JP2011211311A - Optical ultrasonic microphone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical ultrasonic microphone that can measure sound pressure over a wide band up to an ultrasonic region, and has a small size, high sensitivity, and resistance to a change in environment.SOLUTION: The optical ultrasonic microphone includes an acoustic waveguide 60 for receiving sound waves from an opening 71 for propagation, a photoacoustic propagation medium 52 forming at least one portion of a wall surface of the acoustic waveguide 60 and having a sound collecting action, and a laser Doppler vibration meter head 8. The sound waves advancing in the acoustic waveguide 60 are received in the photoacoustic propagation medium 52 for collecting sound in an acoustic focus 57 to generate a change in a refractive index with high efficiency, and the LDV head 8 measures it as an optical modulation signal. When achieving further resistance to environment and a measurement dynamic range, the LDV is replaced with a Mach-Zehnder interferometer and an optical heterodyne interference method.

Description

  The present invention relates to a microphone using light. The present invention relates to an optical ultrasonic microphone that receives an ultrasonic wave propagating in a gas such as air and converts the ultrasonic wave received using light into an electric signal.

As devices for collecting sound waves and converting them into electric signals, dynamic microphones or condenser microphones are widely used in the audible band. In the ultrasonic region, piezoelectric sensors are widely used. These devices make use of the fact that the main component of the energy of sound waves propagates as a dense wave of air, and the minute vibrations excited on the diaphragm by making the sound waves incident on the diaphragm are conductive and electrostatic. Or, it is converted into an electrical signal piezoelectrically.

  In addition, optical systems such as a laser Doppler vibrometer (hereinafter referred to as LDV) that measures fine and high-speed vibration using monochromatic light typified by laser light are widely used. Attempts have been made to collect sound waves.

  Patent Document 1 discloses an optical microphone that uses a diaphragm found in a normal microphone and optical measurement using an optical trigonometry.

  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.

  With reference to FIG. 20, a laser Doppler microphone in Patent Document 2 will be described. The laser Doppler microphone shown in FIG. 20 includes an LDV 121, a pair of reflecting mirrors 122 and 123, a cubic mirror 124, a sound field 126, and a calculation unit 127. The reflecting mirror 122 and the reflecting mirror 123 are arranged in parallel with the sound field 126 interposed therebetween. The LDV 121 and the cubic mirror 124 are provided at both ends of a surface of the reflecting mirror 123 that is orthogonal to the relief facing the reflecting mirror 122.

  The LDV 121 emits laser light at an angle other than 90 degrees toward the reflecting mirror 122. The emitted laser light is reflected by the reflecting mirror 122 and the reflecting mirror 123 a plurality of times, propagates along the laser light path 125, and reaches the cubic mirror 124 provided at the end of the reflecting mirror 123. The laser beam that has reached the LDV 121 is measured by the LDV 121 and the calculation unit 127 for the frequency change of the returned laser beam and the amplitude and phase for each frequency change amount.

  In FIG. 20, when there is no sound wave in the sound field 126, the laser beam is not subjected to Doppler modulation, and therefore the amount of change in the frequency of the laser beam is zero. However, when sound waves are present in the sound field 126, temporal fluctuations occur in the air density of the sound field 126. This temporal fluctuation induces a time variation of the air refractive index. This is equivalent to the temporal expansion and contraction of the optical length of the laser beam path 125, which is the path length converted into the wavelength of the laser beam. Optically, it is as if the cubic mirror 124 has a temporal fluctuation (time) with respect to the traveling direction of the laser beam. This is considered to be a situation where exercise equal to (elongation and contraction) is performed.

  Therefore, since the laser light is Doppler modulated, the Doppler frequency and the amplitude and phase at each Doppler frequency are measured. The time variation of the air refractive index in the sound field 126 is reproduced by Fourier transforming them into a real time signal. The air refractive index at an arbitrary time is calculated by integrating it. As a result, the sound wave in the sound field 126 (the average value of the sound pressure on the laser light path 125) is measured.

  Inventors of the present application disclose an invention relating to an ultrasonic transducer for a gas capable of transmitting and receiving ultrasonic waves in a gas with high sensitivity and broadband using the refraction of ultrasonic waves in gas. Non-Patent Document 1 reports transmission / reception characteristics in an ultrahigh frequency region of 500 kHz.

FIG. 21 shows an ultrasonic transducer according to the invention disclosed in Patent Document 3 and Patent Document 4.
As shown in FIG. 21, the ultrasonic transducer 101 of the invention of Patent Document 3 is provided at least on the ultrasonic transducer 102 and the front surface of the ultrasonic transducer 102, and includes the environmental fluid 104 and the ultrasonic transducer 102. And a propagation medium portion 103 that fills the gap. Here, “environmental fluid” refers to a fluid existing outside the ultrasonic wave receiver 101. The environmental fluid is, for example, air.
Further, the interface between the ultrasonic transducer 102 and the propagation medium part 103 is defined as a first surface area 111, and the interface between the propagation medium part 103 and the environmental fluid 104 is defined as a second surface area 112.

  The ultrasonic receiver 101 is surrounded by an environmental fluid 104. The ultrasonic wave receiver 101 receives the ultrasonic wave propagating in the ultrasonic wave propagation path 105 from the environmental fluid 104 toward the ultrasonic wave receiver 101.

  The ultrasonic transducer 101 of Patent Document 3 transmits and receives ultrasonic waves with high sensitivity by taking ultrasonic waves into a propagation medium portion of a propagation medium with high efficiency from a medium with extremely low acoustic impedance such as air. It is possible.

  Usually, most ultrasonic waves are reflected at the interface of a medium having greatly different acoustic impedances, such as an interface between a gas and a solid, so that it is difficult to transmit with high efficiency and receive with high sensitivity.

In order to transmit the ultrasonic wave propagating in the gas with high efficiency and receive the wave with high sensitivity, the ultrasonic wave transmitter / receiver 101 has a sound velocity propagating through the inside thereof lower than that of the environmental fluid 104, and has a density. Is a dry gel material composed of a silica skeleton larger than the environmental fluid 104 (hereinafter referred to as “silica dry gel”). Silica dry gel is a material that can have various sound speeds and densities depending on the manufacturing process. For example, the conditions of the propagation medium part 103 that can transmit ultrasonic waves with high efficiency such as a density of 200 kg / m ³ and a sound speed of 150 m / s are satisfied.

  The propagation medium part 103 is made of silica dry gel, and as shown in FIG. 21, the angle θ1 formed between the normal line of the second surface region 112 and the ultrasonic wave propagation direction inside the propagation medium part 103, and the environment fluid 104 By appropriately selecting the angle θ2 formed with the ultrasonic wave propagation direction in the case, the reflection of ultrasonic waves in the second surface region 112 can be made substantially zero, and the transmission / reception sensitivity of the ultrasonic transducer can be improved. . Also, since the transmission efficiency in the second surface region 112 is independent of the frequency of the propagating ultrasonic wave, it is possible in principle to realize wideband characteristics and measure various frequencies with high efficiency.

  Specifically, an ultrasonic wave is generated by giving an electric signal to the ultrasonic transducer 102 from a drive circuit (not shown). Here, the XYZ directions are set as shown in FIG. The ultrasonic wave generated by the ultrasonic transducer 102 propagates in the positive direction of the Y axis from the first surface region 111 toward the second surface region 112 in the positive direction of the Y axis. Then, the ultrasonic wave reaching the second surface region 112 changes the propagation direction according to the law of refraction, and propagates toward the fluid 104 in the direction of the ultrasonic propagation path 105 (in this case, the direction opposite to the arrow). To go.

  In the case of receiving the ultrasonic wave, contrary to the case of the wave transmission, the ultrasonic wave propagating through the fluid 104 in the surrounding space is refracted when it reaches the second surface region 112 and is transmitted to the propagation medium part 103. It propagates in the propagation medium portion 103 toward the negative direction of the axis and reaches the ultrasonic transducer 102. The ultrasonic wave that has reached the ultrasonic transducer 102 is detected by a wave receiving circuit (not shown) because the ultrasonic transducer 102 is deformed to generate a potential difference between the electrodes.

  In the ultrasonic transducer 101, even when the environmental fluid 104 is a medium having a very small acoustic impedance (sound speed of material × material density) such as air, ultrasonic waves are incident on the propagation medium unit 103 with high efficiency from the environmental fluid 104. Alternatively, ultrasonic waves can be emitted from the propagation medium unit 103 to the environmental fluid 104 with high efficiency.

In the ultrasonic transducer 101, in order to increase the transmission efficiency of ultrasonic waves, (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) <1 is set. Here, the acoustic velocity C1 of the propagation medium portion 103 of the ultrasonic sound velocity C2 in the ultrasound environment fluid 104, the density of the propagation medium portion 103 .rho.1, showing the density [rho ₂ of environmental fluid 104.

In addition, θ ₁ uses (tan θ ₁ ) ² = {(ρ ₂ / ρ ₁ ) ² − (C ₁ / C ₂ ) ² } / {(C ₁ using C ₁ , C ₂ , ρ ₁ , ρ _2. / C ₂ ) ² -1}
It is set to satisfy.

Also, θ ₁ and θ ₂ are set so as to satisfy (sin θ ₁ / C ₁ ) = (sin θ ₂ / C ₂ ).

  As shown in Patent Document 4, when the above equation is satisfied, the transmission efficiency of ultrasonic waves in the second surface region 112 is approximately 1. Therefore, it is possible to provide the ultrasonic transducer 101 that can transmit and receive ultrasonic waves with high efficiency.

JP 2004-12421 A JP 2004-279259 A International Publication No. 2004/098234 US Patent Application Publication No. 2005/0139013

“Acoustic Properties of Nanofoam Materials and Application to Ultrasonic Sensors (General / Acoustic Imaging)” (Acoustic Properties of Nanofoam Applied Applied Ultrasonic Sensors), Masahiko Hashimoto, Hidetoshi Nagahara, Takeshi Sugiuchi Publication, IEICE technical report, Vol. 105, no. 619, US2005-127 (P.29-34)

  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. However, since the sensitivity rapidly decreases above the resonance frequency, the upper limit of the measurable frequency band as a microphone is limited to the vicinity of the resonance frequency. For this reason, the characteristic guaranteed upper limit frequency of the current condenser microphone using a diaphragm is up to about 100 kHz, and a piezoelectric type is used in the case of more than that. Therefore, it is extremely difficult to construct a microphone having a diaphragm whose band characteristics extend to 100 kHz or more while maintaining sufficient sensitivity.

  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, since the refractive index change 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, an optical path length of 10 m or more is necessary to obtain sufficient S / N. Therefore, it is very difficult to reduce the size of the measurement area. As a result, in the high frequency region, sound wave interference easily occurs in the measurement region, making it difficult to accurately measure the sound pressure. This phenomenon corresponds to mechanical resonance in the case of the diaphragm type, and is called “cavity resonance”.

  That is, the dimension of the measurement range determines the high-frequency limit of the microphone, and since the sound speed of air is slower than the elastic wave velocity of a general diaphragm, if the measurement area and the diaphragm are the same area, the high-frequency limit is The laser Doppler microphone is lower. As described above, the optical microphone of the conventional configuration has a sufficiently wide bandwidth for optical measurement, but the high frequency band is limited by the mechanical resonance and cavity resonance to be used, and it is difficult to operate in an ultrahigh frequency region of 100 kHz or more. It has a problem of becoming.

  Furthermore, the point from which the optical path length of 10 m or more is required produces the subject different from the above. In general, air in free space has spatial and temporal non-uniformities in refractive index distribution due to fluctuations due to temperature and flow. Therefore, when using an optical interferometer such as an LDV to capture sound waves as refractive index fluctuations on the optical path, large spatial and temporal non-uniformities in the refractive index distribution due to the long optical path length are mixed into the refractive index fluctuations due to only the sound waves To do. Therefore, there is also a problem that a signal to be originally excluded is mixed into the measured sound wave signal and correct sound wave measurement becomes difficult.

  While adopting the configuration of Patent Document 2, it is configured with a mechanically strong structure, the optical path is taken in a rare gas atmosphere rich in fluidity such as vacuum and helium, and high alignment accuracy is realized. Although it is possible in principle to solve this problem, the prescription induces another problem that the apparatus as a whole causes a considerable price increase.

  Further, in the ultrasonic transducer 101 of Patent Document 3, since there is no frequency characteristic in the sound wave capturing into the propagation medium unit 103 composed of silica dry gel, it is possible to capture sound waves in a wide frequency range. However, an ultrasonic transducer 102 such as a piezoelectric ceramic is necessary to convert the captured sound wave into an electrical signal.

  When the propagation medium part 103 is made of silica dry gel and the ultrasonic vibrator 102 is made of piezoelectric ceramic, the acoustic impedance value varies by two to three digits. Most of the sound waves that enter the child 102 are reflected at the interface (first surface region 111) between the propagation medium portion 103 and the ultrasonic transducer 102. The reflected sound wave propagates in the propagation medium part 103 in the opposite direction and a part is released to the air, but the rest propagates while being repeatedly reflected at the boundary (second surface region 112) in the propagation medium part 103. It propagates in the medium part 103 and becomes reverberation.

  The reflection phenomenon at the interface (first surface region 111) between the propagation medium unit 103 and the ultrasonic transducer 102 is caused by piezoelectricity as an element for conversion into an electric signal in the propagation medium unit 103 made of silica dry gel. In a configuration in which materials having different acoustic impedances such as ceramics are arranged, it is essentially inevitable. The reverberation accompanying this reflection is superimposed on a sound wave signal that arrives later and causes a decrease in S / N, and there is a problem of deteriorating frequency characteristics such as an unnecessary resonance phenomenon.

  Furthermore, the ultrasonic transducer 101 of Patent Document 3 has a problem that the reception sensitivity is low. Hereinafter, the cause of this problem will be described.

The energy density of the ultrasonic wave that has propagated through the environmental fluid 104 decreases when it is received by the ultrasonic transducer 101. This is the cause of low reception sensitivity. The reason why the reception sensitivity is low will be described with reference to FIG. In FIG. 21, the ultrasonic propagation path 105 is indicated by a solid arrow. As described above, in order for the ultrasonic transducer 101 to receive ultrasonic waves with high efficiency, (ρ ₂ / ρ ₁ ) <(C ₁ / C ₂ ) <1, (tan θ ₁ ) ² = { (Ρ ₂ / ρ ₁ ) ² − (C ₁ / C ₂ ) ² } / {(C ₁ / C ₂ ) ² −1}, (sin θ ₁ / C ₁ ) = (sin θ ₂ / C ₂ ) must be satisfied There is. At this time, in the path of the ultrasonic wave propagating through the environmental fluid 104, the angle formed with the normal line of the second surface region 112 satisfies θ2.

  Accordingly, in FIG. 21, the ultrasonic wave propagates in the range of the length of the fluid 104 (L2 + L3 + L4) toward the ultrasonic transducer 101, and further, the ultrasonic wave propagation path and the normal line of the second surface region 112 Assume that the angle formed satisfies θ2. Here, the range of the length L <b> 2 means a range that is parallel to the ultrasonic propagation path 105 and that does not reach the second surface region 112 at all. The range of the length L3 is a range adjacent to the range of the length L2 and parallel to the ultrasonic wave propagation path 105, and means a range in which the ultrasonic wave can reach all of the second surface region 112. . The range of the length L4 means a range that is adjacent to the range of the length L3 and parallel to the ultrasonic wave propagation path 105 and that the ultrasonic waves do not reach the second surface region 112 at all.

  As shown in FIG. 21, not all the ultrasonic waves propagating in the range of the length (L2 + L3 + L4) of the environmental fluid 104 are received by the ultrasonic transducer 101. The ultrasonic wave propagating through the length L3 reaches the second surface region 112 and is received by the ultrasonic transducer 101. However, the ultrasonic wave propagating through the range of the length L2 and the range of the length L4 cannot reach the second surface region 112 and is not received by the ultrasonic transducer 101.

  That is, a part of the ultrasonic waves (ultrasonic waves propagating through the range of length L3) out of the ultrasonic waves propagating through the environmental fluid 104 (ultrasonic waves propagating through the range of length L2 + L3 + L4) It is received by the sonic transducer 101.

  Then, the ultrasonic wave that has propagated through the range of the length L3 of the environmental fluid 104 passes through the second surface region 112 and is detected by the ultrasonic transducer 102 within the range of the length L1. At this time, since L3 << L1, the ultrasonic wave received by the ultrasonic transducer 101 is diffused by the second surface region 112 and reaches the ultrasonic transducer 102. Therefore, when the ultrasonic wave is received by the ultrasonic transducer 101, the energy density is lowered. Due to the reduction of the energy density of the ultrasonic wave, the wave receiving sensitivity of the ultrasonic transducer 101 is lowered.

  For the above reasons, the wave receiving sensitivity of the ultrasonic transducer 101 is low. That is, since the length L3 of the ultrasonic propagation range that can be received by the second surface region 112 is smaller than the length L1 of the ultrasonic transducer 102, the reception sensitivity of the ultrasonic receiver 101 is low. It has become a thing.

  The object of the present invention has been made in view of the above-mentioned problems, and is capable of measuring sound pressure up to an ultrasonic region that greatly exceeds the high frequency limit of a conventional microphone, and has high resistance and high sensitivity to environmental fluctuations. The present invention provides an optical ultrasonic microphone that achieves both high-efficiency measurement.

The microphone of the present invention is a microphone that receives a sound wave propagating through a surrounding space filled with an environmental fluid, and the first opening through which the sound wave enters and the first opening. A photoacoustic propagation medium provided in the waveguide so as to have an acoustic waveguide through which sound waves propagate and a transmission surface, and the transmission surface forms one surface of the acoustic waveguide along the propagation direction of the ultrasonic waves A part of the ultrasonic wave is transmitted from the transmission surface to the propagation medium part as it propagates through the waveguide, and the transmission surface is configured to converge to a predetermined convergence point, A photoacoustic propagation medium portion disposed with respect to the waveguide, and a light wave that radiates toward the convergence point, and a light wave that is reflected by the radiated light wave propagating through the photoacoustic propagation medium portion is received and received. The sound pressure of the sound wave And a Mel detector, the density [rho _n of the optical acoustic propagation medium portion, the sound velocity C _n in the optical acoustic propagation medium portion, the density [rho _a gas filling the acoustic waveguide, and the gas filling the acoustic waveguide The sound velocity C _a satisfies the relationship (ρ _a / ρ _n ) <(C _n / C _a ) <1, and the propagation direction of the ultrasonic wave on the transmission surface from the first opening of the waveguide the length of the waveguide to the point P is set at an arbitrary position and L _a along, when the length from the point P to the convergence point was L _n, regardless of the position of the point P, L _a / C _a + L _n / C _n is constant.

  According to the optical ultrasonic microphone of the present invention, the sound wave propagating through the gas in the surrounding space is taken into the acoustic waveguide from the opening, and the sound wave traveling in the photoacoustic propagation medium portion from the acoustic waveguide is converted into the Mach-Zehnder. By measuring with an optical interferometer, it is possible to measure up to a high frequency region that greatly exceeds the limits due to mechanical resonance of conventional diaphragms, and the influence of sound wave reflection by conventional electroacoustic transducers such as piezoelectric ceramics In addition, it is possible to provide an optical ultrasonic microphone capable of measuring sound pressure with higher sensitivity and accuracy by realizing small size and high stability against fluctuations in the surrounding environment.

The perspective view which showed schematic structure of the optical ultrasonic microphone 51 of 1st Embodiment. 1 is a cross-sectional view of a part of the optical ultrasonic microphone 51 shown in FIG. 1 cut along a plane parallel to the YZ plane at the center of the convergence portion 77 and the acoustic waveguide member 56 in the X direction. The perspective view which shows a part of base of the optical ultrasonic microphone shown in FIG. 1, and a part of acoustic waveguide member Schematic diagram for explaining the propagation and refraction of sound waves in the optical ultrasonic microphone shown in FIG. The schematic diagram for demonstrating the sound wave convergence of the optical ultrasonic microphone in 1st Embodiment of this invention Sound pressure distribution diagram showing the results of a calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. Sound pressure distribution diagram showing the results of a calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. Sound pressure distribution diagram showing the results of a calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. Sound pressure distribution diagram showing the results of a calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. Sound pressure distribution diagram showing the results of a calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. Sound pressure distribution diagram showing the results of a calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. The graph which shows the time waveform of the input sound wave signal used for the experiment shown in FIGS. Sectional drawing which shows schematic apparatus structure of the optical ultrasonic microphone 51A concerning the modification of 1st Embodiment. Contour map showing measurement result of isophase surface of sound wave propagation of photoacoustic propagation medium part of optical ultrasonic microphone in first embodiment of the present invention Time waveform diagram showing an example of the LDV output waveform (amplitude measurement waveform) 81 of the optical ultrasonic microphone 51 in the vicinity of the acoustic acoustic focus 57 The block diagram which showed the optical system structure of the optical sound pressure measurement part 1300 in 2nd Embodiment of the optical ultrasonic microphone of this invention. The block diagram of the apparatus which showed the experimental apparatus for proof of principle of the optical ultrasonic microphone in 2nd Embodiment of this invention The figure which showed the time waveform of the output signal from the digital oscilloscope 1408, and the time waveform of the input signal to the speaker 1410 measured by the experimental apparatus for proof of principle of the optical ultrasonic microphone of FIG. The block diagram which showed the optical system structure of the optical ultrasonic microphone 1300 with a wide measurement dynamic range and no signal distortion in 3rd Embodiment of this invention. Sectional drawing of the conventional ultrasonic transducer in patent document 2 Configuration diagram of conventional optical microphone in Patent Document 3

The inventors of the present application pay attention to the fact that the silica dry gel has a property close to optical transparency,
For example, it has been found that the rate of change in sound pressure and refractive index in silica dry gel is about an order of magnitude higher than the rate of change in air.

  The rate of change in refractive index due to sound pressure usually increases in the order of solid, liquid, and gas, and is a very unique property that is not found in normal bulk materials.

  The present invention utilizes a basic principle of an interface phenomenon of an ultrasonic receiver capable of propagating ultrasonic waves with high efficiency from an object with extremely low acoustic impedance such as gas to a solid, and a solid satisfying these conditions. The material is that an optical ultrasonic microphone whose band characteristics extend to an extremely high frequency region is formed by using a phenomenon that a very large refractive index change is generated by sound waves.

  In addition, the present invention uses a Mach-Zehnder interferometer to configure the entire optical system with a path length as short as possible, and to change the refractive index fluctuation generated in the photoacoustic propagation medium by the sound pressure as the optical path length fluctuation. This has the advantage of being able to measure directly from the amplitude fluctuation of the interference light or the phase fluctuation of the interference light.

  The optical ultrasonic microphone of the present invention will be described below with reference to the drawings.

(First embodiment)
FIG. 1 shows an optical ultrasonic microphone 51 of the first embodiment. The optical microphone 51 of the first embodiment includes an opening 71 of an acoustic horn that receives an ultrasonic wave that propagates through an environmental fluid 14, an acoustic waveguide 60 through which an ultrasonic wave incident from the opening 71 propagates, and an acoustic waveguide. An LDV head 8 that emits laser light 58 toward a photoacoustic propagation medium portion 52 in which at least a part of ultrasonic waves propagating through 60 propagates, and an acoustic focus 57 in which the ultrasonic waves propagated through the photoacoustic propagation medium portion 52 converge. And an LDV arithmetic processing unit 9 that detects ultrasonic waves from the reflected waves of the emitted laser beam 58.

  An outline of the operation of the optical microphone 51 will be described. Ultrasound enters the inside of the optical microphone 51 along the sound wave propagation direction 55 from the environmental fluid 14 existing outside the optical microphone 51. The ultrasonic wave incident on the inside of the optical microphone 51 propagates through the acoustic waveguide 60 and the photoacoustic propagation medium unit 52 and converges to the acoustic focal point 57. The LDV head 8 emits a laser beam 58 toward the acoustic focal point 57. The LDV head 8 receives the reflected wave of the emitted laser beam 58. The LDV arithmetic processing unit 9 obtains a change in the refractive index of the photoacoustic propagation medium unit 52 due to the ultrasonic wave converged on the acoustic focus 57 from the reflected wave received by the LDV head 8. The optical microphone 51 can detect ultrasonic waves (for example, sound pressure of ultrasonic waves) corresponding to the amount of change in the refractive index.

  Hereinafter, each part of the optical ultrasonic microphone 51 of FIG. 1 will be described in detail.

(Convergence unit 77)
FIG. 2 is a cross-sectional view of a part of the optical ultrasonic microphone 51 shown in FIG. 1 cut along a plane parallel to the YZ plane at the center of the converging portion 77 and the acoustic waveguide member 56 in the X direction.

  The converging unit 77 includes a first opening 71 into which ultrasonic waves propagating from the outside are incident, and a second opening 63 connected to the acoustic waveguide member 56 (acoustic waveguide 60). An inner space 70 in which ultrasonic waves propagate is formed between the first opening and the second opening.

  When the ultrasonic wave propagating through the environmental fluid 14 in the sound wave propagation direction 55 is incident from the first opening 71 and then propagates through the internal space 70, the propagation direction is controlled and the sound pressure is increased ( Compressed). The sound wave whose sound pressure is increased by the converging unit 77 propagates to the acoustic waveguide 60 connected to the converging unit 77.

  In the internal space 70, the cross-sectional area a7 of the plane orthogonal to the propagation direction g7 gradually decreases along the propagation direction g7 in which the sound wave propagates from the first opening 71. More preferably, a converging portion 77 that defines the shape of the inner space 70 so that the cross-sectional area a7 decreases exponentially with respect to the propagation direction g7 from the first opening 71 toward the second opening 63. The inner surface is configured to have a curved surface shape along the propagation direction g7.

  The width dimension in the X direction of the converging portion 77 may be constant, or the width dimension may be gradually reduced. When the width dimension in the X direction of the converging part 77 is constant, it is preferable that the width dimension in the Z direction decreases exponentially with respect to the propagation direction g7. Further, the cross-sectional area a7 may be decreased exponentially by decreasing the width dimension in the X direction and the width dimension in the Z direction of the converging portion 77 in proportion to √e with respect to the propagation direction g7. As the cross-sectional area a7 decreases exponentially in this way, the reflection of the sound wave at the converging part 77 can be suppressed to the minimum, the sound wave can be compressed without the disturbance of the phase, and the sound pressure can be increased.

  For example, the converging portion 77 has a length of 100 mm in the Y direction, and the first opening 71 has a square shape having a length of 50 mm in each of the Z direction and the X direction. The end portion 72 has a square shape with a length of 2 mm in the X direction and the Z direction. In the first embodiment, the length is changed in two directions, the Z direction and the X direction. When the position of the opening 71 is the origin (0) in the Y direction, the position in the Y direction from the origin = 0 mm / 20 mm / 40 mm / 60 mm / 80 mm / 100 mm in the X direction and the Z direction of the inner space 70 at each position (The length in the X direction and the length in the Z direction are the same at each position) are 50.0 mm / 26.3 mm / 13.8 mm / 7.2 mm / 3.8 mm / 2.0 mm.

  According to the converging unit 77 having the above-described size, an effect of increasing the sound pressure of the sound wave of about 10 dB can be obtained as compared with the case where the converging unit 77 is not provided. The shape of the sound pressure waveform that changes with time of the sound pressure hardly changes in the measurement results at the opening 71 and the end 72, and disturbs the sound wave that propagates in the environmental fluid 14 (for example, air). Instead, the sonic energy is compressed at the end 72.

  The converging part 77 can be configured, for example, by machining a metal aluminum plate having a thickness of 5 mm into a predetermined shape by machining. The converging portion 77 may be formed of a material other than aluminum as long as the sound wave propagating through the inner space 70 is hardly transmitted and the density of the sound wave energy can be increased by the shape effect. For example, the converging portion 77 may be configured using a material such as resin or ceramic. Moreover, the converging part 77 does not need to have a horn-shaped outer shape, and the inner space 70 only needs to have a horn shape as described above.

(Acoustic waveguide 60)
Next, the acoustic waveguide 60 that propagates sound waves in a predetermined direction will be described. The acoustic waveguide 60 is configured by an acoustic waveguide member 56. The acoustic waveguide 60 is connected to the second opening 63 of the converging unit 77. A sound wave that enters from the environmental fluid 14 and propagates through the converging portion 77 enters the acoustic waveguide 60 from a portion connected to the second opening 63.

  As shown in FIG. 2, the acoustic waveguide 60 reduces the cross-sectional area of the acoustic waveguide 60 in a plane orthogonal to the sound wave propagation direction. Here, the width dimension in the ZY plane changes with the position along the propagation direction g6 of the ultrasonic wave parallel to the ZY plane. The width dimension in the X-axis direction of the acoustic waveguide 60 is constant, for example, 2 mm. It is also possible to design the width dimension in the X-axis direction to change.

  The reason why the acoustic waveguide 60 has such a shape will be described. The acoustic waveguide 60 includes a transmission surface 61 that is in contact with the photoacoustic propagation medium portion 52 and is defined by an interface with the photoacoustic propagation medium portion 52, and a waveguide outer surface 62 that is defined by the acoustic waveguide member 56. Yes. Further, the near side and the far side in the X direction of the acoustic waveguide 60 are also defined by the acoustic waveguide member 56. As the sound wave propagates through the acoustic waveguide 60, the sound wave gradually infiltrates (propagates) from the transmission surface 61 where the photoacoustic propagation medium unit 52 and the acoustic waveguide 60 are in contact to the photoacoustic propagation medium unit 52. At this time, the propagation direction of the sound wave is refracted on the transmission surface 61. The photoacoustic propagation medium unit 52 may be provided so as to constitute a part of the acoustic waveguide 60.

  As the sound wave propagates through the acoustic waveguide 60, at least a part of the sound wave is transmitted from the transmission surface 61 to the photoacoustic propagation medium unit 52. As a result, the energy of the sound wave propagating through the acoustic waveguide 60 decreases. In order to compensate for the decrease in energy, in order to compress the sound wave (increase the sound pressure of the sound wave), the cross-sectional area of the acoustic waveguide 60 in the plane orthogonal to the propagation direction of the sound wave is reduced.

  Specifically, the acoustic waveguide 60 which is a space between the transmission surface 61 and the waveguide outer surface 62 has a shape in which the width perpendicular to the propagation direction g6 in the YZ plane monotonously decreases with respect to the propagation direction. Further, the waveguide end 64 of the acoustic waveguide 60 is closed. With this shape, the sound wave can be efficiently refracted and transmitted to the photoacoustic propagation medium unit 52 while keeping the energy density of the sound wave propagating through the acoustic waveguide 60 constant. A specific operation until the sound wave propagating through the acoustic waveguide 60 enters the photoacoustic propagation medium unit 52 will be described later.

(Photoacoustic propagation medium part 52)
Next, the photoacoustic propagation medium part 52 that defines a transmission surface 61 through which sound waves are transmitted from the acoustic waveguide 60 to the acoustic propagation medium part 52 will be described. The photoacoustic propagation medium portion 52 is made of a material having a slower propagation speed of sound waves than the environmental fluid 14. That is, when the velocity of the sound wave in the propagation medium is Cn and the sound velocity of the sound wave in the environmental fluid 14 is Ca, (C _n / C _a ) <1 is satisfied. As a material satisfying this condition, there is a dry gel of an inorganic acid compound or an organic polymer. As a dry gel of an inorganic acid compound, it is preferable to use a silica dry gel. Hereinafter, the manufacturing method of a silica dry gel is demonstrated.

  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. “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.

More preferably, the propagation medium of the photoacoustic propagation medium unit 52 has the velocity of sound waves in the propagation medium as Cn, the sound velocity of sound waves in the environmental fluid 14 as Ca, the density of the propagation medium as ρn, and the density of the environmental fluid 14 as ρa. (Ρ _n / ρ _a ) <(C _a / C _n ) <1.

More preferably, the propagation medium of the photoacoustic propagation medium unit 52 has a propagation medium density of ρn of 100 kg / m ₃ or more and a sound wave velocity in the propagation medium of Cn of 300 m / s or less.

The density ρn of the silica dry gel constituting the photoacoustic propagation medium unit 52 used in the first embodiment is 200 kg / m ³ , and the sound velocity Cn in the silica dry gel is 150 m / s. These values are materials that satisfy the refraction propagation phenomenon shown in Patent Document 1. The air density ρa is 1.12 kg / m ³ , and the sound velocity Ca is 340 m / s near room temperature.

  Further, since the photoacoustic propagation medium unit 52 plays a role of propagating the sound wave taken from the environmental fluid 14 to the acoustic focal point 57, if the internal loss is large, the acoustic wave reaching the acoustic focal point 57 is weakened. For this reason, the photoacoustic propagation medium portion 52 is preferably made of a material having a small internal loss. Silica dry gel is a material that satisfies the above-mentioned sound velocity and density conditions and has low internal loss.

(Base 53)
The base 53 that supports the photoacoustic propagation medium unit 52 will be described. Since the silica dry gel which comprises the photoacoustic propagation medium part 52 has a low density, its mechanical strength is also low. For this reason, handling is difficult. Therefore, the base 53 is provided in order to stably support the photoacoustic propagation medium portion 52.

  For ease of understanding, FIG. 1 illustrates that the photoacoustic propagation medium 52 is exposed by breaking a part of the base 53 located on the surface side of the photoacoustic propagation medium 52. Actually, the entire surface of the photoacoustic propagation medium portion 52 may be covered with the base 53 except for a measurement through hole 53a described later. When the entire surface of the photoacoustic propagation medium portion 52 is covered except for the measurement through-hole 53a, it is possible to avoid the incidence of sound waves other than the ultrasonic waves to be detected.

  As shown in FIG. 1, the surface of the photoacoustic propagation medium unit 52 (the back surface, the left side surface, and the bottom surface of the photoacoustic propagation medium unit 52 shown in FIG. 1) is covered with a base 53. Further, the right side surface of the photoacoustic propagation medium portion 52 shown in FIG. 1 is covered with an acoustic waveguide member 56 to constitute a part of the acoustic waveguide 60. As a result, the photoacoustic propagation medium portion 52 is held by the base 53 and the acoustic waveguide member 56.

  The acoustic waveguide member 56 and the base 53 can be configured by the shapes shown in FIGS. 3 (a) and 3 (b). FIG. 3A is a perspective view showing a part of the base 53 of the optical ultrasonic microphone 51 shown in FIG. FIG. 3B is a perspective view showing a part of the acoustic waveguide member of the optical ultrasonic microphone shown in FIG.

  As shown in FIG. 3A, an acoustic waveguide member 56 that defines the acoustic waveguide 60 including the waveguide outer surface 62 is formed using an aluminum member. On the other hand, as shown in FIG. 3B, a base 53 for holding the photoacoustic propagation medium section 52 is prepared. The exposed surface of the photoacoustic propagation medium unit 52 held by the base 53 defines a transmission surface 61.

  For example, the base 53 made of porous ceramics is formed. The base 53 is fitted into a mold whose surface defining the transmission surface 61 is made of a fluorine resin or the like, and a wet gel is introduced into the space. Thereafter, the liquid portion is replaced with liquefied carbon dioxide and dried to obtain a member in which the photoacoustic propagation medium portion 52 and the base 53 are integrated.

  As shown in FIG. 3B, both end portions of A and B of the base 53 holding the photoacoustic propagation medium portion 52 and both end portions of C and D of the acoustic waveguide member 56 shown in FIG. The acoustic waveguide 60 in which the transmission surface 61 is defined by the photoacoustic propagation medium portion 52 is formed by bonding with an adhesive such as an epoxy resin in correspondence with each other.

(Sound wave propagating through acoustic waveguide 60)
Next, the geometrical shape of the acoustic waveguide 60 and the photoacoustic propagation medium portion 52 defined by the acoustic waveguide member 56 and the propagation of sound waves will be described in detail.

  FIG. 4 is an enlarged view of a part of the acoustic waveguide 60 in the optical ultrasonic microphone 51. The propagation and refraction of sound waves will be described with reference to FIG.

  In FIG. 4, the transmission surface 61 and the waveguide outer surface 62 are indicated by dotted lines, and a tangential perpendicular line at an arbitrary point of the transmission surface 61 is indicated by a one-dot chain line. Further, the propagation direction of the sound wave is indicated by an arrow 55a.

  A sound wave incident from the opening 63 and traveling in the acoustic waveguide 60 propagates through the acoustic waveguide 60 while changing the traveling direction according to the shape of the acoustic waveguide 60. The inside of the acoustic waveguide 60 is filled with the environmental fluid 14.

  The component of the sound wave that contacts the transmission surface 61 that is the interface between the acoustic waveguide 60 and the photoacoustic propagation medium 52 is incident on the transmission surface 61 at an angle θa with respect to the normal of the transmission surface 61, and Snell's law is obtained. In order to be satisfied, the light is refracted and transmitted to the photoacoustic propagation medium unit 52 at a certain angle θn with the normal line of the transmission surface 61.

  The angle θn in the propagation direction of the sound wave inside the photoacoustic propagation medium portion 52 is expressed by Equation 1. Here, when the relationship of (Cn / Ca) <1 is satisfied, the angle θn obtained by Equation 1 is a positive value, and the sound wave is refracted and transmitted into the photoacoustic propagation medium portion 52.

  In Equation 1, the velocity of the sound wave in the propagation medium is Cn, the sound velocity of the sound wave in the environmental fluid 14 is Ca, the density of the propagation medium is ρn, and the density of the environmental fluid 14 is ρa.

  On the other hand, the reflectance R at the interface between the acoustic waveguide 60 and the photoacoustic propagation medium portion 52 is expressed by Equation 2.

In order to refract and transmit sound waves from the acoustic waveguide 60 to the photoacoustic propagation medium unit 52 as efficiently as possible, the reflectance R is preferably small. When Cn, Ca, ρn, and ρa satisfy (ρ _n / ρ _a ) <(C _a / C _n ) <1, there are always angles θa and θn at which the numerator of Equation 2 becomes zero. That is, the reflectance R can be made zero.

In the first embodiment, as described above, the density ρn of the silica dry gel is 200 kg / m ³ , the sound velocity Cn in the silica dry gel is 150 m / s, and the air density ρa is 1.12 kg / m. ³ and the sound velocity Ca is 340 m / s near room temperature.

  Substituting these values into Equation 1, the angle θn is about 26 degrees. At this time, if the angle θa is about 89 degrees, the reflectance R is almost zero. Therefore, under the conditions of the first embodiment, the sound wave is high in the direction in which the angle θn is about 26 degrees when the sound wave enters the transmission surface 61 at about 89 degrees with respect to the normal line of the transmission surface 61. The light is transmitted into the photoacoustic propagation medium portion 52 with transmission efficiency.

  When the reflectance R is substantially zero, the refraction angle θn is constant at about 26 degrees. However, by making the transmission surface 61 a curved surface, the sound wave transmitted from a different position of the transmission surface 61 to the photoacoustic propagation medium unit 52. Can be propagated toward the predetermined acoustic focal point 57 to converge the sound wave.

  Further, by bending the acoustic waveguide 60 along the transmission surface 61, a part of the sound wave can be always incident on the transmission surface 61 at a constant angle θa as the sound wave propagates through the acoustic waveguide 60. By utilizing this phenomenon, the sound wave propagating through the acoustic waveguide 60 is gradually refracted and transmitted to the photoacoustic propagation medium unit 52, and the sound wave is converged to one point in the photoacoustic propagation medium unit 52. Is realized.

  Further, the refraction angle θn expressed by Equation 1 and the reflectance R expressed by Equation 2 do not depend on the frequency of the sound wave. Therefore, regardless of the frequency of the propagating sound wave, the sound wave can be transmitted to the photoacoustic propagation medium unit 52 with high transmission efficiency. Therefore, the optical ultrasonic microphone 51 of the first embodiment can detect broadband sound waves with high sensitivity. In other words, according to the first embodiment, it is possible to receive ultrasonic waves in a high frequency region and in a wide band, which has been difficult in the past, with high sensitivity, and a standard microphone having an effective band of 100 kHz or more and high sensitivity can be obtained. It can be realized.

  In the field of optical lenses, for example, Japanese Patent No. 2731389 discloses a structure for converging light emitted from the side surface of an optical waveguide. However, in general, in an optical waveguide, light propagates while being repeatedly reflected at the boundary between the cladding layer and the waveguide, whereas in the acoustic waveguide 60 of the first embodiment, sound waves are transmitted on the outer surface and side surfaces of the acoustic waveguide 60. Does not reflect. For this reason, in the first embodiment, it is important to propagate sound waves having the same phase, whereas the phase of the propagating light is not uniform in the optical waveguide.

(Shapes of transmission surface 61 and waveguide outer surface 62)
Next, the design of the shape of the transmission surface 61 and the waveguide outer surface 62 that define the acoustic waveguide 60 will be described. The shapes of the transmission surface 61 and the waveguide outer surface 62 are designed in the following steps.

  First, from the size of the opening 63 of the acoustic waveguide 60, the length of the acoustic waveguide 60 that allows the sound wave to be efficiently taken into the photoacoustic propagation medium 52 is determined. From the length of the acoustic waveguide 60, the transmission surface 61 is designed as a shape that converges sound waves. Thereafter, the shape of the transmission surface 61 is designed in consideration of the determined shape of the transmission surface 61 and the width required for the acoustic waveguide 60.

  The size of the opening 63 of the acoustic waveguide 60 is preferably less than or equal to ½ of the wavelength of the sound wave to be received. When the width of the waveguide is larger than ½ of the wavelength of the propagating sound wave, the sound wave is likely to be reflected inside the acoustic waveguide 60, and the propagation of the sound wave is disturbed, making it difficult to accurately measure the sound wave. It is.

  In the first embodiment, as an example, since reception of sound waves up to a frequency of 80 kHz is considered, the opening 63 is set to 2.0 mm which is smaller than 2.1 mm which is a half wavelength of the frequency of 80 kHz. It has a square shape of 2.0 mm. The end portion 72 of the converging portion 77 is designed to be the same size as the opening 63.

  The longer the acoustic waveguide 60 is, the more sound waves propagate from the acoustic waveguide 60 to the photoacoustic propagation medium 52. Therefore, it is preferable that the sound wave propagating in the acoustic waveguide 60 has a sufficient length so as to be refracted and transmitted to the photoacoustic propagation medium unit 52.

As described with reference to FIG. 21, in the ultrasonic receiver 101, the sound wave that has propagated through the range of the length L <b> 3 is transmitted through the second surface region 112 into the propagation medium unit 103. . The length L3 and the length L5 in FIG. 21 correspond to the length in the Z direction of the opening 63 in the YZ plane of the acoustic waveguide 60 shown in FIG. 5 and the length in the YZ plane of the transmission surface 61. By sufficiently increasing the length of the transmission surface 61 in the YZ plane, that is, the length of the sound wave propagation direction g6 in the acoustic waveguide 60, the sound wave can be sufficiently transmitted to the photoacoustic propagation medium 52, In the first embodiment, it is possible to improve the receiving sensitivity, reduce the influence of reflection of sound waves that could not be captured, and improve the measurement accuracy. In the first embodiment, the photoacoustic propagation medium 52 in the environmental fluid 14 Since θa (FIG. 4) formed by the normal line and the sound wave propagation direction 55a is approximately 89.3 degrees, the ratio of the length L2 to the length L1 is approximately L1 / L2 = 88. Therefore, ideally, the acoustic waveguide 60 preferably has a length of about 90 times or more than the opening 63. In the first embodiment, the opening 63 of the acoustic waveguide 60 is 2 mm, and the length of the acoustic waveguide 60 is set to 200 mm, which is 100 times the opening 63.

Thus, the lengths of the opening 63 and the acoustic waveguide 60 are determined. Based on the length of the acoustic waveguide 60, the shape of the transmission surface 61 and the shape of the outer surface of the waveguide are designed.
(Sound wave convergence)
FIG. 5 is an enlarged view of the acoustic waveguide 60 and the photoacoustic propagation medium portion 52. It will be described with reference to FIG. 5 that sound waves converge in the optical ultrasonic microphone 51 according to the first embodiment.

  An acoustic focal point 57 for converging the sound wave is set in the photoacoustic propagation medium unit 52. The LDV head 8 is opposed to the acoustic focal point 57, and a sound wave is detected by the LDV head 8 and the LDV arithmetic processing unit 9 using laser light 58.

  5, a point at the opening 63 of the transmission surface 61 is set as a starting point P0, and points P1, P2, P3,. Further, the distance from the point P0 to the point P1 is La1, the distance from the point P1 to the point P2 is La2,..., And the distance from the point Pn-1 to the point Pn is Lan. Further, the distances between the points P1, P2,... Pn and the acoustic focal point 57 are Ln1, Ln2,.

  In order for the sound wave incident from the opening 63, propagating through the acoustic waveguide 60, and refracted and transmitted to the photoacoustic propagation medium 52 to be focused by the acoustic focal point 57, it is necessary to satisfy the following equation (3). is there.

  The fact that the sound wave is focused on the acoustic focal point 57 in the photoacoustic propagation medium unit 52 means that the phase of the acoustic wave is aligned at the acoustic focal point 57. That is, it means that the arrival time of the sound wave from the opening 63 to the acoustic focal point 57 is the same in any route.

  Specifically, in Equation 3, the left side of the leftmost equal sign (La1 / Ca) + (Ln1 / Cn) propagates the sound wave through the environmental fluid 14 by the distance La1, and passes through the photoacoustic propagation medium unit 52. The time until the acoustic focal point 57 is reached by propagating by the distance Ln1 is shown. Further, {(La1 + La2) / Ca} + (Ln2 / Cn), which is the right side of the leftmost equal sign, propagates the sound wave through the environmental fluid 14 by a distance (La1 + La2) and travels through the photoacoustic propagation medium portion 52. The time until reaching the acoustic focal point 57 is shown by propagating by Ln2.

  By the same procedure, the time until the sound wave transmitted from the acoustic waveguide 60 to the photoacoustic propagation medium unit 52 reaches the acoustic focal point 57 at each point Pk can be obtained.

  Equation 3 is generalized, and the distance of the waveguide from the opening 63 of the acoustic waveguide 60 to an arbitrary point Pk along the propagation direction of the sound wave on the transmission surface 61 is Lak, and from the point Pk, the photoacoustic propagation medium When the distance to the acoustic focal point F (57) that is different from the point Pk in the part 52 is Lnk, Equation (3) is (Lak / Ca) + (Lnk / Cn) is expressed as a constant condition.

  (Lak / Ca) + (Lnk / Cn) is constant, as described above, the time required from the opening 63 to the acoustic focus 57 at an arbitrary position of the transmission surface 61 is constant at any point. It shows that there is. In other words, when (Lak / Ca) + (Lnk / Cn) satisfies a certain value, it means that the sound wave taken into the photoacoustic propagation medium unit 52 converges on the sonoacoustic focus 57. In the configuration in FIG. 1, (Lak / Ca) + (Lnk / Cn) satisfies the constant shape of the photoacoustic propagation medium portion 52 on the acoustic waveguide 60 side.

  FIG. 5 is a diagram for explaining that (Lak / Ca) + (Lnk / Cn) is constant. A sound wave that has entered the photoacoustic propagation medium unit 52 made of silica-dried gel from an arbitrary point on the transmission surface 61 of the acoustic waveguide 60 of the photoacoustic propagation medium unit 52, starting from the opening 63 of the acoustic waveguide. Even if there is a transmission surface 61 that satisfies (Lak / Ca) + (Lnk / Cn), it converges on the acoustic focal point 57. This is because the photoacoustic propagation medium portion 52 has a shape constituting a cylindrical (partial cylindrical) wavefront centered on the acoustic focal point 57.

  Strictly speaking, it seems that it is more accurate to calculate the propagation distance of the sound wave propagating through the acoustic waveguide 60 using the central path of the acoustic waveguide 60. However, as will be described below, the width dimension of the acoustic waveguide 60 is sufficiently smaller than its length. For this reason, the above approximation has sufficient accuracy for practical use.

(Light source and detector)
In FIG. 1, only the propagation path of the laser beam 58 in the air is shown for convenience. A detection unit that detects ultrasonic waves converged on the acoustic focal point 57 will be described. In FIG. 1, the light source is configured by an LDV head 8, and the detection unit is configured by an LDV arithmetic processing unit 9.

  By converging the sound wave at the acoustic focal point 57, a standing wave having a large amplitude and density is generated. As a result, the refractive index varies depending on the sound pressure of the acoustic signal received by the converging unit 77 at the acoustic focal point 57.

  The LDV head 8 emits (emits) laser light 58 toward the propagation medium unit 52. Further, after being emitted, the LDV head 8 propagates through the propagation medium portion 52 and receives the reflected laser light 58. The LDV arithmetic processing unit 9 converts the received laser beam 58 into an electrical signal and performs signal processing.

  The laser beam 58 emitted from the LDV head 8 is subjected to frequency modulation by the Doppler frequency corresponding to the rate of change of the refractive index. The modulation frequency is detected by the LDV arithmetic processing unit 9 by a detection method such as self-heterodyne detection.

  For example, using the thickness information of the photoacoustic propagation medium 52 in the direction in which the laser beam 58 is emitted, twice the distance between the detector and the measurement through hole 53a, and the velocity of the laser beam 58 emitted from the detector, the laser is used. The time from when the light 58 is emitted until the reflected wave is received is obtained in advance. From the difference between the obtained time and the actually measured time, the amount of frequency modulation of the laser beam 58 is known. The sound pressure of the sound wave can be obtained from the amount of the frequency modulation.

  The optical ultrasonic microphone 51 according to the first embodiment propagates a sound wave from an environmental fluid 14 having a very small acoustic impedance such as a gas to a solid with high efficiency and converges the sound wave transmitted through the solid inside the solid. The energy density can be increased. Thereby, a sound wave can be received with high sensitivity.

(Experimental result)
6 to 11, a process in which a sound wave propagating through the acoustic waveguide 60 of the optical ultrasonic microphone 51 of the first embodiment is transmitted to the photoacoustic propagation medium unit 52 and converges to the acoustic focal point 57 is obtained by a calculation experiment. The results are shown. 6 to 11 are sound pressure distribution diagrams showing calculation experiment results for sound wave propagation in the optical ultrasonic microphone shown in FIG. FIGS. 6 to 11 show only the acoustic waveguide 60 and the photoacoustic propagation medium 52 of the optical ultrasonic microphone 51 in order to display the position and phase of the sound wave in an easy-to-understand manner.

  6 to 11 show how the sound wave propagates over time. FIG. 6 shows the fastest time and FIG. 11 shows the slowest state. The transmission surface 61 and the waveguide outer surface 62 that define the acoustic waveguide 60 shown in FIGS. 6 to 11 are designed so that the sound wave propagating through the acoustic waveguide 60 converges on the acoustic focal point 57 by the procedure described above. . The opening 63 of the acoustic waveguide 60 is located above, and the closed end is located below. The acoustic waveguide 60 is filled with the environmental fluid 14, here air.

  FIG. 12 shows the time waveform of the input sound wave signal used in the experiments shown in FIGS. Specifically, the waveform of the sound wave incident from the opening 63 is shown. The center frequency of the sound wave is about 40 kHz, and the sound wave has a length of about 5 wavelengths.

  6 to 11, the sound pressure levels of the sound waves inside the photoacoustic propagation medium unit 52 and inside the acoustic waveguide 60 are shown in shades of color. The dark part (black) indicates a sound pressure higher than atmospheric pressure, and the lighter part (white) indicates a sound pressure lower than atmospheric pressure. The same color, for example, between black and black or between white and white indicates 40 kHz, that is, corresponds to one wavelength of the sound wave.

  6 to 11, it is difficult to confirm because the acoustic waveguide 60 is very narrow. However, in the acoustic waveguide 60, since the speed of sound of air is 340 m / s, the distance between the same colors. That is, the distance of one wavelength is about 8.5 mm.

  On the other hand, in the inside of the photoacoustic propagation medium part 52, since the sound speed of the dry gel constituting the photoacoustic propagation medium part 52 is 150 m / s, the distance between the same colors, that is, the distance of one wavelength is about 3 .75 mm.

  FIG. 6 shows the moment when three wavelengths of sound waves propagate from the opening 63 to the acoustic waveguide 60 and the peak of the fourth wave amplitude propagates from the opening 63 into the acoustic waveguide 60. The portion of the sound wave that has propagated inside the acoustic waveguide 60 propagates from the transmission surface 61 in contact with the acoustic waveguide 60 to the photoacoustic propagation medium portion 52. A portion indicated by shading inside the photoacoustic propagation medium portion 52 is a component of a sound wave that is refracted and transmitted from the transmission surface 61 to the photoacoustic propagation medium portion 52.

  FIG. 7 shows a state slightly advanced in time from the state shown in FIG. Inside the acoustic waveguide 60, sound waves propagate along the shape of the acoustic waveguide 60. Further, a state is shown in which a sound wave propagating in the acoustic waveguide 60 is gradually refracted and transmitted to the photoacoustic propagation medium unit 52 and propagates in the photoacoustic propagation medium unit 52. As shown in FIGS. 6 and 7, the sound wave propagating through the acoustic waveguide 60 propagates a longer distance from the opening 63 than the sound wave propagating through the photoacoustic propagation medium unit 52. This indicates that the speed of sound of the air that is the environmental fluid 14 of the acoustic waveguide 60 is faster than the speed of sound of the dry gel that is the propagation medium.

  Similarly, FIG. 8 shows a state in which a part of the sound wave is refracted and transmitted to the photoacoustic propagation medium unit 52 and the sound wave propagates through the photoacoustic propagation medium unit 52 as the sound wave propagates through the acoustic waveguide 60. . Due to refractive transmission, the pattern indicated by black and white shading on the transmission surface 61 is bent, but in the photoacoustic propagation medium section 52, the pattern indicated by black and white shading draws a beautiful curve. It's getting on. This indicates that the phases of sound waves propagating in the photoacoustic propagation medium section 52 are aligned.

  FIG. 9 shows a state of a sound wave propagating substantially near the end of the acoustic waveguide 60 and a sound wave gradually converging toward the acoustic focal point 57 inside the photoacoustic propagation medium portion 52.

  In FIG. 10, the propagation of the sound wave further progresses, and the sound wave propagating inside the acoustic waveguide 60 reaches the end of the waveguide, and is all refracted and transmitted inside the photoacoustic propagation medium unit 52. The sound wave propagating through the interior is further converged toward the acoustic focal point 57.

  In FIG. 11, the first wavefront of the sound wave that has propagated inside the photoacoustic propagation medium unit 52 reaches the acoustic focal point 57. As shown in FIG. 6F, the shades of black are darker, which indicates that the sound wave is converged and the sound pressure is increased at the acoustic focus 57.

  Although specific numerical values are not shown in FIG. 6 to FIG. 11, when the change in the sound pressure due to the sound wave from the atmospheric pressure inside the acoustic waveguide 60 is about 4 Pa, it is found from the experimental results that a large value near the acoustic focal point 57 is obtained. It was found that the change in sound pressure from atmospheric pressure was about 34 Pa. This indicates that the sound pressure of the sound wave has been increased by 8 times or more, and according to the first embodiment, it has become clear that the sound wave in the environmental fluid can be observed with high sensitivity.

  As described above, according to the first embodiment, the sound wave is refracted and transmitted from the environmental fluid 14 to the photoacoustic propagation medium unit 52, thereby suppressing the reflection of the sound wave at the interface having different acoustic impedance, and the sound wave with high efficiency. Can be transmitted through the photoacoustic propagation medium section 52.

  Further, the photoacoustic propagation medium portion 52 is arranged so as to constitute one surface of the acoustic waveguide 60 filled with the environmental fluid 14, and a part of the sound wave is propagated through the acoustic waveguide 60. By designing the shape of the surface that is in contact with the acoustic waveguide 60 so as to pass through and to converge to the predetermined acoustic focal point 57, the phase of the sound wave transmitted to the photoacoustic propagation medium 52 is gradually matched. Can be converged to the acoustic focal point 57. Therefore, the sound wave can be converged using most of the sound wave incident from the opening 63 of the acoustic waveguide 60, and the sound pressure of the received sound wave can be increased. Thereby, a sound wave can be detected with high sensitivity.

  In the optical ultrasonic microphone 51 of the first embodiment, the end of the acoustic waveguide 60 is closed, but the end may be opened. FIG. 13 is a cross-sectional view showing a schematic device configuration of an optical ultrasonic microphone 51A according to a modification of the first embodiment. In the optical ultrasonic microphone 51A shown in FIG. 13, the waveguide terminal end 64A of the acoustic waveguide 60 is opened. When the energy of the sound wave propagating through the acoustic waveguide 60 is relatively high and it is not necessary to capture all the energy, the portion of the sound wave propagating through the acoustic waveguide 60 that has not been transmitted to the photoacoustic propagation medium 52 is terminated. It is preferably removed from the acoustic waveguide 60 so as not to be adversely affected by reflection. According to the acoustic wave receiver 103, since the terminal end 64 of the acoustic waveguide 60 is open, it is possible to remove acoustic waves that have not been transmitted to the photoacoustic propagation medium unit 52. Thereby, the target sound wave can be accurately detected without disturbing the received sound wave. In this case, the length of the acoustic waveguide 60 may be shorter than a preferable length determined in relation to the opening as described above.

  FIG. 14 is a contour diagram showing the measurement results of the isophase surface of the sound wave propagation of the photoacoustic propagation medium portion of the optical ultrasonic microphone according to the first embodiment of the present invention. FIG. 14 shows a result of propagation time measured by two-dimensionally scanning the sound wave propagation inside the photoacoustic propagation medium unit 52 made of silica dry gel with the LDV head 8 in the optical ultrasonic microphone 51. The situation of the equiphase surface (wavefront) 902 of propagation is shown.

  Here, as an example of the photoacoustic propagation medium portion 52, a silica dry gel having a density of 270 kg / m 3 and a sound velocity of 145 m / s was used. In this case, the incident angle at each point is 89.5 degrees and the refraction angle is 26 degrees. A curved surface was designed based on this sound velocity value. It was observed that the acoustic wave propagation direction 901 and the equiphase surface 902 propagated toward the acoustic acoustic focal point 57 using the silica dry gel 52 as a cylindrical wave, and the operation according to the theoretical design was confirmed.

  FIG. 15 is a time waveform diagram showing an example of the LDV output waveform 81 (amplitude measurement waveform) of the optical ultrasonic microphone 51 in the vicinity of the acoustic acoustic focus 57. FIG. 15 shows the result when the sound wave to be measured is radiated from the broadband tweeter at the center frequency of 40 kHz and the drive signal of one wavelength.

  Considering the center frequency of 40 kHz, the width and initial height of the acoustic waveguide 60 (height at the opening 54) are both 4 mm. The thickness of the silica dry gel used as the photoacoustic propagation medium portion 52 is also 4 mm. For the measurement of the laser beam 58 that reciprocates in the photoacoustic propagation medium 52, a heterodyne laser Doppler vibrometer (LDV head 8) using a He—Ne laser with a wavelength of 633 nm is used as an example of a light source and a light detection means. It was.

  The modulation of light by sound waves is frequency modulation. An aluminum material is used for the base 3. The laser light 58 emitted from the He—Ne laser is transmitted through a measurement through hole 53a (a through hole formed in the acoustic focal point 57) 53a of the base 53 located on the surface side of the silica dry gel 52. 52, enters the photoacoustic propagation medium portion 52 in the thickness direction, propagates in the reverse direction of the optical path, reflects off the inner surface of the base 53 on the back side of the photoacoustic propagation medium portion 52, and then again propagates the photoacoustic propagation. The light passes through the medium part 52 in the thickness direction, exits from the through hole 53a of the base 53 located on the surface side of the photoacoustic propagation medium part 52, and returns to the LDV head 8. Accordingly, the optical path for sound wave measurement is 8 mm which is twice the thickness of 4 mm of the photoacoustic propagation medium portion 52.

  From the result of FIG. 15, it can be seen that the reception characteristic has an extremely wide band as in the first embodiment. From the waveform 81 in FIG. 15, the peak displacement is about 5 nm. The converted sound pressure P is about 54.2 Pa. The input equivalent sound pressure at the end of the acoustic horn was about 25 Pa, and a convergence effect of about twice was confirmed. Also in this case, the measured and converted sound pressure and the input converted sound pressure are sufficiently in order, and by appropriately calibrating the measured and converted sound pressure, Extremely accurate sound pressure measurement is possible.

  In the first embodiment, by converging the sound wave inside the photoacoustic propagation medium unit 52, it is possible to receive a broadband with higher sensitivity.

(Second Embodiment)
The optical ultrasonic microphone according to the second embodiment will be described with reference to FIGS.
The difference between the second embodiment and the first embodiment is the configuration of an optical system that optically detects the sound pressure at the acoustic focal point 57. Hereinafter, this optical system is referred to as an optical sound pressure measuring unit 1300.

  In the first embodiment, the optical sound pressure measuring unit 1300 includes the LDV head 8 and the LDV arithmetic processing unit 9. Although LDV is rich in versatility and stability, downsizing is difficult and expensive due to the structure of LDV. Therefore, in order to provide an optical ultrasonic microphone with a small size and a low price, it is desired to make the optical sound pressure measuring unit 1300 small and inexpensive. Hereinafter, the configuration of the optical sound pressure measurement unit 1300 will be described. Also in the second embodiment, illustrations and descriptions are omitted for the configuration that is not changed from the configuration described in the first embodiment.

  FIG. 16 shows an optical system structure of an optical sound pressure measurement unit 1300 in the second embodiment of the optical ultrasonic microphone of the present invention.

  A monochromatic light source 1301 shown in FIG. 16 emits coherent light. The monochromatic light 1302 emitted from the monochromatic light source 1301 is secured by the optical system 1303 to ensure the flatness of the wavefront and to be enlarged or reduced to an appropriate beam size. Thereafter, the monochromatic light 1302 is split into two monochromatic lights 1307 and 1308 by the beam splitter 1304. Then, the monochromatic light 1307 goes to the photoacoustic propagation medium unit 52, and the monochromatic light 1308 goes to the plane mirror 1306.

  The monochromatic light 1307 includes a transparent base 1314 made of a material that can transmit the monochromatic light 1307 and can satisfactorily constrain acoustic vibrations in the photoacoustic propagation medium 52 (for example, a transparent reinforced acrylic plate), and photoacoustic light. The sound passes through the acoustic focal point 57 of the propagation medium unit 52, is reflected by the plane mirror 1310, and then passes through the acoustic focal point 57 again. Then, a part of the light passes through the beam splitter 1304, the shape of the beam is formed by the condensing optical system 1311, and is guided to the light intensity measuring device 1312. The optical path through which the monochromatic light 1307 described above passes is denoted as L11.

  The monochromatic light 1308 is reflected by the plane mirror 1306 and travels in the direction of the beam splitter 1304 again. A part of the monochromatic light 1308 is reflected by the beam splitter 1340 and then condensed by the condensing optical system 1311 as before. Guided to an intensity meter 1312. The optical path through which the monochromatic light 1308 described above passes is denoted as L12.

  The schematic configuration of the optical ultrasonic microphone 1300 and the path through which the monochromatic light 1302 passes have been described above. Next, the operation principle of the optical ultrasonic microphone 1300 will be described with reference to FIG. The wavefronts of the monochromatic light 1307 transmitted through the beam splitter 1304 and the monochromatic light 1308 reflected by the beam splitter 1304 are sufficiently parallel, and both monochromatic lights have high contrast (contrast is the maximum intensity fluctuation amount of interference light in terms of time average intensity). The optical axis of the entire optical system is adjusted so that the interference light can be generated (contrast takes a real value from 0 to 2), and the reflection / transmittance of the beam splitter 1304 is selected. Has been.

  Accordingly, the intensity of the interference light received by the light intensity measuring device 1312 varies with high contrast depending on the optical path length difference between the optical paths L11 and L12 through which the two monochromatic light passes. When there is no time variation of the optical path length difference, the output signal 1313 from the light intensity measuring device 1312 shows a constant value regardless of time. However, the refractive index of the acoustic focal point 57 varies with time due to temporal variation of the intensity of the acoustic signal collected at the acoustic focal point 57 by the photoacoustic propagation medium unit 52, and the optical path length difference varies with time accordingly. Therefore, the output signal 1313 varies with time. Since the refractive index variation depends on the input sound pressure, the sound pressure variation can be observed by analyzing the output signal 1313. The above is the operation of the optical ultrasonic microphone 1300.

  Variations of possible device configurations in the optical ultrasonic microphone 1300 of the second embodiment and considerations during design will be described.

  In FIG. 16, the beam splitter 1304, the transparent base 1314, the plane mirror 1306, and the plane mirror 1310 are all in contact with each other. However, another optical medium such as an air layer is sandwiched between them, and they are separated from each other. Needless to say, they may be arranged. However, the stability of the interference light intensity in this configuration mainly depends on how to suppress the refractive index fluctuation due to the influence other than the acoustic signal due to the air fluctuation that the monochromatic light 1307 and 1308 suffer while passing through different optical paths. . Therefore, in order to obtain a highly stable output signal 1313, the insertion of an air layer or a mechanically unstable optical medium is eliminated as shown in FIG. 16, and the optical paths L11 and L12 follow different paths. It is desirable to make the area as small as possible.

  16 includes an optical system 1303 and a condensing optical system 1311. The monochromatic light 1302 from the monochromatic light source 1301 has sufficient coherence and a beam size necessary for measurement from the monochromatic light source 1301. Needless to say, it can be omitted if the system is provided from the time of emission.

  However, when the focus position shift of the photoacoustic propagation medium unit 52 becomes a problem in the required measurable frequency band of the acoustic signal (that is, the photoacoustic propagation medium unit 52 exhibits “chromatic aberration” with respect to the acoustic signal. If it has, the beam size of the monochromatic light 1302 needs to be large enough to cover the position fluctuation of the acoustic focus 57 due to aberration. This is because if an acoustic signal in a certain frequency band forms a focal point outside the beam spot of the monochromatic light 1302, the acoustic signal in that frequency band is not received, so the acoustic signal is received in the measurable frequency band of the acoustic signal. This is because frequency dependence occurs in the characteristics. If there is frequency dependency in the reception characteristics, so-called signal distortion is generated in which the similarity between the time waveform of the acoustic signal at the acoustic focal point 57 and the signal waveform of the output signal 1313 cannot be secured, which is not preferable in terms of characteristics. Therefore, it is indispensable to avoid this problem in order to secure a wide frequency band capable of acoustic measurement, and it is necessary to expand the monochromatic light 1302 so as to have a certain beam size by inserting the optical system 1303.

  Incidentally, the spectral width required for the monochromatic light 1302 depends on the optical system configuration of the optical ultrasonic microphone 1300. For example, when a parallel plate glass is inserted between the beam splitter 1304 and the plane mirror 1306 and the optical paths L11 and L12 are adjusted to be equal in a situation where no acoustic signal is input to the acoustic focal point 57, the monochromatic light 1302 is obtained. Can use broadband light such as light emitting diode light having maximum intensity at a desired wavelength or white light. As the spectral width of the monochromatic light 1302 becomes narrower, the physical size of the monochromatic light source 1301 increases and becomes expensive. Therefore, when the optical acoustic measurement device is realized in a small size and at a low price, the optical path L11 is as much as possible. , L12 is preferably configured so that the optical path lengths of L12 are equal.

  Further, the transparent base plate 1314 has been described as being made of a transparent material so that the monochromatic light 1307 can be sufficiently transmitted. However, both the transparent base plates 1314 are provided with small openings, and the optical path of the monochromatic light 1307 is the small openings. Needless to say, the transparent base plate 1314 can be applied to the transparent base plate 1314 by allowing it to pass therethrough. As long as the above-mentioned required specifications for interference light intensity stability are satisfied, all configurations in which the monochromatic light 1307 can pass through the transparent base 1314 can be operated equally.

  Next, the results of demonstrating the measurement principle of the optical ultrasonic microphone 1300 shown in FIG. 16 will be described with reference to FIGS.

  The optical ultrasonic microphone 1300 measures the refractive index fluctuation in the acoustic medium generated by the sound pressure as the optical path length fluctuation by the optical interferometry. Therefore, the optical interferometer shown in FIG. 17 was actually constructed and the principle was verified.

  FIG. 17 shows an experimental apparatus for proof of principle of the optical ultrasonic microphone in the second embodiment. In FIG. 17, the laser beam emitted from the He—Ne laser 1400 is divided into two paths L21 and L22 by the beam splitter 1401. The laser beam in the path L21 is folded back by the reflecting mirror 1402, and interferes with the laser beam in the path L22 in the beam splitter 1405.

  The laser light passing through the path L22 interferes with the light beam of the path L21 after being folded by the reflecting mirror 1403 and passing through the acoustic medium 1404. Then, the intensity of the generated interference light is output as an electrical signal by the photodetector 1406. The electrical signal is output as an acoustic reception signal through a digital oscilloscope 1408 through necessary signal processing such as removal of DC components contained in the signal and averaging processing.

  An acoustic medium 1404 is inserted between the reflecting mirror 1403 and the beam splitter 1405 in the path L22. Further, a horn 1409 is connected to the acoustic medium 1404 so that an acoustic signal emitted from the speaker 1410 can be introduced into the acoustic medium 1404. In this experimental apparatus configuration, since the optical path length difference between the path L21 and the path L22 is measured as the interference light intensity by the Mach-Zehnder interferometer, the interference light intensity is a refractive index change generated by the acoustic signal introduced into the acoustic medium 1404. Depends on.

  Next, the actual experimental situation will be described. In the experiment, an input signal to the speaker 1410 was a single sine wave having an amplitude of 0.5 V and a frequency of 40 kHz generated by a function generator (not shown). Further, the distance from the front surface of the speaker 1410 to the opening surface of the horn 1409 was 100 mm, and the acoustic medium 1404 having a thickness of 5 mm (left and right direction with respect to FIG. 17) and a square shape with a side of 20 mm was used. The acoustic medium 1404 has a sound speed of about 70 m with respect to a 40 kHz sine wave.

  FIG. 18 shows the measured results. FIG. 18A is a graph showing a time waveform of an output signal from the digital oscilloscope 1408 measured by an experimental apparatus for proof of principle of the optical ultrasonic microphone of FIG. FIG. 18B is a graph showing a time waveform of an input signal to the speaker 1410. 18A and 18B are arranged so that the time widths on the horizontal axis are the same, and the signal wave fronts coincide with each other.

  As can be seen from FIG. 18, the output signal to the speaker 1410 shown in FIG. 18B is converted into an acoustic signal and propagates through the air, collected by the horn 1409 and introduced into the acoustic medium 1404 and travels as a dense wave. Accordingly, the refractive index fluctuation generated in the intersection region between the acoustic medium 1404 and the optical path L22 is detected as the interference light intensity output from the digital oscilloscope 1408. Therefore, it can be seen from this measurement result that the optical acoustic measurement unit 1300 can actually function.

(Third embodiment)
Next, the optical ultrasonic microphone of the third embodiment will be described with reference to FIG.

  Similarly to the second embodiment, the optical ultrasonic microphone according to the third embodiment also constitutes an optical system that optically detects the sound pressure at the acoustic focal point 57. In the optical ultrasonic microphone 1300 described with reference to FIG. 16, the optical path length difference between two different paths L21 and L22 constituting the Michelson-Morley interferometer is converted into interference light intensity and measured.

  However, when the monochromatic light 1302 is a coherent monochromatic light such as a laser beam, the interference light intensity is converted into the wavelength of the monochromatic light 1302, and all the optical path length difference variations having an integer wavelength difference have the same intensity. Output (ie, pitch skip), if there is an acoustic signal with a strong sound pressure corresponding to such a variation in the optical path length, or if the optical path length difference is already a half-odd wavelength in the absence of an acoustic signal A so-called signal distortion occurs in which the similarity between the time waveform of the acoustic signal at the signal 1313 and the acoustic focal point 57 is lost.

  This problem is caused by the fact that the absolute amount measurable range of the optical path length difference variation amount of the interferometer constituting the optical ultrasonic microphone shown in FIG. 16 is one wavelength or less, and has a wide measurement dynamic range. This is a problem when an optical acoustic microphone is constructed. In order to solve this problem, it is necessary to use an interferometer having a large absolute range measurable range of the optical path length difference. As described below, an optical ultrasonic microphone to which an interferometer having such characteristics is actually applied can be configured.

  With reference to FIG. 19, a configuration of an optical ultrasonic microphone capable of solving the above-described problem will be described. Note that in the third embodiment, illustrations and descriptions of configurations that do not change in the first embodiment are omitted.

  FIG. 19 shows an optical system configuration of an optical ultrasonic microphone 1300 having a wide measurement dynamic range and no signal distortion in the third embodiment. In FIG. 19, the same reference numerals as those in FIG. 16 are applied to components that can be applied without any change from the apparatus configuration in FIG.

  In FIG. 19, reference numeral 1601 denotes a dual-frequency linearly polarized laser light source, which emits laser light 1602 composed of linearly polarized light having mutually orthogonal polarization planes and frequencies of ω and ω + Δω.

  In the following, for the sake of clarity, it is specified that one polarization plane of the two linearly polarized lights is parallel to the paper surface of FIG. 19 and has a frequency ω (so that the other polarization surface is relative to the paper surface of FIG. Vertical and has frequency ω + Δω).

  For the same purpose as the apparatus configuration shown in FIG. 17, the optical system 1303 ensures the flatness of the wavefront and enlarges or reduces the laser beam 1602 to an appropriate beam size (the laser beam 1602 is inserted by inserting the optical system 1303). Changes in the polarization situation are usually very small and can be ignored). The adjusted laser beam 1602 is partially reflected by the non-polarizing beam splitter 1603 for both polarization components, and guided to the light intensity measuring device 1605 by the condensing optical system 1604. Since the reflection is performed on the non-polarized surface, the two polarization components included in the laser beam 1602 immediately after the reflection are the rotation and linear polarization of the polarization surface, and the amplitude intensity ratio of both components is the laser beam 1602 before the reflection. Is the same.

  Incidentally, a polarizing plate 1606 is inserted between the non-polarizing beam splitter 1603 and the condensing optical system 1604. Note that the polarization axis of the polarizing plate 1606 is inclined by 45 ° with respect to the paper surface of FIG. 19, and the polarization axis is equally 45 ° with respect to the two polarization surfaces in the laser light 1602. Since both polarization planes are projected onto the polarization axis by the plate 1606, linearly polarized components having frequencies ω and ω + Δω, which have been non-driable due to the orthogonality of the polarization planes, interfere with each other, and the generated difference frequency The beat light of Δω enters the light intensity measuring device 1605. Therefore, the time waveform of the reference beat signal 1607 is a sine wave having a frequency Δω. Since the linearly polarized wave component having the frequencies ω and ω + Δω always passes through the same path, the reference beat signal 1607 does not include information on the acoustic signal.

  Next, the whereabouts of the laser beam 1602 that has passed through the non-polarizing beam splitter 1603 will be described. The laser beam 1602 that has passed through the non-polarizing beam splitter 1603 is selected by the polarizing beam splitter 1608 to be routed by reflection / transmission according to the polarization plane direction. Here, for the sake of simplicity, it is assumed that the polarization beam splitter 1608 completely reflects linearly polarized light having a polarization plane parallel to the plane of FIG. Accordingly, only linearly polarized light having a polarization plane parallel to the paper surface of FIG. 19 included in the laser light 1602 that has passed through the non-polarizing beam splitter 1603 is reflected toward the acoustic focal point 57, and a straight line having a polarization surface perpendicular to the paper surface of FIG. The polarized light is completely transmitted to the plane mirror 1617.

  First, description will be given of the whereabouts of the laser beam 1609 reflected in the direction of the acoustic focus 57 among the laser beams divided in the polarization plane direction as described above. Since the laser beam 1609 passes through the acoustic focal point 57 twice while being reflected by the plane mirror 1618, the laser beam 1609 suffers from an optical path length variation according to the refractive index variation generated by the acoustic signal input to the photoacoustic propagation medium unit 52. This point is the same as the configuration example shown in FIG. 17, but the configuration in FIG. 19 is different in that it passes through the 1 / 8λ wave plate 1610 twice in the middle of the path. The 1 / 8λ wave plate 1610 has a function of rotating the plane of polarization of light passing therethrough by 45 °. Accordingly, the plane of polarization of the laser light 1609 reflected by the plane mirror 1618 and returning into the polarization beam splitter 1608 is rotated by 90 ° and becomes perpendicular to the paper surface of FIG. 19, and thus passes through the polarization beam splitter 1608 and passes through the polarization plate 1613. Then, it is guided to the light intensity measuring device 1612 by the condensing optical system 1611.

  Next, description will be made regarding the whereabouts of the laser light 1614 directed to the plane mirror 1617 by the laser light whose path is divided in the polarization plane direction by the polarization beam splitter 1608. When the laser beam 1614 is reflected by the plane mirror 1617 and returns to immediately before the reflecting surface of the polarizing beam splitter 1608, the laser beam 1614 passes through the 1 / 8λ wave plate 1615 twice. Become parallel. Accordingly, the laser beam 1614 is reflected by the polarization beam splitter 1608 and is guided to the light intensity measuring device 1612 by the condensing optical system 1611 after passing through the polarizing plate 1613.

  As described above, the two laser beams 1609 and 1614 are integrated into one laser beam whose polarization planes are orthogonal to each other immediately before the polarizing plate 1613, but the polarizing axis of the polarizing plate 1613 is relative to the paper surface of FIG. It is inclined 45 ° and acts in the same manner as the polarizing plate 1606 described above, and after passing through the polarizing plate 1614, the two laser beams 1609 and 1614 become coherent. Accordingly, the laser beams 1609 and 1614 interfere to become beat light having a difference frequency Δω, and a measurement beat signal 1616 having a sine waveform having the same frequency is output from the light intensity measuring device 1612.

  Unlike the reference beat signal 1607, the phase φ of the measurement beat signal 1616 depends on the optical path length difference of the path through which each of the two laser beams 1609 and 1614 passes independently, and the monochromatic light wavelength λ = 2πc / ω (c In terms of the speed of light), the change in the optical path length difference of one wavelength corresponds to the change in 2π of the phase φ. The phase φ is measured by comparing the phase of the reference beat signal 1607 output from the light intensity measuring instruments 1605 and 1612 as an electrical signal with the measured beat signal 1616. The phase comparison can be performed with high accuracy by measuring the phase difference of the measurement beat signal 1616 with reference to the reference beat signal 1607 using, for example, a lock-in amplifier (not shown).

  Since the apparatus configuration of FIG. 19 measures the phase φ, which is a continuous quantity, no signal distortion due to pitch jump, which is one of the problems in FIG. 16, occurs. Further, the measurement of the fluctuation amount of the phase φ exceeding 2π, which is another problem, is performed as follows. First, the phase φ0 in a state where no acoustic signal is input is measured and placed in the photoacoustic propagation medium unit 52, and the phase variation Δφ (that is, φ = φ0 + Δφ) is constantly monitored to thereby exceed the phase φ exceeding 2π. It is possible to measure the amount of fluctuation. As described above, since the phase variation Δφ reflects the refractive index variation at the acoustic focal point 57 caused by the acoustic signal via the optical path length difference variation, the measured phase variation Δφ is analyzed to analyze the above 2 The acoustic signal can be measured while solving one problem.

  In the optical ultrasonic microphone 1300 shown in FIG. 19, variations and changes that can be made in the apparatus configuration will be described. In the configuration of FIG. 19, similarly to FIG. 16, the emitted light from the two-frequency linearly polarized laser light source 1601 can generate the interference wave having sufficient contrast as it is, and the flatness of the wave front, and the acoustic focal point 57. In the case of a beam size that realizes a sufficient coating, the optical system 1303 and the condensing optical systems 1604 and 1611 can be omitted.

  Needless to say, the existing dual-frequency laser light source is equally applicable to the dual-frequency linearly polarized laser light source 1601. For example, a linearly polarized laser beam having a frequency difference Δω is generated by a two-frequency Zeeman laser or an acousto-optic modulator, and the two linearly polarized light beams after modulation so that their planes of polarization are orthogonal to each other. A light source in which wave laser light is integrated into one light beam can be applied.

  Further, in FIG. 19, the polarizing plates 1606 and 1613, the non-polarizing beam splitter 1603, the polarizing beam splitter 1608, the 1 / 8λ wavelength plates 1610 and 1615, the plane mirrors 1617 and 1618, and the photoacoustic propagation medium unit 52 are completely in contact with each other. Although expressed, it goes without saying that a similar function can be produced even if another optical medium such as an air layer is inserted between the respective elements. However, it is better not to insert an air layer or a mechanically weak optical medium in order to achieve high device stability capable of satisfactorily measuring only the refractive index variation generated at the acoustic focus 57 and downsizing of the entire device. .

  In FIG. 19, the polarization planes and polarization axes of the laser beam 1602, the non-polarization beam splitter 1603, the polarization beam splitter 1608, and the polarizing plates 1606 and 1613 are set with reference to the plane of FIG. If so, it goes without saying that the respective polarization planes and polarization axes may be rotated at an arbitrary angle around the optical axis. It goes without saying that the polarization beam splitter 1608 functions similarly even if the polarization selectivity of reflection / transmission is reversed.

  According to the optical ultrasonic microphone of the present invention, it is possible to receive a high-frequency and wide-band ultrasonic wave, which has been difficult in the past, and a standard microphone having an effective band of 100 kHz or more can be realized.

8 LDV head 9 LDV arithmetic processing unit 14, 104 Environmental fluid 51, 51A Optical ultrasonic microphone 52 Photoacoustic propagation medium unit 53 Base 53a Measurement through-hole 55, 901 Sound propagation direction 56 Acoustic waveguide member 57 Acoustic focus 58, 1614, 1602, 1609 Laser light 60 Acoustic waveguide 61 Transmission surface 62 Waveguide outer surface 63 Opening 64, 64A Waveguide termination 70 Inner space 71 Opening 72 End 77 Converging part 81 LDV output waveform 101 Ultrasonic transmitter / receiver 102 Ultrasonic vibrator 103 Propagation medium portion 105 Ultrasonic propagation path 111 First surface region 112 Second surface region 121 LDV
122, 123, 1402, 1403 Reflective mirror 124 Cubic mirror 125 Laser light path 126 Sound field 127 Calculation unit 902 Equiphase surface 1300, 1600 Optical sound pressure measurement unit 1301 Monochromatic light source 1302, 1307, 1308 Monochromatic light 1303 Optical system 1304, 1401, 1405 Beam splitter 1306, 1310, 1617, 1618 Plane mirror 1311, 1604, 1611 Condensing optical system 1312, 1612, 1605 Light intensity measuring device 1313 Output signal 1314 Transparent base 1400 He-Ne laser 1402, 1403 Reflector 1404 Acoustic medium 1406 Photodetector 1407 Digital oscilloscope 1408 Horn 1409 Speaker 1602 Dual-frequency linearly polarized laser light source 1603 Unpolarized beam splitter 1606, 1 13 polarizing plate 1607 reference beat signal 1608 polarizing beam splitter 1610,1615 1 / 8λ wave plate 1616 measurement beat signal

Claims (16)

A microphone that receives a sound wave propagating through a surrounding space filled with an environmental fluid,
A first opening into which the sound wave is incident;
An acoustic waveguide through which a sound wave incident from the first opening propagates;
A photoacoustic propagation medium portion provided in the waveguide so that the transmission surface forms one surface of the acoustic waveguide along a propagation direction of the ultrasonic wave, the transmission surface comprising: As the light propagates, a part of the ultrasonic wave is transmitted from the transmission surface to the propagation medium portion and converges to a predetermined convergence point, and is disposed with respect to the waveguide. A photoacoustic propagation medium section;
A light source that emits a light wave toward the convergence point;
A detector that receives the reflected light wave propagating through the photoacoustic propagation medium portion and that obtains the sound pressure of the sound wave from the received light wave;
The density ρ _{n of} the photoacoustic propagation medium portion, the sound velocity C _n in the photoacoustic propagation medium portion, the density ρ _{a of} the gas filling the acoustic waveguide, and the sound velocity C _{a in the} gas filling the acoustic waveguide are (ρ _a / ρ _n ) <(C _n / C _a ) <1
Wherein the first opening of the waveguide, the length of the waveguide to P the point set to any position along the propagation direction of the ultrasonic wave on the transmitting surface and L _a, wherein from the point P A microphone in which L _a / C _a + L _n / C _n is constant regardless of the position of the point P, _where L _n is the length to the convergence point. The photoacoustic propagation medium part is composed of a dry gel of an inorganic oxide or an organic polymer,
The optical ultrasonic microphone according to claim 1. The dry gel has physical properties of a density of 100 kg / m 3 or more and a sound velocity of 300 m / s or less.
The optical ultrasonic microphone according to claim 2. The solid skeleton of the dried gel is hydrophobized,
The optical ultrasonic microphone according to claim 2 or 3. The light source is a laser light source and emits laser light;
The optical ultrasonic microphone according to any one of claims 1 to 4. The light source and the detection unit are configured by laser Doppler detection means.
The optical ultrasonic microphone according to claim 5. The light source and the detection unit are configured with a Mach-Zehnder optical interferometer,
The optical ultrasonic microphone according to any one of claims 1 to 4. In the Mach-Zehnder type optical interferometer,
The light wave emitted from the light source is a single light beam,
And a beam splitter that splits the one light beam into two light beams,
Only one of the two light beams split by the beam splitter passes through the convergence point and then interferes with the remaining one of the two light beams,
The detection unit obtains a sound pressure of the sound wave based on a change in intensity of the interfered light;
The optical ultrasonic microphone according to claim 7. A difference in optical path length between the two light beams is approximately equal to a coherent length of the light wave emitted from the light source;
The optical ultrasonic microphone according to claim 8. In the Mach-Zehnder type optical interferometer,
The light wave emitted from the light source is one light beam, and the one light beam is composed of two linearly polarized monochromatic lights having different frequencies and orthogonal polarization planes,
And a beam splitter that splits the one light beam into two light beams,
Only one of the two light beams split by the beam splitter passes through the convergence point and then interferes with the remaining one of the two light beams,
The detection unit obtains a sound pressure of the sound wave based on a change in intensity of the interfered light;
The optical ultrasonic microphone according to claim 7. The detection means has a plurality of Mach-Zehnder optical interferometers,
At least one of the plurality of Mach-Zehnder optical interferometers is a reference interferometer, the rest is a measurement interferometer, and the light wave detected by the reference interferometer does not pass through the convergence point,
The lightwave path detected by the measurement interferometer passes through the convergence point,
Detecting the phase difference between the intensity of the interference light output from the reference interferometer and the intensity of the interference light output from the measurement interferometer;
The optical ultrasonic microphone according to claim 10. The two rays propagate in a medium other than air,
The optical ultrasonic microphone according to any one of claims 8 to 12.   The optical ultrasonic microphone according to any one of claims 1 to 12, wherein an acoustic horn is connected to a front end of the first opening of the acoustic waveguide. The height and width of the acoustic waveguide are ½ or less of the wavelength of the ultrasonic wave received by the microphone.
The optical ultrasonic microphone according to any one of claims 1 to 13. The acoustic waveguide is configured by the base and the photoacoustic propagation medium portion so that the height of the acoustic waveguide decreases toward the end side of the acoustic waveguide.
The optical ultrasonic microphone according to any one of claims 1 to 14. The environmental fluid is air;
The optical ultrasonic microphone according to any one of claims 1 to 15.

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