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Claims  |
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I claim:
1. A photoacoustic signal detection method for detecting information about
a characteristic of a surface of a sample and a subsurface of the sample,
comprising the steps of:
splitting a light beam from a single light source into an excitation/probe
light beam and a reference light beam;
intensity-modulating the excitation/probe light beam at a desired intensity
modulation frequency to produce an intensity-modulated excitation/probe
light beam for exciting and probing the surface of the sample and the
subsurface of the sample;
producing an optical frequency difference between the intensity-modulated
excitation/probe light beam and the reference light beam;
focusing the intensity-modulated excitation/probe light beam on the surface
of the sample to generate a photoacoustic effect or a photothermal effect
at the surface of the sample, thereby producing thermal distortions in the
sample, wherein the intensity-modulated excitation/probe light beam is
reflected from the surface of the sample to produce a reflected
intensity-modulated excitation/probe light beam representative of the
thermal distortions in the sample;
causing heterodyne interference to occur between the reflected
intensity-modulated excitation/probe light beam and the reference light
beam to produce heterodyne interference light including a frequency
component having a frequency equal to twice the intensity modulation
frequency, the heterodyne interference occurring as a result of the
optical frequency difference between the excitation/probe light beam and
the reference light beam;
converting the heterodyne interference light to an electric signal
including a frequency component having a frequency equal to twice the
intensity modulation frequency;
extracting from the electric signal at least one of an amplitude and a
phase of the frequency component having the frequency equal to twice the
intensity modulation frequency; and
detecting information about the characteristic of the surface of the sample
and the subsurface of the sample based on the extracted at least one of
the amplitude and the phase.
2. A method according to claim 1, wherein the single light source includes
a laser source for emitting a linearly-polarized light beam.
3. A method according to claim 1, wherein the step of producing an optical
frequency difference between the intensity-modulated excitation/probe
light beam and the reference light beam includes shifting an optical
frequency of the intensity-modulated excitation/probe light beam with an
acousto-optical modulator to produce the optical frequency difference.
4. A method according to claim 1, wherein the step of extracting from the
electric signal at least one of an amplitude and a phase is performed with
a lock-in amplifier operating based on the electric signal and a signal
having a frequency equal to twice the intensity modulation frequency.
5. A method according to claim 1, wherein the optical frequency difference
between the intensity-modulated excitation/probe light beam and the
reference light beam is higher than the intensity modulation frequency.
6. A method according to claim 1, wherein the step of producing an optical
frequency difference between the intensity-modulated excitation/probe
light beam and the reference light beam includes shifting an optical
frequency of only one of the intensity-modulated excitation/probe light
beam and the reference light beam to produce the optical frequency
difference.
7. A method according to claim 1, wherein the step of producing an optical
frequency difference between the intensity-modulated excitation/probe
light beam and the reference light beam includes shifting respective
optical frequencies of the intensity-modulated excitation/probe light beam
and the reference light beam by mutually different amounts to produce the
optical frequency difference.
8. A method according to claim 1, wherein the step of focusing the
intensity-modulated excitation/probe light beam on the surface of the
sample is performed with focusing means;
wherein the step of causing heterodyne interference to occur is performed
with means for causing heterodyne interference to occur; and
wherein the focusing means and the means for causing heterodyne
interference to occur are formed as a confocal optical system.
9. A photoacoustic signal detection apparatus for detecting information
about a characteristic of a surface of a sample and a subsurface of the
sample, comprising:
a single light source for emitting a light beam;
means for splitting the light beam into an excitation/probe light beam and
a reference light beam;
intensity modulating means for intensity-modulating the excitation/probe
light beam at a desired intensity modulation frequency to produce an
intensity modulated excitation/probe light beam for exciting and probing a
surface of a sample and a subsurface of the sample;
means for producing an optical frequency difference between the
intensity-modulated excitation/probe light beam and the reference light
beam;
means for focusing the intensity-modulated excitation/probe light beam on
the surface of the sample to generate a photoacoustic effect or a
photothermal effect at the surface of the sample, thereby producing
thermal distortions in the sample, wherein the excitation/probe light beam
is reflected from the surface of the sample to produce a reflected
intensity-modulated excitation/probe light beam representative of the
thermal distortions in the sample;
means for causing heterodyne interference to occur between the reflected
intensity-modulated excitation/probe light beam and the reference light
beam to produce heterodyne interference light including a frequency
component having a frequency equal to twice the intensity modulation
frequency, the heterodyne interference occurring as a result of the
optical frequency difference between the excitation/probe light beam and
the reference light beam;
means for converting the heterodyne interference light to an electric
signal including a frequency component having a frequency equal to twice
the intensity modulation frequency;
means for extracting from the electric signal at least one of an amplitude
and a phase of the frequency component having the frequency equal to twice
the intensity modulation frequency; and
means for detecting information about the characteristic of the surface of
the sample and the subsurface of the sample based on the extracted at
least one of the amplitude and the phase.
10. An apparatus according to claim 9, wherein the single light source
includes a laser source for emitting a linearly-polarized light beam.
11. An apparatus according to claim 9, wherein the means for producing an
optical frequency difference between the intensity-modulated
excitation/probe light beam and the reference light beam includes means
for shifting an optical frequency of the intensity-modulated
excitation/probe light beam with the intensity modulating means to produce
the optical frequency difference.
12. An apparatus according to claim 9, wherein the means for extracting
from the electric signal at least one of an amplitude and a phase includes
a lock-in amplifier operating based on the electric signal and a signal
having a frequency equal to twice the intensity modulation frequency.
13. An apparatus according to claim 9, wherein the optical frequency
difference between the intensity-modulated excitation/probe light beam and
the reference light beam is higher than the intensity modulation
frequency.
14. An apparatus according to claim 9, wherein the means for producing an
optical frequency difference between the intensity-modulated
excitation/probe light beam and the reference light beam includes means
for shifting an optical frequency of only one of the intensity-modulated
excitation/probe light beam and the reference light beam to produce the
optical frequency difference.
15. An apparatus according to claim 9, wherein the means for producing an
optical frequency difference between the intensity-modulated
excitation/probe light beam and the reference light beam includes means
for shifting respective optical frequencies of the intensity-modulated
excitation/probe light beam and the reference light beam by mutually
different amounts to produce the optical frequency difference.
16. An apparatus according to claim 9, wherein the means for focusing and
the means for causing heterodyne interference to occur are formed as a
confocal optical system.
17. A photoacoustic signal detection method for detecting information about
a characteristic of a surface of a sample and various depths in a
subsurface of the sample, comprising the steps of:
intensity-modulating a light beam at a desired intensity modulation
frequency to produce an intensity-modulated light beam, the intensity
modulation frequency being selected in accordance with a thermal diffusion
length in the sample corresponding to a depth in the subsurface of the
sample about a characteristic of which information is to be detected;
focusing the intensity-modulated light beam on the surface of the sample to
generate a photoacoustic effect or a photothermal effect at the surface of
the sample, thereby producing thermal distortions in the sample;
detecting the thermal distortions in the sample and producing a thermal
distortion signal representative of the thermal distortions in the sample,
the thermal distortion signal including a component having a frequency
related to the intensity modulation frequency, wherein variations in the
intensity modulation frequency produce variations in an amplitude of the
component having a frequency related to the intensity modulation
frequency;
compensating for the variations in the amplitude of the component having a
frequency related to the intensity modulation frequency such that the
amplitude of the component having a frequency related to the intensity
modulation frequency is unaffected by the variations in the intensity
modulation frequency; and
extracting from the thermal distortion signal the amplitude of the
component having a frequency related to the intensity modulation frequency
which is unaffected by the variations in the intensity modulation
frequency.
18. A method according to claim 17, wherein the amplitude of the component
having a frequency related to the intensity modulation frequency has a
sensitivity characteristic which is a function of the intensity modulation
frequency; and
wherein the step of compensating for the variations in the amplitude of the
component having a frequency related to the intensity modulation frequency
includes adjusting an intensity of the intensity-modulated light in
accordance with the intensity modulation frequency such that the
sensitivity characteristic of the amplitude of the component having a
frequency related to the intensity modulation frequency is substantially
constant at each intensity modulation frequency.
19. A method according to claim 17, wherein the amplitude of the component
having a frequency related to the intensity modulation frequency has a
sensitivity characteristic which is a function of the intensity modulation
frequency; and
wherein the step of compensating for the variations in the amplitude of the
component having a frequency related to the intensity modulation frequency
includes adjusting a level of the thermal distortion signal in accordance
with the intensity modulation frequency such that the sensitivity
characteristic of the amplitude of the component having a frequency
related to the intensity modulation frequency is substantially constant at
each intensity modulation frequency.
20. A method according to claim 17, wherein the step of detecting the
thermal distortions in the sample and producing a thermal distortion
signal representative of the thermal distortions in the sample includes
detecting the thermal distortions in the sample with thermal distortion
detecting means having a detection sensitivity characteristic which is a
function of the intensity modulation frequency;
wherein the thermal distortion signal has a sensitivity characteristic
which is a function of the intensity modulation frequency; and
wherein the step of compensating for the variations in the amplitude of the
component having a frequency related to the intensity modulation frequency
includes compensating for the detection sensitivity characteristic of the
thermal distortion detecting means such that the sensitivity
characteristic of the thermal distortion signal is substantially constant
at each intensity modulation frequency.
21. A method according to claim 17, wherein the step of detecting the
thermal distortions in the sample and producing a thermal distortion
signal representative of the thermal distortions in the sample includes
detecting the thermal distortions in the sample on the surface of the
sample with optical interference detecting means.
22. A method according to claim 21, wherein the optical interference
detecting means is formed as a confocal optical system for detecting the
thermal distortions in the sample on the surface of the sample.
23. A method according to claim 17, wherein the step of detecting the
thermal distortions in the sample and producing a thermal distortion
signal representative of the thermal distortions in the sample includes
detecting the thermal distortions in the sample on the surface of the
sample with a piezoelectric transducer.
24. A method according to claim 17, wherein the step of focusing the
intensity-modulated light beam on the surface of the sample to generate a
photoacoustic effect or a photothermal effect at the surface of the sample
includes focusing the intensity-modulated light beam on the surface of the
sample with focusing means which is formed as a confocal optical system.
25. A photoacoustic signal detection apparatus for detecting information
about a characteristic of a surface of a sample and various depths in a
subsurface of the sample, comprising:
means for intensity-modulating a light beam at a desired intensity
modulation frequency to produce an intensity-modulated light beam, the
intensity modulation frequency being selected in accordance with a thermal
diffusion length in a sample corresponding to a depth in a subsurface of
the sample about a characteristic of which information is to be detected;
means for focusing the intensity-modulated light beam on the surface of the
sample to generate a photoacoustic effect or a photothermal effect at the
surface of the sample, thereby producing thermal distortions in the
sample;
means for detecting the thermal distortions in the sample and producing a
thermal distortion signal representative of the thermal distortions in the
sample, the thermal distortion signal including a component having a
frequency related to the intensity modulation frequency, wherein
variations in the intensity modulation frequency produce variations in an
amplitude of the component having a frequency related to the intensity
modulation frequency;
means for compensating for the variations in the amplitude of the component
having a frequency related to the intensity modulation frequency such that
the amplitude of the component having a frequency related to the intensity
modulation frequency is unaffected by the variations in the intensity
modulation frequency; and
means for extracting from the thermal distortion signal the amplitude of
the component having a frequency related to the intensity modulation
frequency which is unaffected by the variations in the intensity
modulation frequency.
26. An apparatus according to claim 25, wherein the amplitude of the
component having a frequency related to the intensity modulation frequency
has a sensitivity characteristic which is a function of the intensity
modulation frequency; and
wherein the means for compensating for the variations in the amplitude of
the component having a frequency related to the intensity modulation
frequency includes means for adjusting an intensity of the
intensity-modulated light in accordance with the intensity modulation
frequency such that the sensitivity characteristic of the amplitude of the
component having a frequency related to the intensity modulation frequency
is substantially constant at each intensity modulation frequency.
27. An apparatus according to claim 25, wherein the amplitude of the
component having a frequency related to the intensity modulation frequency
has a sensitivity characteristic which is a function of the intensity
modulation frequency; and
wherein the means for compensating for the variations in the amplitude of
the component having a frequency related to the intensity modulation
frequency includes means for adjusting a level of the thermal distortion
signal in accordance with the intensity modulation frequency such that the
sensitivity characteristic of the amplitude of the component having a
frequency related to the intensity modulation frequency is substantially
constant at each intensity modulation frequency.
28. An apparatus according to claim 25, wherein the means for detecting the
thermal distortions in the sample and producing a thermal distortion
signal representative of the thermal distortions in the sample includes
thermal detecting distortion means for detecting the thermal distortions
in the sample, the thermal distortion detecting means having a detection
sensitivity characteristic which is a function of the intensity modulation
frequency;
wherein the thermal distortion signal has a sensitivity characteristic
which is a function of the intensity modulation frequency; and
wherein the means for compensating for the variations in the amplitude of
the component having a frequency related to the intensity modulation
frequency includes means for compensating for the detection sensitivity
characteristic of the thermal distortion detecting means such that the
sensitivity characteristic of the thermal distortion signal is
substantially constant at each intensity modulation frequency.
29. An apparatus according to claim 25, wherein the means for detecting the
thermal distortions in the sample and producing a thermal distortion
signal representative of the thermal distortions in the sample includes
optical interference detecting means for detecting the thermal distortions
in the sample on the surface of the sample.
30. An apparatus according to claim 29, wherein the optical interference
detecting means is formed as a confocal optical system for detecting the
thermal distortions in the sample on the surface of the sample.
31. An apparatus according to claim 25, wherein the means for detecting the
thermal distortions in the sample and producing a thermal distortion
signal representative of the thermal distortions in the sample includes a
piezoelectric transducer for detecting the thermal distortions in the
sample on the surface of the sample.
32. An apparatus according to claim 25, wherein the means for focusing the
intensity-modulated light beam on the surface of the sample to generate a
photoacoustic effect or a photothermal effect at the surface of the sample
is formed as a confocal optical system. |
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Claims |
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Description |
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CROSS-REFERENCES TO RELATED APPLICATIONS
U.S. patent applications Ser. No. 479,712 filed on Feb. 14, 1990, now U.S.
Pat. No. 5,062,715, and Ser. No. 567,319 filed on Aug. 14, 1990, both
relate to detecting a photoacoustic signal as does the present application
and are assigned to the assignee of the present application.
BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for detecting a
photoacoustic signal for detecting information relative to the surface and
subsurface of a sample using the photoacoustic or photothermal effect.
PRIOR ART
The photoacoustic effect was discovered by Tyndall, Bele, Rontgen, et al.
in 1881. As shown in FIG. 28, when intensity-modulated light (intermittent
light beam) 19 is irradiated to a sample 7 by focusing the light as an
excitation light with a lens 5, heat is generated in a light absorption
region V.sub.op 21 and periodically diffused through a heat diffusion
region V.sub.th 23 defined by a thermal diffusion length .mu.s 22 so that
the thermal distortion wave thus generated provides a thermoelastic wave.
By detecting this ultrasonic wave i.e. a photoacoustic wave by a
microphone (acoustic-electric converter) or a piezo-electric transducer to
obtain the component in synchronism with the modulation frequency of the
incident light, information relative to the surface and subsurface of the
sample can be obtained. A technique for detecting the above photoacoustic
signal is disclosed, for example, in "HIHAKAI KENSA", Vol. 36, No. 10 pp.
730-736, October 1987 (Showa 62) or IEEE; 1986 ULTRASONIC SYMPOSIUM--pp.
515-516 (1986).
Now, referring to FIG. 27, one example of such a technique will be
explained. A light beam emitted from a laser 1 is intensity-modulated by
an acousto-optical modulator (AOM) 2. The thus obtained intermittent light
is expanded to a parallel beam 19 of a desired diameter by a beam expander
3, which is reflected by a beam splitter or a half mirror 4 and thereafter
focused on the surface of a sample 7 placed on an XY stage 6 by a lens 5.
Then, the heat distortion wave created at a focusing position 21 on a
sample 7 generates a thermoelastic wave and also provides a minute
displacement on the surface of the sample 7. This minute displacement is
detected by a Michelson interferometer explained below. Parallel light
emitted from a laser 8 is expanded to a beam of a desired diameter by a
beam expander 9. This beam is separated into two optical paths by a beam
splitter or a half mirror 10. One is focused on the focusing position 21
on the sample 7 by a lens 5 whereas the other is irradiated to a reference
mirror 11. Then, the light reflected from the sample 7 and the light
reflected from the reference mirror 11 interfere with each other on the
beam splitter 10. The interference pattern thus formed is focused on a
photoelectric converting element 13 such as a photodiode through a lens 12
to provide a photoelectric-converted interference intensity signal. This
interference intensity signal is amplified by a preamplifier 14 and
thereafter applied to a lock-in amplifier 16. The lock-in amplifier 16,
using as a reference signal a modulation frequency signal from an
oscillator 15 used for driving the acousto-optical modulator 2, extracts
only the modulated frequency component contained in the interference
intensity signal. This frequency component has information relative to the
surface or inside of the same according to the frequency. By varying the
modulated frequency, the thermal diffusion length .mu.s 21 can be changed
and the information in a direction of the depth of the sample can be
obtained. If there is a defect such as a crack inside a thermal diffusion
region V.sub.th 23, the modulated frequency component in the interference
intensity signal changes to exhibit a signal change so that the presence
of the defect can be detected. An XY stage shifting signal and an output
signal from the lock-in amplifier 16 are processed by a computer 17.
Accordingly, the photoacoustic signals corresponding to the respective
positions on the sample can be displayed as two-dimensional image
information on a display 18 such as a monitor television.
The above-described prior-art technique is very efficient in that it
enables a photoacoustic signal to be detected in a non-contact and
non-destruction manner, but also has the following two problems.
A first problem is as follows. According to the conventional optical system
as shown in FIG. 27, it is necessary to irradiate the two beams
independently on the sample; an excitation light for generating a
photoacoustic effect on the surface and inside the sample, and a probe
light for detecting minute displacement on the sample surface generated by
the photoacoustic effect. Accordingly, in order to detect the
photoacoustic effect generated by the excitation light at high resolutions
and at high sensitivity, the excitation light and the probe light must be
focused on the same point on the sample with a precision of the submicron
order. However, it is extremely difficult to adjust the two light beams on
the optical axis in this high precision, and, for this purpose, it is
necessary to maintain the stability of the optical system at a high level,
which results in a very complex optical system and a complex peripheral
mechanism.
A second problem is as follows. In general, the signal intensity of a
photoacoustic signal is inversely proportional to the intensity modulation
frequency of an excitation light. The detection sensitivity of the
Michelson interferometer shown in FIG. 27 is inversely proportional to
.sqroot.f, where f is a variable frequency of the surface displacement, or
the intensity-modulated frequency of the excitation light. When a PZT
element is used for detecting a photoacoustic signal, the detection
sensitivity changes in accordance with a frequency characteristic of the
PZT element. Therefore, according to the conventional photoacoustic signal
detector, when defects at different depths within the sample have been
detected by changing the intensity modulation frequency of the excited
light and when an attempt is made to decide a size of each defect based on
the detected photoacoustic signal, it is difficult to quantitatively
decide the size of the defect because the signal intensity of the
photoacoustic signal changes depending on the modulation frequency as
described above.
SUMMARY OF THE INVENTION
In order to solve the above-described first problem, it is a first object
of the present invention to provide a method and an apparatus for
detecting a photoacoustic signal which enables state detection of
information on a surface and inside of a sample with a simple structure
without requiring an adjustment of relative optical axes of an excitation
light for generating a photoacoustic effect and of a probe light for
detecting minute displacement of the sample surface generated by the
photoacoustic effect.
In order to achieve the above object, in accordance with one aspect of the
present invention, of two beams of mutually different frequencies, one
beam is intensity-modulated and focused on a sample to generate a
photoacoustic effect, its reflection beam and the other beam are made to
interfere with each other to detect therefrom a modulation frequency
component which is double that of the other beam so that minute
displacements of the sample surface generated by the photoacoustic effect
are detected, and information of amplitude and phase corresponding to the
intensity-modulated frequency is extracted as a photoacoustic signal from
the detected minute displacement signal. With this arrangement, by using
one beam, it is possible to generate a photoacoustic effect and to detect
minute displacement of the sample surface generated from the photoacoustic
effect, and it is not necessary to carry out a relative adjustment of
optical axes of the excitation light and the probe light. Further, this
simple structure of the optical system enables a stable detection at high
sensitivity of the information relative to the surface and the inside of
the sample.
Further, according to the present invention, in order to achieve the above
object, a difference of frequencies between the two beams is made larger
than the above intensity-modulated frequency of the excitation light so as
to facilitate the extraction of the amplitude and phase information
corresponding to the intensity-modulated frequency from the interference
beam detection signal.
Further, in order to achieve the above object, according to the present
invention, a frequency shift is generated in only one of the excitation
beam and the probe beam to provide a relative optical frequency difference
between the two beams, providing a more simple structure of the optical
system that enables a stable and high-sensitivity detection of information
relative to the surface and inside of the sample.
Further, in order to achieve the above object, according to the present
invention, mutually different frequency shifts are generated in both beams
to provide a relative difference of optical frequency, enabling an
adjustment of an optical frequency difference and a setting of a lower
optical frequency difference, thus enabling a stable and high-sensitivity
detection of information relative to the surface and inside of the sample.
Further, in order to achieve the above object, according to the present
invention, an exciting unit for intensity-modulating one of two beams of
which frequencies are mutually different and focusing the
intensity-modulated beam on the sample and an optical interference
detecting unit for causing an interference between a reflected beam of the
focused beam and the other beam to generate minute displacement on the
surface of the sample by a photoacoustic effect and detecting this minute
displacement, are structured as a confocal optical system, to thereby
improve resolution of the photoacoustic signal in the lateral direction,
detection sensitivity and a signal SN ratio respectively.
In order to solve the second problem, it is a second object of the present
invention to provide a method and an apparatus for detecting a
photoacoustic signal which effectively corrects a signal intensity of the
photoacoustic signal corresponding to each modulation frequency so that
detection sensitivity or signal output intensity is always constant
regardless of the intensity-modulated frequency of the excited beam and
which enables a stable detection of information relative to the surface
and the inside of the sample and a quantitative analysis of the detected
information.
In order to achieve the above object, according to another aspect of the
present invention, the apparatus for detecting an photoacoustic signal
focuses an intensity-modulated beam on a sample, generates a photoacoustic
effect or a photothermal effect either on the surface or inside of the
sample, detects a thermal distortion on the sample surface generated by
the photoacoustic effect or the photothermal effect, detects frequency
components of an intensity modulation frequency from the detected signal,
and then extracts information relative to the surface or inside of the
sample according to the modulation frequency from the frequency
components. Further, the apparatus effectively adjusts and compensates the
detection sensitivity corresponding to each modulation frequency, to
enable a stable detection of information relative to the surface and
inside of the sample and a quantitative analysis of the detected
information.
Further, in order to achieve the above object, according to the present
invention, intensity of the intensity-modulated light is adjusted
corresponding to each modulation frequency so that detection sensitivity
is constant for each modulation frequency, thus reducing an influence of
non-optical noise.
Further, in order to achieve the above object, according to the present
invention, intensity of the detected signal is adjusted corresponding to
each modulation frequency so that detection sensitivity is constant for
each modulation frequency, thus increasing the stability of the optical
system.
Further, in order to achieve the above object, according to the present
invention, detection sensitivity, including modulation frequency
characteristics of a thermal distortion detecting unit for detecting the
thermal distortion, is adjusted so that it becomes constant for each
modulation frequency, thus improving the quantitativeness of the detected
signal.
Further, in order to achieve the above object, according to the present
invention, thermal distortion of the surface of the sample is detected by
using an optical interference so that a photoacoustic signal can be
detected in a non-contact state.
Further, in order to achieve the above object, according to the present
invention, thermal distortion of the surface of the sample is detected by
using a piezoelectric element, to simplify the structure of the detecting
system and improve stability of signal detection.
Further, in order to achieve the above object, according to the present
invention, an exciting unit for focusing an intensity-modulated light on a
sample and generating a photoacoustic effect or a photothermal effect
either on the surface or inside of the sample is structured as a confocal
optical system, thus improving the resolution of the photoacoustic signal
in the lateral direction, detection sensitivity and a signal SN ratio,
respectively.
Further, in order to achieve the above object, according to the present
invention, an optical interference detecting unit for detecting thermal
distortion on the surface of the sample generated by the photoacoustic
effect or the photothermal effect is structured as a confocal optical
system, thus improving the resolution of the photoacoustic signal in the
lateral direction, detection sensitivity and a signal SN ratio.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram for showing a photoacoustic detecting optical system
according to a first embodiment of the present invention;
FIG. 2 is a diagram for showing a polarization direction of an incident
beam in the first embodiment of the present invention;
FIG. 3 is a diagram showing the principle of an acousto-optical modulator;
FIGS. 4(a)-4(c) show waveform diagrams of light signals modulated to be
inputted to the acousto-optical modulator;
FIGS. 5(a)-5(b) are views for explaining a manner of shading the high-order
diffraction light components of a laser spot;
FIG. 6 is a block diagram for showing a phase detecting circuit;
FIG. 7 is a diagram for showing a frequency spectrum of an interference
intensity signal;
FIG. 8 is a diagram for showing a photoacoustic detecting optical system
according to a second embodiment of the present invention;
FIG. 9 is a diagram for showing a modulation signal to be inputted to the
acousto-optical modulator;
FIG. 10 is a diagram for showing a photoacoustic detecting optical system
according to a third embodiment of the present invention;
FIG. 11 is a diagram for showing a state of a two-frequency orthogonal
polarization in the third embodiment of the present invention;
FIG. 12 is a diagram for showing polarization directions of a reflected
beam from a sample, a reference beam and a beam from a polarization plate
in the third embodiment of the present invention;
FIG. 13 is a diagram for showing a photoacoustic detecting optical system
in the fourth embodiment of the present invention;
FIG. 14 is a diagram for showing polarization directions of a reflected
beam from a sample, a reference beam and a beam from a polarization plate
in the fourth embodiment of the present invention;
FIG. 15 is a block diagram for showing a photoacoustic detecting optical
system according to a fifth embodiment of the present invention;
FIG. 16 is a modulation frequency characteristic diagram for showing
intensity of a photoacoustic signal;
FIG. 17 is a modulation frequency characteristic diagram for showing
detection sensitivity of an interference optical system;
FIG. 18 is a characteristic diagram for showing a relationship between an
amplitude of a modulation signal and intensity of a first order
diffraction beam outputted from an acousto-optical modulator;
FIG. 19 is a modulation frequency characteristic diagram for showing
intensity of an excitation beam for compensation of modulation frequency
characteristics of a photoacoustic signal;
FIG. 20 is a modulation frequency characteristic diagram for showing
intensity of a photoacoustic signal after compensation of the sensitivity;
FIG. 21 is a diagram for showing modulation frequency characteristics of
the intensity of a photoacoustic signal before and after compensation of
the sensitivity and modulation frequency characteristics of non-optical
noise;
FIG. 22 is a block diagram for showing a photoacoustic detecting optical
system according to a sixth embodiment of the present invention;
FIGS. 23(a)-23(b) are diagrams showing a continuously variable ND filter
and a transmittance distribution diagram of the continuously variable ND
filter in the sixth embodiment of the present invention;
FIG. 24 is a block diagram for showing a photoacoustic detecting optical
system according to a seventh embodiment of the present invention;
FIG. 25 is a block diagram for showing a photoacoustic detecting optical
system according to an eighth embodiment of the present invention;
FIG. 26 is a modulation frequency characteristic diagram for showing
detection sensitivity of a PZT element in the eighth embodiment of the
present invention;
FIG. 27 is a block diagram for explaining a conventional photoacoustic
detecting optical system; and
FIG. 28 is a diagram for showing the principle of a photoacoustic effect.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments for achieving the first object of the present invention will be
explained with reference to FIGS. 1-3, 4(a)-4(b), 5(a)-5(b), and 6-14.
At first, the first embodiment of the present invention will be explained
with reference to FIGS. 1-3, 4(a)-4(b), and 5-7. FIG. 1 shows the
photoacoustic detecting optical system according to the first embodiment
of the present invention. This optical system includes of a heterodyne
type Mach-Zehender optical interferometric system 301 working as an
excitation optical system and a detecting optical system and a signal
processing system 300. In the system 301, a polarization direction of a
linear polarization beam 32 emitted from an Ar laser 31 is set to have an
angle of 45 degrees with respect to the X axis and Y axis respectively as
shown by 77 in FIG. 2. Assume the vertical direction relative to the paper
surface of FIG. 1 is expressed by the X axis and the direction orthogonal
to the X axis is expressed by the Y axis. Then, by a polarization beam
splitter 33, a p-polarization component 34 shown by 78 in FIG. 2 of an
incident beam 32 passes through the polarization beam splitter 33 and is
applied to an acousto-optical modulator 76. On the other hand, an
s-polarization component 35 shown by 79 in FIG. 2 is reflected by the
polarization beam splitter 33. The acousto-optical modulator 76 has a
structure having a combination of an ultrasonic wave transducer 80 with an
optical medium 86 such as glass or tellurium dioxide (TeO.sub.2), as shown
in FIG. 3. When a sinusoidal wave 83 with a frequency f.sub.B as shown in
FIG | | |
