JP2013101079A - Photoacoustic vibrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoacoustic vibrometer for detecting displacement or vibration of an object.SOLUTION: A photoacoustic vibrometer includes: an acoustic wave source 1 for transmitting first and second acoustic waves at predetermined time intervals; an acoustic lens system 6 for converting first and second scattered waves generated by irradiating an object with the first and second acoustic waves into first and second plane sound waves; a photoacoustic medium part 8 through which the first and second plane sound waves propagate; a light source 11 for emitting at least one plane wave light flux with which two points positioned at an interval matching a distance in which the first plane sound wave propagates through the photoacoustic medium part in a predetermined time are irradiated; a light superimposition part 220 for overlapping first and second diffraction rays of light generated by the photoacoustic medium part by at least one plane wave light flux being diffracted by the first and second plane sound waves, and for generating superimposed diffraction rays of light; an image forming lens system 16 for converging the superimposed diffraction rays of light; and an image reception part 17 for detecting the converged diffraction rays of light, and for outputting an electric signal.

Description

  The present invention relates to a photoacoustic imaging vibrometer that detects displacement and vibration of an object using light and acoustic waves.

  When an acoustic wave is irradiated onto the object and the generated scattered wave is introduced into the photoacoustic medium part, the acoustic wave is a longitudinal wave, so that the medium in the photoacoustic medium part becomes dense and forms a refractive index distribution. For this reason, when light is propagated through the photoacoustic medium, diffracted light affected by the refractive index distribution is generated. That is, when the generated diffracted light is observed, the object can be detected.

  Non-Patent Document 1 discloses a technique for generating Bragg diffracted light and irradiating a target object by irradiating a monochromatic light to a refractive index distribution generated in a photoacoustic medium portion. Specifically, as shown in FIG. 22, Non-Patent Document 1 discloses a technique for projecting an image of an object 1109 onto a screen 1105 using a laser 1101 and an ultrasonic transducer 1111. The monochromatic light beam emitted from the laser 1101 is converted into a monochromatic light beam having a thick beam diameter by the beam expander 1102 and the aperture 1103. When the xyz axis is set as shown in FIG. 22, the monochromatic light beam passes through cylindrical lenses 1104 (a) and 1104 (b) that extend in the x axis and 1104 (c) that extends in the y axis, and the screen 1105. To reach. As described above, the optical system including the three cylindrical lenses is not rotationally symmetric with respect to the optical axis 1113.

  An acoustic cell 1108 filled with water 1107 is disposed between the cylindrical lenses 1104 (a) and 1104 (b), and an object 1109 is disposed in the water 1107. As will be described below, diffracted light is generated when the monochromatic light beam passes through the water 1107. The generated diffracted light has strong astigmatism. Therefore, in order to correct the astigmatism of the generated diffracted light and to form an image on the xz plane and the yz plane at the position of the screen 1105, cylindrical lenses 1104 (a), 1104 (b), 1104 ( The focal lengths of c) are different from each other.

  The cylindrical lens 1104 (a) has a focal length selected so that the monochromatic light beam is focused on the xz plane at the position of the focal plane 1106. Since the image is formed by a cylindrical lens, the focal point is a straight line parallel to the x-axis. The light beam that has passed through the focal plane 1106 diverges on the screen 1105 side from the focal plane 1106, but the divergent light beam is converged by the cylindrical lens 1104 (b) and refocused on the screen 1105. In the yz plane, the monochromatic light beam after passing through the beam expander 1102 is incident on the cylindrical lens 1104 (c) as a parallel beam. Then, the light is focused on the screen 1105 by the condensing action of the cylindrical lens 1104 (c). The selection of the installation position and focal length of each cylindrical lens is performed so that the light beam forms an image on the screen 1105 in both the xz plane and the yz plane, and an image similar to the object 1109 is a first-order diffraction image. 1112 (a) and −1st order diffracted light 1112 (b) are displayed on the screen 1105. As described above, since the optical system is not rotationally symmetric with respect to the optical axis 1113, the first-order diffracted image 1112 (a) and the −1st-order diffracted light 1112 (b) have distortion aberration. Therefore, by using the cylindrical lenses 1104 (b) and 1104 (c), an optical system having a distortion aberration opposite to the distortion aberration of the diffracted light is configured to correct the distortion aberration of the diffracted light, and the target An image similar to the object 1109 is generated on the screen 1105.

  The acoustic cell 1108 is provided with an ultrasonic transducer 1111 that is driven by a signal source 1110, and the object 1109 is irradiated with monochromatic ultrasonic waves from the ultrasonic transducer 1111 through the water 1107. The monochromatic ultrasonic wave means an ultrasonic wave whose sound pressure shows a sinusoidal time variation having a single frequency.

  An ultrasonic scattered wave is generated from the object 1109, and the scattered wave propagates through the passing region of the monochromatic light beam in the water 1107. Since the guided mode of the ultrasonic wave propagating in the water is a dense wave (longitudinal wave), a sound pressure distribution in the water 1107, that is, a refractive index distribution matching the ultrasonic scattered wave is generated in the water 1107. In order to simplify the discussion, it is first assumed that the ultrasonic scattered wave from the object 1109 is a plane wave directed in the positive direction of the y-axis. Since the ultrasonic scattered wave is monochromatic, the refractive index distribution generated in the water 1107 at a certain moment becomes a sinusoidal one-dimensional grating repeated at the ultrasonic wavelength. Therefore, Bragg diffracted light (in the figure, ± 1st order diffracted light beam is expressed) is generated by the one-dimensional grating. The diffracted light appears as one light spot on the screen 1105. The brightness of the light spot is proportional to the amount of change in the refractive index of the one-dimensional grating, that is, the ultrasonic sound pressure.

  Next, the assumed condition that “the ultrasonic scattered wave is a plane wave” is relaxed, and an ultrasonic scattered wave whose wavefront is not a plane is considered. An ultrasonic scattered wave whose wavefront is not plane can be expressed as a superposition of plane waves coming from various directions (in this case, all plane waves have the same frequency). For this reason, when the monochromatic light beam is transmitted through the water 1107 in which the ultrasonic scattered wave whose wavefront is not flat propagates, the light spot of the diffracted light due to each plane wave coming from various directions appears on the screen 1105. The intensity of each light spot is proportional to the amplitude of each plane wave, and the appearance position of each light spot on the screen 1105 is determined by the traveling direction of each plane wave. Therefore, real images of the object 1109 appear on the screen 1105 as a first-order diffraction image 1112 (a) and a −1st-order diffraction image 1112 (b). The fact that an aggregate of light spots on the screen 1105 can be regarded as a real image of the object 1109 is that the relationship between the object and the ± first-order diffraction image is different from that of an object in a general optical camera, except that it is a diffraction phenomenon. It is the same as the real image.

A. Korpel, "Visualization of the cross section of a sound beam by Bragg diffraction of light," Applied Physics Letters, vol.9, no.12, pp.425-427, 15 Dec. 1966.

  However, according to the technique disclosed in Non-Patent Document 1, it is not possible to detect displacement or vibration of the object. An object of the present invention is to solve this conventional problem and to provide a photoacoustic vibrometer that detects displacement and vibration of an object.

  The photoacoustic vibrometer of the present invention includes an acoustic wave source that transmits first and second acoustic waves at a predetermined time interval, and a first generated by irradiating an object with the first and second acoustic waves. And an acoustic lens system that converts the first and second scattered waves into first and second plane acoustic waves in a predetermined convergence state, respectively, and the acoustic plane of the acoustic lens system, and the first and second planes. On the sound axis in the photoacoustic medium part where the sound wave propagates, and on the sound axis in the photoacoustic medium part, the first plane sound wave is positioned at an interval that coincides with the distance propagated through the photoacoustic medium part at the predetermined time. A light source that emits at least one plane wave light beam that irradiates the two points at a non-perpendicular and non-parallel angle with respect to the sound axis, and the at least one plane wave light beam includes the first and second plane sound waves. Diffracted by the photoacoustic medium part. The first and second diffracted lights generated respectively to overlap and generate a superimposed diffracted light, an imaging lens system for condensing the superimposed diffracted light, and the condensed diffracted light And an image receiving unit that outputs an electric signal.

  In a preferred embodiment, the photoacoustic vibrometer further includes an image distortion correction unit that corrects distortion of at least one of the image of the object represented by the superimposed diffracted light and the electric signal.

  In a preferred embodiment, the spectral width of the at least one plane wave light beam is less than 10 nm.

  In a preferred embodiment, the acoustic lens system is a refractive acoustic system.

  In a preferred embodiment, the acoustic lens system is composed of silica nanoporous material or fluorinate.

  In a preferred embodiment, the acoustic lens system includes at least one refracting surface and an antireflection film that prevents reflection of acoustic waves provided on the at least one refracting surface.

  In a preferred embodiment, the acoustic lens system is a reflective acoustic system.

  In a preferred embodiment, the acoustic lens system includes two or more reflecting surfaces.

  In a preferred embodiment, the acoustic lens system includes a focus adjustment mechanism.

  In a preferred embodiment, the light source emits first and second plane wave light beams that irradiate the two points at angles that are non-perpendicular and non-parallel to the sound axis.

  In a preferred embodiment, the light source emits one plane wave light beam that irradiates the two points with an angle that is non-perpendicular and non-parallel to the sound axis.

  In a preferred embodiment, the light superimposing unit includes a non-polarizing beam splitter.

  In a preferred embodiment, the image distortion correction unit includes an optical member that enlarges a cross section of the superimposed diffracted light.

  In a preferred embodiment, the image distortion correction unit includes an optical member that reduces a cross section of the superimposed diffracted light.

  In a preferred embodiment, the optical member is constituted by an anamorphic prism.

  In a preferred embodiment, at least one of the imaging lens system and the optical member includes at least one cylindrical lens.

  In a preferred embodiment, the image distortion correction unit performs image processing based on the electrical signal.

  In a preferred embodiment, the photoacoustic medium part includes at least one of silica nanoporous material, fluorinate, and water.

  In a preferred embodiment, the first and second diffracted lights include a component of Bragg diffracted light having an intensity ratio of 1/2 or more.

  In a preferred embodiment, an optical axis of the at least one plane wave light beam emitted from the light source is adjustable with respect to a sound axis of the acoustic lens system.

  In a preferred embodiment, a trigger circuit for controlling transmission times of the first and second acoustic waves is further included.

  According to the photoacoustic vibrometer of the present invention, the object is irradiated with the first and second acoustic waves transmitted at predetermined time intervals to generate scattered waves at different times, and the acoustic lens system respectively Are converted into plane sound waves and introduced into the photoacoustic medium unit, and diffracted light is generated by the refractive index distribution generated in the photoacoustic medium unit for each plane sound wave. Since the interference is obtained by superimposing the two generated diffracted lights, the displacement and movement of the object at a predetermined time can be detected.

It is a schematic block diagram which shows 1st Embodiment of the photoacoustic vibrometer by this invention. (A) is a schematic diagram explaining how a plane wave light beam is Bragg diffracted by a plane sound wave in the first embodiment, and (b) is a schematic diagram for explaining Bragg diffraction conditions by a one-dimensional diffraction grating. (C) is a schematic diagram for explaining that the sound pressure distribution on the ultrasonic wavefront is transferred to the light amplitude distribution on the diffracted light beam wavefront by Bragg diffraction in the first embodiment. . (A) is a diagram showing that the diffracted light 201 is distorted in the y direction in the first embodiment, and (b) is an analog used as the image distortion correction unit 15 in the first embodiment. It is a figure which shows the structure of a morphic prism. It is a figure for demonstrating the optical path of the light beam in the wedge-shaped prism which comprises an anamorphic prism. (A) is a conceptual diagram for demonstrating operation | movement of the double diffractive optical system in the optical field | area, (b) is a photoacoustic vibrometer of 1st Embodiment by a double diffractive optical system. It is a figure which shows that it can be considered. It is explanatory drawing which shows that the interference fringe depending on the displacement of a target object is superimposed on a real image in the photoacoustic vibrometer of 1st Embodiment. It is a figure explaining the concrete which image | photographs the skin of the drum which is resonating as a target object. It is a figure which shows the image of the image | photographed drum skin. It is a figure explaining the concrete which image | photographs as an object the sphere from which a position and a radius change. It is a figure which shows the image | photographed sphere image. (A) is a figure which shows the incident direction of the 1st and 2nd plane wave light beam in 1st Embodiment, (b) is a figure which shows the other possible incident direction. It is a figure which shows the structure of a cylindrical lens. FIG. 3 is a diagram illustrating an optical system that includes a cylindrical lens and has both an image distortion correction unit and an imaging lens system in the first embodiment. It is a schematic block diagram which shows 2nd Embodiment of the photoacoustic vibrometer by this invention. It is a schematic diagram explaining the concrete of 2nd Embodiment. It is a figure which shows the structure of the acoustic lens system in 3rd Embodiment. It is a figure which shows the structure of the image distortion correction part 15 in 4th Embodiment. It is a figure which shows the structure of the image distortion correction part 15 in 5th Embodiment. It is a schematic block diagram which shows 6th Embodiment of the photoacoustic vibrometer by this invention. It is a schematic block diagram which shows 7th Embodiment of the photoacoustic vibrometer by this invention. It is a schematic block diagram which shows 8th Embodiment of the photoacoustic vibrometer by this invention. It is a schematic diagram which shows the structure of the apparatus described in the nonpatent literature 1.

(First embodiment)
A photoacoustic vibrometer according to a first embodiment of the present invention will be described below. FIG. 1 schematically shows the configuration of the photoacoustic vibrometer 101. The photoacoustic vibrometer 101 includes an acoustic wave source 1, an acoustic lens system 6, a photoacoustic medium unit 8, a light source 11, a light superimposing unit 220, a distortion correcting unit 15, an imaging lens system 16, and an image receiving unit. Unit 17.

  The object 4 is disposed in a medium 3 through which an acoustic wave can propagate. The medium 3 through which the acoustic wave can propagate is, for example, air or water. In addition, the medium 3 may be a body tissue or an elastic body such as metal or concrete. Further, the object 4 may be a body organ such as a person or an animal. In this case, the medium 3 is a human or animal body tissue.

  The acoustic wave source 1 and the acoustic lens system 6 are disposed in the medium 3 or in contact with the medium 3. When the first and second acoustic waves 2a and 2b emitted from the acoustic wave source 1 irradiate the object 4, the surface and / or the internal acoustic impedance of the object 4 (a quantity obtained by multiplying the sound speed by the density) is non-uniform. In this region, the first and second acoustic waves 2a and 2b are reflected, and the first and second scattered waves 5a and 5b are generated. The first and second scattered waves 5 a and 5 b are converted by the acoustic lens system 6 into a predetermined convergence state, in particular, first and second plane sound waves 9 a and 9 b, and enter the photoacoustic medium unit 8. As the first and second plane sound waves 9 a and 9 b propagate through the photoacoustic medium unit 8, a refractive index distribution is generated in the photoacoustic medium unit 8.

  The light source 11 emits a plane wave light beam 14. The plane wave light beam 14 is divided into first and second plane wave light beams 14 a and 14 b by a beam splitter 19, and is incident on the photoacoustic medium unit 8.

  The first and second plane wave light beams 14a and 14b are diffracted by the refractive index distributions of the first and second plane sound waves 9a and 9b of the photoacoustic medium unit 8, and the first and second diffracted beams 201a and 201b are diffracted. Occurs. The diffracted light is superimposed by the light superimposing unit 220 and condensed on the image receiving unit 17 by the imaging lens system 16, whereby the image 18 of the object 4 can be taken. The image 18 is an image similar to the two-dimensional distribution of the elastic coefficient of the object 4 on a plane perpendicular to the sound axis 7 and separated from the acoustic lens system 6 by the focal length f of the acoustic lens system 6. Further, as will be described in detail below, since the second acoustic wave 2b is transmitted after a predetermined time τ from the first acoustic wave 2a, the obtained image 18 includes two objects photographed at different times. The image of the object 4 is superimposed and interfered. This indicates the movement or displacement information of the acoustic lens system 6 in the direction of the sound axis 7 at the time τ of the object 4.

1. Configuration of the photoacoustic vibrometer 100 (1) Acoustic wave source 1
The acoustic wave source 1 irradiates the object 4 with the first and second acoustic waves 2 a and 2 b. The first and second acoustic waves 2a and 2b are preferably ultrasonic waves. Each of the first and second acoustic waves 2a and 2b is preferably a pulse wave including a plurality of sine waves having constant amplitude and frequency. As the wave number increases, the intensity of diffracted light generated in the photoacoustic medium unit 8 increases. Although not shown in FIG. 1, the time when the acoustic wave source 1 generates the first and second acoustic waves 2a and 2b is accurately controlled by the trigger circuit. Preferably, it is controlled with an accuracy of about several ns.

  The first and second acoustic waves 2a and 2b may be plane waves or not plane waves. Preferably, the first and second acoustic waves 2a and 2b irradiate the entire object 4 or the region to be imaged of the object 4 with substantially uniform intensity. That is, it is preferable that the first and second acoustic waves 2a and 2b have an irradiation cross section having a size corresponding to a region to be photographed. The first and second acoustic waves 2a and 2b are reflected and scattered on the surface and inside of the object 4, and the first and second scattered waves 5a having the same frequency as the first and second acoustic waves 2a and 2b. 5b is generated. Since the first and second acoustic waves 2a and 2b are pulse waves each including a plurality of sine waves, the time interval τ at which the first and second acoustic waves 2a and 2b are irradiated is, for example, It is defined by the difference between the rising times of the sine waves of the first and second acoustic waves 2a and 2b. The time interval τ depends on the time interval at which the displacement or movement of the object 4 is to be observed. For example, when the object 4 is a body organ such as a human being or an animal, the time interval τ may be a value in the range of 10 μsec to 1 mmsec.

(2) Acoustic lens system 6
The acoustic lens system 6 converges the first and second scattered waves 5a and 5b to predetermined states, respectively. Specifically, the acoustic lens system 6 has a focal length f in the medium 3. The acoustic lens system 6 may be a refractive acoustic system or a reflective acoustic system. In the present embodiment, a refractive acoustic system acoustic lens system 6 is used. When the acoustic lens system 6 is a refractive acoustic system, the acoustic lens system 6 includes an acoustic lens having at least one refracting surface and transmitting the first and second scattered waves 5a and 5b. The acoustic lens is preferably made of an elastic body having a small acoustic wave propagation loss, such as silica nanoporous material or fluorinate. The refraction of the acoustic wave on the refracting surface is in accordance with Snell's law at an angle determined by the sound speed ratio of the first and second scattered waves 5a and 5b in the material constituting the medium 3 and the acoustic lens. The waves 5a and 5b are refracted. When the acoustic lens system 6 is a reflective acoustic system, the acoustic lens system 6 has at least one reflecting surface made of a material that has a greatly different acoustic impedance from the medium 3 such as metal or glass. Since both the refracting surface and the reflecting surface have the same shape as the optical lens, the first and second scattered waves 5a and 5b can be converged.

  In addition, an antireflection film having the same function as the antireflection film laminated in order to reduce reflection attenuation and stray light generated on the lens refracting surface in the optical field may be provided on the refracting surface. For example, the acoustic impedance equal to the geometric mean value of the acoustic impedances of the medium 3 and the acoustic lens, and a quarter wavelength of the acoustic impedance value (the wavelengths here constitute the first and second acoustic waves 2a and 2b). An antireflection film having a thickness of a sine wave frequency may be provided on the refractive surface.

  The object 4 is preferably located near the focal point of the acoustic lens system 6. Similar to an optical imaging device such as an optical camera, the image 18 of the object 4 blurs as the position of the focal point shifts on the sound axis 7 of the acoustic lens system 6.

  For this reason, when a clear image 18 of the object 4 at a position other than the focal position is obtained, the entire photoacoustic vibrometer 100 is moved so that the object 4 comes to the focal position of the acoustic lens system 6. It is preferable. When it is difficult to move the entire photoacoustic vibrometer 100, or when it is difficult to move the photoacoustic vibrometer 100 in the direction of the sound axis 7 of the acoustic lens system 6, the imaging lens of the optical camera Similarly, the acoustic lens system 6 may further include a focus adjustment mechanism. Further, when the size of the image 18 with respect to the object 4 is made variable, a focal length adjustment function (that is, a zoom function) is provided to one or both of the acoustic lens system 6 and the imaging lens system 16. It may be provided.

  When the object 4 is located at the focal point of the acoustic lens system 6, the acoustic lens system 6 converts the first and second scattered waves 5a and 5b into first and second plane sound waves 9a and 9b.

(3) Photoacoustic medium 8
The photoacoustic medium section 8 has little propagation attenuation with respect to the first and second plane sound waves 9a and 9b, and has translucency with respect to first and second plane wave light beams 14a and 14b described later. It is composed of a isotropic elastic body. As such an elastic body, for example, a nanoporous body formed from silica dry gel, fluorinate, water, or the like can be suitably used. In order to improve the image quality (especially the resolution) of the image 18, it is desirable to apply a translucent elastic body having a sound velocity as low as possible, and it is more preferable to use a silica nanoporous material or fluorinate.

  The photoacoustic medium unit 8 is arranged with respect to the acoustic lens system 6 so that the first and second plane sound waves 9a and 9b converted by the acoustic lens system 6 enter the photoacoustic medium unit 8 with low loss. The acoustic lens system 6 is preferably joined to the photoacoustic medium unit 8. In order to suppress attenuation due to reflection on the joint surface, it is preferable to provide an antireflection film on the joint surface. When the acoustic lens system 6 and the photoacoustic medium part are made of the same material, the acoustic lens system 6 may be provided on a part of the photoacoustic medium part 8 (preferably the boundary surface with the medium 3). As shown in FIG. 1, the first and second plane sound waves 9 a and 9 b have a wavefront perpendicular to the sound axis 7 of the acoustic lens system 6 and are parallel to the sound axis 7 and the photoacoustic medium portion. 8 is propagated.

  As described above, since the first and second acoustic waves 2a and 2b are transmitted at the time interval τ, the first and second plane sound waves 9a and 9b are two independent sound waves as the photoacoustic medium portion. Propagate through 8. If the sound speed of the photoacoustic medium unit 8 is ν, the distances of the first and second plane sound waves 9a and 9b in the photoacoustic medium unit 8 are τ × ν.

(4) Sound wave absorber 10
The first and second plane sound waves 9a and 9b propagated through the photoacoustic medium unit 8 are reflected at the end of the photoacoustic medium unit 8, and the reflected first and second plane sound waves 9a and 9b are the first and second plane sound waves 9a and 9b. In the case of affecting the detection of the second plane sound waves 9 a and 9 b, it is preferable to provide the sound wave absorption unit 10 at the end of the photoacoustic medium unit 8. The sound wave absorber 10 absorbs or attenuates the first and second plane sound waves 9a and 9b without reflection or scattering. Since all the sound waves that reach the sound wave absorbing section 10 are absorbed by the sound wave absorbing section 10, the sound waves existing in the photoacoustic medium section 8 are only the first and second plane sound waves 9a and 9b that propagate in one direction. Become. Thereby, the reflected 1st and 2nd plane sound waves 9a and 9b are detected as noise, and it can suppress that the image quality of the image of the target object 4 falls.

(5) Light source 11
The light source 11 generates a light beam having high coherence. “High coherence” means that the wavelength, traveling direction, and phase of the light beam emitted from the light source 11 are aligned, that is, the light beam is coherent. The spectral width (half width) of the light beam emitted from the light source 11 is preferably less than 10 nm. As the light source 11, for example, a gas laser represented by a He—Ne laser, a solid laser, or a semiconductor laser narrowed by an external resonator can be used. The light source 11 may continuously emit a light beam or may be a pulsed light beam. The wavelength of the light beam emitted from the light source 11 is preferably within a wavelength band with less propagation loss in the photoacoustic medium unit 8. For example, when a silica nanoporous material is used as the photoacoustic medium portion 8, it is preferable to use a laser having a wavelength of 600 nm or more as the light source 11. Thereby, a bright image 18 can be obtained. The light source 11 generates a highly coherent light beam parallel to the optical axis 13. In other words, the light and light in the luminous flux have the same wavelength and phase.

  The light beam emitted from the light source 11 irradiates the photoacoustic medium unit 8 at a non-perpendicular and non-parallel angle with respect to the sound axis 7. More specifically, the luminous flux coincides with the distance that the first plane sound wave 9a propagates through the photoacoustic medium unit 8 during time τ on the sound axis 7 in the photoacoustic medium unit 8, that is, τ × ν. The first and second plane sound waves 9a and 9b that have reached the two points P1 and P2 located at the intervals are applied. It is important that the two points P1 and P2 are separated by τ × ν on the sound axis 7 in the photoacoustic medium unit 8, and the two points P1 or P2 may be anywhere on the sound axis 7. As long as this condition is satisfied, the light beam emitted from the light source 11 may be incident on the photoacoustic medium unit 8 in any state. In this embodiment, the two points P1 and P2 are irradiated with the first and second plane wave light beams 14a and 14b.

  For this purpose, the photoacoustic vibrometer of this embodiment includes a beam expander 12, a beam splitter 19, and a plane mirror 21. The light beam emitted from the light source 11 is first shaped into a plane wave light beam 14 having a larger beam diameter by passing through the beam expander 12. The beam diameter is set so that at least the propagation region of the first and second plane sound waves 9a and 9b propagating in the photoacoustic medium unit 8 is included in the plane wave light beam 14. Such adjustment of the beam diameter is performed by selecting the beam expansion rate of the beam expander 12. By generating a sufficiently thick beam with the beam expander 12 and shielding the periphery of the light beam generated by an aperture stop or the like, a plane wave light beam 14 having uniform light intensity in the light beam cross section can be generated. A better image 18 can be obtained by making the light intensity uniform in the cross section of the plane wave beam 14.

  By passing through the beam splitter 19, the plane wave light beam 14 is divided into first and second plane wave light beams 14a and 14b having substantially the same light intensity and the same cross-sectional diameter. A non-polarizing beam splitter can be suitably used as the beam splitter 19. The non-polarizing beam splitter splits the light flux at a desired light intensity ratio regardless of the polarization of incident light. Non-polarizing beam splitters include prism-type, pellicle-type (thin film is used for the reflective surface), wedge-type, parallel plate type, dielectric multilayer film and metal thin film provided on the reflective surface. It may be used.

  As shown in FIG. 1, the second plane wave light beam 14b is reflected by the plane mirror 21, and the angles of the plane mirror 21 and the beam splitter 19 are adjusted so that the first and second plane wave light beams 14a and 14b are parallel to each other. It is preferable to make it enter into the photoacoustic medium part 8 in the state. The optical axes 23 and 13 and the sound axis 7 of the first and second plane wave light beams 14a and 14b are preferably on the same plane.

  The angles at which the sound axis 7 intersects the optical axis 13 and the optical axis 23 are equal, and each is 90 ° −θ. Here, θ represents the traveling direction of the plane wave light beam, that is, the angle formed by the wavefronts of the optical axis 13 and the optical axis 23 and the first and second plane sound waves 9a and 9b. θ can take any angle except 0 °, 90 °, 180 °, and 270 °. Bragg diffraction occurs in the first and second plane wave light beams 14a and 14b only in the angle range θ, and first and second diffracted beams 201a and 201b are generated. A specific setting method of θ for generating the diffracted light 201 from the first and second diffracted lights 201a and 201b will be described later.

(6) Light superposition unit 220
The first and second diffracted beams 201 a and 201 b are superimposed by the light superimposing unit 220. In the present embodiment, the light superimposing unit 220 includes the plane mirror 22 and the beam splitter 20. The plane mirror 22 reflects the first diffracted light 201a toward the beam splitter 20 through which the second diffracted light 201a passes, and the first and second diffracted lights 201a and 201b are superimposed on the beam splitter 20.

(7) Image distortion correction unit 15
The image distortion correction unit 15 corrects image distortion of the diffracted light superimposed by the light superimposing unit 220. In this embodiment, the distortion of the image represented by the superimposed diffracted light is corrected by enlarging or reducing the cross section of the superimposed diffracted light in one direction. A specific configuration of the image distortion correction unit 15 will be described in detail below.

(8) Imaging lens system 16, image receiving unit 17, and image processing unit 205
The imaging lens system 16 focuses the diffracted light whose distortion has been corrected on the light receiving portion of the image receiving portion 17. The image receiving unit 17 is constituted by a photoelectric conversion element such as a CCD, whereby the image 18 is converted into an electric signal.

  The image processing unit 205 performs image processing based on the electrical signal input from the image receiving unit 17 to form an image 18.

2. Next, the operation of the photoacoustic vibrometer 101 will be described.

  As described above, in the photoacoustic vibrometer 101, the emission times of the first and second acoustic waves 2a and 2b are accurately controlled. For this reason, at the time when the first plane sound wave 9a reaches the point P1, the second plane sound wave 9b reaches the point P2. When the emission interval of the first and second acoustic waves 2a and 2b is controlled with a time accuracy of 1 ns, the first and second acoustic waves 2a and 2b propagating through the photoacoustic medium unit 8 at a sound speed of 50 m / s. The position error is within 50 nm. For example, when a He—Ne laser is used as the light source 11, this positional error corresponds to 0.079 wavelength in terms of the He—Ne laser wavelength of 633 nm. Therefore, by adjusting the emission times of the first and second acoustic waves 2a and 2b, the positions of the first and second plane sound waves 9a and 9b are controlled in the photoacoustic medium unit 8 with very high accuracy. be able to.

  By controlling the positions of the first and second plane sound waves 9a and 9b in this way, the diffracted light of the first plane wave light beam 14a by the first plane sound wave 9a at the point P1 and the second plane sound wave at the point P2 are used. The diffracted light by the second plane wave light beam 14b by 9b is generated under the same conditions. Therefore, hereinafter, the first and second plane acoustic waves 9a and 9b are represented as plane acoustic waves 9, and the first and second plane wave light beams 14a and 14b are represented as plane wave light waves 204, and the generation of diffracted light will be described.

  FIG. 2A schematically shows a state in which the plane wave light beam 204 is Bragg diffracted by the plane sound wave 9 at the moment when the plane sound wave 9 crosses the optical path of the plane wave light beam 204 in the photoacoustic medium unit 8. The plane acoustic wave 9 is a dense elastic wave that propagates through the photoacoustic medium 8. Therefore, a refractive index distribution proportional to the sound pressure distribution of the plane sound wave 9 is generated in the photoacoustic medium unit 8. As described above, since the first and second acoustic waves 2a and 2b are composed of sine waves having a single frequency, the first and second scattered waves 5a and 5b and the plane sound wave 9 are also sine waves having a single frequency. is there. For this reason, the refractive index distribution generated in the photoacoustic medium unit 8 has a period in a direction parallel to the sound axis 7 equal to the wavelength of the plane sound wave 9, and the magnitude of the refractive index changes in a sine wave shape. A periodic structure that is uniform in the direction perpendicular to.

  Such a refractive index distribution functions as a one-dimensional diffraction grating for the plane wave light beam 204. Therefore, when the plane wave light beam 204 is incident on the plane sound wave 9 at an angle θ that satisfies the diffraction conditions described below, diffracted light 201 is generated. Since this one-dimensional diffraction grating has a flat lattice plane and the plane of the plane wave beam 204 is plane, the diffracted light 201 becomes a plane wave beam.

In the photoacoustic vibrometer 101, the first and second acoustic waves 2a and 2b are composed of a sine wave whose number is sufficiently larger than two periods, and therefore the density repetition in the refractive index distribution is two or more. Therefore, the refractive index distribution generated in the photoacoustic medium unit 8 can be regarded as a one-dimensional diffraction grating, and the plane wave light beam 204 is diffracted by Bragg diffraction. In the Bragg diffraction, as shown in FIG. 2A, the angles formed by the plane wave light beam 204 and the diffracted light 201 with respect to the plane sound wave 9 are equal, and each is an angle θ. The angle θ is a discrete value that satisfies the Bragg diffraction condition described below. When the first and second acoustic waves 2a and 2b are composed of a small number of sine waves of about two periods, the diffracted light 201 is mainly generated by Raman-Nath diffraction. Pure Raman-Nath diffraction occurs even if the plane-wave luminous flux 204 and the diffracted light 201 are not equal in angle to the wavefront of the plane acoustic wave 9. Since the Bragg diffraction produces diffracted light 201 having a higher intensity than the Raman-Nath diffraction, the first and second scattered waves 5a and 5b having a smaller sound pressure can be observed, and a high-resolution image 18 can be obtained. Contribute. For this reason, in the photoacoustic vibrometer 101, it is preferable to use the diffracted light 201 generated mainly by Bragg diffraction using the second and second acoustic waves 2a and 2b each having a sine wave having a large wave number. More preferably, the ratio of the intensity of the Bragg diffracted light in the diffracted light 201 is 1/2 or more. For this purpose, it is desirable that the plane sound wave 9 is a pulsed sound wave having a wavefront equal to or greater than the wavefront number N _min represented by the equation (4). In equation (4), _nao is the refractive index of the photoacoustic medium 8, λa is the sound wave wavelength in the photoacoustic medium 8, and λo is the wavelength in the photoacoustic medium 8 of the light emitted from the monochromatic light source. Represent.

For example, when a nanoform having a sound velocity of 50 m / s is applied as the photoacoustic medium 8 and an ultrasonic wave of 5 MHz is used, the refractive index of the nanoform is approximately 1, so N _min = 13. Therefore, in this case, Bragg diffracted light becomes the main diffracted light component if pulsed ultrasonic waves having a wavefront number of 13 waves or more are used.

  The Bragg diffraction condition in the one-dimensional diffraction grating by the refractive index distribution generated by the plane sound wave 9 will be described. As shown in FIG. 2B, the grating interval of the diffraction grating 202 generated by the plane sound wave 9 is equal to the wavelength λa of the plane sound wave 9 propagating in the photoacoustic medium unit 8. One monochromatic light beam in the plane wave light beam 204 is defined as a monochromatic light 203. The wavelength of the monochromatic light 203 is λo. When the monochromatic light 203 is incident on the diffraction grating 202, weak scattered light is generated in each grating. Focusing on the scattered light from the adjacent lattice planes, the optical path length difference (2 × λa × sin θ) of the two rays scattered in the same direction on each lattice plane is an integral multiple of the wavelength λo (m × λ0, m = When equal to ± 1, ± 2,..., The two scattered lights strengthen each other. Since this strengthening also occurs on other lattice planes, high intensity scattered light, that is, diffracted light is generated as a whole. For the above reason, the angle θ at which the diffracted light is observed is expressed by Expression (1).

Equation (1) is a condition for Bragg diffraction, and defines the angle θ between the incident light beam and the outgoing light beam with respect to the lattice plane. sin ⁻¹ represents an inverse sine function. Pure Bragg diffraction refers to a diffraction phenomenon that occurs when the diffraction grating 202 is composed of an infinite number of grating surfaces. As shown in FIG. 2B, the angles of incident light and outgoing light with respect to the lattice plane are equal to θ. In Bragg diffraction, generally, the smaller the order m, the higher the intensity of diffracted light 201 is obtained. Therefore, it is preferable to use the diffracted light 201 of m = ± 1 in order to observe the weaker first and third scattered waves 5a and 5b. In the photoacoustic vibrometer 101 shown in FIG. 1, the diffracted light 201 indicates m = + 1 diffracted light, but the photoacoustic vibrometer 101 using diffracted light of m = −1 may be realized.

  FIG. 2C is a schematic diagram for explaining that the sound pressure distribution on the wavefront of the plane sound wave 9 is reflected in the light intensity distribution in the cross section of the diffracted light 201 by Bragg diffraction. As shown in FIG. 2C, generally, the plane sound wave 9 is a non-uniform sound pressure distribution reflecting the shape of the object 4 and the like (more precisely, the elastic characteristics of the surface of the object 4) in the wavefront. have. Since the spatial distribution of the refractive index change in the photoacoustic medium unit 8 is proportional to the sound pressure distribution of the plane sound wave 9, the in-plane distribution of the refractive index change amount on the grating surface of the diffraction grating 202 is non-uniform. It is assumed that the displacement of the object 4 within the pulse duration of the first and second acoustic waves 2a and 2b is very small and can be regarded as stationary. In this case, since the refractive index distribution on the grating surface of the diffraction grating 202 is the same on all the grating surfaces, the diffraction grating 202 becomes a one-dimensional diffraction grating, and the diffracted light 201 is mainly generated by Bragg diffraction. Since the amplitude of the diffracted light 201 (= 1/2 power of the light intensity) is proportional to the refractive index change amount, the amplitude of the diffracted light 201 is proportional to the sound pressure distribution of the plane sound wave 9. Therefore, the light amplitude distribution on the wavefront of the diffracted light 201 is proportional to the sound pressure distribution of the plane sound wave 9. That is, the diffracted light 201 has an intensity distribution that reflects the shape of the object 4 and the like, similar to the light obtained when the object 4 is directly observed.

  Returning to FIG. 1, the description of the photoacoustic vibrometer 100 will be continued again. As described above, the first and second diffracted beams 201a and 201b are generated at the positions P1 and P2. Since the generation times of the first acoustic wave a and the second acoustic wave 2b are shifted by τ, the first and second diffracted beams 201a and 201b have shapes of the object 4 at two times separated by time τ. It has an intensity distribution that reflects. When the object 4 is not moved or displaced at all, the first and second diffracted beams 201a and 201b have exactly the same intensity distribution.

  As shown in FIG. 1, the first and second diffracted beams 201 a and 201 b are superimposed by a light superimposing unit 220 including a plane mirror 22 and a beam splitter 20. The plane mirror 22 is such that the center position of the first and second diffracted beams 201a and 201b and the wavefront deviation in the entire surface of the light flux wavefront are less than one wavelength in terms of the wavelengths of the first and second plane wave light fluxes 14a and 14b. The position and angle of the beam splitter 20 are adjusted. In order to suppress the superimposition of interference fringes unrelated to the displacement of the object 4 on the image 18, it is preferable that the above-described center position and wavefront deviation are as small as possible.

  The superimposed diffracted light 201 ′ enters the image distortion correction unit 15. The operation of the image distortion correction unit 15 will be described with reference to FIG. FIG. 3A is a schematic diagram showing that the diffracted light 201 ′ is contracted in one direction in the photoacoustic vibrometer 101. For convenience, it is shown that the diffracted light 201 ′ superimposed by the plane sound wave 9 is generated.

  As can be seen from the equation (1), the plane wave light beam 14 must be incident on the plane sound wave 9 obliquely in order to satisfy the diffraction condition. Here, it is assumed that the beam shape of the plane sound wave 9 is a circle having a diameter L, and the diffraction angle of the diffracted light 201 ′ is θ (the definition of θ is the same as described above). As described above, the plane wave light beam 14 has a beam diameter including the plane sound wave 9, and the diffracted light 201 ′ is generated only in the region where the plane sound wave 9 exists. Is an ellipse having a minor axis L × sin θ in the y-axis direction and a major axis L in the x-axis direction in the coordinate system shown in FIG. That is, the light amplitude distribution on the wavefront of the diffracted light 201 ′ is proportional to a distribution obtained by multiplying the sound pressure distribution on the wavefront of the plane sound wave 9 by sin θ in the y-axis direction.

  Therefore, when the diffracted light 201 ′ is imaged by the imaging lens system 16 as it is and the image 18 is generated, the image 18 becomes an optical image distorted in the y-axis direction, and the similarity between the object 4 and the real image 18 is similar. Lost. In other words, the diffracted light 201 'has distortion in the y-axis direction. Therefore, the distortion of the diffracted light 201 ′ is corrected by the image distortion correction unit 15.

In the present embodiment, the image distortion correction unit 15 includes an anamorphic prism 301. FIG. 3B is a schematic diagram showing the configuration and operation of the anamorphic prism 301. As shown in FIG. 3B, the anamorphic prism 301 includes two wedge-shaped prisms 303. The operation of the wedge prism 303 will be described with reference to FIG. FIG. 4 is a ray tracing diagram showing the state of light rays that pass through the wedge-shaped prism 303. The wedge-shaped prism 303 is made of a material that is transparent to the diffracted light 201 having a refractive index n, and has two flat surfaces 303a and 303b. The angle between the plane 303a and the plane 303b is α, the angle at which the light beam enters the plane 303a and the angle at which it exits are θ ₁ and θ ₂ with respect to the normal. Further, the angle at which the light beam is emitted from the plane 303b is θ ₃ with respect to the normal line. In a plane including normal lines of the two planes 303a and 303b, the width of the light beam incident on the plane 303a is Lin, and the width of the light beam emitted from the plane 303b is Lout. At this time, the relationship of Expression (2) is established.

  Further, the beam diameters of the incident light beam and the light beam emitted from the wedge prism 303 in the plane including the normal line of the two planes 303a and 303b are different. The light beam expansion ratio calculated by Lout / Lin is expressed by Equation (3).

As can be seen from the equations (2) and (3), a desired light beam expansion ratio can be realized by appropriately selecting α and n and the angle θ ₁ of the wedge-shaped prism 303. The beam expansion ratio does not change in the direction perpendicular to the plane including the normal line of the two planes 303a and 303b regardless of α, n and the angle θ ₁ , and therefore, if the wedge prism 303 is used, FIG. ) In the y-axis direction can be adjusted.

  As shown in FIG. 3B, the anamorphic prism 301 is configured by combining one or more wedge-shaped prisms 303 shown in FIG. As shown in FIG. 3B, when two wedge-shaped prisms 303 having the same shape are used, incident light and outgoing light to the anamorphic prism 301 can be made parallel, and the optical system can be easily adjusted. .

Thus, the anamorphic prism 301 operates as an optical system for expanding the beam diameter. In the photoacoustic vibrometer 100, α and n of the wedge-shaped prism 303 and the incident angle θ ₁ are selected, and the diffracted light 201 is expanded by 1 / sin θ times in the y-axis direction as shown in FIG. Thereby, the distortion-corrected diffracted light 302 having a circular light beam cross section with a diameter L is obtained. Accordingly, the diffracted light 302 after distortion correction has a light amplitude distribution proportional to the sound pressure distribution on the wavefront of the plane sound wave 9 on its wavefront. That is, although the diffracted light 302 after distortion correction has a wavelength different from that of the plane sound wave 9, the sound pressure distribution on the wavefront of the plane sound wave 9 is entirely reproduced as a light amplitude distribution. 18 can be generated.

  As shown in FIG. 1, the diffracted light 302 after distortion correction is condensed by the imaging lens system 16 having a focal length F. Since the diffracted light 302 after distortion correction is a parallel light beam, it is diffracted on a plane (focal plane) perpendicular to the optical axis that is separated from the imaging lens system 16 on the optical axis of the imaging lens system 16 by the focal length F. The light 302 is condensed to form the real image 18. By arranging the image receiving portion 17 at this position, the real image 18 can be converted into an electric signal.

  The image processing unit 205 performs image processing based on the electrical signal input from the image receiving unit 17 to form a real image 18. In this way, the photoacoustic vibrometer can photograph the object 4.

  Next, the relationship between the size of the object 4 and the image 18 in the photoacoustic vibrometer of the present embodiment will be described. The photoacoustic vibrometer of the present embodiment can be regarded as a modified optical system of a double diffractive optical system that includes two optical lenses having a focal length f and F. FIG. 5A is a schematic diagram for explaining the operation of the double diffractive optical system in the optical field.

  In the double diffractive optical system shown in FIG. 5A, the lens 403 and the lens 404 have focal lengths f and F, respectively. Both lenses are arranged on an optical axis 409 separated by a focal length f + F. Both lens optical axes coincide with the optical axis 409. In general, a convex lens having a focal length fl has a focal point at two points on the optical axis that are separated from the lens by the center of the lens. According to Fourier optics, an object placed at one focus of a convex lens and an optical image at the other focus are in a Fourier transform relationship with each other. Therefore, a Fourier transform image of the object 401 by the lens 403 is formed on the Fourier transform surface 402 which is another focal plane (that is, a plane including the focal point and perpendicular to the optical axis). Since the Fourier transform surface 402 is also the focal plane of the lens 404, a Fourier transform image of the Fourier transform image of the object 401 formed on the Fourier transform surface 402 is formed on the other focal plane of the lens 404. In other words, the optical image formed on the other focal plane of the lens 404 corresponds to the object 401 subjected to Fourier transform twice. Since the two-time Fourier transform is a similarity map (a map in which the size is multiplied by a constant and only the direction of the figure is converted), a real image 405 that is a two-time Fourier transform image of the object 401 is a figure similar to the object 401. It becomes. The real image 405 appears on the focal plane of the lens 404 as an inverted image of the object 401, and the size of the real image 405 is F / f times that of the object 401 because the focal lengths of the lens 403 and the lens 404 are different. . As described above, in the double diffractive optical system of FIG. 5A, an optical image similar to the object 401 appears as a real image 405, and a real image of the lens 404 is formed on an imaging device such as a CCD. If installed on the focal plane, the object 401 can be imaged.

  The photoacoustic vibrometer of this embodiment can be regarded as a double diffractive optical system in which one of the two optical systems is replaced with an acoustic system. As described with reference to FIGS. 2 and 3, the generation of the first and second diffracted beams 201a and 201a and the image distortion correction unit 15 in the photoacoustic vibrometer of this embodiment are plane waves having a wavelength λa. Is converted (transferred) into amplitude distributions (light) of the first and second diffracted lights 202a and 202b after distortion correction, which are plane waves of wavelength λo. It can be regarded as the acousto-optic conversion unit 406. Therefore, the photoacoustic vibrometer of this embodiment is a photoacoustic mixed optical system in which an optical system and an acoustic system are mixed, and the lens 403 and the lens 404 shown in FIG. 4A are shown in FIG. As described above, the acoustic lens system 6 and the imaging lens system 16 are replaced, and the acoustic wave conversion unit 406 that converts the wavelength from λa to λo between these two lens systems converts the acoustic wave to the light wave. The photoacoustic vibrometer of the embodiment performs the same operation as the double diffractive optical system shown in FIG. Therefore, in the photoacoustic mixed optical system shown in FIG. 4B, the focal point of the imaging lens system 16 as an actual image in which an optical image similar to the object 407 is inverted is obtained from Fourier optics as in FIG. 4A. Obtained on the surface.

  However, the wavelength changes from λa to λo before and after the acoustic light conversion unit 406. In the photoacoustic mixed optical system of FIG. 4B, the size of the real image 18 with respect to the object 4 is (F × λo) / (f × λa) times. When λo / λa is extremely small, that is, when the wavelength of the acoustic wave in the photoacoustic medium unit 8 is very long compared to the wavelength of the plane wave light beam 14, F / f is set to be large (F × λo) / It is preferable that the resolution of the optical image obtained by the image receiving unit 17 is not lowered by increasing (f × λa) so that the real image 18 does not become extremely small.

  Next, the image 18 photographed by the photoacoustic vibrometer 101 will be described. As described above, the image 18 is obtained by superimposing optical images of the first and second diffracted lights 201a and 201b generated by the first and second plane sound waves 9a and 9b. Since the first and second plane sound waves 9a and 9b correspond to the first and second scattered waves 5a and 5b generated by the first and second acoustic waves 2a and 2b emitted at different times, the image 18 is different. The optical images of the object 4 at the time overlap. When the object 4 is stationary, the two optical images at different times of the object 4 coincide with each other, and the image 18 is an optical image similar to the object 4. However, when the object 4 is moved or displaced while the first and second acoustic waves 2a and 2b are irradiated, the image 18 corresponds to the amount of displacement in addition to the optical image similar to the object 4. An optical image in which optical interference fringes are superimposed is obtained.

  It is assumed that the object 4 is in two different states at the time when the first and second acoustic waves 2a and 2b are irradiated. It is assumed that the displacement speed of the object 4 is sufficiently slow, and that the object 4 can be regarded as stationary at a time corresponding to the pulse widths constituting the first and second acoustic waves 2a and 2b. This assumption is established, for example, when the displacement speed of the object 4 is sufficiently small as compared with the sound speeds of the first and second acoustic waves 2a and 2b in the medium 3. For example, when observing the motion of an organ in the body, more specifically, the motion state of a heart wall or an arterial blood vessel wall in the body, this condition is satisfied.

  FIG. 6 shows a state in which two different images generated from the first and second diffracted beams 201a and 201b are superimposed. When the first and second plane sound waves 9a and 9b reach the object 4, respectively, it is assumed that the object 4 has an outer shape indicated by an image 141 and an image 142. In FIG. 6, for convenience of explanation, it is assumed that the image 141 is a flat surface, and the image 142 is a curved surface curved in the z-axis direction of the coordinate axis added in the drawing. The image 142 is in contact with the image 141 at the apex (the phase difference between the first and second diffracted beams 201a and 201b is zero at the apex). The z axis is parallel to the optical axis of the imaging lens system 16, and the maximum displacement amount of the image 142 with respect to the image 141 measured in the z axis direction is converted into the wavelength λo of the light beam emitted from the light source 11. Assume that there are about 10 wavelengths. Under this assumption, the deterioration of the image 142 on the image receiving unit 17 surface due to defocus can be ignored. Therefore, the superimposed image 146 corresponding to the image 18 in FIG. These assumptions hold when observing the motion state of the heart wall or arterial vessel wall in the body.

  Light beams 143 and 144 shown in FIG. 6 are spherical waves corresponding to the superimposed diffracted light 201 ′ or diffracted light 302 shown in FIG. 1. These give image points at different positions in the xy plane in the figure.

  The light beam 143 determines the amount of light at the image point 147 on the image 146 after superimposition. In FIG. 6, since the image 141 and the image 142 coincide with each other, the superimposed first and second diffracted beams 201a and 202b are completely overlapped, and the phase difference is zero. Accordingly, the two spherical waves to be superimposed interfere with each other, and the image point 147 exhibits high luminance.

  On the other hand, the light beam 144 includes first and second diffracted beams 201a and 202b, and the optical path length difference 145 measured in the z-axis direction of the first and second diffracted beams 201a and 202b2 is equal to the wavelength λo of the light beam of the light source 11. It is 3/2 wavelength in terms of conversion. Such spherical waves having an optical path length difference 145 that is an odd multiple of ½ wavelength are in phase with each other and are weakened by interference. Therefore, the brightness of the image point 148 on the image 146 after superimposition, in which the light beam 144 determines the amount of light, decreases.

  As described above, in the superimposed image 146, the displacement or movement of the object 4 in the direction of the sound axis 7 at the time τ on the optical image similar to the object 4 is a luminance corresponding to the amount of displacement or movement. Is superimposed. In FIG. 6, since the optical path length difference 145 is constant in the y-axis direction and changes only in the y-axis direction, the superimposed image 146 is an optical image in which interference fringes parallel to the x-axis are superimposed on a planar real image. Become.

  The distribution of the absolute value of the displacement amount of the object 4 at the time τ and the distribution of the absolute value of the velocity of the object 4 obtained by dividing the displacement amount by the time τ are obtained from the density of the image 146 after superimposition. it can. In FIG. 6, the number of interference fringes intersecting with a straight line connecting the image point 147 and the image point 148 is 1.5 periods. Therefore, the optical path length difference between the image 141 and the image 142 at the image point 147 and the image point 148 is 1.5λo. In the photoacoustic vibrometer 101, the length 1.5λo on the image 18 corresponds to 1.5 × f / F × λa on the object 4. From this, it is possible to read the absolute value of the difference between the displacement amounts of the two points on the object 4 corresponding to the image point 147 and the image point 148 at time τ, and 1.5 × f / F × λa. Desired.

  When the image 142 and the image 141 do not touch at the vertex, the phase difference between the first and second diffracted lights 201a and 201b is not zero at the vertex. However, in this case, the phase of the second diffracted light 201a composing the image 142 is only shifted with respect to the first diffracted light 201b composing the image 141 with the same phase difference as a whole. For this reason, the optical path length difference between the image 141 and the image 142 at the image point 147 and the image point 148 is similarly 1.5λo.

  As described above, according to the photoacoustic vibrometer of the present embodiment, an optical image in which images of an object at two times different by time τ are superimposed is obtained. When the shape of the object (more precisely, the spatial distribution of the elastic modulus) changes during this time τ, the two Bragg diffracted lights that are superimposed are converted in the amount of change in shape (acoustic wave propagation wavelength). Phase difference corresponding to the amount of change). Accordingly, the superimposed optical image is detected by the image receiving unit as an image accompanied by an intensity change corresponding to the phase difference due to the interference phenomenon of Bragg diffracted light. For example, when the amount of displacement at a certain position of the object corresponds to an odd multiple of 1/2 wavelength, the light intensity at that position on the image decreases, and when the amount of displacement corresponds to an even multiple of 1/2 wavelength, The light intensity at that position on the image increases. Thus, the optical image acquired by the image receiving unit is an image in which interference fringes corresponding to the amount of change in the shape of the object are superimposed on the image of the object. By counting the number of interference fringes recorded in one acquired image, it is possible to know the shape change amount distribution and the change speed distribution of the object.

  Therefore, the photoacoustic vibrometer of the present embodiment acquires the shape information of the object with a spatial resolution about the propagation wavelength of the acoustic wave, and acquires the shape variation distribution and the change velocity distribution in one imaging. be able to. Such an image is obtained by superimposing two diffracted lights and detecting the light superimposed by any image receiving unit such as an image sensor, and individually acquiring images of objects at two times different by time τ. There is no need for signal processing. Therefore, the photoacoustic vibrometer of this embodiment can measure the displacement and movement of an object moving at high speed with a high spatial resolution with a simple configuration.

  A specific example of the image 18 photographed by the photoacoustic vibrometer 101 will be described. As shown in FIG. 7, a monochromatic sound wave 193 transmitted from a speaker 192 is applied to a square drum skin 191 that resonates as an object 4, and the reflected scattered wave is reflected by a drum skin as indicated by an arrow 194. Detection is performed from a direction perpendicular to 191. The frequency of the monochromatic sound wave 193 matches the resonance frequency of the drum skin 191. Therefore, the drum skin 191 resonates and repeats a single vibration at the same frequency as the frequency of the monochromatic sound wave 193. As a result, the drum skin 191 shows an amplitude distribution as shown in FIG.

  It is assumed that the maximum amplitude of the drum skin 191 is 2.5 × f / F × λa. The monochromatic sound wave 193 is irradiated at two times when the amount of displacement of the drum skin 191 becomes zero in all regions and when the maximum amplitude is shown. At this time, an image 18 of the drum skin 191 photographed by the photoacoustic vibrometer 101 is shown in FIG. The image 18 becomes a real image 18 in which a striped pattern is superimposed on a square drum skin 191.

  The photoacoustic vibrometer 101 exhibits high detection sensitivity to the displacement and movement of the object 4 at time τ in a direction parallel to the sound axis 7 of the acoustic lens system 6, but in a plane perpendicular to the sound axis 7. The detection sensitivity of the displacement and movement of the object 4 decreases.

  Consider the case where the object 4 used as shown in FIG. 9 is a sphere, and the radius of the sphere and the center position of the sphere change over time. Specifically, as indicated by a dotted line, the object 4 whose center is located at the origin of coordinates and whose radius is 11.11 × (f / Fλa) is (−0.56, −0.56) after time τ. , −0.56) × (f / Fλa), the center is located, and the object 4 ′ having a radius of 12.07 × (f / Fλa) is considered.

  The wavelength of the first and second acoustic waves 193a and 193b is λa, and the focal lengths of the acoustic lens system 6 and the imaging lens system 16 are f and F. The first and second acoustic waves 193a and 193b are reflected only on the surface of the sphere, and do not consider reflection inside the sphere. The reflected wave is detected from the positive direction of the x axis.

  FIG. 10 shows an image 18 photographed by the photoacoustic vibrometer 101. As shown in FIG. 10, the image 18 includes images of the objects 4 and 4 ′ and a stripe pattern. The striped pattern is centered on the upper right of the sphere, and the intervals are narrowed toward the periphery. This is because the surfaces of the object 4 and the object 4 'are closest to each other in the (1, 1, 1) direction as shown in FIG. Further, the stripe pattern appears only in the object 4 ′ shown by the dotted line in FIG. 20. This is because interference occurs only in a region where the images of the two objects 4, 4 'overlap in the x-axis direction. That is, the striped pattern indicates the displacement amount in the x-axis direction with respect to the object 4 and the object 4 ′, and the displacement in the y direction and the z direction does not appear as a striped pattern. In this calculation, imaging characteristics such as the resolution of the acoustic lens system 6 and the imaging lens system 16 are not considered. Therefore, the image 18 actually obtained has an influence due to the resolution of both lens systems, so that the outline of the sphere is blurred compared to the image shown in FIG.

  As described above, according to the photoacoustic vibrometer of the present embodiment, the object is irradiated with the first and second acoustic waves transmitted at predetermined time intervals, thereby generating scattered waves at different times and scattering. Diffracted light is generated by the refractive index distribution generated in the photoacoustic medium part by the plane sound wave by the wave. Since the interference is obtained by superimposing the two generated diffracted lights, the displacement and movement of the object at a predetermined time can be detected.

  In this embodiment, the focal length of the acoustic lens system 6 of the photoacoustic vibrometer 101 is fixed. However, as described above, the acoustic lens system 6 has a focusing mechanism (focus adjustment mechanism) like a normal photographic lens. You may have. When the focal point of the acoustic lens system 6 is fixed, a sharp image 18 is obtained in the vicinity of the focal plane of the acoustic lens system 6 (more precisely, the optical characteristics of the acoustic lens system 6 and the pixel size of the image receiving unit 17). It is only the object 4 located within the determined depth of field. Therefore, by providing the acoustic lens system 6 with a mechanism that can adjust the focal position of the acoustic lens system 6, the object 4 can be imaged in the optical axis direction. As described above, by providing the focusing mechanism, it is possible to capture a three-dimensional region.

  Moreover, in this embodiment, as shown to Fig.11 (a), it inclines in the direction of the target object 4 from the sound wave absorption part 10, and irradiates the 1st and 2nd plane wave light beams 14a and 14b. However, as shown in FIG. 11B, the first and second plane wave light beams 14a and 14b may be irradiated while being inclined from the object 4 side toward the sound wave absorption unit 10. However, when the first and second plane wave light beams 14a and 14b are irradiated as shown in FIG. 11B, the paper surface of FIG. 11 is mirror-symmetric with respect to the real image generated by the configuration of FIG. A real image having a mirror image relationship with the surface is obtained. Therefore, in order to obtain the real image 18 of the correct direction of the object 4, the captured image is reflected once by a plane mirror or the like and optically mirror-inverted, or the image processing unit 205 performs mirror-image inversion. There is a need.

  In this embodiment, the anamorphic prism 301 is used as the image distortion correction unit 15, but another optical system having the same optical action may be used. For example, the image distortion correction unit 15 may be configured using two condensing cylindrical lenses, for example. As shown in FIG. 12, the cylindrical lens 151 functions as a condensing lens in a plane parallel to the yz plane of the coordinate system set in the figure, but has a condensing function in a plane parallel to the xz plane. Optical element. As shown in FIG. 13, an optical system in which two cylindrical lenses 161 and 162 whose planes having a condensing action are orthogonal to each other is an optical system having both the functions of the image distortion correction unit 15 and the imaging lens system 16. Function as. As shown in FIG. 13, the cylindrical lens 161 condenses the light in the xy plane on a straight line parallel to the y axis, and the cylindrical lens 162 condenses the light in the yz plane on a straight line parallel to the x axis. Since the cylindrical lens 161 has a longer focal length than the cylindrical lens 162, it functions as an optical system that forms an image at different ratios in the yz plane and the xz plane. If this optical system is arranged in the same direction in the coordinates shown in FIG. 7A, it suitably functions as the image distortion correction unit 15 of the photoacoustic vibrometer 101. Specifically, the focal lengths of both lenses are selected so that the ratio of the images in the y-axis direction and the x-axis direction is 1 / sin θ so as to correct the flattening ratio sin θ of the light beam in FIG. More specifically, the focal length of the cylindrical lens 162 is selected to be sin θ times the focal length of the cylindrical lens 161. In this case, the focal length of the cylindrical lens 161 is determined by the similarity ratio between the object 4 and the real image 18.

(Second Embodiment)
Hereinafter, a second embodiment of the photoacoustic vibrometer according to the present invention will be described. FIG. 14 schematically shows the photoacoustic vibrometer 102 of the present embodiment. The photoacoustic vibrometer 102 uses ultrasonic waves as the first and second acoustic waves 2a and 2b and images body organs such as humans and animals non-invasively. As shown in FIG. 14, the photoacoustic vibrometer 102 has the same configuration as the photoacoustic vibrometer 101 of the first embodiment, but the photoacoustic vibration shown in FIG. 1 is the same as that of the conventional ultrasonic probe. All of the total 101 or a configuration excluding the light source 11 is provided in the probe 213.

  As shown in FIG. 14, the acoustic wave source 1 and the acoustic lens system 6 are disposed on the probe surface 213 a of the probe 213. As shown in FIG. 14, at the time of imaging, the probe surface 213a of the probe 213 is brought into contact with the body surface of the subject 210, and the first and second acoustic waves 2a and 2b generated from the acoustic wave source 1 from within the body Wave inside. At this time, in order to reduce reflection attenuation on the body surface, it is preferable to match the acoustic impedance by interposing a matching gel, cream, or acoustic impedance matching layer between the probe surface 213a and the body surface.

  The first and second acoustic waves 2a and 2b propagate through the body tissue 212, are reflected and scattered by the organ 211, and become first and second scattered waves 5a and 5b. The first and second scattered waves 5a and 5b reach the acoustic lens system 6 and are converted into plane waves by the acoustic lens system 6, and an image of the organ 211 can be obtained as described in the first embodiment. . Imaging of the organ 211 that is in a plane perpendicular to the sound axis 7 (not shown) of the photoacoustic vibrometer 102 and is outside the imaging region is performed by moving the photoacoustic vibrometer 100 on the body surface in the same manner as a conventional ultrasonic probe. Can be done. In addition, organs at different depths in the body can be photographed by adjusting the focal length by the focus adjustment mechanism of the acoustic lens system 6 as described in the first embodiment.

A specific configuration example capable of realizing the photoacoustic vibrometer 102 will be described with reference to FIG.
For example, a burst signal composed of 20 sine waves having a frequency of 13.8 MHz is emitted from the acoustic wave source 1. The signal duration of this burst signal is 1.4 μsec. Further, since the speed of sound in the body tissue 212 is about 1500 m / s, the wavelength of the ultrasonic sine wave in the body tissue 212 is about 110 μm, and the physicality of the burst signal measured in parallel with the traveling direction of the ultrasonic waves. The long signal length is about 2.2 mm. Therefore, in this case, the organ 211 oscillating at a frequency of several hundred kHz at maximum can be imaged with a spatial resolution of several hundred μm.

  As the photoacoustic medium portion 8, a silica nanoporous material having a sound velocity of 50 m / s is used. The silica nanoporous material has a low sound velocity and a short propagation wavelength of ultrasonic waves, so that a large diffraction angle can be obtained. The silica nanoporous material has sufficient translucency with respect to a He—Ne laser beam having a wavelength of 633 nm. In addition, since the fluorinate also has sufficient translucency with respect to the He—Ne laser beam having a wavelength of 633 nm and has sufficient translucency, and the sound velocity of florinate is about 500 m / s, the photoacoustic medium portion 8 is suitable.

  When a He—Ne laser having a wavelength of 633 nm is used as the light source 11, the diffraction angle of the first-order diffracted light is 5 °. In this case, the beam expansion ratio that must be realized by the image distortion correction unit 15 is about 5.74, which is a value that can be corrected by a commercially available anamorphic prism.

  An upper limit is set for the sound pressure of the acoustic wave that can be irradiated into the body for safety. Therefore, the light intensity of the generated diffracted light is weak, and it is desirable that the image receiving unit 17 has high sensitivity. Further, from the viewpoint of image quality and light quantity, the first and second plane sound waves 9a and 9b capture the image 18 at the moment when the first and second plane wave light beams 14a and 14b are crossed. In order to observe the movement of No. 4, it is preferable to use an image sensor capable of imaging at high speed as the image receiving unit 17. For example, a high-speed CCD image sensor (Charge Coupled Device Image Sensor) is used as the image receiving unit 17. When the brightness of the real image 18 is insufficient and it is difficult to capture an image, it is preferable to place an image intensifier immediately in front of the image sensor to increase the brightness of the image 18 or use a light source 11 with higher output.

As described in the description of the acoustic lens system 6, acoustic waves are reflected at the interface between acoustic media having different acoustic impedances, and the brightness and image quality of the image 18 are degraded. The greater the acoustic impedance difference at the interface, the greater the reflection. For this reason, it is preferable to provide an antireflection film at the interface between the acoustic lens system 6 and the medium 3 as shown in FIG. For example, when the lens in contact with the medium 3 (body tissue 212) of the acoustic lens system 6 is composed of a silica nanoporous material having a sound velocity of 50 m / s and a density of 0.11 g / cm ³ , the lens has a thickness of 6.2 μm. It is preferable to form a quarter-wave antireflection film made of a silica nanoporous material having a sound velocity of 340 m / s and a density of 0.2 / cm ^{3 on} the surface of the lens.

  When an image 18 having a size 1/5 that of the object 4 is obtained on the image receiving unit 17, F / f = 1.14. As described in the first embodiment, since the size of the real image 18 with respect to the object 4 is (F × λo) / (f × λa) times, (F × λo) / (f × λa) = 1. The relational expression of / 5 is established. Therefore, F / f = λa / λo / 5, where the wavelength of light λo = 633 nm and the wavelength λa = 3.6 μm in the photoacoustic medium part 8 of the 13.8 MHz ultrasonic wave of the silica nanoporous material with a sound velocity of 50 m / s. If substituted, F / f = 1.14 is obtained. Therefore, when the acoustic lens system 6 having a focal length of 50 mm is used, the imaging lens system 16 having a focal length of 57 mm is used (F = 1.14 × f = 1.14 × 50 mm).

  As described with reference to FIG. 4, when the similarity ratio (F × λo) / (f × λa) of the real image 18 to the object 4 is increased, it is necessary to increase the focal length of the imaging lens system 16. The photoacoustic vibrometer 102 is increased in size. In this case, this problem can be solved by using, for example, a folded reflection optical system typified by a Cassegrain optical system as the imaging lens system 16. By applying the folded reflection optical system, the distance between the imaging lens system 16 and the real image 18 can be arranged closer than the actual focal length F, and the photoacoustic vibrometer 102 can be downsized.

  The photoacoustic vibrometer 102 can also be reduced in size by arranging the distance between the acoustic lens system 6 and the imaging lens system 16 closer than f + F. With reference to FIG. 4, it has been explained that the photoacoustic mixed optical system of the photoacoustic vibrometer 101 can be regarded as a double diffractive optical system in the optical field. In the basic configuration of the double diffractive optical system, the acoustic lens system 6 and the imaging lens system 16 are arranged apart from each other by the sum f + F of the focal lengths of the lenses. However, setting the distance between the acoustic lens system 6 and the imaging lens system 16 to a value other than f + F does not affect the optical image formation of the real image 18. That is, as long as the optical image of the real image 18 is acquired as a light intensity distribution (or as long as the phase distribution information of the real image 18 is not observed), the distance between the acoustic lens system 6 and the imaging lens system 16 can be shortened below f + F. The photoacoustic vibrometer 102 can be further reduced in size.

  In the present embodiment, an example of the photoacoustic vibrometer 102 that images a body organ such as a human or an animal from the body has been described. However, the photoacoustic vibrometer of the present invention can be transmitted through a catheter, an endoscope, a laparoscope, and the like. You may implement | achieve as photoacoustic imaging which images an organ and the blood vessel wall from a body.

(Third embodiment)
A third embodiment of the photoacoustic vibrometer according to the present invention will be described. The photoacoustic vibrometer of the third embodiment is the same as the photoacoustic vibrometer 101 of the first embodiment except that the configuration of the acoustic lens system 6 is different. Therefore, only the configuration of the acoustic lens system 6 will be described. FIG. 16 shows a configuration of the acoustic lens system 6 in the present embodiment.

  In the first embodiment, the acoustic lens system 6 is entirely composed of a silica nanoporous material. The silica nanoporous material has an advantage that the sound velocity of an acoustic wave such as an ultrasonic wave in the silica nanoporous material can be changed in a wide range by adjusting the production conditions. The ratio of the sound speed of the silica nanoporous material to the sound speed of the medium 3 corresponds to the refractive index in the optical system. That is, the silica nanoporous material is a flexible acoustic medium that can easily realize various refractive indexes (for ultrasonic waves). Therefore, when the silica nanoporous material is applied as a constituent member of the acoustic lens system 6, the design flexibility of the acoustic lens system 6 is widened due to the wide selectivity of the refractive index with respect to the acoustic wave. Similarly, each aberration can be corrected well, and the acoustic lens system 6 having a wide image circle can be configured. The image circle means an area on the focal plane where good imaging characteristics can be obtained.

  Although the acoustic lens system 6 of the first embodiment has such advantages, it is necessary to join the silica nanoporous materials to each other, and the following problems associated therewith arise. For example, even if the acoustic lens system 6 has a single lens configuration, when the silica nanoporous material is applied to the photoacoustic medium portion 8 as in the specific example shown in FIG. It becomes. In addition, when the acoustic lens system 6 has a multi-group lens configuration and a laminated lens is required like an achromatic lens in the optical field, the silica nanoporous materials need to be joined to each other.

  The acoustic impedance of silica nanoporous material and air is greatly different. Therefore, in order to suppress the generation of the reflected wave on the joint surface, it is important to make the air layer so as not to be sandwiched between the joint surfaces of the silica nanoporous materials. However, it is extremely difficult to join so as not to sandwich the air layer in the process of producing the silica nanoporous material. Therefore, in the acoustic lens system 6 in the first embodiment, it is difficult to suppress the generation of reflected waves on the cemented surface.

  In order to solve such a problem, the acoustic lens system 6 of the present embodiment is configured by a reflective acoustic system. FIG. 16 is a cross-sectional view of the acoustic lens system 6 in a plane including the sound axis 706. The acoustic lens system 6 includes an acoustic waveguide 705 and a primary mirror 702 and a secondary mirror 701 that are reflection surfaces provided inside the acoustic waveguide 705. Further, a photoacoustic medium portion is formed inside the acoustic waveguide 705. The acoustic waveguide 705 has a mirror image symmetric structure with the plane of FIG. 16 as a mirror image symmetry plane. The cross-sectional structure shown in FIG. 16 is rotated 180 degrees around the sound axis 706. The obtained rotating body is cut along two planes parallel to the plane including the sound axis 706, with the plane of mirror symmetry as the plane of mirror symmetry. Thereby, the three-dimensional shape of the acoustic waveguide 705 is obtained. Such an acoustic waveguide 705 is made of, for example, a metal acoustic waveguide 705 having a reflective surface by cutting or the like, and isotropic silica nanoporous material is enclosed in the created acoustic waveguide, The photoacoustic medium unit 8 and the acoustic lens system 6 are integrally shaped. With such a process, it is possible to obtain the acoustic lens system 6 with good aberration correction while eliminating all the joint portions between the silica nanoporous materials.

  As an example of a reflective optical system suitable for the present invention, there is a Cassegrain type optical system constituted by a primary mirror 702 which is a concave mirror and a secondary mirror 701 which is a convex mirror, as shown in FIG. Furthermore, if a Ritchie-Cretian optical system is applied as the surface shape of the primary mirror 702 and the secondary mirror 701, the residual aberration of the Cassegrain type optical system when the focal length is shortened can be corrected well, and a wide image circle is realized. be able to. Since the curvature of field remains at the focal point 704 in the Ritchie-Cretian optical system, the surface of the silica nanoporous material on the focal side (the surface on which the antireflection film 703 is applied) is subjected to curved surface processing to function as a correction lens. Surface curvature can be corrected. As the reflective optical system, other catadioptric optical systems such as a Gregory optical system using a concave mirror as the secondary mirror 701 and a Schmitt-Cassegrain optical system may be used.

  By applying a reflective optical system as the acoustic lens system 6, an acoustic lens system in which aberrations are corrected satisfactorily with only a single silica nanoporous body without joining a plurality of types of silica nanoporous bodies that are difficult to produce. 6 can be configured. Since no reflected wave is generated in the vicinity of the acoustic lens system 6, it is possible to obtain a real image 18 with high brightness and good image quality. Therefore, according to the present embodiment, it is possible to realize a photoacoustic vibrometer that can obtain an image with higher brightness and higher image quality.

(Fourth embodiment)
A fourth embodiment of a photoacoustic vibrometer according to the present invention will be described. The photoacoustic vibrometer of the fourth embodiment is the same as the photoacoustic vibrometer 101 of the first embodiment except that the configuration of the image distortion correction unit 15 is different. Therefore, only the configuration of the image distortion correction unit 15 will be described. FIG. 17 schematically illustrates the configuration of the image distortion correction unit 15 in the present embodiment.

  In the first embodiment, the image distortion correction unit 15 includes an optical system using an anamorphic prism or a cylindrical lens. On the other hand, the image distortion correction unit 15 of the present embodiment performs predetermined processing on the signal of the real image 801 obtained by the image receiving unit 17 and corrects the real image 801 by image processing.

  As shown in FIG. 17, in the present embodiment, the distorted diffracted light 201 is imaged by the imaging lens system 16 without using an anamorphic prism or a cylindrical lens. In this case, the real image 801 is distorted in the y-axis direction, but the real image 801 is acquired by the image receiving unit 17 in this state. The image processing unit 205 receives an electrical signal indicating the real image 801 from the image receiving unit 17 and removes image distortion of the real image 801 by image processing. For example, in the coordinate system shown in FIG. 17, an image similar to the object 4 is generated by performing image processing for multiplying the real image 801 by 1 / sin θ in the y direction.

  If the image distortion correction part 15 of this embodiment is used, since the number of optical elements required for the structure of a photoacoustic vibrometer can be reduced, it becomes possible to provide an acoustic imaging device small in size and at low cost.

  Note that when the diffraction angle θ is small, the object 4 is photographed on the imaging surface of the image receiving unit 17 while being greatly expanded in the y-axis direction of the coordinates set in FIG. For this reason, the image resolution after image processing differs between the x-axis direction and the y-axis direction. In this case, the photoacoustic vibrometer includes both the optical image distortion correction unit 15 and the image distortion correction unit 15 by the image processing of the present embodiment illustrated in FIG. The resolution can be made almost equal.

  Further, when the anamorphic prism 301 is used as the optical image distortion correction unit 15 shown in FIG. 7 and the image distortion correction unit 15 based on image processing according to the present embodiment is used, the anamorphic of a large number of diffracted lights 201 is used. Since image plane distortion caused by different angles of incidence on the prism 301 occurs, it is preferable to perform the image processing of this embodiment for correcting the aberration.

(Fifth embodiment)
5th Embodiment of the photoacoustic vibrometer by this invention is described. The photoacoustic vibrometer of the fifth embodiment is the same as the photoacoustic vibrometer 101 of the first embodiment except that the configuration of the image distortion correction unit 15 is different. Therefore, only the configuration of the image distortion correction unit 15 will be described. FIG. 18 schematically shows the configuration of the image distortion correction unit 15 in the present embodiment. When the diffraction angle of the diffracted light is θ (the definition of θ is the same as described above), the image distortion correction unit 15 of the present embodiment has the diffracted light 201 in the x-axis direction of the coordinates shown in FIG. A reduction optical system 901 that multiplies the beam width by sin θ is included. If the cross-sectional shape of the sound bundle of the plane sound wave 9 is a circle having a diameter L, the cross-sectional shape of the light beam of the diffracted light 201 is an ellipse of L in the x-axis direction and L × sin θ in the y-axis direction. Since the diffracted light 201 is multiplied by sin θ in the x-axis direction by the reduction optical system 901, the cross-sectional shape of the light beam of the diffracted light 902 after distortion correction is a circle having a diameter L × sin θ. In the first and second embodiments, the image distortion correction unit 15 corrects the diffracted light 201 to a light beam having a diameter L. In this embodiment, the image distortion correction unit 15 corrects the light beam to a light beam having a diameter L × sin θ.

  Similarly to the first embodiment, in this embodiment, the focal length of the acoustic lens system 6 is f, the focal length of the imaging lens system 16 is F, the wavelength of the plane sound wave 9 that is an ultrasonic wave is λa, and monochromatic light. The wavelength of the plane wave light beam 14 is λo, and the diffraction angle is θ. At this time, since the cross-sectional shape of the diffracted light 902 after distortion correction is circular, the real image 18 is similar to the object 4. According to Fourier optics, the similarity ratio is (λa × f) / (λo × F) × sin θ. However, because of the relationship of (Equation 1), when the diffracted light 201 is + 1st order diffracted light, the similarity ratio is 1/2 × (f / F).

  As described above, the reduction optical system 901 makes the similarity ratio independent of the wavelengths of the ultrasonic wave and the monochromatic light. For example, the focal length ratio of the acoustic lens system 6 and the imaging lens system 16 is set so that f / F = 2. Is selected, a real image 18 having the same size as the object 4 can be obtained, and an image of the object 4 can be obtained with high resolution. Further, if f is shortened, F is also shortened, so that the photoacoustic vibrometer can be miniaturized. Further, since the light beam of the diffracted light 902 after distortion correction becomes thin, the aperture diameter of the imaging lens system 16 is reduced, the entire apparatus is downsized, and the imaging lens system 16 does not require high surface accuracy. Disappear.

  In the first and second embodiments, the similarity ratio of the real image 18 to the object 4 is (F × λo) / (f × λa). As described in the specific example shown in FIG. 15, since the ultrasonic wavelength λa is actually considerably longer than the monochromatic light wavelength λo, the imaging lens system 16 having a very long focal length is required to obtain a large real image 18. Is required. For this reason, it is necessary to enlarge the photoacoustic vibrometer 100 or use the imaging lens system 16 having a special optical system configuration. In contrast, according to the present embodiment, by using the reduction optical system 901 as the image distortion correction unit 15, the imaging lens system 16 having a small aperture diameter and a short focal length is not used, but the real image 18 can be displayed with high resolution. It is possible to take a picture, and the photoacoustic vibrometer can be miniaturized.

  In the present embodiment, the reduction optical system 901 is composed of an anamorphic prism, but other reduction optical systems having the same action may be used.

  Further, in this embodiment, when the sound bundle cross-sectional shape of the plane sound wave 9 is a circle having a diameter L, the diffracted light 902 after distortion correction having a circular shape having a light beam cross-sectional shape of L × sin θ is obtained. However, even if the cross-sectional shape of the diffracted light 902 after distortion correction is corrected so as to be a circle of C × L (where C <1), the focal point of the imaging lens system 16 is shortened, and the imaging resolution is reduced. Enhanced. For example, two image distortion correction units 15 may be provided, and in the coordinates shown in FIG. 18, a reduction optical system may be used for the x-axis direction, and a magnification optical system may be used for the y-axis direction. Specifically, the beam reduction rate in the x-axis direction and the beam expansion rate in the y-direction are selected so that the sectional shape of the diffracted light 902 after distortion correction is a circle of C × L (where C <1). do it.

  Moreover, you may implement | achieve the photoacoustic vibrometer provided with the image distortion correction part 15 of this embodiment, and the image distortion correction part 15 of 4th Embodiment. In the coordinate system in which the light beam cross-sectional shape of the diffracted light 902 after distortion correction is set in FIG. 17, the elliptical shape is C × L (where C <1) in the x-axis direction and L × sin θ in the y-axis direction. Thus, the beam reduction ratio of the reduction optical system 901 is set. As a result, the resolution of the captured image can be made substantially equal regardless of the focal plane of the imaging lens system 16.

(Sixth embodiment)
A sixth embodiment of the photoacoustic vibrometer according to the present invention will be described with reference to FIG. The photoacoustic vibrometer 106 of the sixth embodiment shown in FIG. 19 is different from the photoacoustic vibrometer 101 of the first embodiment in that it further includes an angle adjusting unit 1302 and an angle adjusting unit 1303. For this reason, in the description of the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions of the other components are omitted.

  As illustrated in FIG. 19, an optical system including the light source 11, the beam expander 12, the beam splitter 19, and the plane mirror 21 is referred to as a light beam generation optical system 1304. An optical system composed of the beam splitter 20, the plane mirror 22, the image distortion correction unit 15, the imaging lens 16, and the image receiving unit 17 is referred to as a diffracted light imaging optical system 1305. The optical axis 1301 is a straight line that passes through the center of the light beam of the second diffracted light 201b and is parallel to the traveling direction.

  The photoacoustic vibrometer 106 includes an angle adjustment unit 1302 that adjusts an angle formed by the optical axis 13 of the light beam generation optical system 1304 with respect to the sound axis 7, and an optical axis of the diffracted light imaging optical system 1305 with respect to the sound axis 7. And an angle adjusting unit 1303 that adjusts an angle formed by 1301. The angle adjusting unit 1302 and the angle adjusting unit 1303 are interlocked, and the angle is adjusted so that the angle formed by the sound axis 7 and the optical axis 13 is equal to the angle formed by the sound axis 7 and the optical axis 1301.

  As described in the first embodiment, the diffraction angle 90 of the diffracted light 201 with respect to the sound axis 7 is calculated from the frequency of the sine wave constituting the first and second acoustic waves 2 a and 2 b and the wavelength of the light beam of the light source 11. ° -θ is determined. The photoacoustic vibrometer 102 has the sound axes of the optical axes 13 and 1303 so that the angle adjustment unit 1302 and the angle adjustment unit 1303 match the diffraction angles even when the frequencies of the first and second acoustic waves 2a and 2b change. The angle with respect to 7 can be adjusted. Thereby, first, the object 4 can be imaged roughly with a low-frequency acoustic wave, and then the object 4 can be imaged with fine details using a high-frequency acoustic wave. Thereby, the imaging time can be shortened.

(Seventh embodiment)
The seventh embodiment of the photoacoustic vibrometer according to the present invention will be described with reference to FIG. The photoacoustic vibrometer 107 of the seventh embodiment shown in FIG. 20 is different from the photoacoustic vibrometer 101 of the first embodiment in that it further includes a translation unit 1703 and a translation unit 1704. For this reason, in the description of the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions of the other components are omitted.

  In order to measure an accurate displacement amount of the object 4 using the photoacoustic vibrometer of the present invention, it is preferable to generate and acquire an image 18 that correctly reflects the shape and displacement of the object 4. In the first to sixth embodiments, the first and second plane sound waves 9a and 9b are simultaneously and accurately positioned in the region irradiated with the first and second plane wave light beams 14a and 14b. The generation times of the first and second acoustic waves 2a and 2b were controlled.

  However, when the displacement or movement of the object 4 is periodic and an integral multiple of the period matches the transmission time interval of the first and second acoustic waves 2a, 2b, the first and second acoustic waves 2a The displacement or movement position of the object 4 when 2b reaches the object 4 is equal, and the generated image 18 does not have interference fringes indicating the displacement state. That is, the displacement distribution and the velocity distribution of the object 4 cannot be observed.

  Even in such a case, the photoacoustic vibrometer 107 of the present embodiment can observe the displacement amount distribution and the velocity distribution of the object 4. For this purpose, the photoacoustic vibrometer 107 includes a translation unit 1703 and a translation unit 1704. The parallel moving unit 1703 adjusts the irradiation position of the first plane wave light beam 14 a by moving the beam splitter 19 in parallel with the optical axis 1701 of the plane wave light beam 14. Further, the translation unit 1704 moves the plane mirror 22 in parallel with the optical axis 1702 of the first diffracted light 201a traveling from the plane mirror 22 toward the beam splitter 20, thereby shifting the generation position of the first diffracted light 201a. In addition, the first diffracted light 201 a can be correctly incident on the beam splitter 20.

  Thereby, the transmission interval τ is determined so that the transmission interval τ of the first and second acoustic waves 2a, 2b does not coincide with an integral multiple of the displacement or movement period of the object 4, and according to the determined τ. The incident position of the first plane wave light beam 14a on the photoacoustic medium unit 8 is changed. Thereby, the displacement amount distribution and the velocity distribution of the object 4 can be correctly observed.

(Eighth embodiment)
The eighth embodiment of the photoacoustic vibrometer according to the present invention will be described with reference to FIG. The photoacoustic vibrometer 108 of the eighth embodiment shown in FIG. 21 is the first in that one plane wave beam 14 is made incident on the photoacoustic medium unit 8 instead of the first and second plane wave beams 14a and 14b. Different from the photoacoustic vibrometer 101 of the embodiment. For this reason, in the description of the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and descriptions of the other components are omitted.

  In the first embodiment, the first and second plane wave light beams 14a and 14b are made incident on the photoacoustic medium unit 8. However, as described above, the two points P1 positioned at intervals corresponding to τ × ν, What is necessary is just the light source 11 which irradiates the 1st and 2nd plane sound waves 9a and 9b which reached | attained P2 simultaneously.

  For this purpose, the photoacoustic vibrometer 108 generates a plane wave light beam 14 having a thick beam diameter using the beam expander 12 having a large beam expansion ratio. The generated plane wave light beam 14 is reflected by, for example, the plane mirror 21, and the first and second plane sound waves 9a and 9b reaching the two points P1 and P2 are simultaneously irradiated. The plane wave light beam 14 may be directly incident on the photoacoustic medium unit 8 without using the plane mirror 21.

  In the photoacoustic medium unit 8, diffracted light is generated only in the region where the first and second plane sound waves 9a and 9b are present, so even if such a light beam is used, it is the same as in the first embodiment. The 1st and 2nd diffracted lights 201a and 201b can be generated. Therefore, similarly to the first embodiment, the displacement amount distribution and the velocity distribution of the object 4 can be correctly observed.

  According to this embodiment, since the beam splitter 19 is unnecessary, it is not necessary to adjust the position of the beam splitter 19. If the first and second plane sound waves 9a and 9b that have reached the two points P1 and P2 can be simultaneously irradiated, the first and second points can be obtained even if the positions of the two points P1 and P2 are not accurately known. Diffracted light 201a, 201b can be generated. Therefore, according to this embodiment, the configuration of the photoacoustic vibrometer 101 can be simplified, and the adjustment of the optical system can be simplified.

  The photoacoustic vibrometer of the present invention is useful as a probe for an ultrasonic diagnostic apparatus and the like because an image by an acoustic wave can be acquired as an optical image. Further, since the acoustic wave radiated from the vibrating object can be observed as an optical image, it can be applied to uses such as a nondestructive vibration measuring apparatus. Furthermore, it can also be used as a non-contact vibrometer that measures minute periodic movements in a non-contact manner or a vibration mode analysis device that measures the in-plane distribution of vibrations.

DESCRIPTION OF SYMBOLS 1 Ultrasonic wave source 2a 1st acoustic wave 2b 2nd acoustic wave 3 Medium 4 Object 5a 1st scattered wave 5b 2nd scattered wave 6 Acoustic lens system 7 Sound axis 13, 23, 706, 1301, 1701, 1702 Optical axis 8 Photoacoustic medium part 9a First plane sound wave 9b Second plane sound wave 10 Sound wave absorbing part 11 Light source 12 Beam expander 14a First plane wave light beam 14b Second plane wave light beam 204 Plane wave light beam 15 Image distortion correction Unit 16 Imaging lens system 17 Image receiving unit 18, 141, 142, 405, 408, 801 Image 19, 20 Beam splitter 21, 22 Planar mirror 101, 400 Photoacoustic vibration meter 143, 144 Light flux 145 Optical path length difference 146 Real image after superposition 147, 148 image points 151, 161, 162 Cylindrical lens 191 Taiko skin 192 Speaker 193 Monochromatic sound wave 194 Imaging direction 201a First diffracted light 201b Second diffracted light 202 Diffraction grating 203 Monochromatic light 301 Anamorphic prism 302, 902 Diffraction light after distortion compensation 303 Wedge prism 401, 407 Object 402 Fourier transform surface 403 , 404 Lens 406 Acoustic light conversion unit 701 Sub mirror 702 Primary mirror 703 Antireflection film 704 Focus 705 Acoustic waveguide 901 Reduction optical system 1302, 1303 Angle adjustment unit 1304 Beam generation optical system 1305 Diffracted light imaging optical system 1703, 1704 Parallel Moving part

Claims (21)

An acoustic wave source that transmits the first and second acoustic waves at predetermined time intervals;
An acoustic lens system for converting the first and second scattered waves generated by irradiating the object with the first and second acoustic waves into first and second plane acoustic waves in a predetermined convergence state, respectively;
A photoacoustic medium portion disposed so as to include a sound axis of an acoustic lens system, through which the first and second plane sound waves propagate;
On the sound axis in the photoacoustic medium part, two points located at an interval that coincides with the distance that the first plane sound wave propagates through the photoacoustic medium part at the predetermined time with respect to the sound axis. A light source that emits at least one plane wave beam that irradiates at a non-vertical and non-parallel angle;
The at least one plane wave light beam is diffracted by the first and second plane sound waves, thereby superimposing the first and second diffracted lights generated in the photoacoustic medium part, and generating a superimposed diffracted light. A light superimposing unit,
An imaging lens system for condensing the superimposed diffracted light; and
An image receiving unit that detects the collected diffracted light and outputs an electrical signal;
A photoacoustic vibrometer.   The photoacoustic vibrometer according to claim 1, further comprising an image distortion correction unit that corrects distortion of at least one of the image of the object represented by the superimposed diffracted light and the electrical signal.   The photoacoustic vibrometer according to claim 2, wherein the spectral width of each monochromatic light is less than 10 nm.   The photoacoustic vibrometer according to any one of claims 1 to 3, wherein the acoustic lens system is a refractive acoustic system.   The photoacoustic vibrometer according to claim 4, wherein the acoustic lens system is composed of a silica nanoporous material or fluorinate.   The photoacoustic vibrometer as defined in claim 5, wherein the acoustic lens system includes at least one refracting surface and an antireflection film for preventing reflection of an acoustic wave provided on the at least one refracting surface.   The photoacoustic vibrometer according to claim 1, wherein the acoustic lens system is a reflective acoustic system.   The photoacoustic vibrometer according to claim 6, wherein the acoustic lens system includes two or more reflecting surfaces.   The photoacoustic vibrometer according to claim 1, wherein the acoustic lens system includes a focus adjustment mechanism.   10. The light according to claim 1, wherein the light source emits first and second plane wave light beams that irradiate the two points at angles that are non-perpendicular and non-parallel to the sound axis, respectively. Acoustic vibration meter.   The photoacoustic vibrometer according to any one of claims 1 to 9, wherein the light source emits one plane wave light beam that irradiates the two points at an angle that is non-perpendicular and non-parallel to the sound axis.   The photoacoustic vibrometer according to claim 1, wherein the light superimposing unit includes a non-polarizing beam splitter.   The photoacoustic vibrometer according to claim 1, wherein the image distortion correction unit includes an optical member that enlarges a cross section of the superimposed diffracted light.   The photoacoustic vibrometer according to any one of claims 1 to 12, wherein the image distortion correction unit includes an optical member that reduces a cross section of the superimposed diffracted light.   The photoacoustic vibrometer according to claim 13 or 14, wherein the optical member is constituted by an anamorphic prism.   The photoacoustic vibrometer according to claim 13, wherein at least one of the imaging lens system and the optical member includes at least one cylindrical lens.   The photoacoustic vibrometer according to claim 1, wherein the image distortion correction unit performs image processing based on the electrical signal.   The photoacoustic vibrometer according to any one of claims 1 to 17, wherein the photoacoustic medium part includes at least one of a silica nanoporous material, fluorinate, and water.   The photoacoustic vibrometer according to any one of claims 1 to 18, wherein the first and second diffracted lights include a component of Bragg diffracted light having an intensity ratio of 1/2 or more.   19. The photoacoustic vibrometer according to claim 1, wherein an optical axis of the at least one plane wave light beam emitted from the light source is adjustable with respect to a sound axis of the acoustic lens system.   The photoacoustic vibrometer according to any one of claims 1 to 20, further comprising a trigger circuit that controls transmission times of the first and second acoustic waves.

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