As it propagates from the end 9a to the converging end 9b, it is compressed and the energy density is increased, and an ultrasonic wave having a high energy density can be propagated to the opening 1.

(Operation of optical microphone)
The operation of the optical microphone of this embodiment will be described. The optical microphone of the present embodiment takes in the ultrasonic wave 14 propagating from the environmental fluid into the propagation medium unit 2 and detects it as an optical signal by the Fabry-Perot interferometer.

When the ultrasonic wave 14 propagates inside the propagation medium unit 2, a dense wave is generated in the propagation medium unit 2. When a dense wave is generated in the propagation medium part 2, the refractive index changes Δn _{n at} the part where the dense wave is generated. At this time, a relationship of Δn _n = P (n _n −1) / (C _n ² ρ _n ) between the sound pressure P of the ultrasonic wave propagating in the propagation medium portion 2 and the refractive index change amount Δn _n. There is. Here, ρ _n is the density of the propagation medium part 2, C _n is the sound velocity of the propagation medium part 2, and n _n is the refractive index of the propagation medium part 2. Thus, when the ultrasonic wave propagates in the propagation medium part 2, the refractive index changes according to the sound pressure of the ultrasonic wave.

The silica dry gel refractive index that changes due to the propagation of ultrasonic waves is one or more orders of magnitude greater than the amount of change in the refractive index of air or water. For example, when the amount of change in the refractive index was measured using a silica dry gel having a density of 150 kg / m ³ and a sound speed of 90 m / sec, the amount of change in the refractive index with respect to 1 Pa of sound pressure was 1.1 × 10 ⁻⁷ . This value of the refractive index change is very large compared to the refractive index change in air (2.0 × 10 ⁻⁹ ) or the refractive index change of water (1.5 × 10 ⁻¹⁰ ). Therefore, it can be seen that the silica dry gel is a medium having a large amount of refractive index change with respect to the sound pressure.

  When the ultrasonic wave propagates inside the propagation medium part 2, the refractive index has a distribution in the order of the wavelength of the ultrasonic wave in the plane of the propagation medium part 2. 5 and 6 show a state in which the light wave 8 is propagated so as to cross the propagation medium portion 2 where the ultrasonic wave 14 is propagated. In FIG. 5, the propagation direction of the ultrasonic wave 14 and the propagation direction of the light wave 8 are shown to be orthogonal, but at least the propagation direction of the ultrasonic wave 14 and the propagation direction of the light wave 8 do not have to be parallel.

  The difference in the brightness of the propagation medium part in FIG. 5 indicates that the dense wave propagates. If the spot diameter of the light wave 8 incident from the light source 4 is large, the light wave 8 propagates so as to include both a place with high brightness (white part) and a place with low brightness (black part). That is, since the light wave 8 propagates simultaneously in a place where the refractive index changes positively and a place where the refractive index changes negatively, the total amount of change in refractive index which can be detected from the light wave 8 transmitted through the propagation medium portion 2 is canceled out. And get smaller.

  Therefore, as shown in FIG. 6, the spot diameter of the light wave 8 is preferably smaller than the wavelength Λ of the ultrasonic wave propagating through the propagation medium portion 2. Thereby, the light wave 8 can propagate through any part of the place where the refractive index changes positively and the place where the refractive index changes negatively. As a result, compared with the case where the spot diameter of the light wave 8 is large, the sensitivity of the refractive index change detected from the light wave 8 transmitted through the propagation medium portion 2 is improved.

  In addition, when the acoustic horn 9 is provided as shown in FIG. 4, since the propagation direction of the ultrasonic wave propagated from the environmental fluid can be made uniform, the ultrasonic wave propagating through the propagation medium portion 2 is converted into the z-axis of FIG. It can be propagated as a plane wave in the direction of. Therefore, in the y-axis direction, which is the propagation direction of the light wave 8, the refractive index distribution can be ignored.

  Next, the Fabry-Perot interferometer used in the optical microphone of the first embodiment will be described.

  In the optical microphone according to the first embodiment, in the propagation medium portion 2, two partial reflection members 3a and 3b are arranged on both end faces in the direction in which the light wave 8 propagates. There are a plurality of paths until the light wave 8 emitted from the light source 4 passes through the partial reflection member 3 a, the propagation medium unit 2, and the partial reflection member 3 b and propagates to the photoelectric conversion unit 5.

  As shown in FIG. 7, a part of the light wave 8 emitted from the light source 4 passes through the partial reflection member 3 a and propagates through the propagation medium portion 2. A part of the light transmitted through the partial reflection member 3a further transmits through the other partial reflection member 3b.

  A part of the light transmitted through the partial reflection member 3a is reflected a plurality of times between the partial reflection member 3a and the partial reflection member 3b and propagates in the propagation medium part 2, and a part of the light is partially reflected. It penetrates from the member 3b. In the optical microphone of the first embodiment, the Fabry-Perot interferometer is configured by arranging the two partial reflection members 3a and 3b so that the transmitted light of the partial reflection member 3b interferes.

  FIG. 8 shows a calculation result of how the intensity of the output light changes due to the change in the refractive index of the propagation medium unit 2 in the Fabry-Perot interferometer. In FIG. 8, the vertical axis represents the output light intensity, and the horizontal axis represents the refractive index change amount. FIG. 8 shows the optical microphone shown in FIG. 7, in which the wavelength of the light wave 8 is 633 nm, the reflectances of the partial reflection members 3a and 3b are both 50%, and the refractive index of the propagation medium part 2 before propagation of the ultrasonic wave is 1. 07 is a result of calculating the output response when the interval between the two partial reflection members 3 is 5 mm.

  From this, it can be seen that the intensity of light changes according to the change in refractive index. The operating point is preferably selected so that the light intensity changes linearly with respect to the change in refractive index. The operating point can be adjusted by changing the distance between the two partial reflection members 3 or changing the refractive index of the propagation medium section 2.

  FIG. 9 shows the result of calculating how the output response changes when the reflectance of the partial reflection members 3a and 3b changes. In FIG. 9, the vertical axis represents the output light intensity, and the horizontal axis represents the refractive index change amount.

  FIG. 9 shows a calculation result when the reflectances of the partial reflection members 3a and 3b are changed to 30%, 50%, 70%, and 90%, respectively. As a result, the higher the reflectivity of the partial reflection members 3a and 3b, the sharper the change in light intensity. If the operating point is appropriately selected, a large output intensity change can be obtained with respect to the refractive index change, and high sensitivity can be obtained. I understand that However, the sharper the output response, the more severe the setting of the operating point, and a sufficient change in light intensity cannot be obtained with a slight deviation from the optimal point. Considering the accuracy and sensitivity of the operating point adjustment, it is preferable that the reflectance of the partial reflection members 3a and 3b is about 10% to 70%.

  In addition, as shown in FIG. 7, the partial reflection members 3a and 3b may be in contact with the propagation medium portion 2, and as shown in FIGS. 10, 11, and 12, the partial reflection members 3a and 3b are the propagation medium portion. 2 may be separated.

  As shown in FIG. 11, there may be a part of the holding part 7 in addition to the propagation medium part 2 between the two partial reflection members 3. In this case, however, it is necessary to use a transparent material such as an acrylic material or glass as the holding portion 7.

  Further, the holding unit 7 does not necessarily hold the entire surface of the propagation medium unit 2, and if the periphery of the portion where the light wave 8 propagates is not held as shown in FIGS. 7, 10, and 12, the light wave 8 is The light enters the propagation medium part 2 without propagating through the holding part 7. In this case, it is not necessary to use a transparent material for the holding portion 7.

  Further, as shown in FIGS. 7, 12, and 13, the partial reflection member 3 can be a part of the holding portion 7. If a film is formed on the surface of the propagation medium part 2 and partial reflection occurs on the surface of the propagation medium part 2, the film-formed surface can be used instead of the partial reflection member 3.

  Note that the light wave incident on the photoelectric conversion unit 5 in the Fabry-Perot interferometer does not necessarily have to pass through the partial reflection member 3b, and is directed from the partial reflection member 3a to the light source 4 as shown in FIG. A propagating light wave can also be incident. This is because the light wave propagating from the partial reflection member 3a in the direction of the light source 4 is the interference light of the light wave reflected a plurality of times between the two partial reflection members 3a and 3b, similarly to the light wave transmitted through the partial reflection member 3b. Because there is. When performing this, if the beam splitter 12 is used, incident light and interference light can be separated, and the arrangement of the photoelectric conversion unit 5 becomes easy.

  FIG. 15 shows changes in refractive index and output light intensity. FIG. 15 shows the result of calculation when the reflectances of the partially reflecting members 3a and 3b are both 50%. It can be seen from FIG. 15 that the change in the light intensity corresponding to the change in the refractive index can be obtained as in the case where the transmitted light of the partial reflection member 3b is measured. When measuring the reflected light, the reflectance of the partial reflection member 3b can be set to 100%.

  Note that detection of ultrasonic waves using a Fabry-Perot interferometer is also described in Patent Document 2, which reflects reflected light from a subject to the Fabry-Perot interferometer. When reflected by a subject that propagates ultrasonic waves, the frequency of light changes due to the Doppler effect. This frequency change of the light is detected by a Fabry-Perot interferometer.

  In the optical microphone according to the present embodiment, the output of the Fabry-Perot interferometer changes as the refractive index inside the interferometer changes due to the propagation of ultrasonic waves to the propagation medium section 2 in the Fabry-Perot interferometer. Can detect ultrasound. As described above, an optical microphone that detects a change in refractive index accompanying propagation of ultrasonic waves inside the Fabry-Perot interferometer has not been disclosed so far.

  As described above, the ultrasonic wave propagating from the environmental fluid is taken into the propagation medium part 2, and the refractive index change caused by the propagation of the ultrasonic wave in the propagation medium part 2 is detected by the Fabry-Perot interferometer. It can be detected with light.

(Embodiment 2)
The configuration of the optical microphone in this embodiment will be described. FIG. 16 is a sectional view showing the difference between the optical microphone according to the present embodiment and the optical microphone according to the first embodiment. The difference between the optical microphone according to the present embodiment and the optical microphone according to the first embodiment is that the shape of the waveguide 10 connected from the opening 1 and the shape of the propagation medium portion 2 constituting a part of the waveguide 10 are different. is there.

  FIG. 17 shows the shapes of the propagation medium section 2 and the waveguide 10 in the optical microphone of this embodiment. In FIG. 17, a convergence point F for converging ultrasonic waves is set inside the propagation medium section 2.

When an arbitrary point on the side surface where the propagation medium part 2 and the waveguide 10 are in contact is _defined as a point P _k , a distance _Lak from the opening 1 to the point _Pk and a distance L from the point _Pk to the convergence point F _nk is designed to be a curved surface in which L _ak / C _a + L _nk / C _n is constant. Here, the sound velocity C _{a of the} ultrasonic wave propagating through the waveguide 10 and the sound velocity C _{n of the} ultrasonic wave propagating through the propagation medium portion 2 are C _n <C _a .

Part of the ultrasonic wave incident from the opening 1 and propagating through the waveguide 10 is on the side surface (P ₀ , P ₁ ,... P _n ...) Where the propagation medium portion 2 and the waveguide 10 are in contact. The light propagates in the propagation medium portion 2 while being refracted and propagated. When the side surface where the waveguide 10 and the propagation medium portion 2 are in contact with each other is designed so as to satisfy the above equation, any point on the side surface of the propagation medium portion 2 (P ₀ , P ₁ ,... P _n. The time required for the ultrasonic wave transmitted into the propagation medium portion 2 from (.) To reach the focal point 11 becomes equal. As a result, the ultrasonic wave propagating from the waveguide 10 into the propagation medium unit 2 can be focused on the focusing point 11, and a large sound pressure can be obtained at the focusing point 11.

  Note that the width of the waveguide 10 may be constant, or may be designed to become narrower as the ultrasonic wave propagates. Further, the terminal end 13 of the waveguide 10 may be closed or opened.

At the convergence point F, a detection unit that detects ultrasonic waves is arranged. For example, by arranging the photoelectric conversion unit 5 or the like at the convergence point F,
Ultrasonic waves taken into the propagation medium unit 2 are detected using a Fabry-Perot interferometer as in the first embodiment. At that time, if the position where the light wave 8 is incident is set to the focusing point 11, the ultrasonic wave can be detected with high sensitivity.

(Embodiment 3)
The configuration of the optical microphone in this embodiment will be described. FIG. 18 is a perspective view of the optical microphone according to the present embodiment. The optical microphone according to the present embodiment takes ultrasonic waves input from the opening 1 into the propagation medium unit 2. At this time, the propagation medium portion 2 is directly connected to the opening 1 without using the waveguide 10. In the present embodiment, the refractive propagation phenomenon is not used.

The density ρ _{1 of} the propagation medium part 2, the sound speed C ₁ in the propagation medium part 2, the density ρ _{2 of} the environmental fluid, and the sound speed C ₂ in the environmental fluid are (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) < If the material satisfies the relationship 1, the acoustic impedance is small, and the difference from the acoustic impedance of air, which is an environmental fluid, is much smaller than that of a normal solid. Can be captured inside.

The ultrasonic waves taken into the propagation medium unit 2 as described above are detected using a Fabry-Perot interferometer as in the first embodiment.
Example 1
An example of implementation of the optical microphone according to the first embodiment of the present invention will be described below. 19 and 20 are cross-sectional views of the prototype optical microphone.

The horn 9 was produced by machining an aluminum plate. The length of the _horn is L _horn = 100 mm, the size of the horn opening is the width H _in1 = 40 mm, the height H _in2 = 50 mm, and the size of the aperture of the converging portion of the horn is the width H _out1 = 4 mm, the height H _out2 = 5 mm.

The holding part 7 was produced by machining an acrylic plate. As for the size of the holding portion 7, the size of the outer shape was set to L _a1 = 30 mm, L _a2 = 35 mm, and L _a3 = 15 mm, respectively. In addition, the thicknesses t ₁ , t _2, and t ₃ of the holding portion 7 that holds the upper surface, the lower surface, and the side surface of the propagation medium portion 2 are all 5 mm. The width of the waveguide 10 was L _w = 4 mm.

Both the partial reflection members 3a and 3b were the same, and the size was 15 mm for both the vertical L _m1 and the horizontal L _m2 , and the thickness L _m3 was 3 mm. Further, when arranging the partial reflection members 3a and 3b, the partial reflection members 3a and 3b were inserted and fixed in through holes formed in the support portion 7 in advance according to the size of the partial reflection members 3a and 3b.

Further, the propagation medium portion 2 has a length L _n1 and a width L _n2 of 20 mm, and a thickness L _n3 of 5 mm. From the above, the size of the opening 1 is 4 mm in width and 5 mm in height, and this is connected to the converging end 9 b of the horn 9.

As a material of the propagation medium part 2, a silica dry gel having a density of 150 kg / m ³ and a sound velocity of 90 m / sec was used. The silica dry gel can be prepared, for example, by the following method.

  First, a solution in which tetraethoxysilane (hereinafter abbreviated as TEOS), ethanol and aqueous ammonia is mixed, and this is gelled to prepare a wet gel. A wet gel refers to a dry gel in which the pores are filled with a liquid. A silica dry gel is obtained by replacing the liquid portion of the wet gel with liquefied carbon dioxide and removing it by supercritical drying using carbon dioxide. The density of the silica dry gel can be adjusted by changing the mixing ratio of TEOS, ethanol and ammonia water, and the speed of sound changes according to the density.

  Silica dry gel is a material having a fine porous structure of silicon oxide, and the skeleton is hydrophobized. The size of the vacancies and the skeleton is about several nm. When the solvent is directly dried from the state in which the liquid is contained in the pore portion of such a structure, a large force due to capillary action acts when the solvent volatilizes, and the structure of the skeleton portion is easily broken. By using a supercritical drying method in which the surface tension does not work to prevent this breakage, a dry gel body can be obtained without breaking the silica skeleton.

  As the light source 4, a He—Ne laser light source having a wavelength of 633 nm was used. The spot diameter is about 0.6 mm. When the sound velocity of the propagation medium part 2 is 90 m / sec and the detected ultrasonic wave is 40 kHz, the wavelength Λ of the ultrasonic wave in the propagation medium part 2 is 2.25 mm, which is sufficiently smaller than this.

  As the partial reflection members 3a and 3b, a partial reflection mirror made of a dielectric multilayer film having a reflectance of 56% at a wavelength of 633 nm was used, and the reflection surface was arranged to face the propagation medium portion 2. In addition, the non-reflective coating was given to the surface which is not a reflective surface of a partial reflection mirror.

  As the photoelectric conversion unit 5, a Si photodiode having good light receiving sensitivity at a wavelength of 633 nm was used.

  The Fabry-Perot interferometer is configured by arranging the light source 4, the propagation medium unit 2, the partial reflection members 3 a and 3 b, the support unit 7, and the photoelectric conversion unit 5, and outputs the photoelectric conversion unit 5 to the electric signal detection unit 6 input. In the present embodiment, an oscilloscope is used as the electrical signal detector 6.

  Subsequently, ultrasonic waves were output to the air from the tweeter, and a receiving experiment was performed with the prototype optical microphone.

  From the tweeter, one sine wave signal of 40 kHz is output as an ultrasonic signal having a sound pressure amplitude of 13.8 Pa. The oscilloscope was observed after removing the DC component of the signal using a high-pass filter. The measurement results are shown in FIG. Thereby, the detection of ultrasonic waves can be confirmed.

  Moreover, the same signal was input while changing the sound pressure, and the change of the output signal was observed. The results are shown in FIG. From this, it can be confirmed that the output changes in proportion to the sound pressure. As described above, the operation of the optical microphone according to the present invention was confirmed.

  Since the ultrasonic wave taken into the propagation medium part 2 is detected by the Fabry-Perot interferometer without using the diaphragm, there is no frequency limitation due to the resonance of the diaphragm. Therefore, a broadband ultrasonic receiver can be provided.

  In addition, the optical microphone according to the present invention can produce the aperture 1 in the order of millimeters, and can provide an ultrasonic receiver with a small aperture.

  In addition, when a laser Doppler vibrometer is used as in Patent Document 1, the apparatus becomes large because an acousto-optic modulation element, a calculation unit, and the like need to be incorporated in the laser Doppler vibration system. The microphone can also be provided in a small size since the optical microphone according to the present invention can be constituted by the partial reflection members 3a and 3b, the propagation medium unit 2, the light source 4, the holding unit 7, the photoelectric conversion unit 5, and the electric signal detection unit 6. Can do.

  According to the optical microphone of the present invention, ultrasonic waves can be received in a wide frequency band by detecting ultrasonic waves without using a diaphragm.

DESCRIPTION OF SYMBOLS 1 Aperture 2 Propagation medium part 3a, 3b Partial reflection member 4 Light source 5 Photoelectric conversion part 6 Electric signal detection part 7 Holding part 8 Light wave 9 Horn 9a Horn opening end 9b Horn focusing end 10 Waveguide 11 Focusing point 12 Beam splitter 13 Termination 14 Ultrasonic

Claims (18)

An ultrasonic receiver for receiving an ultrasonic wave propagating through a surrounding space filled with an environmental fluid,
A waveguide through which the ultrasonic wave is incident, and a waveguide through which the ultrasonic wave incident on the opening propagates;
A propagation medium part in contact with a plane parallel to the propagation direction of the ultrasonic wave in the waveguide, and a part of the ultrasonic wave propagating through the waveguide is incident;
A light source that emits a light wave that crosses the propagation medium part from a direction different from the propagation direction of the ultrasonic wave;
A partially reflecting member that is disposed at both ends of the propagation medium portion in the propagation direction of the light wave across the propagation medium portion, transmits a part of the light wave, and reflects the remaining part of the light wave;
A photoelectric conversion unit that converts the light wave transmitted through the partial reflection member and the propagation medium unit into an electric signal;
The propagation medium portion, the light source, the partially reflecting member, and the light source constitute a Fabry-Perot interferometer,
The density ρ _{1 of} the propagation medium part, the sound velocity C ₁ in the propagation medium part, the density ρ _{2 of} the environmental fluid, and the sound speed C ₂ in the environmental fluid are (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ). <Ultrasonic wave receiver satisfying the relationship of 1. The ultrasonic wave receiver according to claim 1, wherein the propagation medium portion is formed of a dry gel of an inorganic oxide or an organic polymer. The ultrasonic receiver according to claim 2, wherein the solid skeleton portion of the dry gel is hydrophobized. The ultrasonic receiver according to claim 2, wherein the density of the dried gel is 50 kg / m ³ or more. The ultrasonic receiver according to claim 1, wherein the wavelength of the light wave emitted from the light source is 400 nm or more. The ultrasonic receiver according to claim 1, wherein a spot diameter of the light wave emitted from the light source is smaller than a wavelength of the ultrasonic wave propagating through the propagation medium part. 2. The ultrasonic receiver according to claim 1, wherein a change in refractive index due to propagation of the ultrasonic wave does not have a distribution in a propagation direction of the light wave from the light source in the propagation medium portion. The ultrasonic receiver according to claim 1, wherein the environmental fluid is air. The ultrasonic wave receiver according to claim 1, wherein the propagation medium portion constitutes a part of the waveguide. The length of the waveguide and the propagation medium portion are in contact with the waveguide to arbitrary point P which is set at a position along said propagation direction of the ultrasonic wave on the surface and L _a, the propagation medium from the waveguide When the length from a predetermined convergence point where the ultrasonic wave incident on the part converges to the point P is L _n ,
An ultrasonic receiver in which a surface where the waveguide and the propagation medium portion are in contact with each other is formed so that L _a / C _a + L _n / C _n is constant regardless of the position of the point P. The ultrasonic wave receiver according to claim 1, wherein a size of the waveguide decreases from the opening toward a terminal end of the waveguide. The ultrasonic receiver according to claim 10, wherein a terminal end of the waveguide is closed. The ultrasonic receiver according to claim 10, wherein a terminal end of the waveguide is open. The ultrasonic receiver according to any one of claims 1 to 13, wherein an acoustic horn is connected to the opening of the waveguide. Furthermore, a holding unit that holds the waveguide and the propagation medium unit is provided,
The ultrasonic receiver according to claim 1, wherein the light wave emitted from the light source does not propagate through the holding unit. The ultrasonic receiver according to claim 15, wherein at least a part of the holding portion is configured by the partial reflection member. The ultrasonic receiver according to any one of claims 1 to 16, wherein light transmitted through the Fabry-Perot interferometer is received by the photoelectric conversion unit. The ultrasonic receiver according to any one of claims 1 to 16, wherein light reflected by the Fabry-Perot interferometer is received by the photoelectric conversion unit.

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