Patent Translate Powered by EPO and Google Notice This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate, complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or financial decisions, should not be based on machine-translation output. DESCRIPTION JP2011211311 An optical ultrasonic microphone capable of measuring a wide range of sound pressure up to an ultrasonic region, having a small size, high sensitivity, and being resistant to environmental fluctuations. An optical ultrasonic microphone according to the present invention includes an acoustic waveguide (60) for taking in and propagating a sound wave from an opening (71), and at least a part of a wall of the acoustic waveguide (60). A propagation medium 52 and a laser doppler vibrometer head 8 are provided, and a sound wave traveling through an acoustic waveguide 60 is taken into the photoacoustic propagation medium 52 and collected at the acoustic focal point 57 to generate a change in refractive index with high efficiency. , It is measured as a light modulation signal by the LDV head 8. When achieving further environmental tolerance and measurement dynamic range, replace the LDV with a Mach-Zehnder interferometer and optical heterodyne interferometry. [Selected figure] Figure 1 Optical ultrasonic microphone [0001] The present invention relates to a microphone using light. The present invention relates to an optical ultrasonic microphone that receives ultrasonic waves propagating in a gas such as air, and converts the received ultrasonic waves into electric signals using light. [0002] 05-05-2019 1 Dynamic microphones or condenser microphones are widely used in the audio band as devices for collecting and converting sound waves into electrical signals. In addition, piezoelectric sensors are widely used in the ultrasonic region. These devices make use of the fact that the main component of the energy of the sound wave propagates as a compressional wave of air, and conduct the fine vibration that is excited in the vibration plate by making the sound wave incident on the vibration plate electrically and electrostatically Or piezoelectrically converted into electrical signals. [0003] In addition, an optical system such as a laser Doppler vibrometer (hereinafter referred to as an LDV) that measures fine and high-speed vibration using monochromatic light represented by laser light is widely used, and such an apparatus is Attempts have been made to collect sound waves that have been used. [0004] Patent Document 1 discloses an optical microphone to which a diaphragm found in a normal microphone and light measurement by light trigonometry are applied. [0005] Patent Document 2 discloses a laser Doppler microphone that measures sound pressure by directly propagating laser light into a sound field and directly capturing, with an LDV, a change in refractive index caused in the air by sound waves. [0006] The laser Doppler microphone in Patent Document 2 will be described with reference to FIG. 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 an operation unit 127. The reflecting mirror 122 and the reflecting mirror 123 are disposed in parallel across the sound field 126. 05-05-2019 2 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 facing surface facing the reflecting mirror 122. [0007] The LDV 121 emits laser light toward the reflecting mirror 122 at an angle other than 90 degrees. The emitted laser light is repeatedly reflected a plurality of times by the reflecting mirror 122 and the reflecting mirror 123, propagates along the laser light path 125, and reaches the cubic mirror 124 provided at the end of the reflecting mirror 123. For the laser beam that has reached the LDV 121, the LDV 121 and the calculation unit 127 measure the frequency change of the returned laser beam and the amplitude and phase for each frequency change amount. [0008] In FIG. 20, when no sound wave is present in the sound field 126, the laser light does not receive the Doppler modulation, so the amount of change in the frequency of the laser light is zero. However, when sound waves exist in the sound field 126, temporal fluctuations occur in the air density of the sound field 126. This temporal fluctuation induces temporal fluctuation of the air refractive index. This is the same value as the temporal expansion and contraction of the optical length of the laser light path 125 which is the path length converted to the laser light wavelength, and optically as if the cubic mirror 124 fluctuates temporally with respect to the traveling direction of the laser light (time Motion that is equivalent to the [0009] Therefore, since the laser light is Doppler modulated, the amplitude and phase at the Doppler frequency and each Doppler frequency are measured. By Fourier-transforming them into a realtime signal, the time variation of the air refractive index in the sound field 126 is reproduced. By integrating it, the air refractive index at any time is calculated. As a result, sound waves in the sound field 126 (average value of sound pressure on the laser light path 125) are measured. [0010] 05-05-2019 3 The inventors of the present invention, in Patent Document 3, disclose an invention relating to a gas ultrasonic transducer capable of transmitting and receiving ultrasonic waves in high sensitivity and in a wide band by utilizing the refraction of ultrasonic waves in gas. In Non-Patent Document 1, transmission and reception characteristics in the 500 kHz ultrahigh frequency region are reported. [0011] FIG. 21 shows the ultrasonic transducer of the invention disclosed in Patent Document 3 and Patent Document 4. As shown in FIG. 21, the ultrasonic transducer 101 according to the invention of Patent Document 3 is provided at least on the front surface of the ultrasonic transducer 102 and the ultrasonic transducer 102, and the environmental fluid 104 and the ultrasonic transducer 102 are provided. And a propagation medium portion 103 which fills the space between Here, environmental fluid indicates a fluid present 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 unit 103 is defined as a first surface area 111, and the interface between the propagation medium unit 103 and the environmental fluid 104 is defined as a second surface area 112. [0012] The ultrasonic wave receiver 101 is surrounded by an environmental fluid 104. The ultrasonic wave propagating in the ultrasonic wave propagation path 105 is received by the ultrasonic wave receiver 101 from the environmental fluid 104 toward the ultrasonic wave receiver 101. [0013] 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 very small acoustic impedance such as air. It is possible. [0014] In general, most of the ultrasonic waves are reflected at the interface of media having different acoustic impedances, such as at the interface of gas and solid, so it is difficult to transmit with 05-05-2019 4 high efficiency and to receive with high sensitivity. [0015] As described above, in order to transmit the ultrasonic waves propagating in the gas with high efficiency and to receive the ultrasonic waves with high sensitivity, the ultrasonic transducer 101 has a lower speed of sound velocity propagating inside thereof than the environmental fluid 104, Uses a dry gel material (hereinafter referred to as "silica dry gel") composed of a silica skeleton larger than the environmental fluid 104. Silica dry gel is a material that can have various speeds of sound and density depending on the manufacturing process. For example, as the density 200 kg / m <3> and the sound velocity 150 m / s, the condition of the propagation medium portion 103 capable of transmitting ultrasonic waves with high efficiency is satisfied. [0016] The propagation medium portion 103 is made of silica dry gel, and as shown in FIG. 21, an angle θ1 between the normal line of the second surface region 112 and the ultrasonic wave propagation direction inside the propagation medium portion 103 and the inside of the environmental fluid 104 The transmission / reception sensitivity of the ultrasonic transducer can be improved by making the reflection of the ultrasonic wave in the second surface area 112 almost zero by appropriately selecting the angle θ2 with the ultrasonic wave propagation direction in . In addition, since the transmission efficiency in the second surface area 112 is not related to the frequency of the propagating ultrasonic wave, in principle a wide band characteristic can be realized and various frequencies can be measured with high efficiency. [0017] Specifically, an ultrasonic wave is generated by giving an electric signal to the ultrasonic transducer 102 from a drive circuit (not shown). Here, as shown in FIG. 21, the XYZ directions 05-05-2019 5 are set. The ultrasonic waves generated by the ultrasonic transducer 102 propagate from the first surface area 111 to the second surface area 112 in the propagation medium portion 103 in the positive direction of the Y axis. Then, the ultrasonic wave reaching the second surface area 112 changes the propagation direction according to the law of refraction, and propagates toward the direction of the ultrasonic wave propagation path 105 (in this case, the opposite direction of the arrow) to the fluid 104 To go. [0018] In the case of receiving ultrasonic waves, the ultrasonic waves propagating in the fluid 104 in the surrounding space are refracted and transmitted to the propagation medium portion 103 when reaching the second surface area 112, contrary to the case of transmission. It propagates inside the propagation medium section 103 in 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) in order to generate a potential difference between the electrodes by deforming the ultrasonic transducer 102. [0019] In the ultrasonic transducer 101, even when the environmental fluid 104 is a medium having a very small acoustic impedance (sound velocity of material x density of material) such as air, the ultrasonic wave is incident from the environmental fluid 104 into the propagation medium portion 103 with high efficiency. Alternatively, ultrasonic waves can be emitted from the propagation medium portion 103 to the environmental fluid 104 with high efficiency. [0020] The ultrasonic transducer 101 is set to satisfy (高 く 2 / ρ1) <(C1 / C2) <1 in order to increase the transmission efficiency of ultrasonic waves. Here, the velocity of sound C1 in the propagation medium portion 103 of ultrasonic waves, the velocity of sound C2 in the environmental fluid 104 of ultrasonic waves, the density 11 of the propagation medium portion 103, and the density 22 of the environmental fluid 104 are shown. [0021] 05-05-2019 6 In addition, θ1 can be calculated using (tan θ1) <2> = {(ρ2 / ρ1) <2>-(C1 / C2) <2>} / {(C1 / C2) <2 using C1, C2, 11 and 22. It is set to satisfy> −1}. [0022] Further, θ1 and θ2 are set to satisfy (sin θ1 / C1) = (sin θ2 / C2). [0023] 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 an ultrasonic transducer 101 capable of transmitting and receiving ultrasonic waves with high efficiency. [0024] Patent Document 1: Japanese Patent Application Publication No. 2004-12421 Patent Document 2: Japanese Patent Application Publication No. 2004-279259 Patent Document 2: International Publication No. 2004/098234 Patent Application Publication No. 2005/0139013 [0025] "Acoustic properties of nanofoam materials and their application to ultrasonic sensors (general / acoustic imaging) (Acoustic Properties of Nanofoam Material and its Applied Ultrasonic Sensors)" Masahiko Hashimoto, Hidetoshi Nagahara, Takehiko Suginoi, The Institute of Electronics, Information and Communication Engineers Technical Report of IEICE, Vol. 105, no. 619, US 2005-127 (P. 29-34). [0026] In the optical microphone disclosed in Patent Document 1, the mechanical resonance characteristic of the diaphragm largely affects the frequency band, as in a normal microphone. That is, the frequency lower than the mechanical resonance frequency of the diaphragm has 05-05-2019 7 relatively flat frequency characteristics. However, above the resonance frequency, the sensitivity drops sharply, so the upper limit of the measurable frequency band as the microphone is limited to around the resonance frequency. Therefore, the characteristic guarantee upper limit frequency of the current condenser microphone using the diaphragm is up to about 100 kHz, and in the case of more than that, the piezoelectric type is used. Therefore, it is extremely difficult to construct a microphone having a diaphragm whose band characteristics extend up to 100 kHz or more while maintaining sufficient sensitivity. [0027] Moreover, since the laser doppler microphone disclosed in Patent Document 2 does not have a diaphragm, there is no limitation of the high frequency region due to mechanical resonance. Also, the high frequency limit in the vibration measurement of the LDV being used easily exceeds 1 MHz. However, since the change in refractive index related to the sound pressure of air is small, an extremely long optical path is required to ensure sufficient sensitivity. [0028] In the example disclosed in Patent Document 2, an optical path length of 10 m or more is required to obtain a sufficient S / N. Therefore, miniaturization of the measurement area is extremely difficult. As a result, in the high frequency region, sound wave interference easily occurs in the measurement region, and accurate measurement of the sound pressure becomes difficult. This phenomenon corresponds to mechanical resonance in the diaphragm type, and is called "cavity resonance". [0029] That is, the dimensions of the measurement range determine the upper limit of the microphone, and the speed of sound of air is slower than the elastic wave velocity of a general diaphragm, so if the measurement area and the diaphragm have the same area, the upper limit is The laser 05-05-2019 8 doppler microphone is lower. As described above, in the conventional optical microphone, although the bandwidth of light measurement is sufficiently wide, the high frequency band is limited by the mechanical resonance or cavity resonance to be used, and it is difficult to operate particularly in the super high frequency region of 100 kHz or more Have the task of [0030] Furthermore, the point that an optical path length of 10 m or more is required poses a problem different from the above. In general, air in free space has spatial / temporal inhomogeneity of refractive index distribution due to temperature fluctuation or flow. Therefore, when using a light interferometer such as LDV to capture sound waves as refractive index fluctuations on the optical path, large spatial and temporal nonuniformities in the refractive index distribution due to long optical path lengths are mixed in with refractive index fluctuations due to only sound waves. Do. Therefore, there is also a problem that a signal to be originally excluded is mixed into the measured sound signal, and correct sound measurement becomes difficult. [0031] By applying the configuration of Patent Document 2, it is composed of a mechanically strong structure, the optical path is taken in a dilute gas atmosphere rich in fluidity such as vacuum or helium, and high alignment accuracy is realized. Although it is possible in principle to solve this problem, the prescription induces another problem that the price of the entire apparatus is increased considerably. [0032] Further, in the ultrasonic transducer 101 of Patent Document 3, since there is no frequency characteristic in taking in sound waves into the propagation medium portion 103 made of silica dry gel, it is possible to take in sound waves in a wide frequency range. Although there is a need for an ultrasonic transducer 102, such as a piezoelectric ceramic, to convert the captured sound waves into an electrical signal. [0033] When the propagation medium portion 103 is made of silica dry gel and the ultrasonic transducer 102 is made of piezoelectric ceramic, the acoustic impedance value is different from 2 digits to 3 digits. Most of the sound wave entering the element 102 is reflected at the interface 05-05-2019 9 (first surface region 111) of the propagation medium portion 103 and the ultrasonic transducer 102. The reflected sound wave propagates in the propagation medium portion 103 in the reverse direction and is partially emitted to the air, but the rest is propagated while repeating reflections at the boundary (the second surface region 112) in the propagation medium portion 103 It propagates in the medium section 103 and becomes a reverberation. [0034] The reflection phenomenon at the interface (the first surface region 111) of the propagation medium portion 103 and the ultrasonic transducer 102 is piezoelectric as an element for conversion to an electrical signal in the propagation medium portion 103 made of silica dry gel. In a configuration in which materials having different acoustic impedances, such as ceramic, are disposed, this is essentially unavoidable. The reverberation associated with this reflection causes a reduction in S / N by superimposing on a sound wave signal arriving later, and there is a problem that the frequency characteristic such as an unnecessary resonance phenomenon is deteriorated. [0035] Furthermore, in the ultrasonic transducer 101 of Patent Document 3, there is also a problem that the receiving sensitivity is low. The cause of this problem will be described below. [0036] The energy density of the ultrasonic wave that has propagated the environmental fluid 104 decreases when it is received by the ultrasonic transducer 101. This is the cause of the low receiving sensitivity. The cause of the low receiving sensitivity will be described with reference to FIG. In FIG. 21, the ultrasonic wave 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, (ρ2 // 1) <(C1 / C2) <1 and (tan θ1) <2> = {(ρ2 /) It is necessary to satisfy ρ1) <2>- 05-05-2019 10 (C1 / C2) <2>} / {(C1 / C2) <2> -1}, (sin θ1 / C1) = (sin θ2 / C2). At this time, in the path of the ultrasonic wave propagating through the environmental fluid 104, the angle formed with the normal to the second surface area 112 satisfies θ2. [0037] Therefore, in FIG. 21, the ultrasonic wave propagates in the range of the length (L 2 + L 3 + L 4) of the fluid 104 toward the ultrasonic transducer 101, and further the ultrasonic wave propagation path and the normal of the second surface area 112. It is assumed that the angle to be formed satisfies θ2. Here, the range of the length L2 is a range parallel to the ultrasonic wave propagation path 105 and means a range in which the ultrasonic waves do not reach the second surface area 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, which means a range in which the ultrasonic waves can all reach the second surface area 112. . The range of the length L4 is a range adjacent to the range of the length L3 and parallel to the ultrasonic wave propagation path 105, and means a range in which the ultrasonic waves do not reach all to the second surface area 112. [0038] As shown in FIG. 21, not all 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 waves propagating in the range of the length L3 reach the second surface area 112 and are received by the ultrasonic transducer 101. However, the ultrasonic waves propagating in the range of the length L2 and the range of the length L4 can not reach the second surface area 112 and are not received by the ultrasonic transducer 101. [0039] That is, a part of the ultrasonic waves (the ultrasonic waves propagating in the range of the length L3) of the ultrasonic waves (the ultrasonic waves propagating in the range of the lengths L2 + L3 + L4) that have propagated through the environmental fluid 104 The sound wave is received by the sound wave transmitter / receiver 101. [0040] 05-05-2019 11 Then, the ultrasonic wave propagating in the range of the length L3 of the environmental fluid 104 is transmitted through the second surface area 112 and detected by the ultrasonic transducer 102 in the range of the length L1. At this time, since L 3 << L 1, the ultrasonic waves received by the ultrasonic transducer 101 are diffused in the second surface area 112 and reach the ultrasonic transducer 102. Accordingly, when ultrasonic waves are received by the ultrasonic transducer 101, the energy density thereof is reduced. Due to the decrease in energy density of the ultrasonic waves, the wave receiving sensitivity of the ultrasonic transducer 101 is lowered. [0041] Due to the above reasons, the receiving sensitivity of the ultrasonic transducer 101 is low. That is, since the length L3 of the propagation range of ultrasonic waves 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 wave receiver 101 is low. It has become a thing. [0042] The object of the present invention is made in view of the above-mentioned problems, and it is possible to measure the sound pressure to the ultrasonic region far beyond the high frequency limit of the conventional microphone, and to be highly resistant to environmental fluctuation and high sensitivity. The present invention provides an optical ultrasonic microphone that achieves high efficiency measurement. [0043] The microphone according to the present invention is a microphone for receiving a sound wave propagating in a surrounding space filled with an environmental fluid, and the first sound wave is incident, and the light is incident from the first wave opening. A photoacoustic propagation medium having an acoustic waveguide through which sound waves propagate, and a transmission surface, the transmission surface forming one surface of the acoustic waveguide along the propagation direction of the ultrasonic wave. The transmission surface is configured such that a part of the ultrasonic wave is transmitted from the transmission surface to the propagation medium as it propagates through the waveguide and converges to a predetermined convergence point, A photoacoustic propagation medium portion disposed with respect to a waveguide, and a light wave emitted toward the convergence point, and the emitted light wave 05-05-2019 12 propagating through the photoacoustic propagation medium portion is received and received The sound pressure of the sound wave from the A detection unit, the density .rho.n of the photoacoustic propagation medium portion, the speed of sound Cn in the photoacoustic propagation medium portion, the density .rho.a of a gas filling the acoustic waveguide, and the speed of sound Ca in the gas filling the acoustic waveguide Satisfy the relationship of (ρa / ρn) <(Cn / Ca) <1 and set at an arbitrary position along the propagation direction of the ultrasonic wave on the transmission surface from the first opening of the waveguide When the length of the waveguide up to the point P is La and the length from the point P to the convergence point is Ln, La / Ca + Ln / Cn is constant regardless of the position of the point P. [0044] According to the optical ultrasonic microphone of the present invention, the sound wave propagating in the gas in the surrounding space is taken into the acoustic waveguide from the opening, and the sound wave traveling from the acoustic waveguide to the inside of the photoacoustic propagation medium is Mach-Zehnder By measuring with the use of an optical interferometer, it is possible to measure up to a high frequency region that greatly exceeds the limit due to the mechanical resonance of the conventional diaphragm, and the influence of the reflection of the sound wave by the electroacoustic transducer such as the conventional piezoelectric ceramic By realizing the small size and the high stability against fluctuations in the surrounding environment, it is possible to provide an optical ultrasonic microphone capable of more sensitive and accurate sound pressure measurement. [0045] A perspective view showing a schematic structure of the optical ultrasonic microphone 51 of the first embodiment A portion of the optical ultrasonic microphone 51 shown in FIG. 1 at the center of the focusing portion 77 and the acoustic waveguide member 56 in the X direction, Propagation of sound waves in the optical ultrasonic microphone shown in FIG. 1 which is a sectional view cut along a plane parallel to the YZ plane and which shows a part of the base of the optical ultrasonic microphone shown in FIG. 1 and a part of the acoustic waveguide member. -A schematic diagram for explaining refraction The schematic diagram for explaining the sound wave convergence of the optical ultrasonic microphone in the first embodiment of the present invention The sound showing the result of calculation experiment for the sound wave propagation in the optical ultrasonic microphone shown in FIG. Pressure distribution diagram Sound pressure distribution diagram showing the result of calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. 1 Sound pressure distribution diagram showing the results of calculation experiment for sound wave propagation in the microphone Sound pressure distribution diagram showing the results of calculation experiment for sound wave propagation in the optical ultrasonic microphone shown in FIG. 1 Calculation for the sound wave propagation in the optical ultrasonic microphone shown in FIG. 05-05-2019 13 Sound pressure distribution chart showing experimental results Sound pressure distribution chart showing experimental calculation results for sound wave propagation in the optical ultrasonic microphone shown in FIG. 1 Graph showing the time waveform of the input sound wave signal used in the experiments shown in FIGS. The sectional view showing the outline device composition of optical ultrasonic microphone 51A concerning the modification of a 1st embodiment The measurement of the equal phase side of the sound wave propagation of the photoacoustic propagation medium part of the optical ultrasonic microphone in the 1st embodiment of the present invention LDV output of the optical ultrasonic microphone 51 in the vicinity of the contour line acoustic wave acoustic focus 57 showing the result Waveform diagram showing an example of the waveform (amplitude measurement waveform) 81. Configuration diagram showing the optical system structure of the optical sound pressure measurement unit 1300 in the second embodiment of the optical ultrasonic microphone of the present invention Second embodiment of the present invention Configuration of the device showing the experimental device for demonstrating the principle of the optical ultrasonic microphone in the form the time of the output signal from the digital oscilloscope 1408 measured by the experimental device for demonstrating the principle of the optical ultrasonic microphone in FIG. Figure showing waveform and time waveform of input signal to speaker 1410 Configuration diagram showing optical system configuration of optical ultrasonic microphone 1300 having wide measurement dynamic range and no signal distortion in the third embodiment of the present invention 2 is a cross-sectional view of a conventional ultrasonic transducer at 2 and FIG. 3 is a block diagram of a conventional optical microphone in Patent Document 3 [0046] The present inventors pay attention to the fact that the silica dry gel has a property close to optical transparency, for example, the rate of change of the sound pressure and the refractive index in the silica dry gel relative to the rate of change in air, I found that it was about one digit higher. [0047] The rate of change of refractive index due to sound pressure usually increases in the order of solid, liquid and gas, which is a very unique property not found in normal bulk materials. [0048] The present invention utilizes the basic principle of the interface phenomenon of an ultrasonic wave receiver capable of propagating ultrasonic waves from a very low acoustic impedance object such as gas to a solid with high efficiency, and a solid that satisfies these conditions. The point is that an optical ultrasonic microphone is constructed in which the band characteristics 05-05-2019 14 extend to an extremely high frequency region by using a phenomenon that the material generates a very large refractive index change by the sound wave. [0049] In the present invention, a Mach-Zehnder interferometer is used to construct an entire optical system with the shortest possible path length, and the refractive index fluctuation generated in the photoacoustic propagation medium by the sound pressure is taken as the optical path fluctuation. There is an advantage that it is possible to directly measure from amplitude fluctuation of interference light or phase fluctuation of interference light. [0050] Hereinafter, the optical ultrasonic microphone of the present invention will be described with reference to the drawings. [0051] First Embodiment FIG. 1 shows an optical ultrasonic microphone 51 according to a first embodiment. The optical microphone 51 according to the first embodiment includes an opening 71 of an acoustic horn to which an ultrasonic wave propagating in the environmental fluid 14 is incident, an acoustic waveguide 60 through which an ultrasonic wave incident from the opening 71 propagates, and an acoustic waveguide 60 and an LDV head 8 emitting laser light 58 toward an acoustic focal point 57 where the ultrasonic waves propagating through the photoacoustic propagation medium unit 52 converge. And an LDV arithmetic processing unit 9 for detecting an ultrasonic wave from the reflected wave of the emitted laser beam 58. [0052] An outline of the operation of the optical microphone 51 will be described. Ultrasonic waves enter the inside of the optical microphone 51 along the sound wave propagation direction 55 from the environmental fluid 14 present outside the optical 05-05-2019 15 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 on the acoustic focal point 57. The LDV head 8 emits a laser beam 58 toward the acoustic focus 57. The LDV head 8 receives the reflected wave of the emitted laser beam 58. The LDV arithmetic processing unit 9 obtains, from the reflected wave received by the LDV head 8, the change in the refractive index of the photoacoustic propagation medium unit 52 due to the ultrasonic wave converged on the acoustic focus 57. The optical microphone 51 can detect an ultrasonic wave (for example, the sound pressure of the ultrasonic wave) corresponding to the amount of change of the refractive index. [0053] Hereinafter, each part of the optical ultrasonic microphone 51 of FIG. 1 will be described in detail. [0054] (Converging Unit 77) FIG. 2 is a cross section obtained by cutting a part of the optical ultrasonic microphone 51 shown in FIG. 1 in a plane parallel to the YZ plane at the center of the converging unit 77 and the acoustic waveguide member 56 in the X direction. Figure shows. [0055] The converging unit 77 includes a first opening 71 to which an ultrasonic wave propagating from the outside is incident, and a second opening 63 connected to the acoustic waveguide member 56 (acoustic waveguide 60). 05-05-2019 16 An inner space 70 through which the ultrasonic wave propagates is formed between the first opening and the second opening. [0056] The ultrasonic waves propagating the environmental fluid 14 in the sound wave propagation direction 55 are controlled in the propagation direction when propagating through the inner space 70 after being incident from the first opening 71, and the sound pressure is enhanced ( Compressed). The sound wave whose sound pressure has been increased by the converging unit 77 propagates to the acoustic waveguide 60 connected to the converging unit 77. [0057] In the inner 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 defining 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 side surface of the light source is configured to have a curved shape along the propagation direction g7. [0058] The width dimension of the converging portion 77 in the X direction may be constant, or the width dimension may be gradually reduced. When the width dimension in the X direction of the converging portion 77 is constant, the width dimension in the Z direction is preferably configured to decrease exponentially with respect to 05-05-2019 17 the propagation direction g7. Also, the cross-sectional area a7 may be exponentially reduced 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 manner, the reflection of the sound wave at the converging portion 77 can be minimized, the sound wave can be compressed without phase disturbance, and the sound pressure can be increased. [0059] The converging portion 77 has a length of, for example, 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. Further, the end portion 72 is a square having a length of 2 mm in the X direction and the Z direction. In the first embodiment, the lengths are changed in two directions, the Z direction and the X direction. Assuming that the position of the opening 71 is the origin (0) in the Y direction, the X direction and the Z direction of the inner space 70 at each position of the position in the Y direction = 0 mm / 20 mm / 40 mm / 60 mm / 80 mm / 100 mm from the origin (The length in the X direction and the length in the Z direction are the same at each position. )は、 50.0mm/26.3mm/13.8mm/7.2mm/3.8mm/2.0mmである。 [0060] According to the converging unit 77 having the above-described size, an effect of an increase in 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. In addition, the shape of the sound pressure waveform, which changes with time, hardly changes in the measurement results of the opening 71 and the end 72 and disturbs the sound wave propagating through the environmental fluid 14 (for example, air) Without the sound energy being compressed at the end 72. [0061] The converging portion 77 can be configured, for example, by machining an aluminum plate having a thickness of 5 mm, which is a metal, into a predetermined shape by machining. The convergence portion 77 may be formed of a material other than aluminum, as long as the material can hardly transmit the sound waves propagating in the inner space 70 and can 05-05-2019 18 increase the density of the sound energy by the effect of the shape. For example, the convergence portion 77 may be configured using a material such as resin or ceramic. Moreover, the convergence part 77 does not need to have a horn-type external shape, and the internal space 70 should just have a horn shape which was mentioned above. [0062] (Acoustic Waveguide 60) Next, an acoustic waveguide 60 for propagating a sound wave in a predetermined direction will be described. The acoustic waveguide 60 is constituted 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 incident from the environmental fluid 14 and propagating through the converging portion 77 is incident on the acoustic waveguide 60 from a portion connected to the second opening 63. [0063] 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 propagation direction of the sound wave. Here, the width dimension in the ZY plane changes depending on the position along the ultrasonic wave propagation direction g6 parallel to the ZY plane. The width dimension in the X-axis direction of the acoustic waveguide 60 is constant, for example, 2 mm. The width dimension in the X-axis direction can also be designed to change. [0064] The reason why the acoustic waveguide 60 has such a shape will be described. The acoustic waveguide 60 includes a transmission surface 61 in contact with the photoacoustic propagation medium portion 52 and defined by an interface with the photoacoustic propagation medium portion 52, and a waveguide outer surface 62 defined by the acoustic waveguide member 56. There is. Further, the front and back sides in the X direction of the acoustic waveguide 60 are also defined by the acoustic waveguide member 56. As the sound wave propagates in the acoustic waveguide 60, it gradually infiltrates (propagates) from the transmission surface 61 where the photoacoustic propagation medium portion 52 and the acoustic waveguide 60 are in contact to the photoacoustic propagation medium portion 52. At this time, in the transmission surface 61, the propagation direction of the sound wave is refracted. The photoacoustic 05-05-2019 19 propagation medium portion 52 may be provided to constitute a part of the acoustic waveguide 60. [0065] As the sound wave propagates through the acoustic waveguide 60, as a result of the transmission of at least a part of the sound wave from the transmission surface 61 to the photoacoustic propagation medium portion 52, the energy of the sound wave propagating through the acoustic waveguide 60 decreases. In order to compensate for the reduction in energy, the cross-sectional area of the acoustic waveguide 60 in a plane orthogonal to the propagation direction of the sound wave is reduced in order to compress the sound wave (increase the sound pressure of the sound wave). [0066] 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 monotonically decreases with respect to the propagation direction. Also, 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 portion 52 while keeping the energy density of the sound wave propagating through the acoustic waveguide 60 constant. The specific operation until the sound wave propagating through the acoustic waveguide 60 is incident on the photoacoustic propagation medium portion 52 will be described later. [0067] (Photoacoustic Propagation Medium Portion 52) Next, the photoacoustic propagation medium portion 52 defining the transmission surface 61 through which the sound wave passes from the acoustic waveguide 60 to the acoustic propagation medium portion 52 will be described. The photoacoustic propagation medium portion 52 is made of a material whose propagation velocity of sound waves is slower than that of the environmental fluid 14. That is, assuming that 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, (Cn / Ca) <1 is satisfied. Materials that satisfy this condition include dried gels of inorganic acid compounds or organic polymers. It is preferable to 05-05-2019 20 use a silica dry gel as a dry gel of an inorganic acid compound. Hereinafter, a method for producing a silica dry gel will be described. [0068] First, a mixed solution of tetraethoxysilane (hereinafter abbreviated as TEOS), ethanol and aqueous ammonia is prepared, and this is gelled to prepare a wet gel. The "wet gel" refers to one in which the pore portion of the dried gel is filled with liquid. The liquid portion of this wet gel is replaced with liquefied carbon dioxide gas and removed by supercritical drying using carbon dioxide gas to obtain a silica dry gel. The density of the silica dry gel can be adjusted by changing the mixing ratio of TEOS, ethanol and aqueous ammonia, and the speed of sound changes with the density. [0069] The silica dry gel is a material consisting of a fine porous structure of silicon oxide, and the skeleton is hydrophobized. The size of the pores and the skeleton is about several nm. When the solvent is dried directly from the state in which the liquid is contained in the pore portion of such a structure, a large force by capillary action is exerted when the solvent is volatilized, and the structure of the skeleton portion is easily broken. By using supercritical drying in which no surface tension acts to prevent the breakage, a dried gel can be obtained without breaking the silica skeleton. [0070] More preferably, the propagation medium of the photoacoustic propagation medium portion 52 is C n, the sound velocity of the acoustic fluid in the environmental fluid 14 is Ca, the density of the propagation media is n n, and the density of the environmental fluid 14 is ρ a When (.rho.n / .rho.a) <(Ca / Cn) <1 is satisfied. [0071] The propagation medium of the photoacoustic propagation medium portion 52 more preferably has a density of the propagation medium of 100 kg / m 3 or more and a velocity of sound in the propagation medium of 300 m / s or less. 05-05-2019 21 [0072] The density n n of the silica dry gel constituting the photoacoustic propagation medium portion 52 used in the first embodiment is 200 kg / m <3>, and the speed of sound 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 density 空 気 a of air is 1.12 kg / m <3>, and the speed of sound Ca is 340 m / s at around room temperature. [0073] 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 focus 57, the sound wave reaching the acoustic focus 57 is weakened when the internal loss is large. For this reason, the photoacoustic propagation medium unit 52 is preferably made of a material having a small internal loss. Silica dry gel is a material that meets the above-mentioned conditions of sound velocity and density, and has a small internal loss. [0074] (Base 53) The base 53 supporting the photoacoustic propagation medium unit 52 will be described. Since the silica dry gel which comprises the photoacoustic propagation medium part 52 has low density, mechanical strength is also low. For this reason, handling is difficult. Therefore, a base 53 is provided to support the photoacoustic propagation medium portion 52 stably. [0075] FIG. 1 illustrates that a part of the base 53 located on the surface side of the photoacoustic 05-05-2019 22 propagation medium portion 52 is broken to expose the photoacoustic propagation medium portion 52 for easy understanding. In fact, the surface of the photoacoustic propagation medium portion 52 may be entirely covered with the base 53 except for the measurement through holes 53a described later. When the surface of the photoacoustic propagation medium portion 52 is completely covered except for the measurement through holes 53a, it is possible to avoid the incidence of sound waves other than the ultrasonic waves to be detected. [0076] As shown in FIG. 1, the base 53 covers the surface of the photoacoustic propagation medium portion 52 (the back surface, the left side surface, the bottom surface, etc. of the photoacoustic propagation medium portion 52 shown in FIG. 1). Further, the right side surface of the photoacoustic propagation medium portion 52 shown in FIG. 1 is covered with the acoustic waveguide member 56 and configured as 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. [0077] The acoustic waveguide member 56 and the base 53 can be configured according to 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. [0078] As shown in FIG. 3 (a), an aluminum member is used to form an acoustic waveguide member 56 defining an acoustic waveguide 60 including the waveguide outer surface 62. As shown in FIG. On the other hand, as shown in FIG. 3B, a base 53 for holding the photoacoustic propagation medium portion 52 is prepared. The exposed surface of the photoacoustic propagation medium portion 52 held by the base 53 defines a transmission surface 61. [0079] 05-05-2019 23 For example, a base 53 made of porous ceramic is formed. The base 53 is inserted into a mold in which the surface defining the transmission surface 61 is made of fluorine resin or the like, and the wet gel is introduced into the space. Thereafter, the liquid portion is replaced with liquefied carbon dioxide gas and dried to obtain a member in which the photoacoustic propagation medium portion 52 and the base 53 are integrated. [0080] 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 waveguides 60 of which the transmission surface 61 is defined by the photoacoustic propagation medium portion 52 are formed by respectively bonding them with an adhesive such as epoxy resin. [0081] (Sound Wave Propagating Acoustic Waveguide 60) Next, the geometric shape of the acoustic waveguide 60 and the photoacoustic propagation medium portion 52 defined by the acoustic waveguide member 56 and the propagation of the sound wave will be described in detail. [0082] FIG. 4 is an enlarged view of a part of the acoustic waveguide 60 in the optical ultrasonic microphone 51. As shown in FIG. The propagation and refraction of sound waves will be described with reference to FIG. [0083] In FIG. 4, the transmission surface 61 and the waveguide outer surface 62 are shown by dotted lines, and the perpendiculars of tangents at arbitrary points of the transmission surface 61 are shown by dashed dotted lines. Further, the propagation direction of the sound wave is indicated by an arrow 55a. 05-05-2019 24 [0084] A sound wave incident from the opening 63 and traveling in the acoustic waveguide 60 propagates in the acoustic waveguide 60 while changing the traveling direction according to the shape of the acoustic waveguide 60. The interior of the acoustic waveguide 60 is filled with the environmental fluid 14. [0085] The component of the sound wave contacting the transmission surface 61, which is the interface between the acoustic waveguide 60 and the photoacoustic propagation medium portion 52, is incident on the transmission surface 61 at an angle θa with respect to the normal to the transmission surface 61. As satisfying, the light is refracted and transmitted to the photoacoustic propagation medium portion 52 at a constant angle θ n with the normal to the transmission surface 61. [0086] The angle θ n in the propagation direction of the sound wave in the inside of the photoacoustic propagation medium unit 52 is represented by Equation 1. Here, when the relationship of (Cn / Ca) <1 is satisfied, the angle θn obtained by Equation 1 becomes a positive value, and the sound wave is refracted and transmitted into the photoacoustic propagation medium portion 52. [0087] [0088] 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 nn, and the density of the environmental fluid 14 is ρa. 05-05-2019 25 [0089] 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. [0090] [0091] In order to refract and transmit a sound wave from the acoustic waveguide 60 to the photoacoustic propagation medium portion 52 as efficiently as possible, it is preferable that the reflectance R be as small as possible. When Cn, Ca, nn, ρa satisfy (ρn / ρa) <(Ca / Cn) <1, there always exist angles θa, θn where the numerator of Equation 2 becomes zero. That is, the reflectance R can be made zero. [0092] In the first embodiment, as described above, the density n n of the dried silica gel is 200 kg / m <3>, the speed of sound Cn in the dried silica gel is 150 m / s, and the density a a of air is 1.12 kg. / M <3>, and the sound velocity Ca is 340 m / s near room temperature. [0093] Substituting these values into Equation 1, the angle θ n is approximately 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 higher in the direction in which the angle θ n becomes about 26 degrees by the sound wave entering the transmission surface 61 at about 89 degrees with respect to the normal of the transmission 05-05-2019 26 surface 61 It permeate ¦ transmits the inside of the photoacoustic propagation medium part 52 by transmission efficiency. [0094] When the reflectance R is almost zero, the refraction angle θn is constant at about 26 degrees, but by making the transmission surface 61 a curved surface, the sound waves transmitted to the photoacoustic propagation medium portion 52 from different positions of the transmission surface 61 Can be propagated toward a predetermined acoustic focus 57 to converge the sound wave. [0095] Further, by bending the acoustic waveguide 60 along the transmission surface 61, as the sound wave propagates through the acoustic waveguide 60, a part of the sound wave can be incident on the transmission surface 61 at a constant angle θa. By utilizing this phenomenon, the sound wave propagating through the acoustic waveguide 60 is refracted and transmitted little by little to the photoacoustic propagation medium portion 52, and the acoustic wave is converged to one point in the photoacoustic propagation medium portion 52, thereby achieving high wave receiving sensitivity. To achieve. [0096] Further, the refraction angle θ n represented by Equation 1 and the reflectance R represented 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 a wide band sound wave with high sensitivity. That is, according to the first embodiment, reception of ultrasonic waves in a high frequency range and in a wide band, which was conventionally difficult, becomes possible with high sensitivity, and a standard microphone having an effective band of 100 kHz or more and high sensitivity It will be realized. 05-05-2019 27 [0097] In the field of optical lenses, for example, Japanese Patent No. 2731389 discloses a structure for focusing light emitted from the side surface of an optical waveguide. However, in general, in the 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, the sound wave is on the outer surface or side surface of the acoustic waveguide 60 It does not reflect. For this reason, in the optical waveguide, it is important to propagate the sound wave having the same phase in the first embodiment while the phase of the propagating light is not the same. [0098] (Shapes of Transmission Surface 61 and Waveguide Outer Surface 62) Next, design of shapes of the transmission surface 61 and the waveguide outer surface 62 defining 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. [0099] First, from the size of the opening 63 of the acoustic waveguide 60, the length of the acoustic waveguide 60 capable of efficiently incorporating the sound wave into the photoacoustic propagation medium portion 52 is determined. Due to the length of the acoustic waveguide 60, the transmission surface 61 is designed as a shape for focusing the sound wave. 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. [0100] The size of the opening 63 of the acoustic waveguide 60 is preferably half or less of the wavelength of the received sound wave. If the width of the waveguide is larger than half 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, and accurate 05-05-2019 28 measurement of the sound wave becomes difficult. It is. [0101] In the first embodiment, as an example, since the reception of sound waves up to a frequency of 80 kHz is considered, 2.0 mm smaller than 2.1 mm, which is a half wavelength of 80 kHz, is used, and the opening 63 has one side It has a square shape of 2.0 mm. The end 72 of the converging part 77 is designed to be equal in size to the opening 63. [0102] As the length of the acoustic waveguide 60 increases, the number of sound waves propagating from the acoustic waveguide 60 to the photoacoustic propagation medium 52 increases. 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 portion 52. [0103] As described with reference to FIG. 21, in the ultrasonic wave receiver 101, the sound wave propagating in the range of the length L3 is transmitted to the inside of the propagation medium portion 103 through the second surface region 112. . 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 making the length of the transmission surface 61 in the YZ plane, ie, the length of the sound wave propagation direction g6 in the acoustic waveguide 60 sufficiently long, the sound wave can be sufficiently transmitted to the photoacoustic propagation medium portion 52, In the first embodiment, it is possible to improve the receiving sensitivity, to reduce the influence of reflection of the sound wave that can not be captured, and to improve the measurement accuracy, and in the first embodiment, the photoacoustic propagation medium portion 52 in the environmental fluid 14 Since the angle θa (FIG. 4) formed by the normal line and the sound wave propagation direction 55a is about 89.3 degrees, the ratio of the length L2 to the length L1 is about L1 / L2 = 88. Therefore, ideally, the acoustic waveguide 60 preferably has a length of about 90 times or more of 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, 05-05-2019 29 which is 100 times the opening 63. [0104] Thus, the lengths of the opening 63 and the acoustic waveguide 60 are determined. The shape of the transmission surface 61 and the shape of the outer surface of the waveguide are designed based on the length of the acoustic waveguide 60. (Convergence of Sound Wave) FIG. 5 is an enlarged view of the acoustic waveguide 60 and the photoacoustic propagation medium portion 52. As shown in FIG. The convergence of the sound wave in the optical ultrasonic microphone 51 in the first embodiment will be described with reference to FIG. [0105] An acoustic focus 57 for focusing the sound wave is set in the photoacoustic propagation medium unit 52. The LDV head 8 is made to face the acoustic focus 57, and the LDV head 8 and the LDV arithmetic processing unit 9 detect a sound wave using the laser beam 58. [0106] In FIG. 5, a point at the opening 63 of the transmission surface 61 is set as a start point P 0, and points P 1, P 2, P 3,..., P n are sequentially arranged from the side closer to the opening 63 of the transmission surface 61. 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 focus 57 are Ln1, Ln2, ..., Lnn, respectively. [0107] In order for the sound wave incident from the opening 63 to propagate in the acoustic waveguide 60 and to be refracted and transmitted to the photoacoustic propagation medium portion 52 to be focused at the acoustic focal point 57, it is necessary to satisfy the following Equation 3 is there. [0108] 05-05-2019 30 [0109] The fact that the sound wave is focused on the acoustic focus 57 in the photoacoustic propagation medium portion 52 means that the phase of the sound wave is aligned at the acoustic focus 57. That is, it means that the arrival time of the sound wave from the opening 63 to the acoustic focus 57 is the same no matter which route it passes through. [0110] Specifically, in Equation 3, the left side (La1 / Ca) + (Ln1 / Cn) of the leftmost equal sign causes the sound wave to propagate through the environmental fluid 14 by the distance La1, and the photoacoustic propagation medium portion 52 By propagating by the distance Ln1, the time to reach the acoustic focus 57 is shown. Also, {(La1 + La2) / Ca} + (Ln2 / Cn), which is the right side of the leftmost equal sign, propagates the sound wave by the distance (La1 + La2) in the environmental fluid 14, and the distance in the photoacoustic propagation medium portion 52 By propagating Ln 2, the time to reach the acoustic focal point 57 is shown. [0111] By the same procedure, at each point Pk, the time until the sound wave transmitted from the acoustic waveguide 60 to the photoacoustic propagation medium portion 52 can reach the acoustic focal point 57 can be obtained. [0112] Generalizing equation (3), the distance of the waveguide from the opening 63 of the acoustic waveguide 60 to any point Pk along the propagation direction of the sound wave on the transmission surface 61 is Lak, and from the point Pk to the photoacoustic propagation medium Assuming that the distance to the acoustic focus F (57) different from the point Pk in the part 52 is Lnk, Equation 3 is (Lak / Ca) + (Lnk / for any k from 1 to n. It is expressed as a condition that 05-05-2019 31 Cn) is constant. [0113] The constant (Lak / Ca) + (Lnk / Cn) means that the time required from the opening 63 to the acoustic focal point 57 at any position on the transmission surface 61 is constant, as described above. It shows that there is. In other words, by satisfying (Lak / Ca) + (Lnk / Cn), it means that the sound wave taken into the photoacoustic propagation medium portion 52 converges on the acoustic wave focal point 57. In the configuration in FIG. 1, the shape of the photoacoustic propagation medium unit 52 on the side of the acoustic waveguide 60 satisfies (Lak / Ca) + (Lnk / Cn). [0114] FIG. 5 is a diagram for explaining that (Lak / Ca) + (Lnk / Cn) is constant. The sound wave entered into the photoacoustic propagation medium portion 52 made of silica dry gel from an arbitrary point on the transmission surface 61 of the acoustic waveguide 60 of the photoacoustic propagation medium portion 52 starting from the opening 63 of the acoustic waveguide Even if the transmission surface 61 satisfies (Lak / Ca) + (Lnk / Cn) at a constant level, it converges on the acoustic focus 57. This is because, in the photoacoustic propagation medium portion 52, a cylindrical (partially cylindrical) wavefront centered on the acoustic focus 57 is formed. [0115] Strictly speaking, it is considered more accurate to calculate the propagation distance of the sound wave propagating in the acoustic waveguide 60 by using the central path of the acoustic waveguide 60. However, as described below, the width dimension of the acoustic waveguide 60 is sufficiently smaller than its length. For this reason, the above-mentioned approximation has practically sufficient accuracy. 05-05-2019 32 [0116] (Light Source and Detection Unit) In FIG. 1, for convenience, only the propagation path of the laser beam 58 in the air is described. A detection unit for detecting the ultrasonic wave converged on the acoustic focus 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. [0117] By focusing the sound wave on the acoustic focus 57, a standing wave of a compressional wave with a large amplitude is generated. As a result, a change in refractive index depending on the sound pressure of the acoustic signal received by the converging unit 77 occurs in the acoustic focus 57. [0118] The LDV head 8 emits (fires) the laser beam 58 toward the propagation medium unit 52. Further, after being emitted, the LDV head 8 propagates the propagation medium portion 52 and receives the reflected laser beam 58. The LDV arithmetic processing unit 9 converts the received laser beam 58 into an electrical signal and performs signal processing. [0119] The laser beam 58 emitted from the LDV head 8 is frequency-modulated by the Doppler frequency according to the speed 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. [0120] For example, using the thickness information of the photoacoustic propagation medium portion 52 in the direction in which the laser beam 58 was emitted, twice the distance between the 05-05-2019 33 detecting portion and the measurement through hole 53a, and the velocity of the laser beam 58 emitted from the detecting portion The time from the emission of the light 58 to the reception of the reflected wave is determined in advance. The amount of frequency modulation of the laser beam 58 can be known from the difference between the determined time and the actually measured time. The sound pressure of the sound wave can be determined from the amount of frequency modulation. [0121] The optical ultrasonic microphone 51 according to the first embodiment propagates a sound wave from the environmental fluid 14 having a very small acoustic impedance, such as gas, to the solid with high efficiency, and converges the sound wave transmitted to the solid within the solid. Energy density can be increased. Thereby, sound waves can be received with high sensitivity. [0122] (Experimental Results) FIGS. 6 to 11 show a process in which the 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 portion 52 and is converged to the acoustic focus 57. The results obtained by calculation experiments are shown. 6 to 11 are sound pressure distribution diagrams showing calculation experimental results for sound wave propagation in the optical ultrasonic microphone shown in FIG. 6 to 11 show only the acoustic waveguide 60 and the photoacoustic propagation medium portion 52 of the optical ultrasonic microphone 51 in order to easily display the position and the phase of the sound wave. [0123] 6 to 11 show the propagation of sound waves over time. FIG. 6 shows the earliest in time, and FIG. 11 shows the slowest state. The transmission surface 61 and the waveguide outer surface 62 which define the acoustic waveguide 60 shown in FIGS. 6 to 11 are designed such that the sound wave propagating in the acoustic waveguide 60 converges on the acoustic focus 57 by the above-described procedure. . The opening 63 of the acoustic waveguide 60 is at the top and the closed end is at the bottom. The acoustic waveguide 60 is filled with an environmental fluid 14, 05-05-2019 34 here air. [0124] The time waveform of the input sound wave signal used for FIG. 12 at the experiment shown to FIGS. 6-11 is shown in FIG. 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. [0125] 6 to 11, the sound pressure levels of the sound waves in the photoacoustic propagation medium unit 52 and in the acoustic waveguide 60 are shown by shading of colors. Dark parts (black) indicate sound pressure higher than atmospheric pressure, and light parts (white) indicate sound pressure lower than atmospheric pressure. The same color, for example between black and black or white and white, represents 40 kHz, ie corresponding to one wavelength of the sound wave. [0126] In FIGS. 6 to 11, although it is difficult to confirm because the acoustic waveguide 60 is very narrow, in the acoustic waveguide 60, since the sound velocity of air is 340 m / s, the distance between the same colors is That is, the distance of one wavelength is about 8.5 mm. [0127] On the other hand, since the speed of sound of the dried gel constituting the photoacoustic propagation medium portion 52 is 150 m / s in the photoacoustic propagation medium portion 52, the distance between the same colors, that is, the distance of one wavelength is about 3 It will be .75 mm. [0128] FIG. 6 shows the moment when three wavelengths of the sound wave propagate from the opening 63 to the acoustic waveguide 60 and the peak of the fourth wave propagates from the opening 63 into the acoustic waveguide 60. 05-05-2019 35 The portion of the acoustic wave propagated to the inside of the acoustic waveguide 60 propagates from the transmission surface 61 in contact with the acoustic waveguide 60 to the photoacoustic propagation medium portion 52. The portion shown by shading inside the photoacoustic propagation medium portion 52 is a component of the sound wave refracted and transmitted from the transmission surface 61 to the photoacoustic propagation medium portion 52. [0129] 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 sound wave propagating inside the acoustic waveguide 60 is gradually refracted and transmitted to the photoacoustic propagation medium portion 52, and propagates in the photoacoustic propagation medium portion 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 sound velocity of air, which is the environmental fluid 14 of the acoustic waveguide 60, is faster than the sound velocity of the drying gel, which is the propagation medium. [0130] Similarly, FIG. 8 also shows how, as part of the sound wave propagates through the acoustic waveguide 60, it is refracted and transmitted to the photoacoustic propagation medium portion 52, and the sound wave propagates inside the photoacoustic propagation medium portion 52. . Although the pattern indicated by black and white shades on the transmission surface 61 is bent due to refraction and transmission, the pattern indicated by black and white shades in the photoacoustic propagation medium portion 52 has a beautiful curve. It is getting worse. This indicates that the phases of the sound waves propagating in the photoacoustic propagation medium unit 52 are aligned. [0131] FIG. 9 shows a sound wave propagating near the near end of the acoustic waveguide 60 and a 05-05-2019 36 sound wave gradually converging toward the acoustic focal point 57 inside the photoacoustic propagation medium portion 52. [0132] 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 to the inside of the photoacoustic propagation medium portion 52. The sound wave propagating inside is shown to be converging toward the acoustic focus 57 further. [0133] In FIG. 11, the first wave front of the sound wave propagated inside the photoacoustic propagation medium portion 52 reaches the acoustic focal point 57. As shown in FIG. 6F, the darker shades of black are darker, which indicates that the acoustic wave converges at the acoustic focus 57 and the sound pressure is increased. [0134] Although specific numerical values are not shown in FIGS. 6 to 11, from experimental results, when the change in sound pressure due to the sound wave from the atmospheric pressure inside the acoustic waveguide 60 is about 4 Pa, The change in sound pressure from barometric pressure was found to be about 34 Pa. This indicates that the sound pressure of the sound wave has been increased by eight 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. [0135] As described above, according to the first embodiment, the reflection of the sound wave at the interface different in acoustic impedance is suppressed by refracting the sound wave and transmitting it from the environmental fluid 14 to the photoacoustic propagation medium unit 52, and the sound wave with high efficiency Can be transmitted to the photoacoustic propagation 05-05-2019 37 medium unit 52. [0136] Further, the photoacoustic propagation medium portion 52 is disposed to constitute one surface of the acoustic waveguide 60 filled with the environmental fluid 14, and as the acoustic waveguide 60 propagates, part of the acoustic wave is transmitted to the photoacoustic propagation medium portion 52. By designing the shape of the surface in contact with the acoustic waveguide 60 so as to transmit light to and converge on the predetermined acoustic focus 57, the phases of the sound waves transmitted to the photoacoustic Can be focused on the acoustic focus 57. Therefore, the acoustic wave can be converged using most of the acoustic wave incident from the opening 63 of the acoustic waveguide 60, and the sound pressure of the received acoustic wave can be increased. Thereby, a sound wave can be detected with high sensitivity. [0137] In the optical ultrasonic microphone 51 of the first embodiment, the end of the acoustic waveguide 60 is closed, but the end may be open. 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 end 64A of the acoustic waveguide 60 is open. If the energy of the sound wave propagating through the acoustic waveguide 60 is relatively high and it is not necessary to take in 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 portion 52 terminates It is preferable to remove it from the acoustic waveguide 60 so as not to be reflected and adversely affected. According to the acoustic wave receiver 103, since the terminal end 64 of the acoustic waveguide 60 is open, it is possible to remove the sound wave that has not been transmitted to the photoacoustic propagation medium portion 52. Thereby, the target sound wave can be accurately detected without disturbance of the received sound wave. In this case, the length of the acoustic waveguide 60 may be shorter than the preferred length determined in relation to the opening as described above. [0138] 05-05-2019 38 FIG. 14 is a contour diagram showing the measurement results of the equiphase surface of the sound wave propagation of the photoacoustic propagation medium portion of the optical ultrasonic microphone in the first embodiment of the present invention. FIG. 14 shows the sound wave propagation time measured by two-dimensional scanning with the LDV head 8 of the sound wave propagation inside the photoacoustic propagation medium part 52 made of silica dry gel in the optical ultrasonic microphone 51, as a result of the sound wave The situation of the equiphase surface (wave front) 902 of propagation is shown. [0139] Here, a silica dry gel having a density of 270 kg / m 3 and an acoustic velocity of 145 m / s was used as an example of the photoacoustic propagation medium portion 52. The incident angle at each point in this case is 89.5 degrees, and the refraction angle is 26 degrees. The curved surface was designed based on this sound velocity value. It can be observed that the sound wave propagation direction 901 and the equal phase surface 902 propagate toward the sound wave acoustic focus 57 as the silica dry gel 52 as a cylindrical wave, and the operation as the theoretical design was confirmed. [0140] 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 sound wave acoustic focus 57. As shown in FIG. FIG. 15 shows the result when the broadband tweeter emits a sound wave to be measured at a center frequency of 40 kHz and a drive signal of one wavelength. [0141] In consideration of the center frequency of 40 kHz, both the width and the initial height (height at the opening 54) of the acoustic waveguide 60 were 4 mm. The thickness of the dried silica gel used as the photoacoustic propagation medium portion 52 is also 4 mm. For measurement of the laser beam 58 reciprocating in the photoacoustic propagation medium portion 52, a heterodyne type laser Doppler vibrometer (LDV head 8) using a He̶Ne laser with a wavelength of 633 nm 05-05-2019 39 is used as an example of a light source and light detection means It was. [0142] Modulation of light by sound waves is frequency modulation. An aluminum material is used for the base 3. The laser beam 58 emitted from the He-Ne laser is a photoacoustic propagation medium portion via the measurement through hole (a through hole formed in the acoustic focus 57) 53a of the base 53 located on the surface side of the silica dry gel 52 52, and penetrates the photoacoustic propagation medium portion 52 in the thickness direction, propagates the light path in reverse, reflects on the inner surface of the base 53 on the back side of the photoacoustic propagation medium portion 52, and then propagates photoacoustics again. The light passes through the medium portion 52 in the thickness direction, and is emitted from the through hole 53 a of the base 53 located on the surface side of the photoacoustic propagation medium portion 52, and returns to the LDV head 8. Therefore, the optical path for measuring the sound wave is 8 mm which is twice the thickness 4 mm of the photoacoustic propagation medium portion 52. [0143] From the results shown in FIG. 15, it can be seen that the receiver has extremely wide band reception characteristics as in the first embodiment. From the waveform 81 of FIG. 15, the peak displacement is about 5 nm. The converted sound pressure P is about 54.2 Pa. The input conversion 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 sufficiently match in order, and by properly calibrating the measured and converted sound pressure, Extremely accurate sound pressure measurement is possible. [0144] In the first embodiment, by focusing the acoustic wave inside the photoacoustic propagation medium unit 52, broadband reception can be performed with higher sensitivity. [0145] Second Embodiment The optical ultrasonic microphone of the second embodiment will be 05-05-2019 40 described with reference to FIGS. 16 to 18. The difference between the second embodiment and the first embodiment lies in the configuration of the optical system that optically detects the sound pressure at the acoustic focus 57. Hereinafter, this optical system is referred to as an optical sound pressure measurement unit 1300. [0146] In the first embodiment, the optical sound pressure measurement unit 1300 is configured of the LDV head 8 and the LDV arithmetic processing unit 9. Although LDV is rich in versatility and stability, it is difficult and expensive to miniaturize the LDV due to its configuration. Therefore, in order to provide the optical ultrasonic microphone in a small size and at a low cost, it is desirable to make the optical sound pressure measurement unit 1300 small and inexpensive. Hereinafter, the configuration of the optical sound pressure measurement unit 1300 will be described. Also in the second embodiment, the configuration described in the first embodiment and the ones that are not modified will not be illustrated and described. [0147] FIG. 16 shows the optical system structure of the optical sound pressure measurement unit 1300 in the second embodiment of the optical ultrasonic microphone of the present invention. [0148] The monochromatic light source 1301 shown in FIG. 16 emits coherent light. An optical system 1303 secures the flatness of the wavefront and expands or reduces the monochromatic light 1302 emitted from the monochromatic light source 1301 to an appropriate beam size. Thereafter, the monochromatic light 1302 is split by the beam splitter 1304 into two monochromatic lights 1307 and 1308. Then, the monochromatic light 1307 travels to the photoacoustic propagation medium unit 52, and the monochromatic light 1308 travels to the plane mirror 1306. 05-05-2019 41 [0149] The monochromatic light 1307 is made of a transparent base 1314 made of a material (for example, a transparent reinforced acrylic plate) capable of transmitting the monochromatic light 1307 and capable of well constraining the acoustic vibration in the photoacoustic propagation medium portion 52; The light passes through the acoustic focus 57 of the propagation medium portion 52 and is reflected by the plane mirror 1310 and passes through the acoustic focus 57 again. Then, a part of the light passes through the beam splitter 1304, and the shape of the beam is shaped by the focusing optical system 1311 and is led to the light intensity measuring instrument 1312. The optical path through which the monochromatic light 1307 described above passes is denoted as L11. [0150] The monochromatic light 1308 is reflected by the plane mirror 1306 and travels in the direction of the beam splitter 1304 again, but a part thereof is reflected by the beam splitter 1340 and then condensed by the condensing optical system 1311 in the same manner as before. It is led to the intensity measuring device 1312. The light path through which the monochromatic light 1308 described above passes is denoted as L12. [0151] 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 of interference light with time average intensity Definition by dividing. Therefore, the entire optical system is adjusted in optical axis so as to generate interference light having a real value of 0 to 2), and the reflection / transmittance of the beam splitter 1304 is selected. [0152] 05-05-2019 42 Therefore, the intensity of interference light received by the light intensity measuring instrument 1312 fluctuates with high contrast depending on the difference in optical path length between the optical paths L11 and L12 through which the two monochromatic lights pass. When there is no time fluctuation of the optical path length difference, the output signal 1313 from the light intensity measuring instrument 1312 shows a constant value regardless of time. However, the refractive index of the acoustic focal spot 57 temporally fluctuates due to the temporal fluctuation of the acoustic signal intensity collected to the acoustic focal spot 57 by the photoacoustic propagation medium unit 52, and the optical path length difference temporally fluctuates accordingly Therefore, the output signal 1313 fluctuates in time. Since the refractive index fluctuation depends on the input sound pressure, the sound pressure fluctuation can be observed by analyzing the output signal 1313. The above is the operation of the optical ultrasonic microphone 1300. [0153] A variation of a possible apparatus configuration in the optical ultrasonic microphone 1300 of the second embodiment and points to be considered in design will be described. [0154] In FIG. 16, the beam splitter 1304, the transparent base 1314, the plane mirror 1306, and the plane mirror 1310 are all shown in contact with each other, but another optical medium such as an air layer is sandwiched between them and they are separated from each other It goes without saying that it 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 influences other than the acoustic signal due to air fluctuation etc. which is suffered while the monochromatic light 1307, 1308 passes through different optical paths. . Therefore, in order to obtain the highly stable output signal 1313, as shown in FIG. 16, while the insertion of the air layer and the mechanically unstable optical medium is eliminated as much as possible, the optical paths L11 and L12 follow different paths. It is desirable to configure the area as small as possible. [0155] Although the optical system 1303 and the condensing optical system 1311 are shown in FIG. 16, the monochromatic light 1302 from the monochromatic light source 1301 has sufficient coherency and a beam size required for measurement from the monochromatic light source 05-05-2019 43 1301. It is needless to say that it can be omitted when preparing from the point of emission. [0156] However, in the case where the shift of the focal position of the photoacoustic propagation medium portion 52 is a problem in the measurable frequency band of the required acoustic signal (ie, the photoacoustic propagation medium portion 52 produces chromatic aberration to the acoustic signal. (If it has), the beam size of the monochromatic light 1302 needs to be large enough to cover the positional variation of the acoustic focal spot 57 due to the aberration. This is because, if an acoustic signal in a certain frequency band forms a focus outside the beam spot of the monochromatic light 1302, the acoustic signal in that frequency band is not received, so reception of the acoustic signal in the measurable frequency band of the acoustic signal This is because the frequency dependency occurs in the characteristics. If frequency dependence exists in the reception characteristics, so-called signal distortion occurs in which the similarity between the time waveform of the acoustic signal at the acoustic focus 57 and the signal waveform of the output signal 1313 can not be secured, which is not preferable in terms of characteristics. Therefore, it is essential to avoid this problem in order to secure a wide acoustic measurable frequency band, and it is necessary to insert an optical system 1303 and extend the monochromatic light 1302 to have a certain beam size. [0157] The spectrum width required for the monochromatic light 1302 depends on the optical system configuration of the optical ultrasonic microphone 1300. For example, when parallel flat glass is inserted between the beam splitter 1304 and the plane mirror 1306, and the light paths L11 and L12 are adjusted to be equal in a situation where no acoustic signal is input to the acoustic focus 57, monochromatic light 1302 is used. Can use broadband light or white light such as light emitting diode light having a maximum intensity at a desired wavelength. As the spectral size of the monochromatic light 1302 narrows, the physical size of the monochromatic light source 1301 increases and the cost increases. Therefore, when realizing the optical acoustic measurement device in a small size and at low cost, the optical path L11 is as low as possible. It is desirable to configure the optical system so that the optical path lengths of L., L12 are equal. [0158] 05-05-2019 44 Also, although the transparent base plate 1314 is described as being made of a transparent material so that the monochromatic light 1307 can be sufficiently transmitted, the small openings are provided in both transparent base plates 1314 and the light path of the monochromatic light 1307 is the small opening. It is needless to say that the transparent base plate 1314 can be applied as an opaque one by being able to pass through. All configurations in which the monochromatic light 1307 can pass through the transparent base 1314 are equally operable if the above-mentioned requirements for the interference light intensity stability are satisfied. [0159] Next, results of demonstrating the measurement principle of the optical ultrasonic microphone 1300 depicted in FIG. 16 will be described with reference to FIGS. 17 and 18. [0160] 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 optical interferometry. Therefore, the optical interferometer shown in FIG. 17 was actually configured to demonstrate its principle. [0161] FIG. 17 shows an experimental apparatus for demonstrating the principle of the optical ultrasonic microphone in the second embodiment. In FIG. 17, laser light emitted from a He̶Ne laser 1400 is split by a beam splitter 1401 into two paths L21 and L22. The laser beam in the path L21 is turned back by the reflecting mirror 1402 and interferes with the laser beam in the path L22 in the beam splitter 1405. [0162] 05-05-2019 45 The laser light passing through the path L22 is folded back by the reflecting mirror 1403 and passes through the acoustic medium 1404, and then interferes with the light ray of the path L21. 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 necessary signal processing such as removal of DC components included in the signal and averaging processing by a digital oscilloscope 1408. [0163] An acoustic medium 1404 is inserted between the reflecting mirror 1403 and the beam splitter 1405 in the path L22. Furthermore, a horn 1409 is connected to the acoustic medium 1404 so that an acoustic signal emitted from the speaker 1410 can be introduced to the acoustic medium 1404. In the present 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 [0164] Next, the actual experimental situation will be described. In the experiment, as an input signal to the speaker 1410, one sine wave having an amplitude of 0.5 V and a frequency of 40 kHz generated by a function generator (not shown) was used. The distance from the front surface of the speaker 1410 to the surface of the opening of the horn 1409 is 100 mm, and the acoustic medium 1404 has a thickness of 5 mm (lateral direction with respect to the paper of FIG. 17) and a square shape of 20 mm. The acoustic medium 1404 has an acoustic velocity of approximately 70 m with respect to a 40 kHz sine wave. [0165] FIG. 18 shows the measured results. FIG. 18A is a graph showing the time waveform of the output signal from the digital oscilloscope 1408 measured by the experimental apparatus for demonstrating the principle of the optical ultrasonic microphone of FIG. Further, FIG. 18B is a graph in which a time waveform of an input signal to the speaker 1410 is described. In FIGS. 18A and 18B, the horizontal axis has the same time width, and both signal wave fronts are arranged 05-05-2019 46 to coincide with each other. [0166] As can be seen from FIG. 18, the output signal to the speaker 1410 described in FIG. 18B is converted into an acoustic signal, propagates in the air, is collected by the horn 1409, is introduced into the acoustic medium 1404, and travels as a compressional wave. Accordingly, the refractive index fluctuation generated at the intersection region of the acoustic medium 1404 and the optical path L22 is detected as an interference light intensity output from the digital oscilloscope 1408. Therefore, it can be understood from this measurement result that the optical acoustic measurement unit 1300 can actually function. [0167] Third Embodiment Next, an optical ultrasonic microphone of a third embodiment will be described with reference to FIG. [0168] The optical ultrasonic microphone of the third embodiment also constitutes an optical system for optically detecting the sound pressure at the acoustic focal point 57 as in the second embodiment. In the optical ultrasonic microphone 1300 described with reference to FIG. 16, the optical path length difference between the two different paths L21 and L22 constituting the MichelsonMorley interferometer is converted to interference light intensity and measured. [0169] However, when the monochromatic light 1302 is coherent monochromatic light such as laser light, the interference light intensity is converted to the wavelength of the monochromatic light 1302 and all optical path length variations having a difference of integer wavelengths have the same intensity. An acoustic signal having a strong sound pressure corresponding to such an optical path length difference fluctuation, or an output when the optical path length difference is already a half odd wavelength in the absence of the acoustic signal, to give (ie, pitch jump) A socalled signal distortion occurs in which the similarity of the time waveform of the acoustic signal 05-05-2019 47 at the signal 1313 and the acoustic focus 57 is broken. [0170] 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 1 wavelength or less, and has a wide measurement dynamic range. This is a problem when constructing an optical acoustic microphone. In order to solve this problem, it is necessary to use an interferometer having a large measurable range of the optical path length difference. As described below, it is possible to construct an optical ultrasonic microphone to which an interferometer having such a feature is actually applied. [0171] The configuration of an optical ultrasonic microphone that can solve the above-described problem will be described with reference to FIG. In addition, in the third embodiment, the configuration appearing in the first embodiment and the one without change are omitted from the illustration and the description. [0172] 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 in FIG. 16 apply to the constituent elements applicable from the device configuration in FIG. 16 without any change. [0173] In FIG. 19, reference numeral 1601 denotes a two-frequency linearly polarized laser light source, which emits laser light 1602 composed of linearly polarized light whose polarization planes are orthogonal to each other and whose frequencies are ω and ω + Δω. 05-05-2019 48 [0174] In the following, in order to clarify the explanation, it is defined that one polarization plane of the two linearly polarized light beams has a frequency ω parallel to the plane of FIG. 19 (therefore, the other polarization plane is to the plane of FIG. Vertical with frequency ω + Δω). [0175] For the same purpose as the apparatus configuration shown in FIG. 17, the laser beam 1602 is subjected to ensuring of the flatness of the wave front by the optical system 1303 and enlargement / reduction to an appropriate beam size (the laser beam 1602 by the insertion of the optical system 1303 Changes in polarization conditions are usually very small and negligible. A part of both polarization components of the adjusted laser beam 1602 is reflected by the nonpolarization beam splitter 1603, and is guided to the light intensity measuring instrument 1605 by the condensing optical system 1604. Since the reflection is performed on the non-polarization plane, the two polarization components included in the laser beam 1602 immediately after reflection have rotation of the polarization plane and linear polarization, and the amplitude intensity ratio of both components is the laser beam 1602 before reflection. Is the same as [0176] A polarizing plate 1606 is inserted between the non-polarization beam splitter 1603 and the focusing optical system 1604. The polarization axis of the polarizing plate 1606 is inclined 45 ° with respect to the paper surface of FIG. 19 and the polarization axis is equally 45 ° with respect to two polarization planes in the laser beam 1602. Since both polarization planes are projected to the polarization axis by the plate 1606, linearly polarized components having the frequencies ω and ω + Δω, which were not dried due to the orthogonality of the polarization planes, interfere with each other, and the generated difference frequency is generated. Beat light of Δω is incident on the light intensity measuring instrument 1605. Therefore, the time waveform of the reference beat signal 1607 is a sine wave of frequency Δω. Since the linear polarization components having the frequencies ω and ω + Δω always pass through the same 05-05-2019 49 path, the reference beat signal 1607 does not include information on the acoustic signal. [0177] Next, the direction of the laser beam 1602 that has passed through the non-polarization beam splitter 1603 will be described. The laser beam 1602 having passed through the non-polarization beam splitter 1603 is subjected to path selection by reflection and transmission according to the polarization plane direction by the polarization beam splitter 1608. Here, in order to simplify the description, it is assumed that linearly polarized light having a polarization plane parallel to the paper surface of FIG. 19 is completely reflected by the polarization beam splitter 1608. Accordingly, only linearly polarized light having a polarization plane parallel to the paper surface of FIG. 19 contained in the laser beam 1602 which has passed through the non-polarization beam splitter 1603 is reflected toward the acoustic focal point 57 and a straight line having a polarization plane perpendicular to the paper of FIG. The polarized light is completely transmitted to the plane mirror 1617. [0178] Among the laser beams divided by the polarization plane direction as described above, the way of the laser beam 1609 reflected in the acoustic focal point 57 direction will be described first. The laser beam 1609 passes through the acoustic focus 57 twice while being reflected by the plane mirror 1618, and therefore suffers 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 1/8 λ wave plate 1610 twice in the middle of the path. The 1/8 λ wave plate 1610 has the function of rotating the polarization plane of the light passing therethrough by 45 °. Accordingly, the polarization plane of the laser beam 1609 reflected by the plane mirror 1618 and returned back to the polarization beam splitter 1608 is rotated by 90 ° and becomes perpendicular to the plane of FIG. 19 and passes through the polarization beam splitter 1608 and the polarization plate 1613 Then, it is led to the light intensity measuring instrument 1612 by the condensing optical system 1611. [0179] Next, the direction of the laser beam 1614 directed to the plane mirror 1617 will be described 05-05-2019 50 for the laser beam whose path is split by the polarization beam splitter 1608 in the polarization plane direction. When the laser beam 1614 is reflected by the plane mirror 1617 and returns to the position just before the reflection surface of the polarization beam splitter 1608 again, it passes through the 1/8 λ wavelength plate 1615 twice and the polarization plane is rotated by 90 °. Parallel to each other. Therefore, the laser beam 1614 is reflected by the polarizing beam splitter 1608, passes through the polarizing plate 1613, and is guided to the light intensity measuring instrument 1612 by the focusing optical system 1611. [0180] As described above, the two laser beams 1609 and 1614 are integrated into a single laser beam whose polarization planes are orthogonal to each other again immediately before the polarizing plate 1613, but the polarizing axis of the polarizing plate 1613 is as shown in FIG. The two laser beams 1609 and 1614 become coherent after passing through the polarizing plate 1614. The laser beams 1609 and 1614 are inclined by 45 ° and function in the same manner as the polarizing plate 1606 described above. Accordingly, the laser beams 1609 and 1614 interfere with each other to become beat light of the difference frequency Δω, and a measurement beat signal 1616 having a sine waveform having the same frequency is output from the light intensity measurement device 1612. [0181] 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 The change of the optical path length difference of one wavelength in terms of the speed of light corresponds to the change of 2π of the phase φ. The measurement of the phase φ is performed by phase comparison between the reference beat signal 1607 output from the light intensity measuring instruments 1605 and 1612 as an electrical signal and the measurement beat signal 1616. The phase comparison can be performed with high accuracy by measuring the phase difference of the measurement beat signal 1616 based on the reference beat signal 1607 using, for example, a lock-in amplifier (not shown). [0182] 05-05-2019 51 Since the device 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. The measurement of the variation of the phase φ exceeding 2π, which is another problem, is performed as follows. First, the phase φ0 in the state where there is no input of an acoustic signal in the photoacoustic propagation medium portion 52 is measured and placed, and the phase fluctuation Δφ (that is, φ = φ0 + Δφ) from that is constantly monitored to obtain a phase φ exceeding 2π. It becomes possible to measure the fluctuation amount of As described above, the phase fluctuation Δφ reflects the refractive index fluctuation at the acoustic focal point 57 caused by the acoustic signal via the optical path length difference fluctuation, so that the above-mentioned 2) can be obtained by analyzing the measured phase fluctuation Δφ. The acoustic signal can be measured while solving one problem. [0183] In the optical ultrasonic microphone 1300 shown in FIG. 19, possible variations and modifications in the apparatus configuration will be described. In the configuration of FIG. 19 as well as in FIG. 16, the flatness of the wavefront where the light emitted from the dual-frequency linearly polarized laser light source 1601 can generate interference light with sufficient contrast as it is and the acoustic focus 57 If the beam size is sufficient to realize sufficient coverage, the optical system 1303 and the focusing optical systems 1604 and 1611 can be omitted. [0184] It goes without saying that the existing dual-frequency laser light source is equally applicable to the dual-frequency linear polarization 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 (Acoustic-optic modulator), and two linearly polarized beams after modulation so that their polarization planes are orthogonal to each other. A light source in which the wave laser light is integrated into one light flux can be applied. [0185] Further, in FIG. 19, the polarization plates 1606 and 1613, the non-polarization beam splitter 1603, the polarization beam splitter 1608, the 1/8 λ wavelength plate 1610 and 1615, the plane mirrors 1617 and 1618, and the photoacoustic propagation medium portion 52 are 05-05-2019 52 completely in contact. It goes without saying that similar functions can be produced even if another optical medium such as an air layer is inserted between the elements. However, it is better not to insert an air layer or a mechanically weak optical medium in order to achieve high device stability that can measure only the refractive index variation generated in the acoustic focus 57 and to miniaturize the entire device. . [0186] In FIG. 19, the polarization plane and polarization axis of the laser beam 1602, non-polarization beam splitter 1603, polarization beam splitter 1608, and polarizing plates 1606 and 1613 are set with reference to the paper of FIG. It is needless to say that the respective polarization planes and polarization axes may be simultaneously rotated at an arbitrary angle around the optical axis, as long as they do. Also, it goes without saying that the polarization selectivity of reflection / transmission of the polarization beam splitter 1608 is similarly reversed. [0187] According to the optical ultrasonic microphone of the present invention, it is possible to receive ultrasonic waves in a high frequency range and in a wide band, which was conventionally difficult, and a standard microphone having an effective band of 100 kHz or more can be realized. [0188] DESCRIPTION OF SYMBOLS 8 LDV head 9 LDV arithmetic processing part 14, 104 Environment fluid 51, 51A Optical ultrasonic microphone 52 Photoacoustic propagation medium part 53 Base 53a Through hole for measurement 55, 901 Sound wave propagation direction 56 Acoustic wave guide member 57 Acoustic focus 58, 1614, 1602, 1609 laser light 60 acoustic waveguide 61 transmission surface 62 waveguide outer surface 63 opening 64, 64 A waveguide end 70 space 71 opening 72 end 77 convergence 81 LDV output waveform 101 ultrasonic transducer 102 Ultrasonic transducer 103 Propagation medium portion 105 Ultrasonic wave propagation path 111 First surface region 112 Second surface region 121 LDV 122, 123, 1402, 1403 Reflector 124 Cubic mirror 125 Laser light path 126 Sound field 127 Arithmetic portion 902 Equal phase surface 1300, 1600 optical Pressure measuring unit 1301 Monochromatic light source 1302, 1307, 1308 Monochromatic light 1303 Optical system 1304, 1401, 1405 Beam splitter 1306, 1310, 1617, 1618 Flat mirror 1311, 1604, 1611 Condensing optical system 1312, 1612, 1605 Light intensity measuring instrument 1313 output signal 1314 transparent base 1400 He-Ne laser 1402 and 1403 reflector 1404 acoustic medium 1406 light detection unit 1407 digital oscilloscope 1408 horn 1409 speaker 1602 dual frequency linear polarization laser light source 05-05-2019 53 1603 non-polarization beam splitter 1606 and 1613 polarization plate 1607 Reference beat signal 1608 Polarization beam splitter 1610, 1615 1/8 λ wave plate 1616 Measurement beat signal 05-05-2019 54
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