JPH08285820A - Laser-ultrasonic inspection apparatus - Google Patents

Abstract

PURPOSE: To provide a laser ultrasonic inspection apparatus by which the sensitivity of a Doppler shaft detection means and an S/N ratio after a photoelectric conversion are enhanced and by which an ultrasonic echo can be detected easily. CONSTITUTION: A laser beam 11 for excitation is incident on an intensity distribution uniformization part 33, it is passed through the part so as to be changed into a laser beam 11a whose spectral intensity distribution is uniform, and the laser beam is shone at a surface region 35 on a sample 15. Thereby, wide- band ultrasonic waves are generated in the sample. In addition, the region 35 becomes a mirror face state by the uniform laser beam 11 for excitation. When a laser beam 34a for a probe is emitted to the surface region 35 which has been irradiated with the laser beam 11a for excitation, the laser beam is reflected at a high reflectance because the region has been set to the mirror face state. Consequently, a laser beam 34b, for a probe, which is subjected to the Doppler shift due to an ultrasonic echo when it is reflected is incident on a Doppler shift detection part 31, and the Doppler shift can be detected with high sensitivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser ultrasonic inspection apparatus for detecting an internal state of a sample in a non-contact manner by generating an ultrasonic wave in the sample using a laser beam and optically detecting the ultrasonic wave. It is about.

[0002]

2. Description of the Related Art One of the methods for detecting internal defects of various materials is a so-called laser ultrasonic method. For this, for example ultrasonic TECHNO5
It is described in detail in the monthly issue (vol.5, No.5, p38 (1993) Nippon Kogyo Shuppan). Since the laser ultrasonic method detects an ultrasonic wave using a laser beam, it can be used for a non-contact inspection for examining an internal state of a material or the like which cannot be inspected by contact. Therefore, for example, quality inspection in steel making process, welding inspection in steel frame processing, quality inspection inside ceramics, internal inspection of aircraft parts, other, metal,
It is expected to be applied to quality inspection of composite materials.

FIG. 3 is a schematic block diagram showing the configuration of an example of a laser ultrasonic inspection apparatus using this laser ultrasonic method. In the figure, an excitation laser 10 is a laser for exciting ultrasonic waves on the surface of a sample to be inspected, and a laser capable of emitting a pulsed laser beam having a relatively large output, for example, a Q-switch YAG laser is used. . The laser light 11 emitted from the excitation laser 10 is a beam splitter 12, a mirror 13,
The surface of the sample 15 to be inspected is irradiated via 14 When the surface of the sample 15 is irradiated with the laser light 11, the surface of the sample is instantaneously evaporated, and thermal stress or evaporation reaction force at that time generates a broadband ultrasonic wave. This ultrasonic wave propagates inside the sample, reaches the back surface of the sample, is reflected here, and then returns to the sample surface as an echo again.
A part of the laser light from the excitation laser 10 is guided to the photodetector 16 by the beam splitter 12 and detected, and is used as a trigger signal of the oscilloscope 17.

On the other hand, a probe light source 20 provided separately from the excitation laser 10 is a laser for detecting ultrasonic echoes returning to the surface of the sample 15, for example, a He-Ne laser having a stable frequency. Is used. The laser light 21 emitted from the probe light source 20 passes through the mirror 22, the polarization beam splitter 23, and the quarter wavelength plate 24, and the laser light 1 for excitation on the surface of the sample 15
It is irradiated in the vicinity where 1 is irradiated. This reflected light is
The light enters the Fabry-Perot interferometer 26 via the quarter-wave plate 24, the polarization beam splitter 23, and the lens 25.
The light transmitted through the Fabry-Perot interferometer 26 is converted into an electric signal by a photodetector 27 including a photodiode. This signal is amplified by amplifier 28,
After passing through a high pass filter 29 to remove low frequency noise, it is supplied to the oscilloscope 17.

FIG. 4 is a schematic diagram showing the result of measuring the optical frequency of He-Ne laser light by sweeping the resonator length (horizontal axis) of the Fabry-Perot interferometer, and FIG. It is the schematic which expanded and showed the horizontal axis. That is, the cavity length at which the transmitted light intensity reaches its peak is the wavelength ν of the He-Ne laser light.
Corresponding to (632.8 nm), when the resonator length is changed, the wavelength at which the transmitted light intensity of the interferometer peaks also changes, so the transmitted light intensity at the wavelength ν decreases. Utilizing this characteristic, He-
The transmitted light intensity with respect to the wavelength ν of the Ne laser light is A ′ in FIG.
The resonator length of the Fabry-Perot interferometer 26 is adjusted in advance so that the value becomes a value indicated by a point, and the reflected light from the sample 15 is incident on the Fabry-Perot interferometer 26.

When the surface of the sample 15 is ultrasonically vibrated by the ultrasonic echo reflected on the back surface of the sample, the spatial position of the sample surface is displaced in the cycle of this ultrasonic wave. Therefore, when the laser light from the probe light source 20 is reflected by the sample surface, the laser light undergoes a Doppler shift due to this displacement, and the wavelength changes. As shown in FIG. 5, when the reflected light from the sample 15 undergoes a frequency change of ± Δν ′ due to the Doppler shift, the intensity of the transmitted light also undergoes a change of ± ΔI ′ as shown in the figure. That is, the change in wavelength can be converted into the change in intensity by passing the reflected light from the sample 15 through the Fabry-Perot interferometer.

Therefore, the output signal of the photodetector 27 greatly changes every time an ultrasonic echo repeatedly reflected on the front and back surfaces of the sample 15 returns to the front surface. When this signal is displayed on the oscilloscope 17 with time on the horizontal axis and signal intensity on the vertical axis, if there is no defect in the sample, the signal is output at fixed time intervals determined by the speed of sound in the sample and the thickness of the sample. The intensity changes. On the other hand, if there is a defect such as a cavity or an impurity inside the sample, the ultrasonic wave is reflected by this defect portion, and the signal intensity changes in a time shorter than the above time. Therefore,
By examining this time interval, it is possible to examine the presence or absence of defects inside the sample.

[0008]

By the way, when the inspection object of the laser ultrasonic inspection apparatus is, for example, a steel material in the process of being processed by an iron making line of an iron mill,
The surface is in a so-called rough surface state having irregularities or an oxide film attached. When such a surface is irradiated with probe laser light, most of the light is scattered or absorbed, so that the ratio of reflected light, that is, the reflectance is reduced. When the reflected light is small, the sensitivity of the Doppler shift detecting means is lowered and the S / N ratio after photoelectric conversion is lowered, even if the reflected light is subjected to the Doppler shift due to the ultrasonic vibration of the sample surface. Is difficult to detect.

The present invention has been made based on the above circumstances, and the laser ultrasonic inspection in which the sensitivity of the Doppler shift detecting means and the S / N ratio after photoelectric conversion are improved and the ultrasonic echo can be easily detected. The purpose is to provide a device.

[0010]

According to a first aspect of the invention for solving the above-mentioned problems, the surface of a sample to be inspected is irradiated with excitation laser light to excite ultrasonic waves in the sample. Excitation laser light irradiating means, probe light irradiating means for irradiating a probe light for ultrasonic wave detection to the portion of the sample irradiated with the excitation laser light, and when the probe light is reflected by the sample And a Doppler shift detecting means for detecting the Doppler shift received by the device and converting it into light intensity.

According to a second aspect of the present invention, in the first aspect of the present invention, the excitation laser light irradiation means uniformizes the spatial intensity distribution of the excitation laser light with which the sample is irradiated. It is characterized by including means.

According to a third aspect of the present invention, in the second aspect of the invention, the intensity distribution uniformizing means is composed of a columnar dielectric that propagates the excitation laser light in its axial direction. Is.

According to a fourth aspect of the present invention, in the second aspect of the present invention, the intensity distribution uniformizing means comprises an optical fiber for propagating a laser beam for excitation.

[0014]

According to the invention described in claim 1, when the surface of the sample is irradiated with the exciting laser light, the extremely shallow portion of the surface of the sample is instantaneously evaporated, and the reaction force (evaporation reaction force) causes the sample to move. Ultrasound is generated inside. Also, due to this instantaneous evaporation, the portion of the sample surface irradiated with the excitation laser light is immediately modified from the rough surface state to the mirror surface state. Therefore, when the probe light is applied to the portion irradiated with the excitation laser light, most of the probe light is effectively reflected by the mirror surface. If a Doppler shift due to an ultrasonic echo is received during this reflection, a sufficient signal component is obtained in the Doppler shift detecting means, so that the sensitivity is improved and the S / N ratio of the signal after photoelectric conversion is improved.

According to the second aspect of the present invention, the laser light generally has a Gaussian intensity distribution in which the intensity is high in the central portion and decreases as it goes to the outside. By homogenizing the spatial intensity distribution of the excitation laser light that irradiates the sample surface by the method, compared to when the excitation laser light is irradiated without going through the intensity distribution equalizing means, a wider range of the sample surface can be obtained. Is mirrored. Therefore, the reflection of the probe light is increased, and thus the S / N ratio after photoelectric conversion is further improved.

According to a third aspect of the present invention, when the excitation laser light is incident on the dielectric parallel to the axis from the end of the columnar dielectric, the dielectric acts as a kind of optical waveguide. . That is, the excitation laser light incident with a divergence angle equal to or less than the critical angle for total reflection has several guided modes (0th order, 1st order, 2nd order ...) Determined by the wavelength, the cross-sectional size of the optical waveguide, and the refractive index. , N order).
Laser light in each guided mode propagates inside the dielectric while repeating total reflection at different angles.
As the light propagates, the guided modes are superposed on each other, and a laser beam having a spatial intensity distribution substantially flattened at the emission end face is obtained. When the sample surface is irradiated with the laser light thus flattened, a wide range of the sample surface is uniformly mirror-finished, so that the reflection of the probe light is increased and the S / N ratio after photoelectric conversion is significantly increased. improves.

According to the invention described in claim 4, when the exciting laser light is made incident on the incident end of the optical fiber, the laser light is repeatedly reflected many times on the side surface in the optical fiber and emitted. Reach the edge. For this reason, the laser light, which has different intensity depending on the location before being incident, passes through the optical fiber, and the spatial intensity distribution is substantially flattened. When the sample surface is irradiated with the laser light thus flattened, a wide range of the sample surface is uniformly mirror-finished, so that the reflection of the probe light is increased and the S / N ratio after photoelectric conversion is significantly increased. improves.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of an embodiment of the laser ultrasonic inspection apparatus of the present invention. Incidentally, in FIG. 1, the probe light irradiation unit 30 is the probe light source 2 of FIG.
0, the mirror 22, the polarization beam splitter 23, and the quarter-wave plate 24 are collectively shown. The Doppler shift detector 31 includes the quarter-wave plate 24, the lens 25, and the Fabry-Perot interferometer 26 of FIG. , Photodetector 27, amplifier 28,
The high-pass filter 29, the oscilloscope 17, and the photodetector 16 are shown collectively, and the excitation laser light irradiation unit 3 is shown.
2 is a pumping laser 10 and a beam splitter 1 shown in FIG.
2, the mirrors 13 and 14 are collectively shown. Further, in FIG. 1, an intensity distribution uniformizing section 33 is provided between the excitation laser beam irradiation section 32 and the sample 15. In addition, in this example, the sample 15 is assumed to be a steel material.

In FIG. 1, the excitation laser beam 11 is
It is emitted from the excitation laser 10 of FIG. 3, passes through a predetermined optical path, and then enters an intensity distribution uniformizing section 33 described later. The excitation laser light 11 before entering the intensity distribution uniformizing portion 33 has a Gaussian distribution in which the intensity is high in the central portion and decreases as it goes to the peripheral portion. Excitation laser beam 11a that has passed through the intensity distribution uniformizing section 33
Is a laser beam with a uniform spatial intensity distribution,
5 to the surface area 35. A region 35 of the surface of the sample 15 irradiated with the exciting laser beam 11a is instantaneously vaporized, and a broadband ultrasonic wave is generated by thermal stress or vaporization reaction force at that time.

The probe light irradiating section 30 irradiates the probe laser light 34 on the surface region 35 irradiated with the excitation laser light 11a. This is the main feature of the present invention. In the conventional laser ultrasonic inspection apparatus, since the surface of the sample to be measured is often mirror-polished, surface damage is caused by irradiating the sample with excitation laser light, and the damaged laser beam is used for the probe laser. Irradiation with light causes a decrease in the amount of reflected laser light for a probe. Therefore, the probe laser light is applied to the vicinity of the region 35 to which the excitation laser light 11a is applied. However, the present inventor has
When the probe laser light was irradiated to the excitation laser light irradiation region, it was found that the probe laser light was reflected at a high reflectance. As a result of investigating the reason, the sample surface, which was initially in a rough surface with irregularities,
It was found that this is because the mirror surface state is immediately modified at the moment when the exciting laser beam is irradiated.

Further, the inventor of the present invention has found that the exciting laser beam 11
Was attempted to be irradiated through the intensity distribution uniformizing section 33 shown in FIG. As a result, it was found that the mirror surface state of the sample surface became uniform over the entire region 35 irradiated with the excitation laser light 11a, and the reflectance of the probe laser light 34a was further improved.

The probe laser beam 34a is used for the sample 15
When reflected by the surface of the, it undergoes Doppler shift due to ultrasonic echo, but the reflectance is increased by making the reflecting surface a mirror surface, and therefore, the Doppler shift detection unit 31 detects the Doppler shift with high sensitivity, The final S / N ratio after photoelectric conversion is significantly improved.

Next, the intensity distribution uniformizing section 33 will be described. The applicant of the present invention filed an amorphous Si in a patent application (Japanese Patent Application No. 5-97134) dated March 31, 1993.
From this, we proposed a laser annealing method that can obtain uniform polycrystalline Si without unevenness. Among them, a transparent dielectric column was used as a means for flattening (uniformizing) the spatial intensity of laser light.

When laser light is made incident on one end face of this dielectric pillar at a spread angle of not more than the critical angle for total reflection, the laser light will have several guided modes determined by the wavelength, the cross-sectional size of the optical waveguide and the refractive index. Propagate in. These guided modes propagate in the dielectric column while repeating total reflection at different angles, so they are superposed as the guided modes propagate, and a perfectly flat laser beam is emitted at the exit end face. Is output. The intensity distribution uniformizing section 33 in FIG. 1 is configured by combining this dielectric pillar and an optical system such as a necessary lens.

In addition, the applicant of the present invention, in the patent application (Japanese Patent Application No. 5-264125) dated September 27, 1993,
A laser output uniformizing element as shown in FIGS. 2A to 2C was proposed. In these, a quartz prism is used as a dielectric column, and the incident end thereof is subjected to predetermined processing.

The quartz prism 40 shown in FIG. 2A has a concave lens 41 at the incident end thereof. The laser light 11 reaches the incident end as parallel light, but the concave lens 41
When incident on, the light is refracted at different angles depending on the distance from the central axis. Therefore, the laser light entering the inside of the quartz prism 40 propagates to the emission end 42 while repeating total reflection on the side surface thereof. Thus, by processing the incident end of the quartz prism 40 on the lens surface 41, the focal length of the lens can be made shorter than that of a lens provided separately.

The quartz prism 50 of FIG. 2B differs from that of FIG. 2A in that the incident end is processed into a convex lens 51.
By using a convex lens, the refraction of laser light is the opposite of that of a concave lens, but the laser light is
This is the same as in the case of the concave lens in that it is refracted at different angles depending on the distance from the central axis upon entering and enters the inside of the quartz prism 50, and the laser light is repeatedly totally reflected on the side surface to the exit end 52. Propagate.

The quartz prism 60 of FIG. 2C has a refractive index distribution region 61 instead of a lens at the incident end. The refractive index of the refractive index distribution region 61 is continuously changed and has a different refractive index depending on the place. Although the shape is the same as that of the quartz prism 60, by continuously changing the refractive index, the laser light incident on the refractive index distribution region 61 is refracted in the same manner as when it passes through the lens, and the quartz prism 60 is used.
Of the output end 6 while propagating to the inside of the
Reach 2.

When the quartz prism shown in FIGS. 2A and 2B is used, the reflection of the laser light inside the quartz prism can be brought close to the critical angle, so that the spatial intensity distribution of the laser light can be obtained with a short dimension. There are various advantages such as uniformization. Therefore, such a quartz prism is suitable as the intensity distribution uniformizing part 33 of FIG. In addition to this, for example,
As reported in Optics Comm.88, 59 (1992), the intensity of the central part of the laser beam is deformed so that the intensity of the central part of the laser beam is at both ends, and the intensity of both ends is the center. It is also possible to use a means for flattening the spatial intensity as the intensity distribution uniformizing section 33 by superimposing it on.

Like a conventional laser ultrasonic inspection apparatus,
When the probe laser of about 1 W is irradiated near the portion irradiated with the excitation laser light, the reflected light intensity is 10
It was about μW. On the other hand, as in the present embodiment, when the portion for which the excitation laser beam 11b whose spatial intensity is made uniform by using the intensity distribution uniformizing section 33 is irradiated, is also irradiated with the probe laser of about 1 W, The reflected light intensity was about 10 mW, and the reflectance was improved about 1000 times. With such an improvement in reflectance, the detection sensitivity of ultrasonic echoes in the Doppler shift detecting means and the S / N ratio after photoelectric conversion are greatly improved.

The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the gist thereof.
For example, in the above embodiment, the columnar dielectric is used as the intensity distribution uniformizing means, but instead, the intensity distribution uniformizing means may be configured by combining an optical fiber and a necessary optical system. The action and effect can be obtained. In this case, there is an advantage that the laser light for probe can also be irradiated at an arbitrary position by using the optical fiber.

[0032]

As described above, according to the present invention,
Since the probe light is applied to the portion of the sample surface that is being irradiated with the excitation laser light, the probe light is reflected by the surface that is mirror-finished by the excitation laser light,
The reflectance of the probe light is improved, and accordingly, the detection sensitivity of the ultrasonic echo and the S / N ratio after photoelectric conversion are improved. Furthermore, through the intensity distribution uniformizing means such as a dielectric pillar,
By irradiating the sample laser beam for excitation,
To provide a laser ultrasonic inspection apparatus in which the reflectance of probe light is further improved, and the detection sensitivity of ultrasonic echoes and the S / N ratio after photoelectric conversion are further improved, because the mirror state of the irradiation site becomes uniform. You can

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a quartz prism, which is a laser output uniformizing element.

FIG. 3 is a schematic block diagram showing a configuration of an example of a laser ultrasonic inspection apparatus using a laser ultrasonic method.

FIG. 4 is a schematic diagram showing light transmission characteristics of a Fabry-Perot interferometer.

FIG. 5 is a schematic diagram showing an enlarged characteristic of FIG.

[Explanation of symbols]

 10 Laser for Excitation 12 Beam Splitter 13, 14, 22 Mirror 15 Sample 16, 27 Photodetector 17 Oscilloscope 20 Probe Light Source 23 Polarization Beam Splitter 24 1/4 Wave Plate 25 Lens 26 Fabry-Perot Interferometer 28 Amplifier 29 High-pass Filter 30 Probe light irradiation unit 31 Doppler shift detection unit 32 Excitation laser light irradiation unit 33 Uniform intensity distribution unit

Claims (4)

[Claims] 1. Excitation laser light irradiating means for irradiating the surface of a sample to be inspected with excitation laser light to excite ultrasonic waves in the sample; and probe light for ultrasonic wave detection, A probe light irradiating means for irradiating a portion of the sample irradiated with laser light, and a Doppler shift detecting means for detecting a Doppler shift received when the probe light is reflected by the sample and converting it into a light intensity, A laser ultrasonic inspection apparatus comprising: 2. The excitation laser light irradiating means includes intensity distribution uniformizing means for uniformizing the spatial intensity distribution of the excitation laser light with which the sample is irradiated. Laser ultrasonic inspection equipment. 3. The laser ultrasonic inspection apparatus according to claim 2, wherein the intensity distribution uniformizing means is made of a columnar dielectric that propagates the excitation laser light in its axial direction. 4. The laser ultrasonic inspection apparatus according to claim 2, wherein the intensity distribution uniformizing means comprises an optical fiber for propagating a laser beam for excitation.