JPH06273399A - Acoustic lens - Google Patents

Abstract

PURPOSE:To allow radiation of high strength ultrasonic wave without applying excessive voltage to a transducer and without limiting the working distance of lens. CONSTITUTION:The acoustic lens comprises a lens rod 11, a transducer 12 for transmitting/receiving ultrasonic wave disposed on the end face of the lens rod 11, and means 17, 18 exhibiting electric and magnetic or electric and optical functions for the lens rod 11 depending on the quality thereof and amplifying ultrasonic wave propagating through the lens rod.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acoustic lens used for focusing an ultrasonic wave in an ultrasonic microscope, an ultrasonic flaw detector or the like.

[0002]

2. Description of the Related Art An acoustic lens is a device that converges ultrasonic waves and irradiates an object to be measured and imaged, and is generally used in ultrasonic microscopes, ultrasonic flaw detectors, and the like. .

Generally, an acoustic lens is used for the purpose of converging an ultrasonic wave to the wavelength limit and improving the resolution of the above apparatus. As another application, in an acoustic microscope, an acoustic lens may be used to measure the so-called V (z) curve by providing a distribution of incident angles of incident ultrasonic waves. Also,
In a cryogenic acoustic microscope that uses a low temperature liquid such as liquid nitrogen or liquid helium as a medium, an acoustic lens is used in order to further increase the sound pressure near the focal point and generate the second harmonic to further improve the image resolution. There is.

This acoustic lens is basically constructed as shown in FIG. Reference numeral 1 is a lens rod, which is made of a material such as sapphire or quartz, which has good ultrasonic transparency. At one end of the lens rod 1, a transducer 2 having a conversion function between ultrasonic / electrical signals is formed. The transducer 2 is composed of a piezoelectric vibration plate 3 and electrodes 4a and 4b that are closely attached to the top and bottom of the piezoelectric vibration plate 3. On the other hand, a spherical shell 5 is formed at the other end of the lens rod 1. This spherical shell 5
Converts the plane wave ultrasonic wave transmitted from the transducer 2 into a convergent spherical wave by refraction here, and converts the spherical wave ultrasonic wave reflected from the sample object into a plane wave here. An antireflection film 6 is generally formed on the spherical shell portion 5 in order to improve the ultrasonic wave transmittance from the lens rod 1 to the ultrasonic wave propagating liquid.

[0005]

By the way, in the ultrasonic imaging apparatus using the acoustic lens described above, it is important to obtain a received signal of large intensity in order to improve the S / N ratio of the image. For this purpose, the voltage applied to the transducer 2 may be increased to increase the intensity of the transmitted ultrasonic waves, but this has a limit. This is because the transducer 2 is composed of the extremely thin piezoelectric plate 3, and therefore, when an excessive voltage is applied, a defect such as discharge or destruction occurs. In addition, when the transducer 2 is driven with a large voltage, the piezoelectric plate 3 vibrates in a non-linear manner, and energy escapes to higher harmonics.

[0006] As a solution to the above problem, the sound wave path in the ultrasonic wave propagating liquid is shortened to suppress the attenuation of the ultrasonic wave. For this purpose, it is necessary to reduce the radius of curvature of the spherical shell of the acoustic lens, but this has limitations in processing, and as the radius of curvature of the spherical shell is reduced, the working distance of the acoustic lens is reduced. There is a drawback that d becomes small.

The present invention has been made in view of the above circumstances, and an ultrasonic wave of high intensity can be emitted without applying an excessive voltage to the transducer, and an image obtained from the reflected wave It is an object of the present invention to provide an acoustic lens that can improve the S / N ratio of the above-mentioned item, can increase the lens working distance, and can perform ultrasonic observation inside a deeper sample.

[0008]

In order to achieve the above object, an acoustic lens of the present invention is one in which ultrasonic waves are converged and applied to an object, and the focused ultrasonic wave is emitted from the object while the ultrasonic waves are emitted from the object. A lens rod having a wave transmitting / receiving surface for receiving reflected ultrasonic waves, an ultrasonic wave transmitting / receiving transducer provided on the other end surface of the lens rod, and the lens rod is electrically or magnetically according to the material of the lens rod, Alternatively, it is configured by including an ultrasonic wave amplifying means for amplifying the ultrasonic wave propagating in the lens rod by exerting an electrical and optical effect.

[0009]

In the acoustic lens of the present invention, the ultrasonic wave generated by the ultrasonic wave transmitting / receiving transducer propagates in the lens rod and is emitted as an ultrasonic wave which is refracted and converged at the transmitting / receiving surface. In addition, the reflected ultrasonic waves from the object incident on the transmitting / receiving surface of the lens rod propagate again in the lens rod,
It is converted into an electrical reception signal by the ultrasonic transmission / reception transducer.

When an ultrasonic wave is propagating in the lens rod, the ultrasonic wave amplifying means causes an electric action, a magnetic action, or an electric action on the lens rod depending on the material of the lens rod. When the combined action of the action and the optical action is exerted, the action of amplifying the ultrasonic wave is generated, and the ultrasonic wave of high intensity is emitted from the transmitting / receiving surface.

[0011]

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

FIG. 1 is a block diagram of an acoustic lens according to the first embodiment of the present invention. The acoustic lens of this embodiment is a sound amplification type acoustic lens based on magnetoacoustic parametric interaction.

The acoustic lens of this embodiment is the lens rod 1.
A transducer 12 is formed on one end surface of the No. 1.
The transducer 12 is composed of a piezoelectric vibrating plate 13 and electrodes 14a and 14b that are closely attached to the upper and lower sides of the piezoelectric vibrating plate 13. At the other end of the lens rod 11, there is formed a spherical shell portion 15 which serves as a transmitting / receiving surface of ultrasonic waves. An antireflection film 16 is formed on the spherical shell portion 15 in order to improve the ultrasonic transmittance from the lens rod 11 to the ultrasonic wave propagating liquid. A coil 17 is wound around the outer peripheral surface of the lens rod 11, and an AC power supply 18 is connected between both ends of the coil 17 in order to pass a current having a frequency twice the ultrasonic frequency. The coil 17 and the AC power supply 18 constitute ultrasonic amplification means.

In this embodiment, a nonlinear magnetostrictive material is used as a material for forming the lens rod 11. Examples of the nonlinear magnetostrictive material include ferrite and hematite. This non-linear magnetostrictive material has a non-linear magnetostrictive characteristic with respect to the applied magnetic field. Therefore, if a magnetic field having a frequency double that of the ultrasonic wave propagating in the nonlinear magnetostrictive material is applied, the ultrasonic wave can be amplified by the parametric interaction described later.

Parametric interaction, which is a basic principle for sound wave amplification of this embodiment, will be described below. This parametric interaction is a phenomenon in which two sound waves having an angular frequency ω and a magnetic field having an angular frequency 2ω perform energy exchange in a nonlinear medium. Amplification of an incident wave and generation of a phase-common wave with the energy exchange. Happens. Now the incident wave,
When the phase conjugate wave and the amplitude of the applied magnetic field are Ui, Uc, and W, this interaction is described by the following equation.

[0016]

[Equation 1]

Here, v is the sound velocity of the nonlinear medium, ρ is the density of the nonlinear medium, and h is the nonlinear constant. If this equation is solved under the boundary condition that the phase conjugate wave is 0 at the output end of the interaction region, the solution for the incident wave is Ui (L) = Ui (O) / cos (ωhWL / 2ρv ³ ) (2) is obtained. Where L is the length of the interaction region and U
i (0) is the incident wave amplitude at the input end.

This shows that the incident wave is always amplified.
is doing. Especially ωhWL / 2ρv³ When = π / 2
, Ui (L) becomes infinite. In fact, in this condition
The amplification factor becomes a finite value due to the decrease of the applied magnetic field. So
Also, for example, using ferrite as a nonlinear magnetostrictive material,
Sound wave frequency of 10 MHz, applied magnetic field of 100 Oe
It is possible to obtain an amplification factor of several tens of dB. More than
Based on the principle, this embodiment operates as follows.

That is, an ultrasonic wave Ui (O) is generated from the transducer 12 and the ultrasonic wave Ui (O) is transmitted and received within the lens rod 11.
Propagate in the direction of 5. At that time, a magnetic field is generated in the lens rod 11 by passing a current from the AC power supply 18 to the coil 17. The ultrasonic wave propagating in the lens rod 11 is amplified by the magnetic field applied from the coil 17. The amplified ultrasonic wave has an amplitude Ui (L) represented by the equation (2) on the transmitting / receiving surface 15 of the lens rod 11.
Becomes The power amplification factor R is R = {cos (ωhWL / 2ρv ³ ) ^-2 (3).

The ultrasonic wave amplified by the power amplification factor R is emitted from the wave transmitting / receiving surface 15 of the lens rod 11 and converges on an object (not shown). The reflected wave from the converging point is incident on the wave transmitting / receiving surface 15 again, propagates in the lens rod 11, and is converted into an electric reception signal by the transducer 12.

As described above, according to the present embodiment, the non-linear magnetostrictive material is used for the lens rod 11 of the acoustic lens, and the ultrasonic wave is amplified by the parametric interaction between the sound wave and the magnetic field in the lens rod 11. As a result, the signal strength is increased, so that a high-intensity ultrasonic wave can be emitted without applying an excessive voltage to the transducer 12 or reducing the radius of curvature of the spherical shell portion 15, and a high intensity ultrasonic wave can be emitted. The received signal can be obtained.

Therefore, an image with a high S / N ratio can be obtained by using the received signal. In particular, increase the strength of the magnetic field to increase πρv ³ The amplification factor becomes very large as it approaches / ωhL.

Further, since the frequency of the current for generating the applied magnetic field for amplification is double the ultrasonic frequency, there is an advantage that it does not interfere with the signal and can be easily cut by the frequency filter.

The magnetoacoustic parametric interaction is
Since ultrasonic waves of frequency ω interact with each other in a magnetic field or electric field of frequency 2ω in a very narrow band, there is also an advantage that noise mixing is extremely unlikely to occur. Next, an acoustic lens according to Example 2 of the present invention will be described.

FIG. 2 shows a block diagram of an acoustic lens according to the second embodiment. The same parts as those in the first embodiment described above are designated by the same reference numerals. This example is an example of a sound amplification type acoustic lens using electroacoustic parametric interaction.

The acoustic lens of this embodiment is the lens rod 1.
9 is made of a non-linear piezoelectric material. Examples of nonlinear piezoelectric materials include LiNbO ₃ and PZT ceramics. A pair of electrodes 21a and 21b are formed on both side surfaces of the lens rod 19. Then, one electrode 21a is grounded, and the other electrode 21b is connected to the AC power supply 22, so that the electrodes 21a, 2
An AC voltage having a frequency twice that of the ultrasonic wave can be applied between 1b. The electrodes 21a and 21b and the AC power source 22 constitute an ultrasonic wave amplifying means.

In this embodiment thus constructed, when a voltage is applied from the AC power source 22 to the electrodes 21a, 21b, an AC electric field is generated in the lens rod 19, and the region between the electrodes 21a, 21b of the lens rod 19 is generated. Becomes an interaction region, parametric interaction occurs, and ultrasonic waves are amplified in the lens rod. The principle of ultrasonic amplification is expressed by the formula (1
If the magnetic field is replaced by an electric field in a) to (3), the explanation is exactly the same. According to the present embodiment as described above, it is possible to emit a high-intensity ultrasonic wave, and it is possible to obtain the same effect as that of the first embodiment. FIG. 3 shows a modification of the second embodiment.

Although this modification operates by electroacoustic parametric interaction, the lens rod of the acoustic lens has a constricted shape so that a larger amplification factor can be obtained.

Lens rod 1 made of non-linear piezoelectric material
9'has a diameter that continuously decreases from both ends toward the center, and its upper end surface is spherical. A transducer 12 'is formed on the spherical upper end surface of the lens rod 19'. The transducer 12 'radiates a spherical wave into the lens rod, and the lens rod 11 is shaped so that the diameter is minimized near the beam waist. Electrodes 21a ', 21 near the beam waist
Form b'and apply an electric field.

When the lens rod 11 having such a shape is used, the distance between the electrodes can be greatly reduced as compared with the shape shown in FIG. When a voltage having the same amplitude is applied, the electric field between the electrodes is inversely proportional to the distance between the electrodes, and therefore the amplitude of the applied electric field becomes very large in the configuration of this modification, and a larger amplification factor can be obtained. Next, an acoustic lens according to Example 3 of the present invention will be described.

FIG. 4 shows the configuration of an acoustic lens according to the third example. The present embodiment is an example of a sound wave amplification type acoustic lens utilizing an ultrasonic wave amplification effect by an electrostatic field in a piezoelectric semiconductor.

The acoustic lens of this embodiment is the lens rod 2
A predetermined piezoelectric semiconductor is used as the material of 3. An electrode 24 for applying an electrostatic field is formed on the lower end surface of the lens rod 23 over the entire area of the wave transmitting / receiving surface 25 and its periphery. An insulating coat layer 26 is formed on the surface of the electrode 24 to prevent the electrode 24 from directly contacting the ultrasonic wave propagating liquid. A DC power supply 27 is connected to the electrode 24.

Since the thickness of the electrode 24 and the insulating coat layer 26 formed by the usual vapor deposition method is much smaller than the wavelength of ultrasonic waves, their influence on the propagation of ultrasonic waves is negligible. .

A transducer 29 is formed on the other end surface of the lens rod 23. This transducer 29 includes a piezoelectric vibrating plate 31 and electrodes 3 that are in close contact with the piezoelectric vibrating plate 31.
It consists of 2a and 32b. The electrode 32b arranged on the lens rod side is grounded and also functions as an earth electrode for applying an electrostatic field.

Further, this embodiment is provided with a light source 33 such as a laser, and the light is generated by the light source 33 to generate an appropriate optical system 3.
The light is emitted to the side surface of the lens rod 23 via the beam line 4. The light source 33, the optical system 34, the electrode 24, and the electrode 32b constitute ultrasonic amplification means. The sound wave amplifying effect in this embodiment is completely different from the above-mentioned parametric interaction, and utilizes a phenomenon caused by interaction between conduction electrons in a semiconductor and ultrasonic waves.

Piezoelectric semiconductors used for the material of the lens rod 23 include CdS, CdTe, ZnS and ZnS.
Many substances such as e and ZnTe are targeted. When these materials are irradiated with light, conduction electrons are generated. Further, since these substances have piezoelectricity, when an ultrasonic wave is propagated, an associated electric field in phase with the ultrasonic wave is generated. The conduction electrons generated by light irradiation are rearranged according to the electric field associated with the ultrasonic wave. At this time, if a DC electric field is externally applied to force the electrons to drift, the ultrasonic wave is attenuated or amplified. Acoustic wave amplification occurs when electrons are drifted in the same direction as ultrasonic waves at a speed of sound or higher, and the amplification factor R is given by the following equation. R = (k ² / 2) {ωc / (vs-vd)} [1+ {ωc ² / ((1-vd / vs) ² ω ² )} × {1+ {ω ² / (Ωcωd)} ^{2 -1} (4) where k is the electromechanical coupling constant of the piezoelectric semiconductor, v is the speed of sound,
vd is the electron drift velocity, ωc is the dielectric relaxation frequency, and ωd
Is the cutoff frequency of the device.

It has been confirmed by experiments that ultrasonic waves of several MHz to several tens GHz are amplified by several tens to several hundreds dB / mm by applying an applied electric field of several hundreds V / cm by using the above-mentioned piezoelectric semiconductor.

In the present embodiment constructed as described above, when the lens rod 23 is irradiated with light through the optical system 34, conduction electrons are generated in the lens rod 23, and the conduction electrons are generated between the electrodes 32a and 24. It amplifies the ultrasonic waves that are drifted by the electrostatic field and propagate in the same part. The amplified ultrasonic wave is emitted from the wave transmitting / receiving surface 25 as a convergent ultrasonic wave.

As described above, according to the present embodiment, a high-intensity ultrasonic wave is radiated from the acoustic lens without applying an excessive voltage to the transducer 29 and shortening the working distance of the acoustic lens. it can. Further, since the amplifying action of the ultrasonic waves is performed by the electrostatic field, the S / N ratio of the received signal that is obtained can be further improved without interference with the signal or noise-like mixing. Next, an example in which the acoustic lens of the first embodiment described above is applied to a reflective ultrasonic microscope will be described as a fourth embodiment.

FIG. 4 shows a block diagram of a reflection type ultrasonic microscope according to the fourth embodiment. In this embodiment, the acoustic lens body is composed of the lens rod 11, the transducer 12, the coil 17 and the like in the same manner as in FIG.

The oscillator 31 generates a high-frequency transmission signal having the angular frequency ω, and the transmission signal is divided into two directions by the demultiplexer 32. The gate circuit 6 sends one of the transmitted signals.
It is converted into an rf signal shaped into a burst wave, and the rf signal is passed through a circulator 34 to the transducer 12
Is being applied to. The transducer 12 converts the rf signal into ultrasonic waves.

Further, the other transmission signal outputted from the oscillator 31 and divided by the demultiplexer 32 is converted into a doubled frequency 2ω by the frequency multiplier 35, and then inputted to the gate circuit 36. The gate circuit 36 gates the input signal of frequency 2ω.
Shaped into a burst wave. The waveform-shaped signal is amplified by the amplifier 37 and then applied to the coil 17.

Ultrasonic waves passing through the lens rod 11,
The magnetic field of angular frequency 2ω generated by the coil 17 undergoes parametric interaction as described above, and as a result, is amplified. Due to the sound wave amplification action, a high-intensity ultrasonic wave is applied to the sample S as an object arranged near the converging point of the acoustic lens.

The reflected ultrasonic wave from the sample S is incident on the wave transmitting / receiving surface of the acoustic lens, and is transmitted by the transducer 12 to rf.
Converted to a signal. The rf signal is subjected to the same signal processing as that of a normal ultrasonic microscope by the receiver 38 and the CRT 39.
Is displayed as an image on.

The control circuit 42 controls on / off of the transmission gate circuit 33 and the magnetic field application gate circuit 36 in conjunction with the operation of the acoustic lens scanner 42. The timing is i) when the transmitted pulse passes through the lens rod 11, ii) when the received pulse passes through the lens rod 11, and iii) in both i) and ii).
The control circuits 33 and 36 are controlled to be on.

In this embodiment, ultrasonic waves are amplified in the lens rod 11 by transmission, reception, or both.
Therefore, the S / N ratio of the signal is improved, and higher image quality can be obtained as compared with the conventional device.

Further, since the radius of curvature of the wave transmitting / receiving surface 1 of the acoustic lens is not reduced in order to increase the intensity of the ultrasonic wave,
The working distance of the lens can be increased, and the deeper inside of the sample can be observed.

If a low temperature liquid is used as an ultrasonic wave propagation medium (not shown in the figure) filled between the acoustic lens and the sample S, the sound pressure near the ultrasonic focus can be made extremely large, so that it is easy to Second harmonics are generated and the resolution can be improved.

In this embodiment, the magneto-acoustic parametric interaction is used as the sound wave amplifying effect. However, by using the acoustic lenses of the second and third embodiments described above, the electro-acoustic parametric interaction or the electrostatic field in the piezoelectric semiconductor is used. An ultrasonic microscope can be constructed in exactly the same manner by using the ultrasonic amplification effect. Next, an example of measuring the V (z) curve by using the acoustic lens of the first embodiment described above in a reflective ultrasonic microscope will be described as a fifth embodiment.

FIG. 6 is a block diagram of the reflection type ultrasonic microscope according to the fifth embodiment. The components such as the acoustic lens, the oscillator 31, the demultiplexer 32, the gate circuits 33 and 36, the circulator 34, the frequency multiplier 36, the amplifier 37, and the control circuit 42 are the same as those in the fourth embodiment.

In this embodiment, the distance between the acoustic lens and the sample is continuously changed by the z scanner 42, and the acoustic lens output output from the receiver 38 at that time is measured.
Then, the measurement signal is frequency-decomposed by the Fourier transformer 43, and the surface acoustic wave velocity of the sample is calculated.

Even if the ultrasonic wave is amplified in the lens rod 11 of the acoustic lens, the V (z) curve does not essentially change, and its S /
The N ratio is improved. Further, if the working distance of the acoustic lens is lengthened, the measurement range of z in V (z) measurement becomes wider, and the accuracy is further improved. Furthermore, it is possible to further improve the accuracy of Fourier transform by changing the sound wave amplification factor in the acoustic lens according to the lens-sample distance z.

For this purpose, the amplification factor of the amplifier 37 is fed back according to the displacement of the z scanner 42.
The amplification factor is set so that the peak value of the V (z) curve is almost constant at any z position.

7 (a) and 7 (b), a general V (z) curve obtained by the conventional method and V obtained by this embodiment are shown.
(z) Shown in comparison with the curve. V obtained by conventional method
The (z) curve is shown in (a), and the V (z) curve obtained in this example is shown in (b).

The V (z) curve obtained by the conventional method attenuates as the absolute value of z increases, but the V (z) curve obtained in this embodiment by the above-described operation has a substantially constant amplitude. Only the frequency due to the surface acoustic wave appears. Therefore, if the V (z) curve shown in FIG. 9B is subjected to the Fourier transform, the surface acoustic wave velocity can be determined with higher accuracy than that obtained by performing the Fourier transform of the conventional attenuation waveform curve. In addition, since the ultrasonic amplification methods in the lens rods of the respective embodiments described above have low noise, the S / N ratio higher than that of amplifying the electrical transmission output is obtained.
It can be measured by the ratio.

[0056]

As described above in detail, according to the present invention, a high-intensity ultrasonic wave can be emitted without applying an excessive voltage to the transducer, and the S of the image obtained from the reflected wave can be emitted. It is possible to provide an acoustic lens that can improve the / N ratio, can increase the lens working distance, and can perform ultrasonic observation inside a deeper sample.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an acoustic lens according to Example 1 of the present invention.

FIG. 2 is a configuration diagram of an acoustic lens according to Example 2 of the present invention.

FIG. 3 is a configuration diagram of a modified example of the acoustic lens shown in FIG.

FIG. 4 is a configuration diagram of an acoustic lens according to Example 3 of the present invention.

FIG. 5 is a configuration diagram of a reflective ultrasonic microscope according to a fourth embodiment of the present invention.

FIG. 6 is a configuration diagram of a reflective ultrasonic microscope according to a fifth embodiment of the present invention.

7 is a diagram showing a V (z) curve obtained by a conventional method and a V (z) curve obtained by the reflection acoustic microscope shown in FIG.

FIG. 8 is a configuration diagram of a conventional acoustic lens 8.

[Explanation of symbols]

11, 19, 23 ... Lens rod, 12 ... Transducer, 15 ... Wave transmitting / receiving surface, 17 ... Coil, 18 ... AC power supply, 21a, 21b, 24 ... Electrode, 33 ... Light source, 34 ...
Optical system.

Claims (1)

[Claims] 1. An acoustic lens for converging ultrasonic waves to irradiate an object, the lens rod having one end surface for transmitting and receiving the converged ultrasonic waves and for receiving reflected ultrasonic waves from the object. An ultrasonic wave transmitting / receiving transducer provided on the other end surface of the lens rod, and an electric or magnetic or electrical and optical action depending on the material of the lens rod, and propagating in the lens rod. And an ultrasonic wave amplifying means for amplifying the ultrasonic wave.