 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for detecting a
photoacoustic signal to detect information relative to the surface and the
subsurface of a sample using photoacoustic or photothermal effect, and
more particularly to a method and an apparatus for detecting a
photoacoustic signal, devised to effectively correct abnormality of phase
shifts by phase jumps at specified points of the sample when a
photoacoustic signal is detected at those points of the sample.
The photoacoustic effect was discovered by Tyndall, Bell, Roentgen, et al.
in 1881. As shown in FIG. 2, 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 thermal diffusion
region V.sub.th 23 defined by a thermal diffusion length .mu..sub.S 22 so
that the resulting thermal distortion wave gives rise to a thermoelastic
wave (ultrasonic wave). By detecting this ultrasonic wave, i.e. a
photoacoustic wave by a microphone (acousto-electric converter) or by a
piezo-electric or a light interferometer to thereby obtain a signal
component synchronized with the modulated frequency of the excitation
light, information relative to the surface and subsurface of the sample
can be obtained. Incidentally, the thermal diffusion length .mu..sub.S 22
can be obtained by the following expression (1) from the thermal
conductivity k, density .rho., and specific heat c of the sample 7 when
the modulated frequency of the excitation light is denoted by f.sub.L.
##EQU1##
A technique for detecting the above photoacoustic signal is disclosed, for
example, in "HIHAKAI KENSA", Vol. 36, No. 10 issue, pp. 730-736 October
1987 (Showa 62) or IEEE 1986 ULTRASONIC SYMPOSIUM pp. 515-526 (1986).
Referring to FIG. 1, one example of such a technique will be explained. A
parallel light beam emitted from a laser 1 is intensity-modulated by an
acousto-optical modulator (A0 modulator) 2. The thus obtained intermittent
light is expanded to a parallel beam of a desired diameter by a beam
expander 3, which is reflected by a half mirror 4, and then focused by a
lens 5 on the surface of the sample 7 placed on an X-Y stage 6. The
thermal distortion wave emanating from the focusing position 21 on the
sample 7 generates a thermoelastic wave, thus causing minute displacements
at the surface of the sample. The minute displacements will be detected by
a Michelson interferometer explained below. After the parallel light beam
from the laser 8 is expanded to a desired beam diameter by the beam
expander 9, the beam is separated into two beams traveling along two
optical paths by a beam splitter or a half mirror 10. One is focused at
the focusing position 21 on the sample by the lens 5, while 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 at the half mirror 10. The interference light is focused
by a lens 12 on a photoelectric converting element 13 such as a photodiode
by a lens 12 to provide a photoelectric-converted interference intensity
signal. After amplified by a preamplifier 14, the interference intensity
signal is sent to a lock-in amplifier 16. Using a modulation frequency
signal from an oscillator 15 used to drive the acousto-optical modulator 2
as a reference signal, the lock-in amplifier 16 extracts only the
modulated frequency component contained in the interference intensity
signal. This frequency component has information relative to the surface
or the inside of the sample 7. According to the expression (1), by varying
the modulated frequency, the thermal diffusion length .mu..sub.S 21 can be
changed and information as to the condition through the depth of the
sample can be obtained. If there is a defect such as a crack in the
thermal diffusion region V.sub.th 23, the amplitude of the modulated
frequency component in the interference light intensity signal and the
phase thereof relative to the modulation frequency signal change, by which
the presence of the defect can be known. An X-Y stage shifting signal and
an output signal from the lock-in amplifier 16 are processed by a computer
17. Accordingly, the photoacoustic signal corresponding to the respective
points on the sample can be gathered and displayed as a two-dimensional
image on a display 18 such as a monitor television.
Though the above-mentioned prior-art technique is extremely effective means
for detecting a photoacoustic signal in non-contact and non-destructive
inspection of samples, but has the following problems.
In the conventional photoacoustic detection optical system as shown in FIG.
1, when a two-dimensional internal information of a sample is to be
obtained, it is necessary to perform a two-dimensional scanning of the
surface of a sample by a relative motion of two beams, that is, an
excitation light for generating a photoacoustic effect and a probe light
for detecting minute displacements of the sample surface caused by the
photoacoustic effect. This two-dimensional scanning is the so-called point
scanning by which information is obtained point by point, and therefore,
if one tries to scan the whole surface of the sample, a very large amount
of detection time is required. This necessity for a large amount of
detection time is the greatest reason why the photoacoustic detection
technique has not been applied to internal defect inspection of samples in
the production line. In some samples, the reflectance of the surface
varies with different positions of the sample. In such a case, with the
prior-art technique, the intensity of the reflected light of the probe
light unavoidably contains information relative to the surface reflectance
in addition to information about the internal condition, so that it is
difficult to accurately detect only information about the inside of the
sample. Furthermore, in some samples, the surface is not flat and has
local undulations. In this case, in the prior-art technique, the phase of
the reflected beam of the probe light varies according to the undulations
of the sample surface, so that the reflected light intensity includes
information with regard to the surface undulations in addition to internal
information and, as a result, it is difficult to accurately detect only
internal information about the internal condition.
Moreover, in the conventional photoacoustic signal detection method, there
is no way to cope with the phenomenon called "phase jump". Suppose a case
in which a photoacoustic signal is detected from a sample 200 having a
crack 109 in the surface, as shown in FIG. 3A, for example. FIG. 3B shows
a longitudinal sectional view taken along the line A--A' in FIG. 3A, while
FIG. 4A shows a phase shift image (hereafter referred to as the phase
image) of two-dimensional photoacoustic images of the sample 200. In the
phase image, the thin white region corresponds to the crack 109, and in
this region, there are several dark areas where the phase shift occurs
sharply. FIG. 4B shows a phase shift signal 111 in a section taken along
the line B--B' in FIG. 4A. As is apparent from this figure, there is a
sharp phase jump in the dark areas, and in those areas, the phase signal
111 shows a sharp drop and rebound.
Generally, out of various photoacoustic signals, the phase signal has a
characteristic that when the phase signal exceeds +.pi., the phase changes
-2.pi. relative to the phase value, and similarly, when the phase signal
exceeds -.pi., the phase changes 2.pi. relative to the phase value. In
this patent application, those changes are defined as the so-called "phase
jump". This phase jump phenomenon occurs in the extraction of phase shift
from the photoacoustic signal. More specifically, in the lock-in amplifier
16 in FIG. 1 the photoacoustic signal (interference intensity signal) is
separated into a cosine component X and a sine component Y as shown by the
expressions (1) and (2). The amplitude A and the phase .theta. of the
photoacoustic signal can be obtained by the expressions (3) and (4).
##EQU2##
As shown in the expression (4), the phase shift .theta. can be obtained as
an arctangent of a rate of the sine component Y to the cosine component X.
As is well known, the principal value of the arctangent in this case
exists in the range of +.pi. to -.pi.. Therefore, for example, when the
phase shift .theta. has a value (.pi.+.beta.), which exceeds +.pi., the
value output from the lock-in amplifier is -.pi.+.beta., that is,
.pi.+.beta. added with -2.pi.. Likewise, when the phase shift has a value
(-.pi.-.beta.) less than -.pi., the output value is .pi.-.beta., that is,
-.pi.-.beta. added with +2.pi.. In this way, a phase jump occurs.
Therefore, when a phase jump has occurred at a sampling point, the real
situation is that the phase shift at this sampling point does not contain
a correct phase shift information, and for this reason, information
relative to the surface and the inside of the sample cannot be obtained
securely.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a method and an
apparatus, in a simple arrangement, for detecting a photoacoustic signal,
which are less susceptible to effects of the reflectance distribution and
the undulations distribution and which enable high-speed detection of
two-dimensional internal information of a sample.
In order to achieve the above object, in accordance with the present
invention, an intensity-modulated light is irradiated to a plurality of
points being measured on a surface of a sample to generate a photoacoustic
effect or a photothermal effect either on the surface or the inside of the
plurality of points being measured, and at the same time, a light beam is
irradiated to a plurality of points being measured, the reflected light
and a reference light are made to interfere with each other, the resulting
interference light is detected by a detector in a conjugate relation with
the surface of the sample, said detector comprising a plurality of
photoelectric converting elements corresponding to the respective points
being measured, and from the detected interference light intensity signal,
thermal distortion of the frequency component equal to the above-mentioned
intensity-modulated frequency at the plurality of points being measured is
detected. This method enables almost simultaneous extraction of
information relative to the surface and the inside of a plurality of
points being measured of a sample, and also enables detection of a
photoacoustic signal at far higher speed than in the prior-art detection
method.
Further, in order to achieve the above object, according to the present
invention, the intensity-modulated light irradiated to the sample is
formed as a continuous and straight beam on the sample, and a plurality of
points being measured on the sample can be excited simultaneously, so that
a photoacoustic signal can be detected at far higher speed than in the
prior-art technique.
Further, in order to achieve the above object, according to the present
invention, the intensity-modulated light, irradiated to the sample, is
formed in a straight row of spot beams arranged on the sample, a plurality
of points being measured on the sample can be excited simultaneously, so
that a photoacoustic signal can be detected at far higher speed than in
the prior-art technique.
In order to achieve the above object, according to the present invention,
the above-mentioned spot beams in a row are arranged at such intervals
that thermal diffusion regions by the spot beams do not overlap one
another, thus improving the detection resolution of a photoacoustic image.
In order to achieve the above object, according to the present invention,
by using a detector comprising a plurality of storage type photoelectric
converting elements for detecting an interference light, a photoacoustic
signal can be detected at far higher speed than in the prior-art method.
In order to achieve the above object, according to the present invention,
by using a detector comprising a plurality of non-storage type
photoelectric converting elements for detecting an interference light, a
photoacoustic signal can be detected at far higher speed than in the
prior-art method.
In order to achieve the above object, according to the present invention,
by using a detector which outputs an interference light intensity signal
from a plurality of photoelectric converting elements as a one-dimensional
signal in time sequence, a photoacoustic signal can be detected at far
higher speed than in the prior-art method.
In order to achieve the above object, according to the present invention,
by using a detector which does parallel and simultaneous output of an
interference light intensity signal from a plurality of photoelectric
converting elements, a photoacoustic signal can be detected at far higher
speed than in the prior-art method.
In order to achieve the above object, according to the present invention,
by using a detector comprising a plurality of storage type photoelectric
converting elements to detect an interference light, the interference
light intensity signal, which is stored in a desired storage time and
output from each of the storage type photoelectric converting elements of
the detector, is detected 2n n:arbitrary integer, >0 times by shifting the
phase of the signal by .pi./n for each photoelectric converting element,
and according to 2n pieces of signal data, the thermal distortion of the
frequency component equal to the above-mentioned intensity-modulated
frequency is calculated, and thereby use of a digital frequency filtering
process is made possible instead of an analog frequency filtering process,
so that detection of a photoacoustic signal can be performed with high
sensitivity and high accuracy without being much affected by a high
harmonic component. Furthermore, it is possible with only one detector to
simultaneously detect a total of four items of information relative to the
surface and the inside of the sample, namely, the reflectance distribution
of the sample surface, the undulations distribution of the sample surface,
the amplitude distribution of the photoacoustic signal, and the phase
distribution of the photoacoustic signal, and thereby a high-speed complex
characterization of a sample is made possible. Moreover, according to the
present invention, it is possible to detect a photoacoustic signal in
which corrections have been made to the reflectance distribution of the
sample surface, the undulations distribution of the sample surface, and
the optical path fluctuation, so that information relative to the surface
and the inside of the sample can be detected with high sensitivity and
stability.
In order to achieve the above object, according to the present invention,
as a method for shifting the phase of the interference light intensity
signal output from the detector, comprising the storage type photoelectric
converting elements, by .pi./n for each photoelectric converting element,
a method is used in which a combination of desired values is set for the
difference in light frequency between the reflected light from the
above-mentioned sample surface and the reference light, the
above-mentioned intensity-modulated frequency, and the storage time of the
above-mentioned storage type photoelectric converting elements, and
therefore, by using only one detector, it is possible to simultaneously
detect a total of four items of information as to the surface and the
inside of the sample, including the sample surface reflectance
distribution, the sample surface undulations distribution, the
photoacoustic signal amplitude distribution, and the phase distribution of
the photoacoustic signal, and thereby a high-speed complex
characterization of a sample is made possible. Also, it is possible to
implement photoacoustic signal detection in which corrections have been
made to the reflectance distribution of the sample surface, the
undulations distribution of the sample surface, and the optical path
fluctuation, so that information relative to the surface and the inside of
a sample can be detected with high sensitivity and stability.
In order to achieve the above object, according to the present invention,
from the interference light intensity signal output in parallel form
simultaneously from the detector, the thermal distortion of the frequency
component equal to the above-mentioned intensity-modulated frequency is
detected with a plurality of photoelectric converting elements in parallel
simultaneously.
In order to achieve the above object, according to the present invention,
detection of the information of an interface inside the sample is possible
by setting the intensity-modulated frequency so that the thermal diffusion
length owing to the photoacoustic effect or the photothermal effect is
equal to or longer than the depth of the interface in the inside being
measured of the above-mentioned sample.
In a photoacoustic signal detection apparatus, by irradiating an
intensity-modulated light to a plurality of points being measured on the
surface of a sample, a photoacoustic effect or a photothermal effect can
be generated at a plurality of points being measured, and at the same
time, by irradiating the light to the plurality of points being measured
and causing a reflected light from the points being measured and a
reference light to interfere with each other, the resulting interference
light is detected by a detector, which is in a conjugate relation with the
sample surface, the detector comprising a plurality of photoelectric
converting elements corresponding to the points being measured, and from
the interference light intensity signal detected, the thermal distortion
of a frequency component equal to the above-mentioned intensity-modulated
frequency at the plurality of points being measured, and thereby
information relative to the surface and the inside at the plurality of
points being measured of the sample can be extracted substantially
simultaneously as a photoacoustic signal, so that detection of a
photoacoustic signal can be performed at far higher speed than in the
prior-art method.
Further, since the intensity-modulated light is irradiated to the sample in
a continuous, straight beam on the sample, the plurality of points being
measured on the sample can be excited simultaneously, and detection of a
photoacoustic signal can be performed at far higher speed than in the
prior-art method.
Further, since the intensity-modulated light is irradiated to the sample in
a row of spot beams arranged in a straight line on the sample, the
plurality of points being measured on the sample can be excited
simultaneously, and detection of a photoacoustic signal can be performed
at far higher speed than in the prior-art method.
Further, since the spot beams in a row are arranged at such intervals that
the thermal diffusion regions do not overlap each other, the photoacoustic
signal can be detected independently from the respective points being
measured, thereby improving the detection resolution of a photoacoustic
image.
Further, a detector comprising a plurality of storage type photoelectric
converting elements is used to detect an interference light, and
therefore, the photoacoustic signal at the plurality of points being
measured can be extracted substantially simultaneously, which enables
detection of a photoacoustic signal at far higher speed than in the
prior-art method.
Further, a detector comprising a plurality of non-storage type
photoelectric converting elements are used to detect an interference
light, and therefore, the photoacoustic signal at the plurality of points
being measured is extracted substantially simultaneously, which enables
detection of a photoacoustic signal at far higher speed than in the
prior-art method.
Further, a detector is used which outputs an interference light intensity
signal from a plurality of photoelectric converting elements as a
one-dimensional signal in time sequence, and therefore, the photoacoustic
signal at the plurality of points being measured can be extracted
substantially simultaneously, which enables detection of a photoacoustic
signal be performed at far higher speed than in the prior-art method.
Further, a detector is used which outputs an interference light intensity
signal from a plurality of photoelectric converting elements in parallel
simultaneously, and therefore, the photoacoustic signal at the plurality
of points being measured can be extracted substantially simultaneously,
which enables detection of a photoacoustic signal at far higher speed than
in the prior-art method.
Further, since a detector comprising a plurality of storage type
photoelectric converting elements is used to detect an interference light,
the interference light intensity signal, which is stored in a desired
storage time and output from each of the storage type photoelectric
converting elements of the detector, is detected 2n times by shifting the
phase of the signal by .pi./n for each photoelectric converting element,
and according to 2n pieces of signal data, the thermal distortion of the
frequency component equal to the above-mentioned intensity-modulated
frequency is calculated, and thereby use of a digital frequency filtering
process is made possible instead of an analog frequency filtering process,
so that detection of a photoacoustic signal can be performed with high
sensitivity and high accuracy without being much affected by a high
harmonic component. Furthermore, it is possible with only one detector to
simultaneously detect a total of four items of information relative to the
surface and the inside of the sample, namely, the reflectance distribution
of the sample surface, the undulations distribution of the sample surface,
the amplitude distribution of the photoacoustic signal, and the phase
distribution of the photoacoustic signal, and thereby a high-speed complex
assessment of a sample is made possible. Moreover, it is possible to
detect a photoacoustic signal in which corrections have been made on the
reflectance distribution of the sample surface, the undulations
distribution of the sample surface, and the optical path fluctuation.
Also, information relative to the surface and the inside of the sample can
be detected with high sensitivity and stability.
As a method for shifting the phase of the interference light intensity
signal output from the detector comprising the storage type photoelectric
converting elements by .pi./n for each photoelectric converting element, a
method is used in which a combination of desired values is set for the
difference in light frequency between the reflected light from the
above-mentioned sample surface and the reference light, the
above-mentioned intensity-modulated frequency, and the storage time of the
above-mentioned storage type photoelectric converting elements, and
therefore, by using only one detector, it is possible to simultaneously
detect a total of four items of information relative to the surface and
the inside of the sample, which includes the sample surface reflectance
distribution, the sample surface undulations distribution, the
photoacoustic signal amplitude distribution, and the phase distribution of
the photoacoustic signal, and thereby a high-speed complex
characterization of a sample is made possible. Also, it is possible to
implement photoacoustic signal detection in which corrections have been
made on the reflectance distribution of the sample surface, the
undulations distribution of the sample surface, and the optical path
fluctuation, so that information relative to the surface and the inside of
a sample can be detected with high sensitivity and stability.
Further, from the interference light intensity signal output in parallel
form simultaneously from the detector, the thermal distortion of the
frequency component equal to the above-mentioned intensity-modulated
frequency is detected with a plurality of photoelectric converting
elements in parallel simultaneously.
Further, the information of an interface inside the sample can be detected
by setting the intensity-modulated frequency so that the thermal diffusion
length owing to the photoacoustic effect or the photothermal effect is
equal to or longer than the depth of the interface in the inside being
measured of the above-mentioned sample.
Further, another object of the present invention is to provide a method and
an apparatus for detecting a photoacoustic signal, which enables
information relative to the surface and the inside of a sample to be
detected more stably and with higher accuracy by effectively correcting a
phase jump in a phase shift when the phase shift is extracted from the
photoacoustic signal.
The above object can be achieved by intensity-modulating a light beam from
a light source by a desired frequency, scanning, focusing and irradiating
the intensity-modulated light beam on a sample in two dimensions,
generating a photoacoustic effect or a photothermal effect on the surface
or the inside of the sample to cause a thermal distortion at the sample
surface, detecting the thermal distortion at each sampling point,
detecting the amplitude of the frequency component synchronized with the
above-mentioned intensity-modulated frequency and a phase shift thereof
relative to the above-mentioned intensity-modulated signal from the
detected signal, detecting the presence or absence of a phase jump in the
detected phase shift, correcting the abnormality of the phase shift signal
caused by the phase jump when a phase jump is detected in the phase shift,
and detecting information relative to the surface and the inside of the
sample according to the amplitude and the corrected phase shift.
More specifically, the above object can be achieved by detecting whether a
phase jump has occurred in the phase shift detected in each sampling point
according to a change in the phase shift, and effectively correcting the
abnormality of the phase shift caused by the phase jump at the sampling
point where the phase jump has occurred by adding or subtracting 2.pi. to
or from the phase shift value at the sampling point according to the
change of the phase shift.
Meanwhile, the photoacoustic signal detection apparatus comprises scanning,
focusing and irradiating means for intensity-modulating the light from a
light source by a desired frequency, scanning, focusing and irradiating
the intensity-modulated light on the sample in two dimensions, and causing
a thermal distortion on the sample surface by generating a photoacoustic
effect or a photothermal effect at the surface or the inside of the
sample; thermal distortion detecting means for detecting the thermal
distortion at the sampling point caused by the scanning, focusing and
irradiating means; amplitude and phase shift detecting means for detecting
the amplitude of the frequency component synchronized with the
intensity-modulated frequency and a phase shift thereof relative to the
intensity-modulated signal from the thermal distortion signal detected by
the detecting means; phase jump detecting means for detecting whether a
phase jump has occurred in the phase shift detected by the phase detecting
means; and correcting means for correcting the abnormality caused by the
phase jump when the phase jump is detected in the phase shift by the phase
jump detecting means, wherein information relative to the surface and the
inside of the sample is detected according to the amplitude detected by
the amplitude and detecting means and the phase shift corrected by the
correcting means.
More specifically, the above object can be achieved by arranging for the
phase jump detecting and correcting means of the photoacoustic signal
detection apparatus to effectively correct the abnormality in the phase
shift caused by the phase jump at the sampling point by detecting whether
a phase jump has occurred in the phase shift detected at each sampling
point according to a change of the phase shift, and adding or subtracting
2.pi. to or from the phase shift value at the sampling point where the
phase jump has occurred.
In other words, if the phase shift at some sampling point is recognized as
a sharp drop or rise compared with the phase shift at a nearby sampling
point under a certain condition, a decision is made that a phase jump
occurred in the phase shift at that sampling point, the phase shift at the
sampling point is corrected by adding 2.pi. or -2.pi. to the phase value
at the sampling point. By applying this correction process to the phase
shifts at the sampling points which include a phase jump, from the phase
shifts at those sampling points, information as to the surface and the
subsurface of the sample can be obtained with improved stability and
higher accuracy.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a structural example of the conventional
photoacoustic detection optical system;
FIG. 2 is a diagram showing the principle of a photoacoustic effect;
FIG. 3A is a plan view of a crack in the surface of a sample;
FIG. 3B is a cross-sectional view taken along the line A--A' in FIG. 3A;
FIG. 4A is a two-dimensional photoacoustic image (phase image) of a sample
having a surface crack;
FIG. 4B is a phase signal diagram taken along the line B--B' in FIG. 4A;
FIG. 5A is a structural diagram of the photoacoustic detection unit;
FIG. 5B is a diagram of a structural example of the processing unit in FIG.
5A;
FIGS. 6A, 6B, and 6C are diagrams showing modulating signals input to the
acousto-optical modulator element;
FIGS. 7A and 7B are structural diagrams of the excitation optical system
and the heterodyne interferometric optical system in a first embodiment of
the present invention;
FIG. 8 is a perspective view showing a planar structure of a sample, an
excitation beam and a probe beam in the first embodiment;
FIGS. 9A and 9B are diagrams showing the cross-sectional structures of the
sample and the generated states of a photoacoustic effect by the
excitation beam in a stripe form in the first embodiment;
FIGS. 10A and 10B are diagrams showing the polarization direction of the
laser beam incident on the heterodyne interference optical system and the
orthogonal-polarized beams of two frequencies;
FIGS. 11A and 11B are diagrams showing the incident state on the sample
surface of a probe beam in a stripe form in the first embodiment;
FIG. 12 is a diagram showing the polarization directions of the reflected
light from the sample, a reference light and a polarizing plate;
FIGS. 13A and 13B are structural diagrams of the detection unit of the
heterodyne interferometric optical system in the first embodiment;
FIG. 14 is a diagram showing data structure in a two-dimensional memory;
FIGS. 15A, 15B, and 15C are diagrams showing examples of detection of a
photoacoustic signal in the first embodiment;
FIGS. 16A, 16B, and 16C are diagrams of examples of a photoacoustic signal
showing the effects of phase correction in the first embodiment;
FIG. 17 is a diagram of the photoacoustic detection optical system in a
second embodiment of the present invention;
FIGS. 18A and 18B are diagrams showing structural examples of the
excitation optical system in the second embodiment;
FIG. 19 is a perspective view a planar structure of a sample, an excitation
beam, a probe beam, and a reference beam in the second embodiment;
FIGS. 20A and 20B are diagrams showing cross-sectional structures of the
sample and generated states of a photoacoustic effect by an excitation
beam in a stripe form in the second embodiment;
FIGS. 21A and 21B are diagrams showing incident states on the sample
surface of a probe beam and a reference beam both in a stripe form in the
second embodiment;
FIG. 22 is a structural diagram of the heterodyne interferometric optical
sys | | |
