JP5716250B2 - Resonant photoacoustic imager - Google Patents

Description

The present invention relates to a photoacoustic imaging apparatus applied to nondestructive inspection and spectroscopic imaging of a sample.

The photoacoustic method that irradiates a substance with light and captures the energy converted into heat as a sound wave in the air or a thermoelastic wave in a solid is a non-destructive inspection inside the solid sample and the spectroscopic imaging of the sample. Have been applied to photoacoustic microscopes or photoacoustic imaging methods.

  In an apparatus for photoacoustic imaging, gas = microphone method in which a sample is sealed in a closed cell
Or a method of attaching a piezoelectric element to a sample The two were mainstream. In the former, the sample must be enclosed in a photoacoustic cell, and in the latter, the spectral linearity of the photoacoustic signal becomes a problem. .

  In addition, Helmholtz resonators
And spheroid acoustic resonator Although a photoacoustic sensor using a sensor has been realized, it has been used only as a sensor for spectroscopy closely attached to a sample, and has not been used as an imaging sensor for scanning in a state of being separated from the sample. .

R. L. Thomas, J. J. Pouch, Y. H. Wong, L. D. Favro, P. K. Kuo, and A. Rosencwaig: J. Appl. Phys., 51, 1152 (1980). A. Rosencwaig and G. Busse: Appl. Phys. Lett., 36, 725 (1980). W. Jackson and N. M. Amer: J. Appl. Phys., 51, 3343 (1980). A. Miklos, P. Hess and Z. Bozoki: Rev. Sci. Instr., 72, 1937 (2001). Wada Mori Nao and Matsuda Junichi: IEICE Technical Report MBE2003-61 (2003-09) Masamaki Nishimaki: Electroacoustic Vibration Studies (Corona, 1960). C. T. M. Chang: J. Acoust. Soc. Amer., 49, 611 (1971)

  The present invention is an acoustically open cavity for specimens of micro to large size that is difficult to achieve with piezoelectric elements, without enclosing the object to be measured in a photoacoustic cell. Since a resonator is used in a non-contact manner and does not require an optical window or an optical fiber for introducing light, a photoacoustic imaging means capable of measuring in a very wide wavelength range is realized.

The inventor has proposed a photoacoustic cell incorporating a spheroid acoustic resonator in photoacoustic measurement using a spheroid acoustic resonator that has been used only in a structure in which a sample and a photoacoustic cell are in close contact with each other. As an example of measurement when a structure separated from the sample is taken, the decrease in the photoacoustic signal based on acoustic resonance is as small as about 40% even when the distance between the resonator and the sample is about 1 mm. In addition, it was newly discovered that a shift in resonance frequency occurs on the high frequency side.

Grounded in this fact, the spheroidal type acoustic resonator, dare such that the sample and the photoacoustic cell is between away, by taking a structure in which a leak is acoustically, the two approaches so far Three advantages that cannot be obtained: 1) Realization of free non-contact / non-destructive inspection from micro-size to large-sized structures by encapsulating in photoacoustic cells and attaching piezoelectric elements 2) No need for windows To eliminate the restriction of the measurement wavelength range for 3), and guarantee the spectroscopic linearity for using the gas = microphone method.

  This invention utilizes an acoustic resonator that does not take a sealed structure, and does not enclose a measurement object in a photoacoustic cell, and non-contact measurement of a sample from a small size to a large size is performed in an acoustically open measurement system. There is an effect of realizing possible photoacoustic imaging means.

  Further, since a sealed structure using a window or an optical fiber for introducing light serving as a light source is not adopted, there is an effect of enabling spectral imaging using a light source in a very wide wavelength range.

Further, after the laser light for irradiating the sample is modulated with a pulse or a sine wave having the same period as the acoustic resonance frequency of the detector, the envelope is modulated with 1) a sine wave with another period or 2 ) By modulating with random pulses, in the former case, information on the internal structure of the sample can be obtained from the amplitude and phase of the modulated temperature signal generated from inside the sample, and in the latter case from the amplitude and delay time. It becomes possible.

FIG. 1 explains the claims 1 and 2 of the present invention and shows a basic configuration. FIG. 2 explains the first aspect of the present invention, and shows a specific configuration of the photoacoustic imaging apparatus described in the claims. FIG. 3 explains claim 3 of the present invention, and is an explanation of experimental data. The degree of sealing is "0mm closed" (when sealed), "0mm closed tension" (when the resonator is in close contact with the sample and a wire is inserted between them), "1mm open lap" (the distance between the resonator and the sample is 1mm) (When opened with a lap and closed with a lap) and “1 mm open” (when the distance between the resonator and the sample is 1 mm open), the resonance frequency also shifts in the order of increasing to the high frequency side in this order. FIG. 4 explains claim 3 of the present invention and represents an equivalent circuit representation of the acoustic system [Reference 6]. Voltage represents pressure, current represents volume velocity, and acoustic impedance represents pressure / volume velocity ratio. The RLC parallel resonance circuit indicated as Ycavity in the figure represents a spheroid resonator, and the series circuit indicated as Yhole on the left side shows the radiant impedance associated with the outflow of gas converted to admittance. FIG. 5 shows an aluminum metal plate used for the sample coated with four colors of paint. The colors are blue, red from the top, left to right, red, black, green from the bottom left to right. FIG. 6 is an amplitude image obtained by photoacoustic imaging of the sample shown in FIG. The scanning range is 23 mm × 23 mm.

  Hereinafter, the principle and embodiments of the present invention will be described with reference to the drawings in the case where the resonant photoacoustic image apparatus according to the present invention is applied.

  FIG. 1 shows an embodiment in which a resonant photoacoustic image apparatus according to the present invention is applied to photoacoustic spectroscopic imaging. First, as shown in the figure, the cavity acoustic resonator (1) in which the inside of the rectangular parallelepiped is hollowed out into a spheroid is passed through the entrance opening (5), and the intensity-modulated laser beam (4) Including the case of wavelength conversion by an oscillator) is incident so as to irradiate the sample (3). The photoacoustic signal generated by overheating of the sample reaches the high sensitivity microphone (2) through the acoustic resonator if the distance between the resonator and the sample is a short distance.

  The embodiment of FIG. 2 is an application example when this apparatus is used as a photoacoustic imaging apparatus. After the light of the laser (7) modulated by the oscillator (6) is reflected by the mirror (8), the sample (3) is irradiated through the light introduction opening (5) opened in the spheroid acoustic resonator (1). To do. The sound wave generated by the photoacoustic effect passes through the acoustic resonator and is detected by the high sensitivity microphone (2). The electric signal is synchronously detected by the lock-in amplifier (10) using the modulation signal. The amplitude and phase are transferred to the personal computer (13) via the signal transfer bus line (11). The sample is scanned two-dimensionally by the scanning stage (9), and the control is performed by the stage controller (12) and a personal computer.

Photoacoustic signals generated from each point of scanning are displayed as an amplitude image and a phase image. When a window is attached to the opening and the solid sample to be imaged is brought into close contact with the acoustic resonator, the resonance frequency is the resonance frequency of the spheroid.
It becomes.

However, when the sample is not brought into close contact with the acoustic resonator, the resonance frequency shifts in accordance with the acoustic admittance (volume velocity / pressure ratio) at which the generated sound wave flows out. An actual example of the shift amount of the resonance frequency is shown in FIG. When the length of half of the major axis of the ellipse is 85.5 mm and the ratio of the length of the major axis to the minor axis is 9.0, the resonant frequency of the photoacoustic signal in a completely sealed state is 1360 Hz, the signal voltage However, when the resonator and the sample were separated from each other by 1 mm, 1490 Hz and 83 μV were obtained, resulting in a frequency shift to about 10% on the high frequency side and a signal decrease of about 40%. For this reason, this type of resonance photoacoustic measurement has shown that it is sufficiently possible to image the sample and the resonator apart.

If the resonance of the spheroid is expressed using an acoustic circuit, it can be expressed by the RLC parallel resonance circuit shown in FIG. In the sealed state, only this circuit is connected to the sound source generated by the photoacoustic effect, but if there is leakage in an acoustic open system, the acoustic radiation admittance shown in the figure is added, so this reactance Min (equivalent to inductance) is added to the parallel circuit. Therefore, the resonance frequency shift shifts to the high frequency side, and the amount can be calculated and designed.

4 16 in the a reactance component generated by the presence of holes, which is a cause of a shift of the resonant frequency for the resonant frequency in the absence of opening or hole. When there is no hole, the resonance frequency of the cavity resonator is represented by the following formula (1). However, when the hole is present, the resonance frequency is a high frequency as shown by the following formula (2). Shift to the side.

It has been experimentally obtained that the frequency shift amount is not proportional to the distance between the resonator and the sample but tends to be saturated.

5 and 6 are specific examples of Embodiment 2 of photoacoustic imaging. The photoacoustic signal image obtained when the second harmonic (wavelength of 532 nm) of the YAG laser is used as the light source for the sample of FIG. 5 is displayed in gray scale (white is strong and black is weak) in FIG. Show. At this wavelength, the light absorption of the sample is strong in the order of black> red> blue> green, so that the photoacoustic signal becomes an image in this order. It shows that photoacoustic imaging can be performed even when the sample is not in close contact with the acoustic cell.

  As an industrial application of the present invention, first, spectroscopic detection of a structure inside an invisible sample can be mentioned. In other words, when there is a material that absorbs visible light but transmits infrared light in the surface layer of the sample, and there are structures and components that absorb infrared light that has passed through the surface layer, it is based on internal light absorption. Since a photoacoustic signal is generated, this apparatus functions as a microspectroscopic imaging apparatus capable of measuring the internal spatial distribution while performing spectral analysis over a very wide wavelength range.

The following applications of the present device is expected, the size is combined with a high sensitivity microphone and the semiconductor laser or a high intensity light emitting diode photoacoustic cell Le and small with a built-in acoustic resonators of approximately pen, low in size and weight An inexpensive photoacoustic nondestructive inspection device can be created.

1 spheroid acoustic resonator 2 acoustic sensor (high sensitivity microphone)
3 Measurement Sample 4 Laser Beam 5 Light Introducing Opening 6 Oscillator 7 Laser 8 Mirror 9 Scanning Stage 10 Lock-in Amplifier 11 Signal Transfer Bus Line 12 Stage Controller 13 Personal Computer 14 Volume Velocity Source Meaning Pressure Source Generated by Photoacoustic Effect 15 Real part of radiation impedance (R _H )
16 Reactance part of radiation impedance (L _H )
17 Acoustic resistance of spheroid acoustic resonator (R _A )
18 Inertance of spheroid acoustic resonator (L _A )
19 Acoustic capacity of spheroid acoustic resonator (C _A )
20 Pressure incident on the microphone

Claims (4)

In photoacoustic cell which incorporates an acoustic resonator spheroidal body type, by irradiating laser to the sample and through the opening provided in the acoustic resonator, detected by the acoustic sensor sound waves generated in the sample is passed through an acoustic resonator None as repeats this procedure by scanning the acoustic resonator, a video apparatus for performing acoustic Imaging, by moving and scanning the edge of the cell to the spaced structure about the sample and 1mm , video equipment, characterized in that to enable photoacoustic imaging of the sample. The apparatus according to item 1, wherein wide-band spectroscopic imaging is possible by removing a window for introducing a light beam into a spheroid-type acoustic resonator and removing a restriction on a wavelength of a laser beam to be used. A video device characterized by the above. An apparatus as described in paragraphs 1 and 2, after modulating the laser beam illuminating the sample with a pulse or a sinusoidal wave having the same period as the co-ringing frequency of the acoustic resonator, the envelope 1) Further, by modulating with a sine wave of another period, or 2) modulating with a random pulse, in the former case, the amplitude and phase of the modulated temperature signal generated from inside the sample, and in the latter case, the amplitude A video apparatus characterized in that information on the internal structure of the sample is obtained from the delay time. An apparatus as described in Section 3 of paragraph 1, using a high sensitivity acoustic sensor and the semiconductor laser or high intensity light emitting diodes, compact size the size of the resonator can scan in hand of around pen device.