JPH051988A - Photo-acoustic signal detection method and device therefor - Google Patents

Abstract

PURPOSE:To enable the internal and external information of a sample to be stably detected and quantitatively analyzed by correcting effectively signal detection sensitivity according to each modulated frequency in such a way as keeping the sensitivity constant for the frequency. CONSTITUTION:There is applied a method for changing, the intensity of primary diffracted light 85 or excited light from a photoacoustic modulation element 83 via the change of amplitude of a frequency outputted from an oscillator 69. In detecting a photoacoustic signal by changing a modulated frequency variously, the amplitude of rectangular waveform of a frequency outputted from the oscillator 69, is changed in proportion to 3/2 power of the frequency under control by a computer 74, thereby changing the amplitude of a modulated signal further. Consequently, the intensity of excited light 85 and 92 can be changed in proportion to 3/2 power of a frequency at every modulation. In addition, the intensity of a photo-acoustic signal finally becomes constant in each modulated frequency, and modulated frequency characteristics for a detection signal can be corrected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoacoustic signal detecting method and apparatus for detecting surface and internal information of a sample by utilizing photoacoustic effect.

[0002]

Photoacoustic effects were discovered in 1881 by Tyndall, Bell, Roentgen and others. That is, as shown in FIG. 18, when the intensity-modulated light (intermittent light) 19 is used as excitation light and is focused on the sample 7 by the lens 5 and is irradiated,
This is a phenomenon in which heat is generated in the light absorption region V _op 21 and is periodically diffused in the heat diffusion region V _th 23 given by the heat diffusion length μ _s 22, and an elastic wave (ultrasonic wave) is generated by this thermal strain wave. is there. This ultrasonic wave, that is, a photoacoustic signal is detected by using a microphone (acoustoelectric converter), a piezoelectric element, or an optical interferometer, and a signal component synchronized with the modulation frequency of the excitation light is obtained to obtain the surface and the inside of the sample. Information can be obtained. Regarding the detection method of the photoacoustic signal, for example, the document “Non-destructive inspection; Vol. 36, No. 10, p. 730 to p. 7” is used.
36 (October 1987) "and" I-E-E-1986 Ultra Sonics Symposium; p.
515-526 (1986) (IEEE 1986 UL
TRA-SONICS SYMPOSIUM; p. 51
5-526 (1986) ".

An example thereof will be described with reference to FIG. The collimated light emitted from the laser 1 is intensity-modulated by an acousto-optic modulator (AO modulator) 2, and the intermittent light, that is, excitation light, is collimated by a beam expander 3 into a collimated light 1 having a desired beam diameter.
After setting to 9, the light is reflected by the half mirror 4 and is focused on the surface of the sample 7 on the XY stage 6 by the lens 5. An ultrasonic wave is generated by the thermal strain wave generated from the light condensing part 21 on the sample 7, and at the same time, a minute displacement is generated on the surface of the sample 7. This minute displacement is detected by the Michelson interferometer described below. The parallel light emitted from the laser 8 is expanded to a desired beam diameter by the beam expander 9, and then split into two optical paths by the half mirror 10, and one of them is used as the probe light 24 by the lens 5
The light is focused on the light collecting portion 21 on the sample 7 by. The other side irradiates the reference mirror 11. The reflected light from the sample 7 and the reflected light from the reference mirror 11 interfere with each other on the half mirror 10, and the interference light is condensed by a lens 12 on a photoelectric conversion element 13 such as a photodiode. The photoelectrically converted interference intensity signal is amplified by the preamplifier 14 and then sent to the lock-in amplifier 16. The lock-in amplifier 16 extracts only the modulation frequency component contained in the interference intensity signal, using the modulation frequency signal from the oscillator 15 used to drive the acousto-optic modulator 2 as a reference signal. This frequency component has information on the surface or inside of the sample 7 according to the frequency. By changing the modulation frequency, the thermal diffusion length μ _s
21 can be varied to obtain information on different depths of the sample. If there is a crack or other defect in the thermal diffusion region V _th 23, a signal change appears in the modulation frequency component in the interference intensity signal, so that its presence can be known. X
The Y stage movement signal and the output signal from the lock-in amplifier 16 are processed by the computer 17, and the photoacoustic signal at each point on the sample is output to the display 18 such as a monitor television as two-dimensional image information.

[0004]

The above-mentioned prior art is an extremely effective means for detecting a photoacoustic signal in a non-contact and non-destructive manner, but has the following problems.

That is, generally, the signal intensity of the photoacoustic signal has a property of being inversely proportional to the intensity modulation frequency of the excitation light, and the detection sensitivity of the Michelson interferometer shown in FIG. It has a property of being inversely proportional to √f, where f is the intensity modulation frequency of the excitation light. Further, even when the PZT element is used for detecting the photoacoustic signal, the detection sensitivity changes variously depending on the frequency characteristic of the PZT element. Therefore, in the conventional photoacoustic signal detection device, when the intensity modulation frequency of the excitation light is variously changed and defects of various depths inside the sample are detected, the size of the defect is determined from the detected photoacoustic signal. Even if an attempt is made, since the signal intensity of the photoacoustic signal changes variously according to the modulation frequency as described above, it is difficult to quantitatively grasp the size of the defect.

An object of the present invention is to effectively correct the signal intensity of a photoacoustic signal according to each modulation frequency so that the detection sensitivity is always constant irrespective of the intensity modulation frequency of the excitation light, and the surface and inside of the sample It is an object of the present invention to provide a photoacoustic signal detection method and apparatus capable of stable detection and quantitative analysis of information.

[0007]

In order to achieve the above object, the present invention collects intensity-modulated light on a sample to generate a photoacoustic effect or a photothermal effect on the surface of or inside the sample. Detects the thermal strain of the sample surface caused by the acoustic effect or the photothermal effect, detects the frequency component of the intensity modulation frequency from the detected detection signal, and the sample surface and internal information according to the modulation frequency from the frequency component In the photoacoustic signal detection device for extracting, so that the detection sensitivity of the detection signal is constant for each modulation frequency, by effectively correcting the detection sensitivity according to each modulation frequency, the surface of the sample and It enabled stable detection of internal information and quantitative analysis.

To achieve the above object, the present invention adjusts the intensity of the intensity-modulated light according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency. , Reduced the effects of non-optical noise.

In order to achieve the above object, the present invention adjusts the intensity of the detection signal according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency. The stability of the system was increased.

In order to achieve the above object, the present invention corrects the detection sensitivity including the modulation frequency characteristic of the thermal strain detecting means for detecting the thermal strain so as to be constant for each modulation frequency. By doing so, the quantitativeness of the detection signal was further improved.

Further, in order to achieve the above object, the present invention makes it possible to detect a photoacoustic signal in a non-contact manner by detecting the thermal strain of the sample surface using optical interference.

Further, in order to achieve the above object, according to the present invention, the thermal strain on the sample surface is detected by using a piezoelectric element, so that the structure of the detection system is simplified and the stability of signal detection is improved. .

Further, in order to achieve the above object, the present invention contemplates a confocal optical system as an excitation means for condensing intensity-modulated light on a sample and generating a photoacoustic effect or a photothermal effect on the surface or inside the sample. With this configuration, the lateral resolution of the photoacoustic signal, the detection sensitivity, and the signal-to-noise ratio are improved.

Further, in order to achieve the above object, the present invention comprises an optical interference detecting means for detecting a thermal strain of a sample surface caused by a photoacoustic effect or a photothermal effect as a confocal optical system. The lateral resolution, detection sensitivity, and signal-to-noise ratio of the photoacoustic signal were improved.

[0015]

In the photoacoustic signal detection device, the detection sensitivity correction means is provided, and the detection sensitivity is effectively corrected according to each modulation frequency so that the detection sensitivity of the photoacoustic detection signal becomes constant for each modulation frequency. By doing so, stable detection and quantitative analysis of the surface and internal information of the sample becomes possible.

Further, the detection sensitivity correction means adjusts the intensity of the intensity-modulated light according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency, so that the influence of non-optical noise is affected. It can be reduced.

Further, the detection sensitivity correction means adjusts the intensity of the detection signal according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency, so that the stability of the optical system increases.

Further, the detection sensitivity correction means corrects the sensitivity so that the detection sensitivity becomes constant with respect to each modulation frequency, including the modulation frequency characteristic of the thermal distortion detection means for detecting the above-mentioned thermal distortion. Quantitatively improves.

Further, since the thermal strain detecting means detects the thermal strain on the sample surface by using the optical interference detecting means, the photoacoustic signal can be detected in a non-contact manner.

Further, since the thermal strain detecting means detects the thermal strain on the sample surface by using the piezoelectric element, the structure of the detection system is simplified and the stability of signal detection is improved.

Further, the intensity-modulated light is condensed on the sample,
Since the excitation means for generating the photoacoustic effect or the photothermal effect on the sample surface or inside is configured as a confocal optical system, the lateral resolution of the photoacoustic signal, the detection sensitivity and the signal S.
The N ratio is improved.

Further, since the optical interference detecting means for detecting the thermal strain of the sample surface caused by the photoacoustic effect or the photothermal effect is constructed as a confocal optical system, the lateral resolution of the photoacoustic signal, the detection sensitivity and the signal SN. The ratio is improved.

[0023]

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

First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a photoacoustic detection optical system in the first embodiment. This optical system is an excitation optical system 30.
1. A heterodyne type Mach-Zehnder interference optical system 302 for detecting a photoacoustic signal and a signal processing system 300. Ar laser 81 of excitation optical system 301 (wavelength 515n
m), the parallel beam 82 emitted from the acousto-optic modulator 8
It is incident on 3. Now, referring to FIG.
The sine wave 100 having the frequency f _B shown in (a) and the rectangular wave 101 having the frequency f _L (f _L <f _B ) shown in FIG. By taking the product of the waveforms, the modulated signal 102 shown in FIG. 7C is created and input to the acousto-optic modulator 83. As a result, the acousto-optic modulator 83 intermittently outputs the first-order diffracted light 85 whose frequency is shifted by f _{B at the} frequency f _L. That is,
As the excitation light, the modulation frequency f _L frequency-shifted by f _B
Intensity modulated beam of is obtained. The 0th order light 84 is used for the diaphragm 8
It is shaded at 6. After the intensity-modulated beam 85 is expanded to a desired beam diameter by the beam expander 87, the lens 8
8 is used to focus the light on the rear focus position 89. A pinhole 90 is installed at the rear focal position 89, as shown in FIG.
As shown in (a), the high-order diffracted light components 105a and 105b existing around the peak portion 105 of the focused spot are shielded. As a result, the light intensity distribution after passing through the pinhole 90 is only the peak portion 105, as shown in FIG. Since the focus position 89 is the front focus position of the lens 91, the light flux after passing through the pinhole 90 becomes parallel light 92 after passing through the lens 91. The parallel light 92 is reflected by a dichroic prism (reflected at a wavelength of 600 nm or less and transmitted at a wavelength of 600 nm or more) 93, and then passes through the λ / 4 plate 47 and is collected by the objective lens 48 on its front focal position 50, that is, the sample 51. It becomes a light spot that has the same light intensity distribution as shown in FIG. That is, the front focus position 89 of the lens 91 and the front focus position 50 of the objective lens 48 are
It is conjugate and at the same time confocal. Ultrasonic waves (thermoelastic waves) are generated by the thermal strain wave generated on the sample 51 on the basis of the photoacoustic effect, and at the same time, the condenser 50 on the surface of the sample 51 is slightly displaced. This small displacement is caused by the excitation light 9
It changes periodically with an intensity modulation frequency f _L of 2.

On the other hand, in the Mach-Zehnder interference optical system 302, the deflection direction of the linearly polarized beam 32 emitted from the He-Ne laser 31 (wavelength 633 nm) is 106 in FIG.
The angle is set to 45 ° with respect to the x-axis and the y-axis. Here, the direction perpendicular to the plane of the paper of FIG. 1 is the x-axis, and the direction orthogonal thereto is the y-axis. Polarizing beam splitter 33
Of the incident light beam 32, p shown by 107 in FIG.
The polarization component 34 passes through the polarization beam splitter 33 and enters the acousto-optic modulator 76. Further, the s-polarized component 35 indicated by 108 in FIG. 4 is reflected by the polarization beam splitter 33. The sine wave 100 having the frequency f _B shown in FIG. 2A is input from the oscillator 115 to the acousto-optic modulator 76. As a result, the acousto-optic modulator 76 outputs the first-order diffracted light 37 frequency-shifted by f _B. The 0th order light 36 is blocked by the diaphragm 38. The first-order diffracted light 37 is expanded to a desired beam diameter by the beam expander 39, and then is focused at the rear focal position 41 by the lens 40. A pinhole 42 is provided at the rear focal position 41, and as shown in FIG. 3A, the high-order diffracted light components 105a and 105b existing around the peak portion 105 of the focused spot are shielded. As a result, the light intensity distribution after passing through the pinhole 42 is at the peak portion 10 as shown in FIG.
Only 5 Since the focus position 41 is the front focus position of the lens 43, the light flux after passing through the pinhole 42 becomes parallel light after passing through the lens 43. Since the parallel light is composed of the p-polarized light component, it passes through the polarization beam splitter 45 as it is, is reflected by the mirror 46, and then is reflected by the dichroic prism 9.
After passing through the 3 and λ / 4 plate 47, it becomes circularly polarized light, and is further focused by the objective lens 48 on the front focus position 50, that is, the sample 51.
The light spot is condensed above and has a light intensity distribution similar to that shown in FIG. That is, the front focus position 41 of the lens 43 and the front focus position 50 of the objective lens 48 are conjugate and confocal at the same time. The reflected light from the condensing unit 50 on the sample 51 has a minute displacement amount generated on the surface of the sample 51 as phase information, becomes parallel light after passing through the objective lens 48, and becomes an s-polarized beam after passing through the λ / 4 plate 47. Become. The s-polarized beam passes through the same optical path again, is then reflected by the polarization beam splitter 45, and passes through the non-polarization beam splitter 60.

On the other hand, after the s-polarized beam 35, which is the reference light, reflected by the polarization beam splitter 33 is reflected by the mirror 53, it is expanded to a desired beam diameter by the beam expander 54, and then the rear focus is obtained by the lens 55. Position 5
Focus on 6. A pinhole 57 is provided at the rear focal position 56.
Is installed, and blocks the higher-order diffracted light components 105a and 105b existing around the peak portion 105 of the focused spot, as shown in FIG. 3 (a). As a result, the light intensity distribution after passing through the pinhole 57 is as shown in FIG.
Only the peak portion 105 is provided. The focal position 56 is the lens 58
Since it is at the front focus position of, the light flux after passing through the pinhole 57 becomes the parallel reference light 59 after passing through the lens 58. The reference light 59 is reflected by the non-polarizing beam splitter 60 and then passes through the non-polarizing beam splitter 60.
The reflected light 61 from the surface of No. 1 interferes with each other. The interference light 62 contains, as optical phase information, a minute displacement amount generated on the surface of the sample 51 due to the photoacoustic effect. The interference light 62 is condensed on the rear focal position 64 by the lens 63 and detected by the photoelectric conversion element 67 such as a photodiode.
In the Mach-Zehnder interference optical system 302, the front focal position 41 of the lens 43, the front focal position 56 of the lens 58, the front focal position 50 of the objective lens 48, and the rear focal position 64 of the lens 63 are conjugate with each other and at the same time. There is a confocal relationship. Further, a pinhole 65 is provided at the rear focal position 64 of the lens 63. As a result, stray light generated in the objective lens 48, interference components generated in the transparent thin film on the sample, or high-order diffracted light components generated by minute unevenness on the sample surface can be shielded.

Now, the wavelength of the incident light 32 is λ, the intensity of the reflected light 61 from the surface of the sample 51 is I _s , the intensity of the reference light 59 is I _r , and the phase difference between the two optical paths including time variation is φ. (t),
Assuming that the amplitude of the minute displacement generated on the surface of the sample 51 is a and the phase is θ, the intensity I of the interference light detected by the photoelectric conversion element 67.
Is represented by formula (1).

[0028]

[Equation 1]

Further, the equation above a << λ can be approximately changed to the equation (2).

[0030]

[Equation 2]

Here, a · cos (2πf _L t + θ) is a term representing a minute displacement of the surface of the sample 51 caused by the photoacoustic effect. In this embodiment, f _B = 40 MHz,
f _L = 100 kHz.

In the following, the signal processing system 300 uses the interference light expressed by the equation (1) to generate an amplitude a of a minute displacement of the surface of the sample 51 generated at the modulation frequency f _L based on the photoacoustic effect.
And a method of obtaining the phase θ will be described. The photoelectrically converted interference intensity signal is amplified by the preamplifier 71 and then sent to the phase detection circuit 72. In the phase detection circuit 72, the detected interference intensity signal is separated by the phase holding demultiplexer 94 as shown in FIG. 5, one of which is passed through the bandpass filter 95 of the center frequency f _B , and then the phase shifter 96. The phase is delayed by π / 2. The output signal from the phase shifter 96 is amplified by the amplifier 97, then sent to the mixer 98, and the product with the other interference intensity signal separated by the phase holding demultiplexer 94 is output. The other interference intensity signal I _D1 is expressed by the formula (3), the output signal I _D2 from the amplifier 97 is expressed by the formula (4), and the output signal I _D from the mixer 98 is expressed by the formula (5).

[0033]

[Equation 3]

[0034]

[Equation 4]

[0035]

[Equation 5]

Expression (5) can be approximated to the following expression from a << λ.

[0037]

[Equation 6]

In the equation (6), the first term is the direct current component, the second term is the frequency component of f _{L to} be obtained, the third and fourth terms are the frequency components of f _B , and the fifth and sixth terms are It is a frequency component of 2f _B. As now f _L «f _B, the output signal from the mixer 98, passed through a low-pass filter 99 for cutting off the frequency components above f _B, is input to the lock-in amplifier 73 of FIG. 1. The low-pass filter 99 extracts only the DC component and the frequency component of f _L , and further, the lock-in amplifier 7
In 3, the amplitude and phase of the frequency component of f _L are finally extracted using the rectangular wave signal of frequency f _L output from the oscillator 69 as a reference signal. From this amplitude and phase, sample 5
The amplitude a and the phase θ of the minute displacement on one surface are obtained. Further, the amplitude a and the phase θ are the modulation frequencies f
_{It has} thermal and elastic information within the thermal diffusion region V _th defined by _L. Therefore, if there is an internal defect such as a crack in the thermal diffusion region V _th , the amplitude a and the phase θ change and the existence thereof can be known. The movement signal of the XY stage 52 and the output signal from the lock-in amplifier 73 are processed by the computer 74, and the photoacoustic signal at each point on the sample 51 is monitored by the monitor TV.
It is output as a two-dimensional photoacoustic image on 75. Further, if the frequency f _{L of the} modulation signal output from the oscillator 69 is controlled by the computer 74, various modulation frequencies can be set, and internal information at various depths of the sample 51 can be detected.

Here, for example, when the sample is optically transparent and thermally thin, as shown in FIG. 6, the signal intensity a (f _L ) of the photoacoustic signal is inverse to the intensity modulation frequency f _L of the excitation light. It has a frequency characteristic 110 of being proportional. Further, as shown in FIG. 7, the detection sensitivity b of the Mach-Zehnder interference optical system 302
(f _L ) is a frequency characteristic 1 that is inversely proportional to √ f _L with respect to the fluctuating frequency of the surface displacement (excitation light intensity modulation frequency) f _L.
Have 11. Therefore, in the conventional photoacoustic signal detection device,
When the intensity modulation frequency f _L of the excitation light is variously changed to detect defects at various depths inside the sample, even if an attempt is made to determine the size of the defect from the detected photoacoustic signal, the photoacoustic signal is detected. Since the sensitivity changes variously depending on the modulation frequency, it is difficult to quantitatively grasp the size of the defect. Therefore, in this embodiment, this problem is solved by the following means. First, the modulation frequency characteristics of the photoacoustic signal and the interferometer are obtained in advance theoretically or experimentally for each sample. In this case, the signal intensity of the finally obtained photoacoustic signal is proportional to the modulation frequency f _L −3/2 ((−1) + (− 1/2)), as described above. In order to make the detection sensitivity at the modulation frequency constant, in proportion to the 3/2 power of the modulation frequency f _L which is the reciprocal of the modulation frequency characteristic,
The intensity I _{e of the} excitation light may be changed. Excitation light intensity I _e
In the present embodiment, as shown in FIG. 8, the amplitude V _M of the rectangular wave 101 of the frequency f _L shown in FIG. A method of changing the intensity I _e of the first-order diffracted light 85 output from 83, that is, the excitation light is used. Therefore,
When the photoacoustic signal is detected by changing the modulation frequency f _L variously, the frequency f shown in FIG. 2B output from the oscillator 69 in proportion to the 3/2 power of the modulation frequency f _L under the control of the computer 74. The amplitude V _M of the _L rectangular wave 101 is changed, and further the amplitude V _M of the modulated signal 102 shown in FIG. As a result, as shown in FIG. 9, the excitation light 85 in proportion to the 3/2 power of the modulation frequency f _L for each modulation frequency (9
The intensity I _{e of} 2) can be changed, and finally, as shown in FIG.
As shown by 0, the signal intensity I of the photoacoustic signal becomes constant at each modulation frequency, and the modulation frequency characteristic of the detection sensitivity can be corrected.

According to this embodiment, the detection sensitivity of the photoacoustic signal is always constant regardless of the intensity modulation frequency of the excitation light. Therefore, when the modulation frequency is changed and defects of various depths are detected, It becomes possible to quantitatively determine the size of the defect from the detected photoacoustic signal, stable detection of the surface and internal information of the sample, and the inside of various depths of the sample detected by changing the modulation frequency. It enables quantitative analysis of information.

Further, according to this embodiment, the following great effects are obtained. For example, as shown in FIG. 11, the frequency characteristic of the signal intensity a (f _L ) of the photoacoustic signal is 110, whereas the frequency characteristic of non-optical noise, for example, vibration noise of the stage of the device is 130. If so, at the modulation frequency f _LN , the photoacoustic signal a (f _LN ) becomes smaller than the noise n (f _LN ) and detection becomes impossible. Even in such a case, according to the present embodiment, the intensity I _{e of the} excitation light is changed, the photoacoustic signal intensity increases particularly near f _LN , and becomes constant with respect to the modulation frequency f _L like 110 p. Adjustment, it is possible to increase only the photoacoustic signal intensity a (f _LN ) while maintaining the non-optical noise intensity n (f _LN ),
The photoacoustic signal a (f _LN ) can be detected at the modulation frequency f _LN .

Further, in this embodiment, as a means for changing the intensity I _e of the excitation light, a method of changing the amplitude V _M of the modulation signal 102 of the acousto-optic modulator 83 used for intensity modulation of the excitation light is used. Since it is adopted, there is an effect that the conventional optical system can be used as it is.

Further, according to this embodiment, by constructing the excitation optical system and the interference optical system as a confocal optical system, an ideal high-order diffracted light component does not exist on the sample and the interference light detection means. High-order diffracted light generated by stray light generated in the objective lens 48, interference components generated in the transparent thin film on the sample, or minute unevenness on the sample surface can be formed. The components can be shielded from light. Therefore, the lateral resolution, detection sensitivity, and signal-to-noise ratio of the photoacoustic signal are improved.

Second Embodiment of the Present Invention FIGS. 12 and 13
It will be described based on. FIG. 12 shows a photoacoustic detection optical system in the second embodiment. This optical system includes an excitation optical system 303, a heterodyne type Mach-Zehnder interference optical system 302, and a signal processing system 300. The basic configuration and the function thereof are the same as those of the photoacoustic detection optical system in the first embodiment shown in FIG. 1, so detailed description will be omitted. In the first embodiment, the amplitude V _{M of the} modulation signal 102 shown in FIG.
By changing the intensity I _e of the primary diffracted light 85 output from the acousto-optic modulator 83, that is, the excitation light of the modulation frequency f _L , to correct the modulation frequency characteristic of the detected photoacoustic signal intensity I. ing. On the other hand, in the second embodiment, as shown in FIG. 12, in front of the acousto-optic modulator 83 of the excitation optical system 303, as shown by 123 in FIG. A continuously variable ND filter 120 having a change in transmittance is inserted, and a driving mechanism 121 including a pulse motor and a crank mechanism (not shown) and a controller 122 finely move the ND filter 120 in the arrow direction to excite the intensity I _e of the excitation light. Is changed.
That is, when detecting the photoacoustic signal by variously changing the modulation frequency f _L, by fine movement of the continuous-variable ND filter 120 for each modulation frequency, the modulation frequency f _L as shown in FIG. 9 3 / If the intensity I _{e of the} excitation light is changed in proportion to the square and the modulation frequency characteristic of the photoacoustic signal is corrected,
Finally, similarly to the first embodiment, as shown in FIG. 10, the photoacoustic signal (signal intensity I), that is, the modulation frequency characteristic of the detection sensitivity can be made constant.

According to this embodiment, as in the first embodiment,
Since the detection sensitivity of the photoacoustic signal is always constant regardless of the modulation frequency of the excitation light, when the defects of various depths in the sample are detected by changing the modulation frequency, the size of the defect is detected from the detected photoacoustic signal. It becomes possible to quantitatively discriminate among the samples, and it is possible to stably detect the surface and internal information of the sample, and to quantitatively analyze the internal information of various depths of the sample detected by changing the modulation frequency.

Further, according to this embodiment, similar to the first embodiment, the following great effects are obtained. For example, as shown in FIG. 11, the frequency characteristic of the signal intensity a (f _L ) of the photoacoustic signal is 110, while the frequency characteristic of non-optical noise, such as vibration noise of the stage of the device, is 130. , The photoacoustic signal a at the modulation frequency f _LN
(f _LN ) becomes smaller than noise n (f _LN ) and detection becomes impossible. Even in such a case, according to the present embodiment, if the intensity I _{e of the} excitation light is changed and the signal intensity of the photoacoustic signal is adjusted to be constant with respect to the modulation frequency f _L like 110 p, the modulation is performed. Even at the frequency f _LN , the photoacoustic signal a
(f _LN ) can be detected.

Further, in this embodiment, since the continuously variable ND filter 120 separately provided in the optical path of the excitation optical system 303 is used as a means for changing the intensity I _{e of the} excitation light, the oscillator 69 shown in FIG. Since it is not necessary to control the amplitude V _M of the rectangular wave 101 having the frequency f _L shown in (b), there is an effect that the circuit configuration becomes easy.

Further, according to the present embodiment, as in the first embodiment, by constructing the excitation optical system and the interference optical system as a confocal optical system, it is unnecessary on the sample and the interference light detecting means. It is possible to form a spot light having an ideal peak portion in which a higher-order diffracted light component does not exist, and further, stray light generated in the objective lens 48, an interference component generated in the transparent thin film on the sample, or the sample surface. It is possible to block the high-order diffracted light component generated by the minute unevenness of. Therefore, the lateral resolution of the photoacoustic signal, the detection sensitivity, and the signal SN
The ratio is improved.

A third embodiment of the present invention will be described with reference to FIG. FIG. 14 shows a photoacoustic detection optical system in the third embodiment. This optical system is an excitation optical system 30.
4. A heterodyne type Mach-Zehnder interference optical system 302 and a signal processing system 305. The basic configuration and the function thereof are the same as those of the photoacoustic detection optical system in the first embodiment shown in FIG. 1, so detailed description will be omitted. In the first embodiment, the amplitude V _M of the modulation signal 102 shown in FIG. 2C is changed to change the intensity I _e of the first-order diffracted light 85 output from the acousto-optic modulator 83, that is, the excitation light. , The modulation frequency characteristic of the detected photoacoustic signal intensity I is corrected. On the other hand, in the third embodiment, the modulation frequency characteristic of the photoacoustic signal intensity I is not corrected by adjusting the intensity I _e of the excitation light, but an analog multiplier newly provided in the signal processing system 305. 124. That is, the modulation frequency f _L
When the photoacoustic signal is detected by changing various values, input the correction voltage signal corresponding to FIG. 9 from the computer 74 to the analog multiplier 124 for each modulation frequency, and multiply it by the detected interference intensity signal. For example, finally, as in the first embodiment, the photoacoustic signal (signal intensity I), that is, the modulation frequency characteristic of the detection sensitivity can be made constant as shown in FIG.

According to this embodiment, as in the first embodiment,
Since the detection sensitivity of the photoacoustic signal is always constant regardless of the modulation frequency of the excitation light, when the defects of various depths in the sample are detected by changing the modulation frequency, the size of the defect is detected from the detected photoacoustic signal. It becomes possible to quantitatively discriminate among the samples, and it is possible to stably detect the surface and internal information of the sample, and to quantitatively analyze the internal information of various depths of the sample detected by changing the modulation frequency.

In this embodiment, the signal processing system 30 is used as means for correcting the modulation frequency characteristic of the photoacoustic signal intensity I.
Since it is electrically performed using the analog multiplier 124 provided in FIG. 5, there is no need to provide the pumping light intensity control function in the pumping optical system 304, and the stability of the optical system is increased.

Further, according to the present embodiment, as in the first embodiment, by constructing the excitation optical system and the interference optical system as a confocal optical system, it is unnecessary on the sample and the interference light detecting means. It is possible to form a spot light having an ideal peak portion in which a high-order diffracted light component does not exist, and further, stray light generated in the objective lens 48, an interference component generated in the transparent thin film on the sample, or a minute amount on the sample surface. Higher-order diffracted light components generated by such unevenness can be shielded. Therefore, the lateral resolution, detection sensitivity, and signal-to-noise ratio of the photoacoustic signal are improved.

Fourth Embodiment of the Present Invention FIGS. 15 and 16
It will be described based on. FIG. 15 shows a photoacoustic detection optical system in the fourth embodiment. This optical system includes an excitation optical system 306 and a PZT element 1 for detecting a photoacoustic signal.
It includes a signal processing system 307 including 26. Excitation optical system 3
Since the configuration of 06 is the same as that of the excitation optical system 301 in the first embodiment shown in FIG. 1, detailed description thereof will be omitted.
The mirror 12 is used instead of the dichroic prism 93.
5 is used. In the first to third embodiments, the heterodyne type Mach-Zehnder interference optical system is used to detect the photoacoustic signal, but in this embodiment, the PZT element 126 mounted on the back surface of the sample 51 is used. The output voltage from the PZT element 126 is amplified by the preamplifier 127 and then sent to the lock-in amplifier 73. The lock-in amplifier 73 extracts the amplitude and phase of the frequency component of f _L from the output signal of the PZT element 126 using the rectangular wave signal of the frequency f _L output from the oscillator 69 as a reference signal. From the amplitude and the phase, the amplitude a and the phase θ of the minute displacement on the surface of the sample 51 can be obtained. Further, the amplitude a and the phase θ are the thermal diffusion region V _th defined by the modulation frequency f _L.
It has thermal and elastic information inside. Therefore, the thermal diffusion region V
If there is an internal defect such as a crack within _th , the amplitude a and the phase θ change and the existence thereof can be known. Similar to the first embodiment, the movement signal of the XY stage 52 and the output signal from the lock-in amplifier 73 are processed by the computer 74, and the photoacoustic signal at each point on the sample 51 is displayed on the monitor TV 75 as a two-dimensional photoacoustic image. Is output as. Also, the oscillator 69
By controlling the frequency f _{L of the} modulation signal output from the computer 74, it is possible to set various modulation frequencies, and it is possible to detect internal information at various depths of the sample 51.

In this embodiment, since the PZT element 126 is used to detect the photoacoustic signal, the modulation frequency characteristic 110 of the photoacoustic signal intensity a (f _L ) shown in FIG. 6 and the PZT element shown in FIG. Modulation frequency characteristic of sensitivity p (f _L ) of 128
Both are corrected. As in the first embodiment, the correction method is as follows: first, the photoacoustic signal intensity a shown in FIG.
(f _L) is measured in advance a modulation frequency characteristic 128 theoretically or experimentally in the sensitivity of the PZT element shown to the modulation frequency characteristics 110 and FIG. 16 p (f _L) of. Next, as shown in FIG. 10, the computer 74 obtains the reciprocal (correction coefficient) of each measured frequency characteristic so that the detection sensitivity at each modulation frequency becomes constant. Next, the amplitude V _M of the modulation signal 101 shown in FIG. 2B output from the oscillator 69 is controlled by the computer 74 based on this correction coefficient, and the modulation signal 10 shown in FIG.
By changing the amplitude V _M of 2, the intensity I _e of the first-order diffracted light 85 output from the acousto-optic modulator 83, that is, the excitation light is changed to obtain the modulation frequency characteristic of the detected photoacoustic signal intensity I. to correct.

According to this embodiment, as in the first embodiment,
Since the detection sensitivity of the photoacoustic signal is always constant regardless of the modulation frequency of the excitation light, when the defects of various depths in the sample are detected by changing the modulation frequency, the size of the defect is detected from the detected photoacoustic signal. It becomes possible to quantitatively discriminate among the samples, and it is possible to stably detect the surface and internal information of the sample, and to quantitatively analyze the internal information of various depths of the sample detected by changing the modulation frequency.

Further, according to this embodiment, similar to the first embodiment, the following great effects are obtained. For example, as shown in FIG. 11, the frequency characteristic of the signal intensity a (f _L ) of the photoacoustic signal is 110, while the frequency characteristic of non-optical noise, such as vibration noise of the stage of the device, is 130. In this case, at the modulation frequency f _LN , the photoacoustic signal a (f _LN ) becomes smaller than the noise n (f _LN ) and detection becomes impossible. Even in such a case, according to the present embodiment, the intensity I _{e of the} excitation light is changed to change the signal intensity of the photoacoustic signal to 110 p.
By adjusting to the modulation frequency f _L as to be constant, even modulation frequency f _LN, it is possible to detect the photoacoustic signal a (f _LN).

Further, in this embodiment, as a means for changing the intensity I _e of the excitation light, a method of changing the amplitude V _M of the modulation signal 102 of the acousto-optic modulator 83 used for intensity modulation of the excitation light is used. Since it is adopted, there is an effect that the conventional optical system can be used as it is.

Further, according to the present embodiment, since the PZT element is used for detecting the photoacoustic signal, the optical system is simplified and the optical axis adjustment is facilitated, or the stability of the optical system is increased. There is an effect.

Further, according to the present embodiment, by forming the excitation optical system as a confocal optical system, spot light having an ideal peak portion free of unnecessary higher-order diffracted light components is formed on the sample. Therefore, the lateral resolution of the photoacoustic signal, the detection sensitivity, and the signal-to-noise ratio are improved.

[0060]

According to the present invention, in the photoacoustic signal detecting apparatus, the detection sensitivity of the photoacoustic signal is always constant regardless of the modulation frequency of the excitation light, so that the modulation frequency is changed to various depths of the sample. When a defect of the sample is detected, it becomes possible to quantitatively determine the size of the defect from the detected photoacoustic signal, the surface of the sample and internal information are stably detected, and the sample detected by changing the modulation frequency is detected. It enables quantitative analysis of internal information at various depths.

Further, a confocal optical system is provided with an exciting means for collecting the exciting light on the sample and an optical interference detecting means for detecting a minute displacement (thermal strain) of the sample surface caused by the photoacoustic effect or the photothermal effect. With such a configuration, spot light having an ideal peak portion free of unnecessary higher-order diffracted light components can be formed on the sample and the interference light detection means, and lateral resolution and detection of the photoacoustic signal can be performed. Sensitivity and signal SN
The ratio is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a photoacoustic detection optical system in a first embodiment of the present invention,

FIG. 2 is a waveform diagram of a modulation signal input to an acousto-optic modulator,

FIG. 3 is an explanatory view showing a state in which a high-order diffracted light component of a focused spot is shielded,

FIG. 4 is an explanatory view showing polarization directions of incident light in the interferometers of the first to third embodiments,

FIG. 5 is a block diagram of a phase detection circuit,

FIG. 6 is a modulation frequency characteristic diagram of photoacoustic signal intensity,

FIG. 7 is a modulation frequency characteristic diagram of detection sensitivity of the interference optical system,

FIG. 8 is a characteristic diagram showing the relationship between the amplitude of a modulation signal and the intensity of first-order diffracted light output from an acousto-optic modulator.

FIG. 9 is a modulation frequency characteristic diagram of the excitation light intensity for correcting the modulation frequency characteristic of the photoacoustic signal,

FIG. 10 is a modulation frequency characteristic diagram of photoacoustic signal intensity after sensitivity correction,

FIG. 11 is a modulation frequency characteristic diagram of photoacoustic signal intensity before and after sensitivity correction and a modulation frequency characteristic diagram of non-optical noise.

FIG. 12 is a block diagram showing a photoacoustic detection optical system according to a second embodiment of the invention.

FIG. 13 is a transmittance distribution diagram of the continuously variable ND filter according to the second embodiment,

FIG. 14 is a block diagram showing a photoacoustic detection optical system according to a third embodiment of the invention.

FIG. 15 is a block diagram showing a photoacoustic detection optical system according to a fourth embodiment of the present invention.

FIG. 16 is a modulation frequency characteristic diagram of PZT element detection sensitivity in the fourth embodiment;

FIG. 17 is a block diagram for explaining a conventional photoacoustic detection optical system,

FIG. 18 is a principle diagram of a photoacoustic effect.

[Explanation of symbols]

1, 8 ... Laser, 81 ... Ar laser, 31 ... He-Ne
Laser, 2, 76, 83 ... Acousto-optic modulator, 42, 5
7, 90 ... Pinhole, 5, 48 ... Objective lens, 13,
67 ... Photoelectric conversion element, 126 ... PZT element, 15, 69
... Oscillator, 72 ... Phase detection circuit, 120 ... Continuous variable type N
D filter, 124 ... Analog multiplier, 16, 73 ... Lock-in amplifier, 17, 74 ... Calculator, 7, 51 ... Sample.

Claims (16)

[Claims] Claim: What is claimed is: 1. A light source is intensity-modulated at a desired frequency, the intensity-modulated light is condensed on a sample, and a photoacoustic effect or a photothermal effect is generated on or inside the sample. A photoacoustic signal that detects the thermal strain on the sample surface caused by the effect, detects the frequency component of the intensity modulation frequency from the detected signal, and extracts the sample surface and internal information according to the modulation frequency from the frequency component. In the detection method, the detection sensitivity is effectively corrected according to each modulation frequency so that the detection sensitivity of the detection signal becomes constant for each modulation frequency. 2. The photoacoustic according to claim 1, wherein the detection sensitivity is corrected by adjusting the intensity of the intensity-modulated light according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency. Signal detection method. 3. The correction of the detection sensitivity according to claim 1, wherein the intensity of the detection signal is adjusted according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency. Photoacoustic signal detection method. 4. The correction of the detection sensitivity according to claim 1, wherein the detection sensitivity including the modulation frequency characteristic of the thermal strain detecting means for detecting thermal strain is constant for each modulation frequency. Photoacoustic signal detection method. 5. The method for detecting a photoacoustic signal according to claim 1, wherein the thermal strain on the surface of the sample is detected by using optical interference. 6. The method for detecting a photoacoustic signal according to claim 1, wherein the thermal strain on the surface of the sample is detected by using a piezoelectric element. 7. The photoacoustic signal according to claim 1, wherein the excitation method for condensing the intensity-modulated light on a sample to generate a photoacoustic effect or a photothermal effect on the sample surface or inside is a confocal optical system. Detection method. 8. The photoacoustic signal detection method according to claim 5, wherein the optical interference detection means for detecting thermal strain on the sample surface is configured as a co-focusing optical system. 9. A light source, a modulation means for intensity-modulating the light from the light source at a desired frequency, and the intensity-modulated light is condensed on a sample to produce a photoacoustic effect or a photothermal effect on the sample surface or inside. Excitation means for generating, thermal strain detecting means for detecting thermal strain of the sample surface caused by the photoacoustic effect or photothermal effect, and a frequency component for detecting the frequency component of the intensity modulation frequency from the detected detection signal. In a photoacoustic signal detection device comprising a detection means and an information extraction means for extracting the surface and internal information of the sample according to the modulation frequency from the frequency component, the detection sensitivity of the detection signal is constant for each modulation frequency. As described above, the photoacoustic signal detection apparatus is provided with a detection sensitivity correction unit that effectively corrects the detection sensitivity according to each modulation frequency. 10. The photoacoustic signal according to claim 9, wherein the detection sensitivity correction means adjusts the intensity of the intensity-modulated light according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency. Detection device. 11. The photoacoustic signal detection according to claim 9, wherein the detection sensitivity correction means adjusts the intensity of the detection signal according to each modulation frequency so that the detection sensitivity becomes constant for each modulation frequency. apparatus. 12. The sensitivity correction means according to claim 9, wherein the detection sensitivity including the modulation frequency characteristic of the thermal strain detecting means for detecting thermal strain is constant for each modulation frequency. Photoacoustic signal detector. 13. The photoacoustic signal detecting device according to claim 9, wherein the thermal strain detecting means detects thermal strain on the surface of the sample by using optical interference detecting means. 14. The photoacoustic signal detecting device according to claim 9, wherein the thermal strain detecting means detects a thermal strain on the surface of the sample by using a piezoelectric element. 15. The photoacoustic system according to claim 9, wherein the excitation means for condensing the intensity-modulated light on a sample to generate a photoacoustic effect or a photothermal effect on the sample surface or inside is a confocal optical system. Signal detection device. 16. The photoacoustic signal detection device according to claim 13, wherein the optical interference detection means for detecting thermal strain on the sample surface is configured as a confocal optical system.